Response to Comments on the Data 

Requirements Rule for the 1-Hour Sulfur 

Dioxide Primary National Ambient Air 

Quality Standard 

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Research Triangle Park, NC

August 4, 2015
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Table of Contents

I. Introduction...................................................................................................................7

II. Monitoring and Modeling to Satisfy the Proposed Rule .........................................11

A. Legal authority to require data by proposed schedule ......................................11

1. Proposal conflicts with and modifies the Clean Air Act (CAA) ..........11

2. Proposal bypasses section 107..............................................................13

3. Unclassifiable designations...................................................................14

a. DRR nullifies the "unclassifiable" designation

b. EPA should immediately designate areas unclassifiable

B. Legal authority to find that modeling satisfies the data requirements..............21

1. Conflicts with the SO2 NAAQS............................................................21

2. State responsibility................................................................................23

3. Historic use of modeling for designations............................................25

a. Historic use supports modeling for designations

b. Historic use does not support modeling for designations

4. Repeal of section 171(2).......................................................................29

5. Rulemaking needed for use of modeling for designations....................30

6. Court decisions on use of modeling for designations...........................31

7. Nonattainment area requirements.........................................................33

C. Monitoring and/or modeling for designations ..................................................34

1. Support option to model and/or monitor...............................................34

2. Support monitoring data use only.........................................................35

3. Do not support modeling to make attainment designations..................37

4. Support modeling to make designations...............................................41

5. Rule text regarding monitoring preference ...........................................42

a. Section 51.1201

b. Section 51.1203(b)

6. Bias for modeling rather than monitoring.............................................44

D. Considering all monitoring/modeling data .......................................................46

1. Do not support considering all monitoring/modeling data ...................46

2. Support considering all monitoring/modeling data...............................47

III. Source Coverage and Emission Threshold Options ................................................48

A. Options to identify sources for monitoring/modeling.......................................48

1. Emissions metrics for use in a threshold approach.......................................48

a. Use of annual emissions

b. Emission threshold approach

2. Threshold including population metric.........................................................50

a. Support using a population threshold

b. Do not support using a population threshold

c. Other comments on population thresholds
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3. Proposed option 1 (> 2,000/1,000 tpy) .................................................56

a. Support option 1

b. Do not support option 1

4. Proposed option 2 (> 5,000/2,000 tpy) .................................................58

5. Proposed option 3 (> 10,000/3,000 tpy) ...............................................58

6. Other options to identify sources..........................................................62

a. Refine dispersion modeling

b. 1,000 tpy

c. 2,000 tpy

d. 3,000/5000 tpy

e. 10,000 tpy

f. 10,000/1,000 tpy (Option 1&3 hybrid)

g. 10,000/5,000 tpy

h. 500 pounds per hour and annual limits

i. Sources with seasonal or monthly emission spikes

j. Multiple sources in close proximity that individually are below the 

threshold but whose combined emissions may violate the NAAQS

k. Phased approach

l. Statewide percent reduction target

m. Factors other than population

n. Do not codify the thresholds

7. State option – identify additional sources.............................................75

8. EPA option – identify additional sources .............................................77

9. State discretion to exclude above-threshold sources......................79

IV. Data Requirements and Program Implementation Timeline .................................82

A. Comments on the overall timeline ....................................................................84

1. The proposed timeline may be reasonable............................................84

2. The proposed timeline is not achievable...............................................86

3. Monitoring and modeling timelines should be the same ......................89

B. Requirement to submit list of SO2 sources & election of monitoring or 

modeling by January 15, 2016..........................................................................89

1. List by most recent year of emissions...................................................89

a. General

b. Data sources

c. Expected availability of data for specific years

2. List by average emissions over a three-year period..............................92

3. Unrepresentative emissions ..................................................................94

4. Legal agreements or permit limits ........................................................95

a. Enforceable limits by January 1, 2016

b. Enforceable limits by January 1, 2017

c. Enforceable limits effective by the attainment date 

5. Electricity generation reliability .........................................................103

6. Enforceable limits in SIPs...................................................................104
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7. Election of monitoring or modeling....................................................107

8. Common area analytical approach......................................................109

9. Oppose submitting a list......................................................................111

10. Section 51.1203(a) ..............................................................................113

11. Updated information on EPA’s preliminary list .................................114

12. Sources currently in a nonattainment area ..........................................115

13. New/modified sources or existing sources initially below the 

threshold, subsequently exceeding it ..................................................117

C. January 15, 2016: Requirement to submit modeling protocols for sources 

characterized with modeling...........................................................................118

1. Schedule..............................................................................................118

2. Section 51.1203(d)..............................................................................119

3. EPA oversight of modeling plans.......................................................120

4. Stakeholder participation in air agency characterization efforts.........121

D. July 2016: Inclusion of SO2 monitoring network modifications to satisfy 

DRR ................................................................................................................123

1. Schedule is not feasible.......................................................................123

2. Schedule needs to be changed.............................................................124

E. January 1, 2017: Requirement that SO2 monitors be operational...................125

1. Time between monitoring plans due and monitors operational..........125

a. Insufficient time

b. Reasonable time frame

2. EPA oversight of monitoring plans.....................................................128

a. Timing for EPA determination

b. Eliminate EPA approval of monitoring plans

3. Phased approach..................................................................................132

F. January 13, 2017: Requirement to provide modeling analyses ......................135

1. Schedule..............................................................................................135

a. Schedule may be adequate

b. Schedule is not feasible

2. EPA review of the modeling analyses................................................137

3. Updated designation recommendations..............................................138

a. Procedures

b. March 24, 2011 designations guidance

c. Guidance needed on boundaries and areas without significant SO2

sources

d. Designations as "attainment" or "unclassifiable/attainment"

4. Clarify terms .......................................................................................143

G. December 2017: Intended date for issuance of final designations for majority of 

country ............................................................................................................143

H. May 2020: Required certification of 2019 monitoring data ...........................144

I. December 2020: Intended date for issuance of final designations .................144

V. Technical Considerations.........................................................................................144
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A. Monitoring ......................................................................................................144

1. Monitoring in part 58 ..........................................................................144

2. Move existing monitors to free up resources for the near-source 

network ...............................................................................................145

3. Network design ...................................................................................146

a. Guidance is needed - maximum concentration site

b. Guidance is needed - number of monitors

c. One monitor could be sufficient

d. Evaluate the existing monitoring network

e. Multiple monitors per source are needed

f. Flexibility for states

4. Delayed monitoring--automatically move a source to the modeling 

pathway...............................................................................................154

a. Support automatically moving a source

b. Do not support automatically moving a source

5. Delayed monitoring--evaluate ............................................................157

a. Allow a time extension

b. Data gaps

6. Resources............................................................................................161

a. Costs

b. Industry costs

c. Funding by EPA

d. Partnerships and third party monitoring

7. Monitor shutdowns.............................................................................165

a. Threshold below which monitors may be shut down

b. Multiple monitors

c. Areas initially designated attainment

d. Clean data

e. State discretion

f. Monitors in unclassifiable/attainment areas

g. Near a source that shuts down or reduces emissions

h. Statistical approach

i. Do not shut down monitors

j. Case-by-case monitor shutdowns

8. Post-monitor shutdown annual reporting............................................174

a. Option1: annual report as an appendix to the annual monitoring plan

b. Option 2: independent annual submittal from the state

c. Allow both Options 1 and 2

d. Monitors with data under 50% of the standard

e. Sunset provision for the reporting requirements

f. EPA disapproval of a monitoring plan due to nonattainment

9. Emerging technologies........................................................................180

B. Modeling.........................................................................................................180
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1. Emissions--actual data ........................................................................180

a. Support use of actual data

b. State option to model actual or allowable

c. Intermittent sources

d. Hourly rates for model input

e. Revise the regulatory text

2. Emissions--most recent three years ....................................................186

3. Emissions-- accounting for enforceable limits ...................................188

a. Support accounting for enforceable limits

b. Controls implemented and enforceable as of January 2017

c. Emissions reductions after 2017

4. Background concentrations.................................................................192

5. Stack height.........................................................................................196

6. Meteorological data ............................................................................197

a. One year

b. Five years

c. Concurrent meteorological and emissions data

d. Where meteorological data are available

e. Multiple meteorological stations 

7. Receptor placement.............................................................................200

8. Paired test............................................................................................202

9. AERMOD, EPA's preferred near-field dispersion model ............ 210

a. Performs well

b. Concerns with AERMOD

10. Alternative models and alternative approaches ..................................210

11. Multi-source modeling analyses.........................................................213

12. Demonstrate ongoing data requirement --options...............................216

a. Model once using allowable emissions or adopt limits with the actual 

emissions rates used in the modeling

b. Proposed option 1: model using actual emissions, then assess emissions 

annually and conduct new modeling every 3 years

c. Proposed option 2: if a source's emissions increases over the previous 

year, the state would be required to submit to the EPA an assessment of 

the cause of the increase and provide an initial determination of whether or 

not air quality modeling would be needed

d. Concerns related to choosing either option 1 or 2

e. Proposed option 3: model using actual emissions, then conduct 

screening modeling every 3 years

f. Option for ambient air quality monitoring

g. Option for annual modeling

h. Allow multiple options

i. Clarify verification options

j. Do not support ongoing data requirements 
vi

k. Sunset provision for the ongoing data requirements 

13. Post-attainment designation annual reporting.....................................230

14. Resources............................................................................................233

15. Procedural issues.................................................................................234

VI. Statutory and Executive Order Reviews.................................................................235

A. Paperwork Reduction Act...............................................................................235

B. Executive Order 13175: Consultation and coordination with Indian tribal 

governments ...........................................................................235

VII. TADs for Monitoring and Modeling and Appendix W.........................................239

A. Comments in the DRR docket related to the TADs and Appendix W ...........239

B. TADs are not binding......................................................................................240

C. Procedure for TADs........................................................................................240

VIII. Proposed Consent Decree (CD) ...............................................................................242

A. Comments in the DRR docket related to the proposed CD ............................242

B. Conflicts with the APA...................................................................................243

C. Conflicts with the DRR...................................................................................244

D. Procedural options for the CD and DRR ........................................................246

E. CD 16 month deadline ....................................................................................248

IX. Comments Outside the Scope of this Rulemaking.................................................248

A. Comments on designation policy...................................................249

1. Alternate approaches for designations

2. Designations before 2017

3. Previously submitted SO2 designations in specific states

4. Redesignation to attainment following nonattainment designation based on 

modeling

5. Future monitored violations

6. December 2017: Intended date for issuance of final designations for 

majority of country

7. December 2020: Intended date for issuance of final designations

B. SO2 NAAQS final rule .............................................................255

C. Tenth EPA modeling conference ..................................................255

D. Other comments outside the scope of this rulemaking ........................255

1. SIP requirements

2. Electric rates

X. Other Comments.......................................................................................................255

A. Support other comments.................................................................................257

B. EPA guidance..................................................................................................257

C. Organization of the proposal...........................................................................259
DELIBERATIVE - FOR INTERNAL EPA REVIEW ONLY

I. Introduction 

The EPA proposed a rule on May 13, 2014 that would direct state and tribal air agencies (air 

agencies) to provide data to characterize current air quality in areas with large sources of sulfur 

dioxide (SO2) emissions if such areas do not have sufficient air quality monitoring in place to 

identify maximum 1-hour ambient SO2 concentrations. The proposed rule described criteria for 

identifying the sources around which air agencies would need to characterize SO2 air quality. It 

also described a process and timetables by which air agencies would characterize air quality 

around sources through ambient monitoring and/or air quality modeling techniques and submit 

such data to the EPA.

The public comment period closed on July 14, 2014. A total of 87 comments are included in the 

docket, Table 1. A few of these comments are duplicates: 0076 and 0077; 0096, 0104 & 0105; 

0063 & 0065 (although 0065 has more attachments); and 0086 & 0112. Item 0098 is an agenda 

for a July 17, 2014 EPA meeting with NEDA/CAP. A list of acronyms and frequently used 

abbreviations are contained in Table 2.

The notice of final rulemaking signed by the Administrator August 4, 2015 presents the EPA's 

final regulatory conclusions and rule text, and includes summaries of and responses to several of 

the public comments received during the public comment period. This Response to Comments 

(RTC) document presents further discussion of the public comments received and provides 

additional responses to those comments by the EPA. In some cases, the responses presented in 

this document provide more detail or elaboration than do corresponding responses in the notice 

of final rulemaking. In other cases, the responses in this document repeat or refer to the notice of 

final rulemaking as providing the EPA’s complete response to the public comment at issue.

Table 1. Docket Number and Public Comments

Docket Number Commenter Name

EPA-HQ-OAR-2013-0711-0033 Anonymous Public Comment

EPA-HQ-OAR-2013-0711-0034 Bill Thompson, Chairman, National Tribal Air Assoc. 

(NTAA)

EPA-HQ-OAR-2013-0711-0035 Jack P. Broadbent, APCO, Bay Area Air Quality 

Management District

EPA-HQ-OAR-2013-0711-0036 Arthur N. Marin, Northeast States for Coordinated Air 

Use Management (NESCAUM)

EPA-HQ-OAR-2013-0711-0037 Steve Jackson, Alliant Energy Services, Inc.

EPA-HQ-OAR-2013-0711-0038 Bart Sponseller, Director, Bureau of Air Management, 

Wisconsin Dept. of Natural Resources

EPA-HQ-OAR-2013-0711-0039 Bernie Paul, B Paul Consulting, LLC

EPA-HQ-OAR-2013-0711-0040 Wade Foster, The Fertilizer Institute (TFI)

EPA-HQ-OAR-2013-0711-0041 Lynn Fielder, Acting Chief, Air Quality Division, 

Michigan Dept. of Environmental Quality

EPA-HQ-OAR-2013-0711-0042 Joseph C. Stanko, Jr., Hunton and Williams LLP, for 

NAAQS Implementation Coalition
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EPA-HQ-OAR-2013-0711-0043 Steven M. Pirner, Secretary, South Dakota Dept. of 

Natural Resources

EPA-HQ-OAR-2013-0711-0044 Robert A. Hirsch, Dir., Environmental Affairs, National 

Lime Association (NLA)

EPA-HQ-OAR-2013-0711-0045 Maya Rao, Chief, Air Div., Mississippi Dept. of 

Environmental Quality

EPA-HQ-OAR-2013-0711-0046 Paul Roe, American Forest & Paper Assoc. (AF&PA)

EPA-HQ-OAR-2013-0711-0047 Charles R. Matthews, We Energies

EPA-HQ-OAR-2013-0711-0048 Kenneth Anderson, Ameren Corporation

EPA-HQ-OAR-2013-0711-0049 Thomas Kierspe, Santee Cooper

EPA-HQ-OAR-2013-0711-0050 Douglas J. Fulle, Oglethorpe Power Corp.

EPA-HQ-OAR-2013-0711-0051 Richard A. Hyde, Executive Dir., Texas Commission on 

Environmental Quality

EPA-HQ-OAR-2013-0711-0052 John S. Lyons, Dir., Division for Air Quality, 

Commonwealth of Kentucky

EPA-HQ-OAR-2013-0711-0053 Robert W. Clemens, FCG Environmental Committee, 

Florida Electric Power Coordinating Group, Inc. (FCGEC)

EPA-HQ-OAR-2013-0711-0054 Charles D. Johnson, The Aluminum Association

EPA-HQ-OAR-2013-0711-0055 Grant Rodway, Bison Engineering, Inc, et al.

EPA-HQ-OAR-2013-0711-0056 Patrick Coughlin, Duke Energy

EPA-HQ-OAR-2013-0711-0057 Mark Thomas, Otter Tail Power Co.

EPA-HQ-OAR-2013-0711-0058 W. Danny Herrin, Southern Co.

EPA-HQ-OAR-2013-0711-0059 Dan Holland, Public Commenter

EPA-HQ-OAR-2013-0711-0060 Craig W. Butler, Dir., Ohio EPA

EPA-HQ-OAR-2013-0711-0061 Ray Hendricks, Portland General Electric

EPA-HQ-OAR-2013-0711-0062 Stuart A. Clark, Air Quality Program Mgr., Dept. of 

Ecology, State of Washington

EPA-HQ-OAR-2013-0711-0063 Devi Chandrasekaran, Baker Botts LLP, for BCCA 

Appeal Group

EPA-HQ-OAR-2013-0711-0064 Bill Matthews, Cleco Corp.

EPA-HQ-OAR-2013-0711-0065 Devi Chandrasekaran, Baker Botts LLP, for Texas SO2

Working Group

EPA-HQ-OAR-2013-0711-0066 Michael P. Lebeis, DTE Energy

EPA-HQ-OAR-2013-0711-0067 Stephen R. Gossett, Eastman Chemical Co.

EPA-HQ-OAR-2013-0711-0068 Zachary M. Fabish, Sierra Club

EPA-HQ-OAR-2013-0711-0069 Keith Bentley, Chief, Air Protection Branch, Georgia 

Environmental Protection Div., Georgia Dept. Natural 

Resources

EPA-HQ-OAR-2013-0711-0070 American Electric Power Service Corp. (AEPSC)

EPA-HQ-OAR-2013-0711-0071 Kyra L. Moore, Dir., Missouri Dept. of Natural Resources
DELIBERATIVE - FOR INTERNAL EPA REVIEW ONLY

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EPA-HQ-OAR-2013-0711-0072 Erie C. Massey, Dir., Air Quality Div., Arizona Dept. of 

Environmental Quality (ADEQ)

EPA-HQ-OAR-2013-0711-0073 Lyle Witham, Western Energy Supply & Transmission 

Assoc. (WEST)

EPA-HQ-OAR-2013-0711-0074 Grant Rodway, Bison Engineering Inc. & Larry Zink, 

Montana Sulfur and Chemical Co.

EPA-HQ-OAR-2013-0711-0075 Andrea Field, Hunton & Williams LLP, for Utility Air 

Regulatory Group (UARG)

EPA-HQ-OAR-2013-0711-0076 Maxine Lipeles, Washington Univ. School of Law, for 

Sierra Club 

EPA-HQ-OAR-2013-0711-0077 Maxine Lipeles, Washington Univ. School of Law, for 

Sierra Club Missouri Chapter

EPA-HQ-OAR-2013-0711-0078 Frank L. Kohlasch, Minnesota Pollution Control Agency 

(MPCA)

EPA-HQ-OAR-2013-0711-0079 Bryan Brendle, Portland Cement Assoc. (PCA)

EPA-HQ-OAR-2013-0711-0080 Michael J. Nasi, Counsel for Gulf Coast Lignite Coalition 

(CGLC)

EPA-HQ-OAR-2013-0711-0081 Andrew Hollis, South Carolina Dept. of Health and 

Environmental Control (SCDHEC) Bureau of Air Quality

EPA-HQ-OAR-2013-0711-0082 Christine Wyman, Class of 85 Regulatory Response 

Group

EPA-HQ-OAR-2013-0711-0083 Bob Holmes, Environmental Affairs Admin., Omaha 

Public Power District (OPPD)

EPA-HQ-OAR-2013-0711-0084 James Taylor, Environmental Regional Mgr., 

MeadWestvaco (MWV)

EPA-HQ-OAR-2013-0711-0085 George S. Aburn, Dir., Air & Radiation Mgmt. Admin., 

Maryland Department of the Environment

EPA-HQ-OAR-2013-0711-0086 Jason Bohrer, Pres/CEO, Lignite Energy Council (LEC)

EPA-HQ-OAR-2013-0711-0087 Pamela F. Faggert, Dominion Resources Services

EPA-HQ-OAR-2013-0711-0088 National Environmental Development Association’s Clean 

Air Project NEDA/CAP

EPA-HQ-OAR-2013-0711-0089 Jasmine Mehta, Bureau of Air Quality Planning, State of 

Nevada

EPA-HQ-OAR-2013-0711-0090 Kim Mireles, Luminant

EPA-HQ-OAR-2013-0711-0091 John M. Holloway III, Winston & Strawn LLP, for Westar 

Energy

EPA-HQ-OAR-2013-0711-0092 James D. Jones, Alcoa, Inc.

EPA-HQ-OAR-2013-0711-0093 Ronald W. Gore, Alabama Dept. of Environmental Mgmt. 

(ADEM)

EPA-HQ-OAR-2013-0711-0094 Kimberly S.L. Bauman, Mississippi Lime Co.

EPA-HQ-OAR-2013-0711-0095 Roger Walker, Regulatory Environmental Group for 

Missouri (REGFORM)
DELIBERATIVE - FOR INTERNAL EPA REVIEW ONLY

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EPA-HQ-OAR-2013-0711-0096 Madison B. Cary Miller, Office of the General Counsel 

Office at Tulsa Air Quality Div., Oklahoma Dept. of 

Environmental Quality (ODEQ)

EPA-HQ-OAR-2013-0711-0097 Michael G. Dowd, Virginia Dept. of Environmental 

Quality (VDEQ)

EPA-HQ-OAR-2013-0711-0099 Roger Walker, REGFORM

EPA-HQ-OAR-2013-0711-0100 Steven A. Dietrich, Dept. of Environmental Quality – Air 

Quality Div. (DEQ/AQD), State of Wyoming

EPA-HQ-OAR-2013-0711-0101 Joe L. Citta, Nebraska Public Power District (NPPD)

EPA-HQ-OAR-2013-0711-0102 John W. Mitchell, Div. of Environment, Kansas Dept. of 

Health and Environment

EPA-HQ-OAR-2013-0711-0103 Catharine Fitzsimmons, Air Quality Bureau, Iowa Dept. of 

Natural Resources

EPA-HQ-OAR-2013-0711-0104 Oklahoma Dept. of Environmental Quality

EPA-HQ-OAR-2013-0711-0105 Oklahoma Dept. of Environmental Quality

EPA-HQ-OAR-2013-0711-0106 Terry L. O’Clair, Div. of Air Quality, North Dakota Dept. 

of Health

EPA-HQ-OAR-2013-0711-0107 Sheila Holman, Div. of Air Quality, North Carolina Dept. 

of Environment and Natural Resources (NCDENR)

EPA-HQ-OAR-2013-0711-0108 Eric Bott, Wisconsin Manufacturers and Commerce

(WMC)

EPA-HQ-OAR-2013-0711-0109 Peter Pagano, American Iron and Steel Institute (AISI)

EPA-HQ-OAR-2013-0711-0110 Howard J. Feldman, American Petroleum Institute (API)

EPA-HQ-OAR-2013-0711-0111 Sam L. Phillips, Louisiana Dept. of Environmental Quality

(LDEQ)

EPA-HQ-OAR-2013-0711-0112 Jason Bohrer, Pres/CEO, Lignite Energy Council (LEC)

EPA-HQ-OAR-2013-0711-0113 David L. Klemp, Air Resources Mgmt. Bureau, Montana 

Dept. of Environmental Quality

EPA-HQ-OAR-2013-0711-0114 Thomas N. Effinger, SCANA Environmental Services

EPA-HQ-OAR-2013-0711-0115 Joy Wiecks, Fond du Lac Environmental Program

EPA-HQ-OAR-2013-0711-0116 Paul M. Ling, Kansas City Power & Light Co.

EPA-HQ-OAR-2013-0711-0117 Cindy A. Hollenberg, New Mexico Environment 

Department, Air Quality Bureau

EPA-HQ-OAR-2013-0711-0118 Scott Deloney, Chief, Air Programs Branch, Office of Air 

Quality, Indiana Department of Environmental 

Management (IDEM)

EPA-HQ-OAR-2013-0711-0119 Myra C. Reece, Chief, Bureau of Air Quality, South 

Carolina Department of Health and Environmental Control 

(DHEC)

EPA-HQ-OAR-2013-0711-0120 Tribal Environmental Policy Center
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Table 2. Explanation of Acronyms and Frequently Used Abbreviations

Acronym Long Name

Act Clean Air Act 

Administrator Administrator of the EPA

AERR Air Emissions Reporting Rule

CAA Clean Air Act

CBSA Core Based Statistical Area

CD Consent Decree published on June 2, 2014 by the EPA at 79 Federal 

Register 31325

CEMS Continuous Emissions Monitoring System

CFR Code of Federal Regulations

DRR Data Requirements Rule proposed May 13, 2014

EGU electricity generating unit

EPA U.S. Environmental Protection Agency

FR Federal Register

GEP Good Engineering Practice

km kilometer

MACT Maximum Achievable Control Technology

MATS Mercury and Air Toxics Standards

NAAQS National Ambient Air Quality Standards

NEI National Emissions Inventory

NO2 nitrogen dioxide

NSR New Source Review

ppb parts per billion

Proposed Rule Data Requirements Rule proposed May 13, 2014

PSD Prevention of Significant Deterioration

SIP State Implementation Plan

SO2 sulfur dioxide

tpy tons per year

II. Monitoring and Modeling to Satisfy the Proposed Rule

A. Legal authority to require data by proposed schedule

1. Proposal conflicts with and modifies the Clean Air Act (CAA)

Comment:

Commenters (0040, 0043, 0049, 0050, 0051, 0052, 0053, 0058, 0063, 0065, 0075, 0081, 0085, 

0089, 0103, 0106, and 0109) stated the proposal conflicts with the designation process in the 
DELIBERATIVE - FOR INTERNAL EPA REVIEW ONLY

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CAA. 

Commenters (0058, 0063, 0065, 0089, 0103, 0106) stated that the CAA required the EPA to 

make area designations under the new SO2 standard—either attainment, nonattainment, or 

unclassifiable—no later than June 3, 2013, and that the EPA failed to comply with that 

mandatory obligation.

Commenters (0052, 0075, 0089, 0106) stated that the proposed schedule, by which the EPA 

would issue final designations for the remainder of the country by December 2020, or ten years 

after the revised SO2 NAAQS, conflicts with section 107(d)(l)(B)(i) of the CAA. Commenter 

(0089) stated that, to take a total of ten years from the date that the 1-hour SO2 NAAQS was 

promulgated to issue initial area designations is beyond the EPA's legislatively granted authority. 

Commenter (0089) cited EME Homer City Generation LP v. Envt'l Prot. Agency, 696 F. 3d 7, 27 

(D.C. Cir. 2012) and stated the SO2 Data Requirements Rule (DRR) cannot stand as proposed 

because it fails to follow the mandatory timelines for promulgating area designations, and, 

therefore, exceeds the EPA's legislative authority.

Commenter (0106) stated the EPA must make the initial designations for all areas of the country 

based on the data it now has in its possession. Commenter (0106) noted the EPA could 

theoretically complete two reevaluations of the SO2 NAAQS before making the initial 

designations for the June 2010 standard, given the schedule in the proposal. Commenter (0106) 

stated that, if the EPA decides to change the standard during the reevaluation process, all data 

required by this Proposed Rule would likely be useless and a waste of scarce resources.

Commenter (0052) stated the DRR modifies and revises the statutory provisions established by 

the CAA and imposes additional statutory obligations on air agencies. Commenter (0051) stated 

that requiring states to develop monitoring or modeling data in accordance with this proposal for 

designations ignores the statutory mandate. Commenter (0051) stated that the proposal arbitrarily 

grants the EPA up to an additional seven years to do what the Agency was mandated to do by 

June 2013: designate all areas of the country as either attainment, nonattainment, or 

unclassifiable. Commenter (0053) stated it is not appropriate to replace the CAA's statutory 

directive with extra-statutory provisions like those proposed in the DRR and CD.

Response: 

The EPA has legal authority under sections 110(a)(2)(B), 110(a)(2)(K), and 301(a)(1) of the 

CAA to request data and information from states regarding air quality status within their borders. 

In addition, CAA section 114(a)(1) provides broad authority for the EPA to direct any person to 

provide emissions information. 

The EPA disagrees with commenters’ claims that the rule directing air agencies to gather and 

submit such data and information to the EPA conflicts with the CAA statutory provisions 

governing issuance of area designations under the NAAQS. The DRR in and of itself does not 

modify the schedule for designations, but rather will result in the submittal of information and 

data to the EPA that may be used in appropriate contexts for evaluating air quality in specific 
DELIBERATIVE - FOR INTERNAL EPA REVIEW ONLY

13

areas to inform various future actions, such as area designations, re-designations, clean data 

determinations, or determinations regarding whether areas have timely attained the NAAQS. The 

timing for designations is governed by the statute and not by this rule. As a result of lawsuits 

seeking to compel the EPA to complete issuance of the initial SO2 designations, the EPA entered 

into an enforceable court order issued on March 2, 2015, outlining a schedule for completing the 

national designations process. 

The EPA disagrees that the EME Homer City ruling is relevant to this rulemaking or the 

objection raised by the commenter, as that case did not address the EPA’s authority to direct air 

agencies to gather and submit to the EPA data and information regarding areas’ SO2 ambient 

concentrations. The EPA additionally considers irrelevant and beyond the scope of this 

rulemaking the comments asserting that the EPA may issue designations only based on 

information already in the agency’s possession, and the other objections related to the timing of 

designations and to the contents of the CD entered on March 2, 2015, that now governs that 

process. The information gathered pursuant to this rule will be useful for purposes of informing 

whether areas are in fact meeting or failing to meet the 2010 SO2 NAAQS, as that information is 

presently generally lacking, no matter what particular statutory provision is at that time being 

implemented that involves assessment of the areas’ current status.

2. Proposal bypasses section 107

Comment:

Commenter (0109) asserted states must retain the opportunity to develop their own designations 

without the interim requirement of EPA approval and that the EPA's proposed involvement 

improperly expands the Agency's role beyond what Congress intended when it gave states 

primary authority to craft designations under the CAA.

Commenters (0043, 0052, 0063, 0065) suggested the proposal attempts to bypass section 

107(d)(3)(A), by dismissing states’ designation packages submitted in June 2011. Commenters

(0052, 0063, and 0065) stated the CAA does not allow the Administrator to ignore the list of 

areas submitted by a Governor. Commenter (0052) stated that the Administrator only designated 

certain nonattainment areas and ignored all other areas, including areas with sufficient ambient 

air monitoring data meeting applicable regulatory requirements and the NAAQS. 

Commenters (0063, 0065) stated that the EPA has not identified a statutory basis to delay 

designations pending any revisions to the SO2 monitoring network requirements. Commenters 

(0063, 0065) referred to the 2008 lead NAAQS experience and stated, in a similar fashion, the 

EPA should not allow revisions to the SO2 monitoring network to delay all area designations.

Commenters (0063, 0065) asserted the EPA chose a course of unreasonable deferral when it 

failed to designate all areas of the U.S. for the 1-hour SO2 standard before the statutory deadline. 

Commenters (0063, 0065) claimed the CAA requires the EPA to implement the 1-hour SO2

standard in accordance with the following principles:
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• Each state submits a recommended designation, and the EPA must promulgate the initial 

designations within 3 years of issuing the standard and must give a state 120 days' notice 

if the EPA intends to depart from the state's recommended designations. 

• EPA should continue to use the "unclassifiable" designation where appropriate, and 

should not seek to designate all areas "attainment" or "nonattainment." The CAA 

provides tools for the EPA and a state to address an attainment or unclassifiable area that 

is later found to have air quality that does not meet the standard. 

• Current regulations for characterizing air quality found at 40 CFR § 50.17 and part 58 

provide sufficient information to support area designations. Section 50.17 provides that 

monitoring is the basis for determining attainment. Part 58 was revised along with the 1-

hour SO2 standard, includes detailed rules for SO2 monitor network design, and allows 

case-by-case flexibility where necessary. The Proposed Rule calls for modeling and 

additional monitoring beyond what is necessary for the EPA to comply with the CAA.

• In at least some areas, existing data clearly support attainment designations. An example 

is the Houston, Texas, region, which has several regulatory and privately operated 

monitoring stations (including the Texas Commission on Environmental Quality 's 

monitors, Houston Region al Monitoring, and the Texas City/La Marque Community Air 

Monitoring Network), which have uniformly shown attainment of the 1-hour SO2

standard. Other areas of the country have similar monitoring data. The Proposed Rule 

impermissibly withholds an "attainment" classification for such areas and rewrites the 

rules on how to assess attainment.

Commenter (0053) stated the EPA's decision to defer these otherwise expected unclassifiablearea designations is the basis for the CD, which is a separate and distinct issue from the timing 

and substantive criteria the EPA is promulgating through its DRR to collect additional data and 

information for these areas. 

Response:

As stated previously, the EPA has legal authority under sections 110(a)(2)(B), 110(a)(2)(K), and 

301(a)(1) of the CAA to request data and information from states regarding air quality status 

within their borders. In addition, CAA section 114(a)(1) provides broad authority for EPA to 

direct any person to provide emissions information. The DRR in and of itself does not delay 

designations that are due under the statute and will be due under the court order issued on March 

2, 2015. The comments objecting to the EPA’s schedule for issuing the remaining area 

designations under the 2010 SO2 NAAQS are therefore beyond the scope of this rulemaking and 

warrant no further response.

3. Unclassifiable designations 

a. DRR nullifies the “unclassifiable” designation
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Comment:

Commenter (0053) asserted that the EPA cannot use the DRR to supersede the statutory 

requirement to timely make area designations, including statutorily-appropriate "unclassifiable" 

designations. Commenter (0053) stated the most effective remedy to the EPA's failure to meet 

the statutory deadline is to designate currently undesignated areas as unclassifiable.

Commenters (0040, 0065, 0083, and 0089) stated the Proposed Rule effectively nullifies the 

"unclassifiable" designation, use of which would have allowed the EPA to meet its statutory

deadline. Commenter (0089) cited several court cases and stated that the SO2 DRR essentially 

reads out of the CAA the availability of this "unclassifiable" designation, the very purpose of 

which is to designate areas for which the agency has insufficient information to make a 

designation of attainment or nonattainment. Commenter (0089) provided the following 

additional points.

Timely promulgation of area designations is important for several reasons. First, 

unclassifiable areas have traditionally been treated as attainment, which obviates the need 

for the state of Nevada, along with other states, to develop a more rigorous nonattainment 

state implementation plan (SIP) and unnecessarily impose more stringent controls on 

regulated entities located far from populated areas. Notably, the EPA could make 

unclassifiable designations for those areas for which it believes it lacks sufficient 

information, and at the same time require additional data collection. Making the 

designations does not negate the EPA's ability to require better characterization of SO2

emissions. 

Second, the unclassifiable designation has traditionally recognized the rural nature of the 

state of Nevada, and allowed the state to utilize its limited resources to deploy monitoring 

networks where population centers are located. It obviated the state's need to invest in 

expansive and costly monitoring networks across Nevada's rural landscape. 

Third, it provided certainty to existing and potential industries, so that they would be 

comfortable in investing in capital outlays for equipment that could be used for several, if 

not many, years to come. If industry were to try to locate to Nevada, but the area 

designation is unknown, industry could be required to implement additional or different 

controls after area designations are finally promulgated. 

Fourth, the proposed SO2 DRR contains a timetable by which the EPA must act to make 

area designations, with final area designations to be made no later than December 31, 

2020. Thus, according to this schedule, area designations for the 2010 SO2 NAAQS will 

not be made until after the EPA is required to reevaluate, and possibly lower, the SO2

NAAQS. Such out-of-order rule promulgation creates implementation issues for states 

and muddies the implementation timeline. It also creates considerable uncertainty for 

regulated industries. The delay effectively prohibits Nevada (and other states) from 
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challenging any area designations that the EPA may make until well after the EPA is 

required to revisit the NAAQS. 

Finally, Nevada is concerned that if the EPA is allowed to utilize modeling for this 

standard, it will seek to do so for other standards as they are updated. Modeling is a 

predictive tool, but it is not appropriate for making area designations, which should be 

based on the air quality condition existing at the time of the designation. Modeling is 

susceptible to challenge and is not an accurate predictor for short term standards. For 

these reasons, the SO2 DRR is not an appropriate exercise of the EPA's legislative 

authority.

Commenter (0065) stated the EPA should continue to use the "unclassifiable" designation where

appropriate, and should not seek to designate all areas attainment or nonattainment. Commenter 

(0065) stated the CAA provides adequate tools for EPA and a state to address an attainment or 

unclassifiable area that is later found to have air quality that does not meet the SO2 standard. 

Commenter (0093) stated that the EPA and others must live with the fact that, when a new 

NAAQS is adopted and the availability of monitoring data are lacking, immediate designations 

of areas as attaining or non-attaining is impossible. Commenter (0093) stated this is the reason 

the CAA (CAA) provides for unclassifiable/attainment status.

Commenter (0040) encouraged the EPA, in the preamble to the Final Rule, to explicitly confirm 

that its deviation from mandated CAA NAAQS designation procedures for the 1-hour SO2

NAAQS is the result of unique circumstances and confirm continued applicability of the 

“unclassifiable” category for future NAAQS designations. 

Response:

As stated in the preamble to the final rulemaking, the comments that assert that the EPA has not 

designated areas under the 2010 SO2 NAAQS in a timely manner are beyond the scope of this 

rulemaking, and are not germane to the issue of the EPA’s statutory authority to direct air 

agencies to conduct monitoring or modeling to further characterize ambient air concentrations of 

SO2. Through this rulemaking, the EPA is not establishing or modifying any area designation 

requirements provided for in section 107 of the CAA, nor does any aspect of this final rule 

conflict with any provision of section 107 that directs states and the EPA to take timely action to

issue designations. The purpose and effect of this rulemaking is to require air agencies to 

characterize air quality in priority areas throughout the country where existing ambient monitors 

may not be adequately characterizing peak 1-hour SO2 ambient air concentrations. The air 

quality data obtained as a result of this rulemaking then may be used in future analytical actions 

by the EPA, including designations of any undesignated areas or redesignations of already 

designated areas. It is true that in the proposed rule preamble we discussed how the timing of the 

implementation of this rule would fit with our intended schedule for completing area 

designations, but the proposal did not itself purport to establish a binding schedule for 

completing designations.
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The EPA notes that litigation was filed against the EPA to compel the Agency to complete 

designations under CAA section 107, and on March 2, 2015, the court in one of those cases

issued a ruling that places the EPA on a binding schedule to complete area designations for the 

2010 1-hour SO2 NAAQS. See, Sierra Club, et al. v. McCarthy, Case No. 13-cv-03953-SI (N.D. 

Cal., March 2, 2015) (Order Granting Joint Motion To Approve And Enter Consent Decree And 

Denying Other Motions As Moot; and Consent Decree). Copies of the court’s order and the CD 

setting forth the EPA’s schedule for completing designations have been placed in the docket for 

this rulemaking. Under the schedule ordered by the court, the EPA is required to complete the 

designations in no more than three future rounds. 

The EPA continues to believe that the requirements of this rule for air agencies to submit a list of 

sources where further air quality characterization is needed, and the other data submittal 

requirements found in §51.1203 of this rule, are appropriate steps needed to better understand 

SO2 air quality throughout the country, and are consistent with sections 110(a)(2)(B), 

110(a)(2)(K), 301(a)(1), and 114 of the CAA. The commenters did not challenge this view. 

Section 110(a)(2)(B) indicates that SIPs are to “provide for establishment and operation of 

appropriate devices, methods, systems, and procedures necessary to (i) monitor, compile and 

analyze data on ambient air quality and (ii) upon request, make such data available to the 

Administrator.” Section 110(a)(2)(K) states that SIPs shall “provide for (i) the performance of 

such air quality modeling as the Administrator may prescribe for the purpose of predicting the 

effect on ambient air quality of any emissions of any air pollutant for which the Administrator 

has established a NAAQS and (ii) the submission, upon request, of data related to such air 

quality modeling to the Administrator.” Section 301(a)(1) provides the EPA with general 

authority to establish regulations as necessary to carry out the agency’s functions, which in this 

case includes ensuring the attainment of the SO2 NAAQS throughout each state. This section 

states that “The Administrator is authorized to prescribe such regulations as are necessary to 

carry out his functions under this chapter.”

The EPA often establishes and revises monitoring requirements for implementing NAAQS, 

whether or not those requirements will necessarily generate new information in time to inform 

timely area designations under CAA section 107. See, e.g., 75 FR 81126, 81130, December 27, 

2010. The validity of such rules does not depend upon whether information generated pursuant 

to those requirements will be gathered in time to support designations that are timely under 

section 107. Here, the commenters have raised no objection to the central premise of the rule, 

which is that additional information that better characterizes air quality near larger sources of 

SO2 is warranted and is authorized to be required under sections 110 and 301 of the Act. 

Irrespective of when the EPA uses this information—for example, irrespective of whether the 

EPA promulgates initial designations of “unclassifiable” (and then uses the information collected 

pursuant to this DRR in later redesignations), or whether the EPA promulgates the remaining 

designations after the information required here becomes available—the EPA believes that this 

rule is authorized and is warranted. Therefore, in this final rulemaking, the commenters have 

provided no basis for the EPA to not require air agencies to submit such SO2 monitoring and 

modeling data to the EPA, as proposed. The final rule is fully consistent with the Agency’s broad 
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18

authority under section 110 and 301, as well as with the EPA’s authority under CAA section 

114(a)(1) to direct any person to provide information as is reasonably required to improve 

characterization of ambient air quality near larger sources of SO2.

As previously explained, today’s final rule is not implementing or discharging the EPA’s duties 

under CAA section 107 to designate areas under the 2010 SO2 NAAQS. Therefore, none of the 

comments relating to how or when the EPA must issue designations are relevant or germane to 

this final rule, which directs air agencies to gather and submit to the EPA additional data and 

information regarding their areas’ air quality status that can be used in any appropriate context 

that is assessing such areas’ status. All objections to the use of such data in those future actions 

may be raised at the appropriate stages for those actions. Moreover, at least with respect to 

designations under the 2010 SO2 NAAQS, the EPA is now under an enforceable court order to 

complete those designations no later than the dates provided in the court’s order, and this rule 

does not purport to change that binding schedule.

If EPA in future designations actions uses modeling information generated by implementation of 

this rule, interested persons will be able to comment on that future use at the appropriate time. 

However, at this time the EPA disagrees with the commenter’s suggestion that modeling is an 

inappropriate method by which to assess areas’ status for any purpose, and believes that it is 

reasonable to give states the option under this rule to choose whether to conduct improved 

monitoring at appropriately selected sites of maximum ambient concentrations or to conduct 

actual- or allowable-emissions based modeling to characterize areas. When either monitoring or 

modeling information is used to inform future decisions regarding specific areas will be the 

appropriate time at which to assess whether that specific information is reliable in those cases.

In addition, this rule in no way “nullifies” the unclassifiable designation option that exists under 

CAA section 107(d)(1)(A)(iii), and which remains available under the SO2 and every other 

NAAQS when EPA takes future separate action to issue designations. Nor does the rule 

otherwise establish a precedent for implementation of any other NAAQS. Although comments 

on the merits of the use of interim designations are not germane to this rulemaking and the EPA 

is not taking any final designations actions in this rulemaking, the EPA disagrees that 

promulgation of an interim designation of unclassifiable, with undefined prospects for future 

redesignation either to nonattainment or to attainment, provides greater certainty and better 

facilitates industry investments than promulgating an orderly process for gathering data that may 

later be used for informing designations.

b. EPA should immediately designate areas unclassifiable

Comment:

Commenters (0040, 0048, 0049, 0050, 0051, 0052, 0058, 0063, 0065, 0075, 0081, 0089, 0090, 

0093, 0106, 0111, and 0116) stated that, if the Agency lacked sufficient information to make an 

area designation, it must designate that area as unclassifiable. 
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19

Commenters (0075, 0116) stated the DRR cannot be a substitute for completing the initial area 

designation process within the timeframe set out by Congress. Commenter (0058) stated that this 

rulemaking does not forestall the EPA’s statutory obligation of immediately completing the area 

designations and identifying areas as “unclassifiable” where necessary and appropriate.

Commenter (0070) encouraged the EPA to drop this rulemaking and use its past practice of 

granting attainment or unclassifiable designations to areas that do not have monitored 

nonattainment and are recommended for such designations by the various states.

Commenters (0050, 0058, 0065, 0069, 0070, 0075, 0093, and 0116) urged the EPA to promptly 

designate all as yet undesignated areas, using the “unclassifiable” designation, where 

appropriate. Commenters (0050, 0063, and 0065) provided the following additional points.

Because this option for timely designations already resides in the CAA, the EPA should 

not wait another three years before issuing the required designations. EPA has taken a 

similar approach as recently as 2012, in regard to the 1-hour Nitrogen Dioxide (NO2)

NAAQS. The EPA had promulgated a new NAAQS standard in 2010 and determined 

that it should rely on monitoring data in the area designation process. However, because 

the monitoring network for NO2 was not sufficient to determine whether areas attained 

the new 1-hour NAAQS, the EPA designated all parts of the country as unclassifiable/ 

attainment until a sufficient monitoring network could be established. Following suit, the 

EPA should do the same for the new SO2 NAAQS, designating all remaining areas as 

unclassifiable, using the subsequent data collected pursuant to this Proposed Rule to 

adjust the designations in the future as needed. 

Commenters (0058, 0103, and 0116) stated that, once the EPA has designated areas as 

“unclassifiable,” it may take the time needed to develop the data necessary to reclassify those 

areas as it has proposed in the DRR. Commenter (0058) stated the EPA should complete the area 

designation process either before or at a minimum concurrently with its release of the final DRR. 

Commenter (0093) stated that the EPA and others must live with the fact that, when a new 

NAAQS is adopted and the availability of monitoring data are lacking, immediate designations 

of areas as attaining or non-attaining is impossible. Commenter (0093) stated this is the reason 

the CAA provides for unclassifiable/attainment status and suggested the EPA should 

expeditiously designate the vast majority of the country as unclassifiable.

Response:

The timing and result of designations is beyond the scope of this rulemaking, which directs air 

agencies to gather and submit to the EPA additional data and information regarding their areas’ 

air quality status. That information, if reliable, may then be used in any appropriate context that 

is assessing such areas’ status. It is not the place for this rulemaking to grant the commenters’ 

request to “immediately” issue unclassifiable designations – those decisions will be made in 

future area-specific determinations pursuant to CAA section 107 and in compliance with the 

court order issued on March 2, 2015, directing the EPA to complete designations under the 2010 
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SO2 NAAQS.

The comments that assert that the EPA has not designated areas under the 2010 SO2 NAAQS in 

a timely manner are beyond the scope of this rulemaking, and are not germane to the issue of the 

EPA’s statutory authority to direct air agencies to conduct monitoring or modeling to further 

characterize ambient air concentrations of SO2. Through this rulemaking, the EPA is not 

establishing or modifying any area designation requirements provided for in section 107 of the 

CAA, nor does any aspect of this final rule conflict with any provision of section 107 that directs 

states and the EPA to take timely action to issue designations. The purpose and effect of this 

rulemaking is to require air agencies to characterize air quality in priority areas throughout the 

country where existing ambient monitors may not be adequately characterizing peak 1-hour SO2

ambient air concentrations. The air quality data obtained as a result of this rulemaking then may 

be used in future analytical actions by the EPA, including designations of any undesignated areas 

or redesignations of already designated areas. It is true that in the proposed rule preamble we 

discussed how the timing of the implementation of this rule would fit with our intended schedule 

for completing area designations, but the proposal did not itself purport to establish a binding 

schedule for completing designations. 

The EPA notes that litigation was filed against the EPA to compel the Agency to complete 

designations under CAA section 107, and on March 2, 2015, the court in one of those cases

issued a ruling that places the EPA on a binding schedule to complete area designations for the 

2010 1-hour SO2 NAAQS. See, Sierra Club, et al. v. McCarthy, Case No. 13-cv-03953-SI (N.D. 

Cal., March 2, 2015) (Order Granting Joint Motion To Approve And Enter Consent Decree And 

Denying Other Motions As Moot; and Consent Decree). Copies of the court’s order and the CD

setting forth the EPA’s schedule for completing designations have been placed in the docket for 

this this rulemaking. Under the schedule ordered by the court, the EPA is required to complete 

the designations in no more than three future rounds. 

The EPA continues to believe that the requirements of this rule for air agencies to submit a list of 

sources where further air quality characterization is needed, and the other data submittal 

requirements found in §51.1203 of this rule, are appropriate steps needed to better understand 

SO2 air quality throughout the country, and are consistent with sections 110(a)(2)(B), 

110(a)(2)(K), 301(a)(1), and 114 of the CAA. The commenters did not challenge this view. 

Section 110(a)(2)(B) indicates that SIPs are to “provide for establishment and operation of 

appropriate devices, methods, systems, and procedures necessary to (i) monitor, compile and 

analyze data on ambient air quality and (ii) upon request, make such data available to the 

Administrator.” Section 110(a)(2)(K) states that SIPs shall “provide for (i) the performance of 

such air quality modeling as the Administrator may prescribe for the purpose of predicting the 

effect on ambient air quality of any emissions of any air pollutant for which the Administrator 

has established a NAAQS and (ii) the submission, upon request, of data related to such air 

quality modeling to the Administrator.” Section 301(a)(1) provides the EPA with general 

authority to establish regulations as necessary to carry out the agency’s functions, which in this 

case includes ensuring the attainment of the SO2 NAAQS throughout each state. This section 

states that “The Administrator is authorized to prescribe such regulations as are necessary to 
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21

carry out his functions under this chapter.”

The EPA often establishes and revises monitoring requirements for implementing NAAQS, 

whether or not those requirements will necessarily generate new information in time to inform 

timely area designations under CAA section 107. See, e.g., 75 FR 81126, 81130, December 27, 

2010. The validity of such rules does not depend upon whether information generated pursuant 

to those requirements will be gathered in time to support designations that are timely under 

section 107. Here, the commenters have raised no objection to the central premise of the rule, 

which is that additional information that better characterizes air quality near larger sources of 

SO2 is warranted and is authorized to be required under sections 110 and 301 of the Act. 

Irrespective of when the EPA uses this information—for example, irrespective of whether the 

EPA promulgates initial designations of “unclassifiable” (and then uses the information collected 

pursuant to this DRR in later redesignations), or whether the EPA promulgates the remaining 

designations after the information required here becomes available—the EPA believes that this 

rule is authorized and is warranted. Therefore, in this final rulemaking, the commenters have 

provided no basis for the EPA to not require air agencies to submit such SO2 monitoring and 

modeling data to the EPA, as proposed. The final rule is fully consistent with the Agency’s broad 

authority under section 110 and 301, as well as with the EPA’s authority under CAA section 

114(a)(1) to direct any person to provide information as is reasonably required to improve 

characterization of ambient air quality near larger sources of SO2.

B. Legal authority to find that modeling satisfies the data 

requirements

1. Conflicts with the SO2 NAAQS

Comment:

Commenters (0040, 0046) stated the use of model output to support NAAQS designations 

conflicts with the wording of the SO2 NAAQS, which states that only quality-assured ambient air 

monitoring data may be used. Commenters (0046, 0110) stated the precise wording of 40 CFR 

Part 50.17 establishes ambient air monitoring as the only basis for determining if the SO2

NAAQS is being met because it specifies that:

(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 
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22

A or A-1 of this part, or by a Federal Equivalent Method (FEM) designated in accordance 

with part 53 of this chapter.”

Commenter (0040) asserted that the EPA policy, consistent with the wording of the NAAQS, has 

previously restricted the data that may be used for carrying out NAAQS area designations to 

ambient air measurements that are acquired using an officially designated federal reference 

method (“FRM”), FEM, or approved regional method (“ARM”) that meets the quality assurance

and siting criteria set forth in 40 CFR part 58, Appendix A and Appendix D. Commenter (0040) 

believed the EPA’s tolerance of modeling output uncertainties is incongruous with, and indeed 

undermines, the purpose of the robust program for quality assuring ambient air monitoring data 

that are used for determining NAAQS compliance and stated the following issues must be 

addressed:

• FRMs, FEMs, and ARMs operate according to basic, well-known scientific principles, 

such as the amount of light given off in a specific wavelength when a molecule of SO2 is 

irradiated by light of another wavelength. In contrast, AERMOD is based on a sequence 

of mathematical equations that simplify the complex process of turbulent diffusion in the 

lower atmosphere and the downwind dispersion of a source effluent.

• FRMs, FEMs, and ARMs are calibrated on regular schedules at every regulatory air 

monitoring site using SO2 standards that are traceable to an NIST Standard Reference 

Material or other primary standard. Strict acceptance criteria based on the accuracy and 

linearity of the instrument’s response to known standards is used to calibrate the analyzer 

or trigger corrective actions. In contrast, the EPA has not prescribed any calibration 

requirement or given any guidance pertaining to the onsite calibration of AERMOD in a 

specific environment.

• Standard methods are used to assess the quality of ambient air monitoring data based on 

biweekly precision checks and accuracy assessments by independent auditors. The EPA 

has set data quality objectives of 10 percent accuracy and precision for monitors used to 

evaluate NAAQS compliance. In contrast, the EPA has not prescribed any requirements 

or given any guidance pertaining to quality assessments of modeling output. (0040)

Commenter (0049) stated initial designations under CAA section 107(d) are meant to ensure that 

actual ambient concentration levels in an area meet the air quality standard. Commenter (0049) 

stated there is no room in the statutory language for an initial area designation that may meet or 

not meet the standard based on such hypothetical data, and monitoring is the only method that 

represents actual conditions in an area. Commenter (0049) stated the approach in the DRR is 

contrary to the express language of CAA section 107(d) which does not involve consideration of 

the “potential to cause or contribute to a violation of the NAAQS,” but only requires the 

consideration of whether an area actually meets, or actually does not meet, the NAAQS. 

Commenter (0049) stated the CAA does not specifically authorize the use of modeling as a tool 

to make the initial attainment/nonattainment designations for a NAAQS. The statutory language 
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23

requires only that state governors designate areas as either meeting or not meeting the NAAQS. 

Commenter (0049) stated the CAA specifically requires the EPA to re-designate an area from 

attainment to nonattainment (or vice versa) only on basis air quality data from ambient air 

quality monitors. Commenter (0049) stated that, in the absence of a specific reference to 

modeling data in making initial designations and the reference to only monitoring data in the 

case of re-designations, the EPA should similarly require states to use monitoring data in the 

initial designation process. 

Commenters (0042, 0074) stated the modeling approach has the effect of making the new 

standard more stringent than the lower end of the range of the standard in the Proposed Rule 

because of the conservatism of modeling analyses. Commenter (0074) stated that, while the 2010 

standard is based on known, measured short-term concentrations, it is important to note that the 

standard was not based on modeled concentrations. Commenter (0074) stated it is, therefore, 

inappropriate to now administratively substitute a different estimation technology (modeling) 

with documented inaccuracies and over-predictions as a means of determining that an area fails 

to meet the ambient standard. Commenter (0074) stated such a substitution effectively modifies 

the standard itself, and will certainly produce “non-standard” and non-uniform “results” in 

various geographies across the nation which is not the intention of the authorizing legislation.

Commenters (0072, 0110) stated that because the revised 1-hour SO2 standard is more stringent 

than the previous NAAQS, the margin for error is smaller than it has been in the past, and, when

combined with the conservatism of regulatory models, it is possible that modeled concentrations 

would exceed the standard when monitoring indicates compliance with the standard.

Response:

This final rule does not make any decisions or determinations regarding whether any area is in 

fact meeting or not meeting the NAAQS based on either monitoring or modeling information. 

Those decisions will be made in separate future actions, or have already been made for some 

areas in prior actions. See e.g., 78 FR 47191, August 5, 2013. Therefore, this final rule does not

take final action on the issue of whether it is permissible to implement the commenter’s previous 

quoted provisions of 40 CFR 50.17(a)-(c) based on a combination of both monitoring and 

modeling information where both are available, or exclusively on modeling information where 

appropriate modeling information is available and monitoring is not. The commenters’ 

objections appear to focus on how future-gathered information resulting from the rule may or can 

be used in subsequent NAAQS implementation actions, but the focus of this rule is on the initial 

gathering of the information itself. In future designation, redesignation, or other implementation 

actions, commenters may raise their objections to the validity of information that the EPA relies 

upon in those specific actions, but such objections are beyond the scope of this final rule, and no 

further response to their objections is warranted at this time. 

The commenters appear to be raising objections that were also raised after the EPA’s 

promulgation of the 2010 SO2 NAAQS, in response to the EPA’s final rule preamble discussion 

explaining the Agency’s then-intended implementation approach under the NAAQS. In their 
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petitions for judicial review of the NAAQS, several states claimed that the EPA’s discussion of 

the intended use of modeling in NAAQS implementation contravened the regulatory text of

50.17. However, noting that the petitioners’ claims were addressed to potential final 

implementation actions that had not yet in fact occurred, the U.S. Court of Appeals for the D.C. 

Circuit dismissed the petitioners’ claims without addressing their merits, or lack thereof. See

National Environmental Development Association’s Clean Air Project v. EPA, 686 F.3d 803 

(D.C. Cir. 2012). Likewise here, the EPA is not yet taking any action to apply modeling 

regarding any decision of whether an area is or is not meeting the NAAQS. 

In any event, we note that although 40 CFR 50.17(a)-(c) very clearly sets forth the criteria for 

determining whether the NAAQS is met at a monitoring site, it does not by its terms restrict how 

such decisions may be made more broadly in areas impacted by SO2 sources, including areas 

where there are no monitoring sites or where monitors are not sited at the point of maximum 

ambient concentration. Indeed, it is the relative scarcity of such monitors that has caused the 

EPA to undertake this rulemaking to enable states and the Agency to better understand just what 

the ambient air impacts are from larger sources of SO2, which may not be captured by the current 

limited monitoring network. It is true that past area designations processes for most NAAQS 

(such as for ozone) having violations caused and contributed to by multiple sources over a broad 

region have relied primarily on air quality monitoring data to identify areas that violate the 

standard. However, it is important to note, as the EPA explained in the final 2010 SO2 NAAQS 

preamble, that there is a long history of also using dispersion modeling information to inform 

area designations for the SO2 NAAQS. See, e.g., 75 FR 35520, 35551, June 22, 2010. These 

actions support the view that although the monitoring method in 40 CFR 50, Appendix A-1 or A2 provides a reference method for measurements of SO2 in the atmosphere, the provisions of 40 

CFR 50 do not preclude the use of evidence obtained by other means, in particular evidence 

based on modeling that indicates that ambient SO2 concentrations exceed the standard, as 

grounds for designating areas nonattainment, or for predicting future attainment in SIPs.

2. State responsibility

Comment:

Commenter (0042) stated the modeling approach usurps the role that states are to play when

making designations and thus is inconsistent with Congressional intent. Commenter (0042) 

stated that the EPA must give great weight to the views of states as to how the NAAQS 

implementation process should work because Congress granted states the power to make the 

initial designations of areas within their borders with regard to a new NAAQS (commenter 

attached and referenced a Multiparty Reconsideration Petition at 7-10 and North Dakota and 

South Dakota Reconsideration Petition at 3-7). Commenter (0042) stated that, although Congress 

gave the EPA Administrator authority for making the final designations, Congress made it clear 

that the Administrator was to defer to state designation recommendations in most situations.

Commenter (0042) provided the following additional points.
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When the 1990 Amendments to the CAA were being debated, the power of the states was 

questioned. An original version of the Senate Bill would have ended the states’ role in 

making designations under certain circumstances. In response, a House Report asserted 

that the states are the entities best suited to identify, characterize and report their air 

pollution to EPA. See CAA Amendments: Toxic Air Pollutants: Hearing on H.R. 4, H.R. 

99, H.R. 1212 and H.R. 2585 Before the Subcomm. on Health and the Environment of the 

H. Comm. on Energy and Commerce, 101st Cong. (1989). As a result, the bill was 

amended to leave the states’ power intact. Congress’ intent that the states play a large role 

in NAAQS implementation is therefore clear.

Response:

The EPA refers back to its previous responses explaining why the commenters’ designationsbased objections are not relevant to this final rule, which directs air agencies to gather and 

submit to the EPA additional data and information characterizing air quality status in areas 

within their borders. Indeed, this rule furthers and is fully in the spirit of the values regarding 

state responsibility that the commenter champions, since the rule provides for states to have the 

opportunity to generate and submit to the EPA this additional data and information, which can 

then be used in a variety of contexts in which the EPA may be responding to state requests for 

particular actions, including area designations, re-designations, clean data determinations, and 

determinations regarding whether areas have timely attained the NAAQS. Therefore, the EPA 

disagrees with the assertion that the rule usurps the role of states.

3. Historic use of modeling for designations

a. Historic use supports modeling for designations

Comment:

Commenter (0068) stated this use of modeling is consistent with the EPA’s historic use of air 

dispersion modeling for multiple NAAQS implementation purposes, including for attainment 

designations and provided several references to EPA documents and court citations to 

demonstrate that historic use. 

Response:

Again, this final rule is not a designations rule and does not implement or discharge the EPA’s 

duties under CAA section 107. However, the EPA agrees with the commenter that as a matter of 

fact the EPA has historically used air dispersion modeling in SO2 NAAQS implementation 

contexts, including but not limited to area designations, implementation plan development, and 

determinations of whether SIPs are substantially inadequate to attain or maintain the NAAQS. 

The EPA also agrees that court rulings reviewing these actions have affirmed the use of 

modeling as lawful and reasonable.
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The proposed rule included this brief history of the use of modeling information in past area 

designations for the SO2 NAAQS: 

“The original SO2 NAAQS1 were established in 1971, and the EPA originally designated 

nonattainment areas for the prior SO2 NAAQS in March 1978.2 The Federal Register final rule 

for this action noted that certain areas were designated on the basis of modeling data: “In the 

absence of sufficient monitored air quality data, other evaluation methods were used, including 

air quality dispersion modeling.” In a September 11, 1978, supplement to the March 3, 1978, 

final rule, the EPA responded to commenters and upheld certain designations based on modeling 

information.3 A second supplement to the March 1978 designations notice affirmed the use of 

modeling for SO2 designations and determining air quality status, stating that, “the EPA’s policy 

related to designations for SO2 permit the use of either modeling or monitoring to determine 

attainment status.”4

“Five years later, in 1983, the EPA conducted a review of all section 107 NAAQS designations 

made to date. A related EPA memo, “Section 107 Designation Policy Summary,” identified the 

importance of modeling information for source-oriented pollutants in cases where existing 

monitors did not adequately characterize peak concentrations: “In general, all available 

information relative to the attainment status of the area should be reviewed. These data should 

include the most recent eight consecutive quarters of quality-assured, representative ambient air 

quality data plus evidence of an implemented control strategy that the EPA had fully approved. 

Supplemental information, including air quality modeling, emissions data, etc., should be used to 

determine if the monitoring data accurately characterize the worst case air quality in the area.”5

b. Historic use does not support modeling for designations

Comment:

Commenters (0042, 0046, 0049, 0050, 0052, 0056, 0058, 0064, 0073, 0075, 0080, 0090, 0110, 

0111, and 0116) stated that monitoring has historically been used for designation of areas as 

attainment or nonattainment. Commenter (0042) stated that, not only has the EPA never before 

required states to conduct modeling to make their initial attainment designations, but the EPA

has also historically expressed a preference for reliance on adequate monitoring data, as opposed 

 

1

See 36 FR 8186 (April 30, 1971).

2

See 43 FR 8962 (March 3, 1978).

3

See 43 FR 40416 (September 11, 1978).

4

See 43 FR 40502 (September 12, 1978).

5 Memorandum from Sheldon Myers, Director, EPA Office of Air Quality Planning and 

Standards, to Regional Office Air Division Directors. “Section 107 Designation Policy 

Summary.” April 21, 1983.
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to modeling results, in making designations; for example, EPA’s "SO2 Guideline Document," 

EPA-452/R-94-008 at 2-6 (Feb. 1994).

Commenter (0042) stated that, in the 1978 designation process, there were only a few instances 

in which modeling data were used in addition to or instead of monitoring data. Commenter 

(0046) stated the use of model output to support NAAQS designations conflicts with prior EPA 

Policy, which states that only quality-assured ambient air monitoring data may be used.

Commenter (0042) stated that, in implementing all recently-promulgated NAAQS, states have 

used -- and the EPA has approved the states’ use of -- ambient monitoring data, not computer 

modeling data, to make their initial designations of areas as attaining or not attaining the new 

ambient standards. Commenters (0042, 0049) noted that, although it has not been the EPA’s 

prior practice to require the use of computer modeling analyses in the initial designation process, 

such analyses have been used in the area re-designation process, where it is necessary to 

evaluate ranges of proposed strategies for bringing nonattainment areas into attainment.

Commenters (0042, 0049, 0065) referred to the 2010 1- hour NO2 NAAQS rulemaking as a good 

-- and the most recent -- example of the Agency’s adherence to the use of monitoring data in 

making initial § 107(d) designations. Commenter (0042) stated the EPA recognized that it lacked 

adequate 1-hour NO2 monitoring data on which to base initial § 107(d) designations, however, 

instead of turning to modeling, the EPA decided on an approach that would allow it to develop 

an appropriate set of monitored data on which to base initial designations. Commenter (0042) 

stated the Agency determined that areas without adequate monitoring data would initially be 

designated as unclassifiable, set forth requirements for the placement of additional monitors, 

gave states a reasonable time to install and to collect three years of data from the new monitors, 

and then indicated that areas initially designated unclassifiable could be re-designated as 

attainment or nonattainment based solely on the results of the new monitoring data.

Commenters (0042, 0049) stated that monitoring data were also used in making the initial 

designations for the fine particulate matter (PM2.5) NAAQS that the EPA adopted in 1997, even 

though there was not an abundance of PM2.5 monitoring data when those NAAQS were 

promulgated. Commenter (0042) asserted that, in that case, Congress made clear its support for 

the use of monitoring data to make initial designations: noting that there was a “lack of air 

quality data for fine particle levels, measured as PM2.5 in the United States” and that such data 

were to “provide a basis for designating areas as attainment or nonattainment” for the EPA’s 

1997 PM2.5 NAAQS, Congress enacted § 6101 of the Transportation Equity Act for the 21st 

Century, Pub. L. 105-178 (June 9, 1998), which delayed the initial designation process for the 

new fine particulate matter NAAQS until adequate monitoring data could be gathered and used 

in the initial designation process.

Commenters (0058, 0075) disagreed that the EPA historically relied upon modeling predictions 

in designating areas as attaining or not attaining the SO2 NAAQS. Commenters (0058, 0075) 

provided the following additional points.
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Modeling has only played a limited role in a few states where pre-existing modeling runs 

were used along with monitoring data in a few cases. See EPA Memorandum from 

Sheldon Meyers, Director, OAQPS to Regional Directors, Waste & Air Management 

Divisions (Apr. 21, 1983) (EPA did not require states to submit modeling predictions as 

part of the initial area designation process but modeling information, if available, could 

be submitted in support of an area designation). In addition, modeling has been used in 

some cases in the area re-designation process (from nonattainment to attainment). See Id. 

at 3. (0058)

When the first section 107 area designations were made under the original SO2 standard, 

those area designations were based on air quality monitoring. Specifically, the EPA

directed states to use “the most recent four quarters of monitored ambient . . . data 

available” and if there were no violations then the EPA directed the states to evaluate 

“the previous four quarters of monitoring” to ensure that the attainment finding would not 

be based on unrepresentative meteorological conditions. As a result, virtually all of the 

initial attainment and nonattainment designations under the original SO2 standard were 

based on air quality monitoring data alone, not modeling. (0075)

Prior to the adoption of the 1-hour SO2 NAAQS – ever since section 107(d) was added to 

the CAA in 1977 – EPA consistently based initial area designations, even those for SO2, 

on monitoring data. See Excerpts from the Brief of Petitioners at 29-31, National Envtl. 

Dev. Ass’n’s Clean Air Project v. EPA, 686 F.3d 803 (D.C. Cir. 2012) (No. 10-1252) 

(“SO2 Petitioners’ Brief”) (Attachment 1 to commenter 0075); Excerpts from Petitioners’ 

Reply Brief at 3-4, 6-7, National Envtl. Dev. Ass’n’s Clean Air Project v. EPA (“SO2 

Petitioners’ Reply Brief”) (Attachment 2 to commenter 0075). 

Commenter (0073) was concerned that the EPA is changing the basic character of the NAAQS 

program through greater use of modeling for both area designation and compliance. Commenter 

(0073) stated this is a significant policy question that must be transparently addressed by the 

agency. Commenter (0073) asserted the recent U.S. Supreme Court decision in UARG v. EPA

suggested that the Courts should view skeptically wholesale changes in the implementation of 

the CAA to achieve new policy goals by the agency. Commenter (0073) stated the need for the 

EPA to engage in a broader public dialogue about these significant implementation issues.

Commenter (0110) stated the concept of using air dispersion model results in lieu of ambient air 

monitoring data to set NAAQS area designations is unconventional and unprecedented and that 

area designations for all other NAAQS are based on measurements of ambient air quality 

obtained using appropriate federal reference (FRM), federal equivalent (FEM), or approved 

regional (ARM) methods that meet specific siting and quality assurance criteria (40 CFR part 58, 

Appendix D and Appendix A, respectively).
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Response:

As previously explained in response to similar comments summarized above, this final rule does 

not implement or discharge the EPA’s designation duties under CAA section 107. Therefore, the 

detailed comments how the EPA should or must or has in the past conducted its designations 

actions are not relevant or germane to the requirements the EPA are promulgating in this final 

rule, which direct state air agencies to gather and submit to the EPA their choice of improved 

monitoring data or surrogate modeling information to characterize air quality in areas within 

their borders. The commenters will remain free to raise their designations-based concerns when 

the EPA acts to issue designations that affect the commenters, but this final rule sets no final 

designations policy or regulatory provisions that govern how the EPA or states must implement 

their respective designations responsibilities. 

However, the EPA notes that this rule in fact is the result of a broader public dialogue regarding 

the implementation of the 2010 SO2 NAAQS, as explained in the white paper and strategy paper 

on the EPA’s website. Through 2012 and 2013, the EPA conducted numerous outreach efforts 

with stakeholders from state, local and tribal governments; environmental and public health 

organizations; and associations and companies from the polluting industry sectors. Many of 

those stakeholders requested that the EPA promulgate this very type of rule, to give states the 

option to choose either conducting improved monitoring or actual emissions-based modeling to 

better characterize areas that are not well characterized under the very limited and shrinking SO2

monitoring network, as well as to conserve limited resources. 

c. Repeal of section 171(2)

Comment:

Commenter (0042, attachment A2) stated that Congress effectively made it arbitrary and 

capricious for the EPA to rely entirely on modeling to determine whether an area is meeting the 

NAAQS when it repealed the language from section 171(2) which allowed states to use either 

modeling or monitoring for its attainment designation. Commenter (0042, attachment A2) 

provided the following additional points. 

Prior to the CAA Amendments of 1990, section 171(2) of the Act defined a 

“nonattainment area” as "an area which is shown by monitored data or which is 

calculated by air quality modeling (or other methods determined by the Administrator to 

be reliable) to exceed any [NAAQS]." 42 U.S.C. § 7501 (emphasis added). In 1990, 

Congress deleted any reference to "calculated by air quality modeling (or other methods 

determined by the Administrator to be reliable)" in designating nonattainment areas. In 

explaining this change, the official report issued by the Senate Committee that drafted the 

1990 CAA Amendments states that "EPA may rely for these designations on sound data 

that is available, preferably air quality monitoring data, but in some cases where 

appropriate and necessary, the [EPA] may rely on modeling or on statistical extrapolation 

from monitored concentrations of another pollutant." S. Rep. No. 101-228, at 15 (1989) 
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(emphasis added). Therefore, it is clear that Congress believed that air monitoring was 

preferable to modeling and thus made a conscious effort to change the pre-1990 section 

171(2) that had treated the two equally.

Response:

Again, this final rule is not implementing or discharging the EPA’s CAA section 107 

designations duties. Therefore, to the extent the comment focuses on what information the EPA 

may use to issue a specific area designation, whether it is monitoring data or modeling 

information relied upon in a future action, that issue will be appropriately addressed at that time, 

and we therefore do not respond further to this comment here. However, the EPA notes that an 

argument identical to the commenter’s was presented in recent litigation challenging the EPA’s 

reliance on modeling to find that a state’s implementation plan was substantially inadequate to 

attain or maintain the pre-2010 SO2 NAAQS. The court rejected the challenger’s argument as 

meritless. Montana Sulphur & Chem. Co. v. EPA, 666 F.3d 1174 (9

th Cir. 2012).

In addition, EPA does not agree that in this final rule it would be appropriate to conclude that the 

CAA should be interpreted as having a preference for using monitoring information as the basis 

for designations. Even the commenter’s quotation from the Senate committee report expressly 

makes the point that the EPA has the discretion to rely on modeling information when warranted.

While the EPA in many cases relies on valid and certified monitoring data to identify violations 

of air quality standards for SO2 and other pollutants, the EPA has also found that modeling when 

conducted appropriately can be particularly useful for identifying violations (and identifying the 

scope of violations) of the SO2 standard. Indeed, the EPA has found that modeling in some cases 

provides more reliable information than monitoring, especially where there are significant 

concentration gradients that the monitoring network is inadequate to characterize. Furthermore, 

the text of the CAA, which simplifies the definition of nonattainment in section 171(2) and 

instead provides a more extensive description of a process for identifying nonattainment areas in 

section 107(d), contains no indication of a preference between monitoring and modeling. 

Therefore, the EPA has frequently reasonably considered both modeling and monitoring 

information under the CAA, weighing the relative merits of each on a case-by-case basis. 

The commenter fails to note that an additional element of the Clean Air Act Amendments of 

1990 was the addition of section 110(a)(2)(K). This section newly requires that states provide 

any modeling that the EPA requires for purposes of evaluating whether sources are causing 

violations of any air quality standards.

d. Rulemaking needed for use of modeling for designations

Comment:

Commenters (0042, 0048, 0050, 0053, 0075, and 0116) supported the EPA’s use of notice-andcomment rulemaking through the DRR to address certain major issues. Commenter (0050) stated 

the EPA is statutorily required to use the formal notice-and-comment rulemaking process when
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contemplating any major change in implementation of the CAA and stated the DRR’s proposed 

incorporation of a source threshold and use of actual monitoring and/or modeling to characterize 

air quality and make initial area designations are such major changes. 

Commenter (0042) stated that, prior to the signing of the final 1-hour SO2 NAAQS, the EPA did 

not require the use of computer modeling predictions either in making § 107(d) initial area 

designations and, therefore, the EPA may not now change its prior practice without first 

subjecting any proposed change to notice-and-comment rulemaking. Commenter (0042) stated 

the EPA cannot change that valid past practice through the issuance of guidance, rather, for the 

reasons set out in previous filings with the EPA (see, e.g., the Multiparty Reconsideration 

Petition at 27-28, NEDA/CAP Reconsideration Petition at 4-5, TCEQ Reconsideration Petition 

at 10-14, and State of North Dakota and South Dakota Reconsideration Petition at 7-9), the 

Agency must use the notice-and-comment rulemaking process to consider any such major 

change to the way in which it implements the CAA. Commenter (0042) stated courts have 

recognized that the CAA awards deference to state governors to make initial designations 

pursuant to § 107(d) of the Act, citing Pa. Dep’t of Envtl Prot. v. EPA, 429 F.3d 1125, 1129 

(D.C. Cir. 2005) (explaining that “[a]s EPA observes, the CAA gives great deference to 

governors’ recommendations for areas within their states . . . .”). Commenter (0042) stated that, 

requiring states to deviate from the well-accepted practice of making attainment designations 

based on monitored data and instead requiring them to use far more speculative modeling would 

be an inappropriate infringement on that deference unless and until EPA completes rulemaking 

to make such a change.

Response:

Although this final rule is not implementing or discharging the EPA’s designation duties under 

CAA section 107, it obviously is a notice and comment rulemaking that is establishing the 

requirement that states further characterize air quality in areas within their borders. Thus, the 

objections the commenters raise in the abstract regarding the need for notice and comment 

rulemaking are somewhat moot. Moreover, to the extent that commenters later object to the use 

of data or information gathered pursuant to this rule in subsequent implementation actions –

whether they are area designations, re-designations, clean data determinations, or other notice 

and comment actions, those commenters will have at their disposal a notice and comment 

process, to the extent required by law, in which to challenge the EPA’s use of that data or 

information on a case-by-case basis. Therefore, the EPA disagrees that this final rule itself 

constitutes an inappropriate infringement on any deference due to state governments who will in 

fact lead the data and information gathering steps directed by the final rule. Additionally, the 

EPA has no reason to suspect that subsequent future actions that may rely upon the stategenerated data and information would represent such an infringement either. 

e. Court decisions on use of modeling for designations
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Comment:

Commenter (0068) stated that dispersion modeling is a rigorously verified method for evaluating 

impacts on the SO2 NAAQS, and has a lengthy and court-validated history as an appropriate tool 

for use in designations, including a recent CAA section 126 petition for resolution of cross-state 

impacts: Genon Rema, LLC v. EPA, 722 F.3d 513, 526 (3rd Cir. 2013).

Commenter (0042) cited several cases and stated courts have repeatedly struck down the EPA’s 

use of models as arbitrary and capricious when the models prove to be unreliable. Commenter 

(0042) stated the EPA’s approach is seriously flawed because the modeling tools that it proffers 

have not been fully vetted and are far less reliable than the monitoring tools that the EPA seeks 

to replace. Commenter (0042) stated that, unless and until the EPA takes steps to improve its 

recommended tools for assessing 1-hour SO2 concentrations, those tools will substantially 

overestimate 1-hour concentrations of SO2, which makes them inappropriate for use in 

implementing the 1-hour SO2 NAAQS. Commenter (0042) provided the following additional 

points. 

An agency action is arbitrary if, among other things, the agency “failed to consider an 

important aspect of the problem, offered an explanation for its decision that runs counter 

to the evidence before the agency, or is so implausible that it could not be ascribed to a 

difference in view or the product of agency expertise” [citing Motor Vehicle Mfrs. Ass’n 

v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29, 43 (1983)]. The EPA has already 

recognized that measured air quality data are preferred to modeled data and that the 

former should be used to validate the latter [citing Ohio v. EPA, 784 F.2d 224, 231 (6th 

Cir. 1986) (citing U.S. EPA, GUIDELINE ON AIR QUALITY MODELS (1978)]. When models 

contradict proven, measured ambient concentrations, the EPA may not use those models 

to support agency action. To do so would be to ignore the evidence of actual ambient SO2 

concentrations before the Agency. (0042)

Commenter (0109) stated that, while courts have upheld the use of modeling to determine 

whether an area is nonattainment or attainment in some circumstances, those cases did not 

involve one-hour standards.

Response:

The commenters who either favor or object to the use of monitored data or modeling information 

in any specific future action -- including but not limited to designations, re-designations, or clean 

data determinations – will be able to raise those concerns in those specific actions. The purpose 

of this final rule is to direct states to better characterize air quality in areas within their borders 

that are not currently adequately characterized by the very limited and shrinking SO2 monitoring 

network. It does not implement or discharge the EPA’s duties to issue designations under CAA 

section 107.

However, the EPA notes that it disagrees with commenters who assert that existing models are 

not well suited for use for 1-hour ambient standards, as these models have been shown to 
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perform well in evaluation studies. That being said, the EPA continues to work to improve the 

models and has implemented updates to address concerns from the modeling community such as 

options to address low wind conditions. 

f. Nonattainment area requirements

Comment:

Commenter (0042) stated that the EPA’s new procedures for implementing the §107(d) initial 

designation process essentially conflates the requirements of §107(d), which applies to 

attainment, nonattainment, and unclassifiable areas -- with the requirements of CAA Title I, 

Subpart D, which apply only to nonattainment areas. Commenter (0042) stated the modeling 

analyses that the EPA now requires states to conduct for all areas is essentially equivalent to the 

modeled attainment demonstrations that the EPA has historically demanded that states perform 

only for nonattainment areas.

Response:

The commenter is incorrect. The modeling contemplated by this final rule is intended to 

characterize “current” air quality using recent, actual emissions in an area. This analysis may 

inform decisions regarding the area’s status with respect to attaining or not attaining the 1-hour 

SO2 NAAQS. The modeling traditionally relied upon to support demonstrations of future 

attainment in nonattainment areas, instead, is not based on actual emissions, but instead is a 

projection of what future air quality levels will be based on any additional reductions required by 

new federally enforceable emissions limits for the area that did not exist at the time the area was 

violating the NAAQS and was designated nonattainment. This type of modeling analysis for an 

SO2 NAAQS attainment demonstration must evaluate the air quality levels that would be 

associated with the sources in the area complying with the new allowable emissions limits that 

were adopted. These analyses are completely different kinds of modeling assessments. 

Therefore, the assertion that the DRR, which itself is not a designations rule implementing CAA 

section 107, conflates the requirements of CAA section 107 and the requirements for current 

violating areas to show future attainment is unfounded.

In addition, the commenter appears to be overlooking significant provisions in section 110 and 

elsewhere in Subpart A of Title I that authorize the EPA to require modeling for any location in 

the country, for example, to evaluate whether an area is attaining the air quality standard. 

Especially pertinent is section 110(a)(2)(K), which requires that states provide for conducting 

and submitting results of “such air quality modeling as the Administrator may prescribe for the 

purpose of predicting the effect on ambient air quality of any emissions of any air pollutant for 

which the Administrator has established a national ambient air quality standard.” This paragraph 

of the CAA authorizes the EPA to prescribe that modeling be conducted for the entire country, 

not only for nonattainment areas, for purposes matching those of the DRR. Therefore, this 

paragraph must also be interpreted as authorizing the EPA to require air quality characterization 

either through monitoring or modeling. Additional authority for this rule is given in sections 114 
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and 301(a) of the CAA. Contrary to the comment, these authorities, which are outside Part D of 

Title I of the CAA, authorize EPA to require modeling to assess air quality anywhere in the 

country irrespective of attainment status.

C. Monitoring and/or modeling for designations

1. Support option to model and/or monitor

Comment:

Commenters (0041, 0043, 0048, 0054, 0058, 0062, 0064, 0066, 0071, 0073, 0080, 0082, 0085, 

0090, 0092, 0101, 0115, and 0117) supported the option to use modeling and/or monitoring for 

SO2 as this option provides appropriate flexibility for both the states and affected sources.

Commenter (0054) supported the EPA’s finding that modeling may not be appropriate for all 

SO2 designation scenarios and commended the EPA on its proposal to allow implementing state 

air agencies the flexibility to utilize monitoring data in establishing attainment determinations for 

the SO2 NAAQS. Commenter (0073) stated that, for some locations, the lack of monitors 

presents challenges for making accurate determinations and supported the ability of states to 

choose to evaluate NAAQS attainment through either dispersion modeling or ambient 

monitoring. Commenter (0092) supported the EPA’s encouragement to air agencies that they use 

monitoring to characterize air quality near sources not easily characterized through dispersion 

modeling.

Commenters (0043, 0040, 0048, 0056, 0058, 0066, 0072, 0073, 0089, 0092, 0094, 0095, and

0111) stated that ambient air monitoring data should be the primary basis to designate

nonattainment areas. Commenter (0056) stated that monitoring has historically been used, and 

should continue to be the primary basis for designation of areas as attainment or nonattainment. 

Commenter (0058) stated that, while they support the EPA’s proposal to allow states to elect to 

use modeling for some areas, the EPA must clearly acknowledge that monitoring is superior to 

modeling for characterizing actual ambient air quality. Commenters (0048, 0088, 0094, and

0095) stated that, as modeling is frequently affected by emissions inputs, meteorological data, 

and local geography, it is not as accurate or reliable as real-time, multiple-year monitoring.

Commenter (0089) stated that, in the eyes of the public, a valid result from a representative 

monitoring site always carries greater legitimacy over a modeled value and models are often 

viewed by the public as hypothetical constructs that are unreliable, possessing variable 

uncertainty, and may be subject to manipulation of the input data in order to achieve a desired 

result. Therefore, in order to retain the trust of the public, commenter (0089) suggested the EPA

should provide every opportunity to the states to utilize existing monitoring site data in their area 

designation recommendations, while at the same time ensuring that those monitored results are 

representative, meet quality assurance criteria, and are otherwise scientifically valid.
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Response: 

The EPA agrees with commenters who stressed the need to give air agencies the option to 

characterize SO2 ambient air quality through either enhanced monitoring or modeling, and the 

EPA is maintaining that approach in this final rule. The EPA believes that the commenters have 

not presented any persuasive reasons for changing the basic positions discussed in the preamble 

to the final rule of the 2010 SO2 NAAQS rulemaking, the Strategy Paper, or in the proposed rule 

for why both air quality modeling and ambient monitoring are appropriate tools for 

characterizing ambient air quality. However, as explained in the preamble to the final rule, the 

EPA is not taking final action to make any determinations regarding any area’s status with 

respect to attaining or not attaining the NAAQS, but is only prescribing criteria and a process for 

how and when air agencies are to gather and provide to the EPA additional needed information. 

How the information is used in subsequent actions evaluating the attainment status of specific 

areas will depend upon the information that air agencies collect in the future and what it shows 

about areas’ ambient air quality.

2. Support monitoring data use only

Comment:

Commenters (0037, 0040, 0046, 0049, 0050, 0051, 0057, 0059, 0074, 0084, 0109, and 0116) 

generally stated that nonattainment designations should only be made based on data from 40 

CFR part 58 compliant monitoring showing a violation of the standard.

Commenter (0109) believed that areas can only be properly designated as "nonattainment" based 

on monitoring data because (1) modeled predictions are inherently less accurate and over predict

ambient impacts and (2) nonattainment designations impose significant consequences on local 

economies and limited state resources. Commenter (0059) stated that given the significance 

associated with mis-designating an area, the most prudent approach would include minimal 

uncertainty (i.e., no reliance on air quality modeling), and recommended monitoring be the sole 

approach used for designating the SO2 attainment status.

Commenters (0042, 0063, 0065) stated that, where an area with air quality monitors shows 

compliance with the 1-hour NAAQS, the EPA should not require computer modeling to be 

conducted prior to designating the area “attainment.” Commenter (0042, attachment B) stated 

that where modeling is conducted in an area with at least one monitor, the modeled results

should be compared against the monitored values and, if the modeled concentrations are found to 

over predict at the monitor, then the modeling analysis should either be adjusted to come into 

agreement with the monitored concentrations, or set aside as unworkable.

Commenter (0084) believed modeling could be used in a screening manner to effectively 

identify where monitors should be placed but that to depend on a modeling result to correctly 

designate between attainment and non-attainment is beyond the intended purposes of models.

Commenter (0109) stated that, while predictive modeling can be an appropriate tool for states to 
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use to "screen" areas for attainment designations, monitoring that actually measures ambient 

concentrations must be a required component of all nonattainment designations. Commenter 

(0052) and commenter (0052a, June 2012 comments from Southeastern State Air Pollution 

Control Agencies attached to commenter 0052 comments) stated there must be ambient data 

demonstrating that SO2 levels are above the standard before the EPA initiates designation of any 

area as not attaining the standard.

Commenter (0040) is concerned that the DRR will create a precedent for the EPA to utilize 

modeling for other NAAQS designations. Commenter (0040) asserted most of the prior support 

from stakeholders for using air dispersion modeling output for SO2 NAAQS area designations 

was driven by concerns that state, local, and tribal air agencies did not have sufficient resources 

to monitor the nearly 2,000 sources that the EPA had previously anticipated targeting for impact 

assessments, according to comments summarized in the preamble to the Proposed Rule. Based 

on the much smaller number of sources targeted by the DRR, commenter (0040) stated the gaps 

between anticipated state, local and tribal agency resources and the expected demands of 

characterizing source impacts are not nearly so large. Therefore, commenter (0040) believed 

substituting quality assured ambient air monitoring data with modeling output is no longer 

needed.

Response: 

As stated previously, the EPA agrees with commenters who stressed the need to give air agencies 

the option to characterize SO2 ambient air quality through either enhanced monitoring or 

modeling, and the EPA is maintaining that approach in this final rule. The EPA believes that the 

commenters have not presented any persuasive reasons for changing the basic positions

previously discussed in the preamble to the final rule of the 2010 SO2 NAAQS rulemaking, the 

Strategy Paper, or in the proposed rule for why both air quality modeling and ambient 

monitoring are appropriate tools for characterizing ambient air quality. However, as explained in 

the preamble to the final rule, the EPA is not taking final action to make any determinations 

regarding any area’s status with respect to attaining or not attaining the NAAQS, but is only 

prescribing criteria and a process for how and when air agencies are to gather and provide to the 

EPA additional needed information. How the information is used in subsequent actions 

evaluating the attainment status of specific areas will depend upon the information that air 

agencies collect in the future and what it shows about areas’ ambient air quality.

In addition, concerns about what precedent this rulemaking might set with respect to how the 

EPA will address other pollutants are not germane here, because this rulemaking is only 

addressing SO2. This final rule establishes no precedent for how states would be expected to 

gather and submit information for assessing air quality status under any other criteria pollutant 

NAAQS, and EPA expects the need for, and desirability of, collecting dispersion modeling to 

characterize actual air quality will vary from pollutant to pollutant. As stated in the preamble, the 

EPA believes that the approach set forth in this rule directing air agencies to gather additional 

data to characterize ambient air in the vicinity of larger SO2 sources is uniquely suited for 

implementation of the 1-hour SO2 NAAQS, and the Agency does not anticipate it to be used for 
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other NAAQS pollutants.

3. Do not support modeling to make attainment designations

[Also see section V.B.9. for comments specific to AERMOD.]

Comment:

Commenters (0040, 0042, 0043, 0046, 0049, 0051, 0052, 0053, 0055, 0056, 0057, 0059, 0063, 

0065, 0074, 0075, 0079, 0083, 0087, 0088, 0090, 0095, 0108, 0109, and 0110) generally stated 

they do not support using the air quality models to make attainment designations. Commenters

(0051, 0108) stated that modeling is not appropriate for compliance determinations and therefore 

designations. Commenter (0049) stated modeling data should only be necessary where states 

must make predictions about future circumstances in order to demonstrate compliance with 

specific CAA requirements.

For example, the Prevention of Significant Deterioration (PSD) program prohibits the 

construction of a new or modified major stationary source in an attainment area unless 

the permitting authority can demonstrate that the increased emissions from the source 

will not cause or contribute to an air quality violation within that area. Modeling data are

useful for this purpose because it can predict the air quality effects that could result from 

the added emissions within the general vicinity of the proposed new or modified source. 

In addition, nonattainment SIPs will benefit from modeling, because modeling can 

demonstrate the effectiveness of proposed control strategies to bring a nonattainment area 

back into compliance with the standard. 

Complex terrain and/or multiple sources

Commenters (0046, 0089) stated that a modeling-based assessment would be difficult to 

implement in complex terrain or in areas where there are multiple emission sources without 

hourly emissions and exhaust data.

Concentration gradients

Commenter (0110) stated the EPA’s consideration for basing SO2 NAAQS area designations on 

model output is driven mostly by the EPA’s perception that there are sharp gradients in actual 

“design concentrations” that monitors cannot resolve because of limitations in the number of 

monitors that can be placed in an area. Commenter (0110) stated the EPA’s concerns regarding 

sharp concentration gradients for SO2 concentrations are mitigated by the form of the standard, 

which by design (3-year average of the 99th percentile) results in a more stable form which will 

smooth out any short-term concentration gradients for specific hours. Commenter (0110) 

referenced a study which compared AERMOD modeling and monitored data in North Dakota

and stated it implied that the concentration gradient among the 5 monitors with up to a 20

kilometer (km) separation was insignificant.
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Costs

Commenter (0083, 0088) stated that the fact that modeling is “cheaper” than monitoring is not a 

substitute for the superior accuracy of actual monitored data; nor do cheaper costs of modeling 

offset the regulatory costs and other burdens on sources and communities that result from 

nonattainment designations based on modeling. Commenters (0042, 0051, 0108, and 0110) 

stated the modeling-based approaches for SO2 NAAQS nonattainment designations will likely 

force communities and the facilities that operate in them to take costly steps to combat modeled 

pollution problems that do not exist. 

Over-predicts

Commenters (0042, 0046, 0051, 0072, 0073, 0074, 0080, 0109, and 0110) stated that models

overestimate the monitored values. Commenter (0046) referred to several modeling studies.

Commenter (0110) stated modeling results for over 50 large coal-fired power plants submitted to 

the docket by the Sierra Club were found in many cases to substantially overestimate the 

measured concentrations at the nearest monitors, in some cases, by up to several hundred 

percent; however, a portion of this bias is attributable to the use of allowable instead of actual 

emissions. Commenter (0110) also referenced model results for Detroit that were prepared by the 

Michigan Department of Environmental Quality in which the model overestimated the monitored 

design value at one monitoring site by a factor of two. Commenter (0109) stated that existing 

models vastly over-predict actual ambient 1-hr SO2 levels and should be limited to use as a 

screening tool to confirm attainment status and as a factor to guide state efforts to locate 

monitors. 

Uncertainty

Commenters (0042, 0046, 0063, 0065, and 0110) provided several technical points regarding 

quality assurance, model revisions, and uncertainty which are summarized below.

Model output cannot be quality assured in the same way that monitoring data are. No 

limits exist on the uncertainty of the model output that would be used to set NAAQS 

designations because there is no way to assess model uncertainty for a specific regulatory 

application. If modeling is used as a basis for nonattainment designations, model 

performance should be calibrated/verified at similar locations/terrains for which 

monitoring data are available.

When models are revised, modeling results would need to be re-evaluated with each 

modeling change to ensure that prior conclusions from modeling are valid. This seems to 

be an untenable position for states and sources to be in with respect to attainment or 

nonattainment status.

EPA’s tolerance for uncertainty in model output is incongruous with, and undermines the 
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robust program for quality assuring ambient air monitoring data. The degree of 

uncertainty in the ambient air monitoring data used to set NAAQS area designation is 

routinely assessed using procedures prescribed by 40 CFR part 58. The accuracy of 

model output cannot routinely be assessed and even under the best conditions, model 

accuracy does not meet the data quality objectives of an ambient air monitor operated 

according 40 CFR part 58. 

Commenters (0055, 0074) questioned why the significant efforts to assure monitoring is accurate 

and of high quality are not also applicable to modeling. Commenters (0055, 0074) stated this 

disparity is not logical since modeling depends on many assumptions that are automatically 

controlled for in a monitoring situation (e.g. representative emissions, representative 

meteorology). 

Commenter (0075) disagreed that modeling provides consistently reliable information on which 

to base designations for the 1-hour SO2 NAAQS. Commenters (0094, 0095) stated that modeling 

results are highly affected by emissions data, meteorological data and local geography, all of 

which have proven to be highly subjective in nature and far less reliable than monitoring.

Because of the reliability of monitoring and limitations of modeling, commenter (0053) asked 

that the EPA not force the use of modeling in making nonattainment determinations.

Response: 

As stated previously, the EPA agrees with commenters who stressed the need to give air agencies 

the option to characterize SO2 ambient air quality through either enhanced monitoring or 

modeling, and the EPA is maintaining that approach in this final rule. The EPA believes that the 

commenters have not presented any persuasive reasons for changing the basic positions

previously discussed in the preamble to the final rule of the 2010 SO2 NAAQS rulemaking, the 

Strategy Paper, or in the proposed rule for why both air quality modeling and ambient 

monitoring are appropriate tools for characterizing ambient air quality. 

To the extent that commenters seek to persuade the EPA that modeling should not be included in 

this rule as a characterization option because of alleged reliability issues, the EPA disagrees. The 

EPA has conducted extensive comparisons of modeled versus monitored concentrations, 

demonstrating the reliability of the model estimates, so long as the model inputs correspond to 

the actual circumstances (e.g., the actual emissions) being monitored, and so long as 

concentrations at comparable locations are being compared. However, as explained in the 

preamble to the final rule, the EPA is not taking final action to make any determinations

regarding any area’s status with respect to attaining or not attaining the NAAQS, but is only 

prescribing criteria and a process for how and when air agencies are to gather and provide to the 

EPA additional needed information. How the information is used in subsequent actions 

evaluating the attainment status of specific areas will depend upon the information that air 

agencies collect in the future and what it shows about areas’ ambient air quality.
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The EPA recognizes the uncertainties in modeling, but does not believe that they justify 

exclusion of modeling as a characterization option. To the contrary, the EPA is continuing to 

allow a choice of characterization options, noting that the EPA also recognizes uncertainties in 

monitoring, most notably in assessing maximum concentrations across an area, which the 

commenters may be overlooking. As one example, model concentration estimates in an area with 

complex terrain may have greater uncertainty than model estimates in an area with simple 

terrain, but the use of monitored data at one site (or at a finite network of sites) to extrapolate air 

quality elsewhere in an area would also have greater uncertainty in an area with complex terrain 

than in an area with simple terrain. Similarly, the existence of multiple emission sources would 

increase the uncertainty in both a modeling assessment and an extrapolation of monitoring 

results.

The EPA disputes the implicit contention that the choice between modeling and monitoring is a 

choice between “cheaper” versus “superior accuracy.” Monitoring may be more accurate in 

assessing concentrations at the particular location being monitored, but monitoring may well be 

less accurate in estimating concentrations (or even estimating whether a violation is occurring) at 

other locations within an area of interest. The EPA believes that a modeling analysis using 

reliable actual emission information will in general have similar reliability in assessing whether 

an area has violations as a monitoring assessment based on a well-designed monitoring network. 

The commenter provides no basis for the premise that, were EPA to use such a modeling-based

analysis in supporting a nonattainment designation, that such a designation “will likely” address 

“problems that do not exist.” If, in an action that is informed by modeling, the commenter feels 

that the data identify problems that “do not exist,” the commenter will be free to raise that point 

regarding that specific action.

Concerns with the reliability of models are addressed in section V.B.9. The commentary on the 

Michigan modeling appears to reflect a comparison of a monitored concentration against a 

maximum concentration modeled at a different location. Since Michigan’s modeling of actual 

emissions gave results that were nearly identical to the monitored value, the comments 

demonstrate not the over prediction of the model but rather the underestimation of maximum 

concentrations from monitoring if the monitoring network does not fully reflect the spatial 

variations in concentrations. 

The comments on the reliability of monitoring do not address the reliability of monitors to 

represent concentrations at unmonitored locations. The procedures of part 58 improve the 

reliability of monitored data in representing concentrations at the monitored location, but 

significant errors can arise from assuming that concentrations elsewhere are similar, and in fact 

modeling is sometimes a more reliable means of estimating whether concentrations throughout 

an area exceed the standard. Although area-specific judgments regarding the relative reliability 

of all available evidence will be the subject of separate future actions, the EPA concludes that 

both monitoring (based on a well-designed network) and modeling (using best available inputs), 

when conducted appropriately, can satisfy the purpose of this rule to provide a suitable 

characterization of air quality in priority areas.
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4. Support modeling to make designations

Comment:

Commenters (0039, 0062, 0067, and 0068) supported the EPA’s proposal to allow modeling to 

be used to characterize air quality for making designations.

Commenter (0039) supported the use of modeling if it uses either recent actual emissions or 

enforceable emission limitations. Commenter (0039) believed AERMOD over predicts, but 

added that if the modeling results demonstrate that recent actual emissions and emissions below 

enforceable limits do not predict exceedances of the 1-hour SO2 NAAQS, then it is unlikely that 

ambient monitoring would detect an exceedance of the standard. Commenter (0039) believed 

dispersion modeling can be a reasonable, conservative approach on which to base designation, 

though it would be unreasonable to require states to model potential emissions.

Commenter (0062) agreed with the EPA that modeling is a technically valid and defensible tool 

to assess impacts near SO2 emission sources. Commenter (0062) stated SO2 lends itself to 

modeling because it is a near field, source-oriented pollutant with localized health impacts and 

does not react in the same way as regional pollutants, like ozone or PM2.5. Commenter (0068) 

stated that air dispersion modeling is the best method for evaluating the short-term impacts of 

large SO2 sources. Commenter (0068) stated that modeling, and not monitoring, is the 

appropriate method for ascertaining source-derived impacts on attainment of the NAAQS 

because modeling can capture ambient concentrations across vast areas, and is sensitive to 

minute changes in meteorology that monitors are unable to capture. 

Response: 

The EPA appreciates the support of commenters who agreed with the EPA’s proposal to allow 

modeling to be used as a surrogate for monitoring to assess ambient SO2 concentrations. As 

stated previously, the EPA agrees with commenters who stressed the need to give air agencies 

the option to characterize SO2 ambient air quality through either adequate monitoring or 

modeling, and the EPA is maintaining that approach in this final rule. The EPA believes that 

monitoring and modeling both have advantages and disadvantages. Most notably, monitoring can 

provide a more precise indication of concentrations at a particular location, while modeling can 

provide information at a greater range of locations. Nevertheless, the EPA believes that both 

approaches can provide adequate information in most cases, so that the EPA believes that a rule 

providing air agencies the option to use either method is warranted. The EPA disagrees with the 

assertion that monitoring should not generally be allowed to ascertain impacts under the SO2

NAAQS, and points out that no one challenged the final NAAQS on this basis or the part 58 

monitoring provisions revised when the NAAQS was promulgated. The EPA believes that the 

commenters have not presented any persuasive reasons for changing the basic positions

previously discussed in the preamble to the final rule of the 2010 SO2 NAAQS rulemaking, the 

Strategy Paper, or in the proposed rule for why both air quality modeling and ambient 

monitoring are appropriate tools for characterizing ambient air quality. However, as explained in 
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the preamble to the final rule, the EPA is not taking final action to make any determinations 

regarding any area’s status with respect to attaining or not attaining the NAAQS, but is only 

prescribing criteria and a process for how and when air agencies are to gather and provide to the 

EPA additional needed information. 

5. Rule text regarding monitoring preference

a. Section 51.1201

Comment: 

Commenters (0040, 0044) suggested the text of proposed section 51.1201 be clarified to make 

clear that monitoring rather than modeling is the EPA’s preferred methodology. Commenter 

(0040) suggested the following revisions to the proposed regulations:

§ 51.1201 Purpose. The purpose of this subpart is to require air agencies to take actions to 

develop air quality data characterizing current maximum 1-hour ambient concentrations 

of SO2 more extensively across the United States through either additional ambient air 

quality monitoring or air quality modeling analyses at the air agency’s election. Except as 

otherwise provided in § 51.1203, ambient air quality monitoring is the methodology that 

should be used for this purpose. Such additional monitoring and modeling data may be 

used in future initial area designations by the EPA for the 2010 SO2 NAAQS, or for other 

actions designed to ensure attainment of the 2010 SO2 NAAQS and provide protection of 

the public from the short-term health effects associated with exposure to SO2

concentrations that exceed the NAAQS. 

Commenter (0044) believed that, if the EPA does not intend for the rule to express a preference 

for modeling nor intend to provide air agencies and the Regional Offices with unfettered 

discretion to elect modeling over monitoring, then both the preamble to the final rule and its 

regulatory text should be clarified to say so. To accomplish this, commenter (0044) suggested 

that proposed section 51.1201 be revised to read:

§ 51.1201 Purpose. The purpose of this subpart is to require air agencies to take actions to 

develop air quality data characterizing current maximum 1-hour ambient concentrations 

of SO2 more extensively across the United States through either additional ambient air 

quality monitoring or air quality modeling analyses at the air agency’s election solely for 

the purpose of making designations of attainment of the SO2 NAAQS. Such additional 

monitoring and modeling data may be used in future initial area designations by the EPA 

to ensure attainment of the SO2 NAAQS and provide protection of the public from the 

short-term health effects associated with exposure to SO2 concentrations that exceed the 

NAAQS.
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Response: 

The final rule does not incorporate either of the suggested amendments to proposed section 

51.1201. First, regardless of what its own general preferences might be, the EPA disagrees that it 

is appropriate to express a “preferred methodology” within a rule intended to provide air 

agencies with the choice between using monitoring, or modeling, to characterize ambient SO2

concentrations. Those decisions will be made by air agencies on case by case bases that depend 

upon multiple variables, including source populations, meteorology, area topography, and 

available state resources, which the EPA does not intend to prejudge. Second, the EPA disagrees 

that the information generated by the rule’s future implementation should be explicitly limited 

for use for making attainment designations. It is not reasonable at this point to predetermine what 

specific conclusion that information may reasonably support, or in what NAAQS implementation 

or other context. 

b. Section 51.1203(b)

Comment: 

Commenter (0044) recommended that the regulatory text of section 51.1203(b) be amended to 

include the following sentence which indicates a preference for monitoring: The technique 

which should ordinarily be used for this purpose is ambient air quality monitoring, except where 

the air agency can demonstrate that the use of ambient air quality monitoring is not feasible. 

Commenter (0040) recommended that the regulatory text of section 51.1203(b) be amended to 

include the following sentence which indicates a preference for monitoring: If an air agency 

desires to use air quality modeling in lieu of monitoring, the air agency, in its submission 

required by § 51.1203(d), shall identify why it is unable to proceed with monitoring. EPA shall 

not deny a reasonable air agency request to use modeling in lieu of monitoring.

Response:

Consistent with the recommendations the EPA received during its stakeholder review process, 

the EPA believes that, regardless of what its own general preferences might be, it would be 

inappropriate to identify, in a rule applicable to state, local, or tribal air agencies, a preference for 

either monitoring or modeling. Both approaches have their advantages and disadvantages; for 

example, monitoring likely provides a better indication of actual air quality at a particular 

location, while modeling provides better information regarding concentrations at a range of 

locations. This rule is premised on a finding that either monitoring or modeling provides 

adequate information to characterize air quality, and the EPA believes that the air agencies 

should make their own choice of methods based on their own evaluation of the relative merits of 

the alternatives on a case-by-case basis. Furthermore, the EPA believes that it would be 

inappropriate to establish additional requirements that air agencies provide particular kinds of 

justification for their choice of air quality characterization approach, or to create a presumption 

that air agencies would have to overcome in order to be able to use modeling.
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c. Bias for modeling rather than monitoring

Comment:

Commenters (0051, 0056, 0063, 0065, 0088, and 0107) were critical of what they perceived as a 

bias in the Proposed Rule supporting modeling rather than monitoring SO2 sources for purposes 

of designations as a result of the severely compressed schedules for submission and approval of 

monitoring plans and collection of data. Commenter (0107) stated that the proposed schedule is 

ambitious--especially if a site requires multiple monitors—and, as such, the proposed schedule 

sets states up to fail and potentially forces us into the modeling approach. Commenter (0056) 

stated concerns over the EPA's comment that the agency would force modeling, if monitor 

deployment is delayed, and stated that this approach adds to the uncertainty of taking the 

monitoring approach and makes it a less attractive option.

Commenter (0051) stated the EPA should reconsider the proposed timeline, monitoring-related 

financial burden, and monitoring to modeling penalties as they appear to make modeling the only 

viable option states have for assessing attainment. Commenter (0051) stated the EPA should 

remove the obstacles the Proposed Rule places in the way of reaching its stated goal of providing 

states with options for characterizing air quality. Commenter (0051) stated the multitude of 

uncertainties with and unrealistic timeline for designing and financing a source-oriented SO2

monitoring network make it seem as if the EPA's true intent is to force air agencies to rely solely 

on modeling.

Response: 

The EPA does not specify a preference for modeling as opposed to monitoring, as claimed by 

some commenters. The final rule instead provides flexibility to air agencies to choose which 

approach would be most appropriate on an area-by-area basis. While the timelines that apply to 

both pathways are expeditious, the EPA disagrees that the timelines create a bias toward a 

particular outcome. The rule offers two pathways for characterizing air quality, each of which 

has an array of advantages and disadvantages, the weight of which are in significant part a 

function of case-specific factors such as availability of suitable monitoring sites, spatial 

gradients, availability of resources for monitoring, and the presence of complications such as 

complex terrain or large water bodies. One’s view of the relative merits of the two pathways is 

also a function of one’s views of the reliability of the two pathways for indicating air quality 

throughout an area, as well as whether one considers it advantageous or disadvantageous to 

provide for earlier versus later designations. Thus, a comment that the schedule for monitoring is 

expeditious, without examining whether the schedule for modeling is expeditious, does not 

justify a conclusion that the proposed rule timeline is biased toward one or the other authorized 

pathway.

The EPA believes that both the schedule for monitoring and the schedule for modeling included 

in the final rule are reasonable timelines that provide adequate time for air agencies to complete 

the necessary steps. Air agencies that are weighing their choice of the two methods have the 
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opportunity to consider case-specific factors that may require more or less time for either option 

in particular situations, but the EPA does not believe that a national comparison of the 

expeditiousness of the modeling and the monitoring schedules would indicate a bias in the rule 

toward one or the other option.

Moreover, in response to comments about reconsidering the proposed timeline, the EPA is 

providing additional time for air agencies to notify the EPA of their choice of using modeling or 

monitoring to characterize air quality around specific sources. Accordingly, the deadline for this 

election will be July 1, 2016. The EPA recognizes that evaluating the relative merits of modeling 

and monitoring for any particular area, including identification of funding sources for any new 

monitoring that might be under consideration, warrants more time than was provided under the 

proposed rule. Consistent with this revision, the EPA is also revising the deadline for air 

agencies using modeling to submit modeling protocols for the applicable areas. 

Thus, under the final rule, by July 1, 2016, the air agency must submit its selection of whether 

each area will be characterized through modeling or monitoring and, depending on that selection, 

either must submit a modeling protocol or must include information in the Annual Monitoring 

Network Plan that specifies the monitoring to be conducted to address the requirements of this 

rule. The EPA believes that this revised deadline still provides for timely planning for air quality 

characterization to occur (through modeling) or begin (through monitoring) at the beginning of 

2017. Conversely, the EPA does not agree that any later deadline for selecting the means of 

addressing air quality characterization requirements would provide the time and flexibility to 

address in a timely way any issues that arise after the selection is made. The result would be that 

a later deadline for this selection could jeopardize timely receipt of information characterizing air 

quality. 

Notwithstanding this revision, the Agency encourages air agencies to start their investigation of 

this issue as soon as practicable. The EPA strongly encourages each air agency to consult with its 

respective EPA Regional Office to identify sources exceeding the emission threshold in the final 

rule and any other sources that do not exceed the emission threshold but near which further air 

quality characterization would be warranted. Similarly, the EPA strongly encourages air agencies 

to hold early discussions regarding the manner in which modeling or monitoring might be used. 

As one example, if the air agency believes that the existing monitoring network suffices to 

characterize air quality, early discussions with the EPA would be essential for assuring that the 

intended selection of monitoring is based on appropriate assumptions regarding the network’s 

ability to characterize air quality near the applicable source(s) without further network 

adjustments. 

EPA is also clarifying its expectations should an air agency commit to commence operating new 

monitors by January 1, 2017 but then fail to do so. In this case, the air agency would have failed 

to comply with the DRR, but this failure would not trigger a requirement to meet the 

requirements of the modeling pathway of the DRR. It is worth noting that the court order that 

dictates EPA’s SO2 designation schedule specifies that areas without new monitoring 

commencing by January 1, 2017 shall be designated by December 31, 2017, a mandate that 
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would presumably apply to areas that commit to but then fail to commence timely new 

monitoring. Nevertheless, the DRR itself does not create the uncertainty about the monitoring 

approach that the commenter fears.

D. Considering all monitoring/modeling data

1. Do not support considering all monitoring/modeling data

Comment:

Commenters (0046, 0050, 0078, and 0088) stated the EPA should not rely upon modeling that 

has been performed recently by third parties or non-governmental organizations for large 

electricity generating units (EGUs) in some states that suggests that some areas should be 

classified as nonattainment because it is of very low quality. Commenter (0088) stated that Sierra 

Club’s submission of modeling based on permit allowables is fundamentally flawed since these 

emissions are unlikely to occur at a plant or for that matter, at the same time from multiple plants 

being modeled. Commenter (0088) stated that it is arbitrary and capricious as well as 

unreasonable for “allowable” or plant “PTE” to be used as inputs to describe existing ambient air 

conditions.

Commenter (0078) stated that the EPA should make clear in the rule language or preamble that it 

will only base designation recommendations for sources/areas under the modeling pathway on 

modeling conducted specifically under the parameters of the final DRR (i.e., modeling protocols 

submitted by states in January 2016) and that other modeling will not be considered. Commenter

(0078) stated that they conducted preliminary modeling of sources with SO2 emissions over 100 

tpy in 2011-2012 and that most of these sources would not be included in modeling required by 

the Proposed Rule because they are not over EPA's proposed emissions thresholds and may or 

may not be re-modeled. 

Response:

As indicated in the final rule and elsewhere in this RTC document, this rule only establishes 

requirements to characterize SO2 air quality around priority sources across the country; it does

not govern the procedures for future area designations and how such air quality data may be used 

in that process. For situations where modeling will be used to characterize current air quality, the 

EPA has issued a Technical Assistance Document that describes how such modeling could be 

done with actual emissions. The EPA does not recommend that allowable emissions be used for 

modeling intended to characterize current air quality, except to the extent that states choose to 

model allowable emissions as a means to demonstrate that current and future emissions are 

required to be low enough to provide for current and continued attainment.

The EPA is finalizing its proposed approach of requiring that air agencies choosing modeling to 

characterize ambient air quality be required to submit modeling analyses to the EPA Regional 

Office. Irrespective of whether the state or a third party conducts the modeling, it is the state’s 
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responsibility under the CAA to submit the information that this rule requires. The EPA 

anticipates that any state submittal of third party modeling would reflect a review as to whether 

the state believes that the modeling satisfies applicable requirements. Moreover, the EPA 

anticipates that the submittal would provide adequate information for the EPA to review the 

adequacy of the analysis as well.

2. Support considering all monitoring/modeling data

Comment:

Commenter (0068) stated that, while the draft DRR focuses primarily on requirements to be 

placed on states and tribes to prepare information for designations purposes, any final Rule that 

is issued must make clear that data from other sources, including from environmental and public 

health NGOs, will be considered by the EPA while making designations. Commenter (0068) 

stated that it is a bedrock principle of administrative law that an agency may not ignore 

information put in front of it, such as Sierra Club’s modeling submitted with these comments. 

See generally Motor Vehicle Mfrs. Ass’n v. State Farm Mut. Auto Ins. Co., 463 U.S. 29, 43 

(1983) (holding that it was arbitrary and capricious for the agency to ignore an important aspect 

of an issue placed before it); see also NRDC v. EPA, 571 F.3d 1245, 1254 (D.C. Cir. 2009) 

(restating the EPA’s own statement that additional information presented in a notice-andcomment rulemaking must be considered during the rulemaking by the corresponding state and 

EPA) (citing 70 Fed. Reg. 71,612, 71,655). 

Response: 

The EPA is finalizing its proposed approach of requiring that air agencies choosing either 

modeling or monitoring to characterize ambient air quality be required to submit the monitoring 

or modeling analyses to the EPA Regional Office. Irrespective of whether the state or a third

party conducts the modeling or submits the monitoring data, it is the state’s responsibility under 

the CAA to submit the information that this rule requires. The EPA anticipates that any state 

submittal of third party modeling or monitoring data would reflect a review as to whether it 

believes that the modeling or monitoring data satisfies applicable requirements. Moreover, the 

EPA anticipates that the submittal would provide adequate information for the EPA to review the 

adequacy of the analysis as well.

In addition, the EPA would consider monitoring or modeling conducted by a third party to be an 

appropriate means for air agencies to obtain the data necessary to meet the requirements of this 

rule, provided that the state provides oversight to assure that (1) any monitoring is conducted in a 

manner that is equivalent to SLAMs and quality-assured in accordance with applicable 

requirements, and (2) any modeling analysis that the state submits, even if it was initially 

provided to the state by a third party, is done in a reasonable manner (for example. following the 

recommendations in the modeling TAD).

To the extent that the commenter is addressing the designations process, to urge that the EPA 
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consider third party information in promulgating designations, this rule does not define the 

designations process, and so statements regarding the types of information that the EPA would 

consider in the designations process are not directly germane to this rulemaking. Nevertheless, as 

a general matter, the EPA agrees that it must weigh all information that interested parties provide 

when conducting rulemaking such as to promulgate designations. However, information that 

commenters wish the Agency to consider in issuing specific area designations should be 

submitted to the appropriate EPA Regional office at the appropriate time. Furthermore, to 

facilitate the designations process, commenters are also urged to provide relevant information to 

the pertinent air agency for its consideration in advance of the deadline for the agency to submit 

designation recommendations to the EPA. 

III. Source Coverage and Emission Threshold Options

A. Options to identify sources for monitoring/modeling

1. Emissions metrics for use in a threshold approach

a. Use of annual emissions.

Comment:

Commenters (0041, 0045, 0048, 0050, 0058, 0060, 0071, 0073, 0080, 0081, 0090, 0093, 0096, 

0097, 0107, 0109, 0110, and 0116) supported using annual SO2 emissions for defining source 

thresholds. Commenters (0041, 0058, 0093, 0096, 0097, 0107, 0110, and 0118) stated it is most 

appropriate to evaluate annual emissions, since these data are widely reported to the EPA and are 

readily available to the States. Commenter (0097) stated this data provides a reasonable 

estimation of a facility's overall emissions. Commenter (0118) stated this information is easily

available and treats all areas on a consistent basis and that the use of additional factors makes

this process unnecessarily difficult, time consuming, and more arbitrary.

Commenter (0041) stated that a longer term rate such as tpy is most appropriate for processes 

that are more steady-state, as compared to processes that have more fluctuating emissions which 

are better represented by shorter term limits. Commenter (0045) stated that using a 1-hour SO2

emission rate and the option to incorporate other factors like stack height, proximity to sensitive 

populations and topography would be difficult to implement.

Commenters (0080, 0090) believed this was reasonable approach to establishing a threshold 

level, but stated it is not always an indicator of high hourly SO2 emission rates. For example, 

commenter (0080) noted that generating units may operate at high capacity with annual 

emissions rates that fall within the EPA's recommended threshold and may be equipped with 

state-of-the-art emissions control systems which allow them to operate at very low hourly SO2

rates. 
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Response: 

The EPA agrees with the many commenters who expressed support for using an annual 

emissions metric because annual emissions data are most readily available for all large SO2

sources, whereas 1-hour emissions rate information is not readily available for many SO2

sources. Since the tpy emissions metric is a common denominator in the emissions inventory and 

reporting universe, the EPA believes that the use of this metric is most appropriate to be required 

under a rule that applies broadly to areas with sources that do not already measure 1-hour 

emissions rates. Using tpy will provide air agencies and the regulated community a common, 

easily verifiable, straightforward approach for identifying sources around which air agencies are 

required to characterize air quality. This approach will rely on existing emission inventory 

collection systems that are already in place. An approach based on tons of emissions per year 

also should reduce unforeseen or otherwise uneven application of the requirements for air quality 

characterization that could arise if different metrics are used for different SO2 source sectors to 

identify areas for which air agencies are required to characterize air quality.

The EPA acknowledges that some state commenters suggested inclusion of a 1-hour emissions 

rate-based criterion for identifying certain sources with infrequent, episodic SO2 emissions at 

atypically high rates that could impact nearby populations. The EPA notes that the emissions 

threshold included in the final rule establishes only minimum requirements for identifying 

priority sources. The EPA agrees with state commenters who recommended that air agencies 

should also characterize areas that, based on their knowledge of sources and areas, may be at risk 

of violating the standard. Thus, under this rule air agencies could also require characterization of 

air quality near sources prone to episodic emissions with relatively high rates or amounts, as 

appropriate. However, the EPA believes that no simple indicator can be defined to identify the 

sources with emission spikes that warrant air quality characterization, and a rule with such a 

criterion would be difficult to implement because short-term emissions data are not available for 

all SO2 sources. In addition, the EPA believes that a source that has routinely relatively high 

emissions (and would therefore be covered by the 2,000 tpy threshold) may be more prone to 

cause violations and warrant prioritization than a source with routinely relatively low emissions 

with sporadic emission spikes. For these reasons, the EPA did not include in this rule a minimum 

requirement for identifying source areas needing air quality characterization based on this metric, 

although the rule continues to provide air agencies and the EPA the option to identify sources 

that do not meet the annual emission threshold in the rule as nevertheless warranting air quality 

characterization.

b. Emission threshold approach

Comment:

Commenters (0048, 0075) agreed that using an emissions based threshold approach to 

identifying areas to be evaluated for compliance with the 1-hour SO2 NAAQS would serve to 

make more manageable the demands on local, state and federal resources. Commenter (0088) 

stated that most SO2 emissions are emitted by a relatively small proportion of SO2 emitting 
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sources in the national emissions inventory (NEI) and it would be unreasonable to require states 

to model or require monitoring of small emission sources that would be unlikely to affect air 

quality in any appreciable way based in existing monitoring studies.

Commenter (0088) stated that, since there is no express provision in the CAA that defines how 

modeling and/or monitoring should be conducted for the purpose of NAAQS designations, it is 

within the EPA’s discretion to determine that it is reasonable to focus state resources on the 

sources that are most likely to result in exceedances of the SO2 NAAQS that could result in 

health impacts, provided that the selection of those sources is not arbitrary and that it is 

reasonable. 

Commenter (0110) objected to the EPA’s approach for determining the emission threshold

because it is based upon percent of national SO2 emissions, rather than any direct consideration 

as to the emission level that would reasonably be expected to affect compliance with the SO2

NAAQS.

Commenter (0053) asserted the EPA has not provided an adequate explanation or justification to 

support its proposed thresholds for identifying areas that must be characterized under the DRR. 

Commenter (0053) stated that the EPA provides no analysis of how the 3 options actually relate 

to those areas where exceedances or nonattainment of the 2010 1-hour SO2 NAAQS might 

actually be likely and, for example, the thresholds do not account for factors such as stack height 

that are critical in relation to short-term standards like a 1-hour NAAQS.

Response:

The EPA agrees with commenters that favor use of an emissions based threshold to focus state, 

local, and federal resources on the areas that most warrant evaluation. The resources that are 

available to address these issues are finite, and the EPA believes that a rule requiring air quality 

characterization in the vicinity of sources exceeding the threshold in the final DRR provides for 

focusing an appropriate level of resources for air quality characterization.

The commenter that finds the EPA’s rationale for its thresholds inadequate did not recommend 

any alternative basis for establishing priorities for air quality characterization. This commenter 

appears to recommend that the thresholds should correspond to levels at which violation of the 

“NAAQS might actually be likely,” but the commenter then notes the importance of factors other 

than emissions “such as stack height.” The commenter in effect is arguing that no simple set of 

criteria is appropriate for defining the areas that warrant air quality characterization, although the 

commenter does not expressly argue that a more complicated set of criteria is warranted.

The EPA agrees that air quality is influenced by a variety of factors, including emission levels, 

meteorology, stack height and other release parameters, and in some cases nearby terrain. Given 

the complex influence of these factors, even a complex set of criteria would not provide a 

reliable indicator of the likelihood of violations. On the other hand, a complex set of criteria 

would be very difficult to administer, and would be prone to result in inconsistent application 
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across the country. Therefore, the EPA believes that a simple criterion, namely annual emissions, 

provides a more workable and appropriate basis for determining whether an area should be 

prioritized for air quality characterization. This approach is analogous to the approach to a 

variety of other EPA monitoring regulations, which also rely on criteria (e.g., the population of 

the area) that cannot be expected to be dispositive as to whether a violation is likely to be 

monitored but which provide an appropriate basis for prioritizing areas for air quality 

characterization. It is also consistent with numerous CAA provisions that base applicability on 

source emissions levels, without imposing a burden to show a correlation of those levels with a 

particular likelihood of presenting an environmental or health risk. Finally, the rule in setting the 

emission threshold to identify the sources that at a minimum need to be characterized, does not 

preclude air agencies from additionally characterizing other sources that may pose a risk of 

NAAQS violations. Moreover, nothing in this rule alters either the authority or the responsibility 

EPA and air agencies have under the CAA to address NAAQS violations.

2. Threshold including population metric

a. Support using a population threshold

Comment:

Several commenters (0043, 0046, 0048, 0050, 0051, 0075, 0080, 0081, 0087, 0089, 0090, 0092, 

0094, 0095, 0096, 0106, 0116, 0117, and 0118) supported the application of a lower emission 

threshold in urban areas. Commenters (0051, 0081, and 0106) stated that population centers 

represent locations of higher potential public exposure and, therefore, characterization of air 

quality in these areas would be more representative of the public's SO2 exposure risk. 

Commenter (0081) supported including population and explained that since the 1-hour standard 

is very source specific, and results in relatively small “hotspots” of higher concentrations, a 

tighter threshold for urban areas where a greater population exposure is likely to occur is 

appropriate. Commenter (0051, 0081, 0089, 0117) stated that, as precedent, a population 

threshold has been used to establish the minimum monitoring requirements for other NAAQS, 

including nitrogen dioxide, carbon monoxide, and particulate matter.

Commenters (0043, 0046, 0050, 0051, 0087, and 0092) stated that a threshold approach based 

purely on emissions could inappropriately focus limited resources on areas with limited to no 

public exposure. Commenter (0050) stated that the EPA should consider the population-exposure 

component of the source threshold as it prioritizes areas that may need further scrutiny. 

Commenter (0117) supported this approach, recognizing that because there is greater risk, a 

stricter threshold should apply, and stated this is a common sense approach which is not unduly 

burdensome to air agencies.

Commenter (0118) stated there should be options spelled out (partial county designations, 

removal of counties, etc.) to remove these areas and affected sources after the initial 

determinations where, for example, counties on the edge of a highly populated CBSA have 
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extensive rural areas where such an approach has no benefit because of the relatively localized 

impact of SO2 emissions. 

Commenters (0048, 0094, and 0095) stated many sources located within an existing CBSA are 

located on the edge of the boundary in less populated areas and urged the EPA to consider more 

refined census data based perhaps on population density. Commenter (0048) asserted that using 

population density instead of CBSAs is better as it represents a classification methodology that is 

pertinent to the area affected by the source. Commenter (0048) stated that the EPA’s proposed 

methodology is too broad and needs to be more site- and source-specific, representing actual 

affected populations. Commenter (0048) suggested, for example, the EPA could use population 

density data around the effected facilities out to a radius of 10 km and, if average population 

density from the 2010 census in this area exceeds a certain threshold (e.g. 100 persons/sq km), 

then the lower emissions criteria would be used; for areas less than the population threshold, the 

higher emissions criteria would be used. 

b. Do not support using a population threshold

Comment:

Several commenters (0034, 0036, 0039, 0041, 0045, 0062, 0068, 0078, 0088, 0094, 0095, 0103, 

0115, and 0120) did not support using a population threshold.

Commenters (0034, 0036, 0045, 0062, 0068, 0088, 0103, 0115, and 0120) stated there should be 

no difference in applicability between urban and rural sources since the CAA requires that 

NAAQS apply equally to all populations, regardless of population density. Commenter (0088) 

stated that it seems arbitrary to determine that people living in these communities would 

potentially be more impacted to SO2 emissions than a small group of people living in the 

immediate vicinity of a large SO2 emitting source. Commenters (0115, 0120) stated the residents 

of rural areas deserve the same environmental protection as those in major metropolitan areas.

Commenter (0115) provided the following additional points. 

The Band is concerned that significant sources in areas with fewer than 1,000,000 people 

may will be overlooked under this approach. Since the vast majority of SO2 emissions 

come from large, stationary sources and tend to linger locally (i.e. contributions from 

local traffic and transport are probably not factors), there can be little reason to assume 

that urban residents need a greater level of protection than rural ones in this instance. The 

Band feels that this proposal unfairly places an environmental burden upon tribal 

communities, most of which are located in rural areas, and appears to more highly value 

the lives and health of those living in CBSA s of more than 1,000,000 people. This would

also appear to be unfair treatment of sources that are located in urban areas, as they will 

be required to perform modeling that their counterparts in rural locations may not. If 

urban citizens are exposed to higher background levels of SO2 that require a more 

conservative SO2 designations approach, then please give the details behind this 

assumption.
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Further, the proposed population-based thresholds do not adequately address industryintensive areas that may experience high cumulative rates of emissions or that may be 

located in areas with local meteorology that is not conducive to adequate air dispersion. 

The areas near the Fond du Lac Reservation and the ceded territories include both of 

these types of settings. 

The analysis performed by the EPA regarding its proposed threshold approach does not

adequately address public health concerns. The analysis focuses on what percentage of 

sources would be modeled/monitored, and what percentage of the general population live 

in areas where emission sources would be modeled/monitored. Neither of these factors 

demonstrates whether the NAAQS will be attained in all parts of the country. The 

NAAQS make no exceptions based on population density or whether certain sources emit 

quantities above a certain threshold and what percentage of sources is covered is

irrelevant. Lastly, the proposed 1,000,000 person threshold represents only 55% of the 

US population. The Band does not agree with this approach as it leaves 45% of the 

population unaccounted for, along with affected plant and animal species that may need 

protection.

This approach doesn't specifically account for unique or problematic meteorological 

conditions. The Band described a unique situation in the Duluth/Superior area and stated 

that unique modeling situations like these may be overlooked by a purely population 

based approach. 

Commenters (0034, 0120) stated Indian Country is largely rural in nature and, as such, 

population thresholds unnecessarily disadvantage Indian Country and rural state communities. 

Commenters (0034, 0120) stated the Proposed Rule would inhibit the use of governmental 

resources to characterize air quality data concerning their emissions and learn about such health 

effects. Commenter (0034) stated the Proposed Rule could have a devastating effect on both the 

existing and future resources available to Tribes. Commenters (0034, 0120) stated this is not only 

unfair to Indian Country and its Tribes; this is an environmental justice issue in which Indian 

Country’s best interests are not adequately protected.

Commenter (0120) stated that a lack of resources to support adequate environmental protection 

measures, such as air quality monitoring and dispersion modeling, is not a viable excuse for the 

EPA to promulgate rules that lessen the requirements on air management agencies that have been 

tasked with protecting public health and the environment. Commenter (0068) provided the 

following additional points.

 Nothing in the CAA supports or even contemplates a NAAQS implementation strategy 

that would effectively value air quality in urban areas over that in rural areas.

 Nothing about a source being in an urban area makes it easier to evaluate for its impact

on the NAAQS, and improperly focusing on such sources preferentially is likely to lead 
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to rural sources that do cause exceedances of the NAAQS being overlooked, to the

detriment of the air quality of rural areas and to the health of rural residents. 

 Particularly given the greater levels of poverty in rural areas, and the reality that 

dangerous sources of air pollution tend to be located in poorer and more rural parts of the 

country, the EPA’s proposal to focus the DRR preferentially on urban sources raises 

significant environmental justice concerns. As an example, AERMOD analysis of the 

actual emissions from Schiller Station in Portsmouth, New Hampshire indicates that it 

has, for multi-year periods, caused ambient concentrations of SO2 to exceed the NAAQS.

Although Schiller Station would be included under the EPA’s preferred Option 1 if it

were an urban source, because it is relatively far from a population center like Boston, it 

would be excluded, to the detriment of the over 40,000 people that live in the cities and

towns immediately surrounding Schiller. 

Commenters (0036, 0041, 0078, 0088, 0094, 0095, and 0115) stated that using population as a 

basis for setting thresholds is not necessary because of the nature of the SO2 emission impacts 

distribution. Commenters (0036, 0115) stated the EPA’s population concept for siting monitors 

may make some sense for certain pollutants such as nitrogen dioxide, where there are 

correlations between sources of concern and populations, but with SO2 there is no such 

correlation. Commenter (0088) stated that it is not reasonable to believe that cumulative 

exposure from several SO2 facilities necessarily corresponds to population density. Commenters 

(0094, 0095) stated CBSAs are usually large geographic areas, while the health impacts of SO2

are relatively localized with maximum concentrations expected to be observed within 1-2 miles 

of large power plants or other facilities in flat terrains. Commenter (0041) provided the following 

additional points:

Our analysis of the impacts from large sources in our nonattainment area shows that the 

largest impacts are relatively close to the emitting source, i.e., a localized problem. 

Therefore, to set thresholds on the basis of population in large areas, whether by county 

boundaries or potentially much larger areas such as CBSAs, does not accurately reflect 

the effectiveness or need for control because of the localized nature of SO2 impacts. For 

instance, Michigan has sources located in rural areas that are still part of the greater 

Detroit CBSA with a population of more than one million, while other sources are not in 

a CBSA but may impact more people than the CBSA sources because of the local 

population density.

c. Other comments on population thresholds

Comment:

Commenter (0044) stated that while all three options distinguish between sources “inside” and 

“outside” of CBSAs greater than 1 million, the DRR is silent on what is meant by being outside 

of a CBSA. Commenter (0044) asked if any source that emits SO2 in excess of the 2K, 5K, or 

10K tpy threshold will be subject to the rule irrespective of how many miles distant that source 
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may be to the boundaries of the CBSA or, more importantly, to the 1 million population center 

within the CBSA. Commenter (0044) stated that, at the very least, the final rule needs to define 

what is meant by “outside,” including specifying a distance or other factor that could be used to 

objectively determine when a source would no longer be considered to be “outside” the CBSA.

Commenters (0040, 0044) noted that the health impacts of SO2 are relatively localized, with 

maximum concentrations expected to be observed within 1-2 miles of large power plants and 

other facilities in flat terrains, but that this 1-2 mile dispersal range will also be shortened by 

unique terrain and meteorological conditions. Commenter (0044) recommended that the EPA 

modify the approach to more accurately account for actual population exposure and provided the 

following additional points.

Not every CBSA is geographically small with a high, dense population spread relatively 

evenly across the entirety of the CBSA; such as the New York-Newark-Jersey City 

CBSA. Many CBSAs cover a broad territory and have a population concentrated in one 

or a few small areas, with sources concentrated in another part of the CBSA that is not 

located within the 1-2 miles maximum concentration zone; such as the San Antonio-New 

Braunfels CBSA. While Option 3 may not be precise in this regard, its higher tpy

thresholds would not work to the detriment of populations located in or within proximity 

to CBSA sources located in and around a CBSA like New York-Newark-Jersey City. 

Option 3 would, however, properly exclude sources located within or in proximity to 

CBSAs like San Antonio-New Braunfels and whose emission impacts (i.e., the source’s) 

would only be marginal even under the worst of circumstances.

Response:

The EPA considered the comments received on the issue of whether a lower emissions threshold 

should be included for areas with more dense populations (e.g., CBSAs greater than 1 million 

population). A number of commenters appeared to interpret the inclusion of a lower threshold for 

areas with higher population as being less protective of the public in less populated areas. The 

EPA wants to clarify that this was not the intention behind the population-based options included 

in the proposed rule. The SO2 NAAQS, and all NAAQS, are to provide equal protection for 

citizens throughout the country. The proposed use of both population and emissions thresholds as 

a means to require air quality characterization was simply one approach to focus limited federal 

and state modeling and monitoring resources into characterizing locations where a greater 

coincidence of people and SO2 emissions occur, and thus a potentially greater potential for 

exposure is presented. After reviewing the comments on this issue, however, the EPA has 

decided not to move forward with the proposed preferred approach, and instead to apply 

requirements for air quality characterization based on emissions levels uniformly across the 

country for both more urbanized and less urbanized populations so as to focus primarily on the 

size of the sources.

The EPA agrees with the comment that population density varies among and within CBSAs. 

The EPA believes that a criterion that reflects population density in the immediate vicinity of 
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sources would be difficult to define, justify, and administer. Nevertheless, the fact that sources in 

large population CBSAs are not necessarily in the immediate vicinity of substantial populations 

provides additional rationale for not using a CBSA population criterion in determining what 

areas are established as priority areas for air quality characterization.

 

The EPA acknowledges the comments in support of using a population criterion for prioritizing 

areas to be subject to air quality characterization, and agrees that sources located in populated 

metropolitan areas generally have the potential to cause more substantial population exposure 

than sources outside (i.e., not within) such areas. However, some commenters noted that large 

SO2 sources in CBSAs are sometimes located in relatively lightly populated portions of those 

areas. Others noted that impacts from an SO2 source are relatively localized, regardless of 

whether the source is located within a CBSA or not. 

In considering how to prioritize air quality characterization efforts under this rule, the EPA

considered the possible greater efficiency of a program that gives higher priority to 

characterizing air quality near sources in CBSAs, along with the concerns of some commenters 

that populations located near sources of a similar size to those exceeding the CBSA threshold but 

who live outside of a CBSA would receive unequal air quality protection. The EPA gave greater 

weight to the latter set of concerns and opted for an approach that requires air quality 

characterization near similar-sized sources throughout the country regardless of whether the 

source is located within or outside a CBSA, while still efficiently prioritizing air quality 

characterization for a feasible number of sources.

It should be noted here that any monitoring that occurs pursuant to this rulemaking is potentially 

in addition to, or can possibly help to satisfy, required SO2 monitoring stemming from 40 CFR 

part 58, appendix D, and section 4.4. Those monitors required in 40 CFR part 58, appendix D, 

section 4.4 are determined using a unique metric that accounts for the coincidental occurrences 

of SO2 emissions and population, namely the Population Weighted Emissions Index (PWEI). 

This rulemaking does not supplant or otherwise modify those existing requirements.

3. Proposed option 1 (> 2,000/1,000 tpy)

a. Support option 1 

Comment:

Commenters (0035, 0117) supported the EPA's Option 1 and stated that, based on their

experience; the use of this threshold value strikes an appropriate balance between NAAQS 

protection and the significant resource requirements of modeling and monitoring approaches to 

demonstrating attainment. Commenter (0117) stated this approach offers the highest level of 

protection (90% of emissions coverage) while creating a manageable requirement for air 

agencies. Commenter (0035) stated that, based on the air monitoring data collected in the 

BAAQMD, they believe that it is unlikely that sources with emissions under 1,000 tpy would 

cause or contribute to a NAAQS violation.
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b. Do not support option 1

Comment:

Commenters (0046, 0051, 0056, 0058, 0070, 0075, 0090, 0091, 0092, 0110, 0113, and 0116) 

opposed Option 1, stating that these levels have not been justified and they would be a 

burdensome and inefficient approach.

Commenters (0051, 0110) stated that the Proposed Rule does not justify why sources of 1,000, 

2,000, or 3,000 tpy are likely to or expected to potentially result in a NAAQS violation and 

therefore require monitoring or additional modeling. Commenter (0058) stated the EPA’s 

justification for preferring the thresholds provided in Option 1 over those of Options 2 and 3 is 

arbitrary since the “90 percent threshold” is no magic number and will force States to evaluate 

many more sources than necessary.

Commenters (0046, 0051, 0075, 0091, and 0110) stated Option 1 is very burdensome and would 

be an inefficient means for identifying areas that are not meeting the 1-hour SO2 NAAQS. 

Commenter (0046) stated Option 1 would target 3 times as many sources as Option 3 but would 

cover only 20% more emissions. Commenter (0046) stated Option 1 is anticipated to identify 10 

areas not meeting the SO2 NAAQS that would not be identified by Option 3 but would do so by 

installing a total of 288 additional monitors of which over 200 are projected to have average 99th 

percentile daily maximum 1-hour levels less that do not exceed 38 ppb (i.e., less than 50 percent 

of the level of the NAAQS). Commenter (0051) stated that, because the EPA's proposed Option 

1 requires characterization of 12 more sources in Texas than Option 2 but only equates to 

characterizing air quality for 7.5% more of the SO2 emissions in Texas, the added monetary and 

resource costs associated with modeling or monitoring under Option 1 are not balanced by an 

equal benefit of protecting the public from short-term SO2 exposure.

Commenters (0046, 0091) stated Option 1 would be an inefficient drain on state, local, or tribal 

area resources and may force states to deliver model output instead of quality assured ambient air 

monitoring data to the EPA for setting area designations. Commenter (0091) stated that including 

sources at the Option 1 level of emissions will waste precious agency resources and expend 

unnecessary costs for sources that are already heavily controlled. 

Commenter (0056) stated that in most cases the EPA's preferred Option 1 applicability threshold 

will result in significant expense and time where there is no real issue with NAAQS compliance.

Commenter (0113) referenced monitoring data analysis in Montana which demonstrated 

NAAQS compliance downwind from two local industrial facilities whose individual SO2

emissions exceed the thresholds provided under Option 1 of the DRR for rural locations.

Commenter (0113) stated that since these monitors demonstrate NAAQS compliance and were 

used by the EPA in limiting the geographic extent of the nonattainment area boundary, further 

evaluation of areas with individual sources covered under Option 1 in the DRR is unnecessary 

and inappropriate.
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4. Proposed option 2 (> 5,000/2,000 tpy)

Comment:

Commenters (0054, 0088, 0089, 0092, 0109, 0113, and 0114) supported Option 2 because it 

balances limiting agency resource demand for the implementation of this rule while still allowing 

important SO2 emission sources to be evaluated. Commenters (0088, 0114) stated Option 2 

appears to be the best option because the difference between the number of sources captured by 

Options 1 and 2 is substantial. Commenter (0088) stated that, in comparison to Option 1, which 

would require monitoring or modeling of 443 sources that account for 90% of all SO2 emissions, 

only 270 sources would be affected and account for 82% of emissions. Even with the 

implementation of Option 2, commenter (0054) asserted that a state could always lower the 

threshold on their own if there was concern about impacts from a particular source emitting less 

than 5,000 tpy in an area outside 1,000,000 population CBSA’s.

5. Proposed option 3 (> 10,000/3,000 tpy)

Comment:

Commenters (0040, 0043, 0044, 0046, 0048, 0050, 0051, 0056, 0057, 0058, 0060, 0063, 0064,

0065, 0070, 0075, 0079, 0080, 0084, 0087, 0090, 0093, 0094, 0095, 0106, 0109, 0111, 0113, and

0116) generally supported the EPA Option 3 because it would apply reasonable thresholds

without burdening states with unnecessary modeling or monitoring.

Commenter (0056) stated this option would allow states to focus their limited resources on the 

areas with the largest 211 sources of SO2 emissions, and avoid being forced, based on crude 

criteria, to either monitor or model an additional 285 sources. Commenter (0060) stated that

Option 3 (3,000 and 10,000 tpy) is more than adequate to address Ohio's more significant 

sources. Commenter (0060) stated that the thresholds in Option 1 and 2 are well beyond what is 

necessary to capture significant sources in Ohio as these options would require modeling or 

monitoring of all of Ohio's well-controlled EGUs. Commenter (0111) supported Option 3 since it 

would cover almost 70% of the national SO2 emissions in 2011 (including sources located in 

nonattainment areas).

Commenters (0043, 0111) referenced state experience collecting ambient monitoring data near 

large SO2 sources which indicate that large amounts of SO2 do not necessarily correlate to 

violations of the standard. Commenter (0043) referenced South Dakota's experience in collecting 

air monitoring data around a large coal-fired power plant with sulfur dioxide emissions over 

12,000 tons/yr over a 13 period in which no sulfur dioxide concentrations exceeded the 1-hour 

sulfur dioxide standard even though the testing was conducted where the model indicated the 

maximum sulfur dioxide concentrations would be encountered. Commenter (0043) stated this air 

monitoring data shows that emission levels as great as 12,000 tpy do not have high concentration

impacts in rural areas. Commenter (0111) stated that, given LDEQ operates a number of SO2

monitors located in close proximity to one or more sources which could be impacted by the 
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Proposed Rule, yet each of these monitors is compliant with the 1-hour SO2 NAAQS, it is 

evident that large amounts of SO2 do not necessarily correlate to violations of the standard.

Commenter (0064) stated that areas located outside of a core-based statistical area of population 

greater than a million will be rural and sparsely populated. Thus, commenter (0064) believed a 

threshold of 10,000 tpy SO2 could be viewed as acceptable due to the low risk. Commenter 

(0106) supported Option 3 because of the limited number of people exposed and the amount of 

resources required to comply with the Proposed Rule. 

Commenters (0056, 0080, 0090, and 0093) supported Option 3 and noted that, while there may 

be situations where the Option 3 source applicability thresholds may not be appropriate, the 

Proposed Rule gives the EPA, States and local permitting agencies the authority to include 

sources below the thresholds on a case by case basis. Commenter (0111) stated that, if data 

generated by this rule indicate that exceedances of the 1-hour SO2 NAAQS are prevalent and 

affecting large numbers of people, the EPA could propose to expand the SO2 monitoring or 

modeling requirements at a later date.

Commenters (0046, 0084) stated Option 3 provides increased opportunities to add more (or 

move existing) monitoring stations and reduce the focus on models, while concentrating on 

known nonattainment issues. Commenter (0106) stated that, by adopting Option 3, States' limited 

resources can be better utilized on other programs such as PSD, Regional Haze, and greenhouse 

gas (GHG) controls.

Commenters (0040, 0044) supported Option 3, stating Options 1 and 2 rely on arbitrary 

population densities that will bring in more sources than necessary for characterization of SO2

effects on air quality. Commenters (0048, 0050, 0087, 0094, and 0095) stated that Option 3 is 

consistent with the EPA’s February 6, 2013 strategy paper regarding implementation of the 

designation process. Commenters (0094, 0095) asserted that nowhere in this paper does the EPA 

mention emissions as low as those proposed in Option 1.

Commenter (0050) stated Option 3 would allow the EPA and stakeholders to evaluate and 

determine the effectiveness of the agency’s approach before undertaking more costly and 

expansive efforts. Commenter (0079) supported Option 3, in part, because it provides the 

greatest margin of error in the event a designation is based on modeling, which is not as accurate 

as a stationary air emissions monitor.

Response to comments on proposed options 1, 2, and 3 above:

See the preamble for the response to comments on proposed options 1, 2, and 3. Further relevant 

discussion is also presented in a memo to the DRR docket entitled “Analysis of Source 

Threshold Options for the Final SO2 DRR,” Douglas Solomon and Rich Damberg, EPA Office 

of Air Quality Planning and Standards, docket EPA-HQ-OAR-2013-0711, July 16, 2015. The 

EPA received a wide range of comments on the proposed emission threshold options, as 

indicated previously. Most commenters identified a preference among the three options proposed 
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by the EPA. A few commenters recommended alternate thresholds in the range from 1,000 tpy to 

10,000 tpy, or recommended pairs of thresholds within this range. Some commenters provided

modeling analyses as an indication that sources larger than 12,000 tpy did not cause a violation 

of the standard, while other commenters recommended a single emissions threshold of 1,000 tpy

and provided modeling analyses of different sources as an indication that sources less than 2,000

tpy caused modeled violations. These comments demonstrate that ambient SO2 impacts can be 

variable, and are dependent on many factors other than annual emissions (such as meteorology, 

stack height, local topography and plant operations). These factors can only be assessed through 

analytical approaches, such as ambient monitoring or air quality modeling, which take many of 

these related factors into account simultaneously. These comments demonstrate why air quality 

characterization of the area around these sources is needed to protect public health in the first 

place. 

A number of commenters discussed monitoring evidence in their areas from which they inferred 

the likelihood of violations being observed near sources with various annual emission levels. 

These comments must be considered in the context that concentrations can vary significantly as a 

result of other factors such as stack heights, meteorology, and terrain. For this reason, the EPA 

agrees with the commenter that “large amounts of SO2 do not necessarily correlate to violations 

of the standard,” a statement that should be supplemented with the statement that small amounts 

of SO2 do not necessarily correlate to attainment of the standard. For this reason, an important 

part of this rule is provision for air agencies and for the EPA to conclude that a source (or a 

cluster of sources) that does not meet the threshold of this rule but nevertheless poses sufficient 

concern that air quality characterization in the vicinity of this source (or these sources) should be 

prioritized. Nevertheless, as a general matter, the EPA believes that prioritizing sources emitting 

at least 2,000 tpy provides for an appropriate balance in focusing an adequate amount of air 

agency resources on the area’s most likely to violate the standard. It is common under the CAA 

to tie applicability of requirements to sources’ emissions size, even where such thresholds do not 

necessarily expressly correlate to emissions levels that cause certain risks. In this rule, the EPA 

has not chosen to adopt the most common of those thresholds, the 100 tpy threshold for 

determining whether a source of criteria pollutants is “major” under the CAA, but instead has 

adopted a threshold that the Agency expects can be realistically implemented by resourceconstrained air agencies and that will result in characterizing areas affecting the vast majority of 

SO2 emissions beyond what had previously been required. 

Based on 2011 emissions (the latest emissions available at proposal), 42 states, 1 Territory, and 1 

Tribe had sources above the 2,000 threshold. Most of these (25 states, the 1 Territory, and the 1 

Tribe) had fewer than 10 sources above the threshold. The most sources above the threshold in 

any state is 28. While characterizing air quality around 28 sources (or fewer) in a state is not a 

trivial exercise, the Agency believes it can be realistically done and provides a reasonable 

balance between characterizing air quality in areas most likely to violate the standard and agency 

resources.

For the purposes of establishing a minimum threshold that prioritizes the resources that will be 

devoted to characterizing air quality near SO2 sources nationally, the 2,000 tpy source emissions 
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threshold strikes a reasonable balance between the need to characterize air quality near sources 

that have a higher likelihood of contributing to a NAAQS violation and the analytical burden on 

air agencies. This threshold is on the lower end of the range of thresholds recommended by 

commenters because sources on the lower end of the range have the potential to cause or 

contribute to violations of the NAAQS. As compared to the preferred option in the proposal (i.e., 

1,000 tpy sources in CBSAs over 1 million people; 2,000 tpy sources not in CBSAs over 1 

million people), the 2,000 tpy threshold would mean that, in the aggregate, air agencies would 

need to address air quality near about 35 fewer sources (or 7 percent fewer). Nevertheless, the 

total emissions addressed would still account for 89 percent of the SO2 emissions nationally 

(based on 2011 emissions), very close to the 90 percent level that has been considered to be 

reasonable by many stakeholders in the past.

6 National SO2 emissions have declined by a 

significant amount from 2011 to 2013 (around 1.5 million tons, or more than 20 percent), for 

various reasons. The EPA assessed the number of sources meeting a 2,000 tpy threshold based 

on 2013 emissions data now available for EGUs and 2011 emissions data for non-EGUs. 

Compared to the assessment in the proposal, which assessed the number of sources meeting the 

proposed threshold (1,000 tpy in urban areas/2,000 tpy elsewhere) based solely on 2011 data, the 

EPA now estimates that approximately 70 fewer sources (about 15 percent) will need nearby air 

quality to be characterized than was estimated in the proposal. Based on the updated data, the 

EPA estimates that already-designated sources plus sources currently exceeding the final 

threshold in this rule still would account for 86 percent of national emissions. Under this rule, 

each air agency will be required to identify all sources with annual SO2 emissions that exceed 

2,000 tpy (using emissions data from the most recent calendar year for which such data are 

available) and characterize air quality around such sources according to the timeline in section C 

of this preamble. 

Of course, if the trend in reduction of SO2 emissions continues at individual sources, there will 

also be a corresponding reduction in national emissions, and both kinds of reductions are 

desirable for improving public health, even if that results in fewer source areas becoming subject 

to the emissions characterization requirements in the final rule. Conversely, if the trend reverses 

and source emissions increase, more sources and areas will be required to be characterized under 

the rule. Thus, the exact number of sources and areas that will exceed the promulgated threshold 

when air agencies begin characterizing areas under the rule cannot be precisely known at this 

time, nor can their future percentage share of the national inventory be precisely estimated. 

Nevertheless, the EPA believes that the promulgated threshold strikes a reasonable balance based 

on the information the Agency currently has regarding recent historical SO2 emissions inventory 

levels. 

The circumstances in Montana do not warrant a revision to the rule. If air quality characterization 

 

6 The May 2012 White Paper and high-level summaries of stakeholder meetings are available at: 

http://www.epa.gov/oaqps001/sulfurdioxide/implement.html. These documents and written 

comments received from stakeholders are also included in the docket for this rulemaking.
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is required under this rule, the first step would be to determine the adequacy of the existing 

monitoring network for characterizing air quality near the pertinent sources, which will 

determine the need for further efforts to characterize air quality. The exclusion of sources from a 

nonattainment area cannot be presumed to signify that these sources are not contributing to 

violations of the air quality standard elsewhere.

The EPA does not share the concern of the commenter that objects that lower applicability 

thresholds will require more air agencies to rely on modeling. The EPA finds that modeling and 

monitoring both have advantages and disadvantages, and the EPA believes that the potential

influence of resource constraints to increase reliance on modeling will not have adverse effects 

on the suitability of air quality characterization performed pursuant to this rule. Furthermore, the 

EPA does not share the apparent view of the commenter that reducing the reliance on modeling 

warrants being set as an objective that in turn warrants reducing the number of areas where the 

potential for violations of the air quality standard is evaluated.

The comment objecting to the fact that the 2013 strategy paper did not discuss a 1,000 tpy

threshold is moot, in light of the final rule’s adoption of a general 2,000 tpy threshold. However, 

we note that the absence of discussion of an element in the strategy paper does not render its 

subsequent proposal, or final promulgation, in notice and comment rulemaking unreasonable. 

Numerous aspects of the proposed and final rules were not addressed in the strategy paper, 

including the provisions addressing source incentives to reduce emissions or the ability to 

discontinue monitoring if ambient levels fall a certain amount below the NAAQS, but that does 

not render their adoption in this rule unreasonable.

6. Other options to identify sources

a. Refined dispersion modeling

Comment:

Commenter (0051) stated the EPA should explicitly allow for alternate approaches to identify the 

sources that should be modeled using refined dispersion modeling. Commenter (0051) provided 

the following additional points.

Application of a regional grid model, such as CAMx, at relatively high resolution and 

additional targeted receptors as needed, combined with conservative assumptions about 

emissions and a threshold concentration inversely proportional to the grid cell area should 

suffice to identify possible areas of concern. Advantages of this approach include the 

ability to include additional SO2 sources; use of prognostic (e.g., weather research and 

forecasting (WRF)) wind fields rather than "straight-line" winds from discrete National

Weather Service (NWS) locations; chemical processing of SO2 emissions; treatment of 

wet and dry deposition; and accounting for impacts beyond the 50 kilometer range over 

which AERMOD is viable. Additionally, using a grid model could provide estimates of 

background values that should be more representative than those derived from other 
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63

techniques.

Response: 

The EPA seeks to define the areas required to be characterized on the basis of a relatively simple 

indicator of the potential for violations of the standard, or more precisely a simple indicator as to 

whether any particular area warrants being prioritized for air quality characterization. Aside from 

the inherent disadvantages of regional grid models as even screening tools for identifying areas 

with potential violations of the SO2 standard, such as the inherent averaging over the applicable 

horizontal and vertical grid dimensions (and the infeasibility of including “additional targeted 

receptors”), the use of such models does not provide a practical basis for pursuing a nationally 

consistent set of areas for which to require air quality characterization. Therefore, the EPA is 

promulgating a rule requiring air quality characterization in areas with sources that emit at least 

2,000 tpy, along with any additional areas that are identified as having sufficient potential for 

violations to warrant air quality characterization. Notwithstanding the disadvantages of regional 

grid models for assessing this potential, states and other air agencies already have the option, 

whether implementing this rule or for other purposes, to use these and any other tools they judge 

appropriate to identify additional areas that may warrant characterization.

The EPA also does not support the use of regional grid models for performing the air quality 

characterization that this rule requires, to the extent the commenter would recommend using 

them for that purpose. The Agency believes that a modeling protocol or monitoring network 

design that follows or references the recommended approaches in the TADs is likely to be 

adequate, and will better ensure the success and a timely fulfilment of the requirements of this 

rulemaking. However, air agencies remain free under the final rule to suggest alternative 

approaches to those suggested in the TADs. Whether an agency chooses to follow a TAD or 

suggest an alternative approach does not affect the fact that for every approach chosen, the air 

agency will need to submit its rationale and approach to the EPA for review on a case-by-case 

basis. 

b. 1,000 tpy

Comment:

Commenters (0036, 0062, 0068, 0078, and 0085) urged the EPA to adopt the proposed Option 1 

level of 1,000 tpy, but apply it uniformly, regardless of population in order to ensure a basic level 

of health protection to people who live around the sources. 

Commenters (0068, 0078) stated that because modeling has shown that sources with emissions 

below 2,000 tpy have the potential to cause or contribute to modeled NAAQS violations, an 

emissions threshold of 1,000 tpy is more appropriate to ensure that air quality characterizations 

are accurately capturing potential NAAQS violations. Commenter (0078) referred to preliminary 

modeling conducted within Minnesota. Commenter (0068) provided the following additional 

points. 
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As an example, the Brandon Shores and CP Crane coal-fired power plants in Maryland 

have been modeled using actual emissions, and according to that modeling (exhibit 8, 

attached to their comments), are individually and solely responsible for causing 

exceedances of the SO2 NAAQS. The AERMOD modeling shows peak concentrations 

from Crane of roughly double the NAAQS; however, Crane and Brandon Shores both 

have had annual total emissions lower than some of the options contemplated by the EPA

in the draft DRR. Thus, using a threshold of greater than 1,000 tpy would exclude sources 

known to be emitting at levels that would cause nonattainment of the NAAQS, and thus 

would result in a failure of states and the EPA to adequately implement the SO2 NAAQS. 

Commenter (0068) stated that modeling of the Seward Plant in Pennsylvania provides a 

similar example.

Commenter (0085) stated a threshold of 1,000 tpy of SO2 is a reasonable cutoff for capturing 

both larger sources and some medium-sized sources that contribute to higher SO2 emissions, and 

achieving a total emissions coverage of 90% (79 FR 27456, Table 1-

Summary of Source Threshold Options). Commenter (0085) stated that, as far as identifying 

sources around which to characterize the air quality, ARMA supports the following approach:

Sources with continuous emission monitors (CEMS):

(1) Determine which sources have annual emissions of greater than 1,000 tpy, and 

for those sources with emissions of greater than 1,000 tpy, determine which, if 

any, sources have CEM data showing an hourly average SO2 emissions rate below 

the 1-hour 75 ppb SO2 NAAQS.

(2) Exclude from air quality characterization any sources with hourly average SO2

emissions below the 1-hour 75 ppb NAAQS.

(3) Proceed with characterizing the air quality in the area of the remaining CEM

sources via monitoring and/or modeling.

Sources without continuous emissions monitors:

(1) Determine which of these sources have annual emissions of greater than 1,000

tpy.

(2) Require monitoring or modeling of any non-CEM sources above the 1,000 tpy

threshold. (0085)

Commenter (0115) suggested an applicability approach that considers cumulative emissions 

from all local major sources and the unique meteorology/topography of an area, rather than 

focusing solely on emissions from individual sources and the population of the area. Commenter 

(0115) suggested a threshold of 1,000 tpy, either from one individual source or cumulatively

from all major sources in the immediate area.
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Response:

The EPA is not asserting that areas with sources emitting under its threshold (now 2,000 tpy) 

will in all cases be attaining the standard, just as it is not asserting that areas with sources 

emitting over its threshold will in all cases be violating the standard. Instead, the EPA is merely 

promulgating a rule to prioritize areas that warrant further air quality characterization, and to 

require this air quality characterization in areas that are prioritized. While a number of factors 

influence the air quality impacts of a given quantity of emissions, the EPA seeks to establish 

relatively simple criteria for identifying areas that warrant air quality characterization, along with 

provision for prioritizing additional areas that the air agency or the EPA believes to warrant air 

quality characterization notwithstanding that the sources have emissions lower than the threshold 

established in the rule. The EPA is setting the threshold at 2,000 tpy in order to provide an 

appropriate balance between the goal of assuring that most areas with significant prospects of 

violating the standard are characterized and the goal of minimizing the planning burden of 

conducting air quality characterization. Within the constraints of the need for administrative 

simplicity, the EPA believes that the effort of characterizing air quality near sources emitting 

more than 2,000 tpy is warranted, and the EPA believes that the provision for discretionary 

addition of areas with sources emitting less than 2,000 tpy suitably assures that areas with 

smaller sources that nevertheless warrant air quality characterization are adequately addressed.

A number of commenters discussed monitoring evidence in their areas from which they inferred 

the likelihood of violations being observed near sources with various annual emission levels. 

These comments must be considered in the context that concentrations can vary significantly as a 

result of other factors such as stack heights, meteorology, and terrain. For this reason, the EPA 

agrees with the commenter that “large amounts of SO2 do not necessarily correlate to violations 

of the standard,” a statement that should be supplemented with the statement that small amounts 

of SO2 do not necessarily correlate to attainment of the standard. For this reason, an important 

part of this rule is provision for air agencies and for the EPA to conclude that a source (or a 

cluster of sources) that does not meet the threshold of this rule nevertheless poses sufficient 

concern that air quality characterization in the vicinity of this source (or these sources) should be 

prioritized. Nevertheless, as a general matter, the EPA believes that prioritizing sources emitting 

at least 2,000 tpy provides for an appropriate balance in focusing an adequate amount of air 

agency resources on the area’s most likely to violate the standard. It is common under the CAA 

to tie applicability of requirements to sources’ emissions size, even where such thresholds do not 

necessarily correlate to emissions levels that cause certain risks. In this rule, the EPA has not 

chosen to adopt the most common of those thresholds, the 100 tpy threshold for determining 

whether a source of criteria pollutants is “major” under the CAA, but instead has adopted a 

threshold that the Agency expects can be realistically implemented by resource constrained air 

agencies and that will result in characterizing areas affected the vast majority of SO2 emissions 

beyond what had previously been required. 

c. 2,000 tpy
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Comment:

Commenter (0088) suggested a 2,000 ton threshold should be applied to large sources, without a 

population threshold factor.

Comment:

Commenter (0051) suggested consideration of limits at 5,000 tpy SO2 sources in any area of the

state, and 3,000 tpy SO2 sources in CBSAs greater than 1 million and noted this would provide 

coverage of 18 Texas sources representing 83% of the total 2012 reported point source 

inventory.

Response: 

See above response concerning EPA’s decision on the final threshold for characterization.

d. 3,000/5,000 tpy

Response: 

See above response concerning EPA’s decision on the final threshold for characterization.

e. 10,000 tpy

Comment:

Commenters (0039, 0041, and 0045) supported a 10,000-ton threshold, without a population 

threshold factor. Commenter (0041) stated the AQD found that a 10,000-ton threshold in all 

areas outside the current SO2 nonattainment area would account for five sources out of the 12 

largest (greater than 1,000 tpy) sources and represent 89 percent of SO2 emitted from the 12

sources. Commenter (0041) stated that, because this level of impact is significant, the AQD 

believes a 10,000-ton threshold is appropriate for Michigan. Commenter (0039) stated that this 

threshold would greatly increase the overall effectiveness in identifying and addressing potential 

1-hour SO2 NAAQS issues.

Commenter (0039) stated that a quick review of Indiana’s existing SO2 monitoring network data 

and Indiana SO2 emissions shows a high correlation between measured design values above the 

75 ppb standard with emission sources that exceed 10,000 tpy of actual emissions and that same 

data show no instances of measured design values above 75 ppb where the largest sources are 

less than 10,000 tpy of actual emissions. Commenter (0039) stated this simple evaluation is an 

indicator that focusing the Proposed Rule on emissions sources with less than 10,000 tpy of 

actual emissions may cause states and sources to expend resources ineffectively. 
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Response: 

See the response above concerning source characterization.

f. 10,000/1,000 tpy (Option 1 & 3 hybrid)

Comment:

Commenter (0049) recommended a hybrid of Options 1 and 3 with a 1,000 tpy threshold used 

inside the CBSA with a population greater than one million and a 10,000 tpy threshold for areas 

outside a CBSA with population greater than one million. Commenter (0049) stated this would 

retain the objective of protecting public health in urban settings by focusing on the air quality 

surrounding significant SO2 emission sources in those areas. Commenter (0049) believed this 

would still ensure public health outside urban centers by targeting large sources having the 

greatest potential for causing or contributing to violations of the 1-hr SO2 NAAQS.

Response: 

See the response above concerning source characterization.

g. 10,000/5,000 tpy

Comment:

Commenter (0060) suggested the EPA should consider thresholds above those in Option 3.

Commenters (0051, 0110) proposed an option covering 10,000 tpy SO2 sources in any area of 

the state, and 5,000 tpy SO2 sources in CBSAs greater than 1 million. Commenter (0051) stated 

that, given SO2 emission reduction trends and constraints on resources, the emission threshold

approach should characterize only the largest sources with the greatest potential for relatively 

higher SO2 concentrations. Commenter (0110) stated that with these alternate thresholds, the risk 

for NAAQS violations is still very low, the workload for state agency review is kept more 

manageable, and state resources are not wasted on assessment of many small low-risk SO2

sources. Commenter (0051) stated that the TCEQ's preferred option would allow for more 

thorough and rigorous analysis of the highest emitting SO2 sources in Texas. Commenter (0110) 

stated that it is simply impossible for hundreds of protocols to be carefully considered and 

processed in the time allocated by the proposed DRR; instead, the EPA needs to reduce the 

burden on states by requiring review of NAAQS compliance only for the sources with emissions 

high enough to possibly result in a nonattainment situation.

Commenter (0110) stated that considerations of both modeling and monitoring data indicate that 

an SO2 emissions threshold of 10,000 tpy will have a low probability of an actual NAAQS 

violation. Commenter (0110) stated that, of the 29 areas designated as SO2 nonattainment in the 

August 5, 2013 Federal Register, all but five of these had an SO2 emission source with more 

than 10,000 tpy (the five remaining areas were associated with unique source-monitor issues 
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involving either stack downwash or steep terrain effects). Commenter (0110) concluded that, 

except for unusual situations, the likelihood for a monitored SO2 NAAQS violation for sources 

with SO2 emissions up to 10,000 tpy is very low. Commenter (0110) reviewed the results of 

modeling for tall stack sources to determine the emission levels that are likely to affect 

compliance with 1-hour SO2 NAAQS and provided the following statements.

EPA’s own analysis, provided by Roger Brode of EPA’s Office of Air Quality Planning 

and Standards in support of the NAAQS promulgation, indicated that a tall-stack 

electrical generating unit (EGU) source emitting 12,000 tpy of SO2 would show NAAQS 

attainment, although the results would be somewhat dependent upon terrain and 

background concentration values. 

Another modeling analysis presented at a 2013 Air & Waste Management Association 

specialty conference on air quality modeling by Gilbert, Liebsch, and Dunbar for a 

modeling of a tall stack source indicated that the compliance SO2 emission rate would 

depend upon the stack height and background concentration values. Their overall 

AERMOD modeling results, assuming a background concentration that was 20% of the 

NAAQS, would indicate that the compliant emission rate could be as high as 16,000 tpy

for a Good Engineering Practice (GEP) stack height of 375 feet (2.5 times the building 

height of 150 feet). Thus, an upper SO2 emissions limit threshold of 10,000 tpy as 

proposed under the EPA’s Option 3 is reasonable and protective of the NAAQS.

Response:

See the response above concerning source characterization

h. 500 pounds per hour and annual limits

Comment:

In addition to an annual emissions applicability threshold, commenter (0036) recommended that 

the EPA adopt an hourly emission trigger for rule applicability to address those large emitters of 

SO2 that operate relatively infrequently. Commenter (0036) suggested additional details as 

follows:

The test should include a minimum hours of operation per year and suggested the hourly 

applicability threshold could be for sources that operated more than 500 hours per year 

and for which the 99th percentile of their one-hour emissions values is above 500 pounds 

per hour. All sources that have operated less than 500 hours a year or have an hourly SO2 

allowable emission rate below 500 pounds per hour could be initially eliminated from 

consideration. Hourly data from sources with continuous emissions monitoring systems 

(CEMS) could be used to quantify the hours of operation and the 99th percentile value. 

For sources without CEMS or other means of calculating hourly emissions, either 24-

hour or 30-day average SO2 emissions could be used; these rates could be converted to 
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hourly SO2 emission rates using Tables 1 and 2 in Appendix D of the EPA document 

entitled Guidance for 1-Hour SO2 Nonattainment Area SIP Submissions (April 2014). 

The NESCAUM states further recommend that the EPA revisit 500 hours per year as an 

applicability criterion at some later point in time, once SO2 implementation is underway, 

to reassess its appropriateness as a threshold. 

Response: 

See above response concerning EPA’s decision on the final threshold for characterization.

i. Sources with seasonal or monthly emission spikes

Comment:

Commenter (0062) stated that the EPA should consider adding requirements to characterize SO2 

concentrations near sources with seasonal or monthly emission spikes, sources that exceeded the 

threshold at least once in the past three years, or areas with multiple sources. Commenter (0062) 

stated, for example, if the threshold for the county is set at 1,000 tpy, an area with a source 

whose emissions exceed 250 tons per quarter or 83 tons per month should be characterized. 

Response: 

The EPA believes that a rule with a relatively simple criterion, based on annual emissions, will 

be easier to administer and will help provide greater national consistency in its application. 

Nevertheless, the EPA is also providing discretion to air agencies and to itself to identify 

additional sources near which air quality characterization is warranted. Addition of such sources 

on a case-by-case basis would provide a more appropriate means of assuring that the appropriate 

sources are addressed without having to use complicated applicability criteria that may not be 

necessary in all areas.

j. Multiple sources in close proximity that individually are below the 

threshold but whose combined emissions may violate the NAAQS

Comment:

Commenters (0036, 0071, 0078, 0081, 0082, 0113, 0115, and 0118) stated that the EPA should 

prescribe more specific requirements to address clusters of SO2 sources that would otherwise fall 

below emissions thresholds for required modeling/monitoring.

Commenters (0036, 0078, and 0081) suggested the EPA consider emissions density metrics as 

part of the applicability test. 

 Commenter (0036) stated the metric should ensure that, if two or more major sources are 

located in an area and their combined actual emissions are greater than a specified 

threshold, then an analysis should be conducted on these sources. 
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 Commenter (0078) recommended analysis of sources with actual emissions greater than 

100 tpy in an area 25 km in diameter or less where aggregate emissions within the area 

exceed 1,000 tpy.

 Commenter (0081) stated that, if the EPA wishes to pursue this policy, it would be 

helpful if guidance on a minimum “emission density” threshold for possible source 

cluster evaluation was provided in the final TADs so that states can effectively identify 

possible areas of concern and not be caught off-guard during the designations process. 

Commenter (0081) stated that, recognizing that areas of highest SO2 concentrations are 

likely to localized in relative close proximity around a source, or cluster of sources, the 

Department recommends that the threshold density should be no smaller than the same 

emission rates chosen in the final rule (i.e., Option 1, 2, or 3) aggregated over no larger 

than a 10 km2area.

Commenters (0078, 0082) urged the EPA to clarify that, for an air agency or the EPA Regional 

Administrator to require air quality characterization for sources based solely on their proximity 

to other sources, the combined total emissions from these smaller sources must exceed the 

finalized reporting threshold.

Commenter (0052) suggested that states should be allowed to use their best judgment to identify 

scenarios where a cluster of small sources may cause a problem; i.e., if a demonstration can be 

made of counties with significantly higher emission rates being below the standard, it stands to 

reason that lower emitting counties would also be below the standard and not require any 

analysis.

Commenter (0118) requested that the EPA identify an emissions threshold and domain of such

clusters; e.g., sources of less than 100 tpy or less than 5% of a cluster's total emissions could be 

excluded. Commenter (0118) appreciated the flexibility of the approach, but stated it is better for 

states to work from a basic protocol and makes acceptance of these analyses more uniform 

across different areas. 

Commenter (0113) suggested the EPA could consider an additional option to evaluate NAAQS 

compliance in areas with clusters of sources that cumulatively exceed a larger specified emission 

threshold in areas with significant populations. Commenter (0093) stated that, given the resource 

burden for this modeling/monitoring effort, agencies must focus on the largest sources only in 

the initial effort and that smaller sources and clusters of sources can be reviewed at a later date.

Commenter (0081) stated that, because each situation of possible clusters of sources will be 

different, the Department does not believe it wise, or necessary, to codify a cluster density in the 

final rule. Commenter (0081) stated the professional judgment of the air agency personnel, along 

with consultation with the appropriate EPA Modeling Contact, will be most appropriate in 

evaluating the different source configuration, land use, geographic, and meteorological variables 

involved in this process. Commenter (0081) added that, if the EPA chooses the 2,000 tpy and 

1,000 tpy thresholds currently in the Proposed Rule, about 90 percent of the SO2 inventory in the 
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country will be addressed, leaving few other sources that might qualify as a cluster of sources 

that need to be evaluated.

Commenter (0078) stated that the EPA should include direction for states where cumulative 

impacts may result from cross-state border sources; i.e., sources that would otherwise be below 

the EPA's threshold but which may cumulatively contribute to NAAQS exceedances may be 

located in a neighboring state. Commenter (0078) stated that leaving cluster analyses to the 

discretion of each state air agency without further guidance can result in challenges for states 

seeking to address clusters using different approaches.

Commenter (0115) stated that, while the proposal does urge state agencies to model multiple 

sources located in close proximity, as needed, the Band does not believe that this position is 

strong or clear enough, and may be open to wide interpretation by various facilities and 

regulatory agencies. Commenter (0115) stated that more stringent guidance or oversight in this 

area is needed so that state agencies will have clear authority to require modeling from sources 

with high rates of cumulative emissions. Commenter (0115) provided the following additional 

points.

The proposed thresholds do not adequately address industry-intensive areas that may 

experience high cumulative rates of emissions or that may be located in areas with local 

meteorology that is not conducive to adequate air dispersion. The areas near the Fond du

Lac Reservation and the ceded territories include both of these types of settings. 

Minnesota's Iron Range (a geologically defined area in the Northeastern part of the state) 

has seven active taconite mines and three utilities, all located within roughly 60 miles of 

one another, all of which are major sources of SO2. In 2012, these facilities emitted an 

estimated total of 8,592 tpy of SO2. Only one of these sources emitted more than 2,000 

tpy, however, so only that particular source would have been targeted for modeling under 

the EPA's most favored proposed scenario because these sources are located in an area 

with less than 1,000,000 million residents. Preliminary modeling performed by the 

Minnesota Pollution Control Agency including all of these facilities, however, has shown 

a number of instances of potential modeled non-compliance that are currently being 

further addressed. 

Response:

In general, the EPA has responded to these comments in the preamble to the final rule. The EPA 

is not establishing specific criteria for prioritizing clusters of multiple sources because the factors 

that influence whether particular cases warrant prioritization are too complex and difficult to 

administer to warrant defining generally applicable criteria. 

Emission density can be defined in various ways reflecting various means of defining the area 

over which emissions are averaged. No single definition of emission density best serves the 

range of circumstances that exist. Furthermore, emission density addresses only one factor 

influencing the need to prioritize an area for air quality characterization, since concentrations of 
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SO2 are also influenced by the size of the area, the total emissions of SO2, the geographical 

relationship of the sources (e.g., the proximity of the sources and whether they are aligned in any 

wind direction), and other factors influencing the impacts of the individual sources and the 

combined impacts. Thus, the EPA has concluded that no set of easily administrable criteria could 

appropriately address the range of circumstances that are to be considered. Nevertheless, the 

notice of final rulemaking contemplates the possible impact of below-threshold source clusters, 

notes the possibility that states may choose to characterize, or EPA may further require the 

characterization of, such areas on a case-by-case bases, and discusses the factors that should be 

considered in judging whether particular areas warrant air quality characterization. The rule 

provides for case-by-case judgments as to the areas to be addressed rather than defining any 

particular set of criteria to use in making these judgments.

The degree to which a border area warrants air quality characterization is a function of the 

circumstances on both sides of the border. Thus, in determining whether a border area warrants 

air quality characterization, the affected states should consider all sources on both sides of the 

border and collaborate on reaching a determination on whether to characterize additional belowthreshold source clusters. Where appropriate the EPA will oversee this determination and will 

apply its discretion to require air quality characterization in such areas based on consideration of 

all sources in the area.

A similar response applies to other recommendations for methods for identifying clusters that 

warrant air quality characterization. The existence of two or more sources with combined 

emissions exceeding 2,000 tpy or some other total does not by itself indicate a need for air 

quality characterization, depending on the proximity of the sources and other factors. (Similarly, 

the fact that the combined emissions are less than 2,000 tpy is also not a definitive indication that 

the area does not warrant air quality characterization, given the possibility that the proximity and 

the stack parameters of the sources or other aspects of the situation may warrant prioritizing the 

area.)

k. Phased approach

Comment:

Commenters (0034, 0115) recommended a phased approach. Commenter (0034) recommended a 

phased approach in which an initial round of analyses would be followed by additional analyses 

at a later date, depending on the annual SO2 emissions thresholds of such sources (e.g., 3,000 

tpy, 2,000 tpy, or 1,000 tpy). 

Response: 

The EPA promulgated a revised SO2 standard in 2010, and the EPA believes that appropriate 

efforts to assess the attainment of that standard should be conducted as expeditiously as 

practicable. The EPA does not believe that any portion of this reasonable effort at SO2 NAAQS 

implementation should be deferred. 
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l. Statewide percent reduction target

Comment:

Commenter (0060) suggested the rule should give states an overall total emissions coverage 

percent reduction target. Commenter (0060) recommended that target should not exceed 80% of 

the 3-year average of total state emissions. 

Response: 

The EPA believes that this recommendation would not provide for appropriate, nationally 

consistent targeting of the areas that most warrant air quality characterization. It could, in fact, 

result in certain states being required to address sources that are much smaller and that have 

lesser impacts than sources in other states that might not be required to be characterized based on 

the latter state’s application of such a test. Nevertheless, in the event that the sources covered by 

this rule do not meet an air agency’s own preferred extent of coverage within that particular 

state, this rule provides that the air agency can characterize additional sources as appropriate.

m. Factors other than population

Comment:

Commenter (0044) stated that if the EPA decides that either Option 1 or 2 is preferable, then the 

source threshold needs to be revised to take into account factors in addition to population 

density. Commenter (0044) recommended the final rule require the following additional factors 

be considered, including (but not limited to): the distance a source is located from population 

centers in general and sensitive populations in particular; stack heights; topography and 

meteorological factors unique to the source(s); and economic conditions that will affect a 

source’s expected SO2 emissions. 

Commenter (0044) stated that if the EPA decides that population density is not a threshold factor 

that will need to be considered, the final rule should still designate that, at a minimum, the 

following additional factors be considered, including (but not limited to): the distance a source is 

located from population centers in general and sensitive populations in particular; stack heights; 

topography and meteorological factors unique to the source(s); and economic conditions that will 

affect a source’s expected SO2 emissions. 

Commenter (0044) disagreed with the proposal’s statement explaining why the Agency does not 

believe it necessary for air agencies to consider such factors. Commenter (0044) stated the lack 

of a nationwide database with respect to such factors is irrelevant since the modeling is to 

characterize localized ambient air quality; and the statements in the proposal itself describe the 

benefits of these factors (at 27449 and 27461).
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Response: 

Regarding the comments EPA received expressing preferences on the proposed emission 

threshold options, the EPA notes the wide range of views. A few commenters recommended 

alternate thresholds in the range from 1,000 tpy to 10,000 tpy, or recommended pairs of 

thresholds within this range. Some commenters provided modeling analyses as an indication that

sources larger than 12,000 tpy did not cause a violation of the standard, while other commenters 

recommended a single emissions threshold of 1,000 tpy and provided modeling analyses of 

different sources as an indication that sources less than 2,000 tpy caused modeled violations. 

These comments demonstrate that ambient SO2 impacts can be variable, and are dependent on 

many factors other than annual emissions (such as meteorology, stack height, local topography 

and plant operations). These factors can only be assessed through analytical approaches, such as 

ambient monitoring or air quality modeling, which take many of these related factors into 

account simultaneously. The EPA’s goal in this rule was not to devise a perfect test that requires

characterization of every source that would be found to violate the NAAQS and excludes from 

characterization every source that would be found not to violate the NAAQS. While such a test 

may be ideal, it would be unnecessarily burdensome and complex to administer – essentially 

resulting in an effort that would equate to characterization of every single source in the country. 

That is why this rule promulgates a test that is feasible to administer and establishes a threshold 

that appropriately directs limited characterization resources toward priority sources. These 

comments demonstrate why specific air quality characterization of the area around these sources 

is needed to protect public health in the first place. 

n. Do not codify the thresholds

Comment:

Commenters (0053, 0058) stated that the EPA should not make the threshold criteria mandatory 

so as to hamper a state's ability to reasonably tailor its approach. Commenter (0058) stated the 

EPA should provide clear technical justification for any threshold it recommends and should 

simply submit this recommendation as guidance to the states, rather than codify thresholds in this 

rulemaking. Commenter (0058) stated that, while thresholds can be useful tools, the threshold 

options proposed by the EPA do not account for the myriad other factors that are critical to the 

assessment of a source’s impact (e.g. stack height, emissions parameters, topography of the 

area). [Also see III.A.9, regarding state discretion]

Response:

The EPA disagrees with the commenters who stated that EPA should not make the threshold 

criteria mandatory. The EPA held stakeholder meetings to gather comments on what process 

should be taken to implement the SO2 NAAQS in May and June of 2012. From those stakeholder 

participation meetings we received comments stating that any new modeling or monitoring 

requirements relating to the implementation of the SO2 NAAQS should be established through a 
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notice-and-comment rulemaking process. That is, the EPA must weigh this comment against 

feedback from numerous others that recommended rulemaking, presumptively to make the 

threshold criteria mandatory. The EPA also believes that it is important to codify the 

requirements of this rule to ensure that the appropriate information is provided to characterize air 

quality rounding sources of SO2 to use in future EPA actions such as designations and 

redesignations of areas. While the EPA recognizes that a number of factors influence the 

likelihood that a given quantity of emissions will cause a violation, these factors interact in 

complex ways, such that no simple combination of criteria would necessarily provide a better 

indication of whether a source area warrants air quality characterization than a rule with a simple 

emission threshold. In addition, a rule with a simple emission threshold is significantly easier to 

implement and is more prone to provide for national consistency than a rule with complex 

interactive criteria. Nevertheless, the EPA shares the concern that areas with sources emitting 

less than the promulgated threshold may nevertheless be violating the standard (presumably due 

to a combination of pertinent factors), and the EPA is therefore promulgating the proposed 

provisions for discretionary addition of such areas to the list of areas subject to air quality 

characterization requirements. 

7. State option—identify additional sources

Comment:

Commenters (0051, 0089, and 0107) encouraged the EPA to identify criteria so that the states are 

in a position to know what additional source-oriented areas they may be required to monitor or 

model. Commenter (0051) stated that, to promote national consistency and assist states with 

making viable implementation plans, the EPA should develop a process or criteria to identify any

sources that may be requested for evaluation beyond those determined by the rule's thresholds. 

Commenter (0107) stated this criterion ("or has been identified ... as requiring further air quality 

characterization"), being rather vague and open-ended, creates great uncertainty about how to 

implement the rule. Commenter (0107) recommended either removing the phrase or providing 

specific criteria for applying it. 

Commenters (0035, 0051, and 0062) stated that the rule should make clear that states have the 

option to monitor or model sources below the thresholds established in the final rule. Commenter 

(0062) stated there are many factors involved in determining whether or not an individual facility 

causes a NAAQS violation, including total emissions, temporal variations in emissions, 

meteorology, other sources, etc. Commenter (0062) asserted that states should characterize areas 

that, based on their knowledge of SO2 sources and meteorological characteristics in their states,

could be at risk for exceeding the standard. Commenter (0062) suggested that the EPA, in the 

final rule, add the following italicized and bolded language: 

Under section B. Proposed Source Emission Threshold Options: 

“...This Proposed Rule only presents a minimum set of sources for which surrounding 

ambient air quality would need to be characterized. States should characterize areas 
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that, based on their knowledge of sources and areas, may be at risk for exceeding the 

standard. As discussed in more detail in section IV, the air agency or the EPA Regional 

Administrator may identify other sources that should be characterized beyond the 

minimum requirements of this Proposed Rule.”

Under section IV, subsection A. From Promulgation of this rulemaking to January 15, 

016: 

“... The EPA strongly encourages each air agency to consult with its EPA regional Office 

to identify sources exceeding the emission thresholds in the final rule (as described in 

section III) and any other areas near sources that do not exceed the emissions thresholds 

but which would be appropriate for air quality characterization as well. States are in the 

best position to evaluate their SO2 sources and areas and should characterize areas 

that could be at risk for exceeding the standard.” 

... “Section 51.1203(a) of this rulemaking as proposed, would then require each air 

agency to submit to its EPA Regional Administrator by January 15, 2016, a final list 

identifying... We note that, while a state may not have any sources that exceed the 

minimum source threshold requirements, there may still be a separate need (such as may 

arise in making future designations recommendations) for the air agency to characterize 

air quality for any other sources below the thresholds in this Proposed Rule that the air 

agency or the EPA Regional Administrator deems may have the potential to violate the 

NAAQS. For example, the air agency or the EPA administrator may determine that the 

air quality should be characterized around multiple sources located in close proximity 

that individually are below the threshold but whose combined emissions may violate the 

NAAQS. States should add sources to this list that are below the source thresholds 

that, based on their knowledge, are at risk for exceeding the standard.”

Response: 

The primary criterion for identifying sources near which air quality must be characterized is an 

emission threshold of 2,000 tpy. Presumably these commenters would agree that this criterion 

provides a clear indication for how to identify most of the areas that this rule will require to be 

addressed. Since the rule requires air quality characterization for areas with sources emitting at 

least 2,000 tpy or that the air agency or the EPA identify as requiring further air quality 

characterization, the EPA believes that the rule is clear that air agencies and the EPA have the 

authority under this rule (in addition to authority under section 114(a)(1) of the CAA) to require 

air quality characterization in areas that have no sources emitting 2,000 tpy or more of SO2. 

At issue here is how to identify additional areas beyond those having a source emitting 2,000 tpy

that warrant air quality characterization. Among cases in which no single source meets the 

applicable emission threshold, no simple indicator is available to indicate which of these cases

warrants air quality characterization. For areas with a single source, the areas could warrant air 

quality characterization if the stack height is low, if the plume rise is minimal, if terrain or 

meteorology is conducive to high impacts, and/or if emissions are just slightly below the 
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threshold. For areas with multiple sources, concentrations are influenced not only by these stack, 

terrain and meteorological factors but also by the level of emissions at each source and the 

distances and wind directions between the sources. The EPA appreciates the comments urging 

the establishment of specific criteria in the rule for identifying additional areas that warrant air 

quality characterization, but the EPA finds that these areas are better identified on a case-by-case 

basis reflecting a judgment considering the range of factors that influence the likelihood of 

NAAQS violations. That is, the EPA agrees with the state commenter urging that the rule 

provide for discretionary coverage of additional areas, such that additional areas that in the air 

agency’s (and the EPA’s) judgment have significant potential for violating the NAAQS can be 

made subject to requirements for air quality characterization on case-by-case bases.

The EPA agrees with the commenter that states are often in the best position to know whether an 

area is at risk of violating the SO2 standard. At the same time, EPA is often in the best position to 

assure national consistency, to assure that areas in different states with similar risks of violating 

the SO2 standard are addressed similarly. Therefore, the EPA is retaining the proposed approach, 

in which the air agency and the EPA are given equal authority to determine that areas without 

sources emitting 2,000 tpy or more shall be subject to the requirements for air quality 

characterization.

8. EPA option--identify additional sources

Comment

Commenters (0035, 0089, 0106, and 0107) encouraged the EPA to identify criteria so that the 

states are in a position to know what additional source-oriented areas they may be required to 

monitor or model. Commenter (0106) stated that this requirement is ambiguous and extremely 

subjective and would result in uncertainty as to who has the authority to determine which sources 

are subject to the Proposed Rule. 

Commenter (0035) stated the EPA should clarify that revisions by the Regional Administrator

that add sources to the submitted list are to be based on appropriate data that show that the 

source's actual SO2 emissions exceed the applicable emissions threshold, and not based on other 

criteria.

Commenter (0107) recommended either removing the phrase or providing specific criteria for 

applying it. Commenter (0107) stated that, if the phrase ("or has been identified ... as requiring 

further air quality characterization") remains in the final rule, the word "or" should be changed to 

"and" such that the rule requires a consultative process between the air agency and EPA. The 

revised phrase would read, "... been identified by the air agency and by the EPA Regional 

Administrator as requiring further air quality characterization."

Commenter (0051) stated that, if the Regional Administrator requests the evaluation of 

additional sources, states need sufficient time (likely to be at least three years) so they are not 
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forced into modeling to meet the EPA's deadlines. Commenter (0051) stated it is crucial that 

states have the flexibility to choose which method to use to determine attainment in a way that 

best suits the states' emissions sources and resources and the potential need to evaluate additional 

sources undermines a state's ability to anticipate resource demands and make solid 

implementation plans.

Commenters (0063, 0065, 0085, 0088, 0093, and 0106) did not support allowing EPA Regional 

Administrators the discretion to add sources below the threshold. Commenters (0063, 0065) 

stated last-minute additions by the Regional Administrator would undermine efforts by states and 

sources to plan and prepare for compliance with any final rule and create a path for states to be 

forced into modeling as the result of an unexpected monitoring plan disapproval. 

Commenter (0085) stated the proposed approach will, by its nature, result in inequities in air 

quality across the country; instead, this rule should establish fair and impartial methods.

Commenter (0088) stated that it is not within the EPA’s authority, unless it is administering the 

State’s entire SO2 NAAQS program to condition approval on modeling other sources. 

Commenters (0085, 0093) did not support allowing air agencies the discretion to add sources 

below the threshold since that approach will, by its nature, result in inequities in air quality 

across the country. Commenter (0085) stated this rule should establish fair and impartial methods 

for deciding which sources must have their air quality characterized and that local politics and 

advocacy should not be involved.

Commenter (0088) stated that if the Administrator is advised by the Regional EPA office that a 

source should have been included, then the Administrator can designate the area “unclassifiable” 

pending further air quality characterizations. Commenter (0088) provided the following 

additional points.

There are no limitations on the EPA’s ability to designate such sources, either temporally 

or with procedural notices by which EPA regions would be able to unilaterally require 

data collection plans to be augmented. NEDA/CAP submits that it is up to the state to 

determine if additional sources should be modeled or monitored, and that it is not within 

EPA’s authority unless it is administering the state’s entire SO2 NAAQS program to 

condition approval on modeling other sources. Given the time available for modeling and 

monitoring plans to be developed and implemented, a “late hit” from the EPA would 

delay the process, provide a de facto veto of a monitoring or modeling plan, and interfere 

with the state’s ability to implement the monitoring/modeling plan. 

Response: 

As noted above, no simple criteria exist to identify the areas not identified by the simple criterion 

of emitting at least 2,000 tpy that nevertheless warrant air quality characterization. Nevertheless, 

given the various factors that affect a source’s ambient impact, there can be little question that 

some areas without any source emitting at least 2,000 tpy may be just as prone to be violating the 

SO2 standard as typical areas with a source emitting at least 2,000 tpy.
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Some of these commenters’ recommendations would defeat the purpose of the provision for 

discretionary addition of areas to the list of areas required to characterize air quality. To stipulate 

that additions of areas must be based on data showing emissions exceeding the threshold is 

simply an obfuscated recommendation that the rule make no provision for requiring air quality 

characterization in areas that do not have a source emitting above the primary threshold. To 

provide that only air agencies and not the EPA may identify such areas as requiring air quality 

characterization, or to provide that such areas may be added only if both the air agency and the 

EPA find the addition warranted, would be to provide the EPA no means to assure national 

consistency in how areas in different states with similar risks of violating the standard are 

addressed. Indeed, the EPA shares the commenter’s concern about inequity in application of air 

quality characterization requirements to areas without sources emitting at least 2,000 tpy, and the 

EPA believes that providing itself the authority to identify areas as subject to air quality 

characterization requirements is an important tool for addressing that concern. As such, the EPA 

believes that the rule is clear that both the air agency and the EPA have the discretion to add 

areas to the list of areas for which air quality is to be characterized.

The EPA believes that the discretionarily added areas, i.e. the areas that the air agencies or the 

EPA identify as warranting air quality characterization notwithstanding the absence of a source 

emitting at least 2,000 tpy, may reasonably be expected to be addressed on the same timetable as 

areas that do have sources emitting at least 2,000 tpy. Air agencies that are uncertain as to 

whether they (or the EPA) would consider an area to warrant being subject to air quality 

characterization requirements are advised to consult their EPA regional office, preferably before 

the list of subject areas is due, so that questions as to what areas are to be subject to these 

requirements are resolved early. Even in the absence of such early consultation, i.e. even if the 

applicability of air quality characterization requirements is uncertain even at the time the air 

agency submits its list of the areas it believes are to be addressed, the EPA believes that the rule 

provides adequate opportunity to resolve those questions and still assure the selection of 

appropriate air quality characterization approaches on the schedule provided in the rule. 

Section 114(a)(1) of the CAA already provides the EPA authority and discretion to require 

emissions sources to install, use and maintain monitoring equipment and provide other 

information as the Agency may reasonably require, even in the absence of this DRR. In addition, 

the EPA had several reasons for proposing as part of this rule to reinforce state and the EPA 

discretion to require air quality characterization around additional sources beyond those with 

emissions above the proposed thresholds. Therefore, the EPA has the authority to promulgate by 

rule a requirement that appropriate areas be subject to air quality characterization requirements, 

irrespective of whether the EPA is administering a state’s SO2 NAAQS program.

9. State discretion to exclude above-threshold sources.

Comment
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Commenters (0053, 0058, 0060, 0063, 0065, 0097) suggested the EPA should provide the states 

with the discretion to determine when sources should be added or eliminated from the list based 

on other factors that are important to determining source impacts (e.g. stack height, emissions 

changes, terrain, and population). 

Commenters (0063, 0065) stated the EPA should not seek to limit states' flexibility to tailor their 

respective approaches to establishing their attainment status, because the CAA assigns states the 

primary responsibility and discretion to determine how to implement and enforce the NAAQS.

Commenter (0109) added that states should be allowed to use other available information to 

predict whether the source will be emitting at this level in the future.

Commenter (0058) stated the EPA should provide clear technical justification for any threshold 

it recommends and should simply submit this recommendation as guidance to the states, rather 

than codify thresholds in this rulemaking. [Also see III.A.6.] Commenter (0053) stated that the 

EPA should not make the threshold criteria mandatory so as to hamper a state's ability to 

reasonably tailor its approach. Commenter (0053) encouraged the EPA to provide flexibility for 

states to deviate from the EPA's proposed thresholds if a state shows that they are unnecessarily 

expansive and/ or greatly in excess of available monitoring resources for states that elect 

monitoring as the best and most accurate approach. Commenter (0053) stated, for example, while 

the EPA indicates that states are not required to use modeling, the EPA also acknowledges that 

states may have limited resources to expand monitoring networks, leading to a logical conclusion 

that if resources are inadequate to expand monitoring networks to cover all areas as required by 

mandatory thresholds, the state will be forced to utilize modeling even if that approach is neither 

desired nor necessary.

Commenter (0060) stated the rule should not take an "all in" or "all out" approach based on a 

simple analysis of one year's emissions or even a 3-year average of emissions alone. Commenter 

(0060) stated that the EPA seems to allow, or consider, the potential addition of non-thresholdmeeting sources but does not recognize there may be instances where the air agencies knowledge 

and judgment warrants exclusion of threshold triggering sources. Commenter (0060) stated that

air agencies should be able to take into consideration operational changes during the 3-year 

period to determine if a different methodology is appropriate for determining if a source should 

be a part of the analysis. Commenter (0060) provided the following additional points.

For example, if the period 2010 to 2012 is used and the 3-year average emissions triggers 

the analysis requirement for a source but in 2011 the source installed flue gas 

desulfurization (FGD) controls and an analysis of normal post-FGD emissions does not 

trigger the analysis threshold, an air agency should be able to make the determination that 

the installation of an FGD and the resulting post-FGD emissions do not warrant inclusion 

of the source in the analysis. 

Likewise, if a source's emissions had period(s) of decline during this 3-year period that

resulted in exclusion from the analysis yet historical emissions showed higher levels and 

the decline was not due to an SO2 emission reduction strategy, the air agency should have 
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flexibility to include them in the analysis. 

Commenter (0060) stated that six of Ohio's 10 well-controlled EGU facilities would remain 

subject to this draft rule under Option 3, many of which have only very recently installed their 

state of the art controls for SO2. Commenter (0060) was concerned regarding spending time and 

resources to monitor and/or model such well controlled sources, many of which likely could not 

produce further appreciable reductions short of switching to natural gas.

Commenter (0097) stated that the EPA should evaluate the use of resources for determining SO2

compliance in remote areas that in past efforts have shown modeling and monitoring challenges

and stated it may make sense to allow the use of unclassifiable designations in certain extreme 

situations.

Response: 

As with many other programs under the CAA, the EPA is following an approach in which states 

have substantial flexibility to tailor their approach according to their preference and their 

particular circumstances but in which EPA has an oversight role to assure reasonable national 

consistency. The EPA is promulgating a rule that provides express authority to states and other 

air agencies to add sources to the list that may not meet the promulgated threshold and yet appear 

to warrant air quality characterization, though the EPA intends to review the intended additions 

to assure that these additions do not create serious national inconsistencies in the sources being 

addressed by this rule. The EPA is not offering the discretion to exclude sources that meet the 

threshold for listing but the state judges not to warrant air quality characterization. Although the 

EPA acknowledges that some sources targeted for characterization under this rule may not, in 

fact, turn out to be causing or contributing to NAAQS violations (just as EPA monitors air 

quality in some areas that are not violating a given NAAQS), it is nevertheless important for the 

rule to establish nationally-consistent criteria for the minimum characterization requirements. 

This document has previously discussed the challenges associated with devising and 

implementing a hypothetical perfect test that would not require characterization of a single nonviolating source. Further, the EPA expects that a rule allowing air agencies to exempt sources 

that meet the threshold by applying some more general criteria short of such a test would result 

in a wide variety of rationales for excluding sources, which would be difficult to administer and 

ensure nationally consistent application. . Thus, EPA has chosen to retain the approach in which, 

at a minimum, sources that meet the threshold must be listed. By this means, the listing serves to 

identify priority sources that warrant review as to whether the surrounding area is attaining the 

standard. The commenter noting the possible existence of sources exceeding the threshold but 

not warranting air quality characterization does not identify any types of sources exceeding the 

threshold that would not warrant characterization of nearby air quality. 

Although under this approach any source that exceeds the emission threshold warrants analysis, 

the EPA notes that sources that are less likely to cause violations may well require less effort; for 

example, the modeling TAD explains that if screening modeling, providing a conservative 
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assessment of ambient concentrations, shows the area to be attaining the standard, then more 

resource-intensive modeling would not be necessary. In addition, the rule provides various 

options for addressing priority sources, including not just monitoring or modeling but also 

avoiding air quality characterization requirements by complying with an enforceable emission 

limit below the threshold.

The EPA believes that once a source is identified as a priority source, the source should remain 

listed, and no further efforts should be undertaken using later emissions data to reassess the list. 

Instead, other opportunities are available to consider emission reductions that occur after the list 

is established. First, if a source reduces its emissions below the threshold of this rule (sufficient 

to be judged to warrant not triggering the air quality characterization requirement), and this 

emission reduction is in compliance with a federally enforceable emission limit, the air quality 

characterization requirements do not apply. Similarly, if suitable modeling demonstrates that 

such a limit provides for attainment, then no further air quality characterization is required. 

Second, if air quality characterization shows an area to be violating the standard but then 

implements emission reductions that can be shown to provide for attainment, then these emission 

reductions (consistent with a suitable emission limit) would provide the key element of a 

nonattainment plan for the area.

One commenter expresses concern that resource constraints may force some states to use 

modeling even if use of modeling is “neither desired nor necessary.” The EPA shares the 

commenter’s regret that resources are constrained. Nevertheless, the EPA has identified in this 

rule (with rationale described previously) an approach for prioritizing sources for which 

characterization is desired and necessary to fulfill the CAA mandate of providing for attainment 

of the air quality standards. The EPA aims not to promulgate a rule that requires more resources 

for characterization than are available, and the EPA agrees with stakeholders that have urged the 

EPA to offer a choice between modeling and monitoring as a means to characterize air quality. 

Some commenters expressed concern that use of three years of data would improperly address a 

source that toward the end of those three years installs emission controls. These concerns are 

addressed in part by revision of the rule to use the most recent year of data. These concerns are 

further addressed by provisions in the final rule that a source may be exempted from air quality 

characterization requirements if the source becomes subject to and complies with a limit by 

January 2017 that requires the source to emit less than 2,000 tpy (provided the reduced emission 

level does not still warrant air quality characterization of the area).

The recommendation to apply unclassifiable designations in selected cases appears to suggest 

that the areas should be designated unclassifiable permanently. The EPA is not regulating how it 

will promulgate future designations. Therefore, the EPA is requiring all areas with priority 

sources to provide for air quality characterization, irrespective of the remoteness of the area.

IV. Data Requirements and Program Implementation Timeline
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A. Comments on the overall timeline

1. The proposed timeline may be reasonable

Comment:

Commenter (0060) stated that the feasibility of the schedule will be dependent upon the 

threshold option selected by the EPA. Commenter (0085) supported the timeline that the EPA

has proposed as long as the EPA finalizes this rule by late 2014. Commenter (0085) stated that, if 

the EPA’s finalization of this rule is delayed, the timeline should be adjusted by as many weeks 

or months as the delay in finalizing the rule.

Commenters (0043, 0058, 0064, 0071, 0085, 0091, 0101) agreed that the EPA’s proposed time 

appears reasonable for the acquiring of the data by either modeling or monitoring, for evaluation 

of the submitted data, for issuing designations and submittal of any SIPs.

Commenter (0043) recommended that the EPA include an unclassifiable designation in the 

August and December 2017 timeline for states collecting monitoring data and stated this will 

prevent the EPA from getting sued for not designating these areas.

Commenter (0079) urged the EPA to acknowledge the challenges that certain regulatory 

deadlines would impose on the regulated community when triggered by whether modeling or 

traditional monitoring is used. Commenter (0079) stated, for example, an air agency that chooses 

a modeling scenario would accelerate nonattainment designations to 2017-2018, adding to the 

burdens of regulatory compliance, which undermines business certainty. 

Commenter (0088) urged the EPA to consider whether adding milestones to the implementation 

schedule for the SO2 Data Rule would be beneficial. Commenter (0088) stated such milestones 

would not have hard regulatory consequences, but could be fashioned to provide assistance to 

state agencies in implementing monitoring programs, provide urgency for the required state 

rulemaking that may be necessary for their implementation, and would provide feedback to the 

EPA and to other stakeholders on the due diligence and good faith of state agencies to implement 

monitoring.

Response:

The EPA recognizes the logistical and financial challenges that were identified by commenters

with respect to the timeline. In response, the final rule contains changes to provide additional 

time for air agencies to determine whether to use modeling or monitoring to characterize air 

quality near their affected sources. However, the final rule retains the proposed deadlines for 

commencing monitoring or providing modeling. The Agency acknowledges that these deadlines 

do not provide as much time as some commenters would prefer; however, the EPA believes that 

these deadlines can be achieved with the appropriate planning, coordination, and program 

implementation by air agencies. In addition, the EPA does not believe that additional milestones 
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are required given the time period associated with air agencies meeting the requirements, given 

that the schedule provides a clear indication of when actions should be taken to meet the 

requirements of the rule. The EPA notes, however, that if air agencies conclude that the timeline 

and resource burdens associated with installing and conducting improved monitoring are less 

feasible for particular areas, they may instead choose the modeling approach, which is generally 

less expensive and can be performed more expeditiously than monitoring, to characterize air 

quality. Alternatively, in some cases the source owner and the air agency may be able to 

establish by January 2017 a federally enforceable requirement limiting emissions to less than 

2,000 tpy, with the result that further modeling or monitoring in that area would not be required 

under the rule unless air agencies or EPA Regional Administrators conclude it is otherwise 

warranted. Because the purpose of this rule is to obtain improved air quality information in an 

efficient manner in order that these data may be used in future actions (such as area designations, 

redesignations, or other actions designed to ensure attainment of the 2010 SO2 NAAQS) to 

protect the public from the short-term health effects associated with exposure to SO2

concentrations that exceed the NAAQS, the EPA believes it would not be appropriate to further 

extend the timelines for air quality characterization in the rule. 

The EPA believes that any further delay in air quality characterization around sources identified 

as a result of this rulemaking would delay the implementation of the SO2 NAAQS and therefore 

would impede public health protection in areas that in the future show violations of the standard 

based on the data to be gathered under the rule. The EPA also believes that any significant delays 

in monitors becoming operational past the date of January 1, 2017, will certainly delay the 

potential for monitoring data to be used to inform actions that depend upon ambient 

concentration assessments, possibly past calendar year 2021. Finally, the EPA notes that under 

the terms of the March 2015 CD, in order to avoid the EPA being required to designate an area 

by December 31, 2017, an air agency will need to have installed and begun operating the new 

SO2 monitoring system no later than January 1, 2017. 

The Agency believes that it is very important to maintain the proposed timetable for conducting 

modeling and installing monitoring sites because of the need for these new data to be available to 

support future determinations concerning the attainment status of areas. The EPA encourages 

each air agency to engage in early dialogue with the appropriate EPA Regional Office and with 

the identified applicable facilities in order to meet the requirements of the rule. In particular, in 

light of the reality of the sometimes complex process of identifying potential monitoring 

locations, securing funding, and installing an appropriate number of new sites, if an air agency is 

considering the monitoring approach for one or more areas, early coordination should improve 

the air agency’s potential for success in meeting the timing and requirements of the rule.

The final rule retains the January 15, 2016, date for submittal of a list of sources, because the 

EPA expects that this information is relatively straightforward to obtain, and it is beneficial for 

planning purposes to have this list available as soon as possible. However, as mentioned 

previously, in light of comments, the EPA is promulgating a schedule that provides an additional 

six months for the air agency to specify how it plans to address the area around each listed 

source. 
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The EPA is promulgating a schedule in which July 1, 2016, is the deadline for selecting among

the monitoring approach, the modeling approach, or establishing source emission requirements. 

If the air agency selects the monitoring approach for a source area, it must also include in the 

annual monitoring plan (also due by July 1, 2016) information about any new monitoring sites it 

will establish by January 1, 2017. If the air agency selects the modeling approach for a source 

area, it must also submit a modeling protocol at that time. If the air agency chooses the option of 

establishing an enforceable source limit or limits as an alternative to air quality characterization, 

it must also at that time provide a description of the planned emission limitation, including such 

information as emission rate, averaging time, and expected legal mechanism for making the 

limitation federally enforceable. To suffice as an alternative to the characterization requirement, 

the emission requirements or limits would need to be adopted by the air agency, made federally 

enforceable, and require compliance by January 13. 2017.

Finally, in this rulemaking we are not addressing comments received on the proposed rule 

concerning the designation process because those issues are beyond the scope of this rulemaking.

In addition, given that the court has already provided a decision concerning whether the issue of 

the time period for EPA to promulgate designations in the March 2015 CD, the time period for 

entering a law suit against EPA on the issue has passed.

2. The proposed timeline is not achievable

Comment:

Commenters (0045, 0088) recommended an extension of at least one year on all the proposed 

actions listed in the implementation timeline.

Commenter (0051) stated the proposed timeline is flawed for multiple reasons and is therefore 

not achievable. Commenter (0051) provided the following additional points.

First, critical elements (such as criteria for characterizing sources, emissions thresholds 

for monitoring of sources, and proximity to population) are not anticipated to be provided 

to states until the DRR is finalized in late 2014, just two years before completion of either 

modeling or monitor deployments are required.

Second, the timeline is not sufficient to allow for the lengthy process of funding 

procurement through the legislative process. Texas has biennial legislative sessions. The 

Texas legislature meets in January of odd numbered years for 140 days. Appropriations 

for additional funding can only be requested during this time period and are granted for 

the following biennium. If adequate funding resources were allocated for this purpose, 

those funds would not be disbursed to state agencies earlier than the biennium beginning 

in September 2015, leaving only 16 months to deploy monitors before the deadline. If the 

TCEQ were to wait to request additional funding until adequate details are available to 

provide an accurate estimate of monitoring costs (i.e., after the DRR is final and the 

monitoring network plan approved), the earliest funds could be appropriated to the TCEQ 
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would be September 2017.

Third, numerous monitoring siting challenges are expected, particularly in rural areas 

near larger SO2 sources. These challenges would include proximity to electricity, 

prohibitive terrain, property owner agreement, and accessibility. The TCEQ recently 

experienced similar challenges and implementation delays in locating an area-wide SO2

monitor near a rural source in the Amarillo area. Due to logistical issues, it took over one 

year to deploy the required SO2 monitor, even after numerous reconnaissance trips to the 

area. Based on this and similar experiences, at least two to three years will be required to 

perform the required number of site visits, propose monitor locations, provide public 

notice through the annual monitoring network plan, secure the monitoring locations, 

prepare the new site, install the monitors, and perform system tests.

Fourth, the EPA has not proposed assurances or legal deadlines for responses or 

approvals of the state monitoring network plans that are necessary for states to proceed 

with deployment of SO2 monitors. (0051)

Commenter (0103) stated that, at current levels of funding from the EPA, this schedule is not 

achievable. Commenter (0103) provided the following additional points.

EPA must provide additional funding to implement this rule. States will begin incurring 

significant costs to perform modeling to site monitors beginning in the 2015 federal fiscal 

year. Funding will also be needed after designations to develop control strategies, 

contingency measures, maintenance area plans, and provide for on-going monitoring of 

progress towards reaching attainment. This could result in numerous SO2 nonattainment 

areas needing to develop nonattainment SIPs on an 18 month schedule. 

Commenters (0050, 0109) stated the timeframes in the DRR must provide states with greater 

flexibility to ensure states are able to properly characterize current ambient air quality. 

Commenter (0050) further believed the EPA should include general timeframes for compliance 

while allowing for variances on a case-by-case basis. Commenter (0109) stated the timeline fails 

to account for unavoidable delays associated with physical siting and startup of numerous 

monitors and the EPA’s monitoring network plan pre-approval process. Commenter (0109) 

provided the following additional comments.

The proposed deadlines fail to provide sufficient flexibility to address the highly complex 

and unique circumstances states will face when simultaneously implementing either 

approach on a case-by-case basis at numerous locations across their jurisdiction. 

Significant data gathering and stakeholder involvement will be required to arrive at 

appropriate plans for making area designations. Additionally, states may encounter 

unavoidable delays despite their good faith efforts and the Proposed Rule offers no relief 

in those situations where a reasonable extension of time is necessary. To avoid shortcircuiting state efforts to implement their primary designation responsibility, the EPA

must allow adequate time for states to identify and implement their preferred SO2 data 
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options and provide express regulatory relief when unexpected delays beyond the states' 

control are encountered. (0109)

Commenter (0090) stated that the proposed timeline leads them to conclude that while states 

would be acting responsibly to defer the purchase of monitors, along with acquiring access and 

use of monitoring sites until the EPA has approved the monitoring plan, the risk of being 

"moved" to the modeling pathway is high. Commenter (0090) asserted the EPA does not propose 

any deadline for approving state plans and states have no control of the approval process once 

plans are submitted to the EPA. Commenter (0090) believed the EPA's failure to appropriately 

use its discretion in either designating areas as unclassifiable or allowing a reasonable amount of 

time for plan development and approval and subsequent implementation of the plan appears 

arbitrary and lacks merit.

Response: 

See EPA’s response to the above comment titled “The proposed timeline may be reasonable.” In 

addition, the EPA believes that routinely available funding, supplemented in some cases by third

party funding of monitoring, in most cases should be adequate for air agencies to take the 

necessary steps as defined in this rule for characterizing air quality at priority areas within their 

jurisdictions. States have been conducting modeling, operating monitoring sites, and performing 

related activities for several decades, with assorted shifts in priorities involving increases in some 

program areas and decreases in other program areas, and the commenters provide no evidence 

that the efforts required under this rule are outside the capacity of air agencies. Several of these 

comments are noting the expense of monitoring. However, this rule provides air agencies a 

choice of characterizing air quality through either monitoring or modeling, and so the EPA is 

giving air agencies the flexibility to weigh whether the additional expense of monitoring is a 

sufficient disadvantage (possibly to the point that monitoring could be considered infeasible) that 

makes modeling a preferred option. Similarly, if air agencies find that siting or commencing 

operation of a monitor is likely to be unusually difficult, the rule provides them the flexibility to 

weigh those disadvantages in selecting an approach for characterizing air quality in the area, just 

as states can also weigh the challenges of conducting reliable modeling (e.g. from complex 

terrain) in making this selection. 

The EPA understands the challenges identified with the Texas funding process as it relates to

unusually expensive planning efforts requiring supplemental funding that are not foreseen long 

in advance. The EPA considered such challenges when promulgating a rule with sufficient 

flexibility that should allow Texas to comply with this rule in spite of such challenges.

The EPA recognizes that unforeseen events can cause delays in commencing new monitoring 

sites, but the EPA also recognizes that advance planning can minimize the risk of problems 

arising. Aside from routine contingency planning, air agencies may be well advised to 

commence monitoring somewhat early, for example in December 2016, to work out any 

implementation issues to increase the likelihood that the monitors are operating smoothly on 

January 1, 2017. (This is especially important in light of the potential ramifications that a delay 
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in the monitor start date would have under the court order for when designations are to occur.)

Similarly, air agencies are well advised to consult early with their regional offices on any issues 

regarding monitor siting that arise, to facilitate EPA’s to complete its formal review in timely 

fashion as provided in 40 CFR 58.10. The EPA has clarified elsewhere that an area that fails to 

commence timely monitoring is not “moved” to the modeling pathway, but other consequences 

of delays in monitoring warrant taking appropriate steps to minimize the risk that monitoring will 

begin late.

3. Monitoring and modeling timelines should be the same

Comment:

Commenter (0103) stated that, to avoid the perception that the states choice to model or monitor 

to designate is arbitrary, the EPA should make the designation timeline the same for the 

modeling and monitoring pathways in the final rule. Commenter (0103) provided the following 

additional points.

These provisions of the Proposed Rule place states in an untenable situation. If the state 

chooses to designate a facility with monitoring data instead of modeling data, this will 

allow violations of ambient air standards for three more years relative to the modeling 

pathway. If, however, the state chooses to designate a facility with modeling data instead 

of monitoring data, this will allow the facility three fewer years before designation to 

design and adapt to new air pollution control plans. 

A revised implementation schedule to facilitate such an approach would have the EPA

approve modeling protocols in early 2015, and states would include the modeling results 

in the 2015 network plan. If resources are available for monitor deployment, after a year 

of investigating potential sites for available power, compliance with siting criteria and 

availability of a property lease agreement, states would submit a list of proposed 

monitoring locations in the 2016 network plan. Any sources subject to the rule that would 

not be scheduled to receive a monitor in the 2016 plan would be remodeled with updated 

meteorological and emissions data after three years. The designation recommendations 

resulting from the revised model runs for the source areas that were not monitored would 

be submitted with the designation recommendations from the source areas that were 

monitored. 

Synchronizing the modeling and monitor designations timelines would allow states to 

base all their designations, whether modeled or monitored, on partially shared 

meteorological conditions and more recent regulatory compliance decisions. In the final 

rule, implementing a single designation timeline provides additional benefits by 

eliminating discontinuities in schedules if a planned monitor is not operational by Jan 1, 

2017, as such locations could simply convert to the modeling pathway.
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Response: 

The deadlines in the final DRR reflect the EPA’s intention to characterize air quality in 

prioritized areas as soon as can reasonably be required. Since the collection of three years of new 

monitoring data by definition requires three years following commencement of the monitoring, 

and modeling can be completed much earlier, a schedule which “synchronized” the completion 

of monitoring and modeling efforts would inherently impose significant delays in characterizing 

areas being addressed by modeling. Therefore, the EPA finds that providing a schedule for 

modeling that is made to match the schedule for obtaining three years of new monitoring data 

would unduly delay the efforts to characterize air quality in modeled areas, which in turn would 

unduly delay any efforts to solve any air quality problems that this modeling might identify.

B. Requirement to submit list of SO2 sources and election of 

monitoring or modeling by January 15, 2016

1. List by most recent year of emissions

a. General

Comment:

Commenters (0035, 0050, 0051, 0069, 0082, 0106, and 0107) generally supported the approach 

that the basis for the emissions to be compared to the threshold be the latest one-year of SO2

emissions data. Commenter (0106) stated using the most recent year of data ensures that any 

recent emissions reductions that have occurred will be properly taken into consideration. 

Commenter (0068) stated that using the most recent year as a snapshot may fail to capture 

sources that simply have a low year, but normally do emit at higher levels. Commenter (0068)

recommended that the EPA require that facilities only be excluded under the threshold if, in prior 

years, the facilities had similar low total emissions below the limit.

Response: 

The EPA continues to believe that the most appropriate generally applicable basis for 

determining applicability of the air quality characterization requirements for the purposes of this 

rule is to use the most recent available year of emissions data for a stationary source as reported 

to the EPA in accordance with any existing regulatory requirement. As we have previously 

explained, SO2 emissions are trending downward, due to numerous national and regional 

requirements that have recently been adopted and are taking effect. In prioritizing sources to 

characterize, the Agency believes it is reasonable to account for this trend by basing applicability 

for this DRR on the most recent available year of emissions. Nevertheless, for sources with low 

emissions in the most recent year but higher emissions in prior years and prospects for higher 

emissions in future years, the rule provides the option to list such sources as triggering air quality 

characterization requirements, an option which may be warranted in such cases. 
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b. Data sources

Comment:

Commenter (0051) supported allowing states the option of using either appropriately reviewed 

and publicly available emissions inventory data or the most recent emissions inventory data 

published by the EPA. Commenter (0107) requested that the EPA allow states to use the most 

recent annual emissions inventory data certified by facilities and reported to the EPA under the 

Air Emissions Reporting Rule (AERR). Commenter (0051) asked the EPA to clarify whether 

SO2 monitoring data submitted to the EPA's Clean Air Markets Division could also be used to 

identify applicable sources as this may allow for use of more up-to-date information. Commenter 

(0107) requested that the EPA avoid the exclusive use of the NEI to identify subject sources as it 

was required in the implementation of the 2008 lead standard. 

Response: 

By January 15, 2016, the EPA would expect that 2014 data will be available for all EGU sources,

and 2015 data may be available for many EGUs in accordance with the requirements of the acid 

rain program and other emission trading programs that require data certification soon after the 

end of the year. These sources report hourly emissions data to the EPA on a quarterly basis. 

Emissions data for large SO2 sources also would be available from annual reporting required for 

the AERR. Every 3 years (i.e., 2011, 2014, 2017 and so on), air agencies must submit to EPA 

emissions data for SO2 sources with the potential to emit more than 100 tpy. In other years, the 

AERR requires states to report emissions data for SO2 sources with the potential to emit more 

than 2,500 tpy. These annual reports under the AERR are due 12 months after the end of the 

emissions year. Thus, the EPA would expect that in January 2016, states would have emissions 

data for calendar year 2014 available for non-EGU sources over 100 tons potential to emit. 

Emissions reporting requirements for the acid rain and AERR programs would be expected to 

cover the vast majority, if not all, of the sources subject to the SO2 DRR. 

By considering the most recent emissions data, the air agency and the EPA will be able to take 

into account any recent emissions increases or decreases that would cause a source to be subject 

to the requirements in this rule or not. Although identifying sources based on the most recent 

year of emissions is a reasonable basis for prioritizing limited modeling and monitoring 

resources for characterizing current air quality, the EPA recognizes the concern of some 

commenters that there may be sources that in the most recent year have emissions that are lower 

than normal and are not representative of normal operations. In these cases, i.e., where recent 

emissions are below 2,000 tpy but no controls have been installed and past representative 

emission levels are typically above 2,000 tpy, the state and the EPA should consider using their 

discretion to require additional air quality characterization near such sources. 

c. Expected availability of data for specific years
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Comment:

Commenters (0063, 0065) stated that the EPA should specify that 2013 or 2014 reported SO2

emissions data will be used to identify sources that require new air quality data. Commenter

(0051) stated the 2014 point source emissions inventory will be reviewed by states and reported 

to the EPA by December 31, 2015, so it will be the most recent publicly available inventory 

before the January 15, 2016 deadline. 

Commenters (0069, 0082) urged the EPA to clarify that the Agency will take into account the 

most recent available emissions data when determining whether a source would be subject to the 

requirements of any final rule. Commenter s (0069, 0082) stated that, while the EPA expects 

2014 data to be available for electricity generating units (EGUs) and 2013 data to be available 

for non-EGU sources by January 2016, in some instances, 2014 data may be available for nonEGUs. 

Commenters (0035, 0069) recommended that states should be given the opportunity to revise the 

list of applicable sources using 2015 data should such data become available in a timely manner. 

Commenter (0035) stated that, by proposing a later date in 2016 that provides adequate time to 

receive and review 2015 emissions data, facilities would be incentivized to reduce emissions 

below finalized thresholds in 2015 rather than requiring potentially time consuming and 

expensive modeling analyses based on 2014 emissions data. Commenter (0035) stated the 

current timeframes in the proposal will likely result in the use of 2014 emissions data to be used 

to determine source applicability which provides essentially no time for sources to take action to

reduce emissions to avoid a modeling requirement. Commenter (0069) stated that, within a few 

months after the January 15, 2016, deadline, 2015 EGU data will be available from the EPA's 

Clean Air Markets Division and non-EGUs may be able to submit 2015 sulfur dioxide emissions 

data to states in early 2015.

Response: 

The most appropriate generally applicable basis for determining applicability of the air quality 

characterization requirements is the most recent available year of emissions data for a stationary 

source as reported to the EPA in accordance with any existing regulatory requirement. As we 

have previously explained, SO2 emissions are trending downward, due to numerous national and 

regional requirements that have recently been adopted and are taking effect. The Agency 

believes it is reasonable to account for this trend by basing applicability for this DRR on the 

most recent available year of emissions. 

By January 15, 2016, the EPA would expect that 2014 data will be available for all EGU sources,

and 2015 data may be available for many EGUs in accordance with the requirements of the acid 

rain program and other emission trading programs that require data certification soon after the 

end of the year. These sources report hourly emissions data to the EPA on a quarterly basis. 

Emissions data for large SO2 sources also would be available from annual reporting required for 

the AERR. Every 3 years (i.e., 2011, 2014, 2017 and so on), air agencies must submit to EPA 

emissions data for SO2 sources with the potential to emit more than 100 tpy. In other years, the 
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AERR requires states to report emissions data for SO2 sources with the potential to emit more 

than 2,500 tpy. These annual reports under the AERR are due 12 months after the end of the 

emissions year. Thus, the EPA would expect that in January 2016, states would have emissions 

data for calendar year 2014 available for non-EGU sources over 100 tons potential to emit. 

Emissions reporting requirements for the acid rain and AERR programs would be expected to 

cover the vast majority, if not all, of the sources subject to the SO2 DRR. 

By considering the most recent emissions data, the air agency and the EPA will be able to take 

into account any recent emissions increases or decreases that would cause a source to be subject 

to the requirements in this rule or not. Although identifying sources based on the most recent 

year of emissions is a reasonable basis for prioritizing limited modeling and monitoring 

resources for characterizing current air quality, the EPA recognizes the concern of some 

commenters that there may be sources that in the most recent year have emissions that are lower 

than normal and are not representative of normal operations. In these cases, i.e., where recent 

emissions are below 2,000 tpy but no controls have been installed and past representative 

emission levels are typically above 2,000 tpy, the state and the EPA should consider using their 

discretion to require additional air quality characterization near such sources. 

The EPA also recognizes the concern about sources for which the most recent year’s emissions 

are unrepresentatively high, i.e., that some sources may have recent year emissions above 2,000 

tpy but normally emit below that level. Given the trends in emissions, the EPA believes that 

situation will be relatively rare. Moreover, the existence of such sources does not negate the 

general conclusion that recent emissions data are an appropriate means for targeting limited 

modeling and monitoring resources for characterizing current air quality. 

The EPA does not believe that this rule should provide ongoing opportunities for revising the list 

of prioritized sources. The EPA believes that the list should be established toward the beginning 

of the process, with one occasion for air agencies and the EPA to resolve any issues (such as 

those regarding areas that do not have any sources exceeding 2,000 tpy but may nevertheless 

warrant air quality characterization and those regarding cases where a source has low emissions 

in the most recent year but is expected normally to have relatively high emissions). If a source 

significantly reduces its emissions after being listed, the EPA believes that a more efficient and 

equitable approach is to continue to treat the source as a listed source but provide consideration 

of the emission reduction in other steps under the rule, such as adopting an emission limit 

requiring emissions by January 1, 2017 to be below 2,000 tpy or modeling the reduced emission 

level. (Monitoring will of course reflect the impacts of any emission reductions that have 

occurred.)

2. List by average emissions over a three-year period

Comment:

Commenter (0036) recommended that applicability be based on a source’s emissions over a 

three-year period, specifically, that EGU annual emissions over the 2012 to 2014 timeframe 
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should be examined, and any source whose emissions exceeded the threshold must comply with 

the SO2 implementation rule. Commenter (0036) stated that this approach would allow states and 

the EPA to subject sources with the potential for large emission increases to an ongoing 

attainment verification process in the future; for example, if SO2 emissions were to rise 

dramatically due to a change in the relative cost of coal versus natural gas, the impact of the 

increased emissions from this source could be evaluated for potential NAAQS violations.

Commenter (0060) stated a 3-year average of emissions should be used to determine 

applicability under the emissions threshold. Commenter (0060) stated that, similarly, compliance 

via monitoring with the 1-hour SO2 standard is based on a 3-year average of the annual 99th 

percentile of 1-hour daily maximum concentrations. 

Response: 

In EPA’s judgement the most appropriate generally applicable basis for determining applicability 

of the air quality characterization requirements is the most recent available year of emissions 

data for a stationary source as reported to the EPA in accordance with any existing regulatory 

requirements. As we have previously explained, SO2 emissions are trending downward, due to 

numerous national and regional requirements that have recently been adopted and are taking 

effect. The Agency believes it is reasonable to account for this trend by basing applicability for 

this DRR on the most recent available year of emissions. By basing applicability on the most 

recent available year of data, the rule appropriately excludes sources that may have had relatively 

high emissions on a three-year average basis but have installed controls and are no longer 

expected to exceed the emission threshold. Using the most recent year of data provides for 

prioritizing sources that, continuing through the most recent year, have relatively high emissions.

The EPA recognizes that use of the most recent year of data brings the risk of failing to prioritize 

a source that generally has relatively high emissions, has not installed controls or otherwise acted 

to reduce emissions, but has unrepresentatively low emissions in the year represented by the 

most recent data. As discussed in section IV.C.2.c of the preamble, the EPA believes that such 

situations can and should be addressed by the air agency or the EPA using their discretion to 

require additional air quality characterization near such sources, as appropriate.

As stated previously, by January 15, 2016, the EPA would expect that 2014 data will be 

available for all EGU sources, and 2015 data may be available for many EGUs in accordance 

with the requirements of the acid rain program and other emission trading programs that require 

data certification soon after the end of the year. These sources report hourly emissions data to the 

EPA on a quarterly basis. Emissions data for large SO2 sources also would be available from 

annual reporting required for the AERR. Every 3 years (i.e., 2011, 2014, 2017 and so on), air 

agencies must submit to EPA emissions data for SO2 sources with the potential to emit more than 

100 tpy. In other years, the AERR requires states to report emissions data for SO2 sources with 

the potential to emit more than 2,500 tpy. These annual reports under the AERR are due 12 

months after the end of the emissions year. Thus, the EPA would expect that in January 2016, 

states would have emissions data for calendar year 2014 available for non-EGU sources over 100 
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tons potential to emit. Emissions reporting requirements for the acid rain and AERR programs 

would be expected to cover the vast majority, if not all, of the sources subject to the SO2 DRR.

By considering the most recent emissions data, the air agency and the EPA will be able to take 

into account any recent emissions increases or decreases that would cause a source to be subject 

to the requirements in this rule or not. Although identifying sources based on the most recent 

year of emissions is a reasonable basis for prioritizing limited modeling and monitoring 

resources for characterizing current air quality, the EPA recognizes the concern of some 

commenters that there may be sources that in the most recent year have emissions that are lower 

than normal and are not representative of normal operations. In these cases, i.e., where recent 

emissions are below 2,000 tpy but no controls have been installed and past representative 

emission levels are typically above 2,000 tpy, the state and the EPA should consider using their 

discretion to require additional air quality characterization near such sources. 

The EPA also recognizes the concern about sources for which the most recent year’s emissions 

are unrepresentatively high, i.e., that some sources may have recent year emissions above 2,000 

tpy but normally emit below that level. Given the trends in emissions, the EPA believes that 

situation will be relatively rare. Moreover, the existence of such sources does not negate the 

general conclusion that recent emissions data are an appropriate means for targeting limited 

modeling and monitoring resources for characterizing current air quality. 

The EPA recognizes the merits of allowing for use of 2015 emission data to determine 

applicability of air quality characterization requirements. At the same time, the EPA does not 

believe that it would be appropriate to delay any portion of the schedule for implementation of 

this rule to provide more time to obtain 2015 data. The EPA believes that identification of 

sources to trigger air quality characterization requirements is an important first step that must be 

completed early enough to provide enough time to review the options for characterizing nearby 

air quality and then to conduct the necessary modeling or to commence operation of any new 

monitors needed to provide the air quality characterization.

The EPA believes that a rule that prioritizes resources based on the most recent year’s data is 

more appropriate for a broader range of circumstances. The EPA notes, however, that after a 

source is initially identified, the air quality characterization requirements require air agencies to 

provide at least 3 years of monitoring or modeling data. The availability of such data will 

provide the opportunity to give appropriate consideration to representative emissions when using 

such data, as appropriate to the specific use.

3. Unrepresentative emissions

Comment:

Commenter (0036) recommended that, if a particular year’s emissions were considered 

unrepresentative for a source, then that source could request that the year not be used for the 

applicability test. 
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Commenter (0071), in light of the variability in annual emissions for some sources, 

recommended the option to allow the use of the following: (1) a different year other than the

prescribed year [e.g., the highest annual emissions rate based on the most recent 3 to 5 years of

emissions data] to assess whether the final emission threshold(s) is triggered, (2) an annual

emissions rate based on an average of multiple years [e.g. an average of the highest two years' of

emissions over the past 10 years], or (3) an adjustment that accounts for multiple annual peaks

and/or seasonal peaks that better characterizes emissions from source operations that are highly

variable.

Response:

The EPA recognizes the concern about sources for which the most recent year’s emissions are 

unrepresentatively high, i.e., that some sources may have recent year emissions above 2,000 tpy

but normally emit below that level. Given the trends in emissions, the EPA believes that situation 

will be relatively rare. Moreover, the existence of such sources does not negate the general 

conclusion that recent emissions data are an appropriate means for targeting limited modeling 

and monitoring resources for characterizing current air quality. 

The EPA believes that a rule that prioritizes resources based on the most recent year’s data is

more appropriate for a broader range of circumstances. The EPA notes, however, that after a 

source is initially identified, the air quality characterization requirements require air agencies to 

provide at least 3 years of monitoring or modeling data. The availability of such data will 

provide the opportunity to give appropriate consideration to representative emissions when using 

such data, as appropriate to the specific use. 

4. Legal agreements or permit limits

a. Enforceable limits by January 1, 2016

Comment:

Commenters (0049, 0050, 0075, 0081, 0087, and 0110) stated the EPA should allow facilities to 

opt out of the rule by taking federally enforceable limits before January 2016. Commenters

(0075, 0087) recommended that sources which have entered into legal agreements that would 

reduce their SO2 emissions below the applicable threshold by January 2016 receive the same 

treatment as sources that are scheduled to shut down. Commenter (0049) stated that to the extent 

that electric utility sources are, by January 2016, shutting down or adding/upgrading scrubbers to 

comply with regulations such as the Mercury and Air Toxics Standards (MATS) for power 

plants, these sources should not be subject to the provisions of the DRR. 

Response: 

The EPA finds merit in the above comments that suggest that the rule should allow for similar 
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treatment for sources that become subject to a federally enforceable emission limit as would be 

provided for sources for which the state provides documentation that the source will shut down.

The EPA has revised the final rule accordingly in section 51.1203(b). However, EPA does not 

agree with commenters who suggest that sources should have until the applicable designation 

date, or attainment date for an area that is designated nonattainment, to implement controls that 

were committed to prior to January 13, 2017. Relying on commitments for emission reductions 

to occur after 2017 would not be consistent with the main focus of this rule, which is to provide 

current, updated information on priority SO2 sources to the EPA beginning in early 2017 that can 

be used to, among other things, inform future area designations (now required in December 2017 

and December 2020 per the March 2015 CD). 

As indicated above, a source would be listed for air quality characterization if its most recent 

emissions were above the 2,000 tpy threshold. However, the final rule also allows the air agency 

to meet the requirements of this rule by submitting a federally enforceable emissions limitation 

(e.g., source-specific SIP revision or minor NSR permit revision) to the EPA prior to January 13, 

2017 that requires the affected source to reduce allowable emissions at the source to an annual 

rate below the 2,000 tpy threshold level prior to January 13, 2017. By July 1, 2016, the air 

agency would be required to identify the sources on the list for which it would be using such an 

approach as an alternative to modeling or monitoring. For such a source identified on the list, if 

the affected air agency has adopted and the source has become subject to effective federally 

enforceable requirements for control measures lowering emissions below 2,000 tpy by January 

13, 2017, the air agency will generally not be required to further characterize the impacts from 

the source’s emissions solely due to its size as of January 15, 2016. 

Regarding comments specifically about MATS compliance occurring within this time window, 

the EPA is not explicitly linking these options to EGU actions pursuant to MATS, either to offer 

these options automatically to sources subject to MATS or to offer these options only to sources 

subject to MATS. Nevertheless, these options are prone to be especially useful to EGUs that are 

complying with MATS in these ways. Therefore, while the EPA is not exempting sources solely 

on the basis of an announcement of plans to shut down or on the basis of potential applicability 

of the MATS SO2 emission limit, the EPA agrees that EGUs that become subject to an 

enforceable agreement to shut down or emit less than 2,000 tpy, reflecting reductions anticipated 

or accomplished as a means of complying with MATS, warrant the exclusion provided in this 

rule from air quality characterization requirements.

Although air agencies may follow this option as an alternative to characterizing areas with 

sources that limit their emissions to below the 2,000 tpy size threshold, the EPA believes that air 

agencies and the EPA must apply judgment as to whether there are still reasons to characterize 

these areas due to other factors. As discussed above, some areas where all sources emit less than 

2,000 tpy may nevertheless warrant air quality characterization, for example because the area has 

a cluster of sources with intermediate emission levels or because the characteristics of a source 

or the area warrant it. Thus, some areas with all sources limited to below 2,000 tpy may still 

warrant air quality characterization. Therefore, the EPA urges air agencies to consult early with 

the EPA regarding areas that are under consideration for being addressed in this manner, in order 
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to develop a common understanding as to whether emission limits under consideration would 

suffice as an alternative to air quality characterization for the area. 

In EPA’s judgement, allowance for this alternative emission limit approach is not only consistent 

with the intent of this rule to prioritize resources to focus on the largest sources of SO2, but it 

also has the additional benefit of providing an incentive for early emission reductions to occur 

which will improve air quality in these areas in an expeditious manner. However, we do 

acknowledge the distinction between a formerly large source with no future emissions and a 

source with reduced but continuing emissions. The Agency does not believe it would be 

appropriate to provide that the latter source can be excluded from evaluation in all cases. It may 

be that a source with emissions newly limited to below the applicability threshold – particularly 

one with limits established just below the threshold – may warrant further characterization, just 

as a source with actual emissions below the threshold may warrant characterization in some 

instances. For example, air quality characterization would continue to be warranted in areas with 

other sources over the applicability threshold, and in areas where no single source has emissions 

over the threshold but the combined emissions of multiple sources warrant air quality 

characterization. In evaluating such cases, the air agency should account for all source emissions 

contributing to ambient concentrations in the area, including those remaining emissions from the 

source that has just reduced its levels to below the applicability threshold. For this reason, the 

rule does not automatically exempt sources with emissions limited to less than 2,000 tpy from air 

quality characterization requirements; the rule instead provides that the air agency or the EPA 

may judge that the area should continue to be required to characterize air quality notwithstanding 

the new emission limits. Air agencies are thus advised to consult with their EPA Regional Office 

before pursuing this alternative to air quality characterization for a particular source area.

b. Enforceable limits by January 1, 2017

Comment:

[Also see section V.B.3, regarding enforceable limits as modeling inputs.]

Commenters (0035, 0036, 0043, 0046, 0048, 0053, 0056, 0057, 0058, 0060, 0061, 0066, 0069, 

0075, 0081, 0084, 0096, 0097, 0114, and 0116) supported the removal of sources from the 

preliminary list based on scheduled enforceable limits in operation prior to January 1, 2017.

Commenter (0046) stated implementation of controls required by upcoming rules such as Boiler 

MACT should be taken into account in the applicability determination, not just in the modeling. 

Commenter (0066) believed that the wet flue gas desulfurization (FGD) systems installed on all 

four Monroe Power Plant units have eliminated the need for performing an updated 1-hour SO2

NAAQS analysis. Commenter (0066) stated this analysis was performed when they received an 

SO2 Best Available Control Technology permit to install, including a 1-hour SO2 NAAQS 

compliance demonstration. 
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Commenters (0056, 0058) stated it would be a waste of resources to force States to commit 

significant effort to model or monitor these facilities when they are already committed to 

substantially lowering or essentially eliminating their SO2 emissions.

Commenters (0048, 0053) stated there is no basis to distinguish between shutdowns and 

enforceable limits because in each case the source would no longer meet the criteria for 

characterization. Commenter (0046) stated sources should be able to take federally enforceable 

limits on a tpy year basis prior to January 2017 to avoid coverage under the rule as they can with 

PSD, Title V, and other programs under the CAA (i.e., a “once in, always in” approach should 

not apply).

Commenter (0081) stated that, if a source indicates (through a company public announcement, 

public utilities commission filing, CD, public legal settlement, final state or federal permit filing, 

or other similar means of communication) a significant fuel switch, partial shutdown, or process 

change which would greatly reduce SO2 emissions, then that source should also be considered 

for exclusion from the rule. Commenter (0084) stated there are a number of other regulatory 

programs with upcoming compliance deadlines that will consequently reduce SO2 sulfur dioxide 

emissions, including MACT standards, GHG rules, and transport rules. Commenter (0097) stated 

that, in addition to sources that are/will be shut down, the EPA should include units that are

switching to inherently lower SO2 emitting fuels, such as natural gas and biomass. Commenter 

(0058) stated that sources should be exempt from the DRR if, by January 2017, they are no 

longer considered a coal-fired electric utility steam generating unit as defined in the Mercury and 

Air Toxics Standard. See 40 C.F.R. 63.10042 (July 1, 2013). 

Commenters (0039, 0087) recommended the EPA extend the date from January 1, 2017 to 

January 31 or May 1, 2017 in light of upcoming rule deadlines. Commenter (0087) stated that 

since the MATS rule contains a provision and mechanism by which sources can, if needed, seek 

an additional year (beyond the initial one-year extension) to comply by way of an EPA 

Administrative Order, the EPA should extend the requirement for state agencies to confirm unit 

shutdowns under the proposed DRR from January 1, 2017 to May 1, 2017. Commenter (0039) 

recommended that the DRR exclude either monitoring or modeling evaluations for sources that 

will shut down prior to January 31, 2017 – instead of January 1, 2017 as suggested. Commenter 

(0039) asserted that the actual deadline for industrial coal-fired boilers that will be shut down as 

a result of Boiler MACT is January 31, 2017, if the source has obtained a 1-year extension from 

the typical January 1, 2016 compliance deadline. A January 31, 2017 applicability date for 

shutdown sources will correspond with potential Boiler MACT compliance extensions and 

ensure that states are not including sources that will have no impact on the modeling or 

monitoring results, according to commenter (0039). Commenter (0039) believed a similar 

provision should apply to units converting to natural gas by January 31, 2017 or achieving 

emission levels below the applicable thresholds as a result of installing emission controls by 

January 31, 2017.

Commenter (0096) recommended that the EPA revise the DRR to exempt from the list sources 

that take an enforceable emission limitation below the 2,000 tpy threshold before 2017, even if 
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reductions and applicability of the limitation are only realized within a reasonable time after 

January 17, 2017. Commenter (0096) stated this approach appears to be reasonable and in line 

with the current approach of removing sources from the list shutting down prior to January 2017. 

Commenter (0096) stated that nothing would be gained by proceeding towards designating an

area as nonattainment when the needed reductions (as demonstrated through modeling) will be in

place prior to the expected date for final designations to take effect (February 2018).

Commenter (0038) stated that requiring sources to implement controls prior to submittal of the 

required SIP, as opposed to the attainment date, would also encourage sources to make emission 

reductions while allowing sufficient time to implement these actions.

Response: 

The EPA is finalizing the proposed approach to allow a state with a source that is in operation as 

of January 15, 2016, but that will shut down permanently prior to January 13, 2017 pursuant to a 

federally enforceable mechanism (e.g., source-specific SIP revision or minor NSR permit 

revision submitted to the EPA by January 13, 2017), to avoid being subject to the requirement to 

characterize air quality in the vicinity of the source. 

As a result of comments received on the proposed rule, the EPA is clarifying how this exclusion 

would work relative to the requirement for development and submittal in January 2016 of the list 

of sources near which air quality is to be characterized. The EPA appreciates that there might be 

a source whose most recent year of actual emissions exceeds the threshold for inclusion on the 

list, but for which the state has already adopted, or will soon adopt enforceable requirements to 

shut down by January 2017. Such a source may have significant emissions during the most 

recent available year, or may even still be in operation on January 15, 2016. The EPA has 

determined that the clearest way to implement the exclusion from the air quality characterization 

requirement is to require that the air agency initially identify such a source on its list for SO2 air 

quality characterization because emissions in the previous year, which serve as the basis for 

listing under this rule, exceeded the emissions threshold. However, the final rule now includes 

language in section 51.1203(b) allowing the air agency to indicate by July 1, 2016 that it will 

provide the EPA with a federally enforceable requirement confirming that the source will be

permanently and enforceably shut down by January 13, 2017. For a source with a federally 

enforceable requirement to shut down, the air agency will have no further obligation regarding 

air quality characterization pursuant to this rulemaking. This approach accomplishes the intent of 

the proposal by implementing the approach in a more clear and straightforward manner.

Commenters on the proposed rule also suggested that, in a similar manner, an air agency should 

not be subject to the air quality characterization obligation for any source that is initially on the 

list of sources due in January 15, 2016 (based on most recent actual emissions), but that becomes 

subject to a federally enforceable requirement to limit annual SO2 emissions to below the 2,000 

tpy emissions threshold. The EPA finds merit in those comments that suggest the rule allow for 

similar treatment for sources that become subject to a federally enforceable emission limit as is 

allowed for sources for which the state can provide documentation that the source has elected to 
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shut down. The EPA has revised the final rule accordingly, and provides further discussion 

below. However, EPA does not agree with commenters who suggest that sources should have 

until the applicable designation date, or attainment date for an area that is designated 

nonattainment, to implement controls that were committed to prior to January 13, 2017. Relying 

on commitments for emission reductions to occur after 2017 would not be consistent with the 

main focus of this rule, which is to provide current, updated information on priority SO2 sources 

to the EPA beginning in early 2017 that can be used to, among other things, inform future area 

designations (now required in December 2017 and December 2020 per the March 2015 CD). 

As indicated above, a source would be listed for air quality characterization if its most recent 

emissions were above the 2,000 tpy threshold. However, the final rule also allows the air agency 

to meet the requirements of this rule by submitting a federally enforceable emissions limitation 

(e.g., source-specific SIP revision or minor NSR permit revision) to the EPA prior to January 13, 

2017 that requires the affected source to reduce allowable emissions at the source to an annual 

rate below the 2,000 tpy threshold level prior to January 13, 2017. By July 1, 2016, the air 

agency would be required to identify the sources on the list for which it would be using such an 

approach as an alternative to modeling or monitoring. For such a source identified on the list, if 

the affected air agency has adopted and the source has become subject to effective federally 

enforceable control measures lowering emissions below 2,000 tpy by January 13, 2017, the air 

agency will generally not be required to further characterize the impacts from the source’s 

emissions solely due to its size as of January 15, 2016. 

Although air agencies may follow this option as an alternative to characterizing areas with 

sources that limit their emissions to below the 2,000 tpy size threshold, the EPA believes that air 

agencies and the EPA must apply judgment as to whether there are still reasons to characterize 

these areas due to other factors. As discussed above, some areas where all sources emit less than 

2,000 tpy may nevertheless warrant air quality characterization, for example because the area has 

a cluster of sources with intermediate emission levels or because the characteristics of a source 

or the area warrant it. Thus, some areas with all sources limited to below 2,000 tpy may still 

warrant air quality characterization. Therefore, the EPA urges air agencies to consult early with 

the EPA and distinction between a formerly large source with no future emissions and a source 

with reduced but continuing emissions. The Agency does not believe it would be appropriate to 

provide that the latter source can be excluded from evaluation in all cases. It may be that a source 

with emissions newly limited to below the applicability threshold – particularly one with limits 

established just below the threshold – may warrant further characterization, just as a source with 

actual emissions below the threshold may warrant characterization in some instances. For 

example, air quality characterization would continue to be warranted in areas with other sources 

over the applicability threshold, and in areas where no single source has emissions over the 

threshold but the combined emissions of multiple sources warrant air quality characterization. In 

evaluating such cases, the air agency should account for all source emissions contributing to 

ambient concentrations in the area, including those remaining emissions from the source that has 

just reduced its levels to below the applicability threshold. For this reason, the rule does not 

automatically exempt sources with emissions limited to less than 2,000 tpy from air quality 

characterization requirements; the rule instead provides that the air agency or the EPA may judge 
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that the area should continue to be required to characterize air quality notwithstanding the new 

emission limits. Air agencies are thus advised to consult with their EPA Regional Office before 

pursuing this alternative to air quality characterization for a particular source area.

Regarding the specific comments about MATS or Boiler MACT compliance occurring after 

January 2017, the EPA believes the final rule appropriately handles these circumstances. 

Although the final rule now allows air agencies to adopt enforceable limits that keep emissions 

below 2,000 tpy for applicable sources as an alternative to characterizing them, the EPA does not 

believe that this alternative should be available where the enforceable limit does not take effect 

until after the characterization is due. As a general matter, the rule must weigh in favor of 

characterizing sources identified as priority sources unless it is clear that they are no longer 

priority sources when the characterization deadline passes. The EPA also notes that, given the 

court ordered deadlines for completing area designations, the final rule will better serve a use of 

generating information to inform those actions by maintaining the proposed schedule. The rule 

builds in appropriate relief from ongoing modeling and monitoring requirements when, due to 

reduced emissions, the sources are no longer a priority for characterization. Moreover, nothing in 

this rule prevents an air agency from providing additional relevant information after the 

characterization is due. The MATS and Boiler MACT rules raise specific timing issues, but do 

not outweigh this overarching principle of rule structure, or convince EPA to build in 

complicated rule-specific exceptions to the rule’s deadlines. Nevertheless, EPA does not expect 

the rule to lead to inappropriate outcomes for these standards.

With respect to the Boiler MACT, the EPA believes that most boilers will be in compliance with 

the rule (and therefore likely, in most cases, to have reduced emission levels to below 2,000 tpy)

by the rule’s January 31, 2016 deadline and well in advance of the DRR’s January 13, 2017

deadline for characterization. However, the EPA acknowledges that 40 CFR 63.6 provides 

generic authorization for a full year compliance date extension for sources that need the 

additional time to install controls in order to comply with the Boiler MACT limits. Therefore, a 

modest number of sources may need fully until January 31, 2017 to comply with the Boiler 

MACT, after the characterization of those sources is due (albeit only by a short time). In these 

few cases, the EPA believes that it would not be unreasonable for SO2 limits reflecting the 

Boiler MACT reductions7to take effect by January 13, a few weeks earlier than the latest 

allowable deadline under 40 CFR 63.6 for sources needing extra time to install controls, rather 

than delay the DRR characterization schedule. Alternatively, the air agency could proceed with 

characterization on schedule for the few sources where limits could not take effect by January 

13, 2017. 

With respect to MATS, a source that is not meeting the applicable requirements by April 2016 is 

out of compliance with the rule. Although the EPA has issued guidance addressing special 

 

7 Also, the EPA notes that, while the relevant limits in the Boiler MACT are limits on hydrogen 

chloride, at issue here are SO2 limits that air agencies adopt that reflect the consequences for SO2

emissions of the control strategy that the source will be implementing to meet the hydrogen 

chloride limits.
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circumstances that might arise necessitating noncompliant operation, the EPA cannot agree to 

modify this rule to accommodate noncompliance with MATS. Furthermore, if a source is still 

operating in violation of MATS for a full year past the April 16, 2016 deadline (not May 1, as 

implied by the commenter), then the EPA finds it appropriate for this rule not to exempt such a 

source from air quality characterization requirements and for the rule to require that any 

modeling of the violating source to consider emission rates that may be occurring well into 2017.

c. Enforceable limits effective by the attainment date

Comment:

Commenters (0038, 0108) stated that allowing sources to implement controls after January 1, 

2017 but before the attainment date supports the EPA's desired outcome of achieving emission 

reductions as quickly as possible. Commenters (0038, 0108) stated there is insufficient time for 

sources to take all the actions needed to implement these controls (including conducting 

modeling, determining the required reductions and control strategies, procuring capital funds, 

obtaining permits and installing equipment) under the Proposed Rule. Commenter (0038) stated 

that requiring sources to commit only to enforceable limits and a schedule for compliance by 

January 1, 2017 is reasonable and consistent with the timeframes necessary in developing the 

required modeling information. Commenter (0038) stated that, under the EPA's proposal, sources

unable to have enforceable limits in place by the January 1, 2017 deadline have little incentive to

take any action prior to the monitoring attainment deadline of 2020.

Commenters (0050, 0060, 0061, 0070, 0073, and 0087) requested that the EPA revise the 

applicability section to exclude sources scheduled to permanently shut down within a reasonable 

period. Commenter (0061) suggested a source should be exempt if it is scheduled to be 

permanently shut down shortly after the suggested 2017 deadline and proposed that as long as 

there is a federally enforceable condition in place before January 1, 2017 that will ensure 

attainment of a designated area, that the demonstration be considered complete. Commenter 

(0087) stated that without such an "up-front" exemption, agencies could be forced to conduct 

unnecessary modeling and/or sources could be subject to unnecessary monitoring-related 

expenditures, however, an "upfront" exemption would allow state agencies to focus their limited 

resources to sources that will be operational beyond the 2017 timeframe.

Commenter (0060) stated that, if there is an enforceable mechanism (e.g., legal agreement/CD) 

that ensures a facility will shut down in time to attain as "expeditiously as practicable ... or by 

February 2023," this should be sufficient for the demonstration and further air quality 

characterization should not be needed. Commenter (0061) was concerned that using actual 

emissions from the Boardman Plant's main boiler for future designation of the area would not be 

accurate due to the source's required shutdown by December 31, 2020 and the federally 

enforceable Title V operating permit which contains this requirement. Commenter (0061) stated 

that including the boiler in modeling or monitoring would not be representative of conditions 

beyond 2020 - allowable emissions from the main boiler are zero after the shutdown date. 
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Response: 

The purpose of the DRR is to obtain timely information regarding current air quality from 

sources that the EPA has determined warrant consideration in light of the limits on available 

resources. Elsewhere EPA has explained the basis for the January 2017 deadlines for states to 

take various characterization actions required by this rule (e.g., submitting modeling or 

deploying monitors), and has provided maximum opportunity for sources to limit their emissions 

to below the rule’s applicability threshold levels prior to this date. However, once this date 

passes, the EPA cannot go back in time and say that these deadlines did not apply. Some 

comments suggest that we should forego characterization requirements in 2017 based on what 

will happen in the future, but the EPA believes that the need for timely information about current 

air quality justifies the requirement to characterize these sources in the present (such as is done 

with monitoring networks), even if air quality might improve in the future. 

We do acknowledge that the DRR contains future ongoing characterization requirements that 

may become unnecessary if sources reduce their emissions sufficiently in the future, and we note 

that the rule already provides mechanisms to cease such ongoing requirements, as discussed 

elsewhere in this document. However, actions beyond January of 2017 cannot provide a basis for 

relief from the current source characterization requirements that come due in January 2017. 

Nevertheless, the EPA understands that a deadline for characterizing air quality does not address 

the various ways EPA might use the information it gets. Many of the comments above relate to 

how EPA will use information about current air quality when there is also information about 

future effective limits when EPA is making area designations or evaluating attainment plans 

should an area ultimately be designated nonattainment. Since this rulemaking does not address or 

establish designations requirements or nonattainment planning requirements, comments about 

how EPA should account for future effective limits in making those decisions are beyond the 

scope of this rule. 

5. Electricity generation reliability

Comment:

Commenters (0038, 0041, 0047, 0075, and 0108) stated the EPA should identify an alternate 

compliance path for electrical generation facilities that are required to operate for reliability 

purposes. Commenters (0047, 0075) recommended that a source be excluded from further 

evaluation if the source owner has expressed its intent to retire a source, but has been required to 

delay that retirement by authorities in light of electric system reliability issues. Commenter 

(0075) recommended these sources should not be included on the air agency’s list for SO2 air 

quality characterization, rather, air agencies should, at most, be expected to note their exclusion 

and provide a brief explanation. 

Commenter (0047) stated the exclusion would apply in narrowly defined situations where units 

are required to continue operations temporarily, pending implementation of system reliability 
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solutions. It differs from the current rule proposal, which would only exclude units that are shut 

down by January 2017, even if plans to shut down units in subsequent years have already been 

announced. Commenter (0047) believed this would eliminate the unnecessary agency and utility 

resources and costs related to modeling or monitoring demonstrations for units already 

designated for retirement but where such retirement is delayed due to electric system reliability 

issues. 

Commenter (0038) stated the EPA should acknowledge the potential for these situations to 

occur, describe how states should address these cases, and offer a way for sources to attain and

maintain the NAAQS under these particular conditions. Commenter (0038) provided the 

following additional points.

There exist situations where power plants are required by independent system operators 

(ISOs) to operate at certain levels to ensure grid reliability, regardless of the state or 

source's preference, which can potentially lead to the situation where a generation facility 

is fully controlled for SO2, yet still shown (via modeling or monitoring) to exceed the 

NAAQS. In this case, the best remaining enforceable measure to achieve attainment 

would be for the state to establish operating limits on that source; however, this may 

place the state and source in conflict with the ISO, which could otherwise require the 

plant to operate beyond those limits to ensure grid reliability. A similar conflict could 

arise when the operator has planned to shut down the facility altogether, yet the ISO 

requires its continued operation. 

Commenter (0070) stated that units and plants that have been approved for shutdown or 

retirement through the reliability region process should be granted the same status as units 

covered by permits or CDs as suggested by the EPA.

Response:

The CAA provides no special exemptions for facilities that have proposed to shut down but 

remain open at the request of ISOs. The air quality issues to which such sources contribute exist 

for as long as the source continues to operate, regardless of the reason the source continues to 

operate. Therefore, a source that meets the criteria for listing and continues to operate at the 

request of an ISO warrants listing as a priority source, just as is the case for a source that meets 

the listing criteria and continues to operate for any other reason. As discussed above, this rule, 

addressing air quality characterization requirements near sources that currently warrant such 

requirements, does not address how future planning activities such as designations and 

nonattainment planning will address future changes in source activity such as the shutdown of a 

source subsequent to resolution of an ISO’s reliability concerns. 

6. Enforceable limits in SIPs
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Comment:

Commenters (0052, 0078, 0081, 0089, 0097, and 0106) objected to a requirement that states 

must submit federally enforceable limits into the SIP. Commenters (0078, 0081, 0089, 0106)

referred to a webinar presented by the EPA on May 13, 2014, in which they assert the EPA

specified that these permits would not only need to undergo Title V approval, but would also 

need to be submitted for approval in the state’s SIP (slide 12). Commenters (0081, 0089, 0097) 

stated this is overly burdensome, unnecessary, and for no real environmental gain.

Commenters (0078, 0081, 0097, and 0106) stated that federally enforceable limits, such as Title 

V operating permits, are all that are needed to ensure facility compliance. Commenter (0078) 

stated that facilities in a state operating under already-existing Title V permitted limits that may 

show modeled attainment are not required to be included in the SIP, so making a unique

requirement on facilities with "newer" enforceable limits is problematic, since in both cases, the

facilities would be able to demonstrate modeled compliance under the restrictions of their 

permitted limits. Commenter (0106) asserted that states have for many years established 

federally enforceable limits to assure compliance with the NAAQS in PSD permits, minor source 

preconstruction permits, Title V Permits, and federally enforceable state operating permits which 

are not directly in the SIP. Commenter (0106) stated that inclusion of limits in the SIP is a waste 

of resources and creates an unnecessary and lengthy process if changes to the limits are required.

Commenter (0078) stated that subjecting facilities making early emissions reductions (in order to 

avoid nonattainment area planning requirements) to permit conditions that require SIP revisions 

and approval for future amendments would essentially penalize these facilities and remove the 

incentive for early action. Commenter (0078) stated that the EPA should not require any preattainment-designation permitted limits to be incorporated in the SIP, unless a state's existing 

permitting program is not otherwise "federally enforceable."

Commenter (0089) stated that, if the EPA is going to require such a departure, it needs to do so 

in a formal rulemaking procedure in which states are provided with notice and the opportunity to 

comment on it; such an implementation shift should not be merely a statement in a webinar, on 

which states have no formal opportunity to comment. Commenter (0089) provided the following 

additional points.

To require that emission limits be included in the SIP, not just in a facility's permit,

would be a several month process, at least. Once submitted to the EPA, EPA has 18

months to act on the submittal--and often does not act in a timely manner. Even more

importantly, to change the limits once they are approved into the SIP would take the same

lengthy process (consuming state and EPA resources once again). After the permit is 

revised, there would be a gap between the state permit requirements and the SIP 

requirements until the EPA acts on the revision. The gap could easily last two years at a 

minimum. The Permittee would legally have to continue to comply with the SIP until the 

EPA acts on the revision submittal or take a risk by just complying with the revised air 

quality operating permit. The result is a situation where the emission limits in the current 
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approved SIP are continually out-of-date with the agency-requested emission limits in a 

revised SIP, which may very well result in a situation under which enforceable emission 

limits may be less protective of human health and the environment.

Placing emission limits into the SIP also results in a certain loss of transparency in terms

of public review and comment on emission limits. When wrapped up in a SIP approval 

process, individual facility emission limits are obscured by all the other details in the SIP. 

When placed in an enforceable facility emissions operating permit, however, the public 

can more easily identify, evaluate, and comment on specific emission limits through the 

public comment process required as part of Title V permitting actions. This allows the 

public to focus its attention on the emission limit being imposed on that particular facility 

and the emission reduction control technologies and operational restrictions required to 

meet that limit. The review and comment on appropriate emission limits should remain 

associated with the issuance of an individual facility emissions operating permit.

Additionally, the EPA has not identified a need for the departure from permit emission

limits to SIP emission limits. The EPA has the opportunity to review and comment on 

Title V permits before they are issued, and the permit limits are federally enforceable. 

Since those emission limits are already federally enforceable, what need is there to 

include those emission limits in the SIP itself? (0089)

Response: 

The final rule allows the air agency to meet the requirements of this rule by submitting a 

federally enforceable emissions limitation (e.g., source-specific SIP revision or minor NSR 

permit revision) to the EPA prior to January 13, 2017 that requires the affected source to reduce 

allowable emissions at the source to an annual rate below the 2,000 tpy threshold level prior to 

January 13, 2017. By July 1, 2016, the air agency would be required to identify the sources on 

the list for which it would be using such an approach as an alternative to modeling or monitoring. 

For such a source identified on the list, if the affected air agency has adopted and the source has 

become subject to effective federally enforceable control measures lowering emissions below 

2,000 tpy by January 13, 2017, the air agency will generally not be required to further 

characterize the impacts from the source’s emissions solely due to its size as of January 15, 2016. 

Note that this provision of the final rule was established after consideration of several comments 

suggesting this type of change from the proposal.

Some commenters appear to have interpreted the slides on the proposed rule used for the May 

2014 webinar as requiring that all federally enforceable requirements limiting emissions of 

sources to below 2,000 tpy are to be adopted into the SIP. The EPA wishes to clarify that this is 

not the case. In some states, federally enforceable emission limits can be established through 

mechanisms other than through adoption of a SIP revision. For example, some states are able to 

establish federally enforceable emissions limits through their minor NSR program. 
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7. Election of monitoring or modeling

Comment:

Commenters (0048, 0094, 0095) asserted that states should not be allowed to limit a source’s 

option (monitoring or modeling) in determining an area’s classification and that if a state 

requires a modeling analysis and this shows nonattainment, the source should have the option of 

installing representative monitors to demonstrate attainment. Moreover, commenters (0094, 

0095) believed the rule should not limit a source’s right to install monitors to determine 

compliance with the standard.

Commenter (0052a) stated that, if a source-oriented monitoring requirement is established, the 

EPA should also develop an exception to this requirement where sources would be able to use 

modeling to avoid the monitoring requirement much like what was allowed in the lead source

monitoring program. Commenter (0052a) stated that, if a source models at less than a 

predetermined threshold percentage of the standard, a source-oriented monitor should not have to 

be installed. Commenter (0052a) stated that projected actual emissions after subject sources have 

installed all required emission reduction strategies should be included in the exception analysis 

to determine if source-oriented monitoring is necessary.

Commenters (0038, 0046, 0063, 0065, 0084, and 0108) stated that the EPA should provide a 

more reasonable schedule for states to elect the monitoring option under the Proposed Rule. 

Commenters (0038, 0108) suggested states should have until January 1, 2017 to make this 

determination because they could benefit from using initial modeling results to inform this 

decision, such an allowance would reduce burdens on state regulators, and it could lead to more

accurate determinations, while not impacting the EPA' s proposed attainment dates. Commenter 

(0038) provided the following additional points.

A likely scenario is that a state will conduct modeling as an initial step to gather the 

necessary information to determine the attainment status of potential sources. This initial 

modeling may reveal that the attainment status of a particular source (or cluster of 

sources) cannot be sufficiently determined via modeling; therefore, the state may decide 

that monitoring is required. However, this conclusion may not be evident until the state 

completes its full modeling analysis. If the rule is finalized on January 1, 2015, the EPA

cannot expect the modeling analysis for all potential sources to be completed prior to the 

January 15, 2016 deadline of declaring whether modeling or monitoring will be used to 

determine attainment. Instead, the EPA should simply let the states declare sources that 

must be monitored as part of the state's January 1, 2017 submittal. At a minimum, the 

rule should allow for states to declare a source or area for monitoring after January 15, 

2016 on a case-by-case basis. 

Commenters (0046, 0084) stated that insufficient time (approximately 1 year from final rule) is 

given in the proposal for states to submit a decision on choosing modeling or monitoring (Jan 15, 

2016), given the lack of resources and large amount of data gathering and analysis and
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collaboration with stakeholders that will need to be performed to implement this rule. 

Commenter (0046) noted states will need to assess financial resource considerations related to

adding or relocating monitors before making the decision whether to model, however, the

proposed timeline would compromise this analysis. Commenter (0084) stated the EPA should 

allow for at least 18 months for the logistical elements to be completed for states to submit a 

decision on choosing modeling or monitoring. 

Response:

Upon review of these comments, the EPA agrees that the rule should provide somewhat more 

time to air agencies to review the options for air quality characterization. Consequently, the final 

rule provides approximately six more months for air agencies to select the approach to be used to 

address priority sources. (Consistent with this revision, the EPA is also delaying the deadline for 

submitting a modeling protocol by six months.) At the same time, the EPA believes that it must 

assure expeditious air quality characterization, and so the EPA is promulgating the proposed 

timeline for the actual air quality characterization (i.e., the submittal of modeling or the 

commencement of monitoring) in the beginning of 2017. However, the EPA believes that an 

election of approach by July 1, 2016, would provide a more manageable timeline for obtaining 

the source information needed in support of the election of approach, while still providing 

enough time to meet deadlines for air quality characterization in the beginning of 2017. Further 

discussion of these comments and the EPA’s response are provided in the preamble to the final 

rule.

The source does not have the option to determine an air agency’s selection of whether to monitor

or model source emissions under the final rule. The obligations of this rule fall to air agencies, 

not sources, and it would not be appropriate to allow sources to predetermine how air agencies 

choose to meet those obligations. This is consistent with how the CAA mandates that the state 

submit designation recommendations (after which the EPA is to promulgate designations), and 

also with how the CAA apportions responsibilities in other aspects of air quality planning. 

Consistent with these CAA provisions, this rule provides that the state or other air agency shall 

determine the approach to be used to characterize air quality. 

This rule is designed to prioritize the use of limited resources for air quality characterization. If 

air quality in an area is successfully characterized through modeling, it would be inappropriate 

then to require monitoring, regardless of whether the source dislikes the result of the modeling. 

If the modeling (which notably may be based on actual emissions for the purposes of this rule)

indicates violations, the most appropriate next step would be to designate the area nonattainment 

and to commence planning for providing for attainment.

The comments urging an approach similar to that used for lead are not germane to the proposed 

rule for SO2, in which a state has the option to model or monitor. There is no requirement, or 

implication, that a state must model to relieve itself of potential monitoring activities as is the 

case with lead. The EPA is promulgating a rule that provides options for air quality 

characterization by either a complete modeling analysis or a complete monitoring effort; an 
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incomplete modeling analysis would not provide suitable justification for forgoing a complete 

monitoring effort.

8. Common area analytical approach

Comment:

Commenters (0040, 0044, 0081, 0083, and 0107) stated air agencies should not be required to 

use the same technique to characterize all sources in an area. Commenter (0044) stated this 

provision would restrict the flexibility of air agencies in selecting the technique they wish to use, 

and possibly forcing them to use modeling in locations where monitoring would not only be 

feasible but also preferable. 

Commenter (0044) stated the preamble provides no caveat that once a choice is made either to 

monitor or model that choice will apply to all sources within the affected area and fails to 

advance any justification for why the proposed regulatory text would impose such a limitation. 

Commenter (0044) does not believe such a justification exists. Commenter (0107) stated it is not 

clear why States must use the “same technique” in 51.1203(b).

Commenter (0081) disagreed that it would not be appropriate to choose monitoring for some 

sources and modeling for others in a common area. Commenter (0081) stated that, if it is deemed 

that sources which are in somewhat close proximity to one another do not produce a significant 

cumulative effect on air quality concentrations, there should be an allowance for one facility to 

use modeling to demonstrate their compliance with the standard and the other to use monitoring 

and forego designations until 3 years of monitoring data has been submitted (i.e. use a combined 

modeling/monitoring approach to designate the area). Commenter (0081) provided the following 

additional points.

One situation where this may be appropriate is in an area where the distance between 

facilities is such that there is no overlap in emissions contributions and/or air dispersion 

modeling shows there is not a significant concentration gradient from one or both of the 

facilities in the vicinity of the other facility. One facility may choose to use modeling for 

compliance with the rule and show that concentrations are below the standard, while the 

other may wish to use a monitor at the closest area of high concentration to forego annual 

reporting of emissions. This option should especially be considered if evaluation “areas” 

are presumed to be counties, CBSAs, or MSAs (or any other jurisdictional-based 

boundaries that are arbitrary in terms of source specific air quality levels).

Commenter (0083) stated there may be situations where it will be beneficial for states to work 

together in establishing the status of an area, but a requirement that states must come to an 

agreement on the approach they are to use shouldn't be codified in the rule. Commenter (0083) 

believed each state should be responsible for the applicable sources in their state and the rule 

should not require different states to come to an agreement on an approach, where there is no 

means for resolution if there is an impasse between two states on the best approach to use.
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Commenter (0044) believed the use of the word “shall” in the last sentence of section 51.1203(b) 

should be deleted and substituted with the phrase “is not required.” Commenter (0083) suggested 

that the second sentence of proposed §51.1203(b) be removed from the rule. Commenter (0040) 

suggested the following changes in section 51.1203(b):

(b) For each area containing an applicable source, the air agency shall state by January 

15, 2016, whether it will characterize air quality through ambient air quality monitoring, 

through air quality modeling techniques, or through a combination of both, as 

appropriate.

Response:

The EPA believes that if two or more sources are contributing to concentrations in a single 

common area, then it would be impracticable to address one portion of the area with modeling 

and another portion of the area with monitoring. While the EPA can consider both modeling and 

monitoring information in promulgating designations, the consideration of both is more feasible 

if both methods characterize air quality in the entire area. The commenters do not describe how 

they envision that data addressing only portions of an area would be combined, and the EPA 

believes that its DRR more appropriately requires that the complete area be characterized by one 

method or another, so as to avoid the problems that would be prone to arise if two different 

analytical methods were used for different portions of an area.

Furthermore, in this rule, the choice of methods for characterizing air quality also determines the 

timetable under which air quality characterization is required. The EPA must designate areas as 

an entirety; the EPA does not have the option to designate the modeled portion of an area at one 

time and then designate the monitored portion of the area sometime later. Thus, whether the air 

agency selects the monitoring pathway or the modeling pathway for characterizing air quality, 40 

CFR 51.1203 requires that the information provided under the selected pathway be sufficient to 

characterize air quality in the area, irrespective of whether other air quality characterizing 

information of the other type is or becomes available. That is, the air agency must provide for 

complete characterization of an area by the selected approach, either by monitoring that 

characterizes the entire area or by modeling that characterizes the entire area. The timetable 

under which this information would be required is clearly defined by the rule. The EPA 

welcomes any supplemental information that the air agency may provide, but the DRR cannot be 

considered satisfied unless air quality information based on a single selected pathway sufficient 

to characterize worst case concentrations throughout the area are obtained.

Some of the commenters appear to be objecting to the rule based on a misunderstanding of the 

term “area.” “Area” may be considered to mean a candidate nonattainment area, in the sense of 

including all of the area being evaluated as to whether it is meeting the SO2 standard and all of 

the area that is to be evaluated as to whether it is contributing to SO2 concentrations that might 

be violating the standard. One commenter posits a situation in which sources in close proximity 

are deemed not to have interactive impacts, but the commenter does not explain how such a 
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situation could arise, in particular how the absence of interaction of sources in close proximity 

could be known without the kind of investigations that the DRR requires. Presuming that 

investigation of the interaction of the sources is warranted, the area being investigated would 

include both sources and their areas of impact, and for reasons described above, the air agency 

would need to select either monitoring or modeling to address the area and assure that the 

selected approach is done in a manner that addresses the entire combined area. Conversely, if 

two sources are separated by sufficient distance that potential violations near one source could be 

presumed to reflect no contribution from the other source, such that violations near both facilities 

could be presumed without further investigation to warrant two separate nonattainment areas, 

then the two sources could be considered to be part of two separate areas that could be addressed 

by different approaches. Where two sources may or may not warrant treatment as part of two 

separate areas, air agencies are urged to discuss the situation with their EPA Regional Office.

9. Oppose submitting a list

Comment:

Commenters (0038, 0060, and 0108) opposed the proposed requirement that, by January 15, 

2016, air agencies must submit a list of sources with election of monitoring or modeling. 

Commenters (0038, 0108) stated submitting a list is unnecessary, as it may prematurely identify 

sources as potentially violating the NAAQS (0038) and such a list is already made publicly 

available on an annual basis through the NEI (0038). 

Commenters (0038, 0108) stated it does not make sense to establish a list for SIP submittal that 

is expected to change. Commenter (0108) stated creating a second list is a redundant use of 

limited resources and creates the potential that future regulatory actions be based upon inaccurate 

information. Commenters (0038, 0108) stated that any distinction or listing of a source should 

wait until the January 2017 analysis justifying attainment status for individual sources or areas is 

complete. Commenter (0108) also suggested allowing states to make this determination by 

January 1, 2017 to reduce the burden on the states and lead to more accurate determinations 

without impacting the EPA’s proposed attainment dates.

Commenter (0060) stated there are numerous federal regulations with compliance timelines that

overlap these timelines and, as companies analyze their options in this ever changing landscape 

of regulation, their decisions may change. Commenter (0060) stated there must be flexibility for 

strategies to change prior to an attainment demonstration being due. Commenter (0060) provided

the following additional points.

It is the EPA's role to "issue minimum criteria that any SIP submission must meet." 

[CAA paragraph 101] Creating a requirement for a binding list is far beyond necessary 

and is not in the spirit of the CAA roles for States and EPA to work cooperatively. Ohio 

EPA has worked with EPA Region 5 cooperatively in the past to finalize similar types of 

lists without a binding requirement or an official submittal until such time the SIP

submittal is required (for example, under the 2008 lead standard monitoring
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requirements). A binding list with official submittal in advance of nonattainment

planning was not required for sources being analyzed under the nonattainment area 

requirements. EPA should not burden States even more with additional commitments.

EPA should not be creating multiple binding lists that force air agencies to continually

and further justify addition or removal of sources.

Commenter (0060) did not find merit in the citations the EPA provided in the proposal regarding 

the authority for requiring this list submittal. Commenter (0060) provided the following 

additional points.

The section 110(a)(2) citations address the requirements for SIP submittals by states for 

implementation, maintenance and enforcement of the standard. The EPA notified states on 

April 12, 2012 that the June 2013 Infrastructure SIP submittals that address section 110(a)(1) 

and (2) elements, should focus on those traditional elements and not performing modeling to

ensure future attainment in unclassifiable areas and that those SIP submittals would come 

later. These are the very submittals that are due in the proposed timeline in August of 2019 

and 2022 ("due date for state attainment plans"), a submittal that is due only after "reasonable 

notice and public hearings" by the state. Will air agencies be required to public notice and 

provide hearings for these lists submitted under the authority of this section but prior to the 

actual attainment demonstration? It seems EPA is putting the cart before the horse.

Response: 

The EPA does not agree with commenters who claim that submittal of an initial list of sources 

near which air quality is to be characterized is not needed in January 2016. The EPA believes 

that it is important to receive the list of source areas to be characterized under the rule by January 

15, 2016, because it will provide timely clarity about which areas will be characterized, which is 

necessary for air agencies to generate timely information that may be used to inform future 

designation, redesignation, or other decisions concerning whether sources are affecting the 

attainment of areas. Retaining this deadline will provide early opportunity for the air agency and 

the EPA to resolve questions about whether air quality characterization should be required for an 

area, for example if emissions are low in some years and high in others, if an area has a cluster of 

smaller sources, or if source-specific factors may warrant air quality characterization. Thus, the 

EPA is retaining the January 2016 deadline, as proposed, for submittal of the list of sources in 

order to initiate an orderly process to obtain additional information on ambient SO2

concentrations, and ensure these data are available to support actions taken for the 

implementation of the 2010 SO2 NAAQS. The EPA believes that the schedule for providing the 

list of sources is a relatively straightforward exercise that can be accomplished within the 

required time frame. 

Once this list is provided, and once any issues relating to the list such as whether additional areas 

warrant air quality characterization or how to address a source with variable emissions are 

resolved, the EPA envisions no further changes to the list, and the DRR sets out requirements 
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that apply to the listed sources. To allow the list to continue to change after an established final 

date would result in confusion about how the state is to meet the requirements of the rule. 

Nevertheless, the requirements for listed sources allow for consideration of the kinds of changes 

in circumstances that commenters describe. As one example, a source with emissions recently 

above the threshold that will enforceably be shutting down by the beginning of 2017 will be on 

the list but can avoid air quality characterization requirements. 

The list is needed for implementing the SO2 standard, providing information that sections 110 

and 114 clearly authorize the EPA to require. The EPA is not requiring air agencies to conduct 

public hearings on these lists, as they are not themselves implementation plans subject to the 

requirements of CAA section 110(l). Although much of the source list is based on a rote listing 

of the sources emitting at least 2,000 tpy in the most recent inventory, the EPA anticipates that 

air agencies may wish to use different emission data bases for different source types. For 

example, use of EPA’s Air Markets emission data base may provide for using more recent 

emission data for EGUs than is available for other source types. Furthermore, the listing process 

involves judgment in some cases, where discretion is needed for areas without a source emitting 

2,000 tpy may nevertheless warrant air quality characterization. Such judgment is needed for 

example where a source may be particularly prone to cause high concentrations (e.g. due to low 

stack heights), where multiple sources are in close proximity, and where future emissions are 

expected to be higher than emissions in the most recent year. For all these reasons, the formal 

listing is an essential step to clarify which source areas are subject to the requirements of this 

rule. 

The comments in relation to the Monroe Power Plant do not justify any changes in the rule. The 

commenter does not clarify whether the installation of controls will bring this source below 

2,000 tpy or when the emission reductions occurred; these facts would be necessary to determine 

whether the plant must be listed under the final rule. The commenter suggests that any 

requirements for modeling this source have already been met, which if true would mean that 

there would be no harm to the source to be required to submit modeling that has already been 

done.

10. Section 51.1203(a)

Comment:

Commenter (0093) requested the wording in section 51.1203(a) be revised to state that the list 

should only specify which pathway will be followed and not specify who will be completing the 

task. Commenter (0093) noted they intend to impose the responsibility for modeling and/or 

monitoring on the affected sources and will act in an oversight capacity.

Response: 

The rules that the EPA is promulgating does not require air agencies to identify who is 

completing which tasks. The EPA recognizes that some agencies will prefer to undertake the 
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necessary efforts themselves, and other agencies will prefer for affected sources to complete 

most of the effort subject to air agency oversight. An important caveat is that the final rule, in 

imposing obligations on air agencies but not on sources, requires the air agency to make the 

ultimate submittal, so that even if the affected source does most of the work, the air agency bears 

ultimate responsibility for assuring that its submittal meets applicable requirements. Nothing in 

the rule dictates how air agencies are to allocate the workload involved in meeting the 

requirements of the rule, and the EPA believes that no rule revisions are needed to clarify this 

point.

11. Updated information on EPA’s preliminary list

Comment:

Commenter (0035) provided the following updated information.

Additional emission controls have been installed at the Valero Refining Company in 

recent years, and current SO2 emissions are nearly an order of magnitude less than the 

Option 1 threshold. The Shell Martinez refinery continues to have SO2 emissions that are 

marginally above the Option 1 threshold, but expects to implement a project that would 

reduce SO2 emissions well below this threshold. The Phillips 66 Carbon Plant is currently 

our region's largest source of SO2, with emissions that are about 25% over the Option 1

threshold. Our most recently adopted Clean Air Plan includes a control measure to reduce 

these emissions, and we expect to complete the required rule development for this 

measure within approximately two years. (0035)

Commenter (0054) supplied information on several Primary Aluminum Plant facilities that are 

listed on the 2011 NEI as SO2 emitters that in the interim have been permanently closed. These 

are Ormet Aluminum – Hannibal, OH (permanently closed in October 2013); and Reynolds 

Metals St. Lawrence Reduction Plant (Alcoa Inc. wholly-owned subsidiary) – Massena, NY 

(permanently closed in March 2014).

Commenter (0057) stated that SO2 emissions at Big Stone Plant in South Dakota in 2016 will be 

well under even the Option 1 thresholds and these emissions will be provided by Otter Tail to the 

EPA through continuous emissions monitoring data. 

Commenter (0070) provided a table below which shows the AEP System Facilities currently 

scheduled for shutdown or conversion to an alternative fuel by 2017 for all reasons (MATS 

Compliance, CD, retirement, etc.) and that have been vetted and approved through the 

appropriate reliability region process.

Commenter (0081) attached an updated list of applicable sources based on 2013 NEI data as 

Attachment 1 to its comments. Commenter (0081) stated the attachment also indicates those 

facilities which have announced planned retirements or fuel switches, so that the EPA can amend 

the list of applicable sources in the final rule, assuming the rule is finalized before another year 
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of emissions inventories is made available.

Commenter (0085) attached a table to its comment letter showing 2013 emissions data and 

planned fuel switching and shutdowns for Maryland sources.

Commenter (0097) provided a table listing 8 facilities and several units that are shut down or will 

be shut down in the next several years.

Commenter (0103) requested the EPA to use the 2012 NEI data to update the preliminary list of 

source areas that were included in the docket with the EPA’s Proposed Rule. Commenter (0103) 

attached the 2012 NEI data for the SO2 sources in Iowa (spreadsheet “2012 NEI IA SO2

Emissions”) that exceed the source emissions thresholds included in the EPA’s Proposed Rule.

Commenter (0107) stated there are three facilities on the EPA's list that have closed and two 

facilities that have shut down their coal fired units; these changes should be reflected in the most 

recent 2013 emissions inventory. Commenter (0107) requested that these five facilities be 

removed from the list of affected sources: Duke Energy Progress – Sutton; Duke Energy 

Progress – Lee; Duke Energy – Buck; Duke Energy – Riverbend; and DAK Americas.

Commenter (0118) identified two sources have either shutdown or announced they will shut 

down by the time this rule is effective: American Electric Power-Tanners Creek in Dearborn 

County and State Line Energy LLC, in Lake County.

Commenter (0070) pointed out errors in Table 3 of the spreadsheet that was placed in the docket 

by the EPA: the IPL Harding Street Plant actually located in Indiana, is erroneously shown as 

being located in West Virginia; Kammer Plant is erroneously shown as being located in Indiana, 

when it is actually located in Marshall County, WV.

Response: 

The EPA included in the docket to the proposed rule a preliminary list of sources that appeared

to meet the criteria described in the EPA’s proposed source threshold and requested that air 

agencies provide in their comments any relevant updated information that would support the 

addition or removal of a source from that preliminary list.

The EPA appreciates the comments on the preliminary list of sources that appeared likely to be 

subject to this rule as proposed. The EPA acknowledges that, for various reasons, such a list of 

sources could change up until the time that the list is required to be submitted. Accordingly, such 

a list is not being promulgated as part of this rule. The EPA plans on continuing consultations 

with air agencies regarding the source areas that the final rule will require to be characterized.

12. Sources currently in a nonattainment area
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Comment:

Commenter (0076) urged the EPA to revise the list of covered sources to include Ameren 

Missouri’s Rush Island plant. Commenter (0076) stated that, if the Rush Island plant’s 

substantial SO2 impacts are not adequately addressed – or not addressed at all – during the 

pending SIP process, then the plant should be covered by the DRR. Commenter (0076) stated 

that there are at least two scenarios whereby the SO2 impacts of the Rush Island plant would not 

be adequately addressed, or addressed at all, notwithstanding the plant’s location in the Jefferson 

County nonattainment area and provided the following clarification.

First, the Jefferson County nonattainment area was designated based on a monitor sited 

specifically for a different large source, the Doe Run Herculaneum lead smelter. 

Monitoring performed on behalf of the Sierra Club shows that Rush Island’s impacts 

occur in large areas outside of, as well as inside, the nonattainment area. There is no 

assurance, and no reason to assume, that any emissions limits imposed on the Rush Island 

plant as part of the in-process nonattainment SIP will sufficiently reduce the plant’s SO2 

emissions to eliminate all NAAQS exceedances outside of the existing nonattainment 

area that are attributable to the Rush Island plant.

Second, readings at the Doe Run-oriented monitor have dropped considerably since the 

Herculaneum smelter closed in December 2013. Other sources in the nonattainment area 

are pressing the Missouri Department of Natural Resources (DNR) to abandon the SIP 

process. While we believe that such action would be both legally and factually unfounded 

(and is beyond the scope of this letter), it is conceivable that DNR will fail to prepare a 

nonattainment SIP for the Jefferson County nonattainment area. Should that occur, then 

no emissions reductions would be required of the Rush Island plant – even though 

modeling performed by DNR shows that the Rush Island plant’s actual (and allowable) 

emissions cause SO2 exceedances well above the NAAQS at locations different from the 

site of the Doe Run-oriented monitor. (0076)

Response: 

The first question is whether air agencies are required under this rule to characterize air quality 

near sources in areas that were designated as nonattainment in August 2013. See 78 FR 47191, 

August 5, 2013. In general, we expect the source characterization obligations required as part of 

the nonattainment planning process to provide adequate characterization of the impacts of 

sources within those nonattainment areas. Therefore, we have concluded that an air agency will 

not be automatically required under this rule to characterize air quality around SO2 sources 

already located in a nonattainment area when the list is due. Specifically, we have clarified the 

definition of “applicable source” in section 51.1200 of the rule to be “a stationary source that is 

(1) not located in a designated nonattainment area, and (2) has annual actual SO2 emissions of 

2,000 tons or more, or has been identified by an air agency or by the EPA Regional 

Administrator as requiring further air quality characterization.” Thus, as a general matter, this 

rule does not require the state’s January 2016 list of sources triggering the requirements of this 

rule to include sources located within areas already designated nonattainment.
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However, it may be possible that in some rare cases an SO2 source or group of sources within the 

boundary of an existing nonattainment area can have significant impacts outside the 

nonattainment area, potentially raising concerns that these impacts might not be adequately 

addressed in a nonattainment plan. The EPA notes that for such cases, the air agency and the 

Regional Administrator retain the authority under this rule to require additional characterization 

of air quality around specific sources located in an existing nonattainment area, in the same 

manner that they retain the authority, as warranted, to require characterization of air quality 

around sources that are below the emissions threshold identified in this rule. As the commenter 

acknowledges, comments regarding EPA action in the event of a failure by Missouri to submit a 

nonattainment plan are beyond the scope of this rulemaking.

13. New/modified sources or existing sources initially below the 

threshold, subsequently exceeding it

Comment:

Commenter (0069) recommended the final rule give states the authority to work with sources 

whose emissions subsequently exceed the applicability threshold to either bring the SO2

emissions below the applicability threshold or to conduct modeling or monitoring consistent with 

the provisions of the rule. Commenter (0069) noted the Proposed Rule is silent on what the EPA

expects should a source's actual SO2 emissions exceed the applicability thresholds based on 

actual data either prior to the designations or after the designations are made. 

Commenter (0081) stated the EPA should address how new or modified facilities will be 

incorporated into state monitoring/modeling plans for compliance with this NAAQS.

Response:

This rule is designed to require air quality characterization near sources that currently (or, more 

precisely, based on the most recently available year of data) warrant it, similar to the manner in 

which a source-oriented monitoring network would be designed based on the current (and known 

future) source circumstances. The rule, as with other monitoring rules, is not designed to address 

unknown future source changes that may warrant air quality characterization in the future. 

Nevertheless, this rule does not preclude states from conducting air quality characterization 

under other authorities, and indeed the EPA has other authority and states have other authority to 

conduct monitoring and modeling to evaluate air quality near sources that do not trigger air 

quality characterization requirements under this rule but that exceed the thresholds of this rule in 

the future.

To the extent that new or modified sources will be subject to review for PSD, much of the 

information required under this rule will likely already have been developed. Nevertheless, with 

or without such review, this rule requires review of all sources that meet the threshold of this rule 

and all other sources necessary to meet the air quality characterization requirements of this rule 
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irrespective of whether the relevant sources are new, recently modified, or unmodified.

C. January 15, 2016: Requirement to submit modeling protocols for 

sources characterized with modeling

1. Schedule

Comment:

Commenter (0051) stated the EPA has failed to address the need for modeling protocols prior to 

modeling for monitor placement, assuming that the final rule will still prefer modeling to 

properly sited monitors. Commenter (0051) stated that time for this tedious and somewhat 

complex analysis, as well as the modeling for monitor siting, has not been allotted in the EPA's 

proposed timeline.

Commenter (0069) stated the rule should not preclude states from submitting protocols 

separately as long as they are all submitted by the January 15, 2016, deadline. Commenter (0069) 

stated the proposed language appears to require that all source-specific protocols must be 

submitted together and with the list of sources that will be modeled and monitored. Commenter 

(0069) stated that, in order to complete and submit modeling for all areas in the modeling

pathway by the proposed January 13, 2017 deadline, the agency may elect to stagger modeling 

over an extended period, which would necessitate submitting some modeling protocols earlier 

than others. 

Commenter (0082) stated that the January 15, 2016 deadline is not feasible for those areas that 

do not have (1) meteorological data from the same period as the most recent three years of actual 

SO2 emissions (“concurrent meteorological data”) or (2) representative site-specific 

meteorological data or ASOS meteorological data from any recent three-year period. 

Alternatively, the commenter (0082) urged the EPA to extend the January 15, 2016 deadline so 

that air agencies have sufficient time to develop concurrent meteorological data. [More 

comments on this issue are at V.B.7, meteorological data.]

Commenter (0110) stated the availability of a final model version several months before the 

January 15, 2016 deadline is very important in order for states and affected sources to make 

critical decisions; however, based upon the announced EPA schedule to have an updated 

Appendix W by spring 2016, it is clear that the January 15, 2016 deadline is unworkable.

Commenter (0110) stated, therefore, the EPA must change that deadline to early 2017 at the 

earliest, and this date may slip further into the future if the update to Appendix W is delayed 

beyond spring 2016.

Response:

In response to these comments, the EPA is providing additional time for making the election of 

modeling or monitoring (or, as discussed later, for making the election of an alternative approach 
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that enforceably limits an applicable source’s emissions). Accordingly, the deadline for this 

election will be July 1, 2016. The EPA recognizes that evaluating the relative merits of modeling 

and monitoring for any particular area, including identification of funding sources for any new 

monitoring that might be under consideration, warrants more time than was provided under the 

proposed rule. Consistent with this revision, the EPA is also revising the deadline for air 

agencies using modeling to submit modeling protocols for the applicable areas. Thus, under the 

final rule, by July 1, 2016, the air agency must submit its selection of whether each area will be 

characterized through modeling or monitoring and, depending on that selection, either must 

submit a modeling protocol or must include information in the Annual Monitoring Network Plan 

that specifies the monitoring to be conducted to address the requirements of this rule. While air 

agencies presumably will often decide how they wish to meet the rule’s requirements somewhat 

in advance of preparing plans (a modeling protocol, a monitoring plan, or specification of plans 

for emission limits) for implementing that choice, the EPA does not wish to regulate this 

decision process or mandate a period of time between pathway selection and submittal of 

implementing plans. Therefore, the EPA is establishing the same deadline for the selection of 

pathway as for the plans that implement the selected pathway. This provides the same deadline 

as was proposed for monitoring plans, but this provides six additional months for air agencies to 

address any modeling issues (like those identified by the commenters) that should be addressed 

as part of the modeling protocol. The EPA believes that this revised deadline still provides for 

timely planning for air quality characterization to occur (through modeling) or begin (through 

monitoring) at the beginning of 2017.

Similarly, the EPA strongly encourages air agencies to hold early discussions regarding the 

manner in which modeling or monitoring might be used. As one example, if the air agency 

believes that the existing monitoring network suffices to characterize air quality, early 

discussions with the EPA would be essential for assuring that the intended selection of 

monitoring is based on appropriate assumptions regarding the network’s ability to characterize 

air quality near the applicable source(s) without further network adjustments. 

The EPA recognizes that modeling is often part of the process of developing monitoring plans. 

The EPA welcomes modeling protocols for this purpose that air agencies wish to provide, and 

the EPA believes that the timetable provides sufficient time for air agencies to provide such 

protocols. However, the EPA is not requiring such protocols as part of this rule.

2. Section 51.1203(d)

Comment:

Commenter (0093) stated that the wording in section 51.1203(d)(2) is unnecessary, where the 

section states that if an air agency uses an allowable emissions limit in the modeling that it may 

submit the limit "for the EPA' s approval'' at the time the modeling protocol is submitted.

Commenter (0093) argues this is unnecessary because whether the emissions limit is in an 

existing permit or is being modified, the EPA either had or will have the opportunity to comment 

on that limit during the permitting process.
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Response: 

The EPA agrees that, so long as the limits are federally enforceable and require compliance by 

January 13, 2017, such limits are appropriate to consider in modeling of allowable emissions 

pursuant to section 51.1203(d)(2), without regard to the means by which the federally 

enforceable limit became federally enforceable. The EPA has revised the language of section 

51.1203(d)(2) accordingly. 

3. EPA oversight of modeling plans

Comment:

Commenters (0060, 0088, 0093, 0096, and 0097) objected to EPA oversight of the modeling 

plans because it is unnecessary and could hinder progress under this rule.

Commenters (0060, 0088, and 0093) were concerned that the EPA could not review the plans in 

a timely manner and could cause delays in the process. Commenter (0060) stated there is only 

one year between when this protocol is to be submitted and the final modeling is to be submitted 

according to this proposal, which leaves no room for the EPA to follow up "expeditiously".

Commenter (0060) stated that, if this level of oversight and approval is finalized in this rule, they 

have serious concerns about whether 2 years from promulgation of the final rule is a reasonable 

amount of time for air agencies to prepare the necessary data inputs and conduct such modeling 

for all subject sources. Commenter (0088) stated it is arbitrary and capricious to require the EPA

approval of state monitoring and modeling plans when the EPA’s technical resources are too 

stretched to provide this oversight in a timely manner. [Also see commenter (0088) comments in 

IV.B.3, EPA oversight of monitoring plans.]

Commenters (0060, 0088, and 0093) stated the EPA oversight/approval of model protocols 

should be eliminated. Commenter (0093) stated that model protocols should be submitted to 

states for review and that the EPA can audit as necessary. Commenter (0093) stated it is 

unnecessary and duplicative to provide these protocols to the EPA. Commenter (0060) stated the 

EPA is over stepping the collaboration between state and federal agencies and the states' role to

implement standards. Commenters (0060, 0093) stated air agencies should be able to determine 

the best approach, using the normal course of discussion and cooperation with their regional 

modeling contacts and document that approach with the final submittal to the EPA. 

Commenter (0097) stated the EPA should clarify that states could provide to the EPA a modeling 

protocol framework for review and approval, and that source specific review of protocols should 

be left up to the respective state agency. Commenter (0097) stated this is consistent with past 

practices in PSD SIP approved states as well as past practices supporting BART.

Commenter (0088) recommended that the roles of the EPA experts should be advisory only in 

initial implementation and section 51.1203 should be revised accordingly. Commenter (0088) 
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provided the following additional points.

While it is critical to acknowledge that the EPA regional monitoring experts can be 

helpful in advising states on monitoring (and modeling) plans, there are simply not 

enough of them to supervise, let alone review and act on, the number of state monitoring 

programs necessary to implement the data rule in a timely manner. Most EPA regions 

have only one such monitoring/modeling expert and OAQPS has fewer than five such 

experts. It therefore would be highly unreasonable for the EPA therefore to require as a 

precondition for implementation of a SO2 monitoring plan, EPA review and 

concurrence/approval before it is implemented. Even if the compliance deadlines for such 

plan requirements and implementation were extended by substantial amounts, this 

requirement would still be unreasonable. (0088)

In order to provide a reasonable period to conduct and review the modeling, commenter (0096) 

recommended the DRR require comments on protocols to be provided to states within 30 days of 

submission. Further, commenter (0096) recommended that the DRR provide that the protocol is 

automatically approved unless the EPA acts to approve or disapprove the protocol by no later 

than April 1, 2016.

Response: 

See the preamble at IV.C.d.3.c.

4. Stakeholder participation in air agency characterization efforts

Comment:

Commenters (0040, 0044) stated that the regulatory text of section 51.1203 should be expanded 

to specify that affected sources have a right to participate in the designation process as it unfolds, 

including an opportunity to be made timely aware of an air agency’s choice of modeling 

methodology and, when necessary, an opportunity to dispute the use of allowable emission rates 

rather than the actual emissions. Commenter (0044) provided the following additional points.

The inclusion of such additional instruction and guidance in both the preamble and

regulatory text will be beneficial to the air agencies and to the regions and will help to 

ensure consistency in the implementation of this rule throughout the country. This is 

especially important because proposed section 51.1203 clearly contemplates consultation 

and coordination between the air agencies and the regional offices. Inclusion of such 

instruction and guidance in the preamble and regulatory text would therefore help to 

protect the due process rights of affected sources who might disagree with an air agency 

over the methodology chosen for determining air quality. For example, the additional 

instruction and guidance should provide sources with the means and opportunity to obtain 

and understand an air agency’s reasoning to utilize modeling based on allowable 
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emission rates over actual and thereby provide sources with an ability to determine 

whether an agency’s reasoning has merit.

Commenter (0040) suggested the following regulatory amendments to section 51.1203(d):

(d) Modeling. For each area for which air quality will be characterized through air quality 

modeling, the air agency shall submit by January 15, 2016, a technical protocol for 

conducting such modeling to the Regional Administrator for review. The air agency shall 

consult with the appropriate the [sic] EPA Regional Office, and also the sources that may 

be subject to the modeling, in developing these modeling protocols. The modeling 

protocols submitted by the air agency must identify comments raised by each potentially 

affected source and provide the agency’s responses thereto. . . .

Commenter (0044) recommended the following revisions to section 51.1203(d) and (d)(1):

(d) Modeling. For each area for which air quality will be characterized through air quality 

modeling, the air agency shall submit by January 15, 2016, a technical protocol for 

conducting such modeling to the Regional Administrator for review. The air agency shall 

consult with the appropriate the [sic] EPA Regional Office, and also the sources that may 

be subject to the modeling, in developing these modeling protocols. Sources and other 

interested parties shall also be given an opportunity to review and comment upon the 

modeling protocol prior its approval by the Regional Administrator.

(1) The modeling protocol shall include information about the modeling approach to be 

followed, including but not limited to the model to be used, modeling domain, receptor 

grid, emissions dataset, meteorological dataset, whether the model will be based on actual 

or allowable emission rates, and how the state will account for background SO2

concentrations, and such other factors that the air agency believes will be beneficial in 

making current ambient air quality determinations under this subpart.

Response: 

At the outset, we note that the EPA has in recent years has consistently provided for a public 

participation opportunity in promulgating designations under NAAQS. We expect to continue 

this in completing designations under the SO2 NAAQS. However, this rule does not prescribe 

requirements for conducting designations or other actions that make decisions regarding areas’ 

air quality, so it is beyond the scope of this rule to codify such requirements. In implementing 

this rule that is targeted at characterizing area emissions concentrations generally, the EPA 

agrees with the commenters regarding the desirability of consulting relevant parties when 

performing modeling under this rule (or for that matter monitoring) and in particular encourages 

air agencies to consult with affected sources and other interested parties regarding the selection 

of air quality characterization methods and regarding particular elements of intended modeling 

analyses. Indeed, the EPA believes that most air agencies consult with the range of interested 
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parties as a matter of routine, a practice that the EPA believes provides for better informed 

actions by the air agencies.

On the other hand, the EPA does not agree that the DRR should be revised to include an explicit 

requirement for states conducting modeling under this rule to consult affected sources and other 

interested parties. The CAA provides that SIPs are to be subject to a notice and comment process 

but does not impose any such requirements on air agencies regarding the conduct of technical 

analyses or the establishment of ambient monitors, and the EPA believes that it would be 

inappropriate to impose such requirements in this rule. Air agencies have a challenging task 

meeting the requirements that the CAA does impose, and the EPA believes that imposition of 

further requirements on who the air agencies consult is neither necessary nor warranted.

D. July 2016: Inclusion of SO2 monitoring network modifications to 

satisfy DRR

1. Schedule is not feasible

Comment:

Commenters (0038, 0046, 0056, 0088, and 0108) stated that it is unreasonable for the EPA to 

assume monitoring plans can be submitted by the proposed July 1, 2016 deadline. Commenter 

(0038) stated that (1) it may not be apparent that monitoring is required until after a lengthy 

round of initial modeling is complete and (2) siting monitors is a lengthy process which involves, 

among other steps, working with the EPA to determine where monitors should be located, 

obtaining access to sites, identifying funding, and procuring and installing equipment. 

Commenter (0088) stated the proposed schedule will not provide a reasonable amount of time for 

states and large sources to properly agree on the placement of new monitors, and details 

regarding their operation and maintenance in conformance with the EPA monitoring criteria set 

forth in the TAD. Commenter (0088) stated it is very important to note that these activities are 

performed in consecutive order and it is extremely unlikely for all of these to be completed in 

only 6 months, especially if there are several issues to be resolved between the EPA Regional 

Offices and state air agencies which could take up a significant portion if not all of the 6 month 

period. 

Commenter (0097) stated that, for facilities that do choose to operate monitoring equipment,

additional time will be needed to develop documentation between states and operators to ensure 

that sites are adequately maintained and that data are reported in a timely and complete manner. 

Commenter (0097) stated facility-operated monitoring sites will necessitate that the states create 

a quality assurance program consistent with the EPA requirements for the entire monitoring 

network. 
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Response:

See section IV.C of the preamble to the final rule for a discussion of these issues, where it is 

stated that the EPA recognizes that the logistical and financial burdens of installing an ambient 

air monitoring station can be varied in difficulty and the resources required. However, as noted 

earlier with regard to the overarching timetables effected by this rule, the Agency believes that, 

as with other parts of the implementation schedule, while the schedule for operating monitors is 

expeditious, it can be achieved with appropriate planning, coordination, and program 

implementation by the air agency which will allow monitoring resources to be in place by the 

deadline. The EPA believes that any further delay in air quality characterization around sources 

identified as a result of this rulemaking will delay implementation of the standard and public 

health protection in areas where there may be a violation of the standard. The Agency believes 

that it is most prudent to maintain the proposed timetable for monitoring network installation 

because of the need for use of these new data in a relatively timely manner for use in making 

attainment status decisions concerning SO2 areas in the country.

With respect to comments concerning documentation needing to be developed between states 

and “operators” (which the EPA assumes to be industry or contracted partners), the state is the 

responsible party in any scenario and must take steps to ensure that all data collection, quality 

assurance, and reporting requirements are met just as a state does for its own SLAMS data. 

Therefore, the EPA believes that it is likely that existing infrastructure can be leveraged if 

industry or contractor led monitoring operations are used to satisfy requirements of the SO2

DRR. 

2. Schedule needs to be changed

Comment:

Commenters (0045, 0088) recommended an extension of at least one year on all the proposed 

actions listed in the implementation timeline. Commenter (0088) stated that the EPA is well 

within its legal authority to simply designate states that intend to pursue monitoring as 

“unclassifiable” for the 1-hour SO2 NAAQS without any temporal regulatory requirements for 

collection or submission of any air quality data. In the face of this authority, commenter (0088) 

stated it is more than reasonable for the EPA to give states that indicate they will conduct air 

quality monitoring for the NAAQS, additional time far beyond one year. 

Commenter (0110) stated that resolution of the modeling issues (revision of Appendix W is 

planned for spring 2016) also affects the deadline for monitoring decisions, planning, and 

deployment because modeling can be a tool to determine where to place the monitor and

modeling results that show NAAQS violations may result in a source being moved into the 

monitoring path, but that result can only happen when a final modeling approach is ready and 

implemented.
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Commenters (0056, 0071) stated that the EPA should move up the deadline for submitting a 

monitoring plan to provide more time for state agencies to install the monitoring network after 

the monitoring protocol is submitted. Commenter (0071) stated it might be prudent to move the

deadline for final identification of sources to be monitored to an earlier date, for example, 

September 30, 2015. 

Response: 

As discussed in section IV.C of the preamble to the final rule, the EPA believes that any further 

delay in air quality characterization around sources identified as a result of this rulemaking will 

delay implementation of the standard and public health protection in areas where there may be a 

violation of the standard. Regarding the comments about AERMOD, the EPA believes that the

current AERMOD package available is sufficient for use in aiding in monitor siting exercises. 

The Agency believes that it is most prudent to maintain the proposed timetable for monitoring 

network plan submittal and network installation. A delay in planning and implementing new 

monitors would result in an unnecessary delay in making attainment status decisions concerning 

SO2 areas in the country and thus in addressing the air quality problems that these monitors may 

identify. The date for submitting network plan revisions is designed to match the date for 

submittal of the routine annual plan addressing any other network revisions, and requiring a 

separate plan or changing the date for the routine annual plan would create undue confusion and 

can unduly increase the workload for air agencies. To the extent that an air agency believes that 

additional time is needed to consult with the EPA regarding network plan revisions, the EPA 

encourages such consultation early in the process of developing network plans, irrespective of 

the deadline for network plan submittal. Conversely, if states wish to advance their selection of 

sources to be monitored to better ensure that they have time devise an appropriate monitoring 

plan, they always have this authority, and the final rule does not restrict this. However, we have 

not chosen to mandate such an early submission, given the already expeditious set of milestones 

established in the rule.

E. January 1, 2017: Requirement that SO2 monitors be operational

1. Time between monitoring plans due and monitors operational

a. Insufficient time

Comment:

Commenters (0039, 0045, 0055, 0074, 0084, 0087, 0088, 0107, 0108, 0110, 0113, and 0118) 

generally stated the short time period between the dates when the monitoring plans need to be 

submitted and the monitors are required to be operational is inadequate. Commenter (0088) 

stated it is technically infeasible to implement the Proposed Rule by 2017 and thus the EPA’s 

proposal is arbitrary and capricious. 
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Commenters (0038, 0039, 0051, 0056, 0075, 0083, 0088, 0107, 0110, and 0118) stated that even 

if monitoring plans are submitted on time, the EPA approval process could prevent a state from 

having monitors operational by the 2017 deadline. Commenter (0100) stated that the timing of 

Region 8 approvals of AQD's Network Plan has varied from 4 months to 15 months.

Commenters (0055, 0074) stated that, if one presumes it will take the EPA 3 to 6 months for 

approval (an expeditious schedule), then approval for the plan would not be forthcoming until 

near 2017, which leaves it unreasonable to expect the agency, or other entity, to be able to have 

monitors operational by January 1, 2017. Commenter (0097) also noted that the EPA requires a 

30 day public notice period for all annual monitoring network plans; this may also limit the 

amount of time available for network upgrades.

Commenters (0039, 0084) asserted there are a number of considerations that the EPA has not 

factored into this timeline that require additional time for proper installation of monitoring 

systems. Commenter (0084) noted, for example, proper siting will be required which could 

require easements or land purchases; proper equipment design, procurement, and delivery alone 

will require more than six months. Commenter (0069) stated that the time in which states may 

determine that modeling the source may be inappropriate could delay monitor deployment and 

result in incomplete data capture during first quarter 2017. Commenter (0110) stated that, once 

the monitoring plan is approved, the necessary equipment has to be procured, prepared, and 

installed on a site with considerable preparation needed, which can take at least 3 months, much 

of which is due to delivery times from equipment vendors. Commenter (0110) stated that, if this 

activity is happening simultaneously throughout the U.S. all at the same time, there may be 

equipment shortages and prolonged delivery times that could require additional months after the 

monitoring plan is approved to acquire the needed equipment.

Commenters (0039, 0045, 0087, 0088, 0107, 0108, 0109, and 0110) suggested providing states 

with additional time to begin actual monitoring. Commenters (0039, 0045, 0087, 0088, and 

0110) recommended an extension of at least one year. Commenter (0039) stated the date should 

be extended to January 1, 2018, if not, then the rule should not include a mandatory requirement 

to conduct modeling evaluations if monitoring has not begun by January 1, 2017. Commenter 

(0109) requested that the EPA extend the initial deadline for implementing monitoring programs 

under the Proposed Rule and create a reasonable extension option to address unavoidable 

implementation delays that occur along the way. Commenter (0110) recommended setting a 

schedule that provides for monitoring network implementation up to 12 months after submittal of 

a monitoring plan as a more reasonable approach. Commenter (0107) stated that other recent 

rules (NO2, SO2, and PM2.5) have not required such rapid deployment of monitors, but have each 

allowed at least 1.5 years from submittal of the network plan to operation of the monitor.

Commenter (0087) urged the EPA to consider extending the deadline to January 2019 and 

consider an "unclassifiable" designation until the planned monitoring is completed to allow at

least 1 full year of data before the 2020 designation deadline

Commenter (0038) suggested an alternative option to allow monitoring to begin later, such as 

between May 1, 2017 and July 1, 2017, for all sources being designated under the monitoring 

pathway. This approach would be consistent with an approach allowing states to declare the 
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monitoring pathway for sources up to January 1, 2017. Commenter (0038) recognized this 

approach would likely require delaying the attainment date, since design values are based on 

three calendar years of monitoring data. Commenter (0038) provided the following additional 

points.

EPA approval of state monitoring plans generally occurs in the fall of each year, but in 

some cases has not occurred until very late in the year. This means that EPA approval of 

an SO2 monitoring plan under this proposal may not occur until late 2016. In Wisconsin, 

monitor siting and set-up can generally only occur until late fall due to winter weather 

(snowfall and frozen ground); a late 2016 approval of a monitoring plan by EPA would 

simply not allow sufficient time for Wisconsin and other areas with winter weather

concerns set up a monitoring network by January 1, 2017.

Commenter (0118) stated that conditions associated with the Sierra Club CD may impact the 

feasibility of deploying new monitoring networks because sources or states may not have time 

adequately develop a plan, get approval for the plan, and have the network operational to rely on 

a monitor-based attainment determination. Commenter (0118) stated that conditions of the 

proposed CD do not provide enough time to redeploy monitors to demonstrate whether or not the 

NAAQS is being met. 

Response:

These issues are discussed in section IV.C of the preamble to the final rule, where it is stated that 

the EPA recognizes that the logistical and financial burdens of installing an ambient air 

monitoring station or stations can be varied in difficulty and the resources required. The EPA 

recognizes the potential logistical hurdles inherent in the installation of new monitoring sites, 

including property identification and access, and the installation of power and 

telecommunications, among other examples. However, due to the overarching timetables 

affected by this rule, the Agency believes that, while the schedule for operating monitors is 

expeditious, it can be achieved with appropriate planning, coordination, and program 

implementation by the air agency which will allow monitoring resources to be in place by the 

deadline. The EPA believes that any further delay in air quality characterization around sources 

identified as a result of this rulemaking will delay implementation of the standard and public 

health protection in areas where there may be a violation of the standard. The Agency believes 

that it is most prudent to maintain the proposed timetable for monitoring network installation 

because of the need for use of these new data in a relatively timely manner for use in making 

attainment status decisions concerning SO2 areas in the country. (This includes meeting the

agency’s obligations to complete area designations pursuant to the CD, which for the most part 

will allow those decisions to be informed by quality information that is timely generated by the 

deadlines established by this final rule.)

In response to comments about Regional approval of monitoring plans, the EPA encourages 

states and local air agencies to engage their respective Regional contacts as early as possible on 

monitoring network design. The end result of early engagement between states and EPA Regions 
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will be states developing monitoring plans that are less likely to have difficulty receiving 

approval.

b. Reasonable time frame

Comment:

Commenter (0058) stated the DRR establishes a reasonable timeframe for states to collect the 

data, either through monitoring or modeling, that is needed to characterize air quality in these 

“unclassifiable” areas and determine whether the 1-hour SO2 NAAQS is being met. Commenter 

(0060) stated that the feasibility of this time period will be dependent upon the threshold option 

selected by the EPA. 

Response:

The EPA believes that the threshold chosen in the final rule does not create a situation where 

data collection required to satisfy the rule is infeasible. The final rule, which establishes a 

threshold (2,000 tpy) that is expected to cover fewer sources than proposed, and in EPA’s 

judgment, with proper planning, the timelines are expeditious but manageable.

2. EPA oversight of monitoring plans

a. Timing for EPA determination

Comment:

Commenters (0038, 0039, 0051, 0056, 0075, 0083, 0088, 0107, 0110, and 0118) stated that even 

if monitoring plans are submitted on time, the EPA approval process could prevent a state from 

having monitors operational by the 2017 deadline. 

Commenter (0107) stated that, as states typically submit the network plan by July 1, with EPA

approval coming in late September, it would virtually be impossible to then have the entire SO2

source oriented network operational three months later, especially if there is extensive 

negotiation with EPA's regional office on the number and location of monitors. Commenter 

(0107) stated such negotiation is expected given:

• EPA's Technical Assistance Document (TAD) is vague about monitor siting, deferring to

"case-by-case" analysis of each facility.

• This is a new rule with no prior experience to use to gauge the most appropriate network 

to propose.

• EPA regional approval of network plans may be delayed due to a desire for consistent

decisions among regions, states, and facilities.
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Commenters (0039, 0055, 0074, and 0088) stated it would be imprudent for a state agency or a 

source to invest in a monitoring network until after the EPA’s blessing of the network had been 

obtained and it will simply be impossible for states and sources to begin monitoring by January 

1, 2017. Commenter (0088) stated this provision must be amended to provide any basis for SO2

monitoring plans to be successfully implemented. Commenter (0056) stated that, if monitor 

deployment is delayed, it adds to the uncertainty of taking the monitoring approach and makes it 

a less attractive option. 

Commenters (0051, 0055, 0056, 0074, 0083, 0088, and 0100) stated the EPA needs to establish a 

clear and timely process under which states can obtain a determination from the EPA that an 

existing or new monitoring network is adequate for making attainment designations. Commenter 

(0051) stated the words,"... as expeditiously as practicable ..." for the EPA's action on the 

protocols are not adequate. Commenters (0055, 0074) stated the EPA needs to either allow more 

time for the establishment of the network (additional 6 months or tolling of deadlines until 

approval is granted) or bind itself to a fast review and approval period. Commenter (0083) stated 

the rule should include a section with required EPA actions and associated timeframes.

Commenter (0051) stated that since the states are required to submit protocols by a certain date, 

the EPA Regional Offices should be required to submit their written approvals or disapprovals 

(via deficiency letters) to the states within 60 days of submittal of each protocol and no later than 

March 15, 2016. Commenter (0100) suggested an initial approval from the Region within 90 

days of the submittal of a demonstration in order to allow the state and industry to commence the 

preparatory work in a timely manner. 

Commenter (0100) suggested that the EPA allow agencies and industry an alternate approval 

process and allow monitoring to take place earlier if the approval process has been followed.

Commenter (0113) suggested that the EPA have a site approval mechanism in place independent 

of a network plan so that monitoring agencies have at least a full year to prepare for the newly 

required monitoring sites; otherwise, monitoring may not be a feasible option under the DRR.

Commenter (0051) suggested an appeals process be part of this rule package, for the sake of 

national consistency. In order to help provide transparency and consistency for states, 

commenters (0051) suggested the EPA Regional Office should provide all submitted protocols 

and the approval status of each on an easily accessible agency webpage. Commenter (0051) 

stated the EPA should establish data sharing procedures. 

Response:

As discussed in section IV.C of the preamble to the final rule, the Agency believes that 

monitoring resources can be appropriately put in place by the January 1, 2017 deadline to satisfy 

this rule, particularly if air agencies begin planning as soon as possible. The EPA has encouraged 

air agencies to begin the monitor planning process as early as possible. The EPA believes that 

while the schedule for meeting the requirements of this rule is expeditious, the schedule can be 
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achieved with the appropriate planning, coordination, and program implementation by affected 

air agencies. The EPA strongly encourages air agencies to start their investigation of this issue as 

soon as practicable. The EPA also encourages each air agency to consult with its respective EPA 

Regional Office to identify sources exceeding the emission threshold in the final rule and any 

other sources that do not exceed the emission threshold but which would warrant the

characterization of nearby air quality.

In addition, the EPA believes that it is necessary to establish expeditious timelines to ensure 

timely availability of air quality information. With this in mind, and in light of the many 

logistical concerns raised by commenters and recognized by the EPA, the Agency is encouraging 

air agencies to engage with their respective EPA Regional Offices well in advance of the time by 

which the Annual Monitoring Network Plan is due. Notably, there is nothing preventing a state 

from sharing their draft SO2 network design plan for SO2 monitoring intended to satisfy this rule 

with the EPA and the public in advance of the complete Annual Monitoring Network Plan. The 

end result of early engagement between states and EPA Regions will be states developing 

monitoring plans that are less likely to have difficulty receiving approval.

b. Eliminate EPA approval of monitoring plans

Comment:

[Also see IV.B.3, EPA oversight of modeling plans.]

Commenter (0088) stated EPA oversight/approval of state monitoring plans should be 

eliminated. Commenter (0088) provided the following additional points.

Proposed section 51.1203(c) of the SO2 Data rule appears to require EPA consultation 

and approval of a state’s monitoring plan. NEDA/CAP submits that this EPA oversight 

and approval unnecessarily interferes with state decisions and should be deleted as a 

requirement. Modifications of a monitoring plan can be made after submission of 

monitoring data and “tweaks” to the monitoring plan should be allowed and encouraged 

for the collection of monitoring data up to, during and following the execution of the 

plan. The EPA experts know that this is technically necessary for such monitoring to be 

successful, and therefore it is unreasonable for the agency to pretend such flexibility is 

not critical to a successful monitoring program. Moreover, given the primacy of state air 

programs under the CAA, EPA’s proposal is not consistent with EPA oversight – not 

active participation – in how an individual state will implement the SO2 monitoring 

protocols. The EPA’s authority in this matter is limited to the review of the monitoring 

data.

Commenter (0088) stated it is arbitrary and capricious to require EPA approval of state 

monitoring and modeling plans when the EPA’s technical resources are too stretched to provide 

this oversight in a timely manner. Commenter (0088) recommended that the roles of the EPA
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experts should be advisory only in initial implementation and section 51.1203 should be revised 

accordingly. Commenter (0088) provided the following additional points.

While it is critical to acknowledge that the EPA regional monitoring experts can be 

helpful in advising states on monitoring (and modeling) plans, there are simply not 

enough of them to supervise, let alone review and act on, the number of state monitoring 

programs necessary to implement the data rule in a timely manner. Most EPA regions 

have only one such monitoring/modeling expert and OAQPS has fewer than five such 

experts. It therefore would be highly unreasonable for the EPA therefore to require as a 

precondition for implementation of a SO2 monitoring plan, EPA review and 

concurrence/approval before it is implemented. Even if the compliance deadlines for such 

plan requirements and implementation were extended by substantial amounts, this 

requirement would still be unreasonable. 

Once monitoring data are collected, it would then be appropriate for these experts to 

review collected monitoring data in the context of the overall state monitoring plan and 

goals. If the experts review concludes that monitors should be relocated or additional 

monitors should be sited, this input can be sought and offered in an advisory role so as 

not to prejudice a state’s efforts to comply with the data rule. If adjustments or others 

changes to a state’s monitoring plan are suggested after data are collected and reviewed, 

the regional air office can deem the air quality control to be unclassified until other 

monitors are sited and data collected. This solution would provide an opportunity for 

EPA input without overwhelming states and source’s judgment and ability to meet even 

an extended timeframe for implementing monitors. (0088)

Commenters (0055, 0074) asked if submittal of the (state) agency annual network plan (7/1/16), 

as it relates specifically to this SO2 monitoring, requires EPA approval. Commenters (0055, 

0074) stated that the preamble (for example, p. 27458) seems to encourage agencies to work with 

the EPA Regional Offices in developing the network plans, while the Proposed Rule in section 

51.1203(c)(3) refers to an “approved Annual Monitoring Network Plan.” Commenter (0100) 

stated that since the Proposed Rule in 51.1203(c) only requires consultation, it is not clear how 

the timing of the approval process will work in conjunction with time needed to deploy 

monitoring by January 1, 2017. 

Response:

The EPA Regional Administrator approval of state Annual Monitoring Network Plans is 

required, per language in 40 CFR part 58, 58.14(a), and modification to that regulatory text was 

not proposed, nor did the commenter provide a reason why this longstanding requirement should 

be reversed. The Agency judges plan approval as a crucial step in the interaction between the 

Agency and states, leading to the appropriate installation, modification, operation, and 

maintenance of ambient monitoring networks, and thus there was no proposed action in this 

rulemaking to deviate from that existing process. Notably, in section 58.10(a), it is stated that the 

EPA will respond within 120 days. States are encouraged to work with their respective EPA 
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Regions regarding critical issues and details germane to satisfying the SO2 DRR in advance of 

their monitoring network plan submittal so that when the plan is formally submitted, there is less 

likelihood of difficulty in getting that plan approved. 

3. Phased approach

Comment:

Commenters (0046, 0051, 0052a, 0107) suggested a phased approach for operation of the 

monitoring network. Commenter (0046) suggested that a phased approach for monitoring that 

begins with Option 3 thresholds would minimize the burden, allow for relocation of 

inappropriately placed monitors, and ensure that the high impact sources are adequately 

addressed. Commenter (0052a) stated the implementation schedule should consider available 

resources and suggested a structured phase-in schedule should be developed. Commenter 

(0052a) provided the following details.

The phase-in schedule for source-oriented monitoring should be prioritized based upon 

an actual emissions threshold. The EPA should establish a requirement for a first phase of 

monitor installations to be completed by January 1, 2015 and a second by January 1, 

2017. The emissions thresholds used in this approach should be established through 

rulemaking, and should consider anticipated future SO2 emissions after full 

implementation of major rules impacting these emissions such as the 2011 Cross- State 

Air Pollution Rule (CSAPR), the 2011 MATS Rule, and the reconsidered the Boiler 

MACT standards for industrial, commercial, and institutional boilers along with all

associated installation and operating costs. Further, as implementing agencies prioritize 

the placement of monitors consistent with this emissions threshold approach, they should 

be allowed to consider other factors such as population and source stack height. This 

approach would allow states to focus their resources in areas with the highest risk to 

public health and the largest sources of SO2 emissions. (0052a)

Commenters (0051, 0107) stated the EPA should allow a phased approach similar to the nearroad monitoring implementation schedule. Commenters (0051, 0107) provided the following 

additional points.

Texas currently has a network of 26 stationary SO2 monitors. Most of these monitors are 

in public locations, and the data are representative of concentrations to which the public 

would be exposed. Data from this monitoring network do not suggest a widespread public 

health concern indicative of such an onerous implementation schedule as the one 

proposed. As such, the EPA should extend the implementation timeline to provide states 

more reasonable time frames for revising the existing monitoring program. Phase 1 

would address sources with emissions of 20,000 tpy and greater by January 1, 2017. 

Phase 2 would address source emissions of 7,500 tpy and greater by January 1, 2018. 

Phase 3 would address all remaining source emissions required by January 1, 2019. This 

phased approach would allow for attainment designation by 2020 for the largest SO2
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emitters while at the same time allowing states to procure funds and resources. (0051)

Commenter (0107) stated that, since the state would be establishing at least 10 SO2 sites in little 

more than two years while the EPA is allowing the state more than four years to establish five 

near-road NO2 sites, even the pace of the following suggested alternate SO2 schedule seems a bit 

ambitious. Commenter (0107) recommended the following thresholds and schedule.

 Initial monitor for facilities above 5,000 tons begin operating by Jan 1, 2017;

 Initial monitor for facilities above 3,000 tons begin operating by Jan 1, 2018;

 Initial monitor for facilities above 1,000 tons begin operating by Jan 1, 2019;

 If needed, additional monitors for the same facility begin operating at the rate of one

every six months after the initial monitor.

Response:

The recommendations for phased monitoring would delay the commencement of significant air 

quality characterization required under this rule. As discussed in section IV.C and in subsequent 

discussions in the preamble to the final rule, the issue of timing is discussed, where it is stated 

that the proposed timetable for conducting modeling and installing monitoring sites is critical 

because of the need for these new data to be available to support future determinations 

concerning the attainment status of areas. The EPA believes that any further delay in air quality 

characterization around sources identified as a result of this rulemaking could delay the 

implementation of the SO2 NAAQS and therefore could impede public health protection in areas 

that would show violations of the standard based on the data to be gathered under the rule. The 

EPA also believes that any significant delays in monitors becoming operational past the date of 

January 1, 2017, will certainly delay the potential for monitoring data to be used to inform 

actions that depend upon ambient concentration assessments, possibly past calendar year 2021. 

Finally, the EPA notes that under the terms of the previously discussed CD, in order to avoid the 

EPA being required to designate an area by December 31, 2017, an air agency will need to have 

installed and begun operating the new SO2 monitoring system no later than January 1, 2017.

Comment

Commenter (0097) stated the EPA could adopt an extension approach, similar to that included 

for ozone under section 172(a)(2)(C). If one full year of SO2 monitoring data were obtained, and 

this data showed no exceedance (i.e. the 99th percentile of the data was not above 75 ppb), then 

EPA could offer two years of extensions before making the designation determination, 

contingent on each year's data recording values less than 75 ppb for the 99th percentile each 

year. Such an approach would allow states more time to upgrade their monitoring networks for 

this effort. Commenter (0097) stated that, if an extension option and provisional compliance 

based on 2019 data are offered to states, the timeline EPA discusses within the rule could be 

modified as noted below:

DATE MILESTONE
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4/17/2014 EPA Proposes Data Rule

12/31/2014 EPA Finalizes Data Rule

1/15/2016 Identify Modeled & Monitored Sources

1/15/2016 Submit Modeling Protocols to EPA

7/1/2018 Submit Annual Monitoring Network Plans to EPA

1/1/2019 Commence Monitoring

1/13/2017 Submit Designation Recommendations Based on Modeling

8/31/2017 EPA Proposes Designations Based on Modeling

12/31/2017 EPA Finalizes Designations Based on Modeling

2/28/2018 Designation Effective Date Based on Modeling

8/31/2019 State Submits SO2 SIP for Modeled Areas

12/31/2019 Complete First Year Monitoring

5/1/2020 Submit Designation Recommendations Based on at least 

1 year of monitoring or submit extension request

5/1/2020 Required Certification of 2019 Monitoring Data

8/31/2020 EPA Proposes Designations Based on Monitoring or

publishes extensions

12/31/2020 EPA Finalizes Designations Based on Monitoring for areas

not receiving extensions

2/28/2021 Designation Effective Date Based on Monitoring for areas 

not receiving extensions

2/28/2021 EPA Approves SO2 SIP for Modeled Areas

8/31/2022 State Submits SO2 SIP for Monitored Areas

2/28/2023 Final Attainment for Modeled Areas and designations for successful 

extension requests

2/28/2024 EPA Approves SO2 SIP for Monitored Areas

2/28/2026 Final Attainment for Monitored Areas

Response: 

As previously explained, in this final rule the EPA is not establishing requirements for 

designations or any deadlines for such actions. The timing of designations is now to be governed 

by the CD entered by the court in March 2015. However, the EPA notes that the CAA section 

172(a)(2)(C) provisions for extensions of time for nonattainment areas to attain a NAAQS are 

not applicable or relevant for the timing of EPA promulgation of designations, even if the agency 

were not already subject to a court order setting forth a binding schedule to take those actions.

The EPA believes that any further delay in air quality characterization around sources identified 

as a result of this rulemaking would delay the implementation of the SO2 NAAQS and therefore 

would impede public health protection in areas that in the future show violations of the standard 

based on the data to be gathered under the rule. The EPA also believes that any significant delays 

in monitors becoming operational past the date of January 1, 2017, will certainly delay the 

potential for monitoring data to be used to inform actions that depend upon ambient 
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concentration assessments, possibly past calendar year 2021. Finally, the EPA notes that under 

the terms of the March 2015 CD, in order to avoid the EPA being required to designate an area 

by December 31, 2017, an air agency will need to have installed and begun operating the new 

SO2 monitoring system no later than January 1, 2017. 

The Agency believes that it is very important to maintain the proposed timetable for conducting 

modeling and installing monitoring sites because of the need for these new data to be available to 

support future determinations concerning the attainment status of areas. The EPA encourages 

each air agency to engage in early dialogue with the appropriate EPA Regional Office and with 

the identified applicable facilities in order to meet the requirements of the rule. In particular, in 

light of the reality of the sometimes complex process of identifying potential monitoring 

locations, securing funding, and installing an appropriate number of new sites, if an air agency is 

considering the monitoring approach for one or more areas, early coordination should improve 

the air agency’s potential for success in meeting the timing and requirements of the rule.

F. January 13, 2017: Requirement to provide modeling analyses

1. Schedule

a. Schedule may be adequate

Comment:

Commenter (0041) stated that the January 2017 date to provide modeling is adequate,

particularly if the number of affected sources is relatively small, as the AQD has proposed. 

Commenter (0060) stated that the feasibility of this time period will be dependent upon the 

threshold option selected by the EPA. Commenter (0085) supported the proposed requirement 

that air quality modeling is conducted well in advance of the January 2017 deadline for states to 

submit modeling analyses to the EPA.

Response: 

The EPA believes that the threshold chosen in the final rule does not create a situation where 

modeling or monitoring required to satisfy the rule is infeasible. The final rule, which establishes 

a threshold (2,000 tpy) that is higher than the threshold that EPA proposed for urban areas (1,000 

tpy) and applies the same threshold as was proposed for other parts of the country will ease state 

efforts to comply with the timetable in the rule. (This includes meeting the agency’s obligations 

to complete area designations pursuant to the CD, which for the most part will allow those 

decisions to be informed by quality information that is timely generated by the deadlines 

established by this final rule.)

b. Schedule is not feasible
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Comment:

Commenter (0051) stated the proposed modeling timeline is not feasible and there is insufficient 

time for informed state planning, modeling preparation, and careful modeling analysis.

Commenter (0051) stated that, with the potential challenge of conducting several multi-source 

analyses using approaches typical for permit evaluation and single-source evaluations, two to 

four years may be needed for adequate to complete adequate modeling demonstrations. 

Commenter (0051) provided the following additional points.

The TCEQ estimates there could be a minimum of 12 and as many as 32 modeling 

protocols required. One year is not enough time for the state to develop a strategy, 

consult with sources and the EPA, develop modeling protocols, and obtain the EPA's 

approval of the protocols. Assuming that emissions thresholds, population limits, and 

other tools limit the number of sources to be explicitly modeled, the modeling itself (not 

including emission inventory and meteorological input development; protocol 

development; preparatory work; post-processing of results; final analysis; and report 

development) could be completed in approximately one to two years, with a team of six 

to eight modelers and analysts working full-time. Texas does not have these resources. In 

addition, the EPA significantly underestimates the cost of a refined modeling analysis.

One year is not enough time to complete modeling demonstrations. Depending on the 

scope of the modeling required, it would take two to four years to complete the entire 

process. The modeling time estimate will increase if refined modeling is required to site 

monitors and if the EPA expects the states to submit modeling protocols and not conduct 

any refined modeling to support monitor placement decisions until the EPA approves the 

protocols.

Commenter (0060) stated Ohio could potentially have to model or monitor anywhere from 10 to 

31 facilities under the three options proposed and noted that electing to monitor will require a 

significant modeling component to determine monitor locations. Commenter (0060) stated that, 

depending on the number of sources to address, the strain on modeling resources by the air 

agency may be too great in such a short time. Commenter (0060) noted these efforts will have to 

occur simultaneously with the work air agencies are performing to finalize modeling under the 

nonattainment area requirements for SO2.

Commenter (0052a) stated the availability of onsite data are a key factor to ensuring credible 

model results and, therefore, EPA should allow sufficient time for states and the regulated 

community to collect onsite meteorological and ambient air measurements, and to evaluate 

model performance when necessary. Commenter (0110) stated a requirement to collect three 

years of on-site data are inconsistent with the proposed designations timeline since such a 

requirement would make it impossible to use new on-site data and still adhere to the anticipated 

EPA schedule.

Commenter (0042, attachment B) suggested an extension of the current implementation schedule

may be necessary. Commenter (0042) stated recent revisions to AERMOD may require 
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additional time to ensure proper operation and the AERMOD shakeout period may warrant an 

extension of the current implementation schedule for attainment designations for SO2.

Commenter (0042, attachment A5) stated that, before emissions reductions are sought, states 

should first consult with the affected source to explore improvements or refinements to the initial 

modeling to help ensure that modeled impacts are “real”. Commenter (0042, attachment B) 

stated that many historical modeling inventories were developed for historical requirements;

therefore, existing inventories may require extensive review for use. Commenter (0042, 

attachment B) stated the shape of existing modeling inventories may warrant an extension of the 

current implementation schedule.

Commenters (0060, 0069) stated that, as states will not know if there will be delays beyond their 

control until after the January 15, 2016 modeling protocol deadline or the July 2016 monitoring 

network modification deadline, there will be insufficient time to complete a modeling protocol, 

complete modeling, prepare modeling analyses and, if necessary, determine boundary 

recommendations, by the January 13, 2017 deadline. 

Response: 

As discussed in the preamble to the final rule, the EPA believes that the schedule for completion 

of modeling and commencement of monitoring is expeditious but feasible. The commenters cite 

numbers of sources they expect to need to address, but they otherwise do not explain why these 

analyses will take longer than the timetable provided by the rule. The CAA provides 18 months 

for preparation of SO2 nonattainment plans, irrespective of how many nonattainment areas a state 

has, and the EPA believes that modeling for air quality characterization purposes, which does not 

involve establishment of emission limits, will require less time. Furthermore, the EPA 

anticipates that the modeling approach for multiple areas within a state will have many features 

in common, so that modeling several sources will not necessarily take several times longer. The 

EPA believes that this information must be obtained as expeditiously as practicable in order to 

provide the health protections from implementation of the SO2 standard as early as possible. To 

the extent that collection of additional meteorological data would delay the schedule for 

implementing the standard, the EPA believes that existing data, used in accordance with the 

modeling TAD, should be used. Similarly, concerns about implementing revised versions of 

AERMOD, and about unforeseen difficulties, do not justify promulgating a delayed schedule for 

air quality characterization requirements. EPA also believes that states should invest in 

resources, i.e. software to aid in the development of model runs and post-processing of results for 

multiple sources. These would be software packages that would automate data manipulation and 

calculations of model inputs, such as creating AERMOD input files, and post-processing 

AERMOD results to calculate relevant metrics such as design values. Many software packages, 

such as scripting languages are open source and require minimal training. The EPA anticipates 

that many state modelers are also technically competent in programming languages such as 

FORTRAN that can aid in automation of the model runs.

2. EPA review of the modeling analyses
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Comment:

To ensure that the timeframes for specific agency requirements included in the Proposed Rule

can be met, commenter (0083) stated that the rule should include a section with required EPA 

actions and associated timeframes. Commenter (0106) stated that, if the modeling protocol is 

submitted by January 15, 2016, the Department believes the final modeling analysis should not 

be due until one year after the Regional Administrator has completed review of the protocol; this 

will make EPA a partner in assuring that the modeling analyses are completed in a timely 

manner and afford the States the time necessary to complete the final modeling analysis.

Commenter (0093) disagreed with the requirement that modeling analyses be submitted to EPA. 

Commenter (0093) asserted that these analyses will be reviewed by the state agency and a 

summary of the analyses can be forwarded to the EPA with sufficient information for the EPA to 

evaluate.

Response:

See section IV. C of the preamble on “Data Requirements and Program Implementation 

Timeline” concerning this issue. This section discusses the actions that are required under this 

rule. The EPA does not believe that state responsibilities under this rule should be contingent on 

EPA action; thus, the preamble discusses the benefits of consultations between the air agency 

and the EPA on issues that a modeling protocol would be expected to address, but the EPA is not 

imposing a deadline on itself for action related to a submitted protocol, nor is it making the 

deadline for state action depend on approval or disapproval of a protocol by the EPA. This rule 

does not otherwise impose requirements on the EPA, but the timeline includes the requirements 

on the EPA for designation of areas pursuant to CD.

3. Updated designation recommendations

a. Procedures

Comment:

Commenter (0041) stated that it is important to include the 120-day review by states prior to the 

EPA's final designation of areas, as proposed in the rule. Commenter (0068) stated any final Rule 

must include explicit provisions for public comment periods on proposed designations, to ensure 

that the public has adequate opportunity for input on the critical designations step of SO2 

NAAQS implementation.

Commenter (0103) stated that areas found to be in nonattainment through implementation of the 

Proposed Rule could be brought into a “maintenance” plan. Commenter (0103) stated that this 

approach would be consistent with the requirements of the Act. 
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Response:

In the final rulemaking we are not addressing comments received on the proposed rule 

concerning how the future designation process (or other processes that are informed by data 

generated through implementation of this rule) because those issues are beyond the scope of this 

rulemaking. This rule is not promulgating requirements for how designations, re-designations, 

clean data determinations, or determinations of attainment following SIP implementation shall be 

conducted. Comments on how information should be used in those processes may be raised 

when those future actions occur.

b. March 24, 2011 designation guidance

Comment:

Commenter (0060) stated that, although the proposal requires any updated designation 

recommendations to follow the most recent SO2 designation guidance, that guidance (March 24, 

2011) conflicts with this rule. Commenter (0060) stated, for example, the document contains 

modeling guidance in Attachment 3 which pre-dates the draft modeling TAD. Commenter 

(0060) stated it is inappropriate to require the use of the March 24, 2011 guidance without 

making it clear in the preamble of this rule that the guidelines laid out in Attachment 3 are no 

longer appropriate or required in many cases.

Response:

As explained above, in the final rulemaking we are not addressing comments received on the 

proposed rule concerning the designation process because those issues are beyond the scope of 

this rulemaking. However, we note that the Agency has issued updated designations guidance 

providing recommendations for air agencies. This guidance may be found at 

http://www.epa.gov/airquality/sulfurdioxide/pdfs/20150320SO2designations.pdf.

c. Guidance needed on boundaries and areas without significant SO2

sources

Comment:

Commenters (0051, 0103, and 0106) stated the EPA should issue timely guidance on selecting 

designation levels and boundaries, including guidance on addressing areas without SO2 sources.

Commenter (0051) stated this is needed to promote national consistency for this round of 

boundary recommendations. Commenters (0103, 0106) requested that this guidance be provided 

in time to allow states to complete designation recommendations for these areas on a schedule 

that would be synchronized with the designation schedule that the EPA finalizes in this rule.

Commenter (0106) stated that the EPA must provide this boundary guidance no later than the 

end of 2015, and that, without guidance from the EPA early in the process, delays could occur in 

establishing boundaries and submittal of the final modeling analysis and designation 
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recommendations. 

Commenter (0051) stated the proposal does not clearly explain whether the state must use 

modeling or monitoring data (or both) to identify areas of the state as unclassifiable, or whether 

the state could demonstrate attainment in other ways; for example, areas could be designated 

based on information such as: no stationary sources of SO2 in a county; sources well below the 

threshold; or with statewide screening modeling using a photochemical model that predicts 99th 

percentile concentrations less than the SO2 standard. Commenter (0106) stated the EPA must 

clarify what information it believes is necessary to define the boundaries, as well as what 

information it intends to use for designating areas that are distant from larger SO2 sources.

Commenter (0046) stated that, while the EPA has recently relied on multiple-factor analyses to 

determine the extent of each nonattainment area (emissions, meteorology,

geography/topography, and jurisdictional boundaries), designations based on AERMOD should 

not use this type of analysis. Commenter (0046) stated the AERMOD results inherently account 

for factors such as the source emissions, meteorology, and topography and AERMOD is only 

considered valid over a relatively short distance. Commenters (0046, 0049) stated that 

nonattainment area designations based on the results of AERMOD should be limited to the 

immediate area of the source/receptors modeling nonattainment. Commenter (0046) stated that, 

if AERMOD is the basis of the nonattainment designation, in order to maintain consistency and 

the integrity of using AERMOD results to determine nonattainment, the areas AERMOD 

demonstrates are attaining the standard must be designated as attainment.

Commenter (0049) asserted that the EPA uses the term "source areas" when talking about which 

areas would be identified for air quality characterization but then also talks about characterizing 

the air quality around multiple smaller sources located in close proximity whose combined 

emissions may violate the standard. Commenter (0049) stated that given that the EPA considers 

the 1-hour SO2 NAAQS to be a source-oriented NAAQS instead of a regional NAAQS, such as 

the ozone NAAQS, the defined area of nonattainment for purposes of an area's designation 

should be as closely aligned as possible to the area surrounding a particular source causing a 

violation. Commenter (0049) stated this would fit with the EPA's notion of "source areas" when 

referring to areas that require further air quality characterization and align with the EPA's view 

of the SO2 NAAQS as a source-oriented standard.

Commenter (0060) did not agree with the proposal that an air agency should not recommend a 

designation for any county that includes a source area with new monitoring. Commenter (0060) 

stated there should be flexibility, with appropriate modeling, for an option to request partial 

attainment of counties with new monitors. Commenter (0060) stated air agencies may model 

actual emissions to determine monitor location and use that to inform boundaries for areas 

outside of the potential impact area, especially when these counties may border very rural 

counties with little to no emission sources.

Commenter (0046) stated that no modeling results should be considered where source-oriented 

monitoring data are available, other than possibly to help determine the nonattainment area 
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boundary. Commenter (0049) stated that given the source specific nature of this monitoring 

approach, nonattainment areas should be as small as possible and only contain the area impacted 

by the source.

Commenter (0081) stated that, because the highest 1-hour SO2 concentrations are likely to be 

localized in a relatively small area around applicable sources, CBSAs, metropolitan statistical 

areas (MSAs), or other jurisdictional areas should not be the presumptive boundary for the 

compliance determination. Commenter (0081) stated this approach would be consistent with 

most of the 1-hour SO2 nonattainment areas which were designated in 2013 and would also keep 

the plans which are to be submitted to the EPA by January 2016 succinct as well as accurate, 

leading to a more effective and efficient review and approval process.

Response:

In the final rulemaking we are not addressing comments received on the proposed rule 

concerning the designation process because those issues are beyond the scope of this rulemaking.

However, the comments also raise issues about how modeling should be conducted pursuant to 

this rule, and we direct the commenters to section IV.D.6 of the preamble to the rulemaking for a 

complete response regarding those issues.

d. Designation as "attainment" or “unclassifiable/attainment”

Commenters (0071, 0081, and 0106) asked the EPA to provide clarification. Commenters (0071, 

0106) stated that under this rule, if a state models attainment and recommends an attainment 

designation for an area, it is unclear whether or not the EPA would designate the area attainment, 

and not "unclassifiable/attainment", based solely on modeling. Commenter (0071) stated that, if 

modeling is chosen as the method for characterizing air quality in an area, then the EPA should 

designate that area attainment when the state submits sufficient modeling data to support such a 

designation. Commenter (0071) requested clarification on the criteria the EPA deems necessary 

to achieve an attainment designation.

Commenter (0081) stated more clarification is needed regarding the option to designate “any 

unclassifiable/attainment areas as appropriate.” Commenter (0081) states that as written this

could include all areas for which a modeling demonstration has been submitted and compliance 

with the standard has been proven. Commenter (0081) stated that if the EPA does not expect 

there to be future designation decision options which have not been included in this Proposed 

Rule or the TADs, then the EPA should amend this wording in the final rule to read “any area 

which has demonstrated attainment based on modeling of actual emissions” or clarify otherwise.

Commenter (0106) stated the EPA's discussion on the "unclassifiable/attainment" designation is 

inconsistent and unclear throughout the preamble and casts doubt on what the EPA means when 

it issues an "attainment" designation. Commenter (0106) stated the EPA should clarify under 

what circumstances it is now proposing to issue designations of "attainment" or "unclassifiable", 
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and how different the consequences would be for areas with the two different designations.

Commenter (0106) provided the following additional points. 

The proposal is inconsistent in how it describes the designation of areas for which 

modeling is conducted: at page 27466, the EPA describes the designation of areas for 

which modeling is conducted as being "unclassifiable/attainment" while, immediately 

preceding this passage as well as in the Proposed Rule, the EPA describes the purpose of 

this modeling is "to demonstrate continued attainment for an area around a source." The 

same confusion exists in the Proposed Rule at 79 Fed. Reg. 27459, where the EPA states 

an "unclassifiable/attainment" designation is appropriate for areas of a state that are 

distant from larger SO2 sources. 

Commenter (0106) stated the CAA establishes a clear difference between "attainment" 

and "unclassifiable" and the EPA cannot through this rulemaking rewrite the CAA to 

redefine what "attainment" and "unclassifiable" mean. 

If approved modeling data are supposed to be a surrogate for approved monitoring data, it 

seems logical that the successful conclusion of this process would be a designation of 

"attainment", not "unclassifiable/attainment". Only for areas distant from larger SO2

sources, or with no SO2 sources, could a designation of unclassifiable be justifiable if 

there is no monitoring or modeling data for these areas. It has been assumed till now that 

the conclusion of this process of providing the EPA with additional monitoring or 

modeling data to support an attainment designation would result in an unqualified 

"attainment" designation, at least in the vicinity of modeled sources. This point of 

confusion needs to be explained. 

Commenter (0049) objected to the proposal that an area will be designated as “unclassifiable” 

without modeling data showing attainment of the standard, even if the area’s monitors show no 

violations, based on the assumption that there may be a potential to cause or contribute to a 

violation of the NAAQS. Commenter (0049) stated this new approach is contrary to the express 

language of the CAA as, under CAA section 107(d), an area can only be designated as 

unclassifiable if it “cannot be classified on the basis of available information as meeting or not 

meeting the national primary or secondary ambient air quality standard for the pollutant.”

Commenter (0049) stated monitoring data showing no violations of the standard qualifies as 

“available information” showing that the area is meeting the standard. Commenter (0049) also 

stated the designation process under CAA section 107(d) does not involve consideration of the 

“potential to cause or contribute to a violation of the NAAQS,” but only requires the 

consideration of whether an area actually meets, or actually does not meet, the NAAQS.

Response: 

In the final rulemaking we are not addressing comments received on the proposed rule 

concerning the designation process because those issues are beyond the scope of this rulemaking. 
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4. Clarify terms

Comment:

Commenters (0049, 0081, 0097, 0100, and 0107) requested the EPA to define certain terms, 

including “area”. Commenter (0049) stated that, throughout the Proposed Rule, the EPA uses the 

term “area” without indicating whether geographic boundaries of the area correspond to a 

county, CBSA, Metropolitan Planning Organization (MPO), or some other jurisdictional area.

Commenters (0049, 0081) stated, given that the EPA is focused on the short-term impacts within 

the close proximity of major SO2 sources, any monitored data or modeling analysis indicating a 

potential air quality concern should only result in a nonattainment designation for a geographic 

area that is also limited in scope.

Commenter (0097) stated it would helpful if the EPA were to clarify their intent for the phrases 

"source areas," "sources," "units," and "stacks." Commenter (0097) stated, for example, it is 

unclear if the EPA is expecting by January 13, 2017 a modeling demonstration for a particular 

source or set of sources, for all sources within an entire area, or if either situation might be 

acceptable depending on circumstances. 

Commenter (0100) requested the EPA to clarify if non-applicable sources are required in 

51.1203 (b) to participate in the modeling/monitoring characterization if an area is found to be 

subject to a demonstration. Commenter (0100) stated the language in this sentence first refers to 

"applicable sources" then discusses "all sources" in an area. 

Response:

As a general matter, an “area” refers to the area that might warrant consideration as being 

impacted by emissions from the sources. Thus, air agencies are to provide information based on 

consideration of all locations within reasonable proximity of sources that might be impacted by 

their emissions. “Source area” refers to the area around and impacted by emissions from an 

applicable source as defined by this rule, including any other sources within or nearby that area 

that influence its concentrations. While an area or source area may include only one “applicable 

source,” the air quality characterization that this source triggers must address the concentrations 

resulting from all sources that influence that area’s air quality.

G. December 2017: Intended date for issuance of final designations

for majority of country

Although the preamble for EPA’s proposed rule provided a timeline that identified designation 

dates as well as dates when the proposed rule requirements would be due, for purposes of 

illustrating a key use that the EPA anticipates for the information that this rule requires to be 

obtained, this rule does not address features of the anticipated future designation actions. 

Therefore, comments on the prospective designations are identified in a section below entitled 

“Comments outside the Scope of This Rulemaking.”
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H. May 2020: Required certification of 2019 monitoring data

Although the preamble for EPA’s proposed rule provided a timeline that identified designation 

dates as well as dates when the proposed rule requirements would be due, for purposes of 

illustrating a key use that the EPA anticipates for the information that this rule requires to be 

obtained, this rule does not address features of the anticipated future designation actions. 

Therefore, comments on the prospective designations are identified in a section below entitled 

“Comments outside the Scope of This Rulemaking.”

I. December 2020: Intended date EPA issuance of final 

designations

Although the preamble for EPA’s proposed rule provided a timeline that identified designation 

dates as well as dates when the proposed rule requirements would be due, for purposes of 

illustrating a key use that the EPA anticipates for the information that this rule requires to be 

obtained, this rule does not address features of the anticipated future designation actions. 

Therefore, comments on the prospective designations are identified in a section below entitled 

“Comments outside the Scope of This Rulemaking.”

V. Technical Considerations

A. Monitoring

1. Monitoring in part 58

Comment:

Commenter (0081) requested, in order to simplify the regulatory process and aid in the review of 

applicable requirements for the monitoring plan, that all monitor related information be codified 

under 40 CFR part 58, not part 51.

Commenters (0063, 0065) stated that the monitoring TAD does not offer the flexibility that part 

58 offers. Commenters (0063, 0065) asserted the TAD provides no guidance to states on 

selecting an appropriate alternative to the preferred location, or the basis on which the EPA will 

approve or disapprove states' monitoring plans. Commenters (0063, 0065) stated the TAD 

provides no assurance that states that elect the monitoring option can obtain a timely approval of 

their monitoring plans. Accordingly, commenter (0063, 0065) stated the EPA should allow states 

to continue siting monitors in accordance with part 58, which, unlike the Proposed Rule, does not 

force a state to switch to modeling if the state's part 58 plan is disapproved.
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Response: 

The EPA believes that due to the unique nature of how air quality characterization can be 

achieved to satisfy this rulemaking, where there is flexibility to use modeling and monitoring, the 

associated regulatory text should be grouped together, as it was proposed and is being finalized.

With respect to the TADs, they are non-binding documents and this rulemaking is not taking 

final action to codify them. As is the case with all required ambient monitoring networks, states 

are encouraged to work with their EPA regions to develop appropriate monitoring network 

designs that are reflected in Annual Monitoring Network Plans. 

In response to the comment that siting monitors per part 58 is preferable, the EPA notes that 

existing siting requirements in part 58 are not specific enough to meet the intent of this rule. This 

rule explicitly states where air quality needs to be characterized (around identified SO2 sources) 

and for what purpose (targeting locations of expected maximum concentrations). 

2. Move existing monitors to free up resources for the near-source 

network

Comment:

Commenter (0036) stated that some NESCAUM states run rural SO2 monitors to support 

tracking trends in acid deposition and to measure background concentrations for use in permitrelated dispersion modeling. Commenter (0036) stated that many of these monitors cannot easily 

be shut down and redeployed, and the EPA should recognize this in the guidance.

Commenter (0046) stated that, if only about one-third of the existing SO2 monitors are sourceoriented, resources should be focused on better use of inappropriately sited monitors.

Commenter (0046) stated that modeling could be used as a screening tool to identify areas where 

exceedances of the standard could be occurring and where reliable monitoring data are needed; 

subsequently, states could relocate existing monitors to those areas and develop reliable data that 

support area designations.

Commenters (0060, 0074) stated they could not locate the supporting material and requested the 

EPA provide a list of monitors analyzed along with the conclusion of their adequacy to be used 

as a source-oriented monitor. Commenter (0074) stated the sites composing the 35% value are 

not identified by name or location in either the DRR or the SO2 Network Review and Background 

Memorandum. Commenter (0074) requested that the EPA provide a list of all monitoring sites 

and the methodology used to determine the 35% value and requested that the 488 sites identified 

in Table 2 to the SO2 Network Review and Background Memorandum be provided along with 

their designated SO2 Monitoring Objective.
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Response: 

The EPA notes that states use monitors to meet federal minimum monitoring requirements and 

also to satisfy their own objectives. If, through a network review or other means, a monitor is 

deemed a lower priority, it could be subject for relocation for use in satisfying this rulemaking. 

In such a case, or any, where a SLAMS monitor is considered for relocation, states will need to 

engage their EPA Region. As noted by another commenter, modeling (and possibly other 

means) could be used to aid in determining which monitors might be appropriate for relocation 

and to where. 

The EPA SO2 Network Review and Background document produced during the 2010 NAAQS 

rulemaking did not list individual monitoring sites that could be classified as being sourceoriented. This was accomplished by mining publicly available AQS meta-data for any SO2 site 

that had a monitor objective of either, or both, of “source-oriented” and “highest concentration”. 

It should be noted that a monitor can have multiple monitor objectives in AQS. These meta-data 

are to be used with caution as states do not always use or apply these labels consistently. With 

that in mind, the EPA did not and has not produced a list of sites. Any particular site will need to 

be evaluated in detail to determine whether or not is adequately located at a point of expected 

maximum SO2 concentrations relative to a particular SO2 source in order to meet the intent of 

this rule. 

3. Network design

a. Guidance is needed—maximum concentration site

Comment:

Commenters (0046, 0051, 0071, 0097, 0110, and 0106) expressed concern that the lack of clear 

criteria provided for designing an SO2 source-oriented monitoring network puts air agencies in 

the unreasonable position of designing a monitoring network without knowing whether it will be 

approved by the EPA. 

Commenter (0051) urged the EPA to allow that the monitor, sited to determine attainment or 

nonattainment of the primary SO2 NAAQS, be placed as close as feasible to the predicted 

maximum concentration location at which actual public exposure is reasonably likely to occur.

Commenter (0110) stated that modeling is only one approach that can be used to site a monitor 

and suggested mobile monitoring or use of newly available rapid response passive monitoring 

should be recognized as available to augment modeling in the siting of monitors. Commenter 

(0106) agreed that passive samples provide very little useful data for developing a monitoring 

plan since they do not provide a good indication of peak 1-hour concentrations. Commenter 

(0106) agreed that "sensors" may provide useful data for developing the monitoring plan.

Commenter (0068) stated that attempting to place monitors based on prior monitor location (or 

even the time-consuming process of “exploratory monitoring”) is overwhelmingly unlikely to 
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result in a monitor network that will capture peak concentrations; such a network is effectively 

useless for evaluating a short-term NAAQS.

Commenter (0068) stated that there are numerous considerations that make it unlikely that 

monitors could be sited at ideal modeled locations, including access to the location, power 

hookups, local pollutant effects, and safe from vandalism. Commenter (0068) stated that, if 

monitors are to be used to determine areas of nonattainment, they should be sited based on 

modeling performed to evaluate impacts on the SO2 NAAQS; specifically, modeling conducted 

to determine where the multiple peak concentrations of SO2 occur due to emissions from a 

source.

Commenter (0071) noted that changes at the facility, such as equipment retirement or 

implementation of permanent & enforceable SO2 emission limitations after the siting of a 

monitor, could change the area of maximum concentration. Commenter (0071) recommended 

that, for clean data determination purposes and subsequent NAAQS attainment designations, the 

area of maximum concentration should remain the same throughout the process, i.e., the same as 

when the monitor was initially sited, to avoid confusion when such designations are later 

pursued.

Commenter (0097) suggested that EPA integrate the concepts of source-specific monitoring and 

population-monitoring such that each is clearly defined, and the need for each is clearly 

explained; this effort may lead to the reduction of redundancies in the 2010 SO2 NAAQS 

monitoring network and therefore may lead to a better utilization of network resources at the 

state level. Commenter (0097) stated the EPA should ask that states include the analyses 

supporting the network design (to characterize air quality around each relevant source) in the 5 

year monitoring assessment report required by 40 CFR 58.10, not the annual monitoring network 

review. Commenter (0097) stated noted that the information more closely resembles the 

information included in the 5 year monitoring assessment report. Commenter (0097) stated

monitors used for NAAQS compliance should meet requirements of the SLAMS network.

Response:

The EPA recognizes that network design for characterizing air quality around individual SO2

sources may involve case-specific considerations. Therefore, making a hard and fast set of 

highly detailed rules or guidelines that are nationally applicable is infeasible. The EPA created 

the non-binding monitoring and modeling Technical Assistance Documents to provide states 

with information that might help any effort they undertake to characterize air quality around an 

SO2 source. As discussed in section IV.D of the preamble to the final rule, the relative location 

and number of monitors that might be sufficient to characterize the air quality around a source is 

a case-by-case determination. In general, the main objective is to monitor at, or as near as 

possible to, the location(s) where ambient SO2 concentration maxima are expected to occur. Site 

selection for any monitoring network is subject to logistical hurdles including site access, 

identification or installation of appropriate infrastructure, telecommunications access, and safety, 

and state, local, and tribal air agencies are well versed in the variety of logistics that can be 
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involved in the installation of an ambient air monitoring station. 

These issues undoubtedly can play into what any ambient air monitoring network ultimately 

looks like. However, as is the case with all required ambient air monitoring, responsible air 

agencies are expected to establish a clear rationale for the number and placement of any one or 

more monitors it is using to satisfy the requirements of the rule. In this process, there is 

flexibility for the state to use professional judgment in determining what is appropriate for their 

individual situations, but they are expected to perform due diligence in attempting to locate 

monitors in the most ideal locations possible. Further, the air agency’s recommended number of 

monitors and preliminary rationale should be discussed with the EPA Regional Offices well in 

advance of the development of an Annual Monitoring Network Plan. As discussed in the 

monitoring TAD, the development of a network design and its rationale can be informed by a 

number of types of analyses which can include the use of air quality modeling, exploratory 

monitoring, or analysis of existing data. In any scenario, the state would need to have a 

technically credible rationale that supports the monitoring network design approach that has been 

chosen to satisfy requirements in this rulemaking.

Commenters both for and against the use of exploratory monitoring make good observations 

about the advantages and disadvantages, respectively, of this means of informing network 

design. This technique remains an option for informing the design of monitoring networks being 

established to meet the requirements of this rule. However, given the need to implement the SO2

standard as expeditiously as practicable, and given the resulting need for air agencies to submit 

network plans by July 1, 2016, in most cases the EPA anticipates that air agencies will not have 

time to implement this approach by the deadline that this rule necessarily imposes.

If changes at a facility result in changes in the area of maximum concentrations, the EPA cannot 

ignore this. While this final rule is not establishing requirements regarding how areas will be

designated in the future, or how areas that are designated nonattainment and seek clean data 

determinations or re-designation shall be treated, we note as a general matter that when 

evaluating whether an area is attaining the standard, the EPA appropriately considers whether the 

entire area is attaining, not just whether the location that previously had the maximum 

concentration is attaining, particularly if information suggests that location has changed.

b. Guidance is needed—number of monitors

Comment:

Commenters (0036, 0046, and 0106) stated that clear guidance is needed on the number of 

monitors required. Commenter (0036) stated that, in the monitoring TAD, the EPA does not 

provide clear guidance on the number of monitors required for a facility, and leaves such 

discretion to the EPA Regional Administrator. Commenter (0051) stated this issue should not be 

left up to negotiations with the Regional Office, rather, a procedure should be outlined that will 

provide consistency for all regional offices and states. Commenter (0036) stated that it is difficult 

for states to design or budget for a network with open-ended siting requirements and stated that 
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the EPA should provide more guidance on the number of monitoring locations it considers 

reasonable under various terrain scenarios, and should allow states to make such determinations 

on a case-by-case basis.

Commenter (0046) stated the EPA should give states guidance on whether modeling could be 

used to site a single monitor that would represent the maximum impact from a particular source 

or group of sources. Commenter (0046) stated that the EPA also should reflect on the full range 

of conservatism built into the emissions, future emissions, dispersion, terrain, monitoring 

predictive pathway to actual human exposure and risk in assessing how many monitors actually 

should be required. Commenter (0051) stated the EPA should provide clear guidance with 

examples of the determination of the number of monitors to be sited for each source. 

Response: 

As discussed in section IV.D of the preamble to the final rule, the relative location and number 

of monitors that might be sufficient to characterize the air quality around a source is a case-bycase determination. There are many factors that can impact monitor network design, but in 

general, the main objective is to monitor at, or as near as possible to, the location(s) where 

ambient SO2 concentration maxima are expected to occur. The EPA cannot specify a certain 

number of monitors that will be universally appropriate across all situations that will be 

encountered by states using monitoring to satisfy this rule. The key is that states provide a clear 

rationale (which can reflect some or all of the factors listed by commenter 0046) on the number 

and placement of any monitor(s) used to satisfy this rulemaking. This is the case whether a state 

uses modeling, exploratory monitoring, or existing data to support their network design choices. 

c. One monitor could be sufficient

Comment:

Commenters (0046, 0049, 0051, 0088, and 0110) suggested that one monitor may be sufficient.

Commenter (0046) stated that, absent historical adverse exposure history in the community, one 

monitor should be the default. Commenter (0088) stated that the final data rule should include a 

discussion of the adequacy of one monitor in certain situations. Commenter (0051) stated that, 

based on precedent from implementation of the lead NAAQS, one monitor could be sufficient to 

obtain reasonable worst-case highest concentrations near a source or a concentrated group of 

sources.

Commenter (0051) stated, from an air quality management perspective, the SO2 standard can be 

considered a "source-oriented" NAAQS rather than a "regional" one (i.e., more similar to the 

lead NAAQS than to the ozone NAAQS); thus, the lead monitoring requirements for a single, 

maximum-concentration monitor sited "taking into account the logistics and potential for 

population exposure" are also appropriate criteria for SO2. Commenter (0051) provided the 

following additional points.
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The lead NAAQS rule requires at least one maximum-concentration, source-oriented 

ambient air quality monitor for each source or cluster of sources with actual annual 

emissions equal to or larger than 0.5 - 1.0 tpy, depending on source type. In practice, this

monitoring requirement has resulted in one appropriately placed, maximum 

concentration, source-oriented monitor for sources without ambient monitoring data or 

other history suggesting that nonattainment exists. If the EPA expects this to be the 

practice for SO2, then it should state that a single monitor will be sufficient, unless 

evidence is presented otherwise.

Commenter (0110) stated that, because large gradients in design concentrations for SO2 are 

likely not present to the extent that the EPA may expect, the use of a single monitor to 

demonstrate NAAQS attainment is sufficient in many cases. Commenter (0110) provided the 

following additional points.

EPA initially proposed modeling instead of monitoring because they envisioned large 

concentration gradients due to isolated sources of SO2, which might require deployment 

of many monitors. However, comparisons of the patterns of modeled and monitored 

design concentrations indicate that models tend to exaggerate spatial gradients. This is 

due, in part, to AERMOD’s over-sensitivity to terrain variations. Thus, applying models 

may be misleading in terms of the number of monitors required. Large gradients could 

exist during any given hour, but over the course of multiple years of monitoring, the 

pattern of design concentrations is likely to be more uniform. 

An example of model overestimation of the concentration gradient due to model 

sensitivity to terrain impacts has been reported by AECOM in the North Dakota low wind 

speed evaluation. In that evaluation, the design concentration measured at 5 monitors 

within 10 km of the two central emission sources were within about 5% of the average 

value. However, at one monitor with an elevation about 80 m above stack base (other 

monitors were in relatively flat terrain); AERMOD in default mode indicated nearly a 

factor of two increase in the predicted design concentration. The use of appropriate beta 

options in AERMOD reduced the variability somewhat compared to observations, but 

still overstated the variability relative to the monitored concentrations. In the North 

Dakota case, one monitor would have been sufficient to determine compliance with the 

NAAQS because the change in the design concentration among the monitors is well 

within the +/- 10% allowed uncertainty in biases for SO2 analyzers.

Response: 

As discussed in section IV.D of the preamble to the final rule, the relative location and number 

of monitors that might be sufficient to characterize the air quality around a source is a case-bycase determination. There are many factors that can impact monitor network design, but in 

general, the main objective is to monitor at, or as near as possible to, the location(s) where 

ambient SO2 concentration maxima are expected to occur. The EPA cannot specify a certain 

number of monitors that will be universally appropriate across all situations that will be 
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encountered by states using monitoring to satisfy this rule. The key is that states provide a clear 

rationale on the number and placement of any monitor(s) used to satisfy this rulemaking. 

Contrary to one set of comments, the EPA continues to believe that substantial gradients in SO2

concentrations exist. The EPA continues to believe that evidence from AERMOD properly 

indicates that substantial gradients in peak SO2 concentrations are commonplace.

d. Evaluate the existing monitoring network 

Comment:

Commenters (0075, 0116) stated it is important that the EPA recognize air agencies may 

demonstrate that the existing monitoring network suffices to evaluate the air quality status of 

particular areas, such that monitoring data available by January 2017 are sufficient to justify 

designating the areas as attainment. Commenter (0109) stated that where existing monitoring 

networks include maximum concentration monitors focused on characterizing impacts from 

nearby large SO2 sources, no additional monitors should be imposed by the EPA through the 

DRR. Commenter (0051) stated the existing monitoring network is adequate for making certain 

attainment designations.

Commenter (0070) stated that, while additional monitors may be needed, (1) data from the 

existing network should be fully evaluated prior to the deployment of additional monitors; (2) the 

knowledge gained from this evaluation should then be applied to the determination of where 

additional monitoring may be needed; and (3) the impacts of regulations such as various Boiler

MACT Rules, CSAPR, Visibility Rules, etc. should also be considered in the deployment of 

these additional monitors, due to the changes in emission profiles and facility operations that 

these rules will cause. When determining whether additional monitors are necessary, commenter 

(0109) stated public cost and public benefit analysis is fundamental to proper decision making. 

Commenters (0055, 0074) requested that the EPA clarify whether an ambient monitor in a 

nonattainment area could be used as the defining monitor to designate areas outside of that 

nonattainment area as well and described the following case.

The data in Yellowstone County, Montana is germane to this question. The sole 

(SLAMS) monitor inside the partial-county nonattainment area has already demonstrated 

compliance (since 2010), and is fully expected to demonstrate compliance with the SO2

ambient standard through 2020 and beyond. There is substantial historical evidence that 

this monitor is source-oriented and has been sited at the worst-case (i.e. area of maximum 

concentration) location for the entire County (both inside and outside of the present 

nonattainment boundary). That being the case, the monitor meets the monitoring 

requirements of this Proposed Rule including for areas outside the non-attainment area. 

Because the monitor serves the dual purpose of on-going surveillance for nonattainmentrelated requirements as well as providing readings for the area of maximum concentration 

for the entire county, including areas outside of the nonattainment area, we request that 
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the EPA clarify that such a monitor could serve both purposes. Similarly, if a monitor 

inside a previously designated nonattainment area successfully “...produces a design 

value of no greater than fifty percent of the 1-hour SO2 NAAQS...” should it not also 

become eligible for shutdown unless it is serving some other SIP related purpose? (0055, 

0074)

Response:

The EPA agrees that while most areas appear to have insufficient monitoring to indicate 

maximum concentrations, some areas may already have sufficient networks for characterizing air 

quality for purposes of this rule. However, the EPA is not in this final rulemaking determining 

whether any specific network is sufficient for purposes of this rule, including the single monitor 

In Yellowstone County, Montana that the commenter discussed. Such evaluations must be made 

on a case by case basis.

In the abstract, it appears likely that if a monitor previously measured nonattainment, and a 

source was not judged to be contributing to a previous violation at that monitor (i.e., the source 

was not included in the designated nonattainment area), then the monitor would not be assumed 

to be at an appropriate location to determine whether concentrations to which the source does 

contribute would exceed the standard. However, this judgment must be made on a case-by-case 

basis after evaluation of the specific source’s emissions, area receptors, monitor, and related 

factors. Criteria for shutdown of monitors in nonattainment areas are governed by existing 

provisions of 40 CFR 58 and are not addressed by the provisions of this rule relating to new 

monitors for purposes of this rule.

e. Multiple monitors per source are needed

Comment:

Commenter (0068) stated that a single monitor is unable to assess air quality impacts from a 

source. Commenter (0068) stated modeling will demonstrate multiple areas with high predicted 

impacts from the source, and those locations will likely shift from year to year due to 

fluctuations in winds and weather and emissions. Commenter (0068) concluded that multiple 

monitors per source would have to be employed before monitoring could even begin to approach 

the robust assessment of ambient concentrations aerial dispersion modeling can accomplish.

Commenter (0068) stated that if a state is unable or unwilling to site multiple monitors based on 

the use of modeling, the monitoring pathway contemplated in the draft Rule should not be 

available to it. 

Response:

Please see response above listed under “one monitor could be sufficient.” The EPA would agree 

that the advantage of monitoring is providing reliable results for the particular location(s) that are 

being monitored; however there is merit to the idea that modeling can also provide better 
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characterization of the spatial variability of concentrations. We are not adopting the commenter’s 

suggestion that the final rule make the monitoring option unavailable based on a judgement 

about an air agency’s inability or unwillingness to site multiple monitors. The technical 

evaluation of the appropriateness of single or multiple monitors for specific sites cannot be 

prejudged in this final rulemaking.

f. Flexibility for states

Comment:

Commenters (0050, 0052a, 0106) urged the EPA to provide flexibility to states for the siting of 

monitors. Commenter (0052a) suggested that states should have flexibility to (1) determine the 

location of source-oriented monitors, (2) determine the appropriate number of monitors 

surrounding a given source, (3) use their discretion to use modeling, wind rose information, and 

other tools to help with properly siting the monitors, and (4) relocate existing monitors to other 

sites. Commenter (0106) stated that, in rural areas that are sparsely populated or in rugged

terrain, the lack of electricity and accessibility can severely limit the number of possible

monitoring sites.

Commenter (0050) stated that given the high cost of moving and installing monitors, the EPA

should ease restrictions on monitor requirements so as to facilitate the use of monitors and allow 

states to take a cost-effective approach to monitoring. For instance, commenter (0050) supported

the possibility of utilizing third party SO2 monitors in areas where the monitors are already 

installed, thus making it easier and more financially reasonable for sources to monitor SO2

emissions. Commenter (0050) believed states should also be permitted to experiment with

exploratory monitoring, including the use of mobile and passive monitors and the long-term 

benefits of these approaches are likely to include an increased number and effectiveness of data 

monitors. Commenter (0050) stated the EPA should also reconsider general requirements for the 

long-term operation and maintenance of certain monitoring networks. 

Response:

The is some flexibility for monitor siting, so long as the monitoring network achieves the 

purpose of characterizing location(s) where maximum ambient concentrations are likely to occur.

The EPA has created a source-oriented SO2 monitoring technical assistance document that 

provides suggestions on how an air agency might go about designing a network to characterize 

peak SO2 concentrations around sources such as is required in the DRR.

Some of these commenters are suggesting that the EPA should account for the difficulty of siting 

monitors in some areas by easing the restrictions on monitoring requirements in those areas. 

These comments in effect acknowledge that feasible monitoring sites in such areas are less able 

to characterize maximum concentrations in the area. This raises the issue that that there can be 

areas that might be more appropriately addressed by modeling. Thus, this comment does not 

support relaxation of monitoring requirements but instead suggests that air agencies might base 
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their selection of air quality characterization on careful consideration of the relative merits of 

monitoring and modeling in the context of how effectively the particular monitoring sites that are 

feasible in the area would characterize maximum concentrations in the area. The EPA supports 

the use of third party monitors, so long as quality assurance and network adequacy criteria are 

met. As noted above, the EPA generally supports the use of exploratory monitoring as one of 

multiple approaches air agencies can take to determine where permanent monitoring sites might 

be placed, but notes that states have a limited time in which they might conduct exploratory 

monitoring, given the expeditious implementation timeline of this rule.

4. Delayed monitoring--automatically move a source to the modeling 

pathway

a. Support automatically moving a source

Comment:

Commenter (0068) stated that the modeling pathway offers a swifter, cheaper, and more accurate 

way of assessing air quality, and so did not believe that states that miss deadlines along the 

monitoring pathway should be allowed to further delay designations. Commenter (0068) stated 

that the monitoring pathway already results in designations occurring fully a decade after the 

NAAQS was promulgated, and eight years after the EPA was originally required to issue area 

designations for the entire country; the monitoring pathway thus should be regarded as an 

absolute edge-of-the-envelope approach. Commenter (0068) stated that failure to meet deadlines 

therein should result in areas being treated under the modeling pathway as a default. Commenter 

(0068) stated that setting such a policy clearly in any final DRR would properly incentivize 

actors to transmit information to the EPA in a timely manner.

Response:

The EPA has provided flexibility to states to choose whichever means they think most prudent to 

characterize air quality around an SO2 source identified through this rulemaking by using 

modeling or monitoring. This concept was supported by many stakeholders. Although EPA did 

not propose to establish designations schedules in this rule, the discussion of delays caused by 

missed monitor deadlines may have been interpreted as establishing a designations schedule 

(rather than a data requirement) for source areas that miss their monitoring deadlines. As 

discussed in section IV.D.3 of the preamble to the final rule, in cases where an air agency 

chooses to characterize air quality with new monitors but fails to have the new monitors 

operational by the January 1, 2017, deadline, the CD governing EPA’s completion of 

designations requires EPA to designate such areas no later than December 31, 2017. However, 

the EPA did not propose or promulgate such a schedule in this rule. 

Regarding the need for additional data requirements to apply when monitoring deadlines are 

missed, for reasons explained in the preamble, the final rule still requires such areas to undertake 

and complete the monitoring that the air agency originally chose to conduct, rather than switch to 
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modeling data requirements. However, given the advanced designation deadline under the CD

for areas in which monitors are not timely installed and operated, air agencies may wish to 

seriously consider alternative means of timely developing and providing to the EPA information

(including modeling information) to inform designations of such areas.

b. Do not support automatically moving a source

Comment:

Commenters (0038, 0041, 0043, 0046, 0050, 0051, 0053, 0054, 0056, 0058, 0060, 0064, 0069, 

0070, 0071, 0072, 0075, 0081, 0085, 00088, 0092, 0093, 0116, and 0118) did not agree that a 

source should be automatically moved to the modeling pathway in the event of any delay in 

monitoring. Commenter (0050) stated that rather than implement this rigid approach, the EPA

should examine each situation on a case-by-case basis to determine whether special

circumstances excuse a state from being delayed in installing its monitors, as well as the

projected amount of time that it will take for such monitors to become operational. This type of

flexibility, while causing slight delays in implementation of the rule, commenter (0050) believed 

will lead to more effective strategies throughout each state, increasing the likelihood of more 

accurate characterizations by the states of their ambient air quality.

Challenges in monitor deployment

Commenter (0051) stated the penalty for unanticipated monitoring site delays or data collection 

hiccups is excessive and noted that deployment of air monitoring sites for sources in rural areas 

provides many challenges not faced in urban areas, including proximity to electricity, prohibitive 

terrain, property owner agreement, and accessibility. Commenter (0093) stated there are too 

many uncertainties, such as shortage of consultants or available monitors, which argue against 

such automatic actions.

Commenters (0071, 0092) stated that the proposal does not take into consideration any of the 

underlying reasons why monitoring was originally selected, such as those sources that cannot be 

characterized through dispersion modeling because of model limitations, complex terrain, 

complex meteorology or multiple sources with overlapping plumes. Commenter (0092) stated 

that mandatory modeling in source areas of this type is neither appropriate nor would it provide 

the EPA with credible information for purposes of making the NAAQS designation. 

EPA delay

Commenters (0038, 0075, and 0108) stated that the EPA should make it clear that any EPA delay 

on approval of monitoring plans will not contribute to a state or local agency requiring modeling 

where monitoring had been selected as the state's preferred SO2 approach. [See IV.D.2. regarding 

EPA oversight of monitoring]

Good faith effort
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Commenters (0063, 0065, 0075, 0087, 0088, 0092, 0116) stated the EPA should not 

categorically declare that it will move an area to the “modeling pathway” if, despite an air 

agency’s best efforts and for reasons beyond that agency’s control, operation of a monitor is 

delayed beyond the January 1, 2017 proposed deadline. Commenter (0088) stated that, not only 

is the EPA’s schedule exceedingly aggressive but importantly, there may be valid reasons that all 

monitors that have been installed are not operational by January 1, 2017. Commenter (0088) 

stated that there are dozens of reasons that a monitor may not be operating properly which can be 

fixed. 

Commenter (0088) stated that usurping the state and private resources and other efforts to install 

and operate monitors because a few are not operational is not reasonable. Commenter (0088) 

stated the EPA should not ignore or attempt to deter commitments for significant investments for 

obtaining ambient air quality data so arbitrarily. 

Unclassifiable

Commenters (0064, 0075, 0087, and 0116) suggested the area should be labeled unclassifiable 

until monitor startup is completed. Commenter (0064) stated that an unintentional delay in 

monitor startup should be viewed by the EPA in the same way it would likely view delayed 

startup of any other pollutant monitor, i.e., the area should be labeled unclassifiable until monitor 

startup is completed.

Response: 

This issue is discussed in section IV.D.3, where we clarified that a failure to monitor by the 

January 1, 2017 due date does not force a state (or the EPA) to conduct modeling nor does it 

relieve the state from conducting monitoring. The EPA’s intent in its notice of proposed 

rulemaking was to explain that in these circumstances, where an air agency chooses to 

characterize air quality with new monitors but failed to have the new monitors operational by the 

January 1, 2017, deadline, the EPA envisioned designating such areas in conjunction with areas 

being characterized by modeling (although we did not propose or promulgate a designation 

timing requirement). That is, the EPA did not envision delaying the designation for such areas to 

the envisioned 2020 date when the Agency anticipates promulgating designations for areas 

characterizing air quality through a new monitoring network. The EPA must now comply with a 

court ordered designation schedule, in which the court expressly requires that areas that have not 

begun operation of a new monitoring network by January 1, 2017, must be designated by the end 

of December 2017. The EPA is not able in this final rule to change that court-ordered schedule. 

While the final rule does not require air agencies in cases of late installation and operation of 

monitors to actually shift to conducting modeling for such areas, we note that air agencies in 

those circumstances may wish to timely develop and provide to the EPA alternative information 

(including modeling information) to inform the designations required for them under the 

advanced deadline. 
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5. Delayed monitoring--evaluate

Comment:

Commenters (0041, 0043, 0046, 0051, 0053, 0054, 0056, 0058, 0060, 0069, 0070, 0072, 0075, 

0081, 0085, and 0088) did not agree that a source should be automatically moved to the 

modeling pathway in the event of any delay in monitoring where an agency is acting in good 

faith and suggested several alternative approaches. Commenter (0054) stated that given 

monitoring’s superior value in reflecting the real-world situation, discretion should be exercised 

in allowing monitoring data to be utilized even if the timing of its initiation does not comport to 

strict deadlines.

a. Allow a time extension

Comment

Commenters (0041, 0054, 0070, 0072, 0081, 0085, 0088, 0093, and 0097) recommended the 

EPA allow time extensions as noted below. 

 Commenters (0041, 0093) stated these issues should be evaluated on a case-by-case basis 

and if due diligence can be shown, short extensions to the monitoring deadline should be 

granted. 

 Commenter (0056) recommended that existing monitoring networks which demonstrate 

compliance with the SO2 NAAQS, but may need some adjustments to be approvable by 

the EPA should be afforded time to make the necessary changes and should not trigger a 

requirement to use modeling for making a designation. 

 Commenter (0070) recommended six month grace periods and stated these would fit into 

the EPA schedule without serving to delay the designation process and would also allow 

the use of the significant resources expended in establishing a monitoring network to not 

be wasted by an issue at a single site. Commenter (0070) provided the following 

additional points. 

As long as the delay is not beyond June 30, 2017, the EPA should explicitly allow 

special handling of the situation. For this special handling, a like period of time in 

2020 would be substituted into the calculation of the 3 year design value with the 

annual periods adjusted to the an alternative year (i.e. March 1, 2017 through 

February 29, 2020) that would be broken into three 12 month periods. Each 12 

month period would contain a full year of data (assuming that data quality was 

achieved) and a three year average could be determined. Offering this only for the 

period through June 30, 2017 would allow the extra data from 2020 to be properly 

quality assured and the data from the offset years to be processed.
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 Commenter (0072) stated that, in those cases where a monitor has been sited but not yet 

operational by January 1, 2017, the Regional Administrator should have the option to 

allow for an extension before moving the source to the modeling pathway, if the 

placement of monitors beyond the January 1, 2017 deadline is done in a timely manner.

 Commenter (0081) suggested that the EPA should consider allowing as much as two 

years delay in monitor start-up. Commenter (0081) stated the timeline goals proposed by 

this rule would still be met in this case, because the EPA could designate the area as 

“unclassifiable” should the monitor show no violations in the single year of operation 

before the final round of designations, or could designate as “nonattainment” should the 

monitor be showing a violation. Commenter (0081) stated this is consistent with how the 

EPA has handled these types of situations in the past (e.g., the Greenville, SC PM2.5, 

“unclassifiable” designation) and would avoid the wasted resources caused when an 

agency has already purchased monitoring equipment and buildings and then has to 

abandon those resources to pursue an unanticipated modeling exercise.

 Commenter (0085) suggested a set of two 1-year extensions for the case where an air

agency, or a source/third party that is installing a monitor that the air agency will be 

quality assuring, while acting in good faith experiences a delay outside of its control; 

specifically, the air agency or source could apply up to two times to the EPA for an 

extension of one year to have the monitor operational.

 Commenter (0097) stated the EPA could adopt an extension approach, similar to that 

included for ozone under section 172(a)(2)(C). If one full year of SO2 monitoring data 

were obtained, and this data showed no exceedance (i.e. the 99th percentile of the data 

was not above 75 ppb), then the EPA could offer two years of extensions before making 

the designation determination, contingent on each year's data recording values less than 

75 ppb for the 99th percentile each year. Such an approach would allow states more time 

to upgrade their monitoring networks for this effort.

b. Data gaps

[Also see IV.I.]

Comment

Commenters (0038, 0043, 0054, 0060, 0088, 0092, and 0097) recommended the EPA allow the 

use of mechanisms to replace missing data.

Commenter (0043) stated that, where an agency is acting in good faith to deploy monitors on 

time, but experiences a delay outside of its control, the EPA should consider the data set 

complete if there is valid and complete data from all four calendar quarters even if they are 

before 2017 or after 2019. Commenter (0043) stated that collecting data for three consecutive 

years, at multiple sites, without data loss from equipment malfunction or other failure is a 
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daunting task. Commenter (0043) stated that allowing the substitution of data from before or 

after the test period would provide data for a quarter lost without losing data during that quarter 

or season of the year and stated that, since this concept is acceptable for an air quality 

demonstration for PSD application process, it should be acceptable for a NAAQS attainment 

determination.

Commenter (0060) suggested basing the recommendations for nonattainment for those 

monitored areas on less than three years of data or making recommendations based upon 36-

months of data rather than three calendar years. Commenter (0060) stated it is likely only weeks

to a couple months in most cases when deployment is delayed. Commenter (0060) noted the 

EPA's lead monitoring rule provides for an alternative to three calendar years of data for

determining design values for monitors that are newly deployed. 

Commenters (0054, 0088, and 0092) stated that automatically moving a source from monitoring 

to modeling should not be considered until after it has been demonstrated that the monitoring 

data are incomplete, including the inability of the state to substitute data. Commenter (0054) 

noted that the EPA’s own TAD guidance as referenced in footnote 40 of the Proposed Rule states 

that monitoring intervals are considered complete when at least 75% of the sampling days have 

complete data. Commenters (0088, 0092) suggested there are a number of data conventions that 

the EPA regulations employ to replace missing data when a continuous operating monitor or 

parametric monitor are not operational. Commenters (0054, 0092) stated the EPA has established 

and codified procedures in Appendix T to part 50 that make provisions for incomplete data sets 

and has established procedures for substituting data. 

Commenter (0038) stated that, if a state could not have its monitors operational by the 2017 

deadline, an alternative is to use modeling to estimate values for the period of missing 

monitoring data in order to create a full data period for design value calculation purposes. 

Commenter (0097) stated the EPA should allow data to be used for provisional attainment 

designations, if that data show potential compliance on a yearly basis from January 1 to 

December 31 by 2019.

Designate attainment or unclassifiable

Commenters (0069, 0075, and 0088) stated it would be more appropriate to designate such areas 

as "unclassifiable." Commenter (0069) suggested revising the designation using complete 2018 

to 2020 data, which would be available within months after the EPA's planned 2020 

designations, resulting in a relatively short delay in designations. Commenter (0088) suggested

that if there have been one-two violations and the state has shown good faith, it would appear 

that it would be appropriate to designate the area unclassifiable, and defer the designation of a 

full three year’s data either to when a consecutive 36 months of data are collected or until the 

next year. 
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Commenter (0088) suggested the EPA could consider designating such areas as attainment if 

there are no violations of the one hour SO2 NAAQS in the data that has been collected, provided 

over two years of data has been collected and submitted. 

Modeling as a backstop

Commenter (0041) stated that, as a backstop to any source that pursues the monitoring approach, 

states may perform and submit necessary modeling for all affected sources regardless of whether 

the source chooses to pursue the monitoring route or modeling route. Commenter (0041) stated 

that this would keep the SIP process within states' oversight and not require the EPA direct 

involvement as proposed here in the event of a failure to monitor. 

Revise the schedule

Commenters (0051, 0088) stated that the EPA should provide a more reasonable period for 

implementation that would obviate this issue. Commenter (0051) stated a more flexible timeline, 

with phased implementation, would make the monitoring pathway a more feasible option for 

states while minimizing wasted resources.

Commenter (0103) suggested the monitoring and modeling deadlines be the same and stated that 

implementing a single designation timeline eliminates discontinuities in schedules if a planned 

monitor is not operational by Jan 1, 2017, as such locations could simply convert to the modeling 

pathway.

State discretion

Commenter (0058, 0088, 0092, and 0118) suggested that the EPA should allow the judicious

application of agency discretion. Commenter (0058) stated that, if a state is making a good-faith 

effort to install monitors but encounters unforeseen delay beyond the state’s reasonable control, 

at most, the DRR should request that states provide reasonable justification to the Agency for 

such delays. Commenter (0088) suggested the option of conditionally approving the monitoring.

Commenter (0092) suggested this approach would not lead to wasted agency resources and 

costly equipment purchases. Commenter (0118) recommended the use of a weight of evidence 

approach on a case by case basis in these situations. Commenter (0088) suggested states should 

be allowed to discuss with EPA Regions why some monitors are not operational and in 

conjunction with a policy for waiving a full three years data, provided a valid explanation is 

offered (or utilizing accepted data conventions for substituting data), the EPA should not ignore 

or attempt to deter commitments for significant investments for obtaining ambient air quality 

data so arbitrarily. 

State rulemaking

Commenter (0056) recommended that the EPA should not force the use of modeling if a state 

agency elects to initiate a rulemaking to require sources to install, operate and maintain ambient 
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SO2 monitoring systems for SO2 NAAQS implementation, since such a rulemaking would take 

time to initiate and complete, leaving less time for monitor deployment. 

Response: 

Please see response above to “Do not support automatically moving a source,” in response to the 

comments addressing whether areas for which monitoring is not timely installed and operated 

should thereafter be required to conduct modeling under the rule. The final rule imposes no such 

requirements, but instead continues to require the air agency to complete the monitoring it 

originally opted to conduct. In response to the comments addressing how designations should be 

conducted in such cases, this final rule is not adopting requirements governing designations, 

which instead are now governed by the CD containing the binding schedule under which EPA is 

to complete designations. That court order requires areas for which monitors are not timely 

installed and operated to be designated no later than the end of December 2017. In such cases, air 

agencies may wish to seriously consider timely development and submission to EPA of 

alternative information to inform designations required under that advanced deadline. 

Nevertheless, the rule’s monitoring requirements would still apply, and the data from that 

monitoring, while no longer timely for 2017 designations, would remain useful for other 

purposes.

6. Resources

a. Costs

Comment:

Commenters (0034, 0038, 0051, 0071, 0103, and 0108) expressed concern about the costs 

associated with monitoring for the SO2 NAAQS. 

Commenters (0034, 0120) recommended that the EPA provide assurances to those Tribes having 

sources near to them that emit at least 1,000 tpy of SO2 that they will receive the requisite 

resources for monitoring and modeling to characterize the air quality around such sources. 

Commenters (0034, 0120) stated this will go a long way in helping Tribes --- sovereign nations 

with inherent rights ensured by the U.S. Constitution, treaties and legal precedent --- address any 

health and environmental impacts to their communities caused by these sources.

Commenter (0051) stated Texas does not have the resources to absorb this additional cost.

Commenter (0051) stated the cost to implement monitoring at a single site per source is 

approximately $85,000 for the initial deployment and an additional $45,000 annually and, if 

under the EPA's proposed Option 1, the TCEQ were to deploy a single new monitor at 31 

sources based on 2012 emissions, this would cost approximately $2.6 million for initial 

deployment and $1.4 million annually. 
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Commenter (0038) stated that, assuming EPA would accept just one monitor per source, and 

using the estimate provided in the Proposed Rule, Wisconsin may face $1.3 million in start-up 

costs alone, with no additional funding forthcoming from the EPA to offset this new expense.

Commenter (0103) estimated that for the twelve affected facilities in Iowa, about $2.78 million 

would be required to purchase equipment and monitoring sites for three years to establish 

attainment. Commenter (0108) estimated that the startup costs of a monitoring network in 

Wisconsin could be $1.3 million at a minimum. 

Response: 

The EPA recognizes the fact that there will be costs and resources required to satisfy the 

requirements of this rulemaking. The EPA provides grant funding for this general purpose. As 

suggested by both state and industry stakeholders who attended the May-June 2012 stakeholder 

meetings, in the absence of increased grant funding specifically for this rule, it may be necessary 

for air agencies to rebalance their existing grant funds for this purpose, or to consider alternative 

funding approaches such as working closely with affected sources to assist in funding either the 

modeling or monitoring required to meet the requirements of the rule. Early planning may be 

helpful to address these funding needs. 

Because the CAA assigns to states much of the responsibility for developing air quality 

characterization data, the EPA describes the requirements of this rule in a consistent manner: air 

agencies are the entities with principal responsibility to establish and operate monitors, and 

conduct modeling, and to provide air quality data to the EPA. However, the EPA recognizes that 

other parties (such as facility owners) also may perform significant portions of the work that this 

rule requires. The EPA would consider monitoring or modeling conducted by a third party to be 

an appropriate means for air agencies to obtain the data necessary to meet the requirements of 

this rule, provided that the state provides oversight to assure that (1) any monitoring is conducted 

in a manner that is equivalent to SLAMS in accordance with applicable requirements, and (2) 

any modeling analysis that the state submits, even if it was initially provided to the state by a 

third party, is done in an appropriate manner considering the recommendations in the modeling 

TAD.

Similarly, the EPA cannot commit to provide Tribes with additional resources expressly for the 

purpose of satisfying the requirements of this rule, but the EPA will work with Tribes as

appropriate to help assure that the requirements of this rule are met.

b. Industry costs

Comment:

Commenters (0034, 0120) stated the cost to Tribes for obtaining the necessary monitoring and 

modeling resources should be borne wholly, or in part, by the sources emitting significant 

amounts of SO2 into the atmosphere, akin to the “polluter pays” principle. Commenter (0036) 

recommended that the EPA allow states the option to require facilities to fund either the costs of 
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the monitoring (and modeling) or the cost to states for overseeing the facility-conducted 

monitoring. Commenter (0036) provided the following additional points:

The EPA should recognize that it will require significant state resources to implement a 

facility-supplied monitoring program. For example, the monitoring sites must somehow 

be integrated into state agency network structures; the data must undergo quality 

assurance and quality control by the state, and be formatted and processed for input into 

various state and federal reporting systems such as the EPA’s Air Quality System. States 

must have the ability to charge fees to fully fund its oversight of the facility-supplied 

monitoring, or the EPA should conduct the oversight itself. 

Commenters (0041, 0046, and 0093) stated that, because of funding limitations, any monitoring 

done would likely need to be funded by the affected source. Commenter (0046) stated this rule 

will be yet another burden on the regulated community when facilities are already spending 

capital on emissions reductions projects required by other EPA air quality rules. Commenter 

(0093) stated that the EPA seems to have drafted this proposal with the idea that states, rather 

than regulated sources, would perform the modeling and/or monitoring. Commenter (0093) 

stated that, due to resource constraints, this is not the case in Alabama and in most states, and 

requested that the proposal be rewritten with this in mind.

Commenter (0070) pointed out that the recession has taken a toll on budgets and money that at 

one time may have been available for industry to spend on expanding the monitoring network. 

One commenter (0109) stated that when determining whether additional monitors are necessary, 

the public cost and public benefit analysis is fundamental to proper decision making. Allocating 

monitoring costs to the local stationary source distorts that cost-benefit analysis, and will result 

in poor policy choices that increase the total cost of monitoring where not otherwise justified by 

the benefit derived from gathering the additional data.

Response:

Please see the response above listed under “Costs.” Regarding the concern of some industry 

commenters about industry covering costs for monitoring, the EPA notes that in some states it 

has been considered to be a reasonable practice for an industrial source to take responsibility for 

providing financial resources for the monitoring of pollutant emissions from that source into the 

environment. In addition, the EPA points out that it is not customary, or legally required, that 

the agency conduct a “cost-benefit” analysis when establishing requirements that state air 

agencies or pollution sources further characterize the impacts of emissions. Neither CAA 

sections 110, 114 or 301 impose such a requirement. Instead, for this rulemaking, the EPA has 

reasonably assessed the costs of the various proposed options and has selected a set of final 

requirements that can be feasibly implemented and will generate highly valuable emissions 

impact information that can be used in future NAAQS implementation actions.

c. Funding by EPA
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Comment:

Commenters (0037, 0038, 0062, 0103, and 0108) stated that the EPA should fund monitoring 

activities that states determine are needed to comply with this rule. Commenters (0038, 0062, 

and 0108) noted states have very limited resources to meet an ever-growing burden of federal 

mandates. Commenter (0037) estimated the cost as between 1.2 and 3.7 million dollars in Iowa 

and Wisconsin alone.

Commenters (0038, 0062) stated that, even if sources voluntarily set up and operate their own 

monitors, state and local agencies will still need to dedicate resources to administer the program, 

provide technical assistance, conduct performance audits, ensure data quality, and submit the 

data to the EPA's Air Quality System database each year. Commenters (0038, 0052a, 0108) 

stated initial funding should pass through a section 103 grant in order to establish the monitoring 

sites. Commenters (0038, 0108) stated that ongoing funding should be provided through each 

state's section 105 grant. Commenter (0038) noted that section 105 funding is diminishing, while 

federal requirements continue to mount.

Response: 

Please see the response above listed under “Costs.” Also, please see section IV.E.4 of the 

preamble.

d. Partnerships and third party monitoring

Comment:

Commenter (0039) supported the EPA’s proposal to accept monitoring data from monitoring 

networks that may be owned or operated by regulated entities instead of state-run networks only. 

Commenter (0052a) stated that, if the EPA chooses to advocate monitoring by the sources, states 

should be given full oversight to validate that monitoring is carried out according to acceptable 

practices through establishment of a Quality Assurance Project Plan (QAPP). Commenter 

(0052a) stated that, alternatively, the EPA should provide a mechanism for funding to come to 

the states to support operation of the monitors as part of their own monitoring networks.

Commenters (0055, 0074) asked whether air agencies are required to directly operate the 

monitors or whether the monitors could be operated by a third party as long as the requirements 

listed in §51.1203(c) are met? Commenter (0074) asked that the EPA modify the text of the rule 

so that it clearly permits use of third party monitor data and describes under what specific 

circumstances. 

Commenter (0089) stated the EPA should specify more directly under what circumstances 

industry or third party monitoring infrastructure will be accepted and what requirements would 

apply. Commenter (0089) stated the proposal does not explicitly support the use of existing third 

party monitors, stating that such monitoring data may not meet the requirements of 40 CFR Part 
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58, yet the EPA also states that industry and third party monitoring infrastructure and operations 

could be leveraged to satisfy the requirements of the rule.

Commenter (0089) contended that the proposal lacks sufficient support for the use of data from 

existing monitors that are operated by third parties that could be used to verify modeled results 

and recommended that third party or industry monitors should be allowed to verify modeling 

results and to provide the basis for correction factors to modeled results where appropriate. 

Commenter (0089) stated that existing third party monitoring stations are following PSD 

regulations and guidance in the manner they are operated and audited and, therefore, their 

measurements should be recognized as a critical tool to evaluate the reliability of modeled 

results, even if the monitors are not located precisely in the position of modeled maximum 

concentrations. 

Commenter (0089) stated the rule should make a more explicit accommodation for the use of 

existing, third party SO2 monitors at sites needed to characterize SO2 sources located in isolated 

areas across the western U.S., including portions of Nevada. Commenter (0089) stated there are 

(1) several SO2 sources in Nevada located in some of the most remote terrain in the lower 48 

states, (2) no nearby population centers or physical infrastructure to support additional 

monitoring sites beyond the monitoring sites already located at the source and maintained by the 

operators of the industrial facility, and (3) no additional nearby locations where infrastructure 

could be used to support the collection of model input parameter data (meteorological data) to be 

used in the development of robust and defensible air dispersion models. Commenter (0089) 

stated the existing monitors in place at these sources are already sited in the only logisticallyfeasible locations to measure maximum concentrations, and are following PSD regulations and 

guidance in the manner they are operated and audited. 

Response:

Third party monitoring is permitted to satisfy the requirements of this rulemaking. However, it is 

the state’s responsibility to ensure compliance and provide oversight. Any monitor used to 

satisfy this rule must be reflected in the state’s Annual Monitoring Network Plan, and meet 

requirements listed in 40 CFR part 58 Appendices A, C, and E, along with pertinent data 

collection and reporting requirements under 58.15 and 58.16. These existing requirements which 

govern monitoring generally also govern monitoring under the DRR, and so EPA does not agree 

that there is a need to provide separate requirements specific to the DRR. As noted above, if 

logistical issues significantly reduce the feasibility of monitoring maximum concentrations in an 

area, this may warrant selection of modeling as a more appropriate approach for characterizing 

air quality.

7. Monitor shutdowns

a. Threshold below which monitors may be shut down
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Comment:

Commenters (0072, 0082) agreed that monitors placed pursuant to the SO2 DRR and located in 

areas that are designated as attainment in the anticipated third round of initial designations in

2020 should be eligible for shutdown in 2021. 

Commenters (0039, 0041, 0045, 0046, 0048, 0049, 0050, 0053, 0054, 0056, 0058, 0060, 0062, 

0064, 0070, 0075, 0080, 0081, 0082, 0087, 0088, 0090, 0092, 0093, 0096, 0106, 0107) 

supported the consideration of shutdown for any monitor that for a three-year period has a design

value of no more than 80 percent of the standard. 

Commenters (0041, 0053, 0054, 0056, 0058, 0075, 0082, 0092, 0093, 0096, 0106, and 0107) 

stated that 80 percent of the NAAQS is a strong enough criteria for the shutdown of a SO2

monitor and the 80 percent criteria is consistent with criteria for shutting down most regulatory 

monitors.

Commenter (0092) advocated the EPA’s selection and promulgation of the 80% alternative 

proposal because it was indirectly derived from existing language in 40 CFR part 58.14(c)(1).

Commenter (0046) stated that monitors meeting the 80% criterion should be shut down and 

could be relocated to another area where modeling has shown there could be 1-hour NAAQS 

violations occurring near sensitive populations. Commenter (0082) stated that providing state 

agencies with the flexibility to shut down unneeded monitors allows agencies to allocate their 

limited resources appropriately. Commenter (0070) stated that, if the sources are properly 

controlled and/or limited by permit, the risk of significant increases in design values over time is 

relatively low absent new sources entering the area. Commenter (0070) stated that, with a 20% 

margin on the monitor, this would allow the occasional upset at the facility to occur without 

disqualifying the area from having the ability to request reduced monitoring if air quality is 

acceptable on a continuing basis.

Commenter (0081) supported the 80% option and recommended that the wording be changed in 

this section so that it is less vague as to whether or not a monitor is eligible for shutdown. 

Commenter (0081) recommended that the final rule state “Any SO2 monitor...that produces a 

design value of no greater than fifty (or 80) percent of the 1-hour SO2 NAAQS, will (emphasis 

added) be eligible for shut down.”

Commenter (0096) recommended the EPA modify the language in §58.14(c)(1) if it wishes to 

change the requirements, rather than making indirect exemptions to that section in other parts of 

the CFR. Commenter (0096) stated it is unclear why a departure from the language of 

§58.14(c)(1) would be necessary in the DRR since § 58.14(c)(1) presumably applies in the SO2

monitor shutdown situations to which the DRR refers. Commenter (0096) suggested it could be 

that “local air monitoring stations” do not include air monitoring stations which are local but not 

employed by a local agency (such as in the case of industry or third party monitoring). 
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Commenter (0096) stated there does not seem to be a reason to make the criteria more stringent

than the existing criteria in part 58.

Commenters (0080, 0090) believed the 50% value was too conservative given the relative health 

concerns of ambient lead and SO2 concentrations. Commenters (0080, 0090) asserted that the 

primary standard for SO2 is over 50 times higher than the standard for lead on a gm/m3basis, 

which supports a higher design value level for shutdown eligibility for SO2; thus, the same 

rationale should apply to the cessation of modeling threshold in the DRR.

Commenter (0068) stated that, because emissions can vary significantly from year-to-year, an 

80% threshold is far too high. 

Commenters (0036, 0039, 0043, 0062, 0115, and 0018) supported the consideration of shut down 

for any monitor that for a three-year period has a design value of no more than 50 percent of the 

standard. Commenter (0036) stated the 50% metric is very sensitive to the high end of one-hour 

data distributions, in part because the form of the one-hour SO2 NAAQS is the 99th percentile 

(unlike the 98th percentiles for other daily NAAQS forms). Commenter (0043) supported the use 

of the 50% threshold because the area would require a significant increase in sulfur dioxide 

emission to cause an exceedance of the sulfur dioxide 1-hour standard. 

Commenter (0062) supported the designation monitor removal 50% option because it allows 

states the ability to redirect limited monitoring resources to higher priority monitoring in the 

event that the 1-hour SO2 NAAQS is met. Commenter (0062) recommended the following 

changes: (suggested text italicized) 

Under section V, subsection A. Monitoring, 

“...Any SO2 monitor identified in an approved state annual monitoring network plan to 

satisfy this proposed DRR will be eligible for shutdown in 2021 or later if the following 

criteria are met: 

“...designated attainment in the anticipated third round of initial designations in 

2020 will be eligible for shut down provided the monitor meets certain criteria.” 

Commenters (0068) stated that, if a 50% threshold is used, any final DRR should make clear that 

the design value used is a fourth-highest daily maximum concentration occurring in any of the 

previous three years, and not the average of such fourth-highest daily maximum over the course 

of those three year; thus, three years’ worth of low concentrations would be a minimum 

prerequisite to removing a monitor.

Commenter (0118) stated that, reviewing data from existing monitors that reach this level 

indicates that they generally either have shown many years of data at or below this level or can 

attribute it to the shutdown of nearby sources. Commenter (0118) stated that, in either case, there 

does not appear to be any likelihood that values will go back up, due to permitting and 

compliance activities.
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Commenter (0103) recommended that the threshold of 50% for discontinuing monitoring in the 

Proposed Rule be dropped in the final rule since 40 CFR § 58.14 already contain provisions for 

shutting down a monitor. Commenter (0103) stated that the origin of the 50% threshold is a rule 

of thumb for uncertainty in ambient modeling that has no bearing to ambient monitoring 

uncertainties.

Commenter (0060) did not agree that the EPA can justify a lower level of 50% as being 

consistent with the provision allowing the EPA Regional Administrator to waive lead NAAQS

monitoring requirements. Commenter (0060) stated that shut down of a lead monitor is still 

subject to 40 CFR 58.14(c)(1) and the 80% provision, just as all other NAAQS. Commenter 

(0060) provided the following additional points. Commenter (0060) stated that, if the EPA

wishes to change that criteria, a revision to 40 CFR 58.14(c)(1) should be considered and made 

available for comment. Commenter (0060) provided the following additional points. 

Under the lead NAAQS, sources that emit over a certain threshold are required to monitor 

unless the air agency can model and show their emissions result in no impact over 50% of the

lead standard. This is a very different process and purpose than the shutdown of a monitor 

that has operated for three years and shown attainment. Under this rule, an air agency is 

electing to monitor in lieu of modeling and the results should require the same safety margin 

for shutdown as all other NAAQS. Ohio currently has 12 monitors attaining the standard that 

would not meet the 50% criterion and six of those monitors would meet this criterion if it 

was maintained at 80%. Given the experience we have seen with low monitoring results on 

many of the required lead monitor deployments that were placed based on threshold emission 

level criteria, the EPA should not make it more difficult for an air agency to request

shutdown of a monitor that is not necessary.

Commenters (0075, 0116) stated the requirement for annual reporting of changes in SO2

emissions with the possibility that further monitoring could be required, argues against the more 

stringent criterion discussed in the Proposed Rule that the design value be less than 50 percent of 

the standard.

Commenter (0085) suggested a value in the range of 65-70% of the NAAQS, in order to allow 

for monitors that have a design value that is notably below the NAAQS and of minimal risk to 

public health to be taken off-line, without having to achieve as stringent and potentially 

unachievable a level as 50%.

Commenter (0052a) stated that for other criteria pollutants, the EPA allows monitors to be shut 

down once the ambient concentrations are 85 percent of the standard and that this is an 

appropriate level for discontinuing the operation of a source-oriented SO2 monitor. Commenter 

(0113) stated that, absent appropriate justification, Montana suggests that the threshold for 

shutting down a monitor be consistent with the requirements for other NAAQS pollutants and 

that the EPA has historically allowed discontinuation of monitoring efforts if the design value 

concentrations are below 85% of the NAAQS provided there are no associated nonattainment 

area control plan SIP requirements to monitor in place (40 CFR 58 Appendix D Table D-2, D-5).
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Commenter (0102) stated that, as the SO2 standard has been lowered to a point where it is 

considerably closer to the background concentration, the proposed range is too stringent of a 

threshold for removing monitors. Commenter (0102) suggested that if a monitor demonstrates a 

region is emitting within the 80-85% range of the NAAQS threshold, this should be adequate to 

establish compliance with the standard.

Response: 

The issues of monitor shutdown options and eligibility are fully discussed in section IV.D.4 of 

the preamble to the final rule. 

The Agency took comment on an alternative option using calculated design values less than 80% 

of the NAAQS to qualify individual SO2 monitors specifically operated to satisfy the DRR for 

potential shutdown. Many of the comments reflected confusion about the relationship between 

the 80% monitor shutdown criterion in 40 CFR 58.14 and the 50% criterion proposed here, 

apparently resulting from failure to consider the additional elements of the 80% criterion in 40 

CFR 58.14 that are not part of the 50% criterion proposed here. Much of the support for the 80% 

threshold commented that it was appropriate because of its derivation from existing use as part of 

a broader network modification language in 40 CFR part 58.14. The Agency notes that in that 

language, the 80% threshold is not used in isolation, but as part of a probability analysis that 

requires the evaluation of data over at least 5 years and is intended to anticipate future NAAQS 

violations. The Agency has decided, per the priority expressed in the proposal, and based on 

public comment, that the 80% threshold is too high (particularly in absence of an analysis of the 

probability of future NAAQS violations) and has decided to use the 50% threshold (without 

requirement for probability analysis) as was proposed. Due to the potential variance in emissions 

and meteorology that exists, the Agency has judged that a 50% threshold is more appropriate 

when using that singular metric to allow shutdown of a monitor satisfying the DRR. As was 

noted in section IV.D.4 of the preamble to the final rule, requirements for shutdown, the 50% 

threshold provides relatively greater assurance that any monitor eligible for shutdown will be 

well below the NAAQS after 3 or 4 years of operation. The EPA also notes that the 80% option 

(available after 5 years of data have been obtained) remains in place in 40 CFR 58.14, and the 

EPA is promulgating the 50% option, available after 3 or 4 years, in addition to the 80% option.

In response to the comment comparing SO2 to lead, the Agency notes that the two pollutants are 

very different in terms of make-up (gas versus mostly particulate, respectively), pollutant 

production, emission, dispersion, and exposure risks and pathways (inhalation for SO2 versus

ingestion and inhalation for lead), and using the approach for one standard as a precedent for 

another is not appropriate.

The EPA agrees that with the comment that a monitor shutdown criterion based only on 

calculated design values being less than 80% of the NAAQS is too high, and the EPA has chosen 

a relatively lower threshold of 50%. The issues of monitor shutdown options and eligibility are 

fully discussed in section IV.D.4 of the preamble to the final rule. 
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b. Multiple monitors

Comment:

Commenters (0060, 0081, and 0107) stated that this rule should include consideration of the case 

of multiple monitors around one source. Commenter (0060) stated there needs to be a clear 

method to shut down monitors that fall under this scenario and asked, for example, what if 2-3 

monitors are located around a source and after three years of modeling it is determined that one 

of the monitors is consistently significantly lower than the other(s)? Commenter (0081) stated 

that, if the EPA wishes to pursue a shutdown criterion of 50 percent of the standard, they should 

consider allowing areas with multiple monitors a lower threshold for shutdown of redundant 

monitors. Commenter (0081) stated, for instance, areas with multiple monitors which are 

affected by similar sources, meteorology, and topography should be allowed to remove the lower 

reading monitor(s) should the monitor(s) be below 80 percent of the standard; such an option 

would be consistent with 40 CFR 58.14(c)(2) which allows for the removal of a monitor which 

has consistently measured lower concentrations than another monitor for the same pollutant in 

the same county.

Commenter (0107) recommended that the 50-80 percent criterion apply to each monitor 

individually rather than to the collective network, especially if set at 50 percent.

Response: 

The EPA notes that the eligibility to seek approval to shut down a monitor applies on a monitorby-monitor basis. In a scenario where a monitor is part of a multi-monitor array being used to 

satisfy the DRR, but is not eligible for shutdown by showing a design value below 50% of the 

NAAQS in the first 3 or 4 years of operation, it then becomes subject to the existing shutdown 

criterion of 40 CFR art 58.14. At that point, language in 58.14(c)(2), allows for potential 

shutdown of monitors shown to be lower than other monitors in the same area, subject to EPA 

Regional Administrator approval. The issue of monitor shutdown options and eligibility are fully 

discussed in section IV.D.4 of the preamble to the final rule.

c. Areas initially designated attainment

Comment:

Commenter (0088) stated that the criteria for shutting down or relocating a monitor in an area 

initially designated as attainment should be less stringent than the criteria for shutdown of a 

monitor in an area initially designated as nonattainment. Commenter (0088) stated that if 

monitoring is used to characterize the air quality around a large SO2 source and three years of 

monitoring data (2017-2019) shows no NAAQS violation, then the area will be designated as 

attainment in 2020. Commenter (0088) suggested that, at that time, the monitor should be 
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eligible to be shut down as there are no requirements to continue to characterize air quality in 

designated attainment areas. 

Response: 

The issue of monitor shutdown options and eligibility are fully discussed in section IV.D.4 of the 

preamble to the final rule. Contrary to the suggestion of the commenter, in areas for which air 

agencies choose the monitoring option, and monitoring shows concentrations above the threshold 

promulgated in this rule, the rule requires continued monitoring without regard to whether the 

area has been designated nonattainment or attainment.

d. Clean data

Comment:

Commenters (0038, 0088) encouraged the EPA to allow for shutdown as long as the three-year 

design value is below the NAAQS at any level. Commenter (0088) stated that the EPA should 

amend proposed section 51.1205(c) to provide that a monitor can be shut down at the discretion 

of the state after a “clean data” determination to avoid unnecessary costs of operation of a 

monitor at certain stations in a state during the redesignation process. Commenter (0038) stated 

that each state should be able to simply notify the EPA that the criteria have been met and that 

the monitor is being discontinued. Commenter (0038) stated there should not be a second EPA 

approval process beyond certification of the data. Commenter (0038) stated a more restrictive 

approach is unnecessary, as the rule also proposes that states will have to continue to identify and 

evaluate any increases of SO2 emissions each year in areas designated as attainment. 

Commenter (0074) asked whether a monitor could be shut down if no violations have occurred 

for many years and the requirements for monitoring arise solely from an area’s designation 

status. Commenter (0074) stated the process of an area moving from nonattainment to attainment 

status moves at a glacial pace and it is inappropriate and wasteful of scarce resources to require 

continued monitoring of such areas based solely on the historical non-attainment status, when 

there is no indication of ongoing violation under the present implementation plans.

Commenter (0070) stated that, if industry were to take on some of this monitoring, there must be 

an explicit sunset provision on the length of time a monitor must be operated if there are no 

exceedances of the 1-hour SO2 Standard monitored for several years.

Response: 

The issue of monitor shutdown options and eligibility are fully discussed in section IV.D.4 of the 

preamble to the final rule. In EPA’s experience, the final rule’s approach to monitor shutdowns 

provides an adequate mechanism for removing unnecessary monitors, and we expect it will be 

sufficient for monitoring that is conducted to meet the requirements of this rule. However, the 

EPA believes that monitor shutdowns should continue to involve EPA review, in order to 
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provide for national consistency. Further, 40 CFR part 58.14 requires RA approval prior to the 

shutdown of a routine monitoring site. 

e. State discretion

Comment:

Commenters (0063, 0065, and 0110) stated the shutdown of a monitor should be left to the 

judgment of the state. Commenters (0063, 0065) stated the Proposed Rule should not supplant or 

limit the flexibility provided by the CAA for addressing NAAQS violations.

Response:

The EPA believes that monitor shutdowns should involve EPA review, in order to provide for 

national consistency. Further, 40 CFR part 58.14 requires RA approval prior to the shutdown of a 

routine monitoring site.

f. Monitors in unclassifiable/attainment areas

Comment:

Commenters (0048, 0050, 0075, 0087, 0116) stated monitors located in 

“unclassifiable/attainment” areas should also be considered candidates for shutdown. 

Response: 

The EPA agrees and notes that under the final rule, monitors located in areas designated 

“unclassifiable/attainment” may be considered candidates for shutdown in accordance with 40 

CFR 51.1205 and 40 CFR 58.14.

g. Near a source that shuts down or reduces emissions

Comment:

Commenters (0106, 0107) recommended that, because of the expense of operating monitoring 

sites, the DRR should specify when monitoring around a source may cease and noted that the 

criteria for shutdown of the monitoring site could be based on changes in emissions or changes in 

stack parameters. Commenter (0107) suggested that, in the event that monitoring is being 

conducted near a source that shuts down, the monitors be allowed to be shut down immediately. 

Commenter (0107) suggested that, in the event that monitoring is being conducted near a source 

that achieves permanent emissions reductions such that it no longer meets the minimum

emissions threshold, the monitoring shutdown criteria be followed.
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Response: 

The issue of monitor shutdown options and eligibility are fully discussed in section IV.D.4 of the 

preamble to the final rule. However, in the case where an SO2 facility shuts down, and there are 

no other sources in the area that contributed to having monitoring be required in the area, 

language in 58.14(c) does allow for a state to request for monitor discontinuation “...on a caseby-case basis if discontinuance does not compromise data collection needed for implementation 

of a NAAQS and if the requirements of appendix D...” continue to be met.

h. Statistical approach

Comment:

Commenter (0071) suggested a statistical approach to evaluating a site’s design value trend with 

more emphasis on the variability monitored at the site rather than a more conservative low cut 

point design value threshold. This allows discontinuing sites that have demonstrated stable 

design values over time, showing little risk of exceeding the NAAQS, but keep sites that show a 

greater risk of exceeding the NAAQS.

Response:

The issue of monitor shutdown options and eligibility are fully discussed in section IV.D.4 of the 

preamble to the final rule. The EPA believes that the final approach for allowing monitor 

shutdown allows immediate relief for those monitors well below the NAAQS (no greater than 

50% of the NAAQS for the first three or four years of operation), and that there is also an 

adequate and established approach for shutting down monitors (via 40 CFR part 58.14) that do 

not meet the initial shutdown criteria provided for monitors used specifically to satisfy this 

rulemaking.

i. Do not shut down monitors

Comment:

Commenter (0068) stated new monitors should not be shut down. Commenter (0068) stated that

temporary periods of low gas prices may cause coal combustion to decrease, and thereby lead to 

monitor readings consistent with short-term but not long-term attainment. Commenter (0068) 

stated, for this reason and for the purpose of rebuilding a shattered SO2 monitor network, states 

that choose the monitoring pathway under any DRR should be required to operate those new 

monitors as part of the monitoring network going forward.

Commenter (0068) recommended that monitors not be removed if the concentrations they are 

recording are trending upward, as opposed to trending downward. Commenter (0068) stated data 

indicative of potential future problems, even if no current problems are recorded, mitigates in 

favor of keeping the monitor in place to assess changes in air quality.
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Response: 

The issue of monitor shutdown options and eligibility are fully discussed in section IV.D.4 of the 

preamble to the final rule. The Agency judges that in situations where monitored data indicate 

SO2 concentrations well below the NAAQS (50%) in the first 3 or 4 years of operation, it is 

unnecessary to continue monitoring at that particular location. Beginning in year 5 of monitor 

operations, the monitor is subject to existing shutdown criterion in 40 CFR part 58.14. Ambient 

air quality monitoring is a resource intensive effort and the EPA believes continued monitoring 

to satisfy this rulemaking in locations well below the NAAQS is not warranted.

j. Case-by-case monitor shutdowns

Comment:

Commenter (0062) supported giving Regional Administrators the option of approving a 

threshold higher than 50%, not to exceed 80%, if states can justify it on a case-by-case basis. 

Commenter (0060) stated the EPA needs to provide for the flexibility currently allowed under 

the same rule [40 CFR 58.14(c)(1)] for case-by-case decisions to be made. Commenter (0116) 

recommended that shutdown be considered on a case-by-case basis for any monitor, consistent 

with criteria for shutting down most regulatory monitors.

Response:

The Agency does not agree that a case-by-case approval threshold or alternative criteria for 

monitor shutdowns is appropriate in this rulemaking. The criteria are applied on an individual 

monitoring basis, but there needs to be a general level of consistency to requirements and 

shutdown criteria applied across the whole of any monitoring conducted to satisfy this rule. The 

EPA judges the finalized shutdown criteria to be sufficient and appropriate. See additional 

discussion of this issue in section IV.D.4 of the preamble to the final rule.

8. Post-monitor shutdown annual reporting

Comment:

Commenters (0036, 0038, 0048, 0053, 0064, 0075, 0080, 0090, 0097, and 0116) expressed 

concerns regarding the annual reporting requirements. 

Commenters (0036, 0048, 0064, 0075, 0080, 0090, and 0097) stated the proposed annual 

reporting requirement appears to be unduly burdensome for the states. Commenter (0097) stated 

this information, to be submitted with the annual network plan, would require significant effort

to compile and analyze on a yearly basis.

Commenters (0048, 0075, and 0116) stated that the need for on-going verification has not been 
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demonstrated, requiring verification is unprecedented, and will be an extreme burden on states 

and local agencies. Commenter (0075) stated that, although states submit ambient monitoring 

data to the EPA annually, UARG is unaware of a comparable requirement for other NAAQS that 

air agencies verify and report to the EPA annually whether areas remain in attainment.

Commenter (0116) believed the most that should be required would be an assessment of SO2

emissions to determine if they have increased and, if they have, an assessment of whether that 

increase is sufficiently large that consideration should be given to new monitoring. Commenter 

(0036) recommended that the monitoring plan verification report be limited in scope to solely 

address issues related to the siting and the running of monitors.

Given that SO2 emissions have been declining on a national level for many years particularly 

from the large sources that are the focus of the Proposed Rule, commenter (0116) believed re 

monitoring should seldom, if ever, be appropriate.

Commenters (0080, 0090) opposed the annual reporting requirement, stating that SO2 emissions 

from sources are already available to the EPA and it is not practical or appropriate for the states 

to assess the cause of emissions increases. Commenter (0080) added that the interest of the states 

and the EPA regarding NAAQS status is continued attainment, not the causes of increases in 

emissions that by the nature of unit operations will fluctuate to some degree each year.

Commenters (0064, 0097) suggested an annual report would be too frequent. Commenter (0064) 

recommended a less burdensome process in which this verification would take place every five 

years instead of annually. Commenter (0064) pointed out that the EPA reviews the NAAQS 

every five years and that this verification would be a good item to include in the review.

Commenter (0097) stated that, if monitors were removed that were providing data under 80% of 

the standard, at best such a comparison should be performed every three years, in conjunction 

with the publication of the NEI data on the EPA's website. Commenter (0097) stated another 

option would be to require such assessments be included in the 5 year ambient monitoring 

assessment plan required by 40 CFR 58.10.

Commenter (0036) stated the EPA’s proposed schedule is unrealistic. Commenter (0036) stated 

that the EPA expects the plans to be submitted annually by July 1, but states will not have the 

required quality-assured emissions monitoring data processed by July 1; i.e., after receiving 

those data, states must process them for modeling, run the model, and assess results. 

Commenter (0096) stated that ongoing verification of compliance for areas which have shut 

down monitoring sites would have to start with the year following the years for which the 

monitor was in operation. Commenter (0096) stated that, if the monitor utilized 2017, 2018, and 

2019 data, then the initial verification of ongoing compliance would be for 2020 emissions and 

would be submitted with the July 1, 2021 submittal. Commenter (0096) stated it is unclear in the 

DRR if the determination of whether there was an emission increase should be based on an 

increase greater than the three year average of emissions during the initial monitoring analysis, 

an increase above the highest single year of emissions during the initial monitoring analysis, or 
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some other metric.

Commenters (0097, 0110) stated the potential attainment status of an area should not be based on 

the previous year's activity and emissions, rather, the comparison should be based on some 

compliant level of emissions from the year(s) where the monitor demonstrated attainment with 

the standard, similar to the requirements of a maintenance plan. Commenter (0097) stated 

changes in emissions due to economic slow-downs and recoveries would necessitate significant 

analyses, when in reality the "increase" or "decrease" in emissions of SO2 may have resulted in 

total SO2 emissions levels well below the annual emission rates during the years when 

monitoring data showed compliance. Commenter (0110) stated an extrapolation of potential 

monitored concentrations can be done using a scaling calculation to adjust the monitored 

concentration in proportion to the emissions change.

Commenter (0058) stated that since sources are already required to widely report annual SO2 

emissions, then changes in emissions after a monitor shut down could easily be assessed to 

determine if re-characterization of ambient SO2 concentrations is potentially necessary.

Response:

The EPA has decided not to finalize the proposed requirement that any state with an area for 

which the EPA has approved the air agency’s request for an SO2 monitor to cease operations

under this rule must still assess SO2 emissions changes annually, beginning in the year after the 

monitor ceases operation. The EPA made this decision based upon comments on the proposed 

rule, and in recognition that a cessation of monitoring will not occur unless a monitor has 

measured SO2 concentrations well below the NAAQS for a given time period and an EPA 

Regional Administrator has allowed the shut down. The Agency is persuaded by commenters 

that monitor shutdown provisions, along with generally applicable emissions reporting 

requirements, are of sufficient strength that subsequent additional annual observation and 

reporting of SO2 source emissions profiles by states specifically due to this rulemaking is 

unnecessary. Further, there are means by which monitoring can be reinitiated in the future if the 

unlikely scenario occurs where SO2 emissions rise significantly in an area, or other data indicate 

possible NAAQS violations in an area after a monitor has been shut down, mainly through the 

EPA Regional Administrator authority granted in 40 CFR part 58 appendix D, section 4.4.3.

a. Option 1: annual report as an appendix to the annual monitoring plan

Comment:

Commenters (0043, 0071, 0102, 0115, 0117, and 0118) supported the option of including the 

annual emissions analysis and analysis of monitoring data related to possible monitor shutdown

as components of the annual monitoring network plan. Commenter (0043) indicated that states or 

agencies with only a small number of sources could opt to use the annual air monitoring annual 

plan. Commenter (0071) stated the analysis of emissions is closely related to network planning, 

and this procedure would provide a single document for public inspection and EPA review and 
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approval.

Commenter (0100) stated the Annual Monitoring Plan may not be the best tool or location to 

place modeled data, emission reports, ongoing air quality characterization requirements, and 

requests to cease modeling. Commenter (0100) stated the Annual Monitoring Network Plan 

described in 40 CFR 58.10 (a)(l) is a document created and submitted to the EPA "which shall 

provide for establishment and maintenance of an air quality surveillance system that consists of a 

network of SLAMS, monitoring stations including FRM, FEM and ARM monitors that are part 

of SLAMS, NCore stations, STN stations, state speciation stations, SPM stations and/or, in 

serious, severe and extreme ozone nonattainment areas, PAMS stations, and SPM monitoring 

stations."

Response: 

Please see response above listed under “Post-monitor shutdown annual reporting.” Because this 

report is not being required, these comments are no longer applicable.

b. Option 2: independent annual submittal from the state

Comment:

Commenters (0036, 0041, and 0081) recommended that the monitoring plan verification report 

be considered a separate element. Commenter (0081) recommended that annual emissions 

reporting data be submitted separate from the monitoring plan for ease of processing and for 

public review. Commenter (0081) stated that by combining these reports, very little, if any real 

efficiency will be gained by doing so and requiring monitoring plans to include facility emissions 

as part of this rule may have the effect of delaying the completion of the plan or reporting of 

emissions. 

Commenter (0036) recommended that the monitoring plan verification report should be due later 

in the fall (e.g., November).

Response: 

Please see response above listed under “Post-monitor shutdown annual reporting.” Because this 

report is not being required, these comments are no longer applicable. 

c. Allow both options 1 and 2

Comment:

Commenters (0043, 0069) requested that the EPA allow both options and stated that the 

flexibility of reporting as a separate report following the same public notice may be necessary for 

states tracking multiple sources. Commenter (0060) suggested that if an annual report is 
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necessary, the EPA should require a submittal on July 1 each year and allow it to be part of the 

annual monitoring plan if an air agency chooses; any public notice would just need to clearly 

identify what the document available for comment covers. 

Response:

Please see response above listed under “Post-monitor shutdown annual reporting.” Because this 

report is not being required, these comments are no longer applicable.

d. Monitors with data under 50% of the standard

Comment:

Commenter (0097) suggested that, if the monitors that were removed were providing data under 

50% of the standard, there is no reason to perform such analyses. Commenter (0097) stated that 

an increase in emissions that would result in such a drastic increase in monitored design values 

would surely be associated with changes to operations that would necessitate air permitting, 

which evaluates projects for NAAQS compliance; thus, for areas where the monitoring data are

less than 50%, submitting such evaluations would be needless paperwork.

Response: 

Please see response above listed under “Post-monitor shutdown annual reporting.” Because this 

report is not being required, these comments are no longer applicable. 

Comment:

Commenter (0038) stated the rule should not require additional modeling or monitoring unless 

the states and EPA make a determination that changes in SO2 emission for the source or area in 

question are a significant change in emissions. Commenter (0038) encouraged the EPA to leave 

the procedural options open as to what works best for each state or local agency.

Response: 

Please see response above listed under “Post-monitor shutdown annual reporting.” Because this 

report is not being required, these comments are no longer applicable. 

e. Sunset provision for the reporting requirements 

Comment:

Commenter (0097) stated the EPA should also include in the implementation rule information on

when a state may stop submitting these reports; i.e., the EPA should choose some threshold and 

timeframe to sunset these requirements. Commenter (0097) suggested that, if emissions of SO2

in an area were to drop to less than 50% of the actual emissions inventory for the compliance 
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year for 5 years, increases in emissions to the point where the area may measure a violation of 

the NAAQS would most likely not be accomplished without triggering New Source Review 

(NSR) permitting. 

Response: 

Please see response above listed under “Post-monitor shutdown annual reporting.” Because this 

report is not being required, this comment is no longer applicable.

f. EPA disapproval of a monitoring plan due to nonattainment

Comment:

Commenters (0063, 0065, 0081) disagreed with the requirements proposed in 40 CFR part 

51.1205(d), which appear to state that if modeling or monitoring indicates that an area is not 

attaining the 2010 1-hour SO2 NAAQS, the EPA has an option to disapprove the monitoring 

plan. Commenter (0081) stated this is an unprecedented act; at no time in the past has the EPA 

failed to approve a monitoring plan if violations of other NAAQS have occurred. Commenter 

(0081) stated that, since the monitoring plan is intended to serve as an accounting of the state’s 

obligations under 40 CFR part 58.5, this step is unnecessary and they requested that it not be 

included in final language; rather, the EPA should rely on its authority to issue a SIP call or 

designate an area “nonattainment” if an air agency is negligent in providing the annual emissions 

data as required by this regulation. 

Commenter (0115) stated it is not clear what approach the EPA is recommending, however, if 

actual emissions from these facilities do increase above the levels being emitted while the 

monitor was operational: would the monitors be re-deployed? Would updated modeling be 

conducted instead?

Response: 

The EPA agrees that evidence that an area is violating the air quality standard does not justify 

disapproving a monitoring plan. The EPA has removed this clause from the final language in 

section 51.1205(d). The rule does not address how the EPA would respond to a situation in 

which criteria for terminating operation of a monitor were met but then emissions increase so as 

to present renewed concern as to whether the area is still attaining the standard. The EPA notes 

that this situation is not unique to this rule, and EPA has adequate authority to address any such 

situations. If the emissions increase occurs prior to EPA approval of the request to discontinue 

operating a monitor, EPA could disapprove such a request. If it occurs post-approval and after 

monitoring has been discontinued EPA could address it in its annual review of state monitoring 

network plans. The EPA intends to address such situations on a case-by-case basis. Moreover, 

EPA notes that the identification of such situations is a relatively straightforward exercise using 

routinely provided data.
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9. Emerging technologies

Comment:

Commenter (0036) urged the EPA to continue evaluating emerging, cost-effective monitoring 

technologies that are easier to deploy and can supplement regulatory SO2 monitoring. When such 

technologies become available, we urge the EPA to work with states to develop guidance that 

clearly explains the role of supplemental monitors.

Response: 

The EPA is currently evaluating new “sensor” technologies to keep up with this emerging sector 

and will be looking for ways to leverage them in our ongoing air quality surveillance efforts.

B. Modeling

1. Emissions--actual data

a. Support use of actual data

Comment:

Commenters (0037, 0039, 0041, 0043, 0044, 0045, 0046, 0047, 0050, 0051, 0052, 0052a, 0053, 

0054, 0056, 0058, 0060, 0066, 0067, 0073, 0075, 0082, 0087, 0088, 0089, 0092, 0093, 0097, 

0106, 0108, 0110, 0114, 0117, 0118) supported the EPA’s proposal to base the attainment status 

modeling analyses on actual emissions.

Modeling for designation purposes only

Commenters (0046, 0088) cautioned that the use of actual emissions in the designation modeling 

should not be used for other purposes. Commenter (0046) stated these actual emissions will 

represent reasonable parameters for attainment classification modeling and should be used for 

that purpose only; they should not be a part of any restriction on the operations of the modeled 

source or incorporated into the SIP based solely on a conservative modeling exercise.

Commenter (0088) stated that the use of existing emissions should not result in emission caps on 

stationary sources being modeled because future conditions are not the basis of attainment and 

nonattainment designations – actual ambient air conditions are.

More realistic

Commenters (0042, 0046, 0054, and 0114) stated that the use of allowable emissions would be 

too conservative and may result in designation of inaccurate nonattainment areas. Commenter 

(0046) stated several layers of conservatism are incorporated into regulatory modeling analyses 

in the form of worst-case emissions, worst-case meteorology, worst-case stack and operating 
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conditions, and most conservative definition of ambient air and, with this compounded 

conservatism, a sole reliance on these types of modeling results for designations would yield 

unrealistic outcomes that ultimately would have the unintended consequence of halting industrial 

growth in areas where public health is not at risk. Commenter (0054) stated that given the 

uncertainties with predictive SO2 modeling, the use of actual facility emissions data as model 

input provides the most accuracy in making SO2 designation decisions. 

Commenters (0042, 0088, 0110) stated that using actual emissions is even more important when 

conducting a cumulative impact analysis since the model’s tendency to overestimate ambient air 

impacts is compounded when numerous sources are all modeled at peak emissions at all times, 

leading to unrealistic model estimates. Commenter (0088) stated that since “allowable” or plant 

“PTE” emissions are unlikely to occur at a plant or for that matter, at the same time from 

multiple plants being modeled, it is arbitrary and capricious as well as unreasonable for 

“allowable” or plant “PTE” to be used as inputs to describe existing ambient air conditions. 

Commenter (0087) stated that to the extent the EPA continues suggesting reliance on modeling, 

the Agency's instructions in the proposal for use of a model are generally reasonable and will 

ensure more realistic predictions of ambient air quality than if modeling were done in accordance 

with the Guideline on Air Quality Models, 40 C.F.R. part 51, App. W.

Surrogate to monitoring

Commenters (0041, 0060, 0088, 0092, and 0106) generally supported using actual SO2 emissions 

for attainment status designations since the use of actual emissions, rather than allowable

emissions, will "serve as a surrogate to comprehensive ambient monitoring" and its use as an 

input in the model will "more closely represent ambient monitoring results." Commenter (0092) 

stated the task is to determine and designate the attainment status of source areas within 

jurisdictions, not to forecast or evaluate scenarios. 

Predictive modeling

Commenters (0040, 0044) requested the final rule preamble to include language to confirm 

distinctions between modeling for 1-hour SO2 designation purposes and other CAA predictive 

purposes (e.g., NSR). Commenter (0044) supported the distinction the Proposed Rule makes 

between modeling conducted for characterizing air quality for designation purposes and use of 

predictive modeling conducted for other purposes, but stated the preamble’s discussion of the 

modeling eventually to be carried out under this rulemaking is ambiguous and contains 

contradictory statements. Commenter (0044) stated that while the preamble first appears to move 

toward modeling actual emissions and away from the use of predictive modeling to characterize 

current air quality, the ensuing discussion appears to reverse course and could be interpreted to 

say only that the use of actual emissions data may “presumptively” be the most appropriate 

methodology but that it may be equally appropriate to use a predictive modeling methodology.
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Response:

EPA agrees that the use of actual emissions is generally expected to be more realistic than using 

allowables to represent current air quality conditions. However, in some cases, there can be little 

if any difference between estimated actual and allowable emissions, for example in the case of a 

source that is limited by work practice requirements, rather than by a numerical emissions limit 

that a source operates under with a compliance margin. The EPA also agrees that in some cases 

the use of allowables would be more conservative and could lead to an overestimation of current 

air quality pollutant concentrations. For this reason, EPA continues to advise that modeling of 

actuals may be submitted (consistent with the modeling TAD) to satisfy the requirements of this 

rule. With respect to the comments about use of actuals modeling for other purposes, nothing in 

this rule changes any established rules or guidance regarding modeling for other purposes (such 

as SIP modeling cited in comments).

b. State option to model actual or allowable

Comment:

Commenter (0078) stated that because the state air agency is ultimately responsible for 

implementation of the NAAQS and any ongoing attainment verification requirements associated 

with the Proposed Rule, the EPA should make clear in the final rule that it will defer to state air 

agencies' decisions regarding appropriate emissions rates for designations modeling. Commenter 

(0078) stated that it is reasonable to expect that outside stakeholders may disagree with the 

emissions inputs a state air agency selects and could potentially seek to circumvent state air 

agency decisions and submit additional modeling to the EPA using the alternate emissions input 

in order to support different designation determinations. 

Response:

As stated in the preamble to the final rule, when using actual emissions, the EPA believes the 

most recent 3 years of time varying emissions (e.g., emissions that vary hourly, seasonally, 

monthly, daily, etc.) should be modeled since the air quality modeling is being used as a 

surrogate for monitoring. The EPA does not believe it is necessary to specify by rule a rigid set 

of requirements for determining emissions inputs for modeling to satisfy this rule. However, to 

ensure the data are appropriate for use in implementing the NAAQS (for example, for use in 

designations), we strongly recommend that air agencies follow the modeling TAD

recommendations. The TAD provides recommendations on such topics as inputting hourly 

emissions into AERMOD for those sources with hourly continuous emissions monitoring (CEM) 

data and on inputting time-varying emissions (e.g. seasonally, monthly, etc.) when no hourly 

emissions are available and only annual emissions and data such as production logs or fuel usage 

are available. The final rule does not restrict the ability of air agencies to use more conservative 

allowable emissions in conducting their modeling. In the event that a particular source does not 

have the most recent 3 years of emissions, it may be possible to use the most recently available 

emissions or develop the most recent 3 years of emissions using recommendations in the TAD. 
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The reviewing authority should work with the appropriate EPA Regional Office on the use of 

such emissions. For an application that contains a mix of sources whose emissions data are not 

concurrent with each other, it is possible to model all of these sources together following 

recommendations in the TAD. Once a modeling protocol or modeling analyses have been 

submitted, there is no requirement to revise the protocol or modeling respectively if more recent 

emissions have become available since the submission, and in the best professional judgment of 

the reviewing authority, those emission changes do not warrant a revision to the protocol or 

modeling analyses. If outside stakeholders provide information those conflicts with information 

that a state provides, the EPA will consider all information, without prejudging which 

information is more reliable.

c. Intermittent sources

Comment:

Commenters (0042, attachment A5, 0110) stated that highly intermittent sources, especially 

those that operate when more base-loaded sources are off-line, should continue to be excluded 

from modeling with actual emission inputs. Commenter (0042, attachment A5) concurred with 

the March 1, 2011 the EPA guidance that modeling analyses be based on emission scenarios that 

can logically be assumed to be relatively continuous and not include sources for which operation 

is sufficiently intermittent that they are unlikely to cause a violation. Commenter (0042, 

attachment A5) stated that, in considering the criteria for exempting intermittent sources, the 

EPA also needs to consider the frequency of meteorological conditions, specifically wind 

direction, that along with the intermittency of a source’s emissions, determine whether any 

downwind location could be affected on more than 1% of the days in a year, corresponding to the 

99th percentile SO2 NAAQS.

Commenter (0042, attachment B) stated that additional guidance may be required to deal with 

continuous emission sources that include both a continuous and non-continuous component since 

the low frequency sources (i.e., non-continuous and intermittent sources) may have a 

disproportionate impact on the 99th and 98th percentile value on which compliance with the 1-

hour NAAQS will be judged.

Response:

As part of the December 2013 update to the SO2 NAAQS Designations Modeling Technical 

Assistance Document, EPA recommended in section 5.4 of the modeling TAD to follow the 

guidance discussed in the March 1, 2011 memo “Additional Clarification Regarding Application 

of Appendix W Modeling Guidance for the 1-hour NO2 National Ambient Air Quality 

Standard.” This guidance is applicable to both modeling with actual emissions and allowable 

emissions.

d. Hourly rates for model input
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Comment:

Commenters (0042, attachment A5, 0052) objected to the EPA guidance that suggests that 

sources be modeled at various operating conditions, rather than only at peak load because this 

can become an unmanageable analysis if multiple sources are involved in the modeling.

Commenter (0042, attachment A5) stated it is our experience that in most cases, maximum load 

(associated with maximum emissions) is the worst‐case modeled result, and it should generally 

be the one selected.

Commenter (0049) stated that if modeling is to be used for attainment designations, then use of 

sub hourly and meteorological data must be allowed to better represent SO2 concentrations 

because hourly emissions and meteorological data currently used in AERMOD, causes 

excessively conservative results. 

Commenter (0110) supported using the average hourly emission rate for model inputs based on 

reasonably available data. Commenter (0110) stated that, if data are not reasonably available, and

variation is random, the use of actual annual emissions divided by actual annual operating hours 

is an appropriate way to generate an hourly emission rate modeling input for such sources. 

Commenters (0063, 0065) stated that sources without CEMS data should still be able to use 

actual emissions data, rather than allowable emissions, as the basis for modeling.

Commenter (0042) recommended that the EPA eliminate the requirement that emission limits to 

demonstrate compliance with the 1-hour SO2 NAAQS must be expressed as 1- hour average 

limits. Commenter (0042) stated the EPA has previously -- and appropriately -- recognized that 

the enforcement of longer-term emission limits can ensure the attainment and maintenance of 

shorter-term ambient standards. Commenter (0078) stated that the final rule should indicate 

concurrence with the EPA's "Guidance for 1-Hour SO2 Nonattainment Area SIP Submissions" 

(April 2014), which indicated that emission limits with longer-than-hourly averaging times may 

be appropriate if states can demonstrate that the longer averaging times provide comparable 

stringency to a one-hour limit at the critical emission value. Commenter (0087) stated that, as the 

EPA has not solicited comment on the April 2014 methodology as part of this proposal, they 

reserved the right to provide comment on this methodology based upon further review.

Commenter (0087) encouraged the EPA to allow flexibility for states and affected sources to 

develop emission limits in a form and averaging time that is meaningful for the source being 

evaluated. Commenter (0087) provided the following additional points.

One-hour mass-based emission limits would impose very stringent requirements on 

boilers. Controls to meet such limits would need to be designed to a much higher 

standard of reliability to ensure that even the smallest upsets (minor plugging of one or 

more sorbent injection ports, for example) would not cause an excursion of the limit. To 

the extent that 1-hour limits are required, periods of SSM must be excluded in calculating 

compliance with the limit. 
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Response:

Since the focus of this rule is on characterizing the air quality impacts of actual emissions, this 

rulemaking is not establishing policy with respect to the comments of commenters 0042 and 

0087 regarding emission limits. With regards to the use of sub-hourly meteorological data, subhourly meteorological data can be input into AERMET but is averaged over the hour for input 

into AERMOD. At this time, AERMOD does not accept sub-hourly data. With regards to the 

sub-hourly emissions data, the source may want to work with the reviewing authority and 

Regional Office to determine the best way to utilize those emissions data. In regards to 

Commenter 0110, the modeling TAD states that in the absence of CEM data, simply dividing the 

annual emissions by the number of hours in the year would not be an accurate representation of 

the actual emissions. Facilities, the reviewing authority, and Regional Office should work 

closely to ensure the best quality of emissions. For sources that do not have CEM data, the TAD 

offers several suggested methods to calculate variable emissions rates from annual emissions and 

other information such as throughput, operating schedule, production, etc.

e. Revise the regulatory text

Comment:

Commenter (0040) requested that the EPA add language to both the preamble and codified 

regulatory language in a Final Rule that (1) actual emissions shall be used for air modeling for 

initial designation purposes, but (2) that allowable emissions may be used to demonstrate 

attainment status. Commenter (0040) suggested the following language for section 

51.1203(d)(2):

(2) Modeling analyses shall characterize air quality based on actual 1-hour SO2 emissions 

from the most recent 3 years, or federally enforceable allowable emissions for purposes 

of demonstrating attainment. . . .

Commenter (0044) stated the regulatory text is at best ambivalent on whether actual emission 

should be used, and could be read to give air agencies (and the EPA Regional Administrators 

with whom they must consult) unfettered discretion to model based on allowable emission rates 

as they see fit. Commenter (0044) believed the rule is intended draw a substantive distinction 

between the use of modeling for the purpose of characterizing current ambient air quality (i.e., 

modeling conducted as a surrogate for 1-hour SO2 NAAQS designation monitoring), and 

predictive modeling conducted for purposes of NSR, PSD, and SIPs, then that distinction should 

be reflected unambiguously in both the rule’s preamble and in the regulatory text of the final 

rule, which it does not currently do. Commenter (0044) recommended the following revisions be 

made to the text of section 51.1203(d)(2) for clarification:

(2) Consistent with subpart BB’s purpose of making designations of current air quality 

attainment of the SO2 NAAQS, modeling analyses shall characterize air quality based on 

actual 1-hour SO2 emissions from the most recent 3 years, or federally enforceable 
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allowable emissions which demonstrate that attainment has been achieved. If the air 

agency intends to use allowable emissions limits for this analysis, it may submit such 

allowable emissions limits for the EPA’s approval at the time the modeling protocol is 

submitted.

Response:

The EPA agrees with the commenters that the modeling of actual emissions can be used to 

characterize the emissions around SO2 areas to satisfy the requirements of this rule. When using 

actual emissions, the EPA believes the most recent 3 years of time varying emissions (e.g., 

emissions that vary hourly, seasonally, monthly, daily, etc.) should be modeled since the air 

quality modeling is being used as a surrogate for monitoring. The modeling TAD gives 

recommendations on inputting hourly emissions into AERMOD for those sources with hourly 

continuous emissions monitoring (CEM) data and also gives recommendations on inputting time 

varying emissions (e.g. seasonally, monthly, etc.) when no hourly emissions are available and 

only annual emissions and data such as production logs or fuel usage are available.

On the other hand, the EPA believes that modeling of allowable emissions is also an appropriate 

approach, which is warranted as an alternative to modeling actual emissions. This approach 

indicates the potential for violations, and thus provides an indication of whether further air 

quality planning might be warranted. If modeling of allowable emissions shows the limits to 

provide for attainment, this rule then provides that no further initial characterization based on 

actual emissions is needed, nor is any ongoing review needed to assess whether variations of 

emissions (within allowable levels) might result in violations of the standard (such as would be 

required for areas designated attainment based on actuals modeling). (Although the relationship 

between the results of such modeling and actual air quality would depend on whether the source 

is complying with its emission limits, the EPA believes that, in absence of information about 

compliance status, modeling based on a presumption that allowable emissions are the maximum 

that emissions might be is a reasonable basis for establishing that a source would not violate the 

standard.) In the case where modeling of allowable emissions shows the potential for violations,

this rule provides that such modeling also satisfies the air agency’s initial obligation to 

characterize the source. Although this rulemaking does not address how the air quality 

characterization information required by this rule will be used in future actions (e.g., in future 

designation actions), the EPA concludes that air agencies should be given the option under this 

rule to model allowable emissions and thereby to develop information that characterizes the 

potential for violations. The EPA does note the factual distinction made by commenters between 

modeled actual violations and modeled potential violations, but at the same time, EPA makes a

note of past practice in which information on the potential for violations was in some cases the 

basis for some areas being designated nonattainment, and we do not agree that it would be 

appropriate for this rule to exclude such information as a characterization option or to proscribe 

the uses for such information if it is provided.

2. Emissions--most recent three years
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Comment:

Commenters (0036, 0040, 0046, 0067, 0087, 0092, 0093, and 0114) generally supported EPA’s 

proposal to base the modeling analyses on actual emissions over a 3 year period. Commenter 

(0040) stated that actual emissions from the previous 3 years, coupled with contemporaneous 

meteorological data, helps to ensure accurate modeling output to make designations.

Commenter (0093) stated that the most recent 3 years of representative emissions data should 

normally be used in the modeling; however, if justification can be provided for an alternative 

dataset, it too may be considered for modeling. Commenter (0069) noted that, in situations where 

multiple sources are being modeled, the most recent three years of actual emissions data may not 

be the same for all sources, particularly when one or more sources are EGUs that report to EPA's 

Acid Rain database and one or more sources are non-EGUs that report under the provisions of 

the AERR. 

Commenter (0069) stated the rule should clarify that states shall use the most recent three years 

of emissions data for an area available to the state at the time that a modeling protocol for that 

area is submitted to the EPA. Commenter (0069) stated that revised modeling should not be 

required if more recent emissions data become available after a modeling protocol is submitted 

to the EPA. Commenter (0069) stated that, should an area be designated attainment based on 

modeling protocols and modeling analyses submitted prior to the regulatory deadlines, the 

proposed provisions for assuring continued attainment are sufficient for addressing changes in 

emissions.

Commenter (0036) stated that, if applicability were to be additionally based on maximum actual 

hourly emissions, then the length of time to apply to the initial annual applicability trigger would 

be less important.

Commenter (0062) recommended that the maximum year from the previous three years be 

required for designation modeling, instead of one year because this would make the modeling 

option more equivalent to the monitoring option for designation. Commenter (0060) did not 

agree with the use of methods such as the highest of the most recent three years.

Response:

Responses to comments regarding actual emissions over the most recent three years, alternative 

datasets, and modeling protocols can be found in the Rule preamble in section VI.D.6.b.iii. In 

addition, the modeling TAD provides extensive discussion of recommendations regarding the 

emissions data to use in modeling analyses conducted to satisfy this rule. The commenters make 

several recommendations that must be considered on a case-by-case basis. For example, the need 

to consider changes in emissions that occur after completion of a modeling protocol must 

consider whether the emission changes are sufficiently significant as to render a modeling 

analysis based on the prior emissions no longer indicative of current air quality. Similarly, the 

utility of a modeling analysis that uses data from different years for different sources must be 

weighed as to how reliable this approach is in representing current air quality for the particular 
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situation. (The modeling TAD provides specific recommendations for addressing this situation.)

The EPA generally recommends using data from the most recent three years for purposes of this 

rule, consistent with the air quality standard being defined in terms of the average among the 99th

percentile daily maximum 1-hour SO2 concentrations over three consecutive years.

3. Emissions-- accounting for enforceable limits

[Also see IV.A.5 regarding legal agreements]

a. Support accounting for enforceable limits

Comment:

Commenters (0046, 0069, 0087, 0096, and 0097) generally supported the inclusion of language 

providing the option for states to model emission rates based on enforceable limits.

Commenters (0046, 0069, 0087, 0096) stated future emissions reductions (e.g., Regional Haze, 

CSAPR, and MATS) should be accounted for in the emissions data used to model designations,

in order to decrease the likelihood that areas will be designated nonattainment based on outdated

information. Commenter (0087) stated the EPA should allow the use of emission data that will 

more accurately reflect the operational and emitting characteristics of these sources in the postMATS and post-Boiler MACT timeframe. Commenter (0097) stated it should be noted that this 

practice is consistent with the methodologies used to develop PM2.5, ozone, and regional haze 

planning inventories, where actual emissions, projected forward into the future year using 

reasonable growth estimates, are modeled to make determinations about compliance for these 

criteria pollutants. Commenter (0097) stated, therefore, the use of actual emissions estimates for 

purposes of SO2 compliance is merely following the attainment planning process of these other 

pollutants.

Commenter (0052) stated that, if the installation of scrubbers generally suffices for areas to meet 

the 1-hour SO2 NAAQS, then those units and units with similar emission profiles should be 

exempt from the modeling demonstration.

Commenter (0052a) stated the EPA should allow the use of actual emission rates and statistically

representative emissions profiles when these data are deemed to be sufficiently reliable. This 

approach could include the use of continuous emission monitoring system data. 

Response:

After review of the comments, the EPA believes that it is appropriate for the air agency to 

consult with the affected source(s) and take action to adopt enforceable emissions limitations as 

necessary prior to January 2017. As long as the emissions limitations are in place and 

enforceable by January 2017, the new allowable emission limit may be input into the model 
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instead of the actual emissions of the most recent 3 years.8

The EPA expects that a number of emissions sources may be candidates for this optional 

approach. Many EGUs will need to meet compliance deadlines for the MATS in April 2015 (or 

in some cases, April 2016), and the EPA expects that many will become subject to Title V 

permits that require compliance with MATS SO2 emission limits as the means of demonstrating 

compliance with the MATS requirements related to acid gas emissions. These EGUs may be able 

to adopt control technologies and enforceable emission limits to reduce emissions of SO2 as well 

as mercury. Similarly, industrial boiler operators will commonly be reducing SO2 emissions in 

conjunction with complying with Industrial Boiler MACT requirements, and sources will have 

incentive to assure that the control strategy they implement to comply with the Boiler MACT 

requirements will also assure attainment of the SO2 air quality standard. As a result, many of 

these sources might wish to become subject to limits reflecting these controls. 

It is appropriate, where modeling information is required to be submitted to EPA under this rule, 

for that information to take these limits into account. Therefore, the EPA believes that as long as 

these emissions reductions are implemented and enforceable prior to January 2017, it would be 

appropriate for the new lower allowable emissions to be used in a modeling analysis in place of 

the higher actual emissions. The air quality impacts from such a source would be characterized 

by the new enforceable allowable limit (at worst). Information that includes the effect of such 

limits could be considered, for example, in future determinations regarding areas’ attainment 

status.

Conversely, if the controls are not required to be implemented by January 2017, then it would be 

inappropriate to consider the controls in assessing current emissions. This is because the 

assessments made under this rule are due in 2017 and it is necessary for them to reflect the 

conditions that are in places at the time the assessments are due. This modeling is intended to 

assess current air quality (which is useful, for example, for making determinations of NAAQS 

attainment status at the then-current time that reflect current, not future, air quality). In this 

respect, modeling for satisfying the informational purposes of this rule is different from 

modeling for attainment planning purposes, which generally assesses the benefits of future 

controls.

b. Controls implemented and enforceable as of January 2017

Comment:

Commenters (0035, 0041, 0045, 0046, 0060, 0062, 0070, 0078, 0082, 0085, 0088, 0097, and 

0114) generally supported the inclusion of language providing the option for states to model 

more recent emission rates based on enforceable limits implemented in advance of the January 

 

8 While these comments address the consideration of emissions data used in modeling analyses, 

the final rule also provides for emission limits at sufficiently low levels (generally at below 2,000 

tpy) to warrant being exempted from air quality characterization requirements.
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2017 modeling deadline. Commenter (0035) stated air agencies could notify the EPA of any 

initially listed sources that have reduced emissions below the threshold by July 1, 2017, well in 

advance of the EPA's proposed timeframe for nonattainment designations.

Commenters (0035, 0041, 0097) stated this approach is a reasonable option which would provide 

industry with an incentive to achieve timely emission reductions to meet the regulatory 

requirements while relieving air agencies from the requirements that a nonattainment designation 

entails. Commenters (0046, 0060) asserted that states need flexibility to work with 

sources/groups of sources to reduce emissions and avoid nonattainment designations based on 

models. Commenter (0114) agreed with the proposed approach but requested that the method for 

reducing emissions not be limited to installing controls.

Commenters (0035, 0060) stated the EPA should develop methodologies for air agencies to work 

with sources whose 2015 emissions are above the threshold to establish permanent and 

enforceable emission limitations that show attainment with the SO2 standards prior to the next 

round of designations in 2017. Commenter (0060) stated there must be a process that allows for 

the air agencies discretion under extenuating circumstances in order to account for significant 

changes at a facility that occurred during the most recent three years. 

Commenter (0070) stated the EPA must clarify the rulemaking in a way that would allow states 

that desire to submit a full attainment SIP based on modeling by January 2017, or such other 

time as specified in the Final Rule, to do so; and, the EPA should be permitted to accept the 

modeling as satisfying the requirements necessary to grant a designation of either attainment or 

unclassifiable at the time of issuance of the initial designations.

Commenter (0060) stated that, under the Proposed Rule, a source which installed a control in 

2013 and has 2014-2016 actual emissions that model attainment will not be required to have an 

enforceable limitation, while a source that installed a control in 2015 and has three years of

emissions that don't model attainment because of the pre-control emission levels only, would 

require an enforceable limit. Commenter (0060) suggested modeling with surrogate emissions 

that are based on operational history for the period of time without control and with actual 

emissions for the period of time with control; if the source fails the modeling, it could elect to 

take an early enforceable limitation to avoid nonattainment or elect to be nonattainment and 

subject to the attainment demonstration analysis and requirements due in the future. Commenter 

(0060) stated this is the type of flexibility air agencies need in order to meaningfully apply these 

rules without subjecting sources to inconsistent and overly burdensome requirements.

Response: 

After review of the comments, the EPA contends that it is appropriate for the air agency to 

consult with the affected source(s) and take action to adopt enforceable emissions limitations as 

necessary prior to January 2017. As long as the emissions limitations are in place and 

enforceable by January 2017, the new allowable emission limit may be input into the model 

instead of the actual emissions of the most recent 3 years.
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The EPA expects that a number of emissions sources may be candidates for this optional 

approach. Many EGUs will need to meet compliance deadlines for the MATS in April 2015 (or 

in some cases, April 2016), and the EPA expects that many will become subject to Title V 

permits that require compliance with MATS SO2 emission limits as the means of demonstrating 

compliance with the MATS requirements related to acid gas emissions. These EGUs may be able 

to adopt control technologies and enforceable emission limits to reduce emissions of SO2 as well 

as mercury. Similarly, industrial boiler operators, subject to acid gas limits under the Boiler 

Maximum Achievable Control Technology Standard (MACT), will in many cases be 

substantially reducing their SO2 emission levels, such that an emission limit reflecting this 

control might suffice to demonstrably assure attainment.

Therefore, the EPA believes that as long as these emissions reductions are implemented and 

enforceable prior to January 2017, it would be appropriate for the modeling analysis submitted to 

EPA to satisfy this rule to reflect the new lower allowable emissions in place of the higher actual 

emissions. The results of such analysis could be useful for purposes such as making future 

determinations regarding areas’ attainment status.

The EPA wishes to clarify that, to the extent that the commenter wishes to conduct modeling that 

considers emission reductions that will occur after January 2017, such modeling will not satisfy 

the requirements of this rule. This rule requires consideration of current, not future, emission 

levels, to assess current, not future, air quality, as of the date that the assessment is due to EPA. 

The DRR does not address the future use of this information, but the EPA nevertheless

recognizes the commenters’ desire to minimize resources spent on air quality planning for areas

that currently violate the NAAQS but where future actions are likely to resolve these air quality 

problems. The EPA welcomes discussions of how best to address such situations on a case-bycase basis.

The EPA does not agree with the suggestion for sources that install controls in 2015, for 

example, to be modeled using an inventory constructed as if the controls had been installed in 

2013. Such an approach would be based on a hybrid inventory that fails to reflect actual 

emissions for the three year period and also does not reflect allowable emissions. For a source 

that installs controls for example in 2015 but is not subject to any requirement to maintain the 

resulting emission level, the control level achieved in 2015 is not necessarily a reliable basis for 

determining “current” emission levels, insofar as the 2015 data differ from the 2013 and 2014 

data and are not necessarily indicative of how emissions will be in 2016, 2017, and beyond.

Furthermore, the EPA believes that a source that installs controls in 2015 warrants greater 

scrutiny and warrants stricter prerequisites for modeling post-control emissions (i.e., needing for 

post-control emissions to become enforceable) than a source that installed comparable controls in 

2013 and has a full three years of data reflecting these controls. This approach also provides 

more incentive for early controls, and this approach reflects the fact that the source that 

controlled in 2013 has more evidence that it can be expected on a long-term basis to emit at the 

reduced levels. For these reasons, the suggested approach is not recommended in the modeling 

TAD.
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c. Emission reductions after 2017

Comment:

Commenters (0046, 0069) suggested that, since the designations are to be based on actual, not 

allowable emissions, the rule should allow for actual emissions to be estimated based on the 

future control that will be applied. Commenter (0046) suggested that states should not be locked 

into using emissions from a prior calendar year to determine attainment, but should be able to 

consider emissions projections beyond 2017. Commenter (0046) stated for example, if a scrubber 

that achieves 90% control of SO2 is being installed, the facility should apply 90% control to their

three years of past actual emissions for a modeling demonstration.

Commenter (0069) stated that some facilities may not be able to design, construct, and operate 

controls as quickly as 2017. Commenter (0069) stated the EPA should allow states to model 

controls that will be implemented between January, 2017, and the attainment deadline of 

February, 2023, as long as limits associated with those controls are included in an enforceable 

permit, rule, or similar mechanism by January, 2017.

Response: 

This final rulemaking is not establishing requirements for completing designations, and 

designations decisions will be based on the information before the agency at the times EPA takes 

those future actions. This rule requires consideration of current, not future, emission levels, to 

assess current, not future, air quality, as of the date that the assessment is due to EPA. The DRR 

does not address the future use of this information, but the EPA nevertheless recognizes that 

controls may not be feasible by 2017 in some instances, and that such future controls may 

resolve air quality problems that may be found in an assessment that is current as of the January 

2017 due date. The EPA welcomes discussions of how best to address such situations on a caseby-case basis

4. Background concentrations

Comment:

Commenters (0042, 0046, and 0070) stated that the rule should allow flexibility in determining 

what background value to use. 

Clarification

Commenter (0106) referred to pages 26-27 of the modeling TAD and stated the discussion in 

section 8 on background concentrations seems to refer to both a design value background 

concentration (Page 27 first paragraph) and an annual average background concentration (Page 
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27 second paragraph). Commenter (0106) stated this seeming discrepancy in the discussion of 

background should be addressed.

Commenter (0052) stated that the EPA fails to address how to determine an appropriate

background for areas with multiple monitors.

Double counting

Commenter (0040) stated that for any modeling method, the EPA should address and eliminate 

the potential for “double-counting” of background concentrations of SO2. Commenters (0042, 

0046) stated required use of background SO2 concentrations often double counts the effect of the 

modeled sources and must be addressed. Commenter (0042, attachment A5) stated the approach 

suggested by the EPA that uses data from just one ambient monitor may result in some hours of 

double counting with modeled source impacts. Commenter (0042, attachment A5) stated the 

EPA needs to change its approach to one that favors an unbiased modeling solution, such as a 

consideration for using concurrent monitoring data to determine hourly background (and using 

the minimum value from multiple monitors to avoid double‐counting of modeled impacts).

Monitors near a source

Commenter (0042, attachment B) stated that, in appropriate situations, monitors near a source 

should be allowed to represent the contribution from off-property and background sources (i.e., 

no currently existing sources on-site and no existing off-property sources should be included in 

the modeling because they are already included in the ambient monitor data). Commenter (0042, 

attachment B) stated background concentrations - based on monitors near sources being modeled 

- should be paired in time with the meteorological conditions that produced them. Commenter 

(0042, attachment B) stated ambient monitoring data collected during exceptional events -- even 

those that do not exceed the relevant NAAQS -- should be excluded when determining 

background. 

Modeling data

Commenter (0046) stated modelers should be allowed to use either a “clean” background 

monitor value or perform a back trajectory analysis to determine the hours when the source is 

impacting the monitor and exclude those hours from the 99th percentile calculation.

Commenter (0051) suggested that the EPA specifically allow modeled background concentration 

using a regional scale model such as CAMx to estimate hourly (if necessary) background 

concentrations when determining existing air quality. Commenter (0052) stated that, in lieu of 

using monitoring values to derive a background values for areas in Kentucky, they propose to 

use regional background values that are derived using CMAQ modeling data. Commenter (0052) 

stated that the use of existing air quality monitoring to represent background concentrations can 

be overly conservative because most of these monitors have some impact from the facilities that 

are being explicitly modeled.
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Percentile value

Commenter (0070), referencing several technical papers, stated the papers all clearly 

demonstrated that background was not a blanket 99th percentile value as has been espoused by 

EPA in draft modeling TAD. Commenter (0070) stated that combining the real design value

(99th percentile daily high monitored value) with the 99th percentile daily high modeled value 

generates a value that results in a value far more stringent than the underlying 1-Hour SO2

NAAQS, to which it is being compared. Commenter (0070) stated that adding the 99th percentile 

monitored value to the 99th percentile daily high modeled value does not result in a 99th percentile 

combined value, but a 99.99th percentile value or one exceedance allowed roughly every 9,000 

days, which is significantly more stringent value than the underlying standard to which it is being 

compared. Commenter (0070) stated this clearly violates the statistical integrity of the 

comparison to the ambient standard. 

Commenter (0070) recommended that, based on several studies performed over the past several 

years, the EPA incorporate no higher than a 50th percentile monitored value as a “can use for 

any purpose” option or call for a rigorous case by case review of the available monitoring data as 

suggested in Appendix W as the appropriate methods to establish background values for use in 

an analysis supporting designation of an unclassifiable or attainment area where a modeling 

approach is selected. Commenter (0070) stated that use of a 50th percentile value for background, 

while still more stringent than the form of the standard, better protects the statistical integrity of 

the standard than does the current EPA guidance. Commenter (0070) stated that use of a 50th

percentile value still results in an increase in the stringency of the standard from roughly one 

exceedance day in every hundred to one exceedance day in roughly every 180 days. 

Commenter (0110) stated the EPA’s March 1, 2011 Tier 1 guidance is overly conservative 

because if the modeled and monitored design concentrations are from independent distributions, 

the probability of the combination being exceeded is not 1%, but rather 0.01%. Commenter 

(0110) stated the EPA’s Tier 2 approach is less conservative for many situations, but could be 

more conservative for specific hours and seasons. Commenter (0110) provided the following 

additional points.

The problem with the Tier 1 and 2 approaches is that the background value added is 

always a near-peak value, while the most likely coincident background value would be 

closer to the 50th percentile ranked value, rather than the 99th percentile. The EPA’s 

approaches therefore assume that background and modeled concentrations are highly 

correlated, even if there is no evidence that this is the case. A way to address possible 

correlation is to add hourly modeled concentrations to the (paired in time) hourly 

background concentrations, using representative monitors for which the modeled source 

is not materially influencing the monitored background concentration. The approach 

allows the appropriate meteorological pairing in time of the background concentration 

due to affected air masses to the predicted concentration from the source. EPA’s March 1, 

2011 guidance allows this approach in cases with adequately representative monitoring 
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data, and we recommend that the EPA should allow its use more generally, especially for 

use of modeling for designation of areas with existing sources. The use of actual hourly 

background would be consistent in modeling with the use of actual hourly emission rates 

for input. In areas with multiple monitors and modeled sources that could result in 

“double counting”, the hourly background concentration should be the lowest reading 

among the available monitors unless there is an upwind monitor that represents regional 

background levels. A method of pairing modeled and monitored background 

concentrations should be an available (Tier 3) approach. To be consistent with the use of 

actual hourly emissions for sources being explicitly modeled, the background

concentrations should consist of concurrently hourly values from a representative 

monitor. 

A refinement to the Tier 1 and 2 approaches, similar to the recent PM2.5 modeling 

guidance, would be to determine the concentration level above which the monitoring data 

indicates local impacts, and then to use the peak value (either a single value for Tier 1 or 

an hour-of-day, seasonal value for Tier 2) from the remaining values. This concentration 

level is not necessarily a fixed percentile value, but must be discerned from a ranked plot 

of the monitored values. (0110)

Response: 

In response to comments regarding flexibility, EPA has offered several options in determining 

background concentrations. As stated in the modeling TAD, a first tier approach first described 

in the August 23, 2010 clarification memorandum “Applicability of Appendix W Modeling 

Guidance for the 1-hour SO2 National Ambient Air Quality Standard” was to use the overall 

highest hourly concentration from a representative monitor. The EPA recognized that this 

approach may be overly conservative and in the March 1, 2011 memorandum “Additional 

Clarification Regarding Application of Appendix W Modeling Guidance for the 1-hour NO2

Ambient Air Quality Standard” which is also applicable to SO2, offered alternative approaches of 

using the most recent design value and a background that varies by season and hour of day. In 

response to the comment regarding areas with multiple monitors (commenter 0052), AERMOD 

does allow for varying background by spatial locations that can be used for locations with 

multiple representative monitors and where those monitors are representative of specific areas. 

With regards to “double counting” of background (Commenter 0040), section 8.2.2.b is clear that 

background concentrations are to exclude hours where modeled sources impact the monitor. 

With regards to comments near a source, the modeling TAD states that background 

concentrations should be from a representative monitor. The monitor near a source of interest 

would usually be representative and can represent nearby sources’ impacts. However, the TAD 

and section 8.2.3 of Appendix W recommend modeling nearby sources which cause a significant 

concentration gradient in the vicinity of the source of interest. In those cases, hours at the 

monitor that are impacted from those modeled sources are excluded from the background 

concentration. When determining hours to exclude, in order to eliminate double counting of 

model, use of back trajectories or other analyses are encouraged.
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With regards to percentiles of background, the recommendations in the TAD are that, 

recommendations. If applicants feel that a different percentile is warranted then they should work 

with the appropriate reviewing authority and Regional Office to justify a different methodology 

than outlined in the TAD. Nevertheless, it is worth noting that the commenters that find the 

combination of a 99th percentile modeled result with a 99th percentile background value to yield a 

99.99th percentile value are applying faulty methods of statistical analysis. The percentile of a 

sum cannot be computed by multiplying the percentiles (or 100% minus the percentiles) of the 

component parts. The error of this method can be illustrated by using this method to compute 

other results, equally in disregard of any differences in magnitude or variability of the 

component parts. For example, this method would suggest that the combination of a 99th

percentile modeled result with a 50 percentile background value would be a 99.5th percentile, and 

the only background value that in combination with a 99th percentile modeled value would give a 

99th percentile result would be a zero background value. In any case, these comments speak not 

to questions pertinent to this rulemaking concerning what modeling requirements should be 

established but rather to questions as to how those future modeling analyses are conducted.

5. Stack height

Comment:

Commenters (0037, 0046, 0047, 0050, 0051, 0052, 0053, 0056, 0058, 0073, 0075, 0082, 0089, 

0093, 0097, and 0108) generally supported the use of actual stack height.

Commenters (0046, 0058, 0081, 0089, 0093, and 0097) stated the EPA should allow sources to 

model using actual stack height regardless of whether they are modeling actual emissions or 

allowable emissions since the purpose of the rule is to estimate, as accurately as possible, 

conditions that would be measured at a monitor. Commenters (0046, 0050, 0058, 0081) 

disagreed that the GEP stack height is more appropriate when using allowable emissions because

the GEP height not always a factor in establishing the emissions limit; for example, in the 

context of emission limits that are established based on emission standards, such as the MATS 

rule. Commenter (0081) agreed that the concern about giving inappropriate credit for dispersion 

techniques is not relevant in this situation as section 123 applies only to emission limitation 

controls; the fact that actual height may have not been used when calculating the allowable 

emission rates is irrelevant in the context of this designation modeling.

Response:

After consideration of comments received on the proposed rule, the EPA modeling TAD 

continues to recommend the use of actual stack heights when using actual emissions and the use 

of GEP height when modeling with allowable emissions where such emissions limits are or 

would be subject to CAA section 123 and to the EPA’s corresponding regulations implementing 

GEP requirements. This would include limits established under any CAA provision that are 

intended to be credited in an implementation plan for attaining and maintaining the NAAQS. The 

use of GEP for allowable emissions modeling in such situations is to be consistent with 
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nonattainment planning requirements, which cannot consider any portion of the stack height that 

is above GEP height, since the purpose of modeling based on allowable emissions would be to 

assess whether the limits would satisfy nonattainment planning requirements for assuring 

attainment. 

6. Meteorological data

a. One year

Comment:

Commenters (0067, 0085) supported the use of one year of meteorological data. Commenter 

(0067) stated that use of the most recent three years of actual emission data modeled with one 

year of on-site met data would yield a very robust data set and record for making a modelingbased designation. Commenter (0085) stated that operating an on-site meteorological monitor for 

three years does not provide a significant benefit over collecting one year's on-site 

meteorological data. Commenter (0085) stated that one year of meteorological data are sufficient 

for PSD purposes. 

Commenter (0067) stated the requirement to use the most recent 3 years of met data are not

practical, will prove costly, and there is no real need for it. Commenter (0067) stated the EPA

should not require 3 years of on-site meteorological data for designations modeling and 

requested the TAD be revised to not require 3 years of on-site meteorological data.

Commenter (0067) noted an on-going, extensive modeling study using one year of onsite 

meteorological data, one year of concurrent actual emissions data, and up to one year of 

concurrent monitoring data from five SO2 ambient air monitors and stated it is not reasonable to 

expect such a met station to run continuously for more than a year due to cost, noise and location 

issues.. Commenter (0067) stated that, in cases where a facility is willing to install such an 

elaborate and costly station, the EPA should encourage such an endeavor as it will result in better 

regulatory decisions, however, the EPA should recognize the challenges and allow such systems 

to operate for the one year minimum and allow that year to be non-concurrent with the actual 

emissions being modeled.

Commenter (0085) stated that not many sites already have on-site meteorological monitors, so in 

addition to the financial cost and time required to install them, requiring the collection of three 

years of data would delay the running of AERMOD for designations. Commenter (0110) stated 

the EPA should continue to require only 1 year of on-site meteorological data for attainment 

designation modeling. Commenter (0110) stated a requirement to collect three years of on-site 

data are inconsistent with the proposed designations timeline since such a requirement would 

make it impossible to use new on-site data and still adhere to the anticipated EPA schedule.

Commenter (0110) stated that, while relatively few meteorological databases with three years of 

on-site meteorological data exist; many sources may have already collected a full year of data in 

accordance with Appendix A and have since dismantled their meteorological towers. Commenter 
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(0110) stated these sources should be able to use the 1 year of data already collected without 

starting all over again on an expensive three-year effort. 

b. Five years

Comment:

Commenter (0062) recommended that up to five years of meteorological data be used concurrent 

with the emissions data in modeling to simulate current air quality for designation purposes and 

future verification modeling. Commenter (0062) stated that a single three-year period may not 

provide adequate confidence in the analysis. Commenter (0062) stated that five years will 

provide more three-year combinations that can be compared to the NAAQS and more 

meteorological data improves confidence in the result.

Response: 

The EPA’s recommendation is to use the most recent 3 years of representative site-specific data. 

When site-specific data are not readily available, or it is not feasible or cost-effective to collect 

site-specific data, the most recent 3 years of representative NWS meteorological data or other 

representative data could be used. When the most recent 3 years of representative meteorological 

data are not available, the use of older representative meteorological data can be used. For such 

cases, the modeling TAD offers recommendations on synchronizing the older meteorological 

data with the more recent emissions, especially for those sources utilizing hourly emissions. The 

TAD provides an explanation of the need for 3 years of meteorological data, even if only 1 year 

of on-site meteorological data is available. With regards to the type of meteorological data that 

are available, i.e. site-specific, NWS data, or prognostic data, appendix W should be consulted 

on the latest acceptable forms of meteorological data at the time of the modeling analyses.

c. Concurrent meteorological and emissions data

Comment:

Commenter (0082) urged EPA to explicitly state in the final rule that air agencies need not use 

concurrent meteorological data, given that some sites simply do not have concurrent 

meteorological data. Commenter (0082) agreed with the suggested approach in the draft 

modeling TAD, which recognizes that, in such situations, it “may be feasible to use older 

meteorological data...that has been used in past regulatory applications for the area containing 

the threshold exceeding source, if these datasets are still considered representative of the most 

recent three years of meteorological conditions.” 

Commenter (0106) raised the question of what is meant by the most recent 3 years: would this 

most recent period necessarily be circa 2010-2012 or later. Would the use of an earlier period of 

actual emissions and meteorological data (say 2006-2008) be acceptable as long as a companion 

analysis is provided which demonstrates that actual emissions have decreased (or at least not 
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increased) since that time. Commenter (0106) referred to page 26 of the modeling TAD and 

stated that use of upper-air data from Radiosonde Data of North America CD for the period 

1946-1997 does not seem consistent with the recommended use of the most recent 3 years of 

emissions paired with the most recent 3 years of concurrent meteorological data (i.e., the most 

recent year on the CD would be 1997). 

Response:

The EPA’s recommendation is to use the most recent 3 years of representative site-specific data. 

When site-specific data are not readily available, or it is not feasible or cost-effective to collect 

site-specific data, the most recent 3 years of representative NWS meteorological data or other 

representative data could be used. When the most recent 3 years of representative meteorological 

data are not available, the use of older representative meteorological data can be used. For such 

cases, the modeling TAD offers recommendations on synching the older meteorological data 

with the more recent emissions, especially for those sources utilizing hourly emissions. The TAD 

provides an explanation of the need for 3 years of meteorological data, even if only 1 year of onsite meteorological data is available. With regards to the type of meteorological data that are 

available, i.e. site-specific, NWS data, or prognostic data, appendix W should be consulted on 

the latest acceptable forms of meteorological data at the time of the modeling analyses.

d. Where meteorological data are unavailable

Comment:

Commenter (0042, attachment A5) stated that the EPA suggests that states should apply 

screening modeling when representative meteorological data suitable as input to AERMOD are 

unavailable. Commenter (0042, attachment A5) stated that, if such an overly conservative

approach indicates a predicted NAAQS violation, then the state and affected sources should be 

provided with an opportunity to determine an alternative, less conservative modeling approach, 

and, if needed, provided with adequate time for a more refined modeling analysis, including new 

field data collection, if that option is selected.

Commenter (0082) stated that, given many air agencies will be unable to submit modeling 

protocols to the EPA by January 15, 2016, for sites that lack three years of representative sitespecific meteorological data or ASOS meteorological data, as required by the Proposed Rule and 

the draft modeling TAD, the EPA should approve the use of prognostic meteorological data, in 

lieu of meteorological data from observations or specific meteorological modeling simulations. 

Commenter (0082) stated prognostic meteorological data are routinely used by the NWS in its 

forecast models and can create three years of site-specific meteorological data in six months. 

Commenter (0082) stated that, for prognostic meteorological data to be used by air agencies in 

this context, the EPA would need to develop guidance on its use by December 31, 2014, to allow 

sufficient time for the data to be created. Alternatively, commenter (0082) stated, the EPA

should extend the timeline for state agencies to submit modeling protocols to the EPA for sites 
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without representative site-specific meteorological data or ASOS meteorological data so that 

state agencies will have time to obtain the required data.

Response:

In response to comments regarding areas without representative meteorological data, the 

modeling TAD states that the meteorological data only be representative, which does not 

preclude the use of a station that is representative of conditions in the analysis domain but is not 

nearby. It may be possible to use such stations acting in consultation with the EPA Regional 

Office. With regards to prognostic data, the latest version of Appendix W at the time of modeling 

should be consulted for acceptable forms of meteorological data. To the extent that the 

commenter considers the shortage of time before the modeling protocol deadline to justify usage 

of prognostic data, it is worth noting that the final rule reflects a six-month delay in the deadline 

for the modeling protocol.

e. Multiple meteorological stations

Commenter (0106) referred to page 24 of the modeling TAD regarding meteorological station 

selection ("if the modeling domain is large enough such that conditions vary drastically across 

the domain, then the selection of a single station to represent the domain should be carefully 

considered") and stated the EPA's meaning here is not clear. Commenter (0106) requested 

explanation/clarification. Commenter (0106) suggested that the presence of varying 

meteorological conditions and the need for more than one meteorological station would be more 

appropriately addressed with a dynamic model such as CALPUFF; the dynamic model would 

handle this situation more seamlessly and accurately than possible manipulation of AERMOD 

layers.

Response:

When the modeling domain is large enough such that multiple stations are available or are 

representative of different parts of the domain, the selection of the meteorological station would 

require best professional judgment by the applicant and reviewing authority. The station that best 

represents the conditions for most of the domain would most likely be the station of choice. 

With regards to use of CALPUFF, this would require an alternative model demonstration 

(Section 3.2.2 of Appendix W) as CALPUFF is not a preferred model for near-field modeling.

7. Receptor placement

Comment:

Commenters (0042, 0110) suggested that certain areas be excluded from receptor placement for 

modeling. Commenter (0042, attachment B) stated receptors should not be located where it is 

highly unlikely individuals will remain stationary for an hour; i.e., freeways, waterways and 

extreme slopes. Commenter (0110) stated receptors should be excluded from areas under the 
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ownership of the source being modeled and areas that cannot support monitoring, such as water 

bodies, steep terrain areas, and transportation routes. Commenter (0042, attachment B) stated 

siting of receptors at a property fence line, regardless of whether any members of the public are 

located there, and basing compliance with NAAQS with predicted concentrations at those 

receptors may require sources such as agricultural operations to reduce emissions, without 

producing health or environmental benefits. Commenter (0051) urged the EPA to modify the 

guidance to recognize the type of public exposure the primary SO2 NAAQS is designed to 

prevent: exposure of an exercising asthmatic to short duration SO2 concentrations that can induce 

mild, reversible changes in respiratory function.

Commenters (0042, 0046, 0051, 0073, 0103, and 0110) supported the placement of model 

receptors only in areas where it is feasible to place a monitor since the modeling is acting as a 

surrogate for monitoring. Commenter (0051) stated the EPA should use a consistent receptor 

placement policy for monitor siting and compliance modeling demonstrations and suggested the 

modeling TAD should follow the receptor placement of the monitoring TAD.

Commenter (0103) suggested it may not be reasonable to use the same receptor placement policy 

for both monitor placement and compliance modeling demonstrations. Commenter (0103) 

provided the following additional points.

Historically, one of the hallmarks of regulatory dispersion modelling analysis for 

NAAQS compliance is that any point in ambient air (outside of the facility fence line) is 

eligible for receptor placement. Because the Proposed Rule indicates that receptor points 

in the modeling are to be regarded as surrogates for monitors, this could be construed to 

mean that a modeling receptor point that is not feasible for monitor placement is not 

eligible for inclusion in the monitoring grid for an attainment determination and is 

therefore not protected under the NAAQS. An overly broad reading of the notion of a 

receptor point as a surrogate for a feasible ambient monitoring location could change the 

attainment status determined from the modeling analysis from non-attainment to 

attainment or in an extreme case, might result in a modeling analysis which could not 

determine attainment status near a facility because of the exclusion of receptors that 

represent “infeasible” monitoring sites.

The federal monitor siting rules in 40 CFR part 58 Appendix E suggest avoiding any 

local influences near the site that might alter air flow from the source to monitoring site 

(walls or trees) or create local sources or sinks of pollutants at the monitoring location 

(roads or furnace flues). In practice, there are many areas excluded for monitors in the 

federal monitor siting rules and was concerned with any overly broad reading of the 

notion of a modeling receptor point as a surrogate for a monitoring site. (0103)

Commenter (0103) noted that the modeling TAD gives only one example of this type of receptor 

point exclusion--over a water body--and commenter (0103) requested clarification on other

receptor points that might be excluded from the modeling analysis based on the criteria of 

monitoring feasibility. Commenter (0103) asked if a receptor point be excluded from the 
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modeling grid, if upon investigation of the receptor location: 

• The location was within 10 meters of a tree, too close to a road, or did not meet any of the 

other criteria in 40 CFR part 58 Appendix E? 

• The state could not secure a lease to operate a monitor at the location? 

• The costs associated with trenching the power to the location exceeded the cost of the 

monitoring equipment at the site? 

• The location was in a remote area, and a site operator could not be secured, or could not 

be secured at reasonable price? 

• The location was in an unpopulated area, where the state could not justify the additional 

expense of monitoring relative to modeling based on the human health benefits of 

operating the monitor at the location? 

• The location was near an emitting facility that had declared its intention to switch from 

coal to natural gas during the 3 year monitoring period? 

Response: 

The December 2014 version of the modeling TAD was updated to include the receptor 

placement strategy of the monitoring TAD. It recommends that receptors could be placed where 

a monitor could be placed since the modeling is acting as a surrogate for monitoring. This, for 

example, would avoid requiring placing receptors over bodies of water and areas where it is 

infeasible to place a monitor. This strategy differs from the other regulatory applications of 

dispersion modeling (SIP and NSR/PSD) where receptors should be placed in ambient air, 

regardless of whether a monitor can be placed in that location.

8. Paired test

Comment:

Commenter (0070) stated the EPA has failed to explain why a paired test is useful for 

photochemical modeling of secondary pollutants, but is not useful for the evaluation of modeling 

of directly emitted primary pollutants. Commenter (0070) stated that logic suggests that if 

meteorological data are captured concurrently with the ambient concentration data when 

examining primary pollutants, if the modeling system is properly formulated and functioning, 

obtaining elevated readings at roughly the same time and place in both the model and ambient 

data sets should occur most of the time.

Commenter (0106) referred to page 28 of the modeling TAD ("hour-by-hour pairing is not 

recommended because this approach would assume that the hourly monitored background 
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concentration is spatially uniform for the hour and that monitored values are representative of 

background levels at each receptor for each receptor") and stated this concern regarding the 

uniformity of hourly background concentrations in the context of a steady-state model 

application has no merit. Commenter (0106) stated that, if it is reasonable to assume that hourly 

meteorological data from a single station is spatially uniform for the entire modeling domain and 

if it is reasonable to assume that surface characteristics associated with a single location are 

spatially uniform for the entire modeling domain, then it is likewise reasonable to assume that 

hourly background concentrations from a single location are also spatially uniform for the entire 

modeling domain; especially if the hourly background concentrations reflect an unbiased rural 

monitoring site.

Response:

While this final rule is not the forum in which the agency would make amendments to its nonbinding technical guidance, the EPA continues to recommend against hour-by-hour pairing of 

background with modeling results except in isolated cases. As stated in the March 1, 2011 

memorandum “Additional Clarification Regarding Application of Appendix W Modeling 

Guidance for the 1-hour NO2 National Ambient Air Quality Standard” and referenced in the 

modeling TAD, pairing hour by hour assumes that the background is spatially uniform and that 

the monitored values are fully representative of background at each receptor at each hour. The 

memo states that such an assumption ignores the temporal and spatial variability of ambient 

concentrations across a typical modeling domain on an hourly basis. In this response the EPA is 

not adopting this position as a final, binding matter, and continues to urge commenters who wish 

to recommend amendments to the TAD to direct such comments to [James Thurman of the Air 

Quality Modeling Group, OAQPS, RTP, N.C., 27711 (Thurman.james@epa.gov, 919-541-

2703).

9. AERMOD, EPA’s preferred near-field dispersion model 

a. Performs well

Comment:

Commenter (0054) agreed that for certain conventional SO2 emission scenarios, such as tall 

stacks at coal fired electricity generating units; AERMOD can be at least reasonably predictive.

Commenter (0068) stated that AERMOD modeling performs particularly well in evaluating 

emission sources with one or a handful of large emission points. Commenter (0068) cited a 

declaration of Roger W. Brode in which he stated that AERMOD is capable of accurately 

predicting whether the revised primary SO2 NAAQS is attained and whether individual sources 

cause or contribute to a violation of the SO2 NAAQS. Commenter (0068) stated that AERMOD 

has been tested and performs very well during conditions of low wind speeds, citing comments 

of Camille Sears.
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b. Concerns with AERMOD

[See also VIII and IX, regarding the TADs and Tenth Modeling Conference, and II.C, regarding 

general comments on models.]

Comment:

Commenters (0039, 0040, 0042, 0046, 0048, 0049, 0050, 0051, 0054, 0058, 0059, 0063, 0064, 

0065, 0070, 0073, 0079, 0080, 0090, 0092, 0108, 0109, 0110, 0112, 0114) expressed concern 

with the use of AERMOD. Commenter (0042) recommended that the Agency take the time to 

correct the problems with AERMOD before the EPA considers or requires the use of that model 

in implementing the 1-hour SO2 NAAQS. Commenter (0070) recommended that, until the EPA

can stabilize the treatment of low wind speed conditions in AERMOD, the EPA should avoid the 

use of AERMOD for designation purposes.

Over predicts

Commenters (0039, 0042, 0043, 0046, 0048, 0049, 0050, 0058, 0064, 0063, 0065, 0073, 0080, 

0090, 0108, 0109, 0110, 0112, and 0114) stated that AERMOD over predicts ambient air 

concentrations. Commenter (0042) listed a number of issues with AERMOD that need to be 

addressed because they lead to over predicting SO2 concentrations, including the following.

 The modified building downwash algorithm specified in Model Change Bulletin #4 

(dated 11059) in AERMOD, which has not been validated, contributes to systematic over

predictions by AERMOD.

 AERMOD over predicts concentrations in gradually sloping terrain; the indication in the 

Draft Guidance that terrain less than stack height need not be considered is inadequate to 

resolve the issue. Over predictions of a factor of 2 can occur due to the terrain effect.

 In general, the aspects of AERMOD and EPA’s guidance that contribute to over

prediction of the SO2 concentrations in the context of the 1‐hour NAAQS are 

multiplicative. As an example, if three independent aspects of the model application each 

resulted in an over prediction factor of two, the resulting over prediction would be a 

factor of eight (2x2x2). By adding conservative assumptions in terms of model input and 

the inclusion of conservative background concentrations, the EPA does not take into 

account the built‐in conservatism of AERMOD and the general EPA modeling guidance.

EPA should be open to the development of improved procedures for addressing variable

emissions and meteorological conditions.

Commenter (0048) asserted this over prediction is particularly true when accurate emissions data 

and representative meteorology are not available. Since modeling will be required in many cases 

based on the proposed dates in the DRR, commenter (0048) stated it will be almost impossible to 

obtain representative meteorological data. Commenter (0048) stated onsite meteorological data 
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are generally not readily available, thus, it is likely that AERMOD will not produce 

representative results and areas will be misclassified with a leaning towards over prediction and 

nonattainment classifications.

Commenters (0039, 0042, 0046, 0049, 0058, 0063, 0065, 0079, 0109, and 0110) referenced 

studies that indicate AERMOD over predicts. Commenter (0039) asserted that studies by EPRI 

and others have shown that AERMOD over predicts actual ambient concentrations by a factor of 

two when the concentrations are in the range of the 75 ppb standard. Commenter (0058) stated

that model evaluation studies, using the most recent version of AERMOD, have documented 

AERMOD’s tendency to over predict concentrations relative to actual measured data, sometimes 

by a factor of 2 or more. Commenters (0042, 0063, and 0065) cited a recent AERMOD study by 

AECOM. Commenter (0109) stated AERMOD over predicts actual SO2 emissions by a factor of 

10 when compared to actual IDEM monitoring data. Commenter (0110) stated that a comparison 

of AERMOD modeling and monitoring data for a North Dakota facility indicated that while 5 

ambient monitors measured levels at about 50% of the SO2 NAAQS, modeling using actual 

emissions and an agency-determined background predicted the peak SO2 concentration at a level 

above the NAAQS.

Commenters (0042, 0073, 0090, 0110, and 0112) expressed concern with over prediction during 

periods of low wind speed. Commenter (0042) stated AERMOD over predicts substantially 

during periods of low-wind speed events (0042, attachment 5 to their comments, provides 3 

pages of technical comments on this issue). Commenter (0110) stated that important overprediction issues have been identified with AERMOD in stable light wind conditions (e.g., Paine 

et al., 2010) and that these problems can affect peak predictions and thus can significantly affect 

the compliance status of a source if modeling is to be used instead of monitoring to establish 

whether an area surrounding a source complies with the SO2 NAAQS. Commenter (0112) 

included a June 2014 study prepared for them by AECOM titled “Evaluation of Low Wind 

Modeling Approaches for Tall-Stack Databases.” The study’s findings and conclusions include:

 For the North Dakota evaluation, the AERMOD model versions all over predicted using 

the design concentration as the metric.

 For the relatively low elevation monitors, the results were similar for both the default and 

beta options and are within 5-30% of the monitored concentrations depending on the 

model option.

 The modeling result for the elevated DGC #17 monitor showed that this location is 

sensitive to terrain as the ratio of modeled to monitored concentration is over 2.0, but 

when this location was modeled with the low wind beta option, the ratio was notably 

better, at less than 1.3.

 For the Gibson flat terrain evaluation, AERMOD with hourly averaged meteorological 

data over predicts at three of the four monitors between 30-50 percent, and about 10% at 

the fourth monitor.

 The AERMOD low wind speed options have a minor effect for flat terrain locations.

 These options have a more significant effect with AERMOD modeling for elevated 

terrain locations.
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 The AERMOD sub-hourly modeling results are the most unbiased of all model 

combinations tested for these two databases.

 SCICHEM results indicate some over prediction tendencies for some monitors. The 

model developer has been provided with this information for purposes of further testing 

and evaluation. (0112)

Commenters (0109, 0110) stated that some sources may feature substantial excess heat releases 

that are not currently accounted for by AERMOD (presentation by Paine at 2014 EPA modeling 

workshop). Commenter (0110) stated that in many cases, accounting for this additional heat 

release may involve an urban characterization to the source or a realization that the emissions 

near heated buildings are not subject to downwash effects, but instead “lift off” due to the 

heating effects.

Commenters (0063, 0065) stated that the recommendation in the modeling TAD to model 

sources that are within a few kilometers of each other together may cause AERMOD to over 

predict SO2 concentrations or result in otherwise inaccurate data because co-locating sources will 

tend to produce higher concentrations in the dispersion model than modeling as individual 

sources and even sources within relatively close proximity to each other could have different

dispersion patterns depending on geography, meteorology, or operating schedules.

Commenters (0042, 0046, and 0108) stated that AERMOD was intentionally designed to over

predict SO2 concentrations. Commenter (0042) stated that certain specific modeling assumptions, 

inputs, and techniques exacerbate AERMOD's over prediction tendencies, including lateral 

dispersion at low wind speeds, vertical concentration gradients, and buoyant line sources. 

Commenter (0042, attachment B) stated that AERMOD over prediction is inappropriate given 

the stringency of the new standards and the proposed use of AERMOD for designation purposes.

Commenters (0063, 0065) stated that AERMOD fails to meet QA criteria for air quality data and 

siting criteria in the current regulations prescribed by part 58, Appendices A and D. 

Improvements to AERMOD

Commenters (0042, 0047, 0075, and 0090) encouraged the EPA to continue improving the 

Agency's preferred models. Commenters (0042, 0051, and 0090) stated that any revisions to the

model and data used to make those revisions should be made available to the public (e.g., put on 

the SCRAM website). Commenter (0051) stated that the EPA should provide states with 

documentation of all technical issues and fixes that have been identified with the AERMOD 

system. Commenter (0051) stated that of particular interest is information on how to adjust the 

modeling process or predicted concentrations.

Commenter (0042) stated that, if the EPA wants to require the use of AERMOD for a broader 

range of regulatory purposes, then it should address the problems with AERMOD, many of 

which are identified in an AECOM report attached to their comment letter. Commenter (0064) 
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stated that while improvements were made to the AERMOD model since promulgation of the 1-

hour standard, more improvements are to take place, suggesting that AERMOD is still not an

accurate indicator of the actual ambient air quality.

Commenter (0110) stated that because there are many technical issues that need to be addressed 

in AERMOD, relying on AERMOD before these changes are implemented could result in 

erroneous NAAQS attainment and nonattainment designations. Commenter (0070) 

recommended that, until the EPA can stabilize the treatment of low wind speed conditions in

AERMOD, the EPA should avoid the use of AERMOD for designation purposes, as it may 

incorrectly penalize sources that do not have an adverse impact on actual ambient air quality 

concentrations, but are located in areas with frequent low wind speeds. Commenter (0040) stated 

that if AERMOD is retained in the final rule, the EPA should allow use of low wind speed 

corrections to better reflect the actual conditions around some sources modeled.

Commenters (0042, 0090) stated AERMOD's performance needs to be re-evaluated in light of

multiple recent modifications.

Commenters (0042, 0075, and 0080) recommended that the EPA establish more formal 

procedures for making changes to its preferred models. Commenters (0042, 0075, and 0080) 

stated that they are concerned that none of EPA’s recent actions to improve the performance of 

AERMOD was the subject of a public comment period or noticed for comment in the Federal 

Register. Commenter (0080) stated the EPA continues to release updates and newer versions of 

AERMOD and AERMET without providing a public comment period or notice of comment in 

the Federal Register and that this lack of comprehensive assessment of the impact of changes to 

the model and processor components creates concerns for stakeholders. While commenter (0075) 

supported allowing regulatory agencies and sources to use new modeling tools that may not have 

undergone full notice-and-comment review, they encouraged the EPA to subject improvements 

in the model to broader review (like the review provided by notice-and-comment rulemaking) 

because stakeholders remain concerned about over-reliance on modeling predictions.

Commenter (0110) stated that a number of AERMOD updates are needed in Appendix W to 

improve the accuracy of model estimates, including stable light wind conditions and excess heat 

releases for certain types of sources. Commenter (0110) stated that, while the EPA intends to 

promulgate changes to Appendix W in the spring of 2016, until these AERMOD changes are 

promulgated, it is premature to rely upon the use of AERMOD to determine modeled compliance 

with the SO2 NAAQS.

Form of the standard

Commenters (0042, 0051) expressed concern that AERMOD may not be suited for

demonstrating compliance with a one-hour standard based on the three-year average of the

annual 99th percentile of the daily maximum one-hour average concentrations.

Commenter (0042, attachment A3) stated the EPA's preferred model, as it exists today, is unable 

to directly provide values that can "fit" the probabilistic form of the NAAQS adopted in the final 
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rule, meaning states will have significant difficulty meeting the requirements of the new 

standard, as stated. Commenter (0042, attachment A3) provided the following additional points.

The difficulties integrating modeling and monitoring data due to the form of the SO2

standard adopted by the EPA will require that modeling procedures be refined to 

realistically address the frequency of peak short-term impacts in order to appropriately 

implement the new 1-hour S02 NAAQS. The joint frequency of worst-case cumulative 

emissions and adverse dispersion conditions becomes more important for probabilistic 

ambient standards. The new SO2 NAAQS adopted by the EPA is a probabilistic standard.

AERMOD is a deterministic model (that is, provides point estimates) and is not suitable 

to provide probabilistic short-term concentrations for a NAAQS that requires a 

probabilistic modeling approach. EPA's preferred air dispersion models have not been 

specifically developed or evaluated to predict short-term locations of maximum 

concentration or account for the probabilistic standard.

For cases where the probability of simultaneous occurrence of peak emissions and worstcase meteorology is very low, the standard modeling approach will unrealistically 

exaggerate peak ambient concentrations. This is particularly true for sources that do not 

operate continuously, such as conditions during startup and shutdown, or testing. Adding 

monitored background concentrations to modeled maximum concentrations will most 

likely be overly conservative, which would make those modeled projections 

inappropriate for use as a basis for designations or redesignations under the CAA. (0042, 

attachment 3)

Other technical concerns

Commenters (0042, attachment B, and 0051) stated the EPA should also require comparison of 

predicted concentrations at the time and location of monitored values. Commenter (0042, 

attachment B) stated this will facilitate evaluation of the model’s treatment of atmospheric 

chemistry and meteorology. Commenter (0051) stated that, while AERMOD can predict high 

concentrations that were observed at a nearby monitor from an isolated source, there was no 

attempt during the model's development to match predicted concentrations to monitored 

concentrations at the same time and location of the monitor and there was no attempt to model 

the transformation of SO, or determine deposition downwind. 

Commenter (0049) stated that if modeling is to be used for attainment designations, then use of 

sub hourly and meteorological data must be allowed to better represent SO2 concentrations 

because hourly emissions and meteorological data currently used in AERMOD, causes 

excessively conservative results. 

Commenters (0054, 0092) stated modeling approaches using AERMOD are of particular concern 

at aluminum smelters because AERMOD does not adequately account for heat release (and 

associated plume rise) within these facilities; the effect of numerous merged buoyant plumes; 
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and/or the impact of downwash from large or long buildings. Commenter (0092) stated they have 

previously provided the EPA with documentation (which they did not cite) that clearly 

demonstrates that AERMOD does not reliably predict emissions from aluminum smelters (or 

sources near large or long buildings).

Commenter (0059) asserted AERMOD can be extremely sensitive to small variations to input 

values and that optimum performance for AERMOD occurs with extremely enhanced source 

characterizations (e.g., actual emission rates, true exhaust flows, exhaust temperatures, etc.).

Commenter (0059) stated that unless these two limitations to using air dispersion modeling, and 

AERMOD specifically, can be quantified and minimized, the ramifications of “false” attainment 

designations are too great. Commenter (0059) stated, for example, that small changes to surface 

roughness values input to AERMET, the meteorological preprocessor for AERMOD, have been 

shown to cause modeled concentrations that vary dramatically.

Commenter (0059) asserted that, while the accuracy of AERMOD is documented in various 

AERMOD evaluation studies, the evaluation studies represent optimum datasets that include 

access to monitoring data from one or more ambient air monitoring locations, detailed and actual 

hourly process information for the sources being modeled for the periods corresponding to the 

period of ambient air monitoring data, and detailed meteorological data that can enhance the 

performance of AERMOD. Commenter (0059) stated it is undocumented as to how AERMOD 

performs for situations where the completeness of source data and meteorological data are 

reduced relative to the evaluation studies. 

Response: 

In the final rule, the EPA is not promulgating a requirement that air agencies use AERMOD in 

all cases, but is retaining the existing flexibility otherwise provided by the EPA’s rules for 

agencies to support the use of the best model for a particular case. Therefore, this final rule does 

not address the specific objections to AERMOD that the commenters have raised. The EPA’s 

latest recommendations for making the assessment of which model to use are contained in the 

modeling TAD, but, again, the EPA is also not codifying the TAD or taking final action to make 

its recommendations binding. In most cases, the EPA believes that AERMOD will likely be the 

model of choice by air agencies to address the requirements of this rule, unless the application 

involves a different recommended model, such as the Buoyant Line and Point Source Dispersion 

Model (BLP). Models recommended for particular applications are listed in appendix A of 

appendix W, section 3.2 of appendix W outlines the procedures for use of alternative models for 

those cases where an alternative model may be more appropriate than a preferred model. In 

addition, the modeling TAD also discusses past use of alternative models for particular 

applications.9 The EPA recommends consultation with the appropriate reviewing authority or 

 

9

It is the EPA’s intention to update the modeling and monitoring TADs as necessary to reflect 

any change in policy or to make clarifications that are necessary. Therefore, any comments on 

the TADs themselves that have been submitted in response to the proposed rule will be 

addressed as a part of any updates made to the TADs in the future, rather than in this final rule.
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EPA Regional Office to determine if the use of an alternative model is valid for that application.

In addition, as stated previously, the TADs are documents that provide recommendations but are 

not binding or enforceable and create no obligations on any person. Although the draft TADs are 

referenced as recommended approaches in the preamble to the proposal and in this rulemaking, 

they are not required to be adhered to by any state that is required to characterize air quality 

around an SO2 source identified in this rulemaking. The TADs have been provided in order to 

potentially aid air agencies seeking advice in the air quality characterization process required by 

this rulemaking.

With regards to over predictions of AERMOD, while any model is not perfect, EPA has shown 

on numerous occasions, that when proper inputs are input into AERMOD when using actual 

emissions, AERMOD has been shown to perform well against other models when compared to 

observations. If users follow the recommendations in the TAD regarding source inputs, 

meteorological data (including surface characteristics) and background information, along with 

the use of best professional judgment, EPA believes that the model outputs will be sufficient for 

the stated goals of this rule. That is, EPA finds AERMOD, when run appropriately, to provide 

information with adequate reliability for use in the various planning efforts that this rule is 

intended to support. That being said, EPA is working in contexts outside of this rulemaking to 

alleviate technical concerns with AERMOD such as low winds, building downwash, etc.

Finally, as explained previously, this final rule is not establishing requirements for how future 

designations must be conducted, and does not prejudge the appropriateness of the use of 

AERMOD or an alternative model in such future designations.

10. Alternative models and alternative approaches

Comment:

Commenters (0050, 0051, 0054, 0056, 0060, 0070, 0092, 0093, 0109, 0106, and 0114) 

encouraged EPA to support alternative models and approaches.

Guideline models and Appendix W

Commenter (0050) supported the idea that air modeling for initial designations should not need 

to conform to the EPA’s Guideline on Air Quality Models and that states should have control 

over the types of models they use. Commenter (0051) stated that the EPA prescribed model 

limits the amount and type of data that can be used to characterize current air quality.

Commenter (0060) requested the EPA to note, similar to Appendix W of 40 CFR part 51, that 

approaches/models other than AERMOD may be appropriately used on a case-by-case basis.

Commenter (0109) believes that 40 CFR part 51 Appendix W should expressly include as many 

model refinement options as possible to provide clear guidance to states and regulated entities 

that these modeling options are appropriately considered.
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List of models used in regulatory assessments

Commenter (0051) stated the EPA should provide a list of models, such as CAMx, that have 

been used extensively in regulatory assessment, that states can use without being required to 

submit lengthy justification and model performance studies. Commenter (0051) provided the 

following additional points.

Since models like CAMx have been used by many state agencies and the EPA to provide 

air quality assessments, states should not be required to go through the rigor specified in 

Appendix W, section 3.2.2(e). The EPA's guidance for attainment demonstrations (EPA454/B-07-002) allows states to determine which model is appropriate with input from the 

EPA. The EPA should not require states to follow the onerous alternative model 

justification procedure in section 3.2 of Appendix W of 40 CFR part 51 to use CAMx as 

well as AERMOD for this rule as that would include information well known to EPA and 

amounts to unnecessary work, wasted resources, and lost time. Strictly adhering to the 

requirements in Appendix W. section 3.2.2 would not give the TCEQ sufficient time to 

propose an alternative model to AERMOD. If the EPA were to reject demonstration 

approaches using CAMx, it is probable that the TCEQ could not meet the EPA's 

proposed implementation timeline.

Commenter (0070) recommended that EPA approve the use of alternative models for the 

designations modeling in those cases where the existing emission limits for sources were 

developed using an alternative model or formulation on a blanket basis, based on the existing 

record demonstrating the appropriateness of the alternative model or formulation. Commenter 

(0070) provided the following additional points. 

There are many situations where complex terrain and wind flows exist or mixed types of

sources that are not amenable to modeling in AERMOD co-exist. In a number of cases in 

the past, alternative models have been approved for these applications. In those cases 

where alternative models or alternative model formulations have been used for the 

existing SIP, these alternatives should be allowed for use in the designations modeling 

called for in this rulemaking. This would speed the process in these areas and avoid the 

time consuming need to re perform the model validation work that already exists. (0070)

Specific modeling tools

Commenter (0056) stated that modeling tools such as the AERMOD Low Wind Beta options, 

EPRI's Sub-Hourly AERMOD Processor (SHARP) and Emissions Variability Processor (EMV 

AP) should be considered as potential modeling options available to state and local agencies.

Commenter (0092) stated EPA should correct the DRR’s blanket assertion regarding 

AERMOD’s capabilities and accuracy and make clear in the final rule that AERMOD is not 

universally appropriate and that in certain cases, such as aluminum smelters, the recommended 

model may be a model other than AERMOD, such as the Buoyant Line and Point Source 
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Dispersion Model (BLP). Commenter (0092) stated this correction is necessary to ensure that air 

agencies, Regional Administrators, and the regulated community clearly understand that 

guideline models, other than AERMOD, have been and are recommended for use in the 1-hr SO2

NAAQS designation process.

Commenter (0052) stated that guidance is necessary on how to merge Buoyant Line and Point 

(BLP) modeling with AERMOD modeling or whether this would be a recommended method for 

modeling certain source categories.

Commenter (0106) stated that, depending on the spatial configuration of sources above the (yet 

to be revealed) emission threshold, it is not obvious that AERMOD is the best choice for 

designation modeling. Commenter (0106) described a scenario where two similarly large, abovethreshold sources are located 40 kilometers apart and application of AERMOD may not 

accurately resolve interactions between these two relatively distant sources, in which case 

commenter (0106) suggested the CALPUFF model may be more appropriate. Commenter (0106) 

also described a case in North Dakota where much double-counting would inevitably result from 

the application of multiple AERMOD runs. Commenter (0106) suggested that, rather than 

subjective and conservative manipulation of background concentrations with multiple AERMOD 

runs, a single CALPUFF application would more accurately account for multi-source 

interactions, variable meteorology, and variable land use which would be present in the North 

Dakota domain. Commenter (0106) noted that Appendix W already provides for the use of 

CALPUFF on the local scale.

Commenter (0114) asserted that accuracy is essential when demonstrating compliance with a 1-

hour standard and that while less conservative models have been presented to the EPA; the 

agency has not formally approved the use of any of these alternatives. Commenter (0114) is 

concerned that the EPA is setting deadlines for sources to comply by conducting air dispersion 

modeling without the approval of more realistic compliance tools.

Commenter (0090) stated that more complex models, like 4-D puff, may be better suited for 

estimating concentrations from single and multiple sources. Commenter (0090) asserted that 

direction comparison of short-term (1-hour) observations is highly questionable. Commenter 

(0090) added that large stochastic fluctuations affect short-term concentration values prohibiting 

meaningful comparison between modeled ensemble averages with short-term concentration 

values (John Irwin, AWMA 2013).

State/Source discretion to choose models

Commenters (0050, 0051, 0054, 0056, 0063, 0065, and 0109) stated the EPA should provide 

states the flexibility to use models they determine are appropriate. Commenter (0051) stated the 

EPA should provide states the flexibility to use tools that are better suited to accounting for the 

chemical reactions of SO2 in the atmosphere, such as CAMx. Commenter (0054) stated

flexibility is needed because AERMOD is not an accurate predictor of emissions in scenarios 

such as those found at primary aluminum smelters. Commenter (0056) stated flexibility is 
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needed because AERMOD is very questionable, especially in complex terrain and in urban areas.

Commenter (0109) stated the EPA should allow individual states to use model refinements on a 

case by case basis as the states deem appropriate including, but not limited to, the Heat Island 

Option, low wind speed improvements, building downwash adjustments, and buoyant plume 

adjustments for volume and area sources.

Commenter (0093) stated there should be flexibility provided to the sources with respect to 

models used, characterization of emissions, etc. Commenter (0093) suggested the emphasis on 

the Technical Assistance Documents as guidance documents for affected sources should not limit 

the modeling analyses but rather should provide options for sources.

Response: 

In this final rule, the EPA is not promulgating a requirement that air agencies use AERMOD in 

all cases, but is retaining the existing flexibility otherwise provided by the EPA’s rules for 

agencies to support the use of the best model for a particular case. The EPA’s latest non-binding 

recommendations for making this assessment are contained in the modeling TAD. In most cases, 

the EPA believes that AERMOD will likely be the model of choice by air agencies to address the 

requirements of this rule, unless the application involves a different recommended model, such 

as the Buoyant Line and Point Source Dispersion Model (BLP). Models recommended for 

particular applications are listed in appendix A of appendix W, section 3.2 of appendix W 

outlines the procedures for use of alternative models for those cases where an alternative model 

may be more appropriate than a preferred model. In addition, the modeling TAD also discusses 

past use of alternative models for particular applications.10 The EPA recommends consultation 

with the appropriate reviewing authority or EPA Regional Office to determine if the use of an 

alternative model is valid for that application.

In addition, as stated previously, the TADs are documents that provide recommendations but are 

not binding or enforceable and create no obligations on any person. Although the draft TADs are 

referenced as recommended approaches in the preamble to the proposal and in this rulemaking, 

they are not required to be adhered to by any state that is required to characterize air quality 

around an SO2 source identified in this rulemaking. The TADs have been provided in order to 

potentially aid air agencies seeking advice in the air quality characterization process required by 

this rulemaking. 

11. Multi-source modeling analyses

 

10 It is the EPA’s intention to update the modeling and monitoring TADs as necessary to reflect 

any change in policy or to make clarifications that are necessary. Therefore, any comments on 

the TADs themselves that have been submitted in response to the proposed rule will be 

addressed as a part of any updates made to the TADs in the future, rather than in this final rule.
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Comment:

Commenters (0036, 0051, 0052, and 0114) requested that EPA provide more specific guidance 

on conducting multi-source modeling analyses. Commenter (0051) stated that leaving these 

topics for negotiation with the Regional Office will lead to inconsistent application of guidance 

among states. Commenter (0052) stated that guidance on modeling facilities across state lines 

should be addressed. Commenter (0114) stated that the language in the proposal seems vague 

and stated that, if there are potential areas of concern that would require sources below the 

threshold to model, these should be identified early in the process to give facilities the same 

timeframe to implement changes or conduct air dispersion modeling to comply with the rule.

Commenter (0036) provided the following additional points:

We appreciate the flexibility that EPA is trying to provide to states, but EPA must also 

ensure that the SO2 implementation program is designed to identify areas where several 

sources contribute to an SO2 problem and to require those sources to be analyzed for their 

combined impacts. We request that EPA set specific criteria, such as proximity and total 

emissions per year, to establish a threshold for analyzing combined area emissions. We 

would like guidance on when a source should be modeled by itself and when a source 

should be modeled with other sources in the surrounding area, more detail on the size and 

location of sources that should be included in a multi-source analysis, and who would be 

responsible for conducting this analysis when sources are located in a multi-state area.

Commenter (0051) stated the EPA should not require modeling of sources of emissions that are 

less than the thresholds adopted by rule. Commenter (0046) stated that, where there are multiple 

sources in an area, the rule should limit the number of sources to include in a modeling 

demonstration. Commenter (0046) stated the inclusion of offsite sources should be determined 

on a case-by-case basis. Commenter (0046) provided the following additional points:

Because this is a 1-hour standard, downwind transport distances are constrained and

overlapping, additive impacts with other sources are limited by meteorological conditions 

of hourly wind direction and speed. As mentioned in the “Additional Clarification 

Regarding Application of Appendix W Modeling Guidance for the 1-hour NO2 National 

Ambient Air Quality Standard” memo from EPA, the inclusion of offsite sources should 

be determined on a case-by-case basis. In some cases this might mean using only ambient 

monitor background data instead of modeling offsite sources. In other cases the modeled 

inventory should be limited to only those sources, which could cause a significant 

concentration gradient in and around likely ambient monitor locations.

Commenter (0106) referred to page 7 of the draft modeling TAD and stated that it is not clear 

why the number of sources explicitly modeled should generally be small. Commenter (0106) 

suggested the TAD should simply indicate a distance and emission threshold for inclusion of 

nearby sources, establish an average actual emission rate for such sources, and explicitly include 

all of them in the AERMOD analysis no matter what their number. 
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Commenter (0106) stated the suggested subjective approach to use contour plots, meteorological 

data, wind roses, and AERSCREEN to limit nearby sources for explicit modeling seems 

unnecessarily cumbersome, time-consuming, of questionable accuracy, and seems to suggest that 

AERMOD is not a robust model for multi-source analysis. Commenter (0051) did not agree with 

the procedure in the TAD in which the EPA describes a complicated, resource-intensive, multistep process to model sources below the emissions thresholds that could cause or contribute to a

NAAQS violation. 

Commenters (0042, 0110) supported an express statement in the March 1, 2011 EPA guidance 

that, by default, nearby sources for cumulative modeling should be limited to 10 km because 

sources beyond that distance are expected to have an insignificant concentration gradient near 

the source and should be considered to be included in the regional background. Commenter 

(0042, attachment A5) provided the following additional points:

Although EPA’s Guideline On Air Quality Models (Appendix W, 40 CFR part 51) states 

that the applicability limit is 50 km, this limit is not always technically appropriate to 

assess the cumulative 1‐ hour SO2 NAAQS because it inherently assumes that a plume 

could reach this distance within 1 hour. For this to happen, the wind speed at plume 

height would need to be about 13.9 m/sec, or about 31 miles per hour, which is fairly 

infrequent at most locations in the United States. Requiring modeling for all sources 

within 50 km necessarily requires the model to consider receptors well in excess of 50 

km for some sources in the domain. 

This also inevitably results in double‐counting sources that are already considered in the 

monitoring data used to estimate background concentrations. The limit on how 

background sources that are distant from the source of interest should be included in the 

cumulative modeling addresses two key concerns: the concentration gradient issue and 

the practical limitation of plume transport distances. In cases of very light winds where 

even 10 km is associated with multiple‐hour transport, the distance for considering 

background sources should be reduced even further. (0042, attachment 5)

Commenters (0051, 0093) stated that background concentrations will best approximate the 

impact of other sources in the area. Commenter (0051) stated, alternatively, a grid model, like

CAMx, could be used to estimate background concentrations and provide concentration

gradients. Commenter (0093) stated the inventory of sources used in any modeling analyses 

should be limited to the immediate local area and be developed at the discretion of the state.

Response: 

The determination of whether to include nearby sources in a modeling exercise around a source 

that exceeds the emissions threshold is case specific, and a standardized methodology cannot be 

developed to fit all scenarios. Therefore, the final rule does not promulgate requirements 

addressing nearby sources. The EPA has offered non-binding technical recommendations in the 

modeling TAD, which we believe provides sufficient guidance to enable air agencies, sources, 
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and EPA Region’s to reach reasonable decisions on how to address nearby sources such that no 

additional specificity is required in this final rule. The identification of nearby sources for 

modeling should rely on sound technical reasoning and best professional judgment. The EPA 

emphasizes that not all emissions sources near the source of interest need to be explicitly 

modeled, as in some cases the impacts of those sources can be sufficiently represented by a 

background monitor as discussed in the modeling TAD and section 8.2 of appendix W.

Nevertheless, to be accurate, the analysis would need to consider the impacts of all sources that 

are influencing concentrations in the area; an analysis that failed to consider the impact of 

sources emitting less than 2,000 tpy or failed to consider the impact of sources beyond some 

arbitrary number of modeled sources would likely fail to characterize air quality in the area.

In response to Commenter 0051’s concern about national inconsistencies, review of the 

modeling protocols by the Regional Office, often in consultation with EPA headquarters when 

questions arise, will lead to national consistency. Regional offices and EPA headquarters are in 

constant contact with each other and national consistency arises from that process.

As stated previously, the TADs provide recommendations but are not binding or enforceable and 

create no obligations on any person. Although the draft TADs are referenced as recommended 

approaches in the preamble to the proposal and in this rulemaking, they are not required to be 

adhered to by any air agency that is required to characterize air quality around an SO2 source 

identified in this rulemaking. The TADs have been provided in order to potentially aid air 

agencies seeking advice in the air quality characterization process required by this rulemaking. 

The Agency has indicated that the TADs are meant to be used as possible tools to aid air 

agencies. The EPA is not codifying changes to the TADs in this rulemaking in response to any 

comments received on the proposed rule. The TADs are living documents which the EPA may 

update as necessary.

12. Demonstrate ongoing data requirement --options

a. Model once using allowable emission or adopt limits with the actual 

emissions rates used in the modeling

Comment:

Commenters (0038, 0041, 0060, 0062, 0096, and 0115) agreed with EPA's proposal that 

verification should not be required for sources or areas that are modeled based on the maximum 

allowable emissions or adopted enforceable emission limits. Commenters (0041, 0062) 

recognized the value of modeling with allowables to determine if a source is attaining at its

maximum permitted or potential emission level, so that re-modeling in the future is not needed, 

saving resources.

Commenter (0048) suggested another option: (1) the EPA should allow the use of modeling in 

conjunction with monitored SO2 levels and actual emissions from the affected sources for the 

previous 3 years to evaluate model reliability in simulating the measured concentrations, (2) the 
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model output could then be used with adjustment for bias to determine appropriate allowable 

emission rates for the affected sources, and (3) emission limits set in this way would no longer 

require verification.

Commenter (0042, attachment C) did not support the proposal in the White Paper that the agency 

should set a maximum allowed emission limit at the level of the actual emissions used in such

modeling. Commenter (0042, attachment C) stated that many processes, such as those in the steel 

industry, are highly variable over short time frames (substantially less than one hour), and setting 

instantaneous maximum emission limits at the level of the actual emissions used for modeling 

does not adequately allow for this variability.

Commenter (0060) stated that a feature of the proposal which is too narrow relates to the 

proposal that an air agency "always has the option to adopt for the source(s) in the area federally

enforceable emission limits at levels that are consistent with the actual emissions used in the 

modeling" thereby making additional assessment unnecessary. Commenter (0060) stated this 

should include sources with federally enforceable limits above actual emissions for which the air 

agency modeled attainment.

Response:

Responses to these comments are provided in section IV.D.g of the preamble to the final rule. In 

addition, the recommendation to set emission limits based on “adjustment for bias” is contrary to 

EPA recommendations. The EPA recognizes that emissions of many sources are variable, and 

EPA’s nonattainment planning guidance discusses how this variability can be addressed in a 

nonattainment planning context, but the limits need to be set in a manner that provides 

appropriate assurance that the limits will provide for attainment, in order to reliably eliminate the 

need for continued verification of attainment. The EPA agrees with the commenter that a limit 

that is slightly higher than actual emissions but that nevertheless can be shown via modeling at 

the allowable level to provide for attainment will suffice to avoid the need for ongoing 

assessment.

b. Proposed option 1: model using actual emissions, then assess 

emissions annually and conduct new modeling every 3 years

Support for Option 1

Comment:

Commenters (0071, 0115) supported Option 1. Commenter (0071) stated that ongoing 

verification of attainment of the NAAQS should be performed at regular intervals using the most 

recent SO2 emissions of the large sources, most recent SO2 emissions of surrounding sources, 

current meteorological data, and the most current modeling program. Commenter (0071) stated

that, simply assessing SO2 emissions changes at large sources does not account for variations in 
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meteorological conditions, increased SO2 emissions from interactive sources, or improvements to 

the actual modeling computer program, and does not necessarily reduce the amount of resources 

required to perform such an analysis.

Response:

The EPA appreciates those commenters supporting the modeling of actual emissions every three

years and recognizes the concerns about Options 1 and 2 regarding criteria and consistency and 

review of triannual modeling. The EPA also recognizes and agrees with the comment regarding 

effects of changes in meteorological conditions, changes to the modeling systems, and changes 

to emissions from others sources cannot be determined by simply reviewing emissions changes 

of the sources. The EPA has finalized a combination of Options 1 and 2 in which the details, 

along with some general recommendations can be found in section IV.D.6.g of the preamble to 

the final rule.

Oppose Option 1

Comment:

Commenters (0038, 0039, 0046, 0058, 0060, 0069, 0088, 0096, 0108, and 0117) opposed Option 

1 and stated that modeling assessments should not be conducted on a three-year or any other 

regular basis if such is not supported by a review of the annual emissions data as that approach is 

not a good use of limited state and EPA resources. Commenter (0081) stated Option 1 would 

require the air agency to submit modeling under an arbitrary and artificial schedule without 

taking into account information which might show that undergoing a revised modeling analysis 

is unnecessary. Commenter (0060) provided the following additional points:

Under the 2008 lead standard there is no requirement to remodel every three years to 

determine if a facility can still be exempt from monitoring. Ohio EPA and EPA Region 5 

work cooperatively together to review annual emissions and determine if changes to the 

lead monitoring or modeling is needed. No other standard requires such a detailed ongoing analysis. If emissions haven't increased, or increased enough, to determine 

modeling is necessary on an annual basis, no new modeling should be needed just 

because meteorology may have changed. If a state believes they have such volatile 

changes in meteorology that it may affect prior modeling results in the absence of 

emissions changes, the state should be able to make its own judgment call (along with 

EPA consultation) as to whether modeling is necessitated. It should not be mandated by 

rule.

Commenter (0039) stated that this would be a highly ineffective use of state agency and 

regulated community resources as changes in emission levels from year to year will have little 

impact on predicted and actual attainment with the standard. Commenter (0039) asserted it is 

critical to leave state agencies with the flexibility to make the determination whether further 

modeling is required. Alternatively, commenter (0039) recommended requiring states to review 
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annually emissions from modeled sources with an eye toward identifying substantial emission 

increases above modeled rates, without any automatic emission increase trigger or mandatory 

reevaluation. Commenter (0117) stated that, as long as conditions have remained the same or 

possibly improved in the intervening timeframe; additional modeling will provide no additional 

useful information and requested that EPA consider an exemption from the three-year cycle for 

remodeling where previous analysis has already shown it to be unwarranted.

Commenter (0103) recommended that if either Option 1 or 2 are retained, then specific criteria 

that the EPA will use in its case-by-case reviews should be established in the rule to ensure 

consistency and timeliness of reviews across EPA regions or that the EPA review of the state 

recommendations be dropped from the options altogether. Commenter (0103) was concerned

about the ability of EPA regional staff to review the state submissions in a timely manner with 

existing staffing given the number of potential submissions that could be made under either 

Option 1 or 2. For example, commenter (0103) stated there is a potential of 44 submittals in 

Region 7 that would require EPA review and action annually.

Response:

The EPA appreciates the concerns from the commenters regarding resources and has developed a 

combined approach using Options 1 and 2 which are detailed in section IV.D.6.g of the preamble 

to the final rule. Rules governing implementation of the lead standard address different 

circumstances; different approaches are warranted in implementing the SO2 standard. The EPA 

has determined that the best and most efficient means of obtaining prompt assessment of 

potential for violations in areas addressed by modeling, while minimizing resource usage where 

violations are unlikely, is to require an annual review of emission data for purposes of judging 

whether a full remodeling effort is warranted. The EPA is promulgating an approach in which 

states have flexibility to determine the need for further modeling, with EPA oversight.

c. Proposed option 2: if a source’s emissions increase over the 

previous year, the state would be required to submit to the EPA an

assessment of the cause of the increase and provide an initial 

determination of whether or not air quality modeling would be needed 

Support Option 2

Comment:

Commenters (0038, 0041, 0045, 0046, 0047, 0048, 0049, 0056, 0058, 0069, 0075, 0081, 0087, 

0088, 0096, 0097, 0106, 0107, 0108, 0110, 0114, 0117, and 0118) generally supported Option 2 

because it balances providing the most air quality protection with level of effort from state 

regulatory authorities. Commenter (0048) noted that with SO2 emissions declining on a national 

level, particularly for the sources affected by the DRR, remodeling would not be expected to be 

required. Commenters (0106, 0118) stated that a simple analysis of the change in emissions is 

sufficient to determine the need for additional modeling. 
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Response:

The EPA appreciates and generally agrees with comments for both options 1 and 2 regarding 

state resources. See final recommended approach in section IV.D.6.g of the preamble to the final 

rule. 

Option 2—trigger for further evaluation

Comment:

Commenters (0038, 0041, 0058, and 0114) generally stated that, under Option 2, states should 

have discretion to determine whether the increases in emissions warrant further action.

Commenters suggested factors to help determine when further action is needed.

 Commenter (0038) stated any modeling verifications should be driven by actual emission 

increases or permit applications that are contingent on changes within a source's control. 

 Commenters (0046, 0088) stated that, if modeling is used to determine the designation of 

an area, EPA should only require re-modeling if the emissions increase is on par with the 

difference between the standard and previously modeled impacts.

 Commenter (0097) stated that if there is no emissions increases above attainment year 

levels, the EPA should not require states to expend additional resources. Commenter 

(0097) stated that the NSR program is another backstop in the maintenance process, 

making redundant modeling unnecessary. 

 Commenter (0096) argued that no additional report should be required if emissions from 

the affected sources have either decreased, remained unchanged, or have not increased 

above the thresholds set in the initial baseline modeling.

 Commenters (0096, 0113) recommended establishing a threshold in the DRR, or included 

in the initial modeling analysis, providing clearer guidance regarding what emissions 

increases trigger further evaluation of sources, rather than providing an assessment for 

each source with increased emissions. Commenter (0113) suggested that thresholds could 

be structured as follows: 

o If original modeling is equal to or greater than 90% of the standard - area needs 

remodeling for all increases regardless of magnitude of emissions increase.

o If original modeling is between 50 and 75% of the standard- area needs 

remodeling if emissions increase is above 15% of the previous year.
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o If original modeling is less than 50% of the standard- suspend modeling activities 

(same as Option 1).

Commenter (0096) stated it is not clear what the analysis is required to include and this issue 

must be clarified in the final rule. Commenter (0096) provided the following suggestions. 

The analysis should establish whether there was an increase above the initial emissions 

modeled in the 2017 submittal. Assuming that emissions from 2012, 2013, and 2014 were 

used in the 2017 modeling submittal, the initial verification of ongoing compliance would 

have to be an analysis related to the emissions from the year subsequent to those years. 

The determination of whether there was an emission increase could be based on an 

increase greater than the three year average of emissions during the initial modeling 

analysis, an increase above the highest single year of emissions during the initial 

modeling analysis, or some other metric. (0096)

Response:

The EPA has implemented a combined Option 1 and 2 which invokes several recommendations 

listed above. Details are provided in section IV.D.6.g of the preamble to the final rule. The EPA 

anticipates that air agencies will consider the types of factors these commenters recommend 

considering as they judge whether further modeling is warranted.

Do not support Option 2

Comment:

Commenters (0060, 0071, and 0115) objected to the second option, for the same reasons as 

mentioned above by these commenters for Option 1. Commenter (0115) stated that the proposed 

emissions assessments required in Option #2 are not stringent enough and the document does not

adequately address the details that will be involved in performing these assessments. 

Response:

The EPA appreciates the comments regarding resources and stringency of option 2. The EPA has 

developed a combined approach using Option 1 and 2 with recommended criteria from state 

commenters. Combining these options provides an appropriate balance that provides an annual 

review but focuses ongoing assessment efforts on those areas where emission increases or other 

relevant changes in circumstances warrant the additional efforts. The EPA requires identifying 

causes of changes in emission levels because this information helps characterize the nature of the 

change and whether the change is likely to be temporary (e.g., reflecting normal variations in 

source usage) or permanent (e.g., reflecting changes in source operation levels, control 

equipment, or other source changes). The EPA has determined that the best and most efficient 

means of obtaining prompt assessment of potential for violations in areas addressed by modeling, 

while minimizing resource usage where violations are unlikely, is to require an annual review of 

emission data for purposes of judging whether a full remodeling effort is warranted. Details are 
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provided in section IV.D.6.g of the preamble to the final rule. The EPA believes that the details 

of these assessments must be determined on a case-by-case basis.

d. Concerns related to choosing either option 1 or 2

Comment:

Commenter (0103) recommended that if either Option 1 or 2 are retained, then specific criteria 

that the EPA will use in its case-by-case reviews should be established in the rule to ensure 

consistency and timeliness of reviews across EPA regions or that the EPA review of the state 

recommendations be dropped from the options altogether. Commenter (0103) was concerned

about the ability of EPA regional staff to review the state submissions in a timely manner with 

existing staffing given the number of potential submissions that could be made under either 

Option 1 or 2. For example, commenter (0103) stated there is a potential of 44 submittals in 

Region 7 that would require EPA review and action annually.

Response:

The EPA appreciates the concerns regarding consistency and timeliness of EPA review of state 

assessments of annual SO2 emission changes and subsequent recommendations. After review of 

all comments regarding all three options, the EPA has finalized an approach that combines 

Options 1 and 2. Details about the combined approach, including general criteria for 

determining if additional modeling is needed are found in section IV.D.6.g of the rule preamble. 

Following these general criteria should help in the development of national consistency among 

the regions. Given the range of circumstances requiring judgment involving complex arrays of 

relevant factors, the EPA has concluded that establishment of specific criteria for these 

judgments would be impracticable, and a better approach is to request air agency 

recommendations on the need for further modeling with EPA review. The EPA routinely 

engages in internal consultations involving all regional and headquarters offices, and the EPA 

considers this approach the best means of assuring national consistency in these circumstances. 

In order to facilitate timely review of recommendations, states should strive to develop concise 

recommendations and should communicate with appropriate regional staff during the 

development of the recommendations. 

e. Proposed option 3: model using actual emissions, then conduct 

screening modeling every three years

Support Option 3

Comment:

Commenter (0057) recommended that EPA adopt Option 3, stating Option 1 imposes

disproportionately greater demands on local, state, and federal staff than do the other options, 
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while Option 1 has comparatively small increases in the percent of total SO2 emissions outside of 

areas already designated nonattainment. 

Response:

While EPA appreciate commenters that agree with Option 3, EPA has developed a final 

recommendation using Options 1 and 2, with details listed in section IV.D.6.g of the preamble to 

the final rule.

Oppose Option 3

Comment:

Commenters (0038, 0039, 0069, 0071, 0081, 0113, and 0117) disagreed with having any default 

modeling requirement. Commenter (0071) stated that facilities with complex building 

configurations and/or multiple stacks should not be eligible for a screening analysis as presented 

in Option 3 because the screening tools were not designed to evaluate complex situations.

Commenter (0103) stated Option 3 does not seem practical as most of the sources (facilities) will 

have multiple emission points and AERSCREEN cannot currently handle sources with multiple 

emission points. Commenter (0117) stated that, as long as conditions have remained the same or 

possibly improved in the intervening timeframe, additional modeling will provide no additional 

useful information.

Commenter (0039) disagreed with EPA’s proposed Option 3 for modeling verification, claiming 

the mandatory use of screening models will result in an overly cautious, ineffective approach to 

verification. Commenter (0039) stated that while screening models are less labor intensive for 

state agencies to employ, the results are almost always overly conservative and are likely to 

predict concentrations that trigger the need for full-fledged modeling that will ultimately show 

there is no need for concern—while removing state agency discretion and creating greater 

uncertainty for states and the regulated community. Commenter (0069) stated that EPA fails to 

take into account that screening modeling is almost as complex and time consuming as full-scale

modeling and is not a good use of limited state and EPA resources.

Commenter (0060) stated that any option, if EPA still believes this should be mandated and

prescribed by rule, should allow the use of screening modeling in lieu of full scale modeling. 

Commenter (0103) stated this option could be modified to allow AERSCREEN to be used to 

model the net change in impacts from a change at an emissions point; if the net change is 

predicted by AERSCREEN to exceed the interim 1-hour SO2 significant impact level then a 

complete, full-scale modeling analysis with updated emissions and meteorological data could be 

performed with AERMOD.

Commenter (0068) disagreed with the proposed Option 3 and stated that annual modeling makes 

far more sense from the perspective of protecting the public health. Commenter (0068) stated 

that modeling once every three years is an extremely periodic and slow way of assessing air 

quality, such that people living in the impacted area could be unaware for years, and thus unable 
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to take action to protect themselves, or place pressure on their government to correct the 

problem. Commenter (0068) stated such periodic modeling would only serve to conceal potential 

air quality problems, and to protect polluters. 

Commenter (0115) stated that the proposed emissions assessments required in Option 3 are not 

stringent enough because assessments would only take place every three years. 

Response:

The EPA appreciates those comments disagreeing with Option 3 and recognizes the resource 

considerations of screening modeling and potential refined modeling associated with Option 3. 

The EPA disagrees with the comment suggesting annual modeling. In response to all three 

options and in consideration of state resources EPA has developed a final option using a 

combined option 1 and 2 approach. Details are listed in section IV.D.6.g of the preamble to the 

final rule.

f. Option for ambient air quality monitoring

Comment

Commenters (0063, 0065, and 0082) suggested that a state could use an ambient air quality 

monitor to provide ongoing verification for an area that was initially designated on the basis of 

modeling. Commenter (0082) stated these initial designations may be based on modeling due to 

timing issues, but a state or source may desire to install and operate a monitor (in accordance 

with EPA’s final guidelines for such monitors) as a basis for ongoing verification of an area’s 

attainment status.

Response:

The EPA notes that the ongoing data requirement provisions for states that initially characterized 

an area with modeling require that the state provide an annual emissions report, identification of 

the cause of any increases at the applicable sources, and a recommendation regarding whether 

additional modeling is needed to characterize the area in light of the increase. The EPA then uses 

that information and considers the state recommendation in order to determine whether to require 

additional modeling. The EPA expects that if there are situations where a monitor provides 

information relevant to characterizing air quality in such an area, such monitoring could be used 

to inform the state recommendation as to whether additional modeling is needed. Due to limited 

resources available for modeling, the EPA is very unlikely to require additional modeling where 

an existing monitor adequately characterizes the area.

g. Option for annual modeling
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Comment:

Commenter (0068) stated a requirement for annual modeling would provide a stronger incentive 

to states to set emission limits for SO2-emitting facilities at levels that ensure attainment of the 

NAAQS would be maintained, which would be better for states, EPA, and the public, as all 

stakeholders would have greater confidence in the quality of their air. Commenter (0068) stated 

annual modeling would not be unduly burdensome on the states because modeling is rapid and 

relatively cheap and, for sources that have already been modeled once, periodic remodeling will 

generally be quite straightforward, requiring little in the way of resources or personnel time. 

Commenter (0068) stated that since state air agencies are empowered under the CAA to collect 

permitting fees sufficient to cover the costs of their programs, it should be the emitting sources 

themselves, not the taxpayer, who is covering the cost of assessing the impact on air quality from 

sources emitting SO2. 

Response:

For reasons discussed in section IV.D.6.g of the preamble, the EPA has concluded that the most 

appropriate use of resources for ongoing assessment of air quality for areas assessed through 

modeling is to conduct an annual review of whether further modeling is warranted. 

h. Allow multiple options

Comment:

Commenter (0069) recommended, in lieu of EPA's proposals, a combination of Options 2 and 3, 

in which states could review/evaluate annual SO2 emissions and conduct screening modeling if 

the analysis of annual emissions warrant. Commenter (0069) recommended that, only if the

screening model fails to confirm continued attainment of the NAAQS would full-scale modeling

be required. Commenter (0069) recommended states should also be given the option to skip the 

screening modeling step and perform full-scale modeling.

Commenter (0082) urged EPA to finalize all three options and allow each state to choose which 

ongoing attainment verification option it will employ to accommodate the state’s limited 

resources. Commenter (0082) stated, for example, while one air agency may have insufficient 

funding to undertake modeling every three years as required by Option 1, another may have 

insufficient personnel to undertake the subjective reviews required by Option 2. 

In addition to EPA’s three options, commenter (0082) urged EPA to allow another option based 

on ambient air quality monitoring (described above). Commenter (0082) suggested EPA allow 

each state to choose among the four options which ongoing attainment verification option it will 

employ. 

Response:

The EPA appreciates the comments suggesting combining Options 2 and 3 in which states could 
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review/evaluate annual emissions and conduct screening modeling if needed. In response to the 

comment regarding the use of ambient air monitoring for verification, see the response above for 

“Option for ambient air quality monitoring.” In response to all of the comments, the EPA has 

finalized an approach combining Options 1 and 2 and details are provided in section IV.D.6.g of 

the preamble to the final rule.

i. Clarify verification options

Comment:

Commenter (0036), stated that the EPA should clearly spell out the verification options available 

to states and the EPA should appropriately bound these requirements in order to avoid openended requirements with significant cost implications. 

Commenter (0096) recommended the phrase “most recent emissions data” as applied in this 

option should be specifically defined to mean the most recent annual submission to the national 

emission inventory (NEI) as required by the AERR. Commenter (0096) stated the Proposed Rule

is unclear exactly what type of data fall within this phrase. Commenter (0096) stated the AERR 

submissions to the NEI will have undergone quality control (QC) procedures which make it more 

dependable than other data potentially considered “most recent emissions data” (i.e., data where 

a shorter timeframe may not allow as thorough QC). Commenter (0096) state agencies already 

must meet the December 31st deadline for NEI, thus it would be a reasonable deadline to meet 

without any added burdens. 

Response:

Responses to these comments are provided in section IV.D.4 and IV.D.6.g of the preamble to the 

final rule. More recent data may be available for some sources than for other sources—for 

example; EGUs are prone to have more recent data than other source types. Therefore, to define 

“most recent emissions data” as the most recent submission to the NEI might prevent air 

agencies from using more recent (or otherwise more appropriate) data from other data bases. 

Therefore, the EPA is retaining the proposed rule’s flexibility for air agencies and the EPA to 

make judgments regarding which data provide the most suitable basis for ongoing assessments of 

air quality.

j. Do not support ongoing data requirements

Comment:

Several commenters (0039, 0047, 0048, 0051, 0052, 0053, 0060, 0063, 0064, 0065, 0075, 0087, 

0088, 0089, 0093, and 0097) did not support the proposed ongoing data requirements because 

they are too burdensome. 
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Commenters (0048, 0051, 0052, 0053, 0075, 0087, 0089, 0093, and 0097) stated that the need 

for such on-going verification of attainment has not been demonstrated. Commenter (0053) saw 

no need for mandatory recurring modeling burden upon the states because sources are already 

required to report changes in emissions after initial modeling and this data could be easily 

assessed by EPA or applicable states to determine if re-characterization is potentially necessary. 

Commenters (0051, 0052, 0093, and 0097) stated that the NSR and/or PSD programs provide 

continued adequate assurance of maintenance of the standard. Commenter (0051) stated the 

usual SIP maintenance schedule should be sufficient for ongoing verification of attainment. 

Commenter (0060) did not agree that annual reports on emissions must be submitted to EPA for 

sources that have monitors shut down or for all sources that were modeled. Commenter (0060) 

stated this is creating an additional regulatory process that is not necessary and provided the 

following additional points.

The Ohio EPA has monitored emissions for years as part of our maintenance plans for the 

various NAAQS without a need to submit a special report to EPA annually. Ohio EPA’s

annual review of lead emitting sources that is used to determine if monitoring 

requirements are triggered is conducted successfully every year without the need for a 

special submittal. If we deploy a monitor as a result of a change in emissions at a lead 

source or opt to model for a waiver request, it is included in our annual monitoring plan. 

And that is accomplished without a detailed, limiting regulation dictating all the criteria 

and every step of the process. It appears there is more bureaucracy and oversight being 

created than is necessary. (0060)

Commenter (0088) stated that the proposed language at section 51.1205(b) should be eliminated 

from the final rule. Commenter (0088) stated that it is absolutely unnecessary and the provision 

has the potential to be terrifically burdensome. Commenter (0088) stated that if EPA acquires 

data indicating probable violations, it has sufficient regulatory authority to revise its designation 

or “to call” the state’s implementation plan. Commenter (0088) stated that this additional 

modeling undercuts the Proposed Rule and, since the purpose of the PSD program is to ensure 

against deterioration of an attainment area’s air quality, additional modeling by the states is 

unnecessary.

Commenters (0053, 0087, and 0097) stated inventory efforts and additional modeling exercises 

are unnecessary because emissions of SO2 are decreasing rapidly and will continue to do so. 

Commenter (0097) stated emissions data from large electrical generating units as reported to 

CAMD show a downward trend; heavy duty diesel engines emit significantly less SO2 than in 

the past due to reductions of sulfur in diesel fuel; the Tier 3 rule should continue to reduce SO2

emissions since the rule requires future reductions in gasoline sulfur content. 

If EPA concludes that such verification is warranted, commenters (0047, 0048, 0087, 0089) 

stated the burden on state/local air agencies should be minimized. Commenter (0048) stated that, 

at most, only an assessment of SO2 emissions to determine if they have increased and, if so, an 

assessment of whether that increase is sufficiently large to warrant the consideration of continued 
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monitoring or new modeling should be required. Commenter (0064) stated that if such a program 

is required in a final rule, verification should take place every five years instead of annually. 

Commenter (0064) noted that EPA reviews the NAAQS every five years.

Commenter (0039) stated the verification approach must provide state regulatory authorities with 

significant discretion about when to re-run modeling evaluations. Commenters (0063, 0065) 

stated that states should have the flexibility to choose appropriate ongoing attainment verification 

options without being subject to improper SIP planning requirements. Commenters (0063, 0065) 

asserted that sections 51.1205(b) through (d) effectively impose attainment planning 

requirements on attainment and unclassifiable areas.

Response: 

Responses to these comments are provided in section IV.D.g of the preamble to the final rule.

The EPA believes that the need for ongoing characterization requirements is clear. A monitor 

inherently provides ongoing assessment of air quality through continued operation of the air 

quality monitor. The EPA believes that ongoing air quality assessment is warranted for areas 

addressed by modeling, just as it is for areas addressed by monitoring, and a one-time modeling 

requirement based on recent actuals would not provide any ongoing information in the manner 

that a monitor does. As explained in section IV.D.g, the final rule provides for these needed 

ongoing assessments in a way that minimizes the burden and offers substantial flexibility to 

states (with EPA oversight), consistent with the recommendations of commenters. The EPA does 

not agree that this approach imposes attainment planning requirements on these areas, any more 

than requirements for ongoing monitoring impose attainment planning requirements.

k. Sunset provision for the ongoing data requirements

Comment:

Commenter (0070, 0097, and 0113) stated that the EPA should include within the 

implementation plan a reasonable framework for determining when the ongoing data 

requirements are no longer required. Commenter (0097) stated that when emissions levels in an 

area drop beneath 50% of the levels used within the model to show attainment, the EPA should 

then acknowledge that a PSD program provides ample protection against possible future SO2

NAAQS violations. Commenters (0070, 0113) recommended that, due to the conservatism in the 

air quality models and the tendency of models to significantly overstate air quality impacts from 

sources, the threshold for being allowed to omit future modeling should not be set at 50% of the 

standard. Commenter (0113) suggested that 85% of the NAAQS is a better threshold, 

particularly with the inclusion of an accurate and representative background concentration. 

Commenter (0070) recommended the threshold for being allowed to omit future modeling be set 

at no less than 90% of the standard, based on the currently available modeling platforms. 

Commenter (0070) provided the following additional points:
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On page 27466, the EPA proposed to allow a state to reduce its future modeling if the 

previous modeling demonstration was at or below 50% of the standard. The EPA’s own 

studies show that the current preferred model for near field modeling, AERMOD, is very 

conservative and overstates ambient concentrations significantly. As such, EPA’s 

proposed level of 50% of the standard based on modeling penalizes those states that 

would choose to use modeling in place of monitoring.

The EPA suggests that much of the work necessary to perform the follow-up modeling 

will be completed as part of the original modeling, further reducing the labor necessary to 

perform the follow-up modeling in the future. Such a claim could not be further removed 

from reality, especially with regard to AERMOD. Since 2005, EPA has periodically 

made changes to the AERMOD modeling system that have included error checks that do 

not allow older versions of major input files to be used. The need for this invalidation of 

existing input files is based on significant changes made in the modeling system and the 

incompatibility of the older files input files with these changes. The claim made on page 

27466 of the Federal Register notice completely ignores this fact and also fails to account 

for periodic changes in air quality modeling science that could result in EPA moving to a 

new modeling platform for near field analyses at some future date. When that occurs, it 

would be necessary for the states to completely revamp their modeling systems for this 

program at great cost in time and effort. We believe that this further supports our

recommendation that the modeled criteria be set no lower than 90% of the standard based 

on the modeling platforms available at the time of this rulemaking in 2014.

Response:

The EPA has finalized the approach for the sunset provision for monitors in areas where data 

show concentrations well below the NAAQS, which is outlined in section IV.D.6.g of the 

preamble to the final rule. With respect to modeling, EPA disagrees with the comments that 

AERMOD is very conservative. While some studies have shown over-prediction by AERMOD, 

others have shown under-prediction. The goal is to have AERMOD unbiased when using the 

proper model inputs. The EPA appreciates the concerns of commenters regarding changes to 

AERMOD causing incompatibility with older input files and possibly leading to increases in 

concentrations over previous versions of AERMOD. That being said, EPA does not make 

changes to AERMOD capriciously and the changes are to fix errors within the model code or 

implement changes that will lead to more accurate results and these changes are made after 

exhaustive testing. Furthermore, retyping input files is a small effort relative to the effort of 

determining the appropriate information (e.g., determining building dimensions), so that EPA 

continues to believe that remodeling a source requires substantially less effort than the initial 

source modeling, irrespective of any model changes.

In response to comments, the EPA has redesigned the modeling-based ongoing assessment 

process. In most cases, additional modeling is not necessary, provided that emissions do not 

increase significantly relative to the attainment margin found in the initial modeling (and 

provided that other relevant source characteristics do not change significantly). The EPA is not 
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promulgating a specific criterion for determining when modeling for ongoing assessment 

purposes is required, because the EPA has concluded that the need for such modeling is best 

judged on a case-by-case basis depending on whether and how much emissions have changed, 

depending on the nature of any other changes in source characteristics, and depending on the 

level of attainment margin found in the initial modeling. For this reason, the EPA is neither 

requiring further modeling in all cases where the initial modeling showed concentrations above 

85 or 90 percent, or any other percent, of the standard, nor exempting areas from further 

modeling where the initial modeling showed concentrations below any particular percentage of 

the standard.

The final rule does, however, provide that air agencies may terminate their ongoing modeling 

when modeled concentrations are shown to be below 50 percent of the standard. This criterion 

reflects a judgment that normal annual variations in emissions would not result in a source one 

year causing concentrations to be below that threshold and another year causing concentrations 

to be above the standard. Higher thresholds for discontinuation of annual review, such as 85 or 

90 percent of the standard, would be too prone to exempt areas from review where a reasonable 

possibility of violations may exist. Given that most areas that initially model at 50 to 80 percent 

of the standard, for example, will be found on review not to have the kind of emission increases 

to warrant further modeling, the EPA believes that use of the conservative threshold of 50 

percent for discontinuation of the review requirement will not significantly increase the resources 

necessary for the ongoing assessment process.. 

13. Post-attainment designation annual reporting

Comment:

Annual report

Commenters (0041, 0045, 0085, 0106, and 0107) stated that the assessment should be submitted

separately from the annual monitoring plan. Commenter (0085) stated these documents are not 

related, and accordingly, they should be kept separate. Commenter (0106) stated that, since the 

annual report refers to modeling, it will cause less confusion for the general public if it is a 

separate document from the Annual Monitoring Plan. Commenter (0107) stated that, because the 

annual monitoring plan and the emissions inventory submittals are performed by separate work 

units on different timelines, it would be better to deliver the products separately rather than delay 

one or the other to deliver them together.

Commenters (0048, 0102, 0115, and 0117) recommended this information be included as an 

appendix to the Ambient Air Monitoring Network Plan submitted to the EPA annually, rather 

than as a stand-alone document. Commenter (0117) stated that, since both options have a 

deadline of July 1 each year, a separate document would only add more time and resource use.

Commenters (0060, 0069) stated EPA should allow states to utilize either procedural approach

for ongoing verification of attainment for areas that have been designated
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unclassifiable/attainment based on modeling analyses (deleting the public notice and comment 

requirement in the second option), as this provides states with flexibility and provides the same 

information to the EPA.

Commenter (0049) stated annual emissions reporting is already required for sources meeting the 

threshold options listed in the proposed DRR, so there is no need for an additional emissions 

reporting plan. Commenter (0090) stated that requiring the state to report on annual SO2

emissions of each applicable source and provide an assessment of the cause of any emissions 

increase is not practical or appropriate. Commenter (0090) believed that this requirement is 

unduly burdensome on the state's limited resources and that the interest of the state and the EPA 

regarding NAAQS status is continued attainment, not the causes of increases in emissions that by 

the nature of unit operations will fluctuate to some degree each year.

Public notice

Commenter (0060) suggested that, if an annual report is necessary, any public notice would just 

need to clearly identify what the document available for comment covers. Commenter (0069) 

stated EPA has failed to identify any statutory or regulatory provision that requires public notice 

and comment in accordance with 40 CFR 58.10 for an annual assessment of emissions or a

modeling analysis; nor has EPA provided compelling reasons why such a requirement should be 

mandated through the DRR. 

Reporting frequency

Commenter (0060) suggested that if an annual report is necessary, EPA should require a 

submittal on July 1 each year.

Commenter (0096) stated that, ideally, under Option 2, agencies would have a 2.5 year 

timeframe to complete the entire process of ongoing verification. Commenter (0096) provided 

the following additional points:

The first year would consist of preparing and submitting the NEI for the previous year, a 

preexisting requirement. Six months thereafter agencies would submit a report to EPA 

stating whether air quality modeling is needed. Twelve more months would then be 

permitted to perform any additional modeling deemed necessary. Assuming EPA issues 

final designations in December 2017, ongoing verification would begin with the initial 

verification in July 1, 2018. The initial verification would be for emissions which 

occurred one year after the emissions used in the initial modeling submittal. If emissions 

exceeded the thresholds established by the initial baseline modeling, then agencies would 

assess whether further modeling is needed pursuant to the DRR in the initial verification 

report due by July 1, 2018 (which is subject to public review); and the agencies would 

complete any needed modeling by July 1, 2019. It is not feasible to operate on a more 

stringent timeline. ODEQ strongly and respectfully urges the EPA to give serious

consideration to Option 2, as described and recommended herein.
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Commenter (0097) suggested that these analyses be conducted no more frequently than every 

five years, in conjunction with the submittal of the 5 year monitoring network assessment as 

required by 40 CFR 58.10. Commenter (0097) provided the following additional points:

These analyses should be coordinated with the maintenance/compliance analyses 

required for areas having removed compliant monitors. A frequency greater than every 

five years would necessitate a significant effort in data gathering and assessment. For 

areas where the modeled value is under the standard, there is already a significantly 

conservative factor built into the process that would help ensure any minor increases in 

emissions would not result in an actual violation of the standard. Additionally, the EPA 

should not compare emissions from one year to the next. Rather it should allow the 

comparison of annual emission rates from the modeled values, similar to a maintenance 

demonstration for an ozone maintenance area. According to the 1992 redesignation 

request and maintenance plan guidance, "Procedures for Processing Requests to

Redesignate Areas to Attainment" (EPA, September 4, 1992), ozone maintenance areas

compare emissions to the attainment year levels to demonstrate ongoing compliance with 

a standard. This same methodology is also used in PM2.5 and PM10 maintenance plans. 

Using a similar approach for SO2 would alleviate the need to explain away increases from 

year to year that may have resulted from an economic downturn followed by an increase 

in economic activity the next. Again, this comparison should be required no more than 

every five years, and deadlines should hinge on the submittal of the 5 year monitoring 

assessment.

Response:

After considering the comments received related to both of the proposed options, the EPA finds

that the best approach for the final rule is to allow the affected air agencies the discretion to 

either include the required annual data requirements report for modeled areas either as an 

appendix to the state’s monitoring plan, or as a stand-alone document. The air agency will have 

the flexibility under the final rule to select the approach that best meets the agency’s workload,

schedule, and particular needs. The EPA finds that either of the procedural approaches will be 

sufficient to implement the ongoing data requirements. Regardless of which approach is chosen

by the air agency, the report must be submitted to the respective EPA Regional Office by July 1st

annually and made available for public review and comment. The first report is due on July 1st

of the year after the effective date of the area’s initial designation and additional reports are due 

July 1st of each subsequent year. The EPA requires identifying causes of changes in emission 

levels because this information helps characterize the nature of the change and whether the 

change is likely to be temporary (e.g., reflecting normal variations in source usage) or permanent 

(e.g., reflecting changes in source operation levels, control equipment, or other source changes). 

The EPA has determined that the best and most efficient means of obtaining prompt assessment 

of potential for violations in areas addressed by modeling, while minimizing resource usage 

where violations are unlikely, is to require an annual review of emission data for purposes of 

judging whether a full remodeling effort is warranted.
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14. Resources

Comment:

Commenters (0034, 0052a, 0103) stated that the EPA must provide assurance that additional 

funding to implement this rule will be provided. Commenter (0034) recommended that EPA 

provide assurances to those Tribes having sources near to them that emit at least 1,000 tpy of 

SO2 that they will receive the requisite resources for monitoring and modeling to characterize the 

air quality around such sources. Commenter (0034) stated this will go a long way in helping 

Tribes --- sovereign nations with inherent rights ensured by the U.S. Constitution, treaties and 

legal precedent --- address any health and environmental impacts to their communities caused by 

these sources.

Commenter (0046) asserted state and local permitting agencies do not have the resources to 

perform all of the modeling analyses that will be required by the rule; therefore, this will become 

an additional burden on the regulated community, especially if annual re-verification modeling is 

required. Commenters (0052) stated the proposed iterative process establishes significant

resource demands for air agencies and is unnecessary for EPA to meet its statutory obligations.

Commenters (0046, 0051) stated the modeling cost estimates noted by the EPA in the proposal 

are underestimates for anything other than the simplest modeling scenarios. Commenter (0051) 

stated the cost could easily become five times the stated estimate as the complexities of the

project accumulate. Commenter (0046) stated the cost of doing a thorough modeling analysis for 

purposes of this rule will be far greater than the $10,000 put forth in the preamble at 79 Fed Reg 

27451. Commenter (0046) stated that, for a complex facility where more accurate measurements 

data are needed to correctly characterize actual emissions and exhaust characteristics over 

different operating modes and accurately build a model, the cost of a modeling analysis to fulfill 

the requirements of this rule could exceed $100,000. Commenter (0103) estimated for the twelve 

affected facilities in Iowa, about $78,000 would be needed to model the facilities for attainment

recommendations according to the TAD.

Commenter (0103) stated DNR estimates that the cost of establishing the attainment status of an 

affected facility via monitoring is about 36 times more than to establish the attainment status of 

the facility via modeling. States will begin incurring significant costs to perform modeling to site 

monitors beginning in the 2015 federal fiscal year. Funding will also be needed after 

designations to develop control strategies, contingency measures, maintenance area plans, and 

provide for on-going monitoring of progress towards reaching attainment. This could result in 

numerous SO2 nonattainment areas to develop nonattainment SIPs on an 18 month schedule.

Response: 

The EPA recognizes the fact that there will be costs and resources required to satisfy the 

requirements of this rulemaking. As suggested by both state and industry stakeholders who 

attended the May-June 2012 stakeholder meetings, in the absence of increased grant funding it 
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may be necessary for air agencies to rebalance their existing grant funds for this purpose, or to 

consider alternative funding approaches such as working closely with affected sources to assist 

in funding either the modeling or monitoring required to meet the requirements of the rule. Early 

planning may be helpful to address these funding needs. 

Because the CAA assigns to states much of the responsibility for developing air quality 

characterization data, the EPA describes the requirements of this rule in a consistent manner: air 

agencies are the entities with principal responsibility to establish and operate monitors, and 

conduct modeling, and to provide air quality data to the EPA. However, the EPA recognizes that 

other parties (such as facility owners) also may perform significant portions of the work that this 

rule requires. The EPA would consider monitoring or modeling conducted by a third party to be 

an appropriate means for air agencies to obtain the data necessary to meet the requirements of 

this rule, provided that the state provides oversight to assure that (1) any monitoring is conducted 

in a manner that is equivalent to SLAMs and quality-assured in accordance with applicable 

requirements, and (2) any modeling analysis that the state submits, even if it was initially 

provided to the state by a third party, is done in a reasonable manner and follows the 

recommendations in the modeling TAD.

See also the responses to analogous comments in section about the resources needed to conduct 

monitoring. The EPA recognizes that all air quality planning, including air quality 

characterization, requires resources, but the EPA considers the air quality characterization efforts 

required by this rule to be an appropriate element of a balanced program for assuring that the 

purposes and requirements of the CAA for identifying and then addressing air quality problems 

are met.

15. Procedural issues

Comment:

Commenter (0042, attachment B) offered several suggestions regarding the process of improving 

models.

 The current EPA approach of evaluating improvements to air quality models using 

the Model Clearinghouse review of issues arising in specific situations is unworkable 

as they tend to have a very narrow focus and not to be universally applicable.

 There is no peer review of EPA modeling guidance and formulation. An independent 

peer review panel is needed to provide guidance and recommendations on modeling 

needs.

 The Modeling Conference does not satisfy this need of additional peer review of EPA 

modeling.

 The EPA has not identified modeling needs and research needed to advance air 
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quality modeling performance.

 The EPA should include discussion of industry-supported model development and 

model validation work on the Agency’s substantive agenda rather than relegating 

such to the public comment period. 

 The EPA has instructed state agencies on when and how they must use modeling for

attainment demonstrations for the 1-hour SO2 NAAQS. Although industry may be

affected by these demonstrations, it has not had any input to the current procedures.

Commenter (0042, attachment C) stated there are numerous issues with using models and 

modeling techniques to implement new NAAQS, and, recognizing that addressing all of these 

issues will take time, they recommend EPA identify modeling issues for short-term, mid-term, 

and long-term focus. Commenter (0042, attachment C) stated they would like EPA to develop 

and share with the public a plan detailing the Agency's anticipated dates for and methods to 

address these issues. Commenter (0042, attachment C) recommended several specific issues to 

resolve in the short-term, mid-term, and long-term. Commenter (0042, attachment C) stated that 

one long-term need is a performance re-evaluation of AERMOD, which has been modified since 

it was approved. 

Response:

The EPA disagrees with the comment regarding the Model Clearinghouse. Issues that are raised 

to the Clearinghouse are addressed through a Clearinghouse action, such as a memorandum 

explaining that the action from the Clearinghouse can be applied to similar applications in the 

future, unless noted in the memorandum. With regards to model updates, the EPA is working to 

correct technical issues with AERMOD and has, in the past, taken industry input under 

advisement.

VI. Statutory and Executive Order Reviews

A. Paperwork reduction Act

Comment:

Commenter (0051) agreed that $30,000 is a good estimate for a fairly simple single source, but 

this estimate could easily double as the complexities of multiple sources, temporal allocation 

down to hourly emissions, downwash analyses, clustering analyses, culpability testing, and 

negotiations with regional EPA staff are added to the modeling project.

Commenter (0103) estimated the cost of implementing the monitoring pathway is about 36 times 

more than the cost of implementing the modeling pathway. Commenter (0103) stated the draft 

rule suggests a much lower ratio of costs, with a much higher estimate for the cost of the 

modeling pathway. Commenter (0103) requested a detailed accounting of the two paths in the 
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final rulemaking, including the costs of locating the point of maximum impact for placing an 

ambient monitor.

Commenter (0111) stated that the EPA does not provide an appropriate cost-benefit analysis to 

justify the substantial increase in resources the states will have to expend in order to comply with 

the proposal. Commenter (0111) stated the proposed DRR requires the states to drastically 

increase resources devoted to the rule’s analyses and that these analyses will duplicate 

subsequent action in the PSD permitting program. Commenter (0111) stated they cannot justify, 

the expenditure of these resources as part of the initial designation process.

Response: 

The EPA is requiring air agencies to more extensively characterize ambient SO2 air quality 

concentrations, pursuant to sections 110(a)(2)(B), 110(a)(2)(K), 301(a) and 114 of the CAA. For 

purposes of analysis of the estimated paperwork burden, the EPA assumed that 43 states and 

tribes would take actions to characterize air quality through either air quality modeling or 

ambient monitoring in 412 areas around SO2 sources emitting 2,000 tpy of more across the 

country, and such states would submit the results of these analyses to the EPA. Under this rule, 

the air agency will have the ability to choose, on an area-by-area basis, the analytical approach to 

follow for characterizing air quality around each qualifying source. For this reason, there is no 

way of determining exactly how many areas may be characterized through ambient monitoring 

versus air quality modeling approaches. 

Potential ambient air monitoring costs are estimated based on the assumption that air quality for 

each of the 412 SO2 sources exceeding the 2,000 tpy threshold would be characterized through a 

single newly deployed air monitor. (Note, however, that there may be situations where more than 

one monitor may be appropriate or necessary to properly characterize peak 1-hour SO2

concentrations in certain areas, which would increase costs proportionally.) Estimates are 

provided for a 3-year period and include a calculation for equipment amortization over 7 years. 

For the period of 2016, 2017, and 2018 (monitoring related expenditures would begin in 2016), 

the total approximate average annual monitoring cost, including a calculation for equipment 

amortization is $8,662,110 (total capital, and labor and non-labor operation and maintenance) 

with a total burden of 102,869 hours. The annual labor costs associated with these hours is 

$7,080,572. Included in the $8,662,110 total are other annual costs of non-labor operations and 

maintenance of $706,827 and equipment and contract costs of $874,711. For reference purposes, 

an estimate for initial establishment of a single, new SO2 monitoring station is $92,614 (does not 

include equipment amortization). In addition to the costs that would be incurred by the state and 

local air agencies, there would be an estimated burden to the EPA related to salary cost and 

equipment cost, etc, of a total of 52,717 hours and $776,005. 

Potential air quality modeling costs are estimated based on the assumption that air quality for 

each of the 412 SO2 sources exceeding the 2,000 tpy threshold would be characterized through 

air quality modeling analyses. Based on market research, stakeholder feedback and assumptions 
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about the procedures to follow when conducting modeling for designations purposes,11 an 

estimate of modeling costs for a single modeling run centered on an identified source would be 

approximately $30,000. This cost takes into account analyses to determine the number of sources 

to explicitly model, determining the modeling domain, emissions preparation, meteorological 

data preparation as well as other information needed for modeling as discussed in the modeling 

TAD. While some commenters feel this estimate could double, the EPA feels that this estimate 

is fair and in order to keep costs low, states should consult often with the EPA Regional Office in 

developing the modeling protocols to minimize future costs. If air agencies choose to 

characterize air quality through modeling analyses around all 412 sources expected to be 

identified as exceeding the source threshold, then total national costs for modeling analyses 

would be estimated at $12,360,000. If these costs were incurred over the course of 3 years, then 

the approximate annual cost for each year over that period would be $4,120,000.

B. Executive Order 13175: Consultation and coordination with Indian

tribal governments

Comment:

Commenters (0034, 0120) disagreed with the Proposed Rule statement that the rule does “not 

have tribal implications, as specified in Executive Order 13175.” Commenters (0034, 0120) 

stated the Proposed Rule would redistribute monitoring and modeling resources supported by 

EPA and focus them on emission sources located in CBSAs in excess of 1 million people and on 

other sources whose emissions are extraordinarily large. Commenters (0034, 0120) stated this 

approach would further limit the much needed monitoring and modeling resources that are 

already scarce in Indian Country. As such, commenters (0034, 0120) stated that the EPA must 

develop an accountability process to ensure “meaningful and timely input by development of 

regulatory policies that have tribal implications.” 

Commenters (0034, 0120) appreciated the outreach that EPA has conducted with regards to the 

Proposed Rule such as holding several meetings with Tribal environmental professionals to 

discuss issues associated with the Proposed Rule and providing an opportunity for Tribes and 

stakeholders to provide written comments on the concepts discussed in the White Paper as 

mentioned in the Proposed Rule. Commenters (0034, 0120) stated that, while it is helpful that 

EPA intends to offer consultation with any Tribal government wanting to discuss the Proposed 

Rule, nevertheless, the EPA must do more to consult with Tribes about the Proposed Rule and 

others like it, and to do so much earlier, such as during the rule development process.

Response: 

This action does not have tribal implications, as specified in Executive Order 13175. It would not 

have a substantial direct effect on one or more Indian tribes. Furthermore, this regulation does

 

11 The Draft SO2 NAAQS Designations Modeling Technical Assistance Document can be found 

at: http://www.epa.gov/airquality/sulfurdioxide/pdfs/SO2ModelingTAD.pdf.
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not affect the relationship or distribution of power and responsibilities between the federal 

government and Indian tribes. The CAA and the Tribal Air Rule establish the relationship of the 

federal government and tribes in characterizing air quality and developing plans to attain the 

NAAQS, and this regulation does nothing to modify that relationship. 

In response to the comment that the rule will redistribute monitoring and modeling resources 

supported by the EPA and focus them on emission sources located in CBSAs in excess of 1 

million people and on other sources whose emissions are extraordinarily large, the EPA 

considered comments such as this in making its final decision on the threshold for characterizing 

sources. After reviewing the comments, the EPA decided not to move forward with the proposed 

approach and has decided instead to apply requirements for air quality characterization based on 

emissions levels uniformly across the country for both more urbanized and less urbanized 

populations so as to focus primarily on the size of the sources. In addition, there are no new 

resources associated this rulemaking, nor for the redistribution of existing monitoring, and the 

rule does not provide for diverting of resources as it relates to modeling to characterize sources. 

A number of commenters appeared to interpret the inclusion of a lower threshold for areas with 

higher population as being less protective of the public in less populated areas such as Tribal 

areas. The EPA wants to clarify that this was not the intention behind the population-inclusive 

options included in the proposed rule. The SO2 NAAQS, and all NAAQS, provide equal 

protection for citizens throughout the country. The proposed use of both population and 

emissions thresholds as a means to require air quality characterization was simply one approach 

to focus limited federal and state modeling and monitoring resources into characterizing

locations where a greater coincidence of people and SO2 emissions occur, and thus a potentially 

greater potential for exposure is presented. After reviewing the comments on this issue, however, 

the EPA has decided not to move forward with the proposed approach, and instead to apply 

requirements for air quality characterization based on emissions levels uniformly across the 

country for both more urbanized and less urbanized populations so as to focus primarily on the 

size of the sources.

In response to the comment stating that EPA should provide earlier consolation in the process of 

developing the rulemaking, consistent with the EPA Policy on Consultation and Coordination 

with Indian tribes, the EPA held several meetings with tribal environmental professionals early 

on in the process of developing the rulemaking to discuss issues associated with this rule, 

including discussions at the National Tribal Forum on May 1, 2013, and on National Tribal Air 

Association policy calls. These meetings discussed the SO2 implementation White Paper. The 

EPA provided an opportunity for tribes and stakeholders to provide written comments on the 

concepts discussed in the White Paper. Summaries of these meetings are included in the docket 

for this rule. The EPA also provided information on the proposed rule and conducted 

consultation with the National Tribal Air Association in the form of a briefing on April 24, 2014, 

and a webinar on May 21, 2014. Therefore, the EPA believes that in this instance, the Agency 

provided consultation with Tribal organizations, as well as other stakeholder as early as possible 

in the developmental process related to the rulemaking.
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VII. TADs for Monitoring and Modeling and Appendix W

A. Comments in the DRR docket related to the TADs and appendix 

W

Comment

Comments on the draft TADs and Appendix W found in the docket and relevant to the DRR 

have been incorporated into this RTC. Some commenters (0038, 0051, 0060, 0081, and 0088) 

referred to comments they previously submitted to EPA regarding the draft TADs but did not 

include those comments in the docket; those comments are not included in this RTC. 

Commenter (0060) stated it is challenging to prepare meaningful comments on the DRR since 

much of this Proposed Rule is contingent upon the use of these TADs. Commenter (0060) stated

that because this Proposed Rule and the draft TADs are so intertwined, all three documents 

should be available for comment at the same time. Commenter (0110) stated that, without having 

the DRR in place, it is difficult to provide complete comments on the TADs. Commenter (0110) 

stated that the proposed DRR references and relies upon guidance provided in the modeling and 

monitoring TADs, therefore, these documents are subject to review and comment for this 

Proposed Rule. Commenter (0110) stated that EPA also has dispersion modeling guidelines 

(Appendix W to 40 CFR Part 51) that are referenced by this Proposed Rule. Commenter (0110) 

stated these documents are therefore available for comment in conjunction with this proposed 

SO2 DRR.

Response:

The EPA disagrees with the assertion that the understanding or ability to comment on this 

rulemaking was contingent on the use of the TADs. The Agency reiterates that the TADs provide 

recommendations, but are not binding or enforceable and create no obligations on any person. 

Although the draft TADs are referenced as recommended approaches in the preamble to the 

proposal and in this final rulemaking, they are not required to be adhered to by any air agency

required to characterize air quality around an SO2 source identified in this rulemaking. The EPA 

developed the TADs to aid air agencies seeking advice in the air quality characterization process 

required by this rulemaking. The Agency has indicated that the TADs are meant to be used as

possible tools to aid air agencies. This rulemaking does not codify the TADs, and none of the 

comments on the proposed rule regarding the TADs resulted in changes to the rule itself. The 

TADs are considered to be living documents that the EPA may update as necessary over time. 

The Agency believes that a modeling protocol or monitoring network design that follows or 

references the recommended approaches in the TADs is likely to be adequate, and will better 

ensure the success and a timely fulfilment of the requirements of this rulemaking. However, air 

agencies remain free under the final rule to suggest alternative approaches to those suggested in 

the TADs. Whether an agency chooses to follow a TAD or suggest an alternative approach does 

not affect the fact that for every approach chosen the air agency will need to submit their 

rationale and approach to the EPA for review on a case-by-case basis.
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B. TADs are not binding

Comment: 

Commenters (0040, 0060) questioned whether referring to the expected use of these TADs in a 

binding rule does not in and of itself make the TADs binding. Commenter (0040) stated that 

EPA should clearly state in each TAD, should they remain non-binding guidance, that they have 

no relevance in any enforcement action or citizen suit for any purpose under the CAA and EPA’s 

regulations promulgated pursuant thereto.

Commenters (0040, 0044, 0051, and 0060) urged EPA to be clearer in the final preamble that the 

TADs are not binding. Commenter (0060) urged EPA to explicitly state in the final rule that 

these TADs are only guidance and air agencies retain the ability to use alternative methods to 

those outlined in the TADs. 

Commenter (0109) stated EPA's reliance on technical guidance documents that have not been 

subject to public notice and comment undermines protections guaranteed by the Administrative

Procedure Act and perpetuates EPA's isolation from the input of affected sources and

individuals. Commenter (0052) stated that because the DRR requires the use of "... separate nonbinding draft technical assistance documents ..." and creates significant regulatory uncertainty 

for air agencies, they oppose EPA's proposal.

Response:

The EPA has stated in the TADs themselves, and has reiterated in the preamble to the final rule, 

that the TADs are non-binding. Also see the response to the comment above, titled “Comments 

in the DRR docket related to TADs and Appendix W.” In addition, the TADs themselves have 

been subjected and remain subject to public comment. Therefore, the EPA disagrees with the 

assertion that reference to the TADs is in contravention of the Administrative Procedure Act

(APA). The EPA appreciates that these commenters have taken their opportunity to assure that 

EPA is not isolated from the input of affected sources and individuals.

C. Procedure for TADs

Comment: 

Commenter (0044) recommended that EPA solicit public comment on each TAD and then 

finalize both TADs. Commenter (0044) recommended that the final rule express the procedure 

the EPA intends to use for that purpose, including whether the public will be provided an 

opportunity to comment on the proposed changes before they are “finalized,” along the same 

lines as contained in Appendix W to part 51. Commenter (0044) stated the TADs should be 

codified as appendices to 40 CFR subpart BB. Commenter (0044) stated that if EPA disagrees 

and elects to take a counter approach, then its reasoning for doing so should be explained in the 
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final rule. Commenter (0110) recommended that, once the DRR is finalized, the TADs be 

subjected to public comment again.

Commenter (0044) stated that the incorporation by reference of Appendix W into the modeling 

TAD is an imprecise and confusing way to provide guidance. Commenter (0044) stated the 

TADs numerous references to Appendix W are often quite broad or otherwise ambiguous and 

often fail to make clear what specific guidance from Appendix W should apply to the modeling 

to be conducted under subpart BB. See, e.g., modeling TAD at 24 and 25. In the absence of EPA 

providing greater clarity and specificity regarding the application of specific provisions of 

Appendix W to the modeling TAD, commenter (0044) believed that some references to 

Appendix W in the modeling TAD could be easily misread and/or more broadly applied than 

might have otherwise been intended.

Commenter (0044) believed these deficiencies in the Proposed Rule can readily be cured by 

inserting specifically the relevant text from Appendix W into the modeling TAD, and/or by 

revising Appendix W’s guidance as necessary and inserting it into the appropriate text of the 

modeling TAD. Commenter (0044) asserted either would work and would go a long way to 

avoid the confusion and problems the current draft TAD’s usage of incorporation by reference 

will cause. Commenter stated that the linkage between Appendix W and the modeling TAD, and 

the need to revise the TAD in order to make it clearer, will also provide EPA the opportunity to 

finalize the TAD (and remove its current “draft” status) and at the same time provide the public 

with an opportunity to review and comment upon proposed revisions and clarifications to the 

TAD; preferably before this rule is finalized.

Commenter (0051) stated the EPA should revise this rule and associated guidance in response to 

comments that the TCEQ has previously submitted regarding the modeling and monitoring

TADs. 

Commenter (0051) stated that, because much of the TAD guidance is identical to permit 

modeling approaches which can be conservative, the EPA should reconsider its adherence to 

modeling approaches that may not be effective for designation purposes. 

Commenter (0065) stated the TADs should be withdrawn because they support EPA’s 

designation schedule which conflicts with EPA's obligations under the CAA. Commenter (0065) 

stated that the TADs, which support that schedule, may not be lawfully pursued. 

Response: 

The EPA reiterates that the TADs provide recommendations but are not binding or enforceable 

and create no obligations on any person. The relevant requirements, requiring air quality 

characterization, are requirements of this rule, and are being promulgated based on the full 

notice-and-comment process recommended by the commenter; the TADs themselves do not 

create any new requirements. In this sense, the TADs may be considered to be commentary on 

the judgments that air agencies will need to make in the course of complying with this rule. 
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Although the draft TADs are referenced as recommended approaches in the preamble to the 

proposal and in this final rulemaking, they are not required to be adhered to by any air agency

required to characterize air quality around an SO2 source identified in this rulemaking. The EPA 

developed the TADs to aid air agencies seeking advice in the air quality characterization process 

required by this rulemaking. The Agency has indicated that the TADs are meant to be used as

possible tools to aid air agencies. This rulemaking does not codify the TADs, and none of the 

comments on the proposed rule regarding the TADs resulted in changes to the rule itself. The 

TADs are considered to be living documents that the EPA may update as necessary over time.

The Agency believes that a modeling protocol or monitoring network design that follows or 

references the recommended approaches in the TADs is likely to be adequate, and will better 

ensure the success and a timely fulfilment of the requirements of this rulemaking. However, air 

agencies remain free under the final rule to suggest alternative approaches to those suggested in 

the TADs. Whether an agency chooses to follow a TAD or suggest an alternative approach does 

not affect the fact that for every approach chosen, the air agency will need to submit their 

rationale and approach to the EPA for review on a case-by-case basis.

VIII. Proposed Consent Decree (CD)

A. Comments in the DRR docket related to the proposed CD

Comments on the Proposed CD found in the docket and relevant to the DRR have been 

incorporated into this RTC, primarily in the discussion below. Some commenters (0053, 0088) 

referred to comments they previously submitted to EPA regarding the proposed CD but did not 

include those comments in the docket. As a result, those comments are not included in this RTC. 

The EPA received several comments on the CD in the record for this rule. To the extent that 

those comments address issues only related to the terms or the status of the CD (that the CD is 

entered, agreed to, withdrawn, or other procedure) or are otherwise not directly relevant to this 

rule, those comments are outside the scope of this RTC and are not included. The following 

issues, for example, are generally outside the scope of the DRR.

 Jurisdiction to enter the proposed CD (0065)

 Withhold an "attainment" classification (0065)

 CD circumvents the requirements of the CAA (0058)

 “Sue and settle” strategy (0058)

 Thresholds described in the CD for the 16 month deadline (0058)

 CD establishes policy beyond the proper scope of a CD (0058) 
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 Recommendations for revisions to the CD (0096)

B. Conflicts with the APA

Comment: 

Commenter (0063, 0065, 0096) stated that, by short-circuiting the ongoing public notice and 

comment process for the Proposed Rule, the proposed CD contravenes the notice and comment 

procedures set forth in the APA and undermines a years-long regulatory process culminating in 

the Proposed Rule. Commenter (0096) stated this action may run afoul of the APA’s notice and 

comment rulemaking requirements by implementing a policy (rule) outside of the rulemaking 

process prescribed by the APA. 

Response:

The proposed rule which was developed during 2013, signed on April 17, 2014, and ultimately 

published on May 13, 2014, did not contain any regulatory deadlines for the EPA to complete 

area designations under the 2010 SO2 NAAQS. However, during the time that the EPA was 

developing the proposed rule and the final rule, the agency also became engaged in district court 

litigation from public interest groups and some states and state agencies seeking to have the EPA 

placed on a binding schedule to complete the designations. The parties in these cases filed 

complete briefs in one of these cases, resulting first in the court finding that the EPA was liable 

for having failed to meet the statutory deadline to complete all area designations. Subsequently, 

the EPA and the other parties conducted extensive settlement discussions over the remedy, i.e., 

the schedule by which the EPA would complete its duties. This resulted in a settlement between 

the EPA and the public interest group plaintiffs, which the plaintiff-interveners did not join. 

On June 2, 2014, the EPA published notice of a proposed CD reflecting this settlement (Sierra 

Club et al v. McCarthy, Civil Action No. 3:13-cv-3953-SI (N.D. Cal.)). 79 FR 31325. This 

proposed CD included deadlines for the EPA to complete designations in three phases, the latter 

two of which were due on the same dates that the EPA discussed as its intended designations 

dates in the preamble to the proposed DRR. The EPA received several comments on the notice 

informing the public of the proposed CD itself, and in response to this proposed rule. 

The EPA is not promulgating deadlines for its completion of area designations in this final rule. 

Therefore, any comments directed to the merits of the CD itself are outside the scope of this 

rulemaking, and we will not respond to them here. Instead, as discussed earlier in this notice, on 

March 2, 2015, the court issued an order entering the CD and establishing its deadlines as 

binding on the EPA. Under the CD, the next round of designations must be completed by July 2, 

2016, which falls too early for the implementation of this final rule to generate data that may 

help inform designations in that round. As also explained earlier, the 2017 and 2020 deadlines 

for the latter two stages of designations established by the CD will allow the EPA and states to 

use the new data and information that is timely generated by the implementation of this rule to 
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inform the designations required to be completed by those dates.

The EPA disagrees with commenters who claim that either the court’s entry of the CD or this 

rulemaking contravene procedural requirements of either the CAA or the APA. The CD, which 

established requirements for EPA to complete designations, was published for public notice and 

comment as required by CAA section 113(g) before it was entered by the court several months 

later. The requirements of this rule, which more generally establish duties for air agencies to 

characterize emissions that can be used for multiple purposes, not only designations or redesignations, have undergone all applicable procedural requirements of the APA.

C. Conflicts with the DRR

Comment: 

Commenters (0048, 0051, 0058, 0065, and 0090) stated the CD conflicts with the DRR and 

limits their opportunity to comment on the DRR. Commenter (0051) stated the EPA has not 

given any notice of this CD and its impact on the present proposal. Commenter (0058) stated the 

CD imposes new substantive requirements that are inconsistent with EPA’s pending DRR and, in 

doing so, the proposed CD significantly undermines the pending rulemaking and the public 

review and comment process. Commenter (0090) stated that EPA's apparent intent to enter into a 

CD directly related to this rulemaking will make comments on this Proposed Rule irrelevant.

Commenters (0048, 0065) stated the proposed CD ignores and bypasses the notice and comment

process for the proposed DRR. 

Commenter (0096) stated that, while the DRR proposes to establish by rule the emission 

thresholds and the applicable data requirements, the CD suddenly removes certain EGUs from

the rulemaking process of the DRR, and therefore removes states’ ability to provide input with

respect to the emission threshold and data requirements for those units. Commenters (0101, 

0109) stated the proposed CD impairs the integrity of the rulemaking process and improperly 

creates a new class of source threshold options that the CD mandates must be designated at an 

earlier time than proposed in the DRR. Commenter (0101) stated the proposed CD, in contrast to 

the DRR’s notice and comment process, arbitrarily and without substantial evidence imposes 

rigid thresholds to be employed without any sign as to how they were developed or what 

competing factors were balanced in selecting the more than 16,000 tpy or 0.45 lbs/MMBtu 

limits.

Commenter (0075) stated that the proposed CD would effectively change without notice-andcomment rulemaking the EPA’s longstanding practice of relying on monitoring data for initial 

designations and otherwise prejudge the outcome of the present rulemaking. Commenter (0075) 

stated that, in light of the Agency’s historic practice of basing the §107(d) initial designation 

process on monitoring data, not modeling predictions, the EPA cannot change that previous 

practice through a CD.
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Commenters (0051, 0058, 0065, and 0075) stated the proposed CD effectively requires EPA to 

prejudge the outcome of key substantive elements of the ongoing rulemaking to set a DRR. 

Commenter (0058) stated that the proposed CD effectively amends this Proposed Rule by: (1) 

adding a new, earlier round of area designations and (2) going beyond resetting deadlines which 

were the subject of the litigation to defining the criteria by which areas will be evaluated for 

designation purposes. Commenter (0075) stated that, although the DRR proposes a wide range of 

options for completing the area designation process – and asks for comment on all such options –

the CD picks a winning option and makes it virtually impossible for anyone other than Plaintiffs

to change the “winner.” Commenter (0051) stated that the CD differs significantly from the DRR

and, if entered by the court prior to promulgation of the rule, will take precedence. 

Commenter (0058) stated the Proposed CD references the proposed DRR and would effectively 

codify many of the dates that are outlined in the Proposed Rule. Commenter (0058) stated 

deadlines for collection of data to support future designations should not be ordered by the court 

based on a Proposed Rule that may change as a result of public comment. Commenter (0058) 

stated it is not appropriate or proper for the CD to tie the EPA’s hands with regard to what will 

occur in a pending rulemaking. Commenter (0106) stated that CD Plaintiffs and EPA are 

presupposing that there will be no changes to the DRR as it is currently drafted even though the 

purpose of notice and comment rulemaking is that an agency may receive the benefit of 

interested stakeholder comment on Proposed Rules.

Response:

The EPA is not establishing or modifying any area designation requirements provided for in 

section 107 of the CAA through this rulemaking. The purpose of this rulemaking is to require 

states to characterize air quality in priority areas throughout the country where existing ambient 

monitors may not be adequately characterizing peak 1-hour SO2 concentrations. The air quality 

data obtained as a result of this rulemaking then may be used in future designations or 

redesignations, as appropriate. While the notice of proposed rulemaking described the EPA’s 

anticipated designations schedule, for purposes of explaining the timeline by which the EPA 

anticipates that the data the EPA was proposing to require will be used, the timeline for possible 

future use of these data does not dictate the schedule or the substantive features of the 

requirements for obtaining data for air quality characterization purposes, and the Agency 

believes it will be highly valuable to obtain these data even if that occurs after initial 

designations occur. 

While the notice of proposed rulemaking described the EPA’s expectations that designations for 

areas not already completed in August 2013 would be completed either in 2017 or in 2020, the 

timetables for obtaining additional data are as prompt as the EPA considers reasonable whether 

or not such data can be used to inform the remaining designations, and thus alternate approaches 

and timetables for designations would not result in a different timetable for implementation of 

the rule’s requirements. In particular, whether designations proceed according to the approach 

described in the EPA’s notice of proposed rulemaking, or whether areas are first designated 

unclassifiable and subsequently redesignated to attainment or nonattainment, the same timetable, 
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246

and substance of requirements for data to support more properly informed future judgments 

regarding areas’ attainment status is warranted. Because this rulemaking is not intended to define 

the designations process and did not propose regulatory deadlines for issuing designations, it 

would be inappropriate in this final rulemaking to codify any particular schedule for designations 

action.

The proposed CD referenced by the comments concerns separate legal proceedings that are 

addressing the EPA’s obligations to designate areas under CAA section 107. The commenters 

have not identified why any potential outcome of those proceedings warrants any particular 

revision to the rule, nor have they explained why the validity of the DRR is contingent on use of 

any particular designations approach. While the court’s decision establishing timing 

requirements for the EPA’s designations obligations will of course affect the EPA’s approach to 

designations, including affecting the extent to which the EPA will be able to use the data 

required under the rule at various times in the designations or redesignations processes, these 

effects do not determine the validity of the data collection requirements of the rule. For these 

reasons, the EPA believes that the ability of commenters to address issues relevant to the rule 

was not compromised by the proposed CD and other actions or statements in the proceedings 

regarding the EPA’s timetable for designations, and the EPA finds that re-proposal of the rule is 

not justified.

The comments objecting to the CD and its alleged interference with or undermining of the 

proposed DRR are outside the scope of this rulemaking. However, the EPA notes that some of 

the commenters who objected to the CD and its relationship to the proposed DRR also filed 

remedy briefs in the deadline suits advocating for designations schedules that were vastly more 

in conflict with the proposed DRR. Indeed, those commenters and litigants advocated for a 

judicial ruling on the schedule for designations that would not have allowed for any 

implementation of the DRR to inform those designations. The final CD establishes the deadlines 

for future rounds of designations. 

D. Procedural options for the CD and DRR

Comment: 

Commenters (0050, 0051, 0053, 0058, 0073, 0075, 0088, 0090, 0101, and 0102) urged EPA to

withdraw or withhold consent from the proposed CD so as not to impair, interfere with or 

contradict determinations made in proceedings with the DRR. Commenter (0058) stated the EPA 

must not enter into a CD that could alter the technical aspects of this rule without the required 

public notice and opportunity for comment. Commenter (0073) believed it is important that 

issues concerning the exercise of discretionary authority be resolved through a rulemaking 

proceeding fully informed by public comment rather than in the context of a CD. 

Commenters (0051, 0053, 0080, 0090, 0096, and 0097) suggested that, in order to give all 

interested persons adequate opportunity to comment on the DRR proposal, the EPA should repropose the DRR. Commenter (0051) stated that, if the CD is finally entered by the court and 
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becomes legally enforceable by the plaintiffs in that case the EPA should re-propose the DRR.

Commenter (0090) suggested that EPA should re propose the DRR to reflect additional 

information raised by the CD. Commenters (0090, 0096) stated that, if the CD is entered by the 

court, its terms must be referenced and incorporated into the DRR. Commenter (0053) stated the 

EPA should not allow the proposed CD to usurp this rulemaking process and any significant 

changes to the proposed DRR (such as those in the CD) should be subject to additional 

rulemaking and not finalized as party of a 2-party settlement agreement. Commenter (0097) 

recommended that any requirements in the proposed CD should be included in this Proposed 

Rulemaking; otherwise, the EPA is imposing rule-like requirements without the public 

participation and other legal backstops associated with a rule. Commenter (0080) stated the EPA 

cannot continue with this rulemaking when substantial portions of the DRR are being modified 

through the pending CD.

Commenter (0096) added the following points. 

In particular, the additional milestones imposed by the CD must be incorporated into the 

program implementation timeline set forth in part IV of the DRR. Specifically, those 

additional milestones include (1) a period for states/air agencies to review the data on 

which EPA intends to base designations for those sources covered by the CD, (2) the date 

EPA must submit 120-day letters to states if deviating from agencies’ designation

recommendations, (3) the date for public comment period on proposed designations, and 

(4) the final date by which EPA must publish designations in the Federal Register for 

those sources covered by the CD. It is important that all data requirements and 

implementation timeline milestones for the 2010 SO2 NAAQS be represented in the 

DRR, and any additional requirements imposed by the CD be incorporated into the DRR.

Commenter (0058) stated that, because the proposed CD was not noticed in conjunction with this 

rulemaking and effectively amends this Proposed Rule, EPA must issue a supplemental proposal 

in the context of this rulemaking to allow for full public notice, review and comment on the 

terms and conditions of the designation criteria proposed in the draft CD. 

Commenter (0096) stated that, if the CD is entered there should be a need to reference the DRR’s 

proposed requirements in the CD to clarify and streamline the interplay between the two 

documents. Commenter (0097) stated that a CD is not the appropriate mechanism for imposing 

requirements supporting the implementation of a NAAQS upon states, particularly where there is 

an outstanding rulemaking on the implementation of that standard. 

Response:

See response to the comment above. Neither the CD nor the court order may be considered to 

amend this rule, and this rulemaking was of course powerless to divest the court of jurisdiction to 

issue a remedy requiring the EPA to complete designations by certain dates. The EPA disagrees 

that it is necessary to re-propose this rule simply because the court has now entered the CD and 

placed the EPA under a binding schedule to complete designations, just as the EPA would so 
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disagree if the court had declined to enter the CD and had placed the agency under a binding 

schedule that would not have accommodated implementation of the rule at all before 

designations were due.

E. CD 16 month deadline

Comment: 

Commenters (0058, 0065, 0066, and 0102) stated the short timeline of 16 months for the first 

phase of designations in the proposed CD short-circuits the provisions in the DRR and does not 

allow time for areas to establish monitors and collect 3 years of actual ambient air quality data.

Commenter (0065) stated, by contrast, the proposed DRR would allow states the option to gather 

monitoring data through 2019 to characterize each area's air quality. Commenter (0065) stated 

that the EPA should not limit in this way the flexibility of states to tailor their respective

approaches to establishing their attainment status, because the CAA assigns states the primary 

responsibility and discretion to determine how to implement and enforce the NAAQS.

Commenter (0082) stated the EPA should clarify in the final rule that, for areas required to be 

designated by the 16-month deadline in the proposed CD, it would be appropriate for EPA to 

designate areas as unclassifiable where the necessary monitoring or modeling data are still being 

developed. 

Commenter (0058) stated the method by which AERMOD should be used in the context of 

designations is a subject of the proposed DRR and, if this court were to commit the states and 

EPA to a path of making designations based on dispersion modeling, the appropriate method of 

applying such models for this purpose would not be established until well after the lodging of 

this CD, significantly reducing the 16 month window. Because the draft settlement uses actual 

2012 SO2 source emissions data to identify sources that need to be modeled, commenter (0066) 

believed it forces states to model power plants where sufficient SO2 controls have already been 

installed to meet the 2010 SO2 NAAQS, which is a waste of state resources and those of affected 

large SO2 sources.

Response:

See response to the comments above. In addition, the EPA stresses that this final rule does not 

establish requirements for how designations are to be conducted, and that the CD does not 

require specific analytical methods or substantive results in future designations. The CD simply 

provides the criteria for which areas must be designated by which dates, and the date that the 

EPA must meet in issuing designations. 

IX. Comments outside the Scope of this Rulemaking

A. Comments on designation policy
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1. Alternate approaches for designations

Comment:

Commenter (0046) suggested states should be able to work with sources on a more flexible 

schedule than the rule allows for in order for them to implement emissions reductions in a timely 

manner rather than proposing a nonattainment designation. Commenter (0046) stated the EPA

has done this in the past with the Early Action Compact (EAC) approach for meeting the 1997 8-

hour ozone standard and stated such an approach would be easier in this case, as SO2 is more of a 

source-based pollutant than ozone, and could result in improvements in air quality sooner than a 

2022 attainment date. Commenter (0046) stated this approach would focus the burden associated 

with a nonattainment designation on areas that need longer range planning and solutions to 

achieve attainment.

Commenter (0096) stated, another, more conventional, approach would be to designate areas 

with future emissions reductions as unclassifiable until enforceable emission limitations are 

implemented.

Commenter (0042) recommended that the EPA proceed as it did with the NO2 NAAQS: the 

Agency should (1) determine that areas without adequate monitoring data would be designated 

as unclassifiable; (2) set forth requirements for additional monitors, developing the requirements 

for an adequate monitoring network in consultation with state and local agencies; (3) give states 

a reasonable time to install those monitors and to collect three years of data; and (4) then 

consider whether areas initially designated unclassifiable could be redesignated as attainment or 

nonattainment based on the results of this monitoring. 

Commenter (0046) stated the EPA should expedite NAAQS designations based on monitoring 

by reviewing existing network plans, if requested by the state, and use existing monitoring data, 

as it would for any other NAAQS if the monitoring plan is approved (retroactively).

Commenter (0052a, 0106) stated that areas with no significant sources and not being impacted 

by significant emissions from a source in an adjacent area, should be designated attainment or 

unclassifiable/attainment. Commenter (0052a) stated that, under this scenario, only those

counties with sources whose emissions are above the threshold should be designated 

unclassifiable and they should only be maintained in that classification until ambient monitoring 

determines the area is attaining or violating the SO2 standard. Commenter (0106) stated that, 

where there are no significant anthropogenic sources in an area (e.g. a county) or near the area's 

border, the EPA should provide a mechanism for designating the area "attainment" without 

monitoring or modeling data; e.g., the EPA could provide a lower emission threshold for total 

emissions in a county below which attainment status would be justified without providing 

additional monitoring or modeling data.

Commenter (0052) recommended that the EPA establish a de minimis level regarding area

evaluation and monitor location. Commenter (0052) stated the EPA should declare areas with 
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combined actual annual emissions less than 100 tpy of SO2 as "attainment". Commenter (0052) 

stated an emissions inventory of all SO2 sources within the county or region showing historical

emissions data (i.e. maximum actual emission totals per county from 2007-2010) should suffice.

Commenter (0083) suggested an alternate approach and stated it will avoid requiring areas that 

may not be actually violating the standards from going through the very expensive, lengthy and 

time-consuming processes associated with a nonattainment designation. Commenter (0083) 

suggested the following as an alternate approach to use in completing the initial designations for 

the 1-hour SO2 NAAQS.

(1) The DRR should state that any data from existing monitors located in reasonable 

proximity to an applicable source of SO2 emissions shall be used to designate the area as 

attainment if the monitoring data shows that the area meets the standard and 

nonattainment if the monitoring data shows the area does not meet the standard.

(2) For the initial designation, all areas without sufficient monitoring data should be 

designated unclassifiable. 

(3) Since modeling is known to provide a conservative estimate of ambient pollutant 

concentrations, the DRR should allow for modeling to demonstrate that an area 

surrounding an applicable source of SO2 emissions does not violate the standard. Those 

areas could then be designated attainment with no further monitoring requirements. 

Modeling should not be used to make a nonattainment designation for an area.

(4) For all other areas with applicable sources under the DRR, the DRR should require 

the placement of ambient monitors, which will also provide a more comprehensive 

ambient air monitoring network, giving regulatory agencies, the general public, and other 

stakeholders a better understanding of the actual air quality in their area. 

Response:

This final rule is not taking any designations action and does not establish requirements for how 

designations are to be conducted, or by what date. (As an aside, we note that the schedule for 

designations is now set forth in a court order in any event). In addition, it would not be 

appropriate for this rule to prejudge the substantive outcome of future designations, as many of 

the commenters appear to request. While the proposed rule discussed how the timing of the 

implementation of this rule would fit with the anticipated schedule for completing area 

designations, the proposed rule did not itself purport to establish a binding schedule for 

completing designations. In this rulemaking we are not addressing comments received on the 

proposed rule concerning the designation process, or alternative approaches for conducting 

designations. Commenters will be able to raise their concerns about specific designations issues 

when those actions are taken.
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2. Designations before 2017

Comment:

Commenter (0069) stated the EPA should designate as "attainment" areas that contain properlysited SO2 monitors with attaining data as well as those areas of the country that do not contain 

large sources of SO2. Commenter (0069) stated the EPA should not wait until December 2017, 

rather this should be completed as soon as possible as required by section 107(d)(1)(B) of the 

CAA.

Commenter (0068) stated that the EPA can and should promulgate additional designations before 

the 2017 date in the draft rule. Commenter (0068) provided the following additional points.

Since under the CAA nonattaining areas have up to five years to resolve their air quality 

problems from the date of designations, Commenters are troubled by a framework in 

which all outstanding designations are not finalized until 2017 and 2020, under the 

modeling and monitoring pathways, respectively, contemplated in the draft DRR. 

Accordingly, commenters believe that the schedule laid out in the draft rule could only be 

supportable if it was coupled with an additional, prior round of designations to occur 

before the 2017 modeling pathway round, focusing on areas containing a subset of highpriority sources. While commenters appreciate and support timetables that ensure that 

accurate designations are promulgated, commenters believe that the EPA can and should 

promulgate additional designations before the 2017 date in the draft Rule.

Response: 

The EPA’s schedule for completing designations is now subject to a court order, which in fact 

does provide for a round of designations to occur in June 2016 for areas with newly monitored 

violations of the NAAQS and with sources emitting over certain very large annual or lb/mmBtu 

rates. While the Proposed Rule discussed how the timing of the implementation of this rule 

would fit with the anticipated schedule for completing area designations, the Proposed Rule did 

not itself purport to establish a binding schedule for completing designations. In this rulemaking 

we are not addressing comments received on the Proposed Rule concerning the designation 

process, or alternative approaches for conducting designations because those issues are beyond 

the intended scope of this rulemaking.

3. Previously submitted SO2 designations in specific states

Comment:

Commenters (0038, 0043, 0051, 0103, and 0108) asked the EPA to act on previously submitted 

designation requests.

Commenters (0038, 0108) asked that previously submitted designation requests be considered by 
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the EPA in the context of WDNR's comments on the proposed DRR. Commenter (0038) noted 

the state of Wisconsin has previously made designation recommendations to the EPA for the 1-

hour SO2 NAAQS and stated that, in these letters, the state recommended to the EPA that 

Wisconsin counties currently demonstrating a 3-year design value below the NAAQS, as well as 

those counties reporting less than 20 tpy of SO2 emissions (or with no individual sources 

reporting greater than 20 tpy of SO2 emissions) be designated as attainment. 

Commenter (0043) recommended that the EPA act on South Dakota's original submittal and 

follow the proper procedures outlined in the CAA. Commenter (0043) stated South Dakota's

designation provides sufficient information for the EPA to make a determination that all areas in

South Dakota are in attainment for the 2010 revised sulfur dioxide standard. 

Commenter (0051) stated the EPA should designate 10 Texas counties as attainment. 

Commenter (0051) provided a table with design values and referred to correspondence between 

the state and Regional Office. Commenter (0063, 0065) stated the Houston Texas area should be 

designated attainment since, not only does the area's regulatory monitoring network meet part 58 

requirements, but the area also contains a dense privately-operated monitoring network and all 

the relevant data from these two networks confirm that the area is attaining the SO2 NAAQS. 

Commenter (0063, 0065) provided additional, detailed information about this monitoring 

network in Attachment A to their comment letter. 

Commenter (0052) stated that because the DRR ignores currently available quality-assured 

ambient air data measured by state and local air monitoring systems that meets all applicable 

regulatory requirements, they oppose the EPA's proposal.

Commenter (0106) stated North Dakota timely submitted its initial designations of attainment for 

all areas in the state to the EPA, the Department believes that the monitoring data supports 

designations of attainment for North Dakota, and that the EPA must proceed promptly to 

promulgate designations of attainment for the state.

Response: 

While the Proposed Rule discussed how the timing of the implementation of this rule would fit 

with the anticipated schedule for completing area designations, the Proposed Rule did not itself 

purport to establish a binding schedule for completing designations. In this rulemaking we are 

not addressing comments received on the Proposed Rule concerning the designation process 

because those issues are beyond the intended scope of this rulemaking. The timing of EPA’s 

designations is now governed by a court order, and the substantive decisions of those future 

designations will be made in those future actions, not prejudged in this final rulemaking.

4. Redesignation to attainment following nonattainment designation 

based on modeling
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Comment:

Commenter (0046) stated the EPA must explain how areas may be redesignated to attainment if 

they are initially designated nonattainment based on modeling. Source-oriented monitoring 

should be the methodology to redesignate an area to attainment; a modeling analysis should not 

be required. Commenter (0110) stated that, for areas that have modeling results showing 

NAAQS violations for a specific source, the state should have the opportunity to change to a 3-

year monitoring approach if there is an expectation that the modeling has limitations that could 

result in over predictions.

Commenter (0051) stated the EPA should clarify that areas can be directly designated or 

redesignated to attainment based on qualifying monitoring data in the following instance: where

states are unable to have monitors deployed by the EPA's deadline, areas may subsequently be 

designated as nonattainment based on modeling; then, once monitoring data are available, it may

demonstrate attainment. Commenter (0051) stated that, if this occurs, the EPA should directly 

redesignate the area to attainment, without requiring the state to go through all the 

redesignation/maintenance plan requirements.

Commenter (0081) stated that, in order to comply with the CAA, the EPA should designate all 

areas which cannot currently be categorized by existing monitors as unclassifiable, the EPA 

should then redesignate areas as “attainment” or “nonattainment” based on the modeling and 

monitoring requirements finalized in this rule, and, upon redesignation from “unclassifiable” to 

“attainment,” no maintenance plan should be required. Commenter (0081) stated that such a 

strategy would be consistent with the legal requirements of the CAA.

Response:

While the Proposed Rule discussed how the timing of the implementation of this rule would fit 

with the anticipated schedule for completing area designations, the Proposed Rule did not itself 

purport to establish a binding schedule for completing designations. In this rulemaking we are 

not addressing comments received on the proposed rule concerning the designation or 

redesignation process because those issues are beyond the intended scope of this rulemaking. In 

addition, we note that the EPA in April 2014 issued non-binding guidance on implementation of 

the NAAQS in designated nonattainment areas, including addressing re-designations. That 

guidance may be found at http://www.epa.gov/airquality/sulfurdioxide/implement.html. This 

final rulemaking does not take final action to codify that guidance, nor to otherwise establish 

requirements for how re-designations shall be conducted.

5. Future monitored violations

Comment:

Commenter (0052a) stated the EPA should allow a state to address a source-oriented ambient air 

quality violation using the air program permitting process if a violation is measured in an 

unclassified area after an ambient monitoring network is expanded. Commenter (0052a) stated 
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that this may be the most efficient way to bring an area back into compliance with the NAAQS 

and is an appropriate federally-enforceable instrument to use. Commenter (0052a) stated that, 

given the short term nature of the SO2 standard and the typical cause and effect relationship 

between an individual source of SO2 and a violating monitor, the traditional designation process 

is not well suited to efficiently address such violations. Commenter (0052a) stated that, if a state 

does not address a violation in a timely manner, then the EPA could proceed with designating an 

area as nonattainment and follow the SIP development process at that time.

Response: 

The EPA welcomes any and all timely efforts to address air quality problems as they are 

identified. Nevertheless, such efforts cannot substitute for meeting the requirements of the CAA. 

The EPA does not have the authority to suspend those requirements while the state seeks to 

address the air quality problem. The EPA is not in this final rule establishing any requirements 

for when or how designations are to be conducted, but notes that in the designations process for 

future actions it may in specific cases be appropriate to consider timely efforts by the state that 

result in the area meeting the air quality standard by the time of issuance of the designations. The 

EPA also notes generally that establishment of limits sufficient to assure attainment would 

address the primary requirement for nonattainment plans. However, such efforts by the state 

cannot authorize EPA to avoid promulgating designations, nor exempt a state from any 

requirements that apply to planning for air quality following a designation.

6. December 2017: Intended date for issuance of final designations for 

majority of country

Comment:

Commenter (0069) stated the EPA should provide for a longer period than 2 months--such as 6 

months--between publication of the designations in the Federal Register and the effective date. 

Commenter (0069) stated this would allow states to certify 2017 ambient monitoring data before 

the designations become effective, which would allow consideration of the air quality 

improvements brought about by significant EPA emission reduction requirements, such as the 

MATS Rule, which requires compliance by April 16, 2015, and the Boiler MACT, which 

requires compliance by January 31, 2016. Commenter (0069) stated this would eliminate an 

unnecessary burden on states to develop and the EPA to approve maintenance plans and redesignation requests for areas that attain the standard shortly after designations are made.

Response:

In this rulemaking we are not addressing comments received on the Proposed Rule concerning 

the designation process because those issues are beyond the scope of this rulemaking. The 

effective dates of future designations will be determined in those actions, and it would not be 

appropriate for this rule to prejudge those decisions.
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7. December 2020: Intended date EPA issuance of final designations

Comment:

Commenter (0069) stated (similar to the their comments on finalization of 2017 designations),

the EPA should provide for an effective date for the December 2020 designations that is far 

enough into 2021 to allow states to certify 2020 ambient monitoring data. Commenter (0069) 

stated this would eliminate an unnecessary burden on states to develop and on EPA to approve 

maintenance plans and redesignation requests for areas that attain the standard shortly after 

designations are made. [Also see IV.G.]

Response:

As stated previously, while the Proposed Rule discussed how the timing of the implementation 

of this rule would fit with the anticipated schedule for completing area designations, the 

proposed rule did not itself purport to establish a binding schedule for completing designations. 

In this rulemaking we are not addressing comments received on the Proposed Rule concerning 

the designation process because those issues are beyond the scope of this rulemaking. The 

effective dates of future designations will be determined in those actions.

B. SO2 NAAQS final rule

Comments on the proposed CD found in the docket and relevant to the DRR proposal have been 

incorporated into this RTC. Comments outside the scope of this RTC are not included. The 

following issues, for example, are generally outside the scope of the DRR.

 Timing suggestions submitted in correspondence before the DRR was proposed are not 

addressed as they have been superseded by the DRR proposal.

 Comments regarding consistency between the implementation approach that is set out in 

the preamble to the final SO2 NAAQS rule and language of the final SO2 NAAQS rule.

 Procedural comments regarding the final SO2 NAAQS rule.

C. Tenth EPA modeling conference

Comments on the draft Tenth EPA Modeling Conference found in the docket and relevant to the 

DRR proposal have been incorporated into this RTC. Those comments not outside the scope of 

this rulemaking are addressed earlier in this RTC. Comments outside the scope of this 

rulemaking include modeling conference comments directed at NO2, ozone, PM, and Significant 

Impact Levels.

D. Other comments outside the scope of this rulemaking
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The docket includes comments on EPA’s September 22, 2011 draft guidance, May 22, 2012 

White Paper, and February 15, 2013 Strategy Paper. The following commenters included in the 

docket previously submitted comments on EPA’s draft guidance: 0042, 0052, 0052a, 0052a, 

0097; those comments relevant to the DRR are included in this RTC. Some commenters (0053, 

0109) referred to previously submitted comments on EPA’s draft guidance but did not provide 

copies of such comments in the docket; those comments are not addressed in this RTC. 

Comments not outside the scope of this rulemaking are addressed earlier in this RTC.

The following issues, for example, are generally outside the scope of the DRR.

 Comments on infrastructure SIPs.

 Because the DRR is designed to require air quality characterization, generally to support 

decisions regarding attainment status, comments regarding the SIP requirements for 

future attainment and maintenance are generally not relevant to this rulemaking.

 Comments regarding redesignations to attainment are generally not addressed.

1. SIP requirements

Comment:

Commenter (0041) stated that methods exist to allow use of actual emissions for SIP modeling as 

long as follow-up checks are also required by the SIP, and the AQD has stated this position in 

numerous venues regarding the SO2 SIP for our existing nonattainment area.

Commenter (0097) stated that additional requirements should be included in implementation 

rules so that states and affected facilities have the opportunity to comment on all elements of the 

planning requirements. Commenter (0097) referenced EPA’s "Guidance for 1-Hour SO2

Nonattainment Area SIP Submissions" and stated that many aspects of this memorandum may 

also impact attainment/maintenance areas. Commenter (0097) stated, for example, this 

memorandum discusses the requirements for submitting SIP revisions regarding source-specific 

emission limitations, the methodology for determining emission limitations that are of longer 

duration than 1 hour, and ways facilities may include reductions from rules such as the MATS in 

their source-specific SO2 modeling. 

Response:

Issues concerning guidance for SO2 Nonattainment Areas are outside the scope of this 

rulemaking.

2. Electric rates
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Comment:

Commenter (0033) stated “So this means my electrical rates will go up again another 30%.”

Response:

While the commenter appears to object to this rule, and presumably believes that air agencies 

will identify a substantial number of areas violating the standard that the commenter would wish 

for air agencies not to identify, the commenter does not support the estimate 30% of the impact 

on electricity costs and does not specify any lawful revisions to any particular feature of the 

proposed rule that might mitigate the concern.

X. Other Comments

A. Support other comments

Commenters (0046, 0079) supported comments of commenter (0042).

Commenters (0047, 0048, 0057, 0058, 0066, 0070, 0090, and 0114) supported the comments of 

commenter (0075).

Commenter (0065) attached to their comments a copy of commenter’s (0042) December 2, 2011 

comments.

Commenter (0084) supported the comments of commenters (0042 and 0046).

Commenter (0094) supports the comments the comments submitted by commenters (0044 and 

0095).

B. EPA guidance 

Comment:

Commenter (0036) urged EPA to be more specific as to which state and EPA region would lead 

a multi-state planning process (e.g., confer responsibility to the state in which the source resides).

Commenter (0046) stated that several layers of conservatism are already incorporated into the 

underlying risk analyses and health-effects assessments considered in establishing NAAQS and 

each element of an ambient air quality analysis has uncertainty and measurement error associated 

with it. Commenter (0046) stated that the whole regulatory decision system should be considered 

by EPA in choosing the degree of conservatism deployed when making policy decisions that 

could materially affect the economic health of a community when public health may not actually 

be at risk.
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Commenter (0060) stated that much of the recent guidance from the EPA has not been issued 

expeditiously or timely and this creates significant issues for air agencies that find it necessary to 

use their knowledge, expertise and experience to develop plans to only find out at the last 

moment the approach doesn't meet the needs outlined in less than forth-coming guidance. 

Commenter (0060) provided a list of more recent guidance the EPA has issued relevant to

important SIP planning activities for air agencies along with the amount of time provided

between issuing the guidance and when the SIP is due and stated these amounts of time are not 

sufficient for air agencies to perform analysis, prepare submittals and provide for the required 

comment periods and hearings before submittal.

Commenter (0060) urged the EPA not to set out such specific criteria and requirements in 

regulation for processes that can occur through the collaboration of states and EPA Regions. 

Commenter (0060) stated that this rule is far too prescriptive and cumbersome and will 

inevitably lead to valuable resources being spent assessing situations that fit the letter of the rule 

but when using common sense, judgment and expertise, would not require assessment.

Response:

The EPA believes that where there are cases where multiple states and areas are involved as it 

relates to the characterization of sources, there should be coordination among the states and the 

affected regional offices to determine what sources should be characterized to satisfy the 

requirements of the rulemaking. More detailed commentary on the need for multi-state 

coordination, particularly for assuring that any monitoring plan provide for addressing maximum 

concentration locations in the multi-state area as a whole, and the presumptive need for an area 

without such coordination to be modeled by the state having the primary source, is provided in 

section IV.E.5 of the preamble to the final rule.

The CAA requires that the EPA set NAAQS that incorporate an adequate margin of safety, and 

the CAA requires that for areas with identified violations of those NAAQS, states must develop 

plans, with EPA review, that provide for attainment. The EPA disputes the commenter’s view 

that a violation of the NAAQS does not necessarily represent a risk to public health, and the 

CAA does not authorize the EPA to follow the commenter’s recommendation to take greater 

risks of violating the NAAQS based the conservatism that the CAA requires in setting the 

NAAQS.

The EPA recognizes the need for timely guidance. While constraints on EPA resources and 

complexity of issues often delay the issuance of guidance, the EPA seeks to provide timely 

guidance as the commenter requests, and the EPA in overseeing state efforts recognizes the 

challenges states face. Development of rulemaking involves a more complicated review process 

and almost inevitably takes longer, and states that request that EPA establish requirements 

through rulemaking rather than issuing guidance memoranda should consider the potential 

impact of that request on the timeliness of the guidance. Nevertheless, in the present case, the 

EPA agreed with the majority of commenters during the White Paper phase of the development 
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of this rule that the requirements in this rule needed to be promulgated as regulatory 

requirements.

In any rulemaking such as this, the EPA must find an appropriate balance between establishing 

specific criteria, which often helps assure national consistency, and providing flexibility to 

address disparate situations in ways that the particular circumstances warrant. The appropriate 

balance depends on the degree to which specific criteria can be crafted to address a wide range of 

circumstances, and also depend on the degree to which flexibility can be incorporated without 

risk of creating fundamental inconsistencies in how similar situations are addressed. The EPA 

believes that this rule sets proper criteria for establishment and implementation of requirements 

for air quality characterization, while providing suitable flexibility and exercise of judgment as to 

how these requirements are met.

C. Organization of the proposal

Comment:

Commenter (0085) noted that EPA provides the list of topics as they are organized in this 

Proposed Rule and encouraged EPA to take this a step further and make this organized list of

topics into a regular table of contents, with page number references for each topic. Commenter 

(0085) states that this would ensure ease in finding topics in what are often times lengthy rules, 

26 pages in the Federal Register in this case, but a couple of hundred pages in other cases.

Response: 

The EPA appreciates the comments and has taken this suggestion into consideration. However, 

the EPA does not know the page numbering for the published version of the notice at the time 

that it signs the notice, and therefore, the EPA cannot prepare the table of contents in the format 

that the commenter requests.