Beverly Eaves Perdue 

Governor 

Gwendolyn Keyes Fleming 

Regional Administrator 

USEPA, Region 4 

Sam Nunn Federal Center 

61 Forsyth Street, SW 

Atlanta, Georgia 30303-8960 

Director 

August 13,2012 

Dee Freeman 

Secretary 

Subject: State Implementation Plan Revisions incorporating Nitrogen Dioxide and Sulfur Dioxide National 

Ambient Air Quality Standards (NAAQS) 

Dear Ms. Fleming: 

Recent review of records indicates that US EPA Region 4 does not have North Carolina's submittal of 

its final rules 15A NCAC 02D .0407, Nitrogen Oxides, and .0402, Sulfur Dioxide, adopting the nitrogen dioxide 

(NOx) and sulfur dioxide (S02) NAAQS, respectively. Please find enclosed North Carolina's submittal of its 

final rules adopted by the Environmental Management Commission (EMC) July 14, 2011 . The rules were the 

subject of a public hearing held on May 4, 2011 and were approved by the State Rules Review Commission at its 

August 18, 2011 meeting. The rules became effective September 1, 2011. 

Enclosed is the hearing record for this public hearing and a copy of the rules for which approval is 

requested. The hard copy of the rules and hearing record is identical to the .pdf file recorded on the CD. Please 

proceed to approve the aforementioned rules as part of the federally-approved State Implementation Plan. 

If you need additional information or have any questions regarding this submittal, please contact Ms. 

J oelle Burleson of my staff at (919) 707-8720 or joelle.burleson @ncdenr.gov. 

Enclosures 

SCH/jb 

c: Nacosta Ward 

Lynorae Benjamin 

Sushma Masemore 

Hearing Record File 

1641 Mail Service Center, Raleigh, North Carolina 27699-1641 

217 West Jones Street, Raleigh, NC 27603 

Sincerely, 

~Cr~ Sheila C. Holman 

Phone: 919-707-8401 / FAX 919-715-0718 / lnternet: www.ncair.org 

An Equal Opportuniiy/Affirmative Action Employer 

N~¥th Carolina 

JVaturalllf 
STATE OF NORTH CAROLINA 

DEPARTMENT OF ENVIRONMENT AND NATURAL RESOURCES 

DIVISION OF AIR QUALITY 

REPORT OF PROCEEDINGS OF PUBLIC HEARINGS 

ON PROPOSED AMENDMENTS TO RULES 

15A NCAC 02D .0402 (SULFUR OXIDES), 

15A NCAC 02D .0407 (NITROGEN DIOXIDE) 

 

 

MAY 4, 2011 

RALEIGH, NC 27604 

ENVIRONMENTAL MANAGEMENT COMMISSION 

Attachment A A-1
This page was intentionally left blank. 

A-2
CONTENTS 

CHAPTER I Summaries and Recommendations I-1

Background and Summary I-1

Comments and Responses Thereto I-2

Hearing Officer’s Recommendation I-2

CHAPTER II Regulations Proposed for Adoption II-1

CHAPTER III Report of Proceedings III-1

Introduction III-1

Designation of Hearing Officer III-2

Public Notice III-3

Transcript III-5

CHAPTER IV Exhibits IV- 1

Proposed Regulations With Amendments as Presented at the 

 Hearing IV-2

Hearing Officer Comments at Hearing IV-4

CHAPTER V Written Comments During Comment Period V-1

Index of Commenters V-1

Comments V-2

CHAPTER VI Attachments VI-1

CHAPTER VII Documentation VII-1

A-3
This page was intentionally left blank. 

A-4
CHAPTER I 

BACKGROUND AND SUMMARY 

The Division of Air Quality (DAQ), under section 110 of the Clean Air Act (CAA), 42 U.S.C. 

7410, is required to submit a State Implementation Plan (SIP) for EPA approval that provides for 

the attainment and maintenance of sulfur dioxide (SO2) and nitrogen dioxide (NO2) National 

Ambient Air Quality Standards (NAAQS) through control programs directed at sources of SO2

and NO2 emissions. If DAQ fails to adopt and implement the required SIP by the time periods 

provided in the CAA, the EPA has responsibility under the CAA to adopt a Federal 

Implementation Plan (FIP) to assure that areas attain the NAAQS in an expeditious manner. 

Proposed amendment to Rule 15A NCAC 02D .0402, Sulfur Oxides. 

The purpose of the amendment is to adjust the ambient air quality standard to incorporate the 

United States Environmental Protection Agency (USEPA) standard for SO2. 

The proposed amendment updates the ambient air quality standard for SO2 by incorporating the 

strengthened primary National Ambient Air Quality Standard (NAAQS) for SO2 by replacing the 

current 24-hour and annual standard with a new 1-hour SO2 standard at a level of 75 (ppb), based 

on the 3-year average of the annual 99th percentile of 1-hour daily maximum concentrations. Two 

existing primary standards of 140 ppb that is evaluated over 24-hours, and 30 ppb annual standard 

that is evaluated over an entire year will be revoked one year after the effective date of initial 

designations under Section 107(d) of the Clean Air Act. The standard is attained at an ambient air 

quality monitoring site when the three-year average of the annual (99th percentile) of the daily 

maximum one-hour average concentrations is less than or equal to 75 ppb, as determined in 

accordance with Appendix T of 40 CFR Part 50. 

Proposed amendment to Rule 15A NCAC 02D .0407, Nitrogen Dioxide. 

The purpose of the amendment is to adjust the ambient air quality standard to reflect the United 

States Environmental Protection Agency (USEPA) standard for NO2. The USEPA revised the 

standard January 22, 2010 in 40 CFR 50 and 58. 

The proposed amendment updates the ambient air quality standard for NO2 by supplementing the 

existing annual standard for NO2 of 53 parts per billion (ppb) for a new short-term standard based 

I-1 A-5
on the 3-year average of the 98th percentile of the yearly distribution of 1-hour daily maximum 

concentrations. The level of this new standard is set at 100 ppb. 

PUBLIC COMMENTS AND RESPONSES THERETO

Comment: Mr. Richard A. Schutt, Chief, Air Planning Branch, offered no comments during the 

comment period in his letter dated May 26, 2011. 

CONCLUSION

Only one person submitted a letter offering no comments for the hearing record during the 

comment period. 

HEARING OFFICER'S RECOMMENDATION

The Hearing Officer recommends that the proposed amendments as presented in Chapter II of this 

hearing report be adopted by the Environmental Management Commission. 

I-2 A-6
1 15A NCAC 02D .0402 SULFUR OXIDES 

2 (a) The ambient air quality standards for sulfur oxides measured as sulfur dioxide are: 

3 (1) 80 micrograms per cubic meter (0.03 p.p.m.ppm) annual arithmetic mean, 

4 (2) 365 micrograms per cubic meter (0.14 p.p.m.ppm) maximum 24-hour concentration not to be exceeded 

5 more than once per year, 

6 (3) 1300 micrograms per cubic meter (0.5 p.p.m.ppm) maximum three-hour concentration not to be exceeded 

7 more than once per year. 

8 (b) Sampling and analysis shall be in accordance with procedures in Appendix A or A-1 40 CFR Part 50 or equivalent 

9 methods established under by a Federal Equivalent Method (FEM) designated in accordance with 40 CFR Part 53. 

10 (c) Applicability of the standards listed in Paragraph (a)(1) and (2) of this rule is in effect until one year after the effective 

11 date of initial designations under Section 107(d) of the Clean Air Act for the sulfur dioxide standard in Paragraph (d) of this

12 Rule.

13 (d) The primary one-hour annual ambient air quality standard for oxides of sulfur is 75 parts per billion (ppb, which is 1 part 

14 in 1,000,000,000), measured in the ambient air as sulfur dioxide.

15 (e) The one-hour primary standard is met at an ambient air quality monitoring site when the three-year average of the annual 

16 (99th percentile) of the daily maximum one-hour average concentrations is less than or equal to 75 ppb, as determined in 

17 accordance with Appendix T of 40 CFR Part 50.

 18 

19 History Note: Authority G.S. 143-215.3(a)(1); 143-215.107(a)(3); 

20 Eff. February 1, 1976; 

21 Amended Eff. July 1, 1984; December 1, 1976. 

 22 

 23 

 24 

II-1 CHAPTER II A-7 RULES PROPOSED FOR AMENDMENT
1 15A NCAC 02D .0407 NITROGEN DIOXIDE 

2 (a) The primary annual ambient air quality standard for nitrogen dioxide oxides of nitrogen is 0.053 parts per million 53 parts 

3 per billion (100 micrograms per cubic meter) annual arithmetic mean. average concentration measured in the ambient air as 

4 nitrogen dioxide. 

5 (b) Sampling and analysis shall be in accordance with procedures in Appendix F 40 CFR Part 50 or equivalent methods 

6 established under 40 CFR Part 53.

7 (b) The primary one hour ambient air quality standard for oxides of nitrogen is 100 parts per billion one hour annual average

8 concentration measured in the ambient air as nitrogen dioxide.

9 (c) The standards are attained when the annual arithmetic mean concentration in a calendar year is less than or equal to 0.053 

10 parts per million, rounded to three decimal places (fractional parts equal to or greater than 0.0005 parts per million are 

11 rounded up). To demonstrate attainment, an annual mean must be based on hourly data that are at least 75 percent complete 

12 or on data derived from manual methods that are at least 75 percent complete for the scheduled sampling days in each 

13 calendar quarter. 

14 (c) The secondary ambient air quality standard for nitrogen dioxide is 0.053 parts per million (100 micrograms per cubic 

15 meter) annual arithmetic mean concentration.

16 (d) Sampling and analysis shall be in accordance with:

17 (1) procedures in Appendix F 40 CFR Part 50; or

18 (2) by a Federal Equivalent Method (FEM) designated in accordance with 40 CFR, Part 53.

19 (e) The annual primary standard is attained when the annual average concentration in a calendar year is less than or equal to 

20 53 parts per billion, as determined in accordance with Appendix S of 40 CFR , Part 50 for the annual standard.

21 (f) The one hour primary standard is attained when the three-year average of the annual 98th percentile of the daily maximum 

22 one-hour average concentration is less than or equal to 100 ppb, as determined in accordance with Appendix S of 40 CFR,

23 Part 50 for one hour standard.

24 (g) The secondary standard is attained when the annual arithmetic mean concentration in a calendar year is less than or equal 

25 to 0.053 parts per million, rounded to three decimal places (fractional parts equal to or greater than 0.0005 parts per million

26 are rounded up). To demonstrate attainment, an annual mean must be based on hourly data that are at least 75 percent 

27 complete or on data derived from manual methods that are at least 75 percent complete for the scheduled sampling days in 

28 each calendar quarter. 

29 History Note: Authority G.S. 143-215.3(a)(1); 143-215.107(a)(3); 

30 Eff. February 1, 1976; 

31 Amended Eff. October 1, 1989; July 1, 1984; December 1, 1976. 

 32 

 33 

 34 

II-2 CHAPTER II A-8 RULES PROPOSED FOR AMENDMENT
CHAPTER III 

REPORT OF PROCEEDINGS 

Introduction 

The Department of Environment and Natural Resources, Division of Air Quality, held a public 

hearing on May 4, 2011, at 6:30 P.M. at DENR – Division of Air Quality Annex Training Room 

(AQ 526) 2728 Capital Blvd., Raleigh, NC 27604. 

The hearing considered the proposed amendments to Rules 15A NCAC 02D .0402, Sulfur 

Oxides, and 15A NCAC 02D .0407, Nitrogen Dioxide. The proposed effective date for these 

amendments is projected to be September 1, 2011. 

A public notice announcing this hearing was mailed to each person on the official mailing list for 

rule-making hearings. The public notice was also published in the North Carolina Register at 

least 15 days before the public hearing and in the Charlotte Observer, the Wilmington Star News, 

the Asheville Citizen Times, the Winston Salem Journal, and the Raleigh News and Observer at 

least 30 days prior to the public hearing. 

III-1 A-9
III-2 A-10

ENVIRONMENTAL MANAGEMENT COMMISSION 

NORTH CAROLINA 

DEPARTMENT OF ENVIRONMENT AND NATURAL RESOURCES 

Beverly Eaves Perdue, Governor 

Dee Freeman, Secretary 

MEMORANDUM 

TO: Mayor Darryl D. Moss 

March 24, 2011 

Donnie Brewer 

Yvonne C. Bailey 

Thomas F. Cecich 

Stan L. Crowe 

John S. Curry 

Marion Deerhake 

Tom Ellis 

William L. Hall, Jr. 

Ernest W. Larkin 

From: Stephen T. Smith ~~ ~ ~ 

Subject: Hearing Officer Appointment 

Stephen T. Smith 

Chairman 

Charles Peterson 

Vice Chairman 

Kevin Martin 

David H. Moreau 

Jeffrey V. Morse 

Darryl D. Moss 

David B. Peden 

Thomas H. Percival 

Dickson Phillips Ill 

Forrest R. Westall, Sr. 

A public hearing has been scheduled for May 4, 2011 at 6:30p.m. at the Division of Air Quality central 

office in Raleigh to receive public comments on amendments to the Nitrogen Dioxide and Sulfur Oxides 

rules to incorporate the revised National Ambient Air Quality Standards (NAAQS) for these pollutants 

into the state rules. The attached public notice describes the hearing's purpose. 

I am hereby appointing you to serve as hearing officer for this hearing. Please receive all relevant public 

comment and report your findings and recommendations to the Environmental Management Commission. 

Ms. Joe lie Burleson will provide staff support for you. 

If you have any questions, please feel free to contact Joelle Burleson at (919) 733-14 74, or me. 

SCH/jb 

Attachment 

cc: Sheila Holman 

Lois Thomas 

Hearing Record File 

An Equal Opportunity Affirmative Action Employer 
III-3 A-11

NORTH CAROLINA ENVIRONMENTAL MANAGEMENT COMMISSION 

PUBLIC NOTICE 

Notice is hereby given for a public hearing to be heard by the North Carolina 

Department of Environment and Natural Resources, Quality 

the proposed amendments of air quality 

DATES AND LOCATION: 

COMMENT PROCEDURES: 

INFORMATION: 

To receive comments on amendments 

15A NCAC 02D .0402, Sulfur Oxides, and 

020 Nitrogen incorporate 

the revised primary National Ambient Quality 

Standards (NAAQS) for sulfur ...... '-" ....... ~ ....... 

nitrogen dioxide (N02), promulgated by United "-JLLAILLI...:J 

Environmental Protection Agency (USEPA), into the 

state air quality rules. 

NOTE: The proposed amendments considered in this 

hearing, if adopted, will be effective statewide; if they 

are later adopted by a local air pollution control 

agency, then that agency will enforce them in its area 

of jurisdiction. 

May 4, 2011, 6:30P.M. 

DENR- Division of Air Quality, Central Office 

Air Quality Annex Training Room (AQ 526) 

2728 Capital Blvd., Raleigh, NC 27604 

All persons interested in these matters are invited to 

attend the public hearings. Any person desiring to 

comment is requested to submit a written 

statement for inclusion in the record of 

proceedings at the public hearing. The hearing 

officer may limit oral presentation lengths if many 

people want to speak. The hearing record will remain 

open until May 31, 2011 to receive additional written 

statements. To be included, the statement must be 

received_qy the Divj_§_ion by May_~1, 4_Q11. 

Copies of the proposed rule changes may be 

downloaded at http://daq.state.nc.us/Rules/Hearing/ 

Copies of the proposals may also be reviewed at the 

regional offices of the North Carolina Department of 

Environment and Natural Resources, Division of Air 

Quality, located at the following cities: 

Asheville 828/296-4500 
III-4 A-12

Fayetteville 

Mooresville 

Raleigh 

Washington 

Wilmington 

Winston-Salem 

910/433-3300 

704/663-1699 

919/791-4200 

252/946-6481 

910/796-7215 

336/771-5000 

Comments should be sent to and additional 

information concerning the or the 

may be obtained by contacting: 

Ms. Joelle Burleson 

Division of Air Quality 

1641 Mail Service 

Raleigh, North Carolina 27699-1641 

(919) 733-14 7 4Phone 

(919) 715-7 4 76 Fax 

joelle.burleson@ncdenr.gov 

Director 
Transcript

Transcripts of the May 4, 2011 hearings have not been prepared; however, an audio 

recording of each proceeding will be kept on file with the Division of Air Quality for one 

year from the date of the final actions by the Environmental Management Commission. 

A list of those attending the hearing as follows: 

Hearing Officer: Mr. Darryl D. Moss, member of the Environmental Management 

Commission 

Staff Members at both hearings: 

Ms. Joelle Burleson, DAQ, DENR 

Mr. Glenn Sappie, DAQ, DENR 

Mr. Steve Schliesser, DAQ, DENR 

Mr. Patrick Knowlson, DAQ, DENR 

Members of the General Public didn’t attend the hearing 

III-5 A-13
This page is intentionally blank 

A-14
CHAPTER IV 

EXHIBITS 

EXHIBIT PAGE

Proposed Regulations as Published in the Public Notice and Presented at the Hearing IV-2 

Hearing Officer Comments at the Public Hearing IV-4 

 

IV-1 A-15
PROPOSED RULES 

25:19 NORTH CAROLINA REGISTER APRIL 1, 2011 

2224 

Quality maintains a Geographic Information Systems data layer 

of the UWL. 

(t) The Schedule of Classifications and Water Quality Standards 

for the French Broad River Basin was amended effective 

September 1, 2009 with the reclassification of the entire 

watershed of Big Laurel Creek (Index No. 6-112) from source to 

the French Broad River from Class C Tr to Class C ORW Tr. 

(u) The Schedule of Classifications and Water Quality 

Standards for the French Broad River Basin was amended 

effective September 1, 2009 with the reclassification of the 

entire watershed of Spring Creek [Index No. 6-118-(1) and 6-

118-(27)] from source to the French Broad River from Class C 

Tr and Class C to Class C ORW Tr and Class C ORW. 

(v) The Schedule of Classifications and Water Quality 

Standards for the French Broad River Basin was amended 

July 1, 2011 with the reclassification of Boylston Creek 

[Index No. 6-52-(0.5)], from source to a point 0.3 mile 

upstream of Murray Branch and all tributaries to this 

portion of the Creek, from Class C to Class C Tr. 

(w) The Schedule of Classifications and Water Quality 

Standards for the French Broad River Basin was amended 

December 1, 2011 with the reclassification of a portion of the 

French Broad River [Index No. 6-(54.5)] from the confluence of 

the Mills River to a point 0.2 miles downstream of the 

confluence of the Mills River from Class B to Class WS-IV&B 

CA.

Authority G.S. 143-214.1; 143-215.1; 143-215.3(a)(1). 

* * * * * * * * * * * * * * * * * * 

Notice is hereby given in accordance with G.S. 150B-21.2 that 

the Environmental Management Commission intends to amend 

the rules cited as 15A NCAC 02D .0402 and .0407. 

Proposed Effective Date: September 1, 2011 

Public Hearing: 

Date: May 4, 2011 

Time: 6:30 p.m. 

Location: 2728 Capital Blvd, Air Quality Annex Training Room 

AQ-526, Raleigh , NC 27604 

Reason for Proposed Action: Rules 15A NCAC 02D .0402 and 

.0407 are proposed for amendment to incorporate the revised 

primary National Ambient Air Quality Standard (NAAQS) for 

sulfur dioxide (SO2) and nitrogen dioxide (NO2) promulgated 

by the United States Environmental Protection Agency into the 

state ambient air quality standards rules. 

Procedure by which a person can object to the agency on a 

proposed rule: If you have any objections to the proposed rule, 

please mail a letter including your specific reasons to: Ms. 

Joelle Burleson, Division of Air Quality, 1641 Mail Service 

Center, Raleigh, NC 27699-1641.

Comments may be submitted to: Joelle Burleson, Division of 

Air Quality, 1641 Mail Service Center, Raleigh, NC 27699-

1641; phone (919) 733-1474; fax (919) 715-7476; email 

joelle.burleson@ncdenr.gov 

Comment period ends: May 31, 2011 

Procedure for Subjecting a Proposed Rule to Legislative 

Review: If an objection is not resolved prior to the adoption of 

the rule, a person may also submit written objections to the 

Rules Review Commission after the adoption of the Rule. If the 

Rules Review Commission receives written and signed 

objections after the adoption of the Rule in accordance with G.S. 

150B-21.3(b2) from 10 or more persons clearly requesting 

review by the legislature and the Rules Review Commission 

approves the rule, the rule will become effective as provided in 

G.S. 150B-21.3(b1). The Commission will receive written 

objections until 5:00 p.m. on the day following the day the 

Commission approves the rule. The Commission will receive 

those objections by mail, delivery service, hand delivery, or 

facsimile transmission. If you have any further questions 

concerning the submission of objections to the Commission, 

please call a Commission staff attorney at 919-431-3000. 

Fiscal Impact: A copy of the fiscal note can be obtained 

from the agency.

 State 

 Local 

 Substantial Economic Impact (>$3,000,000) 

 None 

CHAPTER 02 - ENVIRONMENTAL MANAGEMENT 

SUBCHAPTER 02D - AIR POLLUTION CONTROL 

REQUIREMENTS 

SECTION .0400 - AMBIENT AIR QUALITY STANDARDS 

15A NCAC 02D .0402 SULFUR OXIDES 

(a) The ambient air quality standards for sulfur oxides measured 

as sulfur dioxide are: 

(1) 80 micrograms per cubic meter (0.03 p.p.m.

ppm) annual arithmetic mean, 

(2) 365 micrograms per cubic meter (0.14 p.p.m.

ppm) maximum 24-hour concentration not to 

be exceeded more than once per year, 

(3) 1300 micrograms per cubic meter (0.5 p.p.m.

ppm) maximum three-hour concentration not 

to be exceeded more than once per year. 

(b) Sampling and analysis shall be in accordance with 

procedures in Appendix A or A-1 40 CFR Part 50 or equivalent 

methods established under by a Federal Equivalent Method 

(FEM) designated in accordance with 40 CFR Part 53. 

(c) Applicability of the standards listed in Subparagraph (a)(1) 

and (2) of this Rule is in effect until one year after the effective 

date of initial designations under Section 107(d) of the Clean Air 

Act for the sulfur dioxide standard in Paragraph (d) of this Rule.

(d) The primary one-hour annual ambient air quality standard 

for oxides of sulfur is 75 parts per billion (ppb, which is 1 part in 

1,000,000,000), measured in the ambient air as sulfur dioxide.

IV-2 A-16
PROPOSED RULES 

25:19 NORTH CAROLINA REGISTER APRIL 1, 2011 

2225 

(e) The one-hour primary standard is met at an ambient air 

quality monitoring site when the three-year average of the 

annual (99th percentile) of the daily maximum one-hour average 

concentrations is less than or equal to 75 ppb, as determined in 

accordance with Appendix T of 40 CFR Part 50.

Authority G.S. 143-215.3(a)(1); 143-215.107(a)(3). 

15A NCAC 02D .0407 NITROGEN DIOXIDE 

(a) The primary annual ambient air quality standard for nitrogen

dioxide oxides of nitrogen is 0.053 parts per million 53 parts per 

billion (100 micrograms per cubic meter) annual arithmetic 

mean. average concentration measured in the ambient air as 

nitrogen dioxide. 

(b) Sampling and analysis shall be in accordance with 

procedures in Appendix F 40 CFR Part 50 or equivalent methods 

established under 40 CFR Part 53.

(b) The primary one-hour ambient air quality standard for 

oxides of nitrogen is 100 parts per billion one hour annual 

average concentration measured in the ambient air as nitrogen 

dioxide.

(c) The standards are attained when the annual arithmetic mean 

concentration in a calendar year is less than or equal to 0.053 

parts per million, rounded to three decimal places (fractional 

parts equal to or greater than 0.0005 parts per million are 

rounded up). To demonstrate attainment, an annual mean must 

be based on hourly data that are at least 75 percent complete or 

on data derived from manual methods that are at least 75 percent 

complete for the scheduled sampling days in each calendar 

quarter.

(c) The secondary ambient air quality standard for nitrogen 

dioxide is 0.053 parts per million (100 micrograms per cubic 

meter) annual arithmetic mean concentration.

(d) Sampling and analysis shall be in accordance with:

(1) procedures in Appendix F 40 CFR Part 50; or

(2) by a Federal Equivalent Method (FEM) 

designated in accordance with 40 CFR, Part 

53.

(e) The annual primary standard is attained when the annual 

average concentration in a calendar year is less than or equal to 

53 parts per billion, as determined in accordance with Appendix 

S of 40 CFR , Part 50 for the annual standard.

(f) The one hour primary standard is attained when the threeyear average of the annual 98th percentile of the daily maximum 

one-hour average concentration is less than or equal to 100 ppb, 

as determined in accordance with Appendix S of 40 CFR, Part 

50 for one hour standard.

(g) The secondary standard is attained when the annual 

arithmetic mean concentration in a calendar year is less than or

equal to 0.053 parts per million, rounded to three decimal places 

(fractional parts equal to or greater than 0.0005 parts per million 

are rounded up). To demonstrate attainment, an annual mean 

must be based on hourly data that are at least 75 percent 

complete or on data derived from manual methods that are at 

least 75 percent complete for the scheduled sampling days in 

each calendar quarter.

Authority G.S. 143-215.3(a)(1); 143-215.107(a)(3). 

IV-3 A-17
Hearing Officer's Suggested Hearing Comments 

INTRODUCTION

[Hearing officer]: 

 Good evening ladies and gentlemen. My name is Darryl Moss. I am a member of 

the North Carolina Environmental Management Commission. My role as hearing 

officer is to listen to all relevant comment on these proceedings and report them to 

the full commission. Sitting with me is Ms. Joelle Burleson. She is with the North 

Carolina Division of Air Quality, Planning Section. 

 Some of the staff from the Division of Air Quality are here to assist. Ms. Burleson, 

please introduce the staff present. 

[Ms. Burleson] (Introduces staff) 

[Hearing officer]: 

 This evening we are conducting a hearing on amendments to the state ambient air 

quality standards rules to incorporate the revised National Ambient Air Quality 

Standards (NAAQS) for nitrogen dioxide (NO2) and sulfur dioxide (SO2). 

Incorporation of the NAAQS into the state standards is one of the first steps in 

implementation of the revised NAAQS. Separate from tonight’s hearing, the Division 

of Air Quality has been taking comment on another early step in the implementation 

process for SO2 which is development of the State recommendations for attainment 

and nonattainment boundary options. That comment period on the draft 

recommendations closed on April 30, 2011. 

IV-4 A-18
 This hearing will be held according to the North Carolina Administrative 

Procedures Act. The public notice for the hearing has been advertised in the North 

Carolina Register and five newspapers in the state. Notice has been sent to those on 

the official DAQ mailing list. I will enter the public notice and the proposed 

amendments and adoptions into the hearing record without reading them at this 

time. 

 Any person desiring to comment is requested to submit a written statement for 

inclusion into the hearing record. Once called to speak, please come to the podium 

and state your name clearly, identify the rule or rules you are commenting on, and 

whom you represent. 

[HEARING]: 

 I will now open the hearing and take relevant comments on amendments to 

incorporate the revised National Ambient Air Quality Standards for nitrogen dioxide 

into 15A NCAC 02D .0407, Nitrogen Dioxide, and for sulfur dioxide into 15A 

NCAC 02D .0402, Sulfur Oxides. Staff will now provide a brief overview of the 

revised standards. 

[Staff:] Overview Presentation 

[hearing officer:] Additional background information on these proposed rule changes 

is available on the Division of Air Quality website. The focus of tonight’s hearing is 

on incorporation of the revised NAAQS themselves into the state ambient air quality 

standards rules. 

-------------------------------------------------------------------------------------------------------- 

IV-5 A-19
3

[Hearing officer]: Optional Time Limit 

 Many people have requested to speak at this hearing. Due to time constraints, 

speakers’ presentations will be limited to ___ minutes. Speakers are requested to 

submit a written statement by the close of the comment period for inclusion into the 

hearing record. 

-------------------------------------------------------------------------------------------------------- 

[Hearing officer]: 

 I will now take any comments that you may have. 

[SPEAKERS] 

[Hearing officer]: 

 Is there anyone else who would like to comment? If there are no more comments, 

then this hearing is closed. The hearing record will remain open until May 31, 2011, 

for additional written comments. 

 Thank you for participating in tonight’s hearing. 

IV-6 A-20
V-1 

CHAPTER V 

 COMMENTS RECEIVED AT THE PUBLIC HEARING 

 AND DURING THE COMMENT PERIOD 

 INDEX OF COMMENTORS 

NAME REPRESENTING PAGE

Mr. Richard A. 

Schutt USEPA, Region 4 V-2 

A-21
V-2 A-22

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

REGION 4 

Ms. Sheila C. Holman, Director 

Division of Air Quality 

ATLANTA FEDERAL CENTER 

61 FORSYTH STREET 

ATLANTA, GEORGIA 30303-8960 

May 26,2011 

North Carolina Department of Environment 

and Natural Resources 

1641 Mail Service Center 

Raleigh, North Carolina 27699-1641 

Dear Ms. Holman: 

Thank you for your letter dated March 29, 2011, transmitting a prehearing package regarding the 

revisions to the North Carolina State Implementation Plan (SIP) revising 15A NCAC 02D.0402- Sulfur 

Oxides (S02) and 15A NCAC 02D.0407- Nitrogen Dioxide (N02). Specifically, the SIP revision 

incorporates the revised primary National Ambient Air Quality Standards for S02 and N02 promulgated 

by the U.S. Environmental Protection Agency on June 22, 2010, and February 9, 2010, respectively. 

These proposed revisions will be the subject of a public hearing, if requested, with written comments 

due by the close of business on May 31, 2011. We have completed our review of the submittal and offer 

no comments at this time. 

We look forward to continuing to work with you and your staff. If you have any questions, please 

contact Jane Spann, Acting Chief, Regulatory Development Section at (404) 562-9029, or have your 

staff contact Nacosta C. Ward at (404) 562-9140. 

Air Planning Branch 

Internet Address (URL) • http://www.epa.gov 

Recycled/Recyclable • Printed wHh Vegetable Oil Based Inks on Recycled Paper (Minimum 30% Postconsumer) 
VI-1 

CHAPTER VI 

INDEX OF ATTACHMENTS 

ATTACHMENTS PAGE

Sulfur Dioxide Amendments Economic Analysis VI-2 

Nitrogen Dioxide Amendments Economic Analysis VI-4 

Summary Presentation on Nitrogen Dioxide and Sulfur Dioxide Ambient Air 

Quality Standards Amendments 

VI-7 

Federal Register - Final National Ambient Air Quality Standard for Sulfur Dioxide 

Rule 

VI-18 

Federal Register - Final National Ambient Air Quality Standard for Nitrogen 

Dioxide Rule 

VI-103 

A-23
1 

ECONOMIC ASSESSMENT 

Revision of Ambient Standards for Sulfur Dioxide (SO2) 

I. Basic Information 

Contact Information 

1. Division of Air Quality File Number (495)

2. Analyst: Vladimir Zaytsev

vladimir.zaytsev@ncmail.net

3. Office DENR, Air Quality Planning, Rule 

Development Branch

4. Phone 919/733-1468 

5. Comments to Agency Contact Joelle Burleson, 919/733-1474

Joelle.Burleson@ncdenr.gov

6. Title of the Proposed Rule Sulfur dioxide ambient air quality standards

7. Rule Citations 15A NCAC 02D .0402

8. Brief Description of the Proposed Rule 

Changes

Updates to sulfur dioxide ambient air quality 

standards to match changes made by the 

United States Environmental Protection 

Agency

9. Rule Category Division 1 – Regulatory Behavior

10. Authorizing Statute G.S. 143-215.3(a)(1); 143-215.107(a)(3) 

Screening Analysis 

Rule Actions General Statute

Yes 

or 

No

11. Federal Law Certification Required - Does the proposed rule purport 

to implement a federal law requiring a certification statement by the 

rulemaking coordinator? 

NCGS 150B-21(f)(1) Yes

12. Temporary Rules - Does this rule meet the criteria listed relating to 

temporary rules? NCGS 150B-21.1 No

13. Emergency Rules - Does this rule meet the criteria relating to 

emergency rules? NCGS 150B-21.1(a) No

14. Establishes or Increases Fees - Is the agency required to comply with 

the requirements of G.S. 12-3.1?

NCGS 150B-12-3.1

NCGS 150B-21.3(c1) No

15. State Funds - Does the proposed rule require the expenditure or 

distribution of funds subject to the Executive Budget Act, Article 1 of 

Chapter 143?

NCGS 150B-21.4(a) Yes

16. NC DOT Analysis - Does the proposed rule affect NC DOT 

environmental permitting that will result in an increased cost? NCGS 150B-21.4(a1) No

17. Local Government Unit Funds - Does the proposed rule affect the 

expenditures or revenues of a unit of local government? NCGS 150B-21.4(b) Yes

18. Substantial Economic Impact Analysis - Federal Rule Exemption -

Does this rule meet the criterion of Federal Exemption?

NCGS 150B21.4(b1) Yes

19. Technical Change - Are only technical changes being incorporated 

such that public notice and rulemaking hearings are not required? NCGS 150B-21.5 No

20. Repeal of Regulatory Deadwood - Is the rule obsolete? NCGS 150B-2(1b)

NCGS 150B-21.5(b) No

VI-2 A-24
2 

II. Certification Statement 

On June 2, 2010, EPA strengthened the primary National Ambient Air Quality Standard 

(NAAQS) for sulfur dioxide (SO2) by replacing the current 24-hour and annual standard with 

a new 1-hour SO2 standard at a level of 75 (ppb), based on the 3-year average of the annual 

99th percentile of 1-hour daily maximum concentrations. EPA is revoking the two existing 

primary standards of 140 ppb that is evaluated over 24-hours, and 30 ppb annual standard 

that is evaluated over an entire year because there is little health evidence to suggest an 

association between long-term exposure to SO2 and health effects. The new 1-hour standard 

applies only to the primary standard set to protect public health, especially for children, the 

elderly, and people with asthma. More details found on EPA’s regulatory action websites: 

Final Rule (PDF) (85pp, 624k) - Federal Register, June 22, 2010 

Fact Sheet (PDF) (6pp, 43k) 

Regulatory Impact Analysis

Under section 110 of the CAA, 42 U.S.C. 7410, North Carolina is required to submit, for 

EPA approval, State Implementation Plan (SIP) that provide for the attainment and 

maintenance of SO2 standards through control programs directed at sources of SO2

emissions. If North Carolina fails to adopt and implement the required SIP by the time 

periods provided in the CAA, the EPA has responsibility under the CAA to adopt a Federal 

Implementation Plan (FIP) to assure that areas attain the NAAQS in an expeditious manner. 

The Division of Air Quality (DAQ), Ambient Monitoring section estimates that new sulfur 

dioxide monitoring requirements may result in eight required sulfur dioxide population 

weighted emission index (PWEI) monitors in the state of North Carolina. These eight PWEI 

monitors will be located in seven Metropolitan Statistical Areas (MSAs): Asheville, 

Charlotte-Concord-Gastonia, Durham-Chapel Hill, Greensboro-High Point, Hickory, 

Wilmington, and Winston-Salem. The North Carolina Division of Air Quality (NC DAQ) 

expects three existing monitors to meet the requirements for PWEI monitoring. These 

monitors are located in Forsyth, Mecklenburg, and New Hanover Counties. Thus, five new 

PWEI monitoring sites will be required. The NC DAQ plans to use existing sites to house as 

many of these PWEI monitors as possible to reduce the cost of implementation of the rule. 

These five new monitors will be located in the Asheville, Charlotte-Concord-Gastonia, 

Durham-Chapel Hill, Greensboro-High Point, and Hickory MSAs. 

The DAQ has enough equipment to set up five additional sulfur dioxide monitoring sites in 

buildings at existing sites and estimates it will cost $63,450 to set up five PWEI monitoring 

sites. In addition, the NCDAQ estimates it will cost $107,000 a year for supplies and labor to 

operate the five new sites. Sometime within the next five years, DAQ anticipates the 

monitoring equipment will need to be replaced at an estimated cost of $135,000. 

Although it is uncertain whether State or units of local government funds are affected, the 

amendment to the Rule 15A NCAC 02D .0402 does purport to implement a federal 

regulation, and therefore an impact statement is not required (G.S. 150B-21.4(b1). The 

agency submits a certification statement by the rule-making coordinator, accordingly. 

VI-3 A-25
1 

ECONOMIC ASSESSMENT 

Revision of Ambient Standards for Nitrogen Dioxide (NO2) 

I. Basic Information 

Contact Information 

1. Division of Air Quality File Number (494)

2. Analyst: Vladimir Zaytsev

vladimir.zaytsev@ncmail.net

3. Office DENR, Division of Air Quality, Planning 

Section, Rules Development Branch

4. Phone 919/ 715-4398

5. Comments to Agency Contact Joelle Burleson, 919/733-1474

Joelle.burleson@ncdenr.gov

6. Title of the Proposed Rule NITROGEN DIOXIDE

7. Rule Citations 15A NCAC 02D .0407

8. Brief Description of the Proposed Rules

Updates to nitrogen dioxide ambient air quality 

standards to match changes made by the 

United States Environmental Protection 

Agency

9. Rule Category Division 1 – Regulatory Behavior

10. Authorizing Statute G.S. 143-215.3(a)(1); 143-215.107(a)(3)

Screening Analysis 

Rule Actions General Statute

Yes 

or 

No

10. Federal Law Certification Required - Does the proposed rule purport 

to implement a federal law requiring a certification statement by the 

rulemaking coordinator? 

NCGS 150B-21(f)(1) Yes

11. Temporary Rules - Does this rule meet the criteria listed relating to 

temporary rules? NCGS 150B-21.1 No

12. Emergency Rules - Does this rule meet the criteria relating to 

emergency rules? NCGS 150B-21.1(a) No

13. Establishes or Increases Fees - Is the agency required to comply with 

the requirements of G.S. 12-3.1?

NCGS 150B-12-3.1

NCGS 150B-21.3(c1) No

14. State Funds - Does the proposed rule require the expenditure or 

distribution of funds subject to the Executive Budget Act, Article 1 of 

Chapter 143?

NCGS 150B-21.4(a) Yes

15. NC DOT Analysis - Does the proposed rule affect NC DOT 

environmental permitting that will result in an increased cost? NCGS 150B-21.4(a1) No

16. Local Government Unit Funds - Does the proposed rule affect the 

expenditures or revenues of a unit of local government? NCGS 150B-21.4(b) Yes

17. Substantial Economic Impact Analysis - Federal Rule Exemption -

Does this rule meet the criterion of Federal Exemption?

NCGS 150B21.4(b1) Yes

18. Technical Change - Are only technical changes being incorporated 

such that public notice and rulemaking hearings are not required? NCGS 150B-21.5 No

19. Repeal of Regulatory Deadwood - Is the rule obsolete? NCGS 150B-2(1b)

NCGS 150B-21.5(b) No

VI-4 A-26
2 

II. Certification Statement 

On January 22, 2010, Environmental Protection Agency (EPA) strengthened the health-based 

National Ambient Air Quality Standard (NAAQS) for nitrogen dioxide (NO2). EPA is 

supplementing the existing annual standard for NO2 of 53 parts per billion (ppb) by 

establishing a new short-term standard based on the 3-year average of the 98th percentile of 

the yearly distribution of 1-hour daily maximum concentrations. EPA is setting the level of 

this new standard at 100 ppb. EPA is making changes in data handling conventions for NO2

by adding provisions for this new 1-hour primary standard that new standard will protect 

public health, including the health of sensitive populations - people with asthma, children, 

and the elderly. EPA is also retaining, with no change, the current annual average NO2

standard of 53 ppb. 

For additional information on development of its national ambient air quality standards 

(NAAQS) for nitrogen dioxide see the following EPA websites:

Final Rule (PDF) (65pp, 418k) - Federal Register - Feb 9, 2010 

Fact Sheet (PDF) (5pp, 35k)

Regulatory Impact Analysis (RIA) (PDF) (155pp, 2.6 MB)

Under section 110 of the CAA, 42 U.S.C. 7410, North Carolina is required to submit, for 

EPA approval, State Implementation Plan (SIP) that provide for the attainment and 

maintenance of NO2 standards through control programs directed at sources of NO2

emissions. If North Carolina fails to adopt and implement the required SIP by the time 

periods provided in the CAA, the EPA has responsibility under the CAA to adopt a Federal 

Implementation Plan (FIP) to assure that areas attain the NAAQS in an expeditious manner. 

The Division of Air Quality (DAQ), Ambient Monitoring section estimates that new nitrogen 

dioxide monitoring requirements may result in six required nitrogen dioxide monitors in the 

state of North Carolina. Four of these will be near-road monitors in metropolitan statistical 

areas with over 500,000 people. These four monitors will go in Durham, Guilford, 

Mecklenburg, and Wake Counties. The Durham, Guilford and Wake County near-road 

monitors will be located along the I-40 corridor. The Mecklenburg County near-road 

monitor will be located along one of the interstate highways in or near downtown Charlotte. 

The other two required monitors will be area-wide monitors located in metropolitan 

statistical areas with over one million people. There is already a nitrogen dioxide monitor 

operating in Charlotte that should meet the area-wide monitoring requirements so only one 

additional area-wide monitor will be required in Raleigh. This monitor may go at an existing 

site in Raleigh if the existing site meets the area-wide siting requirements. 

The DAQ estimates it may cost $370,000 for new equipment and buildings to set up four 

near-road monitoring sites and to add a nitrogen dioxide monitor to the existing site in 

Raleigh. The United States Environmental Protection Agency (USEPA) is considering 

providing the State and local air program agencies with funds via the 103 Grant process for 

purchasing equipment to establish the network. The 103 Grant does not require matching 

funds and restricts the use of the funds to monitoring activities and expenses. At this time it 

is not clear if the 103 Grant funds will also cover the $28,000 estimated labor costs for 

ordering the equipment and setting up the sites. There may also be additional costs incurred 

by the North Carolina Department of Transportation to make the monitoring sites safe for 

VI-5 A-27
3 

nearby high-speed traffic. In addition, DAQ estimates it may cost $100,000 a year for 

supplies and labor to operate the five new sites. Even if the USEPA increases the 105 Grant 

to provide additional funds to help with these ongoing costs DAQ would need to provide 

matching funds. 

Although it is uncertain whether State or units of local government funds are affected, the 

amendment to the Rule 15A NCAC 02D .0407 does purport to implement a federal 

regulation, and therefore an impact statement is not required (G.S. 150B-21.4(b1). Instead, 

the agency submits a certification statement under NCGS 150B-21(f1) by the rule-making 

coordinator. 

VI-6 A-28
1

Revision of Sulfur Dioxide and 

Nitrogen Dioxide Ambient Air Nitrogen Nitrogen Dioxide Dioxide Ambient Ambient Air

Quality Standards

1

Public Hearing

May 4, 2011

Background

• Under Clean Air Act (CAA) Nitrogen Dioxide (NO2) and Under Clean Air Act (CAA), Nitrogen Dioxide (NO2) and

Sulfur Dioxide (SO2) are “criteria pollutants”

• US EPA sets National Ambient Air Quality Standards 

(NAAQS)

– Primary - protect public health

– Secondary – protect public welfare

St t i d t d l St t I l t ti Pl

2

• States required to develop State Implementation Plan 

(SIP) to attain the standards

• US EPA required periodically review/revise NAAQS as 

necessary

VI-7 VI-7 A-29 A-29
2

Early Steps in Implementing NAAQS

States must:

• Adopt Ambient Air Quality Standards

• Recommend Nonattainment Boundaries 

• Establish Monitoring Network

3

Establish Monitoring Network

Sulfur Dioxide (SO2)

June 2010 - New scientific studies provide stronger 

evidence for link between short evidence for link between short-term SO term SO2 exposures exposures, 

ranging from 5 minutes to 24 hours, and adverse 

respiratory outcomes. 

USEPA Revoked (effective 1 year after designations)

• Annual standard = 30 parts per billion (ppb)

• 24-hr standard = 140 ppb

4

24 hr standard 140 ppb

USEPA Set New 1-hr standard = 75 ppb

• 3-year average of the annual 99th percentile of 1-hour daily max concentrations

VI-8 VI-8 A-30 A-30
3

Current SO2 Design Values

ALLEGHANY

ASHE

CAMDEN

CASWELL

NORTHAMPTON GATES

ROCKINGHAM PERSON PASQUOTANK SURRY STOKES VANCE

ALAMANCE

ALEXANDER

ANSON

AVERY

BEAUFORT

BERTIE

BLADEN

BUNCOMBE

BURKE

CABARRUS

CALDWELL

CARTERET

CATAWBA CHATHAM

CHEROKEE

CHOWAN

CLAY

CLEVELAND

COLUMBUS

CRAVEN

CUMBERLAND

CURRITUCK

DAVIDSON DARE

DAVIE

DUPLIN

DURHAM

EDGECOMBE

FORSYTH

FRANKLIN

GASTON GRAHAM

GRANVILLE

GREENE

GUILFORD

HALIFAX

HARNETT

HAYWOOD

HENDERSON

HERTFORD

HOKE

HYDE

IREDELL

JACKSON

JOHNSTON

JONES

LEE

LENOIR

LINCOLN

MC DOWELL

MACON

MADISON MARTIN

MECKLENBURG

MITCHELL

MONTGOMERY MOORE

NASH

NEW HANOVER

ONSLOW

ORANGE

PAMLICO

PASQUOTANK

PENDER

PERQUIMANS

PERSON

PITT

POLK

RANDOLPH

RICHMOND

ROBESON

ROCKINGHAM

ROWAN

RUTHERFORD

SAMPSON

SCOTLAND

STANLY

SURRY STOKES

SWAIN

TRANSYLVANIA

TYRRELL

UNION

WAKE

WARREN

WASHINGTON

WATAUGA

WAYNE

WILKES

WILSON

YADKIN

YANCEY

31

20

40

110

14

County 2008-2010 Design 

Values

5

BRUNSWICK

New Hanover 110 ppb

Mecklenburg 40 ppb

Forsyth 20 ppb

Beaufort 31 ppb

Wake 14 ppb

Standard = 75 ppb

2009 SO2 Emissions Inventory

New Hanover, Brunswick, Pender Counties

6

VI-9 VI-9 A-31 A-31
4

Sulfur Dioxide (SO2) Nonattainment 

Boundary Recommendations

• NC DAQ developing boundary 

recommendations considering: recommendations considering:

Air Quality Data, Emissions-Related Data, Meteorology, 

Geography/Topography, Jurisdictional Boundaries

• Focus on southeastern area of state (New Hanover, 

Brunswick, Pender County area)

• Made 7 options available for comment

7

• Held public meetings

• Comment period closed April 30, 2011

• Boundary recommendations due to USEPA 

June 3, 2011

Sulfur Dioxide (SO2) Monitoring 

Network

• NC Q DA developing proposed monitoring 

network plan based on USEPA population 

weighted emissions index (PWEI) criteria

• Will be incorporated in annual network 

monitoring plan - 30 day public comment 

period in ne t fe months

8

period in next few months

• EPA reviews and approves plan

• Network operational by January 1, 2013

VI-10 VI-10 A-32 A-32
5

Sulfur Dioxide (SO2)

Deadline Milestone

June 2010 EPA sets new primary SO2 standard

June 2011 State boundary recommendations to EPA

February 2012 EPA feedback on State recommendations

April 2012 State deadline to respond to EPA feedback

June 2012 EPA makes final designations

January 2013 New SO2 monitoring operational

9

June 2013 State Plan for Attainment areas due

February 2014 State Plan for Nonattainment areas due

August 2017 All areas attain the standard

Sulfur Dioxide (SO2)

Challenges:

• Violations in Wilmington

• Permitting

• Monitoring $$$

– NC needs 5 additional SO2 monitors (3 currently running)

• ~$64,000 initial cost... ~$107,000 recurring annual costs

USEPA e pectations for Maintenance

10

• USEPA expectations for Maintenance

Plan due June 2013

– Modeling demonstrations necessary to show all sources 

contributing to (or have the potential to cause or contribute to) 

monitored and modeled violations of the new standard.

– Awaiting USEPA guidance

VI-11 VI-11 A-33 A-33
6

Nitrogen Dioxide (NO2)

NO2 is the indicator for entire NOx Group of Gases

– NO2 i f f hi hl ti k is one of a group of highly reactive gases known 

as oxides of nitrogen (NOx)

– Other oxides of nitrogen include nitrous acid and 

nitric acid

– NO2 is the component of NOx of greatest interest and 

serves as the indicator for the entire NOx family

11

Nitrogen Dioxide (NO2)

January 2010 – USEPA determined the existing 

primary annual average NO2 standard alone is primary annual average NO2 standard alone is

not sufficient to protect public health.

USEPA Retained

• Annual standard = 53 parts per billion (ppb)

12

USEPA Set New 1-hr standard = 100 ppb

• 3-year average of the annual 98th percentile of 1-hour daily max 

concentrations

VI-12 VI-12 A-34 A-34
7

1-hr.

13

Nitrogen Dioxide (NO2)

December 2010 - NCDAQ submitted boundary 

recommendations to USEPA recommendations to USEPA

• Attainment for Mecklenburg and Forsyth Counties

• Attainment / Unclassifiable for counties where 

monitors will be needed

– Wake, Durham, Guilford

14

VI-13 VI-13 A-35 A-35
8

Nitrogen Dioxide (NO2) Monitoring 

Network

• NC Q DA developing proposed monitoring 

network plan based on USEPA criteria

• Near-road or roadside monitors (4)

• Community wide monitors (2)

• Will be incorporated in annual network 

it i l 30 d bli t

15

monitoring plan - 30 day public comment

period in next few months

• EPA reviews and approves plan

• Network operational by January 1, 2013

Nitrogen Dioxide (NO2)

Challenges:

• Permitting

• Monitoring $$$

– NC needs 5 additional NO2 monitors (2 currently)

• ~$400,000 initial cost... ~$100,000 recurring annual costs

– Road-side monitors

16

• Safety?

• Access?

• Purpose... and future use of data?

• USEPA expectations for Maintenance Plan

VI-14 VI-14 A-36 A-36
9

Nitrogen Dioxide (NO2)

Deadline Milestone

January 2010 EPA sets new primary NO2 standard

January 2011 State boundary recommendations to EPA

January 2012 EPA makes final designations

January 2013 New NO2 monitoring operational

January 2015 Next EPA review of NO2 standard

January 2016/17 Nonattainment re-designations based on new monitoring

January 2021/22 All areas attain the standard

17

Secondary Standards for SO2 and NO2

• Public welfare standards

• Proposal by USEPA is scheduled for July 2011 Proposal by USEPA is scheduled for July 2011

18

VI-15 VI-15 A-37 A-37
10

Status of NAAQS

-Both revised primary NO2 and SO2 

NAAQS tl i ff t NAAQS currently in effect

-Litigation

– SO2 Petition for Reconsideration and Motion 

for Stay denied

– Petition for Review of Denial filed

19

Petition for Review of Denial filed

– Court expected to consolidate

– NO2 Petition for Review filed

– Motion filed for extension of Briefing 

Schedule and Granted by Court

Hearing Focus

-Incorporation of NAAQS into state ambient air 

qua ty u es lity rules

-15A NCAC 02D .0402, Sulfur Oxides

– Revokes Annual standard = 30 parts per 

billion (ppb) and 24-hr standard = 140 ppb 1 

year after designations

– Adds New 1-hr standard = 75 ppb

20

Adds New 1-hr standard = 75 ppb

-15A NCAC 02D .0407, Nitrogen Dioxide 

– Adds New 1-hr standard = 100 ppb

VI-16 VI-16 A-38 A-38
11

Contact Information

Comment Period Closes May 31, 2011

Joelle Burleson, EIT

Rules Development Branch Supervisor

NC Division of Air Quality

(919) 733-1474

– Visit our web site:

htt // i /

21

• http://www.ncair.org/

• Joelle.Burleson@ncdenr.gov

VI-17 VI-17 A-39 A-39
Tuesday, 

June 22, 2010 

Part II 

Environmental 

Protection Agency 

40 CFR Parts 50, 53, and 58 

Primary National Ambient Air Quality 

Standard for Sulfur Dioxide; Final Rule 


srobinson on DSKHWCL6B1PROD with RULES2

VI-18 A-40
35520 Federal Register / Vol. 75, No. 119 / Tuesday, June 22, 2010 / Rules and Regulations 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Parts 50, 53, and 58 

[EPA–HQ–OAR–2007–0352; 9160–4] 

RIN 2060–A048 

Primary National Ambient Air Quality 

Standard for Sulfur Dioxide 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Final rule. 

SUMMARY: Based on its review of the air 

quality criteria for oxides of sulfur and 

the primary national ambient air quality 

standard (NAAQS) for oxides of sulfur 

as measured by sulfur dioxide (SO2), 

EPA is revising the primary SO2

NAAQS to provide requisite protection 

of public health with an adequate 

margin of safety. Specifically, EPA is 

establishing a new 1-hour SO2 standard 

at a level of 75 parts per billion (ppb), 

based on the 3-year average of the 

annual 99th percentile of 1-hour daily 

maximum concentrations. The EPA is 

also revoking both the existing 24-hour 

and annual primary SO2 standards. 

DATES: This final rule is effective on 

August 23, 2010. 

ADDRESSES: EPA has established a 

docket for this action under Docket ID 

No. EPA–HQ–OAR–2007–0352. All 

documents in the docket are listed on 

the http://www.regulations.gov Web 

site. Although listed in the index, some 

information is not publicly available, 

e.g., confidential business information 

or other information whose disclosure is 

restricted by statute. Certain other 

material, such as copyrighted material, 

will be publicly available only in hard 

copy form. Publicly available docket 

materials are available either 

electronically through http:// 

www.regulations.gov or in hard copy at 

the Air and Radiation Docket and 

Information Center, EPA/DC, EPA West, 

Room 3334, 1301 Constitution Ave., 

NW., Washington, DC. The Public 

Reading Room is open from 8:30 a.m. to 

4:30 p.m., Monday through Friday, 

excluding legal holidays. The telephone 

number for the Public Reading Room is 

(202) 566–1744 and the telephone 

number for the Air and Radiation 

Docket and Information Center is (202) 

566–1742. 

FOR FURTHER INFORMATION CONTACT: Dr. 

Michael J. Stewart, Health and 

Environmental Impacts Division, Office 

of Air Quality Planning and Standards, 

U.S. Environmental Protection Agency, 

Mail code C504–06, Research Triangle 

Park, NC 27711; telephone: 919–541– 

7524; fax: 919–541–0237; e-mail: 

stewart.michael@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Table of Contents 

The following topics are discussed in 

this preamble: 

I. Background 

A. Summary of Revisions to the SO2

Primary NAAQS 

B. Statutory Requirements 

C. Related SO2 Control Programs 

D. History of Reviews of the Primary 

NAAQS for Sulfur Oxides 

E. Summary of Proposed Revisions to the 

SO2 Primary NAAQS 

F. Organization and Approach to Final SO2

Primary NAAQS Decisions 

II. Rationale for Decisions on the Primary 

Standards 

A. Characterization of SO2 Air Quality 

1. Anthropogenic Sources and Current 

Patterns of SO2 Air Quality 

2. SO2 Monitoring 

B. Health Effects Information 

1. Short-Term (5-Minute to 24-Hour) SO2

Exposure and Respiratory Morbidity 

Effects 

a. Adversity of Short-Term Respiratory 

Morbidity Effects 

2. Health Effects and Long-Term Exposures 

to SO2

3. SO2-Related Impacts on Public Health 

C. Human Exposure and Health Risk 

Characterization 

D. Approach for Determining Whether To 

Retain or Revise the Current Standards 

E. Adequacy of the Current Standards 

1. Rationale for Proposed Decision 

2. Comments on the Adequacy of the 

Current Standards 

a. Comments on EPA’s Interpretation of the 

Epidemiologic Evidence 

b. Comments on EPA’s Interpretation of the 

Controlled Human Exposure Evidence 

c. Comments on EPA’s Characterization of 

SO2-Associated Exposures and Health 

Risks 

3. Conclusions Regarding the Adequacy of 

the Current 24-Hour and Annual 

Standards 

F. Conclusions on the Elements of a New 

Short-Term Standard 

1. Indicator 

a. Rationale for Proposed Decision 

b. Comments on Indicator 

c. Conclusions on Indicator 

2. Averaging Time 

a. Rationale for Proposed Decision 

b. Comments on Averaging Time 

c. Conclusions on Averaging Time 

3. Form 

a. Rationale for Proposed Decision 

b. Comments on Form 

c. Conclusions on Form 

4. Level 

a. Rationale for Proposed Decision 

b. Comments on Level 

c. Conclusions on Level 

5. Retaining or Revoking the Current 24- 

Hour and Annual Standards 

a. Rationale for Proposed Decision 

b. Comments on Retaining or Revoking the 

Current 24-Hour and Annual Standards 

c. Conclusions on Retaining or Revoking 

the Current 24-Hour and Annual 

Standards 

G. Summary of Decisions on Primary 

Standards 

III. Overview of the Approach for Monitoring 

and Implementation 

IV. Amendments to Ambient Monitoring and 

Reporting Requirements 

A. Monitoring Methods 

1. Requirements for SO2 Federal Reference 

Method (FRM) 

a. Proposed Ultraviolet Fluorescence SO2

FRM and Implementation 

b. Public Comments 

c. Conclusions on Ultraviolet Fluorescence 

SO2 FRM and Implementation 

2. Requirements for Automated SO2

Methods 

a. Proposed Performance Specifications for 

Automated Methods 

b. Public Comments 

c. Conclusions for Performance 

Specifications for SO2 Automated 

Methods 

B. Network Design 

1. Approach for Network Design 

a. Proposed Approach for Network Design 

b. Alternative Network Design 

c. Public Comments 

2. Modeling Ambient SO2 Concentrations 

3. Monitoring Objectives 

a. Proposed Monitoring Objectives 

b. Public Comments 

c. Conclusions on Monitoring Objectives 

4. Final Monitoring Network Design 

5. Population Weighted Emissions Index 

a. Proposed Use of the Population 

Weighted Emissions Index 

b. Public Comments 

c. Conclusions on the Use of the 

Population Weighted Emissions Index 

6. Regional Administrator Authority 

a. Proposed Regional Administrator 

Authority 

b. Public Comments 

c. Conclusions on Regional Administrator 

Authority 

7. Monitoring Network Implementation 

a. Proposed Monitoring Network 

Implementation 

b. Public Comments 

c. Conclusions on Monitoring Network 

Implementation 

C. Data Reporting 

1. Proposed Data Reporting 

2. Public Comments 

3. Conclusions on Data Reporting 

V. Initial Designation of Areas for the 1-Hour 

SO2 NAAQS 

A. Clean Air Act Requirements 

1. Approach Described in Proposal 

2. Public Comments 

B. Expected Designations Process 

VI. Clean Air Act Implementation 

Requirements 

A. How This Rule Applies to Tribes 

B. Nonattainment Area Attainment Dates 

1. Attaining the NAAQS 

2. Consequences of a Nonattainment Area 

Failing To Attain by the Statutory 

Attainment Date 

C. Section 110(a)(1) and (2) NAAQS 

Maintenance/Infrastructure 

Requirements 

1. Section 110(a)(1)–(2) Submission 


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1The legislative history of section 109 indicates 

that a primary standard is to be set at ‘‘the 

maximum permissible ambient air level * * * 

which will protect the health of any [sensitive] 

group of the population,’’ and that for this purpose 

‘‘reference should be made to a representative 

sample of persons comprising the sensitive group 

rather than to a single person in such a group.’’ S. 

Rep. No. 91–1196, 91st Cong., 2d Sess. 10 (1970). 

See also American Lung Ass’n v. EPA, 134 F. 3d 

388, 389 (DC Cir. 1998) (‘‘NAAQS must protect not 

only average healthy individuals, but also ‘sensitive 

citizens’—children, for example, or people with 

asthma, emphysema, or other conditions rendering 

them particularly vulnerable to air pollution. If a 

pollutant adversely affects the health of these 

sensitive individuals, EPA must strengthen the 

entire national standard.’’); Coalition of Battery 

Recyclers Ass’n v. EPA, No. 09–1011 (DC Cir. May 

14, 2010) slip op. at 7 (same). 

2EPA is currently conducting a separate review 

of the secondary SO2 NAAQS jointly with a review 

of the secondary NO2 NAAQS (see http:// 

www.epa.gov/ttn/naaqs/standards/no2so2sec/ 

index.html for more information). 

D. Attainment Planning Requirements 

1. SO2 Nonattainment Area SIP 

Requirements 

2. New Source Review and Prevention of 

Significant Deterioration Requirements 

3. General Conformity 

E. Transition From the Existing SO2

NAAQS to a Revised SO2 NAAQS 

VII. Appendix T—Interpretation of the 

Primary NAAQS for Oxides of Sulfur 

and Revisions to the Exceptional Events 

Rule 

A. Interpretation of the NAAQS for Oxides 

of Sulfur 

1. Proposed Interpretation of the Standard 

2. Comments on Interpretation of the 

Standard 

3. Conclusions on Interpretation of the 

Standard 

B. Exceptional Events Information 

Submission Schedule 

VIII. Communication of Public Health 

Information 

IX. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review 

B. Paperwork Reduction Act 

C. Regulatory Flexibility Act 

D. Unfunded Mandates Reform Act 

E. Executive Order 13132: Federalism 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

G. Executive Order 13045: Protection of 

Children From Environmental Health & 

Safety Risks 

H. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution or Use 

I. National Technology Transfer and 

Advancement Act 

J. Executive Order 12898: Federal Actions 

To Address Environmental Justice in 

Minority Populations and Low-Income 

Populations 

References 

I. Background 

A. Summary of Revisions to the SO2

Primary NAAQS 

Based on its review of the air quality 

criteria for oxides of sulfur and the 

primary national ambient air quality 

standard (NAAQS) for oxides of sulfur 

as measured by sulfur dioxide (SO2), 

EPA is making revisions to the primary 

SO2 NAAQS so the standards are 

requisite to protect public health with 

an adequate margin of safety, as 

appropriate under section 109 of the 

Clean Air Act (Act or CAA). 

Specifically, EPA is replacing the 

current 24-hour and annual standards 

with a new short-term standard based 

on the 3-year average of the 99th 

percentile of the yearly distribution of 

1-hour daily maximum SO2

concentrations. EPA is setting the level 

of this new standard at 75 ppb. EPA is 

adding data handling conventions for 

SO2 by adding provisions for this new 

1-hour primary standard. EPA is also 

establishing requirements for an SO2

monitoring network. These new 

provisions require monitors in areas 

where there is an increased coincidence 

of population and SO2 emissions. EPA 

is also making conforming changes to 

the Air Quality Index (AQI). 

B. Statutory Requirements 

Two sections of the Clean Air Act 

(Act or CAA) govern the establishment 

and revision of National Ambient Air 

Quality Standards NAAQS. Section 108 

of the Act directs the Administrator to 

identify and list air pollutants that meet 

certain criteria, including that the air 

pollutant ‘‘in his judgment, cause[s] or 

contribute[s] to air pollution which may 

reasonably be anticipated to endanger 

public health and welfare’’ and ‘‘the 

presence of which in the ambient air 

results from numerous or diverse mobile 

or stationary sources.’’ CAA section 

108(a)(1)(A) and (B). For those air 

pollutants listed, section 108 requires 

the Administrator to issue air quality 

criteria that ‘‘accurately reflect the latest 

scientific knowledge useful in 

indicating the kind and extent of all 

identifiable effects on public health or 

welfare which may be expected from the 

presence of [a] pollutant in ambient air 

***’’ Section 108(a)(2). 

Section 109(a) of the Act directs the 

Administrator to promulgate ‘‘primary’’ 

and ‘‘secondary’’ NAAQS for pollutants 

for which air quality criteria have been 

issued. Section 109(b)(1) defines a 

primary standard as one ‘‘the attainment 

and maintenance of which in the 

judgment of the Administrator, based on 

[the air quality] criteria and allowing an 

adequate margin of safety, are requisite 

to protect the public health.’’ 1 Section 

109(b)(1). A secondary standard, in turn, 

must ‘‘specify a level of air quality the 

attainment and maintenance of which, 

in the judgment of the Administrator, 

based on [the air quality] criteria, is 

requisite to protect the public welfare 

from any known or anticipated adverse 

effects associated with the presence of 

such pollutant in the ambient air.’’ 2

Section 109(b)(2) This rule concerns 

exclusively the primary NAAQS for 

oxides of sulfur. 

The requirement that primary 

standards include an adequate margin of 

safety is intended to address 

uncertainties associated with 

inconclusive scientific and technical 

information available at the time of 

standard setting. It is also intended to 

provide a reasonable degree of 

protection against hazards that research 

has not yet identified. Lead Industries 

Association v. EPA, 647 F.2d 1130, 1154 

(DC Cir 1980), cert. denied, 449 U.S. 

1042 (1980); American Petroleum 

Institute v. Costle, 665 F.2d 1176, 1186 

(DC Cir. 1981), cert. denied, 455 U.S. 

1034 (1982). Both kinds of uncertainties 

are components of the risk associated 

with pollution at levels below those at 

which human health effects can be said 

to occur with reasonable scientific 

certainty. Thus, in selecting primary 

standards that include an adequate 

margin of safety, the Administrator is 

seeking not only to prevent pollution 

levels that have been demonstrated to be 

harmful but also to prevent lower 

pollutant levels that may pose an 

unacceptable risk of harm, even if the 

risk is not precisely identified as to 

nature or degree. The CAA does not 

require the Administrator to establish a 

primary NAAQS at a zero-risk level or 

at background concentration levels, see 

Lead Industries Association v. EPA, 647 

F.2d at 1156 n. 51, but rather at a level 

that reduces risk sufficiently so as to 

protect public health with an adequate 

margin of safety. 

In addressing the requirement for a 

margin of safety, EPA considers such 

factors as the nature and severity of the 

health effects involved, the size of the 

at-risk population(s), and the kind and 

degree of the uncertainties that must be 

addressed. The selection of any 

particular approach to providing an 

adequate margin of safety is a policy 

choice left specifically to the 

Administrator’s judgment. Lead 

Industries Association v. EPA, 647 F.2d 

at 1161–62. 

In setting standards that are 

‘‘requisite’’ to protect public health and 

welfare, as provided in section 109(b), 

EPA’s task is to establish standards that 

are neither more nor less stringent than 

necessary for these purposes. In so 

doing, EPA may not consider the costs 

of implementing the standards. 

Whitman v. American Trucking 


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Associations, 531 U.S. 457, 471, 475–76 

(2001). 

Section 109(d)(1) of the Act requires 

the Administrator to periodically 

undertake a thorough review of the air 

quality criteria published under section 

108 and the NAAQS and to revise the 

criteria and standards as may be 

appropriate. The Act also requires the 

Administrator to appoint an 

independent scientific review 

committee composed of seven members, 

including at least one member of the 

National Academy of Sciences, one 

physician, and one person representing 

State air pollution control agencies, to 

review the air quality criteria and 

NAAQS and to ‘‘recommend to the 

Administrator any new * * * standards 

and revisions of existing criteria and 

standards as may be appropriate under 

section 108 and subsection (b) of this 

section.’’ CAA section 109(d)(2). This 

independent review function is 

performed by the Clean Air Scientific 

Advisory Committee (CASAC) of EPA’s 

Science Advisory Board. 

C. Related SO2 Control Programs 

States are primarily responsible for 

ensuring attainment and maintenance of 

ambient air quality standards once EPA 

has established them. Under section 110 

of the Act, and related provisions, States 

are to submit, for EPA approval, State 

implementation plans (SIPs) that 

provide for the attainment and 

maintenance of such standards through 

control programs directed to sources of 

the pollutants involved. The States, in 

conjunction with EPA, also administer 

the prevention of significant 

deterioration program that covers these 

pollutants. See CAA sections 160–169. 

In addition, Federal programs provide 

for nationwide reductions in emissions 

of these and other air pollutants through 

the Federal motor vehicle and motor 

vehicle fuel control program under title 

II of the Act (CAA sections 202–250) 

which involves controls for emissions 

from all moving sources and controls for 

the fuels used by these sources; new 

source performance standards under 

section 111; and title IV of the Act (CAA 

sections 402–416), which specifically 

provides for major reductions in SO2

emissions. EPA has also promulgated 

the Clean Air Interstate Rule (CAIR) to 

require additional SO2 emission 

reductions needed in the eastern half of 

the United States to address emissions 

which contribute significantly to 

nonattainment with, or interfere with 

maintenance of, the PM NAAQS by 

downwind States in the CAIR region. 

This rule was remanded by the DC 

Circuit, and although it remains in 

effect, EPA is reevaluating it pursuant to 

the court remand. 

Currently, there are several areas 

designated as being in nonattainment of 

the primary SO2 NAAQS (see section 

VI). Moreover, as a result of this final 

rule, additional areas could be classified 

as non-attainment. Certain States would 

then be required to develop SIPs that 

identify and implement specific air 

pollution control measures to reduce 

ambient SO2 concentrations to attain 

and maintain the revised SO2 NAAQS, 

most likely by requiring air pollution 

controls on sources that emit oxides of 

sulfur (SOx). 

D. History of Reviews of the Primary 

NAAQS for Sulfur Oxides 

On April 30, 1971, the EPA 

promulgated primary SO2 NAAQS (36 

FR 8187). These primary standards, 

which were based on the findings 

outlined in the original 1969 Air Quality 

Criteria for Sulfur Oxides, were set at 

0.14 parts per million (ppm) averaged 

over a 24-hour period, not to be 

exceeded more than once per year, and 

0.030 ppm annual arithmetic mean. In 

1982, EPA published the Air Quality 

Criteria for Particulate Matter and Sulfur 

Oxides (EPA, 1982) along with an 

addendum of newly published 

controlled human exposure studies, 

which updated the scientific criteria 

upon which the initial standards were 

based (EPA, 1982). In 1986, EPA 

published a second addendum 

presenting newly available evidence 

from epidemiologic and controlled 

human exposure studies (EPA, 1986). In 

1988, EPA published a proposed 

decision not to revise the existing 

standards (53 FR 14926) (April 26, 

1988). However, EPA specifically 

requested public comment on the 

alternative of revising the current 

standards and adding a new 1-hour 

primary standard of 0.4 ppm (400 ppb) 

to protect asthmatics against 5–10 

minute peak SO2 concentrations. 

As a result of public comments on the 

1988 proposal and other post-proposal 

developments, EPA published a second 

proposal on November 15, 1994 (59 FR 

58958). The 1994 re-proposal was based 

in part on a supplement to the second 

addendum of the criteria document, 

which evaluated new findings on 5–10 

minute SO2 exposures in asthmatics 

(EPA, 1994a; EPA, 1994b). As in the 

1988 proposal, EPA proposed to retain 

the existing 24-hour and annual 

standards. EPA also solicited comment 

on three regulatory alternatives to 

further reduce the health risk posed by 

exposure to high 5-minute peaks of SO2

if additional protection were judged to 

be necessary. The three alternatives 

were: (1) Revising the existing primary 

SO2 NAAQS by adding a new 5-minute 

standard of 0.6 ppm (600 ppb) SO2; (2) 

establishing a new regulatory program 

under section 303 of the Act to 

supplement protection provided by the 

existing NAAQS, with a trigger level of 

0.6 ppm (600 ppb) SO2, one expected 

exceedance; and (3) augmenting 

implementation of existing standards by 

focusing on those sources or source 

types likely to produce high 5-minute 

peak concentrations of SO2. 

On May 22, 1996, EPA announced its 

final decision not to revise the NAAQS 

for SOx (61 FR 25566). EPA found that 

asthmatics—a susceptible population 

group—could be exposed to short-term 

SO2 bursts resulting in repeated 

‘exposure events’ such that tens or 

hundreds of thousands of asthmatics 

could be exposed annually to lung 

function effects ‘‘distinctly exceeding 

* * * [the] typical daily variation in 

lung function’’ that asthmatics routinely 

experience, and found further that 

repeated occurrences should be 

regarded as significant from a public 

health standpoint. 61 FR at 25572, 

25573. Nonetheless, the agency 

concluded that ‘‘the likelihood that 

asthmatic individuals will be exposed 

* * * is very low when viewed from a 

national perspective’’, that ‘‘5-minute 

peak SO[2] levels do not pose a broad 

public health problem when viewed 

from a national perspective’’, and that 

‘‘short-term peak concentrations of SO[2] 

do not constitute the type of ubiquitous 

public health problem for which 

establishing a NAAQS would be 

appropriate.’’ Id. at 25575. EPA 

concluded, therefore, that it would not 

revise the existing standards or add a 

standard to specifically address 5- 

minute exposures. EPA also announced 

an intention to propose guidance, under 

section 303 of the Act, to assist States 

in responding to short-term peaks of 

SO2 and later initiated a rulemaking to 

do so (62 FR 210 (Jan. 2, 1997). 

The American Lung Association and 

the Environmental Defense Fund 

challenged EPA’s decision not to 

establish a 5-minute standard. On 

January 30, 1998, the Court of Appeals 

for the District of Columbia Circuit 

found that EPA had failed to adequately 

explain its determination that no 

revision to the SO2 NAAQS was 

appropriate and remanded the 

determination back to EPA for further 

explanation. American Lung Ass’n v. 

EPA, 134 F. 3d 388 (DC Cir. 1998). 

Specifically, the court held that EPA 

had failed to adequately explain the 

basis for its conclusion that short-term 

SO2 exposures to asthmatics do not 

constitute a public health problem, 


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noting that the agency had failed to 

explain the link between its finding that 

repeated short-term exposures were 

significant, and that there would be tens 

to hundreds of thousands of such 

exposures annually to a susceptible 

subpopulation. 134 F. 3d at 392. The 

court also rejected the explanation that 

short-term SO2 bursts were ‘‘localized, 

infrequent, and site-specific’’ as a 

rational basis for the conclusion that no 

public health problem existed for 

purposes of section 109: ‘‘[N]othing in 

the Final Decision explains why 

‘localized’, ‘site-specific’, or even 

‘infrequent’ events might nevertheless 

create a public health problem, 

particularly since, in some sense, all 

pollution is local and site-specific 

***’’. Id. The court accordingly 

remanded the case to EPA to adequately 

explain its determination or otherwise 

take action in accordance with the 

opinion. In response, EPA has collected 

and analyzed additional air quality data 

focused on 5-minute concentrations of 

SO2. These air quality analyses 

conducted since the last review helped 

inform the current review, which 

(among other things) address the issues 

raised in the court’s remand of the 

Agency’s last decision. 

EPA formally initiated the current 

review of the air quality criteria for 

oxides of sulfur and the SO2 primary 

NAAQS on May 15, 2006 (71 FR 28023) 

with a general call for information. 

EPA’s draft Integrated Review Plan for 

the Primary National Ambient Air 

Quality Standards for Sulfur Dioxide 

(EPA, 2007a) was made available in 

April 2007 for public comment and was 

discussed by the CASAC via a publicly 

accessible teleconference on May 11, 

2007. As noted in that plan, SOX

includes multiple gaseous (e.g., SO3) 

and particulate (e.g., sulfate) species. 

Because the health effects associated 

with particulate species of SOX have 

been considered within the context of 

the health effects of ambient particles in 

the Agency’s review of the NAAQS for 

particulate matter (PM), the current 

review of the primary SO2 NAAQS is 

focused on the gaseous species of SOX

and does not consider health effects 

directly associated with particulate 

species. 

The first draft of the Integrated 

Science Assessment for Oxides of 

Sulfur-Health Criteria (ISA) and the 

Sulfur Dioxide Health Assessment Plan: 

Scope and Methods for Exposure and 

Risk Assessment (EPA, 2007b) were 

reviewed by CASAC at a public meeting 

held on December 5–6, 2007. Based on 

comments received from CASAC and 

from the public, EPA developed the 

second draft of the ISA and the first 

draft of the Risk and Exposure 

Assessment to Support the Review of 

the SO2 Primary National Ambient Air 

Quality Standard (Risk and Exposure 

Assessment (REA)). These documents 

were reviewed by CASAC at a public 

meeting held on July 30–31, 2008. Based 

on comments received from CASAC and 

the public at this meeting, EPA released 

the final ISA in September of 2008 

(EPA, 2008a; henceforth referred to as 

ISA). In addition, comments received 

were considered in developing the 

second draft of the REA. Importantly, 

the second draft of the REA contained 

a draft staff policy assessment that 

considered the evidence presented in 

the final ISA and the air quality, 

exposure, and risk characterization 

results presented in the second draft 

REA, as they related to the adequacy of 

the current SO2 NAAQS and potential 

alternative primary SO2 standards. This 

document was reviewed by CASAC at a 

public meeting held on April 16–17, 

2009. In preparing the final REA report, 

which included the final staff policy 

assessment, EPA considered comments 

received from CASAC and the public at 

and subsequent to that meeting. The 

final REA containing the final staff 

policy assessment was completed in 

August 2009 (EPA 2009a; henceforth 

referred to as REA)). 

On December 8, 2009 EPA published 

its proposed revisions to the primary 

SO2 NAAQS. 74 FR 64810 presented a 

number of conclusions, findings, and 

determinations proposed by the 

Administrator. EPA invited general, 

specific, and/or technical comments on 

all issues involved with this proposal, 

including all such proposed judgments, 

conclusions, findings, and 

determinations. EPA invited specific 

comment on the level, or range of levels, 

appropriate for such a standard, as well 

as on the rationale that would support 

that level or range of levels. These 

comments were carefully considered by 

the Administrator as she made her final 

decisions, as described in this notice, on 

the primary SO2 NAAQS 

The schedule for completion of this 

review is governed by a judicial order 

resolving a lawsuit filed in September 

2005, concerning the timing of the 

current review. Center for Biologic 

Diversity v. Johnson (Civ. No. 05–1814) 

(D.D.C. 2007). The order that now 

governs this review, entered by the 

court in August 2007 and amended in 

December 2008, provides that the 

Administrator will sign, for publication, 

a final rulemaking concerning the 

review of the primary SO2 NAAQS no 

later than June 2, 2010. 

E. Summary of Proposed Revisions to 

the SO2 Primary NAAQS 

For the reasons discussed in the 

preamble of the proposal for the SO2

primary NAAQS, EPA proposed to make 

revisions to the primary SO2 NAAQS 

(and to add SO2 data handling 

conventions) so the standards provide 

requisite protection of public health 

with an adequate margin of safety. 

Specifically, EPA proposed to replace 

the current 24-hour and annual 

standards with a new short-term SO2

standard. EPA proposed that this new 

short-term standard would be based on 

the 3-year average of the 99th percentile 

(or 4th highest) of the yearly 

distribution of 1-hour daily maximum 

SO2 concentrations. EPA proposed to set 

the level of this new 1-hour standard 

within the range of 50 to 100 ppb and 

solicited comment on standard levels as 

high as 150 ppb. EPA also proposed to 

establish requirements for an SO2

monitoring network at locations where 

maximum SO2 concentrations are 

expected to occur and to add a new 

Federal Reference Method (FRM) for 

measuring SO2 in the ambient air. 

Finally, EPA proposed to make 

corresponding changes to the Air 

Quality Index for SO2. 

F. Organization and Approach to Final 

SO2 Primary NAAQS Decisions 

This action presents the 

Administrator’s final decisions 

regarding the need to revise the current 

SO2 primary NAAQS, and what those 

revisions should be. Revisions to the 

primary NAAQS for SO2, and the 

rationale supporting those revisions, are 

described below in section II. 

An overview of the approach for 

monitoring and implementation is 

presented in section III. Requirements 

for the SO2 ambient monitoring network 

and for a new, additional FRM for 

measuring SO2 in the ambient air are 

described in section IV. EPA’s current 

plans for designations and for 

implementing the revised SO2 primary 

NAAQS are discussed in sections V and 

VI respectively. Related requirements 

for data completeness, data handling, 

data reporting, rounding conventions, 

and exceptional events are described in 

section VII. Communication of public 

health information through the AQI is 

discussed in section VIII. A recitation of 

statutory authority and a discussion of 

those executive order reviews which are 

relevant are provided in section IX. 

Today’s final decisions are based on 

a thorough review in the ISA of 

scientific information on known and 

potential human health effects 

associated with exposure to SO2 in the 


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3A small number of sites, 98 total from 1997 to 

2007 of the approximately 500 SO2 monitors, and 

not the same sites in all years, voluntarily reported 

5-minute block average data to AQS (ISA, section 

2.5.2). Of these, 16 reported all twelve 5-minute 

averages in each hour for at least part of the time 

between 1997 and 2007. The remainder reported 

only the maximum 5-minute average in each hour. 

air. These final decisions also take into 

account: (1) Assessments in the REA of 

the most policy-relevant information in 

the ISA as well as quantitative exposure 

and risk analyses based on that 

information; (2) CASAC Panel advice 

and recommendations, as reflected in its 

letters to the Administrator and its 

public discussions of the ISA and REA; 

(3) public comments received during the 

development of the ISA and REA; and 

(4) public comments received on EPA’s 

notice of proposed rulemaking. 

II. Rationale for Decisions on the 

Primary Standards 

This section presents the rationale for 

the Administrator’s decision to revise 

the existing SO2 primary standards by 

replacing the current 24-hour and 

annual standards with a new 1-hour SO2

standard at a level of 75 ppb, based on 

the 3-year average of the annual 99th 

percentile of 1-hour daily maximum 

concentrations. As discussed more fully 

below, this rationale takes into account: 

(1) Judgments and conclusions 

presented in the ISA and the REA; (2) 

CASAC advice and recommendations as 

reflected in the CASAC panel’s 

discussions of drafts of the ISA and REA 

at public meetings, in separate written 

comments, and in letters to the 

Administrator (Henderson 2008a; 

Henderson 2008b; Samet, 2009); (3) 

public comments received at CASAC 

meetings during the development of the 

ISA and the REA; and (4) public 

comments received on the notice of 

proposed rulemaking. 

In reaching this decision, EPA has 

drawn upon an integrative synthesis of 

the entire body of evidence on human 

health effects associated with the 

presence of SO2 in the ambient air, and 

upon the results of the quantitative 

exposure and risk assessments reflecting 

this evidence. As discussed below, this 

body of evidence addresses a broad 

range of health endpoints associated 

with exposure to SO2 in the ambient air. 

In considering this entire body of 

evidence, EPA chose to focus most on 

those health endpoints for which the 

ISA found the strongest evidence of an 

association with SO2 (see section II.B 

below). Thus, the rationale for this final 

decision on the SO2 NAAQS focused 

primarily on respiratory morbidity 

following short-term (5-minutes to 24- 

hours) exposure to SO2, for which the 

ISA found a causal relationship. 

As discussed below, a substantial 

amount of new research has been 

conducted since EPA’s last review of the 

SO2 NAAQS, with important new 

information coming from epidemiologic 

studies in particular. In addition to the 

substantial amount of new 

epidemiologic research, the ISA 

considered a limited number of new 

controlled human exposure studies and 

re-evaluated key older controlled 

human exposure studies. In evaluating 

both the new and key older controlled 

human exposure studies, the ISA 

utilized updated guidelines published 

by the American Thoracic Society (ATS) 

on what constitutes an adverse effect of 

air pollution (see ISA, section 3.1.3; p. 

3–4). Importantly, all controlled human 

exposure and epidemiologic studies 

evaluated in the ISA have undergone 

intensive scrutiny through multiple 

layers of peer review and opportunities 

for public review and comment. Thus, 

the review of this information has been 

extensive and deliberate. 

After a background discussion of the 

principal emitting sources and current 

patterns of SO2 air quality and a 

description of the current SO2

monitoring network from which those 

air quality patterns are obtained (section 

II.A), the remainder of this section 

discusses the Administrator’s rationale 

for her final decisions on the primary 

standards. Section II.B includes an 

overview of the scientific evidence 

related to the respiratory effects 

associated with ambient SO2 exposure. 

This overview includes a discussion of 

the at-risk populations considered in the 

ISA. Section II.C summarizes the key 

approaches taken by EPA to assess 

exposures and health risks associated 

with exposure to ambient SO2. Section 

II.D summarizes the approach that was 

used in the current review of the SO2

NAAQS with regard to consideration of 

the scientific evidence and the air 

quality, exposure, and risk-based results 

related to the adequacy of the current 

standards and potential alternative 

standards. Sections II.E and II.F discuss, 

respectively, the Administrator’s 

decisions regarding the adequacy of the 

current standards and the elements of a 

new short-term standard, taking into 

consideration public comments on the 

proposed decisions. Section II.G 

summarizes the Administrator’s 

decisions with regard to the SO2

primary NAAQS. 

A. Characterization of SO2 Air Quality 

1. Anthropogenic Sources and Current 

Patterns of SO2 Air Quality 

Anthropogenic SO2 emissions 

originate chiefly from point sources, 

with fossil fuel combustion at electric 

utilities (∼66%) and other industrial 

facilities (∼29%) accounting for the 

majority of total emissions (ISA, section 

2.1). Other anthropogenic sources of 

SO2 include both the extraction of metal 

from ore as well as the burning of high 

sulfur-containing fuels by locomotives, 

large ships, and equipment utilizing 

diesel engines. SO2 emissions and 

ambient concentrations follow a strong 

east to west gradient due to the large 

numbers of coal-fired electric generating 

units in the Ohio River Valley and 

upper Southeast regions. In the 12 

Consolidated Metropolitan Statistical 

Areas (CMSAs) that had at least four 

SO2 regulatory monitors from 2003– 

2005, 24-hour average concentrations in 

the continental U.S. ranged from a 

reported low of ∼1 ppb in Riverside, CA 

and San Francisco, CA to a high of ∼12 

ppb in Pittsburgh, PA and Steubenville, 

OH (ISA, section 2.5.1). In addition, 

outside or inside all CMSAs from 2003– 

2005, the annual average SO2

concentration was 4 ppb (ISA, Table 2– 

8). However, spikes in hourly 

concentrations occurred. The mean 1- 

hour maximum concentration outside or 

inside CMSAs was 13 ppb, with a 

maximum value of greater than 600 ppb 

outside CMSAs and greater than 700 

ppb inside CMSAs (ISA, Table 2–8). 

Temporal and spatial patterns of 5- 

minute peaks of SO2 are also important 

given that controlled human exposure 

studies have demonstrated that 

exposure to these peaks can result in 

adverse respiratory effects in exercising 

asthmatics (see section II.B below). For 

those monitors which voluntarily 

reported 5-minute block average data,3

when maximum 5-minute 

concentrations were reported, the 

absolute highest concentration over the 

ten-year period exceeded 4000 ppb, but 

for all individual monitors, the 99th 

percentile was below 200 ppb (ISA, 

section 2.5.2 Table 2–10). Median 

concentrations from these monitors 

reporting 5-minute data ranged from 

1 ppb to 8 ppb, and the average for each 

maximum 5-minute level ranged from 

3 ppb to 17 ppb. Delaware, 

Pennsylvania, Louisiana, and West 

Virginia had mean values for maximum 

5-minute data exceeding 10 ppb. Among 

aggregated within-State data for the 16 

monitors from which all 5-minute 

average intervals were reported, the 

median values ranged from 1 ppb to 5 

ppb, and the means ranged from 3 ppb 

to 11 ppb (ISA, section 2.5.2 at 2–43). 

The highest reported concentration was 

921 ppb, but the 99th percentile values 


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4A causal relationship is based on ‘‘[e]vidence 

[that] is sufficient to conclude that there is a causal 

relationship between relevant pollutant exposures 

and the health outcome. That is, a positive 

association has been observed between the 

pollutant and the outcome in studies in which 

chance, bias, and confounding could be ruled out 

with reasonable confidence. Evidence includes, for 

example, controlled human exposure studies; or 

observational studies that cannot be explain by 

plausible alternatives or are supported by other 

lines of evidence (e.g. animal studies or mechanism 

of action information). Evidence includes replicated 

and consistent high-quality studies by multiple 

investigators.’’ ISA Table 1–2, at 1–11. 

for aggregated within-State data were all 

below 90 ppb (id). 

2. SO2 Monitoring 

Although EPA established the SO2

standards in 1971, uniform minimum 

monitoring network requirements for 

SO2 monitoring were only adopted in 

May 1979. From the time of the 

implementation of the 1979 monitoring 

rule through 2008, the SO2 monitoring 

network has steadily decreased in size 

from approximately 1496 sites in 1980 

to the approximately 488 sites operating 

in 2008. At present, except for SO2

monitoring required at National Core 

Monitoring Stations (NCore stations), 

there are no minimum monitoring 

requirements for SO2 in 40 CFR part 58 

Appendix D, other than a requirement 

for EPA Regional Administrator 

approval before removing any existing 

monitors and a requirement that any 

ongoing SO2 monitoring must have at 

least one monitor sited to measure the 

maximum concentration of SO2 in that 

area. EPA removed the specific 

minimum monitoring requirements for 

SO2 in the 2006 monitoring rule 

revisions, except for monitoring at 

NCore stations, based on the fact that 

there were no SO2 nonattainment areas 

at that time, coupled with trends 

showing an increasing gap between 

national average SO2 concentrations and 

the current 24-hour and annual 

standards. The rule was also intended to 

provide State, local, and Tribal air 

monitoring agencies flexibility in 

meeting perceived higher priority 

monitoring needs for other pollutants, 

or to implement the new multi-pollutant 

sites (NCore network) required by the 

2006 rule revisions (71 FR 61236, 

(October 6, 2006)). More information on 

SO2 monitoring can be found in section 

IV. 

B. Health Effects Information 

The ISA concluded that there was 

sufficient evidence to infer a ‘‘causal 

relationship’’ between respiratory 

morbidity and short-term (5-minutes to 

24-hours) exposure to SO2 (ISA, section 

5.2). Importantly, we note that a ‘‘causal 

relationship’’ is the strongest finding the 

ISA can make.4 This conclusion was 

based on the consistency, coherence, 

and plausibility of findings observed in 

controlled human exposure studies of 

5–10 minutes, epidemiologic studies 

mostly using 1-hour daily maximum 

and 24-hour average SO2

concentrations, and animal toxicological 

studies using exposures of minutes to 

hours (ISA, section 5.2). This evidence 

is briefly summarized below and 

discussed in more detail in the proposal 

(see sections II.B.1 to II.B.5, see 74 FR 

at 64815–821). We also note that the ISA 

judged evidence of an association 

between SO2 exposure and other health 

categories to be less convincing; other 

associations were judged to be 

suggestive but not sufficient to infer a 

causal relationship 

(i.e., short-term exposure to SO2 and 

mortality) or inadequate to infer the 

presence or absence of a causal 

relationship (i.e., short-term exposure to 

SO2 and cardiovascular morbidity, and 

long-term exposure to SO2 and 

respiratory morbidity, other morbidity, 

and mortality). Key conclusions from 

the ISA are described in greater detail in 

Table 5–3 of the ISA. 

1. Short-Term (5-minute to 24-hour) SO2

Exposure and Respiratory Morbidity 

Effects 

The ISA examined numerous 

controlled human exposure studies and 

found that moderate or greater 

decrements in lung function (i.e., ≥ 15% 

decline in Forced Expiratory Volume 

(FEV1) and/or ≥ 100% increase in 

specific airway resistance (sRaw)) occur 

in some exercising asthmatics exposed 

to SO2 concentrations as low as 

200–300 ppb for 5–10 minutes. The ISA 

also found that among asthmatics, both 

the percentage of individuals affected, 

and the severity of the response 

increased with increasing SO2

concentrations. That is, at 5–10 minute 

concentrations ranging from 200–300 

ppb, the lowest levels tested in free 

breathing chamber studies, 

approximately 5–30% percent of 

exercising asthmatics experienced 

moderate or greater decrements in lung 

function (ISA, Table 3–1). At 

concentrations of 400–600 ppb, 

moderate or greater decrements in lung 

function occurred in approximately 20– 

60% of exercising asthmatics, and 

compared to exposures at 200–300 ppb, 

a larger percentage of asthmatics 

experienced severe decrements in lung 

function (i.e., ≥ 20% decrease in FEV1

and/or ≥ 200% increase in sRaw; ISA, 

Table 3–1). Moreover, at SO2

concentrations ≥ 400 ppb (5–10 minute 

exposures), moderate or greater 

decrements in lung function were often 

statistically significant at the group 

mean level and frequently accompanied 

by respiratory symptoms. Id. 

The ISA also found that in locations 

meeting the current SO2 NAAQS, 

numerous epidemiologic studies 

reported positive associations between 

ambient SO2 concentrations and 

respiratory symptoms in children, as 

well as emergency department visits 

and hospitalizations for all respiratory 

causes and asthma across multiple age 

groups. Moreover, the ISA concluded 

that these epidemiologic studies were 

consistent and coherent. This evidence 

was consistent in that associations were 

reported in studies conducted in 

numerous locations and with a variety 

of methodological approaches (ISA, 

section 5.2; p. 5–5). It was coherent in 

that respiratory symptom results from 

epidemiologic studies of short-term 

(predominantly 1-hour daily maximum 

or 24-hour average) SO2 concentrations 

were generally in agreement with 

respiratory symptom results from 

controlled human exposure studies of 

5–10 minutes. These results were also 

coherent in that the respiratory effects 

observed in controlled human exposure 

studies of 5–10 minutes further 

provided a basis for a progression of 

respiratory morbidity that could lead to 

the increased emergency department 

visits and hospital admissions observed 

in epidemiologic studies (ISA, section 

5.2; p. 5–5). In addition, the ISA found 

that when evaluated as a whole, SO2

effect estimates in multi-pollutant 

models generally remained positive and 

relatively unchanged when copollutants were included. Therefore, 

although recognizing the uncertainties 

associated with separating the effects of 

SO2 from those of co-occurring 

pollutants, the ISA concluded that ‘‘the 

limited available evidence indicates that 

the effect of SO2 on respiratory health 

outcomes appears to be generally robust 

and independent of the effects of 

gaseous co-pollutants, including NO2

and O3, as well as particulate copollutants, particularly PM2.5’’ 

(ISA, section 5.3; p. 5–9). 

The ISA also found that the 

respiratory effects of SO2 were 

consistent with the mode of action as it 

is currently understood from animal 

toxicological and controlled human 

exposure studies (ISA, section 5.2; p. 5– 

2). The immediate effect of SO2 on the 

respiratory system is 

bronchoconstriction. This response is 

mediated by chemosensitive receptors 

in the tracheobronchial tree. Activation 

of these receptors triggers central 

nervous system reflexes that result in 


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5We also note that very young children were not 

included in the controlled human exposure studies 

and this absence of data on what is likely to be a 

sensitive life stage is a source of uncertainty for 

children’s susceptibility to SO2. 

bronchoconstriction and respiratory 

symptoms that are often followed by 

rapid shallow breathing (id). The ISA 

noted that asthmatics are likely more 

sensitive to the respiratory effects of SO2

due to pre-existing inflammation 

associated with the disease. For 

example, pre-existing inflammation may 

lead to enhanced release of 

inflammatory mediators, and/or 

enhanced sensitization of the 

chemosensitive receptors (id). 

Taken together, the ISA concluded 

that the controlled human exposure, 

epidemiologic, and toxicological 

evidence supported its determination of 

a causal relationship between 

respiratory morbidity and short-term (5- 

minutes to 24-hours) exposure to SO2. 

a. Adversity of Short-Term Respiratory 

Morbidity Effects 

As discussed more fully in the 

proposal (section II.B.1.c, 74 FR at 

64817) and in section II.E.2.b below, 

based on: (1) American Thoracic Society 

(ATS) guidelines; (2) advice and 

recommendations from CASAC (see 

specific consensus CASAC comments in 

sections II.E.2.b and II.F.4.b below); and 

(3) conclusions from previous NAAQS 

reviews, EPA found that 5–10 minute 

exposures to SO2 concentrations at least 

as low as 200 ppb can result in adverse 

health effects in some asthmatics (i.e., 

5–30% of the tested individuals in 

controlled human exposure studies of 

200–300 ppb). As just mentioned, at SO2

concentrations ≥ 400 ppb, controlled 

human exposure studies have reported 

decrements in lung function that are 

often statistically significant at the 

group mean level, and that are 

frequently accompanied by respiratory 

symptoms. Being mindful that the ATS 

guidelines specifically indicate 

decrements in lung function with 

accompanying respiratory symptoms as 

being adverse (see proposal section 

II.B.1.c, 74 FR at 64817 and section 

II.E.2.b below), exposure to 5–10 minute 

SO2 concentrations ≥ 400 ppb can result 

in health effects that are clearly adverse. 

The ATS also indicated that exposure 

to air pollution that increases the risk of 

an adverse effect to a population is 

adverse, even though it may not 

increase the risk of any individual to an 

unacceptable level (ATS 2000; see 

proposal section II.B.1.c, 74 FR at 

64817). As an example, ATS states: 

A population of children with asthma 

could have a distribution of lung function 

such that no individual child has a level 

associated with significant impairment. 

Exposure to air pollution could shift the 

distribution toward lower levels without 

bringing any individual child to a level that 

is associated with clinically relevant 

consequences. Individuals within the 

population would, however, have 

diminished reserve function and are at 

potentially increased risk if affected by 

another agent, e.g., a viral infection. 

Assuming that the relationship between the 

risk factor and the disease is causal, the 

committee considered that such a shift in the 

risk factor distribution, and hence the risk 

profile of the exposed population, should be 

considered adverse, even in the absence of 

the immediate occurrence of frank illness 

(ATS 2000, p. 668). 

As mentioned above, the ISA reported 

that exposure to SO2 concentrations as 

low as 200–300 ppb for 5–10 minutes 

results in approximately 5–30% of 

exercising asthmatics experiencing 

moderate or greater decrements in lung 

function (defined in terms of a ≥ 15% 

decline in FEV1 or 100% increase in 

sRaw; ISA, Table 3–1). Even though 

these results were not statistically 

significant at the group mean level, in 

light of EPA’s interpretation of how to 

apply the ATS guidelines for defining 

an adverse effect, as described above, 

the REA found that these results could 

reasonably indicate an SO2-induced 

shift in these lung function 

measurements for this subset of the 

population. As a result, an appreciable 

percentage of exercising asthmatics 

exposed to SO2 concentrations as low as 

200 ppb would be expected to have 

diminished reserve lung function and 

would be expected to be at greater risk 

if affected by another respiratory agent, 

for example, viral infection. 

Importantly, as explained immediately 

above, diminished reserve lung function 

in a population that is attributable to air 

pollution is considered an adverse effect 

under ATS guidance. In addition to the 

2000 ATS guidelines, the REA was also 

mindful of previous CASAC 

recommendations (Henderson 2006) and 

NAAQS review conclusions (EPA 2006, 

EPA 2007d) indicating that moderate 

decrements in lung function can be 

clinically significant in some asthmatics 

(discussed in detail below, see section 

II.E.2.b). The REA further considered 

that subjects participating in these 

controlled human exposure studies do 

not include severe asthmatics and that 

it was reasonable to presume that 

persons with more severe asthma than 

the study participants would have a 

more serious health effect from shortterm exposure to 200 ppb SO2.5 Taken 

together, the REA concluded that 

exposure to SO2 concentrations at least 

as low as 200 ppb can result in adverse 

health effects in asthmatics and that this 

conclusion was in agreement with 

consensus CASAC comments and 

recommendations expressed during the 

current SO2 NAAQS review (see 

sections II.E.2.b and II.F.4.b below). 

In addition to the controlled human 

exposure evidence, epidemiologic 

studies also indicate that adverse 

respiratory morbidity effects are 

associated with SO2 (REA, section 4.3). 

As mentioned above, in reaching the 

conclusion of a causal relationship 

between respiratory morbidity and 

short-term SO2 exposure, the ISA 

generally found positive associations 

between ambient SO2 concentrations 

and emergency department visits and 

hospitalizations for all respiratory 

causes and asthma. Notably, emergency 

department visits, hospitalizations, 

episodic respiratory illness, and 

aggravation of respiratory diseases (e.g. 

asthma) attributable to air pollution are 

considered adverse health effects under 

ATS guidelines. 

2. Health Effects and Long-Term 

Exposures to SO2

There were numerous studies 

published since the last review 

examining possible associations 

between long-term SO2 exposure and 

mortality and morbidity (respiratory 

morbidity, carcinogenesis, adverse 

prenatal and neonatal outcomes) 

endpoints. However, the ISA concluded 

that the evidence relating long-term 

(weeks to years) SO2 exposure to 

adverse health effects was ‘‘inadequate 

to infer the presence or absence of a 

causal relationship’’ (ISA, Table 5–3). 

That is, the ISA found the long-term 

health evidence to be of insufficient 

quantity, quality, consistency, or 

statistical power to make a 

determination as to whether SO2 was 

truly associated with these health 

outcomes (ISA, Table 1–2). 

3. SO2-Related Impacts on Public Health 

Interindividual variation in human 

responses to air pollutants indicates that 

some populations are at increased risk 

for the detrimental effects of ambient 

exposure to SO2. The NAAQS are 

intended to provide an adequate margin 

of safety for both the general population 

and susceptible populations that are 

potentially at increased risk for health 

effects in response to exposure to 

ambient air pollution (see footnote 1 

above). To facilitate the identification of 

populations at increased risk for SO2- 

related health effects, studies have 

identified factors that contribute to the 

susceptibility of individuals to SO2.

Susceptible individuals are broadly 

defined as those with a greater 


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6This aspect of susceptibility is referred to as 

vulnerability in the proposal and in the ISA. 

7The ISA cites one chamber study with 

intermittent exercise where healthy and asthmatic 

Continued 

likelihood of an adverse outcome given 

a specific exposure in comparison with 

the general population (American Lung 

Association, 2001). The susceptibility of 

an individual to SO2 can encompass a 

multitude of factors which represent 

normal developmental phases or life 

stages (e.g., age) or biologic attributes 

(e.g., gender); however, other factors 

(e.g., socioeconomic status (SES)) may 

influence the manifestation of disease 

and also increase an individual’s 

susceptibility (American Lung 

Association, 2001). In addition, 

populations may be at increased risk to 

SO2 due to an increase in their exposure 

during certain life stages (e.g., 

childhood or old age) or as a result of 

external factors (e.g., SES) that 

contribute to an individual being 

disproportionately exposed to higher 

concentrations than the general 

population.6 It should be noted that in 

some cases specific populations may be 

affected by multiple susceptibility 

factors. For example, a population that 

is characterized as having low SES may 

have less access to healthcare resulting 

in the manifestation of a disease, which 

increases their susceptibility to SO2, 

while they may also reside in a location 

that results in disproportionately high 

exposure to SO2. 

To examine whether SO2

differentially affects certain 

populations, stratified analyses are often 

conducted in epidemiologic 

investigations to identify the presence 

or absence of effect modification. A 

thorough evaluation of potential effect 

modifiers may help identify susceptible 

populations that are at increased risk to 

SO2 exposure. These analyses are based 

on the proper identification of 

confounders and subsequent adjustment 

for them in statistical models, which 

helps separate a spurious from a true 

causal association. Although the design 

of toxicological and human clinical 

studies does not allow for an extensive 

examination of effect modifiers, the use 

of animal models of disease and the 

study of individuals with underlying 

disease or genetic polymorphisms do 

allow for comparisons between 

subgroups. Therefore, the results from 

these studies, combined with those 

results obtained through stratified 

analyses in epidemiologic studies, 

contribute to the overall weight of 

evidence for the increased susceptibility 

of specific populations to SO2. Those 

populations identified in the ISA to be 

potentially at greater risk of 

experiencing an adverse health effect 

from SO2 were described in detail in the 

proposal (section II.B.5) and include: (1) 

Those with pre-existing respiratory 

disease; (2) children and older adults; 

(3) persons who spend increased time 

outdoors or at elevated ventilation rates; 

(4) persons with lower SES; and (5) 

persons with certain genetic factors. 

As discussed in the proposal (section 

II.B.5.g, 74 FR at 64821), large 

proportions of the U.S. population are 

likely to be at increased risk of 

experiencing SO2-related health effects. 

In the United States, approximately 7% 

of adults and 9% of children have been 

diagnosed with asthma. Notably, the 

prevalence and severity of asthma is 

higher among certain ethnic or racial 

groups such as Puerto Ricans, American 

Indians, Alaskan Natives, and African 

Americans (EPA 2008b). Furthermore, a 

higher prevalence of asthma among 

persons of lower SES and an excess 

burden of asthma hospitalizations and 

mortality in minority and inner-city 

communities have been observed (EPA, 

2008b). In addition, population groups 

based on age comprise substantial 

segments of individuals that may be 

potentially at risk for SO2-related health 

impacts. Based on U.S. census data from 

2000, about 72.3 million (26%) of the 

U.S. population are under 18 years of 

age, 18.3 million (7.4%) are under 5 

years of age, and 35 million (12%) are 

65 years of age or older. There is also 

concern for the large segment of the 

population that is potentially at risk to 

SO2-related health effects because of 

increased time spent outdoors at 

elevated ventilation rates (those who 

work or play outdoors). Overall, the 

considerable size of the population 

groups at risk indicates that exposure to 

ambient SO2 could have a significant 

impact on public health in the United 

States. 

C. Human Exposure and Health Risk 

Characterization 

To put judgments about SO2- 

associated health effects into a broader 

public health context, EPA has drawn 

upon the results of the quantitative 

exposure and risk assessments. 

Judgments reflecting the nature of the 

evidence and the overall weight of the 

evidence are taken into consideration in 

these quantitative exposure and risk 

assessments. These assessments include 

estimates of the likelihood that 

asthmatic children at moderate or 

greater exertion (e.g. while exercising) 

in St. Louis or Greene County, Missouri 

would experience SO2 exposures of 

potential concern. In addition, these 

analyses include an estimate of the 

number and percent of exposed 

asthmatic children in these locations 

likely to experience SO2-induced lung 

function responses (i.e., moderate or 

greater decrements in lung function 

defined in terms of sRaw or FEV1) under 

varying air quality scenarios (i.e., 

current air quality and air quality 

simulated to just meet the current or 

potential alternative standards). These 

assessments also characterize the kind 

and degree of uncertainties inherent in 

such estimates. 

As previously mentioned, the ISA 

concluded that the evidence for an 

association between respiratory 

morbidity and short-term SO2 exposure 

was ‘‘sufficient to infer a causal 

relationship’’ (ISA, section 5.2) and that 

the ‘‘definitive evidence’’ for this 

conclusion was from the results of 5–10 

minute controlled human exposure 

studies demonstrating decrements in 

lung function and/or respiratory 

symptoms in exercising asthmatics (ISA, 

section 5.2). Accordingly, the air quality 

and exposure analyses and their 

associated risk characterizations focused 

on 5-minute concentrations of SO2 in 

excess of potential health effect 

benchmark values derived from the 

controlled human exposure literature 

(see proposal section II.C.1, 74 FR at 

64821, and REA, section 6.2). These 

benchmark levels are not potential 

standards, but rather are SO2 exposure 

concentrations which represent 

‘‘exposures of potential concern’’ which 

are used in these analyses to estimate 

potential exposures and risks associated 

with 5-minute concentrations of SO2. 

The REA considered 5-minute 

benchmark levels of 100, 200, 300, and 

400 ppb in these analyses, but 

especially noted exceedances or 

exposures with respect to the 200 and 

400 ppb 5-minute benchmark levels. 

These benchmark levels were 

highlighted because (1) 400 ppb 

represents the lowest concentration in 

free-breathing controlled human 

exposure studies where moderate or 

greater lung function decrements 

occurred which were often statistically 

significant at the group mean level and 

were frequently accompanied by 

respiratory symptoms; and (2) 200 ppb 

is the lowest level at which moderate or 

greater decrements in lung function in 

free-breathing controlled human 

exposure studies were found in some 

individuals, although these lung 

function changes were not statistically 

significant at the group mean level. 

Notably, 200 ppb is also the lowest level 

that has been tested in free-breathing 

controlled human exposure studies 

(REA, section 4.2.2).7


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children were exposed to 100 ppb SO2 in a mixture 

with ozone and sulfuric acid. The ISA notes that 

compared to exposure to filtered air, exposure to 

the pollutant mix did not result in statistically 

significant changes in lung function or respiratory 

symptoms (ISA, section 3.1.3.4). 

8Benchmark values derived from the controlled 

human exposure literature were associated with a 

5-minute averaging time. However, as noted in 

footnote 3 above, only 98 ambient monitors located 

in 13 States from 1997–2007 reported measured 5- 

minute SO2 concentrations since such monitoring is 

not required (see section II.A.2 and section IV). In 

contrast, 809 monitors in 48 States, DC, Puerto Rico, 

and the Virgin Islands reported 1-hour SO2

concentrations over a similar time period. 

Therefore, to broaden analyses to areas where 

measured 5-minute SO2 concentrations were not 

available, the REA utilized a statistical relationship 

to estimate the highest 5-minute level in an hour, 

given a reported 1-hour average SO2 concentration 

(REA, section 6.4). Then, similar to measured 5- 

minute SO2 concentrations, statistically estimated 

5-minute SO2 concentrations were compared to 5- 

minute potential health effect benchmark values 

(REA, chapters 7 and 8, respectively). 

9EPA recently conducted a complete quality 

assurance review of all individual subject data. The 

results of this review did not substantively change 

any of the entries in ISA, Table 3–1, and did not 

in any way affect the conclusions of the ISA (see 

Johns and Simmons, 2009). 

The REA utilized three approaches to 

characterize health risks. In the first 

approach, for each air quality scenario, 

statistically estimated 5-minute SO2

concentrations 8 and measured ambient 

5-minute SO2 concentrations were 

compared to the 5-minute potential 

health effect benchmark levels 

discussed above (REA, chapter 7). This 

air quality analysis included all 

available ambient monitoring data as 

well as a more detailed analysis in 40 

counties. The air quality analysis was 

considered a broad characterization of 

national air quality and human 

exposures that might be associated with 

these 5-minute SO2 concentrations. An 

advantage of the air quality analysis is 

its relative simplicity; however, there is 

uncertainty associated with the 

assumption that SO2 air quality can 

serve as an adequate surrogate for total 

exposure to ambient SO2. Actual 

exposures might be influenced by 

factors not considered by this approach, 

including small-scale spatial variability 

in ambient SO2 concentrations (which 

might not be represented by the current 

fixed-site ambient monitoring network) 

and spatial/temporal variability in 

human activity patterns. A more 

detailed overview of the air quality 

analysis and its associated limitations 

and uncertainties is provided in the 

proposal (see sections II.C.2, 74 FR at 

64822 and II.C.3, 74 FR at 64823, 

respectively) and the air quality analysis 

is thoroughly described in the REA 

(chapter 7). 

In the second approach, an inhalation 

exposure model was used to generate 

more realistic estimates of personal 

exposures in asthmatics (REA, chapter 

8). This analysis estimated temporally 

and spatially variable 

microenvironmental 5-minute SO2

concentrations and simulated 

asthmatics’ contact with these pollutant 

concentrations while at moderate or 

greater exertion (i.e., while at elevated 

ventilation rates). The approach was 

designed to estimate exposures that are 

not necessarily represented by the 

existing ambient monitoring data and to 

better represent the physiological 

conditions corresponding with the 

respiratory effects reported in controlled 

human exposure studies. AERMOD, an 

EPA dispersion model, was used to 

estimate 1-hour ambient SO2

concentrations using emissions 

estimates from stationary, non-point, 

and where applicable, port sources. The 

Air Pollutants Exposure (APEX) model, 

an EPA human exposure model, was 

then used to estimate population 

exposures using the estimated hourly 

census block level SO2 concentrations. 

From the 1-hour census block 

concentrations, 5-minute maximum SO2

concentrations within each hour were 

estimated by APEX (REA, section 8.7.1) 

using the statistical relationship 

mentioned above in footnote 8. 

Estimated exposures to 5-minute SO2

levels were then compared to the 5- 

minute potential health effect 

benchmark levels discussed above. This 

approach to assessing exposures was 

more resource intensive than using 

ambient levels as an indicator of 

exposure; therefore, the final REA 

included the analysis of two locations: 

St. Louis and Greene County, MO. 

Although the geographic scope of this 

analysis was limited, the approach 

provided estimates of SO2 exposures in 

asthmatics and asthmatic children in St. 

Louis and Greene Counties, and thus 

served to complement the broader air 

quality characterization. A more 

detailed overview of this exposure 

analysis and its associated limitations 

and uncertainties is provided in the 

proposal (see sections II.C.2, 74 FR at 

64822 and II.C.3, 74 FR at 64823, 

respectively) and the exposure analysis 

is thoroughly described in the REA 

(chapter 8). 

The third approach was a quantitative 

risk assessment. This approach 

combined results from the exposure 

analysis (i.e., the number of exposed 

total asthmatics or asthmatic children 

while at moderate or greater exertion) 

with exposure-response functions 

derived from individual level data from 

controlled human exposure studies (see 

ISA, Table 3–1 and Johns (2009) 9) to 

estimate the percentage and number of 

exposed asthmatics and asthmatic 

children in St. Louis and Greene County 

likely to experience a moderate or 

greater lung function response (i.e., 

decrements in lung function defined in 

terms of FEV1 and sRaw) under the air 

quality scenarios mentioned above 

(REA, chapter 9). A more detailed 

overview of this analysis and its 

associated limitations and uncertainties 

is provided in the proposal (see sections 

II.C.2, 74 FR at 64822 and II.C.3, 74 FR 

at 64823, respectively) and the 

quantitative risk analysis is thoroughly 

described in the REA (chapter 9). 

Notably, for the reasons described in 

the REA (REA, section 10.3.3) and the 

proposal (see section II.E.1.b, 74 FR at 

64827), when considering the St. Louis 

and Greene County exposure and risk 

results as they relate to the adequacy of 

the current standards, the REA 

concluded that the St. Louis results 

were more informative in terms of 

ascertaining the extent to which the 

current standards protect against health 

effects linked to the various benchmarks 

(linked in turn to 5-minute SO2

exposures). The results in fact suggested 

that the current standards may not 

adequately protect public health (REA, 

section 10.3.3, p. 364). Moreover, the 

REA judged that the exposure and risk 

estimates for the St. Louis study area 

provided useful insights into exposures 

and risks for other urban areas in the 

U.S. with similar population and SO2

emissions densities (id.). For similar 

reasons, the St. Louis results were more 

informative for ascertaining the 

adequacy of the potential alternative 

standards under consideration. 

Key results of the air quality, 

exposure, and risk analyses were 

presented in the policy assessment 

chapter of the REA (chapter 10) and 

summarized in the proposal (see Tables 

2–4 in the preamble to the proposed 

rule). In considering these results, the 

proposal noted that these analyses 

support that 5-minute SO2 exposures, 

reasonably judged important from a 

public health perspective, were 

associated with air quality adjusted 

upward to simulate just meeting the 

current standards (see proposal, section 

II.E.1.c, 74 FR at 64828). Moreover, 

these results indicated that 99th 

percentile 1-hour daily maximum 

standard levels in the range of 50–100 

ppb could substantially limit exposures 

of asthmatic children at moderate or 

greater exertion from 5-minute SO2

concentrations ≥400 ppb, and 

appreciably limit exposures of these 

children from 5-minute SO2

concentrations ≥200 ppb (REA, p. 392– 

393). Results of these analyses also 

indicated that a 1-hour standard at 150 


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ppb could still substantially limit 

exposures of asthmatic children at 

moderate or greater exertion from 5- 

minute SO2 concentrations ≥400 ppb, 

but would provide these children 

appreciably less protection from 

exposure to 5-minute SO2

concentrations ≥200 ppb (REA, p. 395– 

396). 

D. Approach for Determining Whether 

To Retain or Revise the Current 

Standards 

EPA notes that the final decision on 

retaining or revising the current primary 

SO2 standards is a public health policy 

judgment to be made by the 

Administrator. This judgment has been 

informed by a recognition that the 

available health effects evidence reflects 

a continuum consisting of ambient 

levels of SO2 at which scientists 

generally agree that health effects are 

likely to occur, through lower levels at 

which the likelihood and magnitude of 

the response become increasingly 

uncertain. The Administrator’s final 

decisions draw upon scientific 

information and analyses related to 

health effects, population exposures and 

risks; judgments about the appropriate 

response to the range of uncertainties 

that are inherent in the scientific 

evidence and analyses; and comments 

received from CASAC and the public. 

To evaluate whether the current 

primary SO2 standards are adequate or 

whether revisions are appropriate, EPA 

has used an approach in this review 

described in chapter 10 of the REA 

which builds upon the approaches used 

in reviews of other criteria pollutants, 

including the most recent reviews of the 

NO2, Pb, O3, and PM NAAQS (EPA, 

2008c; EPA, 2007c; EPA, 2007d; EPA, 

2005), and reflects the latest body of 

evidence and information that is 

currently available, as reflected by the 

ISA. As in other recent reviews, EPA 

considered the implications of placing 

more or less weight or emphasis on 

different aspects of the scientific 

evidence and the exposure-/risk-based 

information, recognizing that the weight 

to be given to various elements of the 

evidence and exposure/risk information 

is part of the public health policy 

judgments that the Administrator will 

make in reaching decisions on the 

standard. 

A series of general questions framed 

this approach to considering the 

scientific evidence and exposure-/riskbased information. First, EPA’s 

consideration of the scientific evidence 

and exposure/risk information with 

regard to the adequacy of the current 

standards has been framed by the 

following questions: 

• To what extent does evidence that has 

become available since the last review 

reinforce or call into question evidence for 

SO2-associated effects that were identified in 

the last review? 

• To what extent has evidence for different 

health effects and/or susceptible populations 

become available since the last review? 

• To what extent have uncertainties 

identified in the last review been reduced 

and/or have new uncertainties emerged? 

• To what extent does evidence and 

exposure-/risk-based information that has 

become available since the last review 

reinforce or call into question any of the 

basic elements (indicator, averaging time, 

form, and level) of the current standard? 

To the extent that the available 

evidence and exposure-/risk-based 

information suggests it may be 

appropriate to consider revision of the 

current standards, EPA considers that 

evidence and information with regard to 

its support for consideration of a 

standard that is either more or less 

stringent than the current standards. 

This evaluation is framed by the 

following questions: 

• Is there evidence that associations, 

especially causal or likely causal 

associations, extend to ambient SO2

concentrations as low as, or lower than, the 

concentrations that have previously been 

associated with health effects? If so, what are 

the important uncertainties associated with 

that evidence? 

• Are exposures above benchmark levels 

and/or health risks estimated to occur in 

areas that meet the current standard? If so, 

are the estimated exposures and health risks 

important from a public health perspective? 

What are the important uncertainties 

associated with the estimated risks? 

To the extent that there is support for 

consideration of a revised standard, EPA 

then considers the specific elements of 

the standard (indicator, averaging time, 

form, and level) within the context of 

the currently available information. In 

so doing, the Agency addresses the 

following questions regarding the 

elements of the standard: 

• Does the evidence provide support for 

considering a different indicator for gaseous 

SOX? 

• Does the evidence provide support for 

considering different, or additional averaging 

times? 

• What ranges of levels and forms of 

alternative standards are supported by the 

evidence, and what are the associated 

uncertainties and limitations? 

• To what extent do specific averaging 

times, levels, and forms of alternative 

standards reduce the estimated exposures 

above benchmark levels and risks attributable 

to exposure to ambient SO2, and what are the 

uncertainties associated with the estimated 

exposure and risk reductions? 

The questions outlined above have 

been addressed in the REA. The 

following sections present 

considerations regarding the adequacy 

of the current standards and 

conclusions on the elements of a new 

short-term standard in terms of 

indicator, averaging time, form, and 

level. 

E. Adequacy of the Current Standards 

This section discusses considerations 

related to the decision as to whether the 

current 24-hour and annual SO2 primary 

NAAQS are requisite to protect public 

health with an adequate margin of 

safety. Specifically, section II.E.1 

provides an overview of the rationale 

supporting the Administrator’s proposal 

that the current standards do not 

provide adequate public health 

protection; section II.E.2 discusses 

public comments received on the 

adequacy of the current standards; and 

section II.E.3 discusses the 

Administrator’s final decision on 

whether the current SO2 primary 

NAAQS is requisite to protect public 

health with an adequate margin of 

safety, as required by sections 109(d) 

and (b) of the Act. 

1. Rationale for Proposed Decision 

In the proposal, the Administrator 

initially concluded that the current 24- 

hour and annual SO2 NAAQS were not 

adequate to protect public health with 

an adequate margin of safety (see 

section II.E.4, 74 FR at 64829). In 

reaching this conclusion, she 

considered the: (1) Scientific evidence 

and conclusions in the ISA; (2) exposure 

and risk information presented in the 

REA; (3) conclusions of the policy 

assessment chapter of the REA; and (4) 

views expressed by CASAC. These 

considerations are discussed in detail in 

the proposal (see section II.E., 74 FR at 

64826) and are summarized in this 

section. 

In the proposal the Administrator 

noted the following in considering the 

adequacy of the current 24-hour and 

annual primary SO2 standards: 

• The conclusion of the ISA that the 

results of controlled human exposure 

and epidemiologic studies form a 

plausible and coherent data set that 

supports a causal relationship between 

short-term (5-minutes to 24-hours) SO2

exposures and adverse respiratory 

effects, and that the epidemiologic 

evidence (buttressed by the clinical 

evidence) indicates that the effects seen 

in the epidemiologic studies are 

attributable to exposure to SO2 (ISA, 

section 5.2). 

• The conclusion of the ISA that ‘‘[i]n 

the epidemiologic studies, respiratory 

effects were observed in areas where the 

maximum ambient 24-h avg SO2

concentration was below the current 24- 


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h avg NAAQS level * * *.’’ (ISA, 

section 5.2, p. 5–2.) and so would occur 

at ambient SO2 concentrations that are 

present in locations meeting the current 

24-hour NAAQS. 

• These respiratory effects also 

occurred in areas with annual air 

quality levels considerably lower than 

those allowed by the current annual 

standard, indicating that the current 

annual standard is also not providing 

protection against short-term health 

effects reported in epidemiologic 

studies (ISA, section 5.2). 

• Analyses in the REA supporting 

that 5-minute exposures, reasonably 

judged important from a public health 

perspective (i.e., respiratory effects 

judged to be adverse to the health of 

asthmatics, see sections II.B.1.c above, 

and II.E.2.b below), were associated 

with air quality adjusted upward to 

simulate just meeting the current 24- 

hour and annual standards. 

• CASAC advice ‘‘that the current 

24-hour and annual standards are not 

adequate to protect public health, 

especially in relation to short term 

exposures to SO2 (5–10 minutes) by 

exercising asthmatics’’ (Samet, 2009, 

p. 15). 

Based on these considerations 

(discussed in more detail in the 

proposal, see sections II.E.1 and II.E.2), 

the Administrator proposed that the 

current 24-hour and annual SO2

standards are not requisite to protect 

public health with an adequate margin 

of safety against adverse respiratory 

effects associated with short-term 

(5-minute to 24-hour) SO2 exposures. In 

considering approaches to revising the 

current standards, the Administrator 

initially concluded it appropriate to 

consider setting a new 1-hour standard. 

The Administrator noted that a 1-hour 

standard would likely provide increased 

public health protection, especially for 

members of at-risk groups, from the 

respiratory effects described in both 

epidemiologic and controlled human 

exposure studies. 

2. Comments on the Adequacy of the 

Current Standards 

This section discusses public 

comments on the proposal that either 

supported or opposed the 

Administrator’s proposed decision to 

revise the current SO2 primary NAAQS. 

Comments on the adequacy of the 

current standards that focused on the 

scientific and/or the exposure/risk basis 

for the Administrator’s proposed 

conclusions are discussed in sections 

II.E.2.a–II.E.2.c. Comments on the 

epidemiologic evidence are considered 

in section II.E.2.a. Comments on the 

controlled human exposure evidence 

are considered in section II.E.2.b. 

Comments on human exposure and 

health risk assessments are considered 

in section II.E.2.c. To the extent these 

comments on the evidence and 

information are also used to justify 

commenters’ conclusions on decisions 

related to indicator, averaging time, 

form, or level, they are noted as well in 

the appropriate sections below (II.F.1– 

II.F.4, respectively). The summaries of 

comments, and responses thereto, 

presented below are not exclusive: other 

comments and responses are being 

included in the Response to Comment 

(RTC) Document which is part of the 

record for this rulemaking (EPA, 2010). 

Many public commenters agreed with 

the proposal that based on the available 

information, the current SO2 standards 

are not requisite to protect public health 

with an adequate margin of safety and 

that revisions to the standards are 

therefore appropriate. Among those 

calling for revisions to the standards 

were environmental groups (e.g., Sierra 

Club, WEACT for Environmental 

Justice, Center for Biological Diversity, 

(CBD) Environmental Defense Fund 

(EDF), Natural Resources Defense 

Council (NRDC)); medical/public health 

organizations (e.g., American Lung 

Association (ALA), American Thoracic 

Society (ATS)); State environmental 

organizations (e.g., National Association 

of Clean Air Agencies (NACAA), 

Northeast States for Coordinated Air 

Use Management (NESCAUM); State 

environmental agencies (e.g., such 

agencies in DE, IA, IL, MI, NY, NM, OH, 

PA, TX, VT); the Fond du Lac Band of 

Lake Superior Chippewa (Fond du Lac) 

Tribe, local groups (e.g., HoustonGalveston Area Council, Alexandria 

Department of Transportation and 

Environmental Services) and most 

individual commenters (∼13,000). These 

commenters generally concluded that 

the current SO2 standards need to be 

revised and that a more stringent 

standard is needed to protect the health 

of susceptible population groups. In 

supporting the need to adopt a more 

stringent NAAQS for SO2, these 

commenters often referenced the 

conclusions of CASAC, as well as 

evidence and information presented in 

the proposal. As such, the rationale 

offered by these commenters was 

consistent with that presented in the 

proposal to support the Administrator’s 

proposed decision to revise the current 

SO2 NAAQS. 

Most industry commenters (e.g., 

Utility Air Regulatory Group (UARG), 

American Petroleum Institute (API), 

Arizona Public Service, National 

Petrochemical & Refiners Association 

(NPRA), Montana-Dakota Utilities Co., 

Dominion Resources, Council of 

Industrial Boiler Owners (CIBO), Edison 

Electric Institute (EEI), Duke Energy, 

National Mining Association (NMA)); 

and some organizations (e.g., Texas 

Association of Business, The Annapolis 

Center for Science-Based Public Policy 

(ACSBPP), South Carolina Chamber of 

Commerce) opposed the proposed 

revisions to the SO2 primary NAAQS. In 

supporting their views, industry 

commenters generally concluded that 

EPA did not appropriately consider 

uncertainties associated with the 

epidemiologic and controlled human 

exposure evidence. 

More specifically, with respect to the 

epidemiologic studies, many of these 

commenters concluded that results of 

these studies are confounded by copollutants and thus too uncertain to 

determine whether SO2 is truly 

associated with the health outcomes 

being measured (e.g., hospital 

admissions; Federal Register see 

below). With respect to the controlled 

human exposure studies, many 

commenters were critical of the 5- 

minute benchmark levels that were 

derived from these studies and 

subsequently used by EPA in the air 

quality, exposure, and risk analyses. 

These groups were particularly 

concerned about the Administrator’s 

reliance on the 200 ppb 5-minute 

benchmark level in assessing the 

adequacy of the current and potential 

alternative standards. In general, many 

industry groups maintained that adverse 

respiratory effects did not occur 

following 5–10 minute SO2 exposures 

< 400 ppb (e.g., API, EEI, CIBO) and 

some groups stated that even at SO2

concentrations ≥ 400 ppb, reported 

effects may not be of clinical concern, 

and thus are likely not adverse (e.g., 

UARG). Many industry groups (e.g., 

API, UARG) also disagreed with EPA’s 

(and CASAC’s) conclusions that severe 

asthmatics were not included in these 

controlled human exposure studies, and 

that severe asthmatics would likely have 

a more pronounced response to SO2

exposures at a given level, or would 

respond to even lower levels of SO2. 

In responding to these specific 

comments, we note that the 

Administrator relied in the proposal on 

the evidence, information, and 

judgments contained in the ISA and the 

REA (including the policy assessment 

chapter), as well as on the advice of 

CASAC. In considering the evidence, 

information, and judgments of the ISA 

and the REA, the Agency notes that 

these documents have been reviewed 

and discussed extensively by CASAC at 

multiple public meetings (see above, 

section I.D) and in their letters to the 


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10As noted in the proposal (see sections II.D.1, 74 

FR at 64824–64825 and II.F.4.a, 74 FR at 64835), 

there is special sensitivity in this review in 

disentangling SO2-related effects from PM-related 

effects (especially sulfate PM). 

EPA Administrator. Thus, it is 

important to note that CASAC generally 

accepted the key findings and 

conclusions presented in both the ISA 

and REA (see Henderson 2008a, 

Henderson 2008b, and Samet, 2009). 

a. Comments on EPA’s Interpretation of 

the Epidemiologic Evidence 

Many industry groups (e.g., API, 

UARG, American Chemistry Council 

(ACC), Dominion Resources, 

ExxonMobil, Progress Energy, CIBO, 

The Fertilizer Institute, EEI, Dow 

Chemical Company (Dow), 

MeadWestvaco Corporation (MWV), 

(NMA) and some organizations (e.g., 

ACSBPP) commented that, given the 

presence of numerous co-pollutants in 

the air, the epidemiologic studies do not 

support the contention that SO2 itself is 

causing health effects. For example, 

UARG stated: ‘‘The epidemiological 

evidence cannot determine that SO2 is 

a cause of or a contributor to hospital 

admissions (‘‘HA’’), emergency 

department (‘‘ED’’) visits or respiratory 

symptoms, the effects cited in the 

Proposed Rule.’’ 

Although EPA has recognized that 

multiple factors can contribute to the 

etiology of respiratory disease and that 

more than one air pollutant could 

independently impact respiratory 

health, we continue to judge, as 

discussed in the ISA, that the available 

evidence supports the conclusion that 

there is an independent effect of SO2 on 

respiratory morbidity. In reaching this 

judgment, we recognize that a major 

methodological issue affecting SO2

epidemiologic studies concerns the 

evaluation of the extent to which other 

air pollutants, particular PM2.5,10 may 

confound or modify SO2-related effect 

estimates. The use of multi-pollutant 

regression models is a common 

approach for evaluating potential 

confounding by co-pollutants in 

epidemiologic studies. It is therefore 

important to note that when the ISA 

evaluated U.S. and international 

epidemiologic studies employing multipollutant models, SO2 effect estimates 

generally remained positive and 

relatively unchanged when copollutants, including PM, were included 

(see ISA, p. 5–5). Therefore, although 

recognizing the uncertainties associated 

with separating the effects of SO2 from 

those of co-occurring pollutants, the ISA 

concluded that the limited available 

evidence indicates that the effect of SO2

on respiratory health outcomes appears 

to be generally robust and independent 

of the effects of gaseous co-pollutants, 

including NO2 and O3, as well as 

particulate co-pollutants, particularly 

PM2.5 (ISA, section 5.2; p. 5–9). 

In considering questions of 

confounding and causation, the 

epidemiologic studies should not be 

considered in a vacuum. As emphasized 

by the ISA, and endorsed by CASAC, 

controlled human exposure studies 

provide support for the plausibility of 

the associations reported in 

epidemiologic studies (ISA, section 5–5; 

Henderson 2008a; Henderson 2008b). 

These controlled human exposure 

studies exposed exercising asthmatics to 

5–10 minute peaks of SO2 and reported 

decrements in lung function and/or 

respiratory symptoms in up to 60% of 

these individuals (depending on 

exposure concentration; see ISA, Table 

5–3; p. 5–11). Thus, these experimental 

study results provide strong support for 

an independent contribution of SO2 to 

the respiratory health effects reported in 

epidemiologic studies: ‘‘The effects of 

SO2 on respiratory symptoms, lung 

function, and airway inflammation 

observed in the human clinical studies 

using peak exposures further provides a 

basis for a progression of respiratory 

morbidity resulting in increased 

emergency department visits and 

hospital admissions. Collectively, these 

findings provide biological plausibility 

for the observed association between 

ambient SO2 levels and emergency 

department visits and hospitalizations 

for all respiratory diseases and asthma, 

notably in children and older adults. 

***’’ (ISA, section 5.2 at p. 5–5). 

Thus, EPA is not relying solely on the 

epidemiologic studies to evaluate 

whether associations reported in these 

studies (e.g., associations with 

emergency department visits) are likely 

the result of ambient SO2 exposure. 

b. Comments on EPA’s Interpretation of 

the Controlled Human Exposure 

Evidence 

Many industry groups (e.g., API, ACC, 

Progress Energy, EEI, CIBO) commented 

that adverse health effects do not occur 

following 5–10 minute SO2 exposures 

< 400 ppb. In addition, some groups 

(e.g., UARG) commented that adverse 

respiratory effects do not occur in 

exercising asthmatics following SO2

exposures below 600 ppb. The 

disagreement is not whether effects 

occur in exercising asthmatics at these 

exposure levels and exposure durations. 

Rather, the issue is whether the effects 

experienced can properly be regarded as 

adverse. In general, these groups 

conclude that EPA’s judgment of 

adverse health effects at SO2 exposure 

levels below 600 or 400 ppb is 

inappropriately based on an unsound 

interpretation of ATS guidelines. More 

specifically, these groups generally 

contend that decrements in lung 

function without accompanying 

respiratory symptoms are not adverse 

effects of SO2 exposure, and that 

decrements in lung function in a 

percentage of exercising asthmatics does 

not represent a shift in lung function at 

the population level. Some of these 

groups also contend that EPA followed 

the advice of individual CASAC 

members, rather than consensus CASAC 

written comments on the ISA and REA 

when concluding respiratory effects 

associated with SO2 exposures below 

600 or 400 ppb are adverse. 

Furthermore, some groups contend that 

effects below 400 ppb should not be 

considered adverse because compared 

to the number of asthmatics 

experiencing decrements in lung 

function, there were similar numbers of 

asthmatics experiencing increases in 

lung function. EPA disagrees with these 

comments, and believes that the clinical 

evidence also supports the conclusion 

that the current standards are not 

requisite to protect public health with 

and adequate margin of safety. 

The Agency disagrees that adverse 

respiratory effects do not occur in 

exercising asthmatics following 5–10 

minute SO2 exposures ranging from 

400–600 ppb. As previously mentioned, 

at SO2 concentrations ranging from 400– 

600 ppb, moderate or greater 

decrements in lung function occur in 

approximately 20–60% of exercising 

asthmatics (again, defined in terms of a 

≥ 15% decline in FEV1 or 100% increase 

in sRaw; ISA, Table 3–1). Moreover, at 

concentrations ≥ 400 ppb, decrements in 

lung function are often statistically 

significant at the group mean level, and 

are frequently accompanied by 

respiratory symptoms (ISA, Table 5–1). 

ATS guidelines on what constitutes an 

adverse health effect of air pollution 

clearly state that reversible loss of lung 

function in combination with the 

presence of symptoms should be 

considered adverse (ATS 1985, 2000). 

Moderate or greater decrements in lung 

function accompanied by respiratory 

symptoms fit this description. Thus, the 

Agency’s conclusion of adverse health 

effects associated with SO2

concentrations ≥ 400 ppb is consistent 

with ATS guidelines. 

The Agency also disagrees with 

industry commenters regarding the 

adversity of the respiratory effects seen 

in exercising asthmatics following 5–10 

minute SO2 exposures ranging from 

200–300 ppb. As mentioned above 

(section II.B.1), and discussed more 


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11See hearing transcripts from EPA Clean Air 

Scientific Advisory Committee (CASAC), July 30– 

31 2008, Sulfur Oxides-Health Criteria (part 3 of 4) 

pages 211–213). These transcripts can be found in 

Docket ID No. EPA–HQ–ORD–2006–0260. Available 

at http://www.regulations.gov. 

fully in the proposal (see section II.B.3, 

74 FR at 64819), the ISA reported that 

exposure to SO2 concentrations as low 

as 200–300 ppb for 5–10 minutes results 

in approximately 5–30% of exercising 

asthmatics experiencing moderate or 

greater decrements in lung function. In 

2000, the ATS updated its guidelines on 

‘‘what constitutes an adverse health 

effect of air pollution.’’ These guidelines 

indicated that exposure to air pollution 

that increases the risk of an adverse 

effect to the entire population is 

adverse, even though it may not 

increase the risk of any individual to an 

unacceptable level (ATS 2000). For 

example, ATS notes that a population of 

asthmatics could have a distribution of 

lung function such that no individual 

has a level associated with significant 

impairment. Exposure to air pollution 

could shift the distribution to lower 

levels that still do not bring any 

individual to a level that is associated 

with clinically relevant effects. 

However, this would be considered 

adverse because individuals within the 

population would have diminished 

reserve function, and therefore would be 

at increased risk if affected by another 

agent (ATS 2000). 

Considering the 2000 ATS guidelines, 

the results of the clinical studies 

conducted at 200–300 ppb were 

reasonably interpreted by EPA to 

indicate an SO2-induced shift in these 

lung function measurements for a subset 

of this population. That is, an 

appreciable percentage of this 

population of exercising asthmatics 

would be expected to experience 

moderate or greater decrements in lung 

function in response to SO2

concentrations as low as 200 ppb, and 

thus would be expected to have 

diminished reserve lung function. As a 

result, this sub-population would be at 

greater risk of a more severe response if 

affected by another respiratory agent 

(e.g., viral infection, or O3). 

EPA is also mindful of CASAC 

comments on this issue following the 

second draft ISA. The second draft ISA 

placed relatively little weight on health 

effects associated with SO2 exposures at 

200–300 ppb. CASAC strongly disagreed 

with this characterization of the health 

evidence. Their consensus letter 

following the second draft ISA states: 

Our major concern is the conclusions in 

the ISA regarding the weight of the evidence 

for health effects for short-term exposure to 

low levels of SO2. Although the ISA presents 

evidence from both clinical and 

epidemiological studies that indicate health 

effects occur at 0.2 ppm or lower, the final 

chapter emphasizes health effects at 0.4 ppm 

and above * * * CASAC believes the clinical 

and epidemiological evidence warrants 

stronger conclusions in the ISA regarding the 

available evidence of health effects at 0.2 

ppm or lower concentrations of SO2. The 

selection of a lower bound concentration for 

health effects is very important because the 

ISA sets the stage for EPA’s risk assessment 

decisions. In its draft Risk and Exposure 

Assessment (REA) to Support the Review of 

the SO2 Primary National Ambient Air 

Quality Standards (July 2008), EPA chose a 

range of 0.4 ppm–0.6 ppm SO2

concentrations for its benchmark analysis. As 

CASAC will emphasize in a forthcoming 

letter on the REA, we recommend that a 

lower bound be set at least as low as 0.2 ppm. 

(Henderson 2008a) 

EPA also notes the similar CASAC 

comments on the first draft of the REA. 

The consensus CASAC letter following 

the 1st draft REA states: 

The CASAC believes strongly that the 

weight of clinical and epidemiology evidence 

indicates there are detectable clinically 

relevant health effects in sensitive 

subpopulations down to a level at least as 

low as 0.2 ppm SO2. These sensitive 

subpopulations represent a substantial 

segment of the at-risk population. 

(Henderson 2008b; p. 1) 

See Coalition of Battery Recyclers 

Association v. EPA, No. 09–1011 (DC 

Cir., May 14, 2010), slip opinion at 9, 

holding that it was reasonable for EPA 

to conclude that a two IQ point mean 

population loss is an adverse effect 

based in part on CASAC advice that 

such a decrement is significant. 

CASAC’s strong advice regarding the 

adversity of effects at the 200 ppb level 

similarly supports EPA’s conclusion 

that the observed lung decrements are 

adverse. 

In addition to the considerations 

described above, we also note the 

following key points: 

• In the current SO2 NAAQS review, 

clinicians on the CASAC Panel advised 

that moderate or greater decrements in 

lung function can be clinically 

significant in some individuals with 

respiratory disease.11

• In the last O3 NAAQS review, 

CASAC indicated that moderate 

decrements in lung function can be 

clinically significant in some asthmatics 

(Henderson 2006), and that in the 

context of standard setting, a focus on 

the lower end of the range of moderate 

functional responses is most appropriate 

for estimating potentially adverse lung 

function decrements in people with 

lung disease (e.g., asthma; see 73 FR at 

16463). 

• In the last O3 NAAQS review, the 

Criteria Document and the Staff Paper 

indicated that for many people with 

lung disease (e.g., asthma), even 

moderate decrements in lung function 

or respiratory symptoms would likely 

interfere with normal activities and 

result in additional and more frequent 

use of medication (EPA 2006, EPA 

2007d). 

• Subjects participating controlled 

human exposure studies do not include 

severe asthmatics, and it is reasonable to 

presume that persons with more severe 

asthma than the study participants 

would have a more serious health effect 

from short-term exposure to 200 ppb 

SO2. 

Considering these key points along 

with the ATS guidelines and consensus 

CASAC comments on the draft ISA and 

REA described above, we reasonably 

conclude that 5–10 minute exposures to 

SO2 concentrations at least as low as 

200 ppb can result in adverse health 

effects in exercising asthmatics. 

In addition, as noted above some 

groups (e.g., API) contend that effects 

below 400 ppb should not be considered 

adverse because compared to the 

number of asthmatics experiencing 

decrements in lung function, there were 

similar numbers of asthmatics 

experiencing increases in lung function. 

The commenters correctly point out 

that at the lowest concentration tested 

in free-breathing chamber studies (200 

ppb), there are a similar number of 

asthmatics experiencing a moderate or 

greater decrease in lung function (i.e., ≥ 

100 increase in sRaw or ≥ 15 decrease 

in FEV1) and experiencing what might 

be called a moderate improvement in 

lung function (i.e., ≥ 100 decrease in 

sRaw or ≥ 15 increase in FEV1). This 

observation is consistent with data 

presented in Figures 4–2 and 4–3 of the 

ISA showing essentially no SO2

-induced change in lung function at 200 

ppb when averaged across asthmatics 

participating in the three Lin et al., 

controlled human exposure studies. 

However, these figures also demonstrate 

that asthmatics who are sensitive to SO2

at a higher concentration (600 ppb) 

experience, on average, a greater 

decrement in lung function at lower 

concentrations, including 200 ppb, 

when compared with all subjects 

combined. Therefore, while some 

asthmatics are relatively insensitive to 

SO2-induced respiratory effects even at 

concentrations ≥ 600 ppb, there is clear 

empirical evidence that others 

experience significant 

bronchoconstriction following 

exposures to both relatively high (600 

ppb) and low (200 ppb) SO2

concentrations. Among these SO2- 

sensitive asthmatics, Figures 4–2 and 4– 

3 of the ISA show a clear increase in 


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bronchoconstriction with increasing 

SO2 concentrations from 200–400 ppb. 

Given this clear relationship of exposure 

and effect at all levels in the sensitive 

asthmatics (i.e. those who experienced 

significant decrements in lung function 

at the highest exposure concentration 

used (600 ppb)), EPA does not accept 

the commenter’s premise that controlled 

human exposure studies do not 

demonstrate adverse effects in some 

asthmatics at 5–10 minute levels below 

400 ppb. 

In addition to disagreeing with EPA’s 

proposed finding of adverse health 

effects following 5– 10 minute SO2

exposures as low as 200 ppb, many 

industry groups (e.g., API, UARG, ACC, 

ExxonMobil) also disagreed with EPA 

that severe asthmatics were not 

included in controlled human exposure 

studies. That is, these groups contend 

that EPA is incorrect in assuming that 

severe asthmatics would likely have a 

more pronounced response to SO2

exposures at a given level, or would 

respond to even lower levels of SO2 and 

that this should be taken into account 

when judging the adequacy of the 

current standards. As support for their 

assertion, multiple industry groups cite 

controlled human exposure studies in 

the ISA stating that they included 

‘‘severe asthmatics’’ and also cite a study 

by Linn et al. (1987) which concluded 

that among asthmatics, responses to SO2

exposure are not dependent on the 

clinical severity of asthma and that ‘‘the 

subjects with the highest risk [of 

temporary respiratory disturbances from 

ambient SO2] can be identified only by 

actually measuring their responses to 

SO2’’. 

We disagree with the assertion that 

severe asthmatics have been evaluated 

in 5–10 minute controlled human 

exposure studies. Although studies 

cited in the ISA referred to a group of 

subjects as ‘‘moderate/severe’’ 

asthmatics, these individuals had wellcontrolled asthma, were able to 

withhold medication, were not 

dependent on corticosteroids, and were 

able to engage in moderate to heavy 

levels of exercise. By today’s standards, 

these individuals would clearly be 

classified as moderate asthmatics. EPA 

therefore concludes that persons with 

asthma that is more severe than 

moderate asthma, as that term is 

currently understood, were not included 

in the controlled human exposure 

studies (and understandably so, for 

ethical reasons). 

In addition, EPA agrees with the 

commenters that there is little evidence 

from controlled human exposure studies 

to suggest that the respiratory effects of 

SO2 differ between mild and moderate 

asthmatics (see Linn et al., 1987). 

However, this may very well be due, at 

least in part, to persistence of 

medication among the moderate 

asthmatic subjects. More importantly, 

the moderate asthmatics began the 

exposure with compromised lung 

function relative to the mild asthmatics. 

Therefore, similar functional declines 

from different baselines between mild 

and moderate asthmatics would clearly 

not have the same physiological 

importance. CASAC specifically 

addressed the issue of asthma severity 

in a letter to the Administrator: ‘‘For 

ethical reasons severe asthmatics were 

not part of these clinical studies, but it 

is not unreasonable to presume that they 

would have responded to even a greater 

degree (Henderson 2008a; p. v).’’ It is 

also important to note that in addition 

to the strict health-specific inclusion 

and exclusion criteria for a given 

controlled human exposure study, many 

asthmatics who might otherwise be able 

to participate choose not to participate 

because of anxiety related to what they 

viewed as potential adverse health risks. 

EPA concludes that it is appropriate to 

assume, as CASAC suggested, that 

persons with more severe asthma would 

respond to an even greater degree than 

the moderate asthmatics in the clinical 

studies. 

c. Comments on EPA’s Characterization 

of SO2-Associated Exposures and Health 

Risks 

Several commenters discussed the 

analyses of SO2-associated exposures 

and health risks presented in the REA. 

As in past reviews (EPA 2005, 2007c, 

2007d), EPA has estimated risks 

associated with the current standards to 

inform judgments on the public health 

risks that could exist under different 

standard options. Some industry 

commenters (e.g., API, UARG, Lignite 

Energy Council (LEC), Jackson Walker, 

ASARCO, the National Rural Electric 

Cooperative Association) concluded that 

when considering the adequacy of the 

current standards, the Administrator 

should consider exposures and risks 

associated with actual SO2 air quality 

rather than air quality allowed by the 

current NAAQS. They consequently 

challenged the relevance and 

appropriateness of EPA’s use of SO2

concentrations that have been simulated 

to just meet the current standards in 

assessing the adequacy of the current 

standards. 

In addition to the objections noted 

above, we note that UARG generally 

concluded that the results of EPA’s 

quantitative risk assessment are 

fundamentally flawed in that they 

substantially overestimate risks 

associated with the various air quality 

scenarios. UARG contends that this is 

because EPA did not use proper 

exposure-response functions in 

estimating risks associated with SO2

exposure. Moreover, UARG contends 

EPA further overestimates risk because 

of the use of 50 ppb exposure bins in 

estimating the number of occurrences of 

an adverse lung function response (see 

below). 

With respect to comments that when 

considering the adequacy of the current 

standards, the Administrator should 

consider exposures and risks associated 

with actual SO2 air quality rather than 

that simulated to just meet the current 

standards, these commenters generally 

concluded: (1) It is more relevant to 

assess exposures and risks associated 

with actual SO2 air quality since 

adjusting air quality to just meet the 

current standards require large 

adjustments to air quality that are highly 

uncertain; and (2) NAAQS are intended 

to address actual, rather than highly 

improbable, risks to human health. In 

addition, these groups generally 

concluded that exposure and risk 

estimates presented in the REA suggest 

relatively little health risk associated 

with current levels of SO2, and thus, 

there is no need to revise the current 

SO2 standards. 

We disagree with these commenters 

that exposure- and risk-related 

considerations in the NAAQS reviews 

should rely only on actual air quality, 

and that EPA therefore improperly 

adjusted air quality in its risk and 

exposure analyses to simulate air 

quality allowed by the current primary 

SO2 NAAQS. EPA is required to review 

whether the present standards—not 

present air quality—are requisite to 

protect public health with an adequate 

margin of safety. Section 109(b)(1). In 

making this determination it is relevant 

to consider exposures and risks which 

could be permissible under the current 

standards. See American Trucking 

Associations v. EPA, 283 F.3d 355, 370 

(DC Cir. 2002) (existence of evidence 

showing adverse effects occurring at 

levels allowed by the current standards 

justifies finding that it is appropriate to 

revise the existing NAAQS). 

Consequently, it is at the very least 

reasonable for EPA, in its REA, to make 

air quality adjustments to estimate SO2- 

related exposures and health risks that 

could exist in areas that just meet the 

present standards. Thus, although we 

acknowledge that exposure and health 

risk estimates associated with current 

ambient concentrations are substantially 

smaller than those associated with air 

quality adjusted to just meet the current 

standards, we also note that this is 


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12 In conducting these analyses, EPA is not trying 

to evaluate whether areas would or would not be 

in attainment of the current standards. Again, those 

issues are addressed during the implementation of 

the NAAQS. 

irrelevant to the question of whether the 

current standards are requisite to protect 

public health with an margin of safety. 

In both of these cases, EPA is not 

trying to evaluate whether areas would 

or would not be in attainment of the 

current standards. Those are issues that 

are addressed during the 

implementation of the NAAQS. Instead, 

in this rulemaking EPA is evaluating 

what NAAQS would be appropriate 

under section 109(b)(1), by evaluating 

the impact on or risks to public health 

from air quality that is at the level of the 

current standards, as well as evaluating 

air quality that is at the level of various 

alternative standards. EPA uses this 

information to inform the decision on 

what NAAQS would be requisite to 

protect public health with an adequate 

margin of safety. 

If EPA determines that the current 

standards require revision, EPA is 

further required to determine what 

revisions are appropriate in light of the 

requirement that primary NAAQS be 

requisite to protect public health with 

an adequate margin of safety. Section 

109(d)(1). It is thus similarly reasonable 

for EPA to make air quality adjustments 

to simulate different potential 

alternative standards to provide 

information on exposures and risks 

under these potential alternative 

standards.12

We agree that there are uncertainties 

inherent in making air quality 

adjustments. These uncertainties are 

discussed thoroughly in the REA (REA, 

sections 6.5 and 7.4.2.5). For example, 

the REA noted the following regarding 

adjustment of SO2 concentrations: 

This procedure for adjusting either the 

ambient concentrations (i.e., in the air quality 

characterization) or health effect benchmark 

levels (i.e., in the exposure assessment) was 

necessary to provide insight into the degree 

of exposure and risk which would be 

associated with an increase in ambient SO2

levels such that the levels were just at the 

current standards in the areas analyzed. Staff 

recognizes that it is extremely unlikely that 

SO2 concentrations in any of the selected 

areas where concentrations have been 

adjusted would rise to meet the current 

NAAQS and that there is considerable 

uncertainty associated with the simulation of 

conditions that would just meet the current 

standards. Nevertheless, this procedure was 

necessary to assess the ability of the current 

standards, not current ambient SO2

concentrations, to protect public health 

(REA, section 6.5; p. 64) 

These air quality adjustments are not 

meant to imply an expectation that SO2

concentrations will increase broadly 

across the United States or in any given 

area. Rather, as just noted above, they 

are meant to estimate SO2-related 

exposures and health risks if air quality 

were at the level of the current and 

potential alternative standards. Such 

estimates can inform decisions on 

whether the current standards, or 

particular potential alternative 

standards, provide the requisite 

protection of public health. 

As mentioned above, UARG generally 

concluded that under all air quality 

scenarios, the results of EPA’s 

quantitative risk assessment (the third of 

the analyses conducted in the REA 

(chapter 9), see section II.C above) are 

substantially overestimated because 

EPA did not use proper methods to 

estimate the parameters of the exposureresponse functions used in its analyses. 

UARG contends this is because many of 

the subjects in the controlled human 

exposure studies from which EPA’s 

exposure-response functions were 

derived (see REA, Table 9–3) were 

exposed to more than one SO2

concentration, yet EPA treated each 

exposure event as being independent 

(e.g., if the same subject was exposed to 

200 and 300 ppb SO2, EPA considered 

these as representing two independent 

exposure events). UARG contends that 

observations from the same subject 

exposed to different SO2 concentrations 

are not independent observations and 

should not be treated as such. Notably, 

when UARG derived their own 

exposure-response functions taking into 

account that observations from the same 

subject exposed to different SO2

concentrations are not independent of 

each other, they estimated appreciably 

less risk than that estimated by EPA. 

There are a variety of techniques and/ 

or assumptions that can be used to fit 

individual subject data from the 

controlled human exposure studies (see 

REA, Table 9–3) to exposure-response 

curves. Moreover, any technique or 

assumption utilized will have inherent 

uncertainties. EPA discussed the 

uncertainties associated with our 

quantitative risk assessment in detail in 

the REA (REA, section 9.4); we also gave 

an overview of key uncertainties in the 

proposal (see section II.C.3, 74 FR at 

64824). The approach used to estimate 

the exposure-response functions was 

not first introduced in the SO2 risk 

assessment, it was previously 

recommended to EPA by an applied 

statistician serving on the O3 CASAC 

Panel and used in the O3 risk 

assessment (which had individual 

controlled human exposure data similar 

to that in the current SO2 NAAQS 

review; see EPA 2007d and EPA 2007e). 

Importantly, this approach allowed EPA 

to use all the available individual 

subject data. Moreover, an inspection of 

the estimated exposure-response curve 

and the underlying data suggest that any 

biases in the parameter estimates are 

likely to be slight (see EPA 2010, section 

II.C). Consequently, EPA does not 

accept UARG’s view that the 

methodology used in EPA’s quantitative 

risk assessment was inappropriate. 

We further note that UARG’s 

exposure-response functions do not fit 

the underlying controlled human 

exposure data (the proportions of 

subjects who responded at each 

exposure level) nearly as well as the 

exposure-response functions estimated 

using EPA’s approach. We believe this 

could be due to the methodology used 

in UARG’s reanalysis of the individuallevel data from the controlled human 

exposure studies used in the 

quantitative risk assessment. UARG 

attempted to estimate subject-specific 

exposure-response functions, and to use 

the results of these estimates to obtain 

estimates of the two parameters in the 

population-level exposure-response 

functions. As described in more detail 

in section II.C of the RTC document 

(EPA 2010), EPA does not believe there 

are sufficient data to properly estimate 

the parameters of subject-specific 

exposure-response functions. More 

specifically, UARG chose a threeparameter quadratic function for the 

subject-specific exposure-response 

functions. However, none of the subjects 

had more than three exposures, and 

many had only one or two. EPA believes 

that this information is particularly 

limited for estimating these subjectspecific exposure-response functions, 

especially given that a large percentage 

of the total number of subjects had 

fewer exposures than the number of 

parameters UARG was attempting to 

estimate (i.e., UARG estimated three 

parameters in its exposure-response 

functions, but over fifty percent of 

subjects only had one or two exposures). 

It appears that UARG’s population-level 

exposure-response function estimates 

depended on these subject-specific 

exposure-response function estimates 

and thus could explain why UARG’s 

estimated population-level exposureresponse functions do not fit the 

underlying controlled human exposure 

data nearly as well as the approach used 

by EPA. A more detailed response to 

this comment can be found in section 

II.C of the RTC document (EPA 2010). 

As mentioned above, UARG also 

concluded that EPA further 

overestimates the total number of 

occurrences of an adverse lung function 

response (i.e., total number of 


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13Although in St. Louis, the percent of exposed 

asthmatic children at moderate or greater exertion 

estimated to have at least one defined lung function 

response per year was not appreciably affected, it 

was found that for this same metric, the already 

very low risk estimates in Greene County became 

appreciably lower when the binning issue 

discussed above was considered. However, as noted 

above in section II.C and discussed in more detail 

in the REA (REA, section 10.3.3) and the proposal 

(see section II.E.b, 74 FR at 64827), the St. Louis 

exposure and risk results were found to be more 

informative in addressing the adequacy of the 

current and potential alternative standards. 

Moreover, while the Administrator’s rationale in 

the proposal relied minimally on the St. Louis 

quantitative risk results (see above), she importantly 

placed no weight on any metric from the Greene 

County quantitative risk assessment. 

occurrences of increases in sRaw ≥ 100 

or 200% and/or declines in FEV1 ≥ 15 

or 20%) in its quantitative risk 

assessment. More specifically, UARG 

concluded that the use of 50 ppb bins, 

combined with assigning all exposures 

within a bin the probability of an 

adverse lung function response at the 

midpoint of that bin (e.g., all exposures 

from 0–50 ppb were assigned the 

probability of an adverse lung function 

response occurring at 25 ppb), resulted 

in a substantial overestimate of the total 

number of occurrences of lung function 

responses in asthmatics at moderate or 

greater exertion. UARG generally 

concludes that this is because the vast 

majority of exposures of asthmatics at 

moderate or greater exertion are 

occurring below the midpoint of the 0– 

50 ppb exposure bin (i.e., most 

exposures are occurring below 25 ppb), 

yet EPA is assigning these very low SO2

exposures the higher probability of a 

lung function response associated with 

the midpoint of the 0–50 ppb exposure 

bin. UARG contends that this results in 

a substantial overestimation of the total 

number of occurrences of lung function 

response in asthmatics and asthmatic 

children at moderate or greater exertion. 

UARG further notes that this 

methodological concern was raised in 

its comments on the second draft REA, 

but EPA failed to address this issue and 

relied heavily on this metric in the 

proposal with respect to the adequacy of 

the current and potential alternative 

standards. EPA’s response to this 

comment is discussed below and in 

more detail in section II.C of the RTC 

document (EPA 2010). 

EPA generally agrees with UARG’s 

technical comments that there is a 

substantial overestimation of the total 

occurrences of lung function responses 

because of the binning issues described 

above. However, we strongly disagree 

that: (1) This issue was not 

acknowledged in the final REA; and (2) 

the metric of total occurrences was 

relied on heavily in the policy 

assessment chapter of the REA (REA, 

chapter 10) and in the Administrator’s 

rationale with respect to the adequacy of 

the current and potential alternative 

standards. First, EPA did respond to this 

concern in the final REA. More 

specifically, page 344 of the final REA 

states: 

As noted in public comments on the 2nd 

draft SO2 REA, the assignment of response 

probability to the midpoint of the exposure 

bin combined with the lack of more finely 

divided intervals in this range can lead to 

significant overestimation of risks based on 

total occurrences of a defined lung function 

response. This is because the distribution of 

population exposures for occurrences is not 

evenly distributed across the bin, but rather 

is more heavily weighted toward the lower 

range of the bin. Thus, combining all 

exposures estimated to occur in the lowest 

bin with a response probability assigned to 

the midpoint of the bin results in a 

significant overestimate of the risk. 

Therefore, staff places less weight on the 

estimated number of occurrences of lung 

function responses. 

Thus, as noted in the final REA, less 

weight was placed on this metric in the 

quantitative risk assessment chapter 

(REA, chapter 9), and importantly, no 

weight was placed on this metric in 

either the policy assessment chapter of 

the REA (REA, chapter 10) or in the 

Administrator’s rationale sections of the 

proposal preamble. Rather, the policy 

assessment chapter of the REA and the 

Administrator’s rationale at the proposal 

considered the percent of exposed 

asthmatic children at moderate or 

greater exertion estimated to have at 

least one defined lung function response 

per year in St. Louis. Importantly, this 

metric is not appreciably affected by the 

binning issue raised in UARG’s 

comments. As stated on page 344–345 of 

the final REA: 

This overestimation of total occurrences 

does not impact the risk metric expressed as 

incidence or percent incidence of a defined 

lung function response 1 or more times per 

year because the bulk of the exposures 

contributing to these risk metrics are not 

skewed toward the lower range of the 

reported exposure bins.13

Finally, it is important to note that the 

Administrator’s rationale in the 

proposal regarding the adequacy of the 

current and potential alternative 

standards in general placed only limited 

reliance on the results of the 

quantitative risk assessment in St. 

Louis, with no reliance on the estimates 

of total occurrences. Rather, in addition 

to the substantial weight that she placed 

on the scientific evidence as described 

in the ISA, the Administrator placed 

relatively more weight on the results of 

the St. Louis exposure analysis. For 

example, in discussing the adequacy of 

the current standards, the proposal 

states: ‘‘The Administrator especially 

notes the results of the St. Louis 

exposure analysis which, as 

summarized above, indicates that 

substantial percentages of asthmatic 

children at moderate or greater exertion 

would be exposed, at least once 

annually, to air quality exceeding the 

400 and 200 ppb benchmarks’’ (see 74 

FR at 64829). We note that results of the 

quantitative risk assessment in St. 

Louis, with respect to the percent of 

asthmatic children estimated to have at 

least one lung function response per 

year (using EPA’s exposure-response 

functions), supports the Administrator’s 

overall conclusions in the proposal 

regarding the adequacy of the current 

and potential alternative standards. 

3. Conclusions Regarding the Adequacy 

of the Current 24-Hour and Annual 

Standards 

In reviewing the adequacy of the 

current standards, the Administrator has 

considered the scientific evidence 

assessed in the ISA, the exposure and 

risk results presented in the REA, the 

conclusions of the policy assessment 

chapter of the REA, and comments from 

CASAC and the public. These 

considerations are described below. 

As in the proposal, the Administrator 

accepts and agrees with the ISA’s 

conclusion that the results of controlled 

human exposure and epidemiologic 

studies form a plausible and coherent 

data set that supports a causal 

relationship between short-term (5 

minutes to 24 hours) SO2 exposures and 

adverse respiratory effects. The 

Administrator acknowledges that there 

are uncertainties associated with the 

epidemiologic evidence (e.g., potential 

confounding by co-pollutants). 

However, she agrees that the 

epidemiologic evidence, supported by 

the controlled human exposure 

evidence, generally indicates that the 

effects seen in these studies are 

attributable to exposure to SO2, rather 

than co-pollutants, most notably PM2.5. 

She also accepts and agrees with the 

conclusion of the ISA that ‘‘[i]n the 

epidemiologic studies, respiratory 

effects were observed in areas where the 

maximum ambient 24-h avg SO2

concentration was below the current 24- 

h avg NAAQS level.***’’ (ISA, 

section 5.2, p. 5–2) and so would occur 

at ambient SO2 concentrations that are 

present in locations meeting the current 

24-hour NAAQS. The Administrator 

also notes that these effects occurred in 

areas with annual air quality levels 

considerably lower than those allowed 

by the current annual standard, 

indicating that the annual standard also 


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14We also note that such a standard would, 

among other things, address the deficiency in the 

current NAAQS which occasioned the remand of 

that standard for failing to adequately explain the 

absence of protection from short-term SO2 bursts 

which could cause adverse health effects in 

hundreds of thousands of heavily breathing 

asthmatics. American Lung Ass’n v. EPA, 134 F. 3d 

at 392–93. 

is not providing protection against such 

effects. Existence of epidemiologic 

studies showing adverse effects 

occurring at levels allowed by the 

current standards is an accepted 

justification for finding that it is 

appropriate to revise the existing 

standards. See, e.g. American Trucking 

Associations v. EPA, 283 F. 3d at 370; 

see also American Farm Bureau v. EPA, 

559 F. 3d.512, 521–23 (DC Cir. 2009) 

(effects associated with short-term 

exposure seen in areas with ambient 

concentrations lower than long-term 

standard, so that without further 

explanation, standard does not 

adequately protect against short-term 

exposures). 

With respect to the controlled human 

exposure studies, the Administrator 

judges that effects following 5–10 

minute SO2 exposures ≥ 400 ppb and 

≥ 200 ppb can result in adverse health 

effects to asthmatics. This judgment is 

based on ATS guidelines, explicit 

CASAC consensus written advice and 

recommendations, and judgments made 

by EPA in previous NAAQS reviews. 

Thus, similar to the proposal, she notes 

analyses in the REA supporting that 5- 

minute exposures ≥ 400 ppb and ≥ 200 

ppb were associated with air quality 

adjusted upward to simulate just 

meeting the current standards. The 

Administrator especially notes the 

results of the St. Louis exposure 

analysis which, as summarized in the 

proposal (see section II.E.1.b and Table 

3, see 74 FR at 64841), indicates that 

substantial percentages of asthmatic 

children at moderate or greater exertion 

would be exposed, at least once 

annually, to air quality exceeding the 

400 and 200 ppb 5-minute benchmarks 

given air quality simulated to just meet 

the current standards. The 

Administrator judged these 5-minute 

exposures to be significant from a public 

health perspective due to their 

estimated frequency: Approximately 

24% of child asthmatics at moderate or 

greater exertion in St. Louis are 

estimated to be exposed at least once 

per year to air quality exceeding the 5- 

minute 400 ppb benchmark, a level 

associated with lung function 

decrements in the presence of 

respiratory symptoms. Additionally, 

approximately 73% of child asthmatics 

in St. Louis at moderate or greater 

exertion would be expected to be 

exposed at least once per year to air 

quality exceeding the 5-minute 200 ppb 

benchmark. This health evidence and 

risk-based information underlie 

CASAC’s conclusion that the current 

SO2 standards do not adequately protect 

public health. As discussed in the 

proposal, CASAC stated: ‘‘the current 

24-hour and annual standards are not 

adequate to protect public health, 

especially in relation to short-term 

exposures to SO2 (5–10 minutes) by 

exercising asthmatics’’ (Samet, 2009, p. 

15). The Administrator agrees with this 

conclusion. 

In considering approaches to revising 

the current standards, the Administrator 

concludes that it is appropriate to set a 

new standard, that such standard must 

provide requisite protection with an 

adequate margin of safety to a 

susceptible population (i.e., asthmatics 

at elevated ventilation), and that the 

standard must afford protection from 

short-term exposures to SO2 in order to 

prevent the adverse health effects 

reported in both the controlled human 

exposure and epidemiologic studies. 

The Administrator notes that a 1-hour 

standard could provide increased public 

health protection, especially for 

members of at-risk groups, from health 

effects described in both controlled 

human exposure and epidemiologic 

studies, and hence, health effects 

associated with 5-minute to 24-hour 

exposures to SO2.14 As discussed in 

section II.F.5 below, given the degree of 

protection afforded by such a standard, 

it may be appropriate to replace, and not 

retain, the current 24-hour and annual 

standards in conjunction with setting a 

new short-term standard. 

F. Conclusions on the Elements of a 

New Short-Term Standard 

In considering a revised SO2 primary 

NAAQS, the Administrator notes the 

need to protect at-risk populations from: 

(1) 1-hour daily maximum and 24-hour 

average exposures to SO2 that could 

cause the types of respiratory morbidity 

effects reported in epidemiologic 

studies; and (2) 5–10 minute SO2

exposure concentrations reported in 

controlled human exposure studies to 

result in moderate or greater decrements 

in lung function and/or respiratory 

symptoms. Considerations with regard 

to potential alternative standards and 

the specific conclusions of the 

Administrator are discussed in the 

following sections in terms of indicator, 

averaging time, form, and level (sections 

II.F.1 to II.F.4 below). 

1. Indicator 

a. Rationale for Proposed Decision 

In the last review, EPA focused on 

SO2 as the most appropriate indicator 

for ambient SOX. In making a decision 

in the current review on the most 

appropriate indicator, the Administrator 

has considered the conclusions of the 

ISA and REA as well as the views 

expressed by CASAC and the public. 

The REA noted that, although the 

presence of gaseous SOX species other 

than SO2 has been recognized, no 

alternative to SO2 has been advanced as 

being a more appropriate surrogate for 

ambient gaseous SOX. Controlled 

human exposure studies and animal 

toxicology studies provide specific 

evidence for health effects following 

exposure to SO2. Epidemiologic studies 

also typically report levels of SO2, as 

opposed to other gaseous SOX. Because 

emissions that lead to the formation of 

SO2 generally also lead to the formation 

of other SOX oxidation products, 

measures leading to reductions in 

population exposures to SO2 can 

generally be expected to lead to 

reductions in population exposures to 

other gaseous SOX. Therefore, as noted 

in the proposal, meeting an SO2

standard that protects the public health 

can also be expected to provide 

protection against potential health 

effects that may be independently 

associated with other gaseous SOX even 

though such effects are not discernable 

from currently available studies indexed 

by SO2 alone. See American Petroleum 

Institute v. EPA, 665 F, 2d 1176, 1186 

(DC Cir. 1981) (reasonable for EPA to 

use ozone as the indicator for all 

photochemical oxidants even though 

health information on the other 

photochemical oxidants is unknown; 

regulating ozone alone is reasonable 

since it presents a ‘‘predictable danger’’ 

and in doing so EPA did not abandon 

its responsibility to regulate other 

photochemical oxidants encompassed 

by the determination that 

photochemical oxidants as a class may 

be reasonably anticipated to endanger 

public health or welfare). Given these 

key points, the REA concluded that the 

available evidence supports the 

retention of SO2 as the indicator in the 

current review (REA, section 10.5.1). 

Consistent with this conclusion, CASAC 

stated in a letter to the EPA 

Administrator that: ‘‘for indicator, SO2 is 

clearly the preferred choice’’ (Samet 

2009, p. 14). 

b. Comments on Indicator 

A small number of commenters 

directly addressed the issue of the 

indicator for the standard. These 


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commenters generally endorsed the 

proposal to continue to use SO2 as the 

indicator for ambient SOX.

c. Conclusions on Indicator 

Based on the available information 

discussed above, and consistent with 

the views of CASAC and other 

commenters, the Administrator 

concludes that it is appropriate to 

continue to use SO2 as the indicator for 

a standard that is intended to address 

effects associated with exposure to SO2, 

alone or in combination with other 

gaseous SOX. In so doing, the 

Administrator recognizes that measures 

leading to reductions in population 

exposures to SO2 will also reduce 

population exposures to other oxides of 

sulfur. 

2. Averaging Time 

This section discusses considerations 

related to the averaging time of the SO2

primary NAAQS. Specifically, this 

section summarizes the rationale for the 

Administrator’s proposed decision 

regarding averaging time (II.F.2.a below; 

see section II.F.2 of the proposal for 

more detail at 74 FR 64832–64833), 

discusses public comments and EPA 

responses related to averaging time 

(II.F.2.b), and presents the 

Administrator’s final conclusions 

regarding averaging time (II.F.2.c). 

Notably, public comments and the 

Administrator’s conclusions on whether 

to retain or revoke the current 24-hour 

and/or annual standards given a new 1- 

hour standard are discussed in section 

II.F.5. 

a. Rationale for Proposed Decision 

In considering the most appropriate 

averaging time for the SO2 primary 

NAAQS, the Administrator noted in the 

proposal the conclusions and judgments 

made in the ISA about the available 

scientific evidence, air quality 

correlations discussed in the REA, 

conclusions of the policy assessment 

chapter of the REA, and CASAC 

recommendations (section II.F.2 in the 

proposal). Specifically, she noted the 

following: 

• The REA conclusion that an 

appropriate averaging time should focus 

protection on SO2 exposures from 5- 

minutes to 24-hours (REA, section, 

10.5.2). 

• Air quality, exposure, and risk 

analyses from the REA indicating it is 

likely a 1-hour standard—with the 

appropriate form and level—can 

substantially reduce 5–10 minute peaks 

of SO2 shown in controlled human 

exposure studies to result in respiratory 

symptoms and/or decrements in lung 

function in exercising asthmatics (i.e. 5- 

minute SO2 concentrations ≥ 200 and 

400 ppb). 

• Air quality analyses indicating that 

a 1-hour standard—with the appropriate 

form and level—can substantially 

reduce the upper end of the distribution 

of SO2 levels more likely to be 

associated with adverse respiratory 

effects (see section II.F.3 below); that is: 

(1) 99th percentile 1-hour daily 

maximum air quality concentrations in 

U.S. cities where positive effect 

estimates in epidemiologic studies of 

hospital admissions and emergency 

department visits for all respiratory 

causes and asthma were observed; and 

(2) 99th percentile 24-hour average air 

quality concentrations found in U.S. 

cities where emergency department visit 

and hospitalization studies (for all 

respiratory causes and asthma) reported 

statistically significant associations in 

multi-pollutant models with PM. 

• The REA conclusion that a 5- 

minute averaging time is undesirable 

because it would result in significant 

and unnecessary instability due to the 

likelihood that locations would 

frequently shift in and out of 

attainment—thereby reducing public 

health protection by disrupting an area’s 

ongoing implementation plans and 

associated control programs. 

• CASAC statement addressing 

whether a 1-hour averaging time can 

adequately control 5–10 minute peak 

exposures and whether there should be 

a 5-minute averaging time. CASAC 

stated that the REA’s rationale for a onehour standard was ‘‘convincing’’ (Samet 

2009, p. 16), and that ‘‘a one-hour 

standard is the preferred averaging time’’ 

(Samet 2009, p. 15). 

• CASAC’s statement that they were 

‘‘in agreement with having a short-term 

standard and finds that the REA 

supports a 1-hour standard as protective 

of public health’’ (Samet 2009, p. 1). 

b. Comments on Averaging Time 

A large number of public commenters 

also endorsed the establishment of a 

new standard with a 1-hour averaging 

time (although some groups’ support 

hinged on the accompanying level). 

These included a number of State 

organizations (e.g., NACAA, 

NESCAUM); State environmental 

agencies (e.g., such agencies in IA, IL, 

NY, MI, NM, OH, PA, TX, VT); public 

health and environmental organizations 

(e.g., ALA, ATS, New York Department 

of Health (NYDOH), Sierra Club, EDF); 

the Fond du Lac Tribe; local groups 

(e.g., Houston-Galveston Area Council, 

New York City); and almost all of the 

individual commenters (13,000). The 

supporting rationales offered by these 

commenters often acknowledged the 

recommendations of CASAC and the 

Administrator’s rationale as discussed 

in the proposal. 

Though many industry commenters 

did not support the proposed revisions 

to the SO2 primary NAAQS (as 

discussed above in section II.E.2), a few 

of these groups did express that if a 

short-term standard were to be set, a 1- 

hour averaging time could be 

appropriate, depending on the level and 

form selected (e.g., ExxonMobil, Kean 

Miller). Other industry commenters 

(e.g., ASARCO, RIO Tinto Alcan, 

Association of Battery Recyclers (ABR)) 

and the South Dakota Department of 

Environment and Natural Resources (SD 

DENR) expressed that EPA should have 

considered longer averaging times (e.g., 

3 hours). In addition, although health 

and environmental groups were 

supportive of setting a new 1-hour 

standard to protect against short-term 

exposures to SO2 (again, depending on 

the level of the 1-hour standard 

selected), these groups also commented 

that a 5-minute standard to protect 

susceptible populations from health 

effects associated with 5-minute peaks 

of SO2 would be optimal (e.g., ALA, 

ATS, Sierra Club, EDF). These 

comments, and EPA’s responses, are 

discussed in more detail below. 

As discussed above, industry 

commenters who disagreed with setting 

a new 1-hour standard generally based 

this conclusion on their interpretation 

of the scientific evidence and their 

conclusion that this evidence does not 

support the proposed revisions to the 

current SO2 NAAQS. EPA’s responses to 

these commenters were presented above 

in section II.E.2.a and II.E.2.b. 

Also noted above, some industry 

commenters (e.g., ASARCO, RIO Tinto 

Alcan, ABR) and the SD DENR 

expressed that EPA should have 

considered longer averaging times (e.g., 

3-hour, 8-hour, 24-hour). In general, 

these groups concluded that a standard 

with a longer averaging time could 

potentially provide the same public 

health protection as a 1-hour standard, 

while also providing a more stable 

regulatory target. For example, in its 

comments, the SD DENR states: ‘‘DENR 

recommends EPA evaluate a 3-hour or 

8-hour standard to determine if these 

averaging periods are also protective of 

the public health. If they are, EPA 

should propose a 3-hour or 8-hour 

sulfur dioxide standard instead of a 1- 

hour standard. A longer averaging 

period would smooth out the variability 

of the upper range measurements and 

provide a more stable standard.’’ 

Similarly, Rio Tinto Alcan stated in its 

comments: ‘‘the short-term averaging 

period defined by EPA (i.e., 5 minutes 


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to 24 hours) is not limited to only 5- 

minute, 1-hour and 24-hour averaging 

periods. EPA could explain in more 

detail why these three averaging periods 

were examined when considering 

appropriate averaging periods to limit 

short-term peaks of SO2 * * * a longer 

term average could provide additional 

stability to the standard while at the 

same time effectively protecting public 

health.’’ 

Although we agree that alternative 

averaging times could potentially 

provide similar public health protection 

(assuming an appropriate form and 

level), we believe that a 1-hour 

averaging time is reasonably justified by 

the scientific evidence presented in the 

ISA and by the air quality information 

presented in the REA. As described in 

detail in the proposal (see section 

II.F.2), the controlled human exposure 

evidence presented in the ISA provided 

support for an averaging time that 

protects against 5–10 minute peak SO2

exposures (REA, section 10.5.2, pp. 

371–372), and results from 

epidemiologic studies most directly 

provided support for both 1-hour and 

24-hour averaging times (REA, section 

10.5.2, p. 372). Thus, we found it most 

reasonable to consider these averaging 

times for a revised SO2 NAAQS given 

that there is very little basis in the 

health evidence presented in the ISA to 

consider other averaging times (e.g., 3- 

hour or 8-hour). In so doing, we first 

noted the likelihood that averaging 

times of 1 and 24 hours could provide 

protection against 5-minute peak SO2

exposures. As described in detail in the 

proposal (see section II.F.2, 74 FR at 

64830–64833), it was initially 

concluded that a 1-hour averaging time, 

rather than a 24-hour averaging time, 

would be more appropriate for limiting 

5-minute peaks of SO2. Similarly, we 

concluded that a 1-hour standard, given 

the appropriate form and level, could 

likely limit 99th percentile 24-hour 

average air quality concentrations found 

in U.S. locations where emergency 

department visit and hospitalization 

studies (for all respiratory causes and 

asthma) observed statistically significant 

associations in multi-pollutant models 

with PM (i.e., 99th percentile 24-hour 

average SO2 concentration ≥ 36 ppb). 

Taken together, we reasonably 

concluded that a 1-hour standard, with 

an appropriate form and level, can 

provide adequate protection against the 

range of health outcomes associated 

with averaging times from 5 minutes to 

24 hours (proposal section II.F.2 and 

REA, section 10.5.2.3). We also note that 

our conclusion is in agreement with 

CASAC comments on the second draft 

REA. CASAC stated that they were ‘‘in 

agreement with having a short-term 

standard and finds that the REA 

supports a one-hour standard as 

protective of public health’’ (Samet 

2009, p. 1). In addition, as discussed in 

more detail below in section II.F.3, we 

found that a 1-hour standard in 

combination with the selected form, 

will provide a stable regulatory target. 

As noted above, although health and 

environmental groups were supportive 

of setting a new 1-hour standard to 

protect against short-term exposures to 

SO2 (again, depending on the level of 

the 1-hour standard selected), these 

groups generally commented that a 5- 

minute standard to protect against 

health effects associated with 5-minute 

peaks would be optimal (e.g., ALA, 

Sierra Club, EDF). For example, in their 

combined comments ALA, EDF, NRDC, 

and Sierra Club (ALA et al.,) stated: ‘‘We 

need a short-term SO2 standard, 

optimally a 5-minute standard, to 

protect against bursts of pollution that 

can result from start-up, shutdown, 

upset, malfunction, downwash, 

complex terrain, atmospheric inversion 

conditions, and other situations’’ and 

that ‘‘EPA has over emphasized a 

concern about the stability of a 5-minute 

standard * * * The record does not 

show that any alleged instability of a 5- 

minute standard has any relevance to 

whether such a standard is requisite to 

protect public health.’’ 

We agree that there needs to be a 

short-term standard to protect against 5- 

minute peaks of SO2. However, we do 

not believe setting a 5-minute standard 

to be the best way of accomplishing that 

objective. As in past NAAQS reviews, 

EPA properly considered the stability of 

the design of pollution control programs 

in its review of the elements of a 

NAAQS, since more stable programs are 

more effective, and hence result in 

enhanced public safety. American 

Trucking Associations v. EPA, 283 F. 3d 

at 375 (choice of 98th percentile form 

for 24-hour PM NAAQS, which allows 

a number of high exposure days per year 

to escape regulation under the NAAQS, 

justifiable as ‘‘promot[ing] development 

of more ‘effective [pollution] control 

programs’ ’’, since such programs would 

otherwise be ‘‘less ‘stable’—and hence 

* * * less effective—than programs 

designed to address longer-term average 

conditions’’, and there are other means 

(viz. emergency episode plans) to 

control those high exposure days). In 

this review, there were legitimate 

concerns about the stability of a 

standard using a 5-minute averaging 

time. Specifically, there was concern 

that compared to longer averaging times 

(e.g., 1-hour, 24-hour), year-to-year 

variation in 5-minute SO2

concentrations were likely to be 

substantially more temporally and 

spatially diverse. Thus, it is more likely 

that locations would frequently shift in 

and out of attainment thereby reducing 

public health protection by disrupting 

an area’s ongoing implementation plans 

and associated control programs. 

Consequently, the REA concluded that a 

5-minute averaging time would not 

provide a stable regulatory target and 

therefore would not be the preferred 

approach to provide adequate public 

health protection. A 1-hour averaging 

time does not have these drawbacks. As 

noted in the REA and the proposal (see 

proposal sections II.F.2.a and II.F.2.c), 

air quality, exposure, and risk analyses 

support that a 1-hour averaging time, 

given an appropriate form and level can 

adequately limit 5-minute SO2

exposures and provide a more stable 

regulatory target than setting a 5-minute 

standard. More specifically, based on 

the air quality and exposure analyses 

presented in chapters 7 and 8 of the 

REA, there is also a strong likelihood 

that a 99th percentile 1-hour daily 

maximum standard will limit 5–10 

minute peaks of SO2 shown in 

controlled human exposure studies to 

result in decrements in lung function 

and/or respiratory symptoms in 

exercising asthmatics (see especially 

REA Tables 7–11 to 7–14 and Figure 8– 

19). 

We also note that a 1-hour standard to 

protect against 5-minute exposures is in 

agreement with CASAC advice and 

recommendations. That is, CASAC 

stated that they were ‘‘in agreement with 

having a short-term standard and finds 

that the REA supports a 1-hour standard 

as protective of public health’’ (Samet 

2009, p. 1). Similarly, in a CASAC 

statement addressing whether a 1-hour 

averaging time can adequately control 

5–10 minute peak exposures and 

whether there should be a 5-minute 

averaging time, CASAC stated that the 

REA had presented a ‘‘convincing 

rationale’’ (Samet 2009, p. 16) for a 1- 

hour standard, and that ‘‘a one-hour 

standard is the preferred averaging time’’ 

(Samet 2009, p. 15). 

c. Conclusions on Averaging Time 

In considering the most appropriate 

averaging time(s) for the SO2 primary 

NAAQS, the Administrator notes the 

conclusions and judgments made in the 

ISA about the available scientific 

evidence, air quality considerations 

from the REA, CASAC advice and 

recommendations, and public 

comments received. Based on these 

considerations, the Administrator 

concludes that a new standard based on 


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15As noted above, such a standard also 

satisfactorily addresses the issue raised by the 

reviewing court in the litigation that followed the 

last review of the SO2 NAAQS: Why was no 

protection afforded in the standard for a susceptible 

subpopulation known to experience repeated 

adverse effects from exposure to 5–10 minute SO2

bursts. American Lung Ass’n, 134 F. 3d at 392–93. 

1-hour daily maximum SO2

concentrations will provide increased 

protection against effects associated 

with short-term (5 minutes to 24 hours) 

exposures. The rationale for this 

decision is described below. 

Similar to the proposal (see section 

II.F.2.c), the Administrator first agrees 

with the REA’s conclusion that the 

standard should focus protection on 

short-term SO2 exposures from 5 

minutes to 24 hours. As noted above, 

CASAC’s strong recommendation 

supports this approach as well.15 The 

Administrator further agrees that the 

standard must provide requisite 

protection from 5–10 minute exposure 

events, but believes that this can be 

provided without having a standard 

with a 5-minute averaging time. The 

Administrator agrees with the REA 

conclusion that it is likely a 1-hour 

standard—with the appropriate form 

and level—can substantially reduce 5– 

10 minute peaks of SO2 shown in 

controlled human exposure studies to 

result in respiratory symptoms and/or 

decrements in lung function in 

exercising asthmatics. The 

Administrator further believes that a 5- 

minute averaging time would result in 

significant and unnecessary instability 

and is undesirable for that reason. The 

Administrator also notes the statements 

from CASAC mentioned above 

addressing whether a 1-hour averaging 

time can adequately control 5–10 

minute peak exposures and whether 

there should be a 5-minute averaging 

time. As noted above, addressing this 

question, CASAC stated that the REA 

had presented a ‘‘convincing rationale’’ 

(Samet 2009, p. 16) for a 1-hour 

standard, and that ‘‘a one-hour standard 

is the preferred averaging time’’ (Samet 

2009, p. 15). 

Second, as in the proposal the 

Administrator agrees that a 1-hour 

averaging time (again, with the 

appropriate form and level) would 

provide protection against the range of 

health outcomes associated with 

averaging times of 1 hour to 24 hours. 

Specifically, the Administrator finds 

that a 1-hour standard can substantially 

reduce the upper end of the distribution 

of SO2 levels more likely to be 

associated with adverse respiratory 

effects (see discussion on Form, section 

II.F.3); that is: (1) 99th percentile 1-hour 

daily maximum SO2 air quality 

concentrations in U.S. locations where 

positive SO2 effect estimates were 

reported in epidemiologic studies of 

emergency department visits and 

hospital admissions for all respiratory 

causes and asthma; and (2) 99th 

percentile 24-hour average SO2 air 

quality concentrations found in U.S. 

locations where emergency department 

visit and hospital admission studies 

using multi-pollutant models with PM 

reported statistically significant 

associations (for all respiratory causes or 

asthma) with ambient SO2 (see REA, 

section 10.5.2.2 and proposal section 

II.F.2, 74 FR at 64831). Finally, the 

Administrator again notes that 

establishing a new 1-hour averaging 

time is in agreement with CASAC 

recommendations. As noted above, 

CASAC stated that they were ‘‘in 

agreement with having a short-term 

standard and finds that the REA 

supports a one-hour standard as 

protective of public health’’ (Samet 

2009, p. 1). Moreover, CASAC agreed 

with the REA that a ‘‘one-hour standard 

is the preferred averaging time’’ (Samet 

2009, p.15). 

3. Form 

This section discusses considerations 

related to the form of the 1-hour SO2

primary NAAQS. Specifically, this 

section summarizes the rationale for the 

Administrator’s proposed decision 

regarding form (II.F.3.a; see proposal 

section II.F.3, 74 FR at 64833–64834 of 

the proposal for more detail), discusses 

comments related to form (II.F.3.b), and 

presents the Administrator’s final 

conclusions regarding form (II.F.3.c). 

a. Rationale for Proposed Decision 

In considering the most appropriate 

form for the SO2 primary NAAQS, the 

Administrator noted in the proposal the 

conclusions and judgments made in the 

ISA about available scientific evidence, 

air quality information discussed in the 

REA, conclusions of the policy 

assessment chapter of the REA, and 

CASAC recommendations (see section 

II.F.3, 74 FR at 64833–64834 in the 

proposal). Specifically, the proposal 

referenced the following: 

• Information in the ISA that 

suggested that adverse respiratory 

effects are more likely to occur at the 

upper end of the distribution of ambient 

SO2 concentrations. That is, the ISA 

describes a few studies that reported an 

increase in SO2-related respiratory 

health effects at the upper end of the 

distribution of SO2 concentrations (ISA, 

section 5.3, p. 5–9). 

• The REA conclusion that a 

concentration-based form averaged over 

three years would better reflect the 

continuum of health risks posed by 

increasing SO2 concentrations (i.e. the 

percentage of asthmatics affected and 

the severity of the response increases 

with increasing SO2 concentrations; 

REA, section 10.5.3) by giving 

proportionally greater weight to years 

when 1-hour daily maximum SO2

concentrations are well above the level 

of the standard, than just above the level 

of the standard. 

• Analyses in the REA that suggested 

for a given SO2 standard level, a 99th 

percentile form is appreciably more 

effective at limiting 5-minute peak SO2

concentrations than a 98th percentile 

form (REA, section 10.5.3 and REA, 

Figures 7–27 and 7–28). 

• Analyses in the REA indicating that 

over the last 10 years and for the vast 

majority of the sites examined, there 

appears to be little difference in 98th 

and 99th percentile design value 

stability (REA, section 10.5.3). 

• The REA conclusion that taken 

together, the evidence and air quality 

information indicate that consideration 

should be given primarily to a 1-hour 

daily maximum standard with a 99th 

percentile or 4th highest daily 

maximum form (REA, section 10.5.3.3). 

• CASAC indications that: ‘‘there is 

adequate information to justify the use 

of a concentration-based form averaged 

over 3 years’’ (Samet 2009, p. 16). 

• CASAC recommendations that 

when evaluating 98th vs. 99th 

percentile forms, EPA should consider 

the number of days per year 98th vs. 

99th percentile forms would allow SO2

concentrations to exceed the selected 

standard level. Similarly, CASAC 

recommendations to consider the 

number of exceedences of 5-minute 

benchmarks given 98th vs. 99th 

percentile forms at a given standard 

level (Samet 2009). 

b. Comments on Form 

Most all State organizations and 

agencies (e.g., NAACA, NESCAUM and 

agencies in FL, NM, PA, SC, TX, VT) 

supported a 99th percentile or 4th 

highest form. Similarly, public health 

(e.g., ALA, ATS) and environmental 

organizations (e.g., CBD, WEACT for 

Environmental Justice) and the 

Alexandria Department of 

Transportation and Environmental 

Services preferred either a 99th 

percentile or a more stringent form (e.g., 

no exceedence) to further limit the 

occurrence of SO2 concentrations that 

exceed the standard level in locations 

that attain the standard. In contrast, 

many industry groups (e.g., UARG, 

NAM, LEC, RRI Energy, AirQuality 

Research & Logistics (AQRL)), and the 

SD DENR conditionally supported a 


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16EPA did not propose or seek comment on a 

98th percentile form or a more restrictive form (e.g., 

an exceedence based form). EPA also considered a 

4th highest form, which is generally equivalent to 

the 99th percentile. However, a percentile based 

form is preferred since it results in a sampling from 

the same part of the annual distribution of 1-hour 

daily maximum SO2 concentrations regardless of 

the number of 1-hour daily maximum 

concentrations reported in a given year for a 

particular location. 

98th percentile form if EPA were to set 

a 1-hour standard.16 EPA responses to 

specific comments on the form of the 

standard can be found below and in the 

RTC document (EPA 2010). 

As mentioned above, a number of 

industry groups and the SD DENR 

preferred a 98th percentile form. In 

general, their preference for a 98th 

percentile form was based on their 

conclusion that a form based on the 

98th percentile would be more stable 

than a form based on the 99th 

percentile, and that a 98th percentile 

form is consistent with the forms 

selected in recent NAAQS reviews (i.e. 

PM2.5 and NO2). For example AQRL 

stated: ‘‘The Administrator should 

reconsider her proposal and choose 

instead the 98th percentile (or 

equivalent nth highest value) form of 

the standard for the added reliability 

and stability it offers in determining 

compliance or progress towards 

attainment. This approach has been 

promulgated for recent revisions of the 

PM2.5 and NO2 standards and this 

consistency should be maintained with 

SO2.’’ 

We agree with the commenters that it 

is important that a 1-hour standard have 

a form that is reasonably stable, but we 

disagree that a 98th percentile form is 

significantly more stable than a 99th 

percentile form. We note that the REA 

discussed analyses (also briefly 

described in the proposal; see section 

II.F.3, 74 FR at 64834) comparing trends 

in 98th and 99th percentile design 

values from 54 sites located in the 40 

counties selected for the detailed air 

quality analysis (REA section 10.5.3 and 

Thompson, 2009). These results 

suggested that at the vast majority of 

sites, there would have been similar 

changes in 98th and 99th percentile 

design values over the last ten years (i.e. 

based on evaluating overlapping three 

year intervals over the last ten years; see 

REA, Figure 10–1 and Thompson, 2009). 

As part of this analysis, all of the design 

values over this ten year period for all 

54 sites were aggregated and the 

standard deviation calculated (REA, 

Figure 10–2 and Thompson, 2009). 

Results demonstrated similar standard 

deviations—i.e. similar stability—based 

on aggregated 98th or aggregated 99th 

percentile design values over the ten 

year period (see REA, Figure 10–2 and 

Thompson 2009). Thus, we believe that 

in most locations, there will not be a 

substantial difference in stability 

between 98th and 99th percentile forms. 

We also disagree with the commenters 

that the forms of NAAQS standards 

should be consistent across different 

NAAQS pollutants. This is almost like 

advocating consistent levels or 

averaging times for different NAAQS 

pollutants. Each pollutant is manifestly 

different from another, and the decision 

as to an appropriate standard for each, 

and appropriate elements (including 

form) of each standard and the 

interaction of these elements, 

necessarily is fact-specific. Cf. Sierra 

Club v. EPA, 353 F. 3d 976, 986 (DC Cir. 

2004) (‘‘This court has adopted an ‘every 

tub on its own bottom’ approach to 

EPA’s setting of standards pursuant to 

the CAA, under which the adequacy of 

the underlying justification offered by 

the agency is the pertinent factor—not 

what the agency did on a different 

record concerning a different industry’’) 

(Roberts J.). There is thus no basis to say 

a priori that any element of one NAAQS 

should be consistent with another, 

although if all other things are equal, 

selecting stable forms for each NAAQS 

is a legitimate objective. 

A 99th percentile form, rather than a 

98th percentile form, is also needed for 

the standard to provide requisite public 

health protection. In this review of the 

primary SO2 NAAQS, we considered 

information in the ISA suggesting that 

adverse respiratory effects are more 

likely to occur at the upper end of the 

distribution of ambient SO2

concentrations. That is, the ISA 

described a few studies that reported an 

increase in SO2-related respiratory 

health effects at the upper end of the 

distribution of SO2 concentrations (i.e., 

above 90th percentile SO2

concentrations; ISA, section 5.3, p. 5–9). 

Moreover, we considered the extent to 

which different percentile forms, given 

the same standard level, limit 5-minute 

concentrations of SO2 above benchmark 

levels. As noted above in section 

II.F.3.a, and in more detail in the 

proposal (see section II.F.3.a, 74 FR at 

64834), air quality analyses presented in 

the REA suggested that at a given SO2

standard level, a 99th percentile form is 

appreciably more effective at limiting 5- 

minute peak SO2 concentrations than a 

98th percentile form (REA, section 

10.5.3, and REA, Figures 7–27 and 7– 

28). Taken together with the analyses 

suggesting that 98th and 99th percentile 

forms have similar stabilities, we 

reasonably concluded that a 99th 

percentile form was most appropriate 

for a 1-hour SO2 standard. 

As mentioned above, a number of 

health and environmental groups 

supported a 99th percentile form, but 

expressed that they would prefer a more 

restrictive form, such as a noexceedence based form. In addition, the 

Alexandria Department of 

Transportation and Environmental 

Services only recommended a no, or one 

exceedence based form. In general, these 

groups concluded that a more restrictive 

form would further limit the: (1) 

Number of days an area could exceed 

the standard level and still attain the 

standard; and (2) the occurrence of 5- 

minute peaks of SO2 above benchmark 

levels. 

It is important that the particular form 

selected for a 1-hour daily maximum 

standard reflect the nature of the health 

risks posed by increasing SO2

concentrations. The REA and proposal 

(see section II.F.3, 74 FR at 64833) noted 

that the form of the standard should 

reflect results from controlled human 

exposure studies demonstrating that the 

percentage of asthmatics affected, and 

the severity of the respiratory response 

(i.e. decrements in lung function, 

respiratory symptoms) increases as SO2

concentrations increase. Taking this into 

consideration, EPA staff concluded that 

a concentration-based form, averaged 

over three years, is more appropriate 

than an exceedance-based form (REA, 

section 10.5.3). This is because a 

concentration-based form averaged over 

three years gives proportionally greater 

weight to years when 

1-hour daily maximum SO2

concentrations are well above the level 

of the standard, as it gives to years when 

1-hour daily maximum SO2

concentrations are just above the level 

of the standard. In contrast, an expected 

exceedance form gives the same weight 

to years when 1-hour daily maximum 

SO2 concentrations are just above the 

level of the standard as it gives to years 

when 1-hour daily maximum SO2

concentrations are well above the level 

of the standard. Therefore, we 

concluded that a concentration-based 

form, averaged over three years (which 

also increases the stability of the 

standard) better reflects the continuum 

of health risks posed by increasing SO2

concentrations (i.e. the percentage of 

asthmatics affected and the severity of 

the response increases with increasing 

SO2 concentrations; REA, section 

10.5.3). Moreover, we note that analyses 

in the REA indicate that in most 

locations analyzed, a 99th percentile 

form would correspond to the 4th 

highest daily maximum concentration 

in a year, and that the 99th percentile, 

combined with the standard level 


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selected, will substantially limit 5- 

minute peaks of SO2 above the 200 ppb 

and higher benchmark levels (see below, 

section II.F.4). Finally, we note that a 

concentration based form is in 

agreement with CASAC advice that: 

‘‘there is adequate information to justify 

the use of a concentration-based form 

averaged over 3 years’’ (Samet 2009, 

p. 16). 

c. Conclusions on Form 

The Administrator agrees that the 

form of the standard should reflect the 

health evidence presented in the ISA 

indicating that the percentage of 

asthmatics affected and the severity of 

the response increases with increasing 

SO2 concentrations. The Administrator 

also agrees that it is reasonable to 

consider the standard’s stability as part 

of consideration of the form of the 

standard. Thus, the Administrator 

agrees that the standard should use a 

concentration-based form averaged over 

three years in order to give due weight 

to years when 1-hour SO2

concentrations are well above the level 

of the standard, than to years when 1- 

hour SO2 concentrations are just above 

the level of the standard. She also notes 

that a concentration-based form 

averaged over 3 years would likely be 

appreciably more stable than a noexceedence based form. 

In selecting a specific concentration 

based form, the Administrator first notes 

that a few epidemiologic studies 

described in the ISA reported an 

increase in SO2-related respiratory 

health effects at the upper end of the 

distribution of ambient SO2

concentrations (i.e., above 90th 

percentile SO2 concentrations; see ISA, 

section 5.3, p. 5–9). The Administrator 

notes further that numerous controlled 

human exposure studies have reported 

decrements in lung function and/or 

respiratory symptoms in exercising 

asthmatics exposed to peak 5–10 minute 

SO2 concentrations. The Administrator 

therefore concludes that the form of a 

new 1-hour standard should be 

especially focused on limiting the upper 

end of the distribution of ambient SO2

concentrations (i.e., above 90th 

percentile SO2 concentrations) in order 

to provide protection with an adequate 

margin of safety against effects reported 

in both epidemiologic and controlled 

human exposure studies. 

In further considering specific 

concentration based forms, the 

Administrator notes as outlined above 

in section II.F.3.b, and discussed in 

more detail in the REA (REA, section 

10.5.3) and proposal (see section II.F.3, 

74 FR at 64834), that a 99th percentile 

form is likely to be appreciably more 

effective at limiting 5-minute 

benchmark exposures of concern 

compared to a 98th percentile form. 

Taken together with the considerations 

just discussed above, the Administrator 

has selected a 99th percentile form, 

averaged over 3 years. The 

Administrator concludes that a 99th 

percentile form, given the level selected 

(see section II.F.4 immediately below), 

will limit both the upper end of the 

distribution of ambient SO2

concentrations reported in some 

epidemiologic studies to be associated 

with increased risk of SO2-related 

respiratory morbidity effects (e.g., 

emergency department visits), as well as 

5-minute peak SO2 concentrations 

resulting in decrements in lung function 

and/or respiratory symptoms in 

exercising asthmatics participating in 

controlled human exposure studies. 

4. Level 

As discussed below and in more 

detail in the proposal (section II.F.4, 74 

FR at 64834), the Administrator 

proposed to set a 1-hour standard with 

a 99th percentile form (averaged over 

three years), with a level in the range of 

50 to 100 ppb. The Administrator also 

solicited comment on standard levels 

greater than 100 ppb up to 150 ppb. 

This section summarizes the rationale 

for the Administrator’s proposed range 

of standard levels (II.F.3.a), discusses 

comments related to the range of 

standard levels (II.F.3.b), and presents 

the Administrator’s final conclusions 

regarding the level of a new 1-hour SO2

standard (II.F.3.c). 

a. Rationale for Proposed Decision 

In assessing the level of a 1-hour 

standard with a 99th percentile form 

(averaged over three years), the 

Administrator considered the broad 

range of scientific evidence assessed in 

the ISA, including the epidemiologic 

studies and controlled human exposure 

studies, as well as the results of air 

quality, exposure, and risk analyses 

presented in the REA. In light of this 

body of evidence and analyses, the 

Administrator found it is necessary to 

provide increased public health 

protection for at-risk populations 

against an array of adverse respiratory 

health effects related to short-term (i.e., 

5 minutes to 24 hours) exposures to 

ambient SO2. In considering the most 

appropriate way to provide this 

protection, the Administrator was 

mindful of the extent to which the 

available evidence and analyses could 

inform a decision on the level of a 

standard. The Administrator’s proposed 

decisions on level, as discussed in detail 

in the proposal (see section II.F.4.e), are 

outlined below. 

Given the above considerations, the 

Administrator proposed to set a level for 

a new 99th percentile 1-hour daily 

maximum primary SO2 standard within 

the range from 50 to 100 ppb and took 

comment on levels above 100 ppb, up 

to 150 ppb. In reaching this proposed 

decision, the Administrator considered: 

(1) The evidence-based considerations 

from the final ISA and the final REA; 

(2) the results of the air quality, 

exposure, and risk assessments 

discussed above and in the final REA; 

(3) CASAC advice and 

recommendations on both the ISA and 

REA discussed above and provided in 

CASAC’s letters to the Administrator; 

and (4) public comments received on 

the first and second drafts of the ISA 

and REA. In considering what level of 

a 1-hour SO2 standard is requisite to 

protect public health with an adequate 

margin of safety, the Administrator was 

mindful that this choice requires 

judgments based on an interpretation of 

the evidence and other information that 

neither overstates nor understates the 

strength and limitations of that evidence 

and information. 

As noted above, the Administrator 

selected an upper end of a range of 

levels to propose at 100 ppb. The 

selection of this level focused on the 

results of the controlled human 

exposure studies and is primarily based 

on the results of the air quality and 

exposure analyses which suggest that a 

1-hour standard should be at or below 

100 ppb to appreciably limit 5-minute 

SO2 benchmark concentrations 

≥ 200 ppb (see proposal Tables 2–4, and 

proposal sections II.F.4.a and II.F.4.b). 

That is, as described in the proposal (see 

section II.F.4.e), the 40-county air 

quality analysis estimates that a 100 ppb 

1-hour standard would allow at most 2 

days per year on average when 

estimated 5-minute daily maximum SO2

concentrations exceed the 400 ppb 

benchmark, and at most 13 days per 

year on average when 5-minute daily 

maximum SO2 concentrations exceed 

the 200 ppb benchmark (see proposal 

Table 2). Furthermore, given a 

simulated 1-hour 100 ppb standard 

level, most counties in the air quality 

analysis were estimated to experience 0 

days per year on average when 5-minute 

daily maximum SO2 concentrations 

exceed the 400 ppb benchmark and ≤ 3 

days per year on average when 5-minute 

daily maximum SO2 concentrations 

were estimated to exceed the 200 ppb 

benchmark (see REA, Tables 7–14 and 

7–12). The Administrator also noted 

that the St. Louis exposure analysis 

indicated that a 1-hour standard at 


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100 ppb would still be estimated to 

protect > 99% of asthmatic children at 

moderate or greater exertion from 

experiencing at least one 5-minute SO2

exposure ≥ 400 ppb per year, and about 

97% of these children from exposures ≥ 

200 ppb. In contrast, as described in the 

proposal (see section II.F.4.b), the St. 

Louis exposure analysis estimated that a 

1-hour standard at 150 ppb would likely 

only protect about 88% of asthmatic 

children at moderate or greater exertion 

from experiencing at least one 5-minute 

SO2 exposure ≥ 200 ppb per year. 

As noted above and described in 

detail in the proposal (see section 

II.F.4.e), the Administrator selected 50 

ppb as the lower end of a range of levels 

to propose, which is consistent with 

CASAC’s advice. The selection of this 

level focused in part on the U.S. 

epidemiologic evidence described in 

detail in the proposal (see sections 

II.B.2, II.F.4.a, and II.F.4.e). With respect 

to these epidemiologic studies, seven of 

ten U.S. emergency department visit 

and hospital admission studies 

reporting generally positive associations 

with ambient SO2 were conducted in 

locations where 99th percentile 1-hour 

daily maximum SO2 levels were about 

75–150 ppb, and three of these studies 

observed statistically significant 

positive associations between ambient 

SO2 and respiratory-related emergency 

department visits and hospitalizations 

in multi-pollutant models with PM 

(NYDOH (2006), Ito et al., (2007), and 

Schwartz et. al, (1995)). Thus, the 

Administrator noted that a 99th 

percentile 1-hour daily maximum 

standard set at a level of 50 ppb is well 

below the 99th percentile 1-hour daily 

maximum SO2 concentrations reported 

in locations where these three studies 

were conducted (i.e. well below 99th 

percentile 1-hour daily maximum SO2

levels of 78–150 ppb seen in NYDOH 

(2006), Ito et al., (2007), and Schwartz 

et. al, (1995)). Finally, the Administrator 

noted that two epidemiologic studies 

reported generally positive associations 

between ambient SO2 and emergency 

department visits in cities when 99th 

percentile 1-hour daily maximum SO2

concentrations were approximately 

50 ppb, but did not consider that 

evidence strong enough to propose 

setting a standard level lower than 50 

ppb. 

In considering the results of the air 

quality and exposure analyses, the 

Administrator also noted that the 40- 

county air quality analysis estimates 

that a 99th percentile 1-hour daily 

maximum standard set at a level of 

50 ppb would result in zero days per 

year when estimated 5-minute SO2

concentrations exceed the 400 ppb 5- 

minute benchmark level and at most 2 

days per year when modeled 5-minute 

SO2 concentrations exceed the 200 ppb 

5-minute benchmark level (see proposal 

section II.F.4.b and proposal Table 2). In 

addition, the St. Louis exposure analysis 

estimates that a 99th percentile 1-hour 

daily maximum standard set at a level 

of 50 ppb would likely protect > 99% 

of asthmatic children at moderate or 

greater exertion from experiencing at 

least one 5-minute exposure both ≥ 400 

and > 200 ppb per year (see proposal 

section II.F.4.b and Table 3). In 

addition, although not directly analyzed 

in the REA, the proposal (section 

II.F.4.b) noted that a 1-hour daily 

maximum standard at a level of 75 ppb 

would be bound by the exposure 

estimates from air quality adjusted to 

just meet 99th percentile 1-hour daily 

maximum standards at 50 and 100 ppb. 

Thus, a 1-hour daily maximum standard 

at a level of 75 ppb would be estimated 

to protect > 99% of asthmatic children 

at moderate or greater exertion in St. 

Louis from experiencing at least one 

exposure ≥ 400 ppb per year, and about 

97% to > 99% of these children from 

experiencing at least one exposure ≥ 200 

ppb per year. 

The Administrator thus proposed to 

set the level of a new 1-hour standard 

that would protect public health with an 

adequate margin of safety between 50 

ppb and 100 ppb. In so doing, the 

Administrator relied on reported 

findings from both epidemiologic and 

controlled human exposure studies, as 

well as the results of air quality and 

exposure analyses. The Administrator 

noted that the lower end of the 

proposed range was consistent with 

CASAC advice that there is clearly 

sufficient evidence for consideration of 

standard levels starting at 50 ppb (Samet 

2009, p. 16). With respect to the upper 

end of the proposed range, the 

Administrator noted that CASAC 

concluded that standards up to 150 ppb 

‘‘could be justified under some 

interpretations of weight of evidence, 

uncertainties, and policy choices 

regarding margin of safety’’ (id.), 

although the letter did not provide any 

indication of what interpretations, 

uncertainties, or policy choices might 

support selection of a level as high as 

150 ppb. 

In light of the range of levels included 

in CASAC’s advice, the Administrator 

also solicited comment on setting a 

standard level above 100 ppb and up to 

150 ppb. In so doing, the Administrator 

recognized that there are uncertainties 

with the scientific evidence, such as 

attributing effects reported in 

epidemiologic studies specifically to 

SO2 given the presence of co-occurring 

pollutants, especially PM, and the 

uncertainties associated with using 

ambient SO2 concentrations as a 

surrogate for exposure. However, the 

Administrator noted that compared to 

the proposed range of 50–100 ppb, a 

standard level as high as 150 ppb would 

not comparably limit 5-minute SO2

exposures ≥ 200 ppb. That is, she noted 

that the St. Louis exposure analysis 

estimated that a 150 ppb standard 

would protect approximately 88% of 

asthmatic children at moderate or 

greater exertion from experiencing at 

least one SO2 exposure ≥ 200 ppb per 

year (compared to > 99% and 

approximately 97% given standards at 

50 and 100 ppb respectively; see 

proposal Table 3 at 74FR at 64841). 

b. Comments on Level 

Most State and local agencies and 

organizations that commented on this 

issue expressed support for setting the 

level of a 1-hour SO2 standard 

somewhere within the proposed range 

of 50 to 100 ppb. More specifically, 

State environmental organizations (i.e., 

NACAA and NESCAUM); State 

environmental agencies (e.g., such 

agencies in DE, IL, MI, NY, NM, PA, 

VT), the Fond du Lac Tribe, and local 

groups (e.g., NYDOH, City of Houston, 

New York City, Houston-Galveston Area 

Council) supported a level of a 1-hour 

SO2 standard in the range of 50 to 100 

ppb. In addition, State environmental 

agencies in IA and TX specifically 

supported a standard level of 100 ppb. 

In general, these groups cited the 

conclusions of CASAC and the 

Administrator’s rationale as stated in 

the proposal as a basis for their 

recommendations, though State 

environmental agencies in IA and TX 

generally recommended placing more 

weight on the controlled human 

exposure evidence rather than on the 

epidemiology. 

A number of environmental and 

medical/public health organizations 

(e.g., ALA, ATS, EDF, Sierra Club, 

WEACT for Environmental Justice, 

NRDC, CBD) and some local 

organizations (e.g., Alexandria 

Department of Transportation and 

Environmental Services, and Harris 

County (TX) Public Health & 

Environmental Services) supported 

setting a standard level at or near 50 

ppb. This recommendation was 

typically based on the commenters’ 

interpretation of the controlled human 

exposure and epidemiologic evidence, 

as described below. 

With regard to the controlled human 

exposure evidence, health and 

environmental groups generally 

concluded that a 1-hour SO2 standard 


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no higher than 50 ppb is needed to 

protect against 5-minute SO2 benchmark 

exposures as low as 100 ppb identified 

from mouthpiece exposure studies, 

rather than the 200 ppb 5-minute SO2

benchmark identified from ‘‘free 

breathing’’ controlled human exposure 

studies. More specifically, ALA et al., 

stated: 

In its analysis of data from chamber studies 

in the ISA and in the REA, EPA focuses on 

studies of ‘‘free breathing’’ exposure. In doing 

so, EPA improperly and arbitrarily 

downplays important evidence that reported 

increased airway resistance, a measure of 

bronchoconstriction, in subjects with mild 

asthma at concentrations of 100 ppb. 

Regrettably, EPA does not rely on the 

mouthpiece studies in formulating its 

proposed standards * * * In downplaying 

the mouthpiece studies, EPA ignores the 

large segment of people who rely on oral or 

oronasal breathing some or all of the time. 

The Administrator disagrees with the 

assertion that results from mouthpiece 

studies were improperly downplayed. 

These studies are discussed in the ISA, 

REA, and proposed rule as 

demonstrating respiratory effects of SO2

at concentrations of 100 ppb, the lowest 

concentration tested using a mouthpiece 

exposure system. Nonetheless, these 

mouthpiece studies are not a reasonable 

proxy for actual exposure. In these 

studies, SO2 is delivered directly 

through the mouth, typically in 

conjunction with nasal occlusion. This 

allows a greater fraction of the inhaled 

SO2 to reach the tracheobronchial 

airways. Although we agree with 

commenters that some individuals do 

breathe oronasally both while at rest 

and during exercise, nasal ventilation 

still constitutes a significant percentage 

of total ventilation. The consequence is 

that individuals exposed to SO2 through 

a mouthpiece are likely to experience 

greater respiratory effects from a given 

SO2 exposure than they would in real 

life. Thus, as noted in the REA (REA, 

section 6.2) and in the proposal 

preamble (see section II.B.1.b), these 

mouthpiece studies only provide very 

limited evidence of decrements in lung 

function following exposure to 100 ppb 

SO2. Therefore, the Administrator did 

not place great weight on these 

mouthpiece studies when considering 

the appropriate level of a 1-hour SO2

standard. 

In addition to their interpretation of 

the controlled human exposure 

evidence, health and environmental 

groups (e.g., ALA, ATS, EDF, NRDC, 

Sierra Club, CBD) and the Alexandria 

Department of Transportation and 

Environmental Services generally 

concluded that the epidemiologic 

evidence indicates that a standard no 

higher than 50 ppb is required to protect 

public health. For example, it its 

comments the CBD stated: 

Epidemiologic studies referenced in the 

Proposed Rule showed positive, and in many 

cases statistically significant, relationships 

between ambient SO2 concentrations and 

hospital admissions where 99th percentile 1- 

hour concentrations ranged from 50–460 ppb. 

Of these studies, two showed positive and 

sometimes statistically significant 

relationships in single-pollutant models at 50 

ppb, and three studies showed statistically 

significant correlations at 78–150 ppb in 

multi-pollutant models. These three 

multipollutant studies, moreover, ‘‘lend[] 

strong support * * * to the conclusion that 

SO2 effects are generally independent’’ of 

those of co-pollutants like particulate matter. 

Giving these studies their proper weight, and 

allowing for an adequate margin of safety, 

EPA should set a one-hour NAAQS at a level 

no higher than the lowest concentration at 

which positive, adverse relationships have 

been demonstrated: 50 ppb (note that 

footnotes were omitted). 

The Administrator agrees that the 

epidemiologic studies referenced in the 

proposal need to be considered in 

judging the appropriate level for a new 

99th percentile 1-hour SO2 standard. 

However, she disagrees that when 

considered in total, these studies 

strongly support an SO2 standard no 

higher than 50 ppb. The Administrator 

notes that selecting a standard level of 

50 ppb would place considerable weight 

on the two U.S. emergency department 

visit studies conducted in locations 

where 99th percentile 1-hour SO2

concentrations were approximately 50 

ppb (i.e., Wilson et al., (2005) in 

Portland, ME and Jaffe et al., (2003) in 

Columbus, OH). However, the 

Administrator does not find this 

appropriate given that, importantly, 

neither of these studies evaluated the 

potential for confounding by copollutants through the use of 

multipollutant models and thus, left 

unaddressed the issue of whether the 

effects seen in the studies were partially 

or totally attributable to exposure to 

sulfate PM. In addition, the 

Administrator notes that the overall 

results reported in these studies are 

mixed. It is important to note that mixed 

results do not automatically disqualify 

studies from being used as part of the 

evidence base for setting levels in 

NAAQS reviews. However, in this 

review the Administrator judges that the 

lack of mutipollutant model evaluation 

for potential confounding by PM in two 

locations with the lowest SO2 levels 

combined with the presence of mixed 

emergency department visit results 

renders these two studies inappropriate 

to serve as the primary basis for the 

selection of the level of the SO2

NAAQS. As an additional matter, the 

suggestion in some of the comments that 

EPA should necessarily base the level of 

a NAAQS on the lowest level seen in 

epidemiologic studies has been rejected 

repeatedly. See, e.g. American 

Petroleum Inst. v. EPA, 665 F. 2d at 

1187 (‘‘In so arguing NRDC essentially 

ignores the mixed results of the medical 

studies evident in the record, choosing 

instead to rely only on the studies that 

favor its position. The Administrator, 

however, was required to take into 

account all the relevant studies revealed 

in the record. Because he did so in a 

rational manner, we will not overrule 

his judgment as to the margin of 

safety.’’) Thus, although the 

Administrator finds that these two 

studies provide limited evidence of 

emergency department visits in cities 

where 99th percentile 1-hour daily 

maximum SO2 concentrations are 

approximately 50 ppb, she also 

concludes that these studies do not 

provide enough evidence to warrant a 

standard at this level. 

As discussed above in section, II.E.2, 

a number of industry groups (e.g., ACC, 

UARG) did not support setting a new 1- 

hour SO2 standard. However, several of 

these groups (e.g., UARG, API) and the 

SC Chamber of Commerce concluded 

that, if EPA does choose to set a new 1- 

hour standard, the level of that standard 

should be ≥ 150 ppb. In addition, State 

environmental agencies in SD (SD 

DENR) and OH recommended standard 

levels at 150 ppb. As a basis for this 

recommendation, these groups generally 

emphasized uncertainties in the 

scientific evidence. Specifically, as 

discussed in more detail above (section 

II.E.2.a), these commenters typically 

concluded that the available 

epidemiologic studies do not support 

the conclusion that SO2 causes the 

reported health effects. This was based 

on their assertion that the presence of 

co-pollutants in the ambient air 

precludes the identification of a specific 

SO2 contribution to reported effects. 

Thus, these groups generally concluded 

that weight should not be placed on the 

cluster of three epidemiologic studies 

reporting statistically significant effects 

in multipollutant models with PM (i.e., 

NYDOH 2006; Ito 2007; and Schwartz 

1995). That is, these groups contend that 

these studies do not demonstrate an 

independent effect of SO2. In addition, 

as noted in section II.E.2.b, many of 

these groups also disagreed with the 

Agency’s judgment that adverse 

respiratory effects occur following 5- 

minute exposures to SO2 concentrations 

as low as 200 ppb. These comments and 

EPA’s responses are discussed below 


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and in section II of the RTC document 

(EPA 2010). 

As described in more detail in section 

II.E.2.a, we agree that the interpretation 

of SO2 epidemiologic studies is 

complicated by the fact that SO2 is but 

one component of a complex mixture of 

pollutants present in the ambient air. 

However, the ISA concluded that when 

U.S. and international epidemiologic 

literature is evaluated as a whole, SO2

effect estimates generally remained 

positive and relatively unchanged in 

multi-pollutant models with gaseous or 

particulate co-pollutants. Thus, 

although recognizing the uncertainties 

associated with separating the effects of 

SO2 from those of co-occurring 

pollutants, the ISA concluded that the 

limited available evidence from studies 

employing multi-pollutant models 

indicates that the effect of SO2 on 

respiratory health outcomes appears to 

be generally robust and independent of 

the effects of gaseous co-pollutants, 

including NO2 and O3, as well as 

particulate co-pollutants, particularly 

PM2.5 (ISA, section 5.2; p. 5–9). 

In addition, as described in detail 

above in section II.E.2.a, the ISA 

emphasized that controlled human 

exposure studies provide support for the 

plausibility of the associations reported 

in epidemiologic studies. The ISA noted 

that the results of controlled human 

exposure and epidemiologic studies 

form a plausible and coherent data set 

that supports a causal relationship 

between short-term (5-minutes to 24- 

hours) SO2 exposures and adverse 

respiratory effects, and that the 

epidemiologic evidence (buttressed by 

the clinical evidence) indicates that the 

effects seen in the epidemiologic studies 

are attributable to exposure to SO2 (ISA, 

section 5.2). The ISA in fact made the 

strongest finding possible regarding 

causality: ‘‘[e]valuation of the health 

evidence, with consideration of issues 

related to atmospheric sciences, 

exposure assessment, and dosimetry, 

led to the conclusion that there is a 

causal relationship between respiratory 

morbidity and short-term exposure to 

SO2. This conclusion is supported by 

the consistency, coherence, and 

plausibility of findings observed in the 

human clinical, epidemiologic, and 

animal toxicological studies.’’ ISA p. 

5–2 (emphasis original). 

As mentioned above, many groups 

dispute the ISA conclusion that taken 

together, results from U.S. and 

international epidemiologic studies 

employing multipollutant models 

indicate that SO2 has an independent 

effect on the respiratory health 

outcomes reported in these studies. 

Thus, these groups contend that the 

Administrator should not place weight 

on epidemiologic studies and their 

associated air quality information in 

general, and more specifically, the 

Administrator should not place weight 

on air quality information from the three 

U.S. epidemiologic studies reporting 

statistically significant effects in 

multipollutant models with PM (i.e., 

NYDOH 2006; Ito 2007; and Schwartz 

1995). Specific comments on these three 

epidemiologic studies reporting 

statistically significant effects in multipollutant models with PM, and EPA 

responses are presented below and in 

the RTC document (EPA 2010). 

Industry groups (e.g., API) had several 

comments with respect to the study 

conducted by the NYDOH (NYDOH, 

2006). First, these groups generally 

concluded that the results of this study 

are mixed. That is, while SO2 effect 

estimates were positive and statistically 

significant even in multipollutant 

models with PM2.5 or NO2 in the Bronx, 

SO2 effect estimates were actually 

negative in Manhattan in both single 

and multipollutant models. These 

groups also contend that this report was 

not peer-reviewed and that the authors 

of this study indicated that high 

correlations among pollutants in the 

Bronx made it difficult to confidently 

identify which pollutants are actually 

increasing risks. For these reasons, 

industry groups generally concluded 

that this study should not be relied 

upon by the Administrator. 

We acknowledge that the results of 

the NYDOH analysis are mixed when 

comparing the Bronx and Manhattan 

study areas. However, we disagree that 

the presence of mixed results renders 

this study unreliable. We note that the 

mixed results reported in this study are 

likely to reflect greater statistical power 

for identifying effects in the Bronx, 

where the average daily emergency 

department visits differed substantially 

from those in Manhattan. Specifically, 

daily asthma emergency department 

visits were six times higher in the Bronx 

study area (43 per day) than in the 

Manhattan study area (7.2 per day). 

Thus, the more prominent effects in the 

Bronx likely at least partially reflect 

greater statistical power for identifying 

effects there. To put these numbers in 

perspective, the crude daily rates of 

asthma emergency department visits can 

be estimated by dividing the daily 

asthma counts by the population. The 

mean daily crude rates of asthma 

emergency department visits were over 

eight-fold higher in the Bronx study area 

(16.9 per 100,000 persons) than in the 

Manhattan area (2.02 per 100,000 

persons). Population age structures were 

quite different in the two communities, 

with larger proportions of younger 

persons in the Bronx versus Manhattan. 

There are likely additional differences 

in population structures of the two 

communities, including differences in 

SES, race/ethnicity, and access to 

primary asthma care. These differences 

in the two communities may explain the 

differences in the results, and do not 

prevent EPA from legitimately relying 

on this study. 

As mentioned above, these groups 

also contend that the NYDOH 

epidemiologic study should not be 

relied upon because it was not peerreviewed. We disagree with this 

assertion. The NYDOH study was 

subject to multiple peer-review 

processes. This included reviews by the 

Agency for Toxic Substances and 

Disease Registry (ATSDR), EPA, and 

CASAC. 

Finally, as also mentioned above, 

these groups contend that the NYDOH 

epidemiologic study is unreliable 

because the study authors indicated that 

high correlations among pollutants in 

the Bronx make it difficult to 

confidently identify which pollutants 

are actually increasing risks. In 

response, we note that high correlations 

among ambient air pollutant 

concentrations are not specific to the 

NYDOH study, and may contribute to 

uncertainty in the interpretation of 

many epidemiologic studies of air 

pollution. The approach most 

commonly utilized to disentangle the 

effects of correlated pollutants in air 

pollution epidemiology is the 

copollutant model. The NYDOH uses 

copollutant models and finds that the 

results for SO2 remain significant in 

models considering the simultaneous 

effects of NO2, O3, and PM2.5. This 

indicates an independent effect of SO2

on the asthma emergency department 

visits reported in this study. 

With respect to Ito et al., (2007), 

industry groups generally commented 

that since the SO2 effect estimate did 

not remain statistically significant in 

multipollutant models with NO2, this 

study does not indicate an independent 

effect of SO2 on emergency department 

visits in the NYC study area. API 

specifically commented: 

The RR for an increase of 6 ppb SO2 was 

statistically significant (1.20; 95% CI: 1.13, 

1.28) and remained so when PM2.5, O3, or CO 

was included in the model, but became 

nonsignificant when NO2 was included in 

the model (RR not provided, 95% CI: 0.9, 

1.1). Because associations with SO2 could be 

attributable to NO2, this study cannot be used 

to assess the effects of SO2 on health effects 

with small incremental increases in 

exposure. 


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17See Coalition of Battery Recyclers Association 

v. EPA, No. 09–1011 (DC Cir., May 14, 2010), slip 

opinion at 9, holding that it was reasonable for EPA 

to conclude that a two IQ point mean population 

loss is an adverse effect based in part on 

consideration of comments from the American 

Academy of Pediatrics that such a loss should be 

prevented. 

We disagree with the commenters. We 

believe that this study does demonstrate 

an independent effect of SO2 on 

emergency department visits in NYC. 

We note that evidence from controlled 

human exposure studies has 

demonstrated effects of NO2 (EPA, 

2008b) and SO2 independently on 

respiratory morbidity. Since each of 

these criteria pollutants has an 

independent effect on the respiratory 

system, it is logical that each may be 

responsible for an increase in 

emergency department visits for asthma 

in epidemiologic studies. In addition, 

the authors note that the attenuation of 

the SO2 effect estimate when NO2 is 

included in the model is ‘‘consistent 

with the result of monitor-to-monitor 

correlations, suggesting that NO2 has 

less exposure error than CO or SO2 in 

this data set.’’ Thus, it appears as though 

the high spatial heterogeneity of SO2

compared to NO2, leading to increased 

exposure error, may be causing the 

attenuation of the SO2 effect estimate 

when NO2 is included in the model in 

this study—not that the effects seen in 

the study are attributable to NO2. 

Overall, the results from this study are 

consistent with the SO2 effect on 

respiratory emergency department visits 

and hospital admissions across studies 

and are coherent with the respiratory 

effects observed in controlled human 

exposure studies. This study thus 

provides persuasive evidence of an 

independent effect of short-term SO2

exposure on respiratory morbidity. 

With respect to Schwartz et al., 

(1995), industry groups generally 

commented that the results of this study 

are mixed, and therefore should not be 

considered by the Administrator. More 

specifically, these commenters noted 

that although the results in New Haven 

remained statistically significant in the 

presence of PM10, the SO2 effect 

estimate in Tacoma was reduced and no 

longer statistically significant in the 

presence of PM10. Commenters also 

noted that in both cities, the SO2 effect 

estimate was reduced and no longer 

statistically significant in the presence 

of O3. 

We disagree that the results of this 

study of hospital admissions should not 

be considered by the Administrator. As 

noted by the commenters, this study 

was conducted in two cities, New 

Haven, CT and Tacoma, WA. These 

cities were chosen because they differ in 

several important aspects and the author 

expected the results from the two cities 

to be different due to the inherent 

nature of the study design and study 

locations. ‘‘New Haven has almost twice 

the mean SO2 concentration of Tacoma, 

almost two and a half times the SO2

concentration in the peak winter season, 

and a much larger summer ozone peak 

than Tacoma (Schwartz 1995).’’ Since 

the study was designed to examine the 

differences in these two cities, the fact 

that the results differed in the two cities 

does not invalidate those results. In 

addition, EPA considers the SO2 effect 

to be robust to inclusion of O3 in New 

Haven. The central effect estimate for 

SO2 changed from 1.03 to 1.02 after the 

addition of O3 as a copollutant and 

likely lost statistical significance due to 

a greater than 40% reduction in the 

number of days included because O3

was only measured during the warm 

months. This reduction likely led to 

model instability and a loss of statistical 

significance. To be consistent with how 

results of other studies were interpreted 

in the ISA, and as supported by the 

CASAC, the effect of SO2 is considered 

robust to the inclusion of O3 in New 

Haven. 

In addition to generally concluding 

that the epidemiology is too uncertain to 

demonstrate that SO2 has an 

independent effect on the respiratory 

effects reported in those studies, many 

industry groups (e.g., API, ACC, 

Progress Energy, EEI, CIBO) also 

commented that adverse health effects 

do not occur following 5–10 minute SO2

exposures < 400 ppb in controlled 

human exposure studies (an issue also 

discussed above in section II.E.2.b). 

Thus, these groups generally maintained 

that the level of a 1-hour standard 

should not take into account limiting 5- 

minute peaks as low as 200 ppb. From 

this argument, many of these groups 

further maintained that 1-hour standard 

levels ≥ 150 ppb are requisite to protect 

public health with an adequate margin 

of safety. 

As first discussed in section II.E.2.b 

above, we disagree with the commenters 

that adverse respiratory effects do not 

occur following 5-minute SO2 exposures 

as low as 200 ppb. The ISA reported 

that exposure to SO2 concentrations as 

low as 200–300 ppb for 5–10 minutes 

results in approximately 5–30% of 

exercising asthmatics experiencing 

moderate or greater decrements in lung 

function (defined in terms of a ≥ 15% 

decline in FEV1 or 100% increase in 

sRaw; ISA, Table 3–1). Considering the 

2000 ATS guidelines described in 

section II.E.2.b, we determined that 

these results could reasonably indicate 

an SO2-induced shift in these lung 

function measurements for this subpopulation. Under this scenario, an 

appreciable percentage of exercising 

asthmatics exposed to SO2

concentrations as low as 200 ppb would 

likely have diminished reserve lung 

function and thus would likely be at 

greater risk if affected by another 

respiratory agent (e.g., viral infection). 

Importantly, diminished reserve lung 

function in a population that is 

attributable to air pollution is 

considered an adverse effect under ATS 

guidance.17 Also noted in section 

II.E.2.b, we were mindful of CASAC’s 

pointed comments. The second draft 

ISA placed relatively little weight on 

health effects associated with SO2

exposures at 200–300 ppb. CASAC 

strongly disagreed with this 

characterization of the health evidence. 

Their consensus letter following the 

second draft ISA states: 

Our major concern is the conclusions in 

the ISA regarding the weight of the evidence 

for health effects for short-term exposure to 

low levels of SO2. Although the ISA presents 

evidence from both clinical and 

epidemiological studies that indicate health 

effects occur at 0.2 ppm or lower, the final 

chapter emphasizes health effects at 0.4 ppm 

and above * * * CASAC believes the clinical 

and epidemiological evidence warrants 

stronger conclusions in the ISA regarding the 

available evidence of health effects at 0.2 

ppm or lower concentrations of SO2. The 

selection of a lower bound concentration for 

health effects is very important because the 

ISA sets the stage for EPA’s risk assessment 

decisions. In its draft Risk and Exposure 

Assessment (REA) to Support the Review of 

the SO2 Primary National Ambient Air 

Quality Standards (July 2008), EPA chose a 

range of 0.4 ppm—0.6 ppm SO2

concentrations for its benchmark analysis. As 

CASAC will emphasize in a forthcoming 

letter on the REA, we recommend that a 

lower bound be set at least as low as 0.2 ppm 

(Henderson 2008a). 

Similarly, we were also mindful of 

CASAC comments on the first draft of 

the REA. The consensus CASAC letter 

following the 1st draft REA states: 

The CASAC believes strongly that the 

weight of clinical and epidemiology evidence 

indicates there are detectable clinically 

relevant health effects in sensitive 

subpopulations down to a level at least as 

low as 0.2 ppm SO2. These sensitive 

subpopulations represent a substantial 

segment of the at-risk population (Henderson 

2008b). 

As noted in section II.E.2.b, we were 

also mindful of: (1) Previous CASAC 

recommendations (Henderson 2006) and 

NAAQS review conclusions (EPA 2006, 

EPA 2007d) indicating that moderate 

decrements in lung function can be 

clinically significant in some asthmatics 

(see section II.E.2.b for more detail) and 


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18The ISA concluded that collective evidence 

from key controlled human exposure studies 

considered in the previous review, along with a 

limited number of new controlled human exposure 

studies, consistently indicates that with elevated 

ventilation rates a large percentage of asthmatic 

individuals tested in a given chamber study (up to 

60%, depending on the study) experience moderate 

or greater decrements in lung function, frequently 

accompanied by respiratory symptoms, following 

peak exposures to SO2 at concentrations of 0.4–0.6 

ppm. (ISA, p. 3–9). 

(2) controlled human exposure studies 

not including severe asthmatics and 

thus, that it is reasonable to assume that 

persons with more severe asthma than 

the study participants would have a 

more serious health effect from shortterm exposure to 200 ppb SO2. CASAC 

echoed this concern in its comments on 

the policy assessment chapter of the 

REA: 

Chapter 10 should better address 

uncertainty in identifying alternative NAAQS 

for SO2. In particular, the uncertainties 

discussed in the health risk characterization 

should be considered in specifying a NAAQS 

that provides adequate margin of safety. One 

particular source of uncertainty needing 

acknowledgment is the characteristics of 

persons included in the clinical studies. The 

draft REA acknowledges that clinical studies 

are unlikely to have included severe 

asthmatics that are likely to be potentially at 

greater risk than those persons included in 

the clinical studies (Samet 2009; p. 15). 

Taken together, the Administrator 

concluded that exposure to SO2

concentrations as low as 200 ppb can 

result in adverse health effects in 

asthmatics. Consequently the 

Administrator also concluded that a 

1-hour standard of 150 ppb is not 

requisite to protect public health with 

an adequate margin of safety, even with 

a 99th percentile form. This conclusion 

takes into account the St. Louis 

exposure analysis estimating that only 

88% of asthmatic children at moderate 

or greater exertion would be protected 

from at least one 5-minute SO2 exposure 

≥ 200 ppb per year at a 1-hour standard 

level of 150 ppb, and appropriate weight 

placed on the epidemiologic evidence 

(see section II.F.4.c for a discussion of 

the epidemiologic evidence with respect 

to level). 

c. Conclusions on Standard Level 

Having carefully considered the 

public comments on the appropriate 

level for a 1-hour SO2 standard, as 

discussed above, the Administrator 

believes the fundamental conclusions 

reached in the ISA and REA remain 

valid. In considering the level at which 

the 1-hour primary SO2 standard should 

be set, the Administrator continues to 

place primary emphasis on the body of 

controlled human exposure and 

epidemiologic evidence assessed in the 

ISA, as summarized above in section 

II.B. In addition, the Administrator 

continues to view the results of 

exposure and risk analyses, discussed 

above in section II.C, as providing 

supporting information for her decision. 

In considering the level of a 1-hour 

SO2 standard, the Administrator notes 

that there is no bright line clearly 

mandating the choice of level within the 

reasonable range proposed. Rather, the 

choice of what is appropriate within 

this reasonable range is a public health 

policy judgment entrusted to the 

Administrator. This judgment must 

include consideration of the strengths 

and limitations of the evidence and the 

appropriate inferences to be drawn from 

the evidence and the exposure and risk 

assessments. These considerations and 

the Administrator’s final decision with 

regard to the level of a new 1-hour SO2

standard are discussed below. 

In considering the controlled human 

exposure studies, the Administrator 

notes that these studies provide the 

most direct evidence of respiratory 

effects from exposure to SO2. These 

studies exposed groups of exercising 

asthmatics to defined concentrations of 

SO2 for 5–10 minutes and found adverse 

respiratory effects. As noted above (see 

section II.C), SO2 exposure levels which 

resulted in respiratory effects in these 

studies were considered 5-minute 

benchmark exposures of potential 

concern in the analyses found in the 

REA. With respect to this evidence, the 

Administrator notes the following key 

points: 

• Exposure of exercising asthmatics 

to 5–10 minute SO2 concentrations ≥ 

400 ppb results in moderate or greater 

decrements in lung function (in terms of 

FEV1 or sRaw) in 20–60% of tested 

individuals in these studies. Moreover, 

these decrements in lung function are 

often statistically significant at the 

group mean level and are frequently 

accompanied by respiratory 

symptoms.18 Based on ATS guidelines, 

exposure to SO2 concentrations ≥ 400 

ppb clearly result in adverse respiratory 

effects (i.e., decrements in lung function 

in the presence of respiratory 

symptoms). Therefore, the 

Administrator has concluded it 

appropriate to place weight on the 400 

ppb 5-minute SO2 benchmark 

concentration of concern. 

• Exposure of exercising asthmatics 

to 5–10 minute SO2 concentrations at 

200–300 ppb results in moderate or 

greater decrements in lung function in 

5–30% of the tested individuals in these 

studies. The Administrator notes that 

although these decrements in lung 

function have not been shown to be 

statistically significant at the group 

mean level, or to be frequently 

accompanied by respiratory symptoms, 

she considers effects associated with 

exposures as low as 200 ppb to be 

adverse in light of CASAC advice, 

similar conclusions in prior NAAQS 

reviews, and the ATS guidelines 

described in detail above (see section 

II.E.2.b and II.F.4.b). Therefore, she has 

concluded it appropriate to place weight 

on the 200 ppb 5-minute benchmark 

concentration. 

• There is very limited evidence from 

two mouthpiece exposure studies 

suggesting respiratory effects in 

exercising asthmatics following SO2

exposures at 100 ppb. However, given 

the uncertainties and potential 

unrepresentativeness associated with 

mouthpiece studies (see section II.F.4.b 

above), the Administrator found it 

appropriate not to place weight on this 

5-minute SO2 benchmark concentration. 

The Administrator also considered 

the results of the air quality, exposure, 

and risk analyses, as they serve to 

estimate the extent to which a given 

1-hour standard limits the 5-minute 

benchmark concentrations of concern 

identified from controlled human 

exposure studies (see REA chapters 

7–9, proposal section II.F.4.b, and 

proposal Tables 2–4). In considering 

these results as they relate to limiting 

5-minute SO2 benchmark concentrations 

≥ 200 and 400 ppb, the Administrator 

notes the following key points: 

• The 40-county air quality analysis 

estimates that a 100 ppb 1-hour daily 

maximum standard would allow at most 

2 days per year on average in any 

county when estimated 5-minute daily 

maximum SO2 concentrations exceed 

the 400 ppb benchmark, and at most 13 

days per year on average when 5-minute 

daily maximum SO2 concentrations 

exceed the 200 ppb benchmark (see 

proposal, Table 2, 74 FR at 64840). 

Furthermore, given a simulated 1-hour 

100 ppb standard level, most of the 

counties in that air quality analysis were 

estimated to experience 0 days per year 

on average when 5-minute daily 

maximum SO2 concentrations exceed 

the 400 ppb benchmark and ≤ 3 days per 

year on average when 5-minute daily 

maximum SO2 concentrations were 

estimated to exceed the 200 ppb 

benchmark (see REA, Tables 7–14 and 

7–12). 

• The St. Louis exposure analysis 

estimates that a 99th percentile 1-hour 

daily maximum standard at a level of 

100 ppb would likely protect > 99% of 

asthmatic children in that city at 

moderate or greater exertion from 

experiencing at least one 5-minute 

exposure ≥ 400 ppb per year, and 


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19For example, as noted in the proposal 

(proposal, section II.F.4, 74 FR at 64835) evidence 

of a pattern of results from a group of studies that 

find effect estimates similar in direction and 

magnitude would warrant consideration of and 

reliance on such studies even if the studies did not 

all report statistically significant associations in 

single- or multi-pollutant models. The SO2

epidemiologic studies fit this pattern, and are 

buttressed further by the results of the clinical 

studies. ISA, section 5.2. 

approximately 97% of those asthmatic 

children at moderate or greater exertion 

from experiencing at least one exposure 

≥ 200 ppb per year (see proposal, 

section II.F.4.b). 

• The St. Louis risk assessment 

estimates that a 99th percentile 1-hour 

standard level at 100 ppb would likely 

protect about 97–98% of exposed 

asthmatic children in that city at 

moderate or greater exertion from 

experiencing at least one moderate or 

greater lung function response (defined 

as a ≥ 100% increase in sRaw; see 

proposal, section II.F.4.b). 

Given the above considerations, the 

Administrator concludes that a 1-hour 

standard at a level of 100 ppb would 

appropriately limit 5-minute SO2

benchmark concentrations ≥ 200 or 400 

ppb. Moreover, although the 

Administrator acknowledges that the air 

quality and exposure analyses 

mentioned above suggest that a 50 ppb 

standard may somewhat further limit 5- 

minute SO2 concentrations/exposures in 

excess of the 200 ppb benchmark (see 

proposal section II.F.4.b), she does not 

believe this information alone warrants 

a standard level lower than 100 ppb. 

More specifically, although she 

considers the health effects resulting 

from 5-minute SO2 exposures as low as 

200 ppb to be adverse, she also 

recognizes that such effects are 

appreciably less severe than those at 

SO2 concentrations ≥ 400 ppb. Thus, she 

concludes that there is little difference 

in limiting 5-minute concentrations/ 

exposures ≥ 400 ppb given 1-hour 

standard levels in the range of 50 to 100 

ppb. 

In considering the epidemiologic 

evidence with regard to level, the 

Administrator notes that there have 

been more than 50 peer reviewed 

epidemiologic studies published 

worldwide evaluating SO2 (ISA, Tables 

5–4 and 5–5). These studies have 

generally reported positive, although 

not always statistically significant 

associations between more serious 

health outcomes (i.e. respiratory-related 

emergency department visits and 

hospitalizations) and ambient SO2

concentrations and have generally 

included populations potentially at 

increased risk for SO2-related 

respiratory effects (e.g, children, older 

adults, and those with pre-existing 

respiratory disease). The Administrator 

finds that in assessing the extent to 

which these studies and their associated 

air quality information can inform the 

level of a new 99th percentile 1-hour 

daily maximum standard for the U.S., 

air quality information from the U.S. 

and Canada is most relevant since these 

areas have similar monitor network 

designs and patterns of air quality. 

However, as described in proposal 

section II.F.4.a, SO2 concentrations 

reported for Canadian studies were not 

directly comparable to those reported 

for U.S. studies due to use of different 

monitoring protocols in those studies. 

Thus, the Administrator focused on 

99th percentile air quality information 

from U.S. studies for informing 

potential 1-hour standard levels. She 

concludes that this information 

provides evidence of associations 

between ambient SO2 and emergency 

department visits and hospital 

admissions in U.S. cities with particular 

99th percentile 1-hour SO2 levels, and 

thus provides information that is 

particularly relevant for setting the level 

of a 1-hour SO2 standard. With regard to 

these studies she notes the following 

key points: 

• Ten studies (some conducted in 

multiple locations) reported mostly 

positive, and sometimes statistically 

significant, associations between 

ambient SO2 concentrations and 

emergency department visit and 

hospital admissions in locations where 

99th percentile 1-hour daily maximum 

SO2 levels ranged from approximately 

50–460 ppb. 

• Within this broader range of SO2

concentrations, there is a cluster of three 

epidemiologic studies between 78–150 

ppb (for the 99th percentile of the 1- 

hour SO2 concentrations) where the SO2

effect estimate remained positive and 

statistically significant in multipollutant models with PM (NYDOH 

(2006), Ito et al., (2007), and Schwartz 

et al., (1995)). Notably, although 

statistical significance in multipollutant models is an important 

consideration, it is not the only 

consideration when relying on such 

epidemiologic evidence.19 However, as 

noted earlier, there is special sensitivity 

in this review in disentangling PMrelated effects (especially sulfate PM) 

from SO2-related effects in interpreting 

the epidemiologic studies; thus, these 

studies are of particular relevance here, 

lending strong support both to the 

conclusion that SO2 effects are generally 

independent of PM (ISA, section 5.2) 

and that these independent adverse 

effects of SO2 have occurred in cities 

with 1-hour daily maximum, 99th 

percentile concentrations in the range of 

78–150 ppb. Nor did EPA find the 

comments criticizing these studies 

persuasive, as explained above in 

section II.F.4.b and in the RTC 

document (EPA 2010). The 

Administrator therefore judges it 

appropriate to place substantial weight 

on this cluster of three U.S. 

epidemiologic studies in selecting a 

standard level, as they are a group of 

studies that reported positive and 

statistically significant associations 

between ambient SO2 and emergency 

department visits or hospital admissions 

even when potential confounding by 

PM was considered. 

• The Administrator agrees with the 

finding in the ISA that the controlled 

human exposure evidence lends 

biological plausibility to the effects 

reported in epidemiologic studies (ISA, 

p. 5–9). 

• There is limited evidence from two 

epidemiologic studies employing single 

pollutant models that found generally 

positive associations between ambient 

SO2 and emergency department visits in 

locations where 99th percentile 1-hour 

SO2 concentrations were approximately 

50 ppb (see proposal, Figures 1 and 2). 

However, considering that the results of 

these studies were mixed, and 

importantly, that neither of these two 

studies evaluated the potential for 

confounding by co-pollutants through 

the use of multipollutant models 

(particularly with PM), the 

Administrator judges it appropriate to 

place limited weight on these studies. 

• With regard to the cluster of three 

studies conducted in the Bronx 

(NYDOH 2006), NYC (Ito et al., 2007), 

and New Haven (Scwartz et al., 1995), 

there is a degree of uncertainty as to 

whether the 99th percentile 1-hour daily 

maximum SO2 concentrations reported 

from monitors in these three study areas 

reflect the highest 99th percentile 

1-hour daily maximum SO2

concentration. Our limited qualitative 

analysis suggests that 99th percentile 1- 

hour daily maximum SO2

concentrations reported by monitors in 

these study areas are reasonable 

approximations for the absolute highest 

99th percentile 1-hour daily maximum 

SO2 concentration that can occur across 

the entire area in these studies 

(including the areas where monitors 

were not located) (see Brode, 2010). 

However, although a reasonable 

approximation, it is still likely that 

these monitored concentrations are 

somewhat lower than the absolute 

highest 99th percentile 1-hour daily 

maximum SO2 concentrations occurring 

across these epidemiologic study areas. 


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Weighing all of this evidence, the 

Administrator concludes that the 

epidemiologic studies provide strong 

support for setting a standard that limits 

the 99th percentile of the distribution of 

1-hour daily maximum SO2

concentrations to 75 ppb. This judgment 

takes into account the strong 

determinations in the ISA (and 

endorsed by CASAC), based on a much 

broader body of evidence, that there is 

a causal association between exposure 

to SO2 and the types of respiratory 

morbidity effects reported in these 

studies. The Administrator further 

judges that it is not necessary based on 

existing epidemiologic evidence, to set 

a standard below 75 ppb. That is, the 

Administrator concludes that a standard 

level of 75 ppb is sufficiently below the 

SO2 levels in three cities where 

epidemiologic studies found statistically 

significant effects in multipollutant 

models with PM (i.e., 78, 82, and 150 

ppb) to provide an adequate margin of 

safety given the uncertainty as to 

whether monitors in these study 

locations reflected the highest 1-hour 

daily maximum SO2 concentration 

across the entire study area. Thus, a 

standard set at a level of 75 ppb is likely 

further below the 99th percentile 1-hour 

daily maximum concentrations in these 

three study areas than the bare 

comparison of levels would otherwise 

indicate. Finally, the Administrator 

again notes that epidemiologic evidence 

below 75 ppb is more uncertain because 

studies below 75 ppb did not evaluate 

potential confounding of results in 

multipollutant models, and because 

these studies reported mixed results. 

Given the above considerations and 

the comments received on the proposal, 

the Administrator determines that the 

appropriate judgment, based on the 

entire body of evidence and information 

available in this review, and the related 

uncertainties, is a standard level of 

75 ppb. She concludes that such a 

standard, with a 1-hour averaging time 

and 99th percentile form, will provide 

a significant increase in public health 

protection compared to the current 

standards and would be expected to 

protect against the respiratory effects 

that have been linked with SO2

exposures in both controlled human 

exposure and epidemiologic studies. 

Specifically, she concludes that such a 

standard will limit 1-hour exposures at 

and above 75 ppb for those in 

susceptible populations that are at-risk 

of experiencing adverse health effects 

from short-term exposure to SO2. Such 

a standard will also maintain SO2

concentrations below those in locations 

where key U.S. epidemiologic studies 

have reported that ambient SO2 is 

associated with clearly adverse 

respiratory health effects, as indicated 

by increased hospital admissions and 

emergency department visits. She also 

notes that a 1-hour standard at a level 

of 75 ppb is expected to substantially 

limit asthmatics’ exposure to 5–10 

minute SO2 concentrations ≥ 200 ppb, 

thereby substantially limiting the 

adverse health effects associated with 

such exposures. Finally, the 

Administrator notes that a standard 

level of 75 ppb is consistent with the 

consensus recommendation of CASAC. 

In setting the standard level at 75 ppb 

rather than at a lower level, the 

Administrator notes that a 1-hour 

standard with a level lower than 75 ppb 

would only result in significant further 

public health protection if, in fact, there 

is a continuum of serious, adverse 

health risks caused by exposure to SO2

concentrations below 75 ppb. Based on 

the available evidence, the 

Administrator does not believe that 

such assumptions are warranted. Taking 

into account the uncertainties that 

remain in interpreting the evidence 

from available controlled human 

exposure and epidemiologic studies, the 

Administrator notes that the likelihood 

of obtaining benefits to public health 

with a standard set below 75 ppb 

decreases, while the likelihood of 

requiring reductions in ambient 

concentrations that go beyond those that 

are needed to protect public health 

increases. 

Therefore, the Administrator judges 

that a 1-hour SO2 standard at 75 ppb is 

sufficient to protect public health with 

an adequate margin of safety. This 

includes protection with an adequate 

margin of safety for susceptible 

populations at increased risk for adverse 

respiratory effects from short-term 

exposures to SO2 for which the evidence 

supports a causal relationship with SO2

exposures. The Administrator does not 

believe that a lower standard level is 

needed to provide this degree of 

protection. These conclusions by the 

Administrator appropriately consider 

the requirement for a standard that is 

neither more nor less stringent than 

necessary for this purpose and 

recognizes that the CAA does not 

require that primary NAAQS be set at a 

zero-risk level or to protect the most 

susceptible individual, but rather at a 

level that reduces risk sufficiently so as 

to protect the public health with an 

adequate margin of safety. 

5. Retaining or Revoking the Current 

24-Hour and Annual Standards 

This section discusses considerations 

related to retaining or revoking the 

current 24-hour and annual SO2 primary 

NAAQS. Specifically, this section 

summarizes the rationale for the 

Administrator’s proposed decision 

regarding whether to retain or revoke 

the current standards (section II.F.5.a), 

discusses public comments related to 

whether to retain or revoke the current 

standards (II.F.5.b), and presents the 

Administrator’s final conclusions 

regarding whether to retain or revoke 

the current standards (II.F.5.c). 

a. Rationale for Proposed Decision 

As noted in the proposal (see section 

II.F.5), the REA recognized that the 

particular level selected for a new 99th 

percentile 1-hour daily maximum 

standard would have implications for 

deciding whether to retain or revoke the 

current 24-hour and annual standards. 

That is, with respect to SO2-induced 

respiratory morbidity, the lower the 

level selected for a 99th percentile 

1-hour daily maximum standard, the 

less additional public health protection 

the current standards would be 

expected to provide. CASAC expressed 

a similar view following their review of 

the 2nd draft REA: ‘‘Assuming that EPA 

adopts a one hour standard in the range 

suggested, and if there is evidence 

showing that the short-term standard 

provides equivalent protection of public 

health in the long-term as the annual 

standard, the panel is supportive of the 

REA discussion of discontinuing the 

annual standard’’ (Samet 2009, p. 15). 

With regard to the current 24-hour 

standard, CASAC was generally 

supportive of using the air quality 

analyses in the REA as a means of 

determining whether the current 

24-hour standard was needed in 

addition to a new 1-hour standard to 

protect public health. CASAC stated: 

‘‘The evidence presented [in REA Table 

10–3] was convincing that some of the 

alternative one-hour standards could 

also adequately protect against 

exceedances of the current 24-hour 

standard’’ (Samet 2009, p. 15). 

In accordance with the REA findings 

and CASAC recommendations 

mentioned above, the Administrator 

noted that 1-hour standards in the range 

of 50–100 ppb would have the effect of 

maintaining 24-hour and annual SO2

concentrations generally well below the 

levels of the current 24-hour and annual 

NAAQS (see REA Tables 10–3 and 10– 

4 and REA Appendix Tables D–3 to D– 

6). Thus, if a new 99th percentile 1-hour 

daily maximum standard was set in the 

proposed range of 50–100 ppb, then the 

Administrator proposed to revoke the 

current 24-hour and annual standards. 

However, as noted in the proposal, if a 

standard was set at a level >100 ppb and 


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up to 150 ppb, then the Administrator 

indicated that she would retain the 

existing 24-hour standard, recognizing 

that a 99th percentile 1-hour daily 

maximum standard at 150 ppb would 

not have the effect of maintaining 24- 

hour average SO2 concentrations below 

the level of the current 24-hour standard 

in all locations analyzed (see REA 

Appendix Table D–4). Under this 

scenario, the Administrator would still 

revoke the current annual standard 

recognizing: (1) 99th percentile 1-hour 

daily maximum standards in the range 

of 50–150 ppb would maintain annual 

average SO2 concentrations below the 

level of the current annual standard (see 

REA Table 10–4 and REA Appendix 

tables D–5 and D–6); and (2) the lack of 

sufficient evidence linking long-term 

SO2 exposure to adverse health effects. 

b. Comments on Retaining or Revoking 

the Current 24-Hour and Annual 

Standards 

As noted above, most industry groups 

were opposed to the proposed revisions 

to the SO2 NAAQS. However, some of 

these groups noted that if a 1-hour 

standard was adopted, then they would 

support revoking the current 24-hour 

and annual standards. State agencies 

generally supported revoking the 

current standards if a 1-hour standard 

was set in the proposed range, although 

NAACA, NESCAUM, and VT, while 

supportive of revoking the existing 

standards, also suggested that EPA 

explore setting a new 24-hour standard 

to minimize the potential that multiple 

hours within a day would exceed a 

1-hour standard (see RTC document 

(EPA 2010), section IV). Groups which 

supported revoking the current 24-hour 

and annual standards (if a 1-hour 

standard was set in the proposed 

ranged) generally referenced the 

Administrator’s rationale and CASAC 

advice described in the proposal (see 

section II.F.5). 

Public health (e.g., ALA, ATS) and 

environmental organizations (e.g., CBD, 

WEACT for Environmental Justice) were 

generally opposed to revoking the 

current 24-hour and annual standards. 

These groups generally concluded that 

the 24-hour standard should be revised 

while the annual standard should be 

retained. In support of this position, 

ALA et al., cited air quality information 

from the REA indicating that if air 

quality was simulated to just meet a 

99th percentile 1-hour daily maximum 

standard in the proposed range of 50– 

100 ppb, then in some locations 

analyzed, 99th percentile 24-hour 

average SO2 concentrations would be 

above concentrations (i.e., above 99th 

percentile 24-hour average 

concentrations) in cities where U.S. 

emergency department visit and 

hospital admission studies reported 

positive associations with SO2. In 

addition, many of these groups were 

opposed to revoking the current annual 

standard. In general, these groups 

concluded that given the uncertainties 

associated with SO2 exposure and longterm health effects, EPA should err on 

the side of being health protective and 

retain the existing annual standard. EPA 

responses to comments on whether the 

current standards should be retained or 

revoked are presented below as well as 

in section IV of the RTC document (EPA 

2010). 

As stated in the REA and proposal, 

99th percentile 24-hour average SO2

concentrations in cities where U.S. 

emergency department visit and 

hospital admission studies (for all 

respiratory causes and asthma; 

identified from Table 5–5 of the ISA) 

were conducted ranged from 16 ppb to 

115 ppb (Thompson and Stewart, 2009). 

Moreover, as stated in the REA and 

proposal (see section II.F.2), effect 

estimates that remained statistically 

significant in multi-pollutant models 

with PM were found in cities with 99th 

percentile 24-hour average SO2

concentrations ranging from 

approximately 36 ppb to 64 ppb. In its 

comments, ALA et al., stated (based on 

the air quality information in REA 

Appendix Table D–2) ‘‘with a 1-hour 

50 ppb 99th percentile standard, 7 

counties would experience a 99th 

percentile 24-hour concentration of 16 

ppb or greater, the range found to be 

harmful in epidemiological studies. 

With an hourly standard of 100 ppb, 24 

of 30 counties would have 99th 

percentile 24-hour concentrations above 

16 ppb, with 1 county exceeding 36 

ppb.’’ Thus, these commenters generally 

maintained that a lowered 24-hour 

standard is needed to protect against 

these 24-hour SO2 concentrations. 

We disagree that a lowered 24-hour 

standard is needed to protect against 

24-hour average SO2 concentrations of 

concern identified from cities where 

U.S. emergency department visit and 

hospital admission studies were 

conducted. As noted in detail in the 

REA, there is uncertainty as to whether 

the health effects reported in 

epidemiologic studies using 24-hour 

average SO2 concentrations are in fact 

due to 24-hour average SO2 exposures 

(REA, section 10.5.2). That is, when 

describing epidemiologic studies 

observing positive associations between 

ambient SO2 and respiratory symptoms, 

the ISA stated ‘‘that it is possible that 

these associations are determined in 

large part by peak exposures within a 

24-hour period’’ (ISA, section 5.2 at 

p. 5–5). Similarly, the ISA stated that: 

‘‘The effects of SO2 on respiratory 

symptoms, lung function, and airway 

inflammation observed in the human 

clinical studies using peak exposures 

further provides a basis for a 

progression of respiratory morbidity 

resulting in increased emergency 

department visits and hospital 

admissions’’ and makes the associations 

observed in the epidemiologic studies 

‘‘biologica[lly] plausib[le]’’ (id.). In 

contrast, evidence from controlled 

human exposure studies of 5–10 

minutes and epidemiologic studies 

using 1-hour daily maximum SO2

concentrations provided appreciably 

stronger evidence of respiratory 

morbidity effects following SO2

exposures ≤ 1-hour. 

Given that respiratory morbidity 

effects following SO2 exposure may be 

most related to averaging times ≤1-hour, 

EPA found it most reasonable to 

consider the extent to which a 1-hour 

averaging time, given an appropriate 

form and level (which as discussed 

above, also substantially limits 5-minute 

benchmark exposures of concern; see 

sections II.F.2 and II.F.4), limited 99th 

percentile 24-hour average 

concentrations of SO2 in locations 

where emergency department visit/ 

hospitalization studies reported that the 

SO2 effect estimate remained 

statistically significant in multipollutant models with PM (i.e., 

locations with 99th percentile 24-hour 

average SO2 concentrations ≥36 ppb). 

Considering this, we note that ALA et 

al., identified only one county with 99th 

percentile 24-hour average SO2

concentrations ≥36 ppb given a 99th 

percentile 1-hour daily maximum 

standard at 100 ppb, and no counties 

≥36 ppb given a 99th percentile 1-hour 

daily maximum standard at 50 ppb. 

Thus, given a 99th percentile 1-hour 

daily maximum standard level at 75 ppb 

(i.e., the form and level selected for a 

new 1-hour SO2 standard), it is possible 

that no county in the ALA et al., 

analysis would have had a 99th 

percentile 24-hour average SO2

concentration ≥36 ppb. 

With regard to the annual standard, 

we also disagree that this standard 

needs to be retained. First, the ISA 

found that ‘‘[t]he evidence linking shortterm SO2 exposure and cardiovascular 

effects, and morbidity and mortality 

with long-term exposures to SO2 is 

inadequate to infer a causal 

relationship.’’ ISA, p. 5–10. Thus, an 

annual standard is unnecessary to 

prevent long-term health effects. The 

remaining issue is whether such a 

standard provides further protection 


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against short-term effects, given the new 

one hour standard. We conclude that it 

does not. As noted in the proposal, our 

air quality information indicates that 1- 

hour standard levels in the range of 50– 

100 ppb are estimated to generally keep 

annual SO2 concentrations well below 

the level of the current annual standard. 

CASAC agreed. The panel stated: 

‘‘Assuming that EPA adopts a one hour 

standard in the range suggested, and if 

there is evidence showing that the shortterm standard provides equivalent 

protection of public health in the longterm as the annual standard, the panel 

is supportive of the REA discussion of 

discontinuing the annual standard’’ 

(Samet 2009, p. 15). Taken together, this 

information indicates that retaining the 

annual standard would add no 

additional public health protection. 

c. Administrator’s Conclusions on 

Retaining or Revoking the Current 24- 

Hour and Annual Standards 

In accordance with the REA findings 

and CASAC recommendations 

mentioned above, the Administrator 

concludes that a 1-hour standard at 

level of 75 ppb would have the effect of 

maintaining 24-hour and annual SO2

concentrations generally well below the 

levels of the current 24-hour and annual 

NAAQS (see REA Tables 10–3 and 10– 

4 and REA Appendix Tables D–3 to D– 

6). She also concludes that, as noted 

above in section II.F.2, a 1-hour 

standard at 75 ppb will likely limit 99th 

percentile 24-hour SO2 concentrations 

in U.S. locations where emergency 

department visit and hospital admission 

studies reported statistically significant 

associations in multi-pollutant models 

with PM. Finally, she notes the lack of 

sufficient health evidence to support an 

annual standard to protect against 

health effects associated with long-term 

SO2 exposure. Taken together, the 

Administrator concludes it appropriate 

to revoke the current 24-hour and 

annual standards. 

G. Summary of Decisions on the 

Primary Standards 

For the reasons discussed above, and 

taking into account information and 

assessments presented in the ISA and 

REA as well as the advice and 

recommendations of CASAC, the 

Administrator concludes that the 

current 24-hour and annual primary 

standards are not requisite to protect 

public health with an adequate margin 

of safety. The Administrator also 

concludes that establishing a new 1- 

hour standard will appropriately protect 

public health with an adequate margin 

of safety, and specifically will afford 

requisite increased protection for 

asthmatics and other at-risk populations 

against an array of adverse respiratory 

health effects related to short-term (5 

minutes to 24 hours) SO2 exposure. 

These effects include decrements in 

lung function (defined in terms of sRaw 

and FEV1), increases in respiratory 

symptoms, and related serious 

indicators of respiratory morbidity 

including emergency department visits 

and hospital admissions for respiratory 

causes. 

Specifically, the Administrator is 

establishing a new short-term primary 

SO2 standard with a 1-hour (daily 

maximum) averaging time and a form 

defined as the 3-year average of the 99th 

percentile of the yearly distribution of 1- 

hour daily maximum SO2

concentrations, and a level of 75 ppb. In 

addition to setting a new 1-hour 

standard at 75 ppb, the Administrator is 

revoking the current 24-hour and annual 

standards recognizing that a 1-hour 

standard set at 75 ppb will have the 

effect of generally maintaining 24-hour 

and annual SO2 concentrations well 

below the levels of the current 24-hour 

and annual standards. 

III. Overview of the Approach for 

Monitoring and Implementation 

We received several comments 

regarding the approaches discussed in 

the proposal for monitoring and 

modeling for comparison to the 

proposed new 1-hour SO2 NAAQS, 

designations of areas as either attaining 

or not attaining the NAAQS, and 

implementation of the new NAAQS in 

State implementation plans (SIPs) that 

would ensure ultimate attainment of the 

new NAAQS in transitioning from the 

annual and 24-hour NAAQS in a timely 

manner. These comments raised 

fundamental questions regarding our 

contemplated approaches in all three 

areas, and caused us to re-examine them 

and review their consistency with past 

practice under the SO2 NAAQS 

implementation program. After 

conducting that review, and in response 

to the public comments we are revising 

our general anticipated approach toward 

implementation of the new 1-hour 

NAAQS. This revised approach would 

better address: (1) The unique sourcespecific impacts of SO2 emissions; (2) 

the special challenges SO2 emissions 

present in terms of monitoring shortterm SO2 levels for comparison with the 

NAAQS in many situations; (3) the 

superior utility that modeling offers for 

assessing SO2 concentrations; and (4) 

the most appropriate method for 

ensuring that areas attain and maintain 

the new 1-hour SO2 NAAQS in a 

manner that is as expeditious as 

practicable, taking into account the 

potential for substantial SO2 emissions 

reductions from forthcoming national 

and regional rules that are currently 

underway. 

Below, we provide an overview of our 

revised approach to monitoring, and of 

our expected approaches to designations 

of areas, and implementation of the 

NAAQS. Due to the unique challenges 

presented by SO2, we do not expect that 

the anticipated approaches discussed 

below would be necessarily transferable 

to other NAAQS pollutant situations. 

For NAAQS pollutants other than SO2, 

air quality monitoring is more 

appropriate for determining whether all 

areas are attaining the NAAQS, and 

there is comparatively less dependence 

upon conducting refined modeling. 

Each of these subjects (i.e., our revised 

approach to monitoring, and our 

expected approaches to designations of 

areas, and implementation of the 

NAAQS) is further addressed later in 

the preamble, in sections IV, V and VI, 

respectively. Where specific public 

comments on the proposal are 

addressed and responded to, further 

details of the specific revised 

approaches are explained. In many 

respects, both the overview discussion 

below and the subsequent more detailed 

discussions explain our expected and 

intended future action in implementing 

the new 1-hour NAAQS—in other 

words, they constitute guidance, rather 

than final agency action—and it is 

possible that our approaches may 

continue to evolve as we, States, and 

other stakeholders proceed with actual 

implementation. In other respects, such 

as in the final regulatory provisions 

regarding the promulgated monitoring 

network, we are explaining EPA’s final 

conclusions regarding what is required 

by this rule. We expect to issue further 

guidance regarding implementation, 

particularly concerning issues that may 

arise regarding the application of 

refined dispersion modeling under this 

revised approach for monitoring and 

implementation, and issues that States 

and other stakeholders may also ask us 

to address as we proceed toward various 

stages of ensuring attainment. EPA 

intends to solicit public comment prior 

to finalizing this guidance. 

The main necessary elements of 

implementing the new 1-hour NAAQS 

are: (1) An approach for assessing 

ambient concentrations to determine 

compliance with the NAAQS; (2) a 

process for using these assessments to 

designate areas relative to the new 

standard; and (3) the development of 

State plans that include control 

measures sufficient for ensuring the 

NAAQS is attained everywhere as 

expeditiously as possible, which we 


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believe should be no later than 2017. 

EPA’s revised anticipated approach to 

determining compliance with the new 

SO2 NAAQS is consistent with our 

historical approach to SO2 designations 

and implementation through permits 

and emissions limitations, which 

involves the combined use of 

monitoring and modeling. The emphasis 

we would place on monitoring and 

modeling, compared with each other, 

under the revised expected approach is 

therefore significantly different than 

that in the approach discussed in the 

proposal, which was less in line with 

our historical practice for SO2, as the 

public comments highlighted. 

In the SO2 NAAQS proposal, we 

recommended a monitoring-focused 

approach for comparison to the new 

NAAQS, featuring a two-pronged 

monitoring network design. This 

included monitors in certain CBSAs 

based on a combination of population 

and SO2 emissions coupled with 

additional monitors within a State based 

on that State’s contribution to national 

SO2 emissions. The resulting proposed 

network would have required 

approximately 348 monitors nationwide 

to be sited at the locations of maximum 

concentration. Numerous State and 

local government commenters expressed 

concerns regarding the burdens of 

implementing the proposed monitoring 

network and the sufficiency of its scope 

for purposes of identifying violations. 

These commenters contended that our 

proposed monitoring network was too 

small and insufficient to cover the range 

of SO2 sources, and yet too burdensome 

and expensive to expand to an adequate 

scale. Some of these commenters (the 

City of Alexandria, and the States of 

Delaware, North Carolina and 

Pennsylvania) suggested using modeling 

to determine the scope of monitoring 

requirements, or favored modeling over 

monitoring to determine compliance 

with the NAAQS. 

Partly in response to these comments, 

and after reconsidering the proposal’s 

monitoring-focused approach in light of 

EPA’s historical approach to SO2

NAAQS implementation and area 

designations decisions, we intend to use 

a hybrid analytic approach that would 

combine the use of monitoring and 

modeling to assess compliance with the 

new 1-hour SO2 NAAQS. We believe 

that some type of hybrid approach is 

more consistent with our historical 

approach and longstanding guidance 

toward SO2 than what we originally 

proposed. In addition, we believe that 

for a short-term 1-hour standard it is 

more technically appropriate, efficient, 

and effective to use modeling as the 

principle means of assessing 

compliance for medium to larger 

sources, and to rely more on monitoring 

for groups of smaller sources and 

sources not as conducive to modeling. 

We discuss the details of the final 

revised monitoring network 

requirements in section IV later in the 

preamble, but note here the relationship 

that the revised approach toward 

monitoring and modeling—taken partly 

in response to the public comments 

mentioned above—has to the other two 

general subject areas in implementation 

for which we are providing guidance, 

namely initial area designations and 

development of substantive 

implementation plans that ensure 

timely attainment and maintenance of 

the NAAQS. Our ultimate intention is to 

place greater emphasis on modeling 

than did the proposed rule as the most 

technically appropriate, efficient, and 

readily available method for assessing 

short-term ambient SO2 concentrations 

in areas with large point sources. This 

projected change in approach would 

necessarily result in a lesser emphasis 

on the less appropriate, more expensive, 

and slower to establish monitoring tool 

than did the proposed rule. Therefore, 

the minimum requirements for the SO2

monitoring network in this final rule are 

of a smaller scale than proposed, and we 

do not expect monitoring to become the 

primary method by which ambient 

concentrations are compared to the new 

1-hour SO2 NAAQS. 

Instead, in areas without currently 

operating monitors but with sources that 

might have the potential to cause or 

contribute to violations of the NAAQS, 

we anticipate that the identification of 

NAAQS violations and compliance with 

the 1-hour SO2 NAAQS would primarily 

be done through refined, sourceoriented air quality dispersion modeling 

analyses, supplemented with a new, 

limited network of ambient air quality 

monitors. Historically, we have favored 

dispersion modeling to support SO2

NAAQS compliance determinations for 

areas with sources that have the 

potential to cause an SO2 NAAQS 

violation, and we have explained that 

for an area to be designated as 

‘‘attainment,’’ dispersion modeling 

regarding such sources needs to show 

the absence of violations even if 

monitoring does not show a violation. 

This has been our general position 

throughout the history of 

implementation of the SO2 NAAQS 

program. See, e.g., ‘‘Air Quality Control 

Regions, Criteria, and Control 

techniques; Attainment Status 

Designations,’’ 43 FR 40412, 40415–16 

(Sept. 11, 1978); ‘‘Air Quality Control 

Regions, Criteria, and Control 

Techniques,’’ 43 FR 45993, 46000–02 

(Oct. 5, 1978); ‘‘Air Quality 

Implementation Plans: State 

Implementation Plans; General 

Preamble,’’ 57 FR 13498, 13545, 13547– 

48 (Apr. 16, 1992); ‘‘Approval and 

Promulgation of State Implementation 

Plans; Call for Sulfur Dioxide SIP 

Revisions for Billings/Laurel, MT,’’ 58 

FR 41430 (Aug. 4, 1993); ‘‘Designation of 

Areas for Air Quality Planning 

Purposes; Ohio,’’ 59 FR 12886, 12887 

(Mar. 18, 1994); ‘‘Ambient Air Quality 

Standards, National and 

Implementation Plans for Sulfur Oxides 

(Sulfur Dioxide),’’ 60 FR 12492, 12494– 

95 (Mar. 7, 1995); ‘‘Air Quality 

Implementation Plans; Approval and 

Promulgation: Various States: Montana,’’ 

67 FR 22167, 22170–71, 22183–887 

(May 2, 2002). 

Compared to other NAAQS 

pollutants, we would not consider 

ambient air quality monitoring alone to 

be the most appropriate means of 

determining whether all areas are 

attaining a short-term SO2 NAAQS. Due 

to the generally localized impacts of 

SO2, we have not historically 

considered monitoring alone to be an 

adequate, nor the most appropriate, tool 

to identify all maximum concentrations 

of SO2. In the case of SO2, we further 

believe that monitoring is not the most 

cost-efficient method for identifying all 

areas of maximum concentrations. 

However, for some situations 

monitoring is well suited, and we 

therefore will require it to some extent, 

as further explained in section IV of the 

preamble. For example, monitoring may 

appropriately be relied upon to assess 

compliance with the NAAQS by groups 

of smaller sources and sources that may 

not be as conducive to modeling as are 

larger SO2 sources. 

States will need to make any 

adjustments to the existing monitoring 

network to ensure that monitors meeting 

today’s network design regulations for 

the new 1-hour NAAQS are sited and 

operational by January 1, 2013. We also 

expect to provide additional guidance 

regarding the application of refined 

dispersion modeling under this revised 

expected approach for implementation 

of the new SO2 standard. Appendix A 

to the Guideline on Air Quality Models 

(Appendix W of 40 CFR part 51), 

Summaries of Preferred Air Quality 

Models, provides ‘‘key features of 

refined air quality models preferred for 

specific regulatory applications’’ (see 

Appendix A to Appendix W of Part 51 

at A.0(1)). Refined dispersion modeling, 

following our current Guideline on Air 

Quality Models with appropriate 

flexibility for use in implementation, is 

anticipated to better reflect and account 


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20EPA is authorized by the Clean Air Act to take 

up to 3 years to complete the initial area 

designations in the event that insufficient 

information is available to complete the 

designations within 2 years. 21Since three complete years of data from any 

newly sited monitors meeting the new monitoring 

network design criteria are not expected to be 

obtained until the end of 2015, any newly sited 

monitors will not play a role in EPA’s initial area 

designations. 

22EPA anticipates making the determination of 

when monitoring alone is ‘‘appropriate’’ for a 

specific area on a case-by-case basis, informed by 

that area’s factual record, as part of the designations 

process. EPA would expect to address this issue for 

such areas by examining the historic treatment of 

the area with respect to prior SO2 designations as 

well as whether the area is one in which monitoring 

would be the more technically appropriate tool for 

determining compliance with the new SO2 NAAQS. 

An example of a situation in which monitoring may 

be the more preferred approach is a shipping port 

(non-point source or ‘‘area’’ source) that is not in 

close proximity to other significant stationary SO2

sources. 

for source-specific SO2 impacts than the 

more limited monitoring-focused 

proposal. As noted above, EPA intends 

to solicit public comment prior to 

finalizing this guidance. 

Based on a revised, hybrid approach, 

we expect to implement the new SO2

standard in the following manner. In 

accordance with CAA section 107(d), 

EPA must designate areas as 

‘‘attainment,’’ ‘‘nonattainment’’ or 

‘‘unclassifiable’’ for the new 1-hour SO2

NAAQS by June 2012 (i.e., two years 

following promulgation of the new 

NAAQS).20 State Governors are required 

to submit their initial area designation 

recommendations to EPA no later than 

June 2011. We expect that EPA’s final 

area designation decisions in 2012 

would be based principally on data 

reported from SO2 monitors currently in 

place today, and any refined modeling 

the State chooses to conduct specifically 

for initial area designations.21 For these 

initial designations, we would expect to 

designate an area ‘‘nonattainment’’ if 

either monitoring data or appropriate 

refined modeling results show a 

violation. Any area that has monitoring 

and appropriate modeling data showing 

no violations we would expect to 

designate as ‘‘attainment.’’ 22 All other 

areas, absent monitoring data and air 

quality modeling results showing no 

violations, we would expect to initially 

designate as ‘‘unclassifiable,’’ as required 

by the Clean Air Act. The expected 

presumptive boundary for any area 

designated ‘‘nonattainment’’ would be 

the county boundary associated with the 

violation unless additional information 

provided to EPA demonstrates 

otherwise, as has been our general 

approach for other NAAQS pollutants. 

Any area initially designated 

‘‘nonattainment’’ or ‘‘unclassifiable’’ 

could request redesignation to 

‘‘attainment’’ after an assessment based 

on air quality modeling, conducted in 

accordance with the new guidance, and 

available monitoring data indicates that 

the standard has been met, as well as 

meeting all other requirements of the 

CAA for redesignation to attainment. 

This anticipated approach toward 

initial area designations is a change 

from the approach discussed in the 

proposal, and logically follows from our 

general change in approach to the use 

and utility of monitoring versus 

modeling for determining short-term 

SO2 ambient concentrations. As public 

commenters pointed out, establishment 

and implementation of the proposed 

monitoring network would have been 

both too limited and too late to inform 

initial area designations, and the 

expense and burden of accelerating it 

and expanding it would have been 

severe for State implementing agencies. 

Given the time needed to establish 

monitors, it is not realistic to expect 

either such an expanded monitoring 

network or even the more reasonable 

limited network of the final rule to be 

the chief tool for informing initial 

designations. 

That means that some other approach 

is needed to inform initial designations 

of areas and other implementation 

decisions under the new SO2 NAAQS. 

In addition to using any valid data 

generated by existing monitors, refined 

dispersion modeling may inform 

designation and implementation 

decisions regarding sources that may 

have the potential to cause or contribute 

to a NAAQS violation. In order for 

modeling to be done on the scale 

sufficient to identify all areas that might 

violate the new 1-hour standard, EPA 

anticipates issuing guidance that 

addresses a variety of issues, such as 

how to identify and appropriately assess 

the air quality impacts of small SO2

sources (e.g., those emitting less than 

100 tons of SO2 per year) that may 

potentially cause or contribute to a 

violation of the new SO2 NAAQS. EPA 

expects that it will take more time for 

EPA to issue that guidance than is 

available in order to use it for the initial 

round of attainment designations. In 

addition to any smaller sources that 

might cause or contribute to NAAQS 

violations, States would need to model 

approximately 2000 larger sources 

across the country (i.e., sources that 

emit greater than 100 tons per year and 

are collectively responsible for about 

99% of all SO2 emissions from point 

sources in the U.S.) to determine 

whether areas are attaining or not 

attaining the 1-hour standard. While 

these sources emitting 100 or more tons 

of SO2 per year represent the significant 

fraction of the total emissions from 

point sources in the U.S., smaller 

sources also have the potential to violate 

the new SO2 NAAQS. 

After receiving EPA’s forthcoming 

modeling guidance, States might 

initially focus modeling assessments on 

these larger sources that have been 

subject to permitting requirements and 

are generally better characterized than 

smaller sources. But even this effort 

would entail a substantial burden on 

States, under a compressed timeline 

following EPA’s issuance of further 

modeling guidance. Consequently, EPA 

does not believe that for this new 1-hour 

SO2 NAAQS it would be realistic or 

appropriate to expect States to complete 

such modeling and incorporate the 

results in initial designation 

recommendations, which under CAA 

section 107(d)(1)(A) must be submitted 

to EPA within 1 year of the 

promulgation of the 1-hour standard. 

The remaining issue, then, is how to 

most appropriately use a modified 

hybrid approach, and its constituent 

modeling and monitoring tools, in the 

implementation plan development 

process in order to ensure expeditious 

attainment and maintenance of the 

NAAQS. Under the CAA, all States must 

develop and submit to EPA State 

implementation plans (SIPs) to attain 

and maintain the new 1-hour SO2

NAAQS. CAA section 110(a)(1) requires 

States, regardless of designation status, 

to adopt SIPs that provide for 

implementation, maintenance and 

enforcement of each primary NAAQS. 

Traditionally, for areas that were 

designated ‘‘attainment’’ or 

‘‘unclassifiable’’, we accepted State 

submissions of prevention of significant 

deterioration (PSD) permitting programs 

and other ‘‘infrastructure’’ SIP elements 

contained in CAA section 110(a)(2) as 

being sufficient to satisfy the section 

110(a)(1) SIP submission requirement. 

However, due to our recognition here 

that monitoring is not generally the 

most appropriate or effective tool for 

assessing compliance with the new 1- 

hour SO2 NAAQS, that additional 

guidance from EPA on conducting 

refined modeling for the new 1-hour 

NAAQS is anticipated to support our 

expected implementation approach, and 

that considerable time and resources 

may be needed to fully identify and 

properly characterize all SO2 sources 

(including those emitting less than 100 

tons of SO2 per year) that may 

potentially cause or contribute to a 

violation of the new SO2 NAAQS, we 

also had to assess how and when to best 

use modeling as the primary method in 

implementation. 


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23The schedule for State plans addressing areas 

designated ‘‘nonattainment’’ is governed by CAA 

section 191. The schedule for State plans for all 

other areas, including areas designated 

‘‘unclassifiable’’ and ‘‘attainment,’’ is governed by 

CAA section 110(a)(1). 

The approach that EPA expects to 

take, which is described in sections V 

and VI of the preamble, is consistent 

with the language of the Clean Air Act 

and would accommodate the time 

needed for an accurate assessment of 

ambient air quality levels for the 1-hour 

SO2 standard. Section 107(d)(1) requires 

areas to be designated ‘‘attainment’’ if 

they meet the standard, ‘‘nonattainment’’ 

if they do not meet the standard or 

contribute to a nearby violation, or 

‘‘unclassifiable’’ if they cannot be 

designated on the basis of available 

information. EPA’s expected approach 

would enable us to make the 

appropriate designation decision 

required by the CAA, based on the 

record of information that will be before 

EPA regarding each area. Areas would 

be designated ‘‘nonattainment’’ if either 

available monitoring data or modeling 

shows that a violation exists, or 

‘‘attainment’’ if both available 

monitoring data and modeling indicate 

the area is attaining. All other areas 

would be designated ‘‘unclassifiable,’’ as 

required by section 107(d)(1)(A). 

We currently anticipate that our 

projected post-designation 

implementation approach would look to 

robust CAA section 110(a)(1) SIPs, 

which have sometimes been previously 

referred to as ‘‘maintenance’’ or 

‘‘infrastructure’’ SIPs but for the new 

SO2 NAAQS would serve as substantive 

‘‘attainment’’ SIPs. Our current thinking 

is that, to be approved by EPA, such 

plans would need to provide for 

attainment and maintenance of the new 

1-hour SO2 NAAQS as expeditiously as 

practicable, which we expect to be no 

later than five years after initial 

designation (or approximately August 

2017) in all areas of the State, including 

any area initially designated 

‘‘nonattainment,’’ and also including any 

area designated ‘‘unclassifiable’’ that has 

SO2 sources with the potential to cause 

or contribute to a violation of the 

NAAQS. The CAA establishes deadlines 

for States to submit these plans to 

EPA.23 State plans that address areas 

designated as ‘‘nonattainment’’ (i.e., 

‘‘nonattainment area SIPs’’) are due 

within 18 months from the effective 

date of the designation, under CAA 

section 192. EPA anticipates that this 

deadline would be February 2014. State 

plans addressing all other areas (i.e., 

‘‘maintenance SIPs’’) are due within 3 

years following the promulgation of the 

new NAAQS, or June 2013, under CAA 

section 110(a)(1). 

Section 110(a)(1), unlike section 192, 

does not specify a maximum deadline 

by which States are required to show 

they have met the requirements to 

implement, maintain, and enforce a 

NAAQS. EPA believes, however, that 

August 2017 is the latest date by which 

areas should show they have achieved 

attainment and maintenance of the 

standard because this deadline is the 

same as would be required for areas 

designated nonattainment in June 2012. 

It is therefore presumptively reasonable 

as it is identical to the period Congress 

provided for nonattainment areas to 

reach attainment. Moreover, EPA notes 

that the maintenance SIPs will be due 

in June 2013, rather than in February 

2014, giving States and sources at least 

as much time between SIP development 

and submission and the date by which 

attainment should be achieved as they 

would have had the area been 

designated nonattainment in 2012. 

These section 110(a)(1) SIPs would be 

able to rely on modeling reflecting any 

SO2 reductions that we expect to result 

before the attainment date from 

compliance with the rules EPA expects 

to promulgate before 2013, (including 

technology-based standards under CAA 

section 112(d) for certain source 

categories emitting large amounts of SO2

such as Electric Generating Units and 

industrial boilers, and revised rules 

establishing further limits on SO2

emitted by sources in upwind States 

which contribute significantly to 

downstream States’ inability to attain or 

maintain the PM2.5 NAAS (the so-called 

Clean Air Interstate Replacement rule)). 

Thus, we intend that a State’s section 

110(a)(1) SIP may account for projected 

emissions reductions, including any 

from national and regional rules that are 

promulgated before these SIP 

submissions, provided that those 

reductions occur under a schedule that 

ensures attainment as expeditiously as 

practicable. We expect that date to be no 

later than 5 years from the date of initial 

designation or August 2017. 

Under this anticipated approach, 

attainment SIPs for nonattainment areas 

would have to include enforceable 

emissions limitations, timetables for 

compliance, and appropriate testing/ 

reporting to assure compliance, and 

demonstrate attainment through air 

quality modeling for all sources 

contributing to monitored and modeled 

violations, or that have the potential to 

cause or contribute to a violation of the 

NAAQS. The SIPs under section 

110(a)(1) would need to demonstrate 

through refined air quality modeling 

that any source or group of sources that 

have the potential to cause or contribute 

to a violation of the NAAQS are, or will 

be, sufficiently controlled to ensure 

timely attainment and maintenance of 

the NAAQS. We would expect this to 

include any individual sources with the 

potential to emit 100 or more tons per 

year of SO2, and other sources that may 

also cause or contribute to violations of 

the new SO2 NAAQS. We expect to 

develop guidance for the States’ use on 

how best to identify and assess the 

impact of sources that may have this 

potential. As mentioned previously, we 

intend to provide an opportunity for 

notice and comment on this guidance 

before finalizing it. 

EPA again notes that it anticipates 

several forthcoming national and 

regional rules, such as the pending 

Industrial Boilers MACT standard under 

CAA section 112(d), that are likely to 

require significant reductions in SO2

emissions over the next several years. A 

limited qualitative assessment based on 

the results of preliminary modeling of 

some sample facilities indicates that 

well controlled sources should meet the 

new SO2 NAAQS (see Brode 2010b). 

Exceptions could include unique 

sources with specific characteristics that 

contribute to higher ambient impacts 

(short stack heights, complex terrain, 

etc.). These national and regional rules 

are expected to lead to SO2 reductions 

that will help achieve compliance with 

the new SO2 NAAQS prior to 2017. If, 

upon EPA review of submitted SIPs that 

rely upon those reductions or other 

local controls, it appears that States will 

nevertheless fail to attain the NAAQS as 

expeditiously as practicable (and no 

later than August 2017), the Clean Air 

Act provides authorities for EPA to 

solve such failure, including, as 

appropriate, disapproving submitted 

SIPs, re-designating unclassifiable areas 

to nonattainment, issuing SIP calls, and 

promulgating FIPs. 

For the reasons discussed above, EPA 

has determined that it is appropriate 

and efficient to principally use 

modeling to assess compliance for 

medium to larger sources, and to rely 

more on monitoring for groups of 

smaller sources and sources not as 

conducive to modeling. EPA’s revised 

monitoring network requirements have 

been developed to be consistent with 

this approach. However, EPA is still 

considering how monitoring and 

modeling data would be used together 

in specific situations to define 

attainment and nonattainment 

boundaries and under what 

circumstances it may be appropriate to 

rely on monitoring data alone to make 

attainment determinations. EPA intends 


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to address these issues as it develops 

implementation guidance. 

In light of the new approach that EPA 

intends to take with respect to 

implementation of the SO2 NAAQS, 

EPA intends to solicit public comment 

on guidance regarding modeling, and 

also solicit public comment on 

additional implementation planning 

guidance, including the content of the 

maintenance plans required under 

section 110(a)(1) of the Clean Air Act. 

EPA also notes that State monitoring 

plans and the SIP submissions that 

States will make will also be subject to 

public notice and comment. 

IV. Amendments to Ambient 

Monitoring and Reporting 

Requirements 

In this section of the preamble, we 

describe the proposal, the public 

comments that we received on the 

proposed monitoring and reporting 

requirements, and the final 

requirements for the SO2 monitoring 

network. We are modifying our 

proposed approach to the amount of 

monitoring to require following 

consideration of public comments and a 

review of our historic practice in 

assessing compliance with the SO2

NAAQS. As we explain above in section 

III, we will use a hybrid approach that 

combines monitoring and modeling, 

using each of these analytic tools where 

they are most appropriate and effective. 

This approach and its requirements are 

intended to support the revised SO2

NAAQS, described in section II above. 

For a short-term 1-hour standard, 

dispersion modeling of stationary 

sources will generally be more 

technically appropriate, efficient, and 

effective because it takes into account 

fairly infrequent combinations of 

meteorological and source operating 

conditions that can contribute to peak 

ground-level concentrations of SO2. 

Even an expansive monitoring network 

could fail to identify all such locations. 

Consequently, we have revised the 

scope of the monitoring network, 

reflecting a modified and expanded set 

of objectives. This section also describes 

and explains the final requirements for 

the new SO2 Federal Reference Method 

(FRM), and the SO2 network design, 

monitoring objectives, data reporting, 

and data quality objectives that support 

the revised primary SO2 NAAQS. 

A. Monitoring Methods 

1. Requirements for SO2 Federal 

Reference Method (FRM) 

The proposal to promulgate an 

automated SO2 FRM was based on a 

need to update the cumbersome existing 

manual wet-chemistry (pararosaniline) 

method to a continuous-type automated 

method that can readily provide 1-hour 

SO2 measurement capability. See 74 FR 

at 64846–849. The following paragraphs 

provide background, rationale, and the 

final changes to the automated SO2

Federal Reference Method (FRM) and to 

the associated performance 

specifications for automated SO2

analyzers. 

a. Proposed Ultraviolet Fluorescence 

SO2 FRM and Its Implementation 

FRMs, set forth in several appendices 

to 40 CFR Part 50, serve (1) To provide 

a specified methodology for definitively 

measuring concentrations of ambient air 

pollutants for comparison to the 

NAAQS in Part 50, and (2) to provide 

a standard of comparison for 

determining equivalency of alternative 

pollutant measurement methods that 

can be used in lieu of the FRM for such 

monitoring. 

The FRM for measuring SO2 in the 

ambient air was promulgated on April 

30, 1971 in conjunction with the first 

primary SO2 NAAQS (36 FR 8196). This 

SO2 FRM is specified in Appendix A of 

Part 50 and identified as the 

pararosaniline manual method. See 

generally 74 FR at 64846. In the interim, 

EPA has designated many SO2 methods 

as equivalent methods (FEMs), most of 

which are based on the ultraviolet 

fluorescence (UVF) measuring 

technique. Id. In fact, virtually all SO2

monitoring data are now obtained with 

FEMs that use the UVF technique. 

In light of this, EPA proposed to 

establish a new automated SO2 FRM 

based on UVF—the same measurement 

technique employed by FEM analyzers 

now in widespread use by most State 

and local monitoring agencies and 

having the measurement capability 

needed to implement the proposed 1- 

hour SO2 NAAQS. FRM analyzers using 

this UVF technique can provide the 

needed detection limits, precision, and 

accuracy and fulfill other purposes of an 

FRM, including use as an appropriate 

standard of reference for testing and 

designation of new FEM analyzers. At 

proposal, EPA specified the new 

method in performance-based form, 

describing a generic reference 

measurement principle and associated 

calibration procedure in a new 

Appendix A–1 to 40 CFR Part 50. 

Associated performance requirements 

applicable to candidate automated SO2

analyzers (both FRMs and FEMs) were 

proposed in 40 CFR Part 53. 

EPA also proposed retaining the 

existing manual pararosaniline FRM for 

SO2. Although EPA recognized that the 

existing method is cumbersome for onehour measurements, it is capable of 

making measurements of 1 hour or even 

30 minute periods. 74 FR at 64846; see 

also Part 50 Appendix A at 1.1 (‘‘[t]he 

method is applicable to the 

measurement of ambient SO2

concentrations using sampling periods 

ranging from 30 minutes to 24 hours’’). 

Supersession of the existing manual 

FRM, as defined in § 53.16, would 

require not only withdrawal of that 

existing FRM but also the cancellation 

of the designations of all existing SO2

FEMs. Loss of the use of these FEM 

analyzers would leave State and local 

monitoring agencies with no approved 

SO2 monitors until new FRM and FEM 

analyzers could be designated under the 

new FRM. The resulting costs and 

disruptions to monitoring agencies is 

unnecessary because the current SO2

FEMs readily and accurately measure 

(and report) one-hour ambient 

measurements. See 74 FR at 64847. 

Accordingly, EPA concluded that 

supersession of the existing FRM was 

not warranted, given the costs and 

disruptions which would occur to State 

monitoring programs and the limited 

benefits from such an action given the 

suitability of the in-use FEMs. Id. at 

68646; see also section 53.16(b)(1) 

stating that in exercising its discretion 

as to whether to proceed with 

supersession of an FRM, EPA will 

consider the benefits (in terms of 

requirements and purposes of the Act) 

from specifying a new reference 

method, potential economic 

consequences of such supersession for 

State and local monitoring agencies, and 

disruption to State and local air quality 

monitoring programs. Instead, EPA 

proposed to add the new UVF FRM 

while retaining the existing FRM for 

some period of time to support the 

continued approval of existing SO2 FEM 

analyzers. 

b. Public Comments on the Proposed 

FRM and Implementation 

EPA received comments from State 

and local groups (e.g., City of Houston, 

Houston-Galveston Area Council, KY, 

NC, NY, PA, SC, SD, and WI) and 

industry (e.g., AirQuality Research and 

Logistics (AQRL), Consumers Energy, 

ExxonMobil, Montana Sulfur and 

Chemical Company, Inc. (MSCC), and 

the Utility Air Regulatory Group 

(UARG)), all generally supporting EPA’s 

proposal to adopt the proposed 

automated UVF as an FRM. For 

example, South Dakota supported 

adding the UVF SO2 method as an 

additional FRM and stated that this 

method is currently being used in the 

network and will reduce the cost of 

implementing the new monitoring 


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requirements for this rule. The UARG 

stated that the proposal to specify a 

different FRM to judge compliance is 

entirely reasonable, and UARG 

generally supported the proposed 

specifications for a new FRM but 

maintained that the current FRM could 

not be used along with a new FRM. 

ExxonMobil stated that it supports 

‘‘* * * EPA allowing monitoring 

agencies to choose mobile monitoring 

that meets monitoring quality 

requirements.’’ AQRL stated that ‘‘EPA is 

correct in choosing to designate 

[promulgate] a new (automated) FRM 

for measurement of SO2.’’ 

EPA did not receive any public 

comments opposing the proposed 

automated UVF SO2 FRM but did 

receive a few technical comments on 

specific provisions of the method. EPA 

proposed use of an inlet line particle 

filter as a requirement for new UVF SO2

FRM analyzers, believing that use of a 

particle filter is advantageous to prevent 

interference, malfunction, or damage to 

the analyzer from particles in the 

sampled air. The State of Missouri 

questioned this requirement, noting that 

such a filter can sometimes cause 

problems and that filter requirements 

for other FRM and FEM analyzers have 

been analyzer-specific depending on the 

manufacturer’s stipulation. EPA 

believes, however, that for new SO2

FRM analyzers, the benefits and 

uniformity provided by a mandatory 

filter requirement outweigh possible 

disadvantages of such a filter. 

Missouri also suggested that the 

language of proposed Sections 4.1.1 and 

4.1.2 regarding calibration system flow 

rate requirements were somewhat 

confusing, and that the high (50–100 

ppm) concentration requirement for the 

calibration standard specified in Section 

4.1.6.1 is sometimes a problem. In 

response to these comments, the 

language of Sections 4.1.1 and 4.1.2 has 

been clarified, and the concentration of 

the standard specified in Section 4.1.6.1 

has been reduced to 10 ppm. 

EPA received a number of comments 

from States (e.g., NC, NYSDEC, PA, SC, 

and SD) that supported the EPA 

proposed plan of temporary retention of 

the existing wet-chemistry 

pararosaniline FRM and for FEMs 

approved based on that method. For 

example, Pennsylvania stated ‘‘[t]his 

methodology should enable State and 

local agencies to continue using their 

existing monitoring equipment and 

[thereby] avoid large capital fund 

outlays for samplers and ultimately 

avoid any delays in collecting data that 

would be comparable to the proposed 

new primary sulfur dioxide NAAQS.’’ 

North Carolina requested ‘‘* * * that 

the EPA maintain the current reference 

method for at least an additional 10 

years.’’ Wisconsin and the Center for 

Biological Diversity (CBD) suggested 

expeditiously phasing out the existing 

manual SO2 FRM. 

In contrast, however, EPA also 

received comments from industry that 

opposed the retention of the existing 

pararosaniline FRM while promulgating 

a new automated UVF FRM. In 

particular, UARG stated ‘‘* * * having 

two FRMs specified for a given 

NAAQS—is not viable,’’ pointing out 

that there is only one FRM for each 

NAAQS under the present standards, a 

result UARG appears to believe is 

legally mandated. 

EPA disagrees with this comment. 

First, there is nothing in the Act that 

mandates a single FRM for each 

NAAQS. Section 109 of the Act, in fact, 

does not address this issue at all. 

Second, as noted previously, there are 

sound policy reasons for not 

withdrawing the existing FRM at this 

time. Therefore, EPA sees no legal or 

other obstacle in adding a new 

automated UVF FRM while retaining 

the existing manual FRM. 

UARG further maintained that EPA 

provided no support for its statement 

that the existing FEMs, which constitute 

the bulk of the existing SO2 monitoring 

network, are adequate for the current 

and proposed new SO2 NAAQS. UARG 

also stated that ‘‘although the FEMs may 

be adequate for many other purposes, 

they may only be used to judge 

compliance with the 1-hour NAAQS if 

they are shown to qualify as FRMs or 

FEMs under the new FRM definition.’’ 

EPA disagrees with this comment 

also. In answer to UARG’s second point, 

it is not necessary that these existing 

FEMs be re-designated as FRMs 

pursuant to the new automated FRM to 

continue their approved use. There is no 

legal impediment to such continued 

use, since they are (and will continue to 

be) FEMs approved based on an FRM 

that adequately measures one-hour 

ambient SO2 concentrations. Nor is 

there any technical impediment to the 

continued use of these FEMs, given that 

they are automated continuous 

monitoring methods capable of 

measuring SO2 concentrations ranging 

from a few minutes to a 1-hour period. 

The existing FEMs in the network use 

the same UVF technology as the 

proposed (and now final) automated 

FRM and have been reporting 1-hour 

monitoring data for decades. These 

FRMs have been tested against the test 

and performance requirements of Part 

53, which are designed specifically to 

test such continuous methods. Further, 

the proposed SO2 method performance 

specifications for the standard 

measurement range were derived from 

data submitted in FEM applications for 

analyzers that were subsequently 

designated as FEMs. Therefore, these 

FEMs are technically and legally sound 

to judge compliance with the one-hour 

NAAQS. 

EPA has clarified the regulatory text 

so that the rules state unambiguously 

that both SO2 FRMs apply to the new 

one-hour standard (as well as to the 24- 

hour and annual standards so long as 

they are retained), as do all presentlydesignated FEMs. 

c. Conclusions on Ultraviolet 

Fluorescence SO2 FRM and 

Implementation 

We are finalizing the proposed new 

automated SO2 FRM, which is based on 

UVF technology, with the following 

minor technical changes: The language 

of Sections 4.1.1 and 4.1.2 has been 

clarified, and the minimum 

concentration of the calibration 

standard specified in Section 4.1.6.1 has 

been reduced to 10 ppm. The new FRM 

is codified as Appendix A–1 to 40 CFR 

Part 50 and titled ‘‘Reference 

Measurement Principle and Calibration 

Procedure for the Measurement of 

Sulfur Dioxide in the Atmosphere 

(Ultraviolet Fluorescence Method).’’ 

EPA is retaining the previously existing 

manual pararosaniline SO2 FRM for the 

time being and re-codifying it as 

Appendix A–2 to 40 CFR Part 50. 

However, EPA plans to rescind this 

manual FRM at a future time when new 

SO2 FRM analyzers have adequately 

permeated State monitoring networks. 

2. Requirements for Automated SO2

Methods 

a. Performance Specifications for 

Automated Methods 

In association with the proposal to 

adopt a new automated FRM, EPA 

proposed to update the performancebased designation requirements for FEM 

SO2 analyzers currently specified in 40 

CFR Part 53. As noted in the proposal 

preamble (74 at 64846), these 

requirements were established in the 

1970’s, based primarily on the wetchemical measurement technology 

available at that time. Those initial 

requirements have become significantly 

outdated and need to be modified to 

match current technology, particularly 

because they would apply to new SO2

FRM analyzers under the proposed new 

FRM. The better instrumental 

performance available with the 

proposed new UVF FRM technique 

allows the performance requirements in 

Part 53 to be made more stringent for 


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both FRM and FEM SO2 analyzers. 

Updating these performance 

requirements is needed to ensure that, 

going forward, all new SO2 monitors 

will have improved performance. 

EPA solicited comments on the 

proposed new performance 

requirements for automated SO2

methods that were included in Table 

B–1 (Performance Specifications for 

Automated Methods) of Part 53. We 

proposed revised performance 

specifications for noise, lower 

detectable limit, interference equivalent, 

zero drift, span drift, lag time, rise time, 

fall time, and precision. EPA proposed 

to reduce the allowable noise limit from 

5 to 1 ppb, the lower detectable limit 

from 10 to 2 ppb, the interference 

equivalent limits from ±20 ppb to ±5 

ppb for each interferent, and from 60 

ppb to 20 ppb for the total of all 

interferents, the zero drift limit from ±20 

to ±4 ppb, the lag time limit from 20 to 

2 minutes, both rise and fall time limits 

from 15 to 2 minutes, and the precision 

limits from 15 ppb to 2 percent of the 

upper range limit. EPA further proposed 

to eliminate the requirements for span 

drift at 20% of the upper range limit. In 

addition, to address the need for more 

sensitive, lower measurement ranges for 

SO2 analyzers, EPA proposed a separate 

set of performance requirements that 

would apply specifically to narrower 

measurement ranges, i.e. ranges 

extending from zero to concentrations 

less than 0.5 ppm. Other minor changes 

were proposed in the wording of a few 

sections of Part 53 Subparts A and B, 

including provision for alternate data 

recording devices in § 53.21 to 

supplement the older language relating 

specifically to strip chart recorders. 

b. Public Comments 

EPA received a number of comments 

from industry (AQRL and UARG) and 

from the multi-State organization 

NESCAUM regarding the proposed 

interferent limit requirements listed in 

Table B–1. UARG submitted comments 

supportive of all the proposed 

requirements for the new UVF SO2

FRM, except for the proposed total 

interferent limits of 20 ppb. UARG 

acknowledged that EPA proposed to 

reduce the total interferent level 

substantially from 60 ppb to 20 ppb, but 

maintained that the proposed level of 20 

ppb is still too high because it amounts 

to 20%–40% of the levels being 

considered for the NAAQS (50–150 

ppb). AQRL recommended limiting 

‘‘* * * each interferent to no more than 

±3 ppb and total interference to no more 

than 12 ppb.’’ NESCAUM recommended 

tightening the nitric oxide (NO) 

interference limit from 100:1 to 300:1 

(i.e., one third of the proposed value of 

±5 ppb). NESCAUM states that the 

proposed interferent value of ±5 ppb 

results in substantial NO interference at 

sites with low SO2 levels in urban areas. 

EPA revisited the issue of the 

interferent equivalent limit for SO2

analyzers in context of the above 

comments and reconsidered what is 

reasonably feasible with current 

technology. We reviewed the current 

instrument specifications and test data 

submitted for numerous SO2 FEM 

applications. We also took into account 

that the test concentrations of most of 

these interferents are substantially 

higher than the concentrations normally 

observed in ambient air. EPA 

considered lowering the testing 

concentrations of these interferents, 

which would have correspondingly 

lowered the interferent equivalent for 

each analyte. However, EPA took a more 

conservative approach and retained the 

existing test concentrations for H2S, 

NO2, NO, O3, m-xylene, and water 

vapor. Based on this review, we found 

that it is not feasible to further lower the 

limit requirement for these interferents 

below ±5 ppb. However, in response to 

the NESCAUM comment, EPA 

determined that the interferent 

equivalent limit requirement for NO 

interference could be reduced to ±3 ppb 

(166:1) for the new, lower measurement 

range to reduce possible NO 

interference at sites with low SO2 levels 

in urban area. 

In regard to the total limit for all 

interferent equivalents for SO2

analyzers, EPA notes that many of the 

interferents for which testing is required 

(specified in Table B–3 of Part 53) 

would likely react with each other and 

would thus not co-exist in ambient air 

at the specified test concentrations. 

Therefore, EPA determined that the 

limit requirement for total interference 

equivalent can be eliminated, and Table 

B–1 now reflects this change. 

EPA received comment from AQRL 

on the existing span drift requirement 

for SO2 analyzers specified in Table B1. AQRL recommended lowering the 

span drift requirement at 80% URL to 

2.5%, stating that ‘‘ambient air monitors 

in the 21st century should be able to 

hold span drift to no more than ±2.5% 

under the conditions specified in EPA 

testing * * *.’’ Based on information 

from FEM testing laboratories and 

manufacturers’ data (EPA, 2009c), EPA 

largely agrees with this comment and 

concludes that the span drift 

requirement at 80% can be lowered to 

±3%. Table B–1 has been changed to 

include this revised limit. 

EPA received comment from the State 

of Wisconsin suggesting that the 

proposed revised provisions of section 

53.21 (Test conditions) be further 

changed to more specifically recognize 

use of digital recorders for obtaining test 

results rather than maintaining the tie to 

analog strip chart recorder technology. 

EPA acknowledges that industry has 

moved away from strip chart recording 

technology to digital data recording. 

However, the proposed language of 

§ 53.21 calls for a graphic representation 

of analyzer responses to test 

concentrations to facilitate visual 

examination of test results and allows 

any ‘‘alternative measurement data 

recording device’’ as long as it can 

provide such a graphic representation. 

Describing the analog strip chart 

recorder in this section provides an 

appropriate model to help define the 

type of graphic representation needed 

for the Part 53 tests. EPA believes that 

the proposed language of § 53.21 is 

adequately broad to permit digital or 

other types of data recording devices. 

c. Conclusions for Performance 

Specifications for SO2 Automated 

Methods 

Based on typical performance 

capabilities of current UVF analyzers 

and manufacturers’ actual testing data, 

we are keeping the limit for each 

interference equivalent for SO2

analyzers at ±5 ppb. However, we are 

lowering the interference equivalent 

requirement for NO to ±3 ppb for the 

lower measurement range. A footnote 

denoting this specific requirement is 

being added to Table B–1. We are 

eliminating the total interference 

equivalent requirement for SO2

analyzers, and Table B–1 is being 

revised to incorporate this change. 

The 24-hour span drift at 80% of the 

upper range limit for SO2 analyzers is 

being lowered to ±3% in Table B–1 to 

be in line with current technology. Also, 

unrelated to SO2, a typographical error 

for the noise requirement for CO 

analyzers is being corrected to 0.5 ppm 

in Table B–1. 

Finally, information on generation 

and verification of test concentrations 

for naphthalene was inadvertently 

omitted from Table B–2, Test 

Atmospheres, even though it was added 

as a required interferent test in our 

proposal. Therefore, we are adding that 

information for naphthalene. Also in 

Table B–2, we are correcting the 

verification information for nitric oxide. 

B. Network Design 

Ambient SO2 monitoring data are 

collected by State, local, and Tribal 

monitoring agencies (‘‘monitoring 

agencies’’) in accordance with the 

monitoring requirements contained in 


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24Prior to this rulemaking there were no 

minimum monitoring requirements, except for 

those required at the multi-pollutant National Core 

(NCore) monitoring sites. The monitoring rule 

promulgated in 2006 (71 FR 61236) removed 

minimum monitoring requirements (except for 

those NCore stations). This change was largely 

driven by the fact that there was no longer an SO2

nonattainment problem under the then-existing 

standards. However, this logic does not apply to the 

revised primary SO2 NAAQS. 

25Required monitor estimates were based on 2008 

Census estimates and the 2005 National Emissions 

Inventory. 

26CBSAs are defined by the U.S. Census Bureau, 

and are comprised of both Metropolitan Statistical 

Areas and Micropolitan Statistical Areas (http:// 

www.census.gov). 

40 CFR parts 50, 53, and 58. A 

monitoring network is generally 

designed to measure, report, and 

provide related information on air 

quality data as described in 40 CFR Part 

58. To ensure that the data from the 

network is accurate and reliable, the 

monitors in the network must meet a 

number of requirements including the 

use of monitoring methods that EPA has 

approved as Federal Reference Methods 

(FRMs) or Federal Equivalent Methods 

(FEMs) (discussed in some detail above 

in section IV.A), focusing on particular 

monitoring objectives, and following 

specific siting criteria, data reporting, 

quality assurance and data handling 

rules or procedures. 

With the revision to the SO2 NAAQS, 

which establishes a new 1-hour 

averaging period intended to limit shortterm exposures that may occur 

anywhere in an area, EPA evaluated the 

existing network to determine if it was 

adequate to support the revised SO2

NAAQS. A significant fact for ambient 

SO2 concentrations is that stationary 

sources are the predominant emission 

sources of SO2 and the peak, maximum 

SO2 concentrations that may occur are 

most likely to occur nearer the parent 

stationary source, as noted in the ISA 

(ISA, 2–1), section II.A.1 above, and in 

section IV.B.1 below. According to the 

2005 National Emissions Inventory, 

there are 32,288 sources (facilities) 

emitting SO2, of which 1,928 are 

emitting 100 tons per year (tpy) or more. 

In the proposal (74 FR 64851), EPA had 

anticipated requiring 348 sourceoriented monitors in the network design 

based on a population and emissions 

metric and a State’s emissions 

contribution to the National Emissions 

Inventory (NEI). In response to this 

proposal, EPA received numerous 

comments arguing that the required 

number of monitors in the network 

would be too small. Other commenters 

argued that expanding the monitoring to 

an adequate scale would impose a large 

burden and expense on the States. Some 

commenters referred to SO2 modeling in 

their submissions as an addition or 

alternative to monitoring. Consequently, 

as part of developing a balanced 

response to these comments, we 

revisited how we had historically dealt 

with SO2 for various purposes including 

designations and implementation 

through permitting and emissions 

limitations. As explained in section III, 

this has been realized through a 

combined monitoring and modeling 

approach. As set out below, and in 

sections III, VI, and VII, our ultimate 

intention is to utilize a combined 

monitoring and modeling approach, a 

hybrid analytic approach, to assess 

compliance with the revised SO2

NAAQS. 

As a result of this contemplated 

hybrid analytic approach, the minimum 

number of monitors required in the 

network through this rulemaking is 

reduced to approximately 163 monitors 

from the approximated 348 monitors 

that were proposed. This section of the 

preamble includes a discussion of the 

proposal, the comments received, and 

the details of and the rationale for the 

final changes to the SO2 network design 

requirements. 

1. Approach for Network Design 

a. Proposed Approach for Network 

Design 

To fully support the proposed 

revision to the SO2 NAAQS, EPA 

indicated the need to identify where 

short-term, peak ground-level 

concentrations—i.e., concentrations 

from 5 minutes to one hour (or 

potentially up to 24 hours)—may occur. 

Given that large stationary sources are 

the predominant source of emissions, 

monitoring short-term, peak groundlevel concentrations would require 

monitors to be sited to assess impacts of 

individual or groups of sources and 

therefore be source-oriented in nature. 

As a result, under a monitoring-focused 

approach, EPA proposed a two-pronged 

monitoring network of all sourceoriented monitors. However, due to the 

multiple variables that affect ground 

level SO2 concentrations from 

individual or groups of sources, 

including stack heights, emission 

velocities, stack diameters, terrain, and 

meteorology, EPA could not specify a 

source specific threshold, algorithm, or 

metric by which to require monitoring. 

The design of the proposed network 

represented a primarily monitoringfocused approach to assess compliance 

with the primary SO2 NAAQS. 

In preparation for the SO2 NAAQS 

proposal, EPA conducted an analysis of 

the approximately 488 SO2 monitoring 

sites operating during calendar year 

2008 (Watkins and Thompson, 2009). 

This analysis indicated that 

approximately ∼ 35% of the monitoring 

network was addressing locations of 

maximum (highest) concentrations, 

likely linked to a specific source or 

group of sources. Meanwhile, just under 

half (∼ 46%) of the sites were reported 

to be for the assessment of 

concentrations for general population 

exposure. These data allowed EPA to 

conclude that the network 24 was not 

properly focused to support the revised 

NAAQS (under the assumption that 

source-oriented monitoring data would 

be the primary tool for assessing 

compliance with the NAAQS). As a 

result, EPA proposed a two-pronged 

monitoring network (74 FR 64850), 

based on the premise of a monitoringfocused approach, with minimum 

requirements for: (1) Monitors in urban 

areas where there is a higher 

coincidence of population and 

emissions, utilizing a Population 

Weighted Emissions Index (PWEI), and 

(2) monitors in States based on each 

State’s contributions to the national SO2

emissions inventory. In addition, all the 

monitors in the network would be sited 

at locations of expected maximum 

hourly concentrations and therefore 

likely be source-oriented. This twopronged network would have resulted 

in a minimum of approximately 348 

monitors nationwide 25 providing data 

for comparison with the 1-hour standard 

and supporting its implementation. 

Under the first prong of the network 

design, EPA proposed that the ambient 

SO2 monitoring network account for 

SO2 exposure by requiring monitors in 

locations where population and 

emissions may lead to higher potential 

for population exposure to peak hourly 

SO2 concentrations. In order to do this, 

EPA developed a Population Weighted 

Emissions Index (PWEI) that uses 

population and emissions inventory 

data at the CBSA 26 level to assign 

required monitoring for a given CBSA 

(with population and emissions being 

obvious relevant factors in prioritizing 

numbers of required monitors). The 

PWEI for a particular CBSA was 

proposed to be calculated by 

multiplying the population (using the 

latest Census Bureau estimates) of a 

CBSA by the total amount of SO2

emissions in that CBSA. The CBSA SO2

emission value would be in tons per 

year, and calculated by aggregating the 

county level emissions for each county 

in a CBSA. We would then divide the 

resulting product of CBSA population 

and CBSA SO2 emissions by 1,000,000 

to provide a PWEI value, the units of 


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which would be millions of people-tons 

per year. 

We proposed that the first prong of 

the SO2 network design require 

monitors in CBSAs, according to the 

following criteria. For any CBSA with a 

calculated PWEI value equal to or 

greater than 1,000,000, a minimum of 

three SO2 monitors would be required 

within that CBSA. For any CBSA with 

a calculated PWEI value equal to or 

greater than 10,000, but less than 

1,000,000, a minimum of two SO2

monitors would be required within that 

CBSA. For any CBSA with a calculated 

PWEI value equal to or greater than 

5,000, but less than 10,000, a minimum 

of one SO2 monitor would be required 

within that CBSA. EPA estimated that 

the proposed criteria would have 

resulted in 231 required sites in 131 

CBSAs. 

Under the second prong of the 

network design, EPA proposed to 

require a monitor or monitors in each 

State, allocated by State-level SO2

emissions. This prong of the network 

design was intended to allow a portion 

of the overall required monitors to be 

placed where needed, independent of 

the first prong of the network design, 

inside or outside of CBSAs. EPA 

proposed to require monitors, using 

State boundaries as the geographic unit 

for allocation purposes, in proportion to 

a State’s SO2 emissions, i.e., a State with 

higher emissions would have been 

required to have a proportionally higher 

number of monitors. The proposed 

percent contribution of individual 

States would have been based on the 

most recent NEI, with SO2 emissions 

being aggregated by State. The number 

of required monitors per State would 

correspond to every one percent (after 

rounding) of each State’s contribution to 

the national SO2 inventory. EPA also 

proposed that each State have at least 

one monitor required as part of this 

second prong, even if a particular State 

contributes less than 0.5% of the total 

anthropogenic national emissions 

inventory. As a result, the proposed 

second prong would have required 

approximately 117 monitoring sites 

based on State-level SO2 emissions in 

the most recent NEI, which at the time 

of the proposal, was the 2005 NEI. 

EPA also stated in the proposal that 

the multi-pollutant National Core 

(NCore) monitoring sites would not 

have counted towards meeting the 

proposed monitoring requirements. 

However, data from the NCore would be 

compared to the NAAQS even though 

NAAQS comparisons are not the sole 

objective of NCore monitors. The 

monitoring rule promulgated in 2006 

(71 FR 61236) and codified at 40 CFR 

Part 58 and its Appendices established 

the NCore multi-pollutant network 

requirement to support integrated air 

quality management data needs. In 

particular, NCore sites are intended to 

provide long-term data for air quality 

trends analysis, model evaluation, and, 

for urban sites, tracking metropolitan air 

quality statistics. To do this, NCore sites 

are required to measure various 

pollutants, including SO2, but they are 

not source oriented monitoring sites, 

and therefore are not likely to be the 

location of maximum expected 

concentration in an area. NCore sites are 

intended to provide data representing 

concentrations at the broader 

neighborhood and urban spatial scales. 

These reasons were the rationale 

justifying why SO2 monitors at NCore 

stations would not have been part of the 

minimum monitors required under the 

proposed network. 

b. Alternative Network Design 

EPA also solicited comment on an 

alternative network design, including 

alternative methods to determine the 

minimum number of monitors per State 

(74 FR 64854). EPA requested comment 

on whether a screening approach for 

assessing the likelihood of a NAAQS 

exceedance could be developed and 

serve as a basis for determining the 

number and location of required 

monitors. In particular, EPA requested 

comment on whether it should utilize 

existing screening tools such as 

AERSCREEN or SCREEN3, which use 

parameters such as effective stack height 

and emissions levels to identify 

facilities with the potential to cause an 

exceedance of the proposed standard. 

For that set of sources, EPA could then 

require States to conduct more refined 

modeling (using the American 

Meteorological Society (AMS)/EPA 

Regulatory Model (AERMOD)) to 

determine locations where monitoring 

should be conducted. Any screening or 

refined modeling would likely be 

carried out by States by using EPA 

recommended models and techniques 

referenced by 40 CFR Part 51, Appendix 

W, which provides guidance on air 

quality modeling. Such screening or 

refined modeling uses facility emission 

tonnage, stack heights, stack diameters, 

emission temperatures, emission 

velocities, and accounts for local terrain 

and meteorology in determining where 

expected maximum hourly 

concentrations may occur. In using this 

approach, EPA would then require 

States to locate monitors at the point of 

maximum concentration around sources 

identified as likely causing NAAQS 

exceedances. EPA also noted that this 

alternative approach would not 

distinctly use population as a factor for 

where monitors should be placed. 

c. Public Comments 

EPA received many comments on the 

proposed network design and the 

alternative network design approaches. 

Based on comments that were clear 

enough on the issue, EPA believes the 

commenters’ positions on the network 

design approach generally fell into one 

of three categories: (1) Those who 

supported the two-prong approach, but 

suggested some modification to it, (2) 

those who supported the alternative 

network design, and (3) those who 

suggested other concepts for the 

network instead of the two approaches 

EPA presented in the proposal. 

The commenters who generally 

supported the two-prong network 

design, but suggested some modification 

included some State and local air 

agencies (e.g. NACAA and nine other 

State groups or agency commenters) and 

industry groups (e.g. AQRL, ACC, and 

eight other commenters). Of this group, 

some of the State and local air agencies 

specifically commented on how EPA 

should modify one or both of the prongs 

of the proposed network design. Some 

particular individual suggestions will be 

addressed here and those comments not 

addressed here will be addressed in the 

response to comment document. 

However, one recurring suggestion from 

the State and local agency commenters 

in this group was that the network 

design leads to some duplicative and/or 

unneeded monitoring, and therefore 

they requested that EPA include a 

provision to ‘‘waive’’ the monitoring 

network design requirements in 

situations where minimum monitoring 

requirements appear duplicative or 

unnecessary. In particular, NACAA 

stated that it ‘‘* * * is concerned that 

the two pronged approach in the 

proposed regulation will lead to 

duplicative monitoring in some areas 

and require monitors in areas where 

monitors are not needed. EPA 

recognizes the potential for duplicative 

monitoring, but the proposal does not 

permit the removal of duplicative 

monitors.’’ This NACAA comment was 

echoed by some of the other States who 

commented on the proposed approach 

(e.g. AK, FL, IL, NC, SC, and WI). The 

industry commenters were also 

generally supportive of the two-prong 

approach, with some making general 

suggestions to modify the network 

design. For example, AQRL stated that 

the ‘‘* * * network design proposal 

seems to provide the flexibility for 

States and the EPA regions to work 

together to arrive at the adequate 

monitoring network.’’ AQRL also 


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suggests that ‘‘a State/local area should 

have the option to shutdown or relocate 

any site mandated [by monitoring 

requirements] if measured design values 

at the site are less than 75% of the 

selected standard level.’’ Multiple 

industry commenters (e.g. API, LEC, and 

RRI Energy) expressed concern that the 

proposed network design had no 

monitoring required specifically to 

measure background concentrations of 

SO2. Dow Chemical suggested that EPA 

maintain some of the existing monitors 

that characterize population exposure 

and other non-source oriented sites for 

trends analysis. 

Those commenters who did not 

support the proposed network design, 

and instead generally supported the 

concepts of the alternative network 

design, include public health and 

environmental groups (e.g. ALA, CBD, 

EDF, EJ, NRDC, and SC) and the States 

of Delaware and Iowa. In particular, 

ALA, EDF, NRDC, and SC stated ‘‘* * * 

the proposed 348 monitors are a grossly 

inadequate number to detect peak 

concentrations from the nearly 2,000 

major sources that emit more than 100 

tons per year of sulfur dioxide * * *’’ 

and that ‘‘it is most appropriate to use 

screening tools to site all the monitors 

in the areas of highest expected 

concentration * * *’’ The Center for 

Biological Diversity, with regard to the 

proposed network design, stated that 

‘‘* * * a number of communities with 

very significant SO2 emissions will not 

have any monitoring stations at all 

***’’ Further, the State of Iowa 

claimed that ‘‘the proposed design of the 

SO2 ambient monitoring network 

provides insufficient assurances that the 

public is protected from the health 

effects of SO2 exposure,’’ and suggested 

that ‘‘* * * the final rule contain 

provisions that require monitors to be 

sited only at locations where dispersion 

modeling indicates that the NAAQS is 

violated.’’ 

Commenters also suggested other 

concepts for the monitoring network 

design in lieu of the approaches 

discussed in the proposal. NESCAUM, 

NYSDEC, and PADEP, all suggested 

using an emissions-only approach to 

trigger required monitoring instead of 

using the PWEI to require monitors in 

an area. For example, NYSDEC suggests 

that the proposed approach, using the 

PWEI, is ‘‘* * * not more predictive 

than using emissions data alone.’’ 

NYSDEC went on to recommend that 

monitors be required in CBSAs with 

aggregated emissions of 50,000 tons per 

year or more and that ambient 

monitoring be considered for point 

sources with 20,000 tons per year. 

PADEP made several suggestions on 

network design, including monitoring in 

any CBSA ‘‘where there is a sulfur 

dioxide source or combination of 

sources within 50 miles emitting a total 

of at least 20,000 tons of SO2 per year 

***’’ 

Among all three groups of 

commenters discussed above, there was 

a subset of commenters who specifically 

mentioned using modeling in some 

form. Modeling was a component of the 

alternative network design, where 

monitors would be required based on 

screening models and possibly refined 

modeling of individual sources. EPA 

also expected that under the proposed 

approach, many States would use 

modeling as a quantitative analysis tool 

to site required monitors. Finally, 

source modeling is a critical element for 

PSD and facility permitting. In their 

comments, NESCAUM recommended 

that EPA allow modeling to be used in 

conjunction with monitoring data to 

better determine nonattainment areas. 

North Carolina advocated that EPA 

require SO2 sources, without specifying 

a threshold size for sources, to perform 

modeling to demonstrate that fence-line 

(ambient) air does not exceed the 

NAAQS due to that particular source’s 

emissions. North Carolina went on to 

suggest that if a source’s modeling 

showed an exceedance of the NAAQS, 

the source could ‘‘then be required to 

reduce emissions from the stack, install 

continuous emissions monitoring (CEM) 

in the stack itself, or require a fence-line 

monitor at the target facility.’’ North 

Carolina also stated, in the context of 

discussing its own PSD program, that 

‘‘the costs for modeling are small 

compared to the costs for monitoring.’’ 

Sierra Club stated that EPA should 

‘‘* * * employ modern computer 

models to determine whether areas 

should be designated nonattainment 

because they do not meet the NAAQS in 

areas where there is no monitor.’’ From 

these comments, EPA gathers that some 

public commenters find modeling a 

useful tool and support the use of 

modeling to ascertain ambient 

concentrations of SO2. 

2. Modeling Ambient SO2

Concentrations 

EPA considered the various and 

sometimes competing concerns raised 

by the commenters including 

duplicative monitoring, lack of adequate 

number of monitors, insufficient 

flexibility, the monitoring burden, and 

the modeling suggestions. EPA 

considered its historic practice and the 

analytic tools available to arrive at a 

balanced approach that took into 

account these concerns. In the past, EPA 

used a combination of modeling and 

monitoring for SO2 during permitting, 

designations, and re-designations in 

recognition of the fact that a single 

monitoring site is generally not 

adequate to fully characterize ambient 

concentrations, including the maximum 

ground level concentrations, which 

exist around stationary SO2 sources. 

With representative and appropriate 

meteorological and other input data, 

refined dispersion models are able to 

characterize air quality impacts from the 

modeled sources across the domain of 

interest on an hourly basis with a high 

degree of spatial resolution, overcoming 

the limitations of an approach based 

solely on monitoring. By simulating 

plume dispersion on an hourly basis 

across a grid of receptor locations, 

dispersion models are able to estimate 

the detailed spatial gradients of ambient 

concentrations resulting from SO2

emission sources across a full range of 

meteorological and source operating 

conditions. The 1-hour NAAQS is 

intended to provide protection against 

short-term (5 minute to 24 hour) peak 

exposures, whether they result from 

typical meteorological conditions or not. 

Because ambient monitors are in fixed 

locations and a single monitor can only 

represent impacts which occur at the 

location of the monitor, a single monitor 

cannot identify all instances of peak 

ground-level concentrations if, for 

example, different wind directions on 

various days cause peak ground-level 

concentrations in different areas that do 

not overlap. The uncertainty associated 

with this limitation is much higher for 

an hourly standard than a long-term 

standard due to the higher degree of 

spatial and temporal variability 

associated with peak hourly impacts 

(discussed in ISA chapters 2.4 and 2.5). 

This limitation of ambient monitoring 

may be true even if the source-oriented 

ambient monitor was sited with the aid 

of modeling data, since the model is less 

reliable at predicting the precise 

location of maximum impacts than at 

predicting the distribution of impacts 

across the full modeling domain, and no 

single monitor can be sited in a way to 

always measure the peak ground-level 

SO2 concentrations that may be 

occurring in the area around a source. 

EPA’s Guideline on Air Quality 

Models, Appendix W to 40 CFR Part 51, 

provides recommendations on modeling 

techniques and guidance for estimating 

pollutant concentrations in order to 

assess control strategies and determine 

emission limits. These 

recommendations were originally 

published in April 1978 and were 

incorporated by reference in the PSD 

regulations, 40 CFR sections 51.166 and 


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27Background monitoring can be considered to be 

representative of ambient concentrations upwind of 

(and therefore not typically influenced by) a 

geographic area such as an urban area, or of an 

individual or group of emission sources. 

52.21 in June 1978 (43 FR 26382). The 

purpose of Appendix is to promote 

consistency in the use of modeling 

within the air quality management 

process. Appendix W is periodically 

revised to ensure that new model 

developments or expanded regulatory 

requirements are incorporated. The 

most recent revision to Appendix W 

was published on November 9, 2005 (70 

FR 68218), wherein EPA adopted 

AERMOD as the preferred dispersion 

model for a wide range of regulatory 

applications in all types of terrain. 

AERMOD is a steady-state plume 

dispersion model that employs hourly 

sequential preprocessed meteorological 

data to simulate transport and 

dispersion from multiple point, area, or 

volume sources for averaging times from 

one hour to multiple years, based on an 

advanced characterization of the 

atmospheric boundary layer. AERMOD 

also accounts for building wake effects 

(i.e., downwash) on plume dispersion. 

To support the promulgation of 

AERMOD as the preferred model for 

near-field dispersion (50 km or less), 

EPA evaluated the performance of the 

model across a total of 17 field study 

data bases (Perry, et al., 2005; EPA, 

2003), including several field studies 

based on model-to-monitor comparisons 

of SO2 concentrations from operating 

power plants. 

EPA anticipates that additional 

guidance for States may be needed to 

clarify how to conduct dispersion 

modeling under Appendix W to support 

the implementation of the new 1-hour 

SO2 NAAQS. Although AERMOD is 

identified as the preferred model under 

Appendix W for a wide range of 

applications and will be appropriate for 

most modeling applications to support 

the new SO2 NAAQS, Appendix W 

allows flexibility to consider the use of 

alternative models on a case-by-case 

basis when an adequate demonstration 

can be made that the alternative model 

performs better than, or is more 

appropriate than, the preferred model 

for a particular application. 

In conclusion, EPA believes that a 

hybrid analytic approach that uses a 

combination of modeling and 

monitoring information addresses the 

varying and competing concerns 

expressed by the commenters. Modeling 

large emission sources, along with 

smaller sources with the potential to 

violate the NAAQS, deals effectively 

with the concern that the monitoring 

network is not large enough to account 

for all sources that could have high 

ambient SO2 concentrations. EPA 

believes that more SO2 sources will 

ultimately be directly addressed through 

modeling alone versus the number of 

sources which would have been 

monitored under the proposed network 

design (which proposed a minimum of 

348 monitors). Because modeling 

provides a technically appropriate and 

efficient method to identify locations of 

maximum concentrations attributable to 

the major stationary SO2 sources, in the 

final network design (discussed below 

in section IV.B.4), EPA is not requiring 

that monitors must be in locations of 

expected maximum concentration, and 

thus, typically source-oriented. Instead, 

monitors required under the final 

network design now can address 

multiple monitoring objectives 

(discussed in IV.B.3 below), with fewer 

number of monitors required overall 

than the number estimated in the 

proposal. The flexibility that States now 

have, where relatively fewer required 

monitors may be sited to meet multiple 

objectives, effectively addresses 

concerns about duplicative monitoring 

and the need for waivers, the need for 

measuring background concentrations, 

and that emissions data rather than the 

PWEI could be more predictive of high 

ambient SO2 concentrations as a basis 

on which to require monitoring. The 

comments that suggested the use of 

modeling, along with an examination of 

past practice, resulted in the change to 

a hybrid approach where we use both 

modeling and monitoring to assess 

ambient SO2 concentrations. 

3. Monitoring Objectives 

Because EPA contemplates an 

ultimate approach that combines both 

monitoring and modeling, the monitor 

objectives of the final network design 

are now broadened to include 

assessment of source impacts, highest 

concentration, population exposure, 

general background concentrations, SO2

transport, and long-term trends. The 

following paragraphs provide 

background, rationale, and details for 

the final changes to monitoring 

objectives. 

a. Proposed Monitoring Objectives 

EPA proposed that all minimally 

required monitoring sites in the 

proposed two-prong network design be 

sited at locations of expected maximum 

1-hour concentrations, which would 

also likely discern 5-minute peaks. EPA 

noted that in general, such locations 

would be close to larger emitting 

sources (in tons per year) and/or areas 

of relatively high emissions densities 

where multiple sources may be 

contributing to peak ground-level 

concentrations. As a result, the 

proposed monitoring network would 

have been comprised primarily of 

source-oriented monitors. EPA also 

proposed that when selecting 

monitoring sites from among a pool of 

candidate locations (which would be 

source-oriented under the proposed 

network design), States prioritize these 

sites based on where the maximum 

expected hourly concentrations would 

occur in greater proximity to 

populations. EPA solicited general 

comments on the role of population 

exposure in the site selection process. 

b. Public Comments 

Commenters discussed a variety of 

issues on the subject of monitoring 

objectives including the importance of 

considering population exposure, the 

need for flexibility in monitor 

placement, monitoring for background 

concentrations, monitoring for long term 

trends analysis, and characterizing 

potential long-range transport of SO2. 

EPA received many comments from 

States (e.g., NACAA, DE, IL, IN, MO, 

SD, WI), the public health group ATS, 

and industry (e.g., AQRL, Consumers 

Energy, Dominion, Dow, EPRI, 

ExxonMobil, Montana Sulfur and 

Chemical, NPRA, Portland Cement, Rio 

Tinto, and UARG) suggesting that 

required monitors account for, or be 

focused on, population exposure. EPA 

also received many comments from 

States (e.g., NACAA, NESCAUM, FL, IL, 

IN, IA, MI, OH, SC, and WI) and 

industry (e.g., API, Dow, and TxOGA) 

asking for more flexibility in (sourceoriented) monitor placement with 

regard to both the target source and the 

physical location of a monitor relative to 

that source. For example NACAA stated 

that ‘‘for source oriented monitors, 

placement at the point of 1-hour 

maximum concentration must be 

realistic and flexible. EPA must allow 

agencies to determine the most 

scientifically defensible location, while 

taking into account potential exposures 

and access to locations with adequate 

siting.’’ Wisconsin stated that ‘‘* * * 

monitor siting should be balanced 

toward population-based monitors with 

a preference toward maximum 

exposure.’’ Wisconsin added that ‘‘* * * 

placing monitors at the maximum 

downwind location does not necessarily 

result in effective protection of public 

health.’’ 

EPA received a number of comments 

on background monitoring 27 from 

industry (API, LEC, and RRI Energy) and 

from the State of South Carolina. API 

stated that ‘‘because the monitors 

provide background concentrations 


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28Spatial scales are defined in 40 CFR Part 58 

Appendix D, section 1. Each scale is a description 

of the physical dimensions of an air parcel nearest 

a monitoring site throughout which pollutant 

concentrations are reasonably similar. 

needed to model impacts of new sources 

or sources undergoing major 

modification in addition to providing 

data for judging compliance with the 

NAAQS, it is important that some 

monitors be sited in a manner suitable 

for assessing this background.’’ API went 

on to state that ‘‘* * * EPA should 

encourage States to site an appropriate 

number of area-wide monitors for use in 

establishing ambient background levels 

of SO2.’’ South Carolina states that ‘‘to 

better support the monitoring objectives, 

in particular those improving our 

understanding and context for the 

source oriented monitoring data, the 

monitoring requirements must include 

the ability for States to address the 

needs for area and regional background 

concentration measurements.’’ 

A number of commenters, including 

States (e.g., Missouri, NESCAUM, Ohio, 

and South Carolina), citizens (Valley 

Watch at the Atlanta public hearing), 

the CBD, and Dow, commented on SO2

transport and related cross-boundary 

monitoring. Dow stated that ‘‘SO2

distribution has long been known as an 

interstate issue with the vast majority of 

SO2 sources being power plants and 

other fossil fuel combustion facilities. 

These facilities are more likely to 

impact distant areas than local areas and 

the resultant ground-level 

concentrations are often minimal.’’ Ohio 

stated that, under the proposed 

approach, ‘‘* * * it is likely that OH, 

WV, KY, and IN will find sources along 

the Ohio River which could result in 

monitors being located across the river 

from each other.’’ In such situations, 

Ohio asserts that ‘‘States are capable of 

working with our neighbors to 

determine which State would be in the 

best position to site and operate a 

monitor.’’ 

c. Conclusions on Monitoring Objectives 

A hybrid analytical approach, as 

noted above in section III and IV.B.1 

would ultimately make the most 

appropriate use of available tools such 

as modeling and monitoring. Thus, 

unlike under the proposal, the 

monitoring network will not have to be 

focused solely at locations of expected 

maximum concentration relative to an 

SO2 source given the anticipated 

adoption of a hybrid analytical 

approach. The final network design is 

intended to be flexible to meet multiple 

monitoring objectives, most of which 

were identified in the public comments. 

Ambient monitoring networks are 

generally designed to meet three 

primary monitoring objectives, as listed 

in 40 CFR Part 58 Appendix D, Section 

1, including: (1) Providing air pollution 

data to the general public in a timely 

manner, (2) support compliance with 

ambient air quality standards and 

emissions strategy development, and (3) 

support air pollution research studies 

(which includes health studies and 

research). In order to support these air 

quality management objectives, 

monitoring networks can have a variety 

of monitoring sites that can be sited, as 

necessary, to characterize (a) emission 

sources (i.e., source-oriented 

monitoring), (b) the highest 

concentration in an area, (c) population 

exposure, (d) general background 

concentrations, (e) regional transport, 

and (f) welfare-based impact. 

In light of the approach described in 

section III and further in IV.B.1 above, 

EPA is finalizing an SO2 network 

design, with broadened objectives, 

which EPA believes will address the 

concerns noted in the public comments 

above, particularly those regarding 

siting flexibility, population exposure, 

cross-boundary impacts, and the need 

for the network to address multiple 

monitoring objectives. The final 

network design requires that any SO2

monitors required in a particular CBSA 

as determined based on PWEI values, 

discussed below in section IV.B.4, shall 

satisfy the minimum monitoring 

requirements if they are sited at 

locations where they can meet any one 

or more of the following objectives (see 

Part 58 Appendix D section 4.4.2 as 

added by today’s final rule): 

(1) Source-Oriented Monitoring: This 

is accomplished with a monitor sited to 

determine the impact of significant 

sources or source categories on air 

quality. In some situations, such 

monitoring sites may also be classified 

as high concentration sites (discussed 

below). Examples of source-oriented 

monitors include those sited to capture 

or assess peak ground-level 

concentrations from one or more major 

SO2 sources, or those sited in an area 

with multiple smaller sources with 

overlapping plumes. 

(2) Highest Concentration: This is 

assessed by a monitor sited to measure 

the highest concentrations expected to 

occur in the area covered by the 

network. Such a location may, or may 

not, also be considered a sourceoriented location (discussed above). 

Depending on the case, this location is 

representative of the highest 

concentration occurring across a 

relatively homogeneous area with 

spatial scales typically ranging from 

tens of meters up to four kilometers.28

(3) Population Exposure: This is 

assessed by a monitor sited to measure 

typical concentrations in areas of 

(relatively) high population density. 

Some examples are a monitor placed in 

an area of elevated or high SO2

concentrations that also has a high 

population density, an area that might 

be included in public health studies, or 

in areas with vulnerable and susceptible 

populations. 

(4) General Background: This is 

assessed by placing a monitor in an area 

to determine general background 

concentrations. Such locations might be 

considered to be representative of 

ambient concentrations upwind of (and 

therefore not typically influenced by) a 

geographic area such as an urban area, 

or of an individual or group of emission 

sources. EPA notes that although a 

required monitor is allowed to be sited 

to assess background concentrations, the 

required monitor is not allowed to be 

sited outside of the parent CBSA (whose 

PWEI value triggered required 

monitoring, discussed in section IV.B.4 

and IV.B.5). If a State believes that there 

is a need to conduct background 

monitoring outside of CBSAs with 

required monitoring, EPA notes that 

States always have the prerogative to 

conduct monitoring above the minimum 

requirements in any location the State 

believes is appropriate. 

(5) Regional Transport: This is 

assessed by placing a monitor in a 

location to determine the extent of 

regional pollutant transport. Such 

locations could be either upwind or 

downwind of urban areas, 

characterizing the entry or exit of the 

pollutant in a region, respectively. EPA 

notes that although a required monitor 

is allowed to be sited to assess regional 

transport, the required monitor is not 

allowed to be sited outside of the parent 

CBSA (whose PWEI value triggered 

required monitoring, discussed in 

section IV.B.4 and IV.B.5). If a State 

believes that there is a need to conduct 

background monitoring outside of 

CBSAs with required monitoring, EPA 

notes that States always have the 

prerogative to conduct monitoring above 

the minimum requirements in any 

location the State believes is 

appropriate. 

In regard to the public comments 

expressing concerns on the issue of 

cross-boundary transport, i.e., a source 

on one side of a political boundary 

contributes to peak ground-level 

concentrations on the other side of that 

boundary, EPA will allow a required 

monitor to be placed outside of the 

parent CBSA (whose PWEI value 

triggered monitoring, discussed in 

section IV.B.4 and IV.B.5) under one 


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29The rationale for finalizing the use of the PWEI 

and the number of monitors required through its 

application are discussed in section III.B.4. 

30CBSAs are defined by the U.S. Census Bureau, 

and are comprised of both Metropolitan Statistical 

Areas and Micropolitan Statistical Areas (http:// 

www.census.gov). 

particular condition. A source-oriented 

monitor may be sited outside of the 

parent CBSA, whose PWEI value 

triggered required monitoring, if that 

monitor is characterizing the location of 

expected maximum concentration of a 

source inside that parent CBSA. If a 

State chooses to exercise this flexibility 

in source-oriented monitor siting, the 

State must provide clear rationale for 

their choice in their annual monitoring 

plan, which is subject to EPA regional 

approval. If the source-oriented monitor 

is to be placed in another State, such as 

the example provided by the State of 

Ohio in the public comments above, the 

two States are responsible for 

collaboration on the location and 

operation of that monitoring site. 

Further, due to the broadened 

objectives of the final network design, 

EPA also is finalizing the provision that 

an NCore SO2 monitor within a CBSA 

(where a CBSAs PWEI value triggered 

required monitoring) can be counted 

towards meeting the minimum 

monitoring requirements in this 

rulemaking (discussed in section IV.B.4) 

because they can meet some of the 

expanded objectives of the network. 

NCore sites are intended to provide 

long-term data for air quality trends 

analysis, model evaluation, and, for 

urban sites, tracking metropolitan air 

quality statistics, and therefore are 

appropriate to allow to count towards 

minimum monitoring requirements 

under the revised monitoring scheme. 

Finally, EPA strongly encourages 

State and local air agencies to consider 

using required monitoring, as 

appropriate, to characterize those 

sources which are not as conducive to 

dispersion modeling and to assess 

population exposure. Sources that are 

not conducive to dispersion modeling 

include (1) sources classified as nonpoint sources (a.k.a. ‘‘area-sources’’) 

such as shipping ports, (2) a source 

situated in an area of complex terrain 

and/or situated in a complex 

meteorological regime, and (3) locations 

that have multiple, relatively small 

sources with overlapping plumes. 

4. Final Monitoring Network Design 

The use of a hybrid analytic approach 

(discussed above in section III and 

IV.B.1) makes it unnecessary for the 

final monitoring network design to be 

distinctly focused on monitoring 

locations of expected maximum 

concentration (and thus be primarily 

source-oriented), as discussed in section 

IV.B.3 above. Instead, with the dual use 

of modeling and monitoring for 

designations, the final monitoring 

network is designed to provide 

flexibility for required monitors to 

address the multiple monitoring 

objectives just discussed in the 

preceding section. This flexibility in 

monitoring objectives is in response, in 

part, to the many public comments 

received from States (e.g., NACAA and 

six other States), industry (API, EPRI, 

UARG, and eight other groups), and 

from the American Thoracic Society 

(ATS), urging EPA to ensure that some 

or all of the required monitors be sited 

and suited to characterize population 

exposure and, from many of these same 

commenters, to allow flexibility in 

implementing the siting requirements 

for the monitors. Under a hybrid 

approach, and the different monitoring 

objectives resulting thereof, the final 

monitoring network design also does 

not need to be a two-prong approach 

like the one proposed. Therefore, EPA is 

adopting a modified version of the first 

prong of the proposed network design, 

which will use PWEI values to require 

monitors in certain CBSAs where there 

is increased coincidence of population 

and SO2 emissions. There is no second 

prong in the final network design by 

which monitors are required based on a 

State’s individual contribution to the 

national anthropogenic SO2 inventory, 

as was proposed. 

The final monitoring network design 

requires monitoring in CBSAs based on 

calculated PWEI values, where a PWEI 

shall be calculated (as discussed in 

section IV.B.5 below) for each CBSA. 

For any CBSA with a calculated PWEI 

value equal to or greater than 1,000,000, 

a minimum of three SO2 monitors are 

required within that CBSA. This 

requirement remains the same as 

proposed. For any CBSA with a 

calculated PWEI value equal to or 

greater than 100,000, but less than 

1,000,000, a minimum of two SO2

monitors are required within that CBSA. 

For any CBSA with a calculated PWEI 

value equal to or greater than 5,000, but 

less than 100,000, a minimum of one 

SO2 monitor is required within that 

CBSA. EPA has adjusted the thresholds 

for requiring one or two monitors in a 

CBSA and the rationale for this 

adjustment is explained more fully 

below in section IV.B.5. As just 

explained in section III.B.3, these 

monitors shall be sited to meet one or 

more of a number of monitoring site 

objectives, including the assessment of 

source impacts, highest concentrations, 

population exposure, general 

background, and regional transport. EPA 

believes that the monitors required 

within these PWEI breakpoints provide 

a reasonable minimum number of 

monitors in a CBSA, where there is a 

relatively increased coincidence of 

population and SO2 emissions and 

therefore increased potential for 

exposures, because we are directly 

accounting for both population and 

emissions that exist in individual 

CBSAs.29 EPA estimates that these 

minimum monitoring criteria (based on 

2008 population and 2005 NEI data) 

require 163 monitors within 131 CBSAs. 

EPA also intends for SO2 monitors at 

NCore stations to satisfy these minimum 

monitoring requirements. Based on 

analysis of proposed and approved 

NCore sites (as of April 2010), all of 

which are scheduled to be operational 

no later than January 1, 2011, EPA 

estimates that 52 of the total 80 SO2

monitors at NCore stations are within 

the 131 CBSAs that have required 

monitors based on their PWEI values. 

As a result, EPA estimates that between 

these minimum monitoring 

requirements and the NCore network, 

there will be at least 191 SO2 monitors 

operating across the country. 

5. Population Weighted Emissions Index 

In the proposal, EPA had introduced 

a metric based on population and 

emissions as a basis for locating 

monitors in the network. EPA 

anticipated that this metric would 

characterize the potential for exposure 

based on the proximity of source 

emissions to populations. The following 

paragraphs provide background, 

rationale, and details for the final 

changes of the calculation and use of the 

Population Weighted Emissions Index 

in determining minimum monitoring 

requirements. 

a. Proposed Use of the Population 

Weighted Emissions Index 

In the proposed network design 

approach, which utilized a two-prong 

network design, EPA created the 

Population Weighted Emissions Index 

(PWEI) in an attempt to focus 

monitoring resource where there was a 

higher proximity of population and SO2

emissions. In effect, areas with higher 

PWEI values have higher potential for 

population exposure to short-term SO2

emissions. EPA proposed that the PWEI 

be calculated using population and 

emissions inventory data at the Core 

Based Statistical Area (CBSA) 30 level to 

assign required monitoring for a given 

CBSA, with population and emissions 

being the relevant factors. To calculate 

the PWEI for a particular CBSA, using 


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the latest Census Bureau estimates, the 

population of a CBSA must be 

multiplied by the total amount of SO2

emissions in that CBSA. The CBSA 

emission value is in tons per year (using 

the latest available National Emissions 

Inventory [NEI] data), and is calculated 

by aggregating the county level 

emissions for each county in a CBSA. 

We then divide the resulting product of 

CBSA population and CBSA SO2

emissions by 1,000,000 to provide a 

PWEI value in more manageable units of 

millions of people-tons per year. 

With the change in the approach 

discussed in section III and section 

IV.B.1 above, and considering the final 

monitoring network design discussed in 

IV.B.4 above, the use of the PWEI from 

that which was proposed also changes. 

The following paragraphs discuss some 

of the public comments received on the 

general use and calculation of the PWEI; 

other comments that focused on the 

detailed application of the PWEI as 

proposed will be addressed in the 

response to comments document since 

our approach in applying the PWEI has 

changed. 

b. Public Comments 

EPA received a number of comments 

from State and local groups (e.g., 

NACAA and eight others) and industry 

(e.g., AQRL, ACC, and eight others) who 

generally agreed with the two-pronged 

network design concept which had the 

PWEI as a component. More 

specifically, some State commenters 

(e.g. NACAA, AK, FL, IL, NC, SC, and 

WI) expressed concern that the PWEI 

(along with the second prong of the 

proposed network design) created 

monitoring requirements that were 

‘‘duplicative’’ and also called for 

monitors in areas where they were not 

needed. Even amongst some of the 

commenters who generally agreed with 

the PWEI concept, some provided 

examples of where the PWEI appeared 

to be duplicative in its proposed 

application. One example was provided 

by the State of Florida, ‘‘in the case of 

Homosassa Springs, the [proposed 

network design] requires two monitors 

[in that CBSA as a result of the proposed 

use of the PWEI]. The driving source is 

the Crystal River Power Plant, with 

emissions in 2008 of over 85,000 tons 

per year of SO2. The next largest source 

in the CBSA has emissions of roughly 

two tons per year.’’ EPA believes that 

Florida is asserting that the one large 

source disproportionately drove the 

PWEI too high for that particular CBSA 

and only one monitor was actually 

needed. EPA notes that these particular 

comments on duplicative monitoring 

were made under the premise that all 

proposed required monitors would be 

sited in locations of expected maximum 

concentration, and therefore would be 

source-oriented in nature. As a result, 

these commenters believed it was 

necessary that a waiver provision be 

included if they could show that the 

required number of monitors was too 

many, as in Florida’s example. 

As discussed in section IV.B.4 above, 

a hybrid approach results in a final 

network design with a reduced number 

of required monitors from the number 

proposed, a different application of the 

PWEI, and provides flexibility in 

meeting additional monitoring 

objectives for the required monitors, 

making the need for a waiver from the 

minimally required monitors 

unnecessary. If a CBSA is required to 

have multiple monitors now, those 

monitors are not specifically required to 

be located near sources where 

maximum concentrations of SO2 are 

expected to occur. Instead, they can be 

sited at different locations to fulfill a 

variety of objectives, although, as noted 

in secion IV.B.3 above, EPA is strongly 

encouraging States to consider 

monitoring near sources not conducive 

to dispersion modeling and for 

characterization of population 

exposures. 

EPA received comments from 

Michigan, South Carolina, and CBD 

requesting clarification on the logic 

behind the proposed PWEI thresholds, 

or breakpoints, by which three, two, 

one, or no monitors would be required 

in a given CBSA. In addition, some 

States (e.g., MI, MO, SC, and WI) and 

industry (e.g., LCA, LMOGA, and LPPA) 

suggested specific adjustments to the 

proposed application of the PWEI. For 

example, Michigan suggested that the 

required monitor breakpoint values be 

adjusted to the ‘‘natural breakpoints in 

the overall distribution’’. South Carolina 

suggested EPA identify a way to 

normalize the PWEI stating the PWEI 

would be more appropriate ‘‘* * * if it 

used a value that better addressed 

difference in area, population 

distribution, land use, number, types of 

sources, etc.’’ 

In the proposed network design, EPA 

selected the PWEI values, or 

breakpoints, to require one or more 

monitors based on the overall 

distribution of PWEI values across all 

CBSAs. Based on U.S. Census Bureau 

data (http://www.census.gov), there are 

approximately 939 CBSAs in the 

country. EPA proposed and now 

requires that a PWEI value be calculated 

for each of these CBSAs to determine if 

monitoring is required in that CBSA. 

Based on 2008 census estimates and the 

2005 NEI, the average CBSA PWEI value 

is 21,900 while the median value is only 

121. This indicates that a relatively 

small number of CBSAs with high PWEI 

values are driving the very upper end of 

the PWEI distribution. The proposed 

breakpoint where one monitor was 

required in a CBSA was a PWEI value 

of 5,000. EPA estimated that 131 out of 

939 CBSAs (∼14%) have a PWEI value 

of 5,000 or more. Further, these 131 

CBSAs occupy ∼98% of the sum of 

PWEI values across all 939 CBSAs, 

where high PWEI values indicate 

increased coincidence in population 

and SO2 emissions. Within this group of 

CBSAs with PWEI values of 5,000 or 

more, EPA considered the relative 

amounts of population, emissions, and 

general frequency of occurrence of 

relatively larger SO2 sources (such as 

those that emit 100 tons per year or 

more) in selecting the breakpoints to 

require two and three monitors in a 

CBSA for the proposed network design. 

These considerations were made in an 

effort to apply a nationally applicable 

process by which to require a minimum 

number of monitors for an area, which 

all were to be sited in locations of 

expected maximum concentration, and 

therefore likely source-oriented 

monitors. In regard to the comments 

suggesting modification to the 

calculation or to normalize the PWEI, 

EPA believes that the proposed 

calculation, under a hybrid analytical 

approach, is still most appropriate. 

Under a hybrid analytical approach, 

States have the flexibility to move 

monitoring resources where needed 

within CBSAs that have a high 

coincidence of population and 

emissions instead of only being able to 

site monitors to characterize sources. 

States have the option to consider 

additional factors such as those listed in 

South Carolina’s comments above in 

further identifying where required 

monitoring may be most appropriate in 

their areas with required monitoring. 

Several States (e.g. NESCAUM, 

NYSDEC, and PADEP) suggested 

abandoning the PWEI concept altogether 

and instead using some form of 

emissions-only approach to require 

monitors. For example, NESCAUM, who 

generally supported a ‘‘hot-spot’’ 

monitoring approach, suggested that the 

PWEI be abandoned and EPA instead 

‘‘* * * adopt an emissions-only 

approach, resulting in fewer CBSA 

monitors. We [NESCAUM] suggest a 

threshold of 50,000 tpy CBSA SO2

emissions to trigger the first CBSA 

monitor and a second CBSA monitor 

required when emissions exceed 

200,000 tpy.’’ NESCAUM states that the 

proposed use of the PWEI ‘‘* * * can 


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31 In simulating NYSDEC’s suggested network 

design, EPA assumed that no CBSA would have 

more than one monitor. According to the 2005 NEI, 

there are 162 sources emitting 20,000 tpy or more 

a year. 93 of those sources are estimated to be inside 

CBSAs that have emissions of 50,000 tpy, leaving 

approximately 62 sources that would need a 

monitor to satisfy NYSDEC’s suggested network 

design. 

result in multiple monitors in large 

cities that have relatively small CBSA 

SO2 emissions, or no monitor in a CBSA 

with large emissions.’’ NYSDEC suggests 

that the proposed approach, using the 

PWEI, is ‘‘* * * not more predictive 

than using emissions data alone.’’ 

NYSDEC went on to suggest that 

monitors be required in CBSAs with 

aggregated emissions of 50,000 tons per 

year or more and that ambient 

monitoring be considered for point 

sources with 20,000 tons per year. 

PADEP made several suggestions on 

network design, with one that suggested 

monitoring in any CBSA ‘‘where there is 

a sulfur dioxide source or combination 

of sources within 50 miles emitting a 

total of at least 20,000 tons of SO2 per 

year * * *’’ 

EPA reviewed emissions and 2005 

NEI data and compared the suggestions 

provided by NESCAUM and NYSDEC to 

the requirement of the final network 

design. Under NESCAUM’s suggested 

design, EPA estimates there would be 75 

required monitors in 65 CBSAs. Of these 

65 CBSAs, 6 CBSAs that are not covered 

by the final network design would be 

included; however, 72 CBSAs that will 

have monitors under the final network 

design would otherwise not have 

monitors under NESCAUM’s design. 

EPA believes that the exclusion of those 

72 CBSAs would lead to too sparse a 

network to adequately meet the 

monitoring objectives of the network. 

Under NYSDEC’s suggested network 

design, EPA estimates that there would 

be a minimum of 65 monitors in the 

same 65 CBSAs of the NESCAUM 

suggested design. Further, if States 

ensured that monitors were placed near 

all sources emitting 20,000 tons per year 

(as NYSDEC suggested should be 

‘‘considered’’ for monitoring), there 

could be an additional 69 monitors.31

EPA believes that the final network 

design as discussed above in section 

IV.B.4, with the increased flexibility for 

monitors to meet multiple monitoring 

objectives (discussed in IV.B.3 above) 

including, among others, 

characterization of source impacts or 

population exposure, is better served 

using PWEI values to require monitors 

because it explicitly accounts for 

population to require and distribute 

monitors as compared to an emissionsonly approach. If there is reason for 

concern that other CBSAs or areas not 

included in the final network design, 

such as the six CBSAs that were 

included in the NESCAUM and 

NYSDEC suggested network designs 

noted above, warrant monitoring 

resources, States or the EPA Regional 

Administrator may take action to 

require monitoring in such areas. The 

authority of an EPA Regional 

Administrator to require additional 

monitoring above the minimum 

requirements is discussed in section 

IV.B.6 below. 

EPA received a number of comments 

from States (e.g., IA, NESCAUM, NC, 

NYSDEC, SC, and WI) and industry 

(e.g., CE, Dominion, EEI, LCA, LMOGA, 

LPPA, and UARG) raising concern over 

the way the PWEI is calculated. 

Specifically, many commenters in this 

group indicated that they believed that 

the 2005 NEI would be used in an 

exclusive or permanent fashion to 

calculate the PWEI, and that updated 

NEI data would not be used. For 

example, NESCAUM states that ‘‘EPA 

should not require States to rely solely 

on EPA’s inventories [for calculating the 

PWEI], such as the National Emissions 

Inventory (NEI), as they do not always 

have the updated information that is 

necessary for such regulatory decisions.’’ 

Wisconsin ‘‘* * * believes that States 

should be allowed to use their own 

annual point source inventories instead 

of EPA’s National Emissions Inventory 

(NEI) for evaluating emission sources. 

Wisconsin’s point inventory is updated 

annually and has a reporting threshold 

of five tons per year for SO2, making it 

more sensitive to changes in facility 

operations than the NEI, which is 

updated triennially.’’ UARG stated that 

their ‘‘primary concern with this 

network design is its reliance on old 

emissions data. For electric utilities 

which report their SO2 emissions to 

EPA annually, the use of more recent 

data would be appropriate.’’ 

EPA does not intend for relatively old 

emissions data to be used in calculating 

the PWEI values for individual CBSAs. 

As was detailed in the proposed 

regulatory text for 40 CFR Part 58 

Appendix D (74 FR 64880), EPA stated 

that ‘‘The PWEI shall be calculated by 

multiplying the population of each 

CBSA, using the most current census 

data, by the total amount of SO2 in tons 

per year emitted within the CBSA area, 

using an aggregate of the most recent 

county level emissions data available in 

the National Emissions Inventory for 

each county in each CBSA.’’ Although 

commenters suggested that there may be 

other resources from which emissions 

data may be obtained, particularly at the 

individual State level, the NEI is 

comprised of emissions data which is 

collected by EPA from the States 

themselves. The Air Emissions 

Reporting Requirements (40 CFR Part 

51), by which EPA sets out how States 

are to report their emission inventories, 

was recently revised in December of 

2008. That rulemaking was intended to 

provide enhanced options to States for 

emissions data collection and exchange 

and unify reporting dates for various 

categories of inventories. EPA notes that 

the NEI is updated in full every three 

years and the 2008 NEI is scheduled to 

be available by January 2011. States will 

have submitted their data by May 31, 

2010, before this rule is promulgated 

and published, and EPA will provide 

comment on these submittals during the 

summer of 2010. States will have an 

opportunity to revise their 2008 data 

submissions in the fall of 2010. In the 

triennial update, both point and 

nonpoint data are required to be 

submitted by States and are included in 

the inventory. Further, States are 

required to submit emissions data 

annually for all sources emitting 2,500 

tons per year or more of SO2 as well as 

for sources emitting other pollutants in 

excess of thresholds set for those 

pollutants. In all point source submittals 

to the NEI, States are also allowed to 

submit emissions data for sources of any 

emissions level, but are not required to 

do so. Starting with the 2009 NEI, the 

annual and triennial State NEI 

submittals will be due one year after the 

end of the emissions year. States have 

an additional opportunity to revise their 

submittals based on EPA comment in 

the spring of the following year, with 

EPA publishing the inventory no later 

than 6 months after the inventory 

submittal dates (18 months after the end 

of the emissions year). This approach 

and schedule is accelerated over past 

NEI schedules and has been designed as 

part of the development of the new 

Emission Inventory System (EIS). Rather 

than representing old emissions data, 

the NEI available through EIS represents 

a timely and appropriate source of 

emissions data. 

EPA believes that the process by 

which the NEI will be updated (through 

use of the EIS) will be adjusted in a 

manner that will allow for more 

frequent insertion of State supplied 

emissions data, allowing for a more upto-date inventory. EPA takes this 

opportunity to encourage States to 

supply all of their available emissions 

information to the NEI as soon as 

practicable. Therefore, EPA believes that 

the NEI is an appropriate and nationally 

representative source of emissions data 

by which PWEI calculations may be 


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made. PWEI calculations for all CBSAs 

will use the same year of data at any 

given time, and States, local agencies, 

and Tribes will have uniform 

opportunity for revising their emissions 

data for this purpose. EPA again 

encourages States to view the NEI 

submittals as their opportunity to 

submit their best available SO2 and 

other inventory data with the 

knowledge that it will be used for the 

purpose of PWEI values. 

c. Conclusions on the Use of the 

Population Weighted Emissions Index 

In the final network design, EPA has 

determined that it is appropriate to use 

PWEI values as the mechanism by 

which to require monitors in certain 

CBSAs, similar to its use in the first 

prong of the proposed two-prong 

network design. EPA believes that using 

the PWEI metric to inform where 

monitoring is required is more 

appropriate for the SO2 network design 

than utilizing a population-only or 

emissions-only type of approach, 

because it takes into account not just 

one factor, i.e., only population or only 

emissions, but instead takes into 

account the exposure from SO2

emissions to groups of people who are 

in greater proximity to such emissions. 

In the final rule, EPA is retaining the 

requirement to calculate the PWEI by 

multiplying the population of each 

CBSA, using the most current census 

data/estimates from the U.S. Census 

bureau, by the total amount of SO2 in 

tons per year emitted within the CBSA 

area, using an aggregate of county level 

emissions data available in the most 

recent published version of the National 

Emissions Inventory for each county in 

each CBSA. The resulting product shall 

be divided by one million, providing a 

PWEI value, the units of which are 

million persons-tons per year. For any 

CBSA with a calculated PWEI value 

equal to or greater than 1,000,000, a 

minimum of three SO2 monitors are 

required within that CBSA. For any 

CBSA with a calculated PWEI value 

equal to or greater than 100,000, but less 

than 1,000,000, a minimum of two SO2

monitors are required within that CBSA. 

For any CBSA with a calculated PWEI 

value equal to or greater than 5,000, but 

less than 100,000, a minimum of one 

SO2 monitor is required within that 

CBSA. EPA believes that the monitors 

required within these breakpoints 

provide a reasonable minimum number 

of monitors in a CBSA that considers 

the combination of population and 

emissions that exist in a CBSA. These 

criteria (based on 2008 population and 

2005 NEI data) are estimated to require 

163 monitors within 131 CBSAs. 

EPA has changed the PWEI 

breakpoint in the final rule at which two 

monitors are required in a CBSA to 

100,000 from the breakpoint of 10,000 

in the proposed network design based 

on multiple considerations. First, EPA 

changed the breakpoint because of a 

hybrid analytic approach and attendant 

changes in monitoring objectives (see 

section IV.B.3), with the result being 

that the monitoring network is no longer 

intended to be comprised primarily of 

source-oriented monitors that are sited 

at locations of expected maximum 

concentration. This change in objective 

of the network design allows fewer 

monitors to provide the necessary 

amount of ambient monitoring data EPA 

to meet the multiple monitoring 

objectives. Second, the breakpoint of 

100,000 occurs near a ‘‘natural’’ 

breakpoint in the PWEI distribution, a 

consideration that Michigan suggested, 

where the estimated 28 CBSAs with 

PWEI values of 100,000 or more occupy 

∼87% of the sum of PWEI values across 

all 939 CBSAs. Finally, EPA considered 

commenters’ assertion that the first 

prong of the proposed network design 

created duplicative monitoring in 

certain CBSAs. This duplicative 

monitoring is especially recognized in 

some CBSAs with relatively small 

populations and somewhat large 

emissions which are dominated by a 

single source (such as the Homosassa 

Springs, FL example discussed above). 

Raising the second breakpoint helps to 

alleviate some of the duplicative 

monitoring that many of the State 

commenters noted. 

EPA therefore is keeping the first and 

third breakpoints, which require one 

monitor in a CBSA having a PWEI value 

of 5,000 and three monitors in a CBSA 

having a PWEI value of 1,000,000. EPA 

believes maintaining these breakpoints 

along with the revised 100,000 PWEI 

breakpoint, will (1) ensure that highly 

populated areas will be monitored for 

ambient SO2 concentrations even if the 

emissions in that area are moderate, 

which is appropriate given the fact that 

the greater population creates increased 

potential for exposure to those moderate 

emissions, and (2) that those areas with 

higher emissions or emission densities, 

with moderate or modest populations 

will be monitored because those 

increased emissions are likely to have a 

significant impact on nearby 

populations. 

6. Regional Administrator Authority 

The following paragraphs provide 

background, rationale, and details for 

the final changes to Regional 

Administrator authority to use 

discretion in requiring additional SO2

monitors beyond the minimum network 

requirements. 

a. Proposed Regional Administrator 

Authority 

EPA proposed that the Regional 

Administrators will have discretion to 

require monitoring above the minimum 

requirements, as necessary, to address 

situations where the minimum 

monitoring requirements are not 

sufficient to meet monitoring objectives. 

EPA recognized that the minimum 

required monitors in the proposed twopronged network design were based on 

indicators that may not have always 

provided spatial coverage for all the 

areas that have SO2 sources. Although 

the network design and the objectives of 

the network design have changed from 

those that were proposed because of our 

contemplated use of a hybrid analytical 

approach, EPA believes it is still 

important for Regional Administrators 

to have the discretion, and authority, to 

require monitoring above the minimum 

requirements. Providing the RAs with 

this discretion will allow them to fill 

any identified gaps in meeting the 

monitoring objectives of the network. 

b. Public Comments 

Some commenters (e.g., LCA, 

LMOGA, LPPA, and South Carolina) 

expressed concerns with the proposed 

provision authorizing the Regional 

Administrator to require additional 

monitoring above the minimum 

requirements. The LCA, LMOGA, and 

LPPA stated that ‘‘the EPA’s proposal to 

allow the Regional Administrator 

discretion to require a State to add 

additional monitors is flawed in that it 

provides unfettered discretion. Criteria 

should be added * * * that limit such 

discretion and require the Regional 

Administrator to consider certain 

objective factors when determining 

whether to require any additional 

ambient SO2 monitors to the network.’’ 

South Carolina stated that ‘‘the Regional 

Administrators should not have the 

discretion to require monitoring above 

the requirements described in [the 

proposal for] Part 58 and its 

Appendices. State monitoring 

organizations must be given discretion 

to decide the appropriate use of 

resources to meet uniform monitoring 

requirements. Additional monitoring 

requirements should not be imposed 

without concurrence of the monitoring 

organization and additional funding that 

completely supports the additional 

costs.’’ 


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32Moreover, as explained in section IV.A, the 

existing FEM monitors in operation may continue 

to be used to monitor compliance with the NAAQS. 

c. Conclusions on Regional 

Administrator Authority 

The authority of Regional 

Administrators to require additional 

monitoring above the minimum 

required is not unique to the SO2

NAAQS. For example, Regional 

Administrators have the authority to use 

their discretion to require additional 

NO2 or Pb monitors (40 CFR Part 58 

Appendix D section 4.3.4 and 4.5, 

respectively) and to work with State and 

local air agencies in designing and/or 

maintaining an appropriate ozone 

monitoring network (40 CFR Part 58 

Appendix D section 4.1). EPA believes 

that the nationally applicable final 

network design, although somewhat 

dictated by local factors (population and 

emissions), may not account for all 

locations where monitors should be 

sited, including where potentially high 

concentrations of SO2 may be occurring. 

Examples include locations that have 

the potential to violate or contribute to 

violations of the NAAQS, areas that 

might have high concentrations of SO2

that are not characterized by modeling 

or have sources that are not conducive 

to modeling, and locations with 

susceptible and vulnerable populations. 

As a result, EPA believes it is important 

for Regional Administrators to have the 

authority to address possible gaps in the 

minimally required monitoring network, 

especially near sources or areas that are 

not conducive to modeling by granting 

them authority to require monitoring 

above the minimum requirements. 

However, in response to public 

comments, EPA notes that Regional 

Administrators would use this authority 

in collaboration with State agencies to 

design and/or maintain the most 

appropriate SO2 monitoring network to 

meet the needs of a given area. For all 

the situations where the Regional 

Administrators may require additional 

monitoring, it is expected that the 

Regional Administrators will work on a 

case-by-case basis with State or local air 

agencies. Further, any monitor required 

through the Regional Administrator and 

selected by the State agency, or any new 

monitor proposed by the State itself, is 

not done so with unfettered discretion, 

since any such action would be 

included in the Annual Monitoring 

Network Plan per § 58.10, which must 

be made available for public inspection 

or comment, and approval by the EPA 

Regional Administrator. 

Therefore, EPA is finalizing the 

proposal that Regional Administrators 

may use their authority to require 

monitoring above the minimum 

requirements, as necessary, in any area, 

to address situations where the 

minimally required monitoring network 

is not sufficient to meet monitoring 

objectives. In all cases in which a 

Regional Administrator may consider 

the need for additional monitoring, it is 

expected that the Regional 

Administrators will work with the State 

or local air agencies to evaluate 

evidence or needs to determine if a 

particular area may warrant additional 

monitoring. 

7. Monitoring Network Implementation 

The following paragraphs provide 

background, rationale, and details for 

the final approach for the monitoring 

network implementation. 

a. Proposed Monitoring Network 

Implementation 

EPA proposed that State and, where 

appropriate, local air monitoring 

agencies submit a plan for deploying 

SO2 monitors in accordance with the 

proposed requirements discussed above 

by July 1, 2011. EPA also proposed that 

the SO2 network be physically 

established no later than January 1, 

2013. EPA also proposed that the 

number of sites required to operate as a 

result of the Population Weighted 

Emissions Index (PWEI) values 

calculated for each CBSA be reviewed 

and revised for each CBSA through the 

5-year network assessment cycle 

required in § 58.10. 

b. Public Comments 

EPA received comments from the 

ALA, EDF, NRDC, and SC that 

supported ‘‘* * * a more accelerated 

deployment of new monitoring than the 

2013 target date proposed by EPA. The 

sooner monitors are in place, the sooner 

the public will experience the health 

benefits of the new standard.’’ However, 

EPA received comment from States (e.g., 

IA, MI, NC, SC and WI), industry (e.g., 

LCA, LMOGA, and LPPA) and public 

health and environmental groups (e.g., 

ALA, EDF, NRDC, and SC) expressing 

concern with the proposed deployment 

schedule of the proposed SO2 network 

in that it was too fast or needed to be 

phased in. The States of Iowa, South 

Carolina, and Wisconsin suggested that 

EPA allow the proposed network to 

deploy on a phased schedule. For 

example, South Carolina recommended 

a ‘‘phased implementation with largest 

source/highest probability population 

exposure areas designated for 

implementation in 2013 (some 

proportion of the highest PWEI 

monitors) and establishment of the 

remaining PWEI and the State level 

emissions triggered monitoring required 

the following year.’’ Meanwhile, the 

States of Michigan and North Carolina, 

along with the industry commenters 

LCA, LMOGA, and LPPA, suggested 

EPA reconsider implementation dates in 

light of the multiple rulemakings that 

impose mandates on States that have 

and will be occurring in the future. For 

example, North Carolina stated that 

‘‘EPA must keep in mind that it is 

simultaneously revising numerous 

ambient standards and associated 

monitoring requirements. EPA seems to 

view each of these proposals as 

independent actions; but the State and 

local agencies must consider the 

cumulative impact of EPA’s various 

regulatory actions on their ability to 

comply.’’ North Carolina goes on to say 

that ‘‘EPA must allow States the 

flexibility to prioritize among the new 

requirements to get community based 

monitors in place first and to establish 

the others as funding and personnel 

resources allow.’’ 

EPA believes that with the use of a 

hybrid analytical approach, the 

concerns raised by States and industry 

commenters suggesting a phased or 

delayed implementation are addressed 

because the final network minimum 

design requirements result in fewer 

monitors being required than in the 

proposed network design. EPA’s 

analysis of the existing network had 

indicated that a substantial number of 

monitors were not sited at locations of 

maximum concentrations. These 

monitors would have had to be relocated to count towards minimum 

monitoring requirements under the 

proposed monitoring-focused approach. 

Under a combined modeling and 

monitoring approach, the required 

monitors can be used to satisfy multiple 

monitoring objectives and therefore, 

many of the monitors in the existing 

network will satisfy the requirements in 

the final network design, eliminating 

any need for a phased or delayed 

network implementation. In regard to 

the suggestion by public health and 

environmental groups to speed up 

implementation, EPA notes that under a 

hybrid analytical approach much of the 

existing network will fulfill minimum 

monitoring requirements, and an 

accelerated schedule is not necessary; 

the network implementation date 

provides a balance between ensuring the 

minimally required network is fully in 

place in a reasonable amount of time 

and providing States adequate time to 

fulfill all the requirements in this 

rulemaking.32

EPA received comment on the 

frequency by which the minimally 


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33Note that some commenters supported more 

than one form of reported 5-minute data. 

required network will be reviewed and 

possibly adjusted based on updated 

population and emissions inventories. 

The State commenters listed above, and 

some others including NACAA, 

indicated that they believed that the 

proposal for reviewing the SO2 network 

every five years was intended to be a 

separate review from the required 5-year 

network assessments required in 

§ 58.10(d). NACAA stated ‘‘EPA 

proposes that the SO2 monitoring 

network be evaluated every five years. 

This is an unnecessary duplication of 

effort in light of the current 

requirements for the annual network 

plan and five year network review.’’ 

NACAA went on to say that ‘‘the current 

requirements [in § 58.10] should be 

regarded as the primary source of 

monitoring network information for all 

NAAQS pollutant monitoring, 

regardless of the pollutant.’’ 

EPA concurs with NACAA’s 

statements that the existing 

requirements for network assessment 

are an appropriate primary source of 

monitoring network information. In the 

proposal, EPA did not intend for a 

required 5-year review of the SO2

network to be an additional effort on top 

of the existing required network 

assessments but instead to be included 

as part of the 5-year assessment in 

§ 58.10(d). EPA notes that CBSA 

populations and emissions inventories 

change over time, suggesting a need for 

periodic review of the monitoring 

network. At the same time, EPA 

recognizes the advantages of a stable 

monitoring network. However, after 

considering comments, EPA is not 

finalizing the proposed language for 40 

CFR Part 58 Appendix D, section 

4.4.3(2) which simply referenced back 

to § 58.10. This proposed text it is not 

needed and appears to simply cause 

confusion. EPA asserts that the existing 

requirements in § 58.10 provide a 

sufficient and appropriate mechanism 

for network updates and assessment. 

c. Conclusions on Monitoring Network 

Implementation 

Based on the public comments, and 

due to the contemplated use of a hybrid 

analytical approach, EPA is finalizing, 

as was proposed, that State and, where 

appropriate, local air monitoring 

agencies submit a plan for deploying 

SO2 monitors in accordance with the 

proposed requirements presented below 

by July 1, 2011. Minimally required SO2

monitors shall be physically established 

no later than January 1, 2013. 

C. Data Reporting 

The following paragraphs provide 

background, rationale, and details for 

monitor data reporting requirements. 

a. Proposed Data Reporting 

Controlled human exposure studies 

indicate that exposures to peaks of SO2

on the order of 5 to 10 minutes result 

in moderate or greater decrements in 

lung function and/or respiratory 

symptoms in exercising asthmatics 

(section II.B.1 above, ISA section 5.2, 

REA section 7.2.3, and REA section 

10.3.3.2). As a result, the 1-hour 

standard is intended to protect against 

short term exposures, including 

exposures on the order of 5 minutes up 

to 24 hours, as is discussed in section 

II.F.2 above. Therefore, in support of the 

revised NAAQS and its intent, EPA 

proposed that State and local agencies 

shall report to AQS the maximum 5- 

minute block average of the twelve 5- 

minute block averages of SO2 for each 

hour. This 5-minute block reporting 

requirement is in addition to the 

existing requirement to report the 1- 

hour average. In addition, EPA solicited 

comment on the advantages and 

disadvantages (including associated 

resource burdens) of alternatively 

requiring State and local agencies to 

report all twelve 5-minute SO2 values 

for each hour or the maximum 5-minute 

concentration in an hour based on a 

moving 5-minute averaging period 

rather than time block averaging. 

EPA also proposed Data Quality 

Objectives (DQOs) for the SO2 network. 

DQOs generally specify the tolerable 

levels for potential decision error used 

as a basis for establishing the quality 

and quantity of data needed to support 

the objectives of the monitors. EPA 

proposed the goal for acceptable 

measurement uncertainty for SO2

methods to be defined as an upper 90 

percent confidence limit for the 

coefficient of variation (CV) of 15 

percent for precision and as an upper 95 

percent confidence limit for the absolute 

bias of 15 percent for bias. 

b. Public Comments 

EPA received many comments on the 

reporting of 5-minute data values. The 

comments generally fell into one of the 

following categories: 33 (1) Those State, 

public health, and environmental 

groups who supported the proposed 

requirement to report the maximum 5- 

minute block average of the twelve 5- 

minute block averages of SO2 for each 

hour (e.g., Missouri, NESCAUM, North 

Carolina, ALA, EJ, EDF, NRDC, and SC), 

(2) those State, public health, and 

environmental groups who supported 

the reporting of all twelve 5-minute 

averages of each hour (e.g., Kentucky, 

NYSDEC, AQRL, ALA, ATS, CBD, EJ, 

EDF, NRDC, and SC), (3) those State, 

public health, and environmental 

groups who supported reporting the 

maximum 5-minute concentration in an 

hour based on a moving 5-minute 

average (e.g., South Dakota, ALA, CBD, 

EJ, EDF, NRDC, and SC), and (4) those 

State and industry groups who did not 

support the reporting of any 5-minute 

data (e.g., Iowa, South Carolina, LEC, 

and RRI Energy). 

Public health and environmental 

groups (e.g. ALA, CBD, EJ, EDF, NRDC, 

and SC) supported an approach where 

5-minute data must be reported. 

However, these commenters were 

flexible in their position and supported 

multiple forms or types of 5-minute data 

reporting. The ALA, EJ, EDF, NRDC, and 

SC stated that ‘‘we support the proposed 

requirement for State and local 

monitoring agencies to report both 

hourly average and maximum 5-minute 

averages out of the twelve 5-minute 

block averages of SO2 for each hour.’’ 

They also expressed a preference for 

alternative 5-minute data reporting 

stating that they ‘‘strongly prefer that 

States be required to report the peak 5- 

minute concentrations of SO2 based on 

a rolling average.’’ Similarly, CBD stated 

that ‘‘* * * EPA should require that 

State and local agencies report all 12 

five-minute SO2 values for each hour in 

addition to 1-hour averages. Where 

possible, EPA also should require 

reporting of rolling five-minute averages 

rather than block data * * *’’ 

Missouri generally supported the 

proposed requirement to report the 

maximum 5-minute average in the hour, 

saying ‘‘it is not a problem to report both 

the hourly average and the maximum 5- 

minute block average.’’ Nevertheless, 

Missouri went on to note constraints, 

stating that ‘‘* * * [their] data logger 

and associated software do not have the 

capability to report all twelve 5-minute 

SO2 values for each hour’’ and that they 

‘‘* * * could not do this without 

software being developed for this 

purpose and it could be time intensive 

to validate this data.’’ 

Kentucky did not support the 

proposal to report the maximum 5- 

minute data block in the hour because 

of the limitations in their data 

acquisition systems. They explained 

that ‘‘the data acquisition system used 

by the [State] does not have the 

capability to automatically report the 

maximum 5-minute block of data from 

an hour concentration. [State] personnel 

would have to manually determine that 


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34The REA assessed exposure and risks 

associated with 5-minute SO2 concentrations above 

5-minute health effect benchmark levels derived 

from controlled human exposure studies. In the 

analyses, the REA noted that very few State and 

local agencies report ambient 5-minute SO2 data 

(REA, section 10.3.3.2) and that the lack of 5-minute 

data necessitated the use of statistically estimated 

5-minute SO2 data in order to expand the 

geographic scope of the exposure and risk analyses 

(REA, section 7.2.3). 

value and then manually enter that data 

into AQS.’’ Kentucky goes on to suggest 

that ‘‘the only feasible option for the 

[State] to submit 5-minute data to AQS 

would be to submit all twelve 5-minute 

blocks of data for each hour to AQS.’’ 

South Dakota stated that its ‘‘* * * 

preference would be to report the 

maximum 5-minute average for each 

hour calculated using a 5-minute rolling 

average.’’ South Dakota goes on to state 

that ‘‘* * * while doubling the work 

required to validate data and load the 

data into AQS, the additional data 

should help determine if the selected 

standard concentration level has 

achieved the necessary reduction in 

high concentration 5-minute levels and 

provide the necessary data for further 

study of health impacts * * *’’ 

South Carolina stated that it ‘‘* * * 

does not support mandatory reporting of 

5-minute averages in addition to the 1- 

hour average required for comparison to 

the standard. The validation and 

reporting of 5-minute averages imposes 

a significant additional burden on the 

reporting organization and its Quality 

System.’’ Iowa, who also did not support 

any form of 5-minute data reporting 

stated that ‘‘the five-minute data is not 

used to determine compliance with the 

NAAQS, and represents ancillary data,’’ 

and that ‘‘validating and uploading the 

five-minute data will take at least as 

much staff time as generating the hourly 

data used for compliance.’’ As a result, 

Iowa states that ‘‘if EPA determines that 

five-minute data is needed, we 

recommend that EPA require the 

maximum five-minute average in each 

hour, rather than all twelve five-minute 

averages, in order to reduce the burden 

associated with generation of the 

ancillary data set.’’ 

With regard to the proposed DQOs, 

EPA received comments from some 

States (e.g., Kentucky, North Carolina, 

NYSDEC, and South Carolina) providing 

general support for the goals for 

acceptable measurement uncertainty for 

precision and bias. North Carolina 

stated that the ‘‘* * * precision and bias 

measurement uncertainty criteria 

should emulate those that have been 

established for other recent NAAQS and 

NCore pollutants.’’ NYSDEC stated that 

‘‘the proposal does not seem 

unreasonable, however these statistics 

are now expressed in terms of 

confidence limits: Precision—90% 

confidence of a CV of 15% and Bias— 

95% confidence of a CV of 15%.’’ 

NYSDEC raises concern that ‘‘* * * the 

results are now dependent on the 

number of audits performed. This is 

highly variable because some agencies 

run automatic audits every night, 

[while] others use the old standard of 

once every 2 weeks.’’ 

In regard to comments on the 

proposed DQOs, EPA notes that the 

precision and bias estimation technique 

on which NYSDEC comments were 

focused were proposed and adopted in 

the monitoring rule promulgated on 

October 6, 2006 and EPA did not intend 

to reopen those requirements for 

comment. Moreover, SO2 precision and 

bias estimates have been performed in 

this manner for the past four years and 

there have been no adverse effects on 

data quality at the minimum required 

level of performance checks every two 

weeks. The statistics for the precision 

and bias estimates and the DQO goals 

are based on the accumulation of the 

one-point precision checks aggregated at 

the frequencies required in CFR which 

is every two weeks. Any organization 

performing more frequent checks (such 

as every night) would accumulate more 

data for the precision and bias 

estimates, have higher confidence in the 

data, and would have less potential for 

outliers or higher than normal values 

effecting the precision and bias 

estimate. In addition, monitoring 

organizations running precision checks 

every 24 hours would be more able to 

control data quality to meet the DQO 

goals than organizations running the 

check every two weeks. 

c. Conclusions on Data Reporting 

EPA received a fairly diverse set of 

comments on the appropriateness of 

reporting 5-minute data and in what 

particular format it may be provided in. 

EPA has considered the comments by 

the States regarding validation of 

potentially 13 data values per hour 

(instead of 1 or 2) and some States’ lack 

of data acquisition capacity or 

processing capability to report any 

particular type of 5-minute value. EPA 

believes that in light of these comments, 

adopting a requirement for continuous 

SO2 analyzers to report all twelve 5- 

minute values or a rolling 5-minute 

value does not appear to provide 

enough added value for the potential 

increased burden on States, such as 

increased staff time dedicated to data 

processing and QA, or in improving or 

adjusting data acquisition capabilities. 

However, EPA also believes that 

obtaining some form of 5-minute data is 

appropriate because such data have 

been critical to this NAAQS review, and 

are anticipated to be of high value to 

inform future health studies and, 

subsequently, future SO2 NAAQS 

reviews.34 Indeed, as noted earlier, it 

was EPA’s failure to adequately explain 

the absence of protection from elevated 

short-term (5- to 10-minute exposure) 

SO2 concentrations for heavily breathing 

asthmatics that occasioned the remand 

of the 1996 SO2 primary NAAQS 

(American Lung Association, 134 F.3d 

at 392). This belief is supported further 

by the expectation that a significant 

portion of the monitors operating to 

satisfy the final monitoring network 

design will likely be sited for 

population exposures, which have 

traditionally provided ambient data that 

is often utilized by epidemiologic health 

studies. Therefore, EPA is finalizing the 

requirement that State and local air 

agencies operating continuous SO2

analyzers shall report the maximum 5- 

minute block average out of the twelve 

5-minute block averages in each hour, 

for each hour of the day, and that State 

and local air agencies operating any 

type of SO2 analyzer shall report the 

integrated 1-hour average value, as was 

proposed. EPA encourages States 

capable of reporting all twelve 5-minute 

data blocks in an hour to report such 

data to AQS. AQS is currently set-up to 

take the 5-minute maximum value in an 

hour under parameter code 42406 and 

can take all twelve 5-minute values 

under parameter code 42401 (with a 

duration code of H). EPA notes that if 

a State were to choose to submit all 

twelve 5-minute blocks in the hour, by 

default, they would be submitting the 

maximum 5-minute data block within 

that hour, although they have not 

singled out that particular value. Since 

the 5-minute data is not directly being 

used for comparison to the NAAQS, 

EPA believes that any State electing to 

submit all twelve 5-minute values is 

still satisfying the intent of having the 

maximum 5-minute value reported. 

Therefore, if a State chooses to submit 

all twelve 5-minute values in an hour, 

they will be considered to be satisfying 

the data reporting requirement of 

submitting the maximum 5-minute 

value in an hour, and they do not have 

to separately report the maximum 5- 

minute value from within that set of 

data values to AQS under parameter 

code 42406. 

EPA proposed new regulation text for 

40 CFR Part 58 Appendix C, which 

would have added section 2.1.2 that 

would have required any SO2 FRM or 


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FEM used for making NAAQS decisions 

to be capable of providing both 1-hour 

and 5-minute averaged concentration 

data. EPA is not finalizing this proposed 

language, as the manual wet-chemistry 

pararosaniline reference method cannot 

provide 5-minute data. Therefore, the 

proposed language is inappropriate. 

However, both the UVF FEM and the 

new UVF FRM continuous methods are 

capable of providing 5-minute averaged 

data. As a result, the language in 

58.12(g) and 58.16(g) requiring 5-minute 

SO2 data has been adjusted to 

appropriately specify that only those 

States operating continuous FRM or 

FEMs are required to report the 

maximum 5-minute data value for each 

hour. 

With regard to acceptable 

measurement uncertainties, EPA 

reviewed summary data for each 

Primary Quality Assurance Organization 

(PQAO) in the 2008 Data Quality 

Indicator Report on SO2 data within the 

2008 Criteria Pollutant Quality Indicator 

Summary Report for AQS Data (http:// 

www.epa.gov/ttn/amtic/qareport.html). 

Of the 100 PQAOs in the report, none 

of those organizations had summary CV 

or bias values exceeding 10 percent. 

Thus, EPA believes that the SO2

network can and does easily attain 

measurement uncertainty criteria more 

stringent than the finalized goal values 

and the monitoring required under the 

final network design should be able to 

maintain this level of performance. 

Therefore, in consideration of comments 

and existing quality assurance data, EPA 

is changing the final goals from those 

which were proposed for acceptable 

measurement uncertainty for SO2

methods to be defined for precision as 

an upper 90 percent confidence limit for 

the coefficient of variation (CV) of 10 

percent and for bias as an upper 95 

percent confidence limit for the absolute 

bias of 10 percent. 

V. Initial Designation of Areas for the 

1-Hour SO2 NAAQS 

This section of the preamble further 

addresses the process under which EPA 

intends to identify whether areas of the 

country attain or do not attain or are 

‘‘unclassifiable’’ regarding the new 1- 

hour SO2 NAAQS. After EPA establishes 

a new NAAQS, the CAA directs States 

and EPA to take this first step, known 

as the ‘‘initial area designations,’’ in 

ensuring that the NAAQS is ultimately 

attained. 

We are revising our discussion of an 

expected approach toward issuing 

initial area designations in response to 

comments we received on the proposed 

rule’s treatment of monitoring and 

modeling (both generally and in the 

specific context of designations), and to 

make the expected process more 

consistent with our historical approach 

to implementing the SO2 NAAQS. A 

revised anticipated approach for issuing 

designations logically follows from our 

revised hybrid approach to monitoring 

and modeling as discussed above in 

sections III and IV. It would also affect 

a revised expected implementation 

approach that we later discuss in 

section VI. 1. Designations. 

a. Clean Air Act Requirements 

The CAA requires EPA and the States 

to take steps to ensure that the new 

NAAQS are met following 

promulgation. The first step is for EPA 

to identify whether areas of the country 

meet, do not meet, or cannot yet be 

classified as either meeting or not 

meeting the new NAAQS. Section 

107(d)(1)(A) provides that, ‘‘By such 

date as the Administrator may 

reasonably require, but not later than 1 

year after promulgation of a new or 

revised NAAQS for any pollutant under 

section 109, the Governor of each State 

shall * * * submit to the Administrator 

a list of all areas (or portions thereof) in 

the State’’ that should be designated as 

nonattainment, attainment, or 

unclassifiable for the new NAAQS. 42 

U.S.C. 7407(d)(1)(A)(i)–(iii). Section 

107(d)(1)(B)(i) further provides, ‘‘Upon 

promulgation or revision of a NAAQS, 

the Administrator shall promulgate the 

designations of all areas (or portions 

thereof) * * * as expeditiously as 

practicable, but in no case later than 2 

years from the date of promulgation. 

Such period may be extended for up to 

one year in the event the Administrator 

has insufficient information to 

promulgate the designations within 2 

years.’’ 42 U.S.C. 7407(d)(1)(B)(i). 

Under CAA section 107(d)(1)(B)(ii), 

no later than 120 days prior to 

promulgating designations, EPA is 

required to notify States of any intended 

modifications to their boundaries as 

EPA may deem necessary, and States 

will have an opportunity to comment on 

EPA’s tentative decision. Whether or not 

a State provides a recommendation, the 

EPA must promulgate the designation 

that it deems appropriate. 42 U.S.C. 

7407(d)(1)(B)(ii). 

Accordingly, since the new 1-hour 

SO2 NAAQS is being promulgated 

today, Governors should submit their 

initial SO2 designation 

recommendations to EPA no later than 

June 2, 2011. If the Administrator 

intends to modify any State’s boundary 

recommendation, the EPA will notify 

the Governor no later than 120 days 

prior to designations or, February 2012. 

States that believe the Administrator’s 

modification is inappropriate will have 

an opportunity to demonstrate why they 

believe their recommendation is more 

appropriate before designations are 

finalized in June 2012. 

For initial designations that will be 

finalized in June 2012, States should use 

monitoring data from the existing SO2

network for the years 2008–2010, as 

well as any refined SO2 dispersion 

modeling (see Appendix W to 40 CFR 

Part 51) for sources that may have the 

potential to cause or contribute to a 

NAAQS violation, provided that it is 

recent and available. EPA will then 

issue designations based on the record 

of information for that area. Under our 

anticipated approach, an area that has 

monitoring data or refined modeling 

results showing a violation of the 

NAAQS would be designated as 

‘‘nonattainment.’’ An area that has both 

monitoring data and appropriate 

modeling results showing no violations 

would be designated as ‘‘attainment.’’ 

All other areas, including those with 

SO2 monitors showing no violations but 

without modeling showing no 

violations, would be designated as 

‘‘unclassifiable.’’ Areas with no SO2

monitors at all i.e., ‘‘rest of State,’’ would 

be designated as ‘‘unclassifiable’’ as 

well. 

b. Approach Described in Proposal 

In the proposed rule’s preamble, we 

explained that we had proposed a new 

SO2 ambient monitoring network, with 

new monitors expected to be deployed 

no later than January 2013. We also 

explained that we expected compliance 

with the new NAAQS to be determined 

based on 3 years of complete, quality 

assured, certified monitoring data. We 

further explained that we did not expect 

newly-cited monitors for the proposed 

network to generate sufficient 

monitoring data for us to use in 

determining whether areas complied 

with the new NAAQS by the statutory 

deadline to complete initial 

designations. Therefore, we explained, 

we intended to complete designations 

by June 2012 based on 3 years of 

complete, quality assured, certified air 

quality monitoring data as generated 

from the current monitoring network. 

Consequently, we discussed our 

expectations to base initial designations 

on air quality data from the years 2008– 

2010 or 2009–2011, from SO2 monitors 

operating at current locations, which we 

expected to continue through 2011. 

While those monitors are generally sited 

to measure 24-hour and annual average 

SO2 concentrations, we noted that they 

all report hourly data, and we estimated 

that at least one third of those monitors 

might meet the proposed network 


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design requirements and not need to be 

moved. We explained that if any 

monitor in the current network 

indicated a violation of the new 1-hour 

NAAQS, we would intend to designate 

the area as ‘‘nonattainment.’’ We further 

explained that if a monitor did not 

indicate a violation, our designation 

decision for the area would be made on 

a case-by-case basis, with one 

possibility being a designation of 

‘‘unclassifiable.’’ 

We also explained that while the CAA 

section 107 designation provisions 

specifically address States, we intended 

to follow the same process for Tribes to 

the extent practicable, pursuant to CAA 

section 301(d), 42 U.S.C. 7601(d), and 

the Tribal Authority Rule, 40 CFR part 

49. 

c. Comments 

Several commenters stated that the 

EPA did not provide nonattainment 

boundary guidance in the proposed rule 

and argued that guidance should be 

developed. Commenters also stated that 

EPA should consider boundaries that 

are less than the Core Based Statistical 

Area (CBSA), and perhaps even smaller 

than the county boundary (State of 

Michigan, Sierra Club). 

In response, we note that the CAA 

requires that the EPA designate as 

‘‘nonattainment’’ any area that does not 

meet (or contributes to an area that does 

not meet) the NAAQS. 42 U.S.C. 

7407(d)(1)(A)(i). States with monitored 

or modeled SO2 violations will need to 

recommend an appropriate 

nonattainment boundary that both 

includes sources contributing to that 

violation, as well as informs the public 

of the extent of the violation. For 

purposes of determining nonattainment 

boundaries, the EPA expects to consider 

the county line as the presumptive 

boundary for SO2. This would be 

consistent with our approach under 

other NAAQS. States recommending 

less-than-countywide nonattainment 

boundaries should provide additional 

information along with their 

recommendation, demonstrating why a 

smaller area is more appropriate, as we 

have advised for other NAAQS. If States 

request it, EPA may develop additional 

guidance on the factors that States 

should consider when determining 

nonattainment boundaries. 

In addition, as further discussed in 

section IV.B above, in the SO2 NAAQS 

proposal, we proposed a monitoringfocused approach for comparison to the 

new NAAQS. The proposed network 

would have required approximately 348 

monitors nationwide to be sited at the 

locations of maximum concentration. 

Numerous State and local government 

commenters expressed concerns 

regarding the perceived burdens of 

implementing the proposed monitoring 

network and the sufficiency of its scope 

for purposes of identifying violations. 

Some of these commenters (the City of 

Alexandria, and the States of Delaware, 

North Carolina and Pennsylvania) 

suggested using modeling to determine 

the scope of monitoring requirements, 

or favored modeling over monitoring to 

determine compliance with the NAAQS. 

Partly in response to these comments, 

and after reconsidering the proposal’s 

monitoring-focused approach, 

specifically regarding how we have 

historically implemented SO2

designations, we now anticipate taking 

a revised approach toward designations, 

using a hybrid analytic approach that 

combines the use of monitoring and 

available modeling to assess compliance 

with the new 1-hour SO2 NAAQS. We 

discuss a revised expected approach 

toward designations below, and further 

discuss in section VI how we expect a 

hybrid approach to affect other 

implementation activities. 

d. Expected Designations Process 

As discussed in sections III and IV of 

this preamble, in response to the 

comments and after reviewing our 

historical SO2 implementation practice, 

we intend to use a hybrid analytic 

approach for assessing compliance with 

the new 1-hour SO2 NAAQS for initial 

designations. We also believe that a 

hybrid approach is more consistent with 

our historical approach and 

longstanding guidance toward SO2

NAAQS designations and 

implementation than what we originally 

proposed. Technically, for a short-term 

1-hour standard, it is more appropriate 

and efficient to principally use 

modeling to assess compliance for 

medium to larger sources, and to rely 

more on monitoring for groups of 

smaller sources and sources not as 

conducive to modeling. 

In cases where there is complete air 

quality data from FRM and FEM SO2

monitors, that data would be considered 

by EPA in designating areas as either 

‘‘attainment’’ or ‘‘nonattainment’’ for the 

new SO2 NAAQS. See Appendix T to 

Part 50 section 3b. In addition, in cases 

where a State submits air quality 

modeling data that are consistent with 

our current guidance or our expected 

revisions thereto, and which indicates 

that an area is attaining the standard or 

violating the standard, these data may 

support recommendations of 

‘‘attainment’’ or ‘‘nonattainment.’’ As 

explained in section IV above, we 

would not consider monitoring alone to 

be an adequate, nor the most accurate, 

tool to identify all areas of maximum 

concentrations of SO2. In the case of 

SO2, we further believe that monitoring 

is not the most cost-efficient method for 

identifying all areas of maximum 

concentrations. 

Due to the necessarily limited spatial 

coverage provided by any monitoring 

regime, and the strong source-oriented 

nature of SO2 ambient impacts, we 

recognize that using this more 

traditional approach in designations, 

would be more likely to identify a 

greater number of potential instances of 

nonattainment, if areas were to 

immediately conduct modeling of 

current source emissions, as compared 

to the approach we discussed in the 

proposed rule. As discussed in section 

III, forthcoming national and regional 

rules, such as the pending Industrial 

Boilers ‘‘Maximum Achievable Control 

Technology’’ (MACT) standard under 

CAA section 112(d), are likely to result 

in significant SO2 emissions reductions 

in the next three to four years. A limited 

qualitative assessment of preliminary 

modeling of some sample facilities that 

would be covered by those rules 

indicates that well-controlled facilities 

should meet the new SO2 NAAQS. 

However, there are some exceptions. 

These exceptions include unique 

sources with specific source 

characteristics that contribute to higher 

ambient impacts (short stack heights, 

complex terrain, etc.). 

Again as described in section III, in 

order for States to conduct modeling on 

a large scale for the new 1-hour NAAQS, 

EPA expects additional guidance would 

be needed to clarify how to conduct 

dispersion modeling under Appendix W 

to support the implementation of the 

new 1-hour SO2 NAAQS, and how to 

identify and appropriately assess the air 

quality impacts of sources that 

potentially may cause or contribute to 

violations of the NAAQS. Our 

anticipated modeling guidance will 

provide for refined modeling that will 

better reflect and account for sourcespecific impacts by following our 

current Guideline on Air Quality 

Models, Appendix W to 40 CFR Part 51, 

with appropriate flexibility for use in 

implementation. EPA intends to solicit 

public comment on this modeling 

guidance. We expect it will take some 

time for EPA to issue this guidance, and 

believe that given the timing and 

substantial burden of having to model 

several hundred sources, it would not 

be realistic or appropriate to expect 

States to complete such modeling and 

incorporate the results in designation 

recommendations for the new 1-hour 

SO2 NAAQS that, under CAA section 


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35See SO2 Guideline Document, Office of Air 

Quality Planning and Standards, Research Triangle 

Park, NC 27711, EPA–452/R–94–008, February 

1994. 

107(d), are due to EPA within 1 year of 

the promulgation of the NAAQS. 

Consequently, we expect that in most 

instances, Governors will submit 

designation recommendations of 

‘‘unclassifiable’’ rather than conduct 

large-scale refined modeling of sources 

in advance of receiving our anticipated 

guidance. The absence of monitoring 

data showing violations for most areas, 

combined with the paucity of refined 

modeling of sources that have the 

potential to cause or contribute to 

violations of the NAAQS, will likely 

result in informational records that are 

insufficient to support initial 

designations of either ‘‘attainment’’ or 

‘‘nonattainment.’’ Under the Clean Air 

Act, in such a situation EPA is required 

to issue a designation for the area as 

‘‘unclassifiable.’’ However, we do not 

expect this result to delay expeditious 

attainment and maintenance of the new 

NAAQS, or to cause inappropriate, 

indefinite uncertainty regarding 

whether or not sources cause or 

contribute to NAAQS violations. 

As described more fully in section III 

above and in section VI below, EPA’s 

expected implementation approach 

would rely on the CAA section 110(a)(1) 

SIP obligation to ensure that all areas of 

the country attain and maintain the 

NAAQS on a timely basis even if they 

are designated ‘‘unclassifiable’’ initially. 

This SIP is due under CAA section 

110(a)(1) within 3 years after 

promulgation of the new NAAQS, and 

does not depend upon EPA designating 

an area ‘‘nonattainment’’ based on 

recently monitored or modeled SO2

levels. This period of time would allow 

States to use EPA’s anticipated guidance 

on modeling for the new 1-hour SO2

NAAQS, as well as account for SO2

reduction levels at individual sources 

that are anticipated to result from 

promulgated national and regional rules 

to show attainment. 

Once areas have both appropriate 

monitoring data (if required) and 

modeling data as appropriate, consistent 

with the new guidance, showing no 

violations of the SO2 NAAQS, and have 

met other applicable requirements of 

CAA section 107(d)(3), the Agency 

would consider re-designating them 

from ‘‘unclassifiable’’ or ‘‘nonattainment’’ 

to ‘‘attainment’’ under CAA section 

107(d)(3). 

VI. Clean Air Act Implementation 

Requirements 

This section of the preamble discusses 

the CAA requirements that States and 

emissions sources would need to 

address when implementing the new 1- 

hour SO2 NAAQS based on the structure 

outlined in the CAA and existing rules. 

The EPA believes that existing guidance 

documents and regulations will be 

useful in helping States and sources to 

implement the new SO2 NAAQS, but we 

also expect to develop additional 

guidance on modeling for the new onehour standard and on developing SIPs 

under Section 110(a)(1) of the CAA.35 In 

light of the new approach that EPA 

intends to take with respect to 

implementation of the SO2 NAAQS, 

EPA intends to solicit public comment 

on guidance regarding modeling, and 

also solicit public comment on 

additional implementation planning 

guidance, including the content of the 

maintenance plans required under 

section 110(a)(1) of the Clean Air Act. 

EPA also notes that State monitoring 

plans and the SIP submissions that 

States will make will also be subject to 

public notice and comment.’’ 

In this section, we also further discuss 

how EPA’s modified expected 

approaches toward monitoring and 

modeling and toward initial 

designations under the new SO2

NAAQS (compared to how the proposed 

rule discussed addressing these issues) 

are anticipated to affect the types of SIP 

submissions States will need to provide 

to EPA and the timing of EPA’s actions 

on those submissions leading up to 

attainment and maintenance of the new 

SO2 NAAQS. In section IV above, we 

discuss the final amendments to the 

ambient monitoring and reporting 

requirements, and explain how in 

response to comments received on the 

proposal and after revisiting our 

historical practice in assessing 

compliance with prior SO2 NAAQS, we 

have revised both the scope of the 

revised monitoring network and our 

expectations on how monitoring will be 

used in conjunction with modeling in 

assessing compliance and designating 

areas. In section V above, we discuss 

how we have revised our expected 

approach for issuing designations for 

the new 1-hour SO2 NAAQS, and 

similarly explain how, in response to 

comments and after reviewing our 

historical approach, we have modified 

our expectations as discussed in the 

proposal for how and when monitoring 

and modeling will be used for 

designations. In this section VI, we 

describe in more detail how and when 

we expect States to demonstrate 

attainment, implementation, 

maintenance and enforcement of the 

new one-hour SO2 NAAQS. 

The CAA assigns important roles to 

EPA, States and Tribal governments to 

achieve the NAAQS. States have the 

primary responsibility for developing 

and implementing State implementation 

plans (SIPs) that contain State measures 

necessary to achieve the air quality 

standards in each area once EPA has 

established the NAAQS. EPA provides 

assistance to States and Tribes by 

providing technical tools, assistance, 

and guidance, including information on 

the potential control measures that may 

assist in helping areas attain the 

standards. 

Under section 110 of the CAA, 42 

U.S.C. 7410, and related provisions, 

States are directed to submit, for EPA 

approval, SIPs that provide for the 

attainment, implementation, 

maintenance, and enforcement of such 

standards through control programs 

directed at sources of SO2 emissions. 

See CAA sections 110(a), and 191–192, 

42 U.S.C. 7410(a) and 7514–7514a. If a 

State fails to adopt and implement the 

required SIPs by the time periods 

provided in the CAA, EPA has the 

responsibility under the CAA to adopt 

a Federal implementation plan (FIP) to 

ensure that areas attain the NAAQS in 

an expeditious manner. The States, in 

conjunction with EPA, also administer 

the prevention of significant 

deterioration (PSD) program for SO2. 

See sections 160–169 of the CAA, 42 

U.S.C. 7470–7479. In addition, Federal 

programs provide for nationwide 

reductions in emissions of SO2 and 

other air pollutants under Title II of the 

Act, 42 U.S.C. 7521–7574. These 

programs involve limits on the sulfur 

content of the fuel used by automobiles, 

trucks, buses, motorcycles, non-road 

engines and equipment, marine vessels 

and locomotives. Emissions reductions 

for SO2 are also obtained from 

implementation of the new source 

performance standards (NSPS) for 

stationary sources under sections 111 

and 129 of the CAA, 42 U.S.C. 7411 and 

7429; and the national emission 

standards for hazardous air pollutants 

(NESHAP) for stationary sources under 

section 112 of the CAA, 42 U.S.C. 7412 

(such reductions resulting due to 

control of hazardous air pollutants 

(HAP) such as hydrogen chloride (HCl) 

under those rules). Title IV of the CAA, 

sections 402–416, 42 U.S.C. 7651a– 

7651o, specifically provides for major 

reductions in SO2 emissions. EPA has 

also promulgated the Clean Air 

Interstate Rule (CAIR) to define 

additional SO2 emission reductions 

needed in the Eastern United States to 

eliminate significant contribution of 

upwind States to downwind States’ 


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nonattainment, or inability to maintain, 

the PM2.5 NAAQS pursuant to CAA 

section 110(a)(2)(D), 42 U.S.C. 

7410(a)(2)(D), a rule which EPA is 

reevaluating pursuant to court remand. 

A. How This Rule Applies to Tribes 

CAA section 301(d) authorizes EPA to 

treat eligible Indian Tribes in the same 

manner as States under the CAA and 

requires EPA to promulgate regulations 

specifying the provisions of the statute 

for which such treatment is appropriate. 

EPA has promulgated these 

regulations—known as the Tribal 

Authority Rule or TAR—at 40 CFR Part 

49. See 63 FR 7254 (February 12, 1998). 

The TAR establishes the process for 

Indian Tribes to seek treatment-as-aState eligibility and sets forth the CAA 

functions for which such treatment will 

be available. Under the TAR, eligible 

Tribes may seek approval for all CAA 

and regulatory purposes other than a 

small number of functions enumerated 

at section 49.4. Implementation plans 

under section 110 are included within 

the scope of CAA functions for which 

eligible Tribes may obtain approval. 

Section 110(o) also specifically 

describes Tribal roles in submitting 

implementation plans. Eligible Indian 

Tribes may thus submit implementation 

plans covering their reservations and 

other areas under their jurisdiction. 

The CAA and TAR do not, however, 

direct Tribes to apply for treatment as a 

State or implement any CAA program. 

In promulgating the TAR EPA explicitly 

determined that it was not appropriate 

to treat Tribes similarly to States for 

purposes of, among other things, 

specific plan submittal and 

implementation deadlines for NAAQSrelated requirements. 40 CFR 49.4(a). In 

addition, where Tribes do seek approval 

of CAA programs, including section 110 

implementation plans, the TAR 

provides flexibility and allows them to 

submit partial program elements, so 

long as such elements are reasonably 

severable—i.e., ‘‘not integrally related to 

program elements that are not included 

in the plan submittal, and are consistent 

with applicable statutory and regulatory 

requirements.’’ 40 CFR 49.7. 

To date, very few Tribes have sought 

treatment as a State for purposes of 

section 110 implementation plans. 

However, some Tribes may be interested 

in pursuing such plans to implement 

today’s proposed standard, once it is 

promulgated. 

1. Approach Described in the Proposal 

In the proposed rule preamble, EPA 

described the various roles and 

requirements States would address in 

implementing the proposed NAAQS. 

Such references to States generally 

included eligible Indian Tribes to the 

extent consistent with the flexibility 

provided to Tribes under the TAR. 

Where Tribes do not seek treatment as 

a State for section 110 implementation 

plans, we explained that EPA under its 

discretionary authority will promulgate 

FIPs as ‘‘necessary or appropriate to 

protect air quality.’’ 40 CFR 49.11(a). 

EPA also noted that some Tribes operate 

air quality monitoring networks in their 

areas. We explained that for such 

monitors to be used to measure 

attainment with the proposed revised 

primary NAAQS for SO2, the criteria 

and procedures identified in the 

proposed rule would apply. 

2. Current Approach 

EPA did not receive any comments on 

this issue. However, as discussed 

elsewhere in this preamble, the final 

rule reflects in several respects modified 

expected approaches regarding the use 

of monitoring and modeling, the manner 

in which we expect to issue 

designations under the new SO2

NAAQS, and the types of SIP 

submissions we expect would be 

needed to show attainment, 

implementation, maintenance and 

enforcement of the new NAAQS. Those 

changes in expected approach would, as 

appropriate, also apply to how we 

address data and any other submissions 

from Tribes for purposes of the new SO2

NAAQS. 

B. Nonattainment Area Attainment 

Dates 

The latest date by which an area 

designated as nonattainment is required 

to attain the SO2 NAAQS is determined 

from the effective date of the 

nonattainment designation for the 

affected area. For areas designated 

nonattainment for the revised SO2

NAAQS, SIPs must provide for 

attainment of the NAAQS as 

expeditiously as practicable, but no later 

than 5 years from the effective date of 

the nonattainment designation for the 

area. See section 192(a) of the CAA, 42 

U.S.C. 7651a(a). The EPA expects to 

determine whether an area has 

demonstrated attainment of the new SO2

NAAQS by evaluating air quality 

monitoring and modeling data 

consistent with 40 CFR part 50, 

Appendix T and 40 CFR part 51, 

Appendix W. (Note that this differs from 

how we explained we would expect to 

make such determinations in the 

proposed rule, where we only 

mentioned monitoring as supplying the 

data we would evaluate. This expanded 

and changed discussion reflects the 

contemplated changes in our overall 

approaches to using monitoring and 

modeling, expectations for issuing 

designations, and expectations for 

reviewing SIP submissions showing 

attainment, implementation, 

maintenance, and enforcement of the 

new SO2 NAAQS.) 

1. Attaining the NAAQS 

a. Approach Described in the Proposal 

In the proposal preamble, we set forth 

the basic five conditions provided under 

section 107(d)(3)(E) of the CAA, 42 

U.S.C. 7407(d)(3)(E) that a 

nonattainment area must meet in order 

to be redesignated as attainment: 

• EPA must have determined that the 

area has met the SO2 NAAQS; 

• EPA has fully approved the State’s 

implementation plan; 

• The improvement in air quality in 

the affected area is due to permanent 

and enforceable reductions in 

emissions; 

• EPA has fully approved a 

maintenance plan for the area; and 

• The State(s) containing the area 

have met all applicable requirements 

under section 110 and part D. 

b. Current Approach 

EPA did not receive any comments on 

this aspect of the preamble of the 

proposal. However, in light of the fact 

that in the final rule, in response to 

other comments and consistent with 

historic practice, we are revising our 

proposed anticipated approaches to the 

overall use of monitoring and modeling 

and our expected approaches to issuing 

initial designations and reviewing SIP 

submissions, it follows that the way in 

which a nonattainment area seeks 

redesignation as an attainment area 

would also be affected by the final rule’s 

overall changed approaches. For 

example, for EPA to determine that a 

nonattainment area has met the SO2

NAAQS, we anticipate that the area 

would need to not only provide any 

monitoring data showing such 

compliance (and there would need to be 

an absence of monitoring data showing 

otherwise), but modeling where 

appropriate, consistent with modeling 

guidance that we plan to issue, would 

also need to show that the area is 

attaining and maintaining the NAAQS. 

2. Consequences of a Nonattainment 

Area Failing To Attain by the Statutory 

Attainment Date 

a. Approach Described in the Proposal 

We explained in the proposal that any 

SO2 nonattainment area that fails to 

attain by its statutory attainment date 

would be subject to the requirements of 

sections 179(c) and (d) of the CAA, 42 


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U.S.C. 7509(c) and (d). EPA is required 

to make a finding of failure to attain no 

later than 6 months after the specified 

attainment date and publish a notice in 

the Federal Register. The State would 

then need to submit an implementation 

plan revision no later than one year 

following the effective date of the 

Federal Register notice making the 

determination of the area’s failure to 

attain. This submission must 

demonstrate that the standard will be 

attained as expeditiously as practicable, 

but no later than 5 years from the 

effective date of EPA’s finding that the 

area failed to attain. In addition, section 

179(d)(2) provides that the SIP revision 

must include any specific additional 

measures as may be reasonably 

prescribed by EPA, including ‘‘all 

measures that can be feasibly 

implemented in the area in light of 

technological achievability, costs, and 

any nonair quality and other air qualityrelated health and environmental 

impacts.’’ 

b. Current Approach 

EPA did not receive any comments on 

this aspect of the discussion in the 

preamble to the proposal. However, due 

to the changes in the final rule’s 

discussion of the overall expected 

approaches to monitoring and modeling, 

designations and EPA review of SIP 

submissions, it follows that the 

implementation of CAA sections 179(c) 

and (d) would also be affected by those 

changes. For example, under the 

anticipated approach, a nonattainment 

area’s initial demonstration of 

attainment would need to show through 

modeling consistent with modeling 

guidance that we plan to issue, that the 

area attains and maintains the new SO2

NAAQS. If the area fails to attain on 

time, any remedial implementation plan 

submission would also need to show, 

where appropriate, through modeling 

consistent with modeling guidance that 

we plan to issue, that the area attains 

and maintains the new SO2 NAAQS. 

C. Section 110(a)(1) and (2) NAAQS 

Maintenance/Infrastructure 

Requirements 

We are significantly revising our 

expected approaches to the use of 

monitoring and modeling, expected 

issuance of initial designations, and 

EPA review of SIP submissions. This 

change in anticipated approach has 

particular relevance for how States 

would meet their statutory obligations 

under CAA section 110(a) to implement, 

maintain and enforce the new SO2

NAAQS. In short, under such an 

approach, all areas, whether designated 

as attainment, nonattainment, or 

unclassifiable, would need to submit 

SIPs under CAA section 110(a) that 

show that they are attaining and 

maintaining the 1-hour SO2 NAAQS as 

expeditiously as practicable through 

permanent and enforceable measures. In 

other words, the duty to show 

maintenance of the SO2 NAAQS would 

not be limited to areas that are initially 

designated as nonattainment, but 

instead would apply regardless of 

designation. As has been expected 

historically, areas initially designated 

attainment for SO2 are expected to 

submit to EPA the infrastructure 

elements of the 110(a) SIP, including the 

PSD program. Historically, EPA has 

determined this to be sufficient to 

demonstrate maintenance absent other 

available information to suggest the area 

would have difficulty maintaining the 

NAAQS. 

As required by CAA section 192, 

nonattainment areas must demonstrate 

attainment as expeditiously as 

practicable, and no later than 5 years 

after designation (which would be 

August 2017). Under a hybrid approach 

as we have discussed earlier in sections 

III, IV, and V of this preamble, EPA 

believes that August 2017 would be the 

latest point that could be as 

expeditiously as practicable for 

attainment and unclassifiable areas as 

well, and EPA anticipates establishing 

this date through future rulemaking 

actions on individual SIPs. 

As noted in earlier sections of this 

preamble, in the SO2 NAAQS proposal, 

we recommended a monitoring-focused 

approach for comparison to the NAAQS. 

We received public comments that 

contended our proposed monitoring 

network was too small and insufficient 

to assess the hundreds of areas that 

might violate the new SO2 NAAQS and 

yet too burdensome and expensive to 

expand to an adequate scale. Some 

commenters, especially State air 

agencies, recommended the use of 

modeling either to determine potential 

nonattainment areas or to identify areas 

subject to monitoring requirements. 

Because SO2 is primarily a localized 

pollutant, modeling is the the most 

appropriate tool to accurately predict 

SO2 impacts from large sources, EPA 

has used it in the past to determine SO2

attainment status, and it can be 

performed more quickly and less costly 

than monitoring. Consequently, as part 

of developing a balanced response to the 

numerous comments we received on 

modeling and monitoring, we expect to 

use a hybrid analytic approach that 

combines the use of monitoring and 

modeling to assess compliance with 

respect to the new SO2 NAAQS. 

A hybrid analytic approach for 

assessing compliance with the new SO2

NAAQS would make the most 

appropriate use of available tools and be 

more consistent with our historical 

approach than was what we originally 

proposed. For a short-term 1-hour 

standard, it is more accurate and 

efficient to use modeling to assess 

medium to larger sources and to rely on 

monitoring for groups of smaller sources 

and sources not as conducive to 

modeling. 

We expect that States would initially 

focus performance of attainment 

demonstration modeling on larger 

sources (e.g., those ≥ 100 tons per year 

(tpy) of SO2), and that States would also 

identify and eventually conduct refined 

modeling of any other sources that may 

be anticipated to cause or contribute to 

a violation to determine compliance 

with the new SO2 NAAQS. As discussed 

in Section III, EPA anticipates providing 

additional guidance to States to clarify 

how to conduct dispersion modeling 

under Appendix W to support the 

implementation of the new 1-hour SO2

NAAQS. Prior to issuing this guidance, 

EPA intends to solicit public comment. 

Since determining compliance with 

the SO2 NAAQS will likely be a 

uniquely source-driven analysis, EPA 

explored options to ensure that the SO2

designations process realistically 

accounts for anticipated SO2 reductions 

at those sources that we expect will be 

achieved by current and pending 

national and regional rules. To ensure 

that all areas of the country attain the 

NAAQS on a timely basis, while 

accommodating modeling that is both 

informed by anticipated modeling 

guidance and accounts for those 

anticipated SO2 reductions, EPA’s 

intention is to emphasize the CAA 

section 110(a)(1) requirement that all 

States submit a SIP that shows 

implementation, maintenance and 

enforcement of the NAAQS. This SIP 

would be due under CAA section 

110(a)(1) within 3 years after 

promulgation of the new NAAQS, and 

would not depend upon EPA 

designating an area nonattainment 

based on recently monitored or modeled 

SO2 levels. In addition, like an 

attainment SIP required for a designated 

nonattainment area under CAA section 

192, to show attainment this SIP can 

account for controlled SO2 levels at 

individual sources that will be achieved 

after submission of the SIP but before 

the demonstrated attainment date. EPA 

intends to implement this approach in 

a way that ensures expeditious 

attainment of the NAAQS, under a 

schedule that we explain more fully 

below. 


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36 In the proposed rule preamble, we explained 

that two elements identified in section 110(a)(2) 

were not listed in our summary because, as EPA 

interprets the CAA, SIPs incorporating any 

necessary local nonattainment area controls would 

not be due within 3 years, but rather are generally 

due at the time the nonattainment area planning 

requirements are due. See 74 FR 64860 at n. 39. 

These elements are: (1) Emission limits and other 

control measures, section 110(a)(2)(A), and (2) 

Provisions for meeting part D, section 110(a)(2)(I), 

which requires areas designated as nonattainment 

to meet the applicable nonattainment planning 

requirements of part D, title I of the CAA. To 

implement our revised intended approach in the 

final rule, however, it would be necessary for States 

to include, if relied upon to show attainment and 

maintenance of the new SO2 NAAQS, any necessary 

emission limits and other control measures under 

section 110(a)(2)(A). 

1. Section 110(a)(1)–(2) Submission 

a. Approach Described in the Proposal 

In the preamble to the proposal, we 

explained that section 110(a)(2) of the 

CAA directs all States to develop and 

maintain a solid air quality management 

infrastructure, including enforceable 

emission limitations, an ambient 

monitoring program, an enforcement 

program, air quality modeling 

capabilities, and adequate personnel, 

resources, and legal authority. Section 

110(a)(2)(D) also requires State plans to 

prohibit emissions from within the State 

which contribute significantly to 

nonattainment or maintenance areas in 

any other State, or which interfere with 

programs under part C of the CAA to 

prevent significant deterioration of air 

quality or to achieve reasonable progress 

toward the national visibility goal for 

Federal class I areas (national parks and 

wilderness areas). 

Under sections 110(a)(1) and (2) of the 

CAA, all States are directed to submit 

SIPs to EPA which demonstrate that 

basic program elements have been 

addressed within 3 years of the 

promulgation of any new or revised 

NAAQS. Subsections (A) through (M) of 

section 110(a)(2) set forth the elements 

that a State’s program must contain in 

the SIP.36 The proposed rule listed 

section 110(a)(2) NAAQS 

implementation requirements as the 

following: 

• Ambient air quality monitoring/ 

data system: Section 110(a)(2)(B) 

requires SIPs to provide for setting up 

and operating ambient air quality 

monitors, collecting and analyzing data 

and making these data available to EPA 

upon request. 

• Program for enforcement of control 

measures: Section 110(a)(2)(C) requires 

SIPs to include a program providing for 

enforcement of SIP measures and the 

regulation and permitting of new/ 

modified sources. 

• Interstate transport: Section 

110(a)(2)(D) requires SIPs to include 

provisions prohibiting any source or 

other type of emissions activity in the 

State from contributing significantly to 

nonattainment or interfering with 

maintenance of the NAAQS in another 

State, or from interfering with measures 

required to prevent significant 

deterioration of air quality or to protect 

visibility. 

• Adequate resources: Section 

110(a)(2)(E) directs States to provide 

assurances of adequate funding, 

personnel and legal authority to 

implement their SIPs. 

• Stationary source monitoring 

system: Section 110(a)(2)(F) directs 

States to establish a system to monitor 

emissions from stationary sources and 

to submit periodic emissions reports to 

EPA. 

• Emergency power: Section 

110(a)(2)(G) directs States to include 

contingency plans, and adequate 

authority to implement them, for 

emergency episodes in their SIPs. 

• Provisions for SIP revision due to 

NAAQS changes or findings of 

inadequacies: Section 110(a)(2)(H) 

directs States to provide for revisions of 

their SIPs in response to changes in the 

NAAQS, availability of improved 

methods for attaining the NAAQS, or in 

response to an EPA finding that the SIP 

is inadequate. 

• Consultation with local and Federal 

government officials: Section 110(a)(2)(J) 

directs States to meet applicable local 

and Federal government consultation 

requirements when developing SIPs and 

reviewing preconstruction permits. 

• Public notification of NAAQS 

exceedances: Section 110(a)(2)(J) directs 

States to adopt measures to notify the 

public of instances or areas in which a 

NAAQS is exceeded. 

• PSD and visibility protection: 

Section 110(a)(2)(J) also directs States to 

adopt emissions imitations, and such 

other measures, as may be necessary to 

prevent significant deterioration of air 

quality in attainment areas and protect 

visibility in Federal Class I areas in 

accordance with the requirements of 

CAA Title I, part C. 

• Air quality modeling/data: Section 

110(a)(2)(K) requires that SIPs provide 

for performing air quality modeling for 

predicting effects on air quality of 

emissions of any NAAQS pollutant and 

submission of data to EPA upon request. 

• Permitting fees: Section 110(a)(2)(L) 

requires the SIP to include requirements 

for each major stationary source to pay 

permitting fees to cover the cost of 

reviewing, approving, implementing 

and enforcing a permit. 

• Consultation/participation by 

affected local government: Section 

110(a)(2)(M) directs States to provide for 

consultation and participation by local 

political subdivisions affected by the 

SIP. 

b. Final 

EPA did not receive any comments on 

this aspect of the approached explained 

in the proposal preamble. However, in 

light of the modified approach 

discussed above, EPA is providing 

additional guidance concerning the 

CAA section 110(a)(1) maintenance plan 

requirement as a part of this discussion 

so that States will have sufficient 

information to meet this requirement 

with a SIP submittal three years after 

promulgation of the NAAQS. Section 

110(a)(1) of the CAA states that each 

State, after reasonable notice and public 

hearing, is required to adopt and to 

submit to EPA, within 3 years after 

promulgation of any new or revised 

NAAQS for any pollutant, a SIP which 

provides for the implementation, 

maintenance, and enforcement of any 

new or revised NAAQS in each area of 

the State. As stated previously, in light 

of the new approach that EPA intends 

to take with respect to implementation 

of the SO2 NAAQS, EPA intends to 

solicit public comment on guidance 

regarding modeling, and also solicit 

public comment on additional 

implementation planning guidance, 

including the content of the 

maintenance plans required under 

section 110(a)(1) of the Clean Air Act. 

EPA expects that most areas of the 

country would be designated as 

unclassifiable for the 1-hour NAAQS for 

SO2, due to a lack of both monitoring 

and modeling information concerning 

the attainment status of areas, in 

advance of States conducting further 

refined modeling according to our 

anticipated guidance. For areas that are 

designated unclassifiable, States are 

required to submit section 110(a)(1) 

plans to demonstrate implementation, 

maintenance and enforcement of the 

new SO2 NAAQS. As previously 

explained in section III of the preamble, 

in order to meet the requirements of 

section 110(a)(1) and to ensure timely 

attainment of the NAAQS on a schedule 

that is as expeditious as would be 

required if an area had been designated 

nonattainment, EPA’s current 

expectation is that States would submit 

SIPs which provide for attainment, 

implementation, maintenance, and 

enforcement of the 1-hour SO2 NAAQS 

in all areas as expeditiously as 

practicable, which EPA believes in these 

cases would be no later than 5 years 

from the effective date of the area’s 

designation. The section 110(a)(1) 

maintenance plan would also need to 

contain the following elements: (1) An 


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attainment emissions inventory, (2) a 

control strategy, as appropriate, (3) a 

maintenance demonstration, using an 

EPA approved air quality model as 

appropriate, (4) a contingency plan, and 

(5) a plan for verification of continued 

attainment of the standard. Attainment 

areas that appear to have difficulty 

maintaining attainment may also have 

to submit some of these elements. These 

elements are now explained in detail. 

(1) Attainment Emissions Inventory 

The State should develop an accurate 

attainment emissions inventory to 

identify the level of emissions in the 

area which is sufficient to attain the 1- 

hour SO2 NAAQS. This inventory 

should be consistent with EPA’s most 

recent guidance on emissions 

inventories currently available, and 

should include the emissions for the 

time period associated with the 

modeling and monitoring data showing 

attainment. Major source size thresholds 

for SO2 are currently listed as 100 

ton/yr, however, in cases where sources, 

individually, or collectively, that are 

below this level may potentially cause 

or contribute to a violation of the 

standard, these sources should also be 

included in the emissions inventory for 

the affected area. EPA notes that, unlike 

any monitoring or modeling data used 

in the initial designations context, 

which would be limited to current 

emissions levels, this estimate under a 

hybrid approach we expect to use for 

the new SO2 NAAQS would be able to 

rely on modeled controlled emissions 

levels at sources achieved by 

enforceable national, regional or local 

rules that will be in place within the 

timeframe for demonstrating attainment. 

This is because demonstrations of 

attainment and maintenance of a 

NAAQS, unlike designations, are 

necessarily projections regarding future 

and continuing levels of ambient air 

pollution concentrations given that the 

statutory deadlines for their submission 

are in advance of the required 

achievement of attainment and 

maintenance. See, e.g., CAA sections 

191(a) and 192(a). 

(2) Maintenance Demonstration 

The key element of a section 110(a)(1) 

maintenance plan is a demonstration 

using, as appropriate, refined SO2

dispersion modeling (see Appendix W 

to 40 CFR Part 51) which provides an 

indication of how the area will attain 

and maintain the 1-hour SO2 NAAQS as 

expeditiously as practicable, which EPA 

believes would be within the 5 year 

period following the designation of the 

area. For SO2 the State may generally 

demonstrate maintenance of the 

NAAQS by using refined dispersion 

modeling to show that the future mix of 

sources and emission rates in an area 

will not cause a violation of the 1-hour 

SO2 NAAQS. As a result of applying the 

control strategy, EPA anticipates that 

additional guidance for States may be 

needed to clarify how to conduct 

dispersion modeling under Appendix W 

to support the implementation of the 

new 1-hour SO2 NAAQS. 

As explained above in IV.B, EPA 

believes that for SO2 attainment and 

maintenance demonstrations, 

monitoring data alone is generally not 

adequate to characterize fully short-term 

ambient concentrations around major 

stationary sources of SO2, and as a result 

may not capture the maximum SO2

impacts. With representative and 

appropriate meteorological and other 

input data, refined dispersion models 

are able to characterize air quality 

impacts from the modeled sources 

across the domain of interest on an 

hourly basis with a high degree of 

spatial resolution, overcoming the 

limitations of an approach based solely 

on monitoring. By simulating plume 

dispersion on an hourly basis across a 

grid of receptor locations, dispersion 

models are able to estimate the detailed 

spatial gradients of ambient 

concentrations resulting from SO2

emission sources across a full range of 

meteorological and source operating 

conditions. To capture such results on 

a monitor would normally require a 

prohibitively expansive air quality 

monitoring network. Further, as we 

have observed in prior actions (see., e.g., 

43 FR 45993, 45997, 46000–03 (Oct. 5, 

1978)), monitoring data would not be 

adequate to demonstrate attainment if 

sources are using stacks with heights 

that are greater than good engineering 

practice (GEP), or other prohibited 

dispersion techniques, as section 123 

prohibits credit in an attainment 

demonstration for any such practices. 

Refined dispersion modeling for the 

section 110(a)(1) maintenance plan is 

expected to follow EPA’s Guideline on 

Air Quality Models, Appendix W to 40 

CFR Part 51, which provides 

recommendations on modeling 

techniques and guidance for estimating 

pollutant concentrations in order to 

assess control strategies and determine 

emission limits. These 

recommendations were originally 

published in April 1978 and were 

incorporated by reference in the PSD 

regulations, 40 CFR sections 51.166 and 

52.21 in June 1978 (43 FR 26382– 

26388). The purpose of Appendix W is 

to promote consistency in the use of 

modeling within the air quality 

management process. Appendix W is 

periodically revised to ensure that new 

model developments or expanded 

regulatory requirements are 

incorporated. The most recent revision 

to Appendix W was published on 

November 9, 2005 (70 FR 68218), 

wherein EPA adopted AERMOD as the 

preferred dispersion model for a wide 

range of regulatory applications in all 

types of terrain. To support the 

promulgation of AERMOD as the 

preferred model, EPA evaluated the 

performance of the model across a total 

of 17 field study data bases (Perry, et al., 

2005; EPA, 2003), including several 

field studies based on model-to-monitor 

comparisons of SO2 concentrations from 

operating power plants. AERMOD is a 

steady-state plume dispersion model 

that employs hourly sequential 

preprocessed meteorological data to 

simulate transport and dispersion from 

multiple point, area, or volume sources 

for averaging times from one hour to 

multiple years, based on an advanced 

characterization of the atmospheric 

boundary layer. AERMOD also accounts 

for building wake effects (i.e., 

downwash) on plume dispersion. 

As stated previously, EPA anticipates 

that additional guidance for States, 

Tribal, and local governments is needed 

to clarify how to conduct refined 

dispersion modeling under Appendix W 

to support the implementation of the 

new 1-hour SO2 NAAQS. EPA intends 

to solicit public comment on guidance 

regarding modeling. Although AERMOD 

is identified as the preferred model 

under Appendix W for a wide range of 

applications and will be appropriate for 

most modeling applications to support 

the new SO2 NAAQS, Appendix W 

allows flexibility to consider the use of 

alternative models on a case-by-case 

basis when an adequate demonstration 

can be made that the alternative model 

performs better than, or is more 

appropriate than, the preferred model 

for a particular application. 

(3) Control Strategy 

The EPA believes that in order to 

meet the implementation, maintenance 

and enforcement plan requirements of 

section 110(a)(1) for the new SO2

NAAQS, States should consider all 

control measures that are reasonable to 

implement in light of the attainment 

and maintenance needs for the affected 

area(s). The EPA believes that where 

additional controls are necessary it 

would be appropriate for the level of 

controls in these areas to be similar to 

that required in areas that are 

designated as nonattainment for SO2. 

These controls would provide for the 

attainment and maintenance of the SO2

1-hour standard as expeditiously as 


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practicable. EPA believes that 

expeditious attainment in these areas 

will be within 5 years of the effective 

date of designation of an area. This 

approach would allow States to take 

into consideration emission reductions 

that we expect to be achieved from the 

implementation of future controls from 

national control measures as well as 

regional and local control measures that 

will be in place by the anticipated 

attainment date and are projected to 

help achieve attainment and 

maintenance of the standard. It would 

also reduce the risk of such areas failing 

to meet the NAAQS as expeditiously as 

nonattainment areas must meet it. 

(4) Contingency Plan 

The contingency plan is considered to 

be an enforceable part of the section 

110(a)(1) plan and should ensure that 

there are appropriate contingency 

measures which can be implemented as 

expeditiously as practicable once they 

are triggered. The contingency plan 

should clearly identify the measures to 

be adopted, provide a schedule and 

procedures for adoption and 

implementation, and provide a specific 

time limit for actions by the State. 

The EPA believes that in this case the 

contingency measures implemented 

under the contingency plan requirement 

for the section 110(a)(1) plan in 

unclassifiable areas under a revised 

approach for SO2 should closely 

resemble the contingency measures 

required under section 172(c)(9) of the 

CAA. Section 172(c)(9) of the CAA 

defines contingency measures as 

measures in the SIP which are to be 

implemented in the event that an area 

fails to attain the NAAQS, or fails to 

meet the reasonable further progress 

(RFP) requirement, by the applicable 

attainment date for the area. 

Contingency measures become effective 

without further action by the State or 

EPA, upon determination by EPA that 

the area (1) failed to attain the NAAQS 

by the applicable attainment date, or (2) 

fail to meet RFP. These contingency 

measures should consist of other 

available control measures that are not 

included in the control strategy for the 

SIP. 

The EPA interprets the contingency 

measure provision as primarily directed 

at general control programs which can 

be undertaken on an area-wide basis. 

Since SO2 control measures are based 

on what is directly and quantifiably 

necessary to attain the SO2 NAAQS, it 

would be unlikely for an area to 

implement the necessary emissions 

control yet fail to attain the NAAQS. 

Therefore, for SO2 programs, EPA 

believes that State agencies should have 

a comprehensive program to identify 

sources of violations of the SO2 NAAQS 

and undertake an aggressive follow-up 

for compliance and enforcement, 

including expedited procedures for 

establishing enforceable consent 

agreements pending the adoption of 

revised SIPs. 

Such an approach toward minimum 

contingency measures for SO2 would 

not preclude a State from requiring 

additional contingency measures that 

are enforceable and appropriate for a 

particular source or source category. A 

contingency measure for an SO2 SIP 

might be a consent agreement between 

the State and EPA to reduce emissions 

from a source further in the event that 

the contingency measures are triggered. 

Alternatively, a source might adopt a 

contingency measure such as switching 

to low sulfur coal or reducing load until 

more permanent measures can be put 

into place to correct the problem. In 

either case, the contingency measure 

should be a fully adopted provision in 

the SIP in order for it to become 

effective at the time that EPA 

determines that the area either fails to 

attain the NAAQS or fails to meet RFP. 

As a necessary part of the section 

110(a)(1) plan, the State should also 

identify specific indicators, or triggers, 

which will be used to determine when 

the contingency measures need to be 

implemented. The identification of 

triggers would allow a State an 

opportunity to take early action to 

address potential violations of the 

NAAQS before they occur. By taking 

early action, States may be able to 

prevent any actual violations of the 

NAAQS, and therefore, reduce the need 

on the part of EPA to start the process 

to re-designate the areas as 

nonattainment. An example of a trigger 

would be monitored or modeled 

violations of the NAAQS. The EPA will 

review what constitutes an approvable 

contingency plan on a case-by-case 

basis. 

(5) Verification of Continued 

Attainment 

The submittal should provide an 

indication of how the State will track 

the progress of the section 110(a)(1) 

plan. This is necessary due to the fact 

that the emissions projections made for 

the attainment and maintenance 

demonstrations depend on assumptions 

of point, area, and mobile source 

growth. One option for tracking the 

progress of the attainment and 

maintenance demonstrations, provided 

here as an example, would be for the 

State to update periodically the 

emissions inventory. The attainment 

and maintenance demonstration should 

project maintenance during the five year 

period following the designations for 

the 1-hour SO2 NAAQS, not simply that 

the area will be in attainment in the fifth 

year. 

States should develop interim 

emission projection years to show a 

trend analysis for attainment and 

maintenance of the standard. These 

emission projections can also be used as 

triggers for implementing contingency 

measures. The EPA recognizes that it 

would be difficult and time consuming 

to develop projections for each year of 

the 5 year period. Therefore, the number 

of interim projection years should 

reflect whatever information exists 

regarding the potential for increases in 

emissions in the intervening years. For 

instance, if there is a high probability 

that emissions will increase to such an 

extent as to jeopardize continued 

maintenance of the standard even 

temporarily over the intervening years, 

the number of interim projection 

periods should be sufficient to 

document that such increases will not 

interfere with maintenance of the 1-hour 

SO2 NAAQS. 

When modeling for the attainment 

and maintenance demonstrations, one 

option for tracking progress would also 

be for the State to reevaluate 

periodically the modeling assumptions 

and data input. Such reevaluation, for 

example, could address any delays in 

source compliance with national, 

regional or local rules for which the 

State had previously modeled timely 

SO2 reductions. In any event, the State 

should monitor the indicators for 

triggering the contingency measures on 

a regular basis. 

EPA recognizes that the approach 

discussed above for SO2 SIPs submitted 

under CAA section 110(a)(1)–(2) is 

significantly different from the one 

outlined in the proposal, and from what 

we have applied in the context of other 

criteria pollutants. However, EPA 

anticipates using a revised approach 

under section 110(a)(1)–(2) as part of an 

overall revised hybrid monitoring and 

modeling approach in response to 

comments on the proposed monitoringfocused approach to implementation of 

the new SO2 NAAQS. We believe that 

such an approach would best account 

for the unique source-specific and 

localized impacts inherent to SO2, and 

would be the most reasonable way to 

ensure that all areas of the United States 

timely attain and maintain the new 

NAAQS, while at the same time 

avoiding inappropriately requiring 

immediate refined modeling of all 

sources without appropriate EPA 

guidance. This would also allow 

attainment demonstrations to account 


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for expected substantial SO2 reductions 

that will occur well in advance of the 

attainment deadline. Of course, for such 

a unique SO2 approach to work, it 

would be imperative for all areas to 

timely submit, and for EPA to able to 

approve, adequate attainment, 

implementation, maintenance and 

enforcement SIPs that show attainment 

as expeditiously as practicable, and no 

later than 5 years following initial 

designations. Only by applying such a 

timeframe to the section 110(a)(1) SIP 

approach we are adopting for SO2 could 

the approach be a reasonable one. To 

that end, EPA would not intend to 

approve SIPs that do not meet this 

schedule, and would take necessary and 

appropriate actions in response to any 

submission that would result in 

unacceptable delay of attainment. Such 

actions may include, but are not limited 

to, any combination of SIP disapproval, 

redesignation to nonattainment, and 

promulgation of a Federal 

implementation plan (FIP). Any future 

action establishing an attainment 

deadline will be completed through 

notice-and-comment rulemaking on 

individual SIP submissions. 

The timeline below shows how we 

expect the several steps from 

promulgation of the new NAAQS 

through attainment should proceed, 

whether areas are designated 

nonattainment or unclassifiable, 

assuming timely action at each step: 

• June 2010: EPA issues new SO2

NAAQS, which starts periods within 

which CAA section 107 initial area 

designations must occur and CAA 

section 110(a)(1)–(2) SIPs must be 

submitted. 

• June 2011: States submit initial area 

designations recommendations, based 

on available monitoring data, and on 

any refined modeling performed in 

advance of submitting CAA section 

110(a)(1)–(2) SIPs. 

• June 2012: EPA issues initial area 

designations. Any monitored or 

modeled violations would trigger 

nonattainment designations. (Per below, 

States designated nonattainment would 

submit nonattainment SIPs by February 

2014, relying on refined modeling that 

demonstrates attainment by no later 

than August 2017.) States would be 

designated attainment if they submit 

both monitoring and modeling showing 

adequate evidence of no violations. All 

other cases would be initially 

designated as unclassifiable. 

• June 2013: States submit CAA 

section 110(a)(1)–(2) SIPs. SIPs would 

rely on refined modeling and any 

required monitoring that demonstrates 

attainment and maintenance of the new 

SO2 NAAQS as expeditiously as 

practicable, and no later than August 

2017. For areas within the State 

designated attainment and 

unclassifiable, the section 110(a) SIP 

must contain any additional Federally 

enforceable control measures necessary 

to ensure attainment and maintenance 

of the NAAQS. (Control measures to be 

implemented in designated 

nonattainment areas are due later as part 

of the nonattainment SIP in February 

2014.) 

• February 2014: Any initially 

designated nonattainment areas submit 

CAA section 191–192 SIPs showing 

attainment no later than August 2017. 

• June 2014: EPA approves or 

disapproves submitted CAA section 

110(a)(1)–(2) SIPs. For attainment and 

unclassifiable areas, EPA’s action would 

be based on adequacy of States’ 

modeling (and any required monitoring) 

showing attainment as expeditiously as 

practicable, and no later than August 

2017, in partial reliance on SO2

reductions from national and regional 

standards that are achieved by the 

attainment date. EPA would also have 

discretion to re-designate areas based on 

these SIPs, including to nonattainment 

if SIPs are inadequate, as well as 

promulgate FIPs. 

• February 2015: EPA approves or 

disapproves CAA section 191–192 

attainment SIPs submitted by areas 

initially designated as nonattainment, 

with similar remedies as discussed 

above if SIPs are deficient. 

• June 2016: CAA section 110(c) 

deadline by which EPA must issue a FIP 

for any area whose section 110(a)(1) SIP 

is disapproved in June 2014. 

• February 2017: CAA section 110(c) 

deadline by which EPA must issue a FIP 

for a nonattainment area whose section 

192 SIP is disapproved in February 

2015. 

August 2017: Expected date by which 

all areas, regardless of classification, 

achieve attainment, implementation, 

maintenance and enforcement of the 

new SO2 NAAQS. 

D. Attainment Planning Requirements 

1. SO2 Nonattainment Area SIP 

Requirements 

a. Approach Described in the Proposal 

We explained in the preamble to the 

proposal that any State containing an 

area designated as nonattainment with 

respect to the SO2 NAAQS would need 

to develop for submission to EPA a SIP 

meeting the requirements of part D, 

Title I, of the CAA, providing for 

attainment by the applicable statutory 

attainment date. See sections 191(a) and 

192(a) of the CAA. As indicated in 

section 191(a), all components of the 

SO2 part D SIP must be submitted 

within 18 months of the effective date 

of an area’s designation as 

nonattainment. 

Section 172 of the CAA addresses the 

general requirements for areas 

designated as nonattainment. Section 

172(c) directs States with nonattainment 

areas to submit a SIP which contains an 

attainment demonstration showing that 

the affected area will attain the standard 

by the applicable statutory attainment 

date. The SIP must show that the area 

will attain the standard as expeditiously 

as practicable, and must ‘‘provide for the 

implementation of all Reasonably 

Available Control Measures (RACM) as 

expeditiously as practicable (including 

such reductions in emissions from 

existing sources in the area as may be 

obtained through the adoption, at a 

minimum, of Reasonably Available 

Control Technology (RACT)).’’ 

SIPs required under Part D of the CAA 

must also provide for reasonable further 

progress (RFP). See section 172(c)(2) of 

the CAA. The CAA defines RFP as ‘‘such 

annual incremental reductions in 

emissions of the relevant air pollution 

as are required by part D, or may 

reasonably be required by the 

Administrator for the purpose of 

ensuring attainment of the applicable 

NAAQS by the applicable attainment 

date.’’ See section 171 of the CAA. 

Historically, for some pollutants, RFP 

has been met by showing annual 

incremental emission reductions 

sufficient to maintain generally linear 

progress toward attainment by the 

applicable attainment date. 

All SO2 nonattainment area SIPs must 

include contingency measures which 

must be implemented in the event that 

an area fails to meet RFP or fails to 

attain the standards by its attainment 

date. See section 172(c)(9) of the CAA. 

These contingency measures must be 

fully adopted rules or control measures 

that take effect without further action by 

the State or the Administrator. The EPA 

interprets this requirement to mean that 

the contingency measures must be 

implemented with only minimal further 

action by the State or the affected 

sources with no additional rulemaking 

actions such as public hearings or 

legislative review. 

Emission inventories are also critical 

for the efforts of State, local, and Federal 

agencies to attain and maintain the 

NAAQS that EPA has established for 

criteria pollutants including SO2. 

Section 191(a) in conjunction with 

section 172(c) requires that areas 

designated as nonattainment for SO2

submit an emission inventory to EPA no 

later than 18 months after designation as 

nonattainment. In the case of SO2, 


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37The terms ‘‘major’’ and ‘‘minor’’ define the size 

of a stationary source, for applicability purposes, in 

terms of an annual emissions rate (tons per year, 

tpy) for a pollutant. Generally, a minor source is 

any source that is not ‘‘major.’’ ‘‘Major’’ is defined 

by the applicable regulations—PSD or 

nonattainment NSR. 

38 In addition, the PSD program applies to noncriteria pollutants subject to regulation under the 

Act, except those pollutants regulated under section 

112 and pollutants subject to regulation only under 

section 211(o). 

sections 191(a) and 172(c) also direct 

States to submit periodic emission 

inventories for nonattainment areas. The 

periodic inventory must include 

emissions of SO2 for point, nonpoint, 

mobile, and area sources. 

b. Current Approach 

EPA did not receive any comments on 

this issue. Thus, EPA has no changes to 

make to this discussion. 

2. New Source Review and Prevention 

of Significant Deterioration 

Requirements 

a. Approach Described in the Proposal 

We provided a discussion of the new 

source review and prevention of 

significant deterioration programs in the 

preamble to the proposed rule. The 

Prevention of Significant Deterioration 

(PSD) and nonattainment New Source 

Review (NSR) programs contained in 

parts C and D of Title I of the CAA 

govern preconstruction review of any 

new or modified major stationary 

sources of air pollutants regulated under 

the CAA as well as any precursors to the 

formation of that pollutant when 

identified for regulation by the 

Administrator.37 The EPA rules 

addressing these programs can be found 

at 40 CFR 51.165, 51.166, 52.21, 52.24, 

and Part 51, appendix S. 

The PSD program applies when a 

major source located in an area that is 

designated as attainment or 

unclassifiable for any criteria pollutant 

is constructed or undergoes a major 

modification.38 The nonattainment NSR 

program applies on a pollutant-specific 

basis when a major source constructs or 

modifies in an area that is designated as 

nonattainment for that pollutant. The 

minor NSR program addresses major 

and minor sources that undergo 

construction or modification activities 

that do not qualify as major, and it 

applies, as necessary to assure 

attainment, regardless of the designation 

of the area in which a source is located. 

The PSD requirements include but are 

not limited to the following: 

• Installation of Best Available 

Control Technology (BACT); 

• Air quality monitoring and 

modeling analyses to ensure that a 

project’s emissions will not cause or 

contribute to a violation of any NAAQS 

or maximum allowable pollutant 

increase (PSD increment); 

• Notification of Federal Land 

Manager of nearby Class I areas; and 

public comment on the permit. 

To the extent necessary to address 

these PSD requirements for the new 

1-hour SO2 NAAQS, SIPs are due no 

later than 3 years after the promulgation 

date. Generally, however, the owner or 

operator of any major stationary source 

or major modification obtaining a final 

PSD permit on or after the effective date 

of the new 1-hour SO2 NAAQS will be 

required, as a prerequisite for the PSD 

permit, to demonstrate that the 

emissions increases from the new or 

modified source will not cause or 

contribute to a violation of that new 

NAAQS. The EPA anticipates that 

individual sources will be able to 

complete this demonstration under the 

PSD regulations based on current 

guidance in EPA’s Guideline on Air 

Quality Models, Appendix W of 40 CFR 

Part 51. 

The owner or operator of a new or 

modified source will still be required to 

demonstrate compliance with the 

annual and 24-hour SO2 increments, 

even when their counterpart NAAQS 

are revoked. The annual and 24-hour 

increments are established in the CAA 

and will need to remain in the PSD 

regulations because EPA does not 

interpret the CAA to authorize EPA to 

remove them. It appears necessary for 

Congress to amend the CAA to make 

appropriate changes to the statutory SO2

increments. In 1990, the CAA was 

amended to accommodate PM10

increments in lieu of the statutory TSP 

increments. 

In association with the requirement to 

demonstrate compliance with the 

NAAQS and increments, the owner or 

operator of a new or modified source 

must submit for review and approval a 

source impact analysis and an air 

quality analysis. The source impact 

analysis, primarily a modeling analysis, 

must demonstrate that allowable 

emissions increases from the proposed 

source or modification, in conjunction 

with emissions from other existing 

sources will not cause or contribute to 

either a NAAQS or increment violation. 

The air quality analysis must assess the 

ambient air quality in the area that the 

proposed source or modification would 

affect. 

For the air quality analysis, the owner 

or operator must submit in its permit 

application air quality monitoring data 

that shall have been gathered over a 

period of one year and is representative 

of air quality in the area of the proposed 

project. If existing data representative of 

the area of the proposed project is not 

available, new data may need to be 

collected by the owner or operator of the 

source or modification. Where data is 

already available, it might be necessary 

to evaluate the location of the 

monitoring sites from which the SO2

data were collected in comparison to 

any new siting requirements associated 

with the 1-hour SO2 NAAQS. If existing 

sites are inappropriate for providing the 

necessary representative data, then new 

monitoring data will need to be 

collected by the owner or operator of the 

proposed project. 

Historically, EPA has allowed the use 

of several screening tools to help 

facilitate the implementation of the new 

source review program by reducing the 

permit applicant’s burden, and 

streamlining the permitting process for 

de minimis circumstances. These 

screening tools include a significant 

emissions rate (SER), significant impact 

levels (SILs), and a significant 

monitoring concentration (SMC). The 

SER, as defined in tons per year for each 

regulated pollutant, is used to determine 

whether any proposed source or 

modification will emit sufficient 

amounts of a particular pollutant to 

require the review of that pollutant 

under the NSR permit program. EPA 

will consider whether to evaluate the 

existing SER for SO2 to see if it would 

change substantially based on the 

NAAQS levels for the 1-hour averaging 

period. Historically, for purposes of 

defining the SER, we have defined a de 

minimis pollutant impact as one that 

results in a modeled ambient impact of 

less than approximately 4% of the shortterm NAAQS. The current SER for SO2

(40 tpy) is based on the impact on the 

24-hour SO2 NAAQS. See 45 FR 52676, 

52707 (August 7, 1980). We have 

typically used the most sensitive 

averaging period to calculate the SER, 

and we may want to evaluate the new 

1-hour period for SO2 because it is 

likely to represent the most sensitive 

averaging period for SO2. 

The SIL, expressed as an ambient 

pollutant concentration (ug/m3), is used 

to determine whether the impact of a 

particular pollutant is significant 

enough to warrant a complete air quality 

impact analysis for any applicable 

NAAQS and increments. EPA has 

promulgated regulations under 40 CFR 

51.165(b) which include SILs for SO2 to 

determine whether a source’s impact 

would be considered to cause or 

contribute to a NAAQS violation for the 

3-hour (the secondary NAAQS), 24-hour 

or annual averaging periods. These SILs 

were originally developed in 1978 to 

limit the application of air quality 

dispersion models to a downwind 


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distance of no more than 50 kilometers 

or to ‘‘insignificant levels.’’ See 43 FR 

26398, June 19, 1978. Through 

guidance, EPA has also allowed the use 

of SILs to determine whether or not it 

is necessary for a source to carry out a 

comprehensive source impact analysis 

and to determine the extent of the 

impact area in which the analysis will 

be carried out. The existing SILs for SO2

were not developed on the basis of 

specific SO2 NAAQS levels, so there 

may be no need to revise the existing 

SILs. Even upon revocation of the 

annual and 24-hour NAAQS, the 

corresponding SIL should still be useful 

for increment assessment. A SIL for the 

1-hour averaging period does not exist, 

and would need to be developed for use 

with modeling for 1-hour SO2 NAAQS 

and any 1-hour increments. 

Finally, the SMC, also measured as an 

ambient pollutant concentration 

(μg/m3), is used to determine whether it 

may be appropriate to exempt a 

proposed project from the requirement 

to collect ambient monitoring data for a 

particular pollutant as part of a 

complete permit application. EPA first 

defined SMCs for regulated pollutants 

under the PSD program in 1980. See 45 

FR 52676, 52709–10 (August 7, 1980). 

The existing SMC for SO2, based on a 

24-hour averaging period, may need to 

be re-evaluated to consider the effect of 

basing the SMC on the 1-hour averaging 

period, especially in light of revocation 

of the NAAQS for the 24-hour averaging 

period. Third, even if the 1-hour 

averaging period does not indicate the 

need for a revised SMC for SO2, the fact 

that the original SMC for SO2 is based 

on 1980 monitoring data (Lowest 

Detectable Level, correction factor of 

‘‘5’’), could be a basis for revising the 

existing value. More up-to-date 

monitoring data and statistical analyses 

of monitoring accuracy may yield a 

different—possibly lower—correction 

factor today. The new 1-hour NAAQS 

will not necessarily cause this result, 

but may provide a ‘‘window of 

opportunity’’ to re-evaluate the SMC for 

SO2. 

States which have areas designated as 

nonattainment for the SO2 NAAQS are 

directed to submit, as a part of the SIP 

due 18 months after an area is 

designated as nonattainment, provisions 

requiring permits for the construction 

and operation of new or modified 

stationary sources anywhere in the 

nonattainment area. Prior to adoption of 

the SIP revision addressing major source 

nonattainment NSR for SO2

nonattainment areas, the requirements 

of 40 CFR part 51, appendix S will 

apply. Nonattainment NSR 

requirements include but are not limited 

to: 

• Installation of Lowest Achievable 

Emissions Rate (LAER) control 

technology; 

• Offsetting new emissions with 

creditable emissions reductions; 

• A certification that all major 

sources owned and operated in the State 

by the same owner are in compliance 

with all applicable requirements under 

the CAA; 

• An alternatives and siting analysis 

demonstrating that the benefits of a 

proposed source significantly outweigh 

the environmental and social costs 

imposed as a result of its location, 

construction, or modification; and 

• Public comment on the permit. 

Minor NSR programs must meet the 

statutory requirements in section 

110(a)(2)(C) of the CAA which requires 

‘‘* * * regulation of the modification 

and construction of any stationary 

source * * * as necessary to assure that 

the [NAAQS] are achieved.’’ These 

programs must be established in each 

State within 3 years of the promulgation 

of a new or revised NAAQS. 

b. Comments and Responses 

Several commenters stated that in 

order to avoid confusion and lag time as 

it relates to PSD/NSR and permitting 

activities, which must be taken by States 

following the promulgation of the 

revised NAAQS, EPA must provide 

guidance as soon as possible related to 

these issues. Commenters also stated 

that EPA must develop guidance as soon 

as possible to address the screening 

tools for PSD/NSR such as SILs, SERs, 

SMCs, and the development of 

increments. Several commenters also 

stated that guidance should be provided 

as it relates to the use of AERMOD to 

address PSD issues. 

The EPA acknowledges that a 

decision to promulgate a new short-term 

SO2 NAAQS will have implications for 

the air permitting process. The full 

extent of how a new short-term SO2

NAAQS will affect the NSR process will 

need to be carefully evaluated. First, 

major new and modified sources 

applying for NSR/PSD permits will 

initially be required to demonstrate that 

their proposed emissions increases of 

SO2 will not cause or contribute to a 

violation of any NAAQS or PSD 

increments for SO2, including the new 

1-hour SO2 NAAQS. In addition, we 

believe that section 166(c) of the CAA 

authorizes EPA to consider the need to 

promulgate a new 1-hour increment. 

Historically, EPA has developed 

increments for each applicable 

averaging period for which a NAAQS 

has been promulgated. However, 

increments for a particular pollutant do 

not necessarily need to match the 

averaging periods that have been 

established for NAAQS for the same 

pollutant. Environmental Defense Fund, 

Inc. v. EPA, 898 F.2d 183, 189–190 (DC 

Cir. 1990) (‘‘* * * the ‘goals and 

purposes’ of the PSD program, set forth 

in § 160, are not identical to the criteria 

on which the ambient standards are 

based.’’) Thus, we would need to 

evaluate the need for a new 1-hour SO2

increment in association with the goals 

and purposes of the statutory PSD 

program requirements. 

We agree with the commenters that 

there may be a need for EPA to provide 

additional screening tools or to revise 

existing screening tools that are 

frequently used under the NSR/PSD 

program for reducing the burden of 

completing SO2 ambient air impact 

analyses. These screening tools include 

the SILs, as mentioned by the 

commenter, but also include the SER for 

emissions of SO2 and the SMC for SO2. 

The existing sceening tools apply to the 

averaging periods used to define the 

existing NAAQS for SO2, including the 

annual, 24-hour, and 3-hour averaging 

periods. EPA intends to evaluate the 

need for possible changes or additions 

to each of these useful screening tools 

for SO2 due to the revision of the SO2

NAAQS to provide for a 1-hour 

standard. We believe it is highly likely 

that in order to be most useful for 

implementing the new 1-hour averaging 

period for NSR purposes, new 1-hour 

screening values will be appropriate. 

Finally, in response to the comment 

concerning the need for additional 

guidance as it relates to the use of 

AERMOD to address PSD issues, EPA 

anticipates providing additional 

technical guidance on modeling and 

analysis as a part of the SIP 

demonstration process. As stated 

previously, EPA intends to solicit public 

comment on guidance regarding 

modeling, and also solicit public 

comment on additional implementation 

planning guidance. However, EPA 

believes that the air quality models 

currently required for NSR/PSD 

permitting as provided in the EPA’s 

Guideline on Air Quality Models, 

Appendix W of CFR 40 Part 51 would 

be appropriate for demonstrating 

compliance with the revised SO2

NAAQS under these programs. At this 

time, EPA is not considering modifying 

the AERMOD dispersion model and its 

underlying science for predicting SO2

concentrations to accommodate the 

revised NAAQS for SO2.


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39Criteria pollutants are those pollutants for 

which EPA has established a NAAQS under section 

109 of the CAA. 

40Transportation conformity is required under 

CAA section 176(c) (42 U.S.C. 7506(c) to ensure that 

Federally supported highway and transit project 

activities are consistent with (‘‘conform to’’) the 

purpose of the SIP. Transportation conformity 

applies to areas that are designated nonattainment, 

and those areas redesignated to attainment after 

1990 (‘‘maintenance areas’’ with plans developed 

under CAA section 175A) for transportation-related 

criteria pollutants. Due to the relatively small 

amounts of sulfur in gasoline and on-road diesel 

fuel, transportation conformity does not apply to 

the SO2 NAAQS. 40 CFR 93.102(b)(1). 

c. Current Approach 

In the preamble to the proposed 

regulation, EPA noted that ‘‘PSD permit 

requirements are effective on the 

promulgation date of a new or revised 

standard.’’ However, this statement did 

not reflect an important distinction that 

needs to be clarified here. Under section 

51.166(b)(49)(i) and 52.21(b)(50)(i) of 

EPA’s regulations, a pollutant that has 

not been regulated previously would 

become a ‘‘regulated NSR pollutant’’ 

upon promulgation of a NAAQS. See, 75 

FR 17004, 17018–19. However, in the 

case of pollutants that are already 

‘‘regulated NSR pollutants,’’ at the time 

a new NAAQS is promulgated or an 

existing NAAQS is revised, EPA 

interprets the CAA and EPA regulations 

to require implementation of the new or 

revised standard in the Federal PSD 

permitting process upon the effective 

date of any new or revised standards. 

Section 165(a)(3) of the CAA and 

section 52.21(k) of EPA’s regulations 

require that a permit applicant 

demonstrate that it will not cause or 

contribute to a violation of ‘‘any’’ 

NAAQS. See, Memorandum from 

Stephen D. Page, Director of EPA Office 

of Air Quality Planning and Standards, 

‘‘Applicability of the Federal Prevention 

of Significant Deterioration Permit 

Requirements to New and Revised 

National Ambient Air Quality 

Standards’’ (April 1, 2010). 

Amendments to the existing PSD 

requirements set forth in EPA 

regulations concerning SILs, SERs and 

SMCs may involve notice and comment 

rulemaking which could take at least 

one year to complete. For PM2.5, EPA 

developed SERs under the initial NSR 

implementation requirements for PM2.5. 

See 73 FR 28321, May 16, 2008. The 

SILs and SMC for PM2.5 are being 

developed under a subsequent 

rulemaking simultaneously with the 

promulgation of PM2.5 increments, 

pursuant to a CAA schedule that allows 

EPA 2 years from the promulgation of 

new and revised NAAQS to promulgate 

increments. Under such an approach, 

SILs and SMC are not available until the 

increments are promulgated. States and 

industry have criticized that approach 

because it has left State permitting 

authorities without an EPA-approved de 

minimis value that could be used in 

determining the level of analysis that 

individual PSD sources must undergo, 

and could result in more detailed 

analyses for sources that will have only 

have de miminis impacts on the 

NAAQS. 

To address this concern, we believe it 

is appropriate to proceed with 

development of the PSD screening tools 

in advance of an increment rulemaking 

to hasten their availability. In addition, 

we are assessing the possibility of 

developing interim screening tools that 

can be used by States prior to the 

completion of the SIP-development 

process if the States establish an 

appropriate record for individual 

permitting actions based on the 

supporting technical information 

provided by EPA. It is our expectation, 

that if such interim screening tools are 

appropriate, we would make the interim 

SIL and the supporting record for EPA’s 

assessment available before the effective 

date of the new 1-hour SO2 NAAQS to 

facilitate more efficient PSD permit 

reviews once the new standard becomes 

effective. 

3. General Conformity 

a. Approach Described in the Proposal 

Section 176(c) of the CAA requires 

that all Federal actions conform to an 

applicable implementation plan 

developed pursuant to section 110 and 

part D of the CAA. The EPA rules 

developed under section 176(c) 

prescribe the criteria and procedures for 

demonstrating and assuring conformity 

of Federal actions to a SIP. Each Federal 

agency must determine that any actions 

covered by the general conformity rule 

conform to the applicable SIP before the 

action is taken. The criteria and 

procedures for conformity apply only in 

nonattainment areas and those 

nonattainment areas redesignated to 

attainment since 1990 (‘‘maintenance 

areas’’) with respect to the criteria 

pollutants under the CAA: 39 carbon 

monoxide (CO), lead (Pb), nitrogen 

dioxide (NO2), ozone (O3), particulate 

matter (PM2.5 and PM10), and sulfur 

dioxide (SO2). The general conformity 

rules apply one year following the 

effective date of designations for any 

new or revised NAAQS.40

The general conformity determination 

examines the impacts of direct and 

indirect emissions related to Federal 

actions. The general conformity rule 

provides several options to satisfy air 

quality criteria, such as modeling or 

offsets, and requires the Federal action 

to also meet any applicable SIP 

requirements and emissions milestones. 

The general conformity rule also 

requires that notices of draft and final 

general conformity determinations be 

provided directly to air quality 

regulatory agencies and to the public by 

publication in a local newspaper. 

b. Current Approach 

EPA did not receive any comments on 

this aspect of the discussion in the 

proposal and expects to follow that 

approach. 

E. Transition From the Existing SO2

NAAQS to a Revised SO2 NAAQS 

a. Proposal 

In addition to proposing a short-term 

1-hour SO2 NAAQS, EPA proposed to 

revoke the annual and 24-hour 

standards (annual 0.03 ppm and 24- 

hour 0.14 ppm). Specifically, EPA 

proposed that the level for the 1-hour 

standard for SO2 be a range between 50– 

100 ppb, and took comment on setting 

the level of the standard up to 150 ppb. 

We explained that if the Administrator 

sets the 1-hour standard at 100 ppb or 

lower, EPA proposed to revoke the 24- 

hour standard. If the Administrator set 

the level of the 1-hour standard between 

a range of 100–150 ppb, then EPA 

proposed to retain the 24-hour standard. 

We explained that if EPA revised the 

SO2 NAAQS and revoked either the 

annual or 24-hour standard, EPA would 

need to promulgate adequate antibacksliding provisions. The CAA 

establishes anti-backsliding 

requirements where EPA relaxes a 

NAAQS. Here, in EPA replacing the 

annual and 24-hour standards with a 

short term 1-hour standard, EPA must 

address the section 172(e) antibacksliding provision of the CAA and 

determine whether it applies on its face 

or by analogy, and what provisions are 

appropriate to provide for transition to 

the new standard. States will need to 

insure that the health protection 

provided under the prior SO2 NAAQS 

continues to be achieved as well as 

maintained as States begin to implement 

the new NAAQS. This means that States 

are directed to continue implementing 

attainment and maintenance SIPs 

associated with the prior SO2 NAAQS 

until such time as they are subsumed by 

any new planning and control 

requirements associated with the new 

NAAQS. 

Whether or not section 172(e) directly 

applies to EPA’s final action on the SO2

NAAQS, EPA has previously looked to 

other provisions of the CAA to 

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41The areas that are currently designated as 

nonattainment for the pre-existing SO2 primary 

NAAQS are Hayden, AZ; Armstrong, PA; Laurel, 

MT; Piti, GU; and Tanguisson, GU. The areas that 

are designated nonattainment for both the primary 

and the secondary standards are East Helena, MT, 

Salt Lake Co, MT, Toole Co, UT, and Warren Co, 

NJ. (See http://www.epa.gov/oar/oaqps/greenbk/ 

lnc.html). The Billings/Laurel, MT, area is the only 

area currently subject to a SIP call. 

backsliding. The CAA contains a 

number of provisions that indicate 

Congress’s intent to not allow 

provisions from implementation plans 

to be altered or removed if the plan 

revision would jeopardize the air 

quality protection being provided by the 

existing plan when EPA revises a 

NAAQS to make it more stringent. For 

example, section 110(l) provides that 

EPA may not approve a SIP revision if 

it interferes with any applicable 

requirement concerning attainment and 

RFP, or any other applicable 

requirement under the CAA. In 

addition, section 193 of the CAA 

prohibits the modification of a control, 

or a control requirement, in effect or 

required to be adopted as of November 

15, 1990 (i.e., prior to the promulgation 

of the Clean Air Act Amendments of 

1990), unless such a modification would 

ensure equivalent or greater emissions 

reductions. Further, section 172(e) of 

the CAA specifies that if EPA revises a 

NAAQS to make it less stringent than a 

previous NAAQS, control obligations no 

less stringent than those that apply in 

nonattainment area SIPs may not be 

relaxed, and adopting those controls 

that have not yet been adopted as 

needed may not be avoided. The intent 

of Congress, concerning the 

aforementioned sections of the CAA, 

was confirmed in a recent DC Circuit 

Court opinion on the Phase I ozone 

implementation rule. See South Coast 

Air Quality Management Dist. v. EPA, 

472 F.3d 882 (DC Cir. 2006). 

To ensure that the anti-backsliding 

provisions and principles of section 

172(e) are met and applied upon EPA 

revocation of the annual and 24-hour 

standards, EPA is providing that those 

SO2 NAAQS will remain in effect for 

one year following the effective date of 

the initial designations under section 

107(d)(1) for the new SO2 NAAQS 

before the current NAAQS are revoked 

in most attainment areas. However, any 

existing SIP provisions under CAA 

sections 110, 191 and 192 associated 

with the annual and 24-hour SO2

NAAQS will remain in effect, including 

all currently implemented planning and 

emissions control obligations, including 

both those in the State’s SIP and that 

have been promulgated by EPA in FIPs. 

This will ensure that both the new 

nonattainment NSR requirements and 

the general conformity requirements for 

a revised standard are in place so that 

there will be no gap in the public health 

protections provided by these two 

programs. It will also ensure that all 

nonattainment areas under the annual 

and/or 24-hour NAAQS and all areas for 

which SIP calls have been issued will 

continue to be protected by currently 

required control measures. 

EPA is also providing that the annual 

and 24-hour NAAQS remain in place for 

any current nonattainment area, or any 

area for which a State has not fulfilled 

the requirements of a SIP call, until the 

affected area submits, and EPA 

approves, a SIP with an attainment, 

implementation, maintenance and 

enforcement SIP which fully addresses 

the attainment and maintenance 

requirements of the new SO2 NAAQS. 

This, in combination with the CAA 

mechanisms provided in sections 110(l), 

193, and 172(e) will help to ensure that 

continued progress is made toward 

timely attainment of the SO2 NAAQS. 

Also, in light of the nature of the new 

SO2 NAAQS, the lack of classifications 

(and mandatory controls associated with 

such classifications pursuant to the 

CAA), and the small number of current 

nonattainment areas, and areas subject 

to SIP calls, EPA believes that retaining 

the current standard for a limited period 

of time until attainment and 

maintenance SIPs are approved for the 

new standard in current nonattainment 

areas and SIP call areas, and one year 

after designations in other areas, will 

adequately serve the anti-backsliding 

requirements and goals of the CAA.41

b. Comments and Responses 

Several commenters stated that they 

support EPA’s proposal stating that the 

annual and 24-hour SO2 NAAQS EPA 

would remain in effect for one year 

following the effective date of the initial 

designations under section 107(d)(1) for 

the revised SO2 NAAQS before the 

current NAAQS are revoked in most 

attainment areas. The commenters also 

support EPA’s proposal that any 

existing SIP provisions under CAA 

sections 110, 191 and 192 associated 

with the annual and 24-hour SO2

NAAQS would remain in effect, 

including all currently implemented 

planning and emissions control 

obligations, including both those in the 

State’s SIP and that have been 

promulgated by EPA in FIPs. Several 

commenters also stated that they 

support EPA’s proposal that an area’s 

nonattainment designation and the 

subsequent CAA requirements under 

the current SO2 NAAQS will remain in 

effect until the affected State submits, 

and EPA approves a SIP which meets all 

of the relevant CAA requirements for 

the affected nonattainment area. EPA 

appreciates the support of the 

commenters on its strategy for 

addressing the anti-backsliding 

requirements related to the current and 

revised SO2 standard, pursuant to 

section 172(e) of the CAA. 

One commenter, however, stated that 

while they support EPA’s proposal to 

address the anti-backsliding provisions 

of section 172(e) of the CAA, they 

believe that EPA’s proposal is deficient 

in several respects. The commenter 

stated that EPA’s proposal to not 

terminate the annual and 24-hour 

standards for SO2 in any nonattainment 

area, or any area for which a State has 

not fulfilled the requirements of a SIP 

call, until after the affected area submits 

and EPA approves a SIP with an 

attainment demonstration which fully 

‘‘addresses’’ the attainment requirements 

of the revised SO2 NAAQS is flawed. 

The commenter states that EPA’s use of 

the term ‘‘addresses’’ is impermissibly 

and arbitrarily ambiguous and that the 

agency needs to clarify that ‘‘fully 

addressing’’ the attainment requirements 

of the revised NAAQS actually means 

providing for timely attainment of the 

NAAQS, and the submittal of a SIP that 

fully meets all of the requirements of 

section 110 and part D of Title I of the 

CAA, including sections 172, 173, and 

191–193 of the CAA. 

Another commenter stated that the 

24-hour SO2 standard should not be 

revoked in attainment areas until EPA 

approves section 110(a)(2) 

‘‘infrastructure’’ SIPs under the new 1- 

hour standard for such areas, in order to 

avoid delays in between attainment 

designation and such SIP approvals 

resulting in leaving the public 

unprotected or creating inter-state 

conflict that triggers section 126 

petitions. This commenter further stated 

that the annual SO2 standard should not 

be revoked until EPA approves SIPs in 

attainment areas under the future SO2

secondary standard, which may also be 

based on an annual averaging time. 

EPA agrees with the comment made 

by the commenter regarding the need to 

approve SIPs in nonattainment areas 

(and in SIP call areas) before revoking 

the 24-hour and annual NAAQS for 

such areas. EPA clarifies that for those 

areas designated as nonattainment for 

the current NAAQS, or areas which 

have not met the requirements of a SIP 

call, that the State must submit a SIP 

that meets all of the applicable CAA 

requirements as they relate to section 

110 and part D of Title I of the CAA, 

including sections 110(a), 172, 173, and 

191–193 of the CAA. In addition to the 


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submittal of the SIP related to these 

requirements, EPA must approve the 

submittal for the area before the current 

standard can be revoked for the affected 

area. 

EPA disagrees with the comment. 

This rulemaking concerns only the 

primary standards for SO2. 74 FR at 

64812 n. 2. The annual SO2 standard is 

a primary standard, not a secondary 

standard. See 40 CFR section 50.4 (a). 

The exclusive secondary standard for 

SO2 is the 3-hour standard codified in 

40 CFR section 50.5. EPA is not 

determining the adequacy of this 

secondary standard in this review or 

this rulemaking, as just noted. The 

commenter’s request to retain the 

annual primary standard until SIPs 

reflecting a new secondary standard are 

approved is effectively a request to 

amend the present secondary standard, 

and is therefore inappropriate given the 

scope of this review. In any case, in the 

event that any substantive responsive to 

this comment is required, air quality 

information indicates that a 1-hour 

standard of 75 ppb is estimated to 

generally keep annual SO2

concentrations well below the level of 

the current annual standard. 74 FR at 

64845. Thus, there would be no loss of 

protection to public welfare due to 

revocation of the annual primary 

standard. 

EPA further disagrees with the 

commenter’s request that we not revoke 

the 24-hour standard in attainment areas 

before section 110(a)(2) ‘‘infrastructure’’ 

SIPs are approved under the new 1-hour 

SO2 standard. An area that has shown 

it has attained the 24-hour standard and 

that is not the subject of a SIP call, even 

after revocation of the 24-hour standard, 

will still have in its SIP its prior 

‘‘infrastucture’’ SIP elements. There is no 

need to delay revocation when that will 

not cause the area to become subject to 

a new SIP under the new 1-hour 

NAAQS any faster than the statute 

already requires (i.e., three years from 

the date of promulgation of the new 

NAAQS). Furthermore, as we have 

explained in sections III, IV, V and VI 

of this preamble, all areas are required 

by section 110(a)(1) of the Clean Air Act 

to submit such SIPs by June 2013, and 

we expect that to be approved they will 

all need to show attainment, 

implementation, maintenance and 

enforcement of the new NAAQS as 

expeditiously as practicable, which we 

believe is no later than August 2017. 

EPA believes this anticipated approach 

would more than sufficiently address 

the backsliding concerns raised by the 

commenter. 

c. Final 

EPA is making no changes to the 

proposed rule’s discussion of the 

transition strategy discussion for SO2

with the exception of the clarifications 

noted above. 

VII. Appendix T—Interpretation of the 

Primary NAAQS for Oxides of Sulfur 

and Revisions to the Exceptional Events 

Rule 

EPA proposed to add Appendix T, 

Interpretation of the Primary National 

Ambient Air Quality Standards for 

Oxides of Sulfur, to 40 CFR Part 50 in 

order to provide monitoring data 

handling procedures for the proposed 

SO2 1-hour primary standard. The 

proposed section 50.17 which sets the 

averaging period, level, indicator, and 

form of the NAAQS referred to this 

Appendix T. The proposed Appendix T 

detailed the computations necessary for 

determining when the proposed 1-hour 

primary SO2 NAAQS is met based on 

data from ambient monitoring and also 

addressed monitoring data reporting, 

data completeness considerations, and 

rounding conventions. 

EPA proposed two versions of 

Appendix T. The first applied to a 1- 

hour primary standard based on the 

annual 4th high value form, while the 

second applied to a 1-hour primary 

standard based on the 99th percentile 

daily value form. The final version of 

the Appendix reflects our choice to 

adopt the 99th percentile daily form (see 

section II. E.3 above). 

For the 1-hour primary standard, EPA 

proposed monitoring data handling 

procedures, a cross-reference to the 

Exceptional Events Rule, a grant of 

discretion for the Administrator to 

consider otherwise incomplete 

monitoring data to be complete, and a 

provision addressing the possibility of 

there being multiple SO2 monitors at 

one site. EPA is finalizing these 

proposals, with one change from the 

proposal with regard to the multiple 

monitor provision. 

EPA is also making certain drafting 

changes to the proposed regulatory text 

to clarify certain points and to assure 

that the regulatory text conforms with 

EPA’s intentions as stated in the 

preamble. Specifically, EPA has slightly 

edited the text of the rule from that 

proposed by adding the phrase ‘‘at an 

ambient air monitoring site’’ to section 

50.17 (b) and to section 1.1 of Appendix 

T to part 50, and also by adding a 

section 50.17 (c) stating that the level of 

the standard is to be measured by an 

FRM found in Appendix A or A–1 to 

Part 50, or by a properly designated 

FEM. Both of these provisions are being 

added to conform the text of the new 1- 

hour standard to the language of other 

NAAQS. See. e.g. the text of the 8-hour 

primary standard for ozone in section 

50.10 (a) and (b). The reference to ‘‘at an 

ambient monitoring site’’ makes clear 

that the regulatory text refers to 

situations where compliance with a 

NAAQS is measured by means of 

monitoring. This text does not restrict or 

otherwise address approaches which 

EPA or States may use to implement the 

new 1-hour NAAQS, which may 

include, for example, use of modeling 

(see sections III—VI above). See CAA 

sections 107 (d) (3) (A) (any ‘‘air quality 

data’’ may be used for redesignations); 

110 (a) (1) (which does not address the 

issue of the types of data States may use 

in devising plans for implementation, 

maintenance, and enforcement of a 

primary NAAQS); 192 (a) (which does 

not specify the types of data that may 

support a demonstration that a nonattainment area has attained a NAAQS). 

Similarly, EPA notes that Appendix T 

applies when ambient monitoring data 

is gathered and utilized in support of 

the new 1-hour SO2 NAAQS. As noted 

in sections III, IV, V, and VI above, there 

are circumstances when EPA is 

considering use of modeling in the SO2

NAAQS implementation effort, and 

other considerations would apply if and 

to the extent modeling is utilized. 

The EPA is also making SO2-specific 

changes to the deadlines in 40 CFR 

50.14, by which States must flag 

ambient air data that they believe have 

been affected by exceptional events and 

submit initial descriptions of those 

events, and to the deadlines by which 

States must submit detailed 

justifications to support the exclusion of 

those data from EPA monitoring-based 

determinations of attainment or 

nonattainment with the NAAQS. 

A. Interpretation of the Primary NAAQS 

for Oxides of Sulfur 

The purpose of a monitoring data 

interpretation rule for the SO2 NAAQS 

is to give effect to the form, level, 

averaging time, and indicator specified 

in the regulatory text at 40 CFR 50.17, 

anticipating and resolving in advance 

various future ambiguities that could 

otherwise occur regarding use of 

ambient monitoring data. The new 

Appendix T provides definitions and 

requirements that apply to the new 1- 

hour primary standard for SO2. The 

requirements concern how ambient 

monitoring data are to be reported, what 

ambient monitoring data are to be 

considered (including the issue of 

which of multiple monitors’ data sets 

will be used when more than one 

monitor has operated at a site), and the 


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applicability of the Exceptional Events 

Rule to the primary SO2 NAAQS. 

1. Proposed Interpretation of the 

Standard Based on Data From Ambient 

Monitoring 

With regard to monitoring data 

completeness for the proposed 1-hour 

primary standard, the proposed 

Appendix T followed past EPA practice 

for other NAAQS pollutants by 

requiring that in general at least 75% of 

the monitoring data that should have 

resulted from following the planned 

monitoring schedule in a period must be 

available for the key air quality statistic 

from that period to be considered valid. 

For the 1-hour primary SO2 NAAQS, the 

key air quality statistics are the daily 

maximum 1-hour concentrations in 

three successive years. It is important 

that sampling within a day encompass 

the period when concentrations are 

likely to be highest and that all seasons 

of the year are well represented. Hence, 

the 75% requirement was proposed to 

be applied at the daily and quarterly 

levels. 

Recognizing that there may be years 

with incomplete data, the proposed 

Appendix T for the 99th percentile form 

provided that a design value derived 

from incomplete monitoring data will 

nevertheless be considered valid if the 

relevant one of two diagnostic 

substitution tests validated such a 

design value as being either above the 

NAAQS level or equal to or below the 

NAAQS level. 

The first proposed diagnostic data 

substitution test, relevant when the 

design value derived from incomplete 

data was equal to or below the NAAQS 

level, was intended to identify those 

cases with incomplete monitoring data 

in which it nevertheless is very likely, 

if not virtually certain, that the daily 1- 

hour design value would have been 

observed to be less than or equal to the 

level of the NAAQS if monitoring data 

had been minimally complete. This test 

involved the substitution of a high 

historical concentration for any missing 

data. The second proposed diagnostic 

data substitution test, relevant when the 

design value derived from incomplete 

data was above the NAAQS level, was 

intended to identify those cases with 

incomplete monitoring data in which it 

nevertheless is very likely, if not 

virtually certain, that the daily 1-hour 

design value would have been observed 

to be above the level of the NAAQS if 

monitoring data had been minimally 

complete. This test involved the 

substitution of a low historical 

concentration for any missing data. 

It should be noted that one possible 

outcome of applying the relevant 

proposed substitution test is that a 3- 

year period with incomplete monitoring 

data may nevertheless be determined to 

not have a valid design value and thus 

to be unusable in making 1-hour 

primary NAAQS compliance 

determinations based on monitoring for 

that 3-year period. 

Also, we proposed that the 

Administrator have general discretion to 

use incomplete monitoring data based 

on case specific factors, either at the 

request of a State or at her own 

initiative. Similar provisions existed 

already for some other NAAQS. 

The 99th percentile version of the 

proposed Appendix T provided a table 

for determining which day’s maximum 

1-hour concentration will be used as the 

99th percentile concentration for the 

year. The proposed table is similar to 

one used now for the 24-hour PM2.5

NAAQS and the new 1-hour NO2

NAAQS, which are both based on a 98th 

percentile form, but adjusted to reflect 

a 99th percentile form for the 1-hour 

primary SO2 standard. The proposed 

Appendix T also provided instructions 

for rounding (not truncating) the average 

of three annual 99th percentile hourly 

concentrations before comparison to the 

level of the primary NAAQS. 

2. Comments on Interpretation of the 

Standard 

Several commenters expressed 

support for EPA’s proposed 75% 

completeness requirement for daily and 

quarterly monitoring data. A comment 

was received that the substitution test 

should not be used to make attainment 

or non-attainment designations. This 

commenter also said that the same 

completeness requirement as used for 

nonattainment should be used for 

attainment. Another commenter agreed 

that there should be completeness 

criteria, but thought that monitoring 

data should be substituted to make the 

set only 75% complete. We received one 

comment that the computation of design 

values where multiple monitors are 

present at a site should be averaged and 

not taken from a designated primary 

monitor. We received no comment on 

the provision which would afford the 

Administrator (or her delegee) 

discretion to use incomplete monitoring 

data based on specified factors and 

accordingly are adopting that provision 

as proposed. 

3. Conclusions on Interpretation of the 

Standard 

Consistent with the Administrator’s 

decision to adopt a 99th percentile form 

for the 1-hour NAAQS, the final version 

of Appendix T is based on that form. 

We agree with the three comments 

expressing the view that the 

requirement for 75% monitoring data 

completeness per quarter should apply 

with respect to the 1-hour standard. The 

final rule includes this requirement. 

We agree that nonattainment based on 

data from ambient monitoring should 

not be declared without a very high 

confidence that actual air quality did 

not meet the NAAQS, but we believe the 

proposed (and final) substitution test 

provides this irrefutable proof. In the 

relevant substitution test (Appendix T 

section 3.c.iii), the lowest daily 

maximum concentration observed in the 

same calendar quarter within the 3-year 

period is the value used in the 

substitution. Moreover, to guard against 

the possibility that even this lowest 

observed value is unrepresentative 

because only a small number of days 

that happened to have had poor air 

quality have valid monitoring data, 

substitution is permitted only if there 

are at least 200 days across the three 

matching quarters of the three years 

under consideration for which 75 

percent of the hours in the day have 

reported concentrations. (If less than 

200 days are available, the outcome is 

that no conclusion can be reached based 

on data from monitoring as to whether 

the NAAQS is met, an outcome which 

satisfies the concern expressed by the 

commenter.) While it is conceivable that 

the actual daily maximum concentration 

on the day(s) without sufficiently 

complete data could have been even 

lower than the value selected as the 

substitute value, the value that is 

selected for substitution will be quite 

low, and therefore it is extremely 

unlikely to be a candidate for selection 

as the annual 99th percentile daily 

maximum concentration. The actual 

effect of the data substitution, if any, is 

to change which of the actually 

observed and ranked daily maximum 

concentrations during the year is 

identified as the 99th percentile; the 

direction of the change, if any, will 

always be towards a lower design value. 

For example, if the substitution test of 

section 3.c.iii is used because there is 

one quarter of 92 days is missing 70 of 

its 92 daily maximum concentration 

values; causing there to be only 295 

days with valid daily values for the 

whole year, it would be necessary to 

substitute 47 values to make that quarter 

75 percent complete. This would result 

in 343 days of actual or substituted 

monitoring data for the year. The 

increase from 292 days to 342 days 

would cause the annual 99th percentile 

value to shift from the 3rd highest value 

to the 4th highest. Since a low 


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42Selecting the maximum or minimum observed 

concentration for an hour, the maximum or 

minimum annual 99th percentile, or the maximum 

or minimum three-year design value would 

introduce such a bias. Averaging multiple 1-hour 

measurements when available, designating one 

monitor as primary and using a second monitor’s 

measurement only when the primary monitor fails 

to give a valid measurement, or simply choosing to 

use the data record from only one of the monitors 

(on some basis that is independent of the 

concentration values obtained) would not introduce 

such a bias. 

concentration is being used for the 

substitution, it is impossible for the 4th 

highest value to itself be a substituted 

value. If this shift results in the 3-year 

design value remaining above the 

NAAQS, the failure to meet the NAAQS 

is confirmed. If this shift results in the 

3-year design value changing to be equal 

to or below the NAAQS, under the 

terms of the substitution test the 

outcome is that no conclusion could be 

reached based on this ambient 

monitoring data as to whether the 

NAAQS is met. Since either the same or 

a lower ranking actually measured 

concentration will always be identified, 

it is impossible for the outcome of the 

substitution test of section 3.c.iii to be 

that an area truly meeting the NAAQS 

based on ambient monitoring data is 

determined to not meet it based on 

ambient monitoring data. 

The commenter who said that the 

same completeness requirement should 

be used for nonattainment as for 

attainment appears to have been 

referring to a particular feature of the 

proposed diagnostic substitution test 

rather than to the basic completeness 

requirement of 75%, which in both the 

proposal and the final rule applies 

equally to both attainment and 

nonattainment situations. This 

particular feature is discussed in the 

next paragraph. 

The commenter who said that it is 

appropriate to substitute data to make 

the set only 75% complete appears to 

have taken note that in the proposed 

substitution test relevant in the case of 

an incomplete design value equal to or 

below the NAAQS (section 3.c.ii), data 

are substituted until 100% 

completeness is reached for the affected 

quarter, while in the test relevant in the 

case of an incomplete design value 

above the NAAQS (section 3.c.iii) data 

are substituted only until 75% 

completeness is reached. EPA believes 

this distinction is appropriate, and we 

have retained the 100% substitution 

limit in the final rule. In the case of an 

incomplete design value that is equal to 

or below the NAAQS, the concern is 

that the actual concentrations on the 

days without a valid daily maximum 1- 

hour concentration may have been quite 

high such that the concentration on one 

of those days would have been selected 

as the annual 99th percentile value. To 

be selected as the annual 99th percentile 

value, a daily maximum must be ranked 

no lower than the 4th highest daily 

value for the year. If substitution 

stopped when 75% of the days in a 

quarter had an actual or substituted 

value, there could be a situation in 

which only one, two, or three historical 

high values would need to be 

substituted to reach the 75% limit. It 

would therefore be possible for one of 

the actually measured concentrations 

(for the same or another quarter) to be 

identified as the annual 99th percentile 

value even if the substitution value is 

higher than any value actually 

measured, defeating the very purpose of 

the diagnostic test for an incomplete 

design value below the NAAQS, which 

is to essentially rule out the possibility 

of not meeting the NAAQS (when 

making monitoring-based 

determinations). The simplest way to 

ensure that at least four values are 

substituted (when there are at least four 

missing daily values) is to require 

substitution up to the 100% limit. 

With regard to situations with 

multiple monitors operating at one site, 

we note that there are few cases of this 

situation for SO2 monitoring. Of over 

500 SO2 monitoring sites in operation 

any time during 2007–2009, for 

example, only seven stations reported 1- 

hour data to the Air Quality System 

under two or more distinct Pollutant 

Occurrence Codes (POC). In the same 

period, collocated monitors reported 

data to AQS under distinct POCs for 

only one of over 400 nitrogen dioxide 

sites, for only two of almost 400 carbon 

monoxide sites, and for only eight of 

almost 1300 ozone sites. Even so, we 

believe is it important to have a well 

defined monitor data handling 

procedure for such situations. Also, 

there is a practical advantage in 

implementation if the same or similar 

procedure is used across NAAQS 

pollutants especially for these four 

gaseous pollutants that are measured on 

a 1-hour basis. A procedure that is 

simple to implement also has 

advantages in implementation. Finally, 

the procedure should not introduce any 

upward or downward bias in the 

determination of the design value for 

the monitoring site.42

The proposed procedure for multiple 

SO2 monitors was the same as EPA 

recently proposed and finalized for the 

new 1-hour NAAQS for nitrogen 

dioxide, where there were no adverse 

comments received on the proposal (75 

FR 6474, February 9, 2010). It is also the 

same as recently proposed in the 

reconsideration of the 8-hour ozone 

NAAQS (75 FR 2938, January 19, 2010). 

In the proposed procedure, in general, 

data from two monitors would never be 

mixed within a year but data from 

different monitors in different years 

could be used to calculate the 3-year 

design value. As noted above, one 

commenter on the SO2 proposal 

suggested that instead of designating a 

primary monitor when there are two 

monitors at a site, the measurements for 

an hour from multiple monitors should 

be averaged instead. EPA has also 

received at least one comment 

disagreeing with the recent proposal 

regarding multiple ozone monitors. The 

comment in the ozone rulemaking 

favored hour-by-hour substitution of 

data from a secondary monitor when the 

designated primary monitor has not 

given a value measurement, as opposed 

to the proposed restriction against 

mixing data within a year. These 

comments have caused us to rethink the 

direction set in the final NO2 rule and 

in the proposals for SO2 and ozone. We 

now believe that substitution of 

monitoring data hour-by-hour is an 

acceptable and in some ways superior 

approach to the other possible 

approaches, while averaging hour-byhour would be unduly complex. Also, 

averaging hour-by-hour might not be 

transparent depending on whether the 

averaging is done at the monitoring 

agency before submission to EPA or by 

EPA as part of calculating a design 

value. However, in light of the rarity of 

collocated monitors, it would be an 

unwarranted demand on limited EPA 

resources to develop and maintain 

software for hour-by-hour data 

substitution. Also, an hour-by-hour data 

substitution approach depends on the 

advance designation of a primary 

monitor, which itself could introduce 

confusion and would require software 

changes to EPA’s data system. 

Therefore, EPA believes that the most 

practical, and still a technically valid 

approach, is to allow monitoring 

agencies the option of hour-by-hour 

substitution between secondary and 

primary monitors before submission of 

data to EPA, and for EPA to select for 

use in calculating design values the one 

monitoring data record which has the 

highest degree of completeness for a 

given year. The final rule is based on 

this approach. EPA will also consider 

this approach when finalizing the ozone 

NAAQS reconsideration rule, and when 

proposing data interpretation provisions 

for a planned rulemaking to review the 

carbon monoxide NAAQS. The already 

finalized procedures for nitrogen 

dioxide data interpretation will be 


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implemented as promulgated, but will 

affect only an extremely small number 

of collocated SO2 monitoring situations. 

Finally, as proposed, the final version 

of Appendix T has a cross reference to 

the Exceptional Events Rule (40 CFR 

50.14) with regard to the exclusion of 

monitoring data affected by exceptional 

events. In addition, the specific steps for 

including such data in completeness 

calculations while excluding such data 

from actual design value calculations is 

clarified in Appendix T. 

B. Exceptional Events Information 

Submission Schedule 

The Exceptional Events Rule at 40 

CFR 50.14 contains generic deadlines 

for a State to submit to EPA specified 

information about exceptional events 

and associated air pollutant 

concentration data. A State must 

initially notify EPA that data have been 

affected by an event by July 1 of the 

calendar year following the year in 

which the event occurred; this is done 

by flagging the data in AQS and 

providing an initial event description. 

The State must also, after notice and 

opportunity for public comment, submit 

a demonstration to justify any claim 

within 3 years after the quarter in which 

the data were collected. However, if a 

regulatory decision based on the data 

(for example, a designation action) is 

anticipated, the schedule to flag data in 

AQS and submit complete 

documentation to EPA for review is 

shortened, and all information must be 

submitted to EPA no later than one year 

before the decision is to be made. 

These generic deadlines are suitable 

for the period after initial designations 

have been made under a NAAQS, when 

the decision that may depend on data 

exclusion is a redesignation from 

attainment to nonattainment or from 

nonattainment to attainment. However, 

these deadlines present problems with 

respect to initial designations under a 

newly revised NAAQS. One problem is 

that some of the deadlines, especially 

the deadlines for flagging some relevant 

data, may have already passed by the 

time the revised NAAQS is 

promulgated. Until the level and form of 

the NAAQS have been promulgated a 

State does not know whether the criteria 

for excluding data (which are tied to the 

level and form of the NAAQS) were met 

on a given day. Another problem is that 

it may not be feasible for information on 

some exceptional events that may affect 

final designations to be collected and 

submitted to EPA at least one year in 

advance of the final designation 

decision. This could have the 

unintended consequence of EPA 

designating an area nonattainment 

because of uncontrollable natural or 

other qualified exceptional events. 

The Exceptional Events Rule at 

section 50.14(c)(2)(v) indicates ‘‘when 

EPA sets a NAAQS for a new pollutant, 

or revises the NAAQS for an existing 

pollutant, it may revise or set a new 

schedule for flagging data for initial 

designation of areas for those NAAQS.’’ 

For the specific case of SO2, the 

signature date for the revised SO2

NAAQS is June 2, 2010. State/Tribal 

area designations recommendations will 

be due by June 2, 2011, and EPA will 

make initial area designations under the 

revised NAAQS by June 1, 2012 (since 

June 2, 2012 would be on a Saturday) 

and will be informed by air quality data 

from the years 2008–2010 or 2009–2011 

if there is sufficient data for these data 

years and by any refined modeling that 

is conducted. (See Sections III, V and VI 

above for more detailed discussions of 

the designation schedule and what data 

EPA expects to use.) Because final 

designations would be made by June 1, 

2012, all events to be considered during 

the designations process would have to 

be flagged and fully documented by 

States one year prior to designations, by 

June 1, 2011. A State would not be able 

to flag and submit documentation 

regarding events that occurred between 

June to December 2011 by one year 

before designations are made in June 

2012. 

EPA is adopting revisions to 40 CFR 

50.14 only to change submission dates 

for information supporting claimed 

exceptional events affecting SO2 data. 

The rule text at the end of this notice 

shows the changes that will apply to the 

new 1-hour SO2 NAAQS. For air quality 

data collected in 2008, we are extending 

the generic July 1, 2009 deadline for 

flagging data (and providing a brief 

initial description of the event) to 

October 1, 2010. EPA believes this 

extension will provide adequate time for 

States to review the impact of 

exceptional events from 2008 on the 

revised standard and notify EPA by 

flagging the relevant data in AQS. EPA 

is not changing the foreshortened 

deadline of June 1, 2011 for submitting 

documentation to justify an SO2-related 

exceptional event from 2008. We believe 

the generic deadline provides adequate 

time for States to develop and submit 

proper documentation. 

For data collected in 2009, EPA is 

extending the generic deadline of July 1, 

2010 for flagging data and providing 

initial event descriptions to October 1, 

2010. EPA is retaining the deadline of 

June 1, 2011 for States to submit 

documentation to justify an SO2-related 

exceptional event from 2009. For data 

collected in 2010, EPA is promulgating 

a deadline of June 1, 2011 for flagging 

data and providing initial event 

descriptions and for submitting 

documentation to justify exclusion of 

the flagged data. EPA believes that this 

deadline provides States with adequate 

time to review and identify potential 

exceptional events that occur in 

calendar year 2010, even for those 

events that might occur late in the year. 

EPA believes these deadlines will be 

feasible because experience suggests 

that exceptional events affecting SO2

data are few in number and easily 

assessed, so no State is likely to have a 

large workload. 

If a State intends 2011 data to be 

considered in SO2 designations, 2011 

data must be flagged and detailed event 

documentation submitted 60 days after 

the end of the calendar quarter in which 

the event occurred or by March 31, 

2012, whichever date occurs first. 

Again, EPA believes these deadlines 

will be feasible because experience 

suggest that exceptional events affecting 

SO2 data are few in number and easily 

assessed, so no State is likely to have a 

large workload. 

Table 1 summarizes the designation 

deadlines discussed in this section and 

provides designation schedule 

information from recent, pending or 

prior NAAQS revisions for other 

pollutants. EPA is revising the final SO2

exceptional event flagging and 

documentation submission deadlines 

accordingly to provide States with 

reasonably adequate opportunity to 

review, identify, and document 

exceptional events that may affect an 

area designation under a revised 

NAAQS. 


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TABLE 1—SCHEDULE FOR EXCEPTIONAL EVENT FLAGGING AND DOCUMENTATION SUBMISSION FOR DATA TO BE USED IN

DESIGNATIONS DECISIONS FOR NEW OR REVISED NAAQS 

NAAQS 

pollutant/standard/(level)/promulgation 

date 

Air quality data 

collected for 

calendar year 

Event flagging & initial description 

deadline 

Detailed documentation submission 

deadline 

PM2.5/24-Hr Standard (35 μg/m3) Promulgated October 17, 2006.2004–2006 October 1, 2007 a .................................... April 15, 2008 a. 

Ozone/8-Hr Standard (0.075 ppm) Promulgated March 12, 2008.2005–2007 

2008 

June 18, 2009 a .......................................

June 18, 2009 a .......................................

June 18, 2009 a. 

June 18, 2009 a. 

2009 60 Days after the end of the calendar 

quarter in which the event occurred or 

February 5, 2010, whichever date occurs first b.

60 Days after the end of the calendar 

quarter in which the event occurred or 

February 5, 2010, whichever date occurs first b. 

NO2/1-Hour Standard (80–100 PPB, final 

level TBD).

2008 

2009 

July 1, 2010 a ..........................................

July 1, 2010 a ..........................................

January 22, 2011 a. 

January 22, 2011 a. 

2010 April 1, 2011 a ......................................... July 1, 2011 a. 

SO2/1-Hour Standard (50–100 PPB, final 

level TBD).

2008 

2009 

October 1, 2010 b ....................................

October 1, 2010 b ....................................

June 1, 2011 b. 

June 1, 2011 b. 

2010 June 1, 2011 b ......................................... June 1, 2011 b. 

2011 60 Days after the end of the calendar 

quarter in which the event occurred or 

March 31, 2012, whichever date occurs first b.

60 Days after the end of the calendar 

quarter in which the event occurred or 

March 31, 2012, whichever date occurs first b. 

aThese dates are unchanged from those published in the original rulemaking, and are shown in this table for informational purposes—the 

Agency is not opening these dates for comment under this rulemaking. b Indicates change from general schedule in 40 CFR 50.14. 

Note: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most notably, for data used by EPA for redesignations to attainment. 

Note further that EPA is reprinting 

portions of this Table in section 5014 

but, with respect to the pollutants other 

than SO2, is doing so only for readers’ 

convenience and is not reopening or 

otherwise reconsidering any aspect of 

the rules related to these other 

pollutants. 

VIII. Communication of Public Health 

Information 

Information on the public health 

implications of ambient concentrations 

of criteria pollutants is currently made 

available primarily through EPA’s Air 

Quality Index (AQI) program. The 

current AQI has been in use since its 

inception in 1999 (64 FR 42530). It 

provides accurate, timely, and easily 

understandable information about daily 

levels of pollution (40 CFR 58.50). The 

AQI establishes a nationally uniform 

system of indexing pollution levels for 

nitrogen dioxide, carbon monoxide, 

ozone, particulate matter and SO2. The 

AQI converts pollutant concentrations 

in a community’s air to a number on a 

scale from 0 to 500. Reported AQI 

values enable the public to know 

whether air pollution levels in a 

particular location are characterized as 

good (0–50), moderate (51–100), 

unhealthy for sensitive groups (101– 

150), unhealthy (151–200), very 

unhealthy (201–300), or hazardous 

(300–500). The AQI index value of 100 

typically corresponds to the level of the 

short-term primary NAAQS for each 

pollutant. An AQI value greater than 

100 means that a pollutant is in one of 

the unhealthy categories (i.e., unhealthy 

for sensitive groups, unhealthy, very 

unhealthy, or hazardous) on a given 

day; an AQI value at or below 100 

means that a pollutant concentration is 

in one of the satisfactory categories (i.e., 

moderate or good). Decisions about the 

pollutant concentrations at which to set 

the various AQI breakpoints, that 

delineate the various AQI categories, 

draw directly from the underlying 

health information that supports the 

review of the primary NAAQS. 

The Agency recognizes the 

importance of revising the AQI in a 

timely manner to be consistent with any 

revisions to the primary NAAQS. 

Therefore, EPA proposed to finalize 

conforming changes to the AQI in 

connection with the Agency’s final 

decision on the SO2 NAAQS. 

Conforming changes that were proposed 

include setting the 100 level of the AQI 

at the same level as the revised primary 

SO2 standard if a short-term primary 

standard was promulgated, and revising 

the other AQI breakpoints at the lower 

end of the AQI scale (i.e., AQI values of 

50 and 150). EPA did not propose to 

change breakpoints at the higher end of 

the AQI scale (from 200 to 500), which 

would apply to State contingency plans 

or the Significant Harm Level (40 CFR 

51.16), because the information from 

this review does not inform decisions 

about breakpoints at those higher levels. 

With regard to an AQI value of 50, the 

breakpoint between the good and 

moderate categories, historically this 

value is set at the level of the annual 

NAAQS, if there is one, or one-half the 

level of the short-term NAAQS in the 

absence of an annual NAAQS (63 FR 

67823, Dec. 12, 1998). Taking into 

consideration this practice, EPA 

proposed to set the AQI value of 50 to 

be between 25 and 50 ppb SO2, 1-hour 

average; stating that concentrations 

toward the lower end of this range 

would be appropriate if the standard 

was set at the lower end of the range of 

proposed standard levels, while 

concentrations toward the higher end of 

this range would be more appropriate if 

the standard was set at the higher end 

of the range of proposed standard levels. 

EPA solicited comments on this range 

for an AQI value of 50 and the 

appropriate basis for selecting an AQI 

value of 50. 

With regard to an AQI value of 150, 

the breakpoint between the unhealthy 

for sensitive groups and unhealthy 

categories, historically values between 

the short-term standard and an AQI 

value of 500 are set at levels that are 

approximately equidistant between the 

AQI values of 100 and 500 unless there 

is health evidence that suggests a 

specific level would be appropriate (63 

FR 67829, Dec. 12, 1998). For an AQI 

value of 150, EPA proposed to set the 

breakpoint within the range from 175 to 

200 ppb SO2, 1-hour average, since it 

represents the midpoint between the 

proposed range for the short-term 


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standard and the level of an AQI value 

of 200 (300 ppb SO2, 1-hour average). 

EPA received few comments on the 

proposed breakpoints. Consistent with 

the level of the short-term primary SO2

standard promulgated in this rule, EPA 

is setting the AQI value of 100, the 

breakpoint between the moderate and 

unhealthy for sensitive groups category, 

at 75 ppb, 1-hour average. EPA is setting 

the AQI value of 50, the breakpoint 

between the good and moderate 

categories, at 35 ppb SO2, 1-hour 

average, which is approximately onehalf the level of the new short-term 

standard, since the annual SO2 standard 

is being revoked. EPA is setting the AQI 

value of 150, the breakpoint between the 

unhealthy for sensitive groups and 

unhealthy categories, at 185 ppb SO2, 1- 

hour average, which represents the 

approximate midpoint between the level 

of the new short-term standard (75 ppb 

SO2, 1-hour average) and the level of an 

AQI value of 200 (300 ppb SO2, 1-hour 

average). 

EPA received comments from several 

State environmental organizations and 

organizations of State and local air 

agencies about forecasting and reporting 

the AQI for SO2. These commenters 

expressed the view that forecasting 

hourly SO2 concentrations would be 

difficult. One commenter requested that 

EPA delay the forecasting requirement 

for one year and other agencies 

requested that EPA provide assistance 

in developing a forecast model. Another 

commenter expressed the view that it is 

impractical to incorporate SO2 into its 

forecasting and public health 

notification program because SO2 does 

not behave like a regional pollutant, and 

that exceedances may occur with little 

or no warning and for two hours or less. 

This commenter requested EPA 

consider the resources necessary for 

public communications at the State and 

local levels, particularly in areas where 

other air quality exceedances are 

relatively rare. 

EPA recommends and encourages air 

quality forecasting but it is not required 

(64 FR 42548; August 4, 1999). We agree 

that there will be new challenges 

associated with creating and 

communicating an SO2 forecast, and 

will work with State and local agencies 

that want to develop an SO2 forecasting 

program on issues including, but not 

limited to, forecasting air quality for 

short time periods. We plan to work 

with State and local air agencies to 

figure out the best way to present this 

information to the public using the AQI. 

With respect to the comment that it is 

impractical to incorporate SO2 into a 

forecasting and public health 

notification program because SO2 does 

not behave like a regional pollutant, this 

final rule departs from the proposed 

rule in that it allows for a combined 

monitoring and modeling approach. 

Because of this, the monitoring network 

is not required to be wholly sourceoriented in nature. States have 

flexibility to allow required monitoring 

sites to serve multiple monitoring 

objectives including characterizing 

source impacts, highest concentrations, 

population exposure, background, and 

regional transport. Further, EPA expects 

that much of the existing network will 

be retained by States to satisfy the 

minimum monitoring requirements. 

This means that it is unlikely that AQI 

reporting and forecasting will be heavily 

driven by source-oriented monitors. 

Rather, many of the existing monitors (a 

majority of which are community-wide 

monitors) will remain in place, which 

prevents the need for new geographic 

regions to be delineated. With respect to 

concerns expressed about the resources 

required to report the AQI in areas were 

exceedances of the standard are very 

rare, Appendix G to Part 58 specifies 

that if the index value for a particular 

pollutant remains below 50 for a season 

or year, then a State or local agency may 

exclude the pollutant from the 

calculation of the AQI. 

IX. Statutory and Executive Order 

Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review 

Under section 3(f)(1) of Executive 

Order 12866 (58 FR 51735, October 4, 

1993), this action is an ‘‘economically 

significant regulatory action’’ because it 

is likely to have an annual effect on the 

economy of $100 million or more. 

Accordingly, EPA submitted this action 

to the Office of Management and Budget 

(OMB) for review under EO 12866 and 

any changes made in response to OMB 

recommendations have been 

documented in the docket for this 

action. In addition, EPA prepared a 

Regulatory Impact Analysis (RIA) of the 

potential costs and benefits associated 

with this action. However, the CAA and 

judicial decisions make clear that the 

economic and technical feasibility of 

attaining the national ambient standards 

cannot be considered in setting or 

revising NAAQS, although such factors 

may be considered in the development 

of State implementation plans to 

implement the standards. Accordingly, 

although an RIA has been prepared, the 

results of the RIA have not been 

considered by EPA in developing this 

final rule. 

When estimating the SO2- and PM2.5- 

related human health benefits and 

compliance costs in Table 2 below, EPA 

applied methods and assumptions 

consistent with the state-of-the-science 

for human health impact assessment, 

economics and air quality analysis. EPA 

applied its best professional judgment 

in performing this analysis and believes 

that these estimates provide a 

reasonable indication of the expected 

benefits and costs to the nation of the 

selected SO2 standard and alternatives 

considered by the Agency. The 

Regulatory Impacts Analysis (RIA) 

available in the docket describes in 

detail the empirical basis for EPA’s 

assumptions and characterizes the 

various sources of uncertainties 

affecting the estimates below. 

EPA’s 2009 Integrated Science 

Assessment for Particulate Matter 

concluded, based on the scientific 

literature, that a no-threshold log-linear 

model most adequately portrays the PMmortality concentration-response 

relationship. Nonetheless, consistent 

with historical practice and our 

commitment to characterizing the 

uncertainty in our benefits estimates, 

EPA has included a sensitivity analysis 

with an assumed threshold in the PMmortality health impact function in the 

RIA. EPA has included a sensitivity 

analysis in the RIA to help inform our 

understanding of the health benefits 

which can be achieved at lower air 

quality concentration levels. While the 

primary estimate and the sensitivity 

analysis are not directly comparable, 

due to differences in population data 

and use of different analysis years, as 

well as the difference in the assumption 

of a threshold in the sensitivity analysis, 

comparison of the two results provide a 

rough sense of the proportion of the 

health benefits that occur at lower PM2.5

air quality levels. Using a threshold of 

10 μg/m3 is an arbitrary choice (EPA 

could have assumed 6, 8, or 12 μg/m3

for the sensitivity analysis). Assuming a 

threshold of 10 μg/m3, the sensitivity 

analysis shows that roughly one-third of 

the benefits occur at air quality levels 

below that threshold. Because the 

primary estimates reflect EPA’s current 

methods and data, EPA notes that 

caution should be exercised when 

comparing the results of the primary 

and sensitivity analyses. EPA 

appreciates the value of sensitivity 

analyses in highlighting the uncertainty 

in the benefits estimates and will 

continue to work to refine these 

analyses, particularly in those instances 

in which air quality modeling data are 

available. 

Table 2 shows the results of the cost 

and benefits analysis for each standard 

alternative. As indicated above, 

implementation of the SO2 control 


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measures identified from 

AirControlNET and other sources does 

not result in attainment with the all 

target NAAQS levels in several areas. In 

these areas, additional unspecified 

emission reductions might be necessary 

to reach some alternative standard 

levels. The first part of the table, labeled 

Partial attainment (identified controls), 

shows only those benefits and costs 

from control measures we were able to 

identify. The second part of the table, 

labeled Unidentified Controls, shows 

only additional benefits and costs 

resulting from unidentified controls. 

The third part of the table, labeled Full 

attainment, shows total benefits and 

costs resulting from both identified and 

unidentified controls. It is important to 

emphasize that we were able to identify 

control measures for a significant 

portion of attainment for many of those 

counties that would not fully attain the 

target NAAQS level with identified 

controls. Note also that in addition to 

separating full and partial attainment, 

the table also separates the portion of 

benefits associated with reduced SO2

exposure (i.e., SO2 benefits) from the 

additional benefits associated with 

reducing SO2 emissions, which are 

precursors to PM2.5 formation—(i.e., the 

PM2.5 co-benefits). For instance, for the 

selected standard of 75 ppb, $2.2 

million in benefits are associated with 

reduced SO2 exposure while $15 billion 

to $37 billion are associated with 

reduced PM2.5 exposure. 

TABLE 2—MONETIZED BENEFITS AND COSTS TO ATTAIN ALTERNATE STANDARD LEVELS IN 2020 

[Millions of 2006$] a

Number of 

counties 

fully 

controlled 

Discount 

rate 

(percent) 

Monetized 

SO2 benefits 

Monetized PM2.5

co-benefits,c,d Costs Net benefits 

Partial Attainment (identified controls) 

50 ppb .............................. 40 3 b $30,000 to $74,000 ... $2,600 $27,000 to $71,000. 

........................ 7 ........................ $28,000 to $67,000 ... ........................ $25,000 to $64,000. 

75 ppb .............................. 20 3 b $14,000 to $35,000 ... $960 $13,000 to $34,000. 

........................ 7 ........................ $13,000 to $31,000 ... ........................ $12,000 to $30,000. 

100 ppb ............................ 6 3 b $6,900 to $17,000 ..... $470 $6,400 to $17,000. 

........................ 7 ........................ $6,200 to $15,000 ..... ........................ $5,700 to $15,000. 

Unidentified Controls 

50 ppb .............................. 16 3 b $4,000 to $9,000 ....... $1,800 $2,200 to $7,200. 

........................ 7 ........................ $3,000 to $8,000 ....... ........................ $1,200 to $6,200. 

75 ppb .............................. 4 3 b $1,000 to $3,000 ....... $500 $500 to $1,500. 

........................ 7 ........................ $1,000 to $3,000 ....... ........................ $500 to $2,500. 

100 ppb ............................ 3 3 b $500 to $1,000 .......... $260 $240 to $740. 

........................ 7 ........................ $500 to $1,000 .......... ........................ $240 to $740. 

Full Attainment 

50 ppb .............................. 56 3 $8.50 $34,000 to $83,000 ... $4,400 $30,000 to $79,000. 

........................ 7 ........................ $31,000 to $75,000 ... ........................ $27,000 to $71,000. 

75 ppb .............................. 24 3 $2.20 $15,000 to $37,000 ... $1,500 $14,000 to $36,000 

........................ 7 ........................ $14,000 to $34,000 ... ........................ $13,000 to $33,000. 

100 ppb ............................ 9 3 $0.60 $7,400 to $18,000 ..... $730 $6,700 to $17,000. 

........................ 7 ........................ $6,700 to $16,000 ..... ........................ $6,000 to $15,000. 

a Estimates have been rounded to two significant figures and therefore summation may not match table estimates. b The approach used to simulate air quality changes for SO2 did not provide the data needed to distinguish partial attainment benefits from full 

attainment benefits from reduced SO2 exposure. Therefore, a portion of the SO2 benefits is attributable to the known controls and a portion of the 

SO2 benefits are attributable to the unidentified controls. Because all SO2-related benefits are short-term effects, the results are identical for all 

discount rates. c Benefits are shown as a range from Pope et al. (2002) to Laden et al. (2006). Monetized benefits do not include unquantified benefits, such 

as other health effects, reduced sulfur deposition, or improvements in visibility. d These models assume that all fine particles, regardless of their chemical composition, are equally potent in causing premature mortality because there is no clear scientific evidence that would support the development of differential effects estimates by particle type. Reductions in SO2

emissions from multiple sectors to meet the SO2 NAAQS would primarily reduce the sulfate fraction of PM2.5. Because this rule targets a specific 

particle precursor (i.e., SO2), this introduces some uncertainty into the results of the analysis. 

B. Paperwork Reduction Act 

The information collection 

requirements in this final rule have been 

submitted for approval to the Office of 

Management and Budget (OMB) under 

the Paperwork Reduction Act, 44 U.S.C. 

3501 et seq. The Information Collection 

Request (ICR) document prepared by 

EPA for these revisions to part 58 has 

been assigned EPA ICR number 2370.02. 

The information collected under 40 CFR 

part 53 (e.g., test results, monitoring 

records, instruction manual, and other 

associated information) is needed to 

determine whether a candidate method 

intended for use in determining 

attainment of the NAAQS in 40 CFR 

part 50 will meet the design, 

performance, and/or comparability 

requirements for designation as a 

Federal reference method (FRM) or 

Federal equivalent method (FEM). We 

do not expect the number of FRM or 

FEM determinations to increase over the 

number that is currently used to 

estimate burden associated with SO2

FRM/FEM determinations provided in 

the current ICR for 40 CFR part 53 (EPA 

ICR numbers 2370.01). As such, no 

change in the burden estimate for 40 

CFR part 53 has been made as part of 

this rulemaking. 

The information collected and 

reported under 40 CFR part 58 is needed 

to determine compliance with the 

NAAQS, to characterize air quality and 


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associated health impacts, to develop 

emissions control strategies, and to 

measure progress for the air pollution 

program. The amendments would revise 

the technical requirements for SO2

monitoring sites, require the siting and 

operation of additional SO2 ambient air 

monitors, and the reporting of the 

collected ambient SO2 monitoring data 

to EPA’s Air Quality System (AQS). The 

ICR is estimated to involve 102 

respondents for a total approximate cost 

of $15,203,762 (total capital, and labor 

and non-labor operation and 

maintenance) and a total burden of 

207,662 hours. The labor costs 

associated with these hours is 

$11,130,409. Included in the 

$15,203,762 total are other costs of other 

non-labor operations and maintenance 

of $1,104,377 and equipment and 

contract costs of $2,968,975. In addition 

to the costs at the State and local air 

quality management agencies, there is a 

burden to EPA for a total of 14,749 

hours and $1,060,621. Burden is defined 

at 5 CFR 1320.3(b). State, local, and 

Tribal entities are eligible for State 

assistance grants provided by the 

Federal government under the CAA 

which can be used for monitors and 

related activities. An agency may not 

conduct or sponsor, and a person is not 

required to respond to, a collection of 

information unless it displays a 

currently valid OMB control number. 

The OMB control numbers for EPA’s 

regulations in 40 CFR are listed in 40 

CFR part 9. 

C. Regulatory Flexibility Act 

The Regulatory Flexibility Act (RFA) 

generally requires an agency to prepare 

a regulatory flexibility analysis of any 

rule subject to notice and comment 

rulemaking requirements under the 

Administrative Procedure Act or any 

other statute unless the agency certifies 

that the rule will not have a significant 

economic impact on a substantial 

number of small entities. Small entities 

include small businesses, small 

organizations, and small governmental 

jurisdictions. 

For purposes of assessing the impacts 

of this rule on small entities, small 

entity is defined as: (1) A small business 

that is a small industrial entity as 

defined by the Small Business 

Administration’s (SBA) regulations at 13 

CFR 121.201; (2) a small governmental 

jurisdiction that is a government of a 

city, county, town, school district or 

special district with a population of less 

than 50,000; and (3) a small 

organization that is any not-for-profit 

enterprise which is independently 

owned and operated and is not 

dominant in its field. 

After considering the economic 

impacts of this final rule on small 

entities, I certify that this action will not 

have a significant economic impact on 

a substantial number of small entities. 

This final rule will not impose any 

requirements on small entities. Rather, 

this rule establishes national standards 

for allowable concentrations of SO2 in 

ambient air as required by section 109 

of the CAA. American Trucking Ass’ns 

v. EPA, 175 F.3d 1027, 1044–45 (DC Cir. 

1999) (NAAQS do not have significant 

impacts upon small entities because 

NAAQS themselves impose no 

regulations upon small entities). 

Similarly, the amendments to 40 CFR 

Part 58 address the requirements for 

States to collect information and report 

compliance with the NAAQS and will 

not impose any requirements on small 

entities. 

D. Unfunded Mandates Reform Act 

This action is not subject to the 

requirements of sections 202 and 205 of 

the UMRA. EPA has determined that 

this final rule does not contain a Federal 

mandate that may result in expenditures 

of $100 million or more for State, local, 

and Tribal governments, in the 

aggregate, or the private sector in any 

one year. The revisions to the SO2

NAAQS impose no enforceable duty on 

any State, local or Tribal governments or 

the private sector. The expected costs 

associated with the monitoring 

requirements are described in EPA’s ICR 

document, but those costs are not 

expected to exceed $100 million in the 

aggregate for any year. Furthermore, as 

indicated previously, in setting a 

NAAQS, EPA cannot consider the 

economic or technological feasibility of 

attaining ambient air quality standards. 

Because the CAA prohibits EPA from 

considering the types of estimates and 

assessments described in section 202 

when setting the NAAQS, the UMRA 

does not require EPA to prepare a 

written statement under section 202 for 

the revisions to the SO2 NAAQS. 

With regard to implementation 

guidance, the CAA imposes the 

obligation for States to submit SIPs to 

implement the SO2 NAAQS. In this final 

rule, EPA is merely providing an 

interpretation of those requirements. 

However, even if this rule did establish 

an independent obligation for States to 

submit SIPs, it is questionable whether 

an obligation to submit a SIP revision 

would constitute a Federal mandate in 

any case. The obligation for a State to 

submit a SIP that arises out of section 

110 and section 191 of the CAA is not 

legally enforceable by a court of law, 

and at most is a condition for continued 

receipt of highway funds. Therefore, it 

is possible to view an action requiring 

such a submittal as not creating any 

enforceable duty within the meaning of 

U.S.C. 658 for purposes of the UMRA. 

Even if it did, the duty could be viewed 

as falling within the exception for a 

condition of Federal assistance under 

U.S.C. 658. 

EPA has determined that this final 

rule contains no regulatory 

requirements that might significantly or 

uniquely affect small governments 

because it imposes no enforceable duty 

on any small governments. Therefore, 

this rule is not subject to the 

requirements of section 203 of the 

UMRA. 

E. Executive Order 13132: Federalism 

This final rule does not have 

federalism implications. It will not have 

substantial direct effects on the States, 

on the relationship between the national 

government and the States, or on the 

distribution of power and 

responsibilities among the various 

levels of government, as specified in 

Executive Order 13132. The rule does 

not alter the relationship between the 

Federal government and the States 

regarding the establishment and 

implementation of air quality 

improvement programs as codified in 

the CAA. Under section 109 of the CAA, 

EPA is mandated to establish NAAQS; 

however, CAA section 116 preserves the 

rights of States to establish more 

stringent requirements if deemed 

necessary by a State. Furthermore, this 

rule does not impact CAA section 107 

which establishes that the States have 

primary responsibility for 

implementation of the NAAQS. Finally, 

as noted in section E (above) on UMRA, 

this rule does not impose significant 

costs on State, local, or Tribal 

governments or the private sector. Thus, 

Executive Order 13132 does not apply 

to this rule. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

Executive Order 13175, entitled 

‘‘Consultation and Coordination with 

Indian Tribal Governments’’ (65 FR 

67249, November 9, 2000), requires EPA 

to develop an accountable process to 

ensure ‘‘meaningful and timely input by 

Tribal officials in the development of 

regulatory policies that have Tribal 

implications.’’ This final rule does not 

have Tribal implications, as specified in 

Executive Order 13175. It does not have 

a substantial direct effect on one or 

more Indian Tribes, on the relationship 

between the Federal government and 

Indian Tribes, or on the distribution of 

power and responsibilities between the 


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Federal government and Tribes. The 

rule does not alter the relationship 

between the Federal government and 

Tribes as established in the CAA and 

the TAR. Under section 109 of the CAA, 

EPA is mandated to establish NAAQS; 

however, this rule does not infringe 

existing Tribal authorities to regulate air 

quality under their own programs or 

under programs submitted to EPA for 

approval. Furthermore, this rule does 

not affect the flexibility afforded to 

Tribes in seeking to implement CAA 

programs consistent with the TAR, nor 

does it impose any new obligation on 

Tribes to adopt or implement any 

NAAQS. Finally, as noted in section E 

(above) on UMRA, this rule does not 

impose significant costs on Tribal 

governments. Thus, Executive Order 

13175 does not apply to this rule. 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

and Safety Risks 

This action is subject to Executive 

Order (62 FR 19885, April 23, 1997) 

because it is an economically significant 

regulatory action as defined by 

Executive Order 12866, and we believe 

that the environmental health risk 

addressed by this action has a 

disproportionate effect on children. The 

final rule will establish uniform 

national ambient air quality standards 

for SO2; these standards are designed to 

protect public health with an adequate 

margin of safety, as required by CAA 

section 109. The protection offered by 

these standards may be especially 

important for asthmatics, including 

asthmatic children, because respiratory 

effects in asthmatics are among the most 

sensitive health endpoints for SO2

exposure. Because asthmatic children 

are considered a sensitive population, 

we have evaluated the potential health 

effects of exposure to SO2 pollution 

among asthmatic children. These effects 

and the size of the population affected 

are discussed in chapters 3 and 4 of the 

ISA; chapters 3, 4, 7, 8, 9 of the REA, 

and sections II.A through II.E of this 

preamble. 

H. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution or Use 

This rule is not a ‘‘significant energy 

action’’ as defined in Executive Order 

13211, ‘‘Actions Concerning Regulations 

That Significantly Affect Energy Supply, 

Distribution, or Use’’ (66 FR 28355; May 

22, 2001) because it is not likely to have 

a significant adverse effect on the 

supply, distribution, or use of energy. 

The purpose of this rule is to establish 

revised NAAQS for SO2. The rule does 

not prescribe specific control strategies 

by which these ambient standards will 

be met. Such strategies will be 

developed by States on a case-by-case 

basis, and EPA cannot predict whether 

the control options selected by States 

will include regulations on energy 

suppliers, distributors, or users. Thus, 

EPA concludes that this rule is not 

likely to have any adverse energy 

effects. 

I. National Technology Transfer and 

Advancement Act 

Section 12(d) of the National 

Technology Transfer and Advancement 

Act of 1995 (NTTAA), Public Law 104– 

113, section 12(d) (15 U.S.C. 27) directs 

EPA to use voluntary consensus 

standards in its regulatory activities 

unless to do so would be inconsistent 

with applicable law or otherwise 

impractical. Voluntary consensus 

standards are technical standards (e.g., 

materials specifications, test methods, 

sampling procedures, and business 

practices) that are developed or adopted 

by voluntary consensus standards 

bodies. The NTTAA directs EPA to 

provide Congress, through OMB, 

explanations when the Agency decides 

not to use available and applicable 

voluntary consensus standards. 

This final rulemaking involves 

technical standards with regard to 

ambient monitoring of SO2. The use of 

this voluntary consensus standard 

would be impractical because the 

analysis method does not provide for 

the method detection limits necessary to 

adequately characterize ambient SO2

concentrations for the purpose of 

determining compliance with the final 

revisions to the SO2 NAAQS. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations 

Executive Order 12898 (59 FR 7629; 

Feb. 16, 1994) establishes Federal 

executive policy on environmental 

justice. Its main provision directs 

Federal agencies, to the greatest extent 

practicable and permitted by law, to 

make environmental justice part of their 

mission by identifying and addressing, 

as appropriate, disproportionately high 

and adverse human health or 

environmental effects of their programs, 

policies, and activities on minority 

populations and low-income 

populations in the United States. 

EPA has determined that this final 

rule will not have disproportionately 

high and adverse human health or 

environmental effects on minority or 

low-income populations because it 

increases the level of environmental 

protection for all affected populations 

without having any disproportionately 

high and adverse human health effects 

on any population, including any 

minority or low-income population. The 

final rule will establish uniform 

national standards for SO2 in ambient 

air. 

References 

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air pollution and health inequities: a 

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Perpect. 109(S3):357–374. 

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American Thoracic Society. (2000). What 

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pollution? Am J Respir Crit Care Med. 

161:665–673. 

Brode (2010). Representativeness of Ambient 

Monitoring Data for SO2 Epidemiological 

Studies. Sulfur Dioxide Review Docket. 

Docket ID No. EPA–HQ–OAR–2007– 

0352. Available at http:// 

www.regulations.gov. 

EPA. (1982). Air Quality Criteria for 

Particulate Matter and Sulfur Oxides. 

U.S. EPA, Research Triangle Park, NC: 

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cfpub.epa.gov/ncea/cfm/ 

recordisplay.cfm?deid=46205. 

EPA. (1986). Second Addendum to Air 

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and Sulfur Oxides (1982): Assessment of 

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Information. US EPA, Research Triangle 

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http://cfpub.epa.gov/ncea/cfm/ 

recordisplay.cfm?deid=39469. 

EPA. (1994a). Supplement to the Second 

Addendum (1986) to Air Quality Criteria 

for Particulate Matter and Sulfur Oxides 

(1982). Research Triangle Park, NC: 

Office of Health and Environmental 

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Assessment Office. EPA–600/FP–93/002. 

Available at: http://cfpub.epa.gov/ncea/ 

cfm/recordisplay.cfm?deid=96580. 

EPA. (1994b). Review of the National 

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to the 1986 OAQPS Staff Paper 

Addendum. Research Triangle Park, NC: 

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EPA. (2005). Review of the National Ambient 

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Staff Paper. Research Triangle Park, NC: 

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Standards. EPA–452/R–05–005a. 

Available at: http://www.epa.gov/ttn/ 

naaqs/standards/pm/data/ 

pmstaffpaper_20051221.pdf. 


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EPA (2006). Air Quality Criteria for Ozone 

and Related Photochemical Oxidants 

(Final); Available at: http:// 

www.epa.gov/ttn/naaqs/standards/ 

ozone/s_o3_cr_cd.html. 

EPA. (2007a). SO2 NAAQS Review Plan— 

Draft. US EPA, Research Triangle Park, 

NC: National Center for Environmental 

Assessment. Available at http:// 

www.epa.gov/ttn/naaqs/standards/so2/ 

s_so2_cr_pd.html. 

EPA. (2007b). Sulfur Dioxide Health 

Assessment Plan: Scope and Methods for 

Exposure and Risk Assessment. US EPA 

Research Triangle Park, NC: Office of Air 

Quality Planning and Standards, 

Research Triangle Park. Available at: 

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standards/so2/s_so2_cr_pd.html. 

EPA (2007c). Review of the National Ambient 

Air Quality Standards for Pb: Policy 

Assessment of Scientific and Technical 

Information. OAQPS Staff paper. Office 

of Air Quality Planning and Standards, 

Research Triangle Park, NC. EPA–452/R– 

07–013. Available at: 

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standards/pb/data/ 

20071101_pb_staff.pd. 

EPA. (2007d). Review of the National 

Ambient Air Quality Standards for 

Ozone: Assessment of Scientific and 

Technical Information, OAQPS Staff 

paper. Office of Air Quality Planning and 

Standards, Research Triangle Park, NC. 

EPA–452/R–07–007a. Available at: 

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ozone/s_o3_cr_sp.html. 

EPA. (2007e). Review of the National 

Ambient Air Quality Standards for 

Ozone: Ozone Health Risk Assessment 

for Selected Urban Areas. Office of Air 

Quality Planning and Standards, 

Research Triangle Park, NC. EPA 452/R– 

07–009. Available at: http:// 

www.epa.gov/ttn/naaqs/standards/ 

ozone/s_o3_cr_td.html. 

EPA. (2008a). Integrated Science Assessment 

(ISA) for Sulfur Oxides—Health Criteria 

(Final Report). EPA/600/R–08/047F. 

Available at: http://cfpub.epa.gov/ncea/ 

cfm/recordisplay.cfm?deid=198843. 

EPA. (2008b). Integrated Science Assessment 

(ISA) for Oxides of Nitrogen—Health 

Criteria (Final Report). EPA/600/R–08/ 

071 . Available at: http://www.epa.gov/ 

ttn/naaqs/standards/nox/ 

s_nox_cr_isi.html. 

EPA. (2008c). Risk and Exposure Assessment 

to Support the Review of the NO2

Primary National Ambient Air Quality 

Standard. EPA–452/R–08–008a; 

Available at: http://www.epa.gov/ttn/ 

naaqs/standards/nox/s_nox_cr_rea.html. 

EPA. (2009a). Risk and Exposure Assessment 

to Support the Review of the SO2

Primary National Ambient Air Quality 

Standards- Final Report. EPA–452/R–09– 

007; Available at: http:// 

www.epa.gov/ttn/naaqs/standards/so2/ 

s_so2_cr_rea.html. 

EPA. (2009b). Modern SO2 Instrument 

Performance Data. Spreadsheet of 

performance data for existing UVF 

analyzers. Office of Research and 

Development. Sulfur Dioxide Review 

Docket. Docket ID No. EPA–HQ–OAR– 

2007–0352. Available at http:// 

www.regulations.gov. 

EPA. (2010). Responses to Significant 

Comments on the 2009 Proposed Rule on 

the Primary National Ambient Air 

Quality Standards for Sulfur Dioxide 

(December 8, 2009; 74 FR 64810) Sulfur 

Dioxide Review Docket. Docket ID No. 

EPA–HQ–OAR–2007–0352. Available at 

http://www.regulations.gov. Also 

available at: http://www.epa.gov/ttn/ 

naaqs/standards/so2/s_so2_cr_rc.html. 

Henderson. (2006). Letter to EPA 

Administrator Stephen Johnson: Clean 

Air Scientific Advisory Committee’s 

(CASAC) Peer Review of the Agency’s 

2nd Draft Ozone Staff Paper. EPA– 

CASAC–07–001. October 24, 2006. 

Sulfur Dioxide Review Docket. Docket ID 

No. EPA–HQ–OAR–2007–0352–0044. 

Available at http://www.regulations.gov. 

Henderson. (2008a). Letter to EPA 

Administrator Stephen Johnson: Clean 

Air Scientific Advisory Committee’s 

(CASAC) Peer Review of EPA’s 

Integrated Science Assessment (ISA) for 

Sulfur Oxides—Health Criteria (Second 

External Review Draft, May 2008). EPA– 

CASAC–08–017 August 8, 2008. Sulfur 

Dioxide Review Docket. Available at 

http://www.regulations.gov. 

Henderson. (2008b). Letter to EPA 

Administrator Stephen Johnson: Clean 

Air Scientific Advisory Committee’s 

(CASAC) Peer Review of EPA’s Risk and 

Exposure Assessment to Support the 

Review of the SO2 Primary National 

Ambient Air Quality Standards (First 

Draft, July 2008). EPA–CASAC–08–019. 

August 22, 2008. Sulfur Dioxide Review 

Docket. Docket ID No. EPA–HQ–OAR– 

2007–0352–0034. Available at http:// 

www.regulations.gov. 

Ito K. (2007). Characterization of PM2.5, 

gaseous pollutants, and meteorological 

interactions in the context of time-series 

health effects models. J Expos Sci 

Environ Epidemiol. 17:S45–S60. 

Jaffe DH, Singer ME, Rimm AA. (2003). Air 

pollution and emergency department 

visits for asthma among Ohio medicaid 

recipients, 1991–1996. Environ Res. 

91:21–28. 

Johns. (2009). Presentation and analysis of 

controlled human exposure data 

described in Table 3–1 of the 2008 

Integrated Science Assessment (ISA) for 

Sulfur Oxides; April 29, 2009. Available 

at: http://www.epa.gov/ttn/naaqs/ 

standards/so2/s_so2_cr_rea.html. 

Johns and Simmons (2009). Memorandum to 

the Sulfur Oxides NAAQS Review 

Docket. Quality Assurance Review of 

Individual Subject Data Presented in 

Table 3–1 of the 2008 Integrated Science 

Assessment (ISA) for Sulfur Oxides. Air 

Quality Criteria for Sulfur Oxides 

Docket. Docket ID No. EPA–HQ–ORD– 

2006–0260–0036. Available at http:// 

www.regulations.gov. 

Linn WS, Avol EL, Peng RC, Shamoo DA, 

Hackney JD. (1987). Replicated doseresponse study of sulfur dioxide effects 

in normal, atopic, and asthmatic 

volunteers. Am Rev Respir Dis. 

136:1127–1134. 

NY DOH. (2006). A Study of Ambient Air 

Contaminants and Asthma in New York 

City. ATSDR Final Report #NTIS 

PB2006–113523. Albany, NY; New York 

State Energy Research and Development 

Authority; New York State Department 

of Health, for Atlanta, GA; Agency for 

Toxic Substances and Disease Registry; 

U.S. Department of Health and Human 

Services. 

Samet JM. (2009). Letter to EPA 

Administrator Lisa P. Jackson: Clean Air 

Scientific Advisory Committee’s 

(CASAC) Review of EPA’s Risk and 

Exposure Assessment to Support the 

Review of the SO2 Primary National 

Ambient Air Quality Standards: Second 

Draft. EPA–CASAC–09–007, May 18, 

2009. Sulfur Dioxide Review Docket. 

Docket ID No. EPA–HQ–OAR–2007– 

0352–0035. Available at http:// 

www.regulations.gov. 

Schwartz J. (1995). Short term fluctuations in 

air pollution and hospital admissions of 

the elderly for respiratory disease. 

Thorax. 50:531–538. 

Thompson R. (2009). Sulfur Dioxide 

Descriptive Statistics Tables. Office of 

Air Quality Planning and Standards, U.S. 

Environmental Protection Agency, 

Research Triangle Park, NC. Sulfur 

Dioxide Review Docket. Docket ID No. 

EPA–HQ–OAR–2007–0352–0036. 

Available at http://www.regulations.gov. 

Thompson R and Stewart MJ. (2009). Air 

Quality Statistics for Cities Referenced in 

Key U.S. and Canadian Hospital 

Admission and Emergency Department 

Visits for All Respiratory Causes and 

Asthma. Office of Air Quality Planning 

and Standards, U.S. Environmental 

Protection Agency, Research Triangle 

Park, NC. Sulfur Dioxide Review Docket. 

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0352–0018. Available at http:// 

www.regulations.gov. 

Watkins and Thompson. (2009). SO2 Network 

Review and Background; OAQPS; Office 

of Air Quality Planning and Standards, 

U.S. Environmental Protection Agency, 

Research Triangle Park, NC. Sulfur 

Dioxide NAAQS Review Docket. (OAR– 

2005–0352). Sulfur Dioxide Review 

Docket. Docket ID No. EPA–HQ–OAR– 

2007–0352–0037. Available at http:// 

www.regulations.gov. 

Wilson AM, Wake CP, Kelly T, Salloway JC. 

(2005). Air pollution, weather, and 

respiratory emergency room visits in two 

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Res. 97:312–321. 

List of Subjects 

40 CFR Part 50 

Environmental protection, Air 

pollution control, Carbon monoxide, 

Lead, Nitrogen dioxide, Ozone, 

Particulate matter, Sulfur oxides. 

40 CFR Part 53 

Environmental protection, 

Administrative practice and procedure, 

Air pollution control, Intergovernmental 


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relations, Reporting and recordkeeping 

requirements. 

40 CFR Part 58 

Environmental protection, 

Administrative practice and procedure, 

Air pollution control, Intergovernmental 

relations, Reporting and recordkeeping 

requirements. 

Dated: June 2, 2010. 

Lisa P. Jackson, 

Administrator. 

■ For the reasons stated in the preamble, 

title 40, chapter I of the Code of Federal 

Regulations is amended as follows: 

PART 50—NATIONAL PRIMARY AND 

SECONDARY AMBIENT AIR QUALITY 

STANDARDS 

■ 1. The authority citation for part 50 

continues to read as follows: 

Authority: 42 U.S.C. 7401, et seq. 

■ 2. Section 50.4 is amended by adding 

paragraph (e) to read as follows: 

§ 50.4 National primary ambient air quality 

standards for sulfur oxides (sulfur dioxide). 

* * * * * 

(e) The standards set forth in this 

section will remain applicable to all 

areas notwithstanding the promulgation 

of SO2 national ambient air quality 

standards (NAAQS) in § 50.17. The SO2

NAAQS set forth in this section will no 

longer apply to an area one year after 

the effective date of the designation of 

that area, pursuant to section 107 of the 

Clean Air Act, for the SO2 NAAQS set 

forth in § 50. 17; except that for areas 

designated nonattainment for the SO2

NAAQS set forth in this section as of the 

effective date of § 50. 17, and areas not 

meeting the requirements of a SIP call 

with respect to requirements for the SO2

NAAQS set forth in this section, the SO2

NAAQS set forth in this section will 

apply until that area submits, pursuant 

to section 191 of the Clean Air Act, and 

EPA approves, an implementation plan 

providing for attainment of the SO2

NAAQS set forth in § 50.17. 

■ 3. Section 50.14 is amended by 

revising paragraph (c)(2)(vi) to read as 

follows: 

§ 50.14 Treatment of air quality monitoring 

data influenced by exceptional events. 

* * * * * 

(c) * * * 

(2) * * * 

(vi) When EPA sets a NAAQS for a 

new pollutant or revises the NAAQS for 

an existing pollutant, it may revise or 

set a new schedule for flagging 

exceptional event data, providing initial 

data descriptions and providing detailed 

data documentation in AQS for the 

initial designations of areas for those 

NAAQS. Table 1 provides the schedule 

for submission of flags with initial 

descriptions in AQS and detailed 

documentation. These schedules shall 

apply for those data which will or may 

influence the initial designation of areas 

for those NAAQS. EPA anticipates 

revising Table 1 as necessary to 

accommodate revised data submission 

schedules for new or revised NAAQS. 

TABLE 1—SCHEDULE OF EXCEPTIONAL EVENT FLAGGING AND DOCUMENTATION SUBMISSION FOR DATA TO BE USED IN

DESIGNATIONS DECISIONS FOR NEW OR REVISED NAAQS 

NAAQS 

Pollutant/standard/(level)/promulgation 

date 

Air quality data 

collected for 

calendar year 

Event flagging & initial description 

deadline 

Detailed documentation submission 

deadline 

PM2.5/24-Hr Standard (35 μg/m3) Promulgated October 17, 2006.2004–2006 October 1, 2007 a .................................... April 15, 2008. a

Ozone/8-Hr Standard (0.075 ppm) Promulgated March 12, 2008.2005–2007 

2008 

2009 

June 18, 2009 a .......................................

June 18, 2009 a .......................................

60 days after the end of the calendar 

quarter in which the event occurred or 

February 5, 2010, whichever date occurs first b.

June 18, 2009 a

June 18, 2009 1

60 days after the end of the calendar 

quarter in which the event occurred or 

February 5, 2010, whichever date occurs first.b

NO2/1-Hour Standard (80–100 PPB, final 

level TBD).

2008 

2009 

2010 

July 1, 2010 a ...........................................

July 1, 2010 a ...........................................

April 1, 2011 a ..........................................

January 22, 2011. a

January 22, 2011. a

July 1, 2010. a

SO 2/1-Hour Standard (50–100 PPB, 

final level TBD).

2008 

2009 

2010 

October 1, 2010 b ....................................

October 1, 2010 b ....................................

June 1, 2011. b ........................................

June 1, 2011. b

June 1, 2011. b

June 1, 2011. b

2011 60 days after the end of the calendar 

quarter in which the event occurred or 

March 31, 2012, whichever date occurs first b.

60 days after the end of the calendar 

quarter in which the event occurred or 

March 31, 2012, whichever date occurs first. b

aThese dates are unchanged from those published in the original rulemaking, or are being proposed elsewhere and are shown in this table for 

informational purposes—the Agency is not opening these dates for comment under this rulemaking. b Indicates change from general schedule in 40 CFR 50.14. 

Note: EPA notes that the table of revised 

deadlines only applies to data EPA will use 

to establish the final initial designations for 

new or revised NAAQS. The general 

schedule applies for all other purposes, most 

notably, for data used by EPA for 

redesignations to attainment. 

* * * * * 

■ 4. A new 50.17 is added to read as 

follows: 

§ 50.17 National primary ambient air 

quality standards for sulfur oxides (sulfur 

dioxide). 

(a) The level of the national primary 

1-hour annual ambient air quality 

standard for oxides of sulfur is 75 parts 

per billion (ppb, which is 1 part in 

1,000,000,000), measured in the ambient 

air as sulfur dioxide (SO2). 

(b) The 1-hour primary standard is 

met at an ambient air quality monitoring 

site when the three-year average of the 

annual (99th percentile) of the daily 

maximum 1-hour average 

concentrations is less than or equal to 

75 ppb, as determined in accordance 

with Appendix T of this part. 

(c) The level of the standard shall be 

measured by a reference method based 

on Appendix A or A–1 of this part, or 

by a Federal Equivalent Method (FEM) 


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designated in accordance with part 53 of 

this chapter. 

■ 5. Add Appendix A–1 to Part 50 to 

read as follows: 

Appendix A–1 to Part 50—Reference 

Measurement Principle and Calibration 

Procedure for the Measurement of 

Sulfur Dioxide in the Atmosphere 

(Ultraviolet Fluorescence Method) 

1.0 Applicability 

1.1 This ultraviolet fluorescence (UVF) 

method provides a measurement of the 

concentration of sulfur dioxide (SO2) in 

ambient air for determining compliance with 

the national primary and secondary ambient 

air quality standards for sulfur oxides (sulfur 

dioxide) as specified in § 50.4, § 50.5, and 

§ 50.17 of this chapter. The method is 

applicable to the measurement of ambient 

SO2 concentrations using continuous (realtime) sampling. Additional quality assurance 

procedures and guidance are provided in part 

58, Appendix A, of this chapter and in 

Reference 3. 

2.0 Principle 

2.1 This reference method is based on 

automated measurement of the intensity of 

the characteristic fluorescence released by 

SO2 in an ambient air sample contained in 

a measurement cell of an analyzer when the 

air sample is irradiated by ultraviolet (UV) 

light passed through the cell. The fluorescent 

light released by the SO2 is also in the 

ultraviolet region, but at longer wavelengths 

than the excitation light. Typically, optimum 

instrumental measurement of SO2

concentrations is obtained with an excitation 

wavelength in a band between approximately 

190 to 230 nm, and measurement of the SO2

fluorescence in a broad band around 320 nm, 

but these wavelengths are not necessarily 

constraints of this reference method. 

Generally, the measurement system 

(analyzer) also requires means to reduce the 

effects of aromatic hydrocarbon species, and 

possibly other compounds, in the air sample 

to control measurement interferences from 

these compounds, which may be present in 

the ambient air. References 1 and 2 describe 

UVF method. 

2.2 The measurement system is calibrated 

by referencing the instrumental fluorescence 

measurements to SO2 standard 

concentrations traceable to a National 

Institute of Standards and Technology (NIST) 

primary standard for SO2 (see Calibration 

Procedure below). 

2.3 An analyzer implementing this 

measurement principle is shown 

schematically in Figure 1. Designs should 

include a measurement cell, a UV light 

source of appropriate wavelength, a UV 

detector system with appropriate wave length 

sensitivity, a pump and flow control system 

for sampling the ambient air and moving it 

into the measurement cell, sample air 

conditioning components as necessary to 

minimize measurement interferences, 

suitable control and measurement processing 

capability, and other apparatus as may be 

necessary. The analyzer must be designed to 

provide accurate, repeatable, and continuous 

measurements of SO2 concentrations in 

ambient air, with measurement performance 

as specified in Subpart B of Part 53 of this 

chapter. 

2.4 Sampling considerations: The use of 

a particle filter on the sample inlet line of a 

UVF SO2 analyzer is required to prevent 

interference, malfunction, or damage due to 

particles in the sampled air. 

3.0 Interferences 

3.1 The effects of the principal potential 

interferences may need to be mitigated to 

meet the interference equivalent 

requirements of part 53 of this chapter. 

Aromatic hydrocarbons such as xylene and 

naphthalene can fluoresce and act as strong 

positive interferences. These gases can be 

removed by using a permeation type scrubber 

(hydrocarbon ‘‘kicker’’). Nitrogen oxide (NO) 

in high concentrations can also fluoresce and 

cause positive interference. Optical filtering 

can be employed to improve the rejection of 

interference from high NO. Ozone can absorb 

UV light given off by the SO2 molecule and 

cause a measurement offset. This effect can 

be reduced by minimizing the measurement 

path length between the area where SO2

fluorescence occurs and the photomultiplier 

tube detector (e.g. <5 cm). A hydrocarbon 

scrubber, optical filter and appropriate 

distancing of the measurement path length 

may be required method components to 

reduce interference. 

4.0 Calibration Procedure 

Atmospheres containing accurately known 

concentrations of sulfur dioxide are prepared 

using a compressed gas transfer standard 

diluted with accurately metered clean air 

flow rates. 

4.1 Apparatus: Figure 2 shows a typical 

generic system suitable for diluting a SO2 gas 

cylinder concentration standard with clean 

air through a mixing chamber to produce the 

desired calibration concentration standards. 

A valve may be used to conveniently divert 

the SO2 from the sampling manifold to 

provide clean zero air at the output manifold 

for zero adjustment. The system may be made 

up using common laboratory components, or 

it may be a commercially manufactured 

system. In either case, the principle 

components are as follows: 

4.1.1 SO2 standard gas flow control and 

measurement devices (or a combined device) 

capable of regulating and maintaining the 

standard gas flow rate constant to within ±2 

percent and measuring the gas flow rate 

accurate to within ±2, properly calibrated to 

a NIST-traceable standard. 

4.1.2 Dilution air flow control and 

measurement devices (or a combined device) 

capable of regulating and maintaining the air 

flow rate constant to within ±2 percent and 

measuring the air flow rate accurate to within 

±2, properly calibrated to a NIST-traceable 

standard. 

4.1.3 Mixing chamber, of an inert 

material such as glass and of proper design 

to provide thorough mixing of pollutant gas 

and diluent air streams. 

4.1.4 Sampling manifold, constructed of 

glass, polytetrafluoroethylene (PTFE 

TeflonTM), or other suitably inert material 

and of sufficient diameter to insure a 

minimum pressure drop at the analyzer 

connection, with a vent designed to insure a 

minimum over-pressure (relative to ambient 

air pressure) at the analyzer connection and 

to prevent ambient air from entering the 

manifold. 

4.1.5 Standard gas pressure regulator, of 

clean stainless steel with a stainless steel 

diaphragm, suitable for use with a high 

pressure SO2 gas cylinder. 

4.1.6 Reagents 

4.1.6.1 SO2 gas concentration transfer 

standard having a certified SO2 concentration 

of not less than 10 ppm, in N2, traceable to 

a NIST Standard Reference Material (SRM). 

4.1.6.2 Clean zero air, free of 

contaminants that could cause a detectable 

response or a change in sensitivity of the 

analyzer. Since ultraviolet fluorescence 

analyzers may be sensitive to aromatic 

hydrocarbons and O2-to-N2 ratios, it is 

important that the clean zero air contains less 

than 0.1 ppm aromatic hydrocarbons and O2

and N2 percentages approximately the same 

as in ambient air. A procedure for generating 

zero air is given in reference 1. 

4.2 Procedure 

4.2.1 Obtain a suitable calibration 

apparatus, such as the one shown 

schematically in Figure 1, and verify that all 

materials in contact with the pollutant are of 

glass, TeflonTM, or other suitably inert 

material and completely clean. 

4.2.2 Purge the SO2 standard gas lines 

and pressure regulator to remove any 

residual air. 

4.2.3 Ensure that there are no leaks in the 

system and that the flow measuring devices 

are properly and accurately calibrated under 

the conditions of use against a reliable 

volume or flow rate standard such as a soapbubble meter or a wet-test meter traceable to 

a NIST standard. All volumetric flow rates 

should be corrected to the same reference 

temperature and pressure by using the 

formula below: 

F F c m = ( ) +

298 15

760 273 15

.

.

P

T

m

m

Where: 

Fc = corrected flow rate (L/min at 25 °C and 

760 mm Hg), 

Fm = measured flow rate, (at temperature, Tm

and pressure, Pm), 

Pm = measured pressure in mm Hg, 

(absolute), and 

Tm = measured temperature in degrees 

Celsius. 

4.2.4 Allow the SO2 analyzer under 

calibration to sample zero air until a stable 

response is obtained, then make the proper 

zero adjustment. 

4.2.5 Adjust the airflow to provide an SO2

concentration of approximately 80 percent of 

the upper measurement range limit of the 

SO2 instrument and verify that the total air 

flow of the calibration system exceeds the 

demand of all analyzers sampling from the 

output manifold (with the excess vented). 

4.2.6 Calculate the actual SO2 calibration 

concentration standard as: 


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SO C

F

F

p

t

[ ] 2 =

Where: 

C = the concentration of the SO2 gas standard 

Fp = the flow rate of SO2 gas standard 

Ft = the total air flow rate of pollutant and 

diluent gases 

4.2.7 When the analyzer response has 

stabilized, adjust the SO2 span control to 

obtain the desired response equivalent to the 

calculated standard concentration. If 

substantial adjustment of the span control is 

needed, it may be necessary to re-check the 

zero and span adjustments by repeating steps 

4.2.4 through 4.2.7 until no further 

adjustments are needed. 

4.2.8 Adjust the flow rate(s) to provide 

several other SO2 calibration concentrations 

over the analyzer’s measurement range. At 

least five different concentrations evenly 

spaced throughout the analyzer’s range are 

suggested. 

4.2.9 Plot the analyzer response (vertical 

or Y-axis) versus SO2 concentration 

(horizontal or X-axis). Compute the linear 

regression slope and intercept and plot the 

regression line to verify that no point 

deviates from this line by more than 2 

percent of the maximum concentration 

tested. 

Note: Additional information on 

calibration and pollutant standards is 

provided in Section 12 of Reference 3. 

5.0 Frequency of Calibration 

The frequency of calibration, as well as the 

number of points necessary to establish the 

calibration curve and the frequency of other 

performance checking will vary by analyzer; 

however, the minimum frequency, 

acceptance criteria, and subsequent actions 

are specified in Reference 3, Appendix D: 

Measurement Quality Objectives and 

Validation Template for SO2 (page 9 of 30). 

The user’s quality control program should 

provide guidelines for initial establishment 

of these variables and for subsequent 

alteration as operational experience is 

accumulated. Manufacturers of analyzers 

should include in their instruction/operation 

manuals information and guidance as to 

these variables and on other matters of 

operation, calibration, routine maintenance, 

and quality control. 

6.0 References for SO2 Method 

1. H. Okabe, P. L. Splitstone, and J. J. Ball, 

‘‘Ambient and Source SO2 Detector Based 

on a Fluorescence Method’’, Journal of 

the Air Control Pollution Association, 

vol. 23, p. 514–516 (1973). 

2. F. P. Schwarz, H. Okabe, and J. K. 

Whittaker, ‘‘Fluorescence Detection of 

Sulfur Dioxide in Air at the Parts per 

Billion Level,’’ Analytical Chemistry, vol. 

46, pp. 1024–1028 (1974). 

3. QA Handbook for Air Pollution 

Measurement Systems—Volume II. 

Ambient Air Quality Monitoring 

Programs. U.S. EPA. EPA–454/B–08–003 

(2008). 

BILLING CODE 6560–50–P 


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BILLING CODE 6560–50–C 

■ 6. Appendix A to Part 50 is 

redesignated as Appendix A–2 to Part 

50. 

■ 7. Appendix T to Part 50 is added to 

read as follows: 

Appendix T to Part 50—Interpretation 

of the Primary National Ambient Air 

Quality Standards for Oxides of Sulfur 

(Sulfur Dioxide) 

1. General 

(a) This appendix explains the data 

handling conventions and computations 

necessary for determining when the primary 

national ambient air quality standards for 

Oxides of Sulfur as measured by Sulfur 

Dioxide (‘‘SO2 NAAQS’’) specified in § 50.17 

are met at an ambient air quality monitoring 

site. Sulfur Dioxide (SO2) is measured in the 

ambient air by a Federal reference method 

(FRM) based on appendix A or A–1 to this 

part or by a Federal equivalent method (FEM) 

designated in accordance with part 53 of this 

chapter. Data handling and computation 

procedures to be used in making 

comparisons between reported SO2

concentrations and the levels of the SO2

NAAQS are specified in the following 

sections. 

(b) Decisions to exclude, retain, or make 

adjustments to the data affected by 

exceptional events, including natural events, 

are made according to the requirements and 

process deadlines specified in §§ 50.1, 50.14 

and 51.930 of this chapter. 

(c) The terms used in this appendix are 

defined as follows: 

Daily maximum 1-hour values for SO2

refers to the maximum 1-hour SO2

concentration values measured from 

midnight to midnight (local standard time) 

that are used in NAAQS computations. 

Design values are the metrics (i.e., 

statistics) that are compared to the NAAQS 

levels to determine compliance, calculated as 

specified in section 5 of this appendix. The 

design value for the primary 1-hour NAAQS 

is the 3-year average of annual 99th 

percentile daily maximum 1-hour values for 

a monitoring site (referred to as the ‘‘1-hour 

primary standard design value’’). 

99th percentile daily maximum 1-hour 

value is the value below which nominally 99 

percent of all daily maximum 1-hour 

concentration values fall, using the ranking 

and selection method specified in section 5 

of this appendix. 

Pollutant Occurrence Code (POC) refers to 

a numerical code (1, 2, 3, etc.) used to 

distinguish the data from two or more 

monitors for the same parameter at a single 

monitoring site. 

Quarter refers to a calendar quarter. 

Year refers to a calendar year. 

2. Requirements for Data Used for 

Comparisons With the SO2 NAAQS and Data 

Reporting Considerations 

(a) All valid FRM/FEM SO2 hourly data 

required to be submitted to EPA’s Air Quality 

System (AQS), or otherwise available to EPA, 

meeting the requirements of part 58 of this 

chapter including appendices A, C, and E 

shall be used in design value calculations. 

Multi-hour average concentration values 

collected by wet chemistry methods shall not 

be used. 

(b) Data from two or more monitors from 

the same year at the same site reported to 

EPA under distinct Pollutant Occurrence 

Codes shall not be combined in an attempt 

to meet data completeness requirements. The 

Administrator will combine annual 99th 

percentile daily maximum concentration 

values from different monitors in different 

years, selected as described here, for the 

purpose of developing a valid 1-hour primary 

standard design value. If more than one of 

the monitors meets the completeness 

requirement for all four quarters of a year, the 

steps specified in section 5(a) of this 

appendix shall be applied to the data from 

the monitor with the highest average of the 

four quarterly completeness values to derive 

a valid annual 99th percentile daily 

maximum concentration. If no monitor is 

complete for all four quarters in a year, the 

steps specified in section 3(c) and 5(a) of this 

appendix shall be applied to the data from 

the monitor with the highest average of the 

four quarterly completeness values in an 

attempt to derive a valid annual 99th 

percentile daily maximum concentration. 

This paragraph does not prohibit a 

monitoring agency from making a local 

designation of one physical monitor as the 

primary monitor for a Pollutant Occurrence 

Code and substituting the 1-hour data from 

a second physical monitor whenever a valid 

concentration value is not obtained from the 

primary monitor; if a monitoring agency 

substitutes data in this manner, each 

substituted value must be accompanied by an 


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AQS qualifier code indicating that 

substitution with a value from a second 

physical monitor has taken place. 

(c) Hourly SO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

3. Comparisons With the 1-Hour Primary 

SO2 NAAQS 

(a) The 1-hour primary SO2 NAAQS is met 

at an ambient air quality monitoring site 

when the valid 1-hour primary standard 

design value is less than or equal to 75 parts 

per billion (ppb). 

(b) An SO2 1-hour primary standard design 

value is valid if it encompasses three 

consecutive calendar years of complete data. 

A year meets data completeness requirements 

when all 4 quarters are complete. A quarter 

is complete when at least 75 percent of the 

sampling days for each quarter have 

complete data. A sampling day has complete 

data if 75 percent of the hourly concentration 

values, including State-flagged data affected 

by exceptional events which have been 

approved for exclusion by the Administrator, 

are reported. 

(c) In the case of one, two, or three years 

that do not meet the completeness 

requirements of section 3(b) of this appendix 

and thus would normally not be useable for 

the calculation of a valid 3-year 1-hour 

primary standard design value, the 3-year 1- 

hour primary standard design value shall 

nevertheless be considered valid if one of the 

following conditions is true. 

(i) At least 75 percent of the days in each 

quarter of each of three consecutive years 

have at least one reported hourly value, and 

the design value calculated according to the 

procedures specified in section 5 is above the 

level of the primary 1-hour standard. 

(ii) (A) A 1-hour primary standard design 

value that is equal to or below the level of 

the NAAQS can be validated if the 

substitution test in section 3(c)(ii)(B) results 

in a ‘‘test design value’’ that is below the level 

of the NAAQS. The test substitutes actual 

‘‘high’’ reported daily maximum 1-hour 

values from the same site at about the same 

time of the year (specifically, in the same 

calendar quarter) for unknown values that 

were not successfully measured. Note that 

the test is merely diagnostic in nature, 

intended to confirm that there is a very high 

likelihood that the original design value (the 

one with less than 75 percent data capture of 

hours by day and of days by quarter) reflects 

the true under-NAAQS-level status for that 3- 

year period; the result of this data 

substitution test (the ‘‘test design value’’, as 

defined in section 3(c)(ii)(B)) is not 

considered the actual design value. For this 

test, substitution is permitted only if there 

are at least 200 days across the three 

matching quarters of the three years under 

consideration (which is about 75 percent of 

all possible daily values in those three 

quarters) for which 75 percent of the hours 

in the day, including State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator, have reported concentrations. 

However, maximum 1-hour values from days 

with less than 75 percent of the hours 

reported shall also be considered in 

identifying the high value to be used for 

substitution. 

(B) The substitution test is as follows: Data 

substitution will be performed in all quarter 

periods that have less than 75 percent data 

capture but at least 50 percent data capture, 

including State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator; 

if any quarter has less than 50 percent data 

capture then this substitution test cannot be 

used. Identify for each quarter (e.g., January– 

March) the highest reported daily maximum 

1-hour value for that quarter, excluding Stateflagged data affected by exceptional events 

which have been approved for exclusion by 

the Administrator, looking across those three 

months of all three years under 

consideration. All daily maximum 1-hour 

values from all days in the quarter period 

shall be considered when identifying this 

highest value, including days with less than 

75 percent data capture. If after substituting 

the highest reported daily maximum 1-hour 

value for a quarter for as much of the missing 

daily data in the matching deficient 

quarter(s) as is needed to make them 100 

percent complete, the procedure in section 5 

yields a recalculated 3-year 1-hour standard 

‘‘test design value’’ less than or equal to the 

level of the standard, then the 1-hour primary 

standard design value is deemed to have 

passed the diagnostic test and is valid, and 

the level of the standard is deemed to have 

been met in that 3-year period. As noted in 

section 3(c)(i), in such a case, the 3-year 

design value based on the data actually 

reported, not the ‘‘test design value’’, shall be 

used as the valid design value. 

(iii) (A) A 1-hour primary standard design 

value that is above the level of the NAAQS 

can be validated if the substitution test in 

section 3(c)(iii)(B) results in a ‘‘test design 

value’’ that is above the level of the NAAQS. 

The test substitutes actual ‘‘low’’ reported 

daily maximum 1-hour values from the same 

site at about the same time of the year 

(specifically, in the same three months of the 

calendar) for unknown hourly values that 

were not successfully measured. Note that 

the test is merely diagnostic in nature, 

intended to confirm that there is a very high 

likelihood that the original design value (the 

one with less than 75 percent data capture of 

hours by day and of days by quarter) reflects 

the true above-NAAQS-level status for that 3- 

year period; the result of this data 

substitution test (the ‘‘test design value’’, as 

defined in section 3(c)(iii)(B)) is not 

considered the actual design value. For this 

test, substitution is permitted only if there 

are a minimum number of available daily 

data points from which to identify the low 

quarter-specific daily maximum 1-hour 

values, specifically if there are at least 200 

days across the three matching quarters of the 

three years under consideration (which is 

about 75 percent of all possible daily values 

in those three quarters) for which 75 percent 

of the hours in the day have reported 

concentrations. Only days with at least 75 

percent of the hours reported shall be 

considered in identifying the low value to be 

used for substitution. 

(B) The substitution test is as follows: Data 

substitution will be performed in all quarter 

periods that have less than 75 percent data 

capture. Identify for each quarter (e.g., 

January–March) the lowest reported daily 

maximum 1-hour value for that quarter, 

looking across those three months of all three 

years under consideration. All daily 

maximum 1-hour values from all days with 

at least 75 percent capture in the quarter 

period shall be considered when identifying 

this lowest value. If after substituting the 

lowest reported daily maximum 1-hour value 

for a quarter for as much of the missing daily 

data in the matching deficient quarter(s) as is 

needed to make them 75 percent complete, 

the procedure in section 5 yields a 

recalculated 3-year 1-hour standard ‘‘test 

design value’’ above the level of the standard, 

then the 1-hour primary standard design 

value is deemed to have passed the 

diagnostic test and is valid, and the level of 

the standard is deemed to have been 

exceeded in that 3-year period. As noted in 

section 3(c)(i), in such a case, the 3-year 

design value based on the data actually 

reported, not the ‘‘test design value’’, shall be 

used as the valid design value. 

(d) A 1-hour primary standard design value 

based on data that do not meet the 

completeness criteria stated in 3(b) and also 

do not satisfy section 3(c), may also be 

considered valid with the approval of, or at 

the initiative of, the Administrator, who may 

consider factors such as monitoring site 

closures/moves, monitoring diligence, the 

consistency and levels of the valid 

concentration measurements that are 

available, and nearby concentrations in 

determining whether to use such data. 

(e) The procedures for calculating the 1- 

hour primary standard design values are 

given in section 5 of this appendix. 

4. Rounding Conventions for the 1-Hour 

Primary SO2 NAAQS 

(a) Hourly SO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

(b) Daily maximum 1-hour values and 

therefore the annual 99th percentile of those 

daily values are not rounded. 

(c) The 1-hour primary standard design 

value is calculated pursuant to section 5 and 

then rounded to the nearest whole number or 

1 ppb (decimals 0.5 and greater are rounded 

up to the nearest whole number, and any 

decimal lower than 0.5 is rounded down to 

the nearest whole number). 

5. Calculation Procedures for the 1-Hour 

Primary SO2 NAAQS 

(a) Procedure for identifying annual 99th 

percentile values. When the data for a 

particular ambient air quality monitoring site 

and year meet the data completeness 

requirements in section 3(b), or if one of the 

conditions of section 3(c) is met, or if the 

Administrator exercises the discretionary 

authority in section 3(d), identification of 

annual 99th percentile value is accomplished 

as follows. 

(i) The annual 99th percentile value for a 

year is the higher of the two values resulting 

from the following two procedures. 


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(1) Procedure 1. For the year, determine the 

number of days with at least 75 percent of 

the hourly values reported. 

(A) For the year, determine the number of 

days with at least 75 percent of the hourly 

values reported including State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator. 

(B) For the year, from only the days with 

at least 75 percent of the hourly values 

reported, select from each day the maximum 

hourly value excluding State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator. 

(C) Sort all these daily maximum hourly 

values from a particular site and year by 

descending value. (For example: (x[1], x[2], 

x[3], * * *, x[n]). In this case, x[1] is the 

largest number and x[n] is the smallest 

value.) The 99th percentile is determined 

from this sorted series of daily values which 

is ordered from the highest to the lowest 

number. Using the left column of Table 1, 

determine the appropriate range (i.e., row) for 

the annual number of days with valid data 

for year y (cny). The corresponding ‘‘n’’ value 

in the right column identifies the rank of the 

annual 99th percentile value in the 

descending sorted list of daily site values for 

year y. Thus, P0.99, y = the nth largest value. 

(2) Procedure 2. For the year, determine the 

number of days with at least one hourly 

value reported. 

(A) For the year, determine the number of 

days with at least one hourly value reported 

including State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator. 

(B) For the year, from all the days with at 

least one hourly value reported, select from 

each day the maximum hourly value 

excluding State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator. 

(C) Sort all these daily maximum values 

from a particular site and year by descending 

value. (For example: (x[1], x[2], x[3], * * *, 

x[n]). In this case, x[1] is the largest number 

and x[n] is the smallest value.) The 99th 

percentile is determined from this sorted 

series of daily values which is ordered from 

the highest to the lowest number. Using the 

left column of Table 1, determine the 

appropriate range (i.e., row) for the annual 

number of days with valid data for year y 

(cny). The corresponding ‘‘n’’ value in the 

right column identifies the rank of the annual 

99th percentile value in the descending 

sorted list of daily site values for year y. 

Thus, P0.99,y = the nth largest value. 

(b) The 1-hour primary standard design 

value for an ambient air quality monitoring 

site is mean of the three annual 99th 

percentile values, rounded according to the 

conventions in section 4. 

TABLE 1 

Annual number of 

days with valid data 

for year ‘‘y’’ (cny) 

P0.99,y is the nth 

maximum value of the 

year, where n is the 

listed number 

1–100 ........................ 1 

101–200 .................... 2 

201–300 .................... 3 

301–366 .................... 4 

PART 53–AMBIENT AIR MONITORING 

REFERENCE AND EQUIVALENT 

METHODS 

■ 8. The authority citation for part 53 

continues to read as follows: 

Authority: Sec. 301(a) of the Clean Air Act 

(42 U.S.C. sec. 1857g(a)), as amended by sec. 

15(c)(2) of Pub. L. 91–604, 84 Stat. 1713, 

unless otherwise noted. 

Subpart A—[Amended] 

■ 9. Section 53.2 is amended by revising 

paragraphs (a)(1) and (b) to read as 

follows: 

§ 53.2 General requirements for a 

reference method determination. 

* * * * * 

(a) Manual methods—(1) Sulfur 

dioxide (SO2) and Lead. For measuring 

SO2 and lead, appendixes A–2 and G of 

part 50 of this chapter specify unique 

manual FRM for measuring those 

pollutants. Except as provided in 

§ 53.16, other manual methods for lead 

will not be considered for a reference 

method determination under this part. 

* * * * * 

(b) Automated methods. An 

automated FRM for measuring SO2, CO, 

O3, or NO2 must utilize the 

measurement principle and calibration 

procedure specified in the appropriate 

appendix to part 50 of this chapter 

(appendix A–1 only for SO2 methods) 

and must have been shown in 

accordance with this part to meet the 

requirements specified in this subpart A 

and subpart B of this part. 

■ 10. Section 53.8 is amended by 

revising paragraph (c) to read as follows: 

§ 53.8 Designation of reference and 

equivalent methods. 

* * * * * 

(c) The Administrator will maintain a 

current list of methods designated as 

FRM or FEM in accordance with this 

part and will send a copy of the list to 

any person or group upon request. A 

copy of the list will be available via the 

Internet and may be available from other 

sources. 

■ 11. Table A–1 to Subpart A is revised 

to read as follows: 

TABLE A–1 TO SUBPART A OF PART 53—SUMMARY OF APPLICABLE REQUIREMENTS FOR REFERENCE AND EQUIVALENT

METHODS FOR AIR MONITORING OF CRITERIA POLLUTANTS

Pollutant Reference or 

equivalent Manual or automated Applicable part 50 appendix 

Applicable subparts of part 53 

A B C D E F 

SO2 .......... Reference .................... Manual ......................... A–2 

Automated ................... A–1 ✓ ✓ 

Equivalent .................... Manual ......................... A–1 ✓ ✓ 

Automated ................... A–1 ✓ ✓ ✓ 

CO ............ Reference .................... Automated ................... C ✓ ✓ 

Equivalent .................... Manual ......................... C ✓ ✓ 

Automated ................... C ✓ ✓ ✓ 

O3 ............. Reference .................... Automated ................... D ✓ ✓ 

Equivalent .................... Manual ......................... D ✓ ✓ 

Automated ................... D ✓ ✓ ✓ 

NO2 .......... Reference .................... Automated ................... F ✓ ✓ 

Equivalent .................... Manual ......................... F ✓ ✓ 

Automated ................... F ✓ ✓ ✓ 

Pb ............. Reference .................... Manual ......................... G 

Equivalent .................... Manual ......................... G ✓ ✓ 

Automated ................... G ✓ ✓ 

PM10-Pb ... Reference .................... Manual ......................... Q 

Equivalent .................... Manual ......................... Q ✓ ✓ 


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VI-96 A-118
35598 Federal Register / Vol. 75, No. 119 / Tuesday, June 22, 2010 / Rules and Regulations 

TABLE A–1 TO SUBPART A OF PART 53—SUMMARY OF APPLICABLE REQUIREMENTS FOR REFERENCE AND EQUIVALENT

METHODS FOR AIR MONITORING OF CRITERIA POLLUTANTS—Continued 

Pollutant Reference or 

equivalent Manual or automated Applicable part 50 appendix 

Applicable subparts of part 53 

A B C D E F 

Automated ................... Q ✓ ✓ 

PM10 ......... Reference .................... Manual ......................... J ✓ ✓ 

Equivalent .................... Manual ......................... J ✓ ✓ ✓ 

Automated ................... J ✓ ✓ ✓ 

PM2.5 ........ Reference .................... Manual ......................... L ✓ ✓ 

Equivalent Class I ....... Manual ......................... L ✓ ✓ ✓ 

Equivalent Class II ...... Manual ......................... L1 ✓ ✓2 ✓ ✓1 2

Equivalent Class III ..... Automated ................... L1 ✓ ✓ ✓ ✓1

PM10–2.5 .... Reference .................... Manual ......................... L, O ✓ ✓ 

Equivalent Class I ....... Manual ......................... L, O ✓ ✓ ✓ 

Equivalent Class II ...... Manual ......................... L, O ✓ ✓2 ✓ ✓1 2

Equivalent Class III ..... Automated ................... L1, O1 ✓ ✓ ✓ ✓1

1. Some requirements may apply, based on the nature of each particular candidate method, as determined by the Administrator. 

2. Alternative Class III requirements may be substituted. 

Subpart B—[Amended] 

■ 12. Section 53.20 is amended by 

revising paragraph (b) and Table B–1 in 

paragraph (c) to read as follows: 

§ 53.20 General provisions. 

* * * * * 

(b) For a candidate method having 

more than one selectable measurement 

range, one range must be that specified 

in table B–1 (standard range for SO2), 

and a test analyzer representative of the 

method must pass the tests required by 

this subpart while operated in that 

range. The tests may be repeated for one 

or more broader ranges (i.e., ones 

extending to higher concentrations) than 

the range specified in table B–1, 

provided that the range does not extend 

to concentrations more than four times 

the upper range limit specified in table 

B–1. For broader ranges, only the tests 

for range (calibration), noise at 80% of 

the upper range limit, and lag, rise and 

fall time are required to be repeated. 

The tests may be repeated for one or 

more narrower ranges (ones extending 

to lower concentrations) than that 

specified in table B–1. For SO2 methods, 

table B–1 specifies special performance 

requirements for narrower (lower) 

ranges. For methods other than SO2, 

only the tests for range (calibration), 

noise at 0% of the measurement range, 

and lower detectable limit are required 

to be repeated. If the tests are conducted 

or passed only for the specified range 

(standard range for SO2), any FRM or 

FEM method determination with respect 

to the method will be limited to that 

range. If the tests are passed for both the 

specified range and one or more broader 

ranges, any such determination will 

include the additional range(s) as well 

as the specified range, provided that the 

tests required by subpart C of this part 

(if applicable) are met for the broader 

range(s). If the tests are passed for both 

the specified range and one or more 

narrower ranges, any FRM or FEM 

method determination for the method 

will include the narrower range(s) as 

well as the specified range. Appropriate 

test data shall be submitted for each 

range sought to be included in a FRM 

or FEM method determination under 

this paragraph (b). 

(c) * * * 

TABLE B–1—PERFORMANCE SPECIFICATIONS FOR AUTOMATED METHODS

Performance parameter Units 1

SO 2

O 3 CO NO 2 Definitions and 

test procedures Std. range 3 Lower 

range 2 3

1. Range ......................................... ppm .............. 0–0 .5 <0 .5 0–0 .5 0–50 0–0 .5 Sec. 53.23(a). 

2. Noise .......................................... ppm .............. 0 .001 0 .0005 0 .005 0.5 0 .005 Sec. 53.23(b). 

3. Lower detectable limit ................ ppm .............. 0 .002 0 .001 0 .010 1.0 0 .010 Sec. 53.23(c). 

4. Interference equivalent 

Each interferent ....................... ppm .............. ±0 .005 4±0 .005 ±0 .02 ±1.0 ±0 .02 Sec. 53.23(d). 

Total, all interferents ................ ppm .............. — — 0 .06 1.5 0 .04 Sec. 53.23(d). 

5. Zero drift, 12 and 24 hour .......... ppm .............. ±0 .004 ±0 .002 ±0 .02 ±1.0 ±0 .02 Sec. 53.23(e). 

6. Span drift, 24 hour 

20% of upper range limit ......... Percent ......... — — ±20 .0 ±10.0 ±20 .0 Sec. 53.23(e). 

80% of upper range limit ......... Percent ......... ±3 .0 ±3 .0 ±5 .0 ±2.5 ±5 .0 Sec. 53.23(e). 

7. Lag time ...................................... Minutes ........ 2 2 20 10 20 Sec. 53.23(e). 

8. Rise time .................................... Minutes ........ 2 2 15 5 15 Sec. 53.23(e). 

9. Fall time ...................................... Minutes ........ 2 2 15 5 15 Sec. 53.23(e). 

10. Precision 

20% of upper range limit ......... ppm .............. — — 0 .010 0.5 0 .020 Sec. 53.23(e). 

Percent ......... 2 2 .................. .................... .................. Sec. 53.23(e). 

80% of upper range limit ......... ppm .............. — — 0 .010 0.5 0 .030 Sec. 53.23(e). 

Percent ......... 2 2 — — — Sec. 53.23(e). 

1. To convert from parts per million (ppm) to μg/m3 at 25 °C and 760 mm Hg, multiply by M/0.02447, where M is the molecular weight of the 

gas. Percent means percent of the upper range limit. 

2. Tests for interference equivalent and lag time do not need to be repeated for any lower SO2 range provided the test for the standard range 

shows that the lower range specification is met for each of these test parameters. 


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3. For candidate analyzers having automatic or adaptive time constants or smoothing filters, describe their functional nature, and describe and 

conduct suitable tests to demonstrate their function aspects and verify that performances for calibration, noise, lag, rise, fall times, and precision 

are within specifications under all applicable conditions. For candidate analyzers with operator-selectable time constants or smoothing filters, conduct calibration, noise, lag, rise, fall times, and precision tests at the highest and lowest settings that are to be included in the FRM or FEM designation. 

4. For nitric oxide interference for the SO2 UVF method, interference equivalent is ±0.003 ppm for the lower range. 

* * * * * 

■ 13. Section 53.21 is amended by 

revising paragraph (a) to read as follows: 

§ 53.21 Test conditions. 

(a) Set-up and start-up of the test 

analyzer shall be in strict accordance 

with the operating instructions specified 

in the manual referred to in § 53.4(b)(3). 

Allow adequate warm-up or 

stabilization time as indicated in the 

operating instructions before beginning 

the tests. The test procedures assume 

that the test analyzer has an analog 

measurement signal output that is 

connected to a suitable strip chart 

recorder of the servo, null-balance type. 

This recorder shall have a chart width 

of a least 25 centimeters, chart speeds 

up to 10 cm per hour, a response time 

of 1 second or less, a deadband of not 

more than 0.25 percent of full scale, and 

capability either of reading 

measurements at least 5 percent below 

zero or of offsetting the zero by at least 

5 percent. If the test analyzer does not 

have an analog signal output, or if other 

types of measurement data output are 

used, an alternative measurement data 

recording device (or devices) may be 

used for the tests, provided it is 

reasonably suited to the nature and 

purposes of the tests and an analog 

representation of the analyzer 

measurements for each test can be 

plotted or otherwise generated that is 

reasonably similar to the analog 

measurement recordings that would be 

produced by a conventional chart 

recorder. 

* * * * * 

■ 14. Section 53.22(d) is amended by 

revising Table B–2 to read as follows: 

§ 53.22 Generation of test atmospheres. 

* * * * * 

(d) * * * 

TABLE B–2—TEST ATMOSPHERES

Test gas Generation Verification 

Ammonia .................... Permeation device. Similar to system described in references 1 and 2.Indophenol method, reference 3. 

Carbon dioxide ........... Cylinder of zero air or nitrogen containing CO2 as required 

to obtain the concentration specified in Table B–3.

Use NIST-certified standards whenever possible. If NIST 

standards are not available, obtain 2 standards from 

independent sources which agree within 2 percent, or 

obtain one standard and submit it to an independent 

laboratory for analysis, which must agree within 2 percent of the supplier’s nominal analysis. 

Carbon monoxide ....... Cylinder of zero air or nitrogen containing CO as required 

to obtain the concentration specified in Table B–3.

Use a FRM CO analyzer as described in reference 8. 

Ethane ........................ Cylinder of zero air or nitrogen containing ethane as required to obtain the concentration specified in Table B–3.Gas chromatography, ASTM D2820, reference 10. Use 

NIST-traceable gaseous methane or propane standards 

for calibration. 

Ethylene ..................... Cylinder of pre-purified nitrogen containing ethylene as required to obtain the concentration specified in Table B–3.Do. 

Hydrogen chloride ...... Cylinder1 of pre-purified nitrogen containing approximately 

100 ppm of gaseous HCL. Dilute with zero air to concentration specified in Table B–3.

Collect samples in bubbler containing distilled water and 

analyze by the mercuric thiocyante method, ASTM 

(D612), p. 29, reference 4. 

Hydrogen sulfide ........ Permeation device system described in references 1 and 

2.

Tentative method of analysis for H2S content of the atmosphere, p. 426, reference 5. 

Methane ..................... Cylinder of zero air containing methane as required to obtain the concentration specified in Table B–3.Gas chromatography ASTM D2820, reference 10. Use 

NIST-traceable methane standards for calibration. 

Naphthalene ............... 1. Permeation device as described in references 1 and 2 ..

2. Cylinder of pre-purified nitrogen containing 100 ppm 

naphthalene. Dilute with zero air to concentration specified in Table B–3. 

Use NIST-certified standards whenever possible. If NIST 

standards are not available, obtain 2 standards from 

independent sources which agree within 2 percent, or 

obtain one standard and submit it to an independent 

laboratory for analysis, which must agree within 2 percent of the supplier’s nominal analysis. 

Nitric oxide ................. Cylinder1 of pre-purified nitrogen containing approximately 

100 ppm NO. Dilute with zero air to required concentration.

Use NIST-certified standards whenever possible. If NIST 

standards are not available, obtain 2 standards from 

independent sources which agree within 2 percent, or 

obtain one standard and submit it to an independent 

laboratory for analysis, which must agree within 2 percent of the supplier’s nominal analysis. 

Nitrogen dioxide ......... 1. Gas phase titration as described in reference 6 .............

2. Permeation device, similar to system described in reference 6. 

1. Use an FRM NO2 analyzer calibrated with a gravimetrically calibrated permeation device. 

2. Use an FRM NO2 analyzer calibrated by gas-phase titration as described in reference 6. 

Ozone ......................... Calibrated ozone generator as described in reference 9 .... Use an FEM ozone analyzer calibrated as described in 

reference 9. 

Sulfur dioxide ............. 1. Permeation device as described in references 1 and 2 ..

2. Dynamic dilution of a cylinder containing approximately 

100 ppm SO2 as described in Reference 7. 

Use an SO2 FRM or FEM analyzer as described in reference 7. 


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TABLE B–2—TEST ATMOSPHERES—Continued 

Test gas Generation Verification 

Water .......................... Pass zero air through distilled water at a fixed known temperature between 20° and 30° C such that the air 

stream becomes saturated. Dilute with zero air to concentration specified in Table B–3.

Measure relative humidity by means of a dew-point indicator, calibrated electrolytic or piezo electric hygrometer, 

or wet/dry bulb thermometer. 

Xylene ........................ Cylinder of pre-purified nitrogen containing 100 ppm xylene. Dilute with zero air to concentration specified in 

Table B–3.

Use NIST-certified standards whenever possible. If NIST 

standards are not available, obtain 2 standards from 

independent sources which agree within 2 percent, or 

obtain one standard and submit it to an independent 

laboratory for analysis, which must agree within 2 percent of the supplier’s nominal analysis. 

Zero air ....................... 1. Ambient air purified by appropriate scrubbers or other 

devices such that it is free of contaminants likely to 

cause a detectable response on the analyzer. 

2. Cylinder of compressed zero air certified by the supplier 

or an independent laboratory to be free of contaminants 

likely to cause a detectable response on the analyzer. 

1 Use stainless steel pressure regulator dedicated to the pollutant measured. 

Reference 1. O’Keefe, A. E., and Ortaman, G. C. ‘‘Primary Standards for Trace Gas Analysis,’’ Anal. Chem. 38, 760 (1966). 

Reference 2. Scaringelli, F. P., A. E. Rosenberg, E., and Bell, J. P., ‘‘Primary Standards for Trace Gas Analysis.’’ Anal. Chem. 42, 871 (1970). 

Reference 3. ‘‘Tentative Method of Analysis for Ammonia in the Atmosphere (Indophenol Method)’’, Health Lab Sciences, vol. 10, No. 2, 115– 

118, April 1973. 

Reference 4. 1973 Annual Book of ASTM Standards, American Society for Testing and Materials, 1916 Race St., Philadelphia, PA. 

Reference 5. Methods for Air Sampling and Analysis, Intersociety Committee, 1972, American Public Health Association, 1015. 

Reference 6. 40 CFR 50 Appendix F, ‘‘Measurement Principle and Calibration Principle for the Measurement of Nitrogen Dioxide in the Atmosphere (Gas Phase Chemiluminescence).’’ 

Reference 7. 40 CFR 50 Appendix A–1, ‘‘Measurement Principle and Calibration Procedure for the Measurement of Sulfur Dioxide in the Atmosphere (Ultraviolet FIuorscence).’’ 

Reference 8. 40 CFR 50 Appendix C, ‘‘Measurement Principle and Calibration Procedure for the Measurement of Carbon Monoxide in the Atmosphere’’ (Non-Dispersive Infrared Photometry)’’. 

Reference 9. 40 CFR 50 Appendix D, ‘‘Measurement Principle and Calibration Procedure for the Measurement of Ozone in the Atmosphere’’. 

Reference 10. ‘‘Standard Test Method for C, through C5 Hydrocarbons in the Atmosphere by Gas Chromatography’’, D 2820, 1987 Annual 

Book of Aston Standards, vol 11.03, American Society for Testing and Materials, 1916 Race St., Philadelphia, PA 19103. 

■ 15. Section 53.23(d) is amended by 

revising Table B–3 to read as follows: 

§ 53.23 Test procedures. 

* * * * * 

(d) * * * 

TABLE B–3—INTERFERENT TEST CONCENTRATION,1 PARTS PER MILLION

Pollutant Analyzer type 

Hydrochloric 

acid 

Ammonia 

Hydrogen 

sulfide 

Sulfur 

dioxide 

Nitrogen 

dioxide 

Nitric 

oxide 

Carbon 

dioxide 

Ethylene Ozone Mxylene Water 

vapor 

Carbon 

monoxide 

Methane Ethane Naphthalene 

SO2 .............. Ultraviolet fluorescence ............ ............ 5 0.1 4 0.14 0.5 0.5 ............ ............ 0.5 0.2 20,000 ............ ............ ............ 6 0.05 

SO2 .............. Flame photometric ...... ............ ............ 0.01 4 0.14 ............ ............ 750 ............ ............ ............ 3 20,000 50 ............ ............ ............

SO2 .............. Gas chromatography ... ............ ............ 0.1 4 0.14 ............ ............ 750 ............ ............ ............ 3 20,000 50 ............ ............ ............

SO2 .............. Spectrophotometric-wet 

chemical 

(pararosanaline).

0.2 0.1 0.1 4 0.14 0.5 ............ 750 ............ 0.5 ............ .................... ............ ............ ............ ............

SO2 .............. Electrochemical ........... 0.2 0.1 0.1 4 0.14 0.5 0.5 ............ 0.2 0.5 ............ 3 20,000 ............ ............ ............ ............

SO2 .............. Conductivity ................. 0.2 0.1 ............ 4 0.14 0.5 ............ 750 ............ ............ ............ .................... ............ ............ ............ ............

SO2 .............. Spectrophotometricgas phase, including 

DOAS.

............ ............ ............ 4 0.14 0.5 ............ ............ ............ 0.5 0.2 .................... ............ ............ ............ ............

O3 ................ Chemiluminescent ....... ............ ............ 3 0.1 ............ ............ ............ 750 ............ 4 0.08 ............ 3 20,000 ............ ............ ............ ............

O3 ................ Electrochemical ........... ............ 3 0.1 ............ 0.5 0.5 ............ ............ ............ 4 0.08 ............ .................... ............ ............ ............ ............

O3 ................ Spectrophotometric-wet 

chemical (potassium 

iodide).

............ 3 0.1 ............ 0.5 0.5 3 0.5 ............ ............ 4 0.08 ............ .................... ............ ............ ............ ............

O3 ................ Spectrophotometricgas phase, including 

ultraviolet absorption 

and DOAS.

............ ............ ............ 0.5 0.5 0.5 ............ ............ 4 0.08 0.02 20,000 ............ ............ ............ ............

CO ............... Infrared ........................ ............ ............ ............ ............ ............ ............ 750 ............ ............ ............ 20,000 4 10 ............ ............ ............

CO ............... Gas chromatography 

with flame ionization 

detector.

............ ............ ............ ............ ............ ............ ............ ............ ............ ............ 20,000 4 10 ............ 0.5 ............

CO ............... Electrochemical ........... ............ ............ ............ ............ ............ 0.5 ............ 0.2 ............ ............ 20,000 4 10 ............ ............ ............

CO ............... Catalytic combustionthermal detection............. 0.1 ............ ............ ............ ............ 750 0.2 ............ ............ 20,000 4 10 5.0 0.5 ............

CO ............... IR fluorescence ........... ............ ............ ............ ............ ............ ............ 750 ............ ............ ............ 20,000 4 10 ............ 0.5 ............

CO ............... Mercury replacementUV photometric............. ............ ............ ............ ............ ............ ............ 0.2 ............ ............ .................... 4 10 ............ 0.5 ............

NO2 ............. Chemiluminescent ....... ............ 3 0.1 ............ 0.5 4 0.1 0.5 ............ ............ ............ ............ 20,000 ............ ............ ............ ............

NO2 ............. Spectrophotometric-wet 

chemical (azo-dye 

reaction).

............ ............ ............ 0.5 4 0.1 0.5 750 ............ 0.5 ............ .................... ............ ............ ............ ............

NO2 ............. Electrochemical ........... 0.2 3 0.1 ............ 0.5 4 0.1 0.5 750 ............ 0.5 ............ 20,000 50 ............ ............ ............

NO2 ............. Spectrophotometricgas phase............. 3 0.1 ............ 0.5 4 0.1 0.5 ............ ............ 0.5 ............ 20,000 50 ............ ............ ............

1. Concentrations of interferent listed must be prepared and controlled to ±10 percent of the stated value. 

2. Analyzer types not listed will be considered by the Administrator as special cases. 

3. Do not mix with the pollutant. 


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4. Concentration of pollutant used for test. These pollutant concentrations must be prepared to ±10 percent of the stated value. 

5. If candidate method utilizes an elevated-temperature scrubber for removal of aromatic hydrocarbons, perform this interference test. 

6. If naphthalene test concentration cannot be accurately quantified, remove the scrubber, use a test concentration that causes a full scale response, reattach the scrubber, and evaluate response for interference 

* * * * * 

Subpart C [Amended] 

■ 16. Section 53.32 is amended by 

revising paragraph (e)(2) to read as 

follows: 

§ 53.32 Test procedures for methods for 

SO2, CO, O3, and NO2. 

* * * * * 

(e) * * * 

(2) For a candidate method having 

more than one selectable range, one 

range must be that specified in table B– 

1 of subpart B of this part, and a test 

analyzer representative of the method 

must pass the tests required by this 

subpart while operated on that range. 

The tests may be repeated for one or 

more broader ranges (i.e., ones 

extending to higher concentrations) than 

the one specified in table B–1 of subpart 

B of this part, provided that such a 

range does not extend to concentrations 

more than four times the upper range 

limit specified in table B–1 of subpart B 

of this part and that the test analyzer has 

passed the tests required by subpart B 

of this part (if applicable) for the 

broader range. If the tests required by 

this subpart are conducted or passed 

only for the range specified in table B– 

1 of subpart B of this part, any 

equivalent method determination with 

respect to the method will be limited to 

that range. If the tests are passed for 

both the specified range and a broader 

range (or ranges), any such 

determination will include the broader 

range(s) as well as the specified range. 

Appropriate test data shall be submitted 

for each range sought to be included in 

such a determination. 

* * * * * 

■ 17. Table C–1 to Subpart C is revised 

to read as follows: 

TABLE C–1 TO SUBPART C OF PART 53—TEST CONCENTRATION RANGES, NUMBER OF MEASUREMENTS REQUIRED, AND

MAXIMUM DISCREPANCY SPECIFICATIONS

Pollutant Concentration range, parts per million 

(ppm) 

Simultaneous measurements required Maximum 

discrepancy 

specification, 

parts per million 

1-hour 24-hour 

First set Second set First set Second set 

Ozone ...................... Low 0.06 to 0.10 ........................................ 5 6 .................... .................... 0.02 

Med. 0.15 to 0.25 ...................................... 5 6 .................... .................... 0.03 

High 0.35 to 0.46 ....................................... 4 6 .................... .................... 0.04 

Total .................................................... 14 18 .................... .................... ............................

Carbon monoxide .... Low 7 to 11 ................................................ 5 6 .................... .................... 1.5 

Med. 20 to 30 ............................................ 5 6 .................... .................... 2.0 

High 25 to 45 ............................................. 4 6 .................... .................... 3.0 

Total .................................................... 14 18 .................... .................... ............................

Sulfur dioxide ........... Low 0.02 to 0.05 ........................................ 5 6 3 3 0.02 

Med. 0.10 to 0.15 ...................................... 5 6 2 3 0.03 

High 0.30 to 0.50 ....................................... 4 6 2 2 0.04 

Total .................................................... 14 18 7 8 ............................

Nitrogen dioxide ....... Low 0.02 to 0.08 ........................................ .................... .................... 3 3 0.02

Med. 0.10 to 0.20 ...................................... .................... .................... 2 2 0.02 

High 0.25 ................................................... .................... .................... 2 2 0.03 

Total .................................................... .................... .................... 7 8 ............................

PART 58—AMBIENT AIR QUALITY 

SURVEILLANCE 

■ The authority citation for part 58 

continues to read as follows: 

Authority: 42 U.S.C. 7403, 7410, 7601(a), 

7611, and 7619. 

Subpart B [AMENDED] 

■ 19. Section 58.10, is amended by 

adding paragraph (a)(6) to read as 

follows: 

§ 58.10 Annual monitoring network plan 

and periodic network assessment. 

* * * * * 

(a) * * * 

(6) A plan for establishing SO2

monitoring sites in accordance with the 

requirements of appendix D to this part 

shall be submitted to the EPA Regional 

Administrator by July 1, 2011 as part of 

the annual network plan required in 

paragraph (a) (1). The plan shall provide 

for all required SO2 monitoring sites to 

be operational by January 1, 2013. 

* * * * * 

■ 20. Section 58.12 is amended by 

adding paragraph (g) to read as follows: 

§ 58.12 Operating Schedules 

* * * * * 

(g) For continuous SO2 analyzers, the 

maximum 5-minute block average 

concentration of the twelve 5-minute 

blocks in each hour must be collected 

except as noted in § 58.12 (a). 

* * * * * 

■ 21. Section 58.13 is amended by 

adding paragraph (d) to read as follows: 

§ 58.13 Monitoring network completion. 

* * * * * 

(d) The network of SO2 monitors must 

be physically established no later than 

January 1, 2013, and at that time, must 

be operating under all of the 

requirements of this part, including the 


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requirements of appendices A, C, D, and 

E to this part. 

■ 22. Section 58.16 is amended by 

adding paragraph (g) to read as follows: 

§ 58.16 Data submittal and archiving 

requirements. 

* * * * * 

(g) Any State or, where applicable, 

local agency operating a continuous SO2

analyzer shall report the maximum 5- 

minute SO2 block average of the twelve 

5-minute block averages in each hour, in 

addition to the hourly SO2 average. 

■ 23. Appendix A to Part 58 is amended 

as by adding paragraph 2.3.1.6 to read 

as follows: 

Appendix A to Part 58—Quality 

Assurance Requirements for SLAMS, 

SPMs and PSD Air Monitoring 

* * * * * 

2.3.1.6 Measurement Uncertainty for SO2. 

The goal for acceptable measurement 

uncertainty for precision is defined as an 

upper 90 percent confidence limit for the 

coefficient of variation (CV) of 10 percent and 

for bias as an upper 95 percent confidence 

limit for the absolute bias of 10 percent. 

* * * * * 

■ 24. Appendix D to Part 58 is amended 

as by revising paragraph 4.4 to read as 

follows: 

Appendix D to Part 58—Network 

Design Criteria for Ambient Air Quality 

Monitoring 

* * * * * 

4.4 Sulfur Dioxide (SO2) Design Criteria. 

4.4.1 General Requirements. (a) State and, 

where appropriate, local agencies must 

operate a minimum number of required SO2

monitoring sites as described below. 

4.4.2 Requirement for Monitoring by the 

Population Weighted Emissions Index. (a) 

The population weighted emissions index 

(PWEI) shall be calculated by States for each 

core based statistical area (CBSA) they 

contain or share with another State or States 

for use in the implementation of or 

adjustment to the SO2 monitoring network. 

The PWEI shall be calculated by multiplying 

the population of each CBSA, using the most 

current census data or estimates, and the 

total amount of SO2 in tons per year emitted 

within the CBSA area, using an aggregate of 

the most recent county level emissions data 

available in the National Emissions Inventory 

for each county in each CBSA. The resulting 

product shall be divided by one million, 

providing a PWEI value, the units of which 

are million persons-tons per year. For any 

CBSA with a calculated PWEI value equal to 

or greater than 1,000,000, a minimum of 

three SO2 monitors are required within that 

CBSA. For any CBSA with a calculated PWEI 

value equal to or greater than 100,000, but 

less than 1,000,000, a minimum of two SO2

monitors are required within that CBSA. For 

any CBSA with a calculated PWEI value 

equal to or greater than 5,000, but less than 

100,000, a minimum of one SO2 monitor is 

required within that CBSA. 

(1) The SO2 monitoring site(s) required as 

a result of the calculated PWEI in each CBSA 

shall satisfy minimum monitoring 

requirements if the monitor is sited within 

the boundaries of the parent CBSA and is one 

of the following site types (as defined in 

section 1.1.1 of this appendix): population 

exposure, highest concentration, source 

impacts, general background, or regional 

transport. SO2 monitors at NCore stations 

may satisfy minimum monitoring 

requirements if that monitor is located within 

a CBSA with minimally required monitors 

under this part. Any monitor that is sited 

outside of a CBSA with minimum monitoring 

requirements to assess the highest 

concentration resulting from the impact of 

significant sources or source categories 

existing within that CBSA shall be allowed 

to count towards minimum monitoring 

requirements for that CBSA. 

4.4.3 Regional Administrator Required 

Monitoring. (a) The Regional Administrator 

may require additional SO2 monitoring 

stations above the minimum number of 

monitors required in 4.4.2 of this part, where 

the minimum monitoring requirements are 

not sufficient to meet monitoring objectives. 

The Regional Administrator may require, at 

his/her discretion, additional monitors in 

situations where an area has the potential to 

have concentrations that may violate or 

contribute to the violation of the NAAQS, in 

areas impacted by sources which are not 

conducive to modeling, or in locations with 

susceptible and vulnerable populations, 

which are not monitored under the minimum 

monitoring provisions described above. The 

Regional Administrator and the responsible 

State or local air monitoring agency shall 

work together to design and/or maintain the 

most appropriate SO2 network to provide 

sufficient data to meet monitoring objectives. 

4.4.4 SO2 Monitoring Spatial Scales. (a) 

The appropriate spatial scales for SO2

SLAMS monitors are the microscale, middle, 

neighborhood, and urban scales. Monitors 

sited at the microscale, middle, and 

neighborhood scales are suitable for 

determining maximum hourly concentrations 

for SO2. Monitors sited at urban scales are 

useful for identifying SO2 transport, trends, 

and, if sited upwind of local sources, 

background concentrations. 

(1) Microscale—This scale would typify 

areas in close proximity to SO2 point and 

area sources. Emissions from stationary point 

and area sources, and non-road sources may, 

under certain plume conditions, result in 

high ground level concentrations at the 

microscale. The microscale typically 

represents an area impacted by the plume 

with dimensions extending up to 

approximately 100 meters. 

(2) Middle scale—This scale generally 

represents air quality levels in areas up to 

several city blocks in size with dimensions 

on the order of approximately 100 meters to 

500 meters. The middle scale may include 

locations of expected maximum short-term 

concentrations due to proximity to major SO2

point, area, and/or non-road sources. 

(3) Neighborhood scale—The 

neighborhood scale would characterize air 

quality conditions throughout some 

relatively uniform land use areas with 

dimensions in the 0.5 to 4.0 kilometer range. 

Emissions from stationary point and area 

sources may, under certain plume 

conditions, result in high SO2 concentrations 

at the neighborhood scale. Where a 

neighborhood site is located away from 

immediate SO2 sources, the site may be 

useful in representing typical air quality 

values for a larger residential area, and 

therefore suitable for population exposure 

and trends analyses. 

(4) Urban scale—Measurements in this 

scale would be used to estimate 

concentrations over large portions of an 

urban area with dimensions from 4 to 50 

kilometers. Such measurements would be 

useful for assessing trends in area-wide air 

quality, and hence, the effectiveness of large 

scale air pollution control strategies. Urban 

scale sites may also support other monitoring 

objectives of the SO2 monitoring network 

such as identifying trends, and when 

monitors are sited upwind of local sources, 

background concentrations. 

4.4.5 NCore Monitoring. (a) SO2

measurements are included within the NCore 

multipollutant site requirements as described 

in paragraph (3)(b) of this appendix. NCorebased SO2 measurements are primarily used 

to characterize SO2 trends and assist in 

understanding SO2 transport across 

representative areas in urban or rural 

locations and are also used for comparison 

with the SO2 NAAQS. SO2 monitors at NCore 

sites that exist in CBSAs with minimum 

monitoring requirements per section 4.4.2 

above shall be allowed to count towards 

those minimum monitoring requirements. 

* * * * * 

■ 25. Appendix G to Part 58 is amended 

as by revising Table 2 to read as follows: 

Appendix G to Part 58—Uniform Air 

Quality Index (AQI) and Daily 

Reporting 

* * * * * 


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Federal Register / Vol. 75, No. 119 / Tuesday, June 22, 2010 / Rules and Regulations 35603 

TABLE 2—BREAKPOINTS FOR THE AQI 

These breakpoints Equal these AQI’s 

O3 (ppm) 

8-hour 

O3 (ppm) 

1-hour 1

PM2.5

(μg/m 3) 

PM10

(μg/m 3) CO (ppm) SO2 (ppm) 

1-hour 

NO2 (ppm) 

1-hour AQI Category 

0.000–0.059 .. 0.0–15.4 0–54 0.0–4.4 0–0.035 0–0.053 0–50 Good. 

0.060–0.075 .. 15.5–40.4 55–154 4.5–9.4 0.036–0.075 0.054–0.100 51–100 Moderate. 

0.076–0.095 .. 0.125–0.164 40.5–65.4 155–254 9.5–12.4 0.076–0.185 0.101–0.360 101–150 Unhealthy for Sensitive Groups. 

0.096–0.115 .. 0.165–0.204 3 65.5–150.4 255–354 12.5–15.4 4 0.186–0.304 0.361–0.64 151–200 Unhealthy. 

0.116–0.374 .. 0.205–0.404 3 150.5–250.4 355–424 15.5–30.4 4 0.305–0.604 0.65–1.24 201–300 Very Unhealthy. 

(2) .................. 0.405–0.504 3 250.5–350.4 425–504 30.5–40.4 4 0.605–0.804 1.25–1.64 301–400 

(2) .................. 0.505–0.604 3 350.5–500.4 505–604 40.5–50.4 4 0.805–1.004 1.65–2.04 401–500 Hazardous. 

1Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small number of areas where an AQI based on 1-hour ozone 

values would be more precautionary. In these cases, in addition to calculating the 8-hour ozone index value, the 1-hour ozone index value may be calculated, and the 

maximum of the two values reported. 2 8-hour O3 values do not define higher AQI values (≥301). AQI values of 301 or greater are calculated with 1-hour O3 concentrations. 3 If a different SHL for PM2.5 is promulgated, these numbers will change accordingly. 4 1-hr SO2 values do not define higher AQI values (≥200). AQI values of 200 or greater are calculated with 24-hour SO2 concentrations. 

* * * * * 

[FR Doc. 2010–13947 Filed 6–21–10; 8:45 am] 

BILLING CODE 6560–50–P 


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VI-102 A-124
Tuesday, 

February 9, 2010 

Part III 

Environmental 

Protection Agency 

40 CFR Parts 50 and 58 

Primary National Ambient Air Quality 

Standards for Nitrogen Dioxide; Final 

Rule 


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6474 Federal Register / Vol. 75, No. 26 / Tuesday, February 9, 2010 / Rules and Regulations 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Parts 50 and 58 

[EPA–HQ–OAR–2006–0922; FRL 9107–9] 

RIN 2060–AO19 

Primary National Ambient Air Quality 

Standards for Nitrogen Dioxide 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Final rule. 

SUMMARY: Based on its review of the air 

quality criteria for oxides of nitrogen 

and the primary national ambient air 

quality standard (NAAQS) for oxides of 

nitrogen as measured by nitrogen 

dioxide (NO2), EPA is making revisions 

to the primary NO2 NAAQS in order to 

provide requisite protection of public 

health. Specifically, EPA is establishing 

a new 1-hour standard at a level of 100 

ppb, based on the 3-year average of the 

98th percentile of the yearly distribution 

of 1-hour daily maximum 

concentrations, to supplement the 

existing annual standard. EPA is also 

establishing requirements for an NO2

monitoring network that will include 

monitors at locations where maximum 

NO2 concentrations are expected to 

occur, including within 50 meters of 

major roadways, as well as monitors 

sited to measure the area-wide NO2

concentrations that occur more broadly 

across communities. 

DATES: This final rule is effective on 

April 12, 2010. 

ADDRESSES: EPA has established a 

docket for this action under Docket ID 

No. EPA–HQ–OAR–2006–0922. All 

documents in the docket are listed on 

the http://www.regulations.gov Web 

site. Although listed in the index, some 

information is not publicly available, 

e.g., confidential business information 

or other information whose disclosure is 

restricted by statute. Certain other 

material, such as copyrighted material, 

will be publicly available only in hard 

copy form. Publicly available docket 

materials are available either 

electronically through http:// 

www.regulations.gov or in hard copy at 

the Air and Radiation Docket and 

Information Center, EPA/DC, EPA West, 

Room 3334, 1301 Constitution Ave., 

NW., Washington, DC. The Public 

Reading Room is open from 8:30 a.m. to 

4:30 p.m., Monday through Friday, 

excluding legal holidays. The telephone 

number for the Public Reading Room is 

(202) 566–1744 and the telephone 

number for the Air and Radiation 

Docket and Information Center is (202) 

566–1742. 

FOR FURTHER INFORMATION CONTACT: Dr. 

Scott Jenkins, Health and 

Environmental Impacts Division, Office 

of Air Quality Planning and Standards, 

U.S. Environmental Protection Agency, 

Mail code C504–06, Research Triangle 

Park, NC 27711; telephone: 919–541– 

1167; fax: 919–541–0237; e-mail: 

jenkins.scott@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Table of Contents 

The following topics are discussed in this 

preamble: 

I. Background 

A. Summary of Revisions to the NO2

Primary NAAQS 

B. Legislative Requirements 

C. Related NO2 Control Programs 

D. Review of the Air Quality Criteria and 

Standards for Oxides of Nitrogen 

E. Summary of Proposed Revisions to the 

NO2 Primary NAAQS 

F. Organization and Approach to Final NO2

Primary NAAQS Decisions 

II. Rationale for Final Decisions on the NO2

Primary Standard 

A. Characterization of NO2 Air Quality 

1. Current Patterns of NO2 Air Quality 

2. NO2 Air Quality and Gradients Around 

Roadways 

B. Health Effects Information 

1. Adverse Respiratory Effects and ShortTerm Exposure to NO2

2. Other Effects With Short-Term Exposure 

to NO2

a. Mortality 

b. Cardiovascular Effects 

3. Health Effects With Long-Term Exposure 

to NO2

a. Respiratory Morbidity 

b. Mortality 

c. Carcinogenic, Cardiovascular, and 

Reproductive/Developmental Effects 

4. NO2-Related Impacts on Public Health 

C. Human Exposure and Health Risk 

Characterization 

D. Approach for Reviewing the Need To 

Retain or Revise the Current Standard 

E. Adequacy of the Current Standard 

1. Rationale for Proposed Decision 

2. Comments on the Adequacy of the 

Current Standard 

a. Comments on EPA’s Interpretation of the 

Epidemiologic Evidence 

b. Comments on EPA’s Interpretation of the 

Controlled Human Exposure Evidence 

c. Comments on EPA’s Characterization of 

NO2-Associated Exposures and Health 

Risks 

3. Conclusions on the Adequacy of the 

Current Standard 

F. Elements of a New Short-Term Standard 

1. Indicator 

a. Rationale for Proposed Decision 

b. Comments on Indicator 

c. Conclusions Regarding Indicator 

2. Averaging Time 

a. Rationale for Proposed Decision 

b. Comments on Averaging Time 

c. Conclusions on Averaging Time 

3. Form 

a. Rationale for Proposed Decision 

b. CASAC and Public Comments on Form 

c. Conclusions on Form 

4. Level 

a. Rationale for Proposed Decisions on 

Approach and Level 

b. Rationale for Alternative Decisions on 

Approach and Level 

c. Comments on Approach and Level 

i. CASAC Comments on the Approach to 

Setting the Standard 

ii. Public Comments on the Approach To 

Setting the Standard 

iii. CASAC Comments on Standard Level 

iv. Public Comments on Standard Level 

d. Conclusions on Approach and Standard 

Level 

G. Annual Standard 

H. Summary of Final Decisions on the 

Primary NO2 Standard 

III. Amendments to Ambient Monitoring and 

Reporting Requirements 

A. Monitoring Methods 

1. Chemiluminescence FRM and 

Alternative Methods 

2. Allowable FRM and FEMs for 

Comparison to the NAAQS 

a. Proposed Changes to FRM and FEMs 

That May Be Compared to the NAAQS 

b. Comments 

c. Decisions on Allowable FRM and FEMs 

for Comparison to the NAAQS 

B. Network Design 

1. Two-Tiered Network Design 

a. Proposed Two-Tier Network Design 

b. Comments 

c. Conclusions Regarding the Two-Tier 

Network Design 

2. First Tier (Near-road Monitoring 

Component) of the NO2 Network Design 

a. Proposed First Tier (Near-road 

Monitoring Component) of the Network 

Design 

b. Comments 

c. Conclusions Regarding the First Tier 

(Near-road Monitoring Component) of 

the Network Design 

3. Second Tier (Area-wide Monitoring 

Component) of the Network Design 

a. Proposed Second Tier (Area-wide 

Monitoring Component) of the Network 

Design 

b. Comments 

c. Conclusions on the Second Tier (Areawide Monitoring Component) of the 

Network Design 

4. Regional Administrator Authority 

a. Proposed Regional Administrator 

Authority 

b. Comments 

c. Conclusions on Regional Administrator 

Authority 

5. Monitoring Network Implementation 

a. Proposed Monitoring Network 

Implementation Approach 

b. Comments 

c. Conclusions on Monitoring Network 

Implementation 

6. Near-Road Site Selection 

a. Proposed Near-Road Site Selection 

Criterion 

b. Comments 

c. Conclusions on Near-Road Site Selection 

7. Near-Road Siting Criteria 

a. Proposed Near-Road Siting Criteria 

b. Comments 

c. Conclusions on Near-Road Siting Criteria 

8. Area-wide Monitor Site Selection and 

Siting Criteria 


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Federal Register / Vol. 75, No. 26 / Tuesday, February 9, 2010 / Rules and Regulations 6475 

1EPA notes that as the promulgation of a NAAQS 

is identified in section 307(d)(1) of the Clean Air 

Act, all of the provisions of this rulemaking are 

subject to the requirements of section 307(d) of the 

Clean Air Act. 

2The legislative history of section 109 indicates 

that a primary standard is to be set at ‘‘the 

maximum permissible ambient air level * * * 

which will protect the health of any [sensitive] 

group of the population,’’ and that for this purpose 

‘‘reference should be made to a representative 

sample of persons comprising the sensitive group 

rather than to a single person in such a group.’’ S. 

Rep. No. 91–1196, 91st Cong., 2d Sess. 10(1970). 

3EPA is currently conducting a separate review 

of the secondary NO2 NAAQS jointly with a review 

of the secondary SO2 NAAQS. 

a. Proposed Area-wide Monitor Site 

Selection and Siting Criteria 

b. Comments 

c. Conclusions on Area-Wide Monitor Site 

Selection and Siting Criteria 

9. Meteorological Measurements 

a. Proposed Meteorological Measurements 

b. Comments 

c. Conclusions on Meteorological 

Measurements 

C. Data Reporting 

1. Proposed Data Quality Objectives and 

Measurement Uncertainty 

2. Comments 

3. Conclusions on Data Quality Objectives 

and Measurement Uncertainty 

IV. Appendix S—Interpretation of the 

Primary NAAQS for Oxides of Nitrogen 

and Revisions to the Exceptional Events 

Rule 

A. Interpretation of the Primary NAAQS 

for Oxides of Nitrogen for the Annual 

Primary Standard 

1. Proposed Interpretation of the Annual 

Standard 

2. Comments on Interpretation of the 

Annual Standard 

3. Conclusions on Interpretation of the 

Annual Standard 

B. Interpretation of the Primary NAAQS for 

Oxides of Nitrogen 1-Hour Primary 

Standard 

1. Proposed Interpretation of the 1-Hour 

Standard 

2. Comments on Interpretation of the 

1-Hour Standard 

3. Conclusions on Interpretation of the 

1-Hour Standard 

C. Exceptional Events Information 

Submission Schedule 

V. Designation of Areas 

A. Proposed Process 

B. Public Comments 

C. Final Designations Process 

VI. Clean Air Act Implementation 

Requirements 

A. Classifications 

1. Proposal 

2. Public comments 

3. Final 

B. Attainment Dates 

1. Attaining the NAAQS 

a. Proposal 

b. Final 

2. Consequences of Failing to Attain by the 

Statutory Attainment Date 

a. Proposal 

b. Final 

C. Section 110(a)(2) NAAQS Infrastructure 

Requirements 

1. Proposal 

2. Final 

D. Attainment Planning Requirements 

1. Nonattainment Area SIPs 

a. Proposal 

b. Public Comments 

c. Final 

2. New Source Review and Prevention of 

Significant Deterioration Requirements 

a. Proposal 

b. Public Comments 

c. Final 

3. General Conformity 

a. Proposal 

4. Transportation Conformity 

a. Proposal 

b. Public Comments 

c. Final 

VII. Communication of Public Health 

Information 

VIII. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review 

B. Paperwork Reduction Act 

C. Regulatory Flexibility Act 

D. Unfunded Mandates Reform Act 

E. Executive Order 13132: Federalism 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

G. Executive Order 13045: Protection of 

Children From Environmental Health & 

Safety Risks 

H. Executive Order 13211: Actions 

Concerning Regulations That 

Significantly Affect Energy Supply, 

Distribution or Use 

I. National Technology Transfer and 

Advancement Act 

J. Executive Order 12898: Federal Actions 

To Address Environmental Justice in 

Minority Populations and Low-Income 

Populations 

K. Congressional Review Act 

References 

I. Background 

A. Summary of Revisions to the NO2

Primary NAAQS 

Based on its review of the air quality 

criteria for oxides of nitrogen and the 

primary national ambient air quality 

standard (NAAQS) for oxides of 

nitrogen as measured by nitrogen 

dioxide (NO2), EPA is making revisions 

to the primary NO2 NAAQS in order to 

provide requisite protection of public 

health as appropriate under section 109 

of the Clean Air Act (Act or CAA). 

Specifically, EPA is supplementing the 

existing annual standard for NO2 of 53 

parts per billion (ppb) by establishing a 

new short-term standard based on the 3- 

year average of the 98th percentile of the 

yearly distribution of 1-hour daily 

maximum concentrations. EPA is setting 

the level of this new standard at 100 

ppb. EPA is making changes in data 

handling conventions for NO2 by adding 

provisions for this new 1-hour primary 

standard. EPA is also establishing 

requirements for an NO2 monitoring 

network. These new provisions require 

monitors at locations where maximum 

NO2 concentrations are expected to 

occur, including within 50 meters of 

major roadways, as well as monitors 

sited to measure the area-wide NO2

concentrations that occur more broadly 

across communities. EPA is making 

conforming changes to the air quality 

index (AQI). 

B. Legislative Requirements 

Two sections of the CAA govern the 

establishment and revision of the 

NAAQS. Section 108 of the Act directs 

the Administrator to identify and list air 

pollutants that meet certain criteria, 

including that the air pollutant ‘‘in [her] 

judgment, cause[s] or contribute[s] to air 

pollution which may reasonably be 

anticipated to endanger public health 

and welfare’’ and ‘‘the presence of which 

in the ambient air results from 

numerous or diverse mobile or 

stationary sources.’’ 42 U.S.C. 21 

7408(a)(1)(A) & (B). For those air 

pollutants listed, section 108 requires 

the Administrator to issue air quality 

criteria that ‘‘accurately reflect the latest 

scientific knowledge useful in 

indicating the kind and extent of all 

identifiable effects on public health or 

welfare which may be expected from the 

presence of [a] pollutant in ambient air 

***’’ 42 U.S.C. 7408(2). 

Section 109(a) of the Act directs the 

Administrator to promulgate ‘‘primary’’ 

and ‘‘secondary’’ NAAQS for pollutants 

for which air quality criteria have been 

issued. 42 U.S.C. 7409(1).1 Section 

109(b)(1) defines a primary standard as 

one ‘‘the attainment and maintenance of 

which in the judgment of the 

Administrator, based on [the air quality] 

criteria and allowing an adequate 

margin of safety, are requisite to protect 

the public health.’’ 2 42 U.S.C. 

7409(b)(1). A secondary standard, in 

turn, must ‘‘specify a level of air quality 

the attainment and maintenance of 

which, in the judgment of the 

Administrator, based on [the air quality] 

criteria, is requisite to protect the public 

welfare from any known or anticipated 

adverse effects associated with the 

presence of such pollutant in the 

ambient air.’’ 3 42 U.S.C. 7409(b)(2). 

The requirement that primary 

standards include an adequate margin of 

safety is intended to address 

uncertainties associated with 

inconclusive scientific and technical 

information available at the time of 

standard setting. It is also intended to 

provide a reasonable degree of 

protection against hazards that research 

has not yet identified. Lead Industries 

Association v. EPA, 647 F.2d 1130, 1154 

(DC Cir 1980), cert. denied, 449 U.S. 


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VI-105 A-127
6476 Federal Register / Vol. 75, No. 26 / Tuesday, February 9, 2010 / Rules and Regulations 

4 In this document, the terms ‘‘oxides of nitrogen’’ 

and ‘‘nitrogen oxides’’ (NOX) refer to all forms of 

oxidized nitrogen (N) compounds, including NO, 

NO2, and all other oxidized N-containing 

compounds formed from NO and NO2. This follows 

usage in the Clean Air Act Section 108(c): ‘‘Such 

criteria [for oxides of nitrogen] shall include a 

discussion of nitric and nitrous acids, nitrites, 

nitrates, nitrosamines, and other carcinogenic and 

potentially carcinogenic derivatives of oxides of 

nitrogen.’’ By contrast, within the air pollution 

research and control communities, the terms 

‘‘oxides of nitrogen’’ and ‘‘nitrogen oxides’’ are 

restricted to refer only to the sum of NO and NO2, 

and this sum is commonly abbreviated as NOX. The 

category label used by this community for the sum 

of all forms of oxidized nitrogen compounds 

including those listed in Section 108(c) is NOY. 

1042 (1980); American Petroleum 

Institute v. Costle, 665 F.2d 1176, 1186 

(DC Cir. 1981), cert. denied, 455 U.S. 

1034 (1982). Both kinds of uncertainties 

are components of the risk associated 

with pollution at levels below those at 

which human health effects can be said 

to occur with reasonable scientific 

certainty. Thus, in selecting primary 

standards that include an adequate 

margin of safety, the Administrator is 

seeking not only to prevent pollution 

levels that have been demonstrated to be 

harmful but also to prevent lower 

pollutant levels that may pose an 

unacceptable risk of harm, even if the 

risk is not precisely identified as to 

nature or degree. 

In addressing the requirement for a 

margin of safety, EPA considers such 

factors as the nature and severity of the 

health effects involved, the size of the 

at-risk population(s), and the kind and 

degree of the uncertainties that must be 

addressed. The selection of any 

particular approach to providing an 

adequate margin of safety is a policy 

choice left specifically to the 

Administrator’s judgment. Lead 

Industries Association v. EPA, supra, 

647 F.2d at 1161–62. 

In setting standards that are 

‘‘requisite’’ to protect public health and 

welfare, as provided in section 109(b), 

EPA’s task is to establish standards that 

are neither more nor less stringent than 

necessary for these purposes. In so 

doing, EPA may not consider the costs 

of implementing the standards. 

Whitman v. American Trucking 

Associations, 531 U.S. 457, 471, 475–76 

(2001). 

Section 109(d)(1) of the Act requires 

the Administrator to periodically 

undertake a thorough review of the air 

quality criteria published under section 

108 and the NAAQS and to revise the 

criteria and standards as may be 

appropriate. 42 U.S.C. 7409(d)(1). The 

Act also requires the Administrator to 

appoint an independent scientific 

review committee composed of seven 

members, including at least one member 

of the National Academy of Sciences, 

one physician, and one person 

representing State air pollution control 

agencies, to review the air quality 

criteria and NAAQS and to ‘‘recommend 

to the Administrator any new * * * 

standards and revisions of existing 

criteria and standards as may be 

appropriate under section 108 and 

subsection (b) of this section.’’ 42 U.S.C. 

7409(d)(2). This independent review 

function is performed by the Clean Air 

Scientific Advisory Committee (CASAC) 

of EPA’s Science Advisory Board. 

C. Related NO2 Control Programs 

States are primarily responsible for 

ensuring attainment and maintenance of 

ambient air quality standards once EPA 

has established them. Under section 110 

of the Act, 42 U.S.C. 7410, and related 

provisions, States are to submit, for EPA 

approval, State implementation plans 

(SIPs) that provide for the attainment 

and maintenance of such standards 

through control programs directed to 

sources of the pollutants involved. The 

States, in conjunction with EPA, also 

administer the prevention of significant 

deterioration program that covers these 

pollutants. See 42 U.S.C. 7470–7479. In 

addition, Federal programs provide for 

nationwide reductions in emissions of 

these and other air pollutants under 

Title II of the Act, 42 U.S.C. 7521–7574, 

which involves controls for automobile, 

truck, bus, motorcycle, nonroad engine 

and equipment, and aircraft emissions; 

the new source performance standards 

under section 111 of the Act, 42 U.S.C. 

7411; and the national emission 

standards for hazardous air pollutants 

under section 112 of the Act, 42 U.S.C. 

7412. 

Currently there are no areas in the 

United States that are designated as 

nonattainment of the NO2 NAAQS. With 

the revisions to the NO2 NAAQS that 

result from this review, however, some 

areas could be classified as nonattainment. Certain States will be 

required to develop SIPs that identify 

and implement specific air pollution 

control measures to reduce ambient NO2

concentrations to attain and maintain 

the revised NO2 NAAQS, most likely by 

requiring air pollution controls on 

sources that emit oxides of nitrogen 

(NOX).4

While NOX is emitted from a wide 

variety of source types, the top three 

categories of sources of NOX emissions 

are on-road mobile sources, electricity 

generating units, and non-road mobile 

sources. EPA anticipates that NOX

emissions will decrease substantially 

over the next 20 years as a result of the 

ongoing implementation of mobile 

source emissions standards. In 

particular, Tier 2 NOX emission 

standards for light-duty vehicle 

emissions began phasing into the fleet 

beginning with model year 2004, in 

combination with low-sulfur gasoline 

fuel standards. For heavy-duty engines, 

new NOX standards are phasing in 

between the 2007 and 2010 model years, 

following the introduction of ultra-low 

sulfur diesel fuel. Lower NOX standards 

for nonroad diesel engines, locomotives, 

and certain marine engines are 

becoming effective throughout the next 

decade. In future decades, these lowerNOX vehicles and engines will become 

an increasingly large fraction of in-use 

mobile sources, effecting large NOX

emission reductions. 

D. Review of the Air Quality Criteria and 

Standards for Oxides of Nitrogen 

On April 30, 1971, EPA promulgated 

identical primary and secondary 

NAAQS for NO2 under section 109 of 

the Act. The standards were set at 0.053 

parts per million (ppm) (53 ppb), annual 

average (36 FR 8186). EPA completed 

reviews of the air quality criteria and 

NO2 standards in 1985 and 1996 with 

decisions to retain the standard (50 FR 

25532, June 19, 1985; 61 FR 52852, 

October 8, 1996). 

EPA initiated the current review of 

the air quality criteria for oxides of 

nitrogen and the NO2 primary NAAQS 

on December 9, 2005 (70 FR 73236) with 

a general call for information. EPA’s 

draft Integrated Review Plan for the 

Primary National Ambient Air Quality 

Standard for Nitrogen Dioxide (EPA, 

2007a) was made available in February, 

2007 for public comment and was 

discussed by the CASAC via a publicly 

accessible teleconference on May 11, 

2007. As noted in that plan, NOX

includes multiple gaseous (e.g., NO2, 

NO) and particulate (e.g., nitrate) 

species. Because the health effects 

associated with particulate species of 

NOX have been considered within the 

context of the health effects of ambient 

particles in the Agency’s review of the 

NAAQS for particulate matter (PM), the 

current review of the primary NO2

NAAQS is focused on the gaseous 

species of NOX and is not intended to 

address health effects directly 

associated with particulate species. 

The first draft of the Integrated 

Science Assessment for Oxides of 

Nitrogen-Health Criteria (ISA) and the 

Nitrogen Dioxide Health Assessment 

Plan: Scope and Methods for Exposure 

and Risk Assessment (EPA, 2007b) were 

reviewed by CASAC at a public meeting 

held on October 24–25, 2007. Based on 

comments received from CASAC and 

the public, EPA developed the second 


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5The ‘‘form’’ of a standard defines the air quality 

statistic that is to be compared to the level of the 

standard in determining whether an area attains the 

standard. 

draft of the ISA and the first draft of the 

Risk and Exposure Assessment to 

Support the Review of the NO2 Primary 

National Ambient Air Quality Standard 

(Risk and Exposure Assessment (REA)). 

These documents were reviewed by 

CASAC at a public meeting held on May 

1–2, 2008. Based on comments received 

from CASAC and the public at this 

meeting, EPA released the final ISA in 

July of 2008 (EPA, 2008a). In addition, 

comments received were considered in 

developing the second draft of the REA, 

which was released for public review 

and comment in two parts. The first part 

of this document, containing chapters 

1–7, 9 and appendices A and C as well 

as part of appendix B, was released in 

August 2008. The second part of this 

document, containing chapter 8 

(describing the Atlanta exposure 

assessment) and a completed appendix 

B, was released in October of 2008. This 

document was the subject of CASAC 

reviews at public meetings on 

September 9 and 10, 2008 (for the first 

part) and on October 22, 2008 (for the 

second part). In preparing the final REA 

(EPA, 2008b), EPA considered 

comments received from the CASAC 

and the public at those meetings. 

In the course of reviewing the second 

draft REA, CASAC expressed the view 

that the document would be incomplete 

without the addition of a policy 

assessment chapter presenting an 

integration of evidence-based 

considerations and risk and exposure 

assessment results. CASAC stated that 

such a chapter would be ‘‘critical for 

considering options for the NAAQS for 

NO2’’ (Samet, 2008a). In addition, within 

the period of CASAC’s review of the 

second draft REA, EPA’s Deputy 

Administrator indicated in a letter to the 

chair of CASAC, addressing earlier 

CASAC comments on the NAAQS 

review process, that the risk and 

exposure assessment will include ‘‘a 

broader discussion of the science and 

how uncertainties may effect decisions 

on the standard’’ and ‘‘all analyses and 

approaches for considering the level of 

the standard under review, including 

risk assessment and weight of evidence 

methodologies’’ (Peacock, 2008, p. 3; 

September 8, 2008). 

Accordingly, the final REA included a 

new policy assessment chapter. This 

policy assessment chapter considered 

the scientific evidence in the ISA and 

the exposure and risk characterization 

results presented in other chapters of 

the REA as they relate to the adequacy 

of the current NO2 primary NAAQS and 

potential alternative primary NO2

standards. In considering the current 

and potential alternative standards, the 

policy assessment chapter of the final 

REA focused on the information that is 

most pertinent to evaluating the basic 

elements of national ambient air quality 

standards: Indicator, averaging time, 

form,5 and level. These elements, which 

together serve to define each standard, 

must be considered collectively in 

evaluating the health protection 

afforded. CASAC discussed the final 

version of the REA, with an emphasis 

on the policy assessment chapter, 

during a public teleconference held on 

December 5, 2008. Following that 

teleconference, CASAC offered 

comments and advice on the NO2

primary NAAQS in a letter to the 

Administrator (Samet, 2008b). 

The schedule for completion of this 

review is governed by a judicial order 

resolving a lawsuit filed in September 

2005, concerning the timing of the 

current review. The order that now 

governs this review, entered by the 

court in August 2007 and amended in 

December 2008, provides that the 

Administrator will sign, for publication, 

notices of proposed and final 

rulemaking concerning the review of the 

primary NO2 NAAQS no later than June 

26, 2009 and January 22, 2010, 

respectively. In accordance with this 

schedule, the Administrator signed a 

notice of proposed rulemaking on June 

26, 2009 (FR 74 34404). This action 

presents the Administrator’s final 

decisions on the primary NO2 standard. 

E. Summary of Proposed Revisions to 

the NO2 Primary NAAQS 

For the reasons discussed in the 

preamble of the proposal for the NO2

primary NAAQS (74 FR 34404), EPA 

proposed to make revisions to the 

primary NO2 NAAQS and to make 

related revisions for NO2 data handling 

conventions in order to provide 

requisite protection of public health. 

EPA also proposed to make 

corresponding changes to the AQI for 

NO2. Specifically, EPA proposed to 

supplement the current annual standard 

by establishing a new short-term NO2

standard that would reflect the 

maximum allowable NO2 concentration 

anywhere in an area. EPA proposed that 

this new short-term standard would be 

based on the 3-year average of the 99th 

percentile (or 4th highest) of the yearly 

distribution of 1-hour daily maximum 

NO2 concentrations and solicited 

comment on using the 3-year average of 

the 98th percentile (or 7th or 8th 

highest) of the yearly distribution of 1- 

hour daily maximum NO2

concentrations. EPA proposed to set the 

level of this new 1-hour standard within 

the range of 80 to 100 ppb and solicited 

comment on standard levels as low as 

65 ppb and as high as 150 ppb. EPA 

proposed to specify the level of the 

standard to the nearest ppb. EPA also 

proposed to establish requirements for 

an NO2 monitoring network at locations 

where maximum NO2 concentrations 

are expected to occur, including 

monitors within 50 meters of major 

roadways, as well as area-wide monitors 

sited to measure the NO2 concentrations 

that can occur more broadly across 

communities. EPA also solicited 

comment on the alternative approach of 

setting a 1-hour standard that would 

reflect the allowable area-wide NO2

concentration. 

F. Organization and Approach to Final 

NO2 Primary NAAQS Decisions 

This action presents the 

Administrator’s final decisions 

regarding the need to revise the current 

NO2 primary NAAQS. Revisions to the 

primary NAAQS for NO2, and the 

rationale supporting those revisions, are 

described below in section II. 

Requirements for the NO2 ambient 

monitoring network are described in 

section III. Related requirements for data 

completeness, data handling, data 

reporting, rounding conventions, and 

exceptional events are described in 

section IV. Implementation of the 

revised NO2 primary NAAQS is 

discussed in sections V and VI. 

Communication of public health 

information through the AQI is 

discussed in section VII and a 

discussion of statutory and executive 

order reviews is provided in section 

VIII. 

Today’s final decisions are based on 

a thorough review in the ISA of 

scientific information on known and 

potential human health effects 

associated with exposure to NO2 in the 

air. These final decisions also take into 

account: (1) Assessments in the REA of 

the most policy-relevant information in 

the ISA as well as quantitative exposure 

and risk analyses based on that 

information; (2) CASAC Panel advice 

and recommendations, as reflected in its 

letters to the Administrator and its 

public discussions of the ISA, the REA, 

and the notice of proposed rulemaking; 

(3) public comments received during the 

development of ISA and REA; and (4) 

public comments received on the 

proposed rulemaking. 

Some commenters have referred to 

and discussed individual scientific 

analyses on the health effects of NO2

that were not included in the ISA (EPA, 

2008a) (‘‘new studies’’). In considering 


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6 It should be noted that the ISA (section 2.4.1) 

references a different number of active monitors in 

the NO2 network. The discrepancy between the ISA 

numbers and the number presented here is due to 

differing metrics used in pulling data from AQS. 

The ISA only references SLAMS, NAMS, and 

PAMS sites with defined monitoring objectives, 

while Watkins and Thompson (2008) considered all 

NO2 sites reporting data at any point during the 

year. Based on this approach, Watkins and 

Thompson (2008) also noted that the size of the 

NO2 monitoring network has remained relatively 

stable since the early 1980s. 

and responding to comments for which 

such ‘‘new studies’’ were cited in 

support, EPA has provisionally 

considered the cited studies in the 

context of the findings of the ISA. 

As in prior NAAQS reviews, EPA is 

basing its decision in this review on 

studies and related information 

included in the ISA and staff’s policy 

assessment, which have undergone 

CASAC and public review. In this NO2

NAAQS review, staff’s policy 

assessment was presented in the form of 

a policy assessment chapter of the REA 

(EPA, 2008b). The studies assessed in 

the ISA and REA, and the integration of 

the scientific evidence presented in 

them, have undergone extensive critical 

review by EPA, CASAC, and the public. 

The rigor of that review makes these 

studies, and their integrative 

assessment, the most reliable source of 

scientific information on which to base 

decisions on the NAAQS, decisions that 

all parties recognize as of great import. 

NAAQS decisions can have profound 

impacts on public health and welfare, 

and NAAQS decisions should be based 

on studies that have been rigorously 

assessed in an integrative manner not 

only by EPA but also by the statutorily 

mandated independent advisory 

committee, as well as the public review 

that accompanies this process. EPA’s 

provisional consideration of ‘‘new 

studies’’ did not and could not provide 

that kind of in-depth critical review. 

This decision is consistent with EPA’s 

practice in prior NAAQS reviews and its 

interpretation of the requirements of the 

CAA. Since the 1970 amendments, the 

EPA has taken the view that NAAQS 

decisions are to be based on scientific 

studies and related information that 

have been assessed as a part of the 

pertinent air quality criteria, and has 

consistently followed this approach. 

This longstanding interpretation was 

strengthened by new legislative 

requirements enacted in 1977, which 

added section 109(d)(2) of the Act 

concerning CASAC review of air quality 

criteria. See 71 FR 61144, 61148 

(October 17, 2006) (final decision on 

review of PM NAAQS) for a detailed 

discussion of this issue and EPA’s past 

practice. 

As discussed in EPA’s 1993 decision 

not to revise the NAAQS for ozone (O3), 

‘‘new studies’’ may sometimes be of such 

significance that it is appropriate to 

delay a decision on revision of a 

NAAQS and to supplement the 

pertinent air quality criteria so the 

studies can be taken into account (58 FR 

at 13013–13014, March 9, 1993). In the 

present case, EPA’s provisional 

consideration of ‘‘new studies’’ 

concludes that, taken in context, the 

‘‘new’’ information and findings do not 

materially change any of the broad 

scientific conclusions regarding the 

health effects of NO2 made in the air 

quality criteria. For this reason, 

reopening the air quality criteria review 

would not be warranted even if there 

were time to do so under the court order 

governing the schedule for this 

rulemaking. 

Accordingly, EPA is basing the final 

decisions in this review on the studies 

and related information included in the 

NO2 air quality criteria that have 

undergone CASAC and public review. 

EPA will consider the ‘‘new studies’’ for 

purposes of decision-making in the next 

periodic review of the NO2 NAAQS, 

which will provide the opportunity to 

fully assess these studies through a 

more rigorous review process involving 

EPA, CASAC, and the public. Further 

discussion of these ‘‘new studies’’ can be 

found below, in section II.E, and in the 

Response to Comments document. 

II. Rationale for Final Decisions on the 

NO2 Primary Standard 

This section presents the rationale for 

the Administrator’s decision to revise 

the existing NO2 primary standard by 

supplementing the current annual 

standard with a new 1-hour standard. In 

developing this rationale, EPA has 

drawn upon an integrative synthesis of 

the entire body of evidence on human 

health effects associated with the 

presence of NO2 in the air. As 

summarized below in section II.B, this 

body of evidence addresses a broad 

range of health endpoints associated 

with exposure to NO2. In considering 

this entire body of evidence, EPA 

focuses in particular on those health 

endpoints for which the ISA finds 

associations with NO2 to be causal or 

likely causal. This rationale also draws 

upon the results of quantitative 

exposure and risk assessments, 

summarized below in section II.C. 

As discussed below, a substantial 

amount of new research has been 

conducted since the last review of the 

NO2 NAAQS, with important new 

information coming from epidemiologic 

studies in particular. The newly 

available research studies evaluated in 

the ISA have undergone intensive 

scrutiny through multiple layers of peer 

review and opportunities for public 

review and comment. While important 

uncertainties remain in the qualitative 

and quantitative characterizations of 

health effects attributable to exposure to 

ambient NO2, the review of this 

information has been extensive and 

deliberate. 

The remainder of this section 

provides background information that 

informed the Administrator’s decisions 

on the primary standard and discusses 

the rationale for those decisions. Section 

II.A presents a discussion of NO2 air 

quality. Section II.B includes an 

overview of the scientific evidence 

related to health effects associated with 

NO2 exposure. This overview includes 

discussion of the health endpoints and 

at-risk populations considered in the 

ISA. Section II.C discusses the 

approaches taken by EPA to assess 

exposures and health risks associated 

with NO2, including a discussion of key 

results. Section II.D summarizes the 

approach that was used in the current 

review of the NO2 NAAQS with regard 

to consideration of the scientific 

evidence and exposure-/risk-based 

results related to the adequacy of the 

current standard and potential 

alternative standards. Sections II.E–II.G 

discuss the Administrator’s decisions 

regarding the adequacy of the current 

standard, elements of a new 1-hour 

standard, and retention of the current 

annual standard, respectively, taking 

into consideration public comments on 

the proposed decisions. Section II.H 

summarizes the Administrator’s 

decisions with regard to the NO2

primary NAAQS. 

A. Characterization of NO2 Air Quality 

1. Current Patterns of NO2 Air Quality 

The size of the State and local NO2

monitoring network has remained 

relatively stable since the early 1980s, 

and currently has approximately 400 

monitors reporting data to EPA’s Air 

Quality System (AQS) database.6 At 

present, there are no minimum 

monitoring requirements for NO2 in 40 

CFR part 58 Appendix D, other than a 

requirement for EPA Regional 

Administrator approval before removing 

any existing monitors, and that any 

ongoing NO2 monitoring must have at 

least one monitor sited to measure the 

maximum concentration of NO2 in that 

area (though, as discussed below 

monitors in the current network do not 

measure peak concentrations associated 

with on-road mobile sources that can 

occur near major roadways because the 

network was not designed for this 

purpose). EPA removed the specific 


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minimum monitoring requirements for 

NO2 of two monitoring sites per area 

with a population of 1,000,000 or more 

in the 2006 monitoring rule revisions 

(71 FR 61236), based on the fact that 

there were no NO2 nonattainment areas 

at that time, coupled with trends 

evidence showing an increasing gap 

between national average NO2

concentrations and the current annual 

standard. Additionally, the minimum 

requirements were removed to provide 

State, local, and Tribal air monitoring 

agencies flexibility in meeting higher 

priority monitoring needs for pollutants 

such as O3 and PM2.5, or implementing 

the new multi-pollutant sites (NCore 

network) required by the 2006 rule 

revisions, by allowing them to 

discontinue lower priority monitoring. 

There are requirements in 40 CFR part 

58 Appendix D for NO2 monitoring as 

part of the Photochemical Assessment 

Monitoring Stations (PAMS) network. 

However, of the approximately 400 NO2

monitors currently in operation, only 

about 10 percent may be due to the 

PAMS requirements. 

An analysis of the approximately 400 

monitors comprising the current NO2

monitoring network (Watkins and 

Thompson, 2008) indicates that the 

current NO2 network has largely 

remained unchanged in terms of size 

and target monitor objective categories 

since it was introduced in the May 10, 

1979 monitoring rule (44 FR 27571). 

The review of the current network 

found that the assessment of 

concentrations for general population 

exposure and maximum concentrations 

at neighborhood and larger scales were 

the top objectives. A review of the 

distribution of listed spatial scales of 

representation shows that only 

approximately 3 monitors are described 

as microscale, representing an area on 

the order of several meters to 100 

meters, and approximately 23 monitors 

are described as middle scale, which 

represents an area on the order of 100 

to 500 meters. This low percentage of 

smaller spatially representative scale 

sites within the network of 

approximately 400 monitoring sites 

indicates that the majority of monitors 

have, in fact, been sited to assess areawide exposures on the neighborhood, 

urban, and regional scales, as would be 

expected for a network sited to support 

the current annual NO2 standard and 

PAMS objectives. The current network 

does not include monitors placed near 

major roadways and, therefore, monitors 

in the current network do not 

necessarily measure the maximum 

concentrations that can occur on a 

localized scale near these roadways (as 

discussed in the next section). It should 

be noted that the network not only 

accommodates NAAQS related 

monitoring but also serves other 

monitoring objectives, such as support 

for photochemistry analysis, O3

modeling and forecasting, and 

particulate matter precursor tracking. 

2. NO2 Air Quality and Gradients 

Around Roadways 

On-road and non-road mobile sources 

account for approximately 60% of NOX

emissions (ISA, table 2.2–1) and trafficrelated exposures can dominate 

personal exposures to NO2 (ISA section 

2.5.4). While driving, personal exposure 

concentrations in the cabin of a vehicle 

could be substantially higher than 

ambient concentrations measured 

nearby (ISA, section 2.5.4). For example, 

estimates presented in the REA suggest 

that on/near roadway NO2

concentrations could be approximately 

80% (REA, section 7.3.2) higher on 

average across locations than 

concentrations away from roadways and 

that roadway-associated environments 

could be responsible for the majority of 

1-hour peak NO2 exposures (REA, 

Figures 8–17 and 8–18). Because 

monitors in the current network are not 

sited to measure peak roadwayassociated NO2 concentrations, 

individuals who spend time on and/or 

near major roadways could experience 

NO2 concentrations that are 

considerably higher than indicated by 

monitors in the current area-wide NO2

monitoring network. 

Research suggests that the 

concentrations of on-road mobile source 

pollutants such as NOX, carbon 

monoxide (CO), directly emitted air 

toxics, and certain size distributions of 

particulate matter (PM), such as 

ultrafine PM, typically display peak 

concentrations on or immediately 

adjacent to roads (ISA, section 2.5). This 

situation typically produces a gradient 

in pollutant concentrations, with 

concentrations decreasing with 

increasing distance from the road, and 

concentrations generally decreasing to 

near area-wide ambient levels, or typical 

upwind urban background levels, 

within a few hundred meters 

downwind. While such a concentration 

gradient is present on almost all roads, 

the characteristics of the gradient, 

including the distance from the road 

that a mobile source pollutant signature 

can be differentiated from background 

concentrations, are heavily dependent 

on factors such as traffic volumes, local 

topography, roadside features, 

meteorology, and photochemical 

reactivity conditions (Baldauf, et al., 

2009; Beckerman et al., 2008; Clements 

et al., 2008; Hagler et al., 2009; Janssen 

et al., 2001; Rodes and Holland, 1981; 

Roorda-Knape et al., 1998; Singer et al., 

2004; Zhou and Levy, 2007). 

Because NO2 in the ambient air is due 

largely to the atmospheric oxidation of 

NO emitted from combustion sources 

(ISA, section 2.2.1), elevated NO2

concentrations can extend farther away 

from roadways than the primary 

pollutants also emitted by on-road 

mobile sources. More specifically, 

review of the technical literature 

suggests that NO2 concentrations may 

return to area-wide or typical urban 

background concentrations within 

distances up to 500 meters of roads, 

though the actual distance will vary 

with topography, roadside features, 

meteorology, and photochemical 

reactivity conditions (Baldauf et al., 

2009; Beckerman et al., 2008; Clements 

et al., 2008; Gilbert et al. 2003; Rodes 

and Holland, 1981; Singer et al., 2004; 

Zhou and Levy, 2007). Efforts to 

quantify the extent and slope of the 

concentration gradient that may exist 

from peak near-road concentrations to 

the typical urban background 

concentrations must consider the 

variability that exists across locations 

and for a given location over time. As 

a result, we have identified a range of 

concentration gradients in the technical 

literature which indicate that, on 

average, peak NO2 concentrations on or 

immediately adjacent to roads may 

typically be between 30 and 100 percent 

greater than concentrations monitored 

in the same area but farther away from 

the road (ISA, Section 2.5.4; Beckerman 

et al., 2008; Gilbert et al., 2003; Rodes 

and Holland, 1981; Roorda-Knape et al., 

1998; Singer et al., 2004). This range of 

concentration gradients has 

implications for revising the NO2

primary standard and for the NO2

monitoring network (discussed in 

sections II.F.4 and III). 

B. Health Effects Information 

In the last review of the NO2 NAAQS, 

the 1993 NOX Air Quality Criteria 

Document (1993 AQCD) (EPA, 1993) 

concluded that there were two key 

health effects of greatest concern at 

ambient or near-ambient concentrations 

of NO2 (ISA, section 5.3.1). The first was 

increased airway responsiveness in 

asthmatic individuals after short-term 

exposures. The second was increased 

respiratory illness among children 

associated with longer-term exposures 

to NO2. Evidence also was found for 

increased risk of emphysema, but this 

appeared to be of major concern only 

with exposures to NO2 at levels much 

higher than then current ambient levels 

(ISA, section 5.3.1). Controlled human 


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exposure and animal toxicological 

studies provided qualitative evidence 

for airway hyperresponsiveness and 

lung function changes while 

epidemiologic studies provided 

evidence for increased respiratory 

symptoms with increased indoor NO2

exposures. Animal toxicological 

findings of lung host defense system 

changes with NO2 exposure provided a 

biologically-plausible basis for the 

epidemiologic results. Subpopulations 

considered potentially more susceptible 

to the effects of NO2 exposure included 

persons with preexisting respiratory 

disease, children, and the elderly. The 

epidemiologic evidence for respiratory 

health effects was limited, and no 

studies had considered endpoints such 

as hospital admissions, emergency 

department visits, or mortality (ISA, 

section 5.3.1). 

As summarized below and discussed 

more fully in section II.B of the proposal 

notice, evidence published since the last 

review generally has confirmed and 

extended the conclusions articulated in 

the 1993 AQCD (ISA, section 5.3.2). The 

epidemiologic evidence has grown 

substantially with the addition of field 

and panel studies, intervention studies, 

time-series studies of endpoints such as 

hospital admissions, and a substantial 

number of studies evaluating mortality 

risk associated with short-term NO2

exposures. While not as marked as the 

growth in the epidemiologic literature, a 

number of recent toxicological and 

controlled human exposure studies also 

provide insights into relationships 

between NO2 exposure and health 

effects. This body of evidence focuses 

the current review on NO2-related 

respiratory effects at lower ambient and 

exposure concentrations than 

considered in the previous review. 

1. Adverse Respiratory Effects and 

Short-Term Exposure to NO2

The ISA concluded that the findings 

of epidemiologic, controlled human 

exposure, and animal toxicological 

studies provide evidence that is 

sufficient to infer a likely causal 

relationship for respiratory effects 

following short-term NO2 exposure 

(ISA, sections 3.1.7 and 5.3.2.1). The 

ISA (section 5.4) concluded that the 

strongest evidence for an association 

between NO2 exposure and adverse 

human health effects comes from 

epidemiologic studies of respiratory 

symptoms, emergency department 

visits, and hospital admissions. These 

studies include panel and field studies, 

studies that control for the effects of cooccurring pollutants, and studies 

conducted in areas where the whole 

distribution of ambient 24-hour average 

NO2 concentrations was below the 

current NAAQS level of 53 ppb (annual 

average). With regard to this evidence, 

the ISA concluded that NO2

epidemiologic studies provide ‘‘little 

evidence of any effect threshold’’ (ISA, 

section 5.3.2.9, p. 5–15). In studies that 

have evaluated concentration-response 

relationships, they appear linear within 

the observed range of data (ISA, section 

5.3.2.9). 

Overall, the epidemiologic evidence 

for respiratory effects has been 

characterized in the ISA as consistent, 

in that associations are reported in 

studies conducted in numerous 

locations with a variety of 

methodological approaches, and 

coherent, in that the studies report 

associations with respiratory health 

outcomes that are logically linked 

together. In addition, a number of these 

associations are statistically significant, 

particularly the more precise effect 

estimates (ISA, section 5.3.2.1). These 

epidemiologic studies are supported by 

evidence from toxicological and 

controlled human exposure studies, 

particularly those that evaluated airway 

hyperresponsiveness in asthmatic 

individuals (ISA, section 5.4). The ISA 

concluded that together, the 

epidemiologic and experimental data 

sets form a plausible, consistent, and 

coherent description of a relationship 

between NO2 exposures and an array of 

adverse respiratory health effects that 

range from the onset of respiratory 

symptoms to hospital admissions. 

In considering the uncertainties 

associated with the epidemiologic 

evidence, the ISA (section 5.4) noted 

that it is difficult to determine ‘‘the 

extent to which NO2 is independently 

associated with respiratory effects or if 

NO2 is a marker for the effects of 

another traffic-related pollutant or mix 

of pollutants.’’ On-road vehicle exhaust 

emissions are a widespread source of 

combustion pollutant mixtures that 

include NOX and are an important 

contributor to NO2 levels in near-road 

locations. Although the presence of 

other pollutants from vehicle exhaust 

emissions complicates efforts to 

quantify specific NO2-related health 

effects, a number of epidemiologic 

studies have evaluated associations with 

NO2 in models that also include cooccurring pollutants such as PM, O3, 

CO, and/or SO2. The evidence 

summarized in the ISA indicates that 

NO2 associations generally remain 

robust in these multi-pollutant models 

and supports a direct effect of shortterm NO2 exposure on respiratory 

morbidity (see ISA Figures 3.1–7, 3.1– 

10, 3.1–11). The plausibility and 

coherence of these effects are also 

supported by epidemiologic studies of 

indoor NO2 as well as experimental (i.e., 

toxicological and controlled human 

exposure) studies that have evaluated 

host defense and immune system 

changes, airway inflammation, and 

airway responsiveness (see subsequent 

sections of this proposal and the ISA, 

section 5.3.2.1). The ISA (section 5.4) 

concluded that the robustness of 

epidemiologic findings to adjustment 

for co-pollutants, coupled with data 

from animal and human experimental 

studies, support a determination that 

the relationship between NO2 and 

respiratory morbidity is likely causal, 

while still recognizing the relationship 

between NO2 and other traffic related 

pollutants. 

The epidemiologic and experimental 

studies encompass a number of 

respiratory-related health endpoints, 

including emergency department visits 

and hospitalizations, respiratory 

symptoms, airway hyperresponsiveness, 

airway inflammation, and lung function. 

The findings relevant to these 

endpoints, which provide the rationale 

to support the judgment of a likely 

causal relationship, are described in 

more detail in section II.B.1 of the 

proposal. 

2. Other Effects With Short-Term 

Exposure to NO2

a. Mortality 

The ISA concluded that the 

epidemiologic evidence is suggestive, 

but not sufficient, to infer a causal 

relationship between short-term 

exposure to NO2 and all-cause and 

cardiopulmonary-related mortality (ISA, 

section 5.3.2.3). Results from several 

large United States and European 

multicity studies and a meta-analysis 

study indicate positive associations 

between ambient NO2 concentrations 

and the risk of all-cause (nonaccidental) 

mortality, with effect estimates ranging 

from 0.5 to 3.6% excess risk in mortality 

per standardized increment (20 ppb for 

24-hour averaging time, 30 ppb for 1- 

hour averaging time) (ISA, section 3.3.1, 

Figure 3.3–2, section 5.3.2.3). In general, 

the ISA concluded that NO2 effect 

estimates were robust to adjustment for 

co-pollutants. Both cardiovascular and 

respiratory mortality have been 

associated with increased NO2

concentrations in epidemiologic studies 

(ISA, Figure 3.3–3); however, similar 

associations were observed for other 

pollutants, including PM and SO2. The 

range of risk estimates for excess 

mortality is generally smaller than that 

for other pollutants such as PM. In 

addition, while NO2 exposure, alone or 

in conjunction with other pollutants, 


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may contribute to increased mortality, 

evaluation of the specificity of this 

effect is difficult. Clinical studies 

showing hematologic effects and animal 

toxicological studies showing 

biochemical, lung host defense, 

permeability, and inflammation changes 

with short-term exposures to NO2

provide limited evidence of plausible 

pathways by which risks of mortality 

may be increased, but no coherent 

picture is evident at this time (ISA, 

section 5.3.2.3). 

b. Cardiovascular Effects 

The ISA concluded that the available 

evidence on cardiovascular health 

effects following short-term exposure to 

NO2 is inadequate to infer the presence 

or absence of a causal relationship at 

this time (ISA, section 5.3.2.2). Evidence 

from epidemiologic studies of heart rate 

variability, repolarization changes, and 

cardiac rhythm disorders among heart 

patients with ischemic cardiac disease 

are inconsistent (ISA, section 5.3.2.2). In 

most studies, associations with PM were 

found to be similar or stronger than 

associations with NO2. Generally 

positive associations between ambient 

NO2 concentrations and hospital 

admissions or emergency department 

visits for cardiovascular disease have 

been reported in single-pollutant 

models (ISA, section 5.3.2.2); however, 

most of these effect estimate values were 

diminished in multi-pollutant models 

that also contained CO and PM indices 

(ISA, section 5.3.2.2). Mechanistic 

evidence of a role for NO2 in the 

development of cardiovascular diseases 

from studies of biomarkers of 

inflammation, cell adhesion, 

coagulation, and thrombosis is lacking 

(ISA, section 5.3.2.2). Furthermore, the 

effects of NO2 on various hematological 

parameters in animals are inconsistent 

and, thus, provide little biological 

plausibility for effects of NO2 on the 

cardiovascular system (ISA, section 

5.3.2.2). 

3. Health Effects With Long-Term 

Exposure to NO2

a. Respiratory Morbidity 

The ISA concluded that overall, the 

epidemiologic and experimental 

evidence is suggestive, but not 

sufficient, to infer a causal relationship 

between long-term NO2 exposure and 

respiratory morbidity (ISA, section 

5.3.2.4). The available database 

evaluating the relationship between 

respiratory illness in children and longterm exposures to NO2 has increased 

since the 1996 review of the NO2

NAAQS (see section II.B.3 of the 

proposal for a more detailed 

discussion). A number of epidemiologic 

studies have examined the effects of 

long-term exposure to NO2 and reported 

positive associations with decrements in 

lung function and partially irreversible 

decrements in lung function growth 

(ISA, section 3.4.1, Figures 3.4–1 and 

3.4–2). While animal toxicological 

studies may provide biological 

plausibility for the chronic effects of 

NO2 that have been observed in 

epidemiologic studies (ISA, sections 

3.4.5 and 5.3.2.4), the high correlation 

among traffic-related pollutants in 

epidemiologic studies makes it difficult 

to accurately estimate independent 

effects (ISA, section 5.3.2.4). 

b. Mortality 

The ISA concluded that the 

epidemiologic evidence is inadequate to 

infer the presence or absence of a causal 

relationship between long-term 

exposure to NO2 and mortality (ISA, 

section 5.3.2.6). In the United States and 

European cohort studies examining the 

relationship between long-term 

exposure to NO2 and mortality, results 

have been inconsistent (ISA, section 

5.3.2.6). Further, when associations 

were suggested, they were not specific 

to NO2 but also implicated PM and 

other traffic indicators. The relatively 

high correlations reported between NO2

and PM indices make it difficult to 

interpret these observed associations at 

this time (ISA, section 5.3.2.6). 

c. Carcinogenic, cardiovascular, and 

reproductive/developmental effects 

The ISA concluded that the available 

epidemiologic and toxicological 

evidence is inadequate to infer the 

presence or absence of a causal 

relationship for carcinogenic, 

cardiovascular, and reproductive and 

developmental effects related to longterm NO2 exposure (ISA, section 

5.3.2.5). Epidemiologic studies 

conducted in Europe have shown an 

association between long-term NO2

exposure and increased incidence of 

cancer (ISA, section 5.3.2.5). However, 

the animal toxicological studies have 

provided no clear evidence that NO2

acts as a carcinogen (ISA, section 

5.3.2.5). The very limited epidemiologic 

and toxicological evidence do not 

suggest that long-term exposure to NO2

has cardiovascular effects (ISA, section 

5.3.2.5). The epidemiologic evidence is 

not consistent for associations between 

NO2 exposure and fetal growth 

retardation; however, some evidence is 

accumulating for effects on preterm 

delivery (ISA, section 5.3.2.5). Scant 

animal evidence supports a weak 

association between NO2 exposure and 

adverse birth outcomes and provides 

little mechanistic information or 

biological plausibility for the 

epidemiologic findings. 

4. NO2-related Impacts on Public Health 

Specific groups within the general 

population are likely at increased risk 

for suffering adverse effects from NO2

exposure. This could occur because they 

are affected by lower levels of NO2 than 

the general population or because they 

experience a larger health impact than 

the general population to a given level 

of exposure (susceptibility) and/or 

because they are exposed to higher 

levels of NO2 than the general 

population (vulnerability). The term 

susceptibility generally encompasses 

innate (e.g., genetic or developmental) 

and/or acquired (e.g., age or disease) 

factors that make individuals more 

likely to experience effects with 

exposure to pollutants. The severity of 

health effects experienced by a 

susceptible subgroup may be much 

greater than that experienced by the 

population at large. Factors that may 

influence susceptibility to the effects of 

air pollution include age (e.g., infants, 

children, elderly); gender; race/ 

ethnicity; genetic factors; and preexisting disease/condition (e.g., obesity, 

diabetes, respiratory disease, asthma, 

chronic obstructive pulmonary disease 

(COPD), cardiovascular disease, airway 

hyperresponsiveness, respiratory 

infection, adverse birth outcome) (ISA, 

sections 4.3.1, 4.3.5, and 5.3.2.8). In 

addition, certain groups may experience 

relatively high exposure to NO2, thus 

forming a potentially vulnerable 

population (ISA, section 4.3.6). Factors 

that may influence susceptibility and 

vulnerability to air pollution include 

socioeconomic status (SES), education 

level, air conditioning use, proximity to 

roadways, geographic location, level of 

physical activity, and work environment 

(e.g., indoor versus outdoor) (ISA, 

section 4.3.5). The ISA discussed factors 

that can confer susceptibility and/or 

vulnerability to air pollution with most 

of the discussion devoted to factors for 

which NO2-specific evidence exists 

(ISA, section 4.3). These factors include 

pre-existing disease (e.g., asthma), age 

(i.e., infants, children, older adults), 

genetic factors, gender, socioeconomic 

status, and proximity to roadways (see 

section II.B.4 in proposal for more 

detailed discussion of these factors). 

As discussed in more detail in the 

proposal (section II.B.4), the population 

potentially affected by NO2 is large. A 

considerable fraction of the population 

resides, works, or attends school near 

major roadways, and these individuals 

are likely to have increased exposure to 

NO2 (ISA, section 4.4). Based on data 


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7The most current American Housing Survey 

(http://www.census.gov/hhes/www/housing/ahs/ 

ahs.html) is from 2007 and lists a higher fraction 

of housing units within the 300 foot boundary than 

do prior surveys. According to Table 1A–6 from 

that report (http://www.census.gov/hhes/www/ 

housing/ahs/ahs07/tab1a-6.pdf), out of 128,203,000 

total housing units in the United States, 20,016,000 

were reported by the surveyed occupant or landlord 

as being within 300 feet of a 4-or-more lane 

highway, railroad, or airport. That constitutes 

15.613% of the total housing units in the U.S. 

Assuming equal distributions, with a current 

population of 306,330,199, that means that there 

would be 47.8 million people meeting the 300 foot 

criteria. 

8The study by Tolbert et al. (2007) reported 

positive associations between 1-hour ambient NO2

concentrations and respiratory-related emergency 

department visits. The meta-analysis was included 

in the ISA and reported that short-term exposures 

to NO2 concentrations at or above 100 ppb 

increased airway responsiveness in most 

asthmatics. 

from the 2003 American Housing 

Survey, approximately 36 million 

individuals live within 300 feet (∼90 

meters) of a four-lane highway, railroad, 

or airport (ISA, section 4.4).7

Furthermore, in California, 2.3% of 

schools, with a total enrollment of more 

than 150,000 students were located 

within approximately 500 feet of hightraffic roads, with a higher proportion of 

non-white and economically 

disadvantaged students attending those 

schools (ISA, section 4.4). Of this 

population, asthmatics and members of 

other susceptible groups discussed 

above will have even greater risks of 

experiencing health effects related to 

NO2 exposure. In the United States, 

approximately 10% of adults and 13% 

of children (approximately 22.2 million 

people in 2005) have been diagnosed 

with asthma, and 6% of adults have 

been diagnosed with COPD (ISA, 

section 4.4). The prevalence and 

severity of asthma is higher among 

certain ethnic or racial groups such as 

Puerto Ricans, American Indians, 

Alaskan Natives, and African Americans 

(ISA, section 4.4). A higher prevalence 

of asthma among persons of lower SES 

and an excess burden of asthma 

hospitalizations and mortality in 

minority and inner-city communities 

have been observed (ISA, section 4.4). In 

addition, based on United States census 

data from 2000, about 72.3 million 

(26%) of the United States population 

are under 18 years of age, 18.3 million 

(7.4%) are under 5 years of age, and 35 

million (12%) are 65 years of age or 

older. Therefore, large portions of the 

United States population are in age 

groups that are likely at-risk for health 

effects associated with exposure to 

ambient NO2. The size of the potentially 

at-risk population suggests that 

exposure to ambient NO2 could have a 

significant impact on public health in 

the United States. 

C. Human Exposure and Health Risk 

Characterization 

To put judgments about NO2- 

associated health effects into a broader 

public health context, EPA has drawn 

upon the results of the quantitative 

exposure and risk assessments. 

Judgments reflecting the nature of the 

evidence and the overall weight of the 

evidence are taken into consideration in 

these quantitative exposure and risk 

assessments, discussed below. These 

assessments provide estimates of the 

likelihood that asthmatic individuals 

would experience exposures of potential 

concern and estimates of the incidence 

of NO2-associated respiratory emergency 

department visits under varying air 

quality scenarios (e.g., just meeting the 

current or alternative standards), as well 

as characterizations of the kind and 

degree of uncertainties inherent in such 

estimates. As discussed more fully in 

section II.C of the proposal, this section 

summarizes the approach taken in the 

REA to characterize NO2-related 

exposures and health risks. Goals of the 

REA included estimating short-term 

exposures and potential human health 

risks associated with (1) recent levels of 

ambient NO2; (2) NO2 levels adjusted to 

simulate just meeting the current 

standard; and (3) NO2 levels adjusted to 

simulate just meeting potential 

alternative standards. 

For purposes of the quantitative 

characterization of NO2 health risks, the 

REA determined that it was appropriate 

to focus on endpoints for which the ISA 

concluded that the available evidence is 

sufficient to infer either a causal or a 

likely causal relationship. This was 

generally consistent with judgments 

made in other recent NAAQS reviews 

(e.g., see EPA, 2005). As noted above in 

section II.A, the only health effect 

category for which the evidence was 

judged in the ISA to be sufficient to 

infer either a causal or a likely causal 

relationship is respiratory morbidity 

following short-term NO2 exposure. 

Therefore, for purposes of characterizing 

health risks associated with NO2, the 

REA focused on respiratory morbidity 

endpoints that have been associated 

with short-term NO2 exposures. 

In evaluating the appropriateness of 

specific endpoints for use in the NO2

risk characterization, the REA 

considered both epidemiologic and 

controlled human exposure studies. As 

described in more detail in the proposal 

(section II.C.1), the characterization of 

NO2-associated health risks was based 

on an epidemiology study conducted in 

Atlanta, Georgia by Tolbert et al. (2007) 

and a meta-analysis of controlled 

human exposure studies of NO2 and 

airway responsiveness in asthmatics 

(ISA, Table 3.1–3).8

As noted above, the purpose of the 

assessments described in the REA was 

to characterize air quality, exposures, 

and health risks associated with recent 

ambient levels of NO2, with NO2 levels 

that could be associated with just 

meeting the current NO2 NAAQS, and 

with NO2 levels that could be associated 

with just meeting potential alternative 

standards. To characterize health risks, 

the REA employed three approaches. In 

the first approach, for each air quality 

scenario, NO2 concentrations at fixedsite monitors and simulated 

concentrations on/near roadways were 

compared to potential health effect 

benchmark values derived from the 

controlled human exposure literature. In 

the second approach, modeled estimates 

of exposures in asthmatics were 

compared to potential health effect 

benchmarks. In the third approach, 

concentration-response relationships 

from an epidemiologic study were used 

in conjunction with baseline incidence 

data and recent or simulated ambient 

concentrations to estimate health 

impacts. An overview of the approaches 

to characterizing health risks is 

provided in the proposal (section II.C.2) 

and each approach, along with its 

limitations and uncertainties (see 

proposal, section II.C.3) has been 

described in more detail in the REA 

(chapters 6 through 9). 

Chapters 7–9 of the REA estimated 

exposures and health risks associated 

with recent air quality and with air 

quality, as measured at monitors in the 

current area-wide network, which had 

been adjusted to simulate just meeting 

the current and potential alternative 

standards. The specific standard levels 

evaluated, for an area-wide standard 

based on the 3-year average of the 98th 

and 99th percentile 1-hour daily 

maximum NO2 concentrations, were 50, 

100, 150, and 200 ppb. In interpreting 

these results within the context of the 

current revisions to the NO2 primary 

NAAQS (see below), we note that 

simulation of different standard levels 

was based on adjusting NO2

concentrations at available area-wide 

monitors. Therefore, the standard levels 

referred to above reflect the allowable 

area-wide NO2 concentrations, not the 

maximum allowable concentrations. As 

a consequence, the maximum 

concentrations in an area that just meets 

one of these standard levels would be 

expected to be higher than the standard 

level. For example, given that near-road 


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NO2 concentrations can be 30% to 

100% higher than area-wide 

concentrations (see section II.E.2), an 

area-wide concentration of 50 ppb could 

correspond to near-road concentrations 

from 65 to 100 ppb. 

Key results of the air quality, 

exposure, and risk analyses were 

presented in the policy assessment 

chapter of the REA and summarized in 

the proposal (Table 1 in proposal). In 

considering these results, the policy 

assessment chapter of the REA 

concluded that the risks estimated to be 

associated with just meeting the current 

annual standard can be judged 

important from a public health 

perspective. The results for specific 1- 

hour standard levels estimate that 

limiting the 98th/99th percentile of the 

distribution of 1-hour daily maximum 

NO2 concentrations measured at areawide monitors to 50 or 100 ppb could 

substantially reduce exposures to 

ambient NO2 and associated health risks 

(compared to just meeting the current 

standard). In contrast, limiting these 

area-wide NO2 concentrations to 150 or 

200 ppb is estimated to result in similar, 

or in some cases higher, NO2-associated 

exposures and health risks than just 

meeting the current standard. The 

pattern of results was similar for 

standards just meeting either the 98th or 

the 99th percentile 1-hour daily 

maximum area-wide standards (REA, 

Chapters 7, 8, and 9). 

D. Approach for Reviewing the Need To 

Retain or Revise the Current Standard 

EPA notes that the final decision on 

retaining or revising the current primary 

NO2 standard is a public health policy 

judgment to be made by the 

Administrator. This judgment has been 

informed by a recognition that the 

available health effects evidence reflects 

a continuum consisting of ambient 

levels of NO2 at which scientists 

generally agree that health effects are 

likely to occur, through lower levels at 

which the likelihood and magnitude of 

the response become increasingly 

uncertain. The Administrator’s final 

decisions draw upon scientific 

information and analyses related to 

health effects, population exposures, 

and risks; judgments about the 

appropriate response to the range of 

uncertainties that are inherent in the 

scientific evidence and analyses; and 

comments received from CASAC and 

the public. 

To evaluate whether the current 

primary NO2 standard is requisite or 

whether consideration of revisions is 

appropriate, EPA has used an approach 

in this review that was described in the 

policy assessment chapter of the REA. 

This approach builds upon those used 

in reviews of other criteria pollutants, 

including the most recent reviews of the 

Pb, O3, and PM NAAQS (EPA, 2007c; 

EPA, 2007d; EPA, 2005), and reflects the 

body of evidence and information that 

is currently available. As in other recent 

reviews, EPA’s considerations included 

the implications of placing more or less 

weight or emphasis on different aspects 

of the scientific evidence and the 

exposure/risk-based information, 

recognizing that the weight to be given 

to various elements of the evidence and 

exposure/risk information is part of the 

public health policy judgments that the 

Administrator will make in reaching 

decisions on the standard. 

A series of general questions framed 

this approach to considering the 

scientific evidence and exposure-/riskbased information. First, EPA’s 

consideration of the scientific evidence 

and exposure/risk information with 

regard to the adequacy of the current 

standard has been framed by the 

following questions: 

• To what extent does evidence that has 

become available since the last review 

reinforce or call into question evidence for 

NO2-associated effects that were identified in 

the last review? 

• To what extent has evidence for different 

health effects and/or sensitive populations 

become available since the last review? 

• To what extent have uncertainties 

identified in the last review been reduced 

and/or have new uncertainties emerged? 

• To what extent does evidence and 

exposure-/risk-based information that has 

become available since the last review 

reinforce or call into question any of the 

basic elements of the current standard? 

To the extent that the available 

evidence and exposure-/risk-based 

information suggests it may be 

appropriate to consider revision of the 

current standard, EPA considers that 

evidence and information with regard to 

its support for consideration of a 

standard that is either more or less 

protective than the current standard. 

This evaluation has been framed by the 

following questions: 

• Is there evidence that associations, 

especially causal or likely causal 

associations, extend to ambient NO2

concentrations as low as, or lower than, the 

concentrations that have previously been 

associated with health effects? If so, what are 

the important uncertainties associated with 

that evidence? 

• Are exposures above benchmark levels 

and/or health risks estimated to occur in 

areas that meet the current standard? If so, 

are the estimated exposures and health risks 

important from a public health perspective? 

What are the important uncertainties 

associated with the estimated risks? 

To the extent that there is support for 

consideration of a revised standard, EPA 

then considers the specific elements of 

the standard (indicator, averaging time, 

form, and level) within the context of 

the currently available information. In 

so doing, the Agency has addressed the 

following questions: 

• Does the evidence provide support for 

considering a different indicator for gaseous 

NOX? 

• Does the evidence provide support for 

considering different averaging times? 

• What ranges of levels and forms of 

alternative standards are supported by the 

evidence, and what are the associated 

uncertainties and limitations? 

• To what extent do specific averaging 

times, levels, and forms of alternative 

standards reduce the estimated exposures 

above benchmark levels and risks attributable 

to NO2, and what are the uncertainties 

associated with the estimated exposure and 

risk reductions? 

The questions outlined above have been 

addressed in the REA, the proposal, and 

in this final rulemaking. The following 

sections present the rationale for 

proposed decisions, discussion of 

public comments, and the 

Administrator’s conclusions on the 

adequacy of the current standard and 

potential alternative standards in terms 

of indicator, averaging time, form, and 

level. 

E. Adequacy of the Current Standard 

This section discusses considerations 

related to the decision as to whether the 

current NO2 primary NAAQS is 

requisite to protect public health with 

an adequate margin of safety. 

Specifically, section II.E.1 provides an 

overview of the rationale supporting the 

Administrator’s conclusion in the 

proposal that the current standard alone 

does not provide adequate public health 

protection; section II.E.2 discusses 

comments received on the adequacy of 

the current standard; and section II.E.3 

discusses the Administrator’s final 

decision on whether the current NO2

primary NAAQS is requisite to protect 

public health with an adequate margin 

of safety. 

1. Rationale for Proposed Decision 

In reaching a conclusion regarding the 

adequacy of the current NO2 NAAQS in 

the proposal (section II.E.5), the 

Administrator considered the scientific 

evidence assessed in the ISA and the 

conclusions of the ISA, the exposure 

and risk information presented in the 

REA and the conclusions of the policy 

assessment chapter of the REA, and the 

views expressed by CASAC. These 

considerations are discussed in detail in 

the proposal (II.E.) and are summarized 

in this section. In the proposal, the 


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Administrator noted the following in 

considering the adequacy of the current 

standard: 

• The ISA concluded that the results 

of epidemiologic and experimental 

studies form a plausible and coherent 

data set that supports a relationship 

between NO2 exposures and respiratory 

endpoints, including respiratory 

symptoms and respiratory-related 

hospital admissions and emergency 

department visits, at ambient 

concentrations that are present in areas 

that meet the current NO2 NAAQS (ISA, 

section 5.4). 

• The policy assessment chapter of 

the REA concluded that risks estimated 

to be associated with air quality 

adjusted upward to simulate just 

meeting the current standard can 

reasonably be judged important from a 

public health perspective (REA, section 

10.3.3). 

• The policy assessment chapter of 

the REA concluded that exposure- and 

risk-based results reinforce the scientific 

evidence in supporting the conclusion 

that consideration should be given to 

revising the current NO2 NAAQS so as 

to provide increased public health 

protection, especially for at-risk groups, 

from NO2-related adverse health effects 

associated with short-term, and 

potential long-term, exposures (REA, 

section 10.3.3). 

• CASAC agreed that the current 

annual standard alone is not sufficient 

to protect public health against the 

types of exposures that could lead to 

these health effects. Specifically, in 

their letter to the Administrator on the 

final REA, they stated that ‘‘CASAC 

concurs with EPA’s judgment that the 

current NAAQS does not protect the 

public’s health and that it should be 

revised’’ (Samet, 2008b). 

Based on these considerations 

(discussed in more detail in the 

proposal, section II.E), the 

Administrator concluded in the 

proposal that the current NO2 primary 

NAAQS is not requisite to protect 

public health with an adequate margin 

of safety against adverse respiratory 

effects associated with short-term 

exposures. In considering approaches to 

revising the current standard, the 

Administrator concluded that it is 

appropriate to consider setting a new 

short-term standard in addition to 

retaining the current annual standard. 

The Administrator noted that such a 

short-term standard could provide 

increased public health protection, 

especially for members of at-risk groups, 

from effects described in both 

epidemiologic and controlled human 

exposure studies to be associated with 

short-term exposures to NO2. 

2. Comments on the Adequacy of the 

Current Standard 

This section discusses comments 

received from CASAC and public 

commenters on the proposal that either 

supported or opposed the 

Administrator’s proposed decision to 

revise the current NO2 primary NAAQS. 

Comments on the adequacy of the 

current standard that focused on the 

scientific and/or the exposure/risk basis 

for the Administrator’s proposed 

conclusions are discussed in sections 

II.E.2.a-II.E.2.c. Comments on the 

epidemiologic evidence are considered 

in section II.E.2.a. Comments on the 

controlled human exposure evidence 

are considered in section II.E.2.b. 

Comments on human exposure and 

health risk assessments are considered 

in section II.E.2.c. To the extent these 

comments on the evidence and 

information are also used to justify 

commenters’ conclusions on decisions 

related to indicator, averaging time, 

level, or form, they are noted in the 

appropriate sections below (II.F.1– 

II.F.4). 

In their comments on the proposal 

(Samet, 2009), CASAC reiterated their 

support for the need to revise the 

current annual NO2 NAAQS in order to 

increase public health protection. As 

noted above, in its letter to the 

Administrator on the final REA (Samet, 

2008b) CASAC stated that it ‘‘concurs 

with EPA’s judgment that the current 

NAAQS does not protect the public’s 

health and that it should be revised.’’ In 

supporting adoption of a more stringent 

NAAQS for NO2, CASAC considered the 

assessment of the scientific evidence 

presented in the ISA, the results of 

assessments presented in the REA, and 

the conclusions of the policy assessment 

chapter of the REA. As such, CASAC’s 

rationale for revising the current 

standard was consistent with the 

Administrator’s rationale as discussed 

in the proposal. 

Many public commenters agreed with 

CASAC that, based on the available 

information, the current NO2 standard is 

not requisite to protect public health 

with an adequate margin of safety and 

that revisions to the standard are 

appropriate. Among those calling for 

revisions to the standard were 

environmental groups (e.g., Clean Air 

Council (CAC), Earth Justice (EJ), 

Environmental Defense Fund (EDF), 

Natural Resources Defense Council 

(NRDC), Group Against Smog and 

Pollution (GASP)); medical/public 

health organizations (e.g., American 

Lung Association (ALA), American 

Medical Association (AMA), American 

Thoracic Society (ATS), National 

Association for the Medical Direction of 

Respiratory Care (NAMDRC), National 

Association of Cardiovascular and 

Pulmonary Rehabilitation (NACPR), 

American College of Chest Physicians 

(ACCP)); a large number of State 

agencies and organizations (e.g., 

National Association of Clean Air 

Agencies (NACAA), Northeast States for 

Coordinated Air Use Management 

(NESCAUM), and State or local agencies 

in CA, IA, IL, MI, MO, NC, NM, NY, TX, 

VA, WI); Tribes (e.g., National Tribal Air 

Association (NTAA), Fond du Lac Band 

of Lake Superior Chippewa (Fond du 

Lac)), and a number of individual 

commenters. These commenters 

concluded that the current NO2

standard needs to be revised and that a 

more stringent standard is needed to 

protect the health of sensitive 

population groups. In supporting the 

need to adopt a more stringent NAAQS 

for NO2, these commenters often 

referenced the conclusions of CASAC 

and relied on the evidence and 

information presented in the proposal. 

As such, similar to CASAC, the 

rationale offered by these commenters 

was consistent with that presented in 

the proposal to support the 

Administrator’s proposed decision to 

revise the current NO2 NAAQS. 

Some industry commenters (e.g., 

Alliance of Automobile Manufacturers 

(AAM), American Petroleum Institute 

(API), Interstate Natural Gas Association 

of America (INGAA), Utility Air 

Regulatory Group (UARG)) and one 

State commenter (IN Department of 

Environmental Management) expressed 

support for retaining the current annual 

standard alone. In supporting this view, 

these commenters generally concluded 

that the current standard is requisite to 

protect public health with an adequate 

margin of safety and that the available 

evidence is not sufficient to support 

revision of the standard. For example, 

UARG stated that ‘‘EPA has failed to 

demonstrate that the present NO2

NAAQS is no longer at the level 

requisite to protect public health with 

an adequate margin of safety.’’ In 

addition, INGAA stated that 

‘‘* * * EPA should be compelled to 

retain the current standard and defer a 

decision on a new short-term standard 

until the science is more clearly 

defined.’’ 

In support of their views, these 

commenters provided specific 

comments on the epidemiologic and 

controlled human exposure evidence as 

discussed below. In responding to these 

specific comments, we note that the 

Administrator relied in the proposal on 

the evidence, information and 

judgments contained in the ISA and the 


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REA (including the policy assessment 

chapter) as well as on the advice of 

CASAC. In considering the evidence, 

information, and judgments of the ISA 

and the REA, the Agency notes that 

these documents have been reviewed 

extensively by CASAC and have been 

discussed by CASAC at multiple public 

meetings (see section I.D). In their letter 

to the Administrator regarding the 

second draft ISA (Henderson, 2008), 

CASAC noted the following: 

Panel members concur with the primary 

conclusions reached in the ISA with regard 

to health risks that are associated with NO2

exposure. In particular, the Panel agrees with 

the conclusion that the current scientific 

evidence is ‘‘sufficient to infer a likely causal 

relationship between short-term NO2

exposure and adverse effects on the 

respiratory system.’’ The strongest evidence 

in support of this conclusion comes from 

epidemiology studies that show generally 

positive associations between NO2 and 

respiratory symptoms, hospitalizations or 

emergency department visits, as summarized 

in Figure 5.3.1.’’ 

Similarly, in their letter to the 

Administrator on the final REA (Samet, 

2008b), CASAC noted the following: 

Overall, CASAC found this version of the 

REA satisfactory in its approach to moving 

from the scientific foundation developed in 

the Integrated Science Assessment (ISA) to 

setting out evidence-based options for the 

NAAQS. The REA provides the needed 

bridge from the evidence presented in the 

ISA to a characterization of the exposures 

and the associated risks with different 

profiles of exposure. It draws on toxicological 

and epidemiological evidence and addresses 

risk to an identified susceptible population, 

people with asthmatic conditions. EPA has 

also systematically described uncertainties 

associated with the risk assessments. We 

commend EPA for developing a succinct and 

thoughtfully developed synthesis in chapter 

10. This summary chapter represents a longneeded and transparent model for linking a 

substantial body of scientific evidence to the 

four elements of the NAAQS. 

Therefore, in discussing comments on 

the interpretation of the scientific 

evidence and exposure/risk information, 

we note that CASAC has endorsed the 

approaches and conclusions of the ISA 

and the REA. These approaches and 

conclusions are discussed below in 

more detail, within the context of 

specific public comments. 

a. Comments on EPA’s Interpretation of 

the Epidemiologic Evidence 

Several industry groups (e.g., API, 

National Mining Association (NMA), 

American Chemistry Council (ACC), 

AAM, Annapolis Center for ScienceBased Public Policy (ACSBPP), Engine 

Manufacturers Association (EMA), 

ExxonMobil (Exxon), National 

Association of Manufacturers (NAM)) 

commented that, given the presence of 

numerous co-pollutants in the air, 

epidemiologic studies do not support 

the contention that NO2 itself is causing 

health effects. 

While EPA has recognized that 

multiple factors can contribute to the 

etiology of respiratory disease and that 

more than one air pollutant could 

independently impact respiratory 

health, we continue to judge, as 

discussed in the ISA, that the available 

evidence supports the conclusion that 

there is an independent effect of NO2 on 

respiratory morbidity. In reaching this 

judgment, we recognize that a major 

methodological issue affecting NO2

epidemiologic studies concerns the 

evaluation of the extent to which other 

air pollutants may confound or modify 

NO2-related effect estimates. The use of 

multipollutant regression models is the 

most common approach for controlling 

potential confounding by co-pollutants 

in epidemiologic studies. The issues 

related to confounding and the evidence 

of potential confounding by copollutants has been thoroughly 

reviewed in the ISA (see Figures 3.1–10 

and 3.1–11) and in previous 

assessments (e.g., the criteria document 

for PM) (EPA, 2004). NO2 risk estimates 

for respiratory morbidity endpoints, in 

general, were not sensitive to the 

inclusion of co-pollutants, including 

particulate and gaseous pollutants. As 

observed in Figures 3.1–10 and 3.1–11 

in the ISA, relative risks for hospital 

admissions or emergency department 

visits are generally unchanged, nor is 

their interpretation modified, upon 

inclusion of PM or gaseous copollutants in the models. Similarly, 

associations between short-term NO2

exposure and asthma symptoms are 

generally robust to adjustment for copollutants in multipollutant models, as 

shown in Figures 3.1–5 and 3.1–7 of the 

ISA. These results, in conjunction with 

the results of a randomized intervention 

study evaluating respiratory effects of 

indoor exposure to NO2 (ISA, section 

3.1.4.1), led to the conclusion that the 

effect of NO2 on respiratory health 

outcomes is robust and independent of 

the effects of other ambient copollutants. 

In addition, experimental studies 

conducted in animals and humans 

provide support for the plausibility of 

the associations reported in 

epidemiologic studies. These controlled 

human exposure and animal 

toxicological studies have reported 

effects of NO2 on immune system 

function, lung host defense, airway 

inflammation, and airway 

responsiveness (ISA, section 5.4). These 

experimental study results support an 

independent contribution of NO2 to the 

respiratory health effects reported in 

epidemiologic studies (ISA Section 5.4). 

In considering the entire body of 

evidence, including epidemiologic and 

experimental studies, the ISA (section 

5.4, p. 5–16) concluded the following: 

Although this [presence of co-pollutants] 

complicates the efforts to disentangle specific 

NO2-related health effects, the evidence 

summarized in this assessment indicates that 

NO2 associations generally remain robust in 

multi-pollutant models and supports a direct 

effect of short-term NO2 exposure on 

respiratory morbidity at ambient 

concentrations below the current NAAQS. 

The robustness of epidemiologic findings to 

adjustment for co-pollutants, coupled with 

data from animal and human experimental 

studies, support a determination that the 

relationship between NO2 and respiratory 

morbidity is likely causal, while still 

recognizing the relationship between NO2

and other traffic-related pollutants. 

Comments on specific epidemiologic 

studies are discussed below. 

The National Association of 

Manufacturers (NAM) commented that 

the final REA relied on an 

epidemiologic study (Delfino et al. 

2002) not critically reviewed in the final 

ISA. Contrary to NAM’s contention, the 

study by Delfino et al. (2002) was 

critically reviewed by EPA staff and 

pertinent information was extracted 

from the study. The respiratory health 

effects of NO2 on asthma reported in 

this study are included in Figure 5.3–1, 

Table 5.4–1, and Annex Table AX6.3–2 

of the ISA. While NAM comments on 

the narrative discussion of this study in 

the final ISA, their contention that EPA 

scientists did not critically analyze the 

study while preparing the final ISA is 

incorrect. The inclusion of the study in 

the figures and tables in this ISA, as 

well as inclusion in the 2004 PM AQCD, 

indicate critical analysis of the study 

that was implemented throughout the 

review process. The narrative 

discussion in the ISA focused on 

multicity studies (specifically those by 

Schwartz et al. 1994, Mortimer et al. 

2002 and Schildcrout et al. 2006), 

which provide substantial 

epidemiologic evidence for the 

respiratory health effects of NO2 on 

asthma among children. 

Additional comments from NAM 

contend that EPA’s interpretation of 

three individual epidemiologic studies 

(e.g. Krewski et al. 2000; Schildcrout et 

al. 2006; Mortimer et al. 2002) is 

inconsistent across different NAAQS 

reviews. The NAM comments on all 

three studies are discussed below. 

NAM stated the following regarding 

the study by Krewski et al: 

In the Final ISA, EPA cites the Krewski, et 

al. (2000) study as evidence of a significant 


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association between NO2 exposure and 

mortality. Although EPA acknowledges that 

exposure to NO2 was ‘‘highly correlated’’ with 

other pollutants, including PM2.5 and SO2, 

EPA does not consider the analysis of the 

respective contributions of single pollutants 

in the same study that EPA included in its 

prior Staff Paper for Particulate Matter. In 

that document, EPA stated: ‘‘In singlepollutant models, none of the gaseous copollutants was significantly associated with 

mortality except SO2.’’ If EPA has not altered 

its scientific views concerning this study as 

expressed in the PM Staff Paper, it is entirely 

inappropriate for EPA to suggest that the 

Krewski, et al. (2000) study provides any 

evidence of an association between NO2

exposure and mortality. 

In these comments, NAM fails to 

recognize that the report from Krewski 

et al. (2000) contains a reanalysis of two 

cohort studies, the Harvard Six Cities 

and the American Cancer Society (ACS) 

studies. The characterization in the NOX

ISA of the study by Krewski et al. 

(2000), referenced by NAM in their 

comments, refers to the reanalysis of the 

Harvard Six Cities Study. As stated in 

the NOX ISA (p. 3–74): 

Krewski et al. (2000) conducted a 

sensitivity analysis of the Harvard Six Cities 

study and examined associations between 

gaseous pollutants (i.e., O3, NO2, SO2, CO) 

and mortality. NO2 showed risk estimates 

similar to those for PM2.5 per ‘‘low to high’’ 

range increment with total (1.15 [95% CI: 

1.04, 1.27] per 10-ppb increase), 

cardiopulmonary (1.17 [95% CI: 1.02, 1.34]), 

and lung cancer (1.09 [95% CI: 0.76, 1.57]) 

deaths; however, in this dataset NO2 was 

highly correlated with PM2.5 (r = 0.78), SO4

2¥ (r = 0.78), and SO2 (r = 0.84). 

In contrast, the characterization in the 

PM Staff Paper (EPA, 2005) of the study 

by Krewski et al. (2000), referenced by 

NAM in their comments, refers to the 

results of the ACS study. Therefore, 

NAM appears to have confused the 

conclusions on the results of the 

reanalysis of the Harvard Six Cities 

Study in the NOX ISA with the 

conclusions on the results of the 

reanalysis of the ACS study in the PM 

Staff Paper. 

Further, in considering the reanalysis 

of the ACS study by Krewski et al. 

(2000), the NOX ISA observed that ‘‘NO2

showed no associations with mortality 

outcomes’’ (ISA, p. 3–74). This 

statement is consistent with the 

interpretation of that reanalysis as 

discussed in the PM Staff Paper. Thus, 

there is no inconsistency in the 

interpretation of the results of the study 

by Krewski et al. (2000) in the PM Staff 

Paper (EPA, 2005) and the NOX ISA 

(EPA, 2008a). 

NAM also commented that EPA has 

relied on a study by Schildcrout et al. 

(2006) in the NOX ISA but declined to 

rely on the same study for the previous 

review of the O3 NAAQS. NAM made 

the following comment regarding the 

study by Schildcrout et al: 

Another example of how EPA has reached 

different scientific conclusions in the Final 

ISA than in prior NAAQS documents is 

provided by the Schildcrout, et al. (2006) 

study. In the Final ISA, EPA includes an 

extensive discussion of this study of 

asthmatic children and the relationship 

purportedly found in this study between NO2

and various respiratory symptoms. In 

contrast, as part of the NAAQS review for 

ozone, EPA expressly declined to rely on this 

same study because of specific limitations in 

the study design. Among the limitations EPA 

cites were the fact that the Schildcrout, et al. 

(2006) study included ‘‘children in which the 

severity of their asthma was not clearly 

identified,’’ and the use of a study population 

that was ‘‘not comparable to other large 

multi-city studies.’’ EPA must explain why it 

chose to discount the value of the 

Schildcrout, et al. (2006) study when 

evaluating the effects of ozone, but has relied 

on it extensively in the Final ISA for NO2. 

The study by Schildcrout et al. (2006) 

appeared in the peer-review literature 

too late to be considered in the 2006 O3

AQCD; however, this study was 

included in the O3 Provisional 

Assessment. The purpose of the 

Provisional Assessment was to 

determine if new literature materially 

changed any of the broad scientific 

conclusions regarding the health effects 

of O3 exposure as stated in the 2006 O3

AQCD. EPA concluded that, taken in 

context, the ‘‘new’’ information and 

findings did not materially change any 

of the broad scientific conclusions 

regarding the health effects of O3

exposure made in the O3 AQCD. 

Therefore, NAM’s contention that EPA 

‘‘declined’’ to rely on the Schildcrout 

study for the O3 review because of 

limitations in study design is not 

correct. 

The observations NAM draws from 

the O3 Provisional Assessment regarding 

severity of asthma and the study 

population do not indicate limitations 

that resulted in EPA ‘‘discounting’’ the 

study results. Rather, these observations 

were intended to put the study in 

perspective for purposes of interpreting 

the results within the context of the 

larger body of O3 health effects 

evidence. These observations were 

drawn from comments submitted by Dr. 

Schildcrout regarding the interpretation 

of the results of his study in the 

decision to revise the ozone standards 

(see docket ID EPA–HQ–OAR–2005– 

0172–6991). The results of this study are 

being fully considered in the ongoing 

review of the ozone NAAQS. 

Finally, NAM contends that EPA 

reached differing scientific conclusions 

on the use of self-reported peak 

expiratory flow (PEF) depending on 

regulatory context, particularly in the 

large multi-city trial by Mortimer et al. 

(2002). We disagree with this 

contention. EPA consistently examines 

clinical measurements of lung function, 

which include PEF, forced expiratory 

flow in 1 second (FEV1), forced vital 

capacity (FVC), maximal midexpiratory 

flow (MMEF), maximal expiratory flow 

at 50% (MEF50), maximal expiratory 

flow at 25% (MEF25), and forced 

expiratory flow at 25 to 75% of FVC 

(FEF25–75). Evidence for all of these 

clinical measurements is considered 

before drawing a conclusion related to 

the association of lung function with a 

criteria pollutant. In different reviews, 

there may be more evidence from one of 

these clinical measurements than 

another. In the previous review of the 

O3 NAAQS, EPA identified statistically 

significant associations between 

increased ozone levels and morning 

PEF, which remained significant even 

when concentrations exceeding 0.08 

ppm were excluded from the analysis 

(Mortimer et al. 2002). EPA considered 

this evidence, along with evidence of 

other clinical measurements of changes 

in lung function, in drawing 

conclusions on the relationship between 

ozone and lung function. Using a 

similar approach to weigh the evidence 

pertinent to lung function, including 

studies that produced no statistically 

significant results for PEF, the NOX ISA 

(section 3.1.5.3) states: 

In summary, epidemiologic studies using 

data from supervised lung function 

measurements (spirometry or peak flow 

meters) report small decrements in lung 

function (Hoek and Brunekreef, 1994; Linn et 

al., 1996; Moshammer et al., 2006; Peacock 

et al., 2003; Schindler et al., 2001). No 

significant associations were reported in any 

studies using unsupervised, selfadministered peak flow [PEF] measurements 

with portable devices. 

The evaluation of the evidence in the 

NOX ISA is consistent with the way the 

evidence from multiple clinical 

measures of lung function was used in 

the review of the O3 NAAQS. 

b. Comments on EPA’s Interpretation of 

the Controlled Human Exposure 

Evidence 

A number of industry groups (e.g., 

AAM, ACC, API, Dow Chemical 

Company (Dow), EMA, NAM, UARG) 

disagreed with EPA’s reliance on a 

meta-analysis of controlled human 

exposure studies of airway 

responsiveness in asthmatics. Based on 

this meta-analysis (ISA, Table 3.1–3 for 

results), the ISA concluded that ‘‘small 

but significant increases in nonspecific 

airway hyperresponsiveness were 


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9EPA considers the Goodman study to be a ‘‘new 

study’’ on which, as discussed above in section 1.B, 

it would not be appropriate to base a standard in 

the absence of thorough CASAC and public review 

of the study and its methodology. However, as 

discussed below, EPA has considered the study in 

the context of responding to public comments on 

the proposal and has concluded it does not provide 

a basis to materially change any of the broad 

scientific conclusions regarding the health effects of 

NO2 made in the air quality criteria. 

observed * * * at 0.1 ppm NO2 for 60- 

min exposures in asthmatics’’ (ISA, p. 5– 

11). Industry groups raised a number of 

objections to this analysis and the way 

in which it has been used in the current 

review. 

Several of these industry groups 

concluded that, in relying on this 

analysis, EPA has inappropriately relied 

on a new unpublished meta-analysis 

that has not been peer-reviewed, was 

not reviewed by CASAC, and was not 

conducted in a transparent manner. For 

example, as part of a Request for 

Correction submitted under EPA’s 

Information Quality Guidelines, NAM 

stated that ‘‘EPA’s substantial reliance 

on an unpublished assessment 

described as a ‘‘meta-analysis’’ of the 

relation between NO2 exposure and 

changes in airway responsiveness 

violates EPA Guidelines requiring 

‘‘transparency about data and methods.’’ 

EPA disagrees with this 

characterization of the updated metaanalysis included in the final ISA. As 

described in the ISA (p. 3–16), this 

meta-analysis is based on an earlier 

analysis by Folinsbee (1992) that has 

been subject to peer-review, that was 

published in a scientific journal 

(Toxicol Ind Health. 8:1–11, 1992), and 

that was reviewed by CASAC as part of 

the previous review of the NO2 NAAQS 

(EPA, 1993, Table 15–10). The updates 

to this earlier analysis did not include 

substantive changes to the approach. As 

discussed in the final ISA (p. 3–16), the 

changes made to the analysis were to 

remove the results of one allergen study 

and add results from a non-specific 

responsiveness study, which focused 

the meta-analysis on non-specific 

airway responsiveness, and to discuss 

results for an additional exposure 

concentration (i.e., 100 ppb). The 

information needed to reproduce this 

meta-analysis is provided in the ISA 

(Tables 3.1–2 and 3.1–3, including 

footnotes). 

While the ISA meta-analysis reports 

findings on airway responsiveness in 

asthmatics following exposure to 100 

ppb NO2, a concentration not 

specifically discussed in the findings of 

the original report by Folinsbee (1992), 

this does not constitute a substantive 

change to that original analysis. For 

exposures at rest, four of the studies 

included in the analysis by Folinsbee 

evaluated the effects of exposure to 100 

ppb NO2. In that original meta-analysis, 

these studies were grouped with another 

study that evaluated exposures to 140 

ppb NO2. When analyzed together, 

exposures to NO2 concentrations of 100 

ppb and 140 ppb (grouped together in 

the manuscript and described as less 

than 0.2 ppm) increased airway 

responsiveness in 65% of resting 

asthmatics (p < 0.01). Therefore, 

reporting results at 100 ppb NO2 in the 

ISA meta-analysis reflects a change in 

the way the data are presented and does 

not reflect a substantive change to the 

study. This change in presentation 

allows specific consideration of the 

potential for exposures to 100 ppb NO2

to increase airway responsiveness, 

rather than grouping results at 100 ppb 

with results at other exposure 

concentrations. 

In addition, the updated metaanalysis was considered by CASAC 

during their review of the REA (REA, 

Table 4–5 reports the results of the 

updated meta-analysis), which based 

part of the assessment of NO2-associated 

health risks on the results of the metaanalysis. In their letter to the 

Administrator on the final REA (Samet, 

2008b), CASAC stated that ‘‘[t]he 

evidence reviewed in the REA indicates 

that adverse health effects have been 

documented in clinical studies of 

persons with asthma at 100 ppb’’ and 

that ‘‘CASAC firmly recommends that 

the upper end of the range [of standard 

levels] not exceed 100 ppb, given the 

findings of the REA.’’ In addition, in 

their comments on the proposal, CASAC 

reiterated this advice in their statement 

that ‘‘the level of the one-hour NO2

standard should be within the range of 

80–100 ppb and not above 100 ppb.’’ 

These statements indicate that CASAC 

did specifically consider the results of 

the updated meta-analysis and that they 

used those results to inform their 

recommendations on the range of 

standard levels supported by the 

scientific evidence. 

In summary, we note the following: 

• The original meta-analysis was 

published in a peer-reviewed journal 

and was reviewed by CASAC in the 

previous review of the NO2 NAAQS. 

• The updated meta-analysis does not 

include substantive changes to the 

methodology of this original analysis. 

• The changes that were made are 

clearly described in the ISA. 

• CASAC specifically reviewed and 

considered the ISA meta-analysis in 

making recommendations regarding the 

range of standard levels supported by 

the science. 

Many of these same industry groups 

also referred in their comments to a 

recent meta-analysis of controlled 

human exposure studies evaluating the 

airway response in asthmatics following 

NO2 exposure (Goodman et al., 2009). 

These groups generally recommended 

that EPA rely on this meta-analysis and 

on the authors’ conclusions with regard 

to NO2 and airway responsiveness. 

Specific comments based on the 

manuscript by Goodman et al., as well 

as EPA’s responses, are discussed below 

in more detail.9

Industry commenters generally 

claimed that the meta-analysis by 

Goodman et al. supports the conclusion 

that no adverse effects occur following 

exposures up to 600 ppb NO2. However, 

Table 4 of the Goodman study reports 

that 64% (95% Confidence Interval: 

58%, 71%) of resting asthmatics 

exposed to NO2 experienced an increase 

in airway responsiveness. Furthermore, 

Figure 2a of this manuscript reports that 

for exposures < 0.2 ppm, the fraction 

affected is 0.61 (95% CI: 0.52, 0.70) 

while for exposures of 0.2 ppm to < 0.3 

ppm, the fraction affected is 0.66 (95% 

CI: 0.59, 0.74). These findings are 

consistent with those reported in the 

meta-analysis by Folinsbee and in the 

updated meta-analysis that was 

included in the final ISA. 

Also based on the meta-analysis by 

Goodman et al. (2009), several industry 

commenters concluded that NO2- 

induced airway hyperresponsiveness is 

not adverse and, therefore, should not 

be considered in setting standards. The 

basis for this comment appears to be the 

conclusions reached by Goodman et al. 

that there is no dose-response 

relationship for NO2 and that the 

magnitude of any NO2 effect on airway 

responsiveness is too small to be 

considered adverse. 

Due to differences in study protocols 

in the NO2-airway response literature 

(ISA, section 3.1.3), EPA disagrees with 

the approach taken in the Goodman 

study to use existing data to attempt to 

evaluate the presence of a dose-response 

relationship and to determine the 

magnitude of the NO2 response. 

Examples of differences in the study 

protocols include the NO2 exposure 

method (i.e., mouthpiece versus 

chamber), subject activity level (i.e., rest 

versus exercise) during NO2 exposure, 

choice of airway challenge agent, and 

physiological endpoint used to quantify 

airway responses. Goodman et al. (2009) 

also recognized heterogeneity among 

studies as a limitation in their analyses. 

As a result of these differences, EPA 

judged it appropriate in the ISA metaanalysis to assess only the fraction of 

asthmatics experiencing increased or 

decreased airway responsiveness 


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10Once EPA determines whether to retain or 

revise the current standard, the actual air quality 

levels in various areas of the country are clearly 

relevant under the NAAQS implementation 

provisions for the Act, such as the provision for 

designation of areas based on whether or not they 

attain the required NAAQS. 

following NO2 exposure. We have 

acknowledged in the REA, the proposal, 

and in this final rulemaking that there 

is uncertainty with regard to the 

magnitude and the clinical-significance 

of NO2-induced increases in airway 

responsiveness (see sections II.C.3 and 

II.F.4.a in the proposed rulemaking as 

well as II.F.3 in this final rulemaking). 

The REA stated the following (p. 302): 

[O]ne of the important uncertainties 

associated with these [NO2-induced airway 

hyperresponsiveness] results is that, because 

the meta-analysis evaluated only the 

direction of the change in airway 

responsiveness, it is not possible to discern 

the magnitude of the change from these data. 

This limitation makes it particularly difficult 

to quantify the public health implications of 

these results. 

While we acknowledge this 

uncertainty, EPA disagrees with the 

conclusion that the NO2-induced 

increase in airway responsiveness in 

asthmatics exposed to NO2

concentrations up to 600 ppb is not 

adverse and should not be considered in 

setting standards. Specifically, we note 

that the ISA concluded that ‘‘[t]ransient 

increases in airway responsiveness 

following NO2 exposure have the 

potential to increase symptoms and 

worsen asthma control’’ (ISA, section 

5.4). The uncertainty over the adversity 

of the response reported in controlled 

human exposure studies does not mean 

that the NO2-induced increase in airway 

responsiveness is not adverse. Rather, it 

means that there is a risk of adversity, 

especially for asthmatics with more than 

mild asthma, but that this risk cannot be 

fully characterized based on existing 

studies. The studies of NO2 and airway 

responsiveness included in the metaanalysis have generally evaluated mild 

asthmatics, rather than more severely 

affected asthmatics who could be more 

susceptible to the NO2-induced increase 

in airway responsiveness (ISA, section 

3.1.3.2). Given that this is the case, and 

given the large percentage of asthmatics 

that experienced an NO2-induced 

increase in airway responsiveness in the 

studies and the large size of the 

asthmatic population in the United 

States, the REA concluded that it is 

appropriate to consider NO2-induced 

airway hyperresponsiveness in 

characterizing NO2-associated health 

risks (REA, section 10.3.2). As noted 

above, CASAC endorsed this conclusion 

in their letters to the Administrator on 

the final REA and on the proposal 

(Samet, 2008b; Samet, 2009). 

c. Comments on EPA’s Characterization 

of NO2-Associated Exposures and 

Health Risks 

Several commenters discussed the 

analyses of NO2-associated exposures 

and health risks presented in the REA. 

As in past reviews (EPA 2005, 2007c, 

2007d), EPA has estimated allowable 

risks associated with the current 

standard and potential alternative 

standards to inform judgments on the 

public health risks that could exist 

under different standard options. Some 

industry commenters (e.g., API, NMA) 

concluded that the Administrator 

should consider modeled exposures and 

risks associated with actual NO2 air 

quality rather than with NO2

concentrations adjusted to simulate just 

meeting the current annual standard or 

potential alternative 1-hour standards. 

These commenters pointed out that 

such simulations require large 

adjustments to air quality and are highly 

uncertain and that NAAQS are intended 

to address actual, rather than highly 

improbable, risks to health. 

We disagree with these commenters 

that exposure- and risk-related 

considerations in the NAAQS review 

should rely only on unadjusted air 

quality. In considering whether the 

current standard is requisite to protect 

public health with an adequate margin 

of safety, air quality adjustments allow 

estimates of NO2-related exposures and 

health risks that could exist in areas that 

just meet that standard. That is, these 

adjustments allow consideration of 

exposures and risks that would be 

permissible under the current standard. 

Therefore, such adjustments are clearly 

useful to inform a decision on the issue 

before EPA (i.e., the adequacy of the 

level of public health protection 

associated with allowable NO2 air 

quality under the standard). Similarly, 

air quality adjustments to simulate 

different potential alternative standards 

provide information on exposures and 

risks that would be permissible under 

these alternatives.10 As noted above, in 

their letter to the Administrator on the 

final REA (Samet, 2008b), CASAC 

concluded that ‘‘The REA provides the 

needed bridge from the evidence 

presented in the ISA to a 

characterization of the exposures and 

the associated risks with different 

profiles of exposure.’’ 

We agree that there are uncertainties 

inherent in air quality adjustments. 

These uncertainties are discussed 

thoroughly in the REA (sections 7.4, 

8.12, 9.6, and 10.3.2.1) and in the 

proposed rule (section II.C.3). For 

example, the policy assessment chapter 

of the REA (section 10.3.2.1) noted the 

following regarding adjustment of NO2

concentrations: 

In order to simulate just meeting the 

current annual standard and many of the 

alternative 1-h standards analyzed, an 

upward adjustment of recent ambient NO2

concentrations was required. We note that 

this adjustment does not reflect a judgment 

that levels of NO2 are likely to increase under 

the current standard or any of the potential 

alternative standards under consideration. 

Rather, these adjustments reflect the fact that 

the current standard, as well as some of the 

alternatives under consideration, could allow 

for such increases in ambient NO2

concentrations. In adjusting air quality to 

simulate just meeting these standards, we 

have assumed that the overall shape of the 

distribution of NO2 concentrations would not 

change. While we believe this is a reasonable 

assumption in the absence of evidence 

supporting a different distribution and we 

note that available analyses support this 

approach (Rizzo, 2008), we recognize this as 

an important uncertainty. It may be an 

especially important uncertainty for those 

scenarios where considerable upward 

adjustment is required to simulate just 

meeting one or more of the standards. 

These air quality adjustments are not 

meant to imply an expectation that NO2

concentrations will increase broadly 

across the United States or in any given 

area (REA, section 10.3.2.1). Rather, as 

noted above, they are meant to estimate 

NO2-related exposures and health risks 

that would be permitted under the 

current and potential alternative 

standards. Such estimates can inform 

decisions on whether the current 

standard, or particular potential 

alternative standards, provide the 

requisite protection of public health. 

3. Conclusions Regarding the Adequacy 

of the Current Standard 

In considering the adequacy of the 

current standard, the Administrator has 

considered the scientific evidence 

assessed in the ISA, the exposure and 

risk results presented in the REA, the 

conclusions of the policy assessment 

chapter of the REA, and comments from 

CASAC and the public. These 

considerations are described below. 

In considering the scientific evidence 

as it relates to the adequacy of the 

current standard, the Administrator 

notes that the epidemiologic evidence 

has grown substantially since the last 

review with the addition of field and 

panel studies, intervention studies, and 

time-series studies of effects such as 

emergency department visits and 

hospital admissions associated with 


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short-term NO2 exposures. No 

epidemiologic studies were available in 

1993 assessing relationships between 

NO2 and outcomes such as hospital 

admissions or emergency department 

visits. In contrast, dozens of 

epidemiologic studies on such 

outcomes, conducted at recent and 

current ambient NO2 concentrations, are 

now included in this evaluation (ISA, 

chapter 3). 

As an initial consideration with 

regard to the adequacy of the current 

standard, the Administrator notes that 

the evidence relating long-term (weeks 

to years) NO2 exposures at current 

ambient concentrations to adverse 

health effects was judged in the ISA to 

be either ‘‘suggestive but not sufficient 

to infer a causal relationship’’ 

(respiratory morbidity) or ‘‘inadequate to 

infer the presence or absence of a causal 

relationship’’ (mortality, cancer, 

cardiovascular effects, reproductive/ 

developmental effects) (ISA, sections 

5.3.2.4–5.3.2.6). In contrast, the 

evidence relating short-term (minutes to 

hours) NO2 exposures to respiratory 

morbidity was judged to be ‘‘sufficient to 

infer a likely causal relationship’’ (ISA, 

section 5.3.2.1). This conclusion was 

supported primarily by a large body of 

recent epidemiologic studies that 

evaluated associations of short-term 

NO2 concentrations with respiratory 

symptoms, emergency department 

visits, and hospital admissions. Given 

these conclusions from the ISA, the 

Administrator judges that, at a 

minimum, consideration of the 

adequacy of the current annual standard 

should take into account the extent to 

which that standard provides protection 

against respiratory effects associated 

with short-term NO2 exposures. 

In considering the NO2 epidemiologic 

studies as they relate to the adequacy of 

the current standard, the Administrator 

notes that annual average NO2

concentrations were below the level of 

the current annual NO2 NAAQS in 

many of the locations where positive, 

and often statistically significant, 

associations with respiratory morbidity 

endpoints have been reported (ISA, 

section 5.4). As discussed previously, 

the ISA characterized that evidence for 

respiratory effects as consistent and 

coherent. The evidence is consistent in 

that associations are reported in studies 

conducted in numerous locations and 

with a variety of methodological 

approaches (ISA, section 5.3.2.1). It is 

coherent in the sense that the studies 

report associations with respiratory 

health outcomes that are logically 

linked together (ISA, section 5.3.2.1). 

The ISA noted that when the 

epidemiologic literature is considered as 

a whole, there are generally positive 

associations between NO2 and 

respiratory symptoms, hospital 

admissions, and emergency department 

visits. A number of these associations 

are statistically significant, particularly 

the more precise effect estimates (ISA, 

section 5.3.2.1). 

As discussed in the proposal (II.E.1) 

and above, the Administrator 

acknowledges that the interpretation of 

these NO2 epidemiologic studies is 

complicated by the fact that on-road 

vehicle exhaust emissions are a nearly 

ubiquitous source of combustion 

pollutant mixtures that include NO2. 

She notes that, in order to provide some 

perspective on the uncertainty related to 

the presence of co-pollutants the ISA 

evaluated epidemiologic studies that 

employed multi-pollutant models, 

epidemiologic studies of indoor NO2

exposure, and experimental studies. 

Specifically, the ISA noted that a 

number of NO2 epidemiologic studies 

have attempted to disentangle the 

effects of NO2 from those of cooccurring pollutants by employing 

multi-pollutant models. When evaluated 

as a whole, NO2 effect estimates in these 

models generally remained robust when 

co-pollutants were included. Therefore, 

despite uncertainties associated with 

separating the effects of NO2 from those 

of co-occurring pollutants, the ISA 

(section 5.4, p. 5–16) concluded that 

‘‘the evidence summarized in this 

assessment indicates that NO2

associations generally remain robust in 

multi-pollutant models and supports a 

direct effect of short-term NO2 exposure 

on respiratory morbidity at ambient 

concentrations below the current 

NAAQS.’’ With regard to indoor studies, 

the ISA noted that these studies can test 

hypotheses related to NO2 specifically 

(ISA, section 3.1.4.1). Although 

confounding by indoor combustion 

sources is a concern, indoor studies are 

not confounded by the same mix of copollutants present in the ambient air or 

by the contribution of NO2 to the 

formation of secondary particles or O3

(ISA, section 3.1.4.1). The ISA noted 

that the findings of indoor NO2 studies 

are consistent with those of studies 

using ambient concentrations from 

central site monitors and concluded that 

indoor studies provide evidence of 

coherence for respiratory effects (ISA, 

section 3.1.4.1). With regard to 

experimental studies, the REA noted 

that they have the advantage of 

providing information on health effects 

that are specifically associated with 

exposure to NO2 in the absence of copollutants. The ISA concluded that the 

NO2 epidemiologic literature is 

supported by (1) evidence from 

controlled human exposure studies of 

airway hyperresponsiveness in 

asthmatics, (2) controlled human 

exposure and animal toxicological 

studies of impaired host-defense 

systems and increased risk of 

susceptibility to viral and bacterial 

infection, and (3) controlled human 

exposure and animal toxicological 

studies of airway inflammation (ISA, 

section 5.3.2.1 and 5.4). Given the above 

consideration of the evidence, 

particularly the epidemiologic studies 

reporting NO2-associated health effects 

in locations that meet the current 

standard, the Administrator agrees with 

the conclusion in the policy assessment 

chapter of the REA that the scientific 

evidence calls into question the 

adequacy of the current standard to 

protect public health. 

In addition to the evidence-based 

considerations described above, the 

Administrator has considered the extent 

to which exposure- and risk-based 

information can inform decisions 

regarding the adequacy of the current 

annual NO2 standard. While she 

acknowledges the uncertainties 

associated with adjusting air quality in 

these analyses, she judges that such 

analyses are appropriate for 

consideration in this review of the NO2

primary NAAQS. In reaching this 

conclusion she notes the considerations 

discussed above, particularly the 

endorsement by CASAC of the REA and 

its characterization of NO2-associated 

exposures and health risks. 

In considering the exposure- and riskbased information with regard to the 

adequacy of the current annual NO2

standard to protect the public health, 

the Administrator notes the conclusion 

in the policy assessment chapter of the 

REA that risks estimated to be 

associated with air quality adjusted 

upward to simulate just meeting the 

current standard can reasonably be 

concluded to be important from a public 

health perspective. In particular, a large 

percentage (8–9%) of respiratory-related 

ED visits in Atlanta could be associated 

with short-term NO2 exposures, most 

asthmatics in Atlanta could be exposed 

on multiple days per year to NO2

concentrations at or above 300 ppb, and 

most locations evaluated could 

experience on-/near-road NO2

concentrations above 100 ppb on more 

than half of the days in a given year. 

Therefore, after considering the results 

of the exposure and risk analyses 

presented in the REA the Administrator 

agrees with the conclusion of the policy 

assessment chapter of the REA that 

exposure- and risk-based results 

reinforce the scientific evidence in 


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supporting the conclusion that 

consideration should be given to 

revising the current standard so as to 

provide increased public health 

protection, especially for at-risk groups, 

from NO2-related adverse health effects 

associated with short-term, and 

potential long-term, exposures. 

In reaching a conclusion on the 

adequacy of the current standard, the 

Administrator has also considered 

advice received from CASAC. In their 

comments on the final REA, CASAC 

agreed that the primary concern in this 

review is to protect against health 

effects that have been associated with 

short-term NO2 exposures. CASAC also 

agreed that the current annual standard 

is not sufficient to protect public health 

against the types of exposures that could 

lead to these health effects. As noted in 

their letter to the EPA Administrator, 

‘‘CASAC concurs with EPA’s judgment 

that the current NAAQS does not 

protect the public’s health and that it 

should be revised’’ (Samet, 2008b). 

Based on the considerations discussed 

above, the Administrator concludes that 

the current NO2 primary NAAQS alone 

is not requisite to protect public health 

with an adequate margin of safety. 

Accordingly, she concludes that the 

NO2 primary standard should be revised 

in order to provide increased public 

health protection against respiratory 

effects associated with short-term 

exposures, particularly for susceptible 

populations such as asthmatics, 

children, and older adults. In 

considering approaches to revising the 

current standard, the Administrator 

concludes that it is appropriate to 

consider setting a new short-term 

standard (see below). The Administrator 

notes that such a short-term standard 

could provide increased public health 

protection, especially for members of atrisk groups, from effects described in 

both epidemiologic and controlled 

human exposure studies to be 

associated with short-term exposures to 

NO2. 

F. Elements of a New Short-Term 

Standard 

In considering a revised NO2 primary 

NAAQS, the Administrator notes the 

need to protect at-risk individuals from 

short-term exposures to NO2 air quality 

that could cause the types of respiratory 

morbidity effects reported in 

epidemiologic studies and the need to 

protect at-risk individuals from shortterm exposure to NO2 concentrations 

reported in controlled human exposure 

studies to increase airway 

responsiveness in asthmatics. The 

Administrator’s considerations with 

regard to her decisions are discussed in 

the following sections in terms of 

indicator (II.F.1), averaging time (II.F.2), 

level (II.F.3), and form (II.F.4). 

1. Indicator 

a. Rationale for Proposed Decision 

In past reviews, EPA has focused on 

NO2 as the most appropriate indicator 

for ambient NOX. In making a decision 

in the current review on the most 

appropriate indicator, the Administrator 

considered the conclusions of the ISA 

and the policy assessment chapter of the 

REA as well as the view expressed by 

CASAC. The policy assessment chapter 

of the REA noted that, while the 

presence of NOX species other than NO2

has been recognized, no alternative to 

NO2 has been advanced as being a more 

appropriate surrogate. Controlled 

human exposure studies and animal 

toxicology studies assessed in the ISA 

provide specific evidence for health 

effects following exposure to NO2. 

Epidemiologic studies also typically 

report levels of NO2 though the degree 

to which monitored NO2 reflects actual 

NO2 levels, as opposed to NO2 plus 

other gaseous NOX, can vary (REA, 

section 2.2.3). In addition, because 

emissions that lead to the formation of 

NO2 generally also lead to the formation 

of other NOX oxidation products, 

measures leading to reductions in 

population exposures to NO2 can 

generally be expected to lead to 

reductions in population exposures to 

other gaseous NOX. Therefore, an NO2

standard can also be expected to 

provide some degree of protection 

against potential health effects that may 

be independently associated with other 

gaseous NOX even though such effects 

are not discernable from currently 

available studies indexed by NO2 alone. 

Given these key points, the policy 

assessment chapter of the REA 

concluded that the evidence supports 

retaining NO2 as the indicator. 

Consistent with this conclusion, the 

CASAC Panel stated in its letter to the 

EPA Administrator that it ‘‘concurs with 

retention of NO2 as the indicator’’ 

(Samet, 2008b). In light of the above 

considerations, the Administrator 

proposed to retain NO2 as the indicator 

in the current review. 

b. Comments on Indicator 

A relatively small number of 

comments directly addressed the issue 

of the indicator for the standard 

(CASAC, Dow, API, AAM, and the 

Missouri Department of Natural 

Resources Air Pollution Control 

Program (MODNR)). All of these 

commenters endorsed the proposal to 

continue to use NO2 as the indicator for 

ambient NOX. 

c. Conclusions on Indicator 

Based on the available information 

discussed above, and consistent with 

the views of CASAC and other 

commenters, the Administrator 

concludes that it is appropriate to 

continue to use NO2 as the indicator for 

a standard that is intended to address 

effects associated with exposure to NO2, 

alone or in combination with other 

gaseous NOX. In so doing, the 

Administrator recognizes that measures 

leading to reductions in population 

exposures to NO2 will also reduce 

exposures to other nitrogen oxides. 

2. Averaging Time 

This section discusses considerations 

related to the averaging time of the NO2

primary NAAQS. Specifically, this 

section summarizes the rationale for the 

Administrator’s proposed decision 

regarding averaging time (II.F.2.a; see 

section II.F.2 of the proposal for more 

detail), discusses comments related to 

averaging time (II.F.2.b), and presents 

the Administrator’s final conclusions 

regarding averaging time (II.F.2.c). 

a. Rationale for Proposed Decision 

In considering the most appropriate 

averaging time for the NO2 primary 

NAAQS, the Administrator noted in the 

proposal the conclusions and judgments 

made in the ISA about available 

scientific evidence, air quality 

correlations discussed in the REA, 

conclusions of the policy assessment 

chapter of the REA, and CASAC 

recommendations (section II.F.2 in the 

proposal). Specifically, she noted the 

following: 

• Experimental studies in humans 

and animals have reported respiratory 

effects following NO2 exposures lasting 

from less than 1-hour up to several 

hours. Epidemiologic studies have 

reported associations between 

respiratory effects and both 1 hour and 

24-hour NO2 concentrations. Therefore, 

the experimental evidence provides 

support for an averaging time of shorter 

duration than 24 hours (e.g., 1 hour) 

while the epidemiologic evidence 

provides support for both 1-hour and 

24-hour averaging times. At a minimum, 

this suggests that a primary concern 

with regard to averaging time is the 

level of protection provided against 

1-hour NO2 concentrations. 

• Air quality correlations presented 

in the policy assessment chapter of the 

REA illustrated the relatively high 

degree of variability in the ratios of 

annual average to short-term NO2

concentrations (REA, Table 10–2). This 


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11As discussed below, 98th and 99th percentile 

forms were evaluated in the REA. A 99th percentile 

form corresponds approximately to the 4th highest 

1-hour concentration in a year while a 98th 

percentile form corresponds approximately to the 

7th or 8th highest 1-hour concentration in a year. 

A 4th highest concentration form has been used 

previously in the O3 NAAQS while a 98th 

percentile form has been used previously in the 

PM2.5 NAAQS. 

variability suggests that a standard 

based on annual average NO2

concentrations would not likely be an 

effective or efficient approach to focus 

protection on short-term exposures. 

• These air quality correlations (REA, 

Table 10–1) suggested that a standard 

based on 1-hour daily maximum NO2

concentrations could also be effective at 

protecting against 24-hour NO2

concentrations. 

• The policy assessment chapter of 

the REA concluded that the scientific 

evidence, combined with the air quality 

correlations, support the 

appropriateness of a standard based on 

1-hour daily maximum NO2

concentrations to protect against health 

effects associated with short-term 

exposures. 

• CASAC concurred ‘‘with having a 

short-term NAAQS primary standard for 

oxides of nitrogen and using the onehour maximum NO2 value’’ (Samet, 

2008b). 

Based on these considerations, the 

Administrator proposed to set a new 

standard based on 1-hour daily 

maximum NO2 concentrations. 

b. Comments on averaging time 

As discussed above, CASAC endorsed 

the establishment of a new standard 

with a 1-hour averaging time. CASAC 

stated the following in their comments 

on the proposal (Samet, 2009): 

In reviewing the REA, CASAC supported a 

short-term standard for NO2 and in reviewing 

the proposal, CASAC supports the proposed 

one-hour averaging time in EPA’s proposed 

rule. 

The supporting rationale offered by 

CASAC in support of a new 1-hour 

standard was generally the same as that 

put forward in the final REA and the 

proposal. Specifically, that rationale 

considered the available scientific 

evidence, which supports a link 

between 1-hour NO2 concentrations and 

adverse respiratory effects, and air 

quality information presented in the 

REA, which suggests that a 1-hour 

standard can protect against effects 

linked to short-term NO2 exposures 

while an annual standard would not be 

an effective or efficient approach to 

protecting against these effects. 

A large number of public commenters 

also endorsed the establishment of a 

new standard with a 1-hour averaging 

time. These included a number of State 

agencies and organizations (e.g., 

NACAA, NESCAUM and agencies in 

CA, IL, NM, TX, VA); environmental, 

medical, and public health 

organizations (e.g., ACCP, ALA, AMA, 

ATS, CAC, EDF, EJ, GASP, NACPR, 

NAMDRC, NRDC); and most individual 

commenters. The supporting rationales 

offered by these commenters often 

acknowledged the recommendations of 

CASAC and the Administrator’s 

rationale as discussed in the proposal. 

Though many industry commenters 

recommended not revising the current 

annual standard (as discussed above in 

section II.E.2), several of these groups 

did conclude that if a short-term 

standard were to be set, a 1-hour 

averaging time would be appropriate 

(e.g., Colorado Petroleum Association 

(CPA), Dow, NAM, Petroleum 

Association of Wyoming (PAW), Utah 

Petroleum Association (UPA)). As 

discussed above, industry commenters 

who disagreed with setting a new 1- 

hour standard generally based this 

conclusion on their interpretation of the 

scientific evidence and their conclusion 

that this evidence does not support the 

need to revise the current annual 

standard. These comments, and EPA’s 

responses, are discussed in more detail 

above (section II.E) and in the Response 

to Comments document. 

c. Conclusions on Averaging Time 

In considering the most appropriate 

averaging time for the NO2 primary 

NAAQS, the Administrator notes the 

available scientific evidence as assessed 

in the ISA, the air quality analyses 

presented in the REA, the conclusions 

of the policy assessment chapter of the 

REA, CASAC recommendations, and 

public comments received. These 

considerations are described below. 

When considering averaging time, the 

Administrator notes that the evidence 

relating short-term (minutes to hours) 

NO2 exposures to respiratory morbidity 

was judged in the ISA to be ‘‘sufficient 

to infer a likely causal relationship’’ 

(ISA, section 5.3.2.1) while the evidence 

relating long-term (weeks to years) NO2

exposures to adverse health effects was 

judged to be either ‘‘suggestive but not 

sufficient to infer a causal relationship’’ 

(respiratory morbidity) or ‘‘inadequate to 

infer the presence or absence of a causal 

relationship’’ (mortality, cancer, 

cardiovascular effects, reproductive/ 

developmental effects) (ISA, sections 

5.3.2.4–5.3.2.6). Thus, the Administrator 

concludes that these judgments most 

directly support an averaging time that 

focuses protection on short-term 

exposures to NO2. 

As in past reviews of the NO2

NAAQS, the Administrator notes that it 

is instructive to evaluate the potential 

for a standard based on annual average 

NO2 concentrations, as is the current 

standard, to provide protection against 

short-term NO2 exposures. To this end, 

the Administrator notes that Table 10– 

1 in the REA reported the ratios of shortterm to annual average NO2

concentrations. Ratios of 1-hour daily 

maximum concentrations (98th and 

99th percentile 11) to annual average 

concentrations across 14 locations 

ranged from 2.5 to 8.7 while ratios of 24- 

hour average concentrations to annual 

average concentrations ranged from 1.6 

to 3.8 (see Thompson, 2008 for more 

details). The policy assessment chapter 

of the REA concluded that the 

variability in these ratios across 

locations, particularly those for 1-hour 

concentrations, suggested that a 

standard based on annual average NO2

concentrations would not likely be an 

effective or efficient approach to focus 

protection on short-term NO2 exposures. 

For example, in an area with a relatively 

high ratio (e.g., 8), the current annual 

standard (53 ppb) would be expected to 

allow 1-hour daily maximum NO2

concentrations of about 400 ppb. In 

contrast, in an area with a relatively low 

ratio (e.g., 3), the current standard 

would be expected to allow 1-hour daily 

maximum NO2 concentrations of about 

150 ppb. Thus, for purposes of 

protecting against the range of 1-hour 

NO2 exposures, the REA noted that a 

standard based on annual average 

concentrations would likely require 

more control than necessary in some 

areas and less control than necessary in 

others, depending on the standard level 

selected. 

In considering the level of support 

available for specific short-term 

averaging times, the Administrator notes 

that the policy assessment chapter of the 

REA considered evidence from both 

experimental and epidemiologic 

studies. Controlled human exposure 

studies and animal toxicological studies 

provide evidence that NO2 exposures 

from less than 1-hour up to 3-hours can 

result in respiratory effects such as 

increased airway responsiveness and 

inflammation (ISA, section 5.3.2.7). 

Specifically, the ISA concluded that 

NO2 exposures of 100 ppb for 1-hour (or 

200 ppb to 300 ppb for 30-min) can 

result in small but significant increases 

in nonspecific airway responsiveness 

(ISA, section 5.3.2.1). In contrast, the 

epidemiologic literature provides 

support for short-term averaging times 

ranging from approximately 1-hour up 

to 24-hours (ISA, section 5.3.2.7). A 


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number of epidemiologic studies have 

detected positive associations between 

respiratory morbidity and 1-hour (daily 

maximum) and/or 24-hour NO2

concentrations. A few epidemiologic 

studies have considered both 1-hour 

and 24-hour averaging times, allowing 

comparisons to be made. The ISA 

reported that such comparisons in 

studies that evaluate asthma emergency 

department visits failed to reveal 

differences between effect estimates 

based on a 1-hour averaging time and 

those based on a 24-hour averaging time 

(ISA, section 5.3.2.7). Therefore, the ISA 

concluded that it is not possible, from 

the available epidemiologic evidence, to 

discern whether effects observed are 

attributable to average daily (or multiday) concentrations (24-hour average) or 

high, peak exposures (1-hour maximum) 

(ISA, section 5.3.2.7). 

As noted in the policy assessment 

chapter of the REA, given the above 

conclusions, the experimental evidence 

provides support for an averaging time 

of shorter duration than 24 hours (e.g., 

1–h) while the epidemiologic evidence 

provides support for both 1-hour and 

24-hour averaging times. The 

Administrator concludes that, at a 

minimum, this suggests that a primary 

concern with regard to averaging time is 

the level of protection provided against 

1-hour NO2 concentrations. However, 

she also notes that it is important to 

consider the ability of a 1-hour 

averaging time to protect against 24- 

hour average NO2 concentrations. To 

this end, the Administrator notes that 

Table 10–2 in the REA presented 

correlations between 1-hour daily 

maximum NO2 concentrations and 24- 

hour average NO2 concentrations (98th 

and 99th percentile) across 14 locations 

(see Thompson, 2008 for more detail). 

Typical ratios ranged from 1.5 to 2.0, 

though one ratio (Las Vegas) was 3.1. 

These ratios were far less variable than 

those discussed above for annual 

average concentrations, suggesting that a 

standard based on 1-hour daily 

maximum NO2 concentrations could 

also be effective at protecting against 24- 

hour NO2 concentrations. The REA 

concluded that the scientific evidence, 

combined with the air quality 

correlations described above, support 

the appropriateness of a standard based 

on 1-hour daily maximum NO2

concentrations to protect against health 

effects associated with short-term 

exposures. 

Based on these considerations, the 

Administrator concludes that a standard 

with a 1-hour averaging time can 

effectively limit short-term (i.e., 1- to 24- 

hours) exposures that have been linked 

to adverse respiratory effects. This 

conclusion is based on the observations 

summarized above and in more detail in 

the proposal, particularly that: (1) The 

1-hour averaging time has been directly 

associated with respiratory effects in 

both epidemiologic and experimental 

studies and that (2) results from air 

quality analyses suggest that a 1-hour 

standard could also effectively control 

24-hour NO2 concentrations. In 

addition, the Administrator notes the 

support provided for a 1-hour averaging 

time in comments from CASAC, States, 

environmental groups, and medical/ 

public health groups. The Administrator 

notes that arguments offered by some 

industry groups against setting a 1-hour 

NO2 standard generally focus on 

commenters’ conclusions regarding 

uncertainties in the scientific evidence. 

As discussed in more detail above 

(section II.E.2), the Administrator 

disagrees with the conclusions of these 

commenters regarding the appropriate 

interpretation of the scientific evidence 

and associated uncertainties. Given 

these considerations, the Administrator 

judges that it is appropriate to set a new 

NO2 standard with a 1-hour averaging 

time. 

3. Form 

This section discusses considerations 

related to the form of the 1-hour NO2

primary NAAQS. Specifically, this 

section summarizes the rationale for the 

Administrator’s proposed decision 

regarding form (II.F.4.a; see section 

II.F.3 of the proposal for more detail), 

discusses comments related to form 

(II.F.4.b), and presents the 

Administrator’s final conclusions 

regarding form (II.F.4.c). 

a. Rationale For Proposed Decision 

When considering alternative forms in 

the proposal, the Administrator noted 

the conclusions in the policy 

assessment chapter of the REA. 

Specifically, she noted the conclusion 

that the adequacy of the public health 

protection provided by the combination 

of standard level and form should be the 

foremost consideration. With regard to 

this, she noted that concentration-based 

forms can better reflect pollutantassociated health risks than forms based 

on expected exceedances. This is the 

case because concentration-based forms 

give proportionally greater weight to 

years when pollutant concentrations are 

well above the level of the standard than 

to years when the concentrations are 

just above the standard, while an 

expected exceedance form would give 

the same weight to years with 

concentrations that just exceed the 

standard as to years when 

concentrations greatly exceed the 

standard. The Administrator also 

recognized the conclusion in the policy 

assessment chapter of the REA that it is 

desirable from a public health 

perspective to have a form that is 

reasonably stable and insulated from the 

impacts of extreme meteorological 

events. With regard to this, she noted 

that a form that calls for averaging 

concentrations over three years would 

provide greater regulatory stability than 

a form based on a single year of 

concentrations. Therefore, consistent 

with recent reviews of the O3 and PM 

NAAQS, the proposal focused on 

concentration-based forms averaged 

over 3 years, as evaluated in the REA. 

In considering specific concentrationbased forms, the REA focused on 98th 

and 99th percentile concentrations 

averaged over 3 years. This focus on the 

upper percentiles of the distribution is 

appropriate given the reliance, in part, 

on NO2 health evidence from 

experimental studies, which provide 

information on specific exposure 

concentrations that are linked to 

specific health effects. The REA noted 

that a 99th percentile form for a 1-hour 

daily maximum standard would 

correspond approximately to the 4th 

highest daily maximum concentration 

in a year (which is the form of the 

current O3 NAAQS) while a 98th 

percentile form (which is the form of the 

current short-term PM2.5 NAAQS) 

would correspond approximately to the 

7th or 8th highest daily maximum 

concentration in a year (REA, Table 10– 

4; see Thompson, 2008 for methods). 

Consideration in the REA of an 

appropriate form for a 1-hour standard 

was based on analyses of standard levels 

that reflected the allowable area-wide 

NO2 concentration, not the maximum 

allowable concentration. Therefore, in 

their review of the final REA, CASAC 

did not have the opportunity to 

comment on the appropriateness of 

specific forms in conjunction with a 

standard level that reflects the 

maximum allowable NO2 concentration 

anywhere in an area. Given this, when 

considering alternative forms for the 

1-hour standard in the proposal, the 

Administrator judged that it was 

appropriate to consider both forms 

evaluated in the REA (i.e., 98th and 99th 

percentiles). Therefore, she proposed to 

adopt either a 99th percentile or a 4th 

highest form, averaged over 3 years, and 

she solicited comment on both 98th 

percentile and 7th or 8th highest forms. 

b. CASAC and Public Comments on 

Form 

In their letter to the Administrator, 

CASAC discussed the issue of form 

within the context of the proposed 


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12 In addition, the air quality analyses presented 

in the REA estimated that on-road NO2

concentrations are about 80% higher on average 

than concentrations away from the road (REA, 

section 7.3.2) and that NO2 monitors within 20 m 

Continued 

approach of setting a 1-hour standard 

level that reflects the maximum 

allowable NO2 concentration anywhere 

in an area. CASAC recommended that, 

for such a standard, EPA adopt a form 

based on the 3-year average of the 98th 

percentile of the distribution of 1-hour 

daily maximum NO2 concentrations. 

Specifically, they stated the following in 

their comments on the proposal (Samet, 

2009): 

The 98th percentile is preferred by CASAC 

for the form, given the likely instability of 

measurements at the upper range and the 

absence of data from the proposed two-tier 

approach. 

As indicated in their letter, CASAC 

concluded that the potential instability 

in higher percentile NO2 concentrations 

near major roads argues for a 98th, 

rather than a 99th, percentile form. 

Several State organizations and agencies 

(e.g., NESCAUM and agencies in IN, NC, 

SD, VA) and industry groups (e.g., 

AAM, ACC, API, AirQuality Research 

and Logistics (AQRL), CPA, Dow, 

ExxonMobil, IPAMS, PAW, UPA) also 

recommended a 98th percentile form in 

order to provide regulatory stability. In 

contrast, a small number of State and 

local agencies (e.g., in MO and TX), 

several environmental organizations 

(e.g., EDF, EJ, GASP, NRDC), and 

medical/public health organizations 

(e.g., ALA, ATS) recommended either a 

99th percentile form or a more stringent 

form (e.g., no exceedance) to further 

limit the occurrence of NO2

concentrations that exceed the standard 

level in locations that attain the 

standard. 

c. Conclusions On Form 

The Administrator recognizes that 

there is not a clear health basis for 

selecting one specific form over another. 

She also recognizes that the analyses of 

different forms in the REA are most 

directly relevant to a standard that 

reflects NO2 concentrations permitted to 

occur broadly across a community, 

rather than the maximum concentration 

that can occur anywhere in the area. In 

contrast, as discussed below (section 

II.F.4.c), the Administrator has judged it 

appropriate to set a new 1-hour standard 

that reflects the maximum allowable 

NO2 concentration anywhere in an area. 

In light of this, the Administrator places 

particular emphasis on the comments 

received on form from CASAC relating 

to a 1-hour standard level that reflects 

the maximum allowable NO2

concentration anywhere in an area. In 

particular, the Administrator notes that 

CASAC recommended a 98th percentile 

form averaged over 3 years for such a 

standard, given the potential for 

instability in the higher percentile 

concentrations around major roadways. 

In considering this recommendation, 

the Administrator recognizes that the 

public health protection provided by the 

1-hour NO2 standard is based on the 

approach used to set the standard and 

the level of the standard (see below), in 

conjunction with the form of the 

standard. Given that the Administrator 

is setting a standard that reflects the 

maximum allowable NO2 concentration 

anywhere in an area, rather than a 

standard that reflects the allowable areawide NO2 concentration, she agrees 

with CASAC that an appropriate 

consideration with regard to form is the 

extent to which specific statistics could 

be unstable at locations where 

maximum NO2 concentrations are 

expected, such as near major roads. 

When considering alternative forms for 

the standard, the Administrator notes 

that an unstable form could result in 

areas shifting in and out of attainment, 

potentially disrupting ongoing air 

quality planning without achieving 

public health goals. Given the limited 

available information on the variability 

in peak NO2 concentrations near 

important sources of NO2 such as major 

roadways, and given the 

recommendation from CASAC that the 

potential for instability in the 99th 

percentile concentration is cause for 

supporting a 98th percentile form, the 

Administrator judges it appropriate to 

set the form based on the 3-year average 

of the 98th percentile of the annual 

distribution of 1-hour daily maximum 

NO2 concentrations. 

4. Level 

As discussed below and in more 

detail in the proposal (section II.F.4), 

the Administrator has considered two 

different approaches to setting the 

1-hour NO2 primary NAAQS. In the 

proposal, each of these approaches was 

linked with a different range of standard 

levels. Specifically, the Administrator 

proposed to set a 

1-hour standard reflecting the maximum 

allowable NO2 concentration anywhere 

in an area and to set the level of such 

a standard from 80 to 100 ppb. The 

Administrator also solicited comment 

on the alternative approach of setting a 

standard that reflects the allowable areawide NO2 concentration and setting the 

standard level from 50 to 75 ppb. This 

section summarizes the rationale for the 

Administrator’s proposed approach and 

range of standard levels (II.F.3.a), 

describes the alternative approach and 

range of standard levels (II.F.3.b), 

discusses comments related to each 

approach and range of standard levels 

(II.F.3.c), and presents the 

Administrator’s final conclusions 

regarding the approach and level 

(II.F.3.d). 

a. Rationale For Proposed Decisions on 

Approach and Level 

In assessing the most appropriate 

approach to setting the 1-hour standard 

and the most appropriate range of 

standard levels to propose, the 

Administrator considered the broad 

body of scientific evidence assessed in 

the ISA, including epidemiologic and 

controlled human exposure studies, as 

well as the results of exposure/risk 

analyses presented in the REA. In light 

of the body of available evidence and 

analyses, as described above, the 

Administrator concluded in the 

proposal that it is necessary to provide 

increased public health protection for 

at-risk individuals against an array of 

adverse respiratory health effects linked 

with short-term (i.e., 30 minutes to 24 

hours) exposures to NO2. Such health 

effects have been associated with 

exposure to the distribution of shortterm ambient NO2 concentrations across 

an area, including higher short-term 

(i.e., peak) exposure concentrations, 

such as those that can occur on or near 

major roadways and near other sources 

of NO2, as well as the lower short-term 

exposure concentrations that can occur 

in areas not near major roadways or 

other sources of NO2. The 

Administrator’s proposed decisions on 

approach and level, as discussed in 

detail in the proposal (section II.F.4), are 

outlined below. 

In considering a standard-setting 

approach, the Administrator was 

mindful in the proposal that the 

available evidence and analyses from 

the ISA and REA support the public 

health importance of roadwayassociated NO2 exposures. The exposure 

assessment described in the REA 

estimated that roadway-associated 

exposures account for the majority of 

exposures to peak NO2 concentrations 

(REA, Figures 8–17, 8–18). The ISA 

concluded (section 4.3.6) that NO2

concentrations in heavy traffic or on 

freeways ‘‘can be twice the residential 

outdoor or residential/arterial road 

level.’’ In considering the potential 

variability in the NO2 concentration 

gradient, the proposal noted that 

available monitoring studies suggest 

that NO2 concentrations could be 30 to 

100% higher than those in the same area 

but away from the road.12


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of roads measure NO2 concentrations that are, on 

average across locations, 40% higher than 

concentrations measured by monitors at least 100 

m from the road (REA, compare Tables 7–11 and 

7–13). 

13The 98th percentile concentrations in these 

study locations ranged from 85 to 94 ppb. 

14For a standard of 100 ppb, area-wide 

concentrations would be expected to range from 

approximately 50 ppb (assuming near-road 

concentrations are 100% higher than area-wide 

concentrations) to 75 ppb (assuming near-road 

concentrations are 30% higher than area-wide 

concentrations). 

The Administrator also considered 

that millions of people in the United 

States live, work, and/or attend school 

near important sources of NO2 such as 

major roadways (ISA, section 4.4), and 

that ambient NO2 concentrations in 

these locations vary depending on the 

distance from major roads (i.e., the 

closer to a major road, the higher the 

NO2 concentration) (ISA, section 2.5.4). 

Therefore, these populations, which 

likely include a disproportionate 

number of individuals in groups with 

higher prevalence of asthma and higher 

hospitalization rates for asthma (e.g. 

ethnic or racial minorities and 

individuals of low socioeconomic 

status) (ISA, section 4.4), are likely 

exposed to NO2 concentrations that are 

higher than those occurring away from 

major roadways. 

Given the above considerations, the 

Administrator proposed an approach to 

setting the 1-hour NO2 primary NAAQS 

whereby the standard would reflect the 

maximum allowable NO2 concentration 

anywhere in an area. In many locations, 

this concentration is likely to occur on 

or near a major roadway. EPA proposed 

to set the level of the standard such that, 

when available information regarding 

the concentration gradient around roads 

is considered, appropriate public health 

protection would be provided by 

limiting the higher short-term peak 

exposure concentrations expected to 

occur on and near major roadways, as 

well as the lower short-term exposure 

concentrations expected to occur away 

from those roadways. The Administrator 

concluded that this approach to setting 

the 1-hour NO2 NAAQS would be 

expected to protect public health against 

exposure to the distribution of shortterm NO2 concentrations across an area 

and would provide a relatively high 

degree of confidence regarding the 

protection provided against peak 

exposures to higher NO2 concentrations, 

such as those that can occur around 

major roadways. The remainder of this 

section discusses the proposed range of 

standard levels. 

In considering the appropriate range 

of levels to propose for a standard that 

reflects the maximum allowable NO2

concentration anywhere in an area, the 

Administrator considered the broad 

body of scientific evidence and 

exposure/risk information as well as 

available information on the 

relationship between NO2

concentrations near roads and those 

away from roads. Specifically, she 

considered the extent to which a variety 

of levels would be expected to protect 

at-risk individuals against increased 

airway responsiveness, respiratory 

symptoms, and respiratory-related 

emergency department visits and 

hospital admissions. 

After considering the scientific 

evidence and the exposure/risk 

information (see sections II.B, II.C, and 

II.F.4.a.1 through II.F.4.a.3 in the 

proposal), as well as the available 

information on the NO2 concentration 

gradient around roadways (section 

II.A.2 above and in the proposal), the 

Administrator concluded that the 

strongest support is for a standard level 

at or somewhat below 100 ppb. The 

Administrator’s rationale in reaching 

this proposed conclusion is provided 

below. 

The Administrator noted that a 

standard level at or somewhat below 

100 ppb in conjunction with the 

proposed approach would be expected 

to limit short-term NO2 exposures to 

concentrations that have been reported 

to increase airway responsiveness in 

asthmatics (i.e., at or above 100 ppb). 

While she acknowledged that exposure 

to NO2 concentrations below 100 ppb 

could potentially increase airway 

responsiveness in some asthmatics, the 

Administrator also noted uncertainties 

regarding the magnitude and the clinical 

significance of the NO2-induced 

increase in airway responsiveness, as 

discussed in the policy assessment 

chapter of the REA (section 10.3.2.1, 

discussed in section II.F.4.e in the 

proposal). Given these uncertainties, the 

Administrator concluded in the 

proposal that controlled human 

exposure studies provide support for 

limiting exposures at or somewhat 

below 100 ppb NO2. 

The Administrator also noted that a 

standard level at or somewhat below 

100 ppb in conjunction with the 

proposed approach would be expected 

to maintain peak area-wide NO2

concentrations considerably below 

those measured in locations where key 

U.S. epidemiologic studies have 

reported associations with more serious 

respiratory effects, as indicated by 

increased emergency department visits 

and hospital admissions. Specifically, 

the Administrator noted that 5 key U.S. 

studies provide evidence for such 

associations in locations where the 99th 

percentile of the distribution of 1-hour 

daily maximum NO2 concentrations 

measured at area-wide monitors ranged 

from 93 to 112 ppb (Ito et al., 2007; Jaffe 

et al., 2003; Peel et al., 2005; Tolbert et 

al., 2007; and a study by the New York 

State Department of Health, 2006).13

The Administrator concluded that these 

studies provide support for a 1-hour 

standard that limits the 99th percentile 

of the distribution of 1-hour daily 

maximum area-wide NO2

concentrations to below 90 ppb 

(corresponds to a 98th percentile 

concentration of 85 ppb), and that 

limiting area-wide concentrations to 

considerably below 90 ppb would be 

appropriate in order to provide an 

adequate margin of safety. The 

Administrator noted that, based on 

available information about the NO2

concentration gradient around roads, a 

standard level at or somewhat below 

100 ppb set in conjunction with the 

proposed approach would be expected 

to accomplish this. Specifically, she 

noted that given available information 

regarding NO2 concentration gradients 

around roads (see section II.A.2), a 

standard level at or below 100 ppb (with 

either a 99th or 98th percentile form) 

would be expected to limit peak areawide NO2 concentrations to 

approximately 75 ppb or below.14

Therefore, the Administrator concluded 

that a standard level at or somewhat 

below 100 ppb under the proposed 

approach would be expected to 

maintain peak area-wide NO2

concentrations well below 90 ppb across 

locations despite the expected variation 

in the NO2 concentration gradient that 

can exist around roadways in different 

locations and over time. 

The Administrator also noted that a 

study by Delfino provides mixed 

evidence for effects in a location with 

area-wide 98th and 99th percentile 

1-hour daily maximum NO2

concentrations of 50 and 53 ppb, 

respectively. In that study, NO2 effect 

estimates were positive, but some 

reported 95% confidence limits for the 

odds ratio (OR) that included values less 

than 1.00. Given the mixed results of the 

Delfino study, the Administrator 

concluded that it may not be necessary 

to maintain area-wide NO2

concentrations at or below 50 ppb to 

provide protection against the effects 

reported in epidemiologic studies. 

In addition to these evidence-based 

considerations, the Administrator noted 

that a standard level at or somewhat 

below 100 ppb under the proposed 

approach would be consistent with the 


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15This conclusion assumes that near-road NO2

concentrations are 65% higher than area-wide 

concentrations, reflecting the mid-point in the range 

of 30 to 100%. Based on available information 

suggesting that near-road concentrations can be 30 

to 100% higher than area-wide concentrations, a 

standard level of 80 ppb could limit area-wide 

concentrations to between 40 and 60 ppb. 

16CASAC members were also part of the CASAC 

Panel for the NO2 NAAQS review (i.e., the Oxides 

of Nitrogen Primary National Ambient Air Quality 

Standards Panel). Therefore, references to the 

CASAC Panel include both CASAC members and 

Panel members. 

results of the exposure and risk analyses 

presented in the REA. As discussed in 

section II.C of the proposal, the results 

of these analyses provide support for 

setting a standard that limits 1-hour 

area-wide NO2 concentrations to 

between 50 and 100 ppb. As described 

above, a standard level of 100 ppb that 

reflects the maximum allowable NO2

concentration would be expected to 

maintain area-wide NO2 concentrations 

at or below approximately 75 ppb. 

Given all of these considerations, the 

Administrator concluded in the 

proposal that a standard level at or 

somewhat below 100 ppb (with a 99th 

percentile form), in conjunction with 

the proposed approach, would be 

requisite to protect public health with 

an adequate margin of safety against the 

array of NO2-associated health effects. 

In addition to the considerations 

discussed above, which support setting 

a standard level at or somewhat below 

100 ppb, the Administrator also 

considered the extent to which available 

evidence could support standard levels 

below 100 ppb. The Administrator 

concluded that the evidence could 

support setting the standard level below 

100 ppb to the extent the following were 

emphasized: 

• The possibility that an NO2-induced 

increase in airway responsiveness could 

occur in asthmatics following exposures 

to concentrations below 100 ppb and/or 

the possibility that such an increase 

could be clinically significant. 

• The mixed results reported in the 

study by Delfino et al. (2002) of an 

association between respiratory 

symptoms and the relatively low 

ambient NO2 concentrations measured 

in the study area. 

Specifically, she noted that a standard 

level of 80 ppb (99th percentile form), 

in conjunction with the proposed 

approach, could limit area-wide NO2

concentrations to 50 ppb 15 and would 

be expected to limit exposure 

concentrations to below those that have 

been reported to increase airway 

responsiveness in asthmatics. For the 

reasons stated above, the Administrator 

proposed to set the level of a new 

1-hour standard between 80 ppb and 

100 ppb. 

b. Rationale for the Alternative 

Approach and Range of Levels 

As described above, the Administrator 

proposed to set a 1-hour NO2 NAAQS 

reflecting the maximum allowable NO2

concentration anywhere in an area and 

to set the level of such a standard from 

80 to 100 ppb. However, prior to the 

proposal, the approach of setting a 

1-hour NO2 NAAQS that reflects the 

maximum allowable NO2 concentration 

anywhere in an area had not been 

discussed by EPA in the REA or 

considered by CASAC. Rather, the 

potential alternative standards 

discussed in the REA, and reviewed by 

CASAC, reflected allowable area-wide 

NO2 concentrations (i.e., concentrations 

that occur broadly across communities). 

Given this, the Administrator noted in 

the proposal that comments received on 

the approach to setting the 1-hour 

standard (i.e., from CASAC and from 

members of the public) could provide 

important new information for 

consideration. Therefore, the 

Administrator also solicited comment 

on the alternative approach of setting a 

1-hour NO2 primary NAAQS that would 

reflect the allowable area-wide NO2

concentration, analogous to the 

standards evaluated in the REA, and 

with a level set within the range of 50 

to 75 ppb. In discussing this alternative 

approach with a standard level from 50 

to 75 ppb, the Administrator noted the 

following in the proposal: 

• Such a standard would be expected 

to maintain area-wide NO2

concentrations below peak 1-hour areawide concentrations measured in 

locations where key U.S. epidemiologic 

studies have reported associations with 

respiratory-related emergency 

department visits and hospital 

admissions. 

• Standard levels from the lower end 

of the range would be expected to limit 

roadway-associated exposures to NO2

concentrations that have been reported 

in controlled human exposure studies to 

increase airway responsiveness in 

asthmatics. Specifically, a standard 

level of 50 ppb under this approach 

could limit near-road concentrations to 

between approximately 65 and 100 ppb, 

depending on the relationship between 

near-road NO2 concentrations and areawide concentrations. 

• This alternative approach would 

provide relatively more confidence 

regarding the degree to which a specific 

standard level would limit area-wide 

NO2 concentrations and less confidence 

regarding the degree to which a specific 

standard level would limit the peak NO2

concentrations likely to occur near 

major roadways. 

c. Comments on Approach and Level 

In the proposal, each approach to 

setting the 1-hour standard, and each 

range of standard levels, was linked to 

different requirements for the design of 

the NO2 monitoring network. 

Specifically, in conjunction with the 

proposed approach (i.e., standard 

reflects the maximum allowable NO2

concentration anywhere in an area and 

the level is set within the range of 80 to 

100 ppb), the Administrator proposed to 

establish a 2-tiered monitoring network 

that would include monitors sited to 

measure the maximum NO2

concentrations anywhere in an area, 

including near major roadways, and 

monitors sited to measure maximum 

area-wide NO2 concentrations. In 

conjunction with the alternative 

approach (i.e., standard reflects the 

allowable area-wide NO2 concentration 

and the level is set within the range of 

50 to 75 ppb), the Administrator 

solicited comment on a monitoring 

network that would only include areawide NO2 monitors. Because of these 

linkages in the proposal, most 

commenters combined their comments 

on the approach to setting a 1-hour 

standard and on the standard level with 

their comments on the monitoring 

requirements. In this section, we discuss 

comments from CASAC and public 

commenters on the approach to setting 

a 1-hour standard and on the standard 

level. Comments on the monitoring 

network are also discussed in this 

section to the extent they indicate a 

preference for either the proposed or 

alternative approach to setting the 1- 

hour standard. More specific comments 

on monitor placement and network 

design are discussed below in section 

III.B.2 and in the Response to Comments 

document. EPA responses to technical 

comments on the scientific evidence 

and the exposure/response information 

are discussed above in section II.E.2 and 

in the Response to Comments 

document. The Administrator’s 

response to commenters’ views on the 

approach to setting the 1-hour standard 

and on the standard level is embodied 

in the discussed in section II.F.4.d. 

i. CASAC Comments on the Approach 

to Setting the Standard 

A majority of CASAC and CASAC 

Panel members 16 favored the proposed 

approach of setting a 1-hour standard 

that reflects the maximum allowable 


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NO2 concentration anywhere in an area 

and linking such a standard with a 2- 

tiered monitoring network that would 

include both near-road and area-wide 

monitors, though CASAC did not reach 

consensus on this approach. 

Specifically, in their letter to the 

Administrator (Samet, 2009), CASAC 

stated the following: 

There was a split view on the two 

approaches among both CASAC and CASAC 

panel members with a majority of each 

favoring the Agency’s proposed two-tiered 

monitoring network because they thought 

this approach would be more effective in 

limiting near-roadway exposures that may 

reach levels in the range at which some 

individuals with asthma may be adversely 

affected. Other members acknowledged the 

need for research and development of nearroad monitoring data for criteria pollutants in 

general but favored retention of EPA’s 

current area-wide monitoring for NO2

regulatory purposes, due to the lack of 

epidemiological data based on near-roadway 

exposure measurements and issues related to 

implementing a near-road monitoring system 

for NO2. 

Thus, the recommendation of the 

majority of CASAC Panel members was 

based on their conclusion that the 

proposed approach would be more 

effective than the alternative at limiting 

near-roadway exposures to NO2

concentrations that could adversely 

affect asthmatics. In addition, these 

CASAC Panel members noted important 

uncertainties with the alternative 

approach. Specifically, they stated the 

following (Samet, 2009): 

Panel members also supported the 

proposed two-tiered approach because basing 

regulations on area-wide monitoring alone 

was problematic. Such an approach would 

require EPA to embed uncertainties and 

assumptions about the relationship between 

area-wide and road-side monitoring into the 

area-wide standard. 

A minority of CASAC Panel members 

expressed support for the alternative 

approach of setting a 1-hour standard 

that reflects the allowable area-wide 

NO2 concentration. These CASAC Panel 

members concluded that there would be 

important uncertainties associated with 

the proposed approach. Specifically, 

they noted that the key U.S. NO2

epidemiologic studies relied upon areawide NO2 concentrations. In their view, 

the use of area-wide concentrations in 

these studies introduces uncertainty 

into the selection of a standard level for 

a standard that reflects the maximum 

allowable NO2 concentration anywhere 

in an area and that is linked with a 

requirement to place monitors near 

major roads. As a result of this 

uncertainty, CASAC Panel members 

who favored the alternative approach 

noted that ‘‘it would be better to set the 

standard on the same area-wide 

monitoring basis as employed in the 

epidemiologic studies upon which it 

[the standard] now relies’’ (Samet, 2009). 

These CASAC Panel members also 

strongly supported obtaining monitoring 

data near major roads, while recognizing 

uncertainties associated with 

identifying appropriate monitoring sites 

near roads (see section III.B.2 and the 

Response to Comments document for 

more discussion of CASAC’s monitoring 

comments). 

ii. Public Comments on the Approach to 

Setting the Standard 

Consistent with the views expressed 

by the majority of CASAC members, a 

number of commenters concluded that 

the most appropriate approach would be 

to set a 1-hour standard that reflects the 

maximum allowable NO2 concentration 

anywhere in an area and to couple that 

standard with a requirement that 

monitors be placed in locations where 

maximum concentrations are expected, 

including near major roads. This view 

was expressed by some State and local 

agencies (e.g., in CA, IA, NY, TX, WA, 

WI), by a number of environmental 

organizations (e.g., CAC, EDF, EJ, GASP, 

NRDC), by the ALA, and individual 

commenters. Several additional medical 

and public health organizations (ACCP, 

AMA, ATS, NADRC, NACPR) did not 

explicitly express a recommendation 

regarding the approach though these 

organizations did recommend that, in 

setting a 1-hour standard, particular 

attention should be paid to NOX

concentrations around major roadways. 

In support of their recommendation to 

adopt the proposed approach and to 

focus monitoring around major roads, 

these commenters generally concluded 

that a primary consideration should be 

the extent to which the NO2 NAAQS 

protects at-risk populations that live 

and/or attend school near important 

sources of NO2 such as major roads. As 

such, these comments supported the 

rationale in the proposal for setting a 1- 

hour standard that reflects the 

maximum allowable NO2 concentration 

anywhere in an area. 

A number of State commenters 

expressed the view that area-wide 

monitors should be used for attainment/ 

non-attainment determinations (e.g., 

NACAA, NESCAUM and agencies in IL, 

IN, MI, MS, NC, NM, SC). One State 

commenter (NESCAUM) agreed with 

EPA concerns about near-road 

exposures but concluded that it is 

premature to establish a large near-road 

monitoring network at this time due to 

uncertainty regarding the relationship 

between near-road and area-wide NO2

concentrations and the variability in 

that relationship. NESCAUM 

recommended that EPA work with 

States to establish a targeted monitoring 

program in select urban areas to gather 

data that would inform future 

modifications to the monitoring 

network, but that ‘‘[t]he existing areawide monitoring network should be 

used to identify initial nonattainment 

areas.’’ Other State commenters also 

concluded that the most appropriate 

approach would be to base nonattainment determinations only on areawide monitors. Based on their 

monitoring comments, many of these 

commenters appeared to support setting 

a 1-hour standard that reflects the 

allowable area-wide NO2 concentration. 

State concerns with the proposed 

approach often included uncertainties 

associated with identifying and 

accessing appropriate monitor sites near 

major roads, as well as concerns related 

to implementation and cost to States (as 

discussed further in the Response to 

Comments document, the Administrator 

may not consider cost of 

implementation in decisions on a 

NAAQS). 

One commenter (AAM) concluded 

that the focus of the proposed approach 

on NO2 concentrations around major 

roadways is not justified because the 

REA and the proposal overstate the 

extent to which NO2 concentrations 

near roads are higher than NO2

concentrations farther away from the 

road. This conclusion is based on an 

analysis of 42 existing NO2 monitors in 

6 locations. Comparing NO2

concentrations measured by these 

monitors, some of which are closer to 

roads and others of which are farther 

from roads, AAM concluded that 

‘‘roadside monitors are not measuring 

high NO2 concentrations.’’ 

We agree that there is uncertainty 

associated with estimates of roadwayassociated NO2 concentrations (see REA, 

sections 7.4.6 and 8.4.8.3 for detailed 

discussion of these uncertainties) and in 

identifying locations where maximum 

concentrations are expected to occur. 

However, we note that the 

Administrator’s conclusions regarding 

the relationship between NO2

concentrations near roads and those 

away from roads rely on multiple lines 

of scientific evidence and information. 

Specifically, the Administrator relied in 

the proposal on the following in 

drawing conclusions regarding the 

distribution of NO2 concentrations 

across areas: 

• Monitoring studies discussed in the 

ISA and REA that were designed to 

characterize the NO2 concentration 

gradient around roads, which indicated 

that NO2 concentrations near roads can 


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17To measure maximum concentrations, the 

Administrator proposed monitoring provisions that 

would require monitors within 50 meters of major 

roads and to allow the Regional Administrator to 

require additional monitors in situations where 

maximum concentrations would be expected to 

occur in locations other than near major roads (e.g., 

due to the influence of multiple smaller roads and/ 

or stationary sources). 

be approximately 30 to 100% higher 

than concentrations away from the road 

in the same area. 

• Air quality and exposure analyses 

presented in the REA which estimate 

that, on average across locations, NO2

concentrations on roads could be 80% 

higher than those away from roads and 

that roadway-associated exposures 

account for the majority of exposures to 

NO2 concentrations at or above 100 ppb. 

In contrast, the existing NO2

monitoring network, which was the 

basis for the analysis submitted by 

AAM, was not designed to characterize 

the spatial gradients in NO2

concentrations surrounding roadways. 

Rather, concentrations of NO2 measured 

by existing monitors are likely to reflect 

contributions from a combination of 

mobile and stationary sources, with one 

or the other dominating depending on 

the proximity of these sources to the 

monitors. Therefore, we conclude that 

the analysis submitted by AAM, which 

does not consider other relevant lines of 

evidence and information, does not 

appropriately characterize the 

relationship between NO2

concentrations near roads and those 

away from roads. (See the Response to 

Comments document for a more 

detailed discussion of AAM comments.) 

In addition, we note that, although the 

Administrator concluded in the 

proposal that maximum NO2

concentrations in many areas are likely 

to occur around major roads, she also 

recognized that maximum 

concentrations can occur elsewhere in 

an area. For this reason, she proposed to 

set a 1-hour NO2 standard that reflects 

the maximum allowable NO2

concentration anywhere in an area, 

regardless of where that maximum 

concentration occurs.17 Therefore, the 

proposed approach to setting the 

standard would be expected to limit the 

maximum NO2 concentrations 

anywhere in an area even if in some 

areas, as is contended by AAM, those 

maximum NO2 concentrations do not 

occur near roads. 

iii. CASAC Comments on Standard 

Level 

In commenting on the proposal, 

CASAC discussed both the proposed 

range of standard levels (i.e., 80–100 

ppb) and the alternative range of 

standard levels (i.e., 50–75 ppb). 

CASAC did express the consensus 

conclusion that if the Agency finalizes 

a 1-hour standard in accordance with 

the proposed approach (i.e., standard 

level reflects the maximum allowable 

NO2 concentration anywhere in an 

area), then it is appropriate to consider 

the proposed range of standard levels 

from 80 to 100 ppb. Specifically, the 

CASAC letter to the Administrator on 

the proposal (Samet, 2009) stated the 

following with regard to the proposed 

approach: 

[T]he level of the one-hour NO2 standard 

should be within the range of 80–100 ppb 

and not above 100 ppb. In its letter of 

December 2, 2008, CASAC strongly voiced a 

consensus view that the upper end of the 

range should not exceed 100 ppb, based on 

evidence of risk at that concentration. The 

lower limit of 80 ppb was viewed as 

reasonable by CASAC; selection of a value 

lower than 80 ppb would represent a policy 

judgment based on uncertainty and the 

degree of public health protection sought, 

given the limited health-based evidence at 

concentrations below 100 ppb. 

CASAC also recommended that this 

level be employed with a 98th 

percentile form, in order to promote the 

stability of the standard (see above for 

discussion of form). 

iv. Public Comments on Standard Level 

A number of State and local agencies 

and organizations expressed support for 

setting the level of the 1-hour NO2

standard within the proposed range of 

80 to 100 ppb. While some State and 

local agencies (e.g., in CA, IA, MI, NY, 

TX) made this recommendation in 

conjunction with a recommendation to 

focus monitoring near major roads and 

other important sources of NO2, a 

number of State commenters (e.g., 

NACAA, NESCAUM and agencies in IL, 

NC, NM, TX, VA) recommended a 

standard level from 80 to 100 ppb in 

conjunction with a recommendation 

that only area-wide monitors be 

deployed for purposes of determining 

attainment with the standard. Based on 

these monitoring comments, these State 

commenters appear to favor an 

approach where a standard level from 

80 to 100 ppb would reflect the 

allowable area-wide NO2 concentration. 

As discussed above (and in more detail 

in section III.B.2 and the Response to 

Comments document), State 

commenters often based these 

recommendations on uncertainties 

associated with designing an 

appropriate national near-road 

monitoring network. 

A number of environmental 

organizations (e.g., CAC, EDF, EJ, GASP, 

NRDC) and medical/public health 

organizations (e.g., ACCP, ALA, AMA, 

ATS, NACPR, NAMDRC) supported 

setting a standard level below 80 ppb for 

a standard that reflects the maximum 

allowable NO2 concentration anywhere 

in an area. Several of these groups 

recommended a standard level of 50 

ppb. This recommendation was 

typically based on the commenters’ 

interpretation of the epidemiologic and 

controlled human exposure evidence, as 

described below. 

Some of these commenters noted that 

the 98th percentile area-wide NO2

concentration was below 80 ppb in the 

location of a single key U.S. 

epidemiologic study (i.e., 50 ppb in 

study by Delfino). Given this, 

commenters concluded that the 

standard level should be set at 50 ppb. 

Their comments on the monitoring 

network generally favored a requirement 

to place monitors near major roads and, 

therefore, these commenters appeared to 

favor a standard level as low as 50 ppb 

and to recommend that such a standard 

level reflect the maximum allowable 

NO2 concentration anywhere in an area. 

In their comments, the ALA, EDF, EJ, 

and NRDC stated the following: 

Considering the Delfino study alone on 

EPA’s terms, that is, focusing on the 98th 

percentile of the 1-hour daily maximum 

concentrations, EPA reports a concentration 

of 50 ppb where asthma symptoms were 

observed. Based primarily on this study, EPA 

concluded in the REA that it was appropriate 

to set the lower end of the range at 50 ppb, 

which corresponded to the lowest-observed 

effects level of airway hyperresponsiveness 

in asthmatics. To provide the strongest 

public health protection, we therefore urge 

the level of the standard be set at 50 ppb. 

In some cases, the same commenters 

also appeared to recommend setting a 

standard level below 50 ppb because 

mean area-wide NO2 concentrations 

reported in locations of key U.S. 

epidemiologic studies are below this 

concentration. Specifically, with regard 

to the key U.S. epidemiologic studies, 

these commenters (e.g., ALA, EDF, EJ, 

NRDC) stated the following: 

These studies clearly identify adverse 

health effects such as emergency room visits 

and hospital admissions for respiratory 

causes at concentrations currently occurring 

in the United States. Mean concentrations for 

all but two of these studies are about or 

below 50 ppb, suggesting that the standard 

must be set below this level to allow for a 

margin of safety. 

The Administrator’s consideration of 

the Delfino study as it relates to a 

decision on standard level is discussed 

below (section II.F.4.d). Regarding the 

recommendation to set the level below 

50 ppb based on mean area-wide NO2

concentrations in epidemiologic study 


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18As discussed above, the Administrator has 

selected the 98th percentile as the form for the new 

1-hour NO2 standard. 

locations, we note that the 

Administrator proposed to set a 

standard that reflects the maximum 

allowable NO2 concentration anywhere 

in an area and to set the form of that 

standard at the upper end of the 

distribution of 1-hour daily maximum 

NO2 concentrations.18 As described in 

the proposal, such a standard, with a 

level from the proposed range of 80 to 

100 ppb, would be expected to maintain 

peak area-wide NO2 concentrations 

below the peak area-wide 

concentrations measured in locations 

where key U.S. epidemiologic studies 

have reported associations with 

respiratory-related emergency 

department visits and hospital 

admissions. Because reducing NOX

emissions to meet a 98th percentile NO2

standard should lower the distribution 

of NO2 concentrations, including the 

mean, a standard that limits the 98th 

percentile of the distribution of 1-hour 

daily maximum concentrations would 

also be expected to limit mean 

concentrations. Therefore, although we 

acknowledge that the relationship 

between peak and mean NO2

concentrations will likely vary across 

locations and over time, if peak areawide NO2 concentrations are 

maintained below those in key 

epidemiologic study locations, mean 

area-wide NO2 concentrations would 

also be expected to be maintained below 

the mean area-wide concentrations in 

those locations (see ISA, figure 2.4–13 

for information on the relationship 

between peak and mean NO2

concentrations). 

As discussed above (section, II.E.2), a 

number of industry groups did not 

support setting a new 1-hour NO2

standard. However, several of these 

groups (e.g., AAM, Dow, NAM, NPRA) 

also concluded that, if EPA does choose 

to set a new 1-hour standard, the level 

of that standard should be above 100 

ppb. As a basis for this 

recommendation, these groups 

emphasized uncertainties in the 

scientific evidence. Specifically, as 

discussed in more detail above (section 

II.E.2), these commenters typically 

concluded that available epidemiologic 

studies do not support the conclusion 

that NO2 causes reported health effects. 

This was based on their assertion that 

the presence of co-pollutants in the 

ambient air precludes the identification 

of a specific NO2 contribution to 

reported effects. As a result, these 

commenters recommended that a 1-hour 

standard should be based on the 

controlled human exposure evidence 

and that, in considering that evidence, 

EPA should rely on the meta-analysis of 

NO2 airway responsiveness studies 

conducted by Goodman et al., (2009) 

rather than the meta-analysis included 

in the final ISA. As described above, 

they concluded that in relying on the 

ISA meta-analysis, EPA has 

inappropriately relied on a new 

unpublished meta-analysis that has not 

been peer-reviewed, was not reviewed 

by CASAC, and was not conducted in a 

transparent manner. EPA recognizes the 

uncertainties in the scientific evidence 

that are discussed by these industry 

commenters; however, we strongly 

disagree with their conclusions 

regarding the implications of these 

uncertainties for decisions on the NO2

NAAQS. These comments, and EPA’s 

responses, are discussed in detail above 

(section II.E.2) and in the Response to 

Comments document and are 

summarized briefly below. 

As noted in section II.E.2, we agree 

that the presence of co-pollutants in the 

ambient air complicates the 

interpretation of epidemiologic studies; 

however, our conclusions regarding 

causality are based on consideration of 

the broad body of epidemiologic studies 

(including those employing multipollutant models) as well as animal 

toxicological and controlled human 

exposure studies. The ISA concluded 

that this body of evidence ‘‘supports a 

direct effect of short-term NO2 exposure 

on respiratory morbidity at ambient 

concentrations below the current 

NAAQS level’’ (ISA, p. 5–16). In 

addition, the ISA (p. 5–15) concluded 

the following: 

[T]he strongest evidence for an association 

between NO2 exposure and adverse human 

health effects comes from epidemiologic 

studies of respiratory symptoms and ED 

visits and hospital admissions. These new 

findings were based on numerous studies, 

including panel and field studies, 

multipollutant studies that control for the 

effects of other pollutants, and studies 

conducted in areas where the whole 

distribution of ambient 24-h avg NO2

concentrations was below the current 

NAAQS level of 0.053 ppm (53 ppb) (annual 

average). 

Given that epidemiologic studies 

provide the strongest support for an 

association between NO2 and 

respiratory morbidity, and that a 

number of these studies controlled for 

the presence of other pollutants with 

multi-pollutant models (in which NO2

effect estimates remained robust), we 

disagree that NO2 epidemiologic studies 

should not be used to inform a decision 

on the level of the 1-hour NO2 standard. 

In addition, we agree that uncertainty 

exists regarding the extent to which the 

NO2-induced increase in airway 

responsiveness is adverse (REA, section 

10.3.2.1); however, as discussed in 

detail above (section II.E.2), we disagree 

with the conclusion by many industry 

commenters that this effect is not 

adverse in asthmatics following 

exposures from 100 to 600 ppb NO2. 

Specifically, we do not agree that the 

approach taken in the study by 

Goodman et al. (2009), which was used 

by many industry commenters to 

support their conclusions, was 

appropriate. The authors of the 

Goodman study used data from existing 

NO2 studies to characterize the doseresponse relationship of NO2 and airway 

responsiveness and to calculate the 

magnitude of the NO2 effect. Given the 

protocol differences in existing studies 

of NO2 and airway responsiveness, we 

do not agree that it is appropriate to 

base such an analysis on these studies. 

The Administrator’s consideration of 

these uncertainties, within the context 

of setting a standard level, is discussed 

in the next section. 

d. Conclusions on Approach and 

Standard Level 

Having carefully considered the 

public comments on the appropriate 

approach and level for a 1-hour NO2

standard, as discussed above, the 

Administrator believes the fundamental 

conclusions reached in the ISA and REA 

remain valid. In considering the 

approach, the Administrator continues 

to place primary emphasis on the 

conclusions of the ISA and the analyses 

of the REA, both of which focus 

attention on the importance of roadways 

in contributing to peak NO2 exposures, 

given that roadway-associated 

exposures can dominate personal 

exposures to NO2. In considering the 

level at which the 1-hour primary NO2

standard should be set, the 

Administrator continues to place 

primary emphasis on the body of 

scientific evidence assessed in the ISA, 

as summarized above in section II.B, 

while viewing the results of exposure 

and risk analyses, discussed above in 

section II.C, as providing information in 

support of her decision. 

With regard to her decision on the 

approach to setting the 1-hour standard, 

the Administrator continues to judge it 

appropriate to provide increased public 

health protection for at-risk individuals 

against an array of adverse respiratory 

health effects linked with short-term 

exposures to NO2, where such health 

effects have been associated with 

exposure to the distribution of shortterm ambient NO2 concentrations across 


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19The most current American Housing Survey 

(http://www.census.gov/hhes/www/housing/ahs/ 

ahs.html) is from 2007 and lists a higher fraction 

of housing units within the 300 foot boundary. 

According to Table 1A–6 from that report (http:// 

www.census.gov/hhes/www/housing/ahs/ahs07/ 

tab1a-6.pdf), out of 128.2 million total housing 

units in the United States, about 20 million were 

reported by the surveyed occupant or landlord as 

being within 300 feet of a 4-or-more lane highway, 

railroad, or airport. That constitutes 15.6% of the 

total housing units in the U.S. Assuming equal 

distributions, with a current population of 306.3 

million, that means that there would be 47.8 

million people meeting the 300 foot criteria. 

an area. In protecting public health 

against exposure to the distribution of 

short-term NO2 concentrations across an 

area, the Administrator is placing 

emphasis on providing a relatively high 

degree of confidence regarding the 

protection provided against exposures 

to peak concentrations of NO2, such as 

those that can occur around major 

roadways. Available evidence and 

information suggest that roadways 

account for the majority of exposures to 

peak NO2 concentrations and, therefore, 

are important contributors to NO2- 

associated public health risks. In 

reaching this conclusion, the 

Administrator notes the following: 

• Mobile sources account for the 

majority of NOX emissions (ISA, Table 

2.2–1). 

• The ISA stated that NO2

concentrations in heavy traffic or on 

freeways ‘‘can be twice the residential 

outdoor or residential/arterial road 

level,’’ that ‘‘exposure in traffic can 

dominate personal exposure to NO2,’’ 

and that ‘‘NO2 levels are strongly 

associated with distance from major 

roads (i.e., the closer to a major road, the 

higher the NO2 concentration)’’ (ISA, 

sections 2.5.4, 4.3.6). 

• The exposure assessment presented 

in the REA estimated that roadwayassociated exposures account for the 

majority of exposures to peak NO2

concentrations (REA, Figures 8–17, 8– 

18). 

• Monitoring studies suggest that NO2

concentrations near roads can be 

considerably higher than those in the 

same area but away from roads (e.g., by 

30–100%, see section II.A.2). 

• In their comments on the approach 

to setting the 1-hour NO2 standard, the 

majority of CASAC Panel members 

emphasized the importance of setting a 

standard that limits roadway-associated 

exposures to NO2 concentrations that 

could adversely affect asthmatics. These 

CASAC Panel members favored the 

proposed approach, including its focus 

on roads. 

In addition, the Administrator notes 

that a considerable fraction of the 

population resides, works, or attends 

school near major roadways or other 

sources of NO2 and that these 

populations are likely to have increased 

exposure to NO2 (ISA, section 4.4). 

Based on data from the 2003 American 

Housing Survey, approximately 36 

million individuals live within 300 feet 

(∼90 meters) of a four-lane highway, 

railroad, or airport (ISA, section 4.4).19

Furthermore, in California, 2.3% of 

schools with a total enrollment of more 

than 150,000 students were located 

within approximately 500 feet of hightraffic roads (ISA, section 4.4). Of this 

population, which likely includes a 

disproportionate number of individuals 

in groups with a higher prevalence of 

asthma and higher hospitalization rates 

for asthma (e.g., ethnic or racial 

minorities and individuals of low 

socioeconomic status) (ISA, section 4.4), 

asthmatics and members of other 

susceptible groups (e.g., children, 

elderly) will have the greatest risks of 

experiencing health effects related to 

NO2 exposure. In the United States, 

approximately 10% of adults and 13% 

of children have been diagnosed with 

asthma, and 6% of adults have been 

diagnosed with COPD (ISA, section 4.4). 

In considering the approach to setting 

the 1-hour standard, the Administrator 

also notes that concerns with the 

proposed approach expressed by the 

minority of CASAC Panel members 

included concern with the uncertainty 

in the relationship between near-road 

and area-wide NO2 concentrations, 

given that U.S. epidemiologic studies 

have been based on concentrations 

measured at area-wide monitors. 

However, as discussed by the majority 

of CASAC Panel members, a similar 

uncertainty would be involved in 

setting a standard with the alternative 

approach (Samet, 2009). The 

Administrator agrees with the majority 

of CASAC Panel members and 

concludes that uncertainty in the 

relationship between near-road and 

area-wide NO2 concentrations should be 

considered regardless of the approach 

selected to set the standard. She 

recognizes that this uncertainty can and 

should be taken into consideration 

when considering the level of the 

standard. 

In drawing conclusions on the 

approach, the Administrator has 

considered the extent to which each 

approach, in conjunction with the 

ranges of standard levels discussed in 

the proposal, would be expected to limit 

the distribution of NO2 concentrations 

across an area and, therefore, would be 

expected to protect against risks 

associated with NO2 exposures. 

Specifically, she has considered the 

extent to which a standard set with each 

approach would be expected to limit 

maximum NO2 concentrations and areawide NO2 concentrations. 

With regard to expected maximum 

concentrations, the Administrator notes 

the following: 

• A standard reflecting the maximum 

allowable NO2 concentration anywhere 

in an area would provide a relatively 

high degree of confidence regarding the 

level of protection provided against 

peak exposures, such as those that can 

occur on or near major roadways. A 

standard level from anywhere within 

the proposed range (i.e., 80 to 100 ppb) 

would be expected to limit exposures to 

NO2 concentrations reported to increase 

airway responsiveness in asthmatics. 

• A standard reflecting the allowable 

area-wide NO2 concentration would not 

provide a high degree of confidence 

regarding the extent to which maximum 

NO2 concentrations would be limited. 

Maximum NO2 concentrations would be 

expected to be controlled to varying 

degrees across locations and over time 

depending on the NO2 concentration 

gradient around roads. Given the 

expected variability in gradients across 

locations and over time, most standard 

levels within the range considered in 

the proposal with this option (i.e., 50 to 

75 ppb) would not be expected to 

consistently limit the occurrence of NO2

concentrations that have been reported 

to increase airway responsiveness in 

asthmatics. 

With regard to expected area-wide 

concentrations, the Administrator notes 

the following: 

• The extent to which a standard 

reflecting the maximum allowable NO2

concentration anywhere in an area 

would be expected to limit area-wide 

NO2 concentrations would vary across 

locations, e.g., depending on the NO2

concentration gradient around roads. 

However, in conjunction with a 

standard level from anywhere within 

the proposed range (i.e., 80–100 ppb), 

such an approach would be expected to 

maintain area-wide NO2 concentrations 

below those measured in locations 

where key U.S. epidemiologic studies 

have reported associations between 

ambient NO2 and respiratory-related 

hospital admissions and emergency 

department visits (based on available 

information regarding the NO2

concentration gradient around roads as 

discussed below). 

• A standard reflecting the maximum 

allowable area-wide NO2 concentration 

would provide a relatively high degree 

of certainty regarding the extent to 

which area-wide NO2 concentrations are 

limited. In conjunction with a standard 

level from anywhere within the range of 


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levels discussed in the proposal (i.e., 

50–75 ppb) with this alternative 

approach, such a standard would be 

expected to maintain area-wide NO2

concentrations below those measured in 

locations where key U.S. epidemiologic 

studies have reported associations 

between ambient NO2 and respiratoryrelated hospital admissions and 

emergency department visits. 

Given the above considerations, the 

Administrator concludes that both 

approaches, in conjunction with 

appropriate standard levels, would be 

expected to maintain area-wide NO2

concentrations below those measured in 

locations where key U.S. epidemiologic 

studies have reported associations 

between ambient NO2 and respiratoryrelated hospital admissions and 

emergency department visits. In 

contrast, the Administrator concludes 

that only a standard reflecting the 

maximum allowable NO2 concentration 

anywhere in an area, in conjunction 

with an appropriate standard level, 

would be expected to consistently limit 

exposures, across locations and over 

time, to NO2 concentrations reported to 

increase airway responsiveness in 

asthmatics. After considering the 

evidence and uncertainties, and the 

advice of the CASAC Panel, the 

Administrator judges that the most 

appropriate approach to setting a 1-hour 

standard to protect against the 

distribution of short-term NO2

concentrations across an area, including 

the higher concentrations that can occur 

around roads and result in elevated 

exposure concentrations, is to set a 

standard that reflects the maximum 

allowable NO2 concentration anywhere 

in an area. 

In considering the level of a 1-hour 

NO2 standard that reflects the maximum 

allowable NO2 concentration anywhere 

in an area, the Administrator notes that 

there is no bright line clearly directing 

the choice of level. Rather, the choice of 

what is appropriate is a public health 

policy judgment entrusted to the 

Administrator. This judgment must 

include consideration of the strengths 

and limitations of the evidence and the 

appropriate inferences to be drawn from 

the evidence and the exposure and risk 

assessments. Specifically, the 

Administrator notes the following: 

• Controlled human exposure studies 

have reported that various NO2

exposure concentrations increased 

airway responsiveness in mostly mild 

asthmatics (section II above and II.B.1.d 

in proposal). These studies can inform 

an evaluation of the risks associated 

with exposure to specific NO2

concentrations, regardless of where 

those exposures occur in an area. 

Because concentrations evaluated in 

controlled human exposure studies are 

at the high end of the distribution of 

ambient NO2 concentrations (ISA, 

section 5.3.2.1), these studies most 

directly inform consideration of the 

risks associated with exposure to peak 

short-term NO2 concentrations. 

• Epidemiologic studies (section 

II.B.1.a and b) conducted in the United 

States have reported associations 

between ambient NO2 concentrations 

measured at area-wide monitors in the 

current network and increased 

respiratory symptoms, emergency 

department visits, and hospital 

admissions. Area-wide monitors in the 

urban areas in which these 

epidemiologic studies were conducted 

are not sited in locations where 

localized peak concentrations are likely 

to occur. Thus, they do not measure the 

full range of ambient NO2

concentrations across the area. Rather, 

the area-wide NO2 concentrations 

measured by these monitors are used as 

surrogates for the distribution of 

ambient NO2 concentrations across the 

area, a distribution that includes NO2

concentrations both higher than (e.g., 

around major roadways) and lower than 

the area-wide concentrations measured 

in study locations. Epidemiologic 

studies evaluate whether area-wide NO2

concentrations are associated with the 

risk of respiratory morbidity. Available 

information on NO2 concentration 

gradients around roadways can inform 

estimates of the relationship between 

the area-wide NO2 concentrations 

measured in epidemiologic study 

locations and the higher NO2

concentrations likely to have occurred 

around roads in those locations, which 

can then inform the decision on the 

level of a standard reflecting the 

maximum allowable NO2 concentration 

anywhere in an area. 

• The risk and exposure analyses 

presented in the REA provide 

information on the potential public 

health implications of setting standards 

that limit area-wide NO2 concentrations 

to specific levels. While the 

Administrator acknowledges the 

uncertainties associated with these 

analyses which, as discussed in the 

REA, could result in either over- or 

underestimates of NO2-associated health 

risks, she judges that these analyses are 

informative for considering the relative 

levels of public health protection that 

could be provided by different 

standards. 

The Administrator’s consideration of 

the controlled human exposure 

evidence, epidemiologic evidence, and 

exposure/risk information are discussed 

below specifically with regard to a 

decision on the level of a standard that 

reflects the maximum allowable NO2

concentration anywhere in an area. 

In considering the potential for 

controlled human exposure studies of 

NO2 and airway responsiveness to 

inform a decision on standard level, the 

Administrator notes the following: 

• NO2-induced increases in airway 

responsiveness, as reported in 

controlled human exposure studies, are 

logically linked to the adverse 

respiratory effects that have been 

reported in NO2 epidemiologic studies. 

• The meta-analysis of controlled 

human exposure data in the ISA 

reported increased airway 

responsiveness in a large percentage of 

asthmatics at rest following exposures at 

and above 100 ppb NO2, the lowest NO2

concentration for which airway 

responsiveness data are available in 

humans. 

• This meta-analysis does not provide 

any evidence of a threshold below 

which effects do not occur. The studies 

included in the meta-analysis evaluated 

primarily mild asthmatics while more 

severely affected individuals could 

respond to lower concentrations. 

Therefore, it is possible that exposure to 

NO2 concentrations below 100 ppb 

could increase airway responsiveness in 

some asthmatics. 

In considering the evidence, the 

Administrator recognizes that the NO2- 

induced increases in airway 

responsiveness reported for exposures 

to NO2 concentrations at or above 100 

ppb could be adverse for some 

asthmatics. However, she also notes that 

important uncertainties exist with 

regard to the extent to which NO2- 

induced increases in airway 

responsiveness are adverse. Specifically, 

she notes the following with regard to 

these uncertainties: 

• The magnitude of the NO2-induced 

increase in airway responsiveness, and 

the extent to which it is adverse, cannot 

be quantified from the ISA metaanalysis (REA, section 10.3.2.1). 

• The NO2-induced increase in 

airway responsiveness in resting 

asthmatics was typically not 

accompanied by increased respiratory 

symptoms, even following exposures to 

NO2 concentrations well above 100 ppb 

(ISA, section 3.1.3.3). 

• The increase in airway 

responsiveness that was reported for 

resting asthmatics was not present in 

exercising asthmatics (ISA, Table 3.1–3). 

Taking into consideration all of the 

above, the Administrator concludes that 

existing evidence supports the 

conclusion that the NO2-induced 

increase in airway responsiveness at or 

above 100 ppb presents a risk of adverse 


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20Some of these studies also included susceptible 

and vulnerable populations (e.g., children in Peel 

et al. (2005); poor and minority populations in Ito 

et al., 2007). 

effects for some asthmatics, especially 

those with more serious (i.e., more than 

mild) asthma. The Administrator notes 

that the risks associated with increased 

airway responsiveness cannot be fully 

characterized by these studies, and thus 

she is not able to determine whether the 

increased airway responsiveness 

experienced by asthmatics in these 

studies is an adverse health effect. 

However, based on these studies the 

Administrator concludes that 

asthmatics, particularly those suffering 

from more severe asthma, warrant 

protection from the risk of adverse 

effects associated with the NO2-induced 

increase in airway responsiveness. 

Therefore, the Administrator concludes 

that the controlled human exposure 

evidence supports setting a standard 

level no higher than 100 ppb to reflect 

a cautious approach to the uncertainty 

regarding the adversity of the effect. 

However, those uncertainties lead her to 

also conclude that this evidence does 

not support setting a standard level 

lower than 100 ppb. 

In considering the more serious health 

effects reported in NO2 epidemiologic 

studies, as they relate to the level of a 

standard that reflects the maximum 

allowable NO2 concentration anywhere 

in an area, the Administrator notes the 

following: 

• A cluster of 5 key U.S. 

epidemiologic studies (Ito et al., 2007; 

Jaffe et al., 2003; Peel et al., 2005; 

Tolbert et al., 2007; and a study by the 

New York State Department of Health, 

2006) provide evidence for associations 

between NO2 and respiratory-related 

emergency department visits and 

hospital admissions in locations where 

98th percentile 1-hour daily maximum 

NO2 concentrations measured at areawide monitors ranged from 85 to 94 

ppb. The Administrator judges it 

appropriate to place substantial weight 

on this cluster of key U.S. epidemiologic 

studies in selecting a standard level, as 

they are a group of studies that reported 

positive, and often statistically 

significant, associations between NO2

and respiratory morbidity in multiple 

cities across the United States.20

• A single study (Delfino et al., 2002) 

provides mixed evidence for NO2 effects 

(i.e., respiratory symptoms) in a location 

with a 98th percentile 1-hour daily 

maximum NO2 concentration, as 

measured by an area-wide monitor, of 

50 ppb. In that study, most of the 

reported NO2 effect estimates were 

positive, but not statistically significant. 

Given the variability in the NO2 effect 

estimates in this study, as well as the 

lack of studies in other locations with 

similarly low NO2 concentrations, the 

Administrator judges it appropriate to 

place limited weight on this study, 

compared to the cluster of 5 studies as 

noted above. 

Given these considerations, the 

Administrator concludes that the 

epidemiologic evidence provides strong 

support for setting a standard that limits 

the 98th percentile of the distribution of 

1-hour daily maximum area-wide NO2

concentrations to below 85 ppb. This 

judgment takes into account the 

determinations in the ISA, based on a 

much broader body of evidence, that 

there is a likely causal association 

between exposure to NO2 and the types 

of respiratory morbidity effects reported 

in these studies. Given the 

considerations discussed above, the 

Administrator judges that it is not 

necessary, based on existing evidence, 

to set a standard that maintains peak 

area-wide NO2 concentrations to below 

50 ppb. 

In considering specific standard levels 

supported by the epidemiologic 

evidence, the Administrator notes that a 

level of 100 ppb, for a standard 

reflecting the maximum allowable NO2

concentration anywhere in the area, 

would be expected to maintain areawide NO2 concentrations well below 85 

ppb, which is the lowest 98th percentile 

concentration in the cluster of 5 studies. 

With regard to this, she specifically 

notes the following: 

• If NO2 concentrations near roads are 

100% higher than concentrations away 

from roads, a standard level of 100 ppb 

would limit area-wide concentrations to 

approximately 50 ppb. 

• If NO2 concentrations near roads are 

30% higher than concentrations away 

from roads, a standard level of 100 ppb 

would limit area-wide concentrations to 

approximately 75 ppb. 

The Administrator has also 

considered the NO2 exposure and risk 

information within the context of the 

above conclusions on standard level. 

Specifically, she notes that the results of 

exposure and risk analyses were 

interpreted as providing support for 

limiting area-wide NO2 concentrations 

to no higher than 100 ppb. Specifically, 

these analyses estimated that a standard 

that limits area-wide NO2

concentrations to approximately 100 

ppb or below would be expected to 

result in important reductions in 

respiratory risks, relative to the level of 

risk permitted by the current annual 

standard alone. As discussed above, a 

standard reflecting the maximum 

allowable NO2 concentration with a 

level of 100 ppb would be expected to 

maintain area-wide NO2 concentrations 

to within a range of approximately 50 to 

75 ppb. Given this, the Administrator 

concludes that a standard level of 100 

ppb is consistent with conclusions 

based on the NO2 exposure and risk 

information. 

Finally, the Administrator notes that 

a standard level of 100 ppb is consistent 

with the consensus recommendation of 

CASAC. 

Given the above considerations and 

the comments received on the proposal, 

the Administrator determines that the 

appropriate judgment, based on the 

entire body of evidence and information 

available in this review, and the related 

uncertainties, is a standard level of 100 

ppb (for a standard that reflects the 

maximum allowable NO2 concentration 

anywhere in an area). She concludes 

that such a standard, with the averaging 

time and form discussed above, will 

provide a significant increase in public 

health protection compared to that 

provided by the current annual standard 

alone and would be expected to protect 

against the respiratory effects that have 

been linked with NO2 exposures in both 

controlled human exposure and 

epidemiologic studies. Specifically, she 

concludes that such a standard will 

limit exposures at and above 100 ppb 

for the vast majority of people, 

including those in at-risk groups, and 

will maintain maximum area-wide NO2

concentrations well below those in 

locations where key U.S. epidemiologic 

studies have reported that ambient NO2

is associated with clearly adverse 

respiratory health effects, as indicated 

by increased hospital admissions and 

emergency department visits. 

In setting the standard level at 100 

ppb rather than a lower level, the 

Administrator notes that a 1-hour 

standard with a level lower than 100 

ppb would only result in significant 

further public health protection if, in 

fact, there is a continuum of serious, 

adverse health risks caused by exposure 

to NO2 concentrations below 100 ppb 

and/or associated with area-wide NO2

concentrations well-below those in 

locations where key U.S. epidemiologic 

studies have reported associations with 

respiratory-related emergency 

department visits and hospital 

admissions. Based on the available 

evidence, the Administrator does not 

believe that such assumptions are 

warranted. Taking into account the 

uncertainties that remain in interpreting 

the evidence from available controlled 

human exposure and epidemiologic 

studies, the Administrator notes that the 

likelihood of obtaining benefits to 

public health with a standard set below 


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100 ppb decreases, while the likelihood 

of requiring reductions in ambient 

concentrations that go beyond those that 

are needed to protect public health 

increases. 

Therefore, the Administrator judges 

that a standard reflecting the maximum 

allowable NO2 concentration anywhere 

in an area set at 100 ppb is sufficient to 

protect public health with an adequate 

margin of safety, including the health of 

at-risk populations, from adverse 

respiratory effects that have been linked 

to short-term exposures to NO2 and for 

which the evidence supports a likely 

causal relationship with NO2 exposures. 

The Administrator does not believe that 

a lower standard level is needed to 

provide this degree of protection. These 

conclusions by the Administrator 

appropriately consider the requirement 

for a standard that is neither more nor 

less stringent than necessary for this 

purpose and recognizes that the CAA 

does not require that primary standards 

be set at a zero-risk level or to protect 

the most sensitive individual, but rather 

at a level that reduces risk sufficiently 

so as to protect the public health with 

an adequate margin of safety. 

G. Annual Standard 

In the proposal, the Administrator 

noted that some evidence supports a 

link between long-term exposures to 

NO2 and adverse respiratory effects and 

that CASAC recommended in their 

comments prior to the proposal that, in 

addition to setting a new 1-hour 

standard to increase public health 

protection, the current annual standard 

be retained. CASAC’s recommendation 

was based on the scientific evidence 

and on their conclusion that a 1-hour 

standard might not provide adequate 

protection against exposure to long-term 

NO2 concentrations (Samet, 2008b). 

With regard to an annual standard, 

CASAC and a large number of public 

commenters (e.g., NACAA, NESCAUM; 

agencies from States including CA, IN, 

MO, NC, NY, SC, TX, VA; Tribal 

organizations including Fon du Lac and 

the National Tribal Air Organization; 

environmental/medical/public health 

groups including ACCP, ALA, AMA, 

ATS, CAC, EDF, EJ, GASP, NACPR, 

NAMDRC, NRDC) agreed with the 

proposed decision to maintain an 

annual standard, though their 

recommendations with regard to the 

level of that annual standard differed 

(see below). 

As noted above, CASAC 

recommended ‘‘retaining the current 

standard based on the annual average’’ 

based on the ‘‘limited evidence related 

to potential long-term effects of NO2

exposure and the lack of strong 

evidence of no effect’’ and that ‘‘the 

findings of the REA do not provide 

assurance that a short-term standard 

based on the one-hour maximum will 

necessarily protect the population from 

long-term exposures at levels potentially 

leading to adverse health effects’’ 

(Samet, 2008b). A number of State 

agencies and organizations also 

recommended maintaining the current 

level of the annual standard (i.e., 53 

ppb). This recommendation was based 

on the conclusion that, while some 

evidence supports a link between longterm NO2 exposures and adverse 

respiratory effects, that evidence is not 

sufficient to support a standard level 

either higher or lower than the current 

level. In addition, a number of industry 

groups (e.g., AAM, API, Dow, INGAA, 

UARG) recommended retaining the 

level of the current annual standard but, 

as described above, did so within the 

context of a recommendation that EPA 

should not set a new 1-hour standard. 

In contrast, some environmental 

organizations and medical/public health 

organizations as well as a small number 

of States (e.g., ALA, EDF, EJ, NRDC, and 

organizations in CA) recommended 

setting a lower level for the annual 

standard. These commenters generally 

supported their recommendation by 

pointing to the State of California’s 

annual standard of 30 ppb and to 

studies where long-term ambient NO2

concentrations have been associated 

with adverse respiratory effects such as 

impairments in lung function growth. 

As discussed above (II.B.3), the 

evidence relating long-term NO2

exposures to adverse health effects was 

judged in the ISA to be either 

‘‘suggestive but not sufficient to infer a 

causal relationship’’ (respiratory 

morbidity) or ‘‘inadequate to infer the 

presence or absence of a causal 

relationship’’ (mortality, cancer, 

cardiovascular effects, reproductive/ 

developmental effects) (ISA, sections 

5.3.2.4–5.3.2.6). In the case of 

respiratory morbidity, the ISA (section 

5.3.2.4) concluded that ‘‘The high 

correlation among traffic-related 

pollutants made it difficult to accurately 

estimate the independent effects in 

these long-term exposure studies.’’ 

Given these uncertainties associated 

with the role of long-term NO2

exposures in causing the reported 

effects, the Administrator concluded in 

the proposal that, consistent with the 

CASAC recommendation, existing 

evidence is not sufficient to justify 

setting an annual standard with either a 

higher or lower level than the current 

standard. Commenters have not 

submitted any new analyses or 

information that would change this 

conclusion. Therefore, the 

Administrator does not agree with the 

commenters who recommended a lower 

level for the annual standard. 

The Administrator judges that her 

conclusions in the proposal regarding 

the annual standard remain appropriate. 

Specifically, she continues to agree with 

the conclusion that, though some 

evidence does support the need to limit 

long-term exposures to NO2, the existing 

evidence for adverse health effects 

following long-term NO2 exposures does 

not support either increasing or 

decreasing the level of the annual 

standard. In light of this and 

considering the recommendation from 

CASAC to retain the current level of the 

annual standard, the Administrator 

judges it appropriate to maintain the 

level of the annual standard at 53 ppb. 

H. Summary of Final Decisions on the 

Primary NO2 Standard 

For the reasons discussed above, and 

taking into account information and 

assessments presented in the ISA and 

REA, the advice and recommendations 

of the CASAC, and public comments, 

the Administrator has decided to revise 

the existing primary NO2 standard. 

Specifically, the Administrator has 

determined that the current annual 

standard by itself is not requisite to 

protect public health with an adequate 

margin of safety. In order to provide 

protection for asthmatics and other atrisk populations against an array of 

adverse respiratory health effects related 

to short-term NO2 exposure, the 

Administrator is establishing a shortterm NO2 standard defined by the 3-year 

average of the 98th percentile of the 

yearly distribution of 1-hour daily 

maximum NO2 concentrations. She is 

setting the level of this standard at 100 

ppb, which is to reflect the maximum 

allowable NO2 concentration anywhere 

in an area. In addition to setting a new 

1-hour standard, the Administrator 

retains the current annual standard with 

a level of 53 ppb. The new 1-hour 

standard, in combination with the 

annual standard, will provide protection 

for susceptible groups against adverse 

respiratory health effects associated 

with short-term exposures to NO2 and 

effects potentially associated with longterm exposures to NO2. 

III. Amendments to Ambient 

Monitoring and Reporting 

Requirements 

The EPA is finalizing several changes 

to the ambient air monitoring, reporting, 

and network design requirements for the 

NO2 NAAQS. This section discusses the 

changes we are finalizing which are 

intended to support the proposed 1- 


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21A list of approved FRM and FEMs is 

maintained by EPA’s Office of Research and 

Development, and can be found at: http:// 

www.epa.gov/ttn/amtic/files/ambient/criteria/ 

reference-equivalent-methods-list.pdf. 

hour NAAQS and retention of the 

current annual NAAQS as discussed in 

Section II. Ambient NO2 monitoring 

data are used to determine whether an 

area is in violation of the NO2 NAAQS. 

Ambient NO2 monitoring data are 

collected by State, local, and Tribal 

monitoring agencies (‘‘monitoring 

agencies’’) in accordance with the 

monitoring requirements contained in 

40 CFR parts 50, 53, and 58. 

A. Monitoring Methods 

We are finalizing the proposed 

changes regarding the NO2 Federal 

Reference Method (FRM) or Federal 

Equivalent Method (FEM) analyzers. 

Specifically, we are continuing to use 

the NO2 chemiluminescence FRM and 

are finalizing the requirement that any 

NO2 FRM or FEM used for making 

primary NAAQS decisions must be 

capable of providing hourly averaged 

concentration data. The following 

paragraphs provide background and 

rationale for the continued use of the 

chemiluminescence FRM and the 

decision to finalize the proposed 

changes. 

1. Chemiluminescence FRM and 

Alternative Methods 

The current monitoring method in use 

by most State and local monitoring 

agencies is the gas-phase 

chemiluminescence FRM (40 CFR Part 

50, Appendix F), which was 

implemented into the NO2 monitoring 

network in the early 1980s. EPA did not 

propose to discontinue using the 

chemiluminescence FRM, although we 

received some comments from industry 

(Alliance of Automobile Manufacturers, 

Edison Electric, and the National 

Petrochemical and Refiners Association) 

raising concerns about using a method 

that is subject to known interferences 

from certain species of oxides of 

nitrogen known as NOZ. Important 

components of ambient NOZ include 

nitrous acid (HNO2), nitric acid (HNO3), 

and the peroxyacetyl nitrates (PANs). 

The issue of concern in public 

comments is that the reduction of NO2

to NO on the MoOX converter substrate 

used in chemiluminescence FRMs is not 

specific to NO2; hence, 

chemiluminescence method analyzers 

are subject to varying interferences 

produced by the presence in the air 

sample of the NOZ species listed above 

and others occurring in trace amounts in 

ambient air. This interference is often 

termed a ‘‘positive artifact’’ in the 

reported NO2 concentration since the 

presence of NOZ results in an overestimate in the reported measurement of 

the actual ambient NO2 concentration. 

This interference by NOZ compounds 

has long been known and evaluated 

(Fehsenfeld et al., 1987; Nunnermacker 

et al., 1998; Parrish and Fehsenfeld, 

2000; McClenny et al., 2002; U.S. 

Environmental Protection Agency, 1993, 

2006a). Further, as noted in the ISA 

(ISA Section 2.3), it appears that 

interference by NOZ on 

chemiluminescence FRMs is not more 

than 10 percent of the reported NO2

concentration during most or all of the 

day during winter (cold temperatures), 

but larger interference ranging up to 70 

percent can be found during summer 

(warm temperatures) in the afternoon at 

sites away and downwind from strong 

emission sources. 

The EPA acknowledges that the NOZ

interference in the reported NO2

concentrations collected well 

downwind of NOX source areas and in 

relatively remote areas away from 

concentrated point, area, or mobile 

sources is significantly larger than the 

NOZ interference in NO2 measurements 

taken in urban cores or other areas with 

fresh NOX emissions. To meet the 

primary objective of monitoring 

maximum NO2 concentrations in an 

area, the EPA is requiring NO2 monitors 

to be placed in locations of the expected 

highest concentrations, not in relatively 

remote areas away from NOX sources. 

The required monitors resulting from 

the network design discussed below in 

Section III.B will require monitors to be 

placed near fresh NOX sources or in 

areas of dense NOX emissions, where 

NO2 concentrations are expected to be at 

a maximum, and interference from NOZ

species is at a minimum. Therefore, EPA 

believes that the positive artifact issue, 

although present, is small, relative to 

the actual NO2 being measured. As a 

result EPA believes the 

chemiluminescence FRM is suitable for 

continued use in the ambient NO2

monitoring network, as the potential 

positive bias from NOZ species is not 

significant enough to discontinue using 

the chemiluminescence FRM. 

EPA also received support from some 

industry groups (e.g. Savannah River 

Nuclear Solutions, Teledyne API, and 

the Utility Air Regulatory Groups) and 

States (e.g., MODEQ and NCDENR) to 

further the development of alternative 

methods in determining NO2

concentrations. Such alternative 

methods include the photolyticchemiluminescence method and cavity 

ring-down spectroscopy. As a result, 

EPA will continue working with 

commercial and industrial vendors, to 

identify and evaluate such new 

technologies. These efforts may include 

field testing instruments and further 

characterizing methods in a laboratory 

setting to assess their potential as future 

reference or equivalent methods, and 

their role in more directly measuring 

NO2. 

2. Allowable FRM and FEMs for 

Comparison to the NAAQS 

The current CFR language does not 

prohibit the use of any particular NO2

FRM or FEM to be used in comparison 

to the standard.21 There are designated 

wet chemical methods that are only able 

to report ambient concentration values 

averaged across multiple hours. With 

the establishment of a 1-hour NAAQS, 

any FRM or FEM which is a wet 

chemical based method would not be 

appropriate for use in determining 

compliance of the 1-hour NAAQS 

because they are unable to report hourly 

data. EPA addressed this issue by 

proposing and finalizing that only those 

methods capable of providing 1-hour 

measurements will be comparable to the 

NAAQS. 

a. Proposed Changes to FRM and FEMs 

That May Be Compared to the NAAQS 

EPA proposed that only those FRMs 

or FEMs that are capable of providing 

hourly averaged concentration data may 

be used for comparison to the NAAQS. 

b. Comments 

EPA received comments from some 

State and industry groups (e.g. Missouri, 

North Carolina, and Air Quality 

Research and Logistics) supporting the 

proposed approach to only allowing 

those FRMs or FEMs that are capable of 

providing hourly averaged 

concentration data may be used for 

comparison to both the annual and 1- 

hour NAAQS, and did not receive any 

public comments that objected to the 

proposed approach. 

c. Decisions on Allowable FRM and 

FEMs for Comparison to the NAAQS 

Accordingly, EPA is finalizing the 

proposed changes to 40 CFR Part 58 

Appendix C to allow only data from 

FRM or FEMs that are capable of 

providing hourly data to be used for 

comparison to both the annual and 1- 

hour NAAQS. 

B. Network Design 

With the establishment of a 1-hour 

NO2 NAAQS intended to limit exposure 

to maximum concentrations that may 

occur anywhere in an area, EPA 

recognizes that the data from the current 

NO2 network is inadequate to fully 

assess compliance with the revised 


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NAAQS. As a result, EPA is 

promulgating new NO2 network design 

requirements. The following sections 

provide background, rationale, and 

details for the final changes to the NO2

network design requirements. 

1. Two-Tiered Network Design 

A two-tiered monitoring network is 

appropriate for the NO2 NAAQS 

because one tier (the near-road network) 

reflects the much higher NO2

concentrations that occur near-road and 

the second-tier (area-wide) characterizes 

the NO2 concentrations that occur in a 

larger area such as neighborhood or 

urban areas. The ISA (Section 2.5.4 and 

4.3.6) stated that NO2 concentrations in 

heavy traffic or on freeways ‘‘can be 

twice the residential outdoor or 

residential/arterial road level,’’ that 

‘‘exposure in traffic can dominate 

personal exposure to NO2,’’ and that 

‘‘NO2 levels are strongly associated with 

distance from major roads (i.e., the 

closer to a major road, the higher the 

NO2 concentration).’’ The exposure 

assessment presented in the REA 

estimated that roadway-associated 

exposures account for the majority of 

exposures to peak NO2 concentrations 

(REA, Figures 8–17, 8–18). Monitoring 

studies suggest that NO2 concentrations 

near roads can be considerably higher 

than those in the same area but away 

from the road (e.g., by 30–100%, see 

section II.A.2), where pollutants 

typically display peak concentrations on 

or immediately adjacent to roads, 

producing a gradient in pollutant 

concentrations where concentrations 

decrease with increasing distance from 

roads. Since the intent of the revised 

NAAQS is to limit exposure to peak 

NO2 concentrations that occur anywhere 

in an area, monitors intended to 

measure the maximum allowable NO2

concentration in an area should include 

measurements of the peak 

concentrations that occur on and near 

roads due to on-road mobile sources. 

The first tier of the network design, 

which focuses monitoring near highly 

trafficked roads in urban areas where 

peak NO2 concentrations are likely to 

occur, is intended to measure maximum 

concentrations anywhere in an area, 

particularly those due to on-road mobile 

sources since roadway-associated 

exposures account for the majority of 

exposures to peak NO2 concentrations. 

The basis for the second tier of the 

network design is to measure the 

highest area-wide concentrations to 

characterize the wider area impact of a 

variety of NO2 sources on urban 

populations. Area-wide monitoring of 

NO2 also serves to maintain continuity 

in collecting data to inform long-term 

pollutant concentration trends analysis 

and support ongoing health and 

scientific research. 

This section discusses the two-tier 

network design approach compared to 

the alternative network design which 

was also presented for comment in 

conjunction with a solicitation for 

comment on an alternative NAAQS. The 

alternative network design concept was 

based entirely on requiring only 

monitors that would be considered areawide, while not requiring any near-road 

monitoring sites. The details of the twotier network design, including how 

many monitors are required, where they 

are to be located, and the related siting 

criteria are discussed in subsequent 

sections. 

a. Proposed Two-Tier Network Design 

EPA proposed a two-tier network 

design composed of (1) near-road 

monitors which would be placed in 

locations of expected maximum 1-hour 

NO2 concentrations near heavily 

trafficked roads in urban areas and (2) 

monitors located to characterize areas 

with the highest expected NO2

concentrations at the neighborhood and 

larger spatial scales (also referred to as 

‘‘area-wide’’ monitors). As an alternative, 

and in conjunction with a solicitation 

for comment on an alternative NAAQS, 

EPA solicited comment on a network 

comprised of only area-wide monitors. 

b. Comments 

EPA received many comments on the 

overall two-tier network design, with 

those who made statements with a 

relatively clear position on the issue 

generally falling into four categories: (1) 

Those who support the adoption of the 

proposed two-tier design approach, (2) 

those who support the adoption of the 

two-tier concept, but with 

modifications, (3) those who only 

supported the adoption of the 

alternative network design, and (4) 

those who encourage EPA to commit to 

further research of the near-road 

environment by monitoring near-roads, 

but not to use near-road data for 

regulatory purposes, and therefore 

support the alternative network design 

in which EPA solicited comment on a 

network design composed only of areawide monitors. 

Those commenters who generally 

supported the proposed two-tier 

network, included CASAC (while there 

was not a consensus, a majority were in 

support of the proposed network 

design), public health organizations 

(e.g., AACPR, ACCP, AMA, ATA, and 

NAMDRC), several State groups (e.g., 

the New York City Law Department and 

the Metropolitan Washington Air 

Quality Committee), and some industry 

commenters (e.g., American Chemistry 

Council, The Clean Energy Group, and 

Dow Chemical). 

Those commenters who supported the 

adoption of the two-tier network design 

concept, but suggested modifications to 

the actual design included some health 

and environmental organizations (e.g., 

ALA, EDF, EJ, and the NRDC), some 

States (e.g., California, the Central 

Pennsylvania Clean Air Board, Harris 

County (Texas), Iowa, New York, San 

Joaquin Air Pollution Control District, 

Spokane Regional Clean Air Agency 

(SRCAA), the Texas Commission on 

Environmental Quality, and Wisconsin), 

and some industry commenters, 

including the American Petroleum 

Institute and the Utility Air Regulatory 

Group, who are cited by other industry 

commenters. We believe that although 

these commenters made suggestions to 

modify the proposed two-tier network 

design, they are indicating that it is an 

acceptable approach. Their comments 

and suggestions are discussed in greater 

detail in the following sections. 

Those commenters who only 

supported the adoption of the 

alternative network design included 

State and industry groups (e.g., Indiana 

Department of Environmental 

Management, the New York Department 

of Transportation (NYSDOT), Alliance 

of Automobile Manufacturers, and the 

Engine Manufacturers Association). 

These commenters typically made 

comments on the two-tier network 

design, but did not do so in a way that 

clearly supported near-road research. 

EPA received comments from some 

States or State organizations (e.g., 

National Association of Clean Air 

Agencies (NACAA), the Northeast States 

for Coordinated Air Use Management 

(NESCAUM), and 10 other individual 

States or State groups) and industry 

commenters (e.g., Consumers Energy, 

Edison Electric, and the National 

Association of Manufacturers) that 

encouraged EPA to further research the 

near-road environment, opposing use of 

near-road monitoring data for regulatory 

purposes, and supported the adoption of 

the alternative network design for 

regulatory purposes. For example, with 

regard to implementing the two-tier 

network design that includes near-road 

regulatory monitoring, NACAA stated 

that ‘‘* * *a major new network— 

particularly one that is inherently 

complicated and untried—should not be 

rolled out without the benefit of an 

effective near-road monitoring research 

program that can address many of the 

relevant data questions, and inform the 

specific siting requirements of the rule.’’ 

The NAM stated that ‘‘conducting such 


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a near road [research] monitoring 

program would allow EPA to collect 

necessary data that can be used to better 

understand the health impacts 

associated with short term NO2

exposures.’’ 

The EPA notes that the existing 

scientific research referenced in the 

proposal and throughout this final rule 

show that there are on- and near-road 

peaks of NO2 concentrations, relative to 

upwind or background levels, which 

exist due to on-road mobile source 

emissions. This research, as a body of 

evidence, also identifies the multiple 

local factors that affect how, where, and 

when peak NO2 concentrations occur on 

or near a particular road segment. These 

factors include traffic volume, fleet mix, 

roadway design, congestion patterns, 

terrain, and meteorology. The EPA and 

States have access to such data typically 

through Federal, State, and/or local 

departments of transportation or other 

government organizations, and, as a 

result, are in a position to implement a 

near-roar monitoring network that is 

intended to measure maximum 

expected NO2 concentrations resulting 

from on-road mobile source emissions. 

Further, EPA notes that near-road 

monitoring is not a new objective for the 

ambient air monitoring community as 

near-road carbon monoxide monitoring 

has been a part of ongoing, long-term, 

routine networks for nearly three 

decades. As a result, there is experience 

within EPA (both OAR and ORD) and 

State and local agencies on conducting 

ambient monitoring near-roads. In 

addition, EPA intends to develop 

guidance with input from all 

stakeholders to assist with 

implementation of the monitoring 

requirements, which is discussed in 

section III.B.5. EPA believes that the 

existing science and research provide a 

sufficient base of information to require 

a near-road monitoring network and that 

the collective experience that exists in 

the ambient monitoring community will 

allow for successful implementation of 

that network. EPA also believes that 

through adherence of requirements for 

near-road site selection and siting 

criteria discussed in sections III.B.6 and 

III.B.7, respectively, that the two-tier 

network design will provide a network 

that has a reasonable degree of 

similarity across the country where the 

required near-road monitors are 

targeting the maximum NO2

concentrations in an area attributable to 

on-road mobile sources. 

Some industry commenters (e.g., 

Engine Manufacturers Association, the 

South Carolina Chamber of Commerce, 

and the South Carolina Manufacturers 

Alliance) who supported the adoption 

of the alternative network design 

suggested that monitoring in the nearroad environment would not be 

indicative of exposure for general 

populations, and that EPA should not 

focus on the near-road environment 

when requiring monitoring. For 

example, the South Carolina Chamber of 

Commerce and the South Carolina 

Manufacturers Alliance both state that 

‘‘it appears the proposed monitoring 

network will result in a collection of 

microscale data, which is not at all 

representative of air quality relevant to 

population exposure.’’ 

The EPA notes that the intent of a 

near-road monitoring is to support the 

revised NAAQS by assessing peak NO2

concentrations that may occur anywhere 

in an area. EPA recognizes that there is 

variability in the properties (such as 

traffic counts, fleet mix, and localized 

features) among the road segments that 

may exist in an area, but on the whole, 

roads are ubiquitous, particularly in 

urban environments. Consequently, a 

substantial fraction of the population is 

potentially exposed to relatively higher 

concentrations of NO2 that can occur in 

the near-road environment. The 2007 

American Housing Survey (http:// 

www.census.gov/hhes/www/housing/ 

ahs/ahs07/ahs07.html) estimates that 

over 20 million housing units are within 

300 feet (91 meters) of a 4-lane highway, 

airport, or railroad. Using the same 

survey, and considering that the average 

number of residential occupants in a 

housing unit is approximately 2.25, it is 

estimated that at least 45 million 

American citizens live near 4-lane 

highways, airports, or railroads. 

Although that survey includes airports 

and railroads, roads are the most 

pervasive of the three, indicating that a 

significant amount of the general 

population live near roads. 

Furthermore, the 2008 American Time 

Use Survey (http://www.bls.gov/tus/) 

reported that the average U.S. civilian 

spent over 70 minutes traveling per day. 

Accordingly, EPA concludes that 

monitors near major roads will address 

a component of exposure for a 

significant portion of the general 

population that would otherwise not be 

addressed. 

The majority of State commenters, 

regardless of their position on the 

proposed network design, along with 

some industry commenters, observed 

that there was a need for funding the 

monitoring network. These comments 

urged EPA to provide the resources 

needed to implement and operate the 

required monitoring network. EPA notes 

that it has historically funded part of the 

cost of the installation and operation of 

monitors used to satisfy Federal 

monitoring requirements. EPA 

understands these concerns, although 

the CAA requirements from which this 

final rule derives (CAA sections 110, 

310(a) and 319) are not contingent on 

EPA providing funding to States to 

assist in meeting monitoring 

requirements. However, EPA intends to 

work with NACAA and the State and 

local air agencies in identifying 

available State and Tribal Air Grant 

(STAG) funds and consider the 

increased resource needs that may be 

needed to plan, implement, and operate 

this revised set of minimum 

requirements. 

c. Conclusions Regarding the Two-Tier 

Network Design 

The EPA believes that requiring nearroad monitors in urban areas as part of 

the network design are necessary to 

protect against risks associated with 

exposures to peak concentrations of NO2

anywhere in an area. The combination 

of increased mobile source emissions 

and increased urban population 

densities can lead to increased 

exposures and associated risks, 

therefore urban areas are the appropriate 

areas to concentrate required near-road 

monitoring efforts. The EPA also 

recognizes the need to have monitors in 

neighborhood and larger spatial scale 

locations away from roads that represent 

area-wide concentrations. These types 

of monitors serve multiple important 

monitoring objectives including 

comparison to the NAAQS, 

photochemical pollutant assessment, 

ozone forecasting, characterization of 

point and area source impacts, and by 

providing historical trends data for 

current and future epidemiological 

health research. In some situations, 

when coupled with data from near-road 

monitors, area-wide monitors may also 

assist in the determination of spatial 

variation of NO2 concentrations across a 

given area and provide insight to the 

gradients that exist between near-road 

or stationary source oriented 

concentrations and area-wide 

concentration levels. 

After considering the scientific data 

and the public comments regarding the 

proposed network design, the 

Administrator concludes that a two-tier 

network design composed of (1) nearroad monitors which would be placed 

in locations of expected maximum 1- 

hour NO2 concentrations near heavily 

trafficked roads in urban areas and (2) 

monitors located to characterize areas 

with the maximum expected NO2

concentrations at the neighborhood and 

larger spatial scales (also referred to as 

‘‘area-wide’’ monitors) are needed to 

implement the 1-hour NO2 NAAQS and 


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22Of the 24 additional sites, 22 are estimated to 

be triggered due to a population of 2,500,000 while 

2 (Las Vegas, NV and Sacramento, CA) are 

estimated to be triggered by the presence of one or 

more road segments with 250,000 AADT since they 

do not have a population of 2,500,000 people. 

23AASHTO, NESCAUM, and NYDOT did not 

support the two-tier network design; however they 

provided suggestions on how the network design 

might be modified if the EPA were to finalize 

requirements for near-road monitors. In the case of 

AASHTO and NYDOT, their suggestions were made 

with the suggestion that EPA use a separate 

rulemaking process to require monitors. 

support the annual NAAQS. The details 

of this two-tier network design are 

discussed in the following eight 

sections. 

2. First Tier (Near-Road Monitoring 

Component) of the NO2 Network Design 

This section provides background, 

rationale, and details for the final 

changes to the first tier of the two-tier 

NO2 network design. In particular, this 

section will focus on the thresholds that 

trigger monitoring requirements. Nearroad site selection and siting criteria 

details will be discussed in subsequent 

sections. 

a. Proposed First Tier (Near-Road 

Monitoring Component) of the Network 

Design 

EPA proposed that the first tier of the 

two-tier NO2 monitoring network design 

focus monitors in locations of expected 

maximum 1-hour concentrations near 

major roads in urban areas. As noted in 

the previous section, the exposure 

assessment presented in the REA 

estimated that roadway-associated 

exposures account for the majority of 

exposures to peak NO2 concentrations 

(REA, Figures 8–17, 8–18). Since the 

combination of increased mobile source 

emissions and increased urban 

population densities leads to increased 

exposures and associated risks, the 

Administrator judges that urban areas 

are the appropriate areas in which to 

concentrate required near-road 

monitoring efforts. Therefore, we 

proposed that a minimum of one nearroad NO2 monitor be required in Core 

Based Statistical Areas (CBSAs) with a 

population greater than or equal to 

350,000 persons. Based on 2008 Census 

Bureau statistics, EPA estimated this 

would result in approximately 143 

monitoring sites in as many CBSAs. 

We also proposed that a second nearroad monitor be required in CBSAs with 

a population greater than or equal to 

2,500,000 persons, or in any CBSAs 

with one or more road segments with an 

Annual Average Daily Traffic (AADT) 

count greater than or equal to 250,000. 

Based on 2008 Census Bureau statistics 

and data from the 2007 Highway 

Performance Monitoring System 

(HPMS) maintained by the U.S. 

Department of Transportation (DOT) 

Federal Highway Administration 

(FHWA), this particular element of the 

minimum monitoring requirements 

would have added approximately 24 22

sites to the approximate 143 near-road 

sites in CBSAs that already would have 

had one near-road monitor required due 

to the 350,000 population threshold. 

Overall, the first tier of the proposed 

network design was estimated to require 

167 near-road sites in 143 CBSAs. 

b. Comments 

The EPA received comments from 

some industry and public health 

organizations (e.g. Dow Chemical, ATS, 

and the AMA) supporting the proposed 

approach to use population thresholds 

for triggering minimum near-road 

monitoring requirements. For example, 

Dow Chemical Company stated that 

‘‘Dow comments that the proposed 

population thresholds are reasonable for 

implementation of the new network 

design and that we don’t see a need to 

establish a threshold lower than 350,000 

people for the lower bound.’’ 

The EPA received comments from 

some States and State groups suggesting 

that a combination of population and 

AADT counts or just AADT counts 

should be used to trigger minimum 

near-road monitoring requirements. For 

example, the San Joaquin Air Pollution 

Control District in California suggested 

that we modify minimum monitoring 

requirements so that one near-road NO2

monitor is required for any CBSA with 

a population of 350,000 people which 

also had one or more road segments 

with AADT counts of 125,000 or more. 

In another example, Harris County 

Public Health and Environmental 

Services (HCPHES) suggested that 

‘‘* * * rather than specifying 

population limits for the monitoring, 

HCPHES supports a metric like the 

Annual Average Daily Traffic (AADT) as 

a threshold for requiring a near-road 

monitor. An initial focus on an AADT 

in excess of 250,000 is acceptable as a 

starting point but EPA should revisit 

that level and consider lowering it to 

100,000 in five years.’’ AASHTO 23 and 

NYDOT 23 suggested that EPA could set 

a threshold at 140,000 AADT for 

requiring near-road monitors rather than 

using population thresholds. 

EPA is finalizing the population-only 

threshold approach to trigger near-road 

monitoring, as the first step in the 

process of establishing the first-tier of 

near-road monitors, and for identifying 

the appropriate number and locations 

for siting these monitors. EPA believes 

that the uncertainty in defining specific 

national AADT counts is too great to 

support use in this first step of the 

alternative approaches suggested by the 

commenters. EPA notes that, in general, 

roads with higher AADT counts have 

relatively higher amounts of mobile 

source emissions, leading to an 

increased potential for relatively higher 

on-road and roadside NO2

concentrations. This concept is 

supported, for example, by Gilbert et al., 

2007, who state that the NO2

concentrations analyzed in their study 

are significantly associated with traffic 

counts. In part, these suggestions by 

commenters to include AADT counts as 

part of, or independently as, a threshold 

for requiring monitors appears to be 

aimed at increasing the focus of the 

near-road network to locations where 

NO2 concentrations are expected to be 

highest. However these suggestions 

would also, in effect, reduce the size of 

the required network compared to the 

network that EPA had proposed. The 

differences in fleet mix, roadway design, 

congestion patterns, terrain, and local 

meteorology amongst road segments that 

may have identical AADTs are quite 

variable and affect the NO2

concentrations on and near those 

segments. The available data and related 

technical and scientific quantification of 

what particular AADT count might be 

expected to contribute to some specific 

NO2 concentration is insufficient to 

establish a specific, nationally 

applicable AADT count threshold that 

could be used as part of a populationAADT combination, or a distinct AADT 

count, to require all near-road monitors. 

Therefore, EPA chose not to utilize a 

population-AADT or an AADT-only 

threshold to trigger all minimally 

required near-road monitoring because 

of the lack of a quantitative, nationally 

applicable relationship between a 

certain AADT threshold and an 

expected NO2 concentration. Instead, 

EPA is finalizing the proposed 

population-only threshold approach to 

trigger a minimum of one monitor in a 

CBSA. In larger CBSAs, EPA does 

require, at a minimum, a second 

monitor based on either an AADT count 

of 250,000 or a population threshold of 

2,500,00 or more persons in a CBSA as 

described more fully below. EPA 

believes this approach for siting nearroad monitoring provides a greater 

degree of certainty in covering a large 

segment of the total population (66%, 

which is explained below) and will 

provide data on exposure from 

geographically and spatially diverse 

areas where a larger number of people 


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are likely to be exposed to peak NO2

concentrations. 

Some commenters (e.g., AASHTO,23

NESCAUM,23 NYDEC, NYDOT23) 

suggested focusing multiple near-road 

monitors only in relatively larger CBSAs 

than those which were proposed. For 

example, NYDEC suggested that EPA 

require, at minimum, two near-road 

monitors in any CBSA of 2,500,000 

people or more, but not in CBSAs below 

that population threshold. In their 

comments, they point out the variety of 

near-road environments that exist in the 

larger CBSAs such as New York City. 

EPA notes that the larger CBSAs, such 

as those with a population of 2,500,000 

or more persons, are more likely to have 

a greater number of major roads across 

a potentially larger geographic area, and 

a corresponding increase in potential for 

exposure in different settings 

(evidenced in the U.S. Department of 

Transportation (U.S. DOT) Federal 

Highway Administration (FHWA) 

‘‘Status of the Nation’s Highways, 

Bridges, and Transit: 2006 Conditions 

and Performance’’ document which is 

discussed below). This is the primary 

reasoning behind the requirement for 

two monitors in CBSAs with more than 

2,500,000 people. EPA also believes that 

having multiple monitors in the largest 

CBSAs will allow better understanding 

of the differences that may exist 

between roads in the same CBSA due to 

fleet mix, congestion patterns, terrain, or 

geographic locations. However, EPA 

believes that a network with 

substantially fewer monitors in 

correspondingly fewer CBSAs, as the 

commenters suggested, would lead to an 

insufficient monitoring network lacking 

a balanced approach needed for a 

regulatory network intended to support 

the revised NAAQS on a national basis. 

On a related note to those comments 

that suggested focusing more near-road 

monitors only in the larger CBSAs, EPA 

proposed that any CBSAs with one or 

more road segments with an Annual 

Average Daily Traffic (AADT) count 

greater than or equal to 250,000 must 

have a second monitor if they do not 

already have two near-road monitors 

because of the population threshold. 

Such an AADT-triggered monitor would 

account for situations where a relatively 

less populated area has a very highly 

trafficked road. In this case, EPA notes 

that because those road segments with 

250,000 AADT have been identified by 

U.S. DOT FHWA (http:// 

www.fhwa.dot.gov/policyinformation/ 

tables/02.cfm) as being the top 0.03 

percent of the most traveled public road 

segments, that they are the most heavily 

trafficked roads in the country. Again 

noting that NO2 concentrations are 

significantly associated with traffic 

counts (Gilbert et al. 2007), these roads 

segments likely have the greatest 

potential for high exposures directly 

connected to motor vehicle emissions in 

the entire country. Typically, these very 

highly trafficked roads are in the largest 

populated CBSAs, such as those with 

2,500,000 people or more, and are 

somewhat atypical for CBSAs with less 

than 2,500,000 people. As a result, EPA 

believes it is appropriate to require a 

second monitor in a CBSA that has one 

or more road segments with 250,000 

AADT counts or more if they do not 

already have two near-road monitors 

required due their population. 

EPA received comments requesting 

that EPA explain the rationale for the 

selection of the population thresholds 

that trigger minimum monitoring 

requirements and also to reconsider the 

size of the network. For example, 

NYDOT suggested that this final rule 

explain the basis for the 350,000 and 

2,500,000 population thresholds that 

will establish near-road monitors. In 

another comment, the Clean Air Council 

(CAC) questioned the selected 

population thresholds, noting that they 

believe that the population thresholds 

that were proposed were too high. 

Specifically, CAC stated that ‘‘at 350,000 

persons, numerous metro areas in the 

mid-Atlantic and Northeastern States 

with urban cores and highways running 

through will likely be exempted from 

the new monitors.’’ The Spokane 

Regional Clean Air Agency stated that 

they ‘‘do not believe it is necessary to 

require air quality monitoring for NO2

near major roadways in every 

metropolitan area. It is our [SRCAA’s] 

view that EPA could establish a 

statistically significant number of air 

quality monitoring stations near 

roadways and develop a correlation 

between traffic density and ambient 

NO2 levels.’’ Further, the EPA received 

many State comments suggesting 

reductions to the overall size of the 

near-road network; however the 

commenters did not provide very 

specific suggestions on how EPA should 

accomplish that reduction in size. For 

example, the Regional Air Pollution 

Control Agency, which represents a 

portion of Ohio, stated ‘‘given the fairly 

standard fleet of vehicles on the nation’s 

major highways, we urge EPA to 

consider the need for 142 near-roadway 

monitors. Perhaps a limited number of 

monitors across the country would 

suffice to sufficiently characterize nearroadway NO2 levels.’’ These State 

commenters provided various reasons 

which are discussed throughout this 

document suggesting that the network 

be reduced in size, including funding 

concerns (section III.B.1.b), the 

perceived need to implement a smaller 

near-road research network in lieu of a 

regulatory network (section III.B.1.b), 

safety issues (section III.B.7.b), and 

problems with State implementation 

plans (section VI. D) and designation 

issues (section V). 

EPA notes that the intent of the first 

tier of the network design is to support 

the revised NAAQS in measuring peak 

NO2 exposures in an area by including 

a minimum number of monitors 

resulting in a sufficiently sized national 

near-road monitoring network that will 

provide data from a geographically and 

spatially diverse array of areas, in terms 

of population, potential fleet mixes, 

geographic extent, and geographic 

setting, from across the country. The 

U.S. Department of Transportation (U.S. 

DOT) Federal Highway Administration 

(FHWA) ‘‘Status of the Nation’s 

Highways, Bridges, and Transit: 2006 

Conditions and Performance’’ document 

(http://www.fhwa.dot.gov/policy/ 

2006cpr/es02h.htm) states that ‘‘while 

urban mileage constitutes only 24.9 

percent of total (U.S.) mileage, these 

roads carried 64.1 percent of the 3 

trillion vehicles miles (VMT) travelled 

in the United States in 2004.’’ The 

document also states that ‘‘urban 

interstate highways made up only 0.4 

percent of total (U.S.) mileage but 

carried 15.5 percent of total VMT.’’ 

These statements indicate how much 

more traffic volume exists on roads in 

urban areas versus the more rural areas 

that have significant amounts mileage of 

the total public road inventory. The 

basis for the selection of the proposed 

CBSA population level of 350,000 to 

trigger the requirement of one near-road 

monitor was chosen in an attempt to 

provide near-road monitoring data from 

a diverse array of areas, as noted above. 

However, in response to the significant 

number of comments discussed above, 

which in various ways encouraged at 

least a reduction of the size of the 

required near-road network or the 

implementation of a relatively smaller 

research network, EPA reconsidered the 

population threshold that will require 

one near-road NO2 monitor in a CBSA. 

EPA reviewed the data, such as 

population, geographic, and spatial 

distribution, associated with particular 

CBSA areas that would and would not 

be included in particular CBSA 

population thresholds. According to the 

2008 U.S. Census Bureau estimates 

(http://www.census.gov) there are 143 

CBSAs with 350,000 or more persons 

(including territories) which contain 

approximately 71% of the total 

population (excluding territories). These 


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CBSAs collectively represent territory in 

44 States, the District of Columbia, and 

Puerto Rico. For comparison, there are 

391 CBSAs with 100,000 or more 

persons, which contain approximately 

86% of the total population (excluding 

territories). These particular CBSAs 

collectively represent territory in 49 

States, the District of Columbia, and 

Puerto Rico. Further, there are 102 

CBSAs with 500,000 or more persons, 

which contain approximately 66% of 

the total population (excluding 

territories). These 102 CBSAs 

collectively represent territory in 43 

States, the District of Columbia, and 

Puerto Rico. Finally, there are 22 CBSAs 

with 2,500,000 or more persons, which 

contain approximately 39% of the total 

population, collectively representing 

territory in 19 States, the District of 

Columbia, and Puerto Rico. In 

comparison to the CBSA population 

threshold of 350,000, the 500,000 

population threshold has 41 less CBSAs. 

However, the percentage of the total 

U.S. population residing in these two 

sets of CBSAs differs by only 

approximately 5 percent of the total 

population (e.g., 71% in CBSAs of 

350,000 or more versus 66% in CBSAs 

of 500,000 or more persons). Also, when 

comparing the number of States that 

have some amount of their territory 

included in these CBSAs, the difference 

between the two sets of CBSAs differs 

by only 1 State (Alaska). 

Further, EPA notes that the REA Air 

Quality Analysis, (REA, section 7.3.2) 

estimated the exceedences of health 

benchmark levels across the United 

States, including explicit consideration 

of on- or near- roadway exceedances in 

17 urban areas associated with CBSA 

populations ranging from approximately 

19,000,000 to 540,000. The analysis 

indicated that all 17 of the areas under 

explicit consideration were estimated to 

experience NO2 concentrations on or 

near roads that exceeded health 

benchmark levels. 

c. Conclusions Regarding the First Tier 

(Near-Road Monitoring Component) of 

the Network Design 

After consideration of public 

comments, and in light of the 

information discussed above, the 

Administrator has chosen to finalize the 

CBSA population threshold for 

requiring a minimum of one near-road 

monitor in CBSAs with a population of 

500,000 or more persons. The 

Administrator is finalizing the other 

thresholds that will trigger a second 

near-road monitor as proposed. 

Accordingly, one near-road NO2

monitor is required in CBSAs with a 

population greater than or equal to 

500,000 persons and a second near-road 

monitor is required in CBSAs with a 

population greater than or equal to 

2,500,000 persons, or in any CBSAs 

with one or more road segments with an 

Annual Average Daily Traffic (AADT) 

count greater than or equal to 250,000. 

The Administrator has concluded that 

using a population threshold of 500,000 

to require a minimum of one near-road 

monitor in a CBSA provides a 

sufficiently sized, national network of 

near-road monitors that will provide 

data from a geographically and spatially 

diverse set of CBSAs that supports the 

intent of the revised NAAQS and 

continues to meet the monitoring 

objectives of the network. Combined 

with the forty additional monitors that 

the Regional Administrators are 

required to site, discussed below, the 

monitoring network would cover an 

additional percentage of the total 

population. 

EPA believes that selecting a lower 

population threshold, such as 100,000 

or, to a lesser degree, 350,000, as 

discussed in the above examples, would 

create a much larger network of required 

near-road monitors but would provide 

diminished population coverage per 

monitor, compared to that provided by 

the 500,000 threshold. EPA notes that if 

a particular area, such as one with a 

population less than 500,000 people, 

might warrant a near-road monitor, the 

Regional Administrator has the 

authority to require additional monitors. 

The Regional Administrators’ authority 

is discussed in section III.B.4. Further, 

States have the right to conduct 

additional monitoring above the 

minimum requirements on their own 

initiative. In the Administrator’s 

judgment, selecting a higher threshold, 

such as 2,500,000, as was suggested by 

some commenters, does not provide a 

sufficient geographical and spatially 

diverse near-road network, compared to 

that provided by the 500,000 threshold. 

The selection of the 2,500,000 

population threshold to trigger a second 

near-road monitor, as noted earlier in 

this section, is based on the fact that the 

larger urban areas in the country are 

likely to have a greater number of major 

roads across a potentially larger 

geographic area, and have a 

corresponding increase in potential for 

population exposure to elevated levels 

in different settings. 

Changing the CBSA population 

threshold 350,000 to 500,000 results in 

a near-road monitoring network 

requiring approximately 126 monitors 

distributed within 102 CBSAs. 

Compared to the total number of 

required near-road monitors that would 

have resulted from the proposed CBSA 

population threshold of 350,000 (167 

monitors), an estimated 41 fewer 

monitors are required. EPA has also 

recognized that susceptible and 

vulnerable populations, which include 

asthmatics and disproportionately 

exposed groups, (as discussed in 

sections II.B.4 and II.F.4.d) are at 

particular risk of NO2-related health 

effects. The Administrator is therefore 

requiring the Regional Administrators, 

working in collaboration with States, to 

site forty monitors in appropriate 

locations, focusing primarily on 

protecting such susceptible and 

vulnerable communities. This decision 

is discussed in detail in section III.B.4. 

3. Second Tier (Area-Wide Monitoring 

Component) of the Network Design 

The following paragraphs provide 

background, rationale, and details for 

the final changes to the second tier of 

the two-tier NO2 network design. In 

particular, this section will focus on the 

threshold that triggers area-wide 

monitoring requirements. Area-wide site 

selection and siting criteria details will 

be discussed in a subsequent section. 

a. Proposed Second Tier (Area-Wide 

Monitoring Component) of the Network 

Design 

As the second tier of the proposed 

two-tier network design, EPA proposed 

to require monitors to characterize the 

expected maximum NO2 concentrations 

at the neighborhood and larger (areawide) spatial scales in an area. This 

component of the two-tier network 

design provides information on areawide exposures that may occur due to 

an individual or a group of point, area, 

on-road, and/or non-road sources. 

Further, area-wide sites serve multiple 

monitoring objectives aside from 

NAAQS comparison to both the 1-hour 

and the annual NAAQS, including 

photochemical pollutant assessment, 

aiding in ozone forecasting, aiding in 

particulate matter precursor analysis 

and particulate matter forecasting. We 

proposed to require one area-wide 

monitoring site in each CBSA with a 

population greater than or equal to 

1,000,000. We proposed that these areawide sites were to be sited to represent 

an area of highest concentration at the 

neighborhood or larger spatial scales. 

Based on 2008 Census Bureau statistics, 

there are 52 CBSAs with 1,000,000 

people or more, which would result in 

an estimated 52 area-wide monitors in 

as many CBSAs being minimally 

required. EPA also proposed to allow 

any current photochemical assessment 

monitoring station (PAMS) sites that are 

sited where the highest NO2

concentrations occur in an urban area 


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and represent a neighborhood or urban 

scale to satisfy the area-wide monitoring 

requirement. 

b. Comments 

Most commenters who commented on 

area-wide monitoring supported the 

adoption of the alternative area-wide 

network design and did not specifically 

comment on the area-wide monitoring 

component of the proposed two-tier 

network design. However, EPA did 

receive comments from public health 

organizations on area-wide monitoring 

in the context of the proposed network 

design. The public health group 

commenters, including the ALA, EJ, 

EDF, and the NRDC, stated they ‘‘oppose 

the proposed requirement to retain only 

52 air monitors to measure area-wide 

concentrations of NO2.’’ 

EPA understands the perceived 

concern to be that with this provision, 

EPA is actively reducing the number of 

required area-wide monitors. Prior to 

this rulemaking, the Ambient Air 

Monitoring Regulations, 71 FR 61236 

(Oct. 17, 2006) (2006 monitoring rule) 

removed minimum monitoring 

requirements for NO2, and the rationale 

for that action is explained in that rule; 

however, the 2006 Monitoring rule has 

had a limited impact to date, evidenced 

by the fact that the size of the NO2

network has remained relatively steady 

at around 400 monitors, a majority of 

which are area-wide monitors, that were 

operating in 2008 (Watkins and 

Thompson, 2008). The stability of the 

NO2 network is due in large part to the 

fact that area-wide monitors serve 

multiple monitoring objectives, 

including photochemical pollutant 

assessment, pollutant forecasting, and in 

some cases, support to ongoing health 

research. However, considering the 

objective of this two-tier network 

design, particularly the first tier, of 

supporting the revised NAAQS to 

protect against peak NO2 exposures, 

some shrinkage in the area-wide 

network is appropriate and likely. EPA 

believes that the actual number of areawide monitors that will operate in the 

NO2 network will be greater than the 

minimally required 52 sites, but likely 

less than the current number. States and 

Regional Administrators will work 

together on which area-wide sites may 

warrant retention above the minimum 

required if States request existing areawide sites to be shut down or relocated. 

c. Conclusions on the Second Tier 

(Area-Wide Monitoring Component) of 

the Network Design 

Area-wide monitoring sites serve 

multiple monitoring objectives aside 

from NAAQS comparison to both the 1- 

hour and the annual NAAQS, including 

photochemical pollutant assessment, 

ozone forecasting, particulate matter 

precursor analysis and particulate 

matter forecasting. EPA recognizes that 

a significant portion of the existing NO2

monitoring network can be 

characterized as area-wide monitors and 

that these monitoring sites serve 

multiple monitoring objectives, as noted 

above. In order to ensure that a 

minimum number of area-wide 

monitors continue operating into the 

future, we are finalizing the proposed 

minimum monitoring requirements for 

area-wide monitors, where one areawide monitor is required in any CBSA 

with 1,000,000 people or more. Since 

there were no adverse comments 

received with regard to allowing PAMS 

stations that meet siting criteria to 

satisfy minimum monitoring 

requirements for area-wide monitors, we 

are finalizing that allowance as 

proposed. EPA encourages States to use 

the upcoming 2010 network assessment 

process to review existing area-wide 

NO2 sites to help determine what 

monitors might meet minimum 

monitoring requirements and whether 

or not other existing monitors warrant 

continued operation. 

4. Regional Administrator Authority 

The following paragraphs provide 

background, rationale, and details for 

the final changes to Regional 

Administrator authority to use 

discretion in requiring additional NO2

monitors beyond the minimum network 

requirements. The proposed rule 

estimated that approximately 167 nearroad monitors would be required within 

CBSAs having populations of 350,000 or 

more persons. As discussed above in 

section III.B.2, in response to public 

comments, particularly from States, EPA 

is changing the population threshold for 

siting a minimum of one near-road NO2

monitor from CBSAs with 350,000 or 

more persons to CBSAs with 500,000 or 

more persons. EPA estimates that this 

change in the population threshold will 

result in a reduction in the number of 

minimally required near-road NO2

monitors by approximately forty 

monitors. EPA has also recognized that 

susceptible and vulnerable populations, 

which include asthmatics and 

disproportionately exposed groups (as 

discussed in sections II.B.4 and II.F.4.d) 

are at particular risk of NO2-related 

health effects. The Administrator is 

therefore requiring the Regional 

Administrators, working in 

collaboration with States, to site these 

forty monitors in appropriate locations, 

focusing primarily on protecting 

susceptible and vulnerable 

communities. In addition, the Regional 

Administrators, working with States, 

may take into account other 

considerations described below in using 

their discretion to require additional 

monitors. 

a. Proposed Regional Administrator 

Authority 

EPA proposed that Regional 

Administrators have the authority to 

require monitoring at their discretion in 

particular instances. First, EPA 

proposed that the Regional 

Administrator have discretion to require 

monitoring above the minimum 

requirements as necessary to address 

situations where the required near-road 

monitors do not represent a location or 

locations where the expected maximum 

hourly NO2 concentrations exist in a 

CBSA. Second, EPA proposed to allow 

Regional Administrators the discretion 

to require additional near-road 

monitoring sites to address 

circumstances where minimum 

monitoring requirements are not 

sufficient to meet monitoring objectives, 

such as where exposures to NO2

concentrations vary across an area 

because of varied fleet mixes, 

congestion patterns, terrain, or 

geographic areas within a CBSA. And 

third, EPA proposed that Regional 

Administrators have the discretion to 

require additional area-wide NO2

monitoring sites above the minimum 

requirements for area-wide monitors 

where the minimum requirements are 

not sufficient to meet monitoring 

objectives. 

b. Comments 

EPA received comments from the 

Center on Race, Poverty and 

Environment expressing concern that 

the proposed monitoring provisions fail 

to consider ‘‘disproportionately 

impacted communities’’ which include 

people of color and of lower 

socioeconomic status. The commenter 

argues that this is ‘‘a gaping hole’’ in the 

proposed monitoring system and 

disproportionately impacts minority 

and low income populations in rural 

communities. In addition, the National 

Tribal Air Association stated that 

‘‘Indian Tribes and Alaska Natives are 

highly susceptible to health impacts as 

a result of NO2 exposure’’ and ‘‘the 

prevalence and severity of asthma is 

higher among certain ethnic or racial 

groups such as Indian Tribes and Alaska 

Natives,’’ which is also discussed in 

section II.B.4 and the ISA (ISA, section 

4.4). 

The proposed rule provided the 

Regional Administrators with the 

authority to use their discretion and 


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consider certain factors to require 

monitors above the minimum number in 

a CBSA. The proposal described one 

example where a Regional 

Administrator might require an 

additional near-road monitor where ‘‘a 

particular community or neighborhood 

is significantly or uniquely affected by 

road emissions.’’ EPA recognizes that 

susceptible and vulnerable populations, 

which include asthmatics and 

disproportionately exposed groups, as 

noted in section II.F.4.d, are at 

particular risk of NO2-related health 

effects, both because of increased 

exposure and because these groups have 

a higher prevalence of asthma and 

higher hospitalization rates for asthma. 

As noted above, in conjunction with 

raising the threshold for requiring one 

near-road NO2 monitor in CBSAs with 

500,000 persons or more, EPA is 

requiring the Regional Administrators, 

under their discretionary authority, to 

work with States to site an additional 

forty monitors, nationally, focusing 

primarily on communities where 

susceptible and vulnerable populations 

are located. To address the risks of 

increased exposure to these 

populations, the Administrator has 

determined that it is appropriate and 

necessary, under this provision, to 

ensure these additional forty monitors 

are sited primarily in communities 

where susceptible and vulnerable 

populations are exposed to NO2

concentrations that have the potential to 

exceed the NAAQS (due to emissions 

from motor vehicles, point sources, or 

area sources). As a result of this action, 

the total number of monitors required 

through this rulemaking is generally 

equivalent to the proposed number of 

minimally required monitors. 

EPA received comments from public 

health groups (e.g., ALA, Center on 

Race, Poverty, and the Environment, 

EDF, EJ, NRDC) and the Swinomish 

Tribe, who suggested that EPA expand 

monitoring coverage to address impacts 

from stationary sources outside of urban 

areas. For example, ALA, EDF, EJ, and 

NRDC, stated that ‘‘EPA should require 

States and local offices to review 

inventory data to identify any potential 

NO2 hotspots outside of those large 

metropolitan areas. For instance, if a 

large power plant or any other source is 

creating elevated NO2 levels in 

proximity to homes, schools or other 

sensitive sites, in an area of less than 

one million people, EPA should 

consider requiring a monitor.’’ 

EPA recognizes that there are major 

NO2 sources outside of CBSAs that have 

the potential to contribute to NO2

concentrations approaching or 

exceeding the NAAQS. The issue is 

whether such monitoring should be 

addressed through a more extensive set 

of minimum requirements that might 

include monitoring near all large 

stationary sources such as airports, 

seaports, and power plants, which could 

lead to deploying a large number of 

monitors. EPA believes that a more 

reasonable approach to address 

monitoring needs related to the diverse 

set of point, area, and non-road mobile 

NO2 sources, whether inside or outside 

of CBSAs, is to provide Regional 

Administrators the authority to require 

additional monitoring in areas where 

these impacts could occur. While the 

proposal did not specifically state that 

Regional Administrators could require 

non-area-wide monitors outside of 

CBSAs, EPA believes that it is important 

that Regional Administrators have the 

authority to require NO2 monitoring in 

locations where NO2 concentrations 

may be approaching or exceeding the 

NAAQS, whether located inside or 

outside of CBSAs. Therefore, in the final 

rule, EPA is not limiting the Regional 

Administrators’ discretionary authority 

to require NO2 monitoring only inside 

CBSAs; instead, the EPA is providing 

Regional Administrators the authority to 

site monitors in locations where NO2

concentrations may be approaching or 

exceeding the NAAQS, both inside or 

outside of CBSAs. 

The EPA also received comments 

from some State groups (e.g. the New 

York Department of Environmental 

Conservation (NYSDEC), New York 

Department of Transportation 

(NYSDOT), and the New York City Law 

Department) and an industry group (the 

Council of Industrial Boiler Operators) 

requesting greater clarification on the 

way in which Regional Administrators 

may use their authority to require 

additional monitors above the minimum 

requirements. For example, the Council 

of Industrial Boiler Operators stated that 

‘‘this [Regional Administrator authority] 

unreasonably vests an unbounded 

amount of discretion in EPA to 

determine when ‘‘minimum monitoring 

requirements are not sufficient’’ and 

which neighborhoods are ‘‘uniquely 

affected,’’ and impose additional 

monitoring requirements where all 

applicable monitoring requirements are 

already met by the State and local 

agency.’’ 

The authority of Regional 

Administrators to require additional 

monitoring above the minimum 

required is not unique to NO2. For 

example, Regional Administrators have 

or are proposed to have the authority to 

use their discretion to require additional 

Pb monitors (40 CFR Part 58 Appendix 

D section 4.5), and have the discretion 

to work with States or local agencies in 

designing and/or maintaining an 

appropriate ozone network, per 40 CFR 

Part 58 Appendix D section 4.1. EPA 

believes that while the NO2 monitoring 

network is sufficiently sized and 

focused, a nationally applicable network 

design may not account for all locations 

in which potentially high 

concentrations approaching or 

exceeding the NAAQS exist. Therefore, 

EPA believes it is important for Regional 

Administrators to have the ability to 

address possible gaps in the minimally 

required monitoring network, by 

granting them authority to require 

monitoring above the minimum 

requirements. 

One case in which the Regional 

Administrator may exercise discretion 

in requiring a monitor might be a 

location or community affected by a 

stationary source where the required 

near-road NO2 monitor site is not the 

location of the maximum hourly 

concentration in a CBSA. For any given 

CBSA, there is the possibility that the 

maximum NO2 concentrations could be 

attributed to impacts from one, or a 

combination of, multiple sources that 

could include point, area, and non-road 

source emissions in addition to on-road 

mobile source emissions. As a result, 

the Regional Administrator may choose 

to require monitoring in such a location. 

In addition, there is the possibility that 

a single source or group of sources 

exists which may contribute to 

concentrations approaching or 

exceeding the NAAQS at locations 

inside or outside CBSAs, including rural 

communities. In such cases, Regional 

Administrators, working with States, 

may require a monitor in these 

locations. Further, if there are NO2

sources responsible for producing more 

widespread impacts on a community or 

relatively larger area, Regional 

Administrators may require an areawide monitor to assess wider 

population exposures, or to support 

other monitor objectives served by areawide monitors such as photochemical 

pollutant assessment or pollutant 

forecasting. 

Regional Administrators may also 

require additional monitoring where a 

State or local agency is fulfilling its 

minimum monitoring requirements with 

an appropriate number of near-road 

monitors, but an additional location is 

identified where near-road population 

exposure exists at concentrations 

approaching or exceeding the NAAQS. 

In this case, the exposure may be due to 

differences in fleet mix, congestion 

patterns, terrain, or geographic area, 

relative to any minimally required 

monitoring site(s) in that area. We note 


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that such areas might exist in CBSAs 

with populations less than 500,000 

persons. 

EPA recognizes that high 

concentrations of NO2 that approach or 

exceed the NAAQS could potentially 

occur in a variety of locations in an area, 

and we believe that Regional 

Administrators should have the 

discretion to require additional 

monitoring when a location is identified 

based on the factors discussed in the 

paragraph above. In such situations, 

State or EPA Regional staff is likely to 

have identified these locations through 

data analysis, such as the evaluation of 

existing ambient data and/or emissions 

data, or through air quality modeling. 

Such information may indicate that an 

area has NO2 concentrations that may 

approach or exceed the NAAQS, and 

that there is potential for population 

exposure to those high concentrations. 

The Regional Administrator would 

use this authority in collaboration with 

State agencies. We expect Regional 

Administrators to work with State and 

local agencies to design and/or maintain 

the most appropriate NO2 network to 

meet the needs of a given area. For all 

the situations where a Regional 

Administrator may require additional 

monitoring, including the forty 

additional monitors the Regional 

Administrators are required to site, EPA 

expects Regional Administrators to 

work on a case-by-case basis with 

States. Further, for the forty additional 

monitors that will focus primarily on 

protecting susceptible and vulnerable 

communities, EPA intends to work with 

States to develop criteria to guide site 

selection for those monitors. 

c. Conclusions on Regional 

Administrator Authority 

EPA is requiring Regional 

Administrators to work with States to 

site forty NO2 monitors, above the 

minimum number required in the twotier network design, focused primarily 

in susceptible and vulnerable 

communities exposed to NO2

concentrations that have the potential to 

approach or exceed NAAQS. In 

addition, recognizing that a nationally 

applicable monitoring network design 

will not include all sites with 

potentially high concentrations due to 

variations across locations, and in 

response to public comments, the 

Administrator is providing Regional 

Administrators with the discretion to 

require additional monitors above the 

minimum requirements. 

Regional Administrators may also use 

their discretionary authority to require 

monitoring above the minimum 

requirements as necessary to address 

situations inside or outside of CBSAs in 

which (1) The required near-road 

monitors do not represent all locations 

of expected maximum hourly NO2

concentrations in an area and NO2

concentrations may be approaching or 

exceeding the NAAQS in that area; (2) 

areas that are not required to have a 

monitor in accordance with the 

monitoring requirements and NO2

concentrations may be approaching or 

exceeding the NAAQS; or (3) the 

minimum monitoring requirements for 

area-wide monitors are not sufficient to 

meet monitoring objectives. In all cases 

in which a Regional Administrator may 

consider the need for additional 

monitoring, EPA expects that Regional 

Administrators will work with the State 

or local agencies to evaluate evidence 

that suggests an area may warrant 

additional monitoring. EPA also notes 

that if additional monitoring should be 

required, as negotiated between the 

Regional Administrator and the State, 

the State will modify the information in 

its Annual Monitoring Network Plan to 

include any potential new sites prior to 

approval by the EPA Regional 

Administrator. 

5. Monitoring Network Implementation 

The following paragraphs provide 

background, rationale, and details for 

the final changes to the approach for the 

monitoring network implementation. 

a. Proposed Monitoring Network 

Implementation Approach 

EPA proposed that State and, when 

appropriate, local air monitoring 

agencies provide a plan for deploying 

monitors in accordance with the 

proposed network design by July 1, 

2011. EPA also proposed that the 

proposed NO2 network be physically 

established no later than January 1, 

2013. 

b. Comments 

Most environmental and public health 

group commenters suggested that EPA 

change the implementation date from 

the proposed January 1, 2013 to a date 

that would require the minimum 

required NO2 network to be deployed 

sooner than proposed. Most States and 

State group commenters, along with 

industry group commenters, 

recommended that EPA keep the 

network implementation date as January 

1, 2013, or move it later than proposed. 

Those commenters who suggested 

moving it later noted that issues with 

monitoring site identification, site 

development, and overall lack of 

experience working in the near-road 

environment would make 

implementation difficult under the 

proposed implementation deadline. 

EPA recognizes the challenges 

involved with deploying the two-tier 

network design by the January 1, 2013 

date. We recognize the need for 

additional information and plan to aid 

State agencies in the network 

implementation process, particularly by 

developing guidance in partnership 

with affected stakeholders, ideally 

including at a minimum NACAA and 

the States. EPA agrees with NACAA’s 

suggestion that the CASAC Ambient Air 

Monitoring and Methods subcommittee 

should be consulted as part of 

developing any guidance developed for 

near-road monitoring, and has already 

begun the process by scheduling 

meetings with them regarding near-road 

monitoring. Further, EPA believes that 

collaboration with the States and State 

groups in developing guidance will be 

highly beneficial to the implementation 

process. This would allow for those 

States that do have increased experience 

in near-road monitoring to support the 

guidance development process and 

provide a conduit for sharing 

experiences amongst all stakeholders. 

In perspective, EPA believes that the 

approximate 2 years and 11 months 

between promulgation of this 

rulemaking and the mandated January 1, 

2013 network implementation date 

includes extra time relative to what is 

traditionally allowed for network 

implementation following rulemakings. 

We are also cognizant of the time 

needed to collect complete data that 

would allow data from the two-tier 

network to be considered for 

designations and for use in the next NO2

NAAQS review data from the 2013, 

2014, and 2015 years would provide 

critical information in the next NAAQS 

review, intended to occur on a 5-year 

cycle, and for use in subsequent 

designations. Even with complete data 

from 2013, 2014, and 2015 years 

designations would not occur until 

2017, at the earliest. 

c. Conclusions on Monitoring Network 

Implementation 

EPA is finalizing the date by which 

State and, when appropriate, local air 

monitoring agencies shall establish the 

required NO2 monitoring network as 

January 1, 2013, as was proposed. We 

believe that the allotted time for 

implementation will allow for the 

development of guidance 

documentation, particularly allowing 

for interactions with CASAC and 

NACAA/States, and for the processes 

that will be involved in deploying this 

network. However, EPA recognizes that 

the network implementation process, 


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particularly for near-road monitors, will 

include the assessment of road segments 

in CBSAs to identify locations of 

maximum expected hourly NO2

concentrations, identifying and working 

with other State and local agencies, 

such as transportation officials, as 

needed on issues regarding access and 

safety, and the exchange of information 

and feedback on potential sites with 

EPA, prior to any commitment to 

selecting and presenting new sites in an 

annual monitoring plan. As a result, 

based on feedback received through 

public comments, and to allow for more 

time to process guidance information, to 

carry out the deployment processes, and 

to allow for information exchanges to 

occur, we are changing the date by 

which State and, when appropriate, 

local air monitoring agencies shall 

provide a plan for deploying monitors 

in accordance with required network 

design, including the monitors required 

under the Regional Administrators’ 

discretional authority which are to be 

primarily focused on providing 

protection to susceptible and vulnerable 

populations, as discussed in section 

III.B.4, from July 1, 2011 to July 1, 2012. 

EPA strongly encourages State and local 

air agencies to supply as much 

information as possible on the NO2 sites 

they may be considering, including 

possible site coordinates if available, or 

have possibly selected, to satisfy the 

minimum NO2 network monitoring 

requirements in their Annual 

Monitoring Network Plan submitted 

July 1, 2011. 

6. Near-Road Site Selection 

The following paragraphs provide 

background, rationale, and details for 

the final changes to the approach and 

criteria by which required near-road 

sites shall be selected. 

a. Proposed Near-Road Site Selection 

Criterion 

EPA proposed that the required nearroad NO2 monitoring stations shall be 

selected by ranking all road segments 

within a CBSA by AADT and then 

identifying a location or locations 

adjacent to those highest ranked road 

segments where maximum hourly NO2

concentrations are expected to be 

highest and siting criteria can be met in 

accordance with that proposed for 40 

CFR Part 58 Appendix E (discussed in 

III.B.7). Where a State or local air 

monitoring agency identifies multiple 

acceptable candidate sites where 

maximum hourly NO2 concentrations 

are expected to occur, the monitoring 

agency should consider taking into 

account the potential for population 

exposure in the criteria utilized to select 

the final site location. Where one CBSA 

is required to have two near-road NO2

monitoring stations, we proposed that 

the sites shall be differentiated from 

each other by one or more of the 

following factors: Fleet mix; congestion 

patterns; terrain; geographic area within 

the CBSA; or different route, interstate, 

or freeway designation. 

b. Comments 

EPA received many comments from 

CASAC, public health groups, States 

and State groups, and industry groups 

on the proposed process by which 

States will select near-road sites. 

CASAC, along with some health group 

and State commenters questioned how 

States should select a site near the road 

with the highest ranked AADT possible, 

noting that EPA did not appear to 

require States to account for other 

factors. For example, one CASAC panel 

member noted that siting monitors 

based on traffic counts alone might miss 

locations where maximum NO2

concentrations would occur. They 

proceeded to recommend the use of 

modeling to assist in the site selection 

process. In another example, the ALA, 

EDJ, EJ, and NRDC, stated that ‘‘Nearroad monitor placement should be 

determined not only by the highest 

AADT volumes in a given CBSA, but 

also by the highest heavy-duty truck 

volumes.’’ NACAA also expressed 

concerns on ‘‘* * * basing monitor 

locations on the annual average daily 

traffic (AADT) without regard to vehicle 

mix or dispersion characteristics 

***’’. 

EPA does not intend for AADT counts 

to be the sole basis for choosing a nearroad site. As noted earlier in section 

III.B.2, there is a general relationship 

between AADT and mobile source 

pollution, where higher traffic counts 

correspond to higher mobile source 

emissions. The use of AADT counts is 

intended to be a mechanism for focusing 

on identifying the locations of expected 

maximum NO2 concentrations due to 

mobile sources. There are other factors 

that can influence which road segment 

in a CBSA may be the actual location 

where the maximum NO2

concentrations could occur. These 

factors include vehicle fleet mix, 

roadway design, congestion patterns, 

terrain, and meteorology. When States 

identify their top-ranked road segments 

by AADT, EPA intends for States to 

evaluate all of the factors listed above in 

their site selection process, due to their 

influence on where the location of 

expected maximum NO2 concentration 

may occur. As a result of the comments 

indicating a need for clarification, EPA 

will specifically list the factors that 

must be considered by States in their 

site selection process once a State has 

identified the most heavily trafficked 

roads in a CBSA based on AADT counts. 

In addition, EPA proposed that States 

consider these factors when they are 

required to place two near-road 

monitors in a CBSA, i.e., CBSAs with a 

population of 2,500,000 persons or 

more. EPA notes that these factors will 

be used in differentiating the two 

monitoring sites from each other, 

providing further characterization of 

near-road environments in larger urban 

areas that are more likely to have a 

greater number of major roads across a 

potentially larger geographic area, and a 

corresponding increase in potential for 

exposure in different settings. Finally, 

EPA notes that air quality models, 

which were noted by the CASAC panel 

member to be considered for use in 

near-road site selection, are tools that 

EPA believes will be useful, and likely 

used by some States to inform where 

near-road sites need to be placed. 

EPA received comments from some 

State and industry commenters (e.g. 

Iowa, NY DEC, Edison Electric Institute, 

and Savannah River Nuclear Solutions) 

who suggested that potential population 

exposure should be a first-level metric 

in the near-road monitoring site 

selection process, instead of a secondlevel metric as EPA had proposed. 

EPA notes that the intent of the 

revised primary NO2 NAAQS is to 

protect against the maximum allowable 

NO2 concentration anywhere in an area, 

which includes ambient air on and 

around roads. This would limit 

exposures to peak NO2 concentrations, 

including those due to mobile source 

emissions, across locations (including 

those locations where population 

exposure near roads is greatest) in a 

given CBSA or area, with a relatively 

high degree of confidence. We also note 

the agency’s historical practice has been 

to site ambient air monitors in locations 

of maximum concentration, at the 

appropriate spatial scale. If EPA were to 

allow population, population density, or 

another population weighted metric to 

be a primary factor in the decision on 

where required near-road NO2 monitors 

are to be located, it is possible that the 

required near-road monitors in a CBSA 

would not be located at a site of 

expected maximum hourly near-road 

NO2 concentration. By monitoring in the 

location of expected maximum 1-hour 

concentrations, near-road monitoring 

sites will likely represent the highest 

NO2 concentrations in an area directly 

attributable to mobile sources or a group 

of sources that includes mobile sources. 

The proposed rule did permit, and the 

final rule states, that States are to 


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consider population in the site selection 

process in situations when a State 

identifies multiple candidate sites 

where maximum hourly NO2

concentrations are expected to occur. 

EPA received a comment from 

HCPHES suggesting that required 

monitoring should take into 

consideration the location of other 

major mobile sources for NO2 emissions 

such as airports and seaports. EPA also 

received a comment from the South 

Carolina Department of Health and 

Environmental Control stating that a 

near-road network does not address 

‘‘widespread pollutants from numerous 

and diverse sources.’’ 

EPA recognizes that there are major 

NO2 sources outside of CBSAs that have 

the potential to contribute to NO2

concentrations approaching or 

exceeding the NAAQS. The issue is 

whether such monitoring should be 

addressed through a more extensive set 

of minimum requirements that might 

include monitoring near all large 

stationary sources such as airports, 

seaports, and power plants, which could 

lead to deploying a large number of 

monitors. EPA believes that a more 

reasonable approach to address 

monitoring needs related to the diverse 

set of point, area, and non-road mobile 

NO2 sources, whether inside or outside 

of CBSAs, is to provide Regional 

Administrators the authority to require 

additional monitoring in areas where 

these impacts could occur. Providing 

the Regional Administrators with the 

discretion to require additional 

monitors allows them to effectively 

address such situations, even if that area 

is satisfying minimum monitoring 

requirements. This Regional 

Administrator authority is discussed 

above in section III.B.4. EPA also notes 

that State and local agencies may also 

monitor such locations on their own 

initiative. 

One State commenter, the Wisconsin 

Department of Natural Resources, 

requested that the term ‘‘major road’’ be 

defined and also requested clarification 

on what ‘‘top-ranked’’ means with regard 

to AADT counts on road segments. 

While the term ‘‘major road’’ is widely 

used in literature and can be found to 

be defined differently from one 

scientific study to another, here, EPA is 

using it in its commonly understood 

meaning as a road that is relatively 

heavily trafficked. EPA also does not 

believe it is appropriate to provide a 

bright-line definition for ‘‘top-ranked’’. 

Each CBSA will have a different 

distribution of total road segments and 

corresponding AADT counts on those 

segments. Further, since required nearroad monitors are to be sited in 

locations of expected maximum 

concentrations, a percentile restriction 

on ‘‘top ranked’’ roads is unnecessary. 

The intent of the requirement to rank all 

road segments by AADT counts and 

select a site, considering the other local 

factors noted above, near a ‘‘top-ranked’’ 

road segment is to focus attention on the 

most heavily trafficked roads, around 

which there is higher potential for 

maximum NO2 concentrations to occur. 

c. Conclusions on Near-Road Site 

Selection 

We are finalizing the near-road site 

selection criteria as proposed, and are 

clarifying that the proposal intended the 

selection criteria to include 

consideration of localized factors when 

identifying locations of expected 

maximum concentrations. As a result, 

required near-road NO2 monitoring 

stations shall be selected by ranking all 

road segments within a CBSA by AADT 

and then identifying a location or 

locations adjacent to those highest 

ranked road segments, considering fleet 

mix, roadway design, congestion 

patterns, terrain, and meteorology, 

where maximum hourly NO2

concentrations are expected to occur 

and siting criteria can be met in 

accordance with 40 CFR Part 58 

Appendix E. As was noted in section 

III.B.5 above, EPA will work with States 

to assist with the near-road site 

selection process through the 

development of guidance material and 

through information exchanges amongst 

the air monitoring community. 

We are also finalizing the 

requirement, as proposed, that when 

one CBSA is required to have two nearroad NO2 monitoring stations, the sites 

shall be differentiated from each other 

by one or more of the following factors: 

fleet mix; congestion patterns; terrain; 

geographic area within the CBSA; or 

different route, interstate, or freeway 

designation, as was proposed. 

7. Near-Road Siting Criteria 

The following paragraphs provide 

background, rationale, and details for 

the final changes to the siting criteria for 

required near-road monitoring sites. 

a. Proposed Near-Road Siting Criteria 

EPA proposed that near-road NO2

monitoring stations must be sited so that 

the NO2 monitor probe is no greater 

than 50 meters away, horizontally, from 

the outside nearest edge of the traffic 

lanes of the target road segment, and 

shall have no obstructions in the fetch 

between the monitor probe and roadway 

traffic such as noise barriers or 

vegetation higher than the monitor 

probe height. We solicited comment on, 

but did not propose, having near-road 

sites located on the predominantly 

downwind side of the target roadways. 

EPA proposed that the monitor probe 

shall be located within 2 to 7 meters 

above the ground, as is required for 

microscale PM2.5 and PM10 sites. We 

also proposed that monitor probe 

placement on noise barriers or 

buildings, where the inlet probe height 

is no less than 2 meters and no more 

than 7 meters above the target road, will 

be acceptable, so long as the inlet probe 

is at least 1 meter vertically or 

horizontally away (in the direction of 

the target road) from any supporting 

wall or structure, and the subsequent 

residence time of the pollutant in the 

sample line between the inlet probe and 

the analyzer does not exceed 20 

seconds. 

b. Comments 

EPA received comments from a 

number of States (e.g. Michigan, 

Mississippi, and Tennessee) indicating 

that the near-road network poses 

significant safety issues and a related 

need for increased logistical flexibility 

for installing a monitoring site. For 

example, the Mississippi Department of 

Environmental Quality states that 

‘‘Given the fact that these NO2 sites will 

be required to be housed in shelters that 

are within 50 meters of the road, we 

believe that these buildings could be 

large and pose a serious risk to drivers 

on the road.’’ 

EPA notes that in all instances of field 

work, safety is a top priority. In this 

instance of near-road monitoring, we are 

dealing with the safety of the public 

driving on roads and the monitoring 

staff who may operate the near-road 

monitoring station as well. There are 

various ways to install near-road sites 

while ensuring worker and traffic safety, 

and safety is an important part of the 

logistical considerations that States 

should consider when selecting and 

installing near-road sites. In many cases, 

State and local monitoring agencies may 

be able to work with their State or local 

transportation officials during the site 

selection process to deal with access 

and safety issues. In public comments, 

AASHTO recommended that ‘‘* * * 

State and local air monitoring agencies 

be required to coordinate with State and 

local DOTs for near-road monitoring 

during the establishment of the 

monitoring plan.’’ Although EPA cannot 

require States to coordinate with other 

State or local entities, EPA believes that 

transportation officials would likely be 

able to assist in finding solutions to 

ensure safety while working with 

monitoring agencies in accommodating 

a new near-road monitoring station. An 


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24NESCAUM officially supported the alternative 

network design; however, they made suggestions 

regarding the near-road network in the event EPA 

finalized the proposed two-tier network design. 

25NACAA made a statement containing many 

concerns about the near-road monitoring 

component proposal which included a passage 

regarding the lack of requiring sites to be 

downwind. They expressed concern in ‘‘* * * 

allowing upwind siting of monitors over a wide 

range of horizontal and vertical distances from the 

road * * *’’. 

example of a step that could be taken to 

alleviate safety concerns might be 

purposefully placing a monitoring site 

behind existing barriers like guardrails 

and fencing, or possibly by installing a 

short distance of such barriers to protect 

the site workers, site infrastructure, and 

nearby traffic. In addition, EPA notes 

that the 50m distance proposed is wide 

enough to accommodate a site that 

would satisfy many setback provisions 

that exist for private or commercial 

building permits near roads, and may be 

viewed as a confirmation that our 

proposed siting criteria are safely 

attainable. 

Some State commenters (e.g. 

AASHTO, NYSDOT, and Wisconsin) 

suggested that the allowable maximum 

distance a near-road monitoring probe 

can be from the target road be increased 

from 50 meters to something wider, 

such as 200 meters. Conversely, there 

were some State, environmental, and 

industry commenters (e.g. NESCAUM,24

Group Against Smog and Pollution, and 

Air Quality Research and Logistics) who 

suggested that the proposed range was 

appropriate, or, as suggested by both 

NESCAUM and the Group Against Smog 

and Pollution, the allowable distance 

should be reduced to as close as 30 or 

20 meters to the nearest edge of the 

traffic lanes of the target road segment, 

respectively. 

EPA believes that increasing the 

allowable distance above 50 meters 

would compromise the intent of nearroad monitoring. As was noted in the 

proposal and this document, the ISA 

(2.5.4 and 4.3.6) and REA (7.3.2) 

indicate that on-road, mobile source 

derived NO2 exhibits a peak 

concentration on or very near the source 

road, and those concentrations decay 

over a variable but relatively short 

distance back to near area-wide or 

background (upwind of the target road) 

concentrations. Literature values 

indicate that the distance required for 

NO2 concentrations to return to near 

area-wide or background concentrations 

away from major roadways can range up 

to 500 meters, but the peak 

concentrations are occurring on or very 

near the source roadway. The behavior 

of NO2 concentrations and the actual 

distance over which concentrations 

return to near area-wide or background 

levels is variable, and highly dependent 

on topography, roadside features, 

meteorology, and the related 

photochemical reactivity conditions 

(Baldauf et al., 2008; Beckerman et al., 

2007; Clements et al., 2008; Gilbert et 

al., 2003; Hagler et al., 2009; Rodes and 

Holland, 1980; Singer et al., 2003; Zhou 

and Levy, 2007). Therefore, monitor 

probe placement at increasing distances 

from a road, such as 200 meters, will 

correspondingly decrease the potential 

for sampling maximum concentrations 

of NO2 due to the traffic on the target 

road. Baldauf et al. (2009) indicate that 

monitoring probes would ideally be 

situated between 10 and 20 meters from 

the nearest traffic lane for near-road 

pollutant monitoring. 

Regarding the comments suggesting 

required monitor probes be closer than 

50 meters, EPA believes the allowable 

distance of 50 meters that a near-road 

NO2 probe can be from the target road 

provides enough flexibility for the 

logistical issues that can occur on a 

case-by-case basis, which is inherent in 

monitoring site placement, while not 

sacrificing the potential to monitor the 

peak NO2 concentrations. However, in 

light of the information provided here 

on how NO2 peak concentrations can 

decay over relatively short distances 

away from roads, EPA strongly 

encourages States to place near-road 

sites, or at least monitor probes, as close 

as safely possible to target roads to 

increase the probability of measuring 

the peak NO2 concentrations that occur 

in the near-road environment, again 

noting that Baldauf et al. (2009) indicate 

that monitor probes would ideally be 

situated between 10 and 20 meters from 

the nearest traffic lane for near-road 

pollutant monitoring. 

EPA also proposed that required nearroad NO2 monitor probes shall have no 

obstructions in the fetch between the 

monitor probe and roadway traffic such 

as noise barriers or vegetation higher 

than the monitor probe height. EPA 

expects that when a State makes a 

measurement in determining whether 

an NO2 inlet probe is no greater than 50 

meters away, horizontally, from the 

outside nearest edge of the traffic lanes 

of the target road segment, that the 

measurement would likely represent a 

path to the monitor probe that is normal 

to the target road. However, EPA notes 

that the monitor probe will likely be 

influenced by various parts of the target 

road segment that are at a relative angle 

compared to the normal transect 

between the road and the monitor 

probe. EPA is not adjusting the wording 

of this requirement, but does intend for 

States to consider more than one linear 

pathway between the target road and the 

monitor probe being clear of 

obstructions when considering 

candidate site locations. 

EPA received comments on the 

solicitation for comment on requiring 

near-road monitoring sites to be placed 

on the downwind side of the target road 

where the commenters (e.g. NACAA,25

NESCAUM, and the Clean Air Council) 

encouraged such a requirement. 

Conversely, other commenters (e.g., Air 

Quality and Logistics and NYSDEC 

suggested that such a requirement may 

be overly restrictive and not necessary. 

For example, NYSDEC stated that ‘‘It is 

important to avoid making the monitor 

siting criteria too restrictive. It is very 

likely that in some CBSAs, finding 

suitable locations near the busiest road 

segments will not be possible. It is also 

important to remember that the NO2

monitoring instrumentation provides 

data continuously. Sites located 

downwind of sources will likely be 

impacted more frequently than the sites 

located upwind particularly when the 

sites are more than 50 meters from the 

source, and are preferred, but either side 

of the road will be downwind some of 

the time. Many of the highest NO2

concentrations are also likely to occur 

during inversion periods and during 

calm meteorological conditions when 

the upwind-downwind designations 

have little meaning.’’ 

EPA noted in its proposal that 

research literature indicates that in 

certain cases, mobile source derived 

pollutant concentrations, including 

NO2, can be detected upwind of roads, 

above background levels, due to a 

phenomenon called upwind 

meandering. Kalthoff et al. (2007) 

indicates that mobile source derived 

pollutants can meander upwind on the 

order of tens of meters, mainly due to 

vehicle induced turbulence. Further, 

Beckerman et al. (2008) note that nearroad pollutant concentrations on the 

predominantly upwind side of their 

study sites dropped off to near 

background levels within the first 50 

meters, but were above background in 

this short and variable upwind range, 

which could be due, at least in part, to 

vehicle induced turbulence. This 

upwind meandering characteristic of 

pollutants in the near-road environment 

provides an additional basis for locating 

near-road sites within 50 meters of 

target road segments, but also reduces 

the absolute need to be downwind of 

the road. EPA believes that very few, if 

any, near-road sites would be able to be 

situated in a location that was always 

downwind. For example, a hypothetical 


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site may have winds routinely out of 

several different cardinal directions 

throughout the year, without one being 

a dominant direction. As a result, given 

variable meteorology, for some period of 

a year, a given near-road site may not be 

downwind of the target road, no matter 

which side of the road it is on. 

Therefore, EPA is not finalizing a 

requirement that near-road sites must be 

climatologically downwind of the target 

road segment because of the additional 

limitations this introduces to finding 

potential site candidates in exchange for 

what may be a small increase in the 

opportunity to monitor peak NO2

concentrations. However, EPA 

encourages States to place monitors in 

the climatologically downwind 

direction whenever possible, in an 

attempt to measure the peak NO2

concentrations more often than not. One 

way States may identify where the 

predominantly downwind location 

might be for candidate sites could be to 

use portable meteorological devices to 

characterize meteorological tendencies, 

in addition to evaluating other available 

meteorological data sources. 

EPA proposed that required near-road 

NO2 monitor probes be located within 2 

to 7 meters above the ground, as is 

required for microscale PM2.5 and PM10

sites. EPA also proposed that monitor 

probe placement on noise barriers or 

buildings, where the inlet probe height 

is no less than 2 meters and no more 

than 7 meters above the target road, will 

be acceptable, so long as the inlet probe 

is at least 1 meter vertically or 

horizontally away (in the direction of 

the target road) from any supporting 

wall or structure. NESCAUM 

commented that ‘‘EPA needs to 

reconcile near-roadway NO2 probe 

height requirements with the existing 

micro-scale near-roadway CO probe 

height requirement of 2.5 to 3.5 meters 

above prevailing terrain. NESCAUM 

supports using this existing height for 

all near-roadway pollution monitors, as 

it minimizes probe height effects on 

measurements, and allows for proper 

measurement of collocated particle 

number concentration (which requires a 

very short inlet, i.e., on the order of 

inches) and CO.’’ NYSDEC commented 

that ‘‘The height requirement may not be 

practical for road segments in dense 

urban areas where existing buildings 

heights may exceed 7 meters. The 

requirement to maintain a 1 meter 

clearance from a supporting wall or 

structure may not be adequate for taller 

walls often found in urban areas. These 

walls can create down washing and 

street canyon effects which will make 

the resulting data less representative of 

nearby areas and will make 

interpretation of the resulting data 

difficult. However, there will need to be 

consistency between similar site 

settings.’’ Finally, EPA received 

comments from some health groups 

(e.g., ALA, EJ, EDF, and NRDC) who 

commented that ‘‘the lower end of the 

proposed height of 2 to 7 meters appears 

to capture the highest NO2

concentrations, and more accurately 

represents human exposure at the 

breathing zone.’’ 

In the proposal, EPA noted that nearroad monitoring sites will be adjacent to 

a variety of road types, where some 

target roads will be on an even plane 

with the monitoring station, while 

others may be cut roads (i.e., below the 

plane of the monitoring station) or fill 

and open elevated roads (i.e., where the 

road plane is above the monitoring 

station). EPA recognizes that 

consistency across sites with regard to 

probe height is desirable, and 

consistency with microscale, urban 

canyon CO sites might also be desirable. 

However, as was noted in the earlier 

discussion on ‘‘downwind’’ site 

placements, it is important to avoid 

making the monitor siting criteria too 

restrictive. An allowable range between 

2 and 7 meters provides more flexibility 

in site installation, which EPA 

considers important because of the 

variety of siting situations each State 

may have to deal with for each 

individual site. While EPA agrees that a 

tighter allowable range such as 2.5 to 3.5 

meters would reduce site to site 

variability and keep probes nearer the 

microscale siting requirements of CO, 

the wider range of 2 to 7 meters still 

provides an adequate amount of site to 

site consistency. EPA may also address 

this issue through forthcoming 

guidance, where an increased 

consistency for probe heights in similar 

situations such as urban canyons may 

be a site implementation goal, within 

the required 2 to 7 meter probe height 

range. Further, EPA believes that 

although certain situations, as noted by 

NYSDEC, may exist where the 1 meter 

clearance from walls or structures may 

be problematic near taller buildings or 

walls, this requirement is consistent 

with similar such clearance 

requirements for microscale CO sites in 

similar such situations that exist in 

urban canyons. 

In the proposed rule, EPA proposed in 

the siting criteria language that the 

subsequent residence time of the 

pollutant in the sample line between the 

inlet probe and the analyzer cannot 

exceed 20 seconds. EPA received 

comments from Air Quality Research 

and Logistics regarding guidelines for 

maximum allowable inlet length and 

sample residence time, where they 

stated that ‘‘* * * the fast 

photodynamic O3-NOX equilibrium may 

occur in darkened sample lines at 

residence times of 10–20 seconds 

(Butcher et al. 1971; Ridley et al. 1988; 

Parrish et al. 1990). EPA should correct 

this apparent error by specifying much 

lower maximum residence times (e.g., 

1–2 seconds) or accounting for this 

effect by reporting ‘corrected’ values in 

error by no more than the allowed 

rounding convention (e.g., ±1 ppb).’’ 

EPA notes that in 40 CFR Part 58 

Appendix E, paragraph (9)(c), states that 

sample probes for reactive gas analyzers, 

particularly NOY monitors, at NCore 

monitoring sites must have a sample 

residence time less than 20 seconds. 

EPA believes this rule is also 

appropriate for NO2 monitors, 

particularly if a monitor inlet manifold 

is extended away from the main 

monitoring shelter. EPA does agree that 

shorter sample residence time in the 

inlet manifold is desirable. Although we 

do not believe it appropriate to require 

residence times on the order of 1 to 2 

seconds, and do not believe correcting 

values is appropriate (which was not a 

concept which was proposed), we do 

encourage States to use best practices in 

selecting non-reactive manifold 

materials, and to install sampling 

manifolds in an efficient manner that 

minimizes sample residence time. 

While EPA proposed this concept in the 

preamble to the proposed rule, we did 

not include it in the proposed regulatory 

text. The final rule includes regulatory 

text on this subject at 40 CFR Part 58 

Appendix E, paragraph (9)(c). 

c. Conclusions on Near-Road Siting 

Criteria 

We are finalizing the near-road NO2

monitor siting criteria, as proposed, 

where (1) required near-road NO2

monitor probes shall be as near as 

practicable to the outside nearest edge 

of the traffic lanes of the target road 

segment; but shall not be located at a 

distance greater than 50 meters, in the 

horizontal, from the outside nearest 

edge of the traffic lanes of the target 

road segment, (2) required near-road 

NO2 monitor probes shall have an 

unobstructed air flow, where no 

obstacles exist at or above the height of 

the monitor probe, between the monitor 

probe and the outside nearest edge of 

the traffic lanes of the target road 

segment, (3) required near-road NO2

monitors are required to have sampler 

inlets between 2 and 7 meters above 

ground level, and (4) residence time of 

NO2 in the sample line between the 


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inlet probe and the analyzer does not 

exceed 20 seconds. 

8. Area-Wide Monitor Site Selection and 

Siting Criteria 

The following paragraphs provide 

background, rationale, and details for 

the final changes to the site selection 

and monitor siting criteria for required 

area-wide monitoring sites. 

a. Proposed Area-Wide Monitor Site 

Selection and Siting Criteria 

EPA proposed that sites required as 

part of the second tier of the NO2

monitoring network design, known as 

the area-wide monitoring component, be 

sited to characterize the highest 

expected NO2 concentrations at the 

neighborhood and larger (area-wide) 

spatial scales in a CBSA. 

b. Comments 

While most commenters who 

supported area-wide monitoring did so 

with regard to the adoption of the 

alternative area-wide network design 

rather than as part of the proposed 

approach, only a few commented on the 

actual sites and siting criteria. The Dow 

Chemical Company suggested that areawide sites should be located at least 

1,000 meters away from any major roads 

or intersections to ensure that the 

concentration of NO2 measured is 

representative of an area-wide 

concentration instead of peak near-road 

concentrations. 

EPA notes that in order for an NO2

monitoring site to be classified as a 

neighborhood (or larger) spatial scale 

site, it must meet the roadway set-back 

requirements in Table E–1 of 40 CFR 

Part 58 Appendix E. EPA believes that 

this existing set-back table is 

appropriate to use to ensure that any 

NO2 site that may be intended as an 

area-wide site will be sufficiently 

distanced from any major road. For 

example, an NO2 monitoring site may be 

considered neighborhood scale if it is 10 

or more meters from the edge of the 

nearest traffic lane of a road with 10,000 

or less AADT counts. 

c. Conclusions on Area-Wide Monitor 

Site Selection and Siting Criteria 

We are finalizing the requirement that 

any sites required as part of the second 

tier of the NO2 monitoring network 

design, known as the area-wide 

monitoring component, be sited to 

characterize the highest expected NO2

concentrations at the neighborhood and 

larger (area-wide) spatial scales in a 

CBSA. 

9. Meteorological Measurements 

The following paragraphs provide 

background, rationale, and details for 

the final changes to the requirement of 

meteorological monitoring at near-road 

monitoring sites. 

a. Proposed Meteorological 

Measurements 

In further support of characterizing 

the peak NO2 concentrations occurring 

in the near-road environment, EPA 

proposed to require three-dimensional 

anemometry, providing wind vector 

data in the horizontal and vertical 

planes, along with temperature and 

relative humidity measurements, at all 

required near-road monitoring sites. 

b. Comments 

EPA received comments from the 

South Carolina Department of Health 

and Environmental Control commented 

that the recording of air turbulence data 

at near-road monitoring stations should 

be encouraged but not required. Other 

States (e.g., Alaska, North Carolina, and 

Wisconsin) provided comments that did 

not support the proposed meteorological 

measurement requirements, noting 

issues with costs, problems siting the 

probe nearer to structures and to the 

ground than is typically done, and that 

the averaging period required to better 

understand turbulence (through 

anemometry data) in the near-road 

environment requires a much higher 

frequency than what is typically 

reported. 

EPA is removing the proposed 

requirements that would have required 

meteorological monitoring at near-road 

NO2 monitoring stations. However, EPA 

strongly encourages States to do some 

meteorological monitoring to better 

characterize the conditions under which 

they are acquiring NO2 data. The nearroad microscale environment is 

complex, and understanding the 

turbulent dispersion that may be 

affecting NO2 measurements, along with 

having a basic understanding of from 

which direction the measured NO2

concentrations are coming from, which 

are very informative in the effort to fully 

understand the data being collected. At 

a minimum, basic anemometry data 

would be useful in identifying whether 

the site is upwind, downwind, or 

otherwise oriented, relative to the target 

road. 

c. Conclusions on Meteorological 

Measurements 

We are not finalizing the proposal to 

require three-dimensional anemometry, 

providing wind vector data in the 

horizontal and vertical planes, along 

with temperature and relative humidity 

measurements, at all required near-road 

monitoring sites. 

C. Data Reporting 

The following paragraphs provide 

background, rationale, and details for 

the final changes to the data reporting 

requirements, data quality objectives, 

and measurement uncertainty. 

1. Proposed Data Quality Objectives and 

Measurement Uncertainty 

In the proposal, EPA noted that State 

and local monitoring agencies are 

required to report hourly NO, NO2, and 

NOX data to AQS within 90 days of the 

end of each calendar quarter. We also 

noted that many agencies also 

voluntarily report their pre-validated 

data on an hourly basis to EPA’s real 

time AIRNow data system, where the 

data may be used by air quality 

forecasters to assist in ozone forecasting. 

We believe these data reporting 

procedures are appropriate to support 

the revised primary NO2 NAAQS. 

EPA proposed to develop data quality 

objectives (DQOs) for the proposed NO2

network. We proposed a goal for 

acceptable measurement uncertainty for 

NO2 methods to be defined for precision 

as an upper 90 percent confidence limit 

for the coefficient of variation (CV) of 15 

percent and for bias as an upper 95 

percent confidence limit for the absolute 

bias of 15 percent. 

2. Comments 

EPA received comments from the 

State of Missouri, supporting the 

proposed DQOs and goals for 

measurement uncertainty, and from 

North Carolina, suggesting that 

measurement uncertainty goals match 

those of the NCore multi-pollutant 

network. 

EPA agrees that it is desirable to have 

measurement uncertainty goals that 

match that of other pollutants. EPA 

originally proposed the goals for 

precision and bias under consideration 

that there may be a need to account for 

potential increased uncertainty in 1- 

hour near-road NO2 data. However, we 

agree with the suggestion from the State 

of North Carolina, and are changing the 

goals for acceptable measurement 

uncertainty for NO2 methods to be 

defined for precision as an upper 90 

percent confidence limit for the 

coefficient of variation (CV) of 10 

percent and for bias as an upper 95 

percent confidence limit for the absolute 

bias of 15 percent. These goals match 

the existing goals for NO2 and are 

consistent with historical measurement 

uncertainty goals. 


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3. Conclusions on Data Quality 

Objectives and Measurement 

Uncertainty 

We are finalizing the approach to 

develop data quality objectives, and are 

changing the proposed goal for 

measurement uncertainty, where the 

goals for acceptable measurement 

uncertainty for NO2 methods to be 

defined for precision as an upper 90 

percent confidence limit for the 

coefficient of variation (CV) of 10 

percent and for bias as an upper 95 

percent confidence limit for the absolute 

bias of 15 percent. 

IV. Appendix S—Interpretation of the 

Primary NAAQS for Oxides of Nitrogen 

and Revisions to the Exceptional Events 

Rule 

The EPA proposed to add Appendix 

S, Interpretation of the Primary National 

Ambient Air Quality Standards for 

Oxides of Nitrogen, to 40 CFR part 50 

in order to provide data handling 

procedures for the proposed NO2 1-hour 

primary standard and for the existing 

NO2 annual primary standard. The 

proposed Appendix S detailed the 

computations necessary for determining 

when the proposed 1-hour and existing 

annual primary NO2 NAAQS are met. 

The proposed Appendix S also 

addressed data reporting, data 

completeness considerations, and 

rounding conventions. 

Two versions of Appendix S were 

proposed. The first applied to a 1-hour 

primary standard based on the annual 

4th high value form, while the second 

applied to a 1-hour primary standard 

based on the 99th percentile daily value 

form. 

The final version of Appendix S is 

printed at the end of this notice and 

applies to an annual primary standard 

and a 1-hour primary standard based on 

the 98th percentile daily value form. 

Appendix S is based on the nearroadway approach to the setting the 

level of the 1-hour standard and to 

siting monitors. As such, these versions 

place no geographical restrictions on 

which monitoring sites’ concentration 

data can and will be compared to the 1- 

hour standard when making 

nonattainment determinations and other 

findings related to attainment or 

violation of the standard. 

The EPA is amending and moving the 

provisions of 40 CFR 50.11 related to 

data completeness for the existing 

annual primary standard to the new 

Appendix S, and adding provisions for 

the proposed 1-hour primary standard. 

Substantively, the data handling 

procedures for the annual primary 

standard in Appendix S are the same as 

the existing provisions in 40 CFR 50.11 

for that standard, except for an addition 

of a cross-reference to the Exceptional 

Events Rule, the addition of 

Administrator discretion to consider 

otherwise incomplete data complete, 

and the addition of a provision 

addressing the possibility of there being 

multiple NO2 monitors at one site. The 

procedures for the 1-hour primary 

standard are entirely new. 

The EPA is also making NO2-specific 

changes to the deadlines, in 40 CFR 

50.14, by which States must flag 

ambient air data that they believe have 

been affected by exceptional events and 

submit initial descriptions of those 

events, and the deadlines by which 

States must submit detailed 

justifications to support the exclusion of 

that data from EPA determinations of 

attainment or nonattainment with the 

NAAQS. The deadlines now contained 

in 40 CFR 50.14 are generic, and are not 

always appropriate for NO2 given the 

anticipated schedule for the 

designations of areas under the final 

NO2 NAAQS. 

The purpose of a data interpretation 

appendix in general is to provide the 

practical details on how to make a 

comparison between multi-day and 

possibly multi-monitor ambient air 

concentration data and the level of the 

NAAQS, so that determinations of 

compliance and violation are as 

objective as possible. Data interpretation 

guidelines also provide criteria for 

determining whether there are sufficient 

data to make a NAAQS level 

comparison at all. The regulatory 

language for the pre-existing annual 

NO2 NAAQS, originally adopted in 

1977, contained data interpretation 

instructions only for the issue of data 

completeness. This situation contrasts 

with the situations for ozone, PM2.5, 

PM10, and most recently Pb for which 

there are detailed data interpretation 

appendices in 40 CFR part 50 

addressing more issues that can arise in 

comparing monitoring data to the 

NAAQS. 

A. Interpretation of the Primary NAAQS 

for Oxides of Nitrogen for the Annual 

Primary Standard 

The purpose of a data interpretation 

rule for the NO2 NAAQS is to give effect 

to the form, level, averaging time, and 

indicator specified in the regulatory text 

at 40 CFR 50.11, anticipating and 

resolving in advance various future 

situations that could occur. Appendix S 

provides common definitions and 

requirements that apply to both the 

annual and the 1-hour primary 

standards for NO2. The common 

requirements concern how ambient data 

are to be reported, what ambient data 

are to be considered (including the issue 

of which of multiple monitors’ data sets 

will be used when more than one 

monitor has operated at a site), and the 

applicability of the Exceptional Events 

Rule to the primary NO2 NAAQS. 

The proposed Appendix S also 

addressed several issues in ways which 

are specific to the individual primary 

NO2 standards, as described below. 

1. Proposed Interpretation of the Annual 

Standard 

The proposed data interpretation 

provisions for the annual standard are 

consistent with the pre-existing 

instructions included along with the 

statement of the level and form of the 

standard in 40 CFR 50.11. These are the 

following: (1) At least 75% of the hours 

in the year must have reported 

concentration data. (2) The available 

hourly data are arithmetically averaged, 

and then rounded (not truncated) to 

whole parts per billion. (3) The design 

value is this rounded annual average 

concentration. (4) The design value is 

compared with the level of the annual 

primary standard (expressed in parts per 

billion). 

In the proposal, EPA noted that it 

would be possible to introduce 

additional steps for the annual primary 

standard which in principle could make 

the design value a more reliable 

indicator of actual annual average 

concentration in cases where some 

monitoring data have been lost. For 

example, averaging within a calendar 

quarter first and then averaging across 

quarters could help compensate for 

uneven data capture across the year. For 

some aspects of the data interpretation 

procedures for some other pollutants, 

the current data interpretation 

appendices do contain such additional 

steps. The proposed provisions for the 

proposed 1-hour NO2 standard also 

incorporated some such features. 

2. Comments on Interpretation of the 

Annual Standard 

We received four comments, all from 

State agencies, on data interpretation for 

the annual NO2 standard. Of the four 

commenters, two recommended the use 

of a weighted annual mean to 

appropriately implement the annual 

primary standard. Two other 

commenters asserted that there is no 

strong seasonality in NO2

concentrations, and that therefore there 

is no need to use a weighted annual 

mean or to require data completeness 

quarter-by-quarter. 


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3. Conclusions on Interpretation of the 

Annual Standard 

Upon investigating the issue of NO2

seasonality using data from AQS as part 

of considering the comments, we have 

found that there are notable variations 

in quarterly mean NO2 concentrations. It 

is therefore quite possible that an 

unweighted annual mean calculated 

without a quarter-by-quarter data 

completeness requirement might not 

represent the true annual mean as well 

as a weighted annual mean calculated 

with a quarter-by-quarter completeness 

requirement. However, the current 

practice of requiring 75% completeness 

of all of the hours in the year and 

calculating the annual mean without 

weighting has been retained in the final 

rule, because of its simplicity and 

because we believe it will not interfere 

with effective implementation of the 

annual NAAQS. No area presently is 

nonattainment for or comes close to 

violating the annual standard. 

Therefore, the choice between the two 

approaches can only have a practical 

effect, if any, on whether at some time 

in the future an area is determined to be 

newly violating the annual standard. If 

a monitor has a complete and valid 

design value below the standard using 

the unweighted mean approach (with 

only an annual data completeness 

requirement) but the design value 

would be considered incomplete and 

invalid under a hypothetical weighted 

mean approach (with a quarterly 

completeness requirement), the monitor 

would in either case be considered not 

to be violating and its data would not be 

the basis for a nonattainment 

designation. If a monitor has a design 

value above the standard using the 

unweighted annual mean approach but 

is incomplete with respect to a 

hypothetical quarterly completeness 

requirement, then the two approaches 

would have different implications for 

the determination of a violation. A 

quarterly completeness requirement 

would make a finding of violation 

impossible, unless the Administrator 

chose to treat the data as if complete 

under another provision of the final 

rule. The unweighted annual mean 

approach would allow but not force a 

finding of violation, because the 

Administrator will have discretion to 

make any such findings because there 

will be no mandatory round of 

designations for the annual standard 

given that the annual standard has not 

been revised in this review. The 

Administrator will be able to consider 

the representativeness of the 

unweighted annual mean when 

deciding whether to make a 

discretionary nonattainment 

redesignation. Given that the annual 

standard requires only one year of 

monitoring data for the calculation of a 

design value, little time will be lost if 

the Administrator chooses to work with 

a State to obtain a new design value 

based on more complete and/or 

seasonally balanced monitoring data. 

B. Interpretation of the Primary NAAQS 

for Oxides of Nitrogen 1-Hour Primary 

Standard 

1. Proposed Interpretation of the 1-Hour 

Standard 

With regard to data completeness for 

the 1-hour primary standard with a 4th 

highest daily value form, the proposed 

Appendix followed past EPA practice 

for other NAAQS pollutants by 

requiring that in general at least 75% of 

the monitoring data that should have 

resulted from following the planned 

monitoring schedule in a period must be 

available for the key air quality statistic 

from that period to be considered valid. 

For the 1-hour primary NO2 NAAQS, 

the key air quality statistics are the daily 

maximum 1-hour concentrations in 

three successive years. It is important 

that sampling within a day encompass 

the period when concentrations are 

likely to be highest and that all seasons 

of the year are well represented. Hence, 

the 75% requirement was proposed to 

be applied at the daily and quarterly 

levels. 

Recognizing that there may be years 

with incomplete data, the proposed text 

provided that a design value derived 

from incomplete data would 

nevertheless be considered valid in 

either of two situations. 

First, if the design value calculated 

from at least four days of monitoring 

observations in each of these years 

exceeds the level of the 1-hour primary 

standard, it would be valid. This 

situation could arise if monitoring was 

intermittent but high NO2 levels were 

measured on enough hours and days for 

the mean of the three annual 4th high 

values to exceed the standard. In this 

situation, more complete monitoring 

could not possibly have indicated that 

the standard was actually met. 

Second, we proposed a diagnostic 

data substitution test which was 

intended to identify those cases with 

incomplete data in which it 

nevertheless is very likely, if not 

virtually certain, that the daily 1-hour 

design value would have been observed 

to be below the level of the NAAQS if 

monitoring data had been minimally 

complete. 

It should be noted that one possible 

outcome of applying the proposed 

substitution test is that a year with 

incomplete data may nevertheless be 

determined to not have a valid design 

value and thus to be unusable in making 

1-hour primary NAAQS compliance 

determinations for that 3-year period. 

Also, we proposed that the 

Administrator have general discretion to 

use incomplete data based on casespecific factors, either at the request of 

a State or at her own initiative. Similar 

provisions exist already for some other 

NAAQS. 

The second version of the proposed 

Appendix S contained proposed 

interpretation procedures for a 1-hour 

primary standard based on the 99th 

percentile daily value form. The 4th 

high daily value form and the 99th 

percentile daily value form would yield 

the same design value in a situation in 

which every hour and day of the year 

has reported monitoring data, since the 

99th percentile of 365 daily values is the 

4th highest value. However, the two 

forms diverge if data completeness is 

82% or less, because in that case the 

99th percentile value is the 3rd highest 

(or higher) value, to compensate for the 

lack of monitoring data on days when 

concentrations could also have been 

high. 

Logically, provisions to address 

possible data incompleteness under the 

99th percentile daily value form should 

be somewhat different from those for the 

4th highest form. With a 4th highest 

form, incompleteness should not 

invalidate a design value that exceeds 

the standard, for reasons explained 

above. With the 99th percentile form, 

however, a design value exceeding the 

standard stemming from incomplete 

data should not automatically be 

considered valid, because 

concentrations on the unmonitored days 

could have been relatively low, such 

that the actual 99th percentile value for 

the year could have been lower, and the 

design value could have been below the 

standard. The second proposed version 

of Appendix S accordingly had 

somewhat different provisions for 

dealing with data incompleteness. One 

difference was the addition of another 

diagnostic test based on data 

substitution, which in some cases can 

validate a design value based on 

incomplete data that exceeds the 

standard. 

The second version of the proposed 

Appendix S provided a table for 

determining which day’s maximum 1- 

hour concentration will be used as the 

99th percentile concentration for the 

year. The proposed table is similar to 

one used now for the 24-hour PM2.5

NAAQS, which is based on a 98th 

percentile form, but adjusted to reflect 


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a 99th percentile form for the 1-hour 

primary NO2 standard. The proposed 

Appendix S also provided instructions 

for rounding (not truncating) the average 

of three annual 99th percentile hourly 

concentrations before comparison to the 

level of the primary NAAQS. 

2. Comments on Interpretation of the 1- 

Hour Standard 

Three commenters expressed the view 

that the 75% completion per quarter 

requirement should apply with respect 

to the 1-hour standard. A fourth 

commenter recommended that the 

requirement be increased to 82%. 

Another person commented that the 

requirement of 75% of the hours in a 

day is too stringent. The commenter 

noted that it would be inappropriate not 

to count the day if the maximum 

concentration observed in the hours 

measured is sufficiently high to make a 

difference with regard to compliance 

with the NAAQS. A comment was 

received that the substitution test 

should not be included, on the grounds 

that nonattainment should not be 

declared without irrefutable proof. This 

commenter also said that the same 

completeness requirement as used for 

nonattainment should be used for 

attainment. We received one comment 

that the computation of design values 

where multiple monitors are present at 

a site should be averaged and not taken 

from a designated primary monitor. 

3. Conclusions on Interpretation of the 

1-Hour Standard 

Consistent with the Administrator’s 

decision to adopt a 98th percentile form 

for the 1-hour NAAQS, the final version 

of Appendix S is based on that form. 

Table 1 has been revised from the 

version that was proposed, so that it 

results in the selection of the 98th 

percentile value rather than the 99th 

percentile value. 

We agree with the three comments 

expressing the view that the 

requirement for 75% data completeness 

per quarter should apply with respect to 

the 1-hour standard. A fourth comment 

recommended that the requirement be 

increased to 82%. We believe 82% is 

too stringent because of the number of 

monitors that would not achieve such a 

requirement and we believe that 75% 

captures the season. We agree that an 

incomplete day should be counted if the 

maximum concentration observed in the 

hours measured is sufficiently high to 

make a difference with regard to 

compliance with the NAAQS, and we 

have accounted for that in section 3.2.c.i 

by validating the design value if it is 

above the level of the primary 1-hour 

standard when at least 75 percent of the 

days in each quarter have at least one 

reported hourly value. We agree that 

substitution should not be used for the 

establishment of attainment/ 

nonattainment. The commenter who 

remarked on this issue appears not to 

have understood that the specific 

proposed substitution tests have 

essentially zero probability of making a 

clean area fail the NAAQS, or vice 

versa, because the substituted values are 

chosen to be conservative against such 

an outcome. As noted in section 

3.2(c)(i), when substitution is used, the 

3-year design value based on the data 

actually reported, not the ‘‘test design 

value’’, shall be used as the valid design 

value. 

In the course of considering the above 

comment regarding data substitution 

tests to be used in cases of data 

incompleteness, EPA has realized that 

there could be some cases of data 

incompleteness in which the proposed 

procedure for calculating the 1-hour 

design value might result in an in 

appropriately low design value. As 

proposed, only days with measurements 

for at least 75% of the hours in the day 

would be considered in any way when 

identifying the 99th percentile value 

(99th for purposes of the adopted 

NAAQS). However, there could be 

individual hours in other, incompletely 

monitored days that had measured 

concentrations higher than the 

identified 98th percentile value from the 

complete days. It would be 

inappropriate not to consider those 

hours and days in some way. However, 

if all days with at least one hourly 

concentration were used to identify the 

99th percentile value without any 

regard to their incompleteness, this 

could also result in a design value that 

is biased low because the extra days 

could increase the number of ‘‘annual 

number of days with valid data’’ enough 

to affect which row of Table 1 of 

Appendix S is used. It could, for 

example, result in the 8th highest 

ranked daily maximum concentration 

being identified as the 98th percentile 

value (based on Table 1 of Appendix S) 

rather than a higher ranked 

concentration; this would also be 

inappropriate because days which were 

not monitored intensively enough to 

give a reasonable likelihood of catching 

the maximum hourly concentration 

would in effect be treated as if they had 

such a likelihood. For example, 50 days 

with only one hourly measurement 

during a time of day with lower 

concentrations would ‘‘earn’’ the State 

the right to drop one notch lower in the 

ranking of days when identifying the 

98th percentile day, inappropriately. 

The final version of Appendix S solves 

this problem by providing that two 

procedures be used to identifying the 

98th percentile value, the first based 

only on days with 75% data 

completeness and the second based on 

all days with at least one hourly 

measurement. The final design value is 

the higher of the two values that result 

from these two procedures. 

With regard to situations with 

multiple monitors operating at one site, 

we think as discussed in the proposal, 

that designation of a primary monitor is 

preferable to averaging the data from 

multiple monitors based on 

administrative simplicity and 

transparency for the public, and is 

unbiased with respect to compliance 

outcome provided the State is able to 

make the designation only before any 

data has been collected. 

Finally, as proposed, the final version 

of Appendix S has a cross reference to 

the Exceptional Events Rule (40 CFR 

50.14) with regard to the exclusion of 

data affected by exceptional events. In 

addition, the specific steps for including 

such data in completeness calculations 

while excluding such data from actual 

design value calculations is clarified in 

Appendix S. 

C. Exceptional Events Information 

Submission Schedule 

The Exceptional Events Rule at 40 

CFR 50.14 contains generic deadlines 

for a State to submit to EPA specified 

information about exceptional events 

and associated air pollutant 

concentration data. A State must 

initially notify EPA that data has been 

affected by an event by July 1 of the year 

after the data are collected; this is done 

by flagging the data in AQS and 

providing an initial event description. 

The State must also, after notice and 

opportunity for public comment, submit 

a demonstration to justify any claim 

within 3 years after the quarter in which 

the data were collected. However, if a 

regulatory decision based on the data 

(for example, a designation action) is 

anticipated, the schedule to flag data in 

AQS and submit complete 

documentation to EPA for review is 

foreshortened, and all information must 

be submitted to EPA no later than one 

year before the decision is to be made. 

These generic deadlines are suitable 

for the period after initial designations 

have been made under a NAAQS, when 

the decision that may depend on data 

exclusion is a redesignation from 

attainment to nonattainment or from 

nonattainment to attainment. However, 

these deadlines present problems with 

respect to initial designations under a 

newly revised NAAQS. One problem is 


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that some of the deadlines, especially 

the deadlines for flagging some relevant 

data, may have already passed by the 

time the revised NAAQS is 

promulgated. Until the level and form of 

the NAAQS have been promulgated a 

State does not know whether the criteria 

for excluding data (which are tied to the 

level and form of the NAAQS) were met 

on a given day. The only way a State 

could guard against this possibility is to 

flag all data that could possibly be 

eligible for exclusion under a future 

NAAQS. This could result in flagging 

far more data than will eventually be 

eligible for exclusion. EPA believes this 

is an inefficient use of State and EPA 

resources, and is potentially confusing 

and misleading to the public and 

regulated entities. Another problem is 

that it may not be feasible for 

information on some exceptional events 

that may affect final designations to be 

collected and submitted to EPA at least 

one year in advance of the final 

designation decision. This could have 

the unintended consequence of EPA 

designating an area nonattainment as a 

result of uncontrollable natural or other 

qualified exceptional events. 

When Section 50.14 was revised in 

March 2007, EPA was mindful that 

designations were needed under the 

recently revised PM2.5 NAAQS, so 

exceptions to the generic deadline were 

included for PM2.5. The EPA was also 

mindful that similar issues would arise 

for subsequent new or revised NAAQS. 

The Exceptional Events Rule at section 

50.14(c)(2)(v) indicates ‘‘when EPA sets 

a NAAQS for a new pollutant, or revises 

the NAAQS for an existing pollutant, it 

may revise or set a new schedule for 

flagging data for initial designation of 

areas for those NAAQS.’’ 

EPA proposed revised exceptional 

event data flagging and documentation 

deadlines in FR 34404 [Federal 

Register/Vol. 74, No. 134/Wednesday, 

July 15, 2009/Proposed Rules] and 

invited comments from the public. The 

Agency received no comments related to 

the revised proposed schedule for NO2

exceptional event data flagging and 

documentation deadlines. 

For the specific case of NO2, EPA 

anticipates that initial designations 

under the revised NAAQS may be made 

by January 22, 2012 based on air quality 

data from the years 2008–2010. (See 

Section VI below for more detailed 

discussion of the designation schedule 

and what data EPA intends to use.) If 

final designations are made by January 

22, 2012, all events to be considered 

during the designations process must be 

flagged and fully documented by States 

one year prior to designations, by 

January 22, 2011. This date also 

coincides with the Clean Air Act 

deadline for Governors to submit to EPA 

their recommendations for designating 

all areas of their States. 

The final rule text at the end of this 

notice shows the changes that will 

apply if a revised NO2 NAAQS is 

promulgated by January 22, 2010, and 

designations are made two years after 

promulgation of a NO2 NAAQS revision. 

Table 1 below summarizes the data 

flagging and documentation deadlines 

corresponding to the two year 

designation schedule discussed in this 

section. If the promulgation date for a 

revised NO2 NAAQS occurs on a 

different date than January 22, 2010, 

EPA will revise the final NO2

exceptional event flagging and 

documentation submission deadlines 

accordingly to provide States with 

reasonably adequate opportunity to 

review, identify, and document 

exceptional events that may affect an 

area designation under a revised 

NAAQS. 

TABLE 1—SCHEDULE FOR EXCEPTIONAL EVENT FLAGGING AND DOCUMENTATION SUBMISSION FOR DATA TO BE USED IN

DESIGNATIONS DECISIONS FOR NEW OR REVISED NAAQS 

NAAQS pollutant/standard/(level)/promulgation date 

Air quality data 

collected for 

calendar year 

Event flagging & initial description deadline 

Detailed documentation submission deadline 

NO2/1-Hour Standard (100 PPB) .............................. 2008 July 1, 2010 a ............................................................ January 22, 2011. 

2009 July 1, 2010 .............................................................. January 22, 2011. 

2010 April 1, 2011a ............................................................ July 1, 2011.a

a Indicates change from general schedule in 40 CFR 50.14. 

Note: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most notably, for data used by EPA for redesignations to attainment. 

V. Designation of Areas 

A. Proposed Process 

The CAA requires EPA and the States 

to take steps to ensure that the new or 

revised NAAQS are met following 

promulgation. The first step is to 

identify areas of the country that do not 

meet the new or revised NAAQS. 

Section 107(d)(1) provides that, ‘‘By 

such date as the Administrator may 

reasonably require, but not later than 1 

year after promulgation of a new or 

revised NAAQS for any pollutant under 

section 109, the Governor of each State 

shall * * * submit to the Administrator 

a list of all areas (or portions thereof) in 

the State’’ that should be designated as 

nonattainment, attainment, or 

unclassifiable for the new NAAQS. 

Section 107(d)(1)(B)(i) further provides, 

‘‘Upon promulgation or revision of a 

NAAQS, the Administrator shall 

promulgate the designations of all areas 

(or portions thereof) * * * as 

expeditiously as practicable, but in no 

case later than 2 years from the date of 

promulgation.’’ 

No later than 120 days prior to 

promulgating designations, EPA is 

required to notify States of any intended 

modifications to their designations as 

EPA may deem necessary. States then 

have an opportunity to comment on 

EPA’s tentative decision. Whether or not 

a State provides a recommendation, the 

EPA must promulgate the designation 

that it deems appropriate. 

Accordingly, Governors must submit 

their initial NO2 designation 

recommendations to EPA no later than 

January 2011. If the Administrator 

intends to modify any State’s 

recommendation, the EPA will notify 

the Governor no later than 120 days 

prior to designations in January 2012. 

States that believe the Administrator’s 

modification is inappropriate will have 

an opportunity to demonstrate why they 

believe their recommendation is more 

appropriate before designations are 

finalized. 

B. Public Comments 

Several industry commenters 

requested that EPA slow the timeline for 

implementing a near-roadway 

monitoring network and designating 

roadway areas because they believe EPA 

lacks significant information about the 

implementation and performance of a 

national, near-roadway monitoring 

network. Two commenters also 

requested that if a near-roadway 

monitoring network is deployed, that 1- 

hour NO2 standards be made more 


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26Since EPA is retaining the annual standard 

without revision, the discussion in this section 

relates to implementation of the proposed 1-hour 

standard, rather than the annual standard. 

lenient until the next review period so 

that more information will be available 

about near-roadway NO2 concentrations 

before a stringent standard is selected. 

A response to commenters’ requests 

that EPA slow the monitoring 

implementation schedule and the 

request that EPA make the 1-hour NO2

standard more lenient until the next 

review period are addressed in sections 

III.B.5 and II.F.4.D, respectively. 

Section 110(d)(1)(B) requires the EPA 

to designate areas no later than 2 years 

following promulgation of a new or 

revised NAAQS (i.e., by January 2012). 

While the CAA provides the Agency an 

additional third year from promulgation 

of a NAAQS to complete designations in 

the event that there is insufficient 

information to make NAAQS 

compliance determinations, we 

anticipate that delaying designations for 

an additional year would not result in 

significant new data to inform the initial 

designations. A near-roadway 

monitoring network is not expected to 

be fully deployed until January 2013 

therefore, EPA must proceed with initial 

designations using air quality data from 

the existing NO2 monitoring network. 

Because none of the current NO2

monitors are sited to measure nearroadway ambient air, we expect that 

most areas in the country with current 

NO2 monitors will not violate the new 

NO2 NAAQS. In the event that a current 

NO2 monitor indicates a violation of the 

revised standards, EPA intends to 

designate such areas ‘‘nonattainment’’ no 

later than 2 years following 

promulgation of the revised standards. 

We intend to designate the rest of the 

country as ‘‘unclassifiable’’ for the 

revised NO2 NAAQS until sufficient air 

quality data is collected from a nearroadway monitoring network. Once the 

near-roadway network is fully deployed 

and 3 years of air quality data are 

available, the EPA has authority under 

the CAA to redesignate areas as 

appropriate from ‘‘unclassifiable’’ to 

‘‘attainment’’ or ‘‘nonattainment.’’ We 

anticipate that sufficient data to conduct 

designations would be available after 

2015. 

A number of commenters, largely 

from industry groups, focused on the 

concern that a near-roadway monitoring 

network would lead to regional 

nonattainment on the basis of high NO2

concentrations found near roadways. 

These commenters requested that any 

future nonattainment areas be limited to 

the area directly surrounding roadways 

found to have above-standard NO2

concentrations. 

The CAA requires that any area that 

does not meet a NAAQS or that 

contributes to a violation in a nearby 

area that does not meet the NAAQS be 

designated ‘‘nonattainment.’’ States and 

EPA will need to determine which 

sources and activities contribute to a 

NAAQS violation in each area. 

Depending on the circumstances in each 

area this may include sources and 

activities in areas beyond the area 

directly surrounding a major roadway. 

EPA intends to issue nonattainment area 

boundary guidance after additional 

information is gathered on the probable 

contributors to violating near-roadway 

NO2 monitors. 

C. Final Designations Process 

The EPA intends to promulgate initial 

NO2 designations by January 2012 (2 

years after promulgation of the revised 

NAAQS). Along with today’s action 

EPA is also promulgating new 

monitoring rules that focus on 

roadways. As noted in section III, States 

must site required NO2 near-roadway 

monitors and have them operational by 

January 1, 2013. States will need an 

additional 3 years thereafter to collect 

air quality data in order to determine 

compliance with the revised NAAQS. 

This means that a full set of air quality 

data from the new network will not be 

available until after 2015. Since we 

anticipate that data from the new 

network will not be available prior to 

the CAA designation deadlines 

discussed above, the EPA intends to 

complete initial NO2 designations by 

January 2012 using the 3 most recent 

years of quality-assured air quality data 

from the current monitoring network, 

which would be for the years 2008– 

2010. The EPA will designate as 

‘‘nonattainment’’ any areas with NO2

monitors recording violations of the 

revised NO2 NAAQS. We intend to 

designate all other areas of the country 

as ‘‘unclassifiable’’ to indicate that there 

is insufficient data to determine 

whether or not they are attaining the 

revised NO2 NAAQS. 

Once the NO2 monitors are positioned 

in locations meeting the near-roadway 

siting requirements and monitoring data 

become available, the Agency has 

authority under section 107(d)(3) of the 

CAA to redesignate areas as appropriate 

from ‘‘unclassifiable’’ to ‘‘attainment’’ or 

‘‘nonattainment.’’ The EPA intends to 

issue guidance on the factors that States 

should consider when determining 

nonattainment boundaries after 

additional information is gathered on 

the probable contributors to violating 

near-roadway NO2 monitors. 

VI. Clean Air Act Implementation 

Requirements 

This section of the preamble discusses 

the Clean Air Act (CAA) requirements 

that States and emissions sources must 

address when implementing new or 

revised NO2 NAAQS based on the 

structure outlined in the CAA and 

existing rules.26 EPA may provide 

additional guidance in the future, as 

necessary, to assist States and emissions 

sources to comply with the CAA 

requirements for implementing new or 

revised NO2 NAAQS. 

The CAA assigns important roles to 

EPA, States, and, in specified 

circumstances, Tribal governments to 

achieve the NAAQS. States have the 

primary responsibility for developing 

and implementing State Implementation 

Plans (SIPs) that contain State measures 

necessary to achieve the air quality 

standards in each area. EPA provides 

assistance to States by providing 

technical tools, assistance, and 

guidance, including information on the 

potential control measures that may 

help areas meet the standards. 

States are primarily responsible for 

ensuring attainment and maintenance of 

ambient air quality standards once they 

have been established by EPA. Under 

section 110 of the CAA, 42 U.S.C. 7410, 

and related provisions, States are 

required to submit, for EPA approval, 

SIPs that provide for the attainment and 

maintenance of such standards through 

control programs directed at sources of 

NO2 emissions. If a State fails to adopt 

and implement the required SIPs by the 

time periods provided in the CAA, the 

EPA has responsibility under the CAA 

to adopt a Federal Implementation Plan 

(FIP) to assure that areas attain the 

NAAQS in an expeditious manner. 

The States, in conjunction with EPA, 

also administer the prevention of 

significant deterioration (PSD) program 

for NO2 and nonattainment new source 

review (NSR). See sections 160–169 of 

the CAA. In addition, Federal programs 

provide for nationwide reductions in 

emissions of NO2 and other air 

pollutants under Title II of the Act, 42 

U.S.C. 7521–7574, which involves 

controls for automobiles, trucks, buses, 

motorcycles, nonroad engines, and 

aircraft emissions; the new source 

performance standards (NSPS) for 

stationary sources under section 111 of 

the CAA, 42 U.S.C. 7411. 

CAA Section 301(d) authorizes EPA to 

treat eligible Indian Tribes in the same 

manner as States (TAS) under the CAA 

and requires EPA to promulgate 

regulations specifying the provisions of 

the statute for which such treatment is 

appropriate. EPA has promulgated these 


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regulations—known as the Tribal 

Authority Rule or TAR—at 40 CFR Part 

49. See 63 FR 7254 (February 12, 1998). 

The TAR establishes the process for 

Indian Tribes to seek TAS eligibility and 

sets forth the CAA functions for which 

TAS will be available. Under the TAR, 

eligible Tribes may seek approval for all 

CAA and regulatory purposes other than 

a small number of functions enumerated 

at section 49.4. Implementation plans 

under section 110 are included within 

the scope of CAA functions for which 

eligible Tribes may obtain approval. 

Section 110(o) also specifically 

describes Tribal roles in submitting 

implementation plans. Eligible Indian 

Tribes may thus submit implementation 

plans covering their reservations and 

other areas under their jurisdiction. 

Under the CAA and TAR, Tribes are 

not, however, required to apply for TAS 

or implement any CAA program. In 

promulgating the TAR EPA explicitly 

determined that it was not appropriate 

to treat Tribes similarly to States for 

purposes of, among other things, 

specific plan submittal and 

implementation deadlines for NAAQSrelated requirements. 40 CFR 49.4(a). In 

addition, where Tribes do seek approval 

of CAA programs, including section 110 

implementation plans, the TAR 

provides flexibility and allows them to 

submit partial program elements, so 

long as such elements are reasonably 

severable—i.e., ‘‘not integrally related to 

program elements that are not included 

in the plan submittal, and are consistent 

with applicable statutory and regulatory 

requirements.’’ 40 CFR 49.7. 

To date, very few Tribes have sought 

TAS for purposes of section 110 

implementation plans. However, some 

Tribes may be interested in pursuing 

such plans to implement today’s 

proposed standard. As noted above, 

such Tribes may seek approval of 

partial, reasonably severable plan 

elements, or they may seek to 

implement all relevant components of 

an air quality program for purposes of 

meeting the requirements of the Act. In 

several sections of this preamble, EPA 

describes the various roles and 

requirements States will address in 

implementing today’s proposed 

standard. Such references to States are 

generally intended to include eligible 

Indian Tribes to the extent consistent 

with the flexibility provided to Tribes 

under the TAR. Where Tribes do not 

seek TAS for section 110 

implementation plans, EPA will 

promulgate Federal implementation 

plans as ‘‘necessary or appropriate to 

protect air quality.’’ 40 CFR 49.11(a). 

EPA also notes that some Tribes operate 

air quality monitoring networks in their 

areas. For such monitors to be used to 

measure attainment with this primary 

NAAQS for NO2, the criteria and 

procedures identified in this rule would 

apply. 

A. Classifications 

1. Proposal 

Section 172(a)(1)(A) of the CAA 

authorizes EPA to classify areas 

designated as nonattainment for the 

purpose of applying an attainment date 

pursuant to section 172(a)(2), or for 

other reasons. In determining the 

appropriate classification, EPA may 

consider such factors as the severity of 

the nonattainment problem and the 

availability and feasibility of pollution 

control measures (see section 

172(a)(1)(A) of the CAA). The EPA may 

classify NO2 nonattainment areas, but is 

not required to do so. The primary 

reason to establish classifications is to 

set different deadlines for each class of 

nonattainment area to complete the 

planning process and to provide for 

different attainment dates based upon 

the severity of the nonattainment 

problem for the affected area. However, 

the CAA separately establishes specific 

planning and attainment deadlines for 

certain pollutants including NO2 in 

sections 191 and 192: 18 months from 

nonattainment designation for the 

submittal of an attainment plan, and as 

expeditiously as possible, but no later 

than 5 years from nonattainment 

designation for areas to attain the 

standard. In the proposal, EPA stated its 

belief that classifications are 

unnecessary in light of these relatively 

short deadlines. 

2. Public Comments 

One commenter stated that they 

disagree with EPA’s decision not to 

impose non-attainment classifications 

on areas with measured near-road NO2

concentrations in excess of the new NO2

standard, and urged EPA to provide a 

graduated non-attainment classification 

system for the new standard. According 

to the commenter, ‘‘a classification 

system defining higher levels of nonattainment with increasingly stringent 

requirements at those levels is one that 

allows for finer calibration of air quality 

regulatory response defined at the 

Federal level.’’ 

As stated in the proposed rule, 

Section 192(a), of part D, of the CAA 

specifically provides an attainment date 

for areas designated as nonattainment 

for the NO2 NAAQS. Therefore, EPA has 

legal authority to classify NO2

nonattainment areas, but the 5-year 

attainment date addressed under section 

192(a) cannot be extended pursuant to 

section 172(a)(2)(D). Based on this 

limitation, EPA proposed not to 

establish classifications within the 5- 

year interval for attaining any new or 

revised NO2 NAAQS. It is also EPA’s 

belief that given the short deadlines that 

States have to develop and submit SIP’s 

and for areas to achieve emissions 

reductions in order to attain the 

standard within the 5 year attainment 

period, a graduated classifications 

system would not be appropriate. 

Therefore, EPA is using it’s discretion 

under the CAA not to establish 

classifications. 

3. Final 

EPA is not making any changes to the 

discussion on classifications in the 

proposed rule. Therefore, there will be 

no classifications for the revised NO2

NAAQS. 

B. Attainment Dates 

The maximum deadline by which an 

area is required to attain the NO2

NAAQS is determined from the effective 

date of the nonattainment designation 

for the affected area. For areas 

designated nonattainment for the 

revised NO2 NAAQS, SIPs must provide 

for attainment of the NAAQS as 

expeditiously as practicable, but no later 

than 5 years from the date of the 

nonattainment designation for the area 

(see section 192(a) of the CAA). The 

EPA will determine whether an area has 

demonstrated attainment of the NO2

NAAQS by evaluating air quality 

monitoring data consistent with the 

form of the NAAQS for NO2 if revised, 

which will be codified at 40 CFR part 

50, Appendix F. 

1. Attaining the NAAQS 

a. Proposal 

In order for an area to be redesignated 

as attainment, the State must comply 

with the five requirements as provided 

under section 107(d)(3)(E) of the CAA. 

This section requires that: 

—EPA must have determined that the 

area has met the NO2 NAAQS; 

—EPA has fully approved the State’s 

implementation plan; 

—The improvement in air quality in the 

affected area is due to permanent and 

enforceable reductions in emissions; 

—EPA has fully approved a 

maintenance plan for the area; and 

—The State(s) containing the area have 

met all applicable requirements under 

section 110 and part D. 

b. Final 

EPA did not receive any comments on 

this aspect of the proposed rule and is 

not making any changes to the 


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27Two elements identified in section 110(a)(2) are 

not listed below because, as EPA interprets the 

CAA, SIPs incorporating any necessary local 

nonattainment area controls would not be due 

within 3 years, but rather are due at the time the 

nonattainment area planning requirements are due. 

These elements are: (1) Emission limits and other 

control measures, section 110(a)(2)(A), and (2) 

Provisions for meeting part D, section 110(a)(2)(I), 

which requires areas designated as nonattainment 

to meet the applicable nonattainment planning 

requirements of part D, title I of the CAA. 

discussion on attaining the NAAQS in 

the proposed rule. 

2. Consequences of Failing To Attain by 

the Statutory Attainment Date 

a. Proposal 

Any NO2 nonattainment area that fails 

to attain by its statutory attainment date 

would be subject to the requirements of 

sections 179(c) and (d) of the CAA. EPA 

is required to make a finding of failure 

to attain no later than 6 months after the 

specified attainment date and publish a 

notice in the Federal Register. The State 

would be required to submit an 

implementation plan revision, no later 

than one year following the effective 

date of the Federal Register notice 

making the determination of the area’s 

failure to attain, which demonstrates 

that the standard will be attained as 

expeditiously as practicable, but no later 

than 5 years from the effective date of 

EPA’s finding that the area failed to 

attain. In addition, section 179(d)(2) 

provides that the SIP revision must 

include any specific additional 

measures as may be reasonably 

prescribed by EPA, including ‘‘all 

measures that can be feasibly 

implemented in the area in light of 

technological achievability, costs, and 

any nonair quality and other air qualityrelated health and environmental 

impacts.’’ 

b. Final 

EPA did not receive any comments on 

this aspect of the proposed rule and is 

not making any changes to the 

discussion on consequences of failing to 

attain by the statutory attainment date 

in the proposed rule. 

C. Section 110(a)(2) NAAQS 

Infrastructure Requirements 

1. Proposal 

Section 110(a)(2) of the CAA requires 

all States to develop and maintain a 

solid air quality management 

infrastructure, including enforceable 

emission limitations, an ambient 

monitoring program, an enforcement 

program, air quality modeling, and 

adequate personnel, resources, and legal 

authority. Section 110(a)(2)(D) also 

requires State plans to prohibit 

emissions from within the State which 

contribute significantly to 

nonattainment or maintenance areas in 

any other State, or which interfere with 

programs under part C to prevent 

significant deterioration of air quality or 

to achieve reasonable progress toward 

the national visibility goal for Federal 

class I areas (national parks and 

wilderness areas). 

Under section 110(a)(1) and (2) of the 

CAA, all States are required to submit 

SIPs to EPA which demonstrate that 

basic program elements have been 

addressed within 3 years of the 

promulgation of any new or revised 

NAAQS. Subsections (A) through (M) of 

section 110(a)(2) listed below, set forth 

the elements that a State’s program must 

contain in the SIP.27 The list of section 

110(a)(2) NAAQS implementation 

requirements are the following: 

• Ambient air quality monitoring/ 

data system: Section 110(a)(2)(B) 

requires SIPs to provide for setting up 

and operating ambient air quality 

monitors, collecting and analyzing data 

and making these data available to EPA 

upon request. 

• Program for enforcement of control 

measures: Section 110(a)(2)(C) requires 

SIPs to include a program providing for 

enforcement of measures and regulation 

and permitting of new/modified 

sources. 

• Interstate transport: Section 

110(a)(2)(D) requires SIPs to include 

provisions prohibiting any source or 

other type of emissions activity in the 

State from contributing significantly to 

nonattainment in another State or from 

interfering with measures required to 

prevent significant deterioration of air 

quality or to protect visibility. 

• Adequate resources: Section 

110(a)(2)(E) requires States to provide 

assurances of adequate funding, 

personnel and legal authority for 

implementation of their SIPs. 

• Stationary source monitoring 

system: Section 110(a)(2)(F) requires 

States to establish a system to monitor 

emissions from stationary sources and 

to submit periodic emissions reports to 

EPA. 

• Emergency power: Section 

110(a)(2)(G) requires States to include 

contingency plans, and adequate 

authority to implement them, for 

emergency episodes in their SIPs. 

• Provisions for SIP revision due to 

NAAQS changes or findings of 

inadequacies: Section 110(a)(2)(H) 

requires States to provide for revisions 

of their SIPs in response to changes in 

the NAAQS, availability of improved 

methods for attaining the NAAQS, or in 

response to an EPA finding that the SIP 

is inadequate. 

• Consultation with local and Federal 

government officials: Section 110(a)(2)(J) 

requires States to meet applicable local 

and Federal government consultation 

requirements when developing SIP and 

reviewing preconstruction permits. 

• Public notification of NAAQS 

exceedances: Section 110(a)(2)(J) 

requires States to adopt measures to 

notify the public of instances or areas in 

which a NAAQS is exceeded. 

• PSD and visibility protection: 

Section 110(a)(2)(J) also requires States 

to adopt emissions limitations, and such 

other measures, as may be necessary to 

prevent significant deterioration of air 

quality in attainment areas and protect 

visibility in Federal Class I areas in 

accordance with the requirements of 

CAA Title I, part C. 

• Air quality modeling/data: Section 

110(a)(2)(K) requires that SIPs provide 

for performing air quality modeling for 

predicting effects on air quality of 

emissions of any NAAQS pollutant and 

submission of data to EPA upon request. 

• Permitting fees: Section 110(a)(2)(L) 

requires the SIP to include requirements 

for each major stationary source to pay 

permitting fees to cover the cost of 

reviewing, approving, implementing 

and enforcing a permit. 

• Consultation and participation by 

affected local government: Section 

110(a)(2)(M) requires States to provide 

for consultation and participation by 

local political subdivisions affected by 

the SIP. 

2. Final 

EPA did not receive any comments on 

this aspect of the proposed rule and is 

not making any changes to the 

discussion on section 110(a)(2) NAAQS 

infrastructure requirements in the 

proposed rule. 

D. Attainment Planning Requirements 

1. Nonattainment Area SIPs 

a. Proposal 

Any State containing an area 

designated as nonattainment with 

respect to the NO2 NAAQS must 

develop for submission a SIP meeting 

the requirements of part D, Title I, of the 

CAA, providing for attainment by the 

applicable statutory attainment date (see 

sections 191(a) and 192(a) of the CAA). 

As indicated in section 191(a) all 

components of the NO2 part D SIP must 

be submitted within 18 months of the 

effective date of an area’s designation as 

nonattainment. 

Section 172 of the CAA includes 

general requirements for all designated 

nonattainment areas. Section 172(c)(1) 


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28The terms ‘‘major’’ and ‘‘minor’’ define the size 

of a stationary source, for applicability purposes, in 

terms of an annual emissions rate (tons per year, 

tpy) for a pollutant. Generally, a minor source is 

any source that is not ‘‘major.’’ ‘‘Major’’ is defined 

by the applicable regulations—PSD or 

nonattainment NSR. 

requires that each nonattainment area 

plan ‘‘provide for the implementation of 

all reasonably available control 

measures (RACM) as expeditiously as 

practicable (including such reductions 

in emissions from existing sources in 

the area as may be obtained through the 

adoption, at a minimum, of Reasonably 

Available Control Technology (RACT)), 

and shall provide for attainment of the 

national primary ambient air quality 

standards.’’ States are required to 

implement RACM and RACT in order to 

attain ‘‘as expeditiously as practicable’’. 

Section 172(c) requires States with 

nonattainment areas to submit a SIP for 

these areas which contains an 

attainment demonstration that shows 

that the affected area will attain the 

standard by the applicable statutory 

attainment date. The State must also 

show that the area will attain the 

standards as expeditiously as 

practicable, and it must include an 

analysis of whether implementation of 

reasonably available measures will 

advance the attainment date for the area. 

Part D SIPs must also provide for 

reasonable further progress (RFP) (see 

section 172(c)(2) of the CAA). The CAA 

defines RFP as ‘‘such annual 

incremental reductions in emissions of 

the relevant air pollution as are required 

by part D, or may reasonably be required 

by the Administrator for the purpose of 

ensuring attainment of the applicable 

NAAQS by the applicable attainment 

date.’’ (See section 171 of the CAA.) 

Historically, for some pollutants, RFP 

has been met by showing annual 

incremental emission reductions 

sufficient to maintain generally linear 

progress toward attainment by the 

applicable attainment date. 

All NO2 nonattainment area SIPs must 

include contingency measures which 

must be implemented in the event that 

an area fails to meet RFP or fails to 

attain the standards by its attainment 

date. (See section 172(c)(9).) These 

contingency measures must be fully 

adopted rules or control measures that 

take effect without further action by the 

State or the Administrator. The EPA 

interprets this requirement to mean that 

the contingency measures must be 

implemented with only minimal further 

action by the State or the affected 

sources with no additional rulemaking 

actions such as public hearings or 

legislative review. 

Emission inventories are also critical 

for the efforts of State, local, and Federal 

agencies to attain and maintain the 

NAAQS that EPA has established for 

criteria pollutants including NO2. 

Section 191(a) in conjunction with 

section 172(c) requires that areas 

designated as nonattainment for NO2

submit an emission inventory to EPA no 

later than 18 months after designation as 

nonattainment. In the case of NO2, 

sections 191(a) and 172(c) also require 

that States submit periodic emission 

inventories for nonattainment areas. The 

periodic inventory must include 

emissions of NO2 for point, nonpoint, 

mobile (on-road and non-road), and area 

sources. 

b. Public Comments 

Several commenters indicated that 

EPA should take steps to ensure that 

States actually require mobile source 

emissions reductions in order to attain 

the NO2 NAAQS as opposed to 

controlling point sources. Another 

commenter went further and stated that 

States be required to control on-road 

emissions as opposed to emissions from 

stationary sources and in particular 

EGUs. This commenter also indicated 

that EPA should delay nonattainment 

designations until States had a cost 

effective means of reducing on-road 

emissions of NO2. 

EPA cannot require States to develop 

a SIP that only addresses one type of 

source, in this case on-road mobile 

sources. States may select appropriate 

control measures to attain the NAAQS 

and EPA must approve them if they 

otherwise meet all applicable 

requirements of the Act. See CAA 116. 

EPA expects that States will evaluate a 

range of control measures that will 

reduce NO2 emissions within the time 

allowed to attain the standard. This 

would include the emissions reductions 

attributable to Federal controls on onroad and non-road mobile sources, and 

controls that they have put in place to 

reduce NOX emissions in order to attain 

the 8-hour ozone NAAQS and/or the 

PM2.5 NAAQS. If these existing controls 

are not sufficient for an area to reach 

attainment with the NO2 NAAQS, EPA 

would expect the State to implement 

additional control measures that would 

bring the area into attainment by the 

deadline. For a designation based on 

data from a near roadway monitor EPA 

would expect the States to give primary 

consideration to controlling emissions 

from on-road sources; however, it is 

likely that other types of sources 

contribute to the concentrations that are 

measured at a near roadway monitor 

and a State may decide to implement 

controls on these other contributing 

sources. 

The Clean Air Act requires that EPA 

finalize designations within two years 

after a NAAQS is revised unless the 

available air quality data is insufficient 

to make designations by that time. In 

that case, EPA must finalize 

designations within three years after the 

NAAQS is revised. As discussed 

elsewhere in today’s final rule, EPA 

believes that it has sufficient data to 

make designations within two years and 

that most areas will be designated as 

unclassifiable at that time. Taking the 

additional year provided by the CAA 

would not allow additional data from 

the new near roadway monitors to be 

factored into the designations process in 

any event. Therefore, it is EPA’s 

intention to designate areas within two 

years as required by the Act. EPA 

intends to redesignate areas once it has 

sufficient data from the new monitoring 

network to designate areas as clearly 

attaining or not attaining the standard. 

c. Final 

The EPA is not making any changes 

to the discussion on nonattainment area 

SIPs in the proposed rule. 

2. New Source Review and Prevention 

of Significant Deterioration 

Requirements 

a. Proposal 

The Prevention of Significant 

Deterioration (PSD) and nonattainment 

New Source Review (NSR) programs 

contained in parts C and D of Title I of 

the CAA govern preconstruction review 

of any new or modified major stationary 

sources of air pollutants regulated under 

the CAA as well as any precursors to the 

formation of that pollutant when 

identified for regulation by the 

Administrator.28 The EPA rules 

addressing these programs can be found 

at 40 CFR 51.165, 51.166, 52.21, 52.24, 

and part 51, appendix S. States which 

have areas designated as nonattainment 

for the NO2 NAAQS must submit, as a 

part of the SIP due 18 months after an 

area is designated as nonattainment, 

provisions requiring permits for the 

construction and operation of new or 

modified stationary sources anywhere 

in the nonattainment area. SIPs that 

address the PSD requirements related to 

attainment areas are due no later than 3 

years after the promulgation of a revised 

NAAQS for NO2. 

The NSR program is composed of 

three different permit programs: 

• Prevention of Significant 

Deterioration (PSD). 

• Nonattainment NSR (NA NSR). 

• Minor NSR. 

The PSD program applies when a 

major source, that is located in an area 

that is designated as attainment or 


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29 In addition, the PSD program applies to noncriteria pollutants subject to regulation under the 

Act, except those pollutants regulated under section 

112 and pollutants subject to regulation only under 

section 211(o). 

30Criteria pollutants are those pollutants for 

which EPA has established a NAAQS under section 

109 of the CAA. 

unclassifiable for any criteria pollutant, 

is constructed, or undergoes a major 

modification.29 The nonattainment NSR 

program applies on a pollutant-specific 

basis when a major source constructs or 

modifies in an area that is designated as 

nonattainment for that pollutant. The 

minor source NSR program addresses 

both major and minor sources which 

undergo construction or modification 

activities that do not qualify as major, 

and it applies, as necessary to assure 

attainment, regardless of the designation 

of the area in which a source is located. 

The PSD requirements include but are 

not limited to the following: 

• Installation of Best Available 

Control Technology (BACT); 

• Air quality monitoring and 

modeling analyses to ensure that a 

project’s emissions will not cause or 

contribute to a violation of any NAAQS 

or maximum allowable pollutant 

increase (PSD increment); 

• Notification of Federal Land 

Manager of nearby Class I areas; and 

• Public comment on permit. 

Nonattainment NSR requirements 

include but are not limited to: 

• Installation of Lowest Achievable 

Emissions Rate (LAER) control 

technology; 

• Offsetting new emissions with 

creditable emissions reductions; 

• A certification that all major 

sources owned and operated in the State 

by the same owner are in compliance 

with all applicable requirements under 

the CAA; 

• An alternative siting analysis 

demonstrating that the benefits of a 

proposed source significantly outweigh 

the environmental and social costs 

imposed as a result of its location, 

construction, or modification; and 

• Public comment on the permit. 

Minor NSR programs must meet the 

statutory requirements in section 

110(a)(2)(C) of the CAA which requires 

‘‘* * * regulation of the modification 

and construction of any stationary 

source * * * as necessary to assure that 

the [NAAQS] are achieved.’’ Areas 

which are newly designated as 

nonattainment for the NO2 NAAQS as a 

result of any changes made to the 

NAAQS will be required to adopt a 

nonattainment NSR program to address 

major sources of NO2 where the program 

does not currently exist for the NO2

NAAQS and may need to amend their 

minor source program as well. Prior to 

adoption of the SIP revision addressing 

major source nonattainment NSR for 

NO2 nonattainment areas, the 

requirements of 40 CFR part 51, 

appendix S may apply. 

b. Public Comments 

One commenter claimed that EPA’s 

setting of a more stringent standard, i.e., 

short-term NO2 NAAQS, could have 

important implications for NSR and 

PSD and title V permits. Another 

commenter indicated that the 

promulgation of a new 1-hr NO2 shortterm standard could create the need for 

a short-term PSD increment. Another 

commenter stated that a 1-hr NO2

Significant Impact Level (SIL) should be 

developed. 

The EPA acknowledges that a 

decision to promulgate a new short-term 

NO2 NAAQS will clearly have 

implications for the air permitting 

process. The full extent of how a new 

short-term NO2 NAAAQS will affect the 

NSR process will need to be carefully 

evaluated. First, major new and 

modified sources applying for NSR/PSD 

permits will initially be required to 

demonstrate that their proposed 

emissions increases of NOX will not 

cause or contribute to a violation of 

either the annual or 1-hour NO2 NAAQS 

and the annual PSD increment. In 

addition, we believe that section 166 of 

the CAA authorizes us to consider the 

need to promulgate a new 1-hour 

increment. Historically, EPA has 

developed increments for each 

applicable averaging period for which a 

NAAQS has been promulgated. 

However, increments for a particular 

pollutant do not necessarily need to 

match the averaging periods that have 

been established for NAAQS for the 

same pollutant. Environmental Defense 

Fund, Inc. v. EPA, 898 F.2d 183, 189– 

190 (DC Cir. 1990) (‘‘ * * * the ‘goals 

and purposes’ of the PSD program, set 

forth in 160, are not identical to the 

criteria on which the ambient standards 

are based.’’) Thus, we would need to 

evaluate the need for a new 1-hour NO2

increment in association with the goals 

and purposes of the statutory PSD 

program requirements. 

We also believe that there may be a 

need to revise the screening tools 

currently used under the NSR/PSD 

program for completing NO2 analyses. 

These screening tools include the 

significant impact levels (SILs), as 

mentioned by one commenter, but also 

include the significant emissions rate 

for emissions of NOX and the significant 

monitoring concentration (SMC) for 

NO2. EPA intends to evaluate the need 

for possible changes or additions to each 

of these important screening tools for 

NOX/NO2 due to the addition of a 

1-hour NO2 NAAQS. If changes or 

additions are deemed necessary, EPA 

will propose any such changes for 

public notice and comment in a separate 

action. 

c. Final 

The EPA is not making any changes 

to the discussion concerning the 

requirements for NSR and PSD as stated 

in the proposed rule. 

3. General Conformity 

a. Proposal 

Section 176(c) of the CAA, as 

amended (42 U.S.C. 7401 et seq.), 

requires that all Federal actions conform 

to an applicable implementation plan 

developed pursuant to section 110 and 

part D of the CAA. The EPA rules, 

developed under the authority of 

section 176(c) of the CAA, prescribe the 

criteria and procedures for 

demonstrating and assuring conformity 

of Federal actions to a SIP. Each Federal 

agency must determine that any actions 

covered by the general conformity rule 

conform to the applicable SIP before the 

action is taken. The criteria and 

procedures for conformity apply only in 

nonattainment areas and those areas 

redesignated attainment since 1990 

(‘‘maintenance areas’’) with respect to 

the criteria pollutants under the CAA: 30

carbon monoxide (CO), lead (Pb), 

nitrogen dioxide (NO2), ozone (O3), 

particulate matter (PM2.5 and PM10), and 

sulfur dioxide (SO2). The general 

conformity rules apply one year 

following the effective date of 

designations for any new or revised 

NAAQS. 

The general conformity determination 

examines the impacts of direct and 

indirect emissions related to Federal 

actions. The general conformity rule 

provides several options to satisfy air 

quality criteria, such as modeling or 

offsets, and requires the Federal action 

to also meet any applicable SIP 

requirements and emissions milestones. 

The general conformity rule also 

requires that notices of draft and final 

general conformity determinations be 

provided directly to air quality 

regulatory agencies and to the public by 

publication in a local newspaper. 

b. Final 

EPA did not receive any comments on 

this aspect of the proposed rule and is 

not making any changes to the 

discussion concerning general 

conformity stated in the proposed rule. 


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4. Transportation Conformity 

a. Proposal 

Transportation conformity is required 

under CAA section 176(c) (42 U.S.C. 

7506(c)) to ensure that transportation 

plans, transportation improvement 

programs (TIPs) and Federally 

supported highway and transit projects 

will not cause new air quality 

violations, worsen existing violations, or 

delay timely attainment of the relevant 

NAAQS or interim reductions and 

milestones. Transportation conformity 

applies to areas that are designated 

nonattainment and maintenance for 

transportation-related criteria 

pollutants: Carbon monoxide (CO), 

ozone (O3), nitrogen dioxide (NO2), and 

particulate matter (PM2.5 and PM10). 

Transportation conformity for a revised 

NO2 NAAQS does not apply until one 

year after the effective date of a 

nonattainment designation. (See CAA 

section 176(c)(6) and 40 CFR 93.102(d)). 

EPA’s Transportation Conformity 

Rule (40 CFR 51.390, and Part 93, 

Subpart A establishes the criteria and 

procedures for determining whether 

transportation activities conform to the 

SIP. The EPA is not making changes to 

the Transportation Conformity rule in 

this rulemaking. However, in the future, 

EPA will review the need to conduct a 

rulemaking to establish any new or 

revised transportation conformity tests 

that would apply under a revision to the 

NO2 NAAQS for transportation plans, 

TIPs, and applicable highway and 

transit projects. 

b. Public Comments 

Several commenters stated that 

transportation conformity could stop the 

funding of highway and transit projects 

in NO2 nonattainment areas. These 

commenters stated that if an area fails 

to demonstrate conformity, it enters a 

conformity lapse and only certain types 

of projects can be funded during a lapse. 

The commenters further stated that the 

NO2 NAAQS will require more areas to 

determine conformity for the first time. 

The commenters also expressed concern 

that the NO2 NAAQS proposal did not 

contain sufficient information to 

understand to what extent revisions to 

the NAAQS, and the NO2 monitoring 

requirements, will result in 

transportation conformity requirements 

for individual transportation projects 

such as the need for a hot-spot analysis. 

The commenters further stated that hotspot analyses could result in needless 

delays for transportation improvement 

projects. 

With regard to the comment that more 

areas will have to demonstrate 

conformity for the first time due to the 

revisions to the NO2 NAAQS, given that 

today’s final rule is requiring that near 

roadway monitoring be carried out in 

urban areas with populations greater 

than 350K, EPA believes that most areas 

with such populations that would be 

designated nonattainment for NO2 are 

already designated nonattainment or 

maintenance for one or more of the 

other transportation-related criteria 

pollutants (ozone, PM2.5, PM10 and 

carbon monoxide). As such, these areas 

would have experience in making 

transportation conformity 

determinations. If areas with no 

conformity experience are designated 

nonattainment for the NO2 NAAQS, 

EPA and U.S. DOT would be available 

to assist areas in implementing the 

transportation conformity requirements. 

The commenter expressed concern 

that transportation conformity could 

stop highway and transit funding 

because areas could experience a 

conformity lapse and in such cases only 

certain types of projects could be 

funded. A conformity lapse occurs 

when an area misses a deadline for a 

required conformity determination. A 

new nonattainment area must 

demonstrate conformity within one year 

after the effective date of its designation. 

For any areas designated nonattainment 

for the revised NO2 NAAQS in early2012, they would have to determine 

conformity within one year of the 

effective date of that designation which 

would be in early-2013. If that date was 

missed, a lapse would occur and only 

projects exempt from conformity such 

as safety projects, transportation control 

measures in an approved SIP for the 

area and projects or project phases that 

were approved by U.S. DOT before the 

lapse began can proceed during the 

lapse. EPA’s experience in 

implementing the 1997 ozone and PM2.5

NAAQS shows that nearly all areas 

make their initial conformity 

determinations within the one-year 

grace period. Areas can also lapse if 

they fail to determine conformity by an 

applicable deadline such as determining 

conformity within two years after motor 

vehicle emissions budgets are found 

adequate. However, areas that miss one 

of these conformity deadlines have a 

one-year grace period before the lapse 

goes into effect. During the grace period, 

the area can continue to advance 

projects from the transportation plan 

and transportation improvement 

program. EPA’s experience is that areas 

generally are able to make a conformity 

determination before the end of the 

grace period. 

The commenter expressed concern 

that the NO2 NAAQS proposal did not 

contain sufficient detail concerning 

possible project-level requirements for 

transportation projects and that any 

requirements for hot-spot analyses 

could needlessly delay transportation 

projects. As EPA indicated in the 

NPRM, EPA is considering whether to 

revise the transportation conformity rule 

to establish requirements that would 

apply to transportation plans, 

transportation improvement programs 

and/or transportation projects in NO2

nonattainment and maintenance areas. 

If EPA concludes that the conformity 

rule must be revised in light of the final 

NO2 NAAQS, we will conduct notice 

and comment rulemaking to accomplish 

the revisions. At that time interested 

parties will have the opportunity to 

comment on any transportation 

conformity NPRM. This is the same 

course of action that EPA has taken with 

respect to revising the transportation 

conformity rule for the ozone and PM2.5

NAAQS. 

With regard to the commenter’s 

assertion that a requirement for hot-spot 

analyses for individual projects would 

needlessly delay transportation projects, 

EPA disagrees. First, CAA section 

176(c)(1)(B) requires that transportation 

projects not cause new violations or 

make existing violations worse, or delay 

timely attainment or cause an interim 

milestone to be missed. EPA would only 

impose a hot-spot requirement for 

projects in NO2 nonattainment and 

maintenance areas if they are necessary 

to comply with CAA conformity 

requirements and therefore are needed 

to protect public health by reducing 

exposures to unhealthy levels of NO2

that could be created by the 

implementation of a proposed highway 

or transit project. The public would be 

exposed to unhealthy levels of NO2 if a 

highway or transit project caused a new 

violation of the NO2 NAAQS, made an 

existing violation worse, or delayed 

timely attainment or delayed achieving 

an interim emissions milestone. If any 

delay in the project did occur, it would 

not be viewed as needless as it occurred 

for the important purpose of protecting 

the exposed public’s health. Second, 

EPA does not agree that requiring a hotspot analysis would needlessly delay 

projects in NO2 nonattainment areas. 

Such hot-spot analyses, if they are 

eventually required, generally would be 

done as part of the NEPA process, 

which these projects are already subject 

to; therefore, conducting an NO2 hotspot analysis would not be introducing 

a new step to a project’s approval 

process, but rather would add one 

additional analysis which must be 

completed as part of an existing project 

approval process. 


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c. Final 

EPA is not making any changes to the 

discussion concerning transportation 

conformity as stated in the proposed 

rule. 

VII. Communication of Public Health 

Information 

Information on the public health 

implications of ambient concentrations 

of criteria pollutants is currently made 

available primarily through EPA’s Air 

Quality Index (AQI) program. This 

section describes the conforming 

changes that were proposed, major 

comments received on these changes, 

EPA’s responses to these comments and 

final decisions on the AQI breakpoints. 

Recognizing the importance of revising 

the AQI in a timely manner to be 

consistent with any revisions to the 

NAAQS, EPA proposed conforming 

changes to the AQI in connection with 

the final decision on the NO2 NAAQS 

if revisions to the primary standard 

were promulgated. Conforming changes 

would include setting the 100 level of 

the AQI at the same level as the revised 

primary NO2 NAAQS and also setting 

the other AQI breakpoints at the lower 

end of the AQI scale (i.e., AQI values of 

50 and 150). EPA did not propose to 

change breakpoints at the higher end of 

the AQI scale (from 200 to 500), which 

would apply to State contingency plans 

or the Significant Harm Level (40 CFR 

51.16), because the information from 

this review does not inform decisions 

about breakpoints at those higher levels. 

With regard to an AQI value of 50, the 

breakpoint between the good and 

moderate categories, EPA proposed to 

set this value to be between 0.040 and 

0.053 ppm NO2, 1-hour average. EPA 

proposed that the figure towards the 

lower end of this range would be 

appropriate if the standard is set 

towards the lower end of the proposed 

range for the standard (e.g. 80 ppb), 

while figures towards the higher end of 

the range would be more appropriate for 

standards set at the higher end of the 

range for the standard (e.g., 100 ppb). 

EPA noted that historically this value is 

set at the level of the annual NAAQS, 

if there is one, or one-half the level of 

the short-term NAAQS in the absence of 

an annual NAAQS, and solicited 

comments on this range for an AQI of 

50 and the appropriate basis for 

selecting an AQI of 50 within this range. 

With regard to an AQI value of 150, 

the breakpoint between the unhealthy 

for sensitive groups and unhealthy 

categories, the range of 0.360 to 0.370 

ppm NO2, 1-hour average, represents the 

midpoint between the proposed range 

for the short-term standard and the level 

of an AQI value of 200 (0.64 ppm NO2, 

1-hour average). Therefore, EPA 

proposed to set the AQI value of 150 to 

be between 0.360 and 0.370 ppm NO2, 

1-hour average. 

EPA received comments from several 

State environmental agencies and 

organizations of State and local agencies 

that generally expressed the view that 

the AQI was designed to provide the 

public with information about regional 

air quality and therefore it should be 

based on community-wide monitors. 

These commenters went on to state that 

using near-road NO2 monitors for the 

AQI would present problems because 

they would not represent regional NO2

concentrations and it would be difficult 

to communicate this type of information 

to the public using the AQI. Some 

expressed concern that NO2 measured at 

near-roadway monitors could be the 

critical pollutant and could drive the 

AQI even though it may not represent 

air quality across the area. Other 

agencies expressed concern that there is 

currently no way to forecast ambient 

NO2 levels near roadways. One State 

agency commented that the AQI is 

intended to represent air quality where 

people live, work and play. 

EPA agrees with commenters that the 

AQI should represent regional air 

quality, and that measurements that 

apply to a limited area should not be 

used to characterize air quality across 

the region. Community-wide NO2

monitors should be used to characterize 

air quality across the region. However, 

the AQI reporting requirements 

encourage, but do not require, the 

reporting of index values of sub-areas of 

an MSA. We agree with the commenter 

that stated the view that the AQI is 

intended to represent air quality where 

people live, work and play. To the 

extent that near-roadway monitoring 

occurs in areas where people live, work 

or play, EPA encourages reporting of the 

AQI for that specific sub-area of the 

MSA (64 FR 42548, August 4, 1999). We 

also agree that it may be difficult to 

communicate this type of information 

and we plan to work with State and 

local air agencies to figure out the best 

way to present this information to the 

public using the AQI. Air quality 

forecasting is recommended but not 

required (64 FR 42548, August 4, 1999). 

EPA will work with State agencies that 

want to develop a forecasting program. 

With regard to the proposed 

breakpoints, EPA received few 

comments. The National Association of 

Clean Air Agencies commented that it 

would be confusing to the public to 

have an AQI value of 50 set below the 

level of the annual NO2 standard. We 

agree with this comment, and therefore 

have decided that it is appropriate to set 

the AQI value of 50, the breakpoint 

between the good and moderate ranges, 

set at the numerical level of the annual 

standard, 53 ppb NO2, 1-hour average. 

The AQI value of 100, the breakpoint 

between the moderate and unhealthy for 

sensitive groups category, is set at 100 

ppb, 1-hour average, the level of the 

primary NO2 NAAQS. EPA is setting an 

AQI value of 150, the breakpoint 

between the unhealthy for sensitive 

groups and unhealthy categories, at 

0.360 ppm NO2, 1-hour average. 

VIII. Statutory and Executive Order 

Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review 

Under Executive Order 12866 (58 FR 

51735, October 4, 1993), this action is a 

‘‘significant regulatory action’’ because it 

was deemed to ‘‘raise novel legal or 

policy issues.’’ Accordingly, EPA 

submitted this action to the Office of 

Management and Budget (OMB) for 

review under Executive Order 12866 

and any changes made in response to 

OMB recommendations have been 

documented in the docket for this 

action. In addition, EPA prepared a 

Regulatory Impact Analysis (RIA) of the 

potential costs and benefits associated 

with this action. However, the CAA and 

judicial decisions make clear that the 

economic and technical feasibility of 

attaining ambient standards are not to 

be considered in setting or revising 

NAAQS, although such factors may be 

considered in the development of State 

plans to implement the standards. 

Accordingly, although an RIA has been 

prepared, the results of the RIA have not 

been considered in developing this final 

rule. 

B. Paperwork Reduction Act 

The information collection 

requirements in this final rule have been 

submitted for approval to the Office of 

Management and Budget (OMB) under 

the Paperwork Reduction Act, 44 U.S.C. 

3501 et seq. The information collection 

requirements are not enforceable until 

OMB approves them. 

The Information Collection Request 

(ICR) document prepared by EPA for 

these revisions to part 58 has been 

assigned EPA ICR number 2358.02. 

The information collected under 40 

CFR part 53 (e.g., test results, 

monitoring records, instruction manual, 

and other associated information) is 

needed to determine whether a 

candidate method intended for use in 

determining attainment of the National 

Ambient Air Quality Standards 

(NAAQS) in 40 CFR part 50 will meet 


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6528 Federal Register / Vol. 75, No. 26 / Tuesday, February 9, 2010 / Rules and Regulations 

the design, performance, and/or 

comparability requirements for 

designation as a Federal reference 

method (FRM) or Federal equivalent 

method (FEM). We do not expect the 

number of FRM or FEM determinations 

to increase over the number that is 

currently used to estimate burden 

associated with NO2 FRM/FEM 

determinations provided in the current 

ICR for 40 CFR part 53 (EPA ICR 

numbers 2358.01). As such, no change 

in the burden estimate for 40 CFR part 

53 has been made as part of this 

rulemaking. 

The information collected and 

reported under 40 CFR part 58 is needed 

to determine compliance with the 

NAAQS, to characterize air quality and 

associated health impacts, to develop 

emissions control strategies, and to 

measure progress for the air pollution 

program. The amendments would revise 

the technical requirements for NO2

monitoring sites, require the siting and 

operation of additional NO2 ambient air 

monitors, and the reporting of the 

collected ambient NO2 monitoring data 

to EPA’s Air Quality System (AQS). The 

annual average reporting burden for the 

collection under 40 CFR part 58 

(averaged over the first 3 years of this 

ICR) is $3,261,007. Burden is defined at 

5 CFR 1320.3(b). State, local, and Tribal 

entities are eligible for State assistance 

grants provided by the Federal 

government under the CAA which can 

be used for monitors and related 

activities. 

An agency may not conduct or 

sponsor, and a person is not required to 

respond to, a collection of information 

unless it displays a currently valid OMB 

control number. The OMB control 

numbers for EPA’s regulations in 40 

CFR are listed in 40 CFR part 9. 

C. Regulatory Flexibility Act 

The Regulatory Flexibility Act (RFA) 

generally requires an agency to prepare 

a regulatory flexibility analysis of any 

rule subject to notice and comment 

rulemaking requirements under the 

Administrative Procedure Act or any 

other statute unless the agency certifies 

that the rule will not have a significant 

economic impact on a substantial 

number of small entities. Small entities 

include small businesses, small 

organizations, and small governmental 

jurisdictions. 

For purposes of assessing the impacts 

of this rule on small entities, small 

entity is defined as: (1) A small business 

that is a small industrial entity as 

defined by the Small Business 

Administration’s (SBA) regulations at 13 

CFR 121.201; (2) a small governmental 

jurisdiction that is a government of a 

city, county, town, school district or 

special district with a population of less 

than 50,000; and (3) a small 

organization that is any not-for-profit 

enterprise which is independently 

owned and operated and is not 

dominant in its field. 

After considering the economic 

impacts of this final rule on small 

entities, I certify that this action will not 

have a significant economic impact on 

a substantial number of small entities. 

This final rule will not impose any 

requirements on small entities. Rather, 

this rule establishes national standards 

for allowable concentrations of NO2 in 

ambient air as required by section 109 

of the CAA. American Trucking Ass’ns 

v. EPA, 175 F.3d 1027, 1044–45 (DC cir. 

1999) (NAAQS do not have significant 

impacts upon small entities because 

NAAQS themselves impose no 

regulations upon small entities). 

Similarly, the amendments to 40 CFR 

part 58 address the requirements for 

States to collect information and report 

compliance with the NAAQS and will 

not impose any requirements on small 

entities. 

D. Unfunded Mandates Reform Act 

This rule does not contain a Federal 

mandate that may result in expenditures 

of $100 million or more for State, local, 

and Tribal governments, in the 

aggregate, or the private sector in any 

one year. The revisions to the NO2

NAAQS impose no enforceable duty on 

any State, local or Tribal governments or 

the private sector. The expected costs 

associated with the monitoring 

requirements are described in EPA’s ICR 

document, but those costs are not 

expected to exceed $100 million in the 

aggregate for any year. Furthermore, as 

indicated previously, in setting a 

NAAQS EPA cannot consider the 

economic or technological feasibility of 

attaining ambient air quality standards. 

Because the Clean Air Act prohibits 

EPA from considering the types of 

estimates and assessments described in 

section 202 when setting the NAAQS, 

the UMRA does not require EPA to 

prepare a written statement under 

section 202 for the revisions to the NO2

NAAQS. Thus, this rule is not subject to 

the requirements of sections 202 or 205 

of UMRA. 

With regard to implementation 

guidance, the CAA imposes the 

obligation for States to submit SIPs to 

implement the NO2 NAAQS. In this 

final rule, EPA is merely providing an 

interpretation of those requirements. 

However, even if this rule did establish 

an independent obligation for States to 

submit SIPs, it is questionable whether 

an obligation to submit a SIP revision 

would constitute a Federal mandate in 

any case. The obligation for a State to 

submit a SIP that arises out of section 

110 and section 191 of the CAA is not 

legally enforceable by a court of law, 

and at most is a condition for continued 

receipt of highway funds. Therefore, it 

is possible to view an action requiring 

such a submittal as not creating any 

enforceable duty within the meaning of 

2 U.S.C. 658 for purposes of the UMRA. 

Even if it did, the duty could be viewed 

as falling within the exception for a 

condition of Federal assistance under 

2 U.S.C. 658. 

This rule is also not subject to the 

requirements of section 203 of UMRA 

because it contains no regulatory 

requirements that might significantly or 

uniquely affect small governments 

because it imposes no enforceable duty 

on any small governments. 

E. Executive Order 13132: Federalism 

This action does not have federalism 

implications. It will not have substantial 

direct effects on the States, on the 

relationship between the national 

government and the States, or on the 

distribution of power and 

responsibilities among the various 

levels of government, as specified in 

Executive Order 13132. The rule does 

not alter the relationship between the 

Federal government and the States 

regarding the establishment and 

implementation of air quality 

improvement programs as codified in 

the CAA. Under section 109 of the CAA, 

EPA is mandated to establish NAAQS; 

however, CAA section 116 preserves the 

rights of States to establish more 

stringent requirements if deemed 

necessary by a State. Furthermore, this 

rule does not impact CAA section 107 

which establishes that the States have 

primary responsibility for 

implementation of the NAAQS. Finally, 

as noted in section E (above) on UMRA, 

this rule does not impose significant 

costs on State, local, or Tribal 

governments or the private sector. Thus, 

Executive Order 13132 does not apply 

to this rule. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

This action does not have Tribal 

implications, as specified in Executive 

Order 13175 (65 FR 67249, November 9, 

2000). It does not have a substantial 

direct effect on one or more Indian 

Tribes, on the relationship between the 

Federal government and Indian Tribes, 

or on the distribution of power and 

responsibilities between the Federal 

government and Tribes. The rule does 

not alter the relationship between the 


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Federal government and Tribes as 

established in the CAA and the TAR. 

Under section 109 of the CAA, EPA is 

mandated to establish NAAQS; 

however, this rule does not infringe 

existing Tribal authorities to regulate air 

quality under their own programs or 

under programs submitted to EPA for 

approval. Furthermore, this rule does 

not affect the flexibility afforded to 

Tribes in seeking to implement CAA 

programs consistent with the TAR, nor 

does it impose any new obligation on 

Tribes to adopt or implement any 

NAAQS. Finally, as noted in section E 

(above) on UMRA, this rule does not 

impose significant costs on Tribal 

governments. Thus, Executive Order 

13175 does not apply to this action. 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

Risks and Safety Risks 

This action is subject to Executive 

Order 13045 (62 FR 19885, April 23, 

1997) because it is an economically 

significant regulatory action as defined 

by Executive Order 12866, and EPA 

believes that the environmental health 

or safety risk addressed by this action 

has a disproportionate effect on 

children. The final rule will establish 

uniform national ambient air quality 

standards for NO2; these standards are 

designed to protect public health with 

an adequate margin of safety, as 

required by CAA section 109. The 

protection offered by these standards 

may be especially important for 

asthmatics, including asthmatic 

children, because respiratory effects in 

asthmatics are among the most sensitive 

health endpoints for NO2 exposure. 

Because asthmatic children are 

considered a sensitive population, we 

have evaluated the potential health 

effects of exposure to NO2 pollution 

among asthmatic children. These effects 

and the size of the population affected 

are discussed in chapters 3 and 4 of the 

ISA; chapters 3, 4, and 8 of the REA, 

and sections II.A through II.E of this 

preamble. 

H. Executive Order 13211: Actions 

Concerning Regulations That 

Significantly Affect Energy Supply, 

Distribution or Use 

This action is not a ‘‘significant energy 

action’’ as defined in Executive Order 

13211, ‘‘Actions Concerning Regulations 

That Significantly Affect Energy Supply, 

Distribution, or Use’’ (66 FR 28355 (May 

22, 2001)) because it is not likely to 

have a significant adverse effect on the 

supply, distribution, or use of energy. 

The purpose of this rule is to establish 

revised NAAQS for NO2. The rule does 

not prescribe specific control strategies 

by which these ambient standards will 

be met. Such strategies will be 

developed by States on a case-by-case 

basis, and EPA cannot predict whether 

the control options selected by States 

will include regulations on energy 

suppliers, distributors, or users. Thus, 

EPA concludes that this rule is not 

likely to have any adverse energy 

effects. 

I. National Technology Transfer and 

Advancement Act 

Section 12(d) of the National 

Technology Transfer and Advancement 

Act of 1995 (NTTAA), Public Law 104– 

113, section 12(d) (15 U.S.C. 272 note) 

directs EPA to use voluntary consensus 

standards in its regulatory activities 

unless to do so would be inconsistent 

with applicable law or otherwise 

impractical. Voluntary consensus 

standards are technical standards (e.g., 

materials specifications, test methods, 

sampling procedures, and business 

practices) that are developed or adopted 

by voluntary consensus standards 

bodies. The NTTAA directs EPA to 

provide Congress, through OMB, 

explanations when the Agency decides 

not to use available and applicable 

voluntary consensus standards. 

This final rulemaking involves 

technical standards. Therefore the 

Agency conducted a search to identify 

potential applicable voluntary 

consensus standards. However, we 

identified no such standards, and none 

were brought to our attention in 

comments. Therefore, EPA has decided 

to use the technical standard described 

in Section III.A of the preamble. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations 

Executive Order 12898 (59 FR 7629; 

Feb. 16, 1994) establishes Federal 

executive policy on environmental 

justice. Its main provision directs 

Federal agencies, to the greatest extent 

practicable and permitted by law, to 

make environmental justice part of their 

mission by identifying and addressing, 

as appropriate, disproportionately high 

and adverse human health or 

environmental effects of their programs, 

policies, and activities on minority 

populations and low-income 

populations in the United States. 

EPA has determined that this final 

rule will not have disproportionately 

high and adverse human health or 

environmental effects on minority or 

low-income populations because it 

increases the level of environmental 

protection for all affected populations 

without having any disproportionately 

high and adverse human health effects 

on any population, including any 

minority or low-income population. The 

final rule will establish uniform 

national standards for NO2 in ambient 

air. 

K. Congressional Review Act 

The Congressional Review Act, 5 

U.S.C. 801 et seq., as added by the Small 

Business Regulatory Enforcement 

Fairness Act of 1996, generally provides 

that before a rule may take effect, the 

agency promulgating the rule must 

submit a rule report, which includes a 

copy of the rule, to each House of the 

Congress and to the Comptroller General 

of the United States. EPA will submit a 

report containing this rule and other 

required information to the U.S. Senate, 

the U.S. House of Representatives, and 

the Comptroller General of the United 

States prior to publication of the rule in 

the Federal Register. A Major rule 

cannot take effect until 60 days after it 

is published in the Federal Register. 

This action is a ‘‘major rule’’ as defined 

by 5 U.S.C. 804(2). This rule will be 

effective on April 12, 2010. 

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distance from a major highway. Sci. Total 

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M. (2007). The influence of highway traffic 

on ambient nitrogen dioxide 

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M. (2009). Meta-analysis of nitrogen 

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a major roadway in Raleigh, North 

Carolina: Downwind attenuation and 

correlation with traffic-related pollutants. 

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Henderson, R. (2008). Letter to EPA 

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Draft).’’ EPA–CASAC–08–015, June 25. 

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on respiratory health of Dutch children. 

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models. J. of Expos. Science and Environ. 

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pollution and emergency department visits 

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recipients, 1991–1996. Environ. Res. 

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near motorways. Atmos. Environ. 35:3875– 

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M, Vogel B. (2005). Vehicle-induced 

turbulence near a motorway, Atmospheric 

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Hoover K, Siemiatycki J, Jerrett M, 

Abrahamowicz M, White WH. (2000). 

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particulate air pollution and mortality: a 

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and individual NOZ species. J. Air Waste 

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Redline S, Tager IB. (2002). The effect of 

air pollution on inner-city children with 

asthma. Eur Respir J. 19:699–705. 

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study of ambient air contaminants and 

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and Manhattan. Prepared for: The U.S. 

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Recommendations Concerning EPA’s 

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List of Subjects 

40 CFR Part 50 

Environmental protection, Air 

pollution control, Carbon monoxide, 

Lead, Nitrogen dioxide, Ozone, 

Particulate matter, Sulfur oxides. 

40 CFR Part 58 

Environmental protection, 

Administrative practice and procedure, 

Air pollution control, Intergovernmental 

relations, Reporting and recordkeeping 

requirements. 

Dated: January 22, 2010. 

Lisa P. Jackson, 

Administrator. 

■ For the reasons stated in the preamble, 

title 40, chapter I of the Code of Federal 

Regulations is amended as follows: 

PART 50—NATIONAL PRIMARY AND 

SECONDARY AMBIENT AIR QUALITY 

STANDARDS 

■ 1. The authority citation for part 50 

continues to read as follows: 

Authority: 42 U.S.C. 7401, et seq. 

■ 2. Section 50.11 is revised to read as 

follows: 

§ 50.11 National primary and secondary 

ambient air quality standards for oxides of 

nitrogen (with nitrogen dioxide as the 

indicator). 

(a) The level of the national primary 

annual ambient air quality standard for 

oxides of nitrogen is 53 parts per billion 

(ppb, which is 1 part in 1,000,000,000), 

annual average concentration, measured 

in the ambient air as nitrogen dioxide. 

(b) The level of the national primary 

1-hour ambient air quality standard for 

oxides of nitrogen is 100 ppb, 1-hour 

average concentration, measured in the 

ambient air as nitrogen dioxide. 

(c) The level of the national secondary 

ambient air quality standard for nitrogen 

dioxide is 0.053 parts per million (100 

micrograms per cubic meter), annual 

arithmetic mean concentration. 

(d) The levels of the standards shall 

be measured by: 

(1) A reference method based on 

appendix F to this part; or 

(2) By a Federal equivalent method 

(FEM) designated in accordance with 

part 53 of this chapter. 

(e) The annual primary standard is 

met when the annual average 

concentration in a calendar year is less 

than or equal to 53 ppb, as determined 

in accordance with Appendix S of this 

part for the annual standard. 

(f) The 1-hour primary standard is met 

when the three-year average of the 

annual 98th percentile of the daily 

maximum 1-hour average concentration 

is less than or equal to 100 ppb, as 

determined in accordance with 

Appendix S of this part for the 1-hour 

standard. 

(g) The secondary standard is attained 

when the annual arithmetic mean 

concentration in a calendar year is less 

than or equal to 0.053 ppm, rounded to 

three decimal places (fractional parts 

equal to or greater than 0.0005 ppm 

must be rounded up). To demonstrate 

attainment, an annual mean must be 

based upon hourly data that are at least 

75 percent complete or upon data 

derived from manual methods that are 

at least 75 percent complete for the 

scheduled sampling days in each 

calendar quarter. 

■ 3. Section 50.14 is amended by adding 

an entry to the end of table in paragraph 

(c)(2)(vi) to read as follows: 

§ 50.14 Treatment of air quality monitoring 

data influenced by exceptional events. 

* * * * * 

(c) * * * 

(2) * * * 

(vi) * * * 

TABLE 1—SCHEDULE FOR EXCEPTIONAL EVENT FLAGGING AND DOCUMENTATION SUBMISSION FOR DATA TO BE USED IN

DESIGNATIONS DECISIONS FOR NEW OR REVISED NAAQS 

NAAQS 

pollutant/ 

standard/ 

(level)/ 

promulgation 

date 

Air quality 

data collected 

for calendar 

year 

Event flagging 

& initial 

description 

deadline 

Detailed 

documentation 

submission 

deadline 

*******

NO2/1-Hour Standard (100 PPB) .............................. 2008 July 1, 2010 a ............................................................. January 22, 2011. 

2009 July 1, 2010 ............................................................... January 22, 2011. 

2010 April 1, 2011 a ............................................................ July 1, 2011 a. 

a Indicates change from general schedule in 40 CFR 50.14. 

Note: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most notably, for data used by EPA for redesignations to attainment. 


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* * * * * 

■ 4. Appendix S to Part 50 is added to 

read as follows: 

Appendix S to Part 50—Interpretation 

of the Primary National Ambient Air 

Quality Standards for Oxides of 

Nitrogen (Nitrogen Dioxide) 

1. General 

(a) This appendix explains the data 

handling conventions and computations 

necessary for determining when the primary 

national ambient air quality standards for 

oxides of nitrogen as measured by nitrogen 

dioxide (‘‘NO2 NAAQS’’) specified in 50.11 

are met. Nitrogen dioxide (NO2) is measured 

in the ambient air by a Federal reference 

method (FRM) based on appendix F to this 

part or by a Federal equivalent method (FEM) 

designated in accordance with part 53 of this 

chapter. Data handling and computation 

procedures to be used in making 

comparisons between reported NO2

concentrations and the levels of the NO2

NAAQS are specified in the following 

sections. 

(b) Whether to exclude, retain, or make 

adjustments to the data affected by 

exceptional events, including natural events, 

is determined by the requirements and 

process deadlines specified in 50.1, 50.14 

and 51.930 of this chapter. 

(c) The terms used in this appendix are 

defined as follows: 

Annual mean refers to the annual average 

of all of the 1-hour concentration values as 

defined in section 5.1 of this appendix. 

Daily maximum 1-hour values for NO2

refers to the maximum 1-hour NO2

concentration values measured from 

midnight to midnight (local standard time) 

that are used in NAAQS computations. 

Design values are the metrics (i.e., 

statistics) that are compared to the NAAQS 

levels to determine compliance, calculated as 

specified in section 5 of this appendix. The 

design values for the primary NAAQS are: 

(1) The annual mean value for a monitoring 

site for one year (referred to as the ‘‘annual 

primary standard design value’’). 

(2) The 3-year average of annual 98th 

percentile daily maximum 1-hour values for 

a monitoring site (referred to as the ‘‘1-hour 

primary standard design value’’). 

98th percentile daily maximum 1-hour 

value is the value below which nominally 98 

percent of all daily maximum 1-hour 

concentration values fall, using the ranking 

and selection method specified in section 5.2 

of this appendix. 

Quarter refers to a calendar quarter. 

Year refers to a calendar year. 

2. Requirements for Data Used for 

Comparisons With the NO2 NAAQS and 

Data Reporting Considerations 

(a) All valid FRM/FEM NO2 hourly data 

required to be submitted to EPA’s Air Quality 

System (AQS), or otherwise available to EPA, 

meeting the requirements of part 58 of this 

chapter including appendices A, C, and E 

shall be used in design value calculations. 

Multi-hour average concentration values 

collected by wet chemistry methods shall not 

be used. 

(b) When two or more NO2 monitors are 

operated at a site, the State may in advance 

designate one of them as the primary 

monitor. If the State has not made this 

designation, the Administrator will make the 

designation, either in advance or 

retrospectively. Design values will be 

developed using only the data from the 

primary monitor, if this results in a valid 

design value. If data from the primary 

monitor do not allow the development of a 

valid design value, data solely from the other 

monitor(s) will be used in turn to develop a 

valid design value, if this results in a valid 

design value. If there are three or more 

monitors, the order for such comparison of 

the other monitors will be determined by the 

Administrator. The Administrator may 

combine data from different monitors in 

different years for the purpose of developing 

a valid 1-hour primary standard design value, 

if a valid design value cannot be developed 

solely with the data from a single monitor. 

However, data from two or more monitors in 

the same year at the same site will not be 

combined in an attempt to meet data 

completeness requirements, except if one 

monitor has physically replaced another 

instrument permanently, in which case the 

two instruments will be considered to be the 

same monitor, or if the State has switched the 

designation of the primary monitor from one 

instrument to another during the year. 

(c) Hourly NO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

3. Comparisons With the NO2 NAAQS 

3.1 The Annual Primary NO2 NAAQS 

(a) The annual primary NO2 NAAQS is met 

at a site when the valid annual primary 

standard design value is less than or equal to 

53 parts per billion (ppb). 

(b) An annual primary standard design 

value is valid when at least 75 percent of the 

hours in the year are reported. 

(c) An annual primary standard design 

value based on data that do not meet the 

completeness criteria stated in section 3.1(b) 

may also be considered valid with the 

approval of, or at the initiative of, the 

Administrator, who may consider factors 

such as monitoring site closures/moves, 

monitoring diligence, the consistency and 

levels of the valid concentration 

measurements that are available, and nearby 

concentrations in determining whether to use 

such data. 

(d) The procedures for calculating the 

annual primary standard design values are 

given in section 5.1 of this appendix. 

3.2 The 1-hour Primary NO2 NAAQS 

(a) The 1-hour primary NO2 NAAQS is met 

at a site when the valid 1-hour primary 

standard design value is less than or equal to 

100 parts per billion (ppb). 

(b) An NO2 1-hour primary standard design 

value is valid if it encompasses three 

consecutive calendar years of complete data. 

A year meets data completeness requirements 

when all 4 quarters are complete. A quarter 

is complete when at least 75 percent of the 

sampling days for each quarter have 

complete data. A sampling day has complete 

data if 75 percent of the hourly concentration 

values, including State-flagged data affected 

by exceptional events which have been 

approved for exclusion by the Administrator, 

are reported. 

(c) In the case of one, two, or three years 

that do not meet the completeness 

requirements of section 3.2(b) of this 

appendix and thus would normally not be 

useable for the calculation of a valid 3-year 

1-hour primary standard design value, the 3- 

year 1-hour primary standard design value 

shall nevertheless be considered valid if one 

of the following conditions is true. 

(i) At least 75 percent of the days in each 

quarter of each of three consecutive years 

have at least one reported hourly value, and 

the design value calculated according to the 

procedures specified in section 5.2 is above 

the level of the primary 1-hour standard. 

(ii)(A) A 1-hour primary standard design 

value that is below the level of the NAAQS 

can be validated if the substitution test in 

section 3.2(c)(ii)(B) results in a ‘‘test design 

value’’ that is below the level of the NAAQS. 

The test substitutes actual ‘‘high’’ reported 

daily maximum 1-hour values from the same 

site at about the same time of the year 

(specifically, in the same calendar quarter) 

for unknown values that were not 

successfully measured. Note that the test is 

merely diagnostic in nature, intended to 

confirm that there is a very high likelihood 

that the original design value (the one with 

less than 75 percent data capture of hours by 

day and of days by quarter) reflects the true 

under-NAAQS-level status for that 3-year 

period; the result of this data substitution test 

(the ‘‘test design value’’, as defined in section 

3.2(c)(ii)(B)) is not considered the actual 

design value. For this test, substitution is 

permitted only if there are at least 200 days 

across the three matching quarters of the 

three years under consideration (which is 

about 75 percent of all possible daily values 

in those three quarters) for which 75 percent 

of the hours in the day, including Stateflagged data affected by exceptional events 

which have been approved for exclusion by 

the Administrator, have reported 

concentrations. However, maximum 1-hour 

values from days with less than 75 percent 

of the hours reported shall also be considered 

in identifying the high value to be used for 

substitution. 

(B) The substitution test is as follows: Data 

substitution will be performed in all quarter 

periods that have less than 75 percent data 

capture but at least 50 percent data capture, 

including State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator; 

if any quarter has less than 50 percent data 

capture then this substitution test cannot be 

used. Identify for each quarter (e.g., January– 

March) the highest reported daily maximum 

1-hour value for that quarter, excluding Stateflagged data affected by exceptional events 

which have been approved for exclusion by 

the Administrator, looking across those three 

months of all three years under 

consideration. All daily maximum 1-hour 

values from all days in the quarter period 

shall be considered when identifying this 

highest value, including days with less than 


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75 percent data capture. If after substituting 

the highest non-excluded reported daily 

maximum 1-hour value for a quarter for as 

much of the missing daily data in the 

matching deficient quarter(s) as is needed to 

make them 100 percent complete, the 

procedure in section 5.2 yields a recalculated 

3-year 1-hour standard ‘‘test design value’’ 

below the level of the standard, then the 1- 

hour primary standard design value is 

deemed to have passed the diagnostic test 

and is valid, and the level of the standard is 

deemed to have been met in that 3-year 

period. As noted in section 3.2(c)(i), in such 

a case, the 3-year design value based on the 

data actually reported, not the ‘‘test design 

value’’, shall be used as the valid design 

value. 

(iii)(A) A 1-hour primary standard design 

value that is above the level of the NAAQS 

can be validated if the substitution test in 

section 3.2(c)(iii)(B) results in a ‘‘test design 

value’’ that is above the level of the NAAQS. 

The test substitutes actual ‘‘low’’ reported 

daily maximum 1-hour values from the same 

site at about the same time of the year 

(specifically, in the same three months of the 

calendar) for unknown values that were not 

successfully measured. Note that the test is 

merely diagnostic in nature, intended to 

confirm that there is a very high likelihood 

that the original design value (the one with 

less than 75 percent data capture of hours by 

day and of days by quarter) reflects the true 

above-NAAQS-level status for that 3-year 

period; the result of this data substitution test 

(the ‘‘test design value’’, as defined in section 

3.2(c)(iii)(B)) is not considered the actual 

design value. For this test, substitution is 

permitted only if there are a minimum 

number of available daily data points from 

which to identify the low quarter-specific 

daily maximum 1-hour values, specifically if 

there are at least 200 days across the three 

matching quarters of the three years under 

consideration (which is about 75 percent of 

all possible daily values in those three 

quarters) for which 75 percent of the hours 

in the day have reported concentrations. 

Only days with at least 75 percent of the 

hours reported shall be considered in 

identifying the low value to be used for 

substitution. 

(B) The substitution test is as follows: Data 

substitution will be performed in all quarter 

periods that have less than 75 percent data 

capture. Identify for each quarter (e.g., 

January-March) the lowest reported daily 

maximum 1-hour value for that quarter, 

looking across those three months of all three 

years under consideration. All daily 

maximum 1-hour values from all days with 

at least 75 percent capture in the quarter 

period shall be considered when identifying 

this lowest value. If after substituting the 

lowest reported daily maximum 1-hour value 

for a quarter for as much of the missing daily 

data in the matching deficient quarter(s) as is 

needed to make them 75 percent complete, 

the procedure in section 5.2 yields a 

recalculated 3-year 1-hour standard ‘‘test 

design value’’ above the level of the standard, 

then the 1-hour primary standard design 

value is deemed to have passed the 

diagnostic test and is valid, and the level of 

the standard is deemed to have been 

exceeded in that 3-year period. As noted in 

section 3.2(c)(i), in such a case, the 3-year 

design value based on the data actually 

reported, not the ‘‘test design value’’, shall be 

used as the valid design value. 

(d) A 1-hour primary standard design value 

based on data that do not meet the 

completeness criteria stated in 3.2(b) and also 

do not satisfy section 3.2(c), may also be 

considered valid with the approval of, or at 

the initiative of, the Administrator, who may 

consider factors such as monitoring site 

closures/moves, monitoring diligence, the 

consistency and levels of the valid 

concentration measurements that are 

available, and nearby concentrations in 

determining whether to use such data. 

(e) The procedures for calculating the 1- 

hour primary standard design values are 

given in section 5.2 of this appendix. 

4. Rounding Conventions 

4.1 Rounding Conventions for the Annual 

Primary NO2 NAAQS 

(a) Hourly NO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

(b) The annual primary standard design 

value is calculated pursuant to section 5.1 

and then rounded to the nearest whole 

number or 1 ppb (decimals 0.5 and greater 

are rounded up to the nearest whole number, 

and any decimal lower than 0.5 is rounded 

down to the nearest whole number). 

4.2 Rounding Conventions for the 1-hour 

Primary NO2 NAAQS 

(a) Hourly NO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

(b) Daily maximum 1-hour values are not 

rounded. 

(c) The 1-hour primary standard design 

value is calculated pursuant to section 5.2 

and then rounded to the nearest whole 

number or 1 ppb (decimals 0.5 and greater 

are rounded up to the nearest whole number, 

and any decimal lower than 0.5 is rounded 

down to the nearest whole number). 

5. Calculation Procedures for the Primary 

NO2 NAAQS 

5.1 Procedures for the Annual Primary NO2

NAAQS 

(a) When the data for a site and year meet 

the data completeness requirements in 

section 3.1(b) of this appendix, or if the 

Administrator exercises the discretionary 

authority in section 3.1(c), the annual mean 

is simply the arithmetic average of all of the 

reported 1-hour values. 

(b) The annual primary standard design 

value for a site is the valid annual mean 

rounded according to the conventions in 

section 4.1. 

5.2 Calculation Procedures for the 1-hour 

Primary NO2 NAAQS 

(a) Procedure for identifying annual 98th 

percentile values. When the data for a 

particular site and year meet the data 

completeness requirements in section 3.2(b), 

or if one of the conditions of section 3.2(c) 

is met, or if the Administrator exercises the 

discretionary authority in section 3.2(d), 

identification of annual 98th percentile value 

is accomplished as follows. 

(i) The annual 98th percentile value for a 

year is the higher of the two values resulting 

from the following two procedures. 

(1) Procedure 1. 

(A) For the year, determine the number of 

days with at least 75 percent of the hourly 

values reported including State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator. 

(B) For the year, from only the days with 

at least 75 percent of the hourly values 

reported, select from each day the maximum 

hourly value excluding State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator. 

(C) Sort all these daily maximum hourly 

values from a particular site and year by 

descending value. (For example: (x[1], x[2], 

x[3], * * *, x[n]). In this case, x[1] is the 

largest number and x[n] is the smallest 

value.) The 98th percentile is determined 

from this sorted series of daily values which 

is ordered from the highest to the lowest 

number. Using the left column of Table 1, 

determine the appropriate range (i.e., row) for 

the annual number of days with valid data 

for year y (cny) as determined from step (A). 

The corresponding ‘‘n’’ value in the right 

column identifies the rank of the annual 98th 

percentile value in the descending sorted list 

of daily site values for year y. Thus, P0.98, y

= the nth largest value. 

(2) Procedure 2. 

(A) For the year, determine the number of 

days with at least one hourly value reported 

including State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator. 

(B) For the year, from all the days with at 

least one hourly value reported, select from 

each day the maximum hourly value 

excluding State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator. 

(C) Sort all these daily maximum values 

from a particular site and year by descending 

value. (For example: (x[1], x[2], x[3], * * *, 

x[n]). In this case, x[1] is the largest number 

and x[n] is the smallest value.) The 98th 

percentile is determined from this sorted 

series of daily values which is ordered from 

the highest to the lowest number. Using the 

left column of Table 1, determine the 

appropriate range (i.e., row) for the annual 

number of days with valid data for year y 

(cny) as determined from step (A). The 

corresponding ‘‘n’’ value in the right column 

identifies the rank of the annual 98th 

percentile value in the descending sorted list 

of daily site values for year y. Thus, P0.98, y

= the nth largest value. 

(b) The 1-hour primary standard design 

value for a site is mean of the three annual 

98th percentile values, rounded according to 

the conventions in section 4. 


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TABLE 1 

Annual number 

of days with 

valid data for 

year ‘‘y’’ (cny) 

P0.98, y is the 

nth maximum 

value of the 

year, where n 

is the listed 

number 

1–50 1 

51–100 2 

101–150 3 

151–200 4 

201–250. 5 

251–300 6 

301–350 7 

351–366 8 

PART 58—AMBIENT AIR QUALITY 

SURVEILLANCE 

■ 5. The authority citation for part 58 

continues to read as follows: 

Authority: 42 U.S.C. 7403, 7410, 7601(a), 

7611, and 7619. 

Subpart A—[Amended] 

■ 6. Section 58.1, is amended by adding 

the definitions for ‘‘AADT’’ and ‘‘Nearroad NO2 Monitor’’ in alphabetical order 

to read as follows: 

§ 58.1 Definitions 

* * * * * 

AADT means the annual average daily 

traffic. 

* * * 

Near-road NO2 Monitor means any 

NO2 monitor meeting the specifications 

in 4.3.2 of Appendix D and paragraphs 

2, 4(d), 6.1, and 6.4 of Appendix E of 

this part. 

* * * * * 

Subpart B [Amended] 

■ 7. Section 58.10, is amended by 

adding paragraphs (a)(5) and (b)(12) to 

read as follows: 

§ 58.10 Annual monitoring network plan 

and periodic network assessment. 

(a) * * * 

(5) A plan for establishing NO2

monitoring sites in accordance with the 

requirements of appendix D to this part 

shall be submitted to the Administrator 

by July 1, 2012. The plan shall provide 

for all required monitoring stations to be 

operational by January 1, 2013. 

* * * * * 

(b) * * * 

(12) The identification of required 

NO2 monitors as either near-road or 

area-wide sites in accordance with 

Appendix D, Section 4.3 of this part. 

* * * * * 

■ 8. Section 58.13 is amended by adding 

paragraph (c) to read as follows: 

§ 58.13 Monitoring network completion. 

* * * * * 

(c) The network of NO2 monitors must 

be physically established no later than 

January 1, 2013, and at that time, must 

be operating under all of the 

requirements of this part, including the 

requirements of appendices A, C, D, and 

E to this part. 

■ 9. Section 58.16 is amended by 

revising paragraph (a) to read as follows: 

§ 58.16 Data submittal and archiving 

requirements. 

* * * * * 

(a) The State, or where appropriate, 

local agency, shall report to the 

Administrator, via AQS all ambient air 

quality data and associated quality 

assurance data for SO2; CO; O3; NO2; 

NO; NOY; NOX; Pb–TSP mass 

concentration; Pb–PM10 mass 

concentration; PM10 mass concentration; 

PM2.5mass concentration; for filterbased PM2.5FRM/FEM the field blank 

mass, sampler-generated average daily 

temperature, and sampler-generated 

average daily pressure; chemically 

speciated PM2.5 mass concentration 

data; PM10–2.5 mass concentration; 

chemically speciated PM10–2.5 mass 

concentration data; meteorological data 

from NCore and PAMS sites; average 

daily temperature and average daily 

pressure for Pb sites if not already 

reported from sampler generated 

records; and metadata records and 

information specified by the AQS Data 

Coding Manual (http://www.epa.gov/ 

ttn/airs/airsaqs/manuals/manuals.htm). 

The State, or where appropriate, local 

agency, may report site specific 

meteorological measurements generated 

by onsite equipment (meteorological 

instruments, or sampler generated) or 

measurements from the nearest airport 

reporting ambient pressure and 

temperature. Such air quality data and 

information must be submitted directly 

to the AQS via electronic transmission 

on the specified quarterly schedule 

described in paragraph (b) of this 

section. 

* * * * * 

■ 10. Appendix A to Part 58 is amended 

by adding paragraph 2.3.1.5 to read as 

follows: 

Appendix A to Part 58—Quality 

Assurance Requirements for SLAMS, 

SPMs and PSD Air Monitoring 

* * * * * 

2.3.1.5 Measurement Uncertainty for 

NO2. The goal for acceptable measurement 

uncertainty is defined for precision as an 

upper 90 percent confidence limit for the 

coefficient of variation (CV) of 15 percent and 

for bias as an upper 95 percent confidence 

limit for the absolute bias of 15 percent. 

* * * * * 

■ 11. Appendix C to Part 58 is amended 

by adding paragraph 2.1.1 to read as 

follows: 

Appendix C to Part 58—Ambient Air 

Quality Monitoring Methodology 

* * * * * 

2.1.1 Any NO2 FRM or FEM used for 

making primary NAAQS decisions must be 

capable of providing hourly averaged 

concentration data. 

* * * * * 

■ 12. Appendix D to Part 58 is amended 

by revising paragraph 4.3 to read as 

follows: 

Appendix D to Part 58—Network 

Design Criteria for Ambient Air Quality 

Monitoring 

* * * * * 

4.3 Nitrogen Dioxide (NO2) Design Criteria 

4.3.1 General Requirements 

(a) State and, where appropriate, local 

agencies must operate a minimum number of 

required NO2 monitoring sites as described 

below. 

4.3.2 Requirement for Near-road NO2

Monitors 

(a) Within the NO2 network, there must be 

one microscale near-road NO2 monitoring 

station in each CBSA with a population of 

500,000 or more persons to monitor a 

location of expected maximum hourly 

concentrations sited near a major road with 

high AADT counts as specified in paragraph 

4.3.2(a)(1) of this appendix. An additional 

near-road NO2 monitoring station is required 

for any CBSA with a population of 2,500,000 

persons or more, or in any CBSA with a 

population of 500,000 or more persons that 

has one or more roadway segments with 

250,000 or greater AADT counts to monitor 

a second location of expected maximum 

hourly concentrations. CBSA populations 

shall be based on the latest available census 

figures. 

(1) The near-road NO2 monitoring stations 

shall be selected by ranking all road segments 

within a CBSA by AADT and then 

identifying a location or locations adjacent to 

those highest ranked road segments, 

considering fleet mix, roadway design, 

congestion patterns, terrain, and 

meteorology, where maximum hourly NO2

concentrations are expected to occur and 

siting criteria can be met in accordance with 

appendix E of this part. Where a State or 

local air monitoring agency identifies 

multiple acceptable candidate sites where 

maximum hourly NO2 concentrations are 

expected to occur, the monitoring agency 

shall consider the potential for population 

exposure in the criteria utilized to select the 

final site location. Where one CBSA is 

required to have two near-road NO2

monitoring stations, the sites shall be 

differentiated from each other by one or more 

of the following factors: fleet mix; congestion 

patterns; terrain; geographic area within the 


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CBSA; or different route, interstate, or 

freeway designation. 

(b) Measurements at required near-road 

NO2 monitor sites utilizing 

chemiluminescence FRMs must include at a 

minimum: NO, NO2, and NOX. 

4.3.3 Requirement for Area-wide NO2

Monitoring 

(a) Within the NO2 network, there must be 

one monitoring station in each CBSA with a 

population of 1,000,000 or more persons to 

monitor a location of expected highest NO2

concentrations representing the 

neighborhood or larger spatial scales. PAMS 

sites collecting NO2 data that are situated in 

an area of expected high NO2 concentrations 

at the neighborhood or larger spatial scale 

may be used to satisfy this minimum 

monitoring requirement when the NO2

monitor is operated year round. Emission 

inventories and meteorological analysis 

should be used to identify the appropriate 

locations within a CBSA for locating required 

area-wide NO2 monitoring stations. CBSA 

populations shall be based on the latest 

available census figures. 

4.3.4 Regional Administrator Required 

Monitoring 

(a) The Regional Administrators, in 

collaboration with States, must require a 

minimum of forty additional NO2 monitoring 

stations nationwide in any area, inside or 

outside of CBSAs, above the minimum 

monitoring requirements, with a primary 

focus on siting these monitors in locations to 

protect susceptible and vulnerable 

populations. The Regional Administrators, 

working with States, may also consider 

additional factors described in paragraph (b) 

below to require monitors beyond the 

minimum network requirement. 

(b) The Regional Administrators may 

require monitors to be sited inside or outside 

of CBSAs in which: 

(i) The required near-road monitors do not 

represent all locations of expected maximum 

hourly NO2 concentrations in an area and 

NO2 concentrations may be approaching or 

exceeding the NAAQS in that area; 

(ii) Areas that are not required to have a 

monitor in accordance with the monitoring 

requirements and NO2 concentrations may be 

approaching or exceeding the NAAQS; or 

(iii) The minimum monitoring 

requirements for area-wide monitors are not 

sufficient to meet monitoring objectives. 

(c) The Regional Administrator and the 

responsible State or local air monitoring 

agency should work together to design and/ 

or maintain the most appropriate NO2

network to address the data needs for an area, 

and include all monitors under this provision 

in the annual monitoring network plan. 

4.3.5 NO2 Monitoring Spatial Scales 

(a) The most important spatial scale for 

near-road NO2 monitoring stations to 

effectively characterize the maximum 

expected hourly NO2 concentration due to 

mobile source emissions on major roadways 

is the microscale. The most important spatial 

scales for other monitoring stations 

characterizing maximum expected hourly 

NO2 concentrations are the microscale and 

middle scale. The most important spatial 

scale for area-wide monitoring of high NO2

concentrations is the neighborhood scale. 

(1) Microscale—This scale represents areas 

in close proximity to major roadways or 

point and area sources. Emissions from 

roadways result in high ground level NO2

concentrations at the microscale, where 

concentration gradients generally exhibit a 

marked decrease with increasing downwind 

distance from major roads. As noted in 

appendix E of this part, near-road NO2

monitoring stations are required to be within 

50 meters of target road segments in order to 

measure expected peak concentrations. 

Emissions from stationary point and area 

sources, and non-road sources may, under 

certain plume conditions, result in high 

ground level concentrations at the 

microscale. The microscale typically 

represents an area impacted by the plume 

with dimensions extending up to 

approximately 100 meters. 

(2) Middle scale—This scale generally 

represents air quality levels in areas up to 

several city blocks in size with dimensions 

on the order of approximately 100 meters to 

500 meters. The middle scale may include 

locations of expected maximum hourly 

concentrations due to proximity to major 

NO2 point, area, and/or non-road sources. 

(3) Neighborhood scale—The 

neighborhood scale represents air quality 

conditions throughout some relatively 

uniform land use areas with dimensions in 

the 0.5 to 4.0 kilometer range. Emissions 

from stationary point and area sources may, 

under certain plume conditions, result in 

high NO2 concentrations at the neighborhood 

scale. Where a neighborhood site is located 

away from immediate NO2 sources, the site 

may be useful in representing typical air 

quality values for a larger residential area, 

and therefore suitable for population 

exposure and trends analyses. 

(4) Urban scale—Measurements in this 

scale would be used to estimate 

concentrations over large portions of an 

urban area with dimensions from 4 to 50 

kilometers. Such measurements would be 

useful for assessing trends in area-wide air 

quality, and hence, the effectiveness of large 

scale air pollution control strategies. Urban 

scale sites may also support other monitoring 

objectives of the NO2 monitoring network 

identified in paragraph 4.3.4 above. 

4.3.6 NOy Monitoring 

(a) NO/NOy measurements are included 

within the NCore multi-pollutant site 

requirements and the PAMS program. These 

NO/NOy measurements will produce 

conservative estimates for NO2 that can be 

used to ensure tracking continued 

compliance with the NO2 NAAQS. NO/NOy

monitors are used at these sites because it is 

important to collect data on total reactive 

nitrogen species for understanding O3

photochemistry. 

* * * * * 

■ 13. Appendix E to Part 58 is amended 

as follows: 

■ a. By revising paragraphs 2, and 6.1. 

■ b. By adding paragraphs 4(d) and 6.4. 

■ c. By revising paragraphs 9(c), 11 and 

Table E–4. 

Appendix E to Part 58—Probe and 

Monitoring Path Siting Criteria for 

Ambient Air Quality Monitoring 

* * * * * 

2. Horizontal and Vertical Placement 

The probe or at least 80 percent of the 

monitoring path must be located between 2 

and 15 meters above ground level for all 

ozone and sulfur dioxide monitoring sites, 

and for neighborhood or larger spatial scale 

Pb, PM10, PM10–2.5, PM2.5, NO2 and carbon 

monoxide sites. Middle scale PM10–2.5 sites 

are required to have sampler inlets between 

2 and 7 meters above ground level. 

Microscale Pb, PM10, PM10–2.5 and PM2.5 sites 

are required to have sampler inlets between 

2 and 7 meters above ground level. 

Microscale near-road NO2 monitoring sites 

are required to have sampler inlets between 

2 and 7 meters above ground level. The inlet 

probes for microscale carbon monoxide 

monitors that are being used to measure 

concentrations near roadways must be 3±1⁄2

meters above ground level. The probe or at 

least 90 percent of the monitoring path must 

be at least 1 meter vertically or horizontally 

away from any supporting structure, walls, 

parapets, penthouses, etc., and away from 

dusty or dirty areas. If the probe or a 

significant portion of the monitoring path is 

located near the side of a building or wall, 

then it should be located on the windward 

side of the building relative to the prevailing 

wind direction during the season of highest 

concentration potential for the pollutant 

being measured. 

* * * * * 

4. * * * 

(d) For near-road NO2 monitoring stations, 

the monitor probe shall have an unobstructed 

air flow, where no obstacles exist at or above 

the height of the monitor probe, between the 

monitor probe and the outside nearest edge 

of the traffic lanes of the target road segment. 

* * * * * 

6. * * * 

6.1 Spacing for Ozone Probes and 

Monitoring Paths 

In siting an O3 analyzer, it is important to 

minimize destructive interferences form 

sources of NO, since NO readily reacts with 

O3. Table E–1 of this appendix provides the 

required minimum separation distances 

between a roadway and a probe or, where 

applicable, at least 90 percent of a monitoring 

path for various ranges of daily roadway 

traffic. A sampling site having a point 

analyzer probe located closer to a roadway 

than allowed by the Table E–1 requirements 

should be classified as microscale or middle 

scale, rather than neighborhood or urban 

scale, since the measurements from such a 

site would more closely represent the middle 

scale. If an open path analyzer is used at a 

site, the monitoring path(s) must not cross 

over a roadway with an average daily traffic 

count of 10,000 vehicles per day or more. For 

those situations where a monitoring path 

crosses a roadway with fewer than 10,000 

vehicles per day, monitoring agencies must 

consider the entire segment of the monitoring 


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path in the area of potential atmospheric 

interference from automobile emissions. 

Therefore, this calculation must include the 

length of the monitoring path over the 

roadway plus any segments of the monitoring 

path that lie in the area between the roadway 

and minimum separation distance, as 

determined from the Table E–1 of this 

appendix. The sum of these distances must 

not be greater than 10 percent of the total 

monitoring path length. 

* * * * * 

6.4 Spacing for Nitrogen Dioxide (NO2) 

Probes and Monitoring Paths 

(a) In siting near-road NO2 monitors as 

required in paragraph 4.3.2 of appendix D of 

this part, the monitor probe shall be as near 

as practicable to the outside nearest edge of 

the traffic lanes of the target road segment; 

but shall not be located at a distance greater 

than 50 meters, in the horizontal, from the 

outside nearest edge of the traffic lanes of the 

target road segment. 

(b) In siting NO2 monitors for 

neighborhood and larger scale monitoring, it 

is important to minimize near-road 

influences. Table E–1 of this appendix 

provides the required minimum separation 

distances between a roadway and a probe or, 

where applicable, at least 90 percent of a 

monitoring path for various ranges of daily 

roadway traffic. A sampling site having a 

point analyzer probe located closer to a 

roadway than allowed by the Table E–1 

requirements should be classified as 

microscale or middle scale rather than 

neighborhood or urban scale. If an open path 

analyzer is used at a site, the monitoring 

path(s) must not cross over a roadway with 

an average daily traffic count of 10,000 

vehicles per day or more. For those situations 

where a monitoring path crosses a roadway 

with fewer than 10,000 vehicles per day, 

monitoring agencies must consider the entire 

segment of the monitoring path in the area 

of potential atmospheric interference form 

automobile emissions. Therefore, this 

calculation must include the length of the 

monitoring path over the roadway plus any 

segments of the monitoring path that lie in 

the area between the roadway and minimum 

separation distance, as determined form the 

Table E–1 of this appendix. The sum of these 

distances must not be greater than 10 percent 

of the total monitoring path length. 

* * * * * 

9. * * * 

(c) No matter how nonreactive the 

sampling probe material is initially, after a 

period of use reactive particulate matter is 

deposited on the probe walls. Therefore, the 

time it takes the gas to transfer from the 

probe inlet to the sampling device is also 

critical. Ozone in the presence of nitrogen 

oxide (NO) will show significant losses even 

in the most inert probe material when the 

residence time exceeds 20 seconds.26 Other 

studies 27 28 indicate that a 10 second or 

less residence time is easily achievable. 

Therefore, sampling probes for reactive gas 

monitors at NCore and at NO2 sites must 

have a sample residence time less than 20 

seconds. 

* * * * * 

11. Summary 

Table E–4 of this appendix presents a 

summary of the general requirements for 

probe and monitoring path siting criteria 

with respect to distances and heights. It is 

apparent from Table E–4 that different 

elevation distances above the ground are 

shown for the various pollutants. The 

discussion in this appendix for each of the 

pollutants describes reasons for elevating the 

monitor, probe, or monitoring path. The 

differences in the specified range of heights 

are based on the vertical concentration 

gradients. For CO and near-road NO2

monitors, the gradients in the vertical 

direction are very large for the microscale, so 

a small range of heights are used. The upper 

limit of 15 meters is specified for the 

consistency between pollutants and to allow 

the use of a single manifold or monitoring 

path for monitoring more than one pollutant. 

TABLE E–4 OF APPENDIX E TO PART 58. SUMMARY OF PROBE AND MONITORING PATH SITING CRITERIA

Pollutant 

Scale (maximum 

monitoring path 

length, meters) 

Height from ground to 

probe, inlet or 80% of 

monitoring path 1

Horizontal and 

vertical distance from 

supporting structures2

to probe, inlet or 90% 

of monitoring path1

(meters) 

Distance from trees 

to probe, inlet or 90% 

of monitoring path1

(meters) 

Distance from roadways to probe, inlet 

or monitoring path1

(meters) 

SO2 3,4,5,6 ..................... Middle (300 m) 

Neighborhood 

Urban, and Regional (1 km).

2–15 .......................... >1 .............................. >10 ............................ N/A 

CO 4,5,7 ........................ Micro, middle (300 

m), Neighborhood 

(1 km).

31⁄2: 2–15 .................. >1 .............................. >10 ............................ 2–10; see Table E–2 

of this appendix for 

middle and neighborhood scales. 

O3 3,4,5 ......................... Middle (300 m) 

Neighborhood, 

Urban, and Regional (1 km).

2–15 .......................... >1 .............................. >10 ............................ See Table E–1 of this 

appendix for all 

scales. 

NO2 3,4,5 ...................... Micro (Near-road [50– 

300]).

2–7 (micro); ............... >1 .............................. >10 ............................ ≤50 meters for nearroad microscale. 

Middle (300m) ........... 2–15 (all other 

scales).

................................... ...................................

Neighborhood, Urban, 

and Regional (1 

km).

................................... ................................... ................................... See Table E–1 of this 

appendix for all 

other scales 

Ozone precursors (for 

PAMS) 345.

Neighborhood and 

Urban (1 km).

2–15 .......................... >1 .............................. >10 ............................ See Table E–4 of this 

appendix for all 

scales. 

PM, Pb 3,4,5,6,8 ............. Micro: Middle, Neighborhood, Urban 

and Regional.

2–7 (micro); 2–7 

(middle PM10–2.5); 

2–15 (all other 

scales).

>2 (all scales, horizontal distance 

only).

>10 (all scales) ......... 2–10 (micro); see Figure E–1 of this appendix for all other 

scales. 

N/A—Not applicable. 

1 Monitoring path for open path analyzers is applicable only to middle or neighborhood scale CO monitoring, middle, neighborhood, urban, and 

regional scale NO2 monitoring, and all applicable scales for monitoring SO2,O3, and O3 precursors. 2When probe is located on a rooftop, this separation distance is in reference to walls, parapets, or penthouses located on roof.

3Should be >20 meters from the dripline of tree(s) and must be 10 meters from the dripline when the tree(s) act as an obstruction. 4 Distance from sampler, probe, or 90% of monitoring path to obstacle, such as a building, must be at least twice the height the obstacle protrudes above the sampler, probe, or monitoring path. Sites not meeting this criterion may be classified as middle scale (see text). 5 Must have unrestricted airflow 270 degrees around the probe or sampler; 180 degrees if the probe is on the side of a building or a wall. 


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6The probe, sampler, or monitoring path should be away from minor sources, such as furnace or incineration flues. The separation distance is 

dependent on the height of the minor source’s emission point (such as a flue), the type of fuel or waste burned, and the quality of the fuel (sulfur, 

ash, or lead content). This criterion is designed to avoid undue influences from minor sources. 7For microscale CO monitoring sites, the probe must be >10 meters from a street intersection and preferably at a midblock location. 8 Collocated monitors must be within 4 meters of each other and at least 2 meters apart for flow rates greater than 200 liters/min or at least 1 

meter apart for samplers having flow rates less than 200 liters/min to preclude airflow interference. 

* * * * * 

14. Appendix G to Part 58 is amended 

as by revising paragraph 9 and Table 2 

to read as follows: 

Appendix G to Part 58—Uniform Air 

Quality Index (AQI) and Daily 

Reporting 

* * * * * 

9. How Does the AQI Relate to Air Pollution 

Levels? 

For each pollutant, the AQI transforms 

ambient concentrations to a scale from 0 to 

500. The AQI is keyed as appropriate to the 

national ambient air quality standards 

(NAAQS) for each pollutant. In most cases, 

the index value of 100 is associated with the 

numerical level of the short-term (i.e., 

averaging time of 24-hours or less) standard 

for each pollutant. The index value of 50 is 

associated with one of the following: the 

numerical level of the annual standard for a 

pollutant, if there is one; one-half the level 

of the short-term standard for the pollutant; 

or the level at which it is appropriate to begin 

to provide guidance on cautionary language. 

Higher categories of the index are based on 

increasingly serious health effects that affect 

increasing proportions of the population. An 

index value is calculated each day for each 

pollutant (as described in section 12 of this 

appendix), unless that pollutant is 

specifically excluded (see section 8 of this 

appendix). The pollutant with the highest 

index value for the day is the ‘‘critical’’ 

pollutant, and must be included in the daily 

AQI report. As a result, the AQI for any given 

day is equal to the index value of the critical 

pollutant for that day. For the purposes of 

reporting the AQI, the indexes for PM10 and 

PM2.5 are to be considered separately. 

* * * * * 

TABLE 2—BREAKPOINTS FOR THE AQI 

These breakpoints Equal these AQIs 

O3 (ppm) 

8-hour 

O3 (ppm) 

1-hour1

PM2.5

(μg/m3) 

PM10

(μg/m3) CO (ppm) SO2 (ppm) NO2 (ppm) 1-hour AQI Category 

0.000–0.059 ..................... 0.0–15.4 0–54 0.0–4.4 0.000–0.034 0–0.053 0–50 Good. 

0.060–0.075 ..................... 15.5–40.4 55–154 4.5–9.4 0.035–0.144 0.054–0.100 51–100 Moderate. 

0.076–0.095 0.125–0.164 40.5–65.4 155–254 9.5–12.4 0.145–0.224 0.101–0.360 101–150 Unhealthy for Sensitive Groups. 

0.096–0.115 0.165–0.204 3 65.5–150.4 255–354 12.5–15.4 0.225–0.304 0.361–0.64 151–200 Unhealthy. 

0.116–0.374 0.205–0.404 3 150.5–250.4 355–424 15.5–30.4 0.305–0.604 0.65–1.24 201–300 Very Unhealthy. 

(2) 0.405–0.504 3 250.5–350.4 425–504 30.5–40.4 0.605–0.804 1.25–1.64 301–400 Hazardous. 

(2) 0.505–0.604 3 350.5–500.4 505–604 40.5–50.4 0.805–1.004 1.65–2.04 401–500 Hazardous. 

1Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small number of areas where an AQI 

based on 1-hour ozone values would be more precautionary. In these cases, in addition to calculating the 8-hour ozone index value, the 1-hour 

ozone index value may be calculated, and the maximum of the two values reported. 2 8-hours O3 values do not define higher AQI values (≥301). AQI values of 301 or greater are calculated with 1-hour O3 concentrations. 3 If a different SHL for PM2.5 is promulgated, these numbers will change accordingly. 

[FR Doc. 2010–1990 Filed 2–8–10; 8:45 am] 

BILLING CODE 6560–50–P 


jlentini on DSKJ8SOYB1PROD with RULES2

VI-167 A-189
VII-1 

CHAPTER VII 

The following documentation of filing and notification is incorporated as part of this 

hearing record and is maintained on file: 

1. DENR 101 Internal Approval Form. 

2. Submission for Notice Form and material submitted to the Office of 

Administrative Hearings. 

3. The public notice as it appears in The North Carolina Register Volume 25, 

Issue 19, pages 2224-2225. 

4. Letters to newspapers requesting that the public notice be published. 

5. Affidavits of Publication from newspaper. 

6. Memorandum transmitting hearing notice and proposal to regional offices 

for public inspection. 

7. Memorandum transmitting hearing notice and proposal to local programs. 

8. Letter notifying EPA of hearing. 

9. Submission of Filing Forms and material filed with Office of 

Administrative Hearings. 

 10. Letter transmitting hearing record to EPA. 

A-190