Tuesday, 

June 22, 2010 

Part II 

Environmental 

Protection Agency 

40 CFR Parts 50, 53, and 58 

Primary National Ambient Air Quality 

Standard for Sulfur Dioxide; Final Rule 


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35520 Federal Register / Vol. 75, No. 119 / Tuesday, June 22, 2010 / Rules and Regulations 

ENVIRONMENTAL PROTECTION 

AGENCY 

40 CFR Parts 50, 53, and 58 

[EPA–HQ–OAR–2007–0352; 9160–4] 

RIN 2060–A048 

Primary National Ambient Air Quality 

Standard for Sulfur Dioxide 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Final rule. 

SUMMARY: Based on its review of the air 

quality criteria for oxides of sulfur and 

the primary national ambient air quality 

standard (NAAQS) for oxides of sulfur 

as measured by sulfur dioxide (SO2), 

EPA is revising the primary SO2

NAAQS to provide requisite protection 

of public health with an adequate 

margin of safety. Specifically, EPA is 

establishing a new 1-hour SO2 standard 

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

based on the 3-year average of the 

annual 99th percentile of 1-hour daily 

maximum concentrations. The EPA is 

also revoking both the existing 24-hour 

and annual primary SO2 standards. 

DATES: This final rule is effective on 

August 23, 2010. 

ADDRESSES: EPA has established a 

docket for this action under Docket ID 

No. EPA–HQ–OAR–2007–0352. All 

documents in the docket are listed on 

the http://www.regulations.gov Web 

site. Although listed in the index, some 

information is not publicly available, 

e.g., confidential business information 

or other information whose disclosure is 

restricted by statute. Certain other 

material, such as copyrighted material, 

will be publicly available only in hard 

copy form. Publicly available docket 

materials are available either 

electronically through http:// 

www.regulations.gov or in hard copy at 

the Air and Radiation Docket and 

Information Center, EPA/DC, EPA West, 

Room 3334, 1301 Constitution Ave., 

NW., Washington, DC. The Public 

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

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

excluding legal holidays. The telephone 

number for the Public Reading Room is 

(202) 566–1744 and the telephone 

number for the Air and Radiation 

Docket and Information Center is (202) 

566–1742. 

FOR FURTHER INFORMATION CONTACT: Dr. 

Michael J. Stewart, Health and 

Environmental Impacts Division, Office 

of Air Quality Planning and Standards, 

U.S. Environmental Protection Agency, 

Mail code C504–06, Research Triangle 

Park, NC 27711; telephone: 919–541– 

7524; fax: 919–541–0237; e-mail: 

stewart.michael@epa.gov. 

SUPPLEMENTARY INFORMATION: 

Table of Contents 

The following topics are discussed in 

this preamble: 

I. Background 

A. Summary of Revisions to the SO2

Primary NAAQS 

B. Statutory Requirements 

C. Related SO2 Control Programs 

D. History of Reviews of the Primary 

NAAQS for Sulfur Oxides 

E. Summary of Proposed Revisions to the 

SO2 Primary NAAQS 

F. Organization and Approach to Final SO2

Primary NAAQS Decisions 

II. Rationale for Decisions on the Primary 

Standards 

A. Characterization of SO2 Air Quality 

1. Anthropogenic Sources and Current 

Patterns of SO2 Air Quality 

2. SO2 Monitoring 

B. Health Effects Information 

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

Exposure and Respiratory Morbidity 

Effects 

a. Adversity of Short-Term Respiratory 

Morbidity Effects 

2. Health Effects and Long-Term Exposures 

to SO2

3. SO2-Related Impacts on Public Health 

C. Human Exposure and Health Risk 

Characterization 

D. Approach for Determining Whether To 

Retain or Revise the Current Standards 

E. Adequacy of the Current Standards 

1. Rationale for Proposed Decision 

2. Comments on the Adequacy of the 

Current Standards 

a. Comments on EPA’s Interpretation of the 

Epidemiologic Evidence 

b. Comments on EPA’s Interpretation of the 

Controlled Human Exposure Evidence 

c. Comments on EPA’s Characterization of 

SO2-Associated Exposures and Health 

Risks 

3. Conclusions Regarding the Adequacy of 

the Current 24-Hour and Annual 

Standards 

F. Conclusions on the Elements of a New 

Short-Term Standard 

1. Indicator 

a. Rationale for Proposed Decision 

b. Comments on Indicator 

c. Conclusions on Indicator 

2. Averaging Time 

a. Rationale for Proposed Decision 

b. Comments on Averaging Time 

c. Conclusions on Averaging Time 

3. Form 

a. Rationale for Proposed Decision 

b. Comments on Form 

c. Conclusions on Form 

4. Level 

a. Rationale for Proposed Decision 

b. Comments on Level 

c. Conclusions on Level 

5. Retaining or Revoking the Current 24- 

Hour and Annual Standards 

a. Rationale for Proposed Decision 

b. Comments on Retaining or Revoking the 

Current 24-Hour and Annual Standards 

c. Conclusions on Retaining or Revoking 

the Current 24-Hour and Annual 

Standards 

G. Summary of Decisions on Primary 

Standards 

III. Overview of the Approach for Monitoring 

and Implementation 

IV. Amendments to Ambient Monitoring and 

Reporting Requirements 

A. Monitoring Methods 

1. Requirements for SO2 Federal Reference 

Method (FRM) 

a. Proposed Ultraviolet Fluorescence SO2

FRM and Implementation 

b. Public Comments 

c. Conclusions on Ultraviolet Fluorescence 

SO2 FRM and Implementation 

2. Requirements for Automated SO2

Methods 

a. Proposed Performance Specifications for 

Automated Methods 

b. Public Comments 

c. Conclusions for Performance 

Specifications for SO2 Automated 

Methods 

B. Network Design 

1. Approach for Network Design 

a. Proposed Approach for Network Design 

b. Alternative Network Design 

c. Public Comments 

2. Modeling Ambient SO2 Concentrations 

3. Monitoring Objectives 

a. Proposed Monitoring Objectives 

b. Public Comments 

c. Conclusions on Monitoring Objectives 

4. Final Monitoring Network Design 

5. Population Weighted Emissions Index 

a. Proposed Use of the Population 

Weighted Emissions Index 

b. Public Comments 

c. Conclusions on the Use of the 

Population Weighted Emissions Index 

6. Regional Administrator Authority 

a. Proposed Regional Administrator 

Authority 

b. Public Comments 

c. Conclusions on Regional Administrator 

Authority 

7. Monitoring Network Implementation 

a. Proposed Monitoring Network 

Implementation 

b. Public Comments 

c. Conclusions on Monitoring Network 

Implementation 

C. Data Reporting 

1. Proposed Data Reporting 

2. Public Comments 

3. Conclusions on Data Reporting 

V. Initial Designation of Areas for the 1-Hour 

SO2 NAAQS 

A. Clean Air Act Requirements 

1. Approach Described in Proposal 

2. Public Comments 

B. Expected Designations Process 

VI. Clean Air Act Implementation 

Requirements 

A. How This Rule Applies to Tribes 

B. Nonattainment Area Attainment Dates 

1. Attaining the NAAQS 

2. Consequences of a Nonattainment Area 

Failing To Attain by the Statutory 

Attainment Date 

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

Maintenance/Infrastructure 

Requirements 

1. Section 110(a)(1)–(2) Submission 


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1The legislative history of section 109 indicates 

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

maximum permissible ambient air level * * * 

which will protect the health of any [sensitive] 

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

‘‘reference should be made to a representative 

sample of persons comprising the sensitive group 

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

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

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

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

only average healthy individuals, but also ‘sensitive 

citizens’—children, for example, or people with 

asthma, emphysema, or other conditions rendering 

them particularly vulnerable to air pollution. If a 

pollutant adversely affects the health of these 

sensitive individuals, EPA must strengthen the 

entire national standard.’’); Coalition of Battery 

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

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

2EPA is currently conducting a separate review 

of the secondary SO2 NAAQS jointly with a review 

of the secondary NO2 NAAQS (see http:// 

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

index.html for more information). 

D. Attainment Planning Requirements 

1. SO2 Nonattainment Area SIP 

Requirements 

2. New Source Review and Prevention of 

Significant Deterioration Requirements 

3. General Conformity 

E. Transition From the Existing SO2

NAAQS to a Revised SO2 NAAQS 

VII. Appendix T—Interpretation of the 

Primary NAAQS for Oxides of Sulfur 

and Revisions to the Exceptional Events 

Rule 

A. Interpretation of the NAAQS for Oxides 

of Sulfur 

1. Proposed Interpretation of the Standard 

2. Comments on Interpretation of the 

Standard 

3. Conclusions on Interpretation of the 

Standard 

B. Exceptional Events Information 

Submission Schedule 

VIII. Communication of Public Health 

Information 

IX. Statutory and Executive Order Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review 

B. Paperwork Reduction Act 

C. Regulatory Flexibility Act 

D. Unfunded Mandates Reform Act 

E. Executive Order 13132: Federalism 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

G. Executive Order 13045: Protection of 

Children From Environmental Health & 

Safety Risks 

H. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution or Use 

I. National Technology Transfer and 

Advancement Act 

J. Executive Order 12898: Federal Actions 

To Address Environmental Justice in 

Minority Populations and Low-Income 

Populations 

References 

I. Background 

A. Summary of Revisions to the SO2

Primary NAAQS 

Based on its review of the air quality 

criteria for oxides of sulfur and the 

primary national ambient air quality 

standard (NAAQS) for oxides of sulfur 

as measured by sulfur dioxide (SO2), 

EPA is making revisions to the primary 

SO2 NAAQS so the standards are 

requisite to protect public health with 

an adequate margin of safety, as 

appropriate under section 109 of the 

Clean Air Act (Act or CAA). 

Specifically, EPA is replacing the 

current 24-hour and annual standards 

with a new short-term standard based 

on the 3-year average of the 99th 

percentile of the yearly distribution of 

1-hour daily maximum SO2

concentrations. EPA is setting the level 

of this new standard at 75 ppb. EPA is 

adding data handling conventions for 

SO2 by adding provisions for this new 

1-hour primary standard. EPA is also 

establishing requirements for an SO2

monitoring network. These new 

provisions require monitors in areas 

where there is an increased coincidence 

of population and SO2 emissions. EPA 

is also making conforming changes to 

the Air Quality Index (AQI). 

B. Statutory Requirements 

Two sections of the Clean Air Act 

(Act or CAA) govern the establishment 

and revision of National Ambient Air 

Quality Standards NAAQS. Section 108 

of the Act directs the Administrator to 

identify and list air pollutants that meet 

certain criteria, including that the air 

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

contribute[s] to air pollution which may 

reasonably be anticipated to endanger 

public health and welfare’’ and ‘‘the 

presence of which in the ambient air 

results from numerous or diverse mobile 

or stationary sources.’’ CAA section 

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

pollutants listed, section 108 requires 

the Administrator to issue air quality 

criteria that ‘‘accurately reflect the latest 

scientific knowledge useful in 

indicating the kind and extent of all 

identifiable effects on public health or 

welfare which may be expected from the 

presence of [a] pollutant in ambient air 

***’’ Section 108(a)(2). 

Section 109(a) of the Act directs the 

Administrator to promulgate ‘‘primary’’ 

and ‘‘secondary’’ NAAQS for pollutants 

for which air quality criteria have been 

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

primary standard as one ‘‘the attainment 

and maintenance of which in the 

judgment of the Administrator, based on 

[the air quality] criteria and allowing an 

adequate margin of safety, are requisite 

to protect the public health.’’ 1 Section 

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

must ‘‘specify a level of air quality the 

attainment and maintenance of which, 

in the judgment of the Administrator, 

based on [the air quality] criteria, is 

requisite to protect the public welfare 

from any known or anticipated adverse 

effects associated with the presence of 

such pollutant in the ambient air.’’ 2

Section 109(b)(2) This rule concerns 

exclusively the primary NAAQS for 

oxides of sulfur. 

The requirement that primary 

standards include an adequate margin of 

safety is intended to address 

uncertainties associated with 

inconclusive scientific and technical 

information available at the time of 

standard setting. It is also intended to 

provide a reasonable degree of 

protection against hazards that research 

has not yet identified. Lead Industries 

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

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

1042 (1980); American Petroleum 

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

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

1034 (1982). Both kinds of uncertainties 

are components of the risk associated 

with pollution at levels below those at 

which human health effects can be said 

to occur with reasonable scientific 

certainty. Thus, in selecting primary 

standards that include an adequate 

margin of safety, the Administrator is 

seeking not only to prevent pollution 

levels that have been demonstrated to be 

harmful but also to prevent lower 

pollutant levels that may pose an 

unacceptable risk of harm, even if the 

risk is not precisely identified as to 

nature or degree. The CAA does not 

require the Administrator to establish a 

primary NAAQS at a zero-risk level or 

at background concentration levels, see 

Lead Industries Association v. EPA, 647 

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

that reduces risk sufficiently so as to 

protect public health with an adequate 

margin of safety. 

In addressing the requirement for a 

margin of safety, EPA considers such 

factors as the nature and severity of the 

health effects involved, the size of the 

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

degree of the uncertainties that must be 

addressed. The selection of any 

particular approach to providing an 

adequate margin of safety is a policy 

choice left specifically to the 

Administrator’s judgment. Lead 

Industries Association v. EPA, 647 F.2d 

at 1161–62. 

In setting standards that are 

‘‘requisite’’ to protect public health and 

welfare, as provided in section 109(b), 

EPA’s task is to establish standards that 

are neither more nor less stringent than 

necessary for these purposes. In so 

doing, EPA may not consider the costs 

of implementing the standards. 

Whitman v. American Trucking 


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

(2001). 

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

the Administrator to periodically 

undertake a thorough review of the air 

quality criteria published under section 

108 and the NAAQS and to revise the 

criteria and standards as may be 

appropriate. The Act also requires the 

Administrator to appoint an 

independent scientific review 

committee composed of seven members, 

including at least one member of the 

National Academy of Sciences, one 

physician, and one person representing 

State air pollution control agencies, to 

review the air quality criteria and 

NAAQS and to ‘‘recommend to the 

Administrator any new * * * standards 

and revisions of existing criteria and 

standards as may be appropriate under 

section 108 and subsection (b) of this 

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

independent review function is 

performed by the Clean Air Scientific 

Advisory Committee (CASAC) of EPA’s 

Science Advisory Board. 

C. Related SO2 Control Programs 

States are primarily responsible for 

ensuring attainment and maintenance of 

ambient air quality standards once EPA 

has established them. Under section 110 

of the Act, and related provisions, States 

are to submit, for EPA approval, State 

implementation plans (SIPs) that 

provide for the attainment and 

maintenance of such standards through 

control programs directed to sources of 

the pollutants involved. The States, in 

conjunction with EPA, also administer 

the prevention of significant 

deterioration program that covers these 

pollutants. See CAA sections 160–169. 

In addition, Federal programs provide 

for nationwide reductions in emissions 

of these and other air pollutants through 

the Federal motor vehicle and motor 

vehicle fuel control program under title 

II of the Act (CAA sections 202–250) 

which involves controls for emissions 

from all moving sources and controls for 

the fuels used by these sources; new 

source performance standards under 

section 111; and title IV of the Act (CAA 

sections 402–416), which specifically 

provides for major reductions in SO2

emissions. EPA has also promulgated 

the Clean Air Interstate Rule (CAIR) to 

require additional SO2 emission 

reductions needed in the eastern half of 

the United States to address emissions 

which contribute significantly to 

nonattainment with, or interfere with 

maintenance of, the PM NAAQS by 

downwind States in the CAIR region. 

This rule was remanded by the DC 

Circuit, and although it remains in 

effect, EPA is reevaluating it pursuant to 

the court remand. 

Currently, there are several areas 

designated as being in nonattainment of 

the primary SO2 NAAQS (see section 

VI). Moreover, as a result of this final 

rule, additional areas could be classified 

as non-attainment. Certain States would 

then be required to develop SIPs that 

identify and implement specific air 

pollution control measures to reduce 

ambient SO2 concentrations to attain 

and maintain the revised SO2 NAAQS, 

most likely by requiring air pollution 

controls on sources that emit oxides of 

sulfur (SOx). 

D. History of Reviews of the Primary 

NAAQS for Sulfur Oxides 

On April 30, 1971, the EPA 

promulgated primary SO2 NAAQS (36 

FR 8187). These primary standards, 

which were based on the findings 

outlined in the original 1969 Air Quality 

Criteria for Sulfur Oxides, were set at 

0.14 parts per million (ppm) averaged 

over a 24-hour period, not to be 

exceeded more than once per year, and 

0.030 ppm annual arithmetic mean. In 

1982, EPA published the Air Quality 

Criteria for Particulate Matter and Sulfur 

Oxides (EPA, 1982) along with an 

addendum of newly published 

controlled human exposure studies, 

which updated the scientific criteria 

upon which the initial standards were 

based (EPA, 1982). In 1986, EPA 

published a second addendum 

presenting newly available evidence 

from epidemiologic and controlled 

human exposure studies (EPA, 1986). In 

1988, EPA published a proposed 

decision not to revise the existing 

standards (53 FR 14926) (April 26, 

1988). However, EPA specifically 

requested public comment on the 

alternative of revising the current 

standards and adding a new 1-hour 

primary standard of 0.4 ppm (400 ppb) 

to protect asthmatics against 5–10 

minute peak SO2 concentrations. 

As a result of public comments on the 

1988 proposal and other post-proposal 

developments, EPA published a second 

proposal on November 15, 1994 (59 FR 

58958). The 1994 re-proposal was based 

in part on a supplement to the second 

addendum of the criteria document, 

which evaluated new findings on 5–10 

minute SO2 exposures in asthmatics 

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

1988 proposal, EPA proposed to retain 

the existing 24-hour and annual 

standards. EPA also solicited comment 

on three regulatory alternatives to 

further reduce the health risk posed by 

exposure to high 5-minute peaks of SO2

if additional protection were judged to 

be necessary. The three alternatives 

were: (1) Revising the existing primary 

SO2 NAAQS by adding a new 5-minute 

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

establishing a new regulatory program 

under section 303 of the Act to 

supplement protection provided by the 

existing NAAQS, with a trigger level of 

0.6 ppm (600 ppb) SO2, one expected 

exceedance; and (3) augmenting 

implementation of existing standards by 

focusing on those sources or source 

types likely to produce high 5-minute 

peak concentrations of SO2. 

On May 22, 1996, EPA announced its 

final decision not to revise the NAAQS 

for SOx (61 FR 25566). EPA found that 

asthmatics—a susceptible population 

group—could be exposed to short-term 

SO2 bursts resulting in repeated 

‘exposure events’ such that tens or 

hundreds of thousands of asthmatics 

could be exposed annually to lung 

function effects ‘‘distinctly exceeding 

* * * [the] typical daily variation in 

lung function’’ that asthmatics routinely 

experience, and found further that 

repeated occurrences should be 

regarded as significant from a public 

health standpoint. 61 FR at 25572, 

25573. Nonetheless, the agency 

concluded that ‘‘the likelihood that 

asthmatic individuals will be exposed 

* * * is very low when viewed from a 

national perspective’’, that ‘‘5-minute 

peak SO[2] levels do not pose a broad 

public health problem when viewed 

from a national perspective’’, and that 

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

do not constitute the type of ubiquitous 

public health problem for which 

establishing a NAAQS would be 

appropriate.’’ Id. at 25575. EPA 

concluded, therefore, that it would not 

revise the existing standards or add a 

standard to specifically address 5- 

minute exposures. EPA also announced 

an intention to propose guidance, under 

section 303 of the Act, to assist States 

in responding to short-term peaks of 

SO2 and later initiated a rulemaking to 

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

The American Lung Association and 

the Environmental Defense Fund 

challenged EPA’s decision not to 

establish a 5-minute standard. On 

January 30, 1998, the Court of Appeals 

for the District of Columbia Circuit 

found that EPA had failed to adequately 

explain its determination that no 

revision to the SO2 NAAQS was 

appropriate and remanded the 

determination back to EPA for further 

explanation. American Lung Ass’n v. 

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

Specifically, the court held that EPA 

had failed to adequately explain the 

basis for its conclusion that short-term 

SO2 exposures to asthmatics do not 

constitute a public health problem, 


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noting that the agency had failed to 

explain the link between its finding that 

repeated short-term exposures were 

significant, and that there would be tens 

to hundreds of thousands of such 

exposures annually to a susceptible 

subpopulation. 134 F. 3d at 392. The 

court also rejected the explanation that 

short-term SO2 bursts were ‘‘localized, 

infrequent, and site-specific’’ as a 

rational basis for the conclusion that no 

public health problem existed for 

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

the Final Decision explains why 

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

‘infrequent’ events might nevertheless 

create a public health problem, 

particularly since, in some sense, all 

pollution is local and site-specific 

***’’. Id. The court accordingly 

remanded the case to EPA to adequately 

explain its determination or otherwise 

take action in accordance with the 

opinion. In response, EPA has collected 

and analyzed additional air quality data 

focused on 5-minute concentrations of 

SO2. These air quality analyses 

conducted since the last review helped 

inform the current review, which 

(among other things) address the issues 

raised in the court’s remand of the 

Agency’s last decision. 

EPA formally initiated the current 

review of the air quality criteria for 

oxides of sulfur and the SO2 primary 

NAAQS on May 15, 2006 (71 FR 28023) 

with a general call for information. 

EPA’s draft Integrated Review Plan for 

the Primary National Ambient Air 

Quality Standards for Sulfur Dioxide 

(EPA, 2007a) was made available in 

April 2007 for public comment and was 

discussed by the CASAC via a publicly 

accessible teleconference on May 11, 

2007. As noted in that plan, SOX

includes multiple gaseous (e.g., SO3) 

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

Because the health effects associated 

with particulate species of SOX have 

been considered within the context of 

the health effects of ambient particles in 

the Agency’s review of the NAAQS for 

particulate matter (PM), the current 

review of the primary SO2 NAAQS is 

focused on the gaseous species of SOX

and does not consider health effects 

directly associated with particulate 

species. 

The first draft of the Integrated 

Science Assessment for Oxides of 

Sulfur-Health Criteria (ISA) and the 

Sulfur Dioxide Health Assessment Plan: 

Scope and Methods for Exposure and 

Risk Assessment (EPA, 2007b) were 

reviewed by CASAC at a public meeting 

held on December 5–6, 2007. Based on 

comments received from CASAC and 

from the public, EPA developed the 

second draft of the ISA and the first 

draft of the Risk and Exposure 

Assessment to Support the Review of 

the SO2 Primary National Ambient Air 

Quality Standard (Risk and Exposure 

Assessment (REA)). These documents 

were reviewed by CASAC at a public 

meeting held on July 30–31, 2008. Based 

on comments received from CASAC and 

the public at this meeting, EPA released 

the final ISA in September of 2008 

(EPA, 2008a; henceforth referred to as 

ISA). In addition, comments received 

were considered in developing the 

second draft of the REA. Importantly, 

the second draft of the REA contained 

a draft staff policy assessment that 

considered the evidence presented in 

the final ISA and the air quality, 

exposure, and risk characterization 

results presented in the second draft 

REA, as they related to the adequacy of 

the current SO2 NAAQS and potential 

alternative primary SO2 standards. This 

document was reviewed by CASAC at a 

public meeting held on April 16–17, 

2009. In preparing the final REA report, 

which included the final staff policy 

assessment, EPA considered comments 

received from CASAC and the public at 

and subsequent to that meeting. The 

final REA containing the final staff 

policy assessment was completed in 

August 2009 (EPA 2009a; henceforth 

referred to as REA)). 

On December 8, 2009 EPA published 

its proposed revisions to the primary 

SO2 NAAQS. 74 FR 64810 presented a 

number of conclusions, findings, and 

determinations proposed by the 

Administrator. EPA invited general, 

specific, and/or technical comments on 

all issues involved with this proposal, 

including all such proposed judgments, 

conclusions, findings, and 

determinations. EPA invited specific 

comment on the level, or range of levels, 

appropriate for such a standard, as well 

as on the rationale that would support 

that level or range of levels. These 

comments were carefully considered by 

the Administrator as she made her final 

decisions, as described in this notice, on 

the primary SO2 NAAQS 

The schedule for completion of this 

review is governed by a judicial order 

resolving a lawsuit filed in September 

2005, concerning the timing of the 

current review. Center for Biologic 

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

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

governs this review, entered by the 

court in August 2007 and amended in 

December 2008, provides that the 

Administrator will sign, for publication, 

a final rulemaking concerning the 

review of the primary SO2 NAAQS no 

later than June 2, 2010. 

E. Summary of Proposed Revisions to 

the SO2 Primary NAAQS 

For the reasons discussed in the 

preamble of the proposal for the SO2

primary NAAQS, EPA proposed to make 

revisions to the primary SO2 NAAQS 

(and to add SO2 data handling 

conventions) so the standards provide 

requisite protection of public health 

with an adequate margin of safety. 

Specifically, EPA proposed to replace 

the current 24-hour and annual 

standards with a new short-term SO2

standard. EPA proposed that this new 

short-term standard would be based on 

the 3-year average of the 99th percentile 

(or 4th highest) of the yearly 

distribution of 1-hour daily maximum 

SO2 concentrations. EPA proposed to set 

the level of this new 1-hour standard 

within the range of 50 to 100 ppb and 

solicited comment on standard levels as 

high as 150 ppb. EPA also proposed to 

establish requirements for an SO2

monitoring network at locations where 

maximum SO2 concentrations are 

expected to occur and to add a new 

Federal Reference Method (FRM) for 

measuring SO2 in the ambient air. 

Finally, EPA proposed to make 

corresponding changes to the Air 

Quality Index for SO2. 

F. Organization and Approach to Final 

SO2 Primary NAAQS Decisions 

This action presents the 

Administrator’s final decisions 

regarding the need to revise the current 

SO2 primary NAAQS, and what those 

revisions should be. Revisions to the 

primary NAAQS for SO2, and the 

rationale supporting those revisions, are 

described below in section II. 

An overview of the approach for 

monitoring and implementation is 

presented in section III. Requirements 

for the SO2 ambient monitoring network 

and for a new, additional FRM for 

measuring SO2 in the ambient air are 

described in section IV. EPA’s current 

plans for designations and for 

implementing the revised SO2 primary 

NAAQS are discussed in sections V and 

VI respectively. Related requirements 

for data completeness, data handling, 

data reporting, rounding conventions, 

and exceptional events are described in 

section VII. Communication of public 

health information through the AQI is 

discussed in section VIII. A recitation of 

statutory authority and a discussion of 

those executive order reviews which are 

relevant are provided in section IX. 

Today’s final decisions are based on 

a thorough review in the ISA of 

scientific information on known and 

potential human health effects 

associated with exposure to SO2 in the 


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

2007 of the approximately 500 SO2 monitors, and 

not the same sites in all years, voluntarily reported 

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

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

averages in each hour for at least part of the time 

between 1997 and 2007. The remainder reported 

only the maximum 5-minute average in each hour. 

air. These final decisions also take into 

account: (1) Assessments in the REA of 

the most policy-relevant information in 

the ISA as well as quantitative exposure 

and risk analyses based on that 

information; (2) CASAC Panel advice 

and recommendations, as reflected in its 

letters to the Administrator and its 

public discussions of the ISA and REA; 

(3) public comments received during the 

development of the ISA and REA; and 

(4) public comments received on EPA’s 

notice of proposed rulemaking. 

II. Rationale for Decisions on the 

Primary Standards 

This section presents the rationale for 

the Administrator’s decision to revise 

the existing SO2 primary standards by 

replacing the current 24-hour and 

annual standards with a new 1-hour SO2

standard at a level of 75 ppb, based on 

the 3-year average of the annual 99th 

percentile of 1-hour daily maximum 

concentrations. As discussed more fully 

below, this rationale takes into account: 

(1) Judgments and conclusions 

presented in the ISA and the REA; (2) 

CASAC advice and recommendations as 

reflected in the CASAC panel’s 

discussions of drafts of the ISA and REA 

at public meetings, in separate written 

comments, and in letters to the 

Administrator (Henderson 2008a; 

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

public comments received at CASAC 

meetings during the development of the 

ISA and the REA; and (4) public 

comments received on the notice of 

proposed rulemaking. 

In reaching this decision, EPA has 

drawn upon an integrative synthesis of 

the entire body of evidence on human 

health effects associated with the 

presence of SO2 in the ambient air, and 

upon the results of the quantitative 

exposure and risk assessments reflecting 

this evidence. As discussed below, this 

body of evidence addresses a broad 

range of health endpoints associated 

with exposure to SO2 in the ambient air. 

In considering this entire body of 

evidence, EPA chose to focus most on 

those health endpoints for which the 

ISA found the strongest evidence of an 

association with SO2 (see section II.B 

below). Thus, the rationale for this final 

decision on the SO2 NAAQS focused 

primarily on respiratory morbidity 

following short-term (5-minutes to 24- 

hours) exposure to SO2, for which the 

ISA found a causal relationship. 

As discussed below, a substantial 

amount of new research has been 

conducted since EPA’s last review of the 

SO2 NAAQS, with important new 

information coming from epidemiologic 

studies in particular. In addition to the 

substantial amount of new 

epidemiologic research, the ISA 

considered a limited number of new 

controlled human exposure studies and 

re-evaluated key older controlled 

human exposure studies. In evaluating 

both the new and key older controlled 

human exposure studies, the ISA 

utilized updated guidelines published 

by the American Thoracic Society (ATS) 

on what constitutes an adverse effect of 

air pollution (see ISA, section 3.1.3; p. 

3–4). Importantly, all controlled human 

exposure and epidemiologic studies 

evaluated in the ISA have undergone 

intensive scrutiny through multiple 

layers of peer review and opportunities 

for public review and comment. Thus, 

the review of this information has been 

extensive and deliberate. 

After a background discussion of the 

principal emitting sources and current 

patterns of SO2 air quality and a 

description of the current SO2

monitoring network from which those 

air quality patterns are obtained (section 

II.A), the remainder of this section 

discusses the Administrator’s rationale 

for her final decisions on the primary 

standards. Section II.B includes an 

overview of the scientific evidence 

related to the respiratory effects 

associated with ambient SO2 exposure. 

This overview includes a discussion of 

the at-risk populations considered in the 

ISA. Section II.C summarizes the key 

approaches taken by EPA to assess 

exposures and health risks associated 

with exposure to ambient SO2. Section 

II.D summarizes the approach that was 

used in the current review of the SO2

NAAQS with regard to consideration of 

the scientific evidence and the air 

quality, exposure, and risk-based results 

related to the adequacy of the current 

standards and potential alternative 

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

respectively, the Administrator’s 

decisions regarding the adequacy of the 

current standards and the elements of a 

new short-term standard, taking into 

consideration public comments on the 

proposed decisions. Section II.G 

summarizes the Administrator’s 

decisions with regard to the SO2

primary NAAQS. 

A. Characterization of SO2 Air Quality 

1. Anthropogenic Sources and Current 

Patterns of SO2 Air Quality 

Anthropogenic SO2 emissions 

originate chiefly from point sources, 

with fossil fuel combustion at electric 

utilities (∼66%) and other industrial 

facilities (∼29%) accounting for the 

majority of total emissions (ISA, section 

2.1). Other anthropogenic sources of 

SO2 include both the extraction of metal 

from ore as well as the burning of high 

sulfur-containing fuels by locomotives, 

large ships, and equipment utilizing 

diesel engines. SO2 emissions and 

ambient concentrations follow a strong 

east to west gradient due to the large 

numbers of coal-fired electric generating 

units in the Ohio River Valley and 

upper Southeast regions. In the 12 

Consolidated Metropolitan Statistical 

Areas (CMSAs) that had at least four 

SO2 regulatory monitors from 2003– 

2005, 24-hour average concentrations in 

the continental U.S. ranged from a 

reported low of ∼1 ppb in Riverside, CA 

and San Francisco, CA to a high of ∼12 

ppb in Pittsburgh, PA and Steubenville, 

OH (ISA, section 2.5.1). In addition, 

outside or inside all CMSAs from 2003– 

2005, the annual average SO2

concentration was 4 ppb (ISA, Table 2– 

8). However, spikes in hourly 

concentrations occurred. The mean 1- 

hour maximum concentration outside or 

inside CMSAs was 13 ppb, with a 

maximum value of greater than 600 ppb 

outside CMSAs and greater than 700 

ppb inside CMSAs (ISA, Table 2–8). 

Temporal and spatial patterns of 5- 

minute peaks of SO2 are also important 

given that controlled human exposure 

studies have demonstrated that 

exposure to these peaks can result in 

adverse respiratory effects in exercising 

asthmatics (see section II.B below). For 

those monitors which voluntarily 

reported 5-minute block average data,3

when maximum 5-minute 

concentrations were reported, the 

absolute highest concentration over the 

ten-year period exceeded 4000 ppb, but 

for all individual monitors, the 99th 

percentile was below 200 ppb (ISA, 

section 2.5.2 Table 2–10). Median 

concentrations from these monitors 

reporting 5-minute data ranged from 

1 ppb to 8 ppb, and the average for each 

maximum 5-minute level ranged from 

3 ppb to 17 ppb. Delaware, 

Pennsylvania, Louisiana, and West 

Virginia had mean values for maximum 

5-minute data exceeding 10 ppb. Among 

aggregated within-State data for the 16 

monitors from which all 5-minute 

average intervals were reported, the 

median values ranged from 1 ppb to 5 

ppb, and the means ranged from 3 ppb 

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

The highest reported concentration was 

921 ppb, but the 99th percentile values 


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

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

relationship between relevant pollutant exposures 

and the health outcome. That is, a positive 

association has been observed between the 

pollutant and the outcome in studies in which 

chance, bias, and confounding could be ruled out 

with reasonable confidence. Evidence includes, for 

example, controlled human exposure studies; or 

observational studies that cannot be explain by 

plausible alternatives or are supported by other 

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

of action information). Evidence includes replicated 

and consistent high-quality studies by multiple 

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

for aggregated within-State data were all 

below 90 ppb (id). 

2. SO2 Monitoring 

Although EPA established the SO2

standards in 1971, uniform minimum 

monitoring network requirements for 

SO2 monitoring were only adopted in 

May 1979. From the time of the 

implementation of the 1979 monitoring 

rule through 2008, the SO2 monitoring 

network has steadily decreased in size 

from approximately 1496 sites in 1980 

to the approximately 488 sites operating 

in 2008. At present, except for SO2

monitoring required at National Core 

Monitoring Stations (NCore stations), 

there are no minimum monitoring 

requirements for SO2 in 40 CFR part 58 

Appendix D, other than a requirement 

for EPA Regional Administrator 

approval before removing any existing 

monitors and a requirement that any 

ongoing SO2 monitoring must have at 

least one monitor sited to measure the 

maximum concentration of SO2 in that 

area. EPA removed the specific 

minimum monitoring requirements for 

SO2 in the 2006 monitoring rule 

revisions, except for monitoring at 

NCore stations, based on the fact that 

there were no SO2 nonattainment areas 

at that time, coupled with trends 

showing an increasing gap between 

national average SO2 concentrations and 

the current 24-hour and annual 

standards. The rule was also intended to 

provide State, local, and Tribal air 

monitoring agencies flexibility in 

meeting perceived higher priority 

monitoring needs for other pollutants, 

or to implement the new multi-pollutant 

sites (NCore network) required by the 

2006 rule revisions (71 FR 61236, 

(October 6, 2006)). More information on 

SO2 monitoring can be found in section 

IV. 

B. Health Effects Information 

The ISA concluded that there was 

sufficient evidence to infer a ‘‘causal 

relationship’’ between respiratory 

morbidity and short-term (5-minutes to 

24-hours) exposure to SO2 (ISA, section 

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

relationship’’ is the strongest finding the 

ISA can make.4 This conclusion was 

based on the consistency, coherence, 

and plausibility of findings observed in 

controlled human exposure studies of 

5–10 minutes, epidemiologic studies 

mostly using 1-hour daily maximum 

and 24-hour average SO2

concentrations, and animal toxicological 

studies using exposures of minutes to 

hours (ISA, section 5.2). This evidence 

is briefly summarized below and 

discussed in more detail in the proposal 

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

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

judged evidence of an association 

between SO2 exposure and other health 

categories to be less convincing; other 

associations were judged to be 

suggestive but not sufficient to infer a 

causal relationship 

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

mortality) or inadequate to infer the 

presence or absence of a causal 

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

SO2 and cardiovascular morbidity, and 

long-term exposure to SO2 and 

respiratory morbidity, other morbidity, 

and mortality). Key conclusions from 

the ISA are described in greater detail in 

Table 5–3 of the ISA. 

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

Exposure and Respiratory Morbidity 

Effects 

The ISA examined numerous 

controlled human exposure studies and 

found that moderate or greater 

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

decline in Forced Expiratory Volume 

(FEV1) and/or ≥ 100% increase in 

specific airway resistance (sRaw)) occur 

in some exercising asthmatics exposed 

to SO2 concentrations as low as 

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

also found that among asthmatics, both 

the percentage of individuals affected, 

and the severity of the response 

increased with increasing SO2

concentrations. That is, at 5–10 minute 

concentrations ranging from 200–300 

ppb, the lowest levels tested in free 

breathing chamber studies, 

approximately 5–30% percent of 

exercising asthmatics experienced 

moderate or greater decrements in lung 

function (ISA, Table 3–1). At 

concentrations of 400–600 ppb, 

moderate or greater decrements in lung 

function occurred in approximately 20– 

60% of exercising asthmatics, and 

compared to exposures at 200–300 ppb, 

a larger percentage of asthmatics 

experienced severe decrements in lung 

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

and/or ≥ 200% increase in sRaw; ISA, 

Table 3–1). Moreover, at SO2

concentrations ≥ 400 ppb (5–10 minute 

exposures), moderate or greater 

decrements in lung function were often 

statistically significant at the group 

mean level and frequently accompanied 

by respiratory symptoms. Id. 

The ISA also found that in locations 

meeting the current SO2 NAAQS, 

numerous epidemiologic studies 

reported positive associations between 

ambient SO2 concentrations and 

respiratory symptoms in children, as 

well as emergency department visits 

and hospitalizations for all respiratory 

causes and asthma across multiple age 

groups. Moreover, the ISA concluded 

that these epidemiologic studies were 

consistent and coherent. This evidence 

was consistent in that associations were 

reported in studies conducted in 

numerous locations and with a variety 

of methodological approaches (ISA, 

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

that respiratory symptom results from 

epidemiologic studies of short-term 

(predominantly 1-hour daily maximum 

or 24-hour average) SO2 concentrations 

were generally in agreement with 

respiratory symptom results from 

controlled human exposure studies of 

5–10 minutes. These results were also 

coherent in that the respiratory effects 

observed in controlled human exposure 

studies of 5–10 minutes further 

provided a basis for a progression of 

respiratory morbidity that could lead to 

the increased emergency department 

visits and hospital admissions observed 

in epidemiologic studies (ISA, section 

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

that when evaluated as a whole, SO2

effect estimates in multi-pollutant 

models generally remained positive and 

relatively unchanged when copollutants were included. Therefore, 

although recognizing the uncertainties 

associated with separating the effects of 

SO2 from those of co-occurring 

pollutants, the ISA concluded that ‘‘the 

limited available evidence indicates that 

the effect of SO2 on respiratory health 

outcomes appears to be generally robust 

and independent of the effects of 

gaseous co-pollutants, including NO2

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

(ISA, section 5.3; p. 5–9). 

The ISA also found that the 

respiratory effects of SO2 were 

consistent with the mode of action as it 

is currently understood from animal 

toxicological and controlled human 

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

2). The immediate effect of SO2 on the 

respiratory system is 

bronchoconstriction. This response is 

mediated by chemosensitive receptors 

in the tracheobronchial tree. Activation 

of these receptors triggers central 

nervous system reflexes that result in 


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5We also note that very young children were not 

included in the controlled human exposure studies 

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

sensitive life stage is a source of uncertainty for 

children’s susceptibility to SO2. 

bronchoconstriction and respiratory 

symptoms that are often followed by 

rapid shallow breathing (id). The ISA 

noted that asthmatics are likely more 

sensitive to the respiratory effects of SO2

due to pre-existing inflammation 

associated with the disease. For 

example, pre-existing inflammation may 

lead to enhanced release of 

inflammatory mediators, and/or 

enhanced sensitization of the 

chemosensitive receptors (id). 

Taken together, the ISA concluded 

that the controlled human exposure, 

epidemiologic, and toxicological 

evidence supported its determination of 

a causal relationship between 

respiratory morbidity and short-term (5- 

minutes to 24-hours) exposure to SO2. 

a. Adversity of Short-Term Respiratory 

Morbidity Effects 

As discussed more fully in the 

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

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

based on: (1) American Thoracic Society 

(ATS) guidelines; (2) advice and 

recommendations from CASAC (see 

specific consensus CASAC comments in 

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

(3) conclusions from previous NAAQS 

reviews, EPA found that 5–10 minute 

exposures to SO2 concentrations at least 

as low as 200 ppb can result in adverse 

health effects in some asthmatics (i.e., 

5–30% of the tested individuals in 

controlled human exposure studies of 

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

concentrations ≥ 400 ppb, controlled 

human exposure studies have reported 

decrements in lung function that are 

often statistically significant at the 

group mean level, and that are 

frequently accompanied by respiratory 

symptoms. Being mindful that the ATS 

guidelines specifically indicate 

decrements in lung function with 

accompanying respiratory symptoms as 

being adverse (see proposal section 

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

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

SO2 concentrations ≥ 400 ppb can result 

in health effects that are clearly adverse. 

The ATS also indicated that exposure 

to air pollution that increases the risk of 

an adverse effect to a population is 

adverse, even though it may not 

increase the risk of any individual to an 

unacceptable level (ATS 2000; see 

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

64817). As an example, ATS states: 

A population of children with asthma 

could have a distribution of lung function 

such that no individual child has a level 

associated with significant impairment. 

Exposure to air pollution could shift the 

distribution toward lower levels without 

bringing any individual child to a level that 

is associated with clinically relevant 

consequences. Individuals within the 

population would, however, have 

diminished reserve function and are at 

potentially increased risk if affected by 

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

Assuming that the relationship between the 

risk factor and the disease is causal, the 

committee considered that such a shift in the 

risk factor distribution, and hence the risk 

profile of the exposed population, should be 

considered adverse, even in the absence of 

the immediate occurrence of frank illness 

(ATS 2000, p. 668). 

As mentioned above, the ISA reported 

that exposure to SO2 concentrations as 

low as 200–300 ppb for 5–10 minutes 

results in approximately 5–30% of 

exercising asthmatics experiencing 

moderate or greater decrements in lung 

function (defined in terms of a ≥ 15% 

decline in FEV1 or 100% increase in 

sRaw; ISA, Table 3–1). Even though 

these results were not statistically 

significant at the group mean level, in 

light of EPA’s interpretation of how to 

apply the ATS guidelines for defining 

an adverse effect, as described above, 

the REA found that these results could 

reasonably indicate an SO2-induced 

shift in these lung function 

measurements for this subset of the 

population. As a result, an appreciable 

percentage of exercising asthmatics 

exposed to SO2 concentrations as low as 

200 ppb would be expected to have 

diminished reserve lung function and 

would be expected to be at greater risk 

if affected by another respiratory agent, 

for example, viral infection. 

Importantly, as explained immediately 

above, diminished reserve lung function 

in a population that is attributable to air 

pollution is considered an adverse effect 

under ATS guidance. In addition to the 

2000 ATS guidelines, the REA was also 

mindful of previous CASAC 

recommendations (Henderson 2006) and 

NAAQS review conclusions (EPA 2006, 

EPA 2007d) indicating that moderate 

decrements in lung function can be 

clinically significant in some asthmatics 

(discussed in detail below, see section 

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

that subjects participating in these 

controlled human exposure studies do 

not include severe asthmatics and that 

it was reasonable to presume that 

persons with more severe asthma than 

the study participants would have a 

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

together, the REA concluded that 

exposure to SO2 concentrations at least 

as low as 200 ppb can result in adverse 

health effects in asthmatics and that this 

conclusion was in agreement with 

consensus CASAC comments and 

recommendations expressed during the 

current SO2 NAAQS review (see 

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

In addition to the controlled human 

exposure evidence, epidemiologic 

studies also indicate that adverse 

respiratory morbidity effects are 

associated with SO2 (REA, section 4.3). 

As mentioned above, in reaching the 

conclusion of a causal relationship 

between respiratory morbidity and 

short-term SO2 exposure, the ISA 

generally found positive associations 

between ambient SO2 concentrations 

and emergency department visits and 

hospitalizations for all respiratory 

causes and asthma. Notably, emergency 

department visits, hospitalizations, 

episodic respiratory illness, and 

aggravation of respiratory diseases (e.g. 

asthma) attributable to air pollution are 

considered adverse health effects under 

ATS guidelines. 

2. Health Effects and Long-Term 

Exposures to SO2

There were numerous studies 

published since the last review 

examining possible associations 

between long-term SO2 exposure and 

mortality and morbidity (respiratory 

morbidity, carcinogenesis, adverse 

prenatal and neonatal outcomes) 

endpoints. However, the ISA concluded 

that the evidence relating long-term 

(weeks to years) SO2 exposure to 

adverse health effects was ‘‘inadequate 

to infer the presence or absence of a 

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

That is, the ISA found the long-term 

health evidence to be of insufficient 

quantity, quality, consistency, or 

statistical power to make a 

determination as to whether SO2 was 

truly associated with these health 

outcomes (ISA, Table 1–2). 

3. SO2-Related Impacts on Public Health 

Interindividual variation in human 

responses to air pollutants indicates that 

some populations are at increased risk 

for the detrimental effects of ambient 

exposure to SO2. The NAAQS are 

intended to provide an adequate margin 

of safety for both the general population 

and susceptible populations that are 

potentially at increased risk for health 

effects in response to exposure to 

ambient air pollution (see footnote 1 

above). To facilitate the identification of 

populations at increased risk for SO2- 

related health effects, studies have 

identified factors that contribute to the 

susceptibility of individuals to SO2.

Susceptible individuals are broadly 

defined as those with a greater 


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6This aspect of susceptibility is referred to as 

vulnerability in the proposal and in the ISA. 

7The ISA cites one chamber study with 

intermittent exercise where healthy and asthmatic 

Continued 

likelihood of an adverse outcome given 

a specific exposure in comparison with 

the general population (American Lung 

Association, 2001). The susceptibility of 

an individual to SO2 can encompass a 

multitude of factors which represent 

normal developmental phases or life 

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

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

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

influence the manifestation of disease 

and also increase an individual’s 

susceptibility (American Lung 

Association, 2001). In addition, 

populations may be at increased risk to 

SO2 due to an increase in their exposure 

during certain life stages (e.g., 

childhood or old age) or as a result of 

external factors (e.g., SES) that 

contribute to an individual being 

disproportionately exposed to higher 

concentrations than the general 

population.6 It should be noted that in 

some cases specific populations may be 

affected by multiple susceptibility 

factors. For example, a population that 

is characterized as having low SES may 

have less access to healthcare resulting 

in the manifestation of a disease, which 

increases their susceptibility to SO2, 

while they may also reside in a location 

that results in disproportionately high 

exposure to SO2. 

To examine whether SO2

differentially affects certain 

populations, stratified analyses are often 

conducted in epidemiologic 

investigations to identify the presence 

or absence of effect modification. A 

thorough evaluation of potential effect 

modifiers may help identify susceptible 

populations that are at increased risk to 

SO2 exposure. These analyses are based 

on the proper identification of 

confounders and subsequent adjustment 

for them in statistical models, which 

helps separate a spurious from a true 

causal association. Although the design 

of toxicological and human clinical 

studies does not allow for an extensive 

examination of effect modifiers, the use 

of animal models of disease and the 

study of individuals with underlying 

disease or genetic polymorphisms do 

allow for comparisons between 

subgroups. Therefore, the results from 

these studies, combined with those 

results obtained through stratified 

analyses in epidemiologic studies, 

contribute to the overall weight of 

evidence for the increased susceptibility 

of specific populations to SO2. Those 

populations identified in the ISA to be 

potentially at greater risk of 

experiencing an adverse health effect 

from SO2 were described in detail in the 

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

Those with pre-existing respiratory 

disease; (2) children and older adults; 

(3) persons who spend increased time 

outdoors or at elevated ventilation rates; 

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

persons with certain genetic factors. 

As discussed in the proposal (section 

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

proportions of the U.S. population are 

likely to be at increased risk of 

experiencing SO2-related health effects. 

In the United States, approximately 7% 

of adults and 9% of children have been 

diagnosed with asthma. Notably, the 

prevalence and severity of asthma is 

higher among certain ethnic or racial 

groups such as Puerto Ricans, American 

Indians, Alaskan Natives, and African 

Americans (EPA 2008b). Furthermore, a 

higher prevalence of asthma among 

persons of lower SES and an excess 

burden of asthma hospitalizations and 

mortality in minority and inner-city 

communities have been observed (EPA, 

2008b). In addition, population groups 

based on age comprise substantial 

segments of individuals that may be 

potentially at risk for SO2-related health 

impacts. Based on U.S. census data from 

2000, about 72.3 million (26%) of the 

U.S. population are under 18 years of 

age, 18.3 million (7.4%) are under 5 

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

65 years of age or older. There is also 

concern for the large segment of the 

population that is potentially at risk to 

SO2-related health effects because of 

increased time spent outdoors at 

elevated ventilation rates (those who 

work or play outdoors). Overall, the 

considerable size of the population 

groups at risk indicates that exposure to 

ambient SO2 could have a significant 

impact on public health in the United 

States. 

C. Human Exposure and Health Risk 

Characterization 

To put judgments about SO2- 

associated health effects into a broader 

public health context, EPA has drawn 

upon the results of the quantitative 

exposure and risk assessments. 

Judgments reflecting the nature of the 

evidence and the overall weight of the 

evidence are taken into consideration in 

these quantitative exposure and risk 

assessments. These assessments include 

estimates of the likelihood that 

asthmatic children at moderate or 

greater exertion (e.g. while exercising) 

in St. Louis or Greene County, Missouri 

would experience SO2 exposures of 

potential concern. In addition, these 

analyses include an estimate of the 

number and percent of exposed 

asthmatic children in these locations 

likely to experience SO2-induced lung 

function responses (i.e., moderate or 

greater decrements in lung function 

defined in terms of sRaw or FEV1) under 

varying air quality scenarios (i.e., 

current air quality and air quality 

simulated to just meet the current or 

potential alternative standards). These 

assessments also characterize the kind 

and degree of uncertainties inherent in 

such estimates. 

As previously mentioned, the ISA 

concluded that the evidence for an 

association between respiratory 

morbidity and short-term SO2 exposure 

was ‘‘sufficient to infer a causal 

relationship’’ (ISA, section 5.2) and that 

the ‘‘definitive evidence’’ for this 

conclusion was from the results of 5–10 

minute controlled human exposure 

studies demonstrating decrements in 

lung function and/or respiratory 

symptoms in exercising asthmatics (ISA, 

section 5.2). Accordingly, the air quality 

and exposure analyses and their 

associated risk characterizations focused 

on 5-minute concentrations of SO2 in 

excess of potential health effect 

benchmark values derived from the 

controlled human exposure literature 

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

64821, and REA, section 6.2). These 

benchmark levels are not potential 

standards, but rather are SO2 exposure 

concentrations which represent 

‘‘exposures of potential concern’’ which 

are used in these analyses to estimate 

potential exposures and risks associated 

with 5-minute concentrations of SO2. 

The REA considered 5-minute 

benchmark levels of 100, 200, 300, and 

400 ppb in these analyses, but 

especially noted exceedances or 

exposures with respect to the 200 and 

400 ppb 5-minute benchmark levels. 

These benchmark levels were 

highlighted because (1) 400 ppb 

represents the lowest concentration in 

free-breathing controlled human 

exposure studies where moderate or 

greater lung function decrements 

occurred which were often statistically 

significant at the group mean level and 

were frequently accompanied by 

respiratory symptoms; and (2) 200 ppb 

is the lowest level at which moderate or 

greater decrements in lung function in 

free-breathing controlled human 

exposure studies were found in some 

individuals, although these lung 

function changes were not statistically 

significant at the group mean level. 

Notably, 200 ppb is also the lowest level 

that has been tested in free-breathing 

controlled human exposure studies 

(REA, section 4.2.2).7


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children were exposed to 100 ppb SO2 in a mixture 

with ozone and sulfuric acid. The ISA notes that 

compared to exposure to filtered air, exposure to 

the pollutant mix did not result in statistically 

significant changes in lung function or respiratory 

symptoms (ISA, section 3.1.3.4). 

8Benchmark values derived from the controlled 

human exposure literature were associated with a 

5-minute averaging time. However, as noted in 

footnote 3 above, only 98 ambient monitors located 

in 13 States from 1997–2007 reported measured 5- 

minute SO2 concentrations since such monitoring is 

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

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

and the Virgin Islands reported 1-hour SO2

concentrations over a similar time period. 

Therefore, to broaden analyses to areas where 

measured 5-minute SO2 concentrations were not 

available, the REA utilized a statistical relationship 

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

given a reported 1-hour average SO2 concentration 

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

minute SO2 concentrations, statistically estimated 

5-minute SO2 concentrations were compared to 5- 

minute potential health effect benchmark values 

(REA, chapters 7 and 8, respectively). 

9EPA recently conducted a complete quality 

assurance review of all individual subject data. The 

results of this review did not substantively change 

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

in any way affect the conclusions of the ISA (see 

Johns and Simmons, 2009). 

The REA utilized three approaches to 

characterize health risks. In the first 

approach, for each air quality scenario, 

statistically estimated 5-minute SO2

concentrations 8 and measured ambient 

5-minute SO2 concentrations were 

compared to the 5-minute potential 

health effect benchmark levels 

discussed above (REA, chapter 7). This 

air quality analysis included all 

available ambient monitoring data as 

well as a more detailed analysis in 40 

counties. The air quality analysis was 

considered a broad characterization of 

national air quality and human 

exposures that might be associated with 

these 5-minute SO2 concentrations. An 

advantage of the air quality analysis is 

its relative simplicity; however, there is 

uncertainty associated with the 

assumption that SO2 air quality can 

serve as an adequate surrogate for total 

exposure to ambient SO2. Actual 

exposures might be influenced by 

factors not considered by this approach, 

including small-scale spatial variability 

in ambient SO2 concentrations (which 

might not be represented by the current 

fixed-site ambient monitoring network) 

and spatial/temporal variability in 

human activity patterns. A more 

detailed overview of the air quality 

analysis and its associated limitations 

and uncertainties is provided in the 

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

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

respectively) and the air quality analysis 

is thoroughly described in the REA 

(chapter 7). 

In the second approach, an inhalation 

exposure model was used to generate 

more realistic estimates of personal 

exposures in asthmatics (REA, chapter 

8). This analysis estimated temporally 

and spatially variable 

microenvironmental 5-minute SO2

concentrations and simulated 

asthmatics’ contact with these pollutant 

concentrations while at moderate or 

greater exertion (i.e., while at elevated 

ventilation rates). The approach was 

designed to estimate exposures that are 

not necessarily represented by the 

existing ambient monitoring data and to 

better represent the physiological 

conditions corresponding with the 

respiratory effects reported in controlled 

human exposure studies. AERMOD, an 

EPA dispersion model, was used to 

estimate 1-hour ambient SO2

concentrations using emissions 

estimates from stationary, non-point, 

and where applicable, port sources. The 

Air Pollutants Exposure (APEX) model, 

an EPA human exposure model, was 

then used to estimate population 

exposures using the estimated hourly 

census block level SO2 concentrations. 

From the 1-hour census block 

concentrations, 5-minute maximum SO2

concentrations within each hour were 

estimated by APEX (REA, section 8.7.1) 

using the statistical relationship 

mentioned above in footnote 8. 

Estimated exposures to 5-minute SO2

levels were then compared to the 5- 

minute potential health effect 

benchmark levels discussed above. This 

approach to assessing exposures was 

more resource intensive than using 

ambient levels as an indicator of 

exposure; therefore, the final REA 

included the analysis of two locations: 

St. Louis and Greene County, MO. 

Although the geographic scope of this 

analysis was limited, the approach 

provided estimates of SO2 exposures in 

asthmatics and asthmatic children in St. 

Louis and Greene Counties, and thus 

served to complement the broader air 

quality characterization. A more 

detailed overview of this exposure 

analysis and its associated limitations 

and uncertainties is provided in the 

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

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

respectively) and the exposure analysis 

is thoroughly described in the REA 

(chapter 8). 

The third approach was a quantitative 

risk assessment. This approach 

combined results from the exposure 

analysis (i.e., the number of exposed 

total asthmatics or asthmatic children 

while at moderate or greater exertion) 

with exposure-response functions 

derived from individual level data from 

controlled human exposure studies (see 

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

estimate the percentage and number of 

exposed asthmatics and asthmatic 

children in St. Louis and Greene County 

likely to experience a moderate or 

greater lung function response (i.e., 

decrements in lung function defined in 

terms of FEV1 and sRaw) under the air 

quality scenarios mentioned above 

(REA, chapter 9). A more detailed 

overview of this analysis and its 

associated limitations and uncertainties 

is provided in the proposal (see sections 

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

at 64823, respectively) and the 

quantitative risk analysis is thoroughly 

described in the REA (chapter 9). 

Notably, for the reasons described in 

the REA (REA, section 10.3.3) and the 

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

64827), when considering the St. Louis 

and Greene County exposure and risk 

results as they relate to the adequacy of 

the current standards, the REA 

concluded that the St. Louis results 

were more informative in terms of 

ascertaining the extent to which the 

current standards protect against health 

effects linked to the various benchmarks 

(linked in turn to 5-minute SO2

exposures). The results in fact suggested 

that the current standards may not 

adequately protect public health (REA, 

section 10.3.3, p. 364). Moreover, the 

REA judged that the exposure and risk 

estimates for the St. Louis study area 

provided useful insights into exposures 

and risks for other urban areas in the 

U.S. with similar population and SO2

emissions densities (id.). For similar 

reasons, the St. Louis results were more 

informative for ascertaining the 

adequacy of the potential alternative 

standards under consideration. 

Key results of the air quality, 

exposure, and risk analyses were 

presented in the policy assessment 

chapter of the REA (chapter 10) and 

summarized in the proposal (see Tables 

2–4 in the preamble to the proposed 

rule). In considering these results, the 

proposal noted that these analyses 

support that 5-minute SO2 exposures, 

reasonably judged important from a 

public health perspective, were 

associated with air quality adjusted 

upward to simulate just meeting the 

current standards (see proposal, section 

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

these results indicated that 99th 

percentile 1-hour daily maximum 

standard levels in the range of 50–100 

ppb could substantially limit exposures 

of asthmatic children at moderate or 

greater exertion from 5-minute SO2

concentrations ≥400 ppb, and 

appreciably limit exposures of these 

children from 5-minute SO2

concentrations ≥200 ppb (REA, p. 392– 

393). Results of these analyses also 

indicated that a 1-hour standard at 150 


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ppb could still substantially limit 

exposures of asthmatic children at 

moderate or greater exertion from 5- 

minute SO2 concentrations ≥400 ppb, 

but would provide these children 

appreciably less protection from 

exposure to 5-minute SO2

concentrations ≥200 ppb (REA, p. 395– 

396). 

D. Approach for Determining Whether 

To Retain or Revise the Current 

Standards 

EPA notes that the final decision on 

retaining or revising the current primary 

SO2 standards is a public health policy 

judgment to be made by the 

Administrator. This judgment has been 

informed by a recognition that the 

available health effects evidence reflects 

a continuum consisting of ambient 

levels of SO2 at which scientists 

generally agree that health effects are 

likely to occur, through lower levels at 

which the likelihood and magnitude of 

the response become increasingly 

uncertain. The Administrator’s final 

decisions draw upon scientific 

information and analyses related to 

health effects, population exposures and 

risks; judgments about the appropriate 

response to the range of uncertainties 

that are inherent in the scientific 

evidence and analyses; and comments 

received from CASAC and the public. 

To evaluate whether the current 

primary SO2 standards are adequate or 

whether revisions are appropriate, EPA 

has used an approach in this review 

described in chapter 10 of the REA 

which builds upon the approaches used 

in reviews of other criteria pollutants, 

including the most recent reviews of the 

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

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

2005), and reflects the latest body of 

evidence and information that is 

currently available, as reflected by the 

ISA. As in other recent reviews, EPA 

considered the implications of placing 

more or less weight or emphasis on 

different aspects of the scientific 

evidence and the exposure-/risk-based 

information, recognizing that the weight 

to be given to various elements of the 

evidence and exposure/risk information 

is part of the public health policy 

judgments that the Administrator will 

make in reaching decisions on the 

standard. 

A series of general questions framed 

this approach to considering the 

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

consideration of the scientific evidence 

and exposure/risk information with 

regard to the adequacy of the current 

standards has been framed by the 

following questions: 

• To what extent does evidence that has 

become available since the last review 

reinforce or call into question evidence for 

SO2-associated effects that were identified in 

the last review? 

• To what extent has evidence for different 

health effects and/or susceptible populations 

become available since the last review? 

• To what extent have uncertainties 

identified in the last review been reduced 

and/or have new uncertainties emerged? 

• To what extent does evidence and 

exposure-/risk-based information that has 

become available since the last review 

reinforce or call into question any of the 

basic elements (indicator, averaging time, 

form, and level) of the current standard? 

To the extent that the available 

evidence and exposure-/risk-based 

information suggests it may be 

appropriate to consider revision of the 

current standards, EPA considers that 

evidence and information with regard to 

its support for consideration of a 

standard that is either more or less 

stringent than the current standards. 

This evaluation is framed by the 

following questions: 

• Is there evidence that associations, 

especially causal or likely causal 

associations, extend to ambient SO2

concentrations as low as, or lower than, the 

concentrations that have previously been 

associated with health effects? If so, what are 

the important uncertainties associated with 

that evidence? 

• Are exposures above benchmark levels 

and/or health risks estimated to occur in 

areas that meet the current standard? If so, 

are the estimated exposures and health risks 

important from a public health perspective? 

What are the important uncertainties 

associated with the estimated risks? 

To the extent that there is support for 

consideration of a revised standard, EPA 

then considers the specific elements of 

the standard (indicator, averaging time, 

form, and level) within the context of 

the currently available information. In 

so doing, the Agency addresses the 

following questions regarding the 

elements of the standard: 

• Does the evidence provide support for 

considering a different indicator for gaseous 

SOX? 

• Does the evidence provide support for 

considering different, or additional averaging 

times? 

• What ranges of levels and forms of 

alternative standards are supported by the 

evidence, and what are the associated 

uncertainties and limitations? 

• To what extent do specific averaging 

times, levels, and forms of alternative 

standards reduce the estimated exposures 

above benchmark levels and risks attributable 

to exposure to ambient SO2, and what are the 

uncertainties associated with the estimated 

exposure and risk reductions? 

The questions outlined above have 

been addressed in the REA. The 

following sections present 

considerations regarding the adequacy 

of the current standards and 

conclusions on the elements of a new 

short-term standard in terms of 

indicator, averaging time, form, and 

level. 

E. Adequacy of the Current Standards 

This section discusses considerations 

related to the decision as to whether the 

current 24-hour and annual SO2 primary 

NAAQS are requisite to protect public 

health with an adequate margin of 

safety. Specifically, section II.E.1 

provides an overview of the rationale 

supporting the Administrator’s proposal 

that the current standards do not 

provide adequate public health 

protection; section II.E.2 discusses 

public comments received on the 

adequacy of the current standards; and 

section II.E.3 discusses the 

Administrator’s final decision on 

whether the current SO2 primary 

NAAQS is requisite to protect public 

health with an adequate margin of 

safety, as required by sections 109(d) 

and (b) of the Act. 

1. Rationale for Proposed Decision 

In the proposal, the Administrator 

initially concluded that the current 24- 

hour and annual SO2 NAAQS were not 

adequate to protect public health with 

an adequate margin of safety (see 

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

reaching this conclusion, she 

considered the: (1) Scientific evidence 

and conclusions in the ISA; (2) exposure 

and risk information presented in the 

REA; (3) conclusions of the policy 

assessment chapter of the REA; and (4) 

views expressed by CASAC. These 

considerations are discussed in detail in 

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

64826) and are summarized in this 

section. 

In the proposal the Administrator 

noted the following in considering the 

adequacy of the current 24-hour and 

annual primary SO2 standards: 

• The conclusion of the ISA that the 

results of controlled human exposure 

and epidemiologic studies form a 

plausible and coherent data set that 

supports a causal relationship between 

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

exposures and adverse respiratory 

effects, and that the epidemiologic 

evidence (buttressed by the clinical 

evidence) indicates that the effects seen 

in the epidemiologic studies are 

attributable to exposure to SO2 (ISA, 

section 5.2). 

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

the epidemiologic studies, respiratory 

effects were observed in areas where the 

maximum ambient 24-h avg SO2

concentration was below the current 24- 


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h avg NAAQS level * * *.’’ (ISA, 

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

at ambient SO2 concentrations that are 

present in locations meeting the current 

24-hour NAAQS. 

• These respiratory effects also 

occurred in areas with annual air 

quality levels considerably lower than 

those allowed by the current annual 

standard, indicating that the current 

annual standard is also not providing 

protection against short-term health 

effects reported in epidemiologic 

studies (ISA, section 5.2). 

• Analyses in the REA supporting 

that 5-minute exposures, reasonably 

judged important from a public health 

perspective (i.e., respiratory effects 

judged to be adverse to the health of 

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

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

with air quality adjusted upward to 

simulate just meeting the current 24- 

hour and annual standards. 

• CASAC advice ‘‘that the current 

24-hour and annual standards are not 

adequate to protect public health, 

especially in relation to short term 

exposures to SO2 (5–10 minutes) by 

exercising asthmatics’’ (Samet, 2009, 

p. 15). 

Based on these considerations 

(discussed in more detail in the 

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

the Administrator proposed that the 

current 24-hour and annual SO2

standards are not requisite to protect 

public health with an adequate margin 

of safety against adverse respiratory 

effects associated with short-term 

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

considering approaches to revising the 

current standards, the Administrator 

initially concluded it appropriate to 

consider setting a new 1-hour standard. 

The Administrator noted that a 1-hour 

standard would likely provide increased 

public health protection, especially for 

members of at-risk groups, from the 

respiratory effects described in both 

epidemiologic and controlled human 

exposure studies. 

2. Comments on the Adequacy of the 

Current Standards 

This section discusses public 

comments on the proposal that either 

supported or opposed the 

Administrator’s proposed decision to 

revise the current SO2 primary NAAQS. 

Comments on the adequacy of the 

current standards that focused on the 

scientific and/or the exposure/risk basis 

for the Administrator’s proposed 

conclusions are discussed in sections 

II.E.2.a–II.E.2.c. Comments on the 

epidemiologic evidence are considered 

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

controlled human exposure evidence 

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

Comments on human exposure and 

health risk assessments are considered 

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

comments on the evidence and 

information are also used to justify 

commenters’ conclusions on decisions 

related to indicator, averaging time, 

form, or level, they are noted as well in 

the appropriate sections below (II.F.1– 

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

comments, and responses thereto, 

presented below are not exclusive: other 

comments and responses are being 

included in the Response to Comment 

(RTC) Document which is part of the 

record for this rulemaking (EPA, 2010). 

Many public commenters agreed with 

the proposal that based on the available 

information, the current SO2 standards 

are not requisite to protect public health 

with an adequate margin of safety and 

that revisions to the standards are 

therefore appropriate. Among those 

calling for revisions to the standards 

were environmental groups (e.g., Sierra 

Club, WEACT for Environmental 

Justice, Center for Biological Diversity, 

(CBD) Environmental Defense Fund 

(EDF), Natural Resources Defense 

Council (NRDC)); medical/public health 

organizations (e.g., American Lung 

Association (ALA), American Thoracic 

Society (ATS)); State environmental 

organizations (e.g., National Association 

of Clean Air Agencies (NACAA), 

Northeast States for Coordinated Air 

Use Management (NESCAUM); State 

environmental agencies (e.g., such 

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

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

Lake Superior Chippewa (Fond du Lac) 

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

Department of Transportation and 

Environmental Services) and most 

individual commenters (∼13,000). These 

commenters generally concluded that 

the current SO2 standards need to be 

revised and that a more stringent 

standard is needed to protect the health 

of susceptible population groups. In 

supporting the need to adopt a more 

stringent NAAQS for SO2, these 

commenters often referenced the 

conclusions of CASAC, as well as 

evidence and information presented in 

the proposal. As such, the rationale 

offered by these commenters was 

consistent with that presented in the 

proposal to support the Administrator’s 

proposed decision to revise the current 

SO2 NAAQS. 

Most industry commenters (e.g., 

Utility Air Regulatory Group (UARG), 

American Petroleum Institute (API), 

Arizona Public Service, National 

Petrochemical & Refiners Association 

(NPRA), Montana-Dakota Utilities Co., 

Dominion Resources, Council of 

Industrial Boiler Owners (CIBO), Edison 

Electric Institute (EEI), Duke Energy, 

National Mining Association (NMA)); 

and some organizations (e.g., Texas 

Association of Business, The Annapolis 

Center for Science-Based Public Policy 

(ACSBPP), South Carolina Chamber of 

Commerce) opposed the proposed 

revisions to the SO2 primary NAAQS. In 

supporting their views, industry 

commenters generally concluded that 

EPA did not appropriately consider 

uncertainties associated with the 

epidemiologic and controlled human 

exposure evidence. 

More specifically, with respect to the 

epidemiologic studies, many of these 

commenters concluded that results of 

these studies are confounded by copollutants and thus too uncertain to 

determine whether SO2 is truly 

associated with the health outcomes 

being measured (e.g., hospital 

admissions; Federal Register see 

below). With respect to the controlled 

human exposure studies, many 

commenters were critical of the 5- 

minute benchmark levels that were 

derived from these studies and 

subsequently used by EPA in the air 

quality, exposure, and risk analyses. 

These groups were particularly 

concerned about the Administrator’s 

reliance on the 200 ppb 5-minute 

benchmark level in assessing the 

adequacy of the current and potential 

alternative standards. In general, many 

industry groups maintained that adverse 

respiratory effects did not occur 

following 5–10 minute SO2 exposures 

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

some groups stated that even at SO2

concentrations ≥ 400 ppb, reported 

effects may not be of clinical concern, 

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

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

API, UARG) also disagreed with EPA’s 

(and CASAC’s) conclusions that severe 

asthmatics were not included in these 

controlled human exposure studies, and 

that severe asthmatics would likely have 

a more pronounced response to SO2

exposures at a given level, or would 

respond to even lower levels of SO2. 

In responding to these specific 

comments, we note that the 

Administrator relied in the proposal on 

the evidence, information, and 

judgments contained in the ISA and the 

REA (including the policy assessment 

chapter), as well as on the advice of 

CASAC. In considering the evidence, 

information, and judgments of the ISA 

and the REA, the Agency notes that 

these documents have been reviewed 

and discussed extensively by CASAC at 

multiple public meetings (see above, 

section I.D) and in their letters to the 


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

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

there is special sensitivity in this review in 

disentangling SO2-related effects from PM-related 

effects (especially sulfate PM). 

EPA Administrator. Thus, it is 

important to note that CASAC generally 

accepted the key findings and 

conclusions presented in both the ISA 

and REA (see Henderson 2008a, 

Henderson 2008b, and Samet, 2009). 

a. Comments on EPA’s Interpretation of 

the Epidemiologic Evidence 

Many industry groups (e.g., API, 

UARG, American Chemistry Council 

(ACC), Dominion Resources, 

ExxonMobil, Progress Energy, CIBO, 

The Fertilizer Institute, EEI, Dow 

Chemical Company (Dow), 

MeadWestvaco Corporation (MWV), 

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

ACSBPP) commented that, given the 

presence of numerous co-pollutants in 

the air, the epidemiologic studies do not 

support the contention that SO2 itself is 

causing health effects. For example, 

UARG stated: ‘‘The epidemiological 

evidence cannot determine that SO2 is 

a cause of or a contributor to hospital 

admissions (‘‘HA’’), emergency 

department (‘‘ED’’) visits or respiratory 

symptoms, the effects cited in the 

Proposed Rule.’’ 

Although EPA has recognized that 

multiple factors can contribute to the 

etiology of respiratory disease and that 

more than one air pollutant could 

independently impact respiratory 

health, we continue to judge, as 

discussed in the ISA, that the available 

evidence supports the conclusion that 

there is an independent effect of SO2 on 

respiratory morbidity. In reaching this 

judgment, we recognize that a major 

methodological issue affecting SO2

epidemiologic studies concerns the 

evaluation of the extent to which other 

air pollutants, particular PM2.5,10 may 

confound or modify SO2-related effect 

estimates. The use of multi-pollutant 

regression models is a common 

approach for evaluating potential 

confounding by co-pollutants in 

epidemiologic studies. It is therefore 

important to note that when the ISA 

evaluated U.S. and international 

epidemiologic studies employing multipollutant models, SO2 effect estimates 

generally remained positive and 

relatively unchanged when copollutants, including PM, were included 

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

recognizing the uncertainties associated 

with separating the effects of SO2 from 

those of co-occurring pollutants, the ISA 

concluded that the limited available 

evidence indicates that the effect of SO2

on respiratory health outcomes appears 

to be generally robust and independent 

of the effects of gaseous co-pollutants, 

including NO2 and O3, as well as 

particulate co-pollutants, particularly 

PM2.5 (ISA, section 5.2; p. 5–9). 

In considering questions of 

confounding and causation, the 

epidemiologic studies should not be 

considered in a vacuum. As emphasized 

by the ISA, and endorsed by CASAC, 

controlled human exposure studies 

provide support for the plausibility of 

the associations reported in 

epidemiologic studies (ISA, section 5–5; 

Henderson 2008a; Henderson 2008b). 

These controlled human exposure 

studies exposed exercising asthmatics to 

5–10 minute peaks of SO2 and reported 

decrements in lung function and/or 

respiratory symptoms in up to 60% of 

these individuals (depending on 

exposure concentration; see ISA, Table 

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

study results provide strong support for 

an independent contribution of SO2 to 

the respiratory health effects reported in 

epidemiologic studies: ‘‘The effects of 

SO2 on respiratory symptoms, lung 

function, and airway inflammation 

observed in the human clinical studies 

using peak exposures further provides a 

basis for a progression of respiratory 

morbidity resulting in increased 

emergency department visits and 

hospital admissions. Collectively, these 

findings provide biological plausibility 

for the observed association between 

ambient SO2 levels and emergency 

department visits and hospitalizations 

for all respiratory diseases and asthma, 

notably in children and older adults. 

***’’ (ISA, section 5.2 at p. 5–5). 

Thus, EPA is not relying solely on the 

epidemiologic studies to evaluate 

whether associations reported in these 

studies (e.g., associations with 

emergency department visits) are likely 

the result of ambient SO2 exposure. 

b. Comments on EPA’s Interpretation of 

the Controlled Human Exposure 

Evidence 

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

Progress Energy, EEI, CIBO) commented 

that adverse health effects do not occur 

following 5–10 minute SO2 exposures 

< 400 ppb. In addition, some groups 

(e.g., UARG) commented that adverse 

respiratory effects do not occur in 

exercising asthmatics following SO2

exposures below 600 ppb. The 

disagreement is not whether effects 

occur in exercising asthmatics at these 

exposure levels and exposure durations. 

Rather, the issue is whether the effects 

experienced can properly be regarded as 

adverse. In general, these groups 

conclude that EPA’s judgment of 

adverse health effects at SO2 exposure 

levels below 600 or 400 ppb is 

inappropriately based on an unsound 

interpretation of ATS guidelines. More 

specifically, these groups generally 

contend that decrements in lung 

function without accompanying 

respiratory symptoms are not adverse 

effects of SO2 exposure, and that 

decrements in lung function in a 

percentage of exercising asthmatics does 

not represent a shift in lung function at 

the population level. Some of these 

groups also contend that EPA followed 

the advice of individual CASAC 

members, rather than consensus CASAC 

written comments on the ISA and REA 

when concluding respiratory effects 

associated with SO2 exposures below 

600 or 400 ppb are adverse. 

Furthermore, some groups contend that 

effects below 400 ppb should not be 

considered adverse because compared 

to the number of asthmatics 

experiencing decrements in lung 

function, there were similar numbers of 

asthmatics experiencing increases in 

lung function. EPA disagrees with these 

comments, and believes that the clinical 

evidence also supports the conclusion 

that the current standards are not 

requisite to protect public health with 

and adequate margin of safety. 

The Agency disagrees that adverse 

respiratory effects do not occur in 

exercising asthmatics following 5–10 

minute SO2 exposures ranging from 

400–600 ppb. As previously mentioned, 

at SO2 concentrations ranging from 400– 

600 ppb, moderate or greater 

decrements in lung function occur in 

approximately 20–60% of exercising 

asthmatics (again, defined in terms of a 

≥ 15% decline in FEV1 or 100% increase 

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

concentrations ≥ 400 ppb, decrements in 

lung function are often statistically 

significant at the group mean level, and 

are frequently accompanied by 

respiratory symptoms (ISA, Table 5–1). 

ATS guidelines on what constitutes an 

adverse health effect of air pollution 

clearly state that reversible loss of lung 

function in combination with the 

presence of symptoms should be 

considered adverse (ATS 1985, 2000). 

Moderate or greater decrements in lung 

function accompanied by respiratory 

symptoms fit this description. Thus, the 

Agency’s conclusion of adverse health 

effects associated with SO2

concentrations ≥ 400 ppb is consistent 

with ATS guidelines. 

The Agency also disagrees with 

industry commenters regarding the 

adversity of the respiratory effects seen 

in exercising asthmatics following 5–10 

minute SO2 exposures ranging from 

200–300 ppb. As mentioned above 

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


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35532 Federal Register / Vol. 75, No. 119 / Tuesday, June 22, 2010 / Rules and Regulations 

11See hearing transcripts from EPA Clean Air 

Scientific Advisory Committee (CASAC), July 30– 

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

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

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

at http://www.regulations.gov. 

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

74 FR at 64819), the ISA reported that 

exposure to SO2 concentrations as low 

as 200–300 ppb for 5–10 minutes results 

in approximately 5–30% of exercising 

asthmatics experiencing moderate or 

greater decrements in lung function. In 

2000, the ATS updated its guidelines on 

‘‘what constitutes an adverse health 

effect of air pollution.’’ These guidelines 

indicated that exposure to air pollution 

that increases the risk of an adverse 

effect to the entire population is 

adverse, even though it may not 

increase the risk of any individual to an 

unacceptable level (ATS 2000). For 

example, ATS notes that a population of 

asthmatics could have a distribution of 

lung function such that no individual 

has a level associated with significant 

impairment. Exposure to air pollution 

could shift the distribution to lower 

levels that still do not bring any 

individual to a level that is associated 

with clinically relevant effects. 

However, this would be considered 

adverse because individuals within the 

population would have diminished 

reserve function, and therefore would be 

at increased risk if affected by another 

agent (ATS 2000). 

Considering the 2000 ATS guidelines, 

the results of the clinical studies 

conducted at 200–300 ppb were 

reasonably interpreted by EPA to 

indicate an SO2-induced shift in these 

lung function measurements for a subset 

of this population. That is, an 

appreciable percentage of this 

population of exercising asthmatics 

would be expected to experience 

moderate or greater decrements in lung 

function in response to SO2

concentrations as low as 200 ppb, and 

thus would be expected to have 

diminished reserve lung function. As a 

result, this sub-population would be at 

greater risk of a more severe response if 

affected by another respiratory agent 

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

EPA is also mindful of CASAC 

comments on this issue following the 

second draft ISA. The second draft ISA 

placed relatively little weight on health 

effects associated with SO2 exposures at 

200–300 ppb. CASAC strongly disagreed 

with this characterization of the health 

evidence. Their consensus letter 

following the second draft ISA states: 

Our major concern is the conclusions in 

the ISA regarding the weight of the evidence 

for health effects for short-term exposure to 

low levels of SO2. Although the ISA presents 

evidence from both clinical and 

epidemiological studies that indicate health 

effects occur at 0.2 ppm or lower, the final 

chapter emphasizes health effects at 0.4 ppm 

and above * * * CASAC believes the clinical 

and epidemiological evidence warrants 

stronger conclusions in the ISA regarding the 

available evidence of health effects at 0.2 

ppm or lower concentrations of SO2. The 

selection of a lower bound concentration for 

health effects is very important because the 

ISA sets the stage for EPA’s risk assessment 

decisions. In its draft Risk and Exposure 

Assessment (REA) to Support the Review of 

the SO2 Primary National Ambient Air 

Quality Standards (July 2008), EPA chose a 

range of 0.4 ppm–0.6 ppm SO2

concentrations for its benchmark analysis. As 

CASAC will emphasize in a forthcoming 

letter on the REA, we recommend that a 

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

(Henderson 2008a) 

EPA also notes the similar CASAC 

comments on the first draft of the REA. 

The consensus CASAC letter following 

the 1st draft REA states: 

The CASAC believes strongly that the 

weight of clinical and epidemiology evidence 

indicates there are detectable clinically 

relevant health effects in sensitive 

subpopulations down to a level at least as 

low as 0.2 ppm SO2. These sensitive 

subpopulations represent a substantial 

segment of the at-risk population. 

(Henderson 2008b; p. 1) 

See Coalition of Battery Recyclers 

Association v. EPA, No. 09–1011 (DC 

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

holding that it was reasonable for EPA 

to conclude that a two IQ point mean 

population loss is an adverse effect 

based in part on CASAC advice that 

such a decrement is significant. 

CASAC’s strong advice regarding the 

adversity of effects at the 200 ppb level 

similarly supports EPA’s conclusion 

that the observed lung decrements are 

adverse. 

In addition to the considerations 

described above, we also note the 

following key points: 

• In the current SO2 NAAQS review, 

clinicians on the CASAC Panel advised 

that moderate or greater decrements in 

lung function can be clinically 

significant in some individuals with 

respiratory disease.11

• In the last O3 NAAQS review, 

CASAC indicated that moderate 

decrements in lung function can be 

clinically significant in some asthmatics 

(Henderson 2006), and that in the 

context of standard setting, a focus on 

the lower end of the range of moderate 

functional responses is most appropriate 

for estimating potentially adverse lung 

function decrements in people with 

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

16463). 

• In the last O3 NAAQS review, the 

Criteria Document and the Staff Paper 

indicated that for many people with 

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

moderate decrements in lung function 

or respiratory symptoms would likely 

interfere with normal activities and 

result in additional and more frequent 

use of medication (EPA 2006, EPA 

2007d). 

• Subjects participating controlled 

human exposure studies do not include 

severe asthmatics, and it is reasonable to 

presume that persons with more severe 

asthma than the study participants 

would have a more serious health effect 

from short-term exposure to 200 ppb 

SO2. 

Considering these key points along 

with the ATS guidelines and consensus 

CASAC comments on the draft ISA and 

REA described above, we reasonably 

conclude that 5–10 minute exposures to 

SO2 concentrations at least as low as 

200 ppb can result in adverse health 

effects in exercising asthmatics. 

In addition, as noted above some 

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

below 400 ppb should not be considered 

adverse because compared to the 

number of asthmatics experiencing 

decrements in lung function, there were 

similar numbers of asthmatics 

experiencing increases in lung function. 

The commenters correctly point out 

that at the lowest concentration tested 

in free-breathing chamber studies (200 

ppb), there are a similar number of 

asthmatics experiencing a moderate or 

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

100 increase in sRaw or ≥ 15 decrease 

in FEV1) and experiencing what might 

be called a moderate improvement in 

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

sRaw or ≥ 15 increase in FEV1). This 

observation is consistent with data 

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

ISA showing essentially no SO2

-induced change in lung function at 200 

ppb when averaged across asthmatics 

participating in the three Lin et al., 

controlled human exposure studies. 

However, these figures also demonstrate 

that asthmatics who are sensitive to SO2

at a higher concentration (600 ppb) 

experience, on average, a greater 

decrement in lung function at lower 

concentrations, including 200 ppb, 

when compared with all subjects 

combined. Therefore, while some 

asthmatics are relatively insensitive to 

SO2-induced respiratory effects even at 

concentrations ≥ 600 ppb, there is clear 

empirical evidence that others 

experience significant 

bronchoconstriction following 

exposures to both relatively high (600 

ppb) and low (200 ppb) SO2

concentrations. Among these SO2- 

sensitive asthmatics, Figures 4–2 and 4– 

3 of the ISA show a clear increase in 


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bronchoconstriction with increasing 

SO2 concentrations from 200–400 ppb. 

Given this clear relationship of exposure 

and effect at all levels in the sensitive 

asthmatics (i.e. those who experienced 

significant decrements in lung function 

at the highest exposure concentration 

used (600 ppb)), EPA does not accept 

the commenter’s premise that controlled 

human exposure studies do not 

demonstrate adverse effects in some 

asthmatics at 5–10 minute levels below 

400 ppb. 

In addition to disagreeing with EPA’s 

proposed finding of adverse health 

effects following 5– 10 minute SO2

exposures as low as 200 ppb, many 

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

ExxonMobil) also disagreed with EPA 

that severe asthmatics were not 

included in controlled human exposure 

studies. That is, these groups contend 

that EPA is incorrect in assuming that 

severe asthmatics would likely have a 

more pronounced response to SO2

exposures at a given level, or would 

respond to even lower levels of SO2 and 

that this should be taken into account 

when judging the adequacy of the 

current standards. As support for their 

assertion, multiple industry groups cite 

controlled human exposure studies in 

the ISA stating that they included 

‘‘severe asthmatics’’ and also cite a study 

by Linn et al. (1987) which concluded 

that among asthmatics, responses to SO2

exposure are not dependent on the 

clinical severity of asthma and that ‘‘the 

subjects with the highest risk [of 

temporary respiratory disturbances from 

ambient SO2] can be identified only by 

actually measuring their responses to 

SO2’’. 

We disagree with the assertion that 

severe asthmatics have been evaluated 

in 5–10 minute controlled human 

exposure studies. Although studies 

cited in the ISA referred to a group of 

subjects as ‘‘moderate/severe’’ 

asthmatics, these individuals had wellcontrolled asthma, were able to 

withhold medication, were not 

dependent on corticosteroids, and were 

able to engage in moderate to heavy 

levels of exercise. By today’s standards, 

these individuals would clearly be 

classified as moderate asthmatics. EPA 

therefore concludes that persons with 

asthma that is more severe than 

moderate asthma, as that term is 

currently understood, were not included 

in the controlled human exposure 

studies (and understandably so, for 

ethical reasons). 

In addition, EPA agrees with the 

commenters that there is little evidence 

from controlled human exposure studies 

to suggest that the respiratory effects of 

SO2 differ between mild and moderate 

asthmatics (see Linn et al., 1987). 

However, this may very well be due, at 

least in part, to persistence of 

medication among the moderate 

asthmatic subjects. More importantly, 

the moderate asthmatics began the 

exposure with compromised lung 

function relative to the mild asthmatics. 

Therefore, similar functional declines 

from different baselines between mild 

and moderate asthmatics would clearly 

not have the same physiological 

importance. CASAC specifically 

addressed the issue of asthma severity 

in a letter to the Administrator: ‘‘For 

ethical reasons severe asthmatics were 

not part of these clinical studies, but it 

is not unreasonable to presume that they 

would have responded to even a greater 

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

also important to note that in addition 

to the strict health-specific inclusion 

and exclusion criteria for a given 

controlled human exposure study, many 

asthmatics who might otherwise be able 

to participate choose not to participate 

because of anxiety related to what they 

viewed as potential adverse health risks. 

EPA concludes that it is appropriate to 

assume, as CASAC suggested, that 

persons with more severe asthma would 

respond to an even greater degree than 

the moderate asthmatics in the clinical 

studies. 

c. Comments on EPA’s Characterization 

of SO2-Associated Exposures and Health 

Risks 

Several commenters discussed the 

analyses of SO2-associated exposures 

and health risks presented in the REA. 

As in past reviews (EPA 2005, 2007c, 

2007d), EPA has estimated risks 

associated with the current standards to 

inform judgments on the public health 

risks that could exist under different 

standard options. Some industry 

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

Energy Council (LEC), Jackson Walker, 

ASARCO, the National Rural Electric 

Cooperative Association) concluded that 

when considering the adequacy of the 

current standards, the Administrator 

should consider exposures and risks 

associated with actual SO2 air quality 

rather than air quality allowed by the 

current NAAQS. They consequently 

challenged the relevance and 

appropriateness of EPA’s use of SO2

concentrations that have been simulated 

to just meet the current standards in 

assessing the adequacy of the current 

standards. 

In addition to the objections noted 

above, we note that UARG generally 

concluded that the results of EPA’s 

quantitative risk assessment are 

fundamentally flawed in that they 

substantially overestimate risks 

associated with the various air quality 

scenarios. UARG contends that this is 

because EPA did not use proper 

exposure-response functions in 

estimating risks associated with SO2

exposure. Moreover, UARG contends 

EPA further overestimates risk because 

of the use of 50 ppb exposure bins in 

estimating the number of occurrences of 

an adverse lung function response (see 

below). 

With respect to comments that when 

considering the adequacy of the current 

standards, the Administrator should 

consider exposures and risks associated 

with actual SO2 air quality rather than 

that simulated to just meet the current 

standards, these commenters generally 

concluded: (1) It is more relevant to 

assess exposures and risks associated 

with actual SO2 air quality since 

adjusting air quality to just meet the 

current standards require large 

adjustments to air quality that are highly 

uncertain; and (2) NAAQS are intended 

to address actual, rather than highly 

improbable, risks to human health. In 

addition, these groups generally 

concluded that exposure and risk 

estimates presented in the REA suggest 

relatively little health risk associated 

with current levels of SO2, and thus, 

there is no need to revise the current 

SO2 standards. 

We disagree with these commenters 

that exposure- and risk-related 

considerations in the NAAQS reviews 

should rely only on actual air quality, 

and that EPA therefore improperly 

adjusted air quality in its risk and 

exposure analyses to simulate air 

quality allowed by the current primary 

SO2 NAAQS. EPA is required to review 

whether the present standards—not 

present air quality—are requisite to 

protect public health with an adequate 

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

making this determination it is relevant 

to consider exposures and risks which 

could be permissible under the current 

standards. See American Trucking 

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

(DC Cir. 2002) (existence of evidence 

showing adverse effects occurring at 

levels allowed by the current standards 

justifies finding that it is appropriate to 

revise the existing NAAQS). 

Consequently, it is at the very least 

reasonable for EPA, in its REA, to make 

air quality adjustments to estimate SO2- 

related exposures and health risks that 

could exist in areas that just meet the 

present standards. Thus, although we 

acknowledge that exposure and health 

risk estimates associated with current 

ambient concentrations are substantially 

smaller than those associated with air 

quality adjusted to just meet the current 

standards, we also note that this is 


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12 In conducting these analyses, EPA is not trying 

to evaluate whether areas would or would not be 

in attainment of the current standards. Again, those 

issues are addressed during the implementation of 

the NAAQS. 

irrelevant to the question of whether the 

current standards are requisite to protect 

public health with an margin of safety. 

In both of these cases, EPA is not 

trying to evaluate whether areas would 

or would not be in attainment of the 

current standards. Those are issues that 

are addressed during the 

implementation of the NAAQS. Instead, 

in this rulemaking EPA is evaluating 

what NAAQS would be appropriate 

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

the impact on or risks to public health 

from air quality that is at the level of the 

current standards, as well as evaluating 

air quality that is at the level of various 

alternative standards. EPA uses this 

information to inform the decision on 

what NAAQS would be requisite to 

protect public health with an adequate 

margin of safety. 

If EPA determines that the current 

standards require revision, EPA is 

further required to determine what 

revisions are appropriate in light of the 

requirement that primary NAAQS be 

requisite to protect public health with 

an adequate margin of safety. Section 

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

for EPA to make air quality adjustments 

to simulate different potential 

alternative standards to provide 

information on exposures and risks 

under these potential alternative 

standards.12

We agree that there are uncertainties 

inherent in making air quality 

adjustments. These uncertainties are 

discussed thoroughly in the REA (REA, 

sections 6.5 and 7.4.2.5). For example, 

the REA noted the following regarding 

adjustment of SO2 concentrations: 

This procedure for adjusting either the 

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

characterization) or health effect benchmark 

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

necessary to provide insight into the degree 

of exposure and risk which would be 

associated with an increase in ambient SO2

levels such that the levels were just at the 

current standards in the areas analyzed. Staff 

recognizes that it is extremely unlikely that 

SO2 concentrations in any of the selected 

areas where concentrations have been 

adjusted would rise to meet the current 

NAAQS and that there is considerable 

uncertainty associated with the simulation of 

conditions that would just meet the current 

standards. Nevertheless, this procedure was 

necessary to assess the ability of the current 

standards, not current ambient SO2

concentrations, to protect public health 

(REA, section 6.5; p. 64) 

These air quality adjustments are not 

meant to imply an expectation that SO2

concentrations will increase broadly 

across the United States or in any given 

area. Rather, as just noted above, they 

are meant to estimate SO2-related 

exposures and health risks if air quality 

were at the level of the current and 

potential alternative standards. Such 

estimates can inform decisions on 

whether the current standards, or 

particular potential alternative 

standards, provide the requisite 

protection of public health. 

As mentioned above, UARG generally 

concluded that under all air quality 

scenarios, the results of EPA’s 

quantitative risk assessment (the third of 

the analyses conducted in the REA 

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

substantially overestimated because 

EPA did not use proper methods to 

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

UARG contends this is because many of 

the subjects in the controlled human 

exposure studies from which EPA’s 

exposure-response functions were 

derived (see REA, Table 9–3) were 

exposed to more than one SO2

concentration, yet EPA treated each 

exposure event as being independent 

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

200 and 300 ppb SO2, EPA considered 

these as representing two independent 

exposure events). UARG contends that 

observations from the same subject 

exposed to different SO2 concentrations 

are not independent observations and 

should not be treated as such. Notably, 

when UARG derived their own 

exposure-response functions taking into 

account that observations from the same 

subject exposed to different SO2

concentrations are not independent of 

each other, they estimated appreciably 

less risk than that estimated by EPA. 

There are a variety of techniques and/ 

or assumptions that can be used to fit 

individual subject data from the 

controlled human exposure studies (see 

REA, Table 9–3) to exposure-response 

curves. Moreover, any technique or 

assumption utilized will have inherent 

uncertainties. EPA discussed the 

uncertainties associated with our 

quantitative risk assessment in detail in 

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

an overview of key uncertainties in the 

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

64824). The approach used to estimate 

the exposure-response functions was 

not first introduced in the SO2 risk 

assessment, it was previously 

recommended to EPA by an applied 

statistician serving on the O3 CASAC 

Panel and used in the O3 risk 

assessment (which had individual 

controlled human exposure data similar 

to that in the current SO2 NAAQS 

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

Importantly, this approach allowed EPA 

to use all the available individual 

subject data. Moreover, an inspection of 

the estimated exposure-response curve 

and the underlying data suggest that any 

biases in the parameter estimates are 

likely to be slight (see EPA 2010, section 

II.C). Consequently, EPA does not 

accept UARG’s view that the 

methodology used in EPA’s quantitative 

risk assessment was inappropriate. 

We further note that UARG’s 

exposure-response functions do not fit 

the underlying controlled human 

exposure data (the proportions of 

subjects who responded at each 

exposure level) nearly as well as the 

exposure-response functions estimated 

using EPA’s approach. We believe this 

could be due to the methodology used 

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

exposure studies used in the 

quantitative risk assessment. UARG 

attempted to estimate subject-specific 

exposure-response functions, and to use 

the results of these estimates to obtain 

estimates of the two parameters in the 

population-level exposure-response 

functions. As described in more detail 

in section II.C of the RTC document 

(EPA 2010), EPA does not believe there 

are sufficient data to properly estimate 

the parameters of subject-specific 

exposure-response functions. More 

specifically, UARG chose a threeparameter quadratic function for the 

subject-specific exposure-response 

functions. However, none of the subjects 

had more than three exposures, and 

many had only one or two. EPA believes 

that this information is particularly 

limited for estimating these subjectspecific exposure-response functions, 

especially given that a large percentage 

of the total number of subjects had 

fewer exposures than the number of 

parameters UARG was attempting to 

estimate (i.e., UARG estimated three 

parameters in its exposure-response 

functions, but over fifty percent of 

subjects only had one or two exposures). 

It appears that UARG’s population-level 

exposure-response function estimates 

depended on these subject-specific 

exposure-response function estimates 

and thus could explain why UARG’s 

estimated population-level exposureresponse functions do not fit the 

underlying controlled human exposure 

data nearly as well as the approach used 

by EPA. A more detailed response to 

this comment can be found in section 

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

As mentioned above, UARG also 

concluded that EPA further 

overestimates the total number of 

occurrences of an adverse lung function 

response (i.e., total number of 


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13Although in St. Louis, the percent of exposed 

asthmatic children at moderate or greater exertion 

estimated to have at least one defined lung function 

response per year was not appreciably affected, it 

was found that for this same metric, the already 

very low risk estimates in Greene County became 

appreciably lower when the binning issue 

discussed above was considered. However, as noted 

above in section II.C and discussed in more detail 

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

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

exposure and risk results were found to be more 

informative in addressing the adequacy of the 

current and potential alternative standards. 

Moreover, while the Administrator’s rationale in 

the proposal relied minimally on the St. Louis 

quantitative risk results (see above), she importantly 

placed no weight on any metric from the Greene 

County quantitative risk assessment. 

occurrences of increases in sRaw ≥ 100 

or 200% and/or declines in FEV1 ≥ 15 

or 20%) in its quantitative risk 

assessment. More specifically, UARG 

concluded that the use of 50 ppb bins, 

combined with assigning all exposures 

within a bin the probability of an 

adverse lung function response at the 

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

from 0–50 ppb were assigned the 

probability of an adverse lung function 

response occurring at 25 ppb), resulted 

in a substantial overestimate of the total 

number of occurrences of lung function 

responses in asthmatics at moderate or 

greater exertion. UARG generally 

concludes that this is because the vast 

majority of exposures of asthmatics at 

moderate or greater exertion are 

occurring below the midpoint of the 0– 

50 ppb exposure bin (i.e., most 

exposures are occurring below 25 ppb), 

yet EPA is assigning these very low SO2

exposures the higher probability of a 

lung function response associated with 

the midpoint of the 0–50 ppb exposure 

bin. UARG contends that this results in 

a substantial overestimation of the total 

number of occurrences of lung function 

response in asthmatics and asthmatic 

children at moderate or greater exertion. 

UARG further notes that this 

methodological concern was raised in 

its comments on the second draft REA, 

but EPA failed to address this issue and 

relied heavily on this metric in the 

proposal with respect to the adequacy of 

the current and potential alternative 

standards. EPA’s response to this 

comment is discussed below and in 

more detail in section II.C of the RTC 

document (EPA 2010). 

EPA generally agrees with UARG’s 

technical comments that there is a 

substantial overestimation of the total 

occurrences of lung function responses 

because of the binning issues described 

above. However, we strongly disagree 

that: (1) This issue was not 

acknowledged in the final REA; and (2) 

the metric of total occurrences was 

relied on heavily in the policy 

assessment chapter of the REA (REA, 

chapter 10) and in the Administrator’s 

rationale with respect to the adequacy of 

the current and potential alternative 

standards. First, EPA did respond to this 

concern in the final REA. More 

specifically, page 344 of the final REA 

states: 

As noted in public comments on the 2nd 

draft SO2 REA, the assignment of response 

probability to the midpoint of the exposure 

bin combined with the lack of more finely 

divided intervals in this range can lead to 

significant overestimation of risks based on 

total occurrences of a defined lung function 

response. This is because the distribution of 

population exposures for occurrences is not 

evenly distributed across the bin, but rather 

is more heavily weighted toward the lower 

range of the bin. Thus, combining all 

exposures estimated to occur in the lowest 

bin with a response probability assigned to 

the midpoint of the bin results in a 

significant overestimate of the risk. 

Therefore, staff places less weight on the 

estimated number of occurrences of lung 

function responses. 

Thus, as noted in the final REA, less 

weight was placed on this metric in the 

quantitative risk assessment chapter 

(REA, chapter 9), and importantly, no 

weight was placed on this metric in 

either the policy assessment chapter of 

the REA (REA, chapter 10) or in the 

Administrator’s rationale sections of the 

proposal preamble. Rather, the policy 

assessment chapter of the REA and the 

Administrator’s rationale at the proposal 

considered the percent of exposed 

asthmatic children at moderate or 

greater exertion estimated to have at 

least one defined lung function response 

per year in St. Louis. Importantly, this 

metric is not appreciably affected by the 

binning issue raised in UARG’s 

comments. As stated on page 344–345 of 

the final REA: 

This overestimation of total occurrences 

does not impact the risk metric expressed as 

incidence or percent incidence of a defined 

lung function response 1 or more times per 

year because the bulk of the exposures 

contributing to these risk metrics are not 

skewed toward the lower range of the 

reported exposure bins.13

Finally, it is important to note that the 

Administrator’s rationale in the 

proposal regarding the adequacy of the 

current and potential alternative 

standards in general placed only limited 

reliance on the results of the 

quantitative risk assessment in St. 

Louis, with no reliance on the estimates 

of total occurrences. Rather, in addition 

to the substantial weight that she placed 

on the scientific evidence as described 

in the ISA, the Administrator placed 

relatively more weight on the results of 

the St. Louis exposure analysis. For 

example, in discussing the adequacy of 

the current standards, the proposal 

states: ‘‘The Administrator especially 

notes the results of the St. Louis 

exposure analysis which, as 

summarized above, indicates that 

substantial percentages of asthmatic 

children at moderate or greater exertion 

would be exposed, at least once 

annually, to air quality exceeding the 

400 and 200 ppb benchmarks’’ (see 74 

FR at 64829). We note that results of the 

quantitative risk assessment in St. 

Louis, with respect to the percent of 

asthmatic children estimated to have at 

least one lung function response per 

year (using EPA’s exposure-response 

functions), supports the Administrator’s 

overall conclusions in the proposal 

regarding the adequacy of the current 

and potential alternative standards. 

3. Conclusions Regarding the Adequacy 

of the Current 24-Hour and Annual 

Standards 

In reviewing the adequacy of the 

current standards, the Administrator has 

considered the scientific evidence 

assessed in the ISA, the exposure and 

risk results presented in the REA, the 

conclusions of the policy assessment 

chapter of the REA, and comments from 

CASAC and the public. These 

considerations are described below. 

As in the proposal, the Administrator 

accepts and agrees with the ISA’s 

conclusion that the results of controlled 

human exposure and epidemiologic 

studies form a plausible and coherent 

data set that supports a causal 

relationship between short-term (5 

minutes to 24 hours) SO2 exposures and 

adverse respiratory effects. The 

Administrator acknowledges that there 

are uncertainties associated with the 

epidemiologic evidence (e.g., potential 

confounding by co-pollutants). 

However, she agrees that the 

epidemiologic evidence, supported by 

the controlled human exposure 

evidence, generally indicates that the 

effects seen in these studies are 

attributable to exposure to SO2, rather 

than co-pollutants, most notably PM2.5. 

She also accepts and agrees with the 

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

epidemiologic studies, respiratory 

effects were observed in areas where the 

maximum ambient 24-h avg SO2

concentration was below the current 24- 

h avg NAAQS level.***’’ (ISA, 

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

at ambient SO2 concentrations that are 

present in locations meeting the current 

24-hour NAAQS. The Administrator 

also notes that these effects occurred in 

areas with annual air quality levels 

considerably lower than those allowed 

by the current annual standard, 

indicating that the annual standard also 


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14We also note that such a standard would, 

among other things, address the deficiency in the 

current NAAQS which occasioned the remand of 

that standard for failing to adequately explain the 

absence of protection from short-term SO2 bursts 

which could cause adverse health effects in 

hundreds of thousands of heavily breathing 

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

at 392–93. 

is not providing protection against such 

effects. Existence of epidemiologic 

studies showing adverse effects 

occurring at levels allowed by the 

current standards is an accepted 

justification for finding that it is 

appropriate to revise the existing 

standards. See, e.g. American Trucking 

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

see also American Farm Bureau v. EPA, 

559 F. 3d.512, 521–23 (DC Cir. 2009) 

(effects associated with short-term 

exposure seen in areas with ambient 

concentrations lower than long-term 

standard, so that without further 

explanation, standard does not 

adequately protect against short-term 

exposures). 

With respect to the controlled human 

exposure studies, the Administrator 

judges that effects following 5–10 

minute SO2 exposures ≥ 400 ppb and 

≥ 200 ppb can result in adverse health 

effects to asthmatics. This judgment is 

based on ATS guidelines, explicit 

CASAC consensus written advice and 

recommendations, and judgments made 

by EPA in previous NAAQS reviews. 

Thus, similar to the proposal, she notes 

analyses in the REA supporting that 5- 

minute exposures ≥ 400 ppb and ≥ 200 

ppb were associated with air quality 

adjusted upward to simulate just 

meeting the current standards. The 

Administrator especially notes the 

results of the St. Louis exposure 

analysis which, as summarized in the 

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

3, see 74 FR at 64841), indicates that 

substantial percentages of asthmatic 

children at moderate or greater exertion 

would be exposed, at least once 

annually, to air quality exceeding the 

400 and 200 ppb 5-minute benchmarks 

given air quality simulated to just meet 

the current standards. The 

Administrator judged these 5-minute 

exposures to be significant from a public 

health perspective due to their 

estimated frequency: Approximately 

24% of child asthmatics at moderate or 

greater exertion in St. Louis are 

estimated to be exposed at least once 

per year to air quality exceeding the 5- 

minute 400 ppb benchmark, a level 

associated with lung function 

decrements in the presence of 

respiratory symptoms. Additionally, 

approximately 73% of child asthmatics 

in St. Louis at moderate or greater 

exertion would be expected to be 

exposed at least once per year to air 

quality exceeding the 5-minute 200 ppb 

benchmark. This health evidence and 

risk-based information underlie 

CASAC’s conclusion that the current 

SO2 standards do not adequately protect 

public health. As discussed in the 

proposal, CASAC stated: ‘‘the current 

24-hour and annual standards are not 

adequate to protect public health, 

especially in relation to short-term 

exposures to SO2 (5–10 minutes) by 

exercising asthmatics’’ (Samet, 2009, p. 

15). The Administrator agrees with this 

conclusion. 

In considering approaches to revising 

the current standards, the Administrator 

concludes that it is appropriate to set a 

new standard, that such standard must 

provide requisite protection with an 

adequate margin of safety to a 

susceptible population (i.e., asthmatics 

at elevated ventilation), and that the 

standard must afford protection from 

short-term exposures to SO2 in order to 

prevent the adverse health effects 

reported in both the controlled human 

exposure and epidemiologic studies. 

The Administrator notes that a 1-hour 

standard could provide increased public 

health protection, especially for 

members of at-risk groups, from health 

effects described in both controlled 

human exposure and epidemiologic 

studies, and hence, health effects 

associated with 5-minute to 24-hour 

exposures to SO2.14 As discussed in 

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

protection afforded by such a standard, 

it may be appropriate to replace, and not 

retain, the current 24-hour and annual 

standards in conjunction with setting a 

new short-term standard. 

F. Conclusions on the Elements of a 

New Short-Term Standard 

In considering a revised SO2 primary 

NAAQS, the Administrator notes the 

need to protect at-risk populations from: 

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

average exposures to SO2 that could 

cause the types of respiratory morbidity 

effects reported in epidemiologic 

studies; and (2) 5–10 minute SO2

exposure concentrations reported in 

controlled human exposure studies to 

result in moderate or greater decrements 

in lung function and/or respiratory 

symptoms. Considerations with regard 

to potential alternative standards and 

the specific conclusions of the 

Administrator are discussed in the 

following sections in terms of indicator, 

averaging time, form, and level (sections 

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

1. Indicator 

a. Rationale for Proposed Decision 

In the last review, EPA focused on 

SO2 as the most appropriate indicator 

for ambient SOX. In making a decision 

in the current review on the most 

appropriate indicator, the Administrator 

has considered the conclusions of the 

ISA and REA as well as the views 

expressed by CASAC and the public. 

The REA noted that, although the 

presence of gaseous SOX species other 

than SO2 has been recognized, no 

alternative to SO2 has been advanced as 

being a more appropriate surrogate for 

ambient gaseous SOX. Controlled 

human exposure studies and animal 

toxicology studies provide specific 

evidence for health effects following 

exposure to SO2. Epidemiologic studies 

also typically report levels of SO2, as 

opposed to other gaseous SOX. Because 

emissions that lead to the formation of 

SO2 generally also lead to the formation 

of other SOX oxidation products, 

measures leading to reductions in 

population exposures to SO2 can 

generally be expected to lead to 

reductions in population exposures to 

other gaseous SOX. Therefore, as noted 

in the proposal, meeting an SO2

standard that protects the public health 

can also be expected to provide 

protection against potential health 

effects that may be independently 

associated with other gaseous SOX even 

though such effects are not discernable 

from currently available studies indexed 

by SO2 alone. See American Petroleum 

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

(DC Cir. 1981) (reasonable for EPA to 

use ozone as the indicator for all 

photochemical oxidants even though 

health information on the other 

photochemical oxidants is unknown; 

regulating ozone alone is reasonable 

since it presents a ‘‘predictable danger’’ 

and in doing so EPA did not abandon 

its responsibility to regulate other 

photochemical oxidants encompassed 

by the determination that 

photochemical oxidants as a class may 

be reasonably anticipated to endanger 

public health or welfare). Given these 

key points, the REA concluded that the 

available evidence supports the 

retention of SO2 as the indicator in the 

current review (REA, section 10.5.1). 

Consistent with this conclusion, CASAC 

stated in a letter to the EPA 

Administrator that: ‘‘for indicator, SO2 is 

clearly the preferred choice’’ (Samet 

2009, p. 14). 

b. Comments on Indicator 

A small number of commenters 

directly addressed the issue of the 

indicator for the standard. These 


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commenters generally endorsed the 

proposal to continue to use SO2 as the 

indicator for ambient SOX.

c. Conclusions on Indicator 

Based on the available information 

discussed above, and consistent with 

the views of CASAC and other 

commenters, the Administrator 

concludes that it is appropriate to 

continue to use SO2 as the indicator for 

a standard that is intended to address 

effects associated with exposure to SO2, 

alone or in combination with other 

gaseous SOX. In so doing, the 

Administrator recognizes that measures 

leading to reductions in population 

exposures to SO2 will also reduce 

population exposures to other oxides of 

sulfur. 

2. Averaging Time 

This section discusses considerations 

related to the averaging time of the SO2

primary NAAQS. Specifically, this 

section summarizes the rationale for the 

Administrator’s proposed decision 

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

see section II.F.2 of the proposal for 

more detail at 74 FR 64832–64833), 

discusses public comments and EPA 

responses related to averaging time 

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

Administrator’s final conclusions 

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

Notably, public comments and the 

Administrator’s conclusions on whether 

to retain or revoke the current 24-hour 

and/or annual standards given a new 1- 

hour standard are discussed in section 

II.F.5. 

a. Rationale for Proposed Decision 

In considering the most appropriate 

averaging time for the SO2 primary 

NAAQS, the Administrator noted in the 

proposal the conclusions and judgments 

made in the ISA about the available 

scientific evidence, air quality 

correlations discussed in the REA, 

conclusions of the policy assessment 

chapter of the REA, and CASAC 

recommendations (section II.F.2 in the 

proposal). Specifically, she noted the 

following: 

• The REA conclusion that an 

appropriate averaging time should focus 

protection on SO2 exposures from 5- 

minutes to 24-hours (REA, section, 

10.5.2). 

• Air quality, exposure, and risk 

analyses from the REA indicating it is 

likely a 1-hour standard—with the 

appropriate form and level—can 

substantially reduce 5–10 minute peaks 

of SO2 shown in controlled human 

exposure studies to result in respiratory 

symptoms and/or decrements in lung 

function in exercising asthmatics (i.e. 5- 

minute SO2 concentrations ≥ 200 and 

400 ppb). 

• Air quality analyses indicating that 

a 1-hour standard—with the appropriate 

form and level—can substantially 

reduce the upper end of the distribution 

of SO2 levels more likely to be 

associated with adverse respiratory 

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

(1) 99th percentile 1-hour daily 

maximum air quality concentrations in 

U.S. cities where positive effect 

estimates in epidemiologic studies of 

hospital admissions and emergency 

department visits for all respiratory 

causes and asthma were observed; and 

(2) 99th percentile 24-hour average air 

quality concentrations found in U.S. 

cities where emergency department visit 

and hospitalization studies (for all 

respiratory causes and asthma) reported 

statistically significant associations in 

multi-pollutant models with PM. 

• The REA conclusion that a 5- 

minute averaging time is undesirable 

because it would result in significant 

and unnecessary instability due to the 

likelihood that locations would 

frequently shift in and out of 

attainment—thereby reducing public 

health protection by disrupting an area’s 

ongoing implementation plans and 

associated control programs. 

• CASAC statement addressing 

whether a 1-hour averaging time can 

adequately control 5–10 minute peak 

exposures and whether there should be 

a 5-minute averaging time. CASAC 

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

2009, p. 16), and that ‘‘a one-hour 

standard is the preferred averaging time’’ 

(Samet 2009, p. 15). 

• CASAC’s statement that they were 

‘‘in agreement with having a short-term 

standard and finds that the REA 

supports a 1-hour standard as protective 

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

b. Comments on Averaging Time 

A large number of public commenters 

also endorsed the establishment of a 

new standard with a 1-hour averaging 

time (although some groups’ support 

hinged on the accompanying level). 

These included a number of State 

organizations (e.g., NACAA, 

NESCAUM); State environmental 

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

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

health and environmental organizations 

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

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

the Fond du Lac Tribe; local groups 

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

New York City); and almost all of the 

individual commenters (13,000). The 

supporting rationales offered by these 

commenters often acknowledged the 

recommendations of CASAC and the 

Administrator’s rationale as discussed 

in the proposal. 

Though many industry commenters 

did not support the proposed revisions 

to the SO2 primary NAAQS (as 

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

of these groups did express that if a 

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

hour averaging time could be 

appropriate, depending on the level and 

form selected (e.g., ExxonMobil, Kean 

Miller). Other industry commenters 

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

Association of Battery Recyclers (ABR)) 

and the South Dakota Department of 

Environment and Natural Resources (SD 

DENR) expressed that EPA should have 

considered longer averaging times (e.g., 

3 hours). In addition, although health 

and environmental groups were 

supportive of setting a new 1-hour 

standard to protect against short-term 

exposures to SO2 (again, depending on 

the level of the 1-hour standard 

selected), these groups also commented 

that a 5-minute standard to protect 

susceptible populations from health 

effects associated with 5-minute peaks 

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

ATS, Sierra Club, EDF). These 

comments, and EPA’s responses, are 

discussed in more detail below. 

As discussed above, industry 

commenters who disagreed with setting 

a new 1-hour standard generally based 

this conclusion on their interpretation 

of the scientific evidence and their 

conclusion that this evidence does not 

support the proposed revisions to the 

current SO2 NAAQS. EPA’s responses to 

these commenters were presented above 

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

Also noted above, some industry 

commenters (e.g., ASARCO, RIO Tinto 

Alcan, ABR) and the SD DENR 

expressed that EPA should have 

considered longer averaging times (e.g., 

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

these groups concluded that a standard 

with a longer averaging time could 

potentially provide the same public 

health protection as a 1-hour standard, 

while also providing a more stable 

regulatory target. For example, in its 

comments, the SD DENR states: ‘‘DENR 

recommends EPA evaluate a 3-hour or 

8-hour standard to determine if these 

averaging periods are also protective of 

the public health. If they are, EPA 

should propose a 3-hour or 8-hour 

sulfur dioxide standard instead of a 1- 

hour standard. A longer averaging 

period would smooth out the variability 

of the upper range measurements and 

provide a more stable standard.’’ 

Similarly, Rio Tinto Alcan stated in its 

comments: ‘‘the short-term averaging 

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


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to 24 hours) is not limited to only 5- 

minute, 1-hour and 24-hour averaging 

periods. EPA could explain in more 

detail why these three averaging periods 

were examined when considering 

appropriate averaging periods to limit 

short-term peaks of SO2 * * * a longer 

term average could provide additional 

stability to the standard while at the 

same time effectively protecting public 

health.’’ 

Although we agree that alternative 

averaging times could potentially 

provide similar public health protection 

(assuming an appropriate form and 

level), we believe that a 1-hour 

averaging time is reasonably justified by 

the scientific evidence presented in the 

ISA and by the air quality information 

presented in the REA. As described in 

detail in the proposal (see section 

II.F.2), the controlled human exposure 

evidence presented in the ISA provided 

support for an averaging time that 

protects against 5–10 minute peak SO2

exposures (REA, section 10.5.2, pp. 

371–372), and results from 

epidemiologic studies most directly 

provided support for both 1-hour and 

24-hour averaging times (REA, section 

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

reasonable to consider these averaging 

times for a revised SO2 NAAQS given 

that there is very little basis in the 

health evidence presented in the ISA to 

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

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

noted the likelihood that averaging 

times of 1 and 24 hours could provide 

protection against 5-minute peak SO2

exposures. As described in detail in the 

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

64830–64833), it was initially 

concluded that a 1-hour averaging time, 

rather than a 24-hour averaging time, 

would be more appropriate for limiting 

5-minute peaks of SO2. Similarly, we 

concluded that a 1-hour standard, given 

the appropriate form and level, could 

likely limit 99th percentile 24-hour 

average air quality concentrations found 

in U.S. locations where emergency 

department visit and hospitalization 

studies (for all respiratory causes and 

asthma) observed statistically significant 

associations in multi-pollutant models 

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

average SO2 concentration ≥ 36 ppb). 

Taken together, we reasonably 

concluded that a 1-hour standard, with 

an appropriate form and level, can 

provide adequate protection against the 

range of health outcomes associated 

with averaging times from 5 minutes to 

24 hours (proposal section II.F.2 and 

REA, section 10.5.2.3). We also note that 

our conclusion is in agreement with 

CASAC comments on the second draft 

REA. CASAC stated that they were ‘‘in 

agreement with having a short-term 

standard and finds that the REA 

supports a one-hour standard as 

protective of public health’’ (Samet 

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

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

found that a 1-hour standard in 

combination with the selected form, 

will provide a stable regulatory target. 

As noted above, although health and 

environmental groups were supportive 

of setting a new 1-hour standard to 

protect against short-term exposures to 

SO2 (again, depending on the level of 

the 1-hour standard selected), these 

groups generally commented that a 5- 

minute standard to protect against 

health effects associated with 5-minute 

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

Sierra Club, EDF). For example, in their 

combined comments ALA, EDF, NRDC, 

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

need a short-term SO2 standard, 

optimally a 5-minute standard, to 

protect against bursts of pollution that 

can result from start-up, shutdown, 

upset, malfunction, downwash, 

complex terrain, atmospheric inversion 

conditions, and other situations’’ and 

that ‘‘EPA has over emphasized a 

concern about the stability of a 5-minute 

standard * * * The record does not 

show that any alleged instability of a 5- 

minute standard has any relevance to 

whether such a standard is requisite to 

protect public health.’’ 

We agree that there needs to be a 

short-term standard to protect against 5- 

minute peaks of SO2. However, we do 

not believe setting a 5-minute standard 

to be the best way of accomplishing that 

objective. As in past NAAQS reviews, 

EPA properly considered the stability of 

the design of pollution control programs 

in its review of the elements of a 

NAAQS, since more stable programs are 

more effective, and hence result in 

enhanced public safety. American 

Trucking Associations v. EPA, 283 F. 3d 

at 375 (choice of 98th percentile form 

for 24-hour PM NAAQS, which allows 

a number of high exposure days per year 

to escape regulation under the NAAQS, 

justifiable as ‘‘promot[ing] development 

of more ‘effective [pollution] control 

programs’ ’’, since such programs would 

otherwise be ‘‘less ‘stable’—and hence 

* * * less effective—than programs 

designed to address longer-term average 

conditions’’, and there are other means 

(viz. emergency episode plans) to 

control those high exposure days). In 

this review, there were legitimate 

concerns about the stability of a 

standard using a 5-minute averaging 

time. Specifically, there was concern 

that compared to longer averaging times 

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

variation in 5-minute SO2

concentrations were likely to be 

substantially more temporally and 

spatially diverse. Thus, it is more likely 

that locations would frequently shift in 

and out of attainment thereby reducing 

public health protection by disrupting 

an area’s ongoing implementation plans 

and associated control programs. 

Consequently, the REA concluded that a 

5-minute averaging time would not 

provide a stable regulatory target and 

therefore would not be the preferred 

approach to provide adequate public 

health protection. A 1-hour averaging 

time does not have these drawbacks. As 

noted in the REA and the proposal (see 

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

air quality, exposure, and risk analyses 

support that a 1-hour averaging time, 

given an appropriate form and level can 

adequately limit 5-minute SO2

exposures and provide a more stable 

regulatory target than setting a 5-minute 

standard. More specifically, based on 

the air quality and exposure analyses 

presented in chapters 7 and 8 of the 

REA, there is also a strong likelihood 

that a 99th percentile 1-hour daily 

maximum standard will limit 5–10 

minute peaks of SO2 shown in 

controlled human exposure studies to 

result in decrements in lung function 

and/or respiratory symptoms in 

exercising asthmatics (see especially 

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

19). 

We also note that a 1-hour standard to 

protect against 5-minute exposures is in 

agreement with CASAC advice and 

recommendations. That is, CASAC 

stated that they were ‘‘in agreement with 

having a short-term standard and finds 

that the REA supports a 1-hour standard 

as protective of public health’’ (Samet 

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

statement addressing whether a 1-hour 

averaging time can adequately control 

5–10 minute peak exposures and 

whether there should be a 5-minute 

averaging time, CASAC stated that the 

REA had presented a ‘‘convincing 

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

hour standard, and that ‘‘a one-hour 

standard is the preferred averaging time’’ 

(Samet 2009, p. 15). 

c. Conclusions on Averaging Time 

In considering the most appropriate 

averaging time(s) for the SO2 primary 

NAAQS, the Administrator notes the 

conclusions and judgments made in the 

ISA about the available scientific 

evidence, air quality considerations 

from the REA, CASAC advice and 

recommendations, and public 

comments received. Based on these 

considerations, the Administrator 

concludes that a new standard based on 


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15As noted above, such a standard also 

satisfactorily addresses the issue raised by the 

reviewing court in the litigation that followed the 

last review of the SO2 NAAQS: Why was no 

protection afforded in the standard for a susceptible 

subpopulation known to experience repeated 

adverse effects from exposure to 5–10 minute SO2

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

1-hour daily maximum SO2

concentrations will provide increased 

protection against effects associated 

with short-term (5 minutes to 24 hours) 

exposures. The rationale for this 

decision is described below. 

Similar to the proposal (see section 

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

with the REA’s conclusion that the 

standard should focus protection on 

short-term SO2 exposures from 5 

minutes to 24 hours. As noted above, 

CASAC’s strong recommendation 

supports this approach as well.15 The 

Administrator further agrees that the 

standard must provide requisite 

protection from 5–10 minute exposure 

events, but believes that this can be 

provided without having a standard 

with a 5-minute averaging time. The 

Administrator agrees with the REA 

conclusion that it is likely a 1-hour 

standard—with the appropriate form 

and level—can substantially reduce 5– 

10 minute peaks of SO2 shown in 

controlled human exposure studies to 

result in respiratory symptoms and/or 

decrements in lung function in 

exercising asthmatics. The 

Administrator further believes that a 5- 

minute averaging time would result in 

significant and unnecessary instability 

and is undesirable for that reason. The 

Administrator also notes the statements 

from CASAC mentioned above 

addressing whether a 1-hour averaging 

time can adequately control 5–10 

minute peak exposures and whether 

there should be a 5-minute averaging 

time. As noted above, addressing this 

question, CASAC stated that the REA 

had presented a ‘‘convincing rationale’’ 

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

standard, and that ‘‘a one-hour standard 

is the preferred averaging time’’ (Samet 

2009, p. 15). 

Second, as in the proposal the 

Administrator agrees that a 1-hour 

averaging time (again, with the 

appropriate form and level) would 

provide protection against the range of 

health outcomes associated with 

averaging times of 1 hour to 24 hours. 

Specifically, the Administrator finds 

that a 1-hour standard can substantially 

reduce the upper end of the distribution 

of SO2 levels more likely to be 

associated with adverse respiratory 

effects (see discussion on Form, section 

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

daily maximum SO2 air quality 

concentrations in U.S. locations where 

positive SO2 effect estimates were 

reported in epidemiologic studies of 

emergency department visits and 

hospital admissions for all respiratory 

causes and asthma; and (2) 99th 

percentile 24-hour average SO2 air 

quality concentrations found in U.S. 

locations where emergency department 

visit and hospital admission studies 

using multi-pollutant models with PM 

reported statistically significant 

associations (for all respiratory causes or 

asthma) with ambient SO2 (see REA, 

section 10.5.2.2 and proposal section 

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

Administrator again notes that 

establishing a new 1-hour averaging 

time is in agreement with CASAC 

recommendations. As noted above, 

CASAC stated that they were ‘‘in 

agreement with having a short-term 

standard and finds that the REA 

supports a one-hour standard as 

protective of public health’’ (Samet 

2009, p. 1). Moreover, CASAC agreed 

with the REA that a ‘‘one-hour standard 

is the preferred averaging time’’ (Samet 

2009, p.15). 

3. Form 

This section discusses considerations 

related to the form of the 1-hour SO2

primary NAAQS. Specifically, this 

section summarizes the rationale for the 

Administrator’s proposed decision 

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

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

the proposal for more detail), discusses 

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

presents the Administrator’s final 

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

a. Rationale for Proposed Decision 

In considering the most appropriate 

form for the SO2 primary NAAQS, the 

Administrator noted in the proposal the 

conclusions and judgments made in the 

ISA about available scientific evidence, 

air quality information discussed in the 

REA, conclusions of the policy 

assessment chapter of the REA, and 

CASAC recommendations (see section 

II.F.3, 74 FR at 64833–64834 in the 

proposal). Specifically, the proposal 

referenced the following: 

• Information in the ISA that 

suggested that adverse respiratory 

effects are more likely to occur at the 

upper end of the distribution of ambient 

SO2 concentrations. That is, the ISA 

describes a few studies that reported an 

increase in SO2-related respiratory 

health effects at the upper end of the 

distribution of SO2 concentrations (ISA, 

section 5.3, p. 5–9). 

• The REA conclusion that a 

concentration-based form averaged over 

three years would better reflect the 

continuum of health risks posed by 

increasing SO2 concentrations (i.e. the 

percentage of asthmatics affected and 

the severity of the response increases 

with increasing SO2 concentrations; 

REA, section 10.5.3) by giving 

proportionally greater weight to years 

when 1-hour daily maximum SO2

concentrations are well above the level 

of the standard, than just above the level 

of the standard. 

• Analyses in the REA that suggested 

for a given SO2 standard level, a 99th 

percentile form is appreciably more 

effective at limiting 5-minute peak SO2

concentrations than a 98th percentile 

form (REA, section 10.5.3 and REA, 

Figures 7–27 and 7–28). 

• Analyses in the REA indicating that 

over the last 10 years and for the vast 

majority of the sites examined, there 

appears to be little difference in 98th 

and 99th percentile design value 

stability (REA, section 10.5.3). 

• The REA conclusion that taken 

together, the evidence and air quality 

information indicate that consideration 

should be given primarily to a 1-hour 

daily maximum standard with a 99th 

percentile or 4th highest daily 

maximum form (REA, section 10.5.3.3). 

• CASAC indications that: ‘‘there is 

adequate information to justify the use 

of a concentration-based form averaged 

over 3 years’’ (Samet 2009, p. 16). 

• CASAC recommendations that 

when evaluating 98th vs. 99th 

percentile forms, EPA should consider 

the number of days per year 98th vs. 

99th percentile forms would allow SO2

concentrations to exceed the selected 

standard level. Similarly, CASAC 

recommendations to consider the 

number of exceedences of 5-minute 

benchmarks given 98th vs. 99th 

percentile forms at a given standard 

level (Samet 2009). 

b. Comments on Form 

Most all State organizations and 

agencies (e.g., NAACA, NESCAUM and 

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

supported a 99th percentile or 4th 

highest form. Similarly, public health 

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

organizations (e.g., CBD, WEACT for 

Environmental Justice) and the 

Alexandria Department of 

Transportation and Environmental 

Services preferred either a 99th 

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

no exceedence) to further limit the 

occurrence of SO2 concentrations that 

exceed the standard level in locations 

that attain the standard. In contrast, 

many industry groups (e.g., UARG, 

NAM, LEC, RRI Energy, AirQuality 

Research & Logistics (AQRL)), and the 

SD DENR conditionally supported a 


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16EPA did not propose or seek comment on a 

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

an exceedence based form). EPA also considered a 

4th highest form, which is generally equivalent to 

the 99th percentile. However, a percentile based 

form is preferred since it results in a sampling from 

the same part of the annual distribution of 1-hour 

daily maximum SO2 concentrations regardless of 

the number of 1-hour daily maximum 

concentrations reported in a given year for a 

particular location. 

98th percentile form if EPA were to set 

a 1-hour standard.16 EPA responses to 

specific comments on the form of the 

standard can be found below and in the 

RTC document (EPA 2010). 

As mentioned above, a number of 

industry groups and the SD DENR 

preferred a 98th percentile form. In 

general, their preference for a 98th 

percentile form was based on their 

conclusion that a form based on the 

98th percentile would be more stable 

than a form based on the 99th 

percentile, and that a 98th percentile 

form is consistent with the forms 

selected in recent NAAQS reviews (i.e. 

PM2.5 and NO2). For example AQRL 

stated: ‘‘The Administrator should 

reconsider her proposal and choose 

instead the 98th percentile (or 

equivalent nth highest value) form of 

the standard for the added reliability 

and stability it offers in determining 

compliance or progress towards 

attainment. This approach has been 

promulgated for recent revisions of the 

PM2.5 and NO2 standards and this 

consistency should be maintained with 

SO2.’’ 

We agree with the commenters that it 

is important that a 1-hour standard have 

a form that is reasonably stable, but we 

disagree that a 98th percentile form is 

significantly more stable than a 99th 

percentile form. We note that the REA 

discussed analyses (also briefly 

described in the proposal; see section 

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

in 98th and 99th percentile design 

values from 54 sites located in the 40 

counties selected for the detailed air 

quality analysis (REA section 10.5.3 and 

Thompson, 2009). These results 

suggested that at the vast majority of 

sites, there would have been similar 

changes in 98th and 99th percentile 

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

based on evaluating overlapping three 

year intervals over the last ten years; see 

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

As part of this analysis, all of the design 

values over this ten year period for all 

54 sites were aggregated and the 

standard deviation calculated (REA, 

Figure 10–2 and Thompson, 2009). 

Results demonstrated similar standard 

deviations—i.e. similar stability—based 

on aggregated 98th or aggregated 99th 

percentile design values over the ten 

year period (see REA, Figure 10–2 and 

Thompson 2009). Thus, we believe that 

in most locations, there will not be a 

substantial difference in stability 

between 98th and 99th percentile forms. 

We also disagree with the commenters 

that the forms of NAAQS standards 

should be consistent across different 

NAAQS pollutants. This is almost like 

advocating consistent levels or 

averaging times for different NAAQS 

pollutants. Each pollutant is manifestly 

different from another, and the decision 

as to an appropriate standard for each, 

and appropriate elements (including 

form) of each standard and the 

interaction of these elements, 

necessarily is fact-specific. Cf. Sierra 

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

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

tub on its own bottom’ approach to 

EPA’s setting of standards pursuant to 

the CAA, under which the adequacy of 

the underlying justification offered by 

the agency is the pertinent factor—not 

what the agency did on a different 

record concerning a different industry’’) 

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

a priori that any element of one NAAQS 

should be consistent with another, 

although if all other things are equal, 

selecting stable forms for each NAAQS 

is a legitimate objective. 

A 99th percentile form, rather than a 

98th percentile form, is also needed for 

the standard to provide requisite public 

health protection. In this review of the 

primary SO2 NAAQS, we considered 

information in the ISA suggesting that 

adverse respiratory effects are more 

likely to occur at the upper end of the 

distribution of ambient SO2

concentrations. That is, the ISA 

described a few studies that reported an 

increase in SO2-related respiratory 

health effects at the upper end of the 

distribution of SO2 concentrations (i.e., 

above 90th percentile SO2

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

Moreover, we considered the extent to 

which different percentile forms, given 

the same standard level, limit 5-minute 

concentrations of SO2 above benchmark 

levels. As noted above in section 

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

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

64834), air quality analyses presented in 

the REA suggested that at a given SO2

standard level, a 99th percentile form is 

appreciably more effective at limiting 5- 

minute peak SO2 concentrations than a 

98th percentile form (REA, section 

10.5.3, and REA, Figures 7–27 and 7– 

28). Taken together with the analyses 

suggesting that 98th and 99th percentile 

forms have similar stabilities, we 

reasonably concluded that a 99th 

percentile form was most appropriate 

for a 1-hour SO2 standard. 

As mentioned above, a number of 

health and environmental groups 

supported a 99th percentile form, but 

expressed that they would prefer a more 

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

Alexandria Department of 

Transportation and Environmental 

Services only recommended a no, or one 

exceedence based form. In general, these 

groups concluded that a more restrictive 

form would further limit the: (1) 

Number of days an area could exceed 

the standard level and still attain the 

standard; and (2) the occurrence of 5- 

minute peaks of SO2 above benchmark 

levels. 

It is important that the particular form 

selected for a 1-hour daily maximum 

standard reflect the nature of the health 

risks posed by increasing SO2

concentrations. The REA and proposal 

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

that the form of the standard should 

reflect results from controlled human 

exposure studies demonstrating that the 

percentage of asthmatics affected, and 

the severity of the respiratory response 

(i.e. decrements in lung function, 

respiratory symptoms) increases as SO2

concentrations increase. Taking this into 

consideration, EPA staff concluded that 

a concentration-based form, averaged 

over three years, is more appropriate 

than an exceedance-based form (REA, 

section 10.5.3). This is because a 

concentration-based form averaged over 

three years gives proportionally greater 

weight to years when 

1-hour daily maximum SO2

concentrations are well above the level 

of the standard, as it gives to years when 

1-hour daily maximum SO2

concentrations are just above the level 

of the standard. In contrast, an expected 

exceedance form gives the same weight 

to years when 1-hour daily maximum 

SO2 concentrations are just above the 

level of the standard as it gives to years 

when 1-hour daily maximum SO2

concentrations are well above the level 

of the standard. Therefore, we 

concluded that a concentration-based 

form, averaged over three years (which 

also increases the stability of the 

standard) better reflects the continuum 

of health risks posed by increasing SO2

concentrations (i.e. the percentage of 

asthmatics affected and the severity of 

the response increases with increasing 

SO2 concentrations; REA, section 

10.5.3). Moreover, we note that analyses 

in the REA indicate that in most 

locations analyzed, a 99th percentile 

form would correspond to the 4th 

highest daily maximum concentration 

in a year, and that the 99th percentile, 

combined with the standard level 


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selected, will substantially limit 5- 

minute peaks of SO2 above the 200 ppb 

and higher benchmark levels (see below, 

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

concentration based form is in 

agreement with CASAC advice that: 

‘‘there is adequate information to justify 

the use of a concentration-based form 

averaged over 3 years’’ (Samet 2009, 

p. 16). 

c. Conclusions on Form 

The Administrator agrees that the 

form of the standard should reflect the 

health evidence presented in the ISA 

indicating that the percentage of 

asthmatics affected and the severity of 

the response increases with increasing 

SO2 concentrations. The Administrator 

also agrees that it is reasonable to 

consider the standard’s stability as part 

of consideration of the form of the 

standard. Thus, the Administrator 

agrees that the standard should use a 

concentration-based form averaged over 

three years in order to give due weight 

to years when 1-hour SO2

concentrations are well above the level 

of the standard, than to years when 1- 

hour SO2 concentrations are just above 

the level of the standard. She also notes 

that a concentration-based form 

averaged over 3 years would likely be 

appreciably more stable than a noexceedence based form. 

In selecting a specific concentration 

based form, the Administrator first notes 

that a few epidemiologic studies 

described in the ISA reported an 

increase in SO2-related respiratory 

health effects at the upper end of the 

distribution of ambient SO2

concentrations (i.e., above 90th 

percentile SO2 concentrations; see ISA, 

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

notes further that numerous controlled 

human exposure studies have reported 

decrements in lung function and/or 

respiratory symptoms in exercising 

asthmatics exposed to peak 5–10 minute 

SO2 concentrations. The Administrator 

therefore concludes that the form of a 

new 1-hour standard should be 

especially focused on limiting the upper 

end of the distribution of ambient SO2

concentrations (i.e., above 90th 

percentile SO2 concentrations) in order 

to provide protection with an adequate 

margin of safety against effects reported 

in both epidemiologic and controlled 

human exposure studies. 

In further considering specific 

concentration based forms, the 

Administrator notes as outlined above 

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

more detail in the REA (REA, section 

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

74 FR at 64834), that a 99th percentile 

form is likely to be appreciably more 

effective at limiting 5-minute 

benchmark exposures of concern 

compared to a 98th percentile form. 

Taken together with the considerations 

just discussed above, the Administrator 

has selected a 99th percentile form, 

averaged over 3 years. The 

Administrator concludes that a 99th 

percentile form, given the level selected 

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

will limit both the upper end of the 

distribution of ambient SO2

concentrations reported in some 

epidemiologic studies to be associated 

with increased risk of SO2-related 

respiratory morbidity effects (e.g., 

emergency department visits), as well as 

5-minute peak SO2 concentrations 

resulting in decrements in lung function 

and/or respiratory symptoms in 

exercising asthmatics participating in 

controlled human exposure studies. 

4. Level 

As discussed below and in more 

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

FR at 64834), the Administrator 

proposed to set a 1-hour standard with 

a 99th percentile form (averaged over 

three years), with a level in the range of 

50 to 100 ppb. The Administrator also 

solicited comment on standard levels 

greater than 100 ppb up to 150 ppb. 

This section summarizes the rationale 

for the Administrator’s proposed range 

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

comments related to the range of 

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

the Administrator’s final conclusions 

regarding the level of a new 1-hour SO2

standard (II.F.3.c). 

a. Rationale for Proposed Decision 

In assessing the level of a 1-hour 

standard with a 99th percentile form 

(averaged over three years), the 

Administrator considered the broad 

range of scientific evidence assessed in 

the ISA, including the epidemiologic 

studies and controlled human exposure 

studies, as well as the results of air 

quality, exposure, and risk analyses 

presented in the REA. In light of this 

body of evidence and analyses, the 

Administrator found it is necessary to 

provide increased public health 

protection for at-risk populations 

against an array of adverse respiratory 

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

5 minutes to 24 hours) exposures to 

ambient SO2. In considering the most 

appropriate way to provide this 

protection, the Administrator was 

mindful of the extent to which the 

available evidence and analyses could 

inform a decision on the level of a 

standard. The Administrator’s proposed 

decisions on level, as discussed in detail 

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

outlined below. 

Given the above considerations, the 

Administrator proposed to set a level for 

a new 99th percentile 1-hour daily 

maximum primary SO2 standard within 

the range from 50 to 100 ppb and took 

comment on levels above 100 ppb, up 

to 150 ppb. In reaching this proposed 

decision, the Administrator considered: 

(1) The evidence-based considerations 

from the final ISA and the final REA; 

(2) the results of the air quality, 

exposure, and risk assessments 

discussed above and in the final REA; 

(3) CASAC advice and 

recommendations on both the ISA and 

REA discussed above and provided in 

CASAC’s letters to the Administrator; 

and (4) public comments received on 

the first and second drafts of the ISA 

and REA. In considering what level of 

a 1-hour SO2 standard is requisite to 

protect public health with an adequate 

margin of safety, the Administrator was 

mindful that this choice requires 

judgments based on an interpretation of 

the evidence and other information that 

neither overstates nor understates the 

strength and limitations of that evidence 

and information. 

As noted above, the Administrator 

selected an upper end of a range of 

levels to propose at 100 ppb. The 

selection of this level focused on the 

results of the controlled human 

exposure studies and is primarily based 

on the results of the air quality and 

exposure analyses which suggest that a 

1-hour standard should be at or below 

100 ppb to appreciably limit 5-minute 

SO2 benchmark concentrations 

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

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

That is, as described in the proposal (see 

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

quality analysis estimates that a 100 ppb 

1-hour standard would allow at most 2 

days per year on average when 

estimated 5-minute daily maximum SO2

concentrations exceed the 400 ppb 

benchmark, and at most 13 days per 

year on average when 5-minute daily 

maximum SO2 concentrations exceed 

the 200 ppb benchmark (see proposal 

Table 2). Furthermore, given a 

simulated 1-hour 100 ppb standard 

level, most counties in the air quality 

analysis were estimated to experience 0 

days per year on average when 5-minute 

daily maximum SO2 concentrations 

exceed the 400 ppb benchmark and ≤ 3 

days per year on average when 5-minute 

daily maximum SO2 concentrations 

were estimated to exceed the 200 ppb 

benchmark (see REA, Tables 7–14 and 

7–12). The Administrator also noted 

that the St. Louis exposure analysis 

indicated that a 1-hour standard at 


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100 ppb would still be estimated to 

protect > 99% of asthmatic children at 

moderate or greater exertion from 

experiencing at least one 5-minute SO2

exposure ≥ 400 ppb per year, and about 

97% of these children from exposures ≥ 

200 ppb. In contrast, as described in the 

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

Louis exposure analysis estimated that a 

1-hour standard at 150 ppb would likely 

only protect about 88% of asthmatic 

children at moderate or greater exertion 

from experiencing at least one 5-minute 

SO2 exposure ≥ 200 ppb per year. 

As noted above and described in 

detail in the proposal (see section 

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

ppb as the lower end of a range of levels 

to propose, which is consistent with 

CASAC’s advice. The selection of this 

level focused in part on the U.S. 

epidemiologic evidence described in 

detail in the proposal (see sections 

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

to these epidemiologic studies, seven of 

ten U.S. emergency department visit 

and hospital admission studies 

reporting generally positive associations 

with ambient SO2 were conducted in 

locations where 99th percentile 1-hour 

daily maximum SO2 levels were about 

75–150 ppb, and three of these studies 

observed statistically significant 

positive associations between ambient 

SO2 and respiratory-related emergency 

department visits and hospitalizations 

in multi-pollutant models with PM 

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

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

Administrator noted that a 99th 

percentile 1-hour daily maximum 

standard set at a level of 50 ppb is well 

below the 99th percentile 1-hour daily 

maximum SO2 concentrations reported 

in locations where these three studies 

were conducted (i.e. well below 99th 

percentile 1-hour daily maximum SO2

levels of 78–150 ppb seen in NYDOH 

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

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

noted that two epidemiologic studies 

reported generally positive associations 

between ambient SO2 and emergency 

department visits in cities when 99th 

percentile 1-hour daily maximum SO2

concentrations were approximately 

50 ppb, but did not consider that 

evidence strong enough to propose 

setting a standard level lower than 50 

ppb. 

In considering the results of the air 

quality and exposure analyses, the 

Administrator also noted that the 40- 

county air quality analysis estimates 

that a 99th percentile 1-hour daily 

maximum standard set at a level of 

50 ppb would result in zero days per 

year when estimated 5-minute SO2

concentrations exceed the 400 ppb 5- 

minute benchmark level and at most 2 

days per year when modeled 5-minute 

SO2 concentrations exceed the 200 ppb 

5-minute benchmark level (see proposal 

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

addition, the St. Louis exposure analysis 

estimates that a 99th percentile 1-hour 

daily maximum standard set at a level 

of 50 ppb would likely protect > 99% 

of asthmatic children at moderate or 

greater exertion from experiencing at 

least one 5-minute exposure both ≥ 400 

and > 200 ppb per year (see proposal 

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

addition, although not directly analyzed 

in the REA, the proposal (section 

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

maximum standard at a level of 75 ppb 

would be bound by the exposure 

estimates from air quality adjusted to 

just meet 99th percentile 1-hour daily 

maximum standards at 50 and 100 ppb. 

Thus, a 1-hour daily maximum standard 

at a level of 75 ppb would be estimated 

to protect > 99% of asthmatic children 

at moderate or greater exertion in St. 

Louis from experiencing at least one 

exposure ≥ 400 ppb per year, and about 

97% to > 99% of these children from 

experiencing at least one exposure ≥ 200 

ppb per year. 

The Administrator thus proposed to 

set the level of a new 1-hour standard 

that would protect public health with an 

adequate margin of safety between 50 

ppb and 100 ppb. In so doing, the 

Administrator relied on reported 

findings from both epidemiologic and 

controlled human exposure studies, as 

well as the results of air quality and 

exposure analyses. The Administrator 

noted that the lower end of the 

proposed range was consistent with 

CASAC advice that there is clearly 

sufficient evidence for consideration of 

standard levels starting at 50 ppb (Samet 

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

end of the proposed range, the 

Administrator noted that CASAC 

concluded that standards up to 150 ppb 

‘‘could be justified under some 

interpretations of weight of evidence, 

uncertainties, and policy choices 

regarding margin of safety’’ (id.), 

although the letter did not provide any 

indication of what interpretations, 

uncertainties, or policy choices might 

support selection of a level as high as 

150 ppb. 

In light of the range of levels included 

in CASAC’s advice, the Administrator 

also solicited comment on setting a 

standard level above 100 ppb and up to 

150 ppb. In so doing, the Administrator 

recognized that there are uncertainties 

with the scientific evidence, such as 

attributing effects reported in 

epidemiologic studies specifically to 

SO2 given the presence of co-occurring 

pollutants, especially PM, and the 

uncertainties associated with using 

ambient SO2 concentrations as a 

surrogate for exposure. However, the 

Administrator noted that compared to 

the proposed range of 50–100 ppb, a 

standard level as high as 150 ppb would 

not comparably limit 5-minute SO2

exposures ≥ 200 ppb. That is, she noted 

that the St. Louis exposure analysis 

estimated that a 150 ppb standard 

would protect approximately 88% of 

asthmatic children at moderate or 

greater exertion from experiencing at 

least one SO2 exposure ≥ 200 ppb per 

year (compared to > 99% and 

approximately 97% given standards at 

50 and 100 ppb respectively; see 

proposal Table 3 at 74FR at 64841). 

b. Comments on Level 

Most State and local agencies and 

organizations that commented on this 

issue expressed support for setting the 

level of a 1-hour SO2 standard 

somewhere within the proposed range 

of 50 to 100 ppb. More specifically, 

State environmental organizations (i.e., 

NACAA and NESCAUM); State 

environmental agencies (e.g., such 

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

VT), the Fond du Lac Tribe, and local 

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

New York City, Houston-Galveston Area 

Council) supported a level of a 1-hour 

SO2 standard in the range of 50 to 100 

ppb. In addition, State environmental 

agencies in IA and TX specifically 

supported a standard level of 100 ppb. 

In general, these groups cited the 

conclusions of CASAC and the 

Administrator’s rationale as stated in 

the proposal as a basis for their 

recommendations, though State 

environmental agencies in IA and TX 

generally recommended placing more 

weight on the controlled human 

exposure evidence rather than on the 

epidemiology. 

A number of environmental and 

medical/public health organizations 

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

WEACT for Environmental Justice, 

NRDC, CBD) and some local 

organizations (e.g., Alexandria 

Department of Transportation and 

Environmental Services, and Harris 

County (TX) Public Health & 

Environmental Services) supported 

setting a standard level at or near 50 

ppb. This recommendation was 

typically based on the commenters’ 

interpretation of the controlled human 

exposure and epidemiologic evidence, 

as described below. 

With regard to the controlled human 

exposure evidence, health and 

environmental groups generally 

concluded that a 1-hour SO2 standard 


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no higher than 50 ppb is needed to 

protect against 5-minute SO2 benchmark 

exposures as low as 100 ppb identified 

from mouthpiece exposure studies, 

rather than the 200 ppb 5-minute SO2

benchmark identified from ‘‘free 

breathing’’ controlled human exposure 

studies. More specifically, ALA et al., 

stated: 

In its analysis of data from chamber studies 

in the ISA and in the REA, EPA focuses on 

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

so, EPA improperly and arbitrarily 

downplays important evidence that reported 

increased airway resistance, a measure of 

bronchoconstriction, in subjects with mild 

asthma at concentrations of 100 ppb. 

Regrettably, EPA does not rely on the 

mouthpiece studies in formulating its 

proposed standards * * * In downplaying 

the mouthpiece studies, EPA ignores the 

large segment of people who rely on oral or 

oronasal breathing some or all of the time. 

The Administrator disagrees with the 

assertion that results from mouthpiece 

studies were improperly downplayed. 

These studies are discussed in the ISA, 

REA, and proposed rule as 

demonstrating respiratory effects of SO2

at concentrations of 100 ppb, the lowest 

concentration tested using a mouthpiece 

exposure system. Nonetheless, these 

mouthpiece studies are not a reasonable 

proxy for actual exposure. In these 

studies, SO2 is delivered directly 

through the mouth, typically in 

conjunction with nasal occlusion. This 

allows a greater fraction of the inhaled 

SO2 to reach the tracheobronchial 

airways. Although we agree with 

commenters that some individuals do 

breathe oronasally both while at rest 

and during exercise, nasal ventilation 

still constitutes a significant percentage 

of total ventilation. The consequence is 

that individuals exposed to SO2 through 

a mouthpiece are likely to experience 

greater respiratory effects from a given 

SO2 exposure than they would in real 

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

section 6.2) and in the proposal 

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

mouthpiece studies only provide very 

limited evidence of decrements in lung 

function following exposure to 100 ppb 

SO2. Therefore, the Administrator did 

not place great weight on these 

mouthpiece studies when considering 

the appropriate level of a 1-hour SO2

standard. 

In addition to their interpretation of 

the controlled human exposure 

evidence, health and environmental 

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

Sierra Club, CBD) and the Alexandria 

Department of Transportation and 

Environmental Services generally 

concluded that the epidemiologic 

evidence indicates that a standard no 

higher than 50 ppb is required to protect 

public health. For example, it its 

comments the CBD stated: 

Epidemiologic studies referenced in the 

Proposed Rule showed positive, and in many 

cases statistically significant, relationships 

between ambient SO2 concentrations and 

hospital admissions where 99th percentile 1- 

hour concentrations ranged from 50–460 ppb. 

Of these studies, two showed positive and 

sometimes statistically significant 

relationships in single-pollutant models at 50 

ppb, and three studies showed statistically 

significant correlations at 78–150 ppb in 

multi-pollutant models. These three 

multipollutant studies, moreover, ‘‘lend[] 

strong support * * * to the conclusion that 

SO2 effects are generally independent’’ of 

those of co-pollutants like particulate matter. 

Giving these studies their proper weight, and 

allowing for an adequate margin of safety, 

EPA should set a one-hour NAAQS at a level 

no higher than the lowest concentration at 

which positive, adverse relationships have 

been demonstrated: 50 ppb (note that 

footnotes were omitted). 

The Administrator agrees that the 

epidemiologic studies referenced in the 

proposal need to be considered in 

judging the appropriate level for a new 

99th percentile 1-hour SO2 standard. 

However, she disagrees that when 

considered in total, these studies 

strongly support an SO2 standard no 

higher than 50 ppb. The Administrator 

notes that selecting a standard level of 

50 ppb would place considerable weight 

on the two U.S. emergency department 

visit studies conducted in locations 

where 99th percentile 1-hour SO2

concentrations were approximately 50 

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

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

Columbus, OH). However, the 

Administrator does not find this 

appropriate given that, importantly, 

neither of these studies evaluated the 

potential for confounding by copollutants through the use of 

multipollutant models and thus, left 

unaddressed the issue of whether the 

effects seen in the studies were partially 

or totally attributable to exposure to 

sulfate PM. In addition, the 

Administrator notes that the overall 

results reported in these studies are 

mixed. It is important to note that mixed 

results do not automatically disqualify 

studies from being used as part of the 

evidence base for setting levels in 

NAAQS reviews. However, in this 

review the Administrator judges that the 

lack of mutipollutant model evaluation 

for potential confounding by PM in two 

locations with the lowest SO2 levels 

combined with the presence of mixed 

emergency department visit results 

renders these two studies inappropriate 

to serve as the primary basis for the 

selection of the level of the SO2

NAAQS. As an additional matter, the 

suggestion in some of the comments that 

EPA should necessarily base the level of 

a NAAQS on the lowest level seen in 

epidemiologic studies has been rejected 

repeatedly. See, e.g. American 

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

1187 (‘‘In so arguing NRDC essentially 

ignores the mixed results of the medical 

studies evident in the record, choosing 

instead to rely only on the studies that 

favor its position. The Administrator, 

however, was required to take into 

account all the relevant studies revealed 

in the record. Because he did so in a 

rational manner, we will not overrule 

his judgment as to the margin of 

safety.’’) Thus, although the 

Administrator finds that these two 

studies provide limited evidence of 

emergency department visits in cities 

where 99th percentile 1-hour daily 

maximum SO2 concentrations are 

approximately 50 ppb, she also 

concludes that these studies do not 

provide enough evidence to warrant a 

standard at this level. 

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

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

UARG) did not support setting a new 1- 

hour SO2 standard. However, several of 

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

SC Chamber of Commerce concluded 

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

hour standard, the level of that standard 

should be ≥ 150 ppb. In addition, State 

environmental agencies in SD (SD 

DENR) and OH recommended standard 

levels at 150 ppb. As a basis for this 

recommendation, these groups generally 

emphasized uncertainties in the 

scientific evidence. Specifically, as 

discussed in more detail above (section 

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

concluded that the available 

epidemiologic studies do not support 

the conclusion that SO2 causes the 

reported health effects. This was based 

on their assertion that the presence of 

co-pollutants in the ambient air 

precludes the identification of a specific 

SO2 contribution to reported effects. 

Thus, these groups generally concluded 

that weight should not be placed on the 

cluster of three epidemiologic studies 

reporting statistically significant effects 

in multipollutant models with PM (i.e., 

NYDOH 2006; Ito 2007; and Schwartz 

1995). That is, these groups contend that 

these studies do not demonstrate an 

independent effect of SO2. In addition, 

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

these groups also disagreed with the 

Agency’s judgment that adverse 

respiratory effects occur following 5- 

minute exposures to SO2 concentrations 

as low as 200 ppb. These comments and 

EPA’s responses are discussed below 


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and in section II of the RTC document 

(EPA 2010). 

As described in more detail in section 

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

of SO2 epidemiologic studies is 

complicated by the fact that SO2 is but 

one component of a complex mixture of 

pollutants present in the ambient air. 

However, the ISA concluded that when 

U.S. and international epidemiologic 

literature is evaluated as a whole, SO2

effect estimates generally remained 

positive and relatively unchanged in 

multi-pollutant models with gaseous or 

particulate co-pollutants. Thus, 

although recognizing the uncertainties 

associated with separating the effects of 

SO2 from those of co-occurring 

pollutants, the ISA concluded that the 

limited available evidence from studies 

employing multi-pollutant models 

indicates that the effect of SO2 on 

respiratory health outcomes appears to 

be generally robust and independent of 

the effects of gaseous co-pollutants, 

including NO2 and O3, as well as 

particulate co-pollutants, particularly 

PM2.5 (ISA, section 5.2; p. 5–9). 

In addition, as described in detail 

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

emphasized that controlled human 

exposure studies provide support for the 

plausibility of the associations reported 

in epidemiologic studies. The ISA noted 

that the results of controlled human 

exposure and epidemiologic studies 

form a plausible and coherent data set 

that supports a causal relationship 

between short-term (5-minutes to 24- 

hours) SO2 exposures and adverse 

respiratory effects, and that the 

epidemiologic evidence (buttressed by 

the clinical evidence) indicates that the 

effects seen in the epidemiologic studies 

are attributable to exposure to SO2 (ISA, 

section 5.2). The ISA in fact made the 

strongest finding possible regarding 

causality: ‘‘[e]valuation of the health 

evidence, with consideration of issues 

related to atmospheric sciences, 

exposure assessment, and dosimetry, 

led to the conclusion that there is a 

causal relationship between respiratory 

morbidity and short-term exposure to 

SO2. This conclusion is supported by 

the consistency, coherence, and 

plausibility of findings observed in the 

human clinical, epidemiologic, and 

animal toxicological studies.’’ ISA p. 

5–2 (emphasis original). 

As mentioned above, many groups 

dispute the ISA conclusion that taken 

together, results from U.S. and 

international epidemiologic studies 

employing multipollutant models 

indicate that SO2 has an independent 

effect on the respiratory health 

outcomes reported in these studies. 

Thus, these groups contend that the 

Administrator should not place weight 

on epidemiologic studies and their 

associated air quality information in 

general, and more specifically, the 

Administrator should not place weight 

on air quality information from the three 

U.S. epidemiologic studies reporting 

statistically significant effects in 

multipollutant models with PM (i.e., 

NYDOH 2006; Ito 2007; and Schwartz 

1995). Specific comments on these three 

epidemiologic studies reporting 

statistically significant effects in multipollutant models with PM, and EPA 

responses are presented below and in 

the RTC document (EPA 2010). 

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

comments with respect to the study 

conducted by the NYDOH (NYDOH, 

2006). First, these groups generally 

concluded that the results of this study 

are mixed. That is, while SO2 effect 

estimates were positive and statistically 

significant even in multipollutant 

models with PM2.5 or NO2 in the Bronx, 

SO2 effect estimates were actually 

negative in Manhattan in both single 

and multipollutant models. These 

groups also contend that this report was 

not peer-reviewed and that the authors 

of this study indicated that high 

correlations among pollutants in the 

Bronx made it difficult to confidently 

identify which pollutants are actually 

increasing risks. For these reasons, 

industry groups generally concluded 

that this study should not be relied 

upon by the Administrator. 

We acknowledge that the results of 

the NYDOH analysis are mixed when 

comparing the Bronx and Manhattan 

study areas. However, we disagree that 

the presence of mixed results renders 

this study unreliable. We note that the 

mixed results reported in this study are 

likely to reflect greater statistical power 

for identifying effects in the Bronx, 

where the average daily emergency 

department visits differed substantially 

from those in Manhattan. Specifically, 

daily asthma emergency department 

visits were six times higher in the Bronx 

study area (43 per day) than in the 

Manhattan study area (7.2 per day). 

Thus, the more prominent effects in the 

Bronx likely at least partially reflect 

greater statistical power for identifying 

effects there. To put these numbers in 

perspective, the crude daily rates of 

asthma emergency department visits can 

be estimated by dividing the daily 

asthma counts by the population. The 

mean daily crude rates of asthma 

emergency department visits were over 

eight-fold higher in the Bronx study area 

(16.9 per 100,000 persons) than in the 

Manhattan area (2.02 per 100,000 

persons). Population age structures were 

quite different in the two communities, 

with larger proportions of younger 

persons in the Bronx versus Manhattan. 

There are likely additional differences 

in population structures of the two 

communities, including differences in 

SES, race/ethnicity, and access to 

primary asthma care. These differences 

in the two communities may explain the 

differences in the results, and do not 

prevent EPA from legitimately relying 

on this study. 

As mentioned above, these groups 

also contend that the NYDOH 

epidemiologic study should not be 

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

assertion. The NYDOH study was 

subject to multiple peer-review 

processes. This included reviews by the 

Agency for Toxic Substances and 

Disease Registry (ATSDR), EPA, and 

CASAC. 

Finally, as also mentioned above, 

these groups contend that the NYDOH 

epidemiologic study is unreliable 

because the study authors indicated that 

high correlations among pollutants in 

the Bronx make it difficult to 

confidently identify which pollutants 

are actually increasing risks. In 

response, we note that high correlations 

among ambient air pollutant 

concentrations are not specific to the 

NYDOH study, and may contribute to 

uncertainty in the interpretation of 

many epidemiologic studies of air 

pollution. The approach most 

commonly utilized to disentangle the 

effects of correlated pollutants in air 

pollution epidemiology is the 

copollutant model. The NYDOH uses 

copollutant models and finds that the 

results for SO2 remain significant in 

models considering the simultaneous 

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

indicates an independent effect of SO2

on the asthma emergency department 

visits reported in this study. 

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

industry groups generally commented 

that since the SO2 effect estimate did 

not remain statistically significant in 

multipollutant models with NO2, this 

study does not indicate an independent 

effect of SO2 on emergency department 

visits in the NYC study area. API 

specifically commented: 

The RR for an increase of 6 ppb SO2 was 

statistically significant (1.20; 95% CI: 1.13, 

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

was included in the model, but became 

nonsignificant when NO2 was included in 

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

1.1). Because associations with SO2 could be 

attributable to NO2, this study cannot be used 

to assess the effects of SO2 on health effects 

with small incremental increases in 

exposure. 


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17See Coalition of Battery Recyclers Association 

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

opinion at 9, holding that it was reasonable for EPA 

to conclude that a two IQ point mean population 

loss is an adverse effect based in part on 

consideration of comments from the American 

Academy of Pediatrics that such a loss should be 

prevented. 

We disagree with the commenters. We 

believe that this study does demonstrate 

an independent effect of SO2 on 

emergency department visits in NYC. 

We note that evidence from controlled 

human exposure studies has 

demonstrated effects of NO2 (EPA, 

2008b) and SO2 independently on 

respiratory morbidity. Since each of 

these criteria pollutants has an 

independent effect on the respiratory 

system, it is logical that each may be 

responsible for an increase in 

emergency department visits for asthma 

in epidemiologic studies. In addition, 

the authors note that the attenuation of 

the SO2 effect estimate when NO2 is 

included in the model is ‘‘consistent 

with the result of monitor-to-monitor 

correlations, suggesting that NO2 has 

less exposure error than CO or SO2 in 

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

the high spatial heterogeneity of SO2

compared to NO2, leading to increased 

exposure error, may be causing the 

attenuation of the SO2 effect estimate 

when NO2 is included in the model in 

this study—not that the effects seen in 

the study are attributable to NO2. 

Overall, the results from this study are 

consistent with the SO2 effect on 

respiratory emergency department visits 

and hospital admissions across studies 

and are coherent with the respiratory 

effects observed in controlled human 

exposure studies. This study thus 

provides persuasive evidence of an 

independent effect of short-term SO2

exposure on respiratory morbidity. 

With respect to Schwartz et al., 

(1995), industry groups generally 

commented that the results of this study 

are mixed, and therefore should not be 

considered by the Administrator. More 

specifically, these commenters noted 

that although the results in New Haven 

remained statistically significant in the 

presence of PM10, the SO2 effect 

estimate in Tacoma was reduced and no 

longer statistically significant in the 

presence of PM10. Commenters also 

noted that in both cities, the SO2 effect 

estimate was reduced and no longer 

statistically significant in the presence 

of O3. 

We disagree that the results of this 

study of hospital admissions should not 

be considered by the Administrator. As 

noted by the commenters, this study 

was conducted in two cities, New 

Haven, CT and Tacoma, WA. These 

cities were chosen because they differ in 

several important aspects and the author 

expected the results from the two cities 

to be different due to the inherent 

nature of the study design and study 

locations. ‘‘New Haven has almost twice 

the mean SO2 concentration of Tacoma, 

almost two and a half times the SO2

concentration in the peak winter season, 

and a much larger summer ozone peak 

than Tacoma (Schwartz 1995).’’ Since 

the study was designed to examine the 

differences in these two cities, the fact 

that the results differed in the two cities 

does not invalidate those results. In 

addition, EPA considers the SO2 effect 

to be robust to inclusion of O3 in New 

Haven. The central effect estimate for 

SO2 changed from 1.03 to 1.02 after the 

addition of O3 as a copollutant and 

likely lost statistical significance due to 

a greater than 40% reduction in the 

number of days included because O3

was only measured during the warm 

months. This reduction likely led to 

model instability and a loss of statistical 

significance. To be consistent with how 

results of other studies were interpreted 

in the ISA, and as supported by the 

CASAC, the effect of SO2 is considered 

robust to the inclusion of O3 in New 

Haven. 

In addition to generally concluding 

that the epidemiology is too uncertain to 

demonstrate that SO2 has an 

independent effect on the respiratory 

effects reported in those studies, many 

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

Progress Energy, EEI, CIBO) also 

commented that adverse health effects 

do not occur following 5–10 minute SO2

exposures < 400 ppb in controlled 

human exposure studies (an issue also 

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

Thus, these groups generally maintained 

that the level of a 1-hour standard 

should not take into account limiting 5- 

minute peaks as low as 200 ppb. From 

this argument, many of these groups 

further maintained that 1-hour standard 

levels ≥ 150 ppb are requisite to protect 

public health with an adequate margin 

of safety. 

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

above, we disagree with the commenters 

that adverse respiratory effects do not 

occur following 5-minute SO2 exposures 

as low as 200 ppb. The ISA reported 

that exposure to SO2 concentrations as 

low as 200–300 ppb for 5–10 minutes 

results in approximately 5–30% of 

exercising asthmatics experiencing 

moderate or greater decrements in lung 

function (defined in terms of a ≥ 15% 

decline in FEV1 or 100% increase in 

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

2000 ATS guidelines described in 

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

these results could reasonably indicate 

an SO2-induced shift in these lung 

function measurements for this subpopulation. Under this scenario, an 

appreciable percentage of exercising 

asthmatics exposed to SO2

concentrations as low as 200 ppb would 

likely have diminished reserve lung 

function and thus would likely be at 

greater risk if affected by another 

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

Importantly, diminished reserve lung 

function in a population that is 

attributable to air pollution is 

considered an adverse effect under ATS 

guidance.17 Also noted in section 

II.E.2.b, we were mindful of CASAC’s 

pointed comments. The second draft 

ISA placed relatively little weight on 

health effects associated with SO2

exposures at 200–300 ppb. CASAC 

strongly disagreed with this 

characterization of the health evidence. 

Their consensus letter following the 

second draft ISA states: 

Our major concern is the conclusions in 

the ISA regarding the weight of the evidence 

for health effects for short-term exposure to 

low levels of SO2. Although the ISA presents 

evidence from both clinical and 

epidemiological studies that indicate health 

effects occur at 0.2 ppm or lower, the final 

chapter emphasizes health effects at 0.4 ppm 

and above * * * CASAC believes the clinical 

and epidemiological evidence warrants 

stronger conclusions in the ISA regarding the 

available evidence of health effects at 0.2 

ppm or lower concentrations of SO2. The 

selection of a lower bound concentration for 

health effects is very important because the 

ISA sets the stage for EPA’s risk assessment 

decisions. In its draft Risk and Exposure 

Assessment (REA) to Support the Review of 

the SO2 Primary National Ambient Air 

Quality Standards (July 2008), EPA chose a 

range of 0.4 ppm—0.6 ppm SO2

concentrations for its benchmark analysis. As 

CASAC will emphasize in a forthcoming 

letter on the REA, we recommend that a 

lower bound be set at least as low as 0.2 ppm 

(Henderson 2008a). 

Similarly, we were also mindful of 

CASAC comments on the first draft of 

the REA. The consensus CASAC letter 

following the 1st draft REA states: 

The CASAC believes strongly that the 

weight of clinical and epidemiology evidence 

indicates there are detectable clinically 

relevant health effects in sensitive 

subpopulations down to a level at least as 

low as 0.2 ppm SO2. These sensitive 

subpopulations represent a substantial 

segment of the at-risk population (Henderson 

2008b). 

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

also mindful of: (1) Previous CASAC 

recommendations (Henderson 2006) and 

NAAQS review conclusions (EPA 2006, 

EPA 2007d) indicating that moderate 

decrements in lung function can be 

clinically significant in some asthmatics 

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


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18The ISA concluded that collective evidence 

from key controlled human exposure studies 

considered in the previous review, along with a 

limited number of new controlled human exposure 

studies, consistently indicates that with elevated 

ventilation rates a large percentage of asthmatic 

individuals tested in a given chamber study (up to 

60%, depending on the study) experience moderate 

or greater decrements in lung function, frequently 

accompanied by respiratory symptoms, following 

peak exposures to SO2 at concentrations of 0.4–0.6 

ppm. (ISA, p. 3–9). 

(2) controlled human exposure studies 

not including severe asthmatics and 

thus, that it is reasonable to assume that 

persons with more severe asthma than 

the study participants would have a 

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

echoed this concern in its comments on 

the policy assessment chapter of the 

REA: 

Chapter 10 should better address 

uncertainty in identifying alternative NAAQS 

for SO2. In particular, the uncertainties 

discussed in the health risk characterization 

should be considered in specifying a NAAQS 

that provides adequate margin of safety. One 

particular source of uncertainty needing 

acknowledgment is the characteristics of 

persons included in the clinical studies. The 

draft REA acknowledges that clinical studies 

are unlikely to have included severe 

asthmatics that are likely to be potentially at 

greater risk than those persons included in 

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

Taken together, the Administrator 

concluded that exposure to SO2

concentrations as low as 200 ppb can 

result in adverse health effects in 

asthmatics. Consequently the 

Administrator also concluded that a 

1-hour standard of 150 ppb is not 

requisite to protect public health with 

an adequate margin of safety, even with 

a 99th percentile form. This conclusion 

takes into account the St. Louis 

exposure analysis estimating that only 

88% of asthmatic children at moderate 

or greater exertion would be protected 

from at least one 5-minute SO2 exposure 

≥ 200 ppb per year at a 1-hour standard 

level of 150 ppb, and appropriate weight 

placed on the epidemiologic evidence 

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

the epidemiologic evidence with respect 

to level). 

c. Conclusions on Standard Level 

Having carefully considered the 

public comments on the appropriate 

level for a 1-hour SO2 standard, as 

discussed above, the Administrator 

believes the fundamental conclusions 

reached in the ISA and REA remain 

valid. In considering the level at which 

the 1-hour primary SO2 standard should 

be set, the Administrator continues to 

place primary emphasis on the body of 

controlled human exposure and 

epidemiologic evidence assessed in the 

ISA, as summarized above in section 

II.B. In addition, the Administrator 

continues to view the results of 

exposure and risk analyses, discussed 

above in section II.C, as providing 

supporting information for her decision. 

In considering the level of a 1-hour 

SO2 standard, the Administrator notes 

that there is no bright line clearly 

mandating the choice of level within the 

reasonable range proposed. Rather, the 

choice of what is appropriate within 

this reasonable range is a public health 

policy judgment entrusted to the 

Administrator. This judgment must 

include consideration of the strengths 

and limitations of the evidence and the 

appropriate inferences to be drawn from 

the evidence and the exposure and risk 

assessments. These considerations and 

the Administrator’s final decision with 

regard to the level of a new 1-hour SO2

standard are discussed below. 

In considering the controlled human 

exposure studies, the Administrator 

notes that these studies provide the 

most direct evidence of respiratory 

effects from exposure to SO2. These 

studies exposed groups of exercising 

asthmatics to defined concentrations of 

SO2 for 5–10 minutes and found adverse 

respiratory effects. As noted above (see 

section II.C), SO2 exposure levels which 

resulted in respiratory effects in these 

studies were considered 5-minute 

benchmark exposures of potential 

concern in the analyses found in the 

REA. With respect to this evidence, the 

Administrator notes the following key 

points: 

• Exposure of exercising asthmatics 

to 5–10 minute SO2 concentrations ≥ 

400 ppb results in moderate or greater 

decrements in lung function (in terms of 

FEV1 or sRaw) in 20–60% of tested 

individuals in these studies. Moreover, 

these decrements in lung function are 

often statistically significant at the 

group mean level and are frequently 

accompanied by respiratory 

symptoms.18 Based on ATS guidelines, 

exposure to SO2 concentrations ≥ 400 

ppb clearly result in adverse respiratory 

effects (i.e., decrements in lung function 

in the presence of respiratory 

symptoms). Therefore, the 

Administrator has concluded it 

appropriate to place weight on the 400 

ppb 5-minute SO2 benchmark 

concentration of concern. 

• Exposure of exercising asthmatics 

to 5–10 minute SO2 concentrations at 

200–300 ppb results in moderate or 

greater decrements in lung function in 

5–30% of the tested individuals in these 

studies. The Administrator notes that 

although these decrements in lung 

function have not been shown to be 

statistically significant at the group 

mean level, or to be frequently 

accompanied by respiratory symptoms, 

she considers effects associated with 

exposures as low as 200 ppb to be 

adverse in light of CASAC advice, 

similar conclusions in prior NAAQS 

reviews, and the ATS guidelines 

described in detail above (see section 

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

concluded it appropriate to place weight 

on the 200 ppb 5-minute benchmark 

concentration. 

• There is very limited evidence from 

two mouthpiece exposure studies 

suggesting respiratory effects in 

exercising asthmatics following SO2

exposures at 100 ppb. However, given 

the uncertainties and potential 

unrepresentativeness associated with 

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

above), the Administrator found it 

appropriate not to place weight on this 

5-minute SO2 benchmark concentration. 

The Administrator also considered 

the results of the air quality, exposure, 

and risk analyses, as they serve to 

estimate the extent to which a given 

1-hour standard limits the 5-minute 

benchmark concentrations of concern 

identified from controlled human 

exposure studies (see REA chapters 

7–9, proposal section II.F.4.b, and 

proposal Tables 2–4). In considering 

these results as they relate to limiting 

5-minute SO2 benchmark concentrations 

≥ 200 and 400 ppb, the Administrator 

notes the following key points: 

• The 40-county air quality analysis 

estimates that a 100 ppb 1-hour daily 

maximum standard would allow at most 

2 days per year on average in any 

county when estimated 5-minute daily 

maximum SO2 concentrations exceed 

the 400 ppb benchmark, and at most 13 

days per year on average when 5-minute 

daily maximum SO2 concentrations 

exceed the 200 ppb benchmark (see 

proposal, Table 2, 74 FR at 64840). 

Furthermore, given a simulated 1-hour 

100 ppb standard level, most of the 

counties in that air quality analysis were 

estimated to experience 0 days per year 

on average when 5-minute daily 

maximum SO2 concentrations exceed 

the 400 ppb benchmark and ≤ 3 days per 

year on average when 5-minute daily 

maximum SO2 concentrations were 

estimated to exceed the 200 ppb 

benchmark (see REA, Tables 7–14 and 

7–12). 

• The St. Louis exposure analysis 

estimates that a 99th percentile 1-hour 

daily maximum standard at a level of 

100 ppb would likely protect > 99% of 

asthmatic children in that city at 

moderate or greater exertion from 

experiencing at least one 5-minute 

exposure ≥ 400 ppb per year, and 


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19For example, as noted in the proposal 

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

of a pattern of results from a group of studies that 

find effect estimates similar in direction and 

magnitude would warrant consideration of and 

reliance on such studies even if the studies did not 

all report statistically significant associations in 

single- or multi-pollutant models. The SO2

epidemiologic studies fit this pattern, and are 

buttressed further by the results of the clinical 

studies. ISA, section 5.2. 

approximately 97% of those asthmatic 

children at moderate or greater exertion 

from experiencing at least one exposure 

≥ 200 ppb per year (see proposal, 

section II.F.4.b). 

• The St. Louis risk assessment 

estimates that a 99th percentile 1-hour 

standard level at 100 ppb would likely 

protect about 97–98% of exposed 

asthmatic children in that city at 

moderate or greater exertion from 

experiencing at least one moderate or 

greater lung function response (defined 

as a ≥ 100% increase in sRaw; see 

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

Given the above considerations, the 

Administrator concludes that a 1-hour 

standard at a level of 100 ppb would 

appropriately limit 5-minute SO2

benchmark concentrations ≥ 200 or 400 

ppb. Moreover, although the 

Administrator acknowledges that the air 

quality and exposure analyses 

mentioned above suggest that a 50 ppb 

standard may somewhat further limit 5- 

minute SO2 concentrations/exposures in 

excess of the 200 ppb benchmark (see 

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

believe this information alone warrants 

a standard level lower than 100 ppb. 

More specifically, although she 

considers the health effects resulting 

from 5-minute SO2 exposures as low as 

200 ppb to be adverse, she also 

recognizes that such effects are 

appreciably less severe than those at 

SO2 concentrations ≥ 400 ppb. Thus, she 

concludes that there is little difference 

in limiting 5-minute concentrations/ 

exposures ≥ 400 ppb given 1-hour 

standard levels in the range of 50 to 100 

ppb. 

In considering the epidemiologic 

evidence with regard to level, the 

Administrator notes that there have 

been more than 50 peer reviewed 

epidemiologic studies published 

worldwide evaluating SO2 (ISA, Tables 

5–4 and 5–5). These studies have 

generally reported positive, although 

not always statistically significant 

associations between more serious 

health outcomes (i.e. respiratory-related 

emergency department visits and 

hospitalizations) and ambient SO2

concentrations and have generally 

included populations potentially at 

increased risk for SO2-related 

respiratory effects (e.g, children, older 

adults, and those with pre-existing 

respiratory disease). The Administrator 

finds that in assessing the extent to 

which these studies and their associated 

air quality information can inform the 

level of a new 99th percentile 1-hour 

daily maximum standard for the U.S., 

air quality information from the U.S. 

and Canada is most relevant since these 

areas have similar monitor network 

designs and patterns of air quality. 

However, as described in proposal 

section II.F.4.a, SO2 concentrations 

reported for Canadian studies were not 

directly comparable to those reported 

for U.S. studies due to use of different 

monitoring protocols in those studies. 

Thus, the Administrator focused on 

99th percentile air quality information 

from U.S. studies for informing 

potential 1-hour standard levels. She 

concludes that this information 

provides evidence of associations 

between ambient SO2 and emergency 

department visits and hospital 

admissions in U.S. cities with particular 

99th percentile 1-hour SO2 levels, and 

thus provides information that is 

particularly relevant for setting the level 

of a 1-hour SO2 standard. With regard to 

these studies she notes the following 

key points: 

• Ten studies (some conducted in 

multiple locations) reported mostly 

positive, and sometimes statistically 

significant, associations between 

ambient SO2 concentrations and 

emergency department visit and 

hospital admissions in locations where 

99th percentile 1-hour daily maximum 

SO2 levels ranged from approximately 

50–460 ppb. 

• Within this broader range of SO2

concentrations, there is a cluster of three 

epidemiologic studies between 78–150 

ppb (for the 99th percentile of the 1- 

hour SO2 concentrations) where the SO2

effect estimate remained positive and 

statistically significant in multipollutant models with PM (NYDOH 

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

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

statistical significance in multipollutant models is an important 

consideration, it is not the only 

consideration when relying on such 

epidemiologic evidence.19 However, as 

noted earlier, there is special sensitivity 

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

from SO2-related effects in interpreting 

the epidemiologic studies; thus, these 

studies are of particular relevance here, 

lending strong support both to the 

conclusion that SO2 effects are generally 

independent of PM (ISA, section 5.2) 

and that these independent adverse 

effects of SO2 have occurred in cities 

with 1-hour daily maximum, 99th 

percentile concentrations in the range of 

78–150 ppb. Nor did EPA find the 

comments criticizing these studies 

persuasive, as explained above in 

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

document (EPA 2010). The 

Administrator therefore judges it 

appropriate to place substantial weight 

on this cluster of three U.S. 

epidemiologic studies in selecting a 

standard level, as they are a group of 

studies that reported positive and 

statistically significant associations 

between ambient SO2 and emergency 

department visits or hospital admissions 

even when potential confounding by 

PM was considered. 

• The Administrator agrees with the 

finding in the ISA that the controlled 

human exposure evidence lends 

biological plausibility to the effects 

reported in epidemiologic studies (ISA, 

p. 5–9). 

• There is limited evidence from two 

epidemiologic studies employing single 

pollutant models that found generally 

positive associations between ambient 

SO2 and emergency department visits in 

locations where 99th percentile 1-hour 

SO2 concentrations were approximately 

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

However, considering that the results of 

these studies were mixed, and 

importantly, that neither of these two 

studies evaluated the potential for 

confounding by co-pollutants through 

the use of multipollutant models 

(particularly with PM), the 

Administrator judges it appropriate to 

place limited weight on these studies. 

• With regard to the cluster of three 

studies conducted in the Bronx 

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

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

there is a degree of uncertainty as to 

whether the 99th percentile 1-hour daily 

maximum SO2 concentrations reported 

from monitors in these three study areas 

reflect the highest 99th percentile 

1-hour daily maximum SO2

concentration. Our limited qualitative 

analysis suggests that 99th percentile 1- 

hour daily maximum SO2

concentrations reported by monitors in 

these study areas are reasonable 

approximations for the absolute highest 

99th percentile 1-hour daily maximum 

SO2 concentration that can occur across 

the entire area in these studies 

(including the areas where monitors 

were not located) (see Brode, 2010). 

However, although a reasonable 

approximation, it is still likely that 

these monitored concentrations are 

somewhat lower than the absolute 

highest 99th percentile 1-hour daily 

maximum SO2 concentrations occurring 

across these epidemiologic study areas. 


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Weighing all of this evidence, the 

Administrator concludes that the 

epidemiologic studies provide strong 

support for setting a standard that limits 

the 99th percentile of the distribution of 

1-hour daily maximum SO2

concentrations to 75 ppb. This judgment 

takes into account the strong 

determinations in the ISA (and 

endorsed by CASAC), based on a much 

broader body of evidence, that there is 

a causal association between exposure 

to SO2 and the types of respiratory 

morbidity effects reported in these 

studies. The Administrator further 

judges that it is not necessary based on 

existing epidemiologic evidence, to set 

a standard below 75 ppb. That is, the 

Administrator concludes that a standard 

level of 75 ppb is sufficiently below the 

SO2 levels in three cities where 

epidemiologic studies found statistically 

significant effects in multipollutant 

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

ppb) to provide an adequate margin of 

safety given the uncertainty as to 

whether monitors in these study 

locations reflected the highest 1-hour 

daily maximum SO2 concentration 

across the entire study area. Thus, a 

standard set at a level of 75 ppb is likely 

further below the 99th percentile 1-hour 

daily maximum concentrations in these 

three study areas than the bare 

comparison of levels would otherwise 

indicate. Finally, the Administrator 

again notes that epidemiologic evidence 

below 75 ppb is more uncertain because 

studies below 75 ppb did not evaluate 

potential confounding of results in 

multipollutant models, and because 

these studies reported mixed results. 

Given the above considerations and 

the comments received on the proposal, 

the Administrator determines that the 

appropriate judgment, based on the 

entire body of evidence and information 

available in this review, and the related 

uncertainties, is a standard level of 

75 ppb. She concludes that such a 

standard, with a 1-hour averaging time 

and 99th percentile form, will provide 

a significant increase in public health 

protection compared to the current 

standards and would be expected to 

protect against the respiratory effects 

that have been linked with SO2

exposures in both controlled human 

exposure and epidemiologic studies. 

Specifically, she concludes that such a 

standard will limit 1-hour exposures at 

and above 75 ppb for those in 

susceptible populations that are at-risk 

of experiencing adverse health effects 

from short-term exposure to SO2. Such 

a standard will also maintain SO2

concentrations below those in locations 

where key U.S. epidemiologic studies 

have reported that ambient SO2 is 

associated with clearly adverse 

respiratory health effects, as indicated 

by increased hospital admissions and 

emergency department visits. She also 

notes that a 1-hour standard at a level 

of 75 ppb is expected to substantially 

limit asthmatics’ exposure to 5–10 

minute SO2 concentrations ≥ 200 ppb, 

thereby substantially limiting the 

adverse health effects associated with 

such exposures. Finally, the 

Administrator notes that a standard 

level of 75 ppb is consistent with the 

consensus recommendation of CASAC. 

In setting the standard level at 75 ppb 

rather than at a lower level, the 

Administrator notes that a 1-hour 

standard with a level lower than 75 ppb 

would only result in significant further 

public health protection if, in fact, there 

is a continuum of serious, adverse 

health risks caused by exposure to SO2

concentrations below 75 ppb. Based on 

the available evidence, the 

Administrator does not believe that 

such assumptions are warranted. Taking 

into account the uncertainties that 

remain in interpreting the evidence 

from available controlled human 

exposure and epidemiologic studies, the 

Administrator notes that the likelihood 

of obtaining benefits to public health 

with a standard set below 75 ppb 

decreases, while the likelihood of 

requiring reductions in ambient 

concentrations that go beyond those that 

are needed to protect public health 

increases. 

Therefore, the Administrator judges 

that a 1-hour SO2 standard at 75 ppb is 

sufficient to protect public health with 

an adequate margin of safety. This 

includes protection with an adequate 

margin of safety for susceptible 

populations at increased risk for adverse 

respiratory effects from short-term 

exposures to SO2 for which the evidence 

supports a causal relationship with SO2

exposures. The Administrator does not 

believe that a lower standard level is 

needed to provide this degree of 

protection. These conclusions by the 

Administrator appropriately consider 

the requirement for a standard that is 

neither more nor less stringent than 

necessary for this purpose and 

recognizes that the CAA does not 

require that primary NAAQS be set at a 

zero-risk level or to protect the most 

susceptible individual, but rather at a 

level that reduces risk sufficiently so as 

to protect the public health with an 

adequate margin of safety. 

5. Retaining or Revoking the Current 

24-Hour and Annual Standards 

This section discusses considerations 

related to retaining or revoking the 

current 24-hour and annual SO2 primary 

NAAQS. Specifically, this section 

summarizes the rationale for the 

Administrator’s proposed decision 

regarding whether to retain or revoke 

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

discusses public comments related to 

whether to retain or revoke the current 

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

Administrator’s final conclusions 

regarding whether to retain or revoke 

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

a. Rationale for Proposed Decision 

As noted in the proposal (see section 

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

particular level selected for a new 99th 

percentile 1-hour daily maximum 

standard would have implications for 

deciding whether to retain or revoke the 

current 24-hour and annual standards. 

That is, with respect to SO2-induced 

respiratory morbidity, the lower the 

level selected for a 99th percentile 

1-hour daily maximum standard, the 

less additional public health protection 

the current standards would be 

expected to provide. CASAC expressed 

a similar view following their review of 

the 2nd draft REA: ‘‘Assuming that EPA 

adopts a one hour standard in the range 

suggested, and if there is evidence 

showing that the short-term standard 

provides equivalent protection of public 

health in the long-term as the annual 

standard, the panel is supportive of the 

REA discussion of discontinuing the 

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

With regard to the current 24-hour 

standard, CASAC was generally 

supportive of using the air quality 

analyses in the REA as a means of 

determining whether the current 

24-hour standard was needed in 

addition to a new 1-hour standard to 

protect public health. CASAC stated: 

‘‘The evidence presented [in REA Table 

10–3] was convincing that some of the 

alternative one-hour standards could 

also adequately protect against 

exceedances of the current 24-hour 

standard’’ (Samet 2009, p. 15). 

In accordance with the REA findings 

and CASAC recommendations 

mentioned above, the Administrator 

noted that 1-hour standards in the range 

of 50–100 ppb would have the effect of 

maintaining 24-hour and annual SO2

concentrations generally well below the 

levels of the current 24-hour and annual 

NAAQS (see REA Tables 10–3 and 10– 

4 and REA Appendix Tables D–3 to D– 

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

daily maximum standard was set in the 

proposed range of 50–100 ppb, then the 

Administrator proposed to revoke the 

current 24-hour and annual standards. 

However, as noted in the proposal, if a 

standard was set at a level >100 ppb and 


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up to 150 ppb, then the Administrator 

indicated that she would retain the 

existing 24-hour standard, recognizing 

that a 99th percentile 1-hour daily 

maximum standard at 150 ppb would 

not have the effect of maintaining 24- 

hour average SO2 concentrations below 

the level of the current 24-hour standard 

in all locations analyzed (see REA 

Appendix Table D–4). Under this 

scenario, the Administrator would still 

revoke the current annual standard 

recognizing: (1) 99th percentile 1-hour 

daily maximum standards in the range 

of 50–150 ppb would maintain annual 

average SO2 concentrations below the 

level of the current annual standard (see 

REA Table 10–4 and REA Appendix 

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

sufficient evidence linking long-term 

SO2 exposure to adverse health effects. 

b. Comments on Retaining or Revoking 

the Current 24-Hour and Annual 

Standards 

As noted above, most industry groups 

were opposed to the proposed revisions 

to the SO2 NAAQS. However, some of 

these groups noted that if a 1-hour 

standard was adopted, then they would 

support revoking the current 24-hour 

and annual standards. State agencies 

generally supported revoking the 

current standards if a 1-hour standard 

was set in the proposed range, although 

NAACA, NESCAUM, and VT, while 

supportive of revoking the existing 

standards, also suggested that EPA 

explore setting a new 24-hour standard 

to minimize the potential that multiple 

hours within a day would exceed a 

1-hour standard (see RTC document 

(EPA 2010), section IV). Groups which 

supported revoking the current 24-hour 

and annual standards (if a 1-hour 

standard was set in the proposed 

ranged) generally referenced the 

Administrator’s rationale and CASAC 

advice described in the proposal (see 

section II.F.5). 

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

environmental organizations (e.g., CBD, 

WEACT for Environmental Justice) were 

generally opposed to revoking the 

current 24-hour and annual standards. 

These groups generally concluded that 

the 24-hour standard should be revised 

while the annual standard should be 

retained. In support of this position, 

ALA et al., cited air quality information 

from the REA indicating that if air 

quality was simulated to just meet a 

99th percentile 1-hour daily maximum 

standard in the proposed range of 50– 

100 ppb, then in some locations 

analyzed, 99th percentile 24-hour 

average SO2 concentrations would be 

above concentrations (i.e., above 99th 

percentile 24-hour average 

concentrations) in cities where U.S. 

emergency department visit and 

hospital admission studies reported 

positive associations with SO2. In 

addition, many of these groups were 

opposed to revoking the current annual 

standard. In general, these groups 

concluded that given the uncertainties 

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

the side of being health protective and 

retain the existing annual standard. EPA 

responses to comments on whether the 

current standards should be retained or 

revoked are presented below as well as 

in section IV of the RTC document (EPA 

2010). 

As stated in the REA and proposal, 

99th percentile 24-hour average SO2

concentrations in cities where U.S. 

emergency department visit and 

hospital admission studies (for all 

respiratory causes and asthma; 

identified from Table 5–5 of the ISA) 

were conducted ranged from 16 ppb to 

115 ppb (Thompson and Stewart, 2009). 

Moreover, as stated in the REA and 

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

estimates that remained statistically 

significant in multi-pollutant models 

with PM were found in cities with 99th 

percentile 24-hour average SO2

concentrations ranging from 

approximately 36 ppb to 64 ppb. In its 

comments, ALA et al., stated (based on 

the air quality information in REA 

Appendix Table D–2) ‘‘with a 1-hour 

50 ppb 99th percentile standard, 7 

counties would experience a 99th 

percentile 24-hour concentration of 16 

ppb or greater, the range found to be 

harmful in epidemiological studies. 

With an hourly standard of 100 ppb, 24 

of 30 counties would have 99th 

percentile 24-hour concentrations above 

16 ppb, with 1 county exceeding 36 

ppb.’’ Thus, these commenters generally 

maintained that a lowered 24-hour 

standard is needed to protect against 

these 24-hour SO2 concentrations. 

We disagree that a lowered 24-hour 

standard is needed to protect against 

24-hour average SO2 concentrations of 

concern identified from cities where 

U.S. emergency department visit and 

hospital admission studies were 

conducted. As noted in detail in the 

REA, there is uncertainty as to whether 

the health effects reported in 

epidemiologic studies using 24-hour 

average SO2 concentrations are in fact 

due to 24-hour average SO2 exposures 

(REA, section 10.5.2). That is, when 

describing epidemiologic studies 

observing positive associations between 

ambient SO2 and respiratory symptoms, 

the ISA stated ‘‘that it is possible that 

these associations are determined in 

large part by peak exposures within a 

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

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

‘‘The effects of SO2 on respiratory 

symptoms, lung function, and airway 

inflammation observed in the human 

clinical studies using peak exposures 

further provides a basis for a 

progression of respiratory morbidity 

resulting in increased emergency 

department visits and hospital 

admissions’’ and makes the associations 

observed in the epidemiologic studies 

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

contrast, evidence from controlled 

human exposure studies of 5–10 

minutes and epidemiologic studies 

using 1-hour daily maximum SO2

concentrations provided appreciably 

stronger evidence of respiratory 

morbidity effects following SO2

exposures ≤ 1-hour. 

Given that respiratory morbidity 

effects following SO2 exposure may be 

most related to averaging times ≤1-hour, 

EPA found it most reasonable to 

consider the extent to which a 1-hour 

averaging time, given an appropriate 

form and level (which as discussed 

above, also substantially limits 5-minute 

benchmark exposures of concern; see 

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

percentile 24-hour average 

concentrations of SO2 in locations 

where emergency department visit/ 

hospitalization studies reported that the 

SO2 effect estimate remained 

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

locations with 99th percentile 24-hour 

average SO2 concentrations ≥36 ppb). 

Considering this, we note that ALA et 

al., identified only one county with 99th 

percentile 24-hour average SO2

concentrations ≥36 ppb given a 99th 

percentile 1-hour daily maximum 

standard at 100 ppb, and no counties 

≥36 ppb given a 99th percentile 1-hour 

daily maximum standard at 50 ppb. 

Thus, given a 99th percentile 1-hour 

daily maximum standard level at 75 ppb 

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

new 1-hour SO2 standard), it is possible 

that no county in the ALA et al., 

analysis would have had a 99th 

percentile 24-hour average SO2

concentration ≥36 ppb. 

With regard to the annual standard, 

we also disagree that this standard 

needs to be retained. First, the ISA 

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

effects, and morbidity and mortality 

with long-term exposures to SO2 is 

inadequate to infer a causal 

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

annual standard is unnecessary to 

prevent long-term health effects. The 

remaining issue is whether such a 

standard provides further protection 


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against short-term effects, given the new 

one hour standard. We conclude that it 

does not. As noted in the proposal, our 

air quality information indicates that 1- 

hour standard levels in the range of 50– 

100 ppb are estimated to generally keep 

annual SO2 concentrations well below 

the level of the current annual standard. 

CASAC agreed. The panel stated: 

‘‘Assuming that EPA adopts a one hour 

standard in the range suggested, and if 

there is evidence showing that the shortterm standard provides equivalent 

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

is supportive of the REA discussion of 

discontinuing the annual standard’’ 

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

information indicates that retaining the 

annual standard would add no 

additional public health protection. 

c. Administrator’s Conclusions on 

Retaining or Revoking the Current 24- 

Hour and Annual Standards 

In accordance with the REA findings 

and CASAC recommendations 

mentioned above, the Administrator 

concludes that a 1-hour standard at 

level of 75 ppb would have the effect of 

maintaining 24-hour and annual SO2

concentrations generally well below the 

levels of the current 24-hour and annual 

NAAQS (see REA Tables 10–3 and 10– 

4 and REA Appendix Tables D–3 to D– 

6). She also concludes that, as noted 

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

standard at 75 ppb will likely limit 99th 

percentile 24-hour SO2 concentrations 

in U.S. locations where emergency 

department visit and hospital admission 

studies reported statistically significant 

associations in multi-pollutant models 

with PM. Finally, she notes the lack of 

sufficient health evidence to support an 

annual standard to protect against 

health effects associated with long-term 

SO2 exposure. Taken together, the 

Administrator concludes it appropriate 

to revoke the current 24-hour and 

annual standards. 

G. Summary of Decisions on the 

Primary Standards 

For the reasons discussed above, and 

taking into account information and 

assessments presented in the ISA and 

REA as well as the advice and 

recommendations of CASAC, the 

Administrator concludes that the 

current 24-hour and annual primary 

standards are not requisite to protect 

public health with an adequate margin 

of safety. The Administrator also 

concludes that establishing a new 1- 

hour standard will appropriately protect 

public health with an adequate margin 

of safety, and specifically will afford 

requisite increased protection for 

asthmatics and other at-risk populations 

against an array of adverse respiratory 

health effects related to short-term (5 

minutes to 24 hours) SO2 exposure. 

These effects include decrements in 

lung function (defined in terms of sRaw 

and FEV1), increases in respiratory 

symptoms, and related serious 

indicators of respiratory morbidity 

including emergency department visits 

and hospital admissions for respiratory 

causes. 

Specifically, the Administrator is 

establishing a new short-term primary 

SO2 standard with a 1-hour (daily 

maximum) averaging time and a form 

defined as the 3-year average of the 99th 

percentile of the yearly distribution of 1- 

hour daily maximum SO2

concentrations, and a level of 75 ppb. In 

addition to setting a new 1-hour 

standard at 75 ppb, the Administrator is 

revoking the current 24-hour and annual 

standards recognizing that a 1-hour 

standard set at 75 ppb will have the 

effect of generally maintaining 24-hour 

and annual SO2 concentrations well 

below the levels of the current 24-hour 

and annual standards. 

III. Overview of the Approach for 

Monitoring and Implementation 

We received several comments 

regarding the approaches discussed in 

the proposal for monitoring and 

modeling for comparison to the 

proposed new 1-hour SO2 NAAQS, 

designations of areas as either attaining 

or not attaining the NAAQS, and 

implementation of the new NAAQS in 

State implementation plans (SIPs) that 

would ensure ultimate attainment of the 

new NAAQS in transitioning from the 

annual and 24-hour NAAQS in a timely 

manner. These comments raised 

fundamental questions regarding our 

contemplated approaches in all three 

areas, and caused us to re-examine them 

and review their consistency with past 

practice under the SO2 NAAQS 

implementation program. After 

conducting that review, and in response 

to the public comments we are revising 

our general anticipated approach toward 

implementation of the new 1-hour 

NAAQS. This revised approach would 

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

the special challenges SO2 emissions 

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

NAAQS in many situations; (3) the 

superior utility that modeling offers for 

assessing SO2 concentrations; and (4) 

the most appropriate method for 

ensuring that areas attain and maintain 

the new 1-hour SO2 NAAQS in a 

manner that is as expeditious as 

practicable, taking into account the 

potential for substantial SO2 emissions 

reductions from forthcoming national 

and regional rules that are currently 

underway. 

Below, we provide an overview of our 

revised approach to monitoring, and of 

our expected approaches to designations 

of areas, and implementation of the 

NAAQS. Due to the unique challenges 

presented by SO2, we do not expect that 

the anticipated approaches discussed 

below would be necessarily transferable 

to other NAAQS pollutant situations. 

For NAAQS pollutants other than SO2, 

air quality monitoring is more 

appropriate for determining whether all 

areas are attaining the NAAQS, and 

there is comparatively less dependence 

upon conducting refined modeling. 

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

approach to monitoring, and our 

expected approaches to designations of 

areas, and implementation of the 

NAAQS) is further addressed later in 

the preamble, in sections IV, V and VI, 

respectively. Where specific public 

comments on the proposal are 

addressed and responded to, further 

details of the specific revised 

approaches are explained. In many 

respects, both the overview discussion 

below and the subsequent more detailed 

discussions explain our expected and 

intended future action in implementing 

the new 1-hour NAAQS—in other 

words, they constitute guidance, rather 

than final agency action—and it is 

possible that our approaches may 

continue to evolve as we, States, and 

other stakeholders proceed with actual 

implementation. In other respects, such 

as in the final regulatory provisions 

regarding the promulgated monitoring 

network, we are explaining EPA’s final 

conclusions regarding what is required 

by this rule. We expect to issue further 

guidance regarding implementation, 

particularly concerning issues that may 

arise regarding the application of 

refined dispersion modeling under this 

revised approach for monitoring and 

implementation, and issues that States 

and other stakeholders may also ask us 

to address as we proceed toward various 

stages of ensuring attainment. EPA 

intends to solicit public comment prior 

to finalizing this guidance. 

The main necessary elements of 

implementing the new 1-hour NAAQS 

are: (1) An approach for assessing 

ambient concentrations to determine 

compliance with the NAAQS; (2) a 

process for using these assessments to 

designate areas relative to the new 

standard; and (3) the development of 

State plans that include control 

measures sufficient for ensuring the 

NAAQS is attained everywhere as 

expeditiously as possible, which we 


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believe should be no later than 2017. 

EPA’s revised anticipated approach to 

determining compliance with the new 

SO2 NAAQS is consistent with our 

historical approach to SO2 designations 

and implementation through permits 

and emissions limitations, which 

involves the combined use of 

monitoring and modeling. The emphasis 

we would place on monitoring and 

modeling, compared with each other, 

under the revised expected approach is 

therefore significantly different than 

that in the approach discussed in the 

proposal, which was less in line with 

our historical practice for SO2, as the 

public comments highlighted. 

In the SO2 NAAQS proposal, we 

recommended a monitoring-focused 

approach for comparison to the new 

NAAQS, featuring a two-pronged 

monitoring network design. This 

included monitors in certain CBSAs 

based on a combination of population 

and SO2 emissions coupled with 

additional monitors within a State based 

on that State’s contribution to national 

SO2 emissions. The resulting proposed 

network would have required 

approximately 348 monitors nationwide 

to be sited at the locations of maximum 

concentration. Numerous State and 

local government commenters expressed 

concerns regarding the burdens of 

implementing the proposed monitoring 

network and the sufficiency of its scope 

for purposes of identifying violations. 

These commenters contended that our 

proposed monitoring network was too 

small and insufficient to cover the range 

of SO2 sources, and yet too burdensome 

and expensive to expand to an adequate 

scale. Some of these commenters (the 

City of Alexandria, and the States of 

Delaware, North Carolina and 

Pennsylvania) suggested using modeling 

to determine the scope of monitoring 

requirements, or favored modeling over 

monitoring to determine compliance 

with the NAAQS. 

Partly in response to these comments, 

and after reconsidering the proposal’s 

monitoring-focused approach in light of 

EPA’s historical approach to SO2

NAAQS implementation and area 

designations decisions, we intend to use 

a hybrid analytic approach that would 

combine the use of monitoring and 

modeling to assess compliance with the 

new 1-hour SO2 NAAQS. We believe 

that some type of hybrid approach is 

more consistent with our historical 

approach and longstanding guidance 

toward SO2 than what we originally 

proposed. In addition, we believe that 

for a short-term 1-hour standard it is 

more technically appropriate, efficient, 

and effective to use modeling as the 

principle means of assessing 

compliance for medium to larger 

sources, and to rely more on monitoring 

for groups of smaller sources and 

sources not as conducive to modeling. 

We discuss the details of the final 

revised monitoring network 

requirements in section IV later in the 

preamble, but note here the relationship 

that the revised approach toward 

monitoring and modeling—taken partly 

in response to the public comments 

mentioned above—has to the other two 

general subject areas in implementation 

for which we are providing guidance, 

namely initial area designations and 

development of substantive 

implementation plans that ensure 

timely attainment and maintenance of 

the NAAQS. Our ultimate intention is to 

place greater emphasis on modeling 

than did the proposed rule as the most 

technically appropriate, efficient, and 

readily available method for assessing 

short-term ambient SO2 concentrations 

in areas with large point sources. This 

projected change in approach would 

necessarily result in a lesser emphasis 

on the less appropriate, more expensive, 

and slower to establish monitoring tool 

than did the proposed rule. Therefore, 

the minimum requirements for the SO2

monitoring network in this final rule are 

of a smaller scale than proposed, and we 

do not expect monitoring to become the 

primary method by which ambient 

concentrations are compared to the new 

1-hour SO2 NAAQS. 

Instead, in areas without currently 

operating monitors but with sources that 

might have the potential to cause or 

contribute to violations of the NAAQS, 

we anticipate that the identification of 

NAAQS violations and compliance with 

the 1-hour SO2 NAAQS would primarily 

be done through refined, sourceoriented air quality dispersion modeling 

analyses, supplemented with a new, 

limited network of ambient air quality 

monitors. Historically, we have favored 

dispersion modeling to support SO2

NAAQS compliance determinations for 

areas with sources that have the 

potential to cause an SO2 NAAQS 

violation, and we have explained that 

for an area to be designated as 

‘‘attainment,’’ dispersion modeling 

regarding such sources needs to show 

the absence of violations even if 

monitoring does not show a violation. 

This has been our general position 

throughout the history of 

implementation of the SO2 NAAQS 

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

Regions, Criteria, and Control 

techniques; Attainment Status 

Designations,’’ 43 FR 40412, 40415–16 

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

Regions, Criteria, and Control 

Techniques,’’ 43 FR 45993, 46000–02 

(Oct. 5, 1978); ‘‘Air Quality 

Implementation Plans: State 

Implementation Plans; General 

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

48 (Apr. 16, 1992); ‘‘Approval and 

Promulgation of State Implementation 

Plans; Call for Sulfur Dioxide SIP 

Revisions for Billings/Laurel, MT,’’ 58 

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

Areas for Air Quality Planning 

Purposes; Ohio,’’ 59 FR 12886, 12887 

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

Standards, National and 

Implementation Plans for Sulfur Oxides 

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

95 (Mar. 7, 1995); ‘‘Air Quality 

Implementation Plans; Approval and 

Promulgation: Various States: Montana,’’ 

67 FR 22167, 22170–71, 22183–887 

(May 2, 2002). 

Compared to other NAAQS 

pollutants, we would not consider 

ambient air quality monitoring alone to 

be the most appropriate means of 

determining whether all areas are 

attaining a short-term SO2 NAAQS. Due 

to the generally localized impacts of 

SO2, we have not historically 

considered monitoring alone to be an 

adequate, nor the most appropriate, tool 

to identify all maximum concentrations 

of SO2. In the case of SO2, we further 

believe that monitoring is not the most 

cost-efficient method for identifying all 

areas of maximum concentrations. 

However, for some situations 

monitoring is well suited, and we 

therefore will require it to some extent, 

as further explained in section IV of the 

preamble. For example, monitoring may 

appropriately be relied upon to assess 

compliance with the NAAQS by groups 

of smaller sources and sources that may 

not be as conducive to modeling as are 

larger SO2 sources. 

States will need to make any 

adjustments to the existing monitoring 

network to ensure that monitors meeting 

today’s network design regulations for 

the new 1-hour NAAQS are sited and 

operational by January 1, 2013. We also 

expect to provide additional guidance 

regarding the application of refined 

dispersion modeling under this revised 

expected approach for implementation 

of the new SO2 standard. Appendix A 

to the Guideline on Air Quality Models 

(Appendix W of 40 CFR part 51), 

Summaries of Preferred Air Quality 

Models, provides ‘‘key features of 

refined air quality models preferred for 

specific regulatory applications’’ (see 

Appendix A to Appendix W of Part 51 

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

following our current Guideline on Air 

Quality Models with appropriate 

flexibility for use in implementation, is 

anticipated to better reflect and account 


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20EPA is authorized by the Clean Air Act to take 

up to 3 years to complete the initial area 

designations in the event that insufficient 

information is available to complete the 

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

newly sited monitors meeting the new monitoring 

network design criteria are not expected to be 

obtained until the end of 2015, any newly sited 

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

designations. 

22EPA anticipates making the determination of 

when monitoring alone is ‘‘appropriate’’ for a 

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

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

process. EPA would expect to address this issue for 

such areas by examining the historic treatment of 

the area with respect to prior SO2 designations as 

well as whether the area is one in which monitoring 

would be the more technically appropriate tool for 

determining compliance with the new SO2 NAAQS. 

An example of a situation in which monitoring may 

be the more preferred approach is a shipping port 

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

close proximity to other significant stationary SO2

sources. 

for source-specific SO2 impacts than the 

more limited monitoring-focused 

proposal. As noted above, EPA intends 

to solicit public comment prior to 

finalizing this guidance. 

Based on a revised, hybrid approach, 

we expect to implement the new SO2

standard in the following manner. In 

accordance with CAA section 107(d), 

EPA must designate areas as 

‘‘attainment,’’ ‘‘nonattainment’’ or 

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

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

following promulgation of the new 

NAAQS).20 State Governors are required 

to submit their initial area designation 

recommendations to EPA no later than 

June 2011. We expect that EPA’s final 

area designation decisions in 2012 

would be based principally on data 

reported from SO2 monitors currently in 

place today, and any refined modeling 

the State chooses to conduct specifically 

for initial area designations.21 For these 

initial designations, we would expect to 

designate an area ‘‘nonattainment’’ if 

either monitoring data or appropriate 

refined modeling results show a 

violation. Any area that has monitoring 

and appropriate modeling data showing 

no violations we would expect to 

designate as ‘‘attainment.’’ 22 All other 

areas, absent monitoring data and air 

quality modeling results showing no 

violations, we would expect to initially 

designate as ‘‘unclassifiable,’’ as required 

by the Clean Air Act. The expected 

presumptive boundary for any area 

designated ‘‘nonattainment’’ would be 

the county boundary associated with the 

violation unless additional information 

provided to EPA demonstrates 

otherwise, as has been our general 

approach for other NAAQS pollutants. 

Any area initially designated 

‘‘nonattainment’’ or ‘‘unclassifiable’’ 

could request redesignation to 

‘‘attainment’’ after an assessment based 

on air quality modeling, conducted in 

accordance with the new guidance, and 

available monitoring data indicates that 

the standard has been met, as well as 

meeting all other requirements of the 

CAA for redesignation to attainment. 

This anticipated approach toward 

initial area designations is a change 

from the approach discussed in the 

proposal, and logically follows from our 

general change in approach to the use 

and utility of monitoring versus 

modeling for determining short-term 

SO2 ambient concentrations. As public 

commenters pointed out, establishment 

and implementation of the proposed 

monitoring network would have been 

both too limited and too late to inform 

initial area designations, and the 

expense and burden of accelerating it 

and expanding it would have been 

severe for State implementing agencies. 

Given the time needed to establish 

monitors, it is not realistic to expect 

either such an expanded monitoring 

network or even the more reasonable 

limited network of the final rule to be 

the chief tool for informing initial 

designations. 

That means that some other approach 

is needed to inform initial designations 

of areas and other implementation 

decisions under the new SO2 NAAQS. 

In addition to using any valid data 

generated by existing monitors, refined 

dispersion modeling may inform 

designation and implementation 

decisions regarding sources that may 

have the potential to cause or contribute 

to a NAAQS violation. In order for 

modeling to be done on the scale 

sufficient to identify all areas that might 

violate the new 1-hour standard, EPA 

anticipates issuing guidance that 

addresses a variety of issues, such as 

how to identify and appropriately assess 

the air quality impacts of small SO2

sources (e.g., those emitting less than 

100 tons of SO2 per year) that may 

potentially cause or contribute to a 

violation of the new SO2 NAAQS. EPA 

expects that it will take more time for 

EPA to issue that guidance than is 

available in order to use it for the initial 

round of attainment designations. In 

addition to any smaller sources that 

might cause or contribute to NAAQS 

violations, States would need to model 

approximately 2000 larger sources 

across the country (i.e., sources that 

emit greater than 100 tons per year and 

are collectively responsible for about 

99% of all SO2 emissions from point 

sources in the U.S.) to determine 

whether areas are attaining or not 

attaining the 1-hour standard. While 

these sources emitting 100 or more tons 

of SO2 per year represent the significant 

fraction of the total emissions from 

point sources in the U.S., smaller 

sources also have the potential to violate 

the new SO2 NAAQS. 

After receiving EPA’s forthcoming 

modeling guidance, States might 

initially focus modeling assessments on 

these larger sources that have been 

subject to permitting requirements and 

are generally better characterized than 

smaller sources. But even this effort 

would entail a substantial burden on 

States, under a compressed timeline 

following EPA’s issuance of further 

modeling guidance. Consequently, EPA 

does not believe that for this new 1-hour 

SO2 NAAQS it would be realistic or 

appropriate to expect States to complete 

such modeling and incorporate the 

results in initial designation 

recommendations, which under CAA 

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

to EPA within 1 year of the 

promulgation of the 1-hour standard. 

The remaining issue, then, is how to 

most appropriately use a modified 

hybrid approach, and its constituent 

modeling and monitoring tools, in the 

implementation plan development 

process in order to ensure expeditious 

attainment and maintenance of the 

NAAQS. Under the CAA, all States must 

develop and submit to EPA State 

implementation plans (SIPs) to attain 

and maintain the new 1-hour SO2

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

States, regardless of designation status, 

to adopt SIPs that provide for 

implementation, maintenance and 

enforcement of each primary NAAQS. 

Traditionally, for areas that were 

designated ‘‘attainment’’ or 

‘‘unclassifiable’’, we accepted State 

submissions of prevention of significant 

deterioration (PSD) permitting programs 

and other ‘‘infrastructure’’ SIP elements 

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

being sufficient to satisfy the section 

110(a)(1) SIP submission requirement. 

However, due to our recognition here 

that monitoring is not generally the 

most appropriate or effective tool for 

assessing compliance with the new 1- 

hour SO2 NAAQS, that additional 

guidance from EPA on conducting 

refined modeling for the new 1-hour 

NAAQS is anticipated to support our 

expected implementation approach, and 

that considerable time and resources 

may be needed to fully identify and 

properly characterize all SO2 sources 

(including those emitting less than 100 

tons of SO2 per year) that may 

potentially cause or contribute to a 

violation of the new SO2 NAAQS, we 

also had to assess how and when to best 

use modeling as the primary method in 

implementation. 


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23The schedule for State plans addressing areas 

designated ‘‘nonattainment’’ is governed by CAA 

section 191. The schedule for State plans for all 

other areas, including areas designated 

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

CAA section 110(a)(1). 

The approach that EPA expects to 

take, which is described in sections V 

and VI of the preamble, is consistent 

with the language of the Clean Air Act 

and would accommodate the time 

needed for an accurate assessment of 

ambient air quality levels for the 1-hour 

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

areas to be designated ‘‘attainment’’ if 

they meet the standard, ‘‘nonattainment’’ 

if they do not meet the standard or 

contribute to a nearby violation, or 

‘‘unclassifiable’’ if they cannot be 

designated on the basis of available 

information. EPA’s expected approach 

would enable us to make the 

appropriate designation decision 

required by the CAA, based on the 

record of information that will be before 

EPA regarding each area. Areas would 

be designated ‘‘nonattainment’’ if either 

available monitoring data or modeling 

shows that a violation exists, or 

‘‘attainment’’ if both available 

monitoring data and modeling indicate 

the area is attaining. All other areas 

would be designated ‘‘unclassifiable,’’ as 

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

We currently anticipate that our 

projected post-designation 

implementation approach would look to 

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

which have sometimes been previously 

referred to as ‘‘maintenance’’ or 

‘‘infrastructure’’ SIPs but for the new 

SO2 NAAQS would serve as substantive 

‘‘attainment’’ SIPs. Our current thinking 

is that, to be approved by EPA, such 

plans would need to provide for 

attainment and maintenance of the new 

1-hour SO2 NAAQS as expeditiously as 

practicable, which we expect to be no 

later than five years after initial 

designation (or approximately August 

2017) in all areas of the State, including 

any area initially designated 

‘‘nonattainment,’’ and also including any 

area designated ‘‘unclassifiable’’ that has 

SO2 sources with the potential to cause 

or contribute to a violation of the 

NAAQS. The CAA establishes deadlines 

for States to submit these plans to 

EPA.23 State plans that address areas 

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

‘‘nonattainment area SIPs’’) are due 

within 18 months from the effective 

date of the designation, under CAA 

section 192. EPA anticipates that this 

deadline would be February 2014. State 

plans addressing all other areas (i.e., 

‘‘maintenance SIPs’’) are due within 3 

years following the promulgation of the 

new NAAQS, or June 2013, under CAA 

section 110(a)(1). 

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

does not specify a maximum deadline 

by which States are required to show 

they have met the requirements to 

implement, maintain, and enforce a 

NAAQS. EPA believes, however, that 

August 2017 is the latest date by which 

areas should show they have achieved 

attainment and maintenance of the 

standard because this deadline is the 

same as would be required for areas 

designated nonattainment in June 2012. 

It is therefore presumptively reasonable 

as it is identical to the period Congress 

provided for nonattainment areas to 

reach attainment. Moreover, EPA notes 

that the maintenance SIPs will be due 

in June 2013, rather than in February 

2014, giving States and sources at least 

as much time between SIP development 

and submission and the date by which 

attainment should be achieved as they 

would have had the area been 

designated nonattainment in 2012. 

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

able to rely on modeling reflecting any 

SO2 reductions that we expect to result 

before the attainment date from 

compliance with the rules EPA expects 

to promulgate before 2013, (including 

technology-based standards under CAA 

section 112(d) for certain source 

categories emitting large amounts of SO2

such as Electric Generating Units and 

industrial boilers, and revised rules 

establishing further limits on SO2

emitted by sources in upwind States 

which contribute significantly to 

downstream States’ inability to attain or 

maintain the PM2.5 NAAS (the so-called 

Clean Air Interstate Replacement rule)). 

Thus, we intend that a State’s section 

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

emissions reductions, including any 

from national and regional rules that are 

promulgated before these SIP 

submissions, provided that those 

reductions occur under a schedule that 

ensures attainment as expeditiously as 

practicable. We expect that date to be no 

later than 5 years from the date of initial 

designation or August 2017. 

Under this anticipated approach, 

attainment SIPs for nonattainment areas 

would have to include enforceable 

emissions limitations, timetables for 

compliance, and appropriate testing/ 

reporting to assure compliance, and 

demonstrate attainment through air 

quality modeling for all sources 

contributing to monitored and modeled 

violations, or that have the potential to 

cause or contribute to a violation of the 

NAAQS. The SIPs under section 

110(a)(1) would need to demonstrate 

through refined air quality modeling 

that any source or group of sources that 

have the potential to cause or contribute 

to a violation of the NAAQS are, or will 

be, sufficiently controlled to ensure 

timely attainment and maintenance of 

the NAAQS. We would expect this to 

include any individual sources with the 

potential to emit 100 or more tons per 

year of SO2, and other sources that may 

also cause or contribute to violations of 

the new SO2 NAAQS. We expect to 

develop guidance for the States’ use on 

how best to identify and assess the 

impact of sources that may have this 

potential. As mentioned previously, we 

intend to provide an opportunity for 

notice and comment on this guidance 

before finalizing it. 

EPA again notes that it anticipates 

several forthcoming national and 

regional rules, such as the pending 

Industrial Boilers MACT standard under 

CAA section 112(d), that are likely to 

require significant reductions in SO2

emissions over the next several years. A 

limited qualitative assessment based on 

the results of preliminary modeling of 

some sample facilities indicates that 

well controlled sources should meet the 

new SO2 NAAQS (see Brode 2010b). 

Exceptions could include unique 

sources with specific characteristics that 

contribute to higher ambient impacts 

(short stack heights, complex terrain, 

etc.). These national and regional rules 

are expected to lead to SO2 reductions 

that will help achieve compliance with 

the new SO2 NAAQS prior to 2017. If, 

upon EPA review of submitted SIPs that 

rely upon those reductions or other 

local controls, it appears that States will 

nevertheless fail to attain the NAAQS as 

expeditiously as practicable (and no 

later than August 2017), the Clean Air 

Act provides authorities for EPA to 

solve such failure, including, as 

appropriate, disapproving submitted 

SIPs, re-designating unclassifiable areas 

to nonattainment, issuing SIP calls, and 

promulgating FIPs. 

For the reasons discussed above, EPA 

has determined that it is appropriate 

and efficient to principally use 

modeling to assess compliance for 

medium to larger sources, and to rely 

more on monitoring for groups of 

smaller sources and sources not as 

conducive to modeling. EPA’s revised 

monitoring network requirements have 

been developed to be consistent with 

this approach. However, EPA is still 

considering how monitoring and 

modeling data would be used together 

in specific situations to define 

attainment and nonattainment 

boundaries and under what 

circumstances it may be appropriate to 

rely on monitoring data alone to make 

attainment determinations. EPA intends 


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to address these issues as it develops 

implementation guidance. 

In light of the new approach that EPA 

intends to take with respect to 

implementation of the SO2 NAAQS, 

EPA intends to solicit public comment 

on guidance regarding modeling, and 

also solicit public comment on 

additional implementation planning 

guidance, including the content of the 

maintenance plans required under 

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

EPA also notes that State monitoring 

plans and the SIP submissions that 

States will make will also be subject to 

public notice and comment. 

IV. Amendments to Ambient 

Monitoring and Reporting 

Requirements 

In this section of the preamble, we 

describe the proposal, the public 

comments that we received on the 

proposed monitoring and reporting 

requirements, and the final 

requirements for the SO2 monitoring 

network. We are modifying our 

proposed approach to the amount of 

monitoring to require following 

consideration of public comments and a 

review of our historic practice in 

assessing compliance with the SO2

NAAQS. As we explain above in section 

III, we will use a hybrid approach that 

combines monitoring and modeling, 

using each of these analytic tools where 

they are most appropriate and effective. 

This approach and its requirements are 

intended to support the revised SO2

NAAQS, described in section II above. 

For a short-term 1-hour standard, 

dispersion modeling of stationary 

sources will generally be more 

technically appropriate, efficient, and 

effective because it takes into account 

fairly infrequent combinations of 

meteorological and source operating 

conditions that can contribute to peak 

ground-level concentrations of SO2. 

Even an expansive monitoring network 

could fail to identify all such locations. 

Consequently, we have revised the 

scope of the monitoring network, 

reflecting a modified and expanded set 

of objectives. This section also describes 

and explains the final requirements for 

the new SO2 Federal Reference Method 

(FRM), and the SO2 network design, 

monitoring objectives, data reporting, 

and data quality objectives that support 

the revised primary SO2 NAAQS. 

A. Monitoring Methods 

1. Requirements for SO2 Federal 

Reference Method (FRM) 

The proposal to promulgate an 

automated SO2 FRM was based on a 

need to update the cumbersome existing 

manual wet-chemistry (pararosaniline) 

method to a continuous-type automated 

method that can readily provide 1-hour 

SO2 measurement capability. See 74 FR 

at 64846–849. The following paragraphs 

provide background, rationale, and the 

final changes to the automated SO2

Federal Reference Method (FRM) and to 

the associated performance 

specifications for automated SO2

analyzers. 

a. Proposed Ultraviolet Fluorescence 

SO2 FRM and Its Implementation 

FRMs, set forth in several appendices 

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

a specified methodology for definitively 

measuring concentrations of ambient air 

pollutants for comparison to the 

NAAQS in Part 50, and (2) to provide 

a standard of comparison for 

determining equivalency of alternative 

pollutant measurement methods that 

can be used in lieu of the FRM for such 

monitoring. 

The FRM for measuring SO2 in the 

ambient air was promulgated on April 

30, 1971 in conjunction with the first 

primary SO2 NAAQS (36 FR 8196). This 

SO2 FRM is specified in Appendix A of 

Part 50 and identified as the 

pararosaniline manual method. See 

generally 74 FR at 64846. In the interim, 

EPA has designated many SO2 methods 

as equivalent methods (FEMs), most of 

which are based on the ultraviolet 

fluorescence (UVF) measuring 

technique. Id. In fact, virtually all SO2

monitoring data are now obtained with 

FEMs that use the UVF technique. 

In light of this, EPA proposed to 

establish a new automated SO2 FRM 

based on UVF—the same measurement 

technique employed by FEM analyzers 

now in widespread use by most State 

and local monitoring agencies and 

having the measurement capability 

needed to implement the proposed 1- 

hour SO2 NAAQS. FRM analyzers using 

this UVF technique can provide the 

needed detection limits, precision, and 

accuracy and fulfill other purposes of an 

FRM, including use as an appropriate 

standard of reference for testing and 

designation of new FEM analyzers. At 

proposal, EPA specified the new 

method in performance-based form, 

describing a generic reference 

measurement principle and associated 

calibration procedure in a new 

Appendix A–1 to 40 CFR Part 50. 

Associated performance requirements 

applicable to candidate automated SO2

analyzers (both FRMs and FEMs) were 

proposed in 40 CFR Part 53. 

EPA also proposed retaining the 

existing manual pararosaniline FRM for 

SO2. Although EPA recognized that the 

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

making measurements of 1 hour or even 

30 minute periods. 74 FR at 64846; see 

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

method is applicable to the 

measurement of ambient SO2

concentrations using sampling periods 

ranging from 30 minutes to 24 hours’’). 

Supersession of the existing manual 

FRM, as defined in § 53.16, would 

require not only withdrawal of that 

existing FRM but also the cancellation 

of the designations of all existing SO2

FEMs. Loss of the use of these FEM 

analyzers would leave State and local 

monitoring agencies with no approved 

SO2 monitors until new FRM and FEM 

analyzers could be designated under the 

new FRM. The resulting costs and 

disruptions to monitoring agencies is 

unnecessary because the current SO2

FEMs readily and accurately measure 

(and report) one-hour ambient 

measurements. See 74 FR at 64847. 

Accordingly, EPA concluded that 

supersession of the existing FRM was 

not warranted, given the costs and 

disruptions which would occur to State 

monitoring programs and the limited 

benefits from such an action given the 

suitability of the in-use FEMs. Id. at 

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

stating that in exercising its discretion 

as to whether to proceed with 

supersession of an FRM, EPA will 

consider the benefits (in terms of 

requirements and purposes of the Act) 

from specifying a new reference 

method, potential economic 

consequences of such supersession for 

State and local monitoring agencies, and 

disruption to State and local air quality 

monitoring programs. Instead, EPA 

proposed to add the new UVF FRM 

while retaining the existing FRM for 

some period of time to support the 

continued approval of existing SO2 FEM 

analyzers. 

b. Public Comments on the Proposed 

FRM and Implementation 

EPA received comments from State 

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

Houston-Galveston Area Council, KY, 

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

industry (e.g., AirQuality Research and 

Logistics (AQRL), Consumers Energy, 

ExxonMobil, Montana Sulfur and 

Chemical Company, Inc. (MSCC), and 

the Utility Air Regulatory Group 

(UARG)), all generally supporting EPA’s 

proposal to adopt the proposed 

automated UVF as an FRM. For 

example, South Dakota supported 

adding the UVF SO2 method as an 

additional FRM and stated that this 

method is currently being used in the 

network and will reduce the cost of 

implementing the new monitoring 


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requirements for this rule. The UARG 

stated that the proposal to specify a 

different FRM to judge compliance is 

entirely reasonable, and UARG 

generally supported the proposed 

specifications for a new FRM but 

maintained that the current FRM could 

not be used along with a new FRM. 

ExxonMobil stated that it supports 

‘‘* * * EPA allowing monitoring 

agencies to choose mobile monitoring 

that meets monitoring quality 

requirements.’’ AQRL stated that ‘‘EPA is 

correct in choosing to designate 

[promulgate] a new (automated) FRM 

for measurement of SO2.’’ 

EPA did not receive any public 

comments opposing the proposed 

automated UVF SO2 FRM but did 

receive a few technical comments on 

specific provisions of the method. EPA 

proposed use of an inlet line particle 

filter as a requirement for new UVF SO2

FRM analyzers, believing that use of a 

particle filter is advantageous to prevent 

interference, malfunction, or damage to 

the analyzer from particles in the 

sampled air. The State of Missouri 

questioned this requirement, noting that 

such a filter can sometimes cause 

problems and that filter requirements 

for other FRM and FEM analyzers have 

been analyzer-specific depending on the 

manufacturer’s stipulation. EPA 

believes, however, that for new SO2

FRM analyzers, the benefits and 

uniformity provided by a mandatory 

filter requirement outweigh possible 

disadvantages of such a filter. 

Missouri also suggested that the 

language of proposed Sections 4.1.1 and 

4.1.2 regarding calibration system flow 

rate requirements were somewhat 

confusing, and that the high (50–100 

ppm) concentration requirement for the 

calibration standard specified in Section 

4.1.6.1 is sometimes a problem. In 

response to these comments, the 

language of Sections 4.1.1 and 4.1.2 has 

been clarified, and the concentration of 

the standard specified in Section 4.1.6.1 

has been reduced to 10 ppm. 

EPA received a number of comments 

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

and SD) that supported the EPA 

proposed plan of temporary retention of 

the existing wet-chemistry 

pararosaniline FRM and for FEMs 

approved based on that method. For 

example, Pennsylvania stated ‘‘[t]his 

methodology should enable State and 

local agencies to continue using their 

existing monitoring equipment and 

[thereby] avoid large capital fund 

outlays for samplers and ultimately 

avoid any delays in collecting data that 

would be comparable to the proposed 

new primary sulfur dioxide NAAQS.’’ 

North Carolina requested ‘‘* * * that 

the EPA maintain the current reference 

method for at least an additional 10 

years.’’ Wisconsin and the Center for 

Biological Diversity (CBD) suggested 

expeditiously phasing out the existing 

manual SO2 FRM. 

In contrast, however, EPA also 

received comments from industry that 

opposed the retention of the existing 

pararosaniline FRM while promulgating 

a new automated UVF FRM. In 

particular, UARG stated ‘‘* * * having 

two FRMs specified for a given 

NAAQS—is not viable,’’ pointing out 

that there is only one FRM for each 

NAAQS under the present standards, a 

result UARG appears to believe is 

legally mandated. 

EPA disagrees with this comment. 

First, there is nothing in the Act that 

mandates a single FRM for each 

NAAQS. Section 109 of the Act, in fact, 

does not address this issue at all. 

Second, as noted previously, there are 

sound policy reasons for not 

withdrawing the existing FRM at this 

time. Therefore, EPA sees no legal or 

other obstacle in adding a new 

automated UVF FRM while retaining 

the existing manual FRM. 

UARG further maintained that EPA 

provided no support for its statement 

that the existing FEMs, which constitute 

the bulk of the existing SO2 monitoring 

network, are adequate for the current 

and proposed new SO2 NAAQS. UARG 

also stated that ‘‘although the FEMs may 

be adequate for many other purposes, 

they may only be used to judge 

compliance with the 1-hour NAAQS if 

they are shown to qualify as FRMs or 

FEMs under the new FRM definition.’’ 

EPA disagrees with this comment 

also. In answer to UARG’s second point, 

it is not necessary that these existing 

FEMs be re-designated as FRMs 

pursuant to the new automated FRM to 

continue their approved use. There is no 

legal impediment to such continued 

use, since they are (and will continue to 

be) FEMs approved based on an FRM 

that adequately measures one-hour 

ambient SO2 concentrations. Nor is 

there any technical impediment to the 

continued use of these FEMs, given that 

they are automated continuous 

monitoring methods capable of 

measuring SO2 concentrations ranging 

from a few minutes to a 1-hour period. 

The existing FEMs in the network use 

the same UVF technology as the 

proposed (and now final) automated 

FRM and have been reporting 1-hour 

monitoring data for decades. These 

FRMs have been tested against the test 

and performance requirements of Part 

53, which are designed specifically to 

test such continuous methods. Further, 

the proposed SO2 method performance 

specifications for the standard 

measurement range were derived from 

data submitted in FEM applications for 

analyzers that were subsequently 

designated as FEMs. Therefore, these 

FEMs are technically and legally sound 

to judge compliance with the one-hour 

NAAQS. 

EPA has clarified the regulatory text 

so that the rules state unambiguously 

that both SO2 FRMs apply to the new 

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

hour and annual standards so long as 

they are retained), as do all presentlydesignated FEMs. 

c. Conclusions on Ultraviolet 

Fluorescence SO2 FRM and 

Implementation 

We are finalizing the proposed new 

automated SO2 FRM, which is based on 

UVF technology, with the following 

minor technical changes: The language 

of Sections 4.1.1 and 4.1.2 has been 

clarified, and the minimum 

concentration of the calibration 

standard specified in Section 4.1.6.1 has 

been reduced to 10 ppm. The new FRM 

is codified as Appendix A–1 to 40 CFR 

Part 50 and titled ‘‘Reference 

Measurement Principle and Calibration 

Procedure for the Measurement of 

Sulfur Dioxide in the Atmosphere 

(Ultraviolet Fluorescence Method).’’ 

EPA is retaining the previously existing 

manual pararosaniline SO2 FRM for the 

time being and re-codifying it as 

Appendix A–2 to 40 CFR Part 50. 

However, EPA plans to rescind this 

manual FRM at a future time when new 

SO2 FRM analyzers have adequately 

permeated State monitoring networks. 

2. Requirements for Automated SO2

Methods 

a. Performance Specifications for 

Automated Methods 

In association with the proposal to 

adopt a new automated FRM, EPA 

proposed to update the performancebased designation requirements for FEM 

SO2 analyzers currently specified in 40 

CFR Part 53. As noted in the proposal 

preamble (74 at 64846), these 

requirements were established in the 

1970’s, based primarily on the wetchemical measurement technology 

available at that time. Those initial 

requirements have become significantly 

outdated and need to be modified to 

match current technology, particularly 

because they would apply to new SO2

FRM analyzers under the proposed new 

FRM. The better instrumental 

performance available with the 

proposed new UVF FRM technique 

allows the performance requirements in 

Part 53 to be made more stringent for 


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both FRM and FEM SO2 analyzers. 

Updating these performance 

requirements is needed to ensure that, 

going forward, all new SO2 monitors 

will have improved performance. 

EPA solicited comments on the 

proposed new performance 

requirements for automated SO2

methods that were included in Table 

B–1 (Performance Specifications for 

Automated Methods) of Part 53. We 

proposed revised performance 

specifications for noise, lower 

detectable limit, interference equivalent, 

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

fall time, and precision. EPA proposed 

to reduce the allowable noise limit from 

5 to 1 ppb, the lower detectable limit 

from 10 to 2 ppb, the interference 

equivalent limits from ±20 ppb to ±5 

ppb for each interferent, and from 60 

ppb to 20 ppb for the total of all 

interferents, the zero drift limit from ±20 

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

2 minutes, both rise and fall time limits 

from 15 to 2 minutes, and the precision 

limits from 15 ppb to 2 percent of the 

upper range limit. EPA further proposed 

to eliminate the requirements for span 

drift at 20% of the upper range limit. In 

addition, to address the need for more 

sensitive, lower measurement ranges for 

SO2 analyzers, EPA proposed a separate 

set of performance requirements that 

would apply specifically to narrower 

measurement ranges, i.e. ranges 

extending from zero to concentrations 

less than 0.5 ppm. Other minor changes 

were proposed in the wording of a few 

sections of Part 53 Subparts A and B, 

including provision for alternate data 

recording devices in § 53.21 to 

supplement the older language relating 

specifically to strip chart recorders. 

b. Public Comments 

EPA received a number of comments 

from industry (AQRL and UARG) and 

from the multi-State organization 

NESCAUM regarding the proposed 

interferent limit requirements listed in 

Table B–1. UARG submitted comments 

supportive of all the proposed 

requirements for the new UVF SO2

FRM, except for the proposed total 

interferent limits of 20 ppb. UARG 

acknowledged that EPA proposed to 

reduce the total interferent level 

substantially from 60 ppb to 20 ppb, but 

maintained that the proposed level of 20 

ppb is still too high because it amounts 

to 20%–40% of the levels being 

considered for the NAAQS (50–150 

ppb). AQRL recommended limiting 

‘‘* * * each interferent to no more than 

±3 ppb and total interference to no more 

than 12 ppb.’’ NESCAUM recommended 

tightening the nitric oxide (NO) 

interference limit from 100:1 to 300:1 

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

±5 ppb). NESCAUM states that the 

proposed interferent value of ±5 ppb 

results in substantial NO interference at 

sites with low SO2 levels in urban areas. 

EPA revisited the issue of the 

interferent equivalent limit for SO2

analyzers in context of the above 

comments and reconsidered what is 

reasonably feasible with current 

technology. We reviewed the current 

instrument specifications and test data 

submitted for numerous SO2 FEM 

applications. We also took into account 

that the test concentrations of most of 

these interferents are substantially 

higher than the concentrations normally 

observed in ambient air. EPA 

considered lowering the testing 

concentrations of these interferents, 

which would have correspondingly 

lowered the interferent equivalent for 

each analyte. However, EPA took a more 

conservative approach and retained the 

existing test concentrations for H2S, 

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

vapor. Based on this review, we found 

that it is not feasible to further lower the 

limit requirement for these interferents 

below ±5 ppb. However, in response to 

the NESCAUM comment, EPA 

determined that the interferent 

equivalent limit requirement for NO 

interference could be reduced to ±3 ppb 

(166:1) for the new, lower measurement 

range to reduce possible NO 

interference at sites with low SO2 levels 

in urban area. 

In regard to the total limit for all 

interferent equivalents for SO2

analyzers, EPA notes that many of the 

interferents for which testing is required 

(specified in Table B–3 of Part 53) 

would likely react with each other and 

would thus not co-exist in ambient air 

at the specified test concentrations. 

Therefore, EPA determined that the 

limit requirement for total interference 

equivalent can be eliminated, and Table 

B–1 now reflects this change. 

EPA received comment from AQRL 

on the existing span drift requirement 

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

span drift requirement at 80% URL to 

2.5%, stating that ‘‘ambient air monitors 

in the 21st century should be able to 

hold span drift to no more than ±2.5% 

under the conditions specified in EPA 

testing * * *.’’ Based on information 

from FEM testing laboratories and 

manufacturers’ data (EPA, 2009c), EPA 

largely agrees with this comment and 

concludes that the span drift 

requirement at 80% can be lowered to 

±3%. Table B–1 has been changed to 

include this revised limit. 

EPA received comment from the State 

of Wisconsin suggesting that the 

proposed revised provisions of section 

53.21 (Test conditions) be further 

changed to more specifically recognize 

use of digital recorders for obtaining test 

results rather than maintaining the tie to 

analog strip chart recorder technology. 

EPA acknowledges that industry has 

moved away from strip chart recording 

technology to digital data recording. 

However, the proposed language of 

§ 53.21 calls for a graphic representation 

of analyzer responses to test 

concentrations to facilitate visual 

examination of test results and allows 

any ‘‘alternative measurement data 

recording device’’ as long as it can 

provide such a graphic representation. 

Describing the analog strip chart 

recorder in this section provides an 

appropriate model to help define the 

type of graphic representation needed 

for the Part 53 tests. EPA believes that 

the proposed language of § 53.21 is 

adequately broad to permit digital or 

other types of data recording devices. 

c. Conclusions for Performance 

Specifications for SO2 Automated 

Methods 

Based on typical performance 

capabilities of current UVF analyzers 

and manufacturers’ actual testing data, 

we are keeping the limit for each 

interference equivalent for SO2

analyzers at ±5 ppb. However, we are 

lowering the interference equivalent 

requirement for NO to ±3 ppb for the 

lower measurement range. A footnote 

denoting this specific requirement is 

being added to Table B–1. We are 

eliminating the total interference 

equivalent requirement for SO2

analyzers, and Table B–1 is being 

revised to incorporate this change. 

The 24-hour span drift at 80% of the 

upper range limit for SO2 analyzers is 

being lowered to ±3% in Table B–1 to 

be in line with current technology. Also, 

unrelated to SO2, a typographical error 

for the noise requirement for CO 

analyzers is being corrected to 0.5 ppm 

in Table B–1. 

Finally, information on generation 

and verification of test concentrations 

for naphthalene was inadvertently 

omitted from Table B–2, Test 

Atmospheres, even though it was added 

as a required interferent test in our 

proposal. Therefore, we are adding that 

information for naphthalene. Also in 

Table B–2, we are correcting the 

verification information for nitric oxide. 

B. Network Design 

Ambient SO2 monitoring data are 

collected by State, local, and Tribal 

monitoring agencies (‘‘monitoring 

agencies’’) in accordance with the 

monitoring requirements contained in 


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24Prior to this rulemaking there were no 

minimum monitoring requirements, except for 

those required at the multi-pollutant National Core 

(NCore) monitoring sites. The monitoring rule 

promulgated in 2006 (71 FR 61236) removed 

minimum monitoring requirements (except for 

those NCore stations). This change was largely 

driven by the fact that there was no longer an SO2

nonattainment problem under the then-existing 

standards. However, this logic does not apply to the 

revised primary SO2 NAAQS. 

25Required monitor estimates were based on 2008 

Census estimates and the 2005 National Emissions 

Inventory. 

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

and are comprised of both Metropolitan Statistical 

Areas and Micropolitan Statistical Areas (http:// 

www.census.gov). 

40 CFR parts 50, 53, and 58. A 

monitoring network is generally 

designed to measure, report, and 

provide related information on air 

quality data as described in 40 CFR Part 

58. To ensure that the data from the 

network is accurate and reliable, the 

monitors in the network must meet a 

number of requirements including the 

use of monitoring methods that EPA has 

approved as Federal Reference Methods 

(FRMs) or Federal Equivalent Methods 

(FEMs) (discussed in some detail above 

in section IV.A), focusing on particular 

monitoring objectives, and following 

specific siting criteria, data reporting, 

quality assurance and data handling 

rules or procedures. 

With the revision to the SO2 NAAQS, 

which establishes a new 1-hour 

averaging period intended to limit shortterm exposures that may occur 

anywhere in an area, EPA evaluated the 

existing network to determine if it was 

adequate to support the revised SO2

NAAQS. A significant fact for ambient 

SO2 concentrations is that stationary 

sources are the predominant emission 

sources of SO2 and the peak, maximum 

SO2 concentrations that may occur are 

most likely to occur nearer the parent 

stationary source, as noted in the ISA 

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

section IV.B.1 below. According to the 

2005 National Emissions Inventory, 

there are 32,288 sources (facilities) 

emitting SO2, of which 1,928 are 

emitting 100 tons per year (tpy) or more. 

In the proposal (74 FR 64851), EPA had 

anticipated requiring 348 sourceoriented monitors in the network design 

based on a population and emissions 

metric and a State’s emissions 

contribution to the National Emissions 

Inventory (NEI). In response to this 

proposal, EPA received numerous 

comments arguing that the required 

number of monitors in the network 

would be too small. Other commenters 

argued that expanding the monitoring to 

an adequate scale would impose a large 

burden and expense on the States. Some 

commenters referred to SO2 modeling in 

their submissions as an addition or 

alternative to monitoring. Consequently, 

as part of developing a balanced 

response to these comments, we 

revisited how we had historically dealt 

with SO2 for various purposes including 

designations and implementation 

through permitting and emissions 

limitations. As explained in section III, 

this has been realized through a 

combined monitoring and modeling 

approach. As set out below, and in 

sections III, VI, and VII, our ultimate 

intention is to utilize a combined 

monitoring and modeling approach, a 

hybrid analytic approach, to assess 

compliance with the revised SO2

NAAQS. 

As a result of this contemplated 

hybrid analytic approach, the minimum 

number of monitors required in the 

network through this rulemaking is 

reduced to approximately 163 monitors 

from the approximated 348 monitors 

that were proposed. This section of the 

preamble includes a discussion of the 

proposal, the comments received, and 

the details of and the rationale for the 

final changes to the SO2 network design 

requirements. 

1. Approach for Network Design 

a. Proposed Approach for Network 

Design 

To fully support the proposed 

revision to the SO2 NAAQS, EPA 

indicated the need to identify where 

short-term, peak ground-level 

concentrations—i.e., concentrations 

from 5 minutes to one hour (or 

potentially up to 24 hours)—may occur. 

Given that large stationary sources are 

the predominant source of emissions, 

monitoring short-term, peak groundlevel concentrations would require 

monitors to be sited to assess impacts of 

individual or groups of sources and 

therefore be source-oriented in nature. 

As a result, under a monitoring-focused 

approach, EPA proposed a two-pronged 

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

multiple variables that affect ground 

level SO2 concentrations from 

individual or groups of sources, 

including stack heights, emission 

velocities, stack diameters, terrain, and 

meteorology, EPA could not specify a 

source specific threshold, algorithm, or 

metric by which to require monitoring. 

The design of the proposed network 

represented a primarily monitoringfocused approach to assess compliance 

with the primary SO2 NAAQS. 

In preparation for the SO2 NAAQS 

proposal, EPA conducted an analysis of 

the approximately 488 SO2 monitoring 

sites operating during calendar year 

2008 (Watkins and Thompson, 2009). 

This analysis indicated that 

approximately ∼ 35% of the monitoring 

network was addressing locations of 

maximum (highest) concentrations, 

likely linked to a specific source or 

group of sources. Meanwhile, just under 

half (∼ 46%) of the sites were reported 

to be for the assessment of 

concentrations for general population 

exposure. These data allowed EPA to 

conclude that the network 24 was not 

properly focused to support the revised 

NAAQS (under the assumption that 

source-oriented monitoring data would 

be the primary tool for assessing 

compliance with the NAAQS). As a 

result, EPA proposed a two-pronged 

monitoring network (74 FR 64850), 

based on the premise of a monitoringfocused approach, with minimum 

requirements for: (1) Monitors in urban 

areas where there is a higher 

coincidence of population and 

emissions, utilizing a Population 

Weighted Emissions Index (PWEI), and 

(2) monitors in States based on each 

State’s contributions to the national SO2

emissions inventory. In addition, all the 

monitors in the network would be sited 

at locations of expected maximum 

hourly concentrations and therefore 

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

in a minimum of approximately 348 

monitors nationwide 25 providing data 

for comparison with the 1-hour standard 

and supporting its implementation. 

Under the first prong of the network 

design, EPA proposed that the ambient 

SO2 monitoring network account for 

SO2 exposure by requiring monitors in 

locations where population and 

emissions may lead to higher potential 

for population exposure to peak hourly 

SO2 concentrations. In order to do this, 

EPA developed a Population Weighted 

Emissions Index (PWEI) that uses 

population and emissions inventory 

data at the CBSA 26 level to assign 

required monitoring for a given CBSA 

(with population and emissions being 

obvious relevant factors in prioritizing 

numbers of required monitors). The 

PWEI for a particular CBSA was 

proposed to be calculated by 

multiplying the population (using the 

latest Census Bureau estimates) of a 

CBSA by the total amount of SO2

emissions in that CBSA. The CBSA SO2

emission value would be in tons per 

year, and calculated by aggregating the 

county level emissions for each county 

in a CBSA. We would then divide the 

resulting product of CBSA population 

and CBSA SO2 emissions by 1,000,000 

to provide a PWEI value, the units of 


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which would be millions of people-tons 

per year. 

We proposed that the first prong of 

the SO2 network design require 

monitors in CBSAs, according to the 

following criteria. For any CBSA with a 

calculated PWEI value equal to or 

greater than 1,000,000, a minimum of 

three SO2 monitors would be required 

within that CBSA. For any CBSA with 

a calculated PWEI value equal to or 

greater than 10,000, but less than 

1,000,000, a minimum of two SO2

monitors would be required within that 

CBSA. For any CBSA with a calculated 

PWEI value equal to or greater than 

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

of one SO2 monitor would be required 

within that CBSA. EPA estimated that 

the proposed criteria would have 

resulted in 231 required sites in 131 

CBSAs. 

Under the second prong of the 

network design, EPA proposed to 

require a monitor or monitors in each 

State, allocated by State-level SO2

emissions. This prong of the network 

design was intended to allow a portion 

of the overall required monitors to be 

placed where needed, independent of 

the first prong of the network design, 

inside or outside of CBSAs. EPA 

proposed to require monitors, using 

State boundaries as the geographic unit 

for allocation purposes, in proportion to 

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

higher emissions would have been 

required to have a proportionally higher 

number of monitors. The proposed 

percent contribution of individual 

States would have been based on the 

most recent NEI, with SO2 emissions 

being aggregated by State. The number 

of required monitors per State would 

correspond to every one percent (after 

rounding) of each State’s contribution to 

the national SO2 inventory. EPA also 

proposed that each State have at least 

one monitor required as part of this 

second prong, even if a particular State 

contributes less than 0.5% of the total 

anthropogenic national emissions 

inventory. As a result, the proposed 

second prong would have required 

approximately 117 monitoring sites 

based on State-level SO2 emissions in 

the most recent NEI, which at the time 

of the proposal, was the 2005 NEI. 

EPA also stated in the proposal that 

the multi-pollutant National Core 

(NCore) monitoring sites would not 

have counted towards meeting the 

proposed monitoring requirements. 

However, data from the NCore would be 

compared to the NAAQS even though 

NAAQS comparisons are not the sole 

objective of NCore monitors. The 

monitoring rule promulgated in 2006 

(71 FR 61236) and codified at 40 CFR 

Part 58 and its Appendices established 

the NCore multi-pollutant network 

requirement to support integrated air 

quality management data needs. In 

particular, NCore sites are intended to 

provide long-term data for air quality 

trends analysis, model evaluation, and, 

for urban sites, tracking metropolitan air 

quality statistics. To do this, NCore sites 

are required to measure various 

pollutants, including SO2, but they are 

not source oriented monitoring sites, 

and therefore are not likely to be the 

location of maximum expected 

concentration in an area. NCore sites are 

intended to provide data representing 

concentrations at the broader 

neighborhood and urban spatial scales. 

These reasons were the rationale 

justifying why SO2 monitors at NCore 

stations would not have been part of the 

minimum monitors required under the 

proposed network. 

b. Alternative Network Design 

EPA also solicited comment on an 

alternative network design, including 

alternative methods to determine the 

minimum number of monitors per State 

(74 FR 64854). EPA requested comment 

on whether a screening approach for 

assessing the likelihood of a NAAQS 

exceedance could be developed and 

serve as a basis for determining the 

number and location of required 

monitors. In particular, EPA requested 

comment on whether it should utilize 

existing screening tools such as 

AERSCREEN or SCREEN3, which use 

parameters such as effective stack height 

and emissions levels to identify 

facilities with the potential to cause an 

exceedance of the proposed standard. 

For that set of sources, EPA could then 

require States to conduct more refined 

modeling (using the American 

Meteorological Society (AMS)/EPA 

Regulatory Model (AERMOD)) to 

determine locations where monitoring 

should be conducted. Any screening or 

refined modeling would likely be 

carried out by States by using EPA 

recommended models and techniques 

referenced by 40 CFR Part 51, Appendix 

W, which provides guidance on air 

quality modeling. Such screening or 

refined modeling uses facility emission 

tonnage, stack heights, stack diameters, 

emission temperatures, emission 

velocities, and accounts for local terrain 

and meteorology in determining where 

expected maximum hourly 

concentrations may occur. In using this 

approach, EPA would then require 

States to locate monitors at the point of 

maximum concentration around sources 

identified as likely causing NAAQS 

exceedances. EPA also noted that this 

alternative approach would not 

distinctly use population as a factor for 

where monitors should be placed. 

c. Public Comments 

EPA received many comments on the 

proposed network design and the 

alternative network design approaches. 

Based on comments that were clear 

enough on the issue, EPA believes the 

commenters’ positions on the network 

design approach generally fell into one 

of three categories: (1) Those who 

supported the two-prong approach, but 

suggested some modification to it, (2) 

those who supported the alternative 

network design, and (3) those who 

suggested other concepts for the 

network instead of the two approaches 

EPA presented in the proposal. 

The commenters who generally 

supported the two-prong network 

design, but suggested some modification 

included some State and local air 

agencies (e.g. NACAA and nine other 

State groups or agency commenters) and 

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

eight other commenters). Of this group, 

some of the State and local air agencies 

specifically commented on how EPA 

should modify one or both of the prongs 

of the proposed network design. Some 

particular individual suggestions will be 

addressed here and those comments not 

addressed here will be addressed in the 

response to comment document. 

However, one recurring suggestion from 

the State and local agency commenters 

in this group was that the network 

design leads to some duplicative and/or 

unneeded monitoring, and therefore 

they requested that EPA include a 

provision to ‘‘waive’’ the monitoring 

network design requirements in 

situations where minimum monitoring 

requirements appear duplicative or 

unnecessary. In particular, NACAA 

stated that it ‘‘* * * is concerned that 

the two pronged approach in the 

proposed regulation will lead to 

duplicative monitoring in some areas 

and require monitors in areas where 

monitors are not needed. EPA 

recognizes the potential for duplicative 

monitoring, but the proposal does not 

permit the removal of duplicative 

monitors.’’ This NACAA comment was 

echoed by some of the other States who 

commented on the proposed approach 

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

industry commenters were also 

generally supportive of the two-prong 

approach, with some making general 

suggestions to modify the network 

design. For example, AQRL stated that 

the ‘‘* * * network design proposal 

seems to provide the flexibility for 

States and the EPA regions to work 

together to arrive at the adequate 

monitoring network.’’ AQRL also 


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suggests that ‘‘a State/local area should 

have the option to shutdown or relocate 

any site mandated [by monitoring 

requirements] if measured design values 

at the site are less than 75% of the 

selected standard level.’’ Multiple 

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

RRI Energy) expressed concern that the 

proposed network design had no 

monitoring required specifically to 

measure background concentrations of 

SO2. Dow Chemical suggested that EPA 

maintain some of the existing monitors 

that characterize population exposure 

and other non-source oriented sites for 

trends analysis. 

Those commenters who did not 

support the proposed network design, 

and instead generally supported the 

concepts of the alternative network 

design, include public health and 

environmental groups (e.g. ALA, CBD, 

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

of Delaware and Iowa. In particular, 

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

the proposed 348 monitors are a grossly 

inadequate number to detect peak 

concentrations from the nearly 2,000 

major sources that emit more than 100 

tons per year of sulfur dioxide * * *’’ 

and that ‘‘it is most appropriate to use 

screening tools to site all the monitors 

in the areas of highest expected 

concentration * * *’’ The Center for 

Biological Diversity, with regard to the 

proposed network design, stated that 

‘‘* * * a number of communities with 

very significant SO2 emissions will not 

have any monitoring stations at all 

***’’ Further, the State of Iowa 

claimed that ‘‘the proposed design of the 

SO2 ambient monitoring network 

provides insufficient assurances that the 

public is protected from the health 

effects of SO2 exposure,’’ and suggested 

that ‘‘* * * the final rule contain 

provisions that require monitors to be 

sited only at locations where dispersion 

modeling indicates that the NAAQS is 

violated.’’ 

Commenters also suggested other 

concepts for the monitoring network 

design in lieu of the approaches 

discussed in the proposal. NESCAUM, 

NYSDEC, and PADEP, all suggested 

using an emissions-only approach to 

trigger required monitoring instead of 

using the PWEI to require monitors in 

an area. For example, NYSDEC suggests 

that the proposed approach, using the 

PWEI, is ‘‘* * * not more predictive 

than using emissions data alone.’’ 

NYSDEC went on to recommend that 

monitors be required in CBSAs with 

aggregated emissions of 50,000 tons per 

year or more and that ambient 

monitoring be considered for point 

sources with 20,000 tons per year. 

PADEP made several suggestions on 

network design, including monitoring in 

any CBSA ‘‘where there is a sulfur 

dioxide source or combination of 

sources within 50 miles emitting a total 

of at least 20,000 tons of SO2 per year 

***’’ 

Among all three groups of 

commenters discussed above, there was 

a subset of commenters who specifically 

mentioned using modeling in some 

form. Modeling was a component of the 

alternative network design, where 

monitors would be required based on 

screening models and possibly refined 

modeling of individual sources. EPA 

also expected that under the proposed 

approach, many States would use 

modeling as a quantitative analysis tool 

to site required monitors. Finally, 

source modeling is a critical element for 

PSD and facility permitting. In their 

comments, NESCAUM recommended 

that EPA allow modeling to be used in 

conjunction with monitoring data to 

better determine nonattainment areas. 

North Carolina advocated that EPA 

require SO2 sources, without specifying 

a threshold size for sources, to perform 

modeling to demonstrate that fence-line 

(ambient) air does not exceed the 

NAAQS due to that particular source’s 

emissions. North Carolina went on to 

suggest that if a source’s modeling 

showed an exceedance of the NAAQS, 

the source could ‘‘then be required to 

reduce emissions from the stack, install 

continuous emissions monitoring (CEM) 

in the stack itself, or require a fence-line 

monitor at the target facility.’’ North 

Carolina also stated, in the context of 

discussing its own PSD program, that 

‘‘the costs for modeling are small 

compared to the costs for monitoring.’’ 

Sierra Club stated that EPA should 

‘‘* * * employ modern computer 

models to determine whether areas 

should be designated nonattainment 

because they do not meet the NAAQS in 

areas where there is no monitor.’’ From 

these comments, EPA gathers that some 

public commenters find modeling a 

useful tool and support the use of 

modeling to ascertain ambient 

concentrations of SO2. 

2. Modeling Ambient SO2

Concentrations 

EPA considered the various and 

sometimes competing concerns raised 

by the commenters including 

duplicative monitoring, lack of adequate 

number of monitors, insufficient 

flexibility, the monitoring burden, and 

the modeling suggestions. EPA 

considered its historic practice and the 

analytic tools available to arrive at a 

balanced approach that took into 

account these concerns. In the past, EPA 

used a combination of modeling and 

monitoring for SO2 during permitting, 

designations, and re-designations in 

recognition of the fact that a single 

monitoring site is generally not 

adequate to fully characterize ambient 

concentrations, including the maximum 

ground level concentrations, which 

exist around stationary SO2 sources. 

With representative and appropriate 

meteorological and other input data, 

refined dispersion models are able to 

characterize air quality impacts from the 

modeled sources across the domain of 

interest on an hourly basis with a high 

degree of spatial resolution, overcoming 

the limitations of an approach based 

solely on monitoring. By simulating 

plume dispersion on an hourly basis 

across a grid of receptor locations, 

dispersion models are able to estimate 

the detailed spatial gradients of ambient 

concentrations resulting from SO2

emission sources across a full range of 

meteorological and source operating 

conditions. The 1-hour NAAQS is 

intended to provide protection against 

short-term (5 minute to 24 hour) peak 

exposures, whether they result from 

typical meteorological conditions or not. 

Because ambient monitors are in fixed 

locations and a single monitor can only 

represent impacts which occur at the 

location of the monitor, a single monitor 

cannot identify all instances of peak 

ground-level concentrations if, for 

example, different wind directions on 

various days cause peak ground-level 

concentrations in different areas that do 

not overlap. The uncertainty associated 

with this limitation is much higher for 

an hourly standard than a long-term 

standard due to the higher degree of 

spatial and temporal variability 

associated with peak hourly impacts 

(discussed in ISA chapters 2.4 and 2.5). 

This limitation of ambient monitoring 

may be true even if the source-oriented 

ambient monitor was sited with the aid 

of modeling data, since the model is less 

reliable at predicting the precise 

location of maximum impacts than at 

predicting the distribution of impacts 

across the full modeling domain, and no 

single monitor can be sited in a way to 

always measure the peak ground-level 

SO2 concentrations that may be 

occurring in the area around a source. 

EPA’s Guideline on Air Quality 

Models, Appendix W to 40 CFR Part 51, 

provides recommendations on modeling 

techniques and guidance for estimating 

pollutant concentrations in order to 

assess control strategies and determine 

emission limits. These 

recommendations were originally 

published in April 1978 and were 

incorporated by reference in the PSD 

regulations, 40 CFR sections 51.166 and 


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27Background monitoring can be considered to be 

representative of ambient concentrations upwind of 

(and therefore not typically influenced by) a 

geographic area such as an urban area, or of an 

individual or group of emission sources. 

52.21 in June 1978 (43 FR 26382). The 

purpose of Appendix is to promote 

consistency in the use of modeling 

within the air quality management 

process. Appendix W is periodically 

revised to ensure that new model 

developments or expanded regulatory 

requirements are incorporated. The 

most recent revision to Appendix W 

was published on November 9, 2005 (70 

FR 68218), wherein EPA adopted 

AERMOD as the preferred dispersion 

model for a wide range of regulatory 

applications in all types of terrain. 

AERMOD is a steady-state plume 

dispersion model that employs hourly 

sequential preprocessed meteorological 

data to simulate transport and 

dispersion from multiple point, area, or 

volume sources for averaging times from 

one hour to multiple years, based on an 

advanced characterization of the 

atmospheric boundary layer. AERMOD 

also accounts for building wake effects 

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

To support the promulgation of 

AERMOD as the preferred model for 

near-field dispersion (50 km or less), 

EPA evaluated the performance of the 

model across a total of 17 field study 

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

2003), including several field studies 

based on model-to-monitor comparisons 

of SO2 concentrations from operating 

power plants. 

EPA anticipates that additional 

guidance for States may be needed to 

clarify how to conduct dispersion 

modeling under Appendix W to support 

the implementation of the new 1-hour 

SO2 NAAQS. Although AERMOD is 

identified as the preferred model under 

Appendix W for a wide range of 

applications and will be appropriate for 

most modeling applications to support 

the new SO2 NAAQS, Appendix W 

allows flexibility to consider the use of 

alternative models on a case-by-case 

basis when an adequate demonstration 

can be made that the alternative model 

performs better than, or is more 

appropriate than, the preferred model 

for a particular application. 

In conclusion, EPA believes that a 

hybrid analytic approach that uses a 

combination of modeling and 

monitoring information addresses the 

varying and competing concerns 

expressed by the commenters. Modeling 

large emission sources, along with 

smaller sources with the potential to 

violate the NAAQS, deals effectively 

with the concern that the monitoring 

network is not large enough to account 

for all sources that could have high 

ambient SO2 concentrations. EPA 

believes that more SO2 sources will 

ultimately be directly addressed through 

modeling alone versus the number of 

sources which would have been 

monitored under the proposed network 

design (which proposed a minimum of 

348 monitors). Because modeling 

provides a technically appropriate and 

efficient method to identify locations of 

maximum concentrations attributable to 

the major stationary SO2 sources, in the 

final network design (discussed below 

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

that monitors must be in locations of 

expected maximum concentration, and 

thus, typically source-oriented. Instead, 

monitors required under the final 

network design now can address 

multiple monitoring objectives 

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

number of monitors required overall 

than the number estimated in the 

proposal. The flexibility that States now 

have, where relatively fewer required 

monitors may be sited to meet multiple 

objectives, effectively addresses 

concerns about duplicative monitoring 

and the need for waivers, the need for 

measuring background concentrations, 

and that emissions data rather than the 

PWEI could be more predictive of high 

ambient SO2 concentrations as a basis 

on which to require monitoring. The 

comments that suggested the use of 

modeling, along with an examination of 

past practice, resulted in the change to 

a hybrid approach where we use both 

modeling and monitoring to assess 

ambient SO2 concentrations. 

3. Monitoring Objectives 

Because EPA contemplates an 

ultimate approach that combines both 

monitoring and modeling, the monitor 

objectives of the final network design 

are now broadened to include 

assessment of source impacts, highest 

concentration, population exposure, 

general background concentrations, SO2

transport, and long-term trends. The 

following paragraphs provide 

background, rationale, and details for 

the final changes to monitoring 

objectives. 

a. Proposed Monitoring Objectives 

EPA proposed that all minimally 

required monitoring sites in the 

proposed two-prong network design be 

sited at locations of expected maximum 

1-hour concentrations, which would 

also likely discern 5-minute peaks. EPA 

noted that in general, such locations 

would be close to larger emitting 

sources (in tons per year) and/or areas 

of relatively high emissions densities 

where multiple sources may be 

contributing to peak ground-level 

concentrations. As a result, the 

proposed monitoring network would 

have been comprised primarily of 

source-oriented monitors. EPA also 

proposed that when selecting 

monitoring sites from among a pool of 

candidate locations (which would be 

source-oriented under the proposed 

network design), States prioritize these 

sites based on where the maximum 

expected hourly concentrations would 

occur in greater proximity to 

populations. EPA solicited general 

comments on the role of population 

exposure in the site selection process. 

b. Public Comments 

Commenters discussed a variety of 

issues on the subject of monitoring 

objectives including the importance of 

considering population exposure, the 

need for flexibility in monitor 

placement, monitoring for background 

concentrations, monitoring for long term 

trends analysis, and characterizing 

potential long-range transport of SO2. 

EPA received many comments from 

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

SD, WI), the public health group ATS, 

and industry (e.g., AQRL, Consumers 

Energy, Dominion, Dow, EPRI, 

ExxonMobil, Montana Sulfur and 

Chemical, NPRA, Portland Cement, Rio 

Tinto, and UARG) suggesting that 

required monitors account for, or be 

focused on, population exposure. EPA 

also received many comments from 

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

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

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

asking for more flexibility in (sourceoriented) monitor placement with 

regard to both the target source and the 

physical location of a monitor relative to 

that source. For example NACAA stated 

that ‘‘for source oriented monitors, 

placement at the point of 1-hour 

maximum concentration must be 

realistic and flexible. EPA must allow 

agencies to determine the most 

scientifically defensible location, while 

taking into account potential exposures 

and access to locations with adequate 

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

monitor siting should be balanced 

toward population-based monitors with 

a preference toward maximum 

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

placing monitors at the maximum 

downwind location does not necessarily 

result in effective protection of public 

health.’’ 

EPA received a number of comments 

on background monitoring 27 from 

industry (API, LEC, and RRI Energy) and 

from the State of South Carolina. API 

stated that ‘‘because the monitors 

provide background concentrations 


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28Spatial scales are defined in 40 CFR Part 58 

Appendix D, section 1. Each scale is a description 

of the physical dimensions of an air parcel nearest 

a monitoring site throughout which pollutant 

concentrations are reasonably similar. 

needed to model impacts of new sources 

or sources undergoing major 

modification in addition to providing 

data for judging compliance with the 

NAAQS, it is important that some 

monitors be sited in a manner suitable 

for assessing this background.’’ API went 

on to state that ‘‘* * * EPA should 

encourage States to site an appropriate 

number of area-wide monitors for use in 

establishing ambient background levels 

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

better support the monitoring objectives, 

in particular those improving our 

understanding and context for the 

source oriented monitoring data, the 

monitoring requirements must include 

the ability for States to address the 

needs for area and regional background 

concentration measurements.’’ 

A number of commenters, including 

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

and South Carolina), citizens (Valley 

Watch at the Atlanta public hearing), 

the CBD, and Dow, commented on SO2

transport and related cross-boundary 

monitoring. Dow stated that ‘‘SO2

distribution has long been known as an 

interstate issue with the vast majority of 

SO2 sources being power plants and 

other fossil fuel combustion facilities. 

These facilities are more likely to 

impact distant areas than local areas and 

the resultant ground-level 

concentrations are often minimal.’’ Ohio 

stated that, under the proposed 

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

WV, KY, and IN will find sources along 

the Ohio River which could result in 

monitors being located across the river 

from each other.’’ In such situations, 

Ohio asserts that ‘‘States are capable of 

working with our neighbors to 

determine which State would be in the 

best position to site and operate a 

monitor.’’ 

c. Conclusions on Monitoring Objectives 

A hybrid analytical approach, as 

noted above in section III and IV.B.1 

would ultimately make the most 

appropriate use of available tools such 

as modeling and monitoring. Thus, 

unlike under the proposal, the 

monitoring network will not have to be 

focused solely at locations of expected 

maximum concentration relative to an 

SO2 source given the anticipated 

adoption of a hybrid analytical 

approach. The final network design is 

intended to be flexible to meet multiple 

monitoring objectives, most of which 

were identified in the public comments. 

Ambient monitoring networks are 

generally designed to meet three 

primary monitoring objectives, as listed 

in 40 CFR Part 58 Appendix D, Section 

1, including: (1) Providing air pollution 

data to the general public in a timely 

manner, (2) support compliance with 

ambient air quality standards and 

emissions strategy development, and (3) 

support air pollution research studies 

(which includes health studies and 

research). In order to support these air 

quality management objectives, 

monitoring networks can have a variety 

of monitoring sites that can be sited, as 

necessary, to characterize (a) emission 

sources (i.e., source-oriented 

monitoring), (b) the highest 

concentration in an area, (c) population 

exposure, (d) general background 

concentrations, (e) regional transport, 

and (f) welfare-based impact. 

In light of the approach described in 

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

EPA is finalizing an SO2 network 

design, with broadened objectives, 

which EPA believes will address the 

concerns noted in the public comments 

above, particularly those regarding 

siting flexibility, population exposure, 

cross-boundary impacts, and the need 

for the network to address multiple 

monitoring objectives. The final 

network design requires that any SO2

monitors required in a particular CBSA 

as determined based on PWEI values, 

discussed below in section IV.B.4, shall 

satisfy the minimum monitoring 

requirements if they are sited at 

locations where they can meet any one 

or more of the following objectives (see 

Part 58 Appendix D section 4.4.2 as 

added by today’s final rule): 

(1) Source-Oriented Monitoring: This 

is accomplished with a monitor sited to 

determine the impact of significant 

sources or source categories on air 

quality. In some situations, such 

monitoring sites may also be classified 

as high concentration sites (discussed 

below). Examples of source-oriented 

monitors include those sited to capture 

or assess peak ground-level 

concentrations from one or more major 

SO2 sources, or those sited in an area 

with multiple smaller sources with 

overlapping plumes. 

(2) Highest Concentration: This is 

assessed by a monitor sited to measure 

the highest concentrations expected to 

occur in the area covered by the 

network. Such a location may, or may 

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

Depending on the case, this location is 

representative of the highest 

concentration occurring across a 

relatively homogeneous area with 

spatial scales typically ranging from 

tens of meters up to four kilometers.28

(3) Population Exposure: This is 

assessed by a monitor sited to measure 

typical concentrations in areas of 

(relatively) high population density. 

Some examples are a monitor placed in 

an area of elevated or high SO2

concentrations that also has a high 

population density, an area that might 

be included in public health studies, or 

in areas with vulnerable and susceptible 

populations. 

(4) General Background: This is 

assessed by placing a monitor in an area 

to determine general background 

concentrations. Such locations might be 

considered to be representative of 

ambient concentrations upwind of (and 

therefore not typically influenced by) a 

geographic area such as an urban area, 

or of an individual or group of emission 

sources. EPA notes that although a 

required monitor is allowed to be sited 

to assess background concentrations, the 

required monitor is not allowed to be 

sited outside of the parent CBSA (whose 

PWEI value triggered required 

monitoring, discussed in section IV.B.4 

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

is a need to conduct background 

monitoring outside of CBSAs with 

required monitoring, EPA notes that 

States always have the prerogative to 

conduct monitoring above the minimum 

requirements in any location the State 

believes is appropriate. 

(5) Regional Transport: This is 

assessed by placing a monitor in a 

location to determine the extent of 

regional pollutant transport. Such 

locations could be either upwind or 

downwind of urban areas, 

characterizing the entry or exit of the 

pollutant in a region, respectively. EPA 

notes that although a required monitor 

is allowed to be sited to assess regional 

transport, the required monitor is not 

allowed to be sited outside of the parent 

CBSA (whose PWEI value triggered 

required monitoring, discussed in 

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

believes that there is a need to conduct 

background monitoring outside of 

CBSAs with required monitoring, EPA 

notes that States always have the 

prerogative to conduct monitoring above 

the minimum requirements in any 

location the State believes is 

appropriate. 

In regard to the public comments 

expressing concerns on the issue of 

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

on one side of a political boundary 

contributes to peak ground-level 

concentrations on the other side of that 

boundary, EPA will allow a required 

monitor to be placed outside of the 

parent CBSA (whose PWEI value 

triggered monitoring, discussed in 

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


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29The rationale for finalizing the use of the PWEI 

and the number of monitors required through its 

application are discussed in section III.B.4. 

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

and are comprised of both Metropolitan Statistical 

Areas and Micropolitan Statistical Areas (http:// 

www.census.gov). 

particular condition. A source-oriented 

monitor may be sited outside of the 

parent CBSA, whose PWEI value 

triggered required monitoring, if that 

monitor is characterizing the location of 

expected maximum concentration of a 

source inside that parent CBSA. If a 

State chooses to exercise this flexibility 

in source-oriented monitor siting, the 

State must provide clear rationale for 

their choice in their annual monitoring 

plan, which is subject to EPA regional 

approval. If the source-oriented monitor 

is to be placed in another State, such as 

the example provided by the State of 

Ohio in the public comments above, the 

two States are responsible for 

collaboration on the location and 

operation of that monitoring site. 

Further, due to the broadened 

objectives of the final network design, 

EPA also is finalizing the provision that 

an NCore SO2 monitor within a CBSA 

(where a CBSAs PWEI value triggered 

required monitoring) can be counted 

towards meeting the minimum 

monitoring requirements in this 

rulemaking (discussed in section IV.B.4) 

because they can meet some of the 

expanded objectives of the network. 

NCore sites are intended to provide 

long-term data for air quality trends 

analysis, model evaluation, and, for 

urban sites, tracking metropolitan air 

quality statistics, and therefore are 

appropriate to allow to count towards 

minimum monitoring requirements 

under the revised monitoring scheme. 

Finally, EPA strongly encourages 

State and local air agencies to consider 

using required monitoring, as 

appropriate, to characterize those 

sources which are not as conducive to 

dispersion modeling and to assess 

population exposure. Sources that are 

not conducive to dispersion modeling 

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

such as shipping ports, (2) a source 

situated in an area of complex terrain 

and/or situated in a complex 

meteorological regime, and (3) locations 

that have multiple, relatively small 

sources with overlapping plumes. 

4. Final Monitoring Network Design 

The use of a hybrid analytic approach 

(discussed above in section III and 

IV.B.1) makes it unnecessary for the 

final monitoring network design to be 

distinctly focused on monitoring 

locations of expected maximum 

concentration (and thus be primarily 

source-oriented), as discussed in section 

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

of modeling and monitoring for 

designations, the final monitoring 

network is designed to provide 

flexibility for required monitors to 

address the multiple monitoring 

objectives just discussed in the 

preceding section. This flexibility in 

monitoring objectives is in response, in 

part, to the many public comments 

received from States (e.g., NACAA and 

six other States), industry (API, EPRI, 

UARG, and eight other groups), and 

from the American Thoracic Society 

(ATS), urging EPA to ensure that some 

or all of the required monitors be sited 

and suited to characterize population 

exposure and, from many of these same 

commenters, to allow flexibility in 

implementing the siting requirements 

for the monitors. Under a hybrid 

approach, and the different monitoring 

objectives resulting thereof, the final 

monitoring network design also does 

not need to be a two-prong approach 

like the one proposed. Therefore, EPA is 

adopting a modified version of the first 

prong of the proposed network design, 

which will use PWEI values to require 

monitors in certain CBSAs where there 

is increased coincidence of population 

and SO2 emissions. There is no second 

prong in the final network design by 

which monitors are required based on a 

State’s individual contribution to the 

national anthropogenic SO2 inventory, 

as was proposed. 

The final monitoring network design 

requires monitoring in CBSAs based on 

calculated PWEI values, where a PWEI 

shall be calculated (as discussed in 

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

For any CBSA with a calculated PWEI 

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

a minimum of three SO2 monitors are 

required within that CBSA. This 

requirement remains the same as 

proposed. For any CBSA with a 

calculated PWEI value equal to or 

greater than 100,000, but less than 

1,000,000, a minimum of two SO2

monitors are required within that CBSA. 

For any CBSA with a calculated PWEI 

value equal to or greater than 5,000, but 

less than 100,000, a minimum of one 

SO2 monitor is required within that 

CBSA. EPA has adjusted the thresholds 

for requiring one or two monitors in a 

CBSA and the rationale for this 

adjustment is explained more fully 

below in section IV.B.5. As just 

explained in section III.B.3, these 

monitors shall be sited to meet one or 

more of a number of monitoring site 

objectives, including the assessment of 

source impacts, highest concentrations, 

population exposure, general 

background, and regional transport. EPA 

believes that the monitors required 

within these PWEI breakpoints provide 

a reasonable minimum number of 

monitors in a CBSA, where there is a 

relatively increased coincidence of 

population and SO2 emissions and 

therefore increased potential for 

exposures, because we are directly 

accounting for both population and 

emissions that exist in individual 

CBSAs.29 EPA estimates that these 

minimum monitoring criteria (based on 

2008 population and 2005 NEI data) 

require 163 monitors within 131 CBSAs. 

EPA also intends for SO2 monitors at 

NCore stations to satisfy these minimum 

monitoring requirements. Based on 

analysis of proposed and approved 

NCore sites (as of April 2010), all of 

which are scheduled to be operational 

no later than January 1, 2011, EPA 

estimates that 52 of the total 80 SO2

monitors at NCore stations are within 

the 131 CBSAs that have required 

monitors based on their PWEI values. 

As a result, EPA estimates that between 

these minimum monitoring 

requirements and the NCore network, 

there will be at least 191 SO2 monitors 

operating across the country. 

5. Population Weighted Emissions Index 

In the proposal, EPA had introduced 

a metric based on population and 

emissions as a basis for locating 

monitors in the network. EPA 

anticipated that this metric would 

characterize the potential for exposure 

based on the proximity of source 

emissions to populations. The following 

paragraphs provide background, 

rationale, and details for the final 

changes of the calculation and use of the 

Population Weighted Emissions Index 

in determining minimum monitoring 

requirements. 

a. Proposed Use of the Population 

Weighted Emissions Index 

In the proposed network design 

approach, which utilized a two-prong 

network design, EPA created the 

Population Weighted Emissions Index 

(PWEI) in an attempt to focus 

monitoring resource where there was a 

higher proximity of population and SO2

emissions. In effect, areas with higher 

PWEI values have higher potential for 

population exposure to short-term SO2

emissions. EPA proposed that the PWEI 

be calculated using population and 

emissions inventory data at the Core 

Based Statistical Area (CBSA) 30 level to 

assign required monitoring for a given 

CBSA, with population and emissions 

being the relevant factors. To calculate 

the PWEI for a particular CBSA, using 


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the latest Census Bureau estimates, the 

population of a CBSA must be 

multiplied by the total amount of SO2

emissions in that CBSA. The CBSA 

emission value is in tons per year (using 

the latest available National Emissions 

Inventory [NEI] data), and is calculated 

by aggregating the county level 

emissions for each county in a CBSA. 

We then divide the resulting product of 

CBSA population and CBSA SO2

emissions by 1,000,000 to provide a 

PWEI value in more manageable units of 

millions of people-tons per year. 

With the change in the approach 

discussed in section III and section 

IV.B.1 above, and considering the final 

monitoring network design discussed in 

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

that which was proposed also changes. 

The following paragraphs discuss some 

of the public comments received on the 

general use and calculation of the PWEI; 

other comments that focused on the 

detailed application of the PWEI as 

proposed will be addressed in the 

response to comments document since 

our approach in applying the PWEI has 

changed. 

b. Public Comments 

EPA received a number of comments 

from State and local groups (e.g., 

NACAA and eight others) and industry 

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

generally agreed with the two-pronged 

network design concept which had the 

PWEI as a component. More 

specifically, some State commenters 

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

WI) expressed concern that the PWEI 

(along with the second prong of the 

proposed network design) created 

monitoring requirements that were 

‘‘duplicative’’ and also called for 

monitors in areas where they were not 

needed. Even amongst some of the 

commenters who generally agreed with 

the PWEI concept, some provided 

examples of where the PWEI appeared 

to be duplicative in its proposed 

application. One example was provided 

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

Homosassa Springs, the [proposed 

network design] requires two monitors 

[in that CBSA as a result of the proposed 

use of the PWEI]. The driving source is 

the Crystal River Power Plant, with 

emissions in 2008 of over 85,000 tons 

per year of SO2. The next largest source 

in the CBSA has emissions of roughly 

two tons per year.’’ EPA believes that 

Florida is asserting that the one large 

source disproportionately drove the 

PWEI too high for that particular CBSA 

and only one monitor was actually 

needed. EPA notes that these particular 

comments on duplicative monitoring 

were made under the premise that all 

proposed required monitors would be 

sited in locations of expected maximum 

concentration, and therefore would be 

source-oriented in nature. As a result, 

these commenters believed it was 

necessary that a waiver provision be 

included if they could show that the 

required number of monitors was too 

many, as in Florida’s example. 

As discussed in section IV.B.4 above, 

a hybrid approach results in a final 

network design with a reduced number 

of required monitors from the number 

proposed, a different application of the 

PWEI, and provides flexibility in 

meeting additional monitoring 

objectives for the required monitors, 

making the need for a waiver from the 

minimally required monitors 

unnecessary. If a CBSA is required to 

have multiple monitors now, those 

monitors are not specifically required to 

be located near sources where 

maximum concentrations of SO2 are 

expected to occur. Instead, they can be 

sited at different locations to fulfill a 

variety of objectives, although, as noted 

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

encouraging States to consider 

monitoring near sources not conducive 

to dispersion modeling and for 

characterization of population 

exposures. 

EPA received comments from 

Michigan, South Carolina, and CBD 

requesting clarification on the logic 

behind the proposed PWEI thresholds, 

or breakpoints, by which three, two, 

one, or no monitors would be required 

in a given CBSA. In addition, some 

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

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

suggested specific adjustments to the 

proposed application of the PWEI. For 

example, Michigan suggested that the 

required monitor breakpoint values be 

adjusted to the ‘‘natural breakpoints in 

the overall distribution’’. South Carolina 

suggested EPA identify a way to 

normalize the PWEI stating the PWEI 

would be more appropriate ‘‘* * * if it 

used a value that better addressed 

difference in area, population 

distribution, land use, number, types of 

sources, etc.’’ 

In the proposed network design, EPA 

selected the PWEI values, or 

breakpoints, to require one or more 

monitors based on the overall 

distribution of PWEI values across all 

CBSAs. Based on U.S. Census Bureau 

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

approximately 939 CBSAs in the 

country. EPA proposed and now 

requires that a PWEI value be calculated 

for each of these CBSAs to determine if 

monitoring is required in that CBSA. 

Based on 2008 census estimates and the 

2005 NEI, the average CBSA PWEI value 

is 21,900 while the median value is only 

121. This indicates that a relatively 

small number of CBSAs with high PWEI 

values are driving the very upper end of 

the PWEI distribution. The proposed 

breakpoint where one monitor was 

required in a CBSA was a PWEI value 

of 5,000. EPA estimated that 131 out of 

939 CBSAs (∼14%) have a PWEI value 

of 5,000 or more. Further, these 131 

CBSAs occupy ∼98% of the sum of 

PWEI values across all 939 CBSAs, 

where high PWEI values indicate 

increased coincidence in population 

and SO2 emissions. Within this group of 

CBSAs with PWEI values of 5,000 or 

more, EPA considered the relative 

amounts of population, emissions, and 

general frequency of occurrence of 

relatively larger SO2 sources (such as 

those that emit 100 tons per year or 

more) in selecting the breakpoints to 

require two and three monitors in a 

CBSA for the proposed network design. 

These considerations were made in an 

effort to apply a nationally applicable 

process by which to require a minimum 

number of monitors for an area, which 

all were to be sited in locations of 

expected maximum concentration, and 

therefore likely source-oriented 

monitors. In regard to the comments 

suggesting modification to the 

calculation or to normalize the PWEI, 

EPA believes that the proposed 

calculation, under a hybrid analytical 

approach, is still most appropriate. 

Under a hybrid analytical approach, 

States have the flexibility to move 

monitoring resources where needed 

within CBSAs that have a high 

coincidence of population and 

emissions instead of only being able to 

site monitors to characterize sources. 

States have the option to consider 

additional factors such as those listed in 

South Carolina’s comments above in 

further identifying where required 

monitoring may be most appropriate in 

their areas with required monitoring. 

Several States (e.g. NESCAUM, 

NYSDEC, and PADEP) suggested 

abandoning the PWEI concept altogether 

and instead using some form of 

emissions-only approach to require 

monitors. For example, NESCAUM, who 

generally supported a ‘‘hot-spot’’ 

monitoring approach, suggested that the 

PWEI be abandoned and EPA instead 

‘‘* * * adopt an emissions-only 

approach, resulting in fewer CBSA 

monitors. We [NESCAUM] suggest a 

threshold of 50,000 tpy CBSA SO2

emissions to trigger the first CBSA 

monitor and a second CBSA monitor 

required when emissions exceed 

200,000 tpy.’’ NESCAUM states that the 

proposed use of the PWEI ‘‘* * * can 


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31 In simulating NYSDEC’s suggested network 

design, EPA assumed that no CBSA would have 

more than one monitor. According to the 2005 NEI, 

there are 162 sources emitting 20,000 tpy or more 

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

CBSAs that have emissions of 50,000 tpy, leaving 

approximately 62 sources that would need a 

monitor to satisfy NYSDEC’s suggested network 

design. 

result in multiple monitors in large 

cities that have relatively small CBSA 

SO2 emissions, or no monitor in a CBSA 

with large emissions.’’ NYSDEC suggests 

that the proposed approach, using the 

PWEI, is ‘‘* * * not more predictive 

than using emissions data alone.’’ 

NYSDEC went on to suggest that 

monitors be required in CBSAs with 

aggregated emissions of 50,000 tons per 

year or more and that ambient 

monitoring be considered for point 

sources with 20,000 tons per year. 

PADEP made several suggestions on 

network design, with one that suggested 

monitoring in any CBSA ‘‘where there is 

a sulfur dioxide source or combination 

of sources within 50 miles emitting a 

total of at least 20,000 tons of SO2 per 

year * * *’’ 

EPA reviewed emissions and 2005 

NEI data and compared the suggestions 

provided by NESCAUM and NYSDEC to 

the requirement of the final network 

design. Under NESCAUM’s suggested 

design, EPA estimates there would be 75 

required monitors in 65 CBSAs. Of these 

65 CBSAs, 6 CBSAs that are not covered 

by the final network design would be 

included; however, 72 CBSAs that will 

have monitors under the final network 

design would otherwise not have 

monitors under NESCAUM’s design. 

EPA believes that the exclusion of those 

72 CBSAs would lead to too sparse a 

network to adequately meet the 

monitoring objectives of the network. 

Under NYSDEC’s suggested network 

design, EPA estimates that there would 

be a minimum of 65 monitors in the 

same 65 CBSAs of the NESCAUM 

suggested design. Further, if States 

ensured that monitors were placed near 

all sources emitting 20,000 tons per year 

(as NYSDEC suggested should be 

‘‘considered’’ for monitoring), there 

could be an additional 69 monitors.31

EPA believes that the final network 

design as discussed above in section 

IV.B.4, with the increased flexibility for 

monitors to meet multiple monitoring 

objectives (discussed in IV.B.3 above) 

including, among others, 

characterization of source impacts or 

population exposure, is better served 

using PWEI values to require monitors 

because it explicitly accounts for 

population to require and distribute 

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

concern that other CBSAs or areas not 

included in the final network design, 

such as the six CBSAs that were 

included in the NESCAUM and 

NYSDEC suggested network designs 

noted above, warrant monitoring 

resources, States or the EPA Regional 

Administrator may take action to 

require monitoring in such areas. The 

authority of an EPA Regional 

Administrator to require additional 

monitoring above the minimum 

requirements is discussed in section 

IV.B.6 below. 

EPA received a number of comments 

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

NYSDEC, SC, and WI) and industry 

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

LPPA, and UARG) raising concern over 

the way the PWEI is calculated. 

Specifically, many commenters in this 

group indicated that they believed that 

the 2005 NEI would be used in an 

exclusive or permanent fashion to 

calculate the PWEI, and that updated 

NEI data would not be used. For 

example, NESCAUM states that ‘‘EPA 

should not require States to rely solely 

on EPA’s inventories [for calculating the 

PWEI], such as the National Emissions 

Inventory (NEI), as they do not always 

have the updated information that is 

necessary for such regulatory decisions.’’ 

Wisconsin ‘‘* * * believes that States 

should be allowed to use their own 

annual point source inventories instead 

of EPA’s National Emissions Inventory 

(NEI) for evaluating emission sources. 

Wisconsin’s point inventory is updated 

annually and has a reporting threshold 

of five tons per year for SO2, making it 

more sensitive to changes in facility 

operations than the NEI, which is 

updated triennially.’’ UARG stated that 

their ‘‘primary concern with this 

network design is its reliance on old 

emissions data. For electric utilities 

which report their SO2 emissions to 

EPA annually, the use of more recent 

data would be appropriate.’’ 

EPA does not intend for relatively old 

emissions data to be used in calculating 

the PWEI values for individual CBSAs. 

As was detailed in the proposed 

regulatory text for 40 CFR Part 58 

Appendix D (74 FR 64880), EPA stated 

that ‘‘The PWEI shall be calculated by 

multiplying the population of each 

CBSA, using the most current census 

data, by the total amount of SO2 in tons 

per year emitted within the CBSA area, 

using an aggregate of the most recent 

county level emissions data available in 

the National Emissions Inventory for 

each county in each CBSA.’’ Although 

commenters suggested that there may be 

other resources from which emissions 

data may be obtained, particularly at the 

individual State level, the NEI is 

comprised of emissions data which is 

collected by EPA from the States 

themselves. The Air Emissions 

Reporting Requirements (40 CFR Part 

51), by which EPA sets out how States 

are to report their emission inventories, 

was recently revised in December of 

2008. That rulemaking was intended to 

provide enhanced options to States for 

emissions data collection and exchange 

and unify reporting dates for various 

categories of inventories. EPA notes that 

the NEI is updated in full every three 

years and the 2008 NEI is scheduled to 

be available by January 2011. States will 

have submitted their data by May 31, 

2010, before this rule is promulgated 

and published, and EPA will provide 

comment on these submittals during the 

summer of 2010. States will have an 

opportunity to revise their 2008 data 

submissions in the fall of 2010. In the 

triennial update, both point and 

nonpoint data are required to be 

submitted by States and are included in 

the inventory. Further, States are 

required to submit emissions data 

annually for all sources emitting 2,500 

tons per year or more of SO2 as well as 

for sources emitting other pollutants in 

excess of thresholds set for those 

pollutants. In all point source submittals 

to the NEI, States are also allowed to 

submit emissions data for sources of any 

emissions level, but are not required to 

do so. Starting with the 2009 NEI, the 

annual and triennial State NEI 

submittals will be due one year after the 

end of the emissions year. States have 

an additional opportunity to revise their 

submittals based on EPA comment in 

the spring of the following year, with 

EPA publishing the inventory no later 

than 6 months after the inventory 

submittal dates (18 months after the end 

of the emissions year). This approach 

and schedule is accelerated over past 

NEI schedules and has been designed as 

part of the development of the new 

Emission Inventory System (EIS). Rather 

than representing old emissions data, 

the NEI available through EIS represents 

a timely and appropriate source of 

emissions data. 

EPA believes that the process by 

which the NEI will be updated (through 

use of the EIS) will be adjusted in a 

manner that will allow for more 

frequent insertion of State supplied 

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

opportunity to encourage States to 

supply all of their available emissions 

information to the NEI as soon as 

practicable. Therefore, EPA believes that 

the NEI is an appropriate and nationally 

representative source of emissions data 

by which PWEI calculations may be 


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made. PWEI calculations for all CBSAs 

will use the same year of data at any 

given time, and States, local agencies, 

and Tribes will have uniform 

opportunity for revising their emissions 

data for this purpose. EPA again 

encourages States to view the NEI 

submittals as their opportunity to 

submit their best available SO2 and 

other inventory data with the 

knowledge that it will be used for the 

purpose of PWEI values. 

c. Conclusions on the Use of the 

Population Weighted Emissions Index 

In the final network design, EPA has 

determined that it is appropriate to use 

PWEI values as the mechanism by 

which to require monitors in certain 

CBSAs, similar to its use in the first 

prong of the proposed two-prong 

network design. EPA believes that using 

the PWEI metric to inform where 

monitoring is required is more 

appropriate for the SO2 network design 

than utilizing a population-only or 

emissions-only type of approach, 

because it takes into account not just 

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

emissions, but instead takes into 

account the exposure from SO2

emissions to groups of people who are 

in greater proximity to such emissions. 

In the final rule, EPA is retaining the 

requirement to calculate the PWEI by 

multiplying the population of each 

CBSA, using the most current census 

data/estimates from the U.S. Census 

bureau, by the total amount of SO2 in 

tons per year emitted within the CBSA 

area, using an aggregate of county level 

emissions data available in the most 

recent published version of the National 

Emissions Inventory for each county in 

each CBSA. The resulting product shall 

be divided by one million, providing a 

PWEI value, the units of which are 

million persons-tons per year. For any 

CBSA with a calculated PWEI value 

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

minimum of three SO2 monitors are 

required within that CBSA. For any 

CBSA with a calculated PWEI value 

equal to or greater than 100,000, but less 

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

monitors are required within that CBSA. 

For any CBSA with a calculated PWEI 

value equal to or greater than 5,000, but 

less than 100,000, a minimum of one 

SO2 monitor is required within that 

CBSA. EPA believes that the monitors 

required within these breakpoints 

provide a reasonable minimum number 

of monitors in a CBSA that considers 

the combination of population and 

emissions that exist in a CBSA. These 

criteria (based on 2008 population and 

2005 NEI data) are estimated to require 

163 monitors within 131 CBSAs. 

EPA has changed the PWEI 

breakpoint in the final rule at which two 

monitors are required in a CBSA to 

100,000 from the breakpoint of 10,000 

in the proposed network design based 

on multiple considerations. First, EPA 

changed the breakpoint because of a 

hybrid analytic approach and attendant 

changes in monitoring objectives (see 

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

that the monitoring network is no longer 

intended to be comprised primarily of 

source-oriented monitors that are sited 

at locations of expected maximum 

concentration. This change in objective 

of the network design allows fewer 

monitors to provide the necessary 

amount of ambient monitoring data EPA 

to meet the multiple monitoring 

objectives. Second, the breakpoint of 

100,000 occurs near a ‘‘natural’’ 

breakpoint in the PWEI distribution, a 

consideration that Michigan suggested, 

where the estimated 28 CBSAs with 

PWEI values of 100,000 or more occupy 

∼87% of the sum of PWEI values across 

all 939 CBSAs. Finally, EPA considered 

commenters’ assertion that the first 

prong of the proposed network design 

created duplicative monitoring in 

certain CBSAs. This duplicative 

monitoring is especially recognized in 

some CBSAs with relatively small 

populations and somewhat large 

emissions which are dominated by a 

single source (such as the Homosassa 

Springs, FL example discussed above). 

Raising the second breakpoint helps to 

alleviate some of the duplicative 

monitoring that many of the State 

commenters noted. 

EPA therefore is keeping the first and 

third breakpoints, which require one 

monitor in a CBSA having a PWEI value 

of 5,000 and three monitors in a CBSA 

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

believes maintaining these breakpoints 

along with the revised 100,000 PWEI 

breakpoint, will (1) ensure that highly 

populated areas will be monitored for 

ambient SO2 concentrations even if the 

emissions in that area are moderate, 

which is appropriate given the fact that 

the greater population creates increased 

potential for exposure to those moderate 

emissions, and (2) that those areas with 

higher emissions or emission densities, 

with moderate or modest populations 

will be monitored because those 

increased emissions are likely to have a 

significant impact on nearby 

populations. 

6. Regional Administrator Authority 

The following paragraphs provide 

background, rationale, and details for 

the final changes to Regional 

Administrator authority to use 

discretion in requiring additional SO2

monitors beyond the minimum network 

requirements. 

a. Proposed Regional Administrator 

Authority 

EPA proposed that the Regional 

Administrators will have discretion to 

require monitoring above the minimum 

requirements, as necessary, to address 

situations where the minimum 

monitoring requirements are not 

sufficient to meet monitoring objectives. 

EPA recognized that the minimum 

required monitors in the proposed twopronged network design were based on 

indicators that may not have always 

provided spatial coverage for all the 

areas that have SO2 sources. Although 

the network design and the objectives of 

the network design have changed from 

those that were proposed because of our 

contemplated use of a hybrid analytical 

approach, EPA believes it is still 

important for Regional Administrators 

to have the discretion, and authority, to 

require monitoring above the minimum 

requirements. Providing the RAs with 

this discretion will allow them to fill 

any identified gaps in meeting the 

monitoring objectives of the network. 

b. Public Comments 

Some commenters (e.g., LCA, 

LMOGA, LPPA, and South Carolina) 

expressed concerns with the proposed 

provision authorizing the Regional 

Administrator to require additional 

monitoring above the minimum 

requirements. The LCA, LMOGA, and 

LPPA stated that ‘‘the EPA’s proposal to 

allow the Regional Administrator 

discretion to require a State to add 

additional monitors is flawed in that it 

provides unfettered discretion. Criteria 

should be added * * * that limit such 

discretion and require the Regional 

Administrator to consider certain 

objective factors when determining 

whether to require any additional 

ambient SO2 monitors to the network.’’ 

South Carolina stated that ‘‘the Regional 

Administrators should not have the 

discretion to require monitoring above 

the requirements described in [the 

proposal for] Part 58 and its 

Appendices. State monitoring 

organizations must be given discretion 

to decide the appropriate use of 

resources to meet uniform monitoring 

requirements. Additional monitoring 

requirements should not be imposed 

without concurrence of the monitoring 

organization and additional funding that 

completely supports the additional 

costs.’’ 


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

existing FEM monitors in operation may continue 

to be used to monitor compliance with the NAAQS. 

c. Conclusions on Regional 

Administrator Authority 

The authority of Regional 

Administrators to require additional 

monitoring above the minimum 

required is not unique to the SO2

NAAQS. For example, Regional 

Administrators have the authority to use 

their discretion to require additional 

NO2 or Pb monitors (40 CFR Part 58 

Appendix D section 4.3.4 and 4.5, 

respectively) and to work with State and 

local air agencies in designing and/or 

maintaining an appropriate ozone 

monitoring network (40 CFR Part 58 

Appendix D section 4.1). EPA believes 

that the nationally applicable final 

network design, although somewhat 

dictated by local factors (population and 

emissions), may not account for all 

locations where monitors should be 

sited, including where potentially high 

concentrations of SO2 may be occurring. 

Examples include locations that have 

the potential to violate or contribute to 

violations of the NAAQS, areas that 

might have high concentrations of SO2

that are not characterized by modeling 

or have sources that are not conducive 

to modeling, and locations with 

susceptible and vulnerable populations. 

As a result, EPA believes it is important 

for Regional Administrators to have the 

authority to address possible gaps in the 

minimally required monitoring network, 

especially near sources or areas that are 

not conducive to modeling by granting 

them authority to require monitoring 

above the minimum requirements. 

However, in response to public 

comments, EPA notes that Regional 

Administrators would use this authority 

in collaboration with State agencies to 

design and/or maintain the most 

appropriate SO2 monitoring network to 

meet the needs of a given area. For all 

the situations where the Regional 

Administrators may require additional 

monitoring, it is expected that the 

Regional Administrators will work on a 

case-by-case basis with State or local air 

agencies. Further, any monitor required 

through the Regional Administrator and 

selected by the State agency, or any new 

monitor proposed by the State itself, is 

not done so with unfettered discretion, 

since any such action would be 

included in the Annual Monitoring 

Network Plan per § 58.10, which must 

be made available for public inspection 

or comment, and approval by the EPA 

Regional Administrator. 

Therefore, EPA is finalizing the 

proposal that Regional Administrators 

may use their authority to require 

monitoring above the minimum 

requirements, as necessary, in any area, 

to address situations where the 

minimally required monitoring network 

is not sufficient to meet monitoring 

objectives. In all cases in which a 

Regional Administrator may consider 

the need for additional monitoring, it is 

expected that the Regional 

Administrators will work with the State 

or local air agencies to evaluate 

evidence or needs to determine if a 

particular area may warrant additional 

monitoring. 

7. Monitoring Network Implementation 

The following paragraphs provide 

background, rationale, and details for 

the final approach for the monitoring 

network implementation. 

a. Proposed Monitoring Network 

Implementation 

EPA proposed that State and, where 

appropriate, local air monitoring 

agencies submit a plan for deploying 

SO2 monitors in accordance with the 

proposed requirements discussed above 

by July 1, 2011. EPA also proposed that 

the SO2 network be physically 

established no later than January 1, 

2013. EPA also proposed that the 

number of sites required to operate as a 

result of the Population Weighted 

Emissions Index (PWEI) values 

calculated for each CBSA be reviewed 

and revised for each CBSA through the 

5-year network assessment cycle 

required in § 58.10. 

b. Public Comments 

EPA received comments from the 

ALA, EDF, NRDC, and SC that 

supported ‘‘* * * a more accelerated 

deployment of new monitoring than the 

2013 target date proposed by EPA. The 

sooner monitors are in place, the sooner 

the public will experience the health 

benefits of the new standard.’’ However, 

EPA received comment from States (e.g., 

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

LCA, LMOGA, and LPPA) and public 

health and environmental groups (e.g., 

ALA, EDF, NRDC, and SC) expressing 

concern with the proposed deployment 

schedule of the proposed SO2 network 

in that it was too fast or needed to be 

phased in. The States of Iowa, South 

Carolina, and Wisconsin suggested that 

EPA allow the proposed network to 

deploy on a phased schedule. For 

example, South Carolina recommended 

a ‘‘phased implementation with largest 

source/highest probability population 

exposure areas designated for 

implementation in 2013 (some 

proportion of the highest PWEI 

monitors) and establishment of the 

remaining PWEI and the State level 

emissions triggered monitoring required 

the following year.’’ Meanwhile, the 

States of Michigan and North Carolina, 

along with the industry commenters 

LCA, LMOGA, and LPPA, suggested 

EPA reconsider implementation dates in 

light of the multiple rulemakings that 

impose mandates on States that have 

and will be occurring in the future. For 

example, North Carolina stated that 

‘‘EPA must keep in mind that it is 

simultaneously revising numerous 

ambient standards and associated 

monitoring requirements. EPA seems to 

view each of these proposals as 

independent actions; but the State and 

local agencies must consider the 

cumulative impact of EPA’s various 

regulatory actions on their ability to 

comply.’’ North Carolina goes on to say 

that ‘‘EPA must allow States the 

flexibility to prioritize among the new 

requirements to get community based 

monitors in place first and to establish 

the others as funding and personnel 

resources allow.’’ 

EPA believes that with the use of a 

hybrid analytical approach, the 

concerns raised by States and industry 

commenters suggesting a phased or 

delayed implementation are addressed 

because the final network minimum 

design requirements result in fewer 

monitors being required than in the 

proposed network design. EPA’s 

analysis of the existing network had 

indicated that a substantial number of 

monitors were not sited at locations of 

maximum concentrations. These 

monitors would have had to be relocated to count towards minimum 

monitoring requirements under the 

proposed monitoring-focused approach. 

Under a combined modeling and 

monitoring approach, the required 

monitors can be used to satisfy multiple 

monitoring objectives and therefore, 

many of the monitors in the existing 

network will satisfy the requirements in 

the final network design, eliminating 

any need for a phased or delayed 

network implementation. In regard to 

the suggestion by public health and 

environmental groups to speed up 

implementation, EPA notes that under a 

hybrid analytical approach much of the 

existing network will fulfill minimum 

monitoring requirements, and an 

accelerated schedule is not necessary; 

the network implementation date 

provides a balance between ensuring the 

minimally required network is fully in 

place in a reasonable amount of time 

and providing States adequate time to 

fulfill all the requirements in this 

rulemaking.32

EPA received comment on the 

frequency by which the minimally 


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33Note that some commenters supported more 

than one form of reported 5-minute data. 

required network will be reviewed and 

possibly adjusted based on updated 

population and emissions inventories. 

The State commenters listed above, and 

some others including NACAA, 

indicated that they believed that the 

proposal for reviewing the SO2 network 

every five years was intended to be a 

separate review from the required 5-year 

network assessments required in 

§ 58.10(d). NACAA stated ‘‘EPA 

proposes that the SO2 monitoring 

network be evaluated every five years. 

This is an unnecessary duplication of 

effort in light of the current 

requirements for the annual network 

plan and five year network review.’’ 

NACAA went on to say that ‘‘the current 

requirements [in § 58.10] should be 

regarded as the primary source of 

monitoring network information for all 

NAAQS pollutant monitoring, 

regardless of the pollutant.’’ 

EPA concurs with NACAA’s 

statements that the existing 

requirements for network assessment 

are an appropriate primary source of 

monitoring network information. In the 

proposal, EPA did not intend for a 

required 5-year review of the SO2

network to be an additional effort on top 

of the existing required network 

assessments but instead to be included 

as part of the 5-year assessment in 

§ 58.10(d). EPA notes that CBSA 

populations and emissions inventories 

change over time, suggesting a need for 

periodic review of the monitoring 

network. At the same time, EPA 

recognizes the advantages of a stable 

monitoring network. However, after 

considering comments, EPA is not 

finalizing the proposed language for 40 

CFR Part 58 Appendix D, section 

4.4.3(2) which simply referenced back 

to § 58.10. This proposed text it is not 

needed and appears to simply cause 

confusion. EPA asserts that the existing 

requirements in § 58.10 provide a 

sufficient and appropriate mechanism 

for network updates and assessment. 

c. Conclusions on Monitoring Network 

Implementation 

Based on the public comments, and 

due to the contemplated use of a hybrid 

analytical approach, EPA is finalizing, 

as was proposed, that State and, where 

appropriate, local air monitoring 

agencies submit a plan for deploying 

SO2 monitors in accordance with the 

proposed requirements presented below 

by July 1, 2011. Minimally required SO2

monitors shall be physically established 

no later than January 1, 2013. 

C. Data Reporting 

The following paragraphs provide 

background, rationale, and details for 

monitor data reporting requirements. 

a. Proposed Data Reporting 

Controlled human exposure studies 

indicate that exposures to peaks of SO2

on the order of 5 to 10 minutes result 

in moderate or greater decrements in 

lung function and/or respiratory 

symptoms in exercising asthmatics 

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

REA section 7.2.3, and REA section 

10.3.3.2). As a result, the 1-hour 

standard is intended to protect against 

short term exposures, including 

exposures on the order of 5 minutes up 

to 24 hours, as is discussed in section 

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

revised NAAQS and its intent, EPA 

proposed that State and local agencies 

shall report to AQS the maximum 5- 

minute block average of the twelve 5- 

minute block averages of SO2 for each 

hour. This 5-minute block reporting 

requirement is in addition to the 

existing requirement to report the 1- 

hour average. In addition, EPA solicited 

comment on the advantages and 

disadvantages (including associated 

resource burdens) of alternatively 

requiring State and local agencies to 

report all twelve 5-minute SO2 values 

for each hour or the maximum 5-minute 

concentration in an hour based on a 

moving 5-minute averaging period 

rather than time block averaging. 

EPA also proposed Data Quality 

Objectives (DQOs) for the SO2 network. 

DQOs generally specify the tolerable 

levels for potential decision error used 

as a basis for establishing the quality 

and quantity of data needed to support 

the objectives of the monitors. EPA 

proposed the goal for acceptable 

measurement uncertainty for SO2

methods to be defined as an upper 90 

percent confidence limit for the 

coefficient of variation (CV) of 15 

percent for precision and as an upper 95 

percent confidence limit for the absolute 

bias of 15 percent for bias. 

b. Public Comments 

EPA received many comments on the 

reporting of 5-minute data values. The 

comments generally fell into one of the 

following categories: 33 (1) Those State, 

public health, and environmental 

groups who supported the proposed 

requirement to report the maximum 5- 

minute block average of the twelve 5- 

minute block averages of SO2 for each 

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

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

(2) those State, public health, and 

environmental groups who supported 

the reporting of all twelve 5-minute 

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

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

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

public health, and environmental 

groups who supported reporting the 

maximum 5-minute concentration in an 

hour based on a moving 5-minute 

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

EJ, EDF, NRDC, and SC), and (4) those 

State and industry groups who did not 

support the reporting of any 5-minute 

data (e.g., Iowa, South Carolina, LEC, 

and RRI Energy). 

Public health and environmental 

groups (e.g. ALA, CBD, EJ, EDF, NRDC, 

and SC) supported an approach where 

5-minute data must be reported. 

However, these commenters were 

flexible in their position and supported 

multiple forms or types of 5-minute data 

reporting. The ALA, EJ, EDF, NRDC, and 

SC stated that ‘‘we support the proposed 

requirement for State and local 

monitoring agencies to report both 

hourly average and maximum 5-minute 

averages out of the twelve 5-minute 

block averages of SO2 for each hour.’’ 

They also expressed a preference for 

alternative 5-minute data reporting 

stating that they ‘‘strongly prefer that 

States be required to report the peak 5- 

minute concentrations of SO2 based on 

a rolling average.’’ Similarly, CBD stated 

that ‘‘* * * EPA should require that 

State and local agencies report all 12 

five-minute SO2 values for each hour in 

addition to 1-hour averages. Where 

possible, EPA also should require 

reporting of rolling five-minute averages 

rather than block data * * *’’ 

Missouri generally supported the 

proposed requirement to report the 

maximum 5-minute average in the hour, 

saying ‘‘it is not a problem to report both 

the hourly average and the maximum 5- 

minute block average.’’ Nevertheless, 

Missouri went on to note constraints, 

stating that ‘‘* * * [their] data logger 

and associated software do not have the 

capability to report all twelve 5-minute 

SO2 values for each hour’’ and that they 

‘‘* * * could not do this without 

software being developed for this 

purpose and it could be time intensive 

to validate this data.’’ 

Kentucky did not support the 

proposal to report the maximum 5- 

minute data block in the hour because 

of the limitations in their data 

acquisition systems. They explained 

that ‘‘the data acquisition system used 

by the [State] does not have the 

capability to automatically report the 

maximum 5-minute block of data from 

an hour concentration. [State] personnel 

would have to manually determine that 


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34The REA assessed exposure and risks 

associated with 5-minute SO2 concentrations above 

5-minute health effect benchmark levels derived 

from controlled human exposure studies. In the 

analyses, the REA noted that very few State and 

local agencies report ambient 5-minute SO2 data 

(REA, section 10.3.3.2) and that the lack of 5-minute 

data necessitated the use of statistically estimated 

5-minute SO2 data in order to expand the 

geographic scope of the exposure and risk analyses 

(REA, section 7.2.3). 

value and then manually enter that data 

into AQS.’’ Kentucky goes on to suggest 

that ‘‘the only feasible option for the 

[State] to submit 5-minute data to AQS 

would be to submit all twelve 5-minute 

blocks of data for each hour to AQS.’’ 

South Dakota stated that its ‘‘* * * 

preference would be to report the 

maximum 5-minute average for each 

hour calculated using a 5-minute rolling 

average.’’ South Dakota goes on to state 

that ‘‘* * * while doubling the work 

required to validate data and load the 

data into AQS, the additional data 

should help determine if the selected 

standard concentration level has 

achieved the necessary reduction in 

high concentration 5-minute levels and 

provide the necessary data for further 

study of health impacts * * *’’ 

South Carolina stated that it ‘‘* * * 

does not support mandatory reporting of 

5-minute averages in addition to the 1- 

hour average required for comparison to 

the standard. The validation and 

reporting of 5-minute averages imposes 

a significant additional burden on the 

reporting organization and its Quality 

System.’’ Iowa, who also did not support 

any form of 5-minute data reporting 

stated that ‘‘the five-minute data is not 

used to determine compliance with the 

NAAQS, and represents ancillary data,’’ 

and that ‘‘validating and uploading the 

five-minute data will take at least as 

much staff time as generating the hourly 

data used for compliance.’’ As a result, 

Iowa states that ‘‘if EPA determines that 

five-minute data is needed, we 

recommend that EPA require the 

maximum five-minute average in each 

hour, rather than all twelve five-minute 

averages, in order to reduce the burden 

associated with generation of the 

ancillary data set.’’ 

With regard to the proposed DQOs, 

EPA received comments from some 

States (e.g., Kentucky, North Carolina, 

NYSDEC, and South Carolina) providing 

general support for the goals for 

acceptable measurement uncertainty for 

precision and bias. North Carolina 

stated that the ‘‘* * * precision and bias 

measurement uncertainty criteria 

should emulate those that have been 

established for other recent NAAQS and 

NCore pollutants.’’ NYSDEC stated that 

‘‘the proposal does not seem 

unreasonable, however these statistics 

are now expressed in terms of 

confidence limits: Precision—90% 

confidence of a CV of 15% and Bias— 

95% confidence of a CV of 15%.’’ 

NYSDEC raises concern that ‘‘* * * the 

results are now dependent on the 

number of audits performed. This is 

highly variable because some agencies 

run automatic audits every night, 

[while] others use the old standard of 

once every 2 weeks.’’ 

In regard to comments on the 

proposed DQOs, EPA notes that the 

precision and bias estimation technique 

on which NYSDEC comments were 

focused were proposed and adopted in 

the monitoring rule promulgated on 

October 6, 2006 and EPA did not intend 

to reopen those requirements for 

comment. Moreover, SO2 precision and 

bias estimates have been performed in 

this manner for the past four years and 

there have been no adverse effects on 

data quality at the minimum required 

level of performance checks every two 

weeks. The statistics for the precision 

and bias estimates and the DQO goals 

are based on the accumulation of the 

one-point precision checks aggregated at 

the frequencies required in CFR which 

is every two weeks. Any organization 

performing more frequent checks (such 

as every night) would accumulate more 

data for the precision and bias 

estimates, have higher confidence in the 

data, and would have less potential for 

outliers or higher than normal values 

effecting the precision and bias 

estimate. In addition, monitoring 

organizations running precision checks 

every 24 hours would be more able to 

control data quality to meet the DQO 

goals than organizations running the 

check every two weeks. 

c. Conclusions on Data Reporting 

EPA received a fairly diverse set of 

comments on the appropriateness of 

reporting 5-minute data and in what 

particular format it may be provided in. 

EPA has considered the comments by 

the States regarding validation of 

potentially 13 data values per hour 

(instead of 1 or 2) and some States’ lack 

of data acquisition capacity or 

processing capability to report any 

particular type of 5-minute value. EPA 

believes that in light of these comments, 

adopting a requirement for continuous 

SO2 analyzers to report all twelve 5- 

minute values or a rolling 5-minute 

value does not appear to provide 

enough added value for the potential 

increased burden on States, such as 

increased staff time dedicated to data 

processing and QA, or in improving or 

adjusting data acquisition capabilities. 

However, EPA also believes that 

obtaining some form of 5-minute data is 

appropriate because such data have 

been critical to this NAAQS review, and 

are anticipated to be of high value to 

inform future health studies and, 

subsequently, future SO2 NAAQS 

reviews.34 Indeed, as noted earlier, it 

was EPA’s failure to adequately explain 

the absence of protection from elevated 

short-term (5- to 10-minute exposure) 

SO2 concentrations for heavily breathing 

asthmatics that occasioned the remand 

of the 1996 SO2 primary NAAQS 

(American Lung Association, 134 F.3d 

at 392). This belief is supported further 

by the expectation that a significant 

portion of the monitors operating to 

satisfy the final monitoring network 

design will likely be sited for 

population exposures, which have 

traditionally provided ambient data that 

is often utilized by epidemiologic health 

studies. Therefore, EPA is finalizing the 

requirement that State and local air 

agencies operating continuous SO2

analyzers shall report the maximum 5- 

minute block average out of the twelve 

5-minute block averages in each hour, 

for each hour of the day, and that State 

and local air agencies operating any 

type of SO2 analyzer shall report the 

integrated 1-hour average value, as was 

proposed. EPA encourages States 

capable of reporting all twelve 5-minute 

data blocks in an hour to report such 

data to AQS. AQS is currently set-up to 

take the 5-minute maximum value in an 

hour under parameter code 42406 and 

can take all twelve 5-minute values 

under parameter code 42401 (with a 

duration code of H). EPA notes that if 

a State were to choose to submit all 

twelve 5-minute blocks in the hour, by 

default, they would be submitting the 

maximum 5-minute data block within 

that hour, although they have not 

singled out that particular value. Since 

the 5-minute data is not directly being 

used for comparison to the NAAQS, 

EPA believes that any State electing to 

submit all twelve 5-minute values is 

still satisfying the intent of having the 

maximum 5-minute value reported. 

Therefore, if a State chooses to submit 

all twelve 5-minute values in an hour, 

they will be considered to be satisfying 

the data reporting requirement of 

submitting the maximum 5-minute 

value in an hour, and they do not have 

to separately report the maximum 5- 

minute value from within that set of 

data values to AQS under parameter 

code 42406. 

EPA proposed new regulation text for 

40 CFR Part 58 Appendix C, which 

would have added section 2.1.2 that 

would have required any SO2 FRM or 


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FEM used for making NAAQS decisions 

to be capable of providing both 1-hour 

and 5-minute averaged concentration 

data. EPA is not finalizing this proposed 

language, as the manual wet-chemistry 

pararosaniline reference method cannot 

provide 5-minute data. Therefore, the 

proposed language is inappropriate. 

However, both the UVF FEM and the 

new UVF FRM continuous methods are 

capable of providing 5-minute averaged 

data. As a result, the language in 

58.12(g) and 58.16(g) requiring 5-minute 

SO2 data has been adjusted to 

appropriately specify that only those 

States operating continuous FRM or 

FEMs are required to report the 

maximum 5-minute data value for each 

hour. 

With regard to acceptable 

measurement uncertainties, EPA 

reviewed summary data for each 

Primary Quality Assurance Organization 

(PQAO) in the 2008 Data Quality 

Indicator Report on SO2 data within the 

2008 Criteria Pollutant Quality Indicator 

Summary Report for AQS Data (http:// 

www.epa.gov/ttn/amtic/qareport.html). 

Of the 100 PQAOs in the report, none 

of those organizations had summary CV 

or bias values exceeding 10 percent. 

Thus, EPA believes that the SO2

network can and does easily attain 

measurement uncertainty criteria more 

stringent than the finalized goal values 

and the monitoring required under the 

final network design should be able to 

maintain this level of performance. 

Therefore, in consideration of comments 

and existing quality assurance data, EPA 

is changing the final goals from those 

which were proposed for acceptable 

measurement uncertainty for SO2

methods to be defined for precision as 

an upper 90 percent confidence limit for 

the coefficient of variation (CV) of 10 

percent and for bias as an upper 95 

percent confidence limit for the absolute 

bias of 10 percent. 

V. Initial Designation of Areas for the 

1-Hour SO2 NAAQS 

This section of the preamble further 

addresses the process under which EPA 

intends to identify whether areas of the 

country attain or do not attain or are 

‘‘unclassifiable’’ regarding the new 1- 

hour SO2 NAAQS. After EPA establishes 

a new NAAQS, the CAA directs States 

and EPA to take this first step, known 

as the ‘‘initial area designations,’’ in 

ensuring that the NAAQS is ultimately 

attained. 

We are revising our discussion of an 

expected approach toward issuing 

initial area designations in response to 

comments we received on the proposed 

rule’s treatment of monitoring and 

modeling (both generally and in the 

specific context of designations), and to 

make the expected process more 

consistent with our historical approach 

to implementing the SO2 NAAQS. A 

revised anticipated approach for issuing 

designations logically follows from our 

revised hybrid approach to monitoring 

and modeling as discussed above in 

sections III and IV. It would also affect 

a revised expected implementation 

approach that we later discuss in 

section VI. 1. Designations. 

a. Clean Air Act Requirements 

The CAA requires EPA and the States 

to take steps to ensure that the new 

NAAQS are met following 

promulgation. The first step is for EPA 

to identify whether areas of the country 

meet, do not meet, or cannot yet be 

classified as either meeting or not 

meeting the new NAAQS. Section 

107(d)(1)(A) provides that, ‘‘By such 

date as the Administrator may 

reasonably require, but not later than 1 

year after promulgation of a new or 

revised NAAQS for any pollutant under 

section 109, the Governor of each State 

shall * * * submit to the Administrator 

a list of all areas (or portions thereof) in 

the State’’ that should be designated as 

nonattainment, attainment, or 

unclassifiable for the new NAAQS. 42 

U.S.C. 7407(d)(1)(A)(i)–(iii). Section 

107(d)(1)(B)(i) further provides, ‘‘Upon 

promulgation or revision of a NAAQS, 

the Administrator shall promulgate the 

designations of all areas (or portions 

thereof) * * * as expeditiously as 

practicable, but in no case later than 2 

years from the date of promulgation. 

Such period may be extended for up to 

one year in the event the Administrator 

has insufficient information to 

promulgate the designations within 2 

years.’’ 42 U.S.C. 7407(d)(1)(B)(i). 

Under CAA section 107(d)(1)(B)(ii), 

no later than 120 days prior to 

promulgating designations, EPA is 

required to notify States of any intended 

modifications to their boundaries as 

EPA may deem necessary, and States 

will have an opportunity to comment on 

EPA’s tentative decision. Whether or not 

a State provides a recommendation, the 

EPA must promulgate the designation 

that it deems appropriate. 42 U.S.C. 

7407(d)(1)(B)(ii). 

Accordingly, since the new 1-hour 

SO2 NAAQS is being promulgated 

today, Governors should submit their 

initial SO2 designation 

recommendations to EPA no later than 

June 2, 2011. If the Administrator 

intends to modify any State’s boundary 

recommendation, the EPA will notify 

the Governor no later than 120 days 

prior to designations or, February 2012. 

States that believe the Administrator’s 

modification is inappropriate will have 

an opportunity to demonstrate why they 

believe their recommendation is more 

appropriate before designations are 

finalized in June 2012. 

For initial designations that will be 

finalized in June 2012, States should use 

monitoring data from the existing SO2

network for the years 2008–2010, as 

well as any refined SO2 dispersion 

modeling (see Appendix W to 40 CFR 

Part 51) for sources that may have the 

potential to cause or contribute to a 

NAAQS violation, provided that it is 

recent and available. EPA will then 

issue designations based on the record 

of information for that area. Under our 

anticipated approach, an area that has 

monitoring data or refined modeling 

results showing a violation of the 

NAAQS would be designated as 

‘‘nonattainment.’’ An area that has both 

monitoring data and appropriate 

modeling results showing no violations 

would be designated as ‘‘attainment.’’ 

All other areas, including those with 

SO2 monitors showing no violations but 

without modeling showing no 

violations, would be designated as 

‘‘unclassifiable.’’ Areas with no SO2

monitors at all i.e., ‘‘rest of State,’’ would 

be designated as ‘‘unclassifiable’’ as 

well. 

b. Approach Described in Proposal 

In the proposed rule’s preamble, we 

explained that we had proposed a new 

SO2 ambient monitoring network, with 

new monitors expected to be deployed 

no later than January 2013. We also 

explained that we expected compliance 

with the new NAAQS to be determined 

based on 3 years of complete, quality 

assured, certified monitoring data. We 

further explained that we did not expect 

newly-cited monitors for the proposed 

network to generate sufficient 

monitoring data for us to use in 

determining whether areas complied 

with the new NAAQS by the statutory 

deadline to complete initial 

designations. Therefore, we explained, 

we intended to complete designations 

by June 2012 based on 3 years of 

complete, quality assured, certified air 

quality monitoring data as generated 

from the current monitoring network. 

Consequently, we discussed our 

expectations to base initial designations 

on air quality data from the years 2008– 

2010 or 2009–2011, from SO2 monitors 

operating at current locations, which we 

expected to continue through 2011. 

While those monitors are generally sited 

to measure 24-hour and annual average 

SO2 concentrations, we noted that they 

all report hourly data, and we estimated 

that at least one third of those monitors 

might meet the proposed network 


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design requirements and not need to be 

moved. We explained that if any 

monitor in the current network 

indicated a violation of the new 1-hour 

NAAQS, we would intend to designate 

the area as ‘‘nonattainment.’’ We further 

explained that if a monitor did not 

indicate a violation, our designation 

decision for the area would be made on 

a case-by-case basis, with one 

possibility being a designation of 

‘‘unclassifiable.’’ 

We also explained that while the CAA 

section 107 designation provisions 

specifically address States, we intended 

to follow the same process for Tribes to 

the extent practicable, pursuant to CAA 

section 301(d), 42 U.S.C. 7601(d), and 

the Tribal Authority Rule, 40 CFR part 

49. 

c. Comments 

Several commenters stated that the 

EPA did not provide nonattainment 

boundary guidance in the proposed rule 

and argued that guidance should be 

developed. Commenters also stated that 

EPA should consider boundaries that 

are less than the Core Based Statistical 

Area (CBSA), and perhaps even smaller 

than the county boundary (State of 

Michigan, Sierra Club). 

In response, we note that the CAA 

requires that the EPA designate as 

‘‘nonattainment’’ any area that does not 

meet (or contributes to an area that does 

not meet) the NAAQS. 42 U.S.C. 

7407(d)(1)(A)(i). States with monitored 

or modeled SO2 violations will need to 

recommend an appropriate 

nonattainment boundary that both 

includes sources contributing to that 

violation, as well as informs the public 

of the extent of the violation. For 

purposes of determining nonattainment 

boundaries, the EPA expects to consider 

the county line as the presumptive 

boundary for SO2. This would be 

consistent with our approach under 

other NAAQS. States recommending 

less-than-countywide nonattainment 

boundaries should provide additional 

information along with their 

recommendation, demonstrating why a 

smaller area is more appropriate, as we 

have advised for other NAAQS. If States 

request it, EPA may develop additional 

guidance on the factors that States 

should consider when determining 

nonattainment boundaries. 

In addition, as further discussed in 

section IV.B above, in the SO2 NAAQS 

proposal, we proposed a monitoringfocused approach for comparison to the 

new NAAQS. The proposed network 

would have required approximately 348 

monitors nationwide to be sited at the 

locations of maximum concentration. 

Numerous State and local government 

commenters expressed concerns 

regarding the perceived burdens of 

implementing the proposed monitoring 

network and the sufficiency of its scope 

for purposes of identifying violations. 

Some of these commenters (the City of 

Alexandria, and the States of Delaware, 

North Carolina and Pennsylvania) 

suggested using modeling to determine 

the scope of monitoring requirements, 

or favored modeling over monitoring to 

determine compliance with the NAAQS. 

Partly in response to these comments, 

and after reconsidering the proposal’s 

monitoring-focused approach, 

specifically regarding how we have 

historically implemented SO2

designations, we now anticipate taking 

a revised approach toward designations, 

using a hybrid analytic approach that 

combines the use of monitoring and 

available modeling to assess compliance 

with the new 1-hour SO2 NAAQS. We 

discuss a revised expected approach 

toward designations below, and further 

discuss in section VI how we expect a 

hybrid approach to affect other 

implementation activities. 

d. Expected Designations Process 

As discussed in sections III and IV of 

this preamble, in response to the 

comments and after reviewing our 

historical SO2 implementation practice, 

we intend to use a hybrid analytic 

approach for assessing compliance with 

the new 1-hour SO2 NAAQS for initial 

designations. We also believe that a 

hybrid approach is more consistent with 

our historical approach and 

longstanding guidance toward SO2

NAAQS designations and 

implementation than what we originally 

proposed. Technically, for a short-term 

1-hour standard, it is more appropriate 

and efficient to principally use 

modeling to assess compliance for 

medium to larger sources, and to rely 

more on monitoring for groups of 

smaller sources and sources not as 

conducive to modeling. 

In cases where there is complete air 

quality data from FRM and FEM SO2

monitors, that data would be considered 

by EPA in designating areas as either 

‘‘attainment’’ or ‘‘nonattainment’’ for the 

new SO2 NAAQS. See Appendix T to 

Part 50 section 3b. In addition, in cases 

where a State submits air quality 

modeling data that are consistent with 

our current guidance or our expected 

revisions thereto, and which indicates 

that an area is attaining the standard or 

violating the standard, these data may 

support recommendations of 

‘‘attainment’’ or ‘‘nonattainment.’’ As 

explained in section IV above, we 

would not consider monitoring alone to 

be an adequate, nor the most accurate, 

tool to identify all areas of maximum 

concentrations of SO2. In the case of 

SO2, we further believe that monitoring 

is not the most cost-efficient method for 

identifying all areas of maximum 

concentrations. 

Due to the necessarily limited spatial 

coverage provided by any monitoring 

regime, and the strong source-oriented 

nature of SO2 ambient impacts, we 

recognize that using this more 

traditional approach in designations, 

would be more likely to identify a 

greater number of potential instances of 

nonattainment, if areas were to 

immediately conduct modeling of 

current source emissions, as compared 

to the approach we discussed in the 

proposed rule. As discussed in section 

III, forthcoming national and regional 

rules, such as the pending Industrial 

Boilers ‘‘Maximum Achievable Control 

Technology’’ (MACT) standard under 

CAA section 112(d), are likely to result 

in significant SO2 emissions reductions 

in the next three to four years. A limited 

qualitative assessment of preliminary 

modeling of some sample facilities that 

would be covered by those rules 

indicates that well-controlled facilities 

should meet the new SO2 NAAQS. 

However, there are some exceptions. 

These exceptions include unique 

sources with specific source 

characteristics that contribute to higher 

ambient impacts (short stack heights, 

complex terrain, etc.). 

Again as described in section III, in 

order for States to conduct modeling on 

a large scale for the new 1-hour NAAQS, 

EPA expects additional guidance would 

be needed to clarify how to conduct 

dispersion modeling under Appendix W 

to support the implementation of the 

new 1-hour SO2 NAAQS, and how to 

identify and appropriately assess the air 

quality impacts of sources that 

potentially may cause or contribute to 

violations of the NAAQS. Our 

anticipated modeling guidance will 

provide for refined modeling that will 

better reflect and account for sourcespecific impacts by following our 

current Guideline on Air Quality 

Models, Appendix W to 40 CFR Part 51, 

with appropriate flexibility for use in 

implementation. EPA intends to solicit 

public comment on this modeling 

guidance. We expect it will take some 

time for EPA to issue this guidance, and 

believe that given the timing and 

substantial burden of having to model 

several hundred sources, it would not 

be realistic or appropriate to expect 

States to complete such modeling and 

incorporate the results in designation 

recommendations for the new 1-hour 

SO2 NAAQS that, under CAA section 


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35See SO2 Guideline Document, Office of Air 

Quality Planning and Standards, Research Triangle 

Park, NC 27711, EPA–452/R–94–008, February 

1994. 

107(d), are due to EPA within 1 year of 

the promulgation of the NAAQS. 

Consequently, we expect that in most 

instances, Governors will submit 

designation recommendations of 

‘‘unclassifiable’’ rather than conduct 

large-scale refined modeling of sources 

in advance of receiving our anticipated 

guidance. The absence of monitoring 

data showing violations for most areas, 

combined with the paucity of refined 

modeling of sources that have the 

potential to cause or contribute to 

violations of the NAAQS, will likely 

result in informational records that are 

insufficient to support initial 

designations of either ‘‘attainment’’ or 

‘‘nonattainment.’’ Under the Clean Air 

Act, in such a situation EPA is required 

to issue a designation for the area as 

‘‘unclassifiable.’’ However, we do not 

expect this result to delay expeditious 

attainment and maintenance of the new 

NAAQS, or to cause inappropriate, 

indefinite uncertainty regarding 

whether or not sources cause or 

contribute to NAAQS violations. 

As described more fully in section III 

above and in section VI below, EPA’s 

expected implementation approach 

would rely on the CAA section 110(a)(1) 

SIP obligation to ensure that all areas of 

the country attain and maintain the 

NAAQS on a timely basis even if they 

are designated ‘‘unclassifiable’’ initially. 

This SIP is due under CAA section 

110(a)(1) within 3 years after 

promulgation of the new NAAQS, and 

does not depend upon EPA designating 

an area ‘‘nonattainment’’ based on 

recently monitored or modeled SO2

levels. This period of time would allow 

States to use EPA’s anticipated guidance 

on modeling for the new 1-hour SO2

NAAQS, as well as account for SO2

reduction levels at individual sources 

that are anticipated to result from 

promulgated national and regional rules 

to show attainment. 

Once areas have both appropriate 

monitoring data (if required) and 

modeling data as appropriate, consistent 

with the new guidance, showing no 

violations of the SO2 NAAQS, and have 

met other applicable requirements of 

CAA section 107(d)(3), the Agency 

would consider re-designating them 

from ‘‘unclassifiable’’ or ‘‘nonattainment’’ 

to ‘‘attainment’’ under CAA section 

107(d)(3). 

VI. Clean Air Act Implementation 

Requirements 

This section of the preamble discusses 

the CAA requirements that States and 

emissions sources would need to 

address when implementing the new 1- 

hour SO2 NAAQS based on the structure 

outlined in the CAA and existing rules. 

The EPA believes that existing guidance 

documents and regulations will be 

useful in helping States and sources to 

implement the new SO2 NAAQS, but we 

also expect to develop additional 

guidance on modeling for the new onehour standard and on developing SIPs 

under Section 110(a)(1) of the CAA.35 In 

light of the new approach that EPA 

intends to take with respect to 

implementation of the SO2 NAAQS, 

EPA intends to solicit public comment 

on guidance regarding modeling, and 

also solicit public comment on 

additional implementation planning 

guidance, including the content of the 

maintenance plans required under 

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

EPA also notes that State monitoring 

plans and the SIP submissions that 

States will make will also be subject to 

public notice and comment.’’ 

In this section, we also further discuss 

how EPA’s modified expected 

approaches toward monitoring and 

modeling and toward initial 

designations under the new SO2

NAAQS (compared to how the proposed 

rule discussed addressing these issues) 

are anticipated to affect the types of SIP 

submissions States will need to provide 

to EPA and the timing of EPA’s actions 

on those submissions leading up to 

attainment and maintenance of the new 

SO2 NAAQS. In section IV above, we 

discuss the final amendments to the 

ambient monitoring and reporting 

requirements, and explain how in 

response to comments received on the 

proposal and after revisiting our 

historical practice in assessing 

compliance with prior SO2 NAAQS, we 

have revised both the scope of the 

revised monitoring network and our 

expectations on how monitoring will be 

used in conjunction with modeling in 

assessing compliance and designating 

areas. In section V above, we discuss 

how we have revised our expected 

approach for issuing designations for 

the new 1-hour SO2 NAAQS, and 

similarly explain how, in response to 

comments and after reviewing our 

historical approach, we have modified 

our expectations as discussed in the 

proposal for how and when monitoring 

and modeling will be used for 

designations. In this section VI, we 

describe in more detail how and when 

we expect States to demonstrate 

attainment, implementation, 

maintenance and enforcement of the 

new one-hour SO2 NAAQS. 

The CAA assigns important roles to 

EPA, States and Tribal governments to 

achieve the NAAQS. States have the 

primary responsibility for developing 

and implementing State implementation 

plans (SIPs) that contain State measures 

necessary to achieve the air quality 

standards in each area once EPA has 

established the NAAQS. EPA provides 

assistance to States and Tribes by 

providing technical tools, assistance, 

and guidance, including information on 

the potential control measures that may 

assist in helping areas attain the 

standards. 

Under section 110 of the CAA, 42 

U.S.C. 7410, and related provisions, 

States are directed to submit, for EPA 

approval, SIPs that provide for the 

attainment, implementation, 

maintenance, and enforcement of such 

standards through control programs 

directed at sources of SO2 emissions. 

See CAA sections 110(a), and 191–192, 

42 U.S.C. 7410(a) and 7514–7514a. If a 

State fails to adopt and implement the 

required SIPs by the time periods 

provided in the CAA, EPA has the 

responsibility under the CAA to adopt 

a Federal implementation plan (FIP) to 

ensure that areas attain the NAAQS in 

an expeditious manner. The States, in 

conjunction with EPA, also administer 

the prevention of significant 

deterioration (PSD) program for SO2. 

See sections 160–169 of the CAA, 42 

U.S.C. 7470–7479. In addition, Federal 

programs provide for nationwide 

reductions in emissions of SO2 and 

other air pollutants under Title II of the 

Act, 42 U.S.C. 7521–7574. These 

programs involve limits on the sulfur 

content of the fuel used by automobiles, 

trucks, buses, motorcycles, non-road 

engines and equipment, marine vessels 

and locomotives. Emissions reductions 

for SO2 are also obtained from 

implementation of the new source 

performance standards (NSPS) for 

stationary sources under sections 111 

and 129 of the CAA, 42 U.S.C. 7411 and 

7429; and the national emission 

standards for hazardous air pollutants 

(NESHAP) for stationary sources under 

section 112 of the CAA, 42 U.S.C. 7412 

(such reductions resulting due to 

control of hazardous air pollutants 

(HAP) such as hydrogen chloride (HCl) 

under those rules). Title IV of the CAA, 

sections 402–416, 42 U.S.C. 7651a– 

7651o, specifically provides for major 

reductions in SO2 emissions. EPA has 

also promulgated the Clean Air 

Interstate Rule (CAIR) to define 

additional SO2 emission reductions 

needed in the Eastern United States to 

eliminate significant contribution of 

upwind States to downwind States’ 


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35572 Federal Register / Vol. 75, No. 119 / Tuesday, June 22, 2010 / Rules and Regulations 

nonattainment, or inability to maintain, 

the PM2.5 NAAQS pursuant to CAA 

section 110(a)(2)(D), 42 U.S.C. 

7410(a)(2)(D), a rule which EPA is 

reevaluating pursuant to court remand. 

A. How This Rule Applies to Tribes 

CAA section 301(d) authorizes EPA to 

treat eligible Indian Tribes in the same 

manner as States under the CAA and 

requires EPA to promulgate regulations 

specifying the provisions of the statute 

for which such treatment is appropriate. 

EPA has promulgated these 

regulations—known as the Tribal 

Authority Rule or TAR—at 40 CFR Part 

49. See 63 FR 7254 (February 12, 1998). 

The TAR establishes the process for 

Indian Tribes to seek treatment-as-aState eligibility and sets forth the CAA 

functions for which such treatment will 

be available. Under the TAR, eligible 

Tribes may seek approval for all CAA 

and regulatory purposes other than a 

small number of functions enumerated 

at section 49.4. Implementation plans 

under section 110 are included within 

the scope of CAA functions for which 

eligible Tribes may obtain approval. 

Section 110(o) also specifically 

describes Tribal roles in submitting 

implementation plans. Eligible Indian 

Tribes may thus submit implementation 

plans covering their reservations and 

other areas under their jurisdiction. 

The CAA and TAR do not, however, 

direct Tribes to apply for treatment as a 

State or implement any CAA program. 

In promulgating the TAR EPA explicitly 

determined that it was not appropriate 

to treat Tribes similarly to States for 

purposes of, among other things, 

specific plan submittal and 

implementation deadlines for NAAQSrelated requirements. 40 CFR 49.4(a). In 

addition, where Tribes do seek approval 

of CAA programs, including section 110 

implementation plans, the TAR 

provides flexibility and allows them to 

submit partial program elements, so 

long as such elements are reasonably 

severable—i.e., ‘‘not integrally related to 

program elements that are not included 

in the plan submittal, and are consistent 

with applicable statutory and regulatory 

requirements.’’ 40 CFR 49.7. 

To date, very few Tribes have sought 

treatment as a State for purposes of 

section 110 implementation plans. 

However, some Tribes may be interested 

in pursuing such plans to implement 

today’s proposed standard, once it is 

promulgated. 

1. Approach Described in the Proposal 

In the proposed rule preamble, EPA 

described the various roles and 

requirements States would address in 

implementing the proposed NAAQS. 

Such references to States generally 

included eligible Indian Tribes to the 

extent consistent with the flexibility 

provided to Tribes under the TAR. 

Where Tribes do not seek treatment as 

a State for section 110 implementation 

plans, we explained that EPA under its 

discretionary authority will promulgate 

FIPs as ‘‘necessary or appropriate to 

protect air quality.’’ 40 CFR 49.11(a). 

EPA also noted that some Tribes operate 

air quality monitoring networks in their 

areas. We explained that for such 

monitors to be used to measure 

attainment with the proposed revised 

primary NAAQS for SO2, the criteria 

and procedures identified in the 

proposed rule would apply. 

2. Current Approach 

EPA did not receive any comments on 

this issue. However, as discussed 

elsewhere in this preamble, the final 

rule reflects in several respects modified 

expected approaches regarding the use 

of monitoring and modeling, the manner 

in which we expect to issue 

designations under the new SO2

NAAQS, and the types of SIP 

submissions we expect would be 

needed to show attainment, 

implementation, maintenance and 

enforcement of the new NAAQS. Those 

changes in expected approach would, as 

appropriate, also apply to how we 

address data and any other submissions 

from Tribes for purposes of the new SO2

NAAQS. 

B. Nonattainment Area Attainment 

Dates 

The latest date by which an area 

designated as nonattainment is required 

to attain the SO2 NAAQS is determined 

from the effective date of the 

nonattainment designation for the 

affected area. For areas designated 

nonattainment for the revised SO2

NAAQS, SIPs must provide for 

attainment of the NAAQS as 

expeditiously as practicable, but no later 

than 5 years from the effective date of 

the nonattainment designation for the 

area. See section 192(a) of the CAA, 42 

U.S.C. 7651a(a). The EPA expects to 

determine whether an area has 

demonstrated attainment of the new SO2

NAAQS by evaluating air quality 

monitoring and modeling data 

consistent with 40 CFR part 50, 

Appendix T and 40 CFR part 51, 

Appendix W. (Note that this differs from 

how we explained we would expect to 

make such determinations in the 

proposed rule, where we only 

mentioned monitoring as supplying the 

data we would evaluate. This expanded 

and changed discussion reflects the 

contemplated changes in our overall 

approaches to using monitoring and 

modeling, expectations for issuing 

designations, and expectations for 

reviewing SIP submissions showing 

attainment, implementation, 

maintenance, and enforcement of the 

new SO2 NAAQS.) 

1. Attaining the NAAQS 

a. Approach Described in the Proposal 

In the proposal preamble, we set forth 

the basic five conditions provided under 

section 107(d)(3)(E) of the CAA, 42 

U.S.C. 7407(d)(3)(E) that a 

nonattainment area must meet in order 

to be redesignated as attainment: 

• EPA must have determined that the 

area has met the SO2 NAAQS; 

• EPA has fully approved the State’s 

implementation plan; 

• The improvement in air quality in 

the affected area is due to permanent 

and enforceable reductions in 

emissions; 

• EPA has fully approved a 

maintenance plan for the area; and 

• The State(s) containing the area 

have met all applicable requirements 

under section 110 and part D. 

b. Current Approach 

EPA did not receive any comments on 

this aspect of the preamble of the 

proposal. However, in light of the fact 

that in the final rule, in response to 

other comments and consistent with 

historic practice, we are revising our 

proposed anticipated approaches to the 

overall use of monitoring and modeling 

and our expected approaches to issuing 

initial designations and reviewing SIP 

submissions, it follows that the way in 

which a nonattainment area seeks 

redesignation as an attainment area 

would also be affected by the final rule’s 

overall changed approaches. For 

example, for EPA to determine that a 

nonattainment area has met the SO2

NAAQS, we anticipate that the area 

would need to not only provide any 

monitoring data showing such 

compliance (and there would need to be 

an absence of monitoring data showing 

otherwise), but modeling where 

appropriate, consistent with modeling 

guidance that we plan to issue, would 

also need to show that the area is 

attaining and maintaining the NAAQS. 

2. Consequences of a Nonattainment 

Area Failing To Attain by the Statutory 

Attainment Date 

a. Approach Described in the Proposal 

We explained in the proposal that any 

SO2 nonattainment area that fails to 

attain by its statutory attainment date 

would be subject to the requirements of 

sections 179(c) and (d) of the CAA, 42 


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U.S.C. 7509(c) and (d). EPA is required 

to make a finding of failure to attain no 

later than 6 months after the specified 

attainment date and publish a notice in 

the Federal Register. The State would 

then need to submit an implementation 

plan revision no later than one year 

following the effective date of the 

Federal Register notice making the 

determination of the area’s failure to 

attain. This submission must 

demonstrate that the standard will be 

attained as expeditiously as practicable, 

but no later than 5 years from the 

effective date of EPA’s finding that the 

area failed to attain. In addition, section 

179(d)(2) provides that the SIP revision 

must include any specific additional 

measures as may be reasonably 

prescribed by EPA, including ‘‘all 

measures that can be feasibly 

implemented in the area in light of 

technological achievability, costs, and 

any nonair quality and other air qualityrelated health and environmental 

impacts.’’ 

b. Current Approach 

EPA did not receive any comments on 

this aspect of the discussion in the 

preamble to the proposal. However, due 

to the changes in the final rule’s 

discussion of the overall expected 

approaches to monitoring and modeling, 

designations and EPA review of SIP 

submissions, it follows that the 

implementation of CAA sections 179(c) 

and (d) would also be affected by those 

changes. For example, under the 

anticipated approach, a nonattainment 

area’s initial demonstration of 

attainment would need to show through 

modeling consistent with modeling 

guidance that we plan to issue, that the 

area attains and maintains the new SO2

NAAQS. If the area fails to attain on 

time, any remedial implementation plan 

submission would also need to show, 

where appropriate, through modeling 

consistent with modeling guidance that 

we plan to issue, that the area attains 

and maintains the new SO2 NAAQS. 

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

Maintenance/Infrastructure 

Requirements 

We are significantly revising our 

expected approaches to the use of 

monitoring and modeling, expected 

issuance of initial designations, and 

EPA review of SIP submissions. This 

change in anticipated approach has 

particular relevance for how States 

would meet their statutory obligations 

under CAA section 110(a) to implement, 

maintain and enforce the new SO2

NAAQS. In short, under such an 

approach, all areas, whether designated 

as attainment, nonattainment, or 

unclassifiable, would need to submit 

SIPs under CAA section 110(a) that 

show that they are attaining and 

maintaining the 1-hour SO2 NAAQS as 

expeditiously as practicable through 

permanent and enforceable measures. In 

other words, the duty to show 

maintenance of the SO2 NAAQS would 

not be limited to areas that are initially 

designated as nonattainment, but 

instead would apply regardless of 

designation. As has been expected 

historically, areas initially designated 

attainment for SO2 are expected to 

submit to EPA the infrastructure 

elements of the 110(a) SIP, including the 

PSD program. Historically, EPA has 

determined this to be sufficient to 

demonstrate maintenance absent other 

available information to suggest the area 

would have difficulty maintaining the 

NAAQS. 

As required by CAA section 192, 

nonattainment areas must demonstrate 

attainment as expeditiously as 

practicable, and no later than 5 years 

after designation (which would be 

August 2017). Under a hybrid approach 

as we have discussed earlier in sections 

III, IV, and V of this preamble, EPA 

believes that August 2017 would be the 

latest point that could be as 

expeditiously as practicable for 

attainment and unclassifiable areas as 

well, and EPA anticipates establishing 

this date through future rulemaking 

actions on individual SIPs. 

As noted in earlier sections of this 

preamble, in the SO2 NAAQS proposal, 

we recommended a monitoring-focused 

approach for comparison to the NAAQS. 

We received public comments that 

contended our proposed monitoring 

network was too small and insufficient 

to assess the hundreds of areas that 

might violate the new SO2 NAAQS and 

yet too burdensome and expensive to 

expand to an adequate scale. Some 

commenters, especially State air 

agencies, recommended the use of 

modeling either to determine potential 

nonattainment areas or to identify areas 

subject to monitoring requirements. 

Because SO2 is primarily a localized 

pollutant, modeling is the the most 

appropriate tool to accurately predict 

SO2 impacts from large sources, EPA 

has used it in the past to determine SO2

attainment status, and it can be 

performed more quickly and less costly 

than monitoring. Consequently, as part 

of developing a balanced response to the 

numerous comments we received on 

modeling and monitoring, we expect to 

use a hybrid analytic approach that 

combines the use of monitoring and 

modeling to assess compliance with 

respect to the new SO2 NAAQS. 

A hybrid analytic approach for 

assessing compliance with the new SO2

NAAQS would make the most 

appropriate use of available tools and be 

more consistent with our historical 

approach than was what we originally 

proposed. For a short-term 1-hour 

standard, it is more accurate and 

efficient to use modeling to assess 

medium to larger sources and to rely on 

monitoring for groups of smaller sources 

and sources not as conducive to 

modeling. 

We expect that States would initially 

focus performance of attainment 

demonstration modeling on larger 

sources (e.g., those ≥ 100 tons per year 

(tpy) of SO2), and that States would also 

identify and eventually conduct refined 

modeling of any other sources that may 

be anticipated to cause or contribute to 

a violation to determine compliance 

with the new SO2 NAAQS. As discussed 

in Section III, EPA anticipates providing 

additional guidance to States to clarify 

how to conduct dispersion modeling 

under Appendix W to support the 

implementation of the new 1-hour SO2

NAAQS. Prior to issuing this guidance, 

EPA intends to solicit public comment. 

Since determining compliance with 

the SO2 NAAQS will likely be a 

uniquely source-driven analysis, EPA 

explored options to ensure that the SO2

designations process realistically 

accounts for anticipated SO2 reductions 

at those sources that we expect will be 

achieved by current and pending 

national and regional rules. To ensure 

that all areas of the country attain the 

NAAQS on a timely basis, while 

accommodating modeling that is both 

informed by anticipated modeling 

guidance and accounts for those 

anticipated SO2 reductions, EPA’s 

intention is to emphasize the CAA 

section 110(a)(1) requirement that all 

States submit a SIP that shows 

implementation, maintenance and 

enforcement of the NAAQS. This SIP 

would be due under CAA section 

110(a)(1) within 3 years after 

promulgation of the new NAAQS, and 

would not depend upon EPA 

designating an area nonattainment 

based on recently monitored or modeled 

SO2 levels. In addition, like an 

attainment SIP required for a designated 

nonattainment area under CAA section 

192, to show attainment this SIP can 

account for controlled SO2 levels at 

individual sources that will be achieved 

after submission of the SIP but before 

the demonstrated attainment date. EPA 

intends to implement this approach in 

a way that ensures expeditious 

attainment of the NAAQS, under a 

schedule that we explain more fully 

below. 


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36 In the proposed rule preamble, we explained 

that two elements identified in section 110(a)(2) 

were not listed in our summary because, as EPA 

interprets the CAA, SIPs incorporating any 

necessary local nonattainment area controls would 

not be due within 3 years, but rather are generally 

due at the time the nonattainment area planning 

requirements are due. See 74 FR 64860 at n. 39. 

These elements are: (1) Emission limits and other 

control measures, section 110(a)(2)(A), and (2) 

Provisions for meeting part D, section 110(a)(2)(I), 

which requires areas designated as nonattainment 

to meet the applicable nonattainment planning 

requirements of part D, title I of the CAA. To 

implement our revised intended approach in the 

final rule, however, it would be necessary for States 

to include, if relied upon to show attainment and 

maintenance of the new SO2 NAAQS, any necessary 

emission limits and other control measures under 

section 110(a)(2)(A). 

1. Section 110(a)(1)–(2) Submission 

a. Approach Described in the Proposal 

In the preamble to the proposal, we 

explained that section 110(a)(2) of the 

CAA directs all States to develop and 

maintain a solid air quality management 

infrastructure, including enforceable 

emission limitations, an ambient 

monitoring program, an enforcement 

program, air quality modeling 

capabilities, and adequate personnel, 

resources, and legal authority. Section 

110(a)(2)(D) also requires State plans to 

prohibit emissions from within the State 

which contribute significantly to 

nonattainment or maintenance areas in 

any other State, or which interfere with 

programs under part C of the CAA to 

prevent significant deterioration of air 

quality or to achieve reasonable progress 

toward the national visibility goal for 

Federal class I areas (national parks and 

wilderness areas). 

Under sections 110(a)(1) and (2) of the 

CAA, all States are directed to submit 

SIPs to EPA which demonstrate that 

basic program elements have been 

addressed within 3 years of the 

promulgation of any new or revised 

NAAQS. Subsections (A) through (M) of 

section 110(a)(2) set forth the elements 

that a State’s program must contain in 

the SIP.36 The proposed rule listed 

section 110(a)(2) NAAQS 

implementation requirements as the 

following: 

• Ambient air quality monitoring/ 

data system: Section 110(a)(2)(B) 

requires SIPs to provide for setting up 

and operating ambient air quality 

monitors, collecting and analyzing data 

and making these data available to EPA 

upon request. 

• Program for enforcement of control 

measures: Section 110(a)(2)(C) requires 

SIPs to include a program providing for 

enforcement of SIP measures and the 

regulation and permitting of new/ 

modified sources. 

• Interstate transport: Section 

110(a)(2)(D) requires SIPs to include 

provisions prohibiting any source or 

other type of emissions activity in the 

State from contributing significantly to 

nonattainment or interfering with 

maintenance of the NAAQS in another 

State, or from interfering with measures 

required to prevent significant 

deterioration of air quality or to protect 

visibility. 

• Adequate resources: Section 

110(a)(2)(E) directs States to provide 

assurances of adequate funding, 

personnel and legal authority to 

implement their SIPs. 

• Stationary source monitoring 

system: Section 110(a)(2)(F) directs 

States to establish a system to monitor 

emissions from stationary sources and 

to submit periodic emissions reports to 

EPA. 

• Emergency power: Section 

110(a)(2)(G) directs States to include 

contingency plans, and adequate 

authority to implement them, for 

emergency episodes in their SIPs. 

• Provisions for SIP revision due to 

NAAQS changes or findings of 

inadequacies: Section 110(a)(2)(H) 

directs States to provide for revisions of 

their SIPs in response to changes in the 

NAAQS, availability of improved 

methods for attaining the NAAQS, or in 

response to an EPA finding that the SIP 

is inadequate. 

• Consultation with local and Federal 

government officials: Section 110(a)(2)(J) 

directs States to meet applicable local 

and Federal government consultation 

requirements when developing SIPs and 

reviewing preconstruction permits. 

• Public notification of NAAQS 

exceedances: Section 110(a)(2)(J) directs 

States to adopt measures to notify the 

public of instances or areas in which a 

NAAQS is exceeded. 

• PSD and visibility protection: 

Section 110(a)(2)(J) also directs States to 

adopt emissions imitations, and such 

other measures, as may be necessary to 

prevent significant deterioration of air 

quality in attainment areas and protect 

visibility in Federal Class I areas in 

accordance with the requirements of 

CAA Title I, part C. 

• Air quality modeling/data: Section 

110(a)(2)(K) requires that SIPs provide 

for performing air quality modeling for 

predicting effects on air quality of 

emissions of any NAAQS pollutant and 

submission of data to EPA upon request. 

• Permitting fees: Section 110(a)(2)(L) 

requires the SIP to include requirements 

for each major stationary source to pay 

permitting fees to cover the cost of 

reviewing, approving, implementing 

and enforcing a permit. 

• Consultation/participation by 

affected local government: Section 

110(a)(2)(M) directs States to provide for 

consultation and participation by local 

political subdivisions affected by the 

SIP. 

b. Final 

EPA did not receive any comments on 

this aspect of the approached explained 

in the proposal preamble. However, in 

light of the modified approach 

discussed above, EPA is providing 

additional guidance concerning the 

CAA section 110(a)(1) maintenance plan 

requirement as a part of this discussion 

so that States will have sufficient 

information to meet this requirement 

with a SIP submittal three years after 

promulgation of the NAAQS. Section 

110(a)(1) of the CAA states that each 

State, after reasonable notice and public 

hearing, is required to adopt and to 

submit to EPA, within 3 years after 

promulgation of any new or revised 

NAAQS for any pollutant, a SIP which 

provides for the implementation, 

maintenance, and enforcement of any 

new or revised NAAQS in each area of 

the State. As stated previously, in light 

of the new approach that EPA intends 

to take with respect to implementation 

of the SO2 NAAQS, EPA intends to 

solicit public comment on guidance 

regarding modeling, and also solicit 

public comment on additional 

implementation planning guidance, 

including the content of the 

maintenance plans required under 

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

EPA expects that most areas of the 

country would be designated as 

unclassifiable for the 1-hour NAAQS for 

SO2, due to a lack of both monitoring 

and modeling information concerning 

the attainment status of areas, in 

advance of States conducting further 

refined modeling according to our 

anticipated guidance. For areas that are 

designated unclassifiable, States are 

required to submit section 110(a)(1) 

plans to demonstrate implementation, 

maintenance and enforcement of the 

new SO2 NAAQS. As previously 

explained in section III of the preamble, 

in order to meet the requirements of 

section 110(a)(1) and to ensure timely 

attainment of the NAAQS on a schedule 

that is as expeditious as would be 

required if an area had been designated 

nonattainment, EPA’s current 

expectation is that States would submit 

SIPs which provide for attainment, 

implementation, maintenance, and 

enforcement of the 1-hour SO2 NAAQS 

in all areas as expeditiously as 

practicable, which EPA believes in these 

cases would be no later than 5 years 

from the effective date of the area’s 

designation. The section 110(a)(1) 

maintenance plan would also need to 

contain the following elements: (1) An 


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attainment emissions inventory, (2) a 

control strategy, as appropriate, (3) a 

maintenance demonstration, using an 

EPA approved air quality model as 

appropriate, (4) a contingency plan, and 

(5) a plan for verification of continued 

attainment of the standard. Attainment 

areas that appear to have difficulty 

maintaining attainment may also have 

to submit some of these elements. These 

elements are now explained in detail. 

(1) Attainment Emissions Inventory 

The State should develop an accurate 

attainment emissions inventory to 

identify the level of emissions in the 

area which is sufficient to attain the 1- 

hour SO2 NAAQS. This inventory 

should be consistent with EPA’s most 

recent guidance on emissions 

inventories currently available, and 

should include the emissions for the 

time period associated with the 

modeling and monitoring data showing 

attainment. Major source size thresholds 

for SO2 are currently listed as 100 

ton/yr, however, in cases where sources, 

individually, or collectively, that are 

below this level may potentially cause 

or contribute to a violation of the 

standard, these sources should also be 

included in the emissions inventory for 

the affected area. EPA notes that, unlike 

any monitoring or modeling data used 

in the initial designations context, 

which would be limited to current 

emissions levels, this estimate under a 

hybrid approach we expect to use for 

the new SO2 NAAQS would be able to 

rely on modeled controlled emissions 

levels at sources achieved by 

enforceable national, regional or local 

rules that will be in place within the 

timeframe for demonstrating attainment. 

This is because demonstrations of 

attainment and maintenance of a 

NAAQS, unlike designations, are 

necessarily projections regarding future 

and continuing levels of ambient air 

pollution concentrations given that the 

statutory deadlines for their submission 

are in advance of the required 

achievement of attainment and 

maintenance. See, e.g., CAA sections 

191(a) and 192(a). 

(2) Maintenance Demonstration 

The key element of a section 110(a)(1) 

maintenance plan is a demonstration 

using, as appropriate, refined SO2

dispersion modeling (see Appendix W 

to 40 CFR Part 51) which provides an 

indication of how the area will attain 

and maintain the 1-hour SO2 NAAQS as 

expeditiously as practicable, which EPA 

believes would be within the 5 year 

period following the designation of the 

area. For SO2 the State may generally 

demonstrate maintenance of the 

NAAQS by using refined dispersion 

modeling to show that the future mix of 

sources and emission rates in an area 

will not cause a violation of the 1-hour 

SO2 NAAQS. As a result of applying the 

control strategy, EPA anticipates that 

additional guidance for States may be 

needed to clarify how to conduct 

dispersion modeling under Appendix W 

to support the implementation of the 

new 1-hour SO2 NAAQS. 

As explained above in IV.B, EPA 

believes that for SO2 attainment and 

maintenance demonstrations, 

monitoring data alone is generally not 

adequate to characterize fully short-term 

ambient concentrations around major 

stationary sources of SO2, and as a result 

may not capture the maximum SO2

impacts. With representative and 

appropriate meteorological and other 

input data, refined dispersion models 

are able to characterize air quality 

impacts from the modeled sources 

across the domain of interest on an 

hourly basis with a high degree of 

spatial resolution, overcoming the 

limitations of an approach based solely 

on monitoring. By simulating plume 

dispersion on an hourly basis across a 

grid of receptor locations, dispersion 

models are able to estimate the detailed 

spatial gradients of ambient 

concentrations resulting from SO2

emission sources across a full range of 

meteorological and source operating 

conditions. To capture such results on 

a monitor would normally require a 

prohibitively expansive air quality 

monitoring network. Further, as we 

have observed in prior actions (see., e.g., 

43 FR 45993, 45997, 46000–03 (Oct. 5, 

1978)), monitoring data would not be 

adequate to demonstrate attainment if 

sources are using stacks with heights 

that are greater than good engineering 

practice (GEP), or other prohibited 

dispersion techniques, as section 123 

prohibits credit in an attainment 

demonstration for any such practices. 

Refined dispersion modeling for the 

section 110(a)(1) maintenance plan is 

expected to follow EPA’s Guideline on 

Air Quality Models, Appendix W to 40 

CFR Part 51, which provides 

recommendations on modeling 

techniques and guidance for estimating 

pollutant concentrations in order to 

assess control strategies and determine 

emission limits. These 

recommendations were originally 

published in April 1978 and were 

incorporated by reference in the PSD 

regulations, 40 CFR sections 51.166 and 

52.21 in June 1978 (43 FR 26382– 

26388). The purpose of Appendix W is 

to promote consistency in the use of 

modeling within the air quality 

management process. Appendix W is 

periodically revised to ensure that new 

model developments or expanded 

regulatory requirements are 

incorporated. The most recent revision 

to Appendix W was published on 

November 9, 2005 (70 FR 68218), 

wherein EPA adopted AERMOD as the 

preferred dispersion model for a wide 

range of regulatory applications in all 

types of terrain. To support the 

promulgation of AERMOD as the 

preferred model, EPA evaluated the 

performance of the model across a total 

of 17 field study data bases (Perry, et al., 

2005; EPA, 2003), including several 

field studies based on model-to-monitor 

comparisons of SO2 concentrations from 

operating power plants. AERMOD is a 

steady-state plume dispersion model 

that employs hourly sequential 

preprocessed meteorological data to 

simulate transport and dispersion from 

multiple point, area, or volume sources 

for averaging times from one hour to 

multiple years, based on an advanced 

characterization of the atmospheric 

boundary layer. AERMOD also accounts 

for building wake effects (i.e., 

downwash) on plume dispersion. 

As stated previously, EPA anticipates 

that additional guidance for States, 

Tribal, and local governments is needed 

to clarify how to conduct refined 

dispersion modeling under Appendix W 

to support the implementation of the 

new 1-hour SO2 NAAQS. EPA intends 

to solicit public comment on guidance 

regarding modeling. Although AERMOD 

is identified as the preferred model 

under Appendix W for a wide range of 

applications and will be appropriate for 

most modeling applications to support 

the new SO2 NAAQS, Appendix W 

allows flexibility to consider the use of 

alternative models on a case-by-case 

basis when an adequate demonstration 

can be made that the alternative model 

performs better than, or is more 

appropriate than, the preferred model 

for a particular application. 

(3) Control Strategy 

The EPA believes that in order to 

meet the implementation, maintenance 

and enforcement plan requirements of 

section 110(a)(1) for the new SO2

NAAQS, States should consider all 

control measures that are reasonable to 

implement in light of the attainment 

and maintenance needs for the affected 

area(s). The EPA believes that where 

additional controls are necessary it 

would be appropriate for the level of 

controls in these areas to be similar to 

that required in areas that are 

designated as nonattainment for SO2. 

These controls would provide for the 

attainment and maintenance of the SO2

1-hour standard as expeditiously as 


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practicable. EPA believes that 

expeditious attainment in these areas 

will be within 5 years of the effective 

date of designation of an area. This 

approach would allow States to take 

into consideration emission reductions 

that we expect to be achieved from the 

implementation of future controls from 

national control measures as well as 

regional and local control measures that 

will be in place by the anticipated 

attainment date and are projected to 

help achieve attainment and 

maintenance of the standard. It would 

also reduce the risk of such areas failing 

to meet the NAAQS as expeditiously as 

nonattainment areas must meet it. 

(4) Contingency Plan 

The contingency plan is considered to 

be an enforceable part of the section 

110(a)(1) plan and should ensure that 

there are appropriate contingency 

measures which can be implemented as 

expeditiously as practicable once they 

are triggered. The contingency plan 

should clearly identify the measures to 

be adopted, provide a schedule and 

procedures for adoption and 

implementation, and provide a specific 

time limit for actions by the State. 

The EPA believes that in this case the 

contingency measures implemented 

under the contingency plan requirement 

for the section 110(a)(1) plan in 

unclassifiable areas under a revised 

approach for SO2 should closely 

resemble the contingency measures 

required under section 172(c)(9) of the 

CAA. Section 172(c)(9) of the CAA 

defines contingency measures as 

measures in the SIP which are to be 

implemented in the event that an area 

fails to attain the NAAQS, or fails to 

meet the reasonable further progress 

(RFP) requirement, by the applicable 

attainment date for the area. 

Contingency measures become effective 

without further action by the State or 

EPA, upon determination by EPA that 

the area (1) failed to attain the NAAQS 

by the applicable attainment date, or (2) 

fail to meet RFP. These contingency 

measures should consist of other 

available control measures that are not 

included in the control strategy for the 

SIP. 

The EPA interprets the contingency 

measure provision as primarily directed 

at general control programs which can 

be undertaken on an area-wide basis. 

Since SO2 control measures are based 

on what is directly and quantifiably 

necessary to attain the SO2 NAAQS, it 

would be unlikely for an area to 

implement the necessary emissions 

control yet fail to attain the NAAQS. 

Therefore, for SO2 programs, EPA 

believes that State agencies should have 

a comprehensive program to identify 

sources of violations of the SO2 NAAQS 

and undertake an aggressive follow-up 

for compliance and enforcement, 

including expedited procedures for 

establishing enforceable consent 

agreements pending the adoption of 

revised SIPs. 

Such an approach toward minimum 

contingency measures for SO2 would 

not preclude a State from requiring 

additional contingency measures that 

are enforceable and appropriate for a 

particular source or source category. A 

contingency measure for an SO2 SIP 

might be a consent agreement between 

the State and EPA to reduce emissions 

from a source further in the event that 

the contingency measures are triggered. 

Alternatively, a source might adopt a 

contingency measure such as switching 

to low sulfur coal or reducing load until 

more permanent measures can be put 

into place to correct the problem. In 

either case, the contingency measure 

should be a fully adopted provision in 

the SIP in order for it to become 

effective at the time that EPA 

determines that the area either fails to 

attain the NAAQS or fails to meet RFP. 

As a necessary part of the section 

110(a)(1) plan, the State should also 

identify specific indicators, or triggers, 

which will be used to determine when 

the contingency measures need to be 

implemented. The identification of 

triggers would allow a State an 

opportunity to take early action to 

address potential violations of the 

NAAQS before they occur. By taking 

early action, States may be able to 

prevent any actual violations of the 

NAAQS, and therefore, reduce the need 

on the part of EPA to start the process 

to re-designate the areas as 

nonattainment. An example of a trigger 

would be monitored or modeled 

violations of the NAAQS. The EPA will 

review what constitutes an approvable 

contingency plan on a case-by-case 

basis. 

(5) Verification of Continued 

Attainment 

The submittal should provide an 

indication of how the State will track 

the progress of the section 110(a)(1) 

plan. This is necessary due to the fact 

that the emissions projections made for 

the attainment and maintenance 

demonstrations depend on assumptions 

of point, area, and mobile source 

growth. One option for tracking the 

progress of the attainment and 

maintenance demonstrations, provided 

here as an example, would be for the 

State to update periodically the 

emissions inventory. The attainment 

and maintenance demonstration should 

project maintenance during the five year 

period following the designations for 

the 1-hour SO2 NAAQS, not simply that 

the area will be in attainment in the fifth 

year. 

States should develop interim 

emission projection years to show a 

trend analysis for attainment and 

maintenance of the standard. These 

emission projections can also be used as 

triggers for implementing contingency 

measures. The EPA recognizes that it 

would be difficult and time consuming 

to develop projections for each year of 

the 5 year period. Therefore, the number 

of interim projection years should 

reflect whatever information exists 

regarding the potential for increases in 

emissions in the intervening years. For 

instance, if there is a high probability 

that emissions will increase to such an 

extent as to jeopardize continued 

maintenance of the standard even 

temporarily over the intervening years, 

the number of interim projection 

periods should be sufficient to 

document that such increases will not 

interfere with maintenance of the 1-hour 

SO2 NAAQS. 

When modeling for the attainment 

and maintenance demonstrations, one 

option for tracking progress would also 

be for the State to reevaluate 

periodically the modeling assumptions 

and data input. Such reevaluation, for 

example, could address any delays in 

source compliance with national, 

regional or local rules for which the 

State had previously modeled timely 

SO2 reductions. In any event, the State 

should monitor the indicators for 

triggering the contingency measures on 

a regular basis. 

EPA recognizes that the approach 

discussed above for SO2 SIPs submitted 

under CAA section 110(a)(1)–(2) is 

significantly different from the one 

outlined in the proposal, and from what 

we have applied in the context of other 

criteria pollutants. However, EPA 

anticipates using a revised approach 

under section 110(a)(1)–(2) as part of an 

overall revised hybrid monitoring and 

modeling approach in response to 

comments on the proposed monitoringfocused approach to implementation of 

the new SO2 NAAQS. We believe that 

such an approach would best account 

for the unique source-specific and 

localized impacts inherent to SO2, and 

would be the most reasonable way to 

ensure that all areas of the United States 

timely attain and maintain the new 

NAAQS, while at the same time 

avoiding inappropriately requiring 

immediate refined modeling of all 

sources without appropriate EPA 

guidance. This would also allow 

attainment demonstrations to account 


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for expected substantial SO2 reductions 

that will occur well in advance of the 

attainment deadline. Of course, for such 

a unique SO2 approach to work, it 

would be imperative for all areas to 

timely submit, and for EPA to able to 

approve, adequate attainment, 

implementation, maintenance and 

enforcement SIPs that show attainment 

as expeditiously as practicable, and no 

later than 5 years following initial 

designations. Only by applying such a 

timeframe to the section 110(a)(1) SIP 

approach we are adopting for SO2 could 

the approach be a reasonable one. To 

that end, EPA would not intend to 

approve SIPs that do not meet this 

schedule, and would take necessary and 

appropriate actions in response to any 

submission that would result in 

unacceptable delay of attainment. Such 

actions may include, but are not limited 

to, any combination of SIP disapproval, 

redesignation to nonattainment, and 

promulgation of a Federal 

implementation plan (FIP). Any future 

action establishing an attainment 

deadline will be completed through 

notice-and-comment rulemaking on 

individual SIP submissions. 

The timeline below shows how we 

expect the several steps from 

promulgation of the new NAAQS 

through attainment should proceed, 

whether areas are designated 

nonattainment or unclassifiable, 

assuming timely action at each step: 

• June 2010: EPA issues new SO2

NAAQS, which starts periods within 

which CAA section 107 initial area 

designations must occur and CAA 

section 110(a)(1)–(2) SIPs must be 

submitted. 

• June 2011: States submit initial area 

designations recommendations, based 

on available monitoring data, and on 

any refined modeling performed in 

advance of submitting CAA section 

110(a)(1)–(2) SIPs. 

• June 2012: EPA issues initial area 

designations. Any monitored or 

modeled violations would trigger 

nonattainment designations. (Per below, 

States designated nonattainment would 

submit nonattainment SIPs by February 

2014, relying on refined modeling that 

demonstrates attainment by no later 

than August 2017.) States would be 

designated attainment if they submit 

both monitoring and modeling showing 

adequate evidence of no violations. All 

other cases would be initially 

designated as unclassifiable. 

• June 2013: States submit CAA 

section 110(a)(1)–(2) SIPs. SIPs would 

rely on refined modeling and any 

required monitoring that demonstrates 

attainment and maintenance of the new 

SO2 NAAQS as expeditiously as 

practicable, and no later than August 

2017. For areas within the State 

designated attainment and 

unclassifiable, the section 110(a) SIP 

must contain any additional Federally 

enforceable control measures necessary 

to ensure attainment and maintenance 

of the NAAQS. (Control measures to be 

implemented in designated 

nonattainment areas are due later as part 

of the nonattainment SIP in February 

2014.) 

• February 2014: Any initially 

designated nonattainment areas submit 

CAA section 191–192 SIPs showing 

attainment no later than August 2017. 

• June 2014: EPA approves or 

disapproves submitted CAA section 

110(a)(1)–(2) SIPs. For attainment and 

unclassifiable areas, EPA’s action would 

be based on adequacy of States’ 

modeling (and any required monitoring) 

showing attainment as expeditiously as 

practicable, and no later than August 

2017, in partial reliance on SO2

reductions from national and regional 

standards that are achieved by the 

attainment date. EPA would also have 

discretion to re-designate areas based on 

these SIPs, including to nonattainment 

if SIPs are inadequate, as well as 

promulgate FIPs. 

• February 2015: EPA approves or 

disapproves CAA section 191–192 

attainment SIPs submitted by areas 

initially designated as nonattainment, 

with similar remedies as discussed 

above if SIPs are deficient. 

• June 2016: CAA section 110(c) 

deadline by which EPA must issue a FIP 

for any area whose section 110(a)(1) SIP 

is disapproved in June 2014. 

• February 2017: CAA section 110(c) 

deadline by which EPA must issue a FIP 

for a nonattainment area whose section 

192 SIP is disapproved in February 

2015. 

August 2017: Expected date by which 

all areas, regardless of classification, 

achieve attainment, implementation, 

maintenance and enforcement of the 

new SO2 NAAQS. 

D. Attainment Planning Requirements 

1. SO2 Nonattainment Area SIP 

Requirements 

a. Approach Described in the Proposal 

We explained in the preamble to the 

proposal that any State containing an 

area designated as nonattainment with 

respect to the SO2 NAAQS would need 

to develop for submission to EPA a SIP 

meeting the requirements of part D, 

Title I, of the CAA, providing for 

attainment by the applicable statutory 

attainment date. See sections 191(a) and 

192(a) of the CAA. As indicated in 

section 191(a), all components of the 

SO2 part D SIP must be submitted 

within 18 months of the effective date 

of an area’s designation as 

nonattainment. 

Section 172 of the CAA addresses the 

general requirements for areas 

designated as nonattainment. Section 

172(c) directs States with nonattainment 

areas to submit a SIP which contains an 

attainment demonstration showing that 

the affected area will attain the standard 

by the applicable statutory attainment 

date. The SIP must show that the area 

will attain the standard as expeditiously 

as practicable, and must ‘‘provide for the 

implementation of all Reasonably 

Available Control Measures (RACM) as 

expeditiously as practicable (including 

such reductions in emissions from 

existing sources in the area as may be 

obtained through the adoption, at a 

minimum, of Reasonably Available 

Control Technology (RACT)).’’ 

SIPs required under Part D of the CAA 

must also provide for reasonable further 

progress (RFP). See section 172(c)(2) of 

the CAA. The CAA defines RFP as ‘‘such 

annual incremental reductions in 

emissions of the relevant air pollution 

as are required by part D, or may 

reasonably be required by the 

Administrator for the purpose of 

ensuring attainment of the applicable 

NAAQS by the applicable attainment 

date.’’ See section 171 of the CAA. 

Historically, for some pollutants, RFP 

has been met by showing annual 

incremental emission reductions 

sufficient to maintain generally linear 

progress toward attainment by the 

applicable attainment date. 

All SO2 nonattainment area SIPs must 

include contingency measures which 

must be implemented in the event that 

an area fails to meet RFP or fails to 

attain the standards by its attainment 

date. See section 172(c)(9) of the CAA. 

These contingency measures must be 

fully adopted rules or control measures 

that take effect without further action by 

the State or the Administrator. The EPA 

interprets this requirement to mean that 

the contingency measures must be 

implemented with only minimal further 

action by the State or the affected 

sources with no additional rulemaking 

actions such as public hearings or 

legislative review. 

Emission inventories are also critical 

for the efforts of State, local, and Federal 

agencies to attain and maintain the 

NAAQS that EPA has established for 

criteria pollutants including SO2. 

Section 191(a) in conjunction with 

section 172(c) requires that areas 

designated as nonattainment for SO2

submit an emission inventory to EPA no 

later than 18 months after designation as 

nonattainment. In the case of SO2, 


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37The terms ‘‘major’’ and ‘‘minor’’ define the size 

of a stationary source, for applicability purposes, in 

terms of an annual emissions rate (tons per year, 

tpy) for a pollutant. Generally, a minor source is 

any source that is not ‘‘major.’’ ‘‘Major’’ is defined 

by the applicable regulations—PSD or 

nonattainment NSR. 

38 In addition, the PSD program applies to noncriteria pollutants subject to regulation under the 

Act, except those pollutants regulated under section 

112 and pollutants subject to regulation only under 

section 211(o). 

sections 191(a) and 172(c) also direct 

States to submit periodic emission 

inventories for nonattainment areas. The 

periodic inventory must include 

emissions of SO2 for point, nonpoint, 

mobile, and area sources. 

b. Current Approach 

EPA did not receive any comments on 

this issue. Thus, EPA has no changes to 

make to this discussion. 

2. New Source Review and Prevention 

of Significant Deterioration 

Requirements 

a. Approach Described in the Proposal 

We provided a discussion of the new 

source review and prevention of 

significant deterioration programs in the 

preamble to the proposed rule. The 

Prevention of Significant Deterioration 

(PSD) and nonattainment New Source 

Review (NSR) programs contained in 

parts C and D of Title I of the CAA 

govern preconstruction review of any 

new or modified major stationary 

sources of air pollutants regulated under 

the CAA as well as any precursors to the 

formation of that pollutant when 

identified for regulation by the 

Administrator.37 The EPA rules 

addressing these programs can be found 

at 40 CFR 51.165, 51.166, 52.21, 52.24, 

and Part 51, appendix S. 

The PSD program applies when a 

major source located in an area that is 

designated as attainment or 

unclassifiable for any criteria pollutant 

is constructed or undergoes a major 

modification.38 The nonattainment NSR 

program applies on a pollutant-specific 

basis when a major source constructs or 

modifies in an area that is designated as 

nonattainment for that pollutant. The 

minor NSR program addresses major 

and minor sources that undergo 

construction or modification activities 

that do not qualify as major, and it 

applies, as necessary to assure 

attainment, regardless of the designation 

of the area in which a source is located. 

The PSD requirements include but are 

not limited to the following: 

• Installation of Best Available 

Control Technology (BACT); 

• Air quality monitoring and 

modeling analyses to ensure that a 

project’s emissions will not cause or 

contribute to a violation of any NAAQS 

or maximum allowable pollutant 

increase (PSD increment); 

• Notification of Federal Land 

Manager of nearby Class I areas; and 

public comment on the permit. 

To the extent necessary to address 

these PSD requirements for the new 

1-hour SO2 NAAQS, SIPs are due no 

later than 3 years after the promulgation 

date. Generally, however, the owner or 

operator of any major stationary source 

or major modification obtaining a final 

PSD permit on or after the effective date 

of the new 1-hour SO2 NAAQS will be 

required, as a prerequisite for the PSD 

permit, to demonstrate that the 

emissions increases from the new or 

modified source will not cause or 

contribute to a violation of that new 

NAAQS. The EPA anticipates that 

individual sources will be able to 

complete this demonstration under the 

PSD regulations based on current 

guidance in EPA’s Guideline on Air 

Quality Models, Appendix W of 40 CFR 

Part 51. 

The owner or operator of a new or 

modified source will still be required to 

demonstrate compliance with the 

annual and 24-hour SO2 increments, 

even when their counterpart NAAQS 

are revoked. The annual and 24-hour 

increments are established in the CAA 

and will need to remain in the PSD 

regulations because EPA does not 

interpret the CAA to authorize EPA to 

remove them. It appears necessary for 

Congress to amend the CAA to make 

appropriate changes to the statutory SO2

increments. In 1990, the CAA was 

amended to accommodate PM10

increments in lieu of the statutory TSP 

increments. 

In association with the requirement to 

demonstrate compliance with the 

NAAQS and increments, the owner or 

operator of a new or modified source 

must submit for review and approval a 

source impact analysis and an air 

quality analysis. The source impact 

analysis, primarily a modeling analysis, 

must demonstrate that allowable 

emissions increases from the proposed 

source or modification, in conjunction 

with emissions from other existing 

sources will not cause or contribute to 

either a NAAQS or increment violation. 

The air quality analysis must assess the 

ambient air quality in the area that the 

proposed source or modification would 

affect. 

For the air quality analysis, the owner 

or operator must submit in its permit 

application air quality monitoring data 

that shall have been gathered over a 

period of one year and is representative 

of air quality in the area of the proposed 

project. If existing data representative of 

the area of the proposed project is not 

available, new data may need to be 

collected by the owner or operator of the 

source or modification. Where data is 

already available, it might be necessary 

to evaluate the location of the 

monitoring sites from which the SO2

data were collected in comparison to 

any new siting requirements associated 

with the 1-hour SO2 NAAQS. If existing 

sites are inappropriate for providing the 

necessary representative data, then new 

monitoring data will need to be 

collected by the owner or operator of the 

proposed project. 

Historically, EPA has allowed the use 

of several screening tools to help 

facilitate the implementation of the new 

source review program by reducing the 

permit applicant’s burden, and 

streamlining the permitting process for 

de minimis circumstances. These 

screening tools include a significant 

emissions rate (SER), significant impact 

levels (SILs), and a significant 

monitoring concentration (SMC). The 

SER, as defined in tons per year for each 

regulated pollutant, is used to determine 

whether any proposed source or 

modification will emit sufficient 

amounts of a particular pollutant to 

require the review of that pollutant 

under the NSR permit program. EPA 

will consider whether to evaluate the 

existing SER for SO2 to see if it would 

change substantially based on the 

NAAQS levels for the 1-hour averaging 

period. Historically, for purposes of 

defining the SER, we have defined a de 

minimis pollutant impact as one that 

results in a modeled ambient impact of 

less than approximately 4% of the shortterm NAAQS. The current SER for SO2

(40 tpy) is based on the impact on the 

24-hour SO2 NAAQS. See 45 FR 52676, 

52707 (August 7, 1980). We have 

typically used the most sensitive 

averaging period to calculate the SER, 

and we may want to evaluate the new 

1-hour period for SO2 because it is 

likely to represent the most sensitive 

averaging period for SO2. 

The SIL, expressed as an ambient 

pollutant concentration (ug/m3), is used 

to determine whether the impact of a 

particular pollutant is significant 

enough to warrant a complete air quality 

impact analysis for any applicable 

NAAQS and increments. EPA has 

promulgated regulations under 40 CFR 

51.165(b) which include SILs for SO2 to 

determine whether a source’s impact 

would be considered to cause or 

contribute to a NAAQS violation for the 

3-hour (the secondary NAAQS), 24-hour 

or annual averaging periods. These SILs 

were originally developed in 1978 to 

limit the application of air quality 

dispersion models to a downwind 


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distance of no more than 50 kilometers 

or to ‘‘insignificant levels.’’ See 43 FR 

26398, June 19, 1978. Through 

guidance, EPA has also allowed the use 

of SILs to determine whether or not it 

is necessary for a source to carry out a 

comprehensive source impact analysis 

and to determine the extent of the 

impact area in which the analysis will 

be carried out. The existing SILs for SO2

were not developed on the basis of 

specific SO2 NAAQS levels, so there 

may be no need to revise the existing 

SILs. Even upon revocation of the 

annual and 24-hour NAAQS, the 

corresponding SIL should still be useful 

for increment assessment. A SIL for the 

1-hour averaging period does not exist, 

and would need to be developed for use 

with modeling for 1-hour SO2 NAAQS 

and any 1-hour increments. 

Finally, the SMC, also measured as an 

ambient pollutant concentration 

(μg/m3), is used to determine whether it 

may be appropriate to exempt a 

proposed project from the requirement 

to collect ambient monitoring data for a 

particular pollutant as part of a 

complete permit application. EPA first 

defined SMCs for regulated pollutants 

under the PSD program in 1980. See 45 

FR 52676, 52709–10 (August 7, 1980). 

The existing SMC for SO2, based on a 

24-hour averaging period, may need to 

be re-evaluated to consider the effect of 

basing the SMC on the 1-hour averaging 

period, especially in light of revocation 

of the NAAQS for the 24-hour averaging 

period. Third, even if the 1-hour 

averaging period does not indicate the 

need for a revised SMC for SO2, the fact 

that the original SMC for SO2 is based 

on 1980 monitoring data (Lowest 

Detectable Level, correction factor of 

‘‘5’’), could be a basis for revising the 

existing value. More up-to-date 

monitoring data and statistical analyses 

of monitoring accuracy may yield a 

different—possibly lower—correction 

factor today. The new 1-hour NAAQS 

will not necessarily cause this result, 

but may provide a ‘‘window of 

opportunity’’ to re-evaluate the SMC for 

SO2. 

States which have areas designated as 

nonattainment for the SO2 NAAQS are 

directed to submit, as a part of the SIP 

due 18 months after an area is 

designated as nonattainment, provisions 

requiring permits for the construction 

and operation of new or modified 

stationary sources anywhere in the 

nonattainment area. Prior to adoption of 

the SIP revision addressing major source 

nonattainment NSR for SO2

nonattainment areas, the requirements 

of 40 CFR part 51, appendix S will 

apply. Nonattainment NSR 

requirements include but are not limited 

to: 

• Installation of Lowest Achievable 

Emissions Rate (LAER) control 

technology; 

• Offsetting new emissions with 

creditable emissions reductions; 

• A certification that all major 

sources owned and operated in the State 

by the same owner are in compliance 

with all applicable requirements under 

the CAA; 

• An alternatives and siting analysis 

demonstrating that the benefits of a 

proposed source significantly outweigh 

the environmental and social costs 

imposed as a result of its location, 

construction, or modification; and 

• Public comment on the permit. 

Minor NSR programs must meet the 

statutory requirements in section 

110(a)(2)(C) of the CAA which requires 

‘‘* * * regulation of the modification 

and construction of any stationary 

source * * * as necessary to assure that 

the [NAAQS] are achieved.’’ These 

programs must be established in each 

State within 3 years of the promulgation 

of a new or revised NAAQS. 

b. Comments and Responses 

Several commenters stated that in 

order to avoid confusion and lag time as 

it relates to PSD/NSR and permitting 

activities, which must be taken by States 

following the promulgation of the 

revised NAAQS, EPA must provide 

guidance as soon as possible related to 

these issues. Commenters also stated 

that EPA must develop guidance as soon 

as possible to address the screening 

tools for PSD/NSR such as SILs, SERs, 

SMCs, and the development of 

increments. Several commenters also 

stated that guidance should be provided 

as it relates to the use of AERMOD to 

address PSD issues. 

The EPA acknowledges that a 

decision to promulgate a new short-term 

SO2 NAAQS will have implications for 

the air permitting process. The full 

extent of how a new short-term SO2

NAAQS will affect the NSR process will 

need to be carefully evaluated. First, 

major new and modified sources 

applying for NSR/PSD permits will 

initially be required to demonstrate that 

their proposed emissions increases of 

SO2 will not cause or contribute to a 

violation of any NAAQS or PSD 

increments for SO2, including the new 

1-hour SO2 NAAQS. In addition, we 

believe that section 166(c) of the CAA 

authorizes EPA to consider the need to 

promulgate a new 1-hour increment. 

Historically, EPA has developed 

increments for each applicable 

averaging period for which a NAAQS 

has been promulgated. However, 

increments for a particular pollutant do 

not necessarily need to match the 

averaging periods that have been 

established for NAAQS for the same 

pollutant. Environmental Defense Fund, 

Inc. v. EPA, 898 F.2d 183, 189–190 (DC 

Cir. 1990) (‘‘* * * the ‘goals and 

purposes’ of the PSD program, set forth 

in § 160, are not identical to the criteria 

on which the ambient standards are 

based.’’) Thus, we would need to 

evaluate the need for a new 1-hour SO2

increment in association with the goals 

and purposes of the statutory PSD 

program requirements. 

We agree with the commenters that 

there may be a need for EPA to provide 

additional screening tools or to revise 

existing screening tools that are 

frequently used under the NSR/PSD 

program for reducing the burden of 

completing SO2 ambient air impact 

analyses. These screening tools include 

the SILs, as mentioned by the 

commenter, but also include the SER for 

emissions of SO2 and the SMC for SO2. 

The existing sceening tools apply to the 

averaging periods used to define the 

existing NAAQS for SO2, including the 

annual, 24-hour, and 3-hour averaging 

periods. EPA intends to evaluate the 

need for possible changes or additions 

to each of these useful screening tools 

for SO2 due to the revision of the SO2

NAAQS to provide for a 1-hour 

standard. We believe it is highly likely 

that in order to be most useful for 

implementing the new 1-hour averaging 

period for NSR purposes, new 1-hour 

screening values will be appropriate. 

Finally, in response to the comment 

concerning the need for additional 

guidance as it relates to the use of 

AERMOD to address PSD issues, EPA 

anticipates providing additional 

technical guidance on modeling and 

analysis as a part of the SIP 

demonstration process. As stated 

previously, EPA intends to solicit public 

comment on guidance regarding 

modeling, and also solicit public 

comment on additional implementation 

planning guidance. However, EPA 

believes that the air quality models 

currently required for NSR/PSD 

permitting as provided in the EPA’s 

Guideline on Air Quality Models, 

Appendix W of CFR 40 Part 51 would 

be appropriate for demonstrating 

compliance with the revised SO2

NAAQS under these programs. At this 

time, EPA is not considering modifying 

the AERMOD dispersion model and its 

underlying science for predicting SO2

concentrations to accommodate the 

revised NAAQS for SO2.


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39Criteria pollutants are those pollutants for 

which EPA has established a NAAQS under section 

109 of the CAA. 

40Transportation conformity is required under 

CAA section 176(c) (42 U.S.C. 7506(c) to ensure that 

Federally supported highway and transit project 

activities are consistent with (‘‘conform to’’) the 

purpose of the SIP. Transportation conformity 

applies to areas that are designated nonattainment, 

and those areas redesignated to attainment after 

1990 (‘‘maintenance areas’’ with plans developed 

under CAA section 175A) for transportation-related 

criteria pollutants. Due to the relatively small 

amounts of sulfur in gasoline and on-road diesel 

fuel, transportation conformity does not apply to 

the SO2 NAAQS. 40 CFR 93.102(b)(1). 

c. Current Approach 

In the preamble to the proposed 

regulation, EPA noted that ‘‘PSD permit 

requirements are effective on the 

promulgation date of a new or revised 

standard.’’ However, this statement did 

not reflect an important distinction that 

needs to be clarified here. Under section 

51.166(b)(49)(i) and 52.21(b)(50)(i) of 

EPA’s regulations, a pollutant that has 

not been regulated previously would 

become a ‘‘regulated NSR pollutant’’ 

upon promulgation of a NAAQS. See, 75 

FR 17004, 17018–19. However, in the 

case of pollutants that are already 

‘‘regulated NSR pollutants,’’ at the time 

a new NAAQS is promulgated or an 

existing NAAQS is revised, EPA 

interprets the CAA and EPA regulations 

to require implementation of the new or 

revised standard in the Federal PSD 

permitting process upon the effective 

date of any new or revised standards. 

Section 165(a)(3) of the CAA and 

section 52.21(k) of EPA’s regulations 

require that a permit applicant 

demonstrate that it will not cause or 

contribute to a violation of ‘‘any’’ 

NAAQS. See, Memorandum from 

Stephen D. Page, Director of EPA Office 

of Air Quality Planning and Standards, 

‘‘Applicability of the Federal Prevention 

of Significant Deterioration Permit 

Requirements to New and Revised 

National Ambient Air Quality 

Standards’’ (April 1, 2010). 

Amendments to the existing PSD 

requirements set forth in EPA 

regulations concerning SILs, SERs and 

SMCs may involve notice and comment 

rulemaking which could take at least 

one year to complete. For PM2.5, EPA 

developed SERs under the initial NSR 

implementation requirements for PM2.5. 

See 73 FR 28321, May 16, 2008. The 

SILs and SMC for PM2.5 are being 

developed under a subsequent 

rulemaking simultaneously with the 

promulgation of PM2.5 increments, 

pursuant to a CAA schedule that allows 

EPA 2 years from the promulgation of 

new and revised NAAQS to promulgate 

increments. Under such an approach, 

SILs and SMC are not available until the 

increments are promulgated. States and 

industry have criticized that approach 

because it has left State permitting 

authorities without an EPA-approved de 

minimis value that could be used in 

determining the level of analysis that 

individual PSD sources must undergo, 

and could result in more detailed 

analyses for sources that will have only 

have de miminis impacts on the 

NAAQS. 

To address this concern, we believe it 

is appropriate to proceed with 

development of the PSD screening tools 

in advance of an increment rulemaking 

to hasten their availability. In addition, 

we are assessing the possibility of 

developing interim screening tools that 

can be used by States prior to the 

completion of the SIP-development 

process if the States establish an 

appropriate record for individual 

permitting actions based on the 

supporting technical information 

provided by EPA. It is our expectation, 

that if such interim screening tools are 

appropriate, we would make the interim 

SIL and the supporting record for EPA’s 

assessment available before the effective 

date of the new 1-hour SO2 NAAQS to 

facilitate more efficient PSD permit 

reviews once the new standard becomes 

effective. 

3. General Conformity 

a. Approach Described in the Proposal 

Section 176(c) of the CAA requires 

that all Federal actions conform to an 

applicable implementation plan 

developed pursuant to section 110 and 

part D of the CAA. The EPA rules 

developed under section 176(c) 

prescribe the criteria and procedures for 

demonstrating and assuring conformity 

of Federal actions to a SIP. Each Federal 

agency must determine that any actions 

covered by the general conformity rule 

conform to the applicable SIP before the 

action is taken. The criteria and 

procedures for conformity apply only in 

nonattainment areas and those 

nonattainment areas redesignated to 

attainment since 1990 (‘‘maintenance 

areas’’) with respect to the criteria 

pollutants under the CAA: 39 carbon 

monoxide (CO), lead (Pb), nitrogen 

dioxide (NO2), ozone (O3), particulate 

matter (PM2.5 and PM10), and sulfur 

dioxide (SO2). The general conformity 

rules apply one year following the 

effective date of designations for any 

new or revised NAAQS.40

The general conformity determination 

examines the impacts of direct and 

indirect emissions related to Federal 

actions. The general conformity rule 

provides several options to satisfy air 

quality criteria, such as modeling or 

offsets, and requires the Federal action 

to also meet any applicable SIP 

requirements and emissions milestones. 

The general conformity rule also 

requires that notices of draft and final 

general conformity determinations be 

provided directly to air quality 

regulatory agencies and to the public by 

publication in a local newspaper. 

b. Current Approach 

EPA did not receive any comments on 

this aspect of the discussion in the 

proposal and expects to follow that 

approach. 

E. Transition From the Existing SO2

NAAQS to a Revised SO2 NAAQS 

a. Proposal 

In addition to proposing a short-term 

1-hour SO2 NAAQS, EPA proposed to 

revoke the annual and 24-hour 

standards (annual 0.03 ppm and 24- 

hour 0.14 ppm). Specifically, EPA 

proposed that the level for the 1-hour 

standard for SO2 be a range between 50– 

100 ppb, and took comment on setting 

the level of the standard up to 150 ppb. 

We explained that if the Administrator 

sets the 1-hour standard at 100 ppb or 

lower, EPA proposed to revoke the 24- 

hour standard. If the Administrator set 

the level of the 1-hour standard between 

a range of 100–150 ppb, then EPA 

proposed to retain the 24-hour standard. 

We explained that if EPA revised the 

SO2 NAAQS and revoked either the 

annual or 24-hour standard, EPA would 

need to promulgate adequate antibacksliding provisions. The CAA 

establishes anti-backsliding 

requirements where EPA relaxes a 

NAAQS. Here, in EPA replacing the 

annual and 24-hour standards with a 

short term 1-hour standard, EPA must 

address the section 172(e) antibacksliding provision of the CAA and 

determine whether it applies on its face 

or by analogy, and what provisions are 

appropriate to provide for transition to 

the new standard. States will need to 

insure that the health protection 

provided under the prior SO2 NAAQS 

continues to be achieved as well as 

maintained as States begin to implement 

the new NAAQS. This means that States 

are directed to continue implementing 

attainment and maintenance SIPs 

associated with the prior SO2 NAAQS 

until such time as they are subsumed by 

any new planning and control 

requirements associated with the new 

NAAQS. 

Whether or not section 172(e) directly 

applies to EPA’s final action on the SO2

NAAQS, EPA has previously looked to 

other provisions of the CAA to 

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41The areas that are currently designated as 

nonattainment for the pre-existing SO2 primary 

NAAQS are Hayden, AZ; Armstrong, PA; Laurel, 

MT; Piti, GU; and Tanguisson, GU. The areas that 

are designated nonattainment for both the primary 

and the secondary standards are East Helena, MT, 

Salt Lake Co, MT, Toole Co, UT, and Warren Co, 

NJ. (See http://www.epa.gov/oar/oaqps/greenbk/ 

lnc.html). The Billings/Laurel, MT, area is the only 

area currently subject to a SIP call. 

backsliding. The CAA contains a 

number of provisions that indicate 

Congress’s intent to not allow 

provisions from implementation plans 

to be altered or removed if the plan 

revision would jeopardize the air 

quality protection being provided by the 

existing plan when EPA revises a 

NAAQS to make it more stringent. For 

example, section 110(l) provides that 

EPA may not approve a SIP revision if 

it interferes with any applicable 

requirement concerning attainment and 

RFP, or any other applicable 

requirement under the CAA. In 

addition, section 193 of the CAA 

prohibits the modification of a control, 

or a control requirement, in effect or 

required to be adopted as of November 

15, 1990 (i.e., prior to the promulgation 

of the Clean Air Act Amendments of 

1990), unless such a modification would 

ensure equivalent or greater emissions 

reductions. Further, section 172(e) of 

the CAA specifies that if EPA revises a 

NAAQS to make it less stringent than a 

previous NAAQS, control obligations no 

less stringent than those that apply in 

nonattainment area SIPs may not be 

relaxed, and adopting those controls 

that have not yet been adopted as 

needed may not be avoided. The intent 

of Congress, concerning the 

aforementioned sections of the CAA, 

was confirmed in a recent DC Circuit 

Court opinion on the Phase I ozone 

implementation rule. See South Coast 

Air Quality Management Dist. v. EPA, 

472 F.3d 882 (DC Cir. 2006). 

To ensure that the anti-backsliding 

provisions and principles of section 

172(e) are met and applied upon EPA 

revocation of the annual and 24-hour 

standards, EPA is providing that those 

SO2 NAAQS will remain in effect for 

one year following the effective date of 

the initial designations under section 

107(d)(1) for the new SO2 NAAQS 

before the current NAAQS are revoked 

in most attainment areas. However, any 

existing SIP provisions under CAA 

sections 110, 191 and 192 associated 

with the annual and 24-hour SO2

NAAQS will remain in effect, including 

all currently implemented planning and 

emissions control obligations, including 

both those in the State’s SIP and that 

have been promulgated by EPA in FIPs. 

This will ensure that both the new 

nonattainment NSR requirements and 

the general conformity requirements for 

a revised standard are in place so that 

there will be no gap in the public health 

protections provided by these two 

programs. It will also ensure that all 

nonattainment areas under the annual 

and/or 24-hour NAAQS and all areas for 

which SIP calls have been issued will 

continue to be protected by currently 

required control measures. 

EPA is also providing that the annual 

and 24-hour NAAQS remain in place for 

any current nonattainment area, or any 

area for which a State has not fulfilled 

the requirements of a SIP call, until the 

affected area submits, and EPA 

approves, a SIP with an attainment, 

implementation, maintenance and 

enforcement SIP which fully addresses 

the attainment and maintenance 

requirements of the new SO2 NAAQS. 

This, in combination with the CAA 

mechanisms provided in sections 110(l), 

193, and 172(e) will help to ensure that 

continued progress is made toward 

timely attainment of the SO2 NAAQS. 

Also, in light of the nature of the new 

SO2 NAAQS, the lack of classifications 

(and mandatory controls associated with 

such classifications pursuant to the 

CAA), and the small number of current 

nonattainment areas, and areas subject 

to SIP calls, EPA believes that retaining 

the current standard for a limited period 

of time until attainment and 

maintenance SIPs are approved for the 

new standard in current nonattainment 

areas and SIP call areas, and one year 

after designations in other areas, will 

adequately serve the anti-backsliding 

requirements and goals of the CAA.41

b. Comments and Responses 

Several commenters stated that they 

support EPA’s proposal stating that the 

annual and 24-hour SO2 NAAQS EPA 

would remain in effect for one year 

following the effective date of the initial 

designations under section 107(d)(1) for 

the revised SO2 NAAQS before the 

current NAAQS are revoked in most 

attainment areas. The commenters also 

support EPA’s proposal that any 

existing SIP provisions under CAA 

sections 110, 191 and 192 associated 

with the annual and 24-hour SO2

NAAQS would remain in effect, 

including all currently implemented 

planning and emissions control 

obligations, including both those in the 

State’s SIP and that have been 

promulgated by EPA in FIPs. Several 

commenters also stated that they 

support EPA’s proposal that an area’s 

nonattainment designation and the 

subsequent CAA requirements under 

the current SO2 NAAQS will remain in 

effect until the affected State submits, 

and EPA approves a SIP which meets all 

of the relevant CAA requirements for 

the affected nonattainment area. EPA 

appreciates the support of the 

commenters on its strategy for 

addressing the anti-backsliding 

requirements related to the current and 

revised SO2 standard, pursuant to 

section 172(e) of the CAA. 

One commenter, however, stated that 

while they support EPA’s proposal to 

address the anti-backsliding provisions 

of section 172(e) of the CAA, they 

believe that EPA’s proposal is deficient 

in several respects. The commenter 

stated that EPA’s proposal to not 

terminate the annual and 24-hour 

standards for SO2 in any nonattainment 

area, or any area for which a State has 

not fulfilled the requirements of a SIP 

call, until after the affected area submits 

and EPA approves a SIP with an 

attainment demonstration which fully 

‘‘addresses’’ the attainment requirements 

of the revised SO2 NAAQS is flawed. 

The commenter states that EPA’s use of 

the term ‘‘addresses’’ is impermissibly 

and arbitrarily ambiguous and that the 

agency needs to clarify that ‘‘fully 

addressing’’ the attainment requirements 

of the revised NAAQS actually means 

providing for timely attainment of the 

NAAQS, and the submittal of a SIP that 

fully meets all of the requirements of 

section 110 and part D of Title I of the 

CAA, including sections 172, 173, and 

191–193 of the CAA. 

Another commenter stated that the 

24-hour SO2 standard should not be 

revoked in attainment areas until EPA 

approves section 110(a)(2) 

‘‘infrastructure’’ SIPs under the new 1- 

hour standard for such areas, in order to 

avoid delays in between attainment 

designation and such SIP approvals 

resulting in leaving the public 

unprotected or creating inter-state 

conflict that triggers section 126 

petitions. This commenter further stated 

that the annual SO2 standard should not 

be revoked until EPA approves SIPs in 

attainment areas under the future SO2

secondary standard, which may also be 

based on an annual averaging time. 

EPA agrees with the comment made 

by the commenter regarding the need to 

approve SIPs in nonattainment areas 

(and in SIP call areas) before revoking 

the 24-hour and annual NAAQS for 

such areas. EPA clarifies that for those 

areas designated as nonattainment for 

the current NAAQS, or areas which 

have not met the requirements of a SIP 

call, that the State must submit a SIP 

that meets all of the applicable CAA 

requirements as they relate to section 

110 and part D of Title I of the CAA, 

including sections 110(a), 172, 173, and 

191–193 of the CAA. In addition to the 


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submittal of the SIP related to these 

requirements, EPA must approve the 

submittal for the area before the current 

standard can be revoked for the affected 

area. 

EPA disagrees with the comment. 

This rulemaking concerns only the 

primary standards for SO2. 74 FR at 

64812 n. 2. The annual SO2 standard is 

a primary standard, not a secondary 

standard. See 40 CFR section 50.4 (a). 

The exclusive secondary standard for 

SO2 is the 3-hour standard codified in 

40 CFR section 50.5. EPA is not 

determining the adequacy of this 

secondary standard in this review or 

this rulemaking, as just noted. The 

commenter’s request to retain the 

annual primary standard until SIPs 

reflecting a new secondary standard are 

approved is effectively a request to 

amend the present secondary standard, 

and is therefore inappropriate given the 

scope of this review. In any case, in the 

event that any substantive responsive to 

this comment is required, air quality 

information indicates that a 1-hour 

standard of 75 ppb is estimated to 

generally keep annual SO2

concentrations well below the level of 

the current annual standard. 74 FR at 

64845. Thus, there would be no loss of 

protection to public welfare due to 

revocation of the annual primary 

standard. 

EPA further disagrees with the 

commenter’s request that we not revoke 

the 24-hour standard in attainment areas 

before section 110(a)(2) ‘‘infrastructure’’ 

SIPs are approved under the new 1-hour 

SO2 standard. An area that has shown 

it has attained the 24-hour standard and 

that is not the subject of a SIP call, even 

after revocation of the 24-hour standard, 

will still have in its SIP its prior 

‘‘infrastucture’’ SIP elements. There is no 

need to delay revocation when that will 

not cause the area to become subject to 

a new SIP under the new 1-hour 

NAAQS any faster than the statute 

already requires (i.e., three years from 

the date of promulgation of the new 

NAAQS). Furthermore, as we have 

explained in sections III, IV, V and VI 

of this preamble, all areas are required 

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

to submit such SIPs by June 2013, and 

we expect that to be approved they will 

all need to show attainment, 

implementation, maintenance and 

enforcement of the new NAAQS as 

expeditiously as practicable, which we 

believe is no later than August 2017. 

EPA believes this anticipated approach 

would more than sufficiently address 

the backsliding concerns raised by the 

commenter. 

c. Final 

EPA is making no changes to the 

proposed rule’s discussion of the 

transition strategy discussion for SO2

with the exception of the clarifications 

noted above. 

VII. Appendix T—Interpretation of the 

Primary NAAQS for Oxides of Sulfur 

and Revisions to the Exceptional Events 

Rule 

EPA proposed to add Appendix T, 

Interpretation of the Primary National 

Ambient Air Quality Standards for 

Oxides of Sulfur, to 40 CFR Part 50 in 

order to provide monitoring data 

handling procedures for the proposed 

SO2 1-hour primary standard. The 

proposed section 50.17 which sets the 

averaging period, level, indicator, and 

form of the NAAQS referred to this 

Appendix T. The proposed Appendix T 

detailed the computations necessary for 

determining when the proposed 1-hour 

primary SO2 NAAQS is met based on 

data from ambient monitoring and also 

addressed monitoring data reporting, 

data completeness considerations, and 

rounding conventions. 

EPA proposed two versions of 

Appendix T. The first applied to a 1- 

hour primary standard based on the 

annual 4th high value form, while the 

second applied to a 1-hour primary 

standard based on the 99th percentile 

daily value form. The final version of 

the Appendix reflects our choice to 

adopt the 99th percentile daily form (see 

section II. E.3 above). 

For the 1-hour primary standard, EPA 

proposed monitoring data handling 

procedures, a cross-reference to the 

Exceptional Events Rule, a grant of 

discretion for the Administrator to 

consider otherwise incomplete 

monitoring data to be complete, and a 

provision addressing the possibility of 

there being multiple SO2 monitors at 

one site. EPA is finalizing these 

proposals, with one change from the 

proposal with regard to the multiple 

monitor provision. 

EPA is also making certain drafting 

changes to the proposed regulatory text 

to clarify certain points and to assure 

that the regulatory text conforms with 

EPA’s intentions as stated in the 

preamble. Specifically, EPA has slightly 

edited the text of the rule from that 

proposed by adding the phrase ‘‘at an 

ambient air monitoring site’’ to section 

50.17 (b) and to section 1.1 of Appendix 

T to part 50, and also by adding a 

section 50.17 (c) stating that the level of 

the standard is to be measured by an 

FRM found in Appendix A or A–1 to 

Part 50, or by a properly designated 

FEM. Both of these provisions are being 

added to conform the text of the new 1- 

hour standard to the language of other 

NAAQS. See. e.g. the text of the 8-hour 

primary standard for ozone in section 

50.10 (a) and (b). The reference to ‘‘at an 

ambient monitoring site’’ makes clear 

that the regulatory text refers to 

situations where compliance with a 

NAAQS is measured by means of 

monitoring. This text does not restrict or 

otherwise address approaches which 

EPA or States may use to implement the 

new 1-hour NAAQS, which may 

include, for example, use of modeling 

(see sections III—VI above). See CAA 

sections 107 (d) (3) (A) (any ‘‘air quality 

data’’ may be used for redesignations); 

110 (a) (1) (which does not address the 

issue of the types of data States may use 

in devising plans for implementation, 

maintenance, and enforcement of a 

primary NAAQS); 192 (a) (which does 

not specify the types of data that may 

support a demonstration that a nonattainment area has attained a NAAQS). 

Similarly, EPA notes that Appendix T 

applies when ambient monitoring data 

is gathered and utilized in support of 

the new 1-hour SO2 NAAQS. As noted 

in sections III, IV, V, and VI above, there 

are circumstances when EPA is 

considering use of modeling in the SO2

NAAQS implementation effort, and 

other considerations would apply if and 

to the extent modeling is utilized. 

The EPA is also making SO2-specific 

changes to the deadlines in 40 CFR 

50.14, by which States must flag 

ambient air data that they believe have 

been affected by exceptional events and 

submit initial descriptions of those 

events, and to the deadlines by which 

States must submit detailed 

justifications to support the exclusion of 

those data from EPA monitoring-based 

determinations of attainment or 

nonattainment with the NAAQS. 

A. Interpretation of the Primary NAAQS 

for Oxides of Sulfur 

The purpose of a monitoring data 

interpretation rule for the SO2 NAAQS 

is to give effect to the form, level, 

averaging time, and indicator specified 

in the regulatory text at 40 CFR 50.17, 

anticipating and resolving in advance 

various future ambiguities that could 

otherwise occur regarding use of 

ambient monitoring data. The new 

Appendix T provides definitions and 

requirements that apply to the new 1- 

hour primary standard for SO2. The 

requirements concern how ambient 

monitoring data are to be reported, what 

ambient monitoring data are to be 

considered (including the issue of 

which of multiple monitors’ data sets 

will be used when more than one 

monitor has operated at a site), and the 


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applicability of the Exceptional Events 

Rule to the primary SO2 NAAQS. 

1. Proposed Interpretation of the 

Standard Based on Data From Ambient 

Monitoring 

With regard to monitoring data 

completeness for the proposed 1-hour 

primary standard, the proposed 

Appendix T followed past EPA practice 

for other NAAQS pollutants by 

requiring that in general at least 75% of 

the monitoring data that should have 

resulted from following the planned 

monitoring schedule in a period must be 

available for the key air quality statistic 

from that period to be considered valid. 

For the 1-hour primary SO2 NAAQS, the 

key air quality statistics are the daily 

maximum 1-hour concentrations in 

three successive years. It is important 

that sampling within a day encompass 

the period when concentrations are 

likely to be highest and that all seasons 

of the year are well represented. Hence, 

the 75% requirement was proposed to 

be applied at the daily and quarterly 

levels. 

Recognizing that there may be years 

with incomplete data, the proposed 

Appendix T for the 99th percentile form 

provided that a design value derived 

from incomplete monitoring data will 

nevertheless be considered valid if the 

relevant one of two diagnostic 

substitution tests validated such a 

design value as being either above the 

NAAQS level or equal to or below the 

NAAQS level. 

The first proposed diagnostic data 

substitution test, relevant when the 

design value derived from incomplete 

data was equal to or below the NAAQS 

level, was intended to identify those 

cases with incomplete monitoring data 

in which it nevertheless is very likely, 

if not virtually certain, that the daily 1- 

hour design value would have been 

observed to be less than or equal to the 

level of the NAAQS if monitoring data 

had been minimally complete. This test 

involved the substitution of a high 

historical concentration for any missing 

data. The second proposed diagnostic 

data substitution test, relevant when the 

design value derived from incomplete 

data was above the NAAQS level, was 

intended to identify those cases with 

incomplete monitoring data in which it 

nevertheless is very likely, if not 

virtually certain, that the daily 1-hour 

design value would have been observed 

to be above the level of the NAAQS if 

monitoring data had been minimally 

complete. This test involved the 

substitution of a low historical 

concentration for any missing data. 

It should be noted that one possible 

outcome of applying the relevant 

proposed substitution test is that a 3- 

year period with incomplete monitoring 

data may nevertheless be determined to 

not have a valid design value and thus 

to be unusable in making 1-hour 

primary NAAQS compliance 

determinations based on monitoring for 

that 3-year period. 

Also, we proposed that the 

Administrator have general discretion to 

use incomplete monitoring data based 

on case specific factors, either at the 

request of a State or at her own 

initiative. Similar provisions existed 

already for some other NAAQS. 

The 99th percentile version of the 

proposed Appendix T provided a table 

for determining which day’s maximum 

1-hour concentration will be used as the 

99th percentile concentration for the 

year. The proposed table is similar to 

one used now for the 24-hour PM2.5

NAAQS and the new 1-hour NO2

NAAQS, which are both based on a 98th 

percentile form, but adjusted to reflect 

a 99th percentile form for the 1-hour 

primary SO2 standard. The proposed 

Appendix T also provided instructions 

for rounding (not truncating) the average 

of three annual 99th percentile hourly 

concentrations before comparison to the 

level of the primary NAAQS. 

2. Comments on Interpretation of the 

Standard 

Several commenters expressed 

support for EPA’s proposed 75% 

completeness requirement for daily and 

quarterly monitoring data. A comment 

was received that the substitution test 

should not be used to make attainment 

or non-attainment designations. This 

commenter also said that the same 

completeness requirement as used for 

nonattainment should be used for 

attainment. Another commenter agreed 

that there should be completeness 

criteria, but thought that monitoring 

data should be substituted to make the 

set only 75% complete. We received one 

comment that the computation of design 

values where multiple monitors are 

present at a site should be averaged and 

not taken from a designated primary 

monitor. We received no comment on 

the provision which would afford the 

Administrator (or her delegee) 

discretion to use incomplete monitoring 

data based on specified factors and 

accordingly are adopting that provision 

as proposed. 

3. Conclusions on Interpretation of the 

Standard 

Consistent with the Administrator’s 

decision to adopt a 99th percentile form 

for the 1-hour NAAQS, the final version 

of Appendix T is based on that form. 

We agree with the three comments 

expressing the view that the 

requirement for 75% monitoring data 

completeness per quarter should apply 

with respect to the 1-hour standard. The 

final rule includes this requirement. 

We agree that nonattainment based on 

data from ambient monitoring should 

not be declared without a very high 

confidence that actual air quality did 

not meet the NAAQS, but we believe the 

proposed (and final) substitution test 

provides this irrefutable proof. In the 

relevant substitution test (Appendix T 

section 3.c.iii), the lowest daily 

maximum concentration observed in the 

same calendar quarter within the 3-year 

period is the value used in the 

substitution. Moreover, to guard against 

the possibility that even this lowest 

observed value is unrepresentative 

because only a small number of days 

that happened to have had poor air 

quality have valid monitoring data, 

substitution is permitted only if there 

are at least 200 days across the three 

matching quarters of the three years 

under consideration for which 75 

percent of the hours in the day have 

reported concentrations. (If less than 

200 days are available, the outcome is 

that no conclusion can be reached based 

on data from monitoring as to whether 

the NAAQS is met, an outcome which 

satisfies the concern expressed by the 

commenter.) While it is conceivable that 

the actual daily maximum concentration 

on the day(s) without sufficiently 

complete data could have been even 

lower than the value selected as the 

substitute value, the value that is 

selected for substitution will be quite 

low, and therefore it is extremely 

unlikely to be a candidate for selection 

as the annual 99th percentile daily 

maximum concentration. The actual 

effect of the data substitution, if any, is 

to change which of the actually 

observed and ranked daily maximum 

concentrations during the year is 

identified as the 99th percentile; the 

direction of the change, if any, will 

always be towards a lower design value. 

For example, if the substitution test of 

section 3.c.iii is used because there is 

one quarter of 92 days is missing 70 of 

its 92 daily maximum concentration 

values; causing there to be only 295 

days with valid daily values for the 

whole year, it would be necessary to 

substitute 47 values to make that quarter 

75 percent complete. This would result 

in 343 days of actual or substituted 

monitoring data for the year. The 

increase from 292 days to 342 days 

would cause the annual 99th percentile 

value to shift from the 3rd highest value 

to the 4th highest. Since a low 


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42Selecting the maximum or minimum observed 

concentration for an hour, the maximum or 

minimum annual 99th percentile, or the maximum 

or minimum three-year design value would 

introduce such a bias. Averaging multiple 1-hour 

measurements when available, designating one 

monitor as primary and using a second monitor’s 

measurement only when the primary monitor fails 

to give a valid measurement, or simply choosing to 

use the data record from only one of the monitors 

(on some basis that is independent of the 

concentration values obtained) would not introduce 

such a bias. 

concentration is being used for the 

substitution, it is impossible for the 4th 

highest value to itself be a substituted 

value. If this shift results in the 3-year 

design value remaining above the 

NAAQS, the failure to meet the NAAQS 

is confirmed. If this shift results in the 

3-year design value changing to be equal 

to or below the NAAQS, under the 

terms of the substitution test the 

outcome is that no conclusion could be 

reached based on this ambient 

monitoring data as to whether the 

NAAQS is met. Since either the same or 

a lower ranking actually measured 

concentration will always be identified, 

it is impossible for the outcome of the 

substitution test of section 3.c.iii to be 

that an area truly meeting the NAAQS 

based on ambient monitoring data is 

determined to not meet it based on 

ambient monitoring data. 

The commenter who said that the 

same completeness requirement should 

be used for nonattainment as for 

attainment appears to have been 

referring to a particular feature of the 

proposed diagnostic substitution test 

rather than to the basic completeness 

requirement of 75%, which in both the 

proposal and the final rule applies 

equally to both attainment and 

nonattainment situations. This 

particular feature is discussed in the 

next paragraph. 

The commenter who said that it is 

appropriate to substitute data to make 

the set only 75% complete appears to 

have taken note that in the proposed 

substitution test relevant in the case of 

an incomplete design value equal to or 

below the NAAQS (section 3.c.ii), data 

are substituted until 100% 

completeness is reached for the affected 

quarter, while in the test relevant in the 

case of an incomplete design value 

above the NAAQS (section 3.c.iii) data 

are substituted only until 75% 

completeness is reached. EPA believes 

this distinction is appropriate, and we 

have retained the 100% substitution 

limit in the final rule. In the case of an 

incomplete design value that is equal to 

or below the NAAQS, the concern is 

that the actual concentrations on the 

days without a valid daily maximum 1- 

hour concentration may have been quite 

high such that the concentration on one 

of those days would have been selected 

as the annual 99th percentile value. To 

be selected as the annual 99th percentile 

value, a daily maximum must be ranked 

no lower than the 4th highest daily 

value for the year. If substitution 

stopped when 75% of the days in a 

quarter had an actual or substituted 

value, there could be a situation in 

which only one, two, or three historical 

high values would need to be 

substituted to reach the 75% limit. It 

would therefore be possible for one of 

the actually measured concentrations 

(for the same or another quarter) to be 

identified as the annual 99th percentile 

value even if the substitution value is 

higher than any value actually 

measured, defeating the very purpose of 

the diagnostic test for an incomplete 

design value below the NAAQS, which 

is to essentially rule out the possibility 

of not meeting the NAAQS (when 

making monitoring-based 

determinations). The simplest way to 

ensure that at least four values are 

substituted (when there are at least four 

missing daily values) is to require 

substitution up to the 100% limit. 

With regard to situations with 

multiple monitors operating at one site, 

we note that there are few cases of this 

situation for SO2 monitoring. Of over 

500 SO2 monitoring sites in operation 

any time during 2007–2009, for 

example, only seven stations reported 1- 

hour data to the Air Quality System 

under two or more distinct Pollutant 

Occurrence Codes (POC). In the same 

period, collocated monitors reported 

data to AQS under distinct POCs for 

only one of over 400 nitrogen dioxide 

sites, for only two of almost 400 carbon 

monoxide sites, and for only eight of 

almost 1300 ozone sites. Even so, we 

believe is it important to have a well 

defined monitor data handling 

procedure for such situations. Also, 

there is a practical advantage in 

implementation if the same or similar 

procedure is used across NAAQS 

pollutants especially for these four 

gaseous pollutants that are measured on 

a 1-hour basis. A procedure that is 

simple to implement also has 

advantages in implementation. Finally, 

the procedure should not introduce any 

upward or downward bias in the 

determination of the design value for 

the monitoring site.42

The proposed procedure for multiple 

SO2 monitors was the same as EPA 

recently proposed and finalized for the 

new 1-hour NAAQS for nitrogen 

dioxide, where there were no adverse 

comments received on the proposal (75 

FR 6474, February 9, 2010). It is also the 

same as recently proposed in the 

reconsideration of the 8-hour ozone 

NAAQS (75 FR 2938, January 19, 2010). 

In the proposed procedure, in general, 

data from two monitors would never be 

mixed within a year but data from 

different monitors in different years 

could be used to calculate the 3-year 

design value. As noted above, one 

commenter on the SO2 proposal 

suggested that instead of designating a 

primary monitor when there are two 

monitors at a site, the measurements for 

an hour from multiple monitors should 

be averaged instead. EPA has also 

received at least one comment 

disagreeing with the recent proposal 

regarding multiple ozone monitors. The 

comment in the ozone rulemaking 

favored hour-by-hour substitution of 

data from a secondary monitor when the 

designated primary monitor has not 

given a value measurement, as opposed 

to the proposed restriction against 

mixing data within a year. These 

comments have caused us to rethink the 

direction set in the final NO2 rule and 

in the proposals for SO2 and ozone. We 

now believe that substitution of 

monitoring data hour-by-hour is an 

acceptable and in some ways superior 

approach to the other possible 

approaches, while averaging hour-byhour would be unduly complex. Also, 

averaging hour-by-hour might not be 

transparent depending on whether the 

averaging is done at the monitoring 

agency before submission to EPA or by 

EPA as part of calculating a design 

value. However, in light of the rarity of 

collocated monitors, it would be an 

unwarranted demand on limited EPA 

resources to develop and maintain 

software for hour-by-hour data 

substitution. Also, an hour-by-hour data 

substitution approach depends on the 

advance designation of a primary 

monitor, which itself could introduce 

confusion and would require software 

changes to EPA’s data system. 

Therefore, EPA believes that the most 

practical, and still a technically valid 

approach, is to allow monitoring 

agencies the option of hour-by-hour 

substitution between secondary and 

primary monitors before submission of 

data to EPA, and for EPA to select for 

use in calculating design values the one 

monitoring data record which has the 

highest degree of completeness for a 

given year. The final rule is based on 

this approach. EPA will also consider 

this approach when finalizing the ozone 

NAAQS reconsideration rule, and when 

proposing data interpretation provisions 

for a planned rulemaking to review the 

carbon monoxide NAAQS. The already 

finalized procedures for nitrogen 

dioxide data interpretation will be 


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implemented as promulgated, but will 

affect only an extremely small number 

of collocated SO2 monitoring situations. 

Finally, as proposed, the final version 

of Appendix T has a cross reference to 

the Exceptional Events Rule (40 CFR 

50.14) with regard to the exclusion of 

monitoring data affected by exceptional 

events. In addition, the specific steps for 

including such data in completeness 

calculations while excluding such data 

from actual design value calculations is 

clarified in Appendix T. 

B. Exceptional Events Information 

Submission Schedule 

The Exceptional Events Rule at 40 

CFR 50.14 contains generic deadlines 

for a State to submit to EPA specified 

information about exceptional events 

and associated air pollutant 

concentration data. A State must 

initially notify EPA that data have been 

affected by an event by July 1 of the 

calendar year following the year in 

which the event occurred; this is done 

by flagging the data in AQS and 

providing an initial event description. 

The State must also, after notice and 

opportunity for public comment, submit 

a demonstration to justify any claim 

within 3 years after the quarter in which 

the data were collected. However, if a 

regulatory decision based on the data 

(for example, a designation action) is 

anticipated, the schedule to flag data in 

AQS and submit complete 

documentation to EPA for review is 

shortened, and all information must be 

submitted to EPA no later than one year 

before the decision is to be made. 

These generic deadlines are suitable 

for the period after initial designations 

have been made under a NAAQS, when 

the decision that may depend on data 

exclusion is a redesignation from 

attainment to nonattainment or from 

nonattainment to attainment. However, 

these deadlines present problems with 

respect to initial designations under a 

newly revised NAAQS. One problem is 

that some of the deadlines, especially 

the deadlines for flagging some relevant 

data, may have already passed by the 

time the revised NAAQS is 

promulgated. Until the level and form of 

the NAAQS have been promulgated a 

State does not know whether the criteria 

for excluding data (which are tied to the 

level and form of the NAAQS) were met 

on a given day. Another problem is that 

it may not be feasible for information on 

some exceptional events that may affect 

final designations to be collected and 

submitted to EPA at least one year in 

advance of the final designation 

decision. This could have the 

unintended consequence of EPA 

designating an area nonattainment 

because of uncontrollable natural or 

other qualified exceptional events. 

The Exceptional Events Rule at 

section 50.14(c)(2)(v) indicates ‘‘when 

EPA sets a NAAQS for a new pollutant, 

or revises the NAAQS for an existing 

pollutant, it may revise or set a new 

schedule for flagging data for initial 

designation of areas for those NAAQS.’’ 

For the specific case of SO2, the 

signature date for the revised SO2

NAAQS is June 2, 2010. State/Tribal 

area designations recommendations will 

be due by June 2, 2011, and EPA will 

make initial area designations under the 

revised NAAQS by June 1, 2012 (since 

June 2, 2012 would be on a Saturday) 

and will be informed by air quality data 

from the years 2008–2010 or 2009–2011 

if there is sufficient data for these data 

years and by any refined modeling that 

is conducted. (See Sections III, V and VI 

above for more detailed discussions of 

the designation schedule and what data 

EPA expects to use.) Because final 

designations would be made by June 1, 

2012, all events to be considered during 

the designations process would have to 

be flagged and fully documented by 

States one year prior to designations, by 

June 1, 2011. A State would not be able 

to flag and submit documentation 

regarding events that occurred between 

June to December 2011 by one year 

before designations are made in June 

2012. 

EPA is adopting revisions to 40 CFR 

50.14 only to change submission dates 

for information supporting claimed 

exceptional events affecting SO2 data. 

The rule text at the end of this notice 

shows the changes that will apply to the 

new 1-hour SO2 NAAQS. For air quality 

data collected in 2008, we are extending 

the generic July 1, 2009 deadline for 

flagging data (and providing a brief 

initial description of the event) to 

October 1, 2010. EPA believes this 

extension will provide adequate time for 

States to review the impact of 

exceptional events from 2008 on the 

revised standard and notify EPA by 

flagging the relevant data in AQS. EPA 

is not changing the foreshortened 

deadline of June 1, 2011 for submitting 

documentation to justify an SO2-related 

exceptional event from 2008. We believe 

the generic deadline provides adequate 

time for States to develop and submit 

proper documentation. 

For data collected in 2009, EPA is 

extending the generic deadline of July 1, 

2010 for flagging data and providing 

initial event descriptions to October 1, 

2010. EPA is retaining the deadline of 

June 1, 2011 for States to submit 

documentation to justify an SO2-related 

exceptional event from 2009. For data 

collected in 2010, EPA is promulgating 

a deadline of June 1, 2011 for flagging 

data and providing initial event 

descriptions and for submitting 

documentation to justify exclusion of 

the flagged data. EPA believes that this 

deadline provides States with adequate 

time to review and identify potential 

exceptional events that occur in 

calendar year 2010, even for those 

events that might occur late in the year. 

EPA believes these deadlines will be 

feasible because experience suggests 

that exceptional events affecting SO2

data are few in number and easily 

assessed, so no State is likely to have a 

large workload. 

If a State intends 2011 data to be 

considered in SO2 designations, 2011 

data must be flagged and detailed event 

documentation submitted 60 days after 

the end of the calendar quarter in which 

the event occurred or by March 31, 

2012, whichever date occurs first. 

Again, EPA believes these deadlines 

will be feasible because experience 

suggest that exceptional events affecting 

SO2 data are few in number and easily 

assessed, so no State is likely to have a 

large workload. 

Table 1 summarizes the designation 

deadlines discussed in this section and 

provides designation schedule 

information from recent, pending or 

prior NAAQS revisions for other 

pollutants. EPA is revising the final SO2

exceptional event flagging and 

documentation submission deadlines 

accordingly to provide States with 

reasonably adequate opportunity to 

review, identify, and document 

exceptional events that may affect an 

area designation under a revised 

NAAQS. 


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TABLE 1—SCHEDULE FOR EXCEPTIONAL EVENT FLAGGING AND DOCUMENTATION SUBMISSION FOR DATA TO BE USED IN

DESIGNATIONS DECISIONS FOR NEW OR REVISED NAAQS 

NAAQS 

pollutant/standard/(level)/promulgation 

date 

Air quality data 

collected for 

calendar year 

Event flagging & initial description 

deadline 

Detailed documentation submission 

deadline 

PM2.5/24-Hr Standard (35 μg/m3) Promulgated October 17, 2006.2004–2006 October 1, 2007 a .................................... April 15, 2008 a. 

Ozone/8-Hr Standard (0.075 ppm) Promulgated March 12, 2008.2005–2007 

2008 

June 18, 2009 a .......................................

June 18, 2009 a .......................................

June 18, 2009 a. 

June 18, 2009 a. 

2009 60 Days after the end of the calendar 

quarter in which the event occurred or 

February 5, 2010, whichever date occurs first b.

60 Days after the end of the calendar 

quarter in which the event occurred or 

February 5, 2010, whichever date occurs first b. 

NO2/1-Hour Standard (80–100 PPB, final 

level TBD).

2008 

2009 

July 1, 2010 a ..........................................

July 1, 2010 a ..........................................

January 22, 2011 a. 

January 22, 2011 a. 

2010 April 1, 2011 a ......................................... July 1, 2011 a. 

SO2/1-Hour Standard (50–100 PPB, final 

level TBD).

2008 

2009 

October 1, 2010 b ....................................

October 1, 2010 b ....................................

June 1, 2011 b. 

June 1, 2011 b. 

2010 June 1, 2011 b ......................................... June 1, 2011 b. 

2011 60 Days after the end of the calendar 

quarter in which the event occurred or 

March 31, 2012, whichever date occurs first b.

60 Days after the end of the calendar 

quarter in which the event occurred or 

March 31, 2012, whichever date occurs first b. 

aThese dates are unchanged from those published in the original rulemaking, and are shown in this table for informational purposes—the 

Agency is not opening these dates for comment under this rulemaking. b Indicates change from general schedule in 40 CFR 50.14. 

Note: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most notably, for data used by EPA for redesignations to attainment. 

Note further that EPA is reprinting 

portions of this Table in section 5014 

but, with respect to the pollutants other 

than SO2, is doing so only for readers’ 

convenience and is not reopening or 

otherwise reconsidering any aspect of 

the rules related to these other 

pollutants. 

VIII. Communication of Public Health 

Information 

Information on the public health 

implications of ambient concentrations 

of criteria pollutants is currently made 

available primarily through EPA’s Air 

Quality Index (AQI) program. The 

current AQI has been in use since its 

inception in 1999 (64 FR 42530). It 

provides accurate, timely, and easily 

understandable information about daily 

levels of pollution (40 CFR 58.50). The 

AQI establishes a nationally uniform 

system of indexing pollution levels for 

nitrogen dioxide, carbon monoxide, 

ozone, particulate matter and SO2. The 

AQI converts pollutant concentrations 

in a community’s air to a number on a 

scale from 0 to 500. Reported AQI 

values enable the public to know 

whether air pollution levels in a 

particular location are characterized as 

good (0–50), moderate (51–100), 

unhealthy for sensitive groups (101– 

150), unhealthy (151–200), very 

unhealthy (201–300), or hazardous 

(300–500). The AQI index value of 100 

typically corresponds to the level of the 

short-term primary NAAQS for each 

pollutant. An AQI value greater than 

100 means that a pollutant is in one of 

the unhealthy categories (i.e., unhealthy 

for sensitive groups, unhealthy, very 

unhealthy, or hazardous) on a given 

day; an AQI value at or below 100 

means that a pollutant concentration is 

in one of the satisfactory categories (i.e., 

moderate or good). Decisions about the 

pollutant concentrations at which to set 

the various AQI breakpoints, that 

delineate the various AQI categories, 

draw directly from the underlying 

health information that supports the 

review of the primary NAAQS. 

The Agency recognizes the 

importance of revising the AQI in a 

timely manner to be consistent with any 

revisions to the primary NAAQS. 

Therefore, EPA proposed to finalize 

conforming changes to the AQI in 

connection with the Agency’s final 

decision on the SO2 NAAQS. 

Conforming changes that were proposed 

include setting the 100 level of the AQI 

at the same level as the revised primary 

SO2 standard if a short-term primary 

standard was promulgated, and revising 

the other AQI breakpoints at the lower 

end of the AQI scale (i.e., AQI values of 

50 and 150). EPA did not propose to 

change breakpoints at the higher end of 

the AQI scale (from 200 to 500), which 

would apply to State contingency plans 

or the Significant Harm Level (40 CFR 

51.16), because the information from 

this review does not inform decisions 

about breakpoints at those higher levels. 

With regard to an AQI value of 50, the 

breakpoint between the good and 

moderate categories, historically this 

value is set at the level of the annual 

NAAQS, if there is one, or one-half the 

level of the short-term NAAQS in the 

absence of an annual NAAQS (63 FR 

67823, Dec. 12, 1998). Taking into 

consideration this practice, EPA 

proposed to set the AQI value of 50 to 

be between 25 and 50 ppb SO2, 1-hour 

average; stating that concentrations 

toward the lower end of this range 

would be appropriate if the standard 

was set at the lower end of the range of 

proposed standard levels, while 

concentrations toward the higher end of 

this range would be more appropriate if 

the standard was set at the higher end 

of the range of proposed standard levels. 

EPA solicited comments on this range 

for an AQI value of 50 and the 

appropriate basis for selecting an AQI 

value of 50. 

With regard to an AQI value of 150, 

the breakpoint between the unhealthy 

for sensitive groups and unhealthy 

categories, historically values between 

the short-term standard and an AQI 

value of 500 are set at levels that are 

approximately equidistant between the 

AQI values of 100 and 500 unless there 

is health evidence that suggests a 

specific level would be appropriate (63 

FR 67829, Dec. 12, 1998). For an AQI 

value of 150, EPA proposed to set the 

breakpoint within the range from 175 to 

200 ppb SO2, 1-hour average, since it 

represents the midpoint between the 

proposed range for the short-term 


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standard and the level of an AQI value 

of 200 (300 ppb SO2, 1-hour average). 

EPA received few comments on the 

proposed breakpoints. Consistent with 

the level of the short-term primary SO2

standard promulgated in this rule, EPA 

is setting the AQI value of 100, the 

breakpoint between the moderate and 

unhealthy for sensitive groups category, 

at 75 ppb, 1-hour average. EPA is setting 

the AQI value of 50, the breakpoint 

between the good and moderate 

categories, at 35 ppb SO2, 1-hour 

average, which is approximately onehalf the level of the new short-term 

standard, since the annual SO2 standard 

is being revoked. EPA is setting the AQI 

value of 150, the breakpoint between the 

unhealthy for sensitive groups and 

unhealthy categories, at 185 ppb SO2, 1- 

hour average, which represents the 

approximate midpoint between the level 

of the new short-term standard (75 ppb 

SO2, 1-hour average) and the level of an 

AQI value of 200 (300 ppb SO2, 1-hour 

average). 

EPA received comments from several 

State environmental organizations and 

organizations of State and local air 

agencies about forecasting and reporting 

the AQI for SO2. These commenters 

expressed the view that forecasting 

hourly SO2 concentrations would be 

difficult. One commenter requested that 

EPA delay the forecasting requirement 

for one year and other agencies 

requested that EPA provide assistance 

in developing a forecast model. Another 

commenter expressed the view that it is 

impractical to incorporate SO2 into its 

forecasting and public health 

notification program because SO2 does 

not behave like a regional pollutant, and 

that exceedances may occur with little 

or no warning and for two hours or less. 

This commenter requested EPA 

consider the resources necessary for 

public communications at the State and 

local levels, particularly in areas where 

other air quality exceedances are 

relatively rare. 

EPA recommends and encourages air 

quality forecasting but it is not required 

(64 FR 42548; August 4, 1999). We agree 

that there will be new challenges 

associated with creating and 

communicating an SO2 forecast, and 

will work with State and local agencies 

that want to develop an SO2 forecasting 

program on issues including, but not 

limited to, forecasting air quality for 

short time periods. We plan to work 

with State and local air agencies to 

figure out the best way to present this 

information to the public using the AQI. 

With respect to the comment that it is 

impractical to incorporate SO2 into a 

forecasting and public health 

notification program because SO2 does 

not behave like a regional pollutant, this 

final rule departs from the proposed 

rule in that it allows for a combined 

monitoring and modeling approach. 

Because of this, the monitoring network 

is not required to be wholly sourceoriented in nature. States have 

flexibility to allow required monitoring 

sites to serve multiple monitoring 

objectives including characterizing 

source impacts, highest concentrations, 

population exposure, background, and 

regional transport. Further, EPA expects 

that much of the existing network will 

be retained by States to satisfy the 

minimum monitoring requirements. 

This means that it is unlikely that AQI 

reporting and forecasting will be heavily 

driven by source-oriented monitors. 

Rather, many of the existing monitors (a 

majority of which are community-wide 

monitors) will remain in place, which 

prevents the need for new geographic 

regions to be delineated. With respect to 

concerns expressed about the resources 

required to report the AQI in areas were 

exceedances of the standard are very 

rare, Appendix G to Part 58 specifies 

that if the index value for a particular 

pollutant remains below 50 for a season 

or year, then a State or local agency may 

exclude the pollutant from the 

calculation of the AQI. 

IX. Statutory and Executive Order 

Reviews 

A. Executive Order 12866: Regulatory 

Planning and Review 

Under section 3(f)(1) of Executive 

Order 12866 (58 FR 51735, October 4, 

1993), this action is an ‘‘economically 

significant regulatory action’’ because it 

is likely to have an annual effect on the 

economy of $100 million or more. 

Accordingly, EPA submitted this action 

to the Office of Management and Budget 

(OMB) for review under EO 12866 and 

any changes made in response to OMB 

recommendations have been 

documented in the docket for this 

action. In addition, EPA prepared a 

Regulatory Impact Analysis (RIA) of the 

potential costs and benefits associated 

with this action. However, the CAA and 

judicial decisions make clear that the 

economic and technical feasibility of 

attaining the national ambient standards 

cannot be considered in setting or 

revising NAAQS, although such factors 

may be considered in the development 

of State implementation plans to 

implement the standards. Accordingly, 

although an RIA has been prepared, the 

results of the RIA have not been 

considered by EPA in developing this 

final rule. 

When estimating the SO2- and PM2.5- 

related human health benefits and 

compliance costs in Table 2 below, EPA 

applied methods and assumptions 

consistent with the state-of-the-science 

for human health impact assessment, 

economics and air quality analysis. EPA 

applied its best professional judgment 

in performing this analysis and believes 

that these estimates provide a 

reasonable indication of the expected 

benefits and costs to the nation of the 

selected SO2 standard and alternatives 

considered by the Agency. The 

Regulatory Impacts Analysis (RIA) 

available in the docket describes in 

detail the empirical basis for EPA’s 

assumptions and characterizes the 

various sources of uncertainties 

affecting the estimates below. 

EPA’s 2009 Integrated Science 

Assessment for Particulate Matter 

concluded, based on the scientific 

literature, that a no-threshold log-linear 

model most adequately portrays the PMmortality concentration-response 

relationship. Nonetheless, consistent 

with historical practice and our 

commitment to characterizing the 

uncertainty in our benefits estimates, 

EPA has included a sensitivity analysis 

with an assumed threshold in the PMmortality health impact function in the 

RIA. EPA has included a sensitivity 

analysis in the RIA to help inform our 

understanding of the health benefits 

which can be achieved at lower air 

quality concentration levels. While the 

primary estimate and the sensitivity 

analysis are not directly comparable, 

due to differences in population data 

and use of different analysis years, as 

well as the difference in the assumption 

of a threshold in the sensitivity analysis, 

comparison of the two results provide a 

rough sense of the proportion of the 

health benefits that occur at lower PM2.5

air quality levels. Using a threshold of 

10 μg/m3 is an arbitrary choice (EPA 

could have assumed 6, 8, or 12 μg/m3

for the sensitivity analysis). Assuming a 

threshold of 10 μg/m3, the sensitivity 

analysis shows that roughly one-third of 

the benefits occur at air quality levels 

below that threshold. Because the 

primary estimates reflect EPA’s current 

methods and data, EPA notes that 

caution should be exercised when 

comparing the results of the primary 

and sensitivity analyses. EPA 

appreciates the value of sensitivity 

analyses in highlighting the uncertainty 

in the benefits estimates and will 

continue to work to refine these 

analyses, particularly in those instances 

in which air quality modeling data are 

available. 

Table 2 shows the results of the cost 

and benefits analysis for each standard 

alternative. As indicated above, 

implementation of the SO2 control 


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measures identified from 

AirControlNET and other sources does 

not result in attainment with the all 

target NAAQS levels in several areas. In 

these areas, additional unspecified 

emission reductions might be necessary 

to reach some alternative standard 

levels. The first part of the table, labeled 

Partial attainment (identified controls), 

shows only those benefits and costs 

from control measures we were able to 

identify. The second part of the table, 

labeled Unidentified Controls, shows 

only additional benefits and costs 

resulting from unidentified controls. 

The third part of the table, labeled Full 

attainment, shows total benefits and 

costs resulting from both identified and 

unidentified controls. It is important to 

emphasize that we were able to identify 

control measures for a significant 

portion of attainment for many of those 

counties that would not fully attain the 

target NAAQS level with identified 

controls. Note also that in addition to 

separating full and partial attainment, 

the table also separates the portion of 

benefits associated with reduced SO2

exposure (i.e., SO2 benefits) from the 

additional benefits associated with 

reducing SO2 emissions, which are 

precursors to PM2.5 formation—(i.e., the 

PM2.5 co-benefits). For instance, for the 

selected standard of 75 ppb, $2.2 

million in benefits are associated with 

reduced SO2 exposure while $15 billion 

to $37 billion are associated with 

reduced PM2.5 exposure. 

TABLE 2—MONETIZED BENEFITS AND COSTS TO ATTAIN ALTERNATE STANDARD LEVELS IN 2020 

[Millions of 2006$] a

Number of 

counties 

fully 

controlled 

Discount 

rate 

(percent) 

Monetized 

SO2 benefits 

Monetized PM2.5

co-benefits,c,d Costs Net benefits 

Partial Attainment (identified controls) 

50 ppb .............................. 40 3 b $30,000 to $74,000 ... $2,600 $27,000 to $71,000. 

........................ 7 ........................ $28,000 to $67,000 ... ........................ $25,000 to $64,000. 

75 ppb .............................. 20 3 b $14,000 to $35,000 ... $960 $13,000 to $34,000. 

........................ 7 ........................ $13,000 to $31,000 ... ........................ $12,000 to $30,000. 

100 ppb ............................ 6 3 b $6,900 to $17,000 ..... $470 $6,400 to $17,000. 

........................ 7 ........................ $6,200 to $15,000 ..... ........................ $5,700 to $15,000. 

Unidentified Controls 

50 ppb .............................. 16 3 b $4,000 to $9,000 ....... $1,800 $2,200 to $7,200. 

........................ 7 ........................ $3,000 to $8,000 ....... ........................ $1,200 to $6,200. 

75 ppb .............................. 4 3 b $1,000 to $3,000 ....... $500 $500 to $1,500. 

........................ 7 ........................ $1,000 to $3,000 ....... ........................ $500 to $2,500. 

100 ppb ............................ 3 3 b $500 to $1,000 .......... $260 $240 to $740. 

........................ 7 ........................ $500 to $1,000 .......... ........................ $240 to $740. 

Full Attainment 

50 ppb .............................. 56 3 $8.50 $34,000 to $83,000 ... $4,400 $30,000 to $79,000. 

........................ 7 ........................ $31,000 to $75,000 ... ........................ $27,000 to $71,000. 

75 ppb .............................. 24 3 $2.20 $15,000 to $37,000 ... $1,500 $14,000 to $36,000 

........................ 7 ........................ $14,000 to $34,000 ... ........................ $13,000 to $33,000. 

100 ppb ............................ 9 3 $0.60 $7,400 to $18,000 ..... $730 $6,700 to $17,000. 

........................ 7 ........................ $6,700 to $16,000 ..... ........................ $6,000 to $15,000. 

a Estimates have been rounded to two significant figures and therefore summation may not match table estimates. b The approach used to simulate air quality changes for SO2 did not provide the data needed to distinguish partial attainment benefits from full 

attainment benefits from reduced SO2 exposure. Therefore, a portion of the SO2 benefits is attributable to the known controls and a portion of the 

SO2 benefits are attributable to the unidentified controls. Because all SO2-related benefits are short-term effects, the results are identical for all 

discount rates. c Benefits are shown as a range from Pope et al. (2002) to Laden et al. (2006). Monetized benefits do not include unquantified benefits, such 

as other health effects, reduced sulfur deposition, or improvements in visibility. d These models assume that all fine particles, regardless of their chemical composition, are equally potent in causing premature mortality because there is no clear scientific evidence that would support the development of differential effects estimates by particle type. Reductions in SO2

emissions from multiple sectors to meet the SO2 NAAQS would primarily reduce the sulfate fraction of PM2.5. Because this rule targets a specific 

particle precursor (i.e., SO2), this introduces some uncertainty into the results of the analysis. 

B. Paperwork Reduction Act 

The information collection 

requirements in this final rule have been 

submitted for approval to the Office of 

Management and Budget (OMB) under 

the Paperwork Reduction Act, 44 U.S.C. 

3501 et seq. The Information Collection 

Request (ICR) document prepared by 

EPA for these revisions to part 58 has 

been assigned EPA ICR number 2370.02. 

The information collected under 40 CFR 

part 53 (e.g., test results, monitoring 

records, instruction manual, and other 

associated information) is needed to 

determine whether a candidate method 

intended for use in determining 

attainment of the NAAQS in 40 CFR 

part 50 will meet the design, 

performance, and/or comparability 

requirements for designation as a 

Federal reference method (FRM) or 

Federal equivalent method (FEM). We 

do not expect the number of FRM or 

FEM determinations to increase over the 

number that is currently used to 

estimate burden associated with SO2

FRM/FEM determinations provided in 

the current ICR for 40 CFR part 53 (EPA 

ICR numbers 2370.01). As such, no 

change in the burden estimate for 40 

CFR part 53 has been made as part of 

this rulemaking. 

The information collected and 

reported under 40 CFR part 58 is needed 

to determine compliance with the 

NAAQS, to characterize air quality and 


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associated health impacts, to develop 

emissions control strategies, and to 

measure progress for the air pollution 

program. The amendments would revise 

the technical requirements for SO2

monitoring sites, require the siting and 

operation of additional SO2 ambient air 

monitors, and the reporting of the 

collected ambient SO2 monitoring data 

to EPA’s Air Quality System (AQS). The 

ICR is estimated to involve 102 

respondents for a total approximate cost 

of $15,203,762 (total capital, and labor 

and non-labor operation and 

maintenance) and a total burden of 

207,662 hours. The labor costs 

associated with these hours is 

$11,130,409. Included in the 

$15,203,762 total are other costs of other 

non-labor operations and maintenance 

of $1,104,377 and equipment and 

contract costs of $2,968,975. In addition 

to the costs at the State and local air 

quality management agencies, there is a 

burden to EPA for a total of 14,749 

hours and $1,060,621. Burden is defined 

at 5 CFR 1320.3(b). State, local, and 

Tribal entities are eligible for State 

assistance grants provided by the 

Federal government under the CAA 

which can be used for monitors and 

related activities. An agency may not 

conduct or sponsor, and a person is not 

required to respond to, a collection of 

information unless it displays a 

currently valid OMB control number. 

The OMB control numbers for EPA’s 

regulations in 40 CFR are listed in 40 

CFR part 9. 

C. Regulatory Flexibility Act 

The Regulatory Flexibility Act (RFA) 

generally requires an agency to prepare 

a regulatory flexibility analysis of any 

rule subject to notice and comment 

rulemaking requirements under the 

Administrative Procedure Act or any 

other statute unless the agency certifies 

that the rule will not have a significant 

economic impact on a substantial 

number of small entities. Small entities 

include small businesses, small 

organizations, and small governmental 

jurisdictions. 

For purposes of assessing the impacts 

of this rule on small entities, small 

entity is defined as: (1) A small business 

that is a small industrial entity as 

defined by the Small Business 

Administration’s (SBA) regulations at 13 

CFR 121.201; (2) a small governmental 

jurisdiction that is a government of a 

city, county, town, school district or 

special district with a population of less 

than 50,000; and (3) a small 

organization that is any not-for-profit 

enterprise which is independently 

owned and operated and is not 

dominant in its field. 

After considering the economic 

impacts of this final rule on small 

entities, I certify that this action will not 

have a significant economic impact on 

a substantial number of small entities. 

This final rule will not impose any 

requirements on small entities. Rather, 

this rule establishes national standards 

for allowable concentrations of SO2 in 

ambient air as required by section 109 

of the CAA. American Trucking Ass’ns 

v. EPA, 175 F.3d 1027, 1044–45 (DC Cir. 

1999) (NAAQS do not have significant 

impacts upon small entities because 

NAAQS themselves impose no 

regulations upon small entities). 

Similarly, the amendments to 40 CFR 

Part 58 address the requirements for 

States to collect information and report 

compliance with the NAAQS and will 

not impose any requirements on small 

entities. 

D. Unfunded Mandates Reform Act 

This action is not subject to the 

requirements of sections 202 and 205 of 

the UMRA. EPA has determined that 

this final rule does not contain a Federal 

mandate that may result in expenditures 

of $100 million or more for State, local, 

and Tribal governments, in the 

aggregate, or the private sector in any 

one year. The revisions to the SO2

NAAQS impose no enforceable duty on 

any State, local or Tribal governments or 

the private sector. The expected costs 

associated with the monitoring 

requirements are described in EPA’s ICR 

document, but those costs are not 

expected to exceed $100 million in the 

aggregate for any year. Furthermore, as 

indicated previously, in setting a 

NAAQS, EPA cannot consider the 

economic or technological feasibility of 

attaining ambient air quality standards. 

Because the CAA prohibits EPA from 

considering the types of estimates and 

assessments described in section 202 

when setting the NAAQS, the UMRA 

does not require EPA to prepare a 

written statement under section 202 for 

the revisions to the SO2 NAAQS. 

With regard to implementation 

guidance, the CAA imposes the 

obligation for States to submit SIPs to 

implement the SO2 NAAQS. In this final 

rule, EPA is merely providing an 

interpretation of those requirements. 

However, even if this rule did establish 

an independent obligation for States to 

submit SIPs, it is questionable whether 

an obligation to submit a SIP revision 

would constitute a Federal mandate in 

any case. The obligation for a State to 

submit a SIP that arises out of section 

110 and section 191 of the CAA is not 

legally enforceable by a court of law, 

and at most is a condition for continued 

receipt of highway funds. Therefore, it 

is possible to view an action requiring 

such a submittal as not creating any 

enforceable duty within the meaning of 

U.S.C. 658 for purposes of the UMRA. 

Even if it did, the duty could be viewed 

as falling within the exception for a 

condition of Federal assistance under 

U.S.C. 658. 

EPA has determined that this final 

rule contains no regulatory 

requirements that might significantly or 

uniquely affect small governments 

because it imposes no enforceable duty 

on any small governments. Therefore, 

this rule is not subject to the 

requirements of section 203 of the 

UMRA. 

E. Executive Order 13132: Federalism 

This final rule does not have 

federalism implications. It will not have 

substantial direct effects on the States, 

on the relationship between the national 

government and the States, or on the 

distribution of power and 

responsibilities among the various 

levels of government, as specified in 

Executive Order 13132. The rule does 

not alter the relationship between the 

Federal government and the States 

regarding the establishment and 

implementation of air quality 

improvement programs as codified in 

the CAA. Under section 109 of the CAA, 

EPA is mandated to establish NAAQS; 

however, CAA section 116 preserves the 

rights of States to establish more 

stringent requirements if deemed 

necessary by a State. Furthermore, this 

rule does not impact CAA section 107 

which establishes that the States have 

primary responsibility for 

implementation of the NAAQS. Finally, 

as noted in section E (above) on UMRA, 

this rule does not impose significant 

costs on State, local, or Tribal 

governments or the private sector. Thus, 

Executive Order 13132 does not apply 

to this rule. 

F. Executive Order 13175: Consultation 

and Coordination With Indian Tribal 

Governments 

Executive Order 13175, entitled 

‘‘Consultation and Coordination with 

Indian Tribal Governments’’ (65 FR 

67249, November 9, 2000), requires EPA 

to develop an accountable process to 

ensure ‘‘meaningful and timely input by 

Tribal officials in the development of 

regulatory policies that have Tribal 

implications.’’ This final rule does not 

have Tribal implications, as specified in 

Executive Order 13175. It does not have 

a substantial direct effect on one or 

more Indian Tribes, on the relationship 

between the Federal government and 

Indian Tribes, or on the distribution of 

power and responsibilities between the 


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35590 Federal Register / Vol. 75, No. 119 / Tuesday, June 22, 2010 / Rules and Regulations 

Federal government and Tribes. The 

rule does not alter the relationship 

between the Federal government and 

Tribes as established in the CAA and 

the TAR. Under section 109 of the CAA, 

EPA is mandated to establish NAAQS; 

however, this rule does not infringe 

existing Tribal authorities to regulate air 

quality under their own programs or 

under programs submitted to EPA for 

approval. Furthermore, this rule does 

not affect the flexibility afforded to 

Tribes in seeking to implement CAA 

programs consistent with the TAR, nor 

does it impose any new obligation on 

Tribes to adopt or implement any 

NAAQS. Finally, as noted in section E 

(above) on UMRA, this rule does not 

impose significant costs on Tribal 

governments. Thus, Executive Order 

13175 does not apply to this rule. 

G. Executive Order 13045: Protection of 

Children From Environmental Health 

and Safety Risks 

This action is subject to Executive 

Order (62 FR 19885, April 23, 1997) 

because it is an economically significant 

regulatory action as defined by 

Executive Order 12866, and we believe 

that the environmental health risk 

addressed by this action has a 

disproportionate effect on children. The 

final rule will establish uniform 

national ambient air quality standards 

for SO2; these standards are designed to 

protect public health with an adequate 

margin of safety, as required by CAA 

section 109. The protection offered by 

these standards may be especially 

important for asthmatics, including 

asthmatic children, because respiratory 

effects in asthmatics are among the most 

sensitive health endpoints for SO2

exposure. Because asthmatic children 

are considered a sensitive population, 

we have evaluated the potential health 

effects of exposure to SO2 pollution 

among asthmatic children. These effects 

and the size of the population affected 

are discussed in chapters 3 and 4 of the 

ISA; chapters 3, 4, 7, 8, 9 of the REA, 

and sections II.A through II.E of this 

preamble. 

H. Executive Order 13211: Actions That 

Significantly Affect Energy Supply, 

Distribution or Use 

This rule is not a ‘‘significant energy 

action’’ as defined in Executive Order 

13211, ‘‘Actions Concerning Regulations 

That Significantly Affect Energy Supply, 

Distribution, or Use’’ (66 FR 28355; May 

22, 2001) because it is not likely to have 

a significant adverse effect on the 

supply, distribution, or use of energy. 

The purpose of this rule is to establish 

revised NAAQS for SO2. The rule does 

not prescribe specific control strategies 

by which these ambient standards will 

be met. Such strategies will be 

developed by States on a case-by-case 

basis, and EPA cannot predict whether 

the control options selected by States 

will include regulations on energy 

suppliers, distributors, or users. Thus, 

EPA concludes that this rule is not 

likely to have any adverse energy 

effects. 

I. National Technology Transfer and 

Advancement Act 

Section 12(d) of the National 

Technology Transfer and Advancement 

Act of 1995 (NTTAA), Public Law 104– 

113, section 12(d) (15 U.S.C. 27) directs 

EPA to use voluntary consensus 

standards in its regulatory activities 

unless to do so would be inconsistent 

with applicable law or otherwise 

impractical. Voluntary consensus 

standards are technical standards (e.g., 

materials specifications, test methods, 

sampling procedures, and business 

practices) that are developed or adopted 

by voluntary consensus standards 

bodies. The NTTAA directs EPA to 

provide Congress, through OMB, 

explanations when the Agency decides 

not to use available and applicable 

voluntary consensus standards. 

This final rulemaking involves 

technical standards with regard to 

ambient monitoring of SO2. The use of 

this voluntary consensus standard 

would be impractical because the 

analysis method does not provide for 

the method detection limits necessary to 

adequately characterize ambient SO2

concentrations for the purpose of 

determining compliance with the final 

revisions to the SO2 NAAQS. 

J. Executive Order 12898: Federal 

Actions To Address Environmental 

Justice in Minority Populations and 

Low-Income Populations 

Executive Order 12898 (59 FR 7629; 

Feb. 16, 1994) establishes Federal 

executive policy on environmental 

justice. Its main provision directs 

Federal agencies, to the greatest extent 

practicable and permitted by law, to 

make environmental justice part of their 

mission by identifying and addressing, 

as appropriate, disproportionately high 

and adverse human health or 

environmental effects of their programs, 

policies, and activities on minority 

populations and low-income 

populations in the United States. 

EPA has determined that this final 

rule will not have disproportionately 

high and adverse human health or 

environmental effects on minority or 

low-income populations because it 

increases the level of environmental 

protection for all affected populations 

without having any disproportionately 

high and adverse human health effects 

on any population, including any 

minority or low-income population. The 

final rule will establish uniform 

national standards for SO2 in ambient 

air. 

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Samet JM. (2009). Letter to EPA 

Administrator Lisa P. Jackson: Clean Air 

Scientific Advisory Committee’s 

(CASAC) Review of EPA’s Risk and 

Exposure Assessment to Support the 

Review of the SO2 Primary National 

Ambient Air Quality Standards: Second 

Draft. EPA–CASAC–09–007, May 18, 

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0352–0035. Available at http:// 

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List of Subjects 

40 CFR Part 50 

Environmental protection, Air 

pollution control, Carbon monoxide, 

Lead, Nitrogen dioxide, Ozone, 

Particulate matter, Sulfur oxides. 

40 CFR Part 53 

Environmental protection, 

Administrative practice and procedure, 

Air pollution control, Intergovernmental 


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relations, Reporting and recordkeeping 

requirements. 

40 CFR Part 58 

Environmental protection, 

Administrative practice and procedure, 

Air pollution control, Intergovernmental 

relations, Reporting and recordkeeping 

requirements. 

Dated: June 2, 2010. 

Lisa P. Jackson, 

Administrator. 

■ For the reasons stated in the preamble, 

title 40, chapter I of the Code of Federal 

Regulations is amended as follows: 

PART 50—NATIONAL PRIMARY AND 

SECONDARY AMBIENT AIR QUALITY 

STANDARDS 

■ 1. The authority citation for part 50 

continues to read as follows: 

Authority: 42 U.S.C. 7401, et seq. 

■ 2. Section 50.4 is amended by adding 

paragraph (e) to read as follows: 

§ 50.4 National primary ambient air quality 

standards for sulfur oxides (sulfur dioxide). 

* * * * * 

(e) The standards set forth in this 

section will remain applicable to all 

areas notwithstanding the promulgation 

of SO2 national ambient air quality 

standards (NAAQS) in § 50.17. The SO2

NAAQS set forth in this section will no 

longer apply to an area one year after 

the effective date of the designation of 

that area, pursuant to section 107 of the 

Clean Air Act, for the SO2 NAAQS set 

forth in § 50. 17; except that for areas 

designated nonattainment for the SO2

NAAQS set forth in this section as of the 

effective date of § 50. 17, and areas not 

meeting the requirements of a SIP call 

with respect to requirements for the SO2

NAAQS set forth in this section, the SO2

NAAQS set forth in this section will 

apply until that area submits, pursuant 

to section 191 of the Clean Air Act, and 

EPA approves, an implementation plan 

providing for attainment of the SO2

NAAQS set forth in § 50.17. 

■ 3. Section 50.14 is amended by 

revising paragraph (c)(2)(vi) to read as 

follows: 

§ 50.14 Treatment of air quality monitoring 

data influenced by exceptional events. 

* * * * * 

(c) * * * 

(2) * * * 

(vi) When EPA sets a NAAQS for a 

new pollutant or revises the NAAQS for 

an existing pollutant, it may revise or 

set a new schedule for flagging 

exceptional event data, providing initial 

data descriptions and providing detailed 

data documentation in AQS for the 

initial designations of areas for those 

NAAQS. Table 1 provides the schedule 

for submission of flags with initial 

descriptions in AQS and detailed 

documentation. These schedules shall 

apply for those data which will or may 

influence the initial designation of areas 

for those NAAQS. EPA anticipates 

revising Table 1 as necessary to 

accommodate revised data submission 

schedules for new or revised NAAQS. 

TABLE 1—SCHEDULE OF EXCEPTIONAL EVENT FLAGGING AND DOCUMENTATION SUBMISSION FOR DATA TO BE USED IN

DESIGNATIONS DECISIONS FOR NEW OR REVISED NAAQS 

NAAQS 

Pollutant/standard/(level)/promulgation 

date 

Air quality data 

collected for 

calendar year 

Event flagging & initial description 

deadline 

Detailed documentation submission 

deadline 

PM2.5/24-Hr Standard (35 μg/m3) Promulgated October 17, 2006.2004–2006 October 1, 2007 a .................................... April 15, 2008. a

Ozone/8-Hr Standard (0.075 ppm) Promulgated March 12, 2008.2005–2007 

2008 

2009 

June 18, 2009 a .......................................

June 18, 2009 a .......................................

60 days after the end of the calendar 

quarter in which the event occurred or 

February 5, 2010, whichever date occurs first b.

June 18, 2009 a

June 18, 2009 1

60 days after the end of the calendar 

quarter in which the event occurred or 

February 5, 2010, whichever date occurs first.b

NO2/1-Hour Standard (80–100 PPB, final 

level TBD).

2008 

2009 

2010 

July 1, 2010 a ...........................................

July 1, 2010 a ...........................................

April 1, 2011 a ..........................................

January 22, 2011. a

January 22, 2011. a

July 1, 2010. a

SO 2/1-Hour Standard (50–100 PPB, 

final level TBD).

2008 

2009 

2010 

October 1, 2010 b ....................................

October 1, 2010 b ....................................

June 1, 2011. b ........................................

June 1, 2011. b

June 1, 2011. b

June 1, 2011. b

2011 60 days after the end of the calendar 

quarter in which the event occurred or 

March 31, 2012, whichever date occurs first b.

60 days after the end of the calendar 

quarter in which the event occurred or 

March 31, 2012, whichever date occurs first. b

aThese dates are unchanged from those published in the original rulemaking, or are being proposed elsewhere and are shown in this table for 

informational purposes—the Agency is not opening these dates for comment under this rulemaking. b Indicates change from general schedule in 40 CFR 50.14. 

Note: EPA notes that the table of revised 

deadlines only applies to data EPA will use 

to establish the final initial designations for 

new or revised NAAQS. The general 

schedule applies for all other purposes, most 

notably, for data used by EPA for 

redesignations to attainment. 

* * * * * 

■ 4. A new 50.17 is added to read as 

follows: 

§ 50.17 National primary ambient air 

quality standards for sulfur oxides (sulfur 

dioxide). 

(a) The level of the national primary 

1-hour annual ambient air quality 

standard for oxides of sulfur is 75 parts 

per billion (ppb, which is 1 part in 

1,000,000,000), measured in the ambient 

air as sulfur dioxide (SO2). 

(b) The 1-hour primary standard is 

met at an ambient air quality monitoring 

site when the three-year average of the 

annual (99th percentile) of the daily 

maximum 1-hour average 

concentrations is less than or equal to 

75 ppb, as determined in accordance 

with Appendix T of this part. 

(c) The level of the standard shall be 

measured by a reference method based 

on Appendix A or A–1 of this part, or 

by a Federal Equivalent Method (FEM) 


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designated in accordance with part 53 of 

this chapter. 

■ 5. Add Appendix A–1 to Part 50 to 

read as follows: 

Appendix A–1 to Part 50—Reference 

Measurement Principle and Calibration 

Procedure for the Measurement of 

Sulfur Dioxide in the Atmosphere 

(Ultraviolet Fluorescence Method) 

1.0 Applicability 

1.1 This ultraviolet fluorescence (UVF) 

method provides a measurement of the 

concentration of sulfur dioxide (SO2) in 

ambient air for determining compliance with 

the national primary and secondary ambient 

air quality standards for sulfur oxides (sulfur 

dioxide) as specified in § 50.4, § 50.5, and 

§ 50.17 of this chapter. The method is 

applicable to the measurement of ambient 

SO2 concentrations using continuous (realtime) sampling. Additional quality assurance 

procedures and guidance are provided in part 

58, Appendix A, of this chapter and in 

Reference 3. 

2.0 Principle 

2.1 This reference method is based on 

automated measurement of the intensity of 

the characteristic fluorescence released by 

SO2 in an ambient air sample contained in 

a measurement cell of an analyzer when the 

air sample is irradiated by ultraviolet (UV) 

light passed through the cell. The fluorescent 

light released by the SO2 is also in the 

ultraviolet region, but at longer wavelengths 

than the excitation light. Typically, optimum 

instrumental measurement of SO2

concentrations is obtained with an excitation 

wavelength in a band between approximately 

190 to 230 nm, and measurement of the SO2

fluorescence in a broad band around 320 nm, 

but these wavelengths are not necessarily 

constraints of this reference method. 

Generally, the measurement system 

(analyzer) also requires means to reduce the 

effects of aromatic hydrocarbon species, and 

possibly other compounds, in the air sample 

to control measurement interferences from 

these compounds, which may be present in 

the ambient air. References 1 and 2 describe 

UVF method. 

2.2 The measurement system is calibrated 

by referencing the instrumental fluorescence 

measurements to SO2 standard 

concentrations traceable to a National 

Institute of Standards and Technology (NIST) 

primary standard for SO2 (see Calibration 

Procedure below). 

2.3 An analyzer implementing this 

measurement principle is shown 

schematically in Figure 1. Designs should 

include a measurement cell, a UV light 

source of appropriate wavelength, a UV 

detector system with appropriate wave length 

sensitivity, a pump and flow control system 

for sampling the ambient air and moving it 

into the measurement cell, sample air 

conditioning components as necessary to 

minimize measurement interferences, 

suitable control and measurement processing 

capability, and other apparatus as may be 

necessary. The analyzer must be designed to 

provide accurate, repeatable, and continuous 

measurements of SO2 concentrations in 

ambient air, with measurement performance 

as specified in Subpart B of Part 53 of this 

chapter. 

2.4 Sampling considerations: The use of 

a particle filter on the sample inlet line of a 

UVF SO2 analyzer is required to prevent 

interference, malfunction, or damage due to 

particles in the sampled air. 

3.0 Interferences 

3.1 The effects of the principal potential 

interferences may need to be mitigated to 

meet the interference equivalent 

requirements of part 53 of this chapter. 

Aromatic hydrocarbons such as xylene and 

naphthalene can fluoresce and act as strong 

positive interferences. These gases can be 

removed by using a permeation type scrubber 

(hydrocarbon ‘‘kicker’’). Nitrogen oxide (NO) 

in high concentrations can also fluoresce and 

cause positive interference. Optical filtering 

can be employed to improve the rejection of 

interference from high NO. Ozone can absorb 

UV light given off by the SO2 molecule and 

cause a measurement offset. This effect can 

be reduced by minimizing the measurement 

path length between the area where SO2

fluorescence occurs and the photomultiplier 

tube detector (e.g. <5 cm). A hydrocarbon 

scrubber, optical filter and appropriate 

distancing of the measurement path length 

may be required method components to 

reduce interference. 

4.0 Calibration Procedure 

Atmospheres containing accurately known 

concentrations of sulfur dioxide are prepared 

using a compressed gas transfer standard 

diluted with accurately metered clean air 

flow rates. 

4.1 Apparatus: Figure 2 shows a typical 

generic system suitable for diluting a SO2 gas 

cylinder concentration standard with clean 

air through a mixing chamber to produce the 

desired calibration concentration standards. 

A valve may be used to conveniently divert 

the SO2 from the sampling manifold to 

provide clean zero air at the output manifold 

for zero adjustment. The system may be made 

up using common laboratory components, or 

it may be a commercially manufactured 

system. In either case, the principle 

components are as follows: 

4.1.1 SO2 standard gas flow control and 

measurement devices (or a combined device) 

capable of regulating and maintaining the 

standard gas flow rate constant to within ±2 

percent and measuring the gas flow rate 

accurate to within ±2, properly calibrated to 

a NIST-traceable standard. 

4.1.2 Dilution air flow control and 

measurement devices (or a combined device) 

capable of regulating and maintaining the air 

flow rate constant to within ±2 percent and 

measuring the air flow rate accurate to within 

±2, properly calibrated to a NIST-traceable 

standard. 

4.1.3 Mixing chamber, of an inert 

material such as glass and of proper design 

to provide thorough mixing of pollutant gas 

and diluent air streams. 

4.1.4 Sampling manifold, constructed of 

glass, polytetrafluoroethylene (PTFE 

TeflonTM), or other suitably inert material 

and of sufficient diameter to insure a 

minimum pressure drop at the analyzer 

connection, with a vent designed to insure a 

minimum over-pressure (relative to ambient 

air pressure) at the analyzer connection and 

to prevent ambient air from entering the 

manifold. 

4.1.5 Standard gas pressure regulator, of 

clean stainless steel with a stainless steel 

diaphragm, suitable for use with a high 

pressure SO2 gas cylinder. 

4.1.6 Reagents 

4.1.6.1 SO2 gas concentration transfer 

standard having a certified SO2 concentration 

of not less than 10 ppm, in N2, traceable to 

a NIST Standard Reference Material (SRM). 

4.1.6.2 Clean zero air, free of 

contaminants that could cause a detectable 

response or a change in sensitivity of the 

analyzer. Since ultraviolet fluorescence 

analyzers may be sensitive to aromatic 

hydrocarbons and O2-to-N2 ratios, it is 

important that the clean zero air contains less 

than 0.1 ppm aromatic hydrocarbons and O2

and N2 percentages approximately the same 

as in ambient air. A procedure for generating 

zero air is given in reference 1. 

4.2 Procedure 

4.2.1 Obtain a suitable calibration 

apparatus, such as the one shown 

schematically in Figure 1, and verify that all 

materials in contact with the pollutant are of 

glass, TeflonTM, or other suitably inert 

material and completely clean. 

4.2.2 Purge the SO2 standard gas lines 

and pressure regulator to remove any 

residual air. 

4.2.3 Ensure that there are no leaks in the 

system and that the flow measuring devices 

are properly and accurately calibrated under 

the conditions of use against a reliable 

volume or flow rate standard such as a soapbubble meter or a wet-test meter traceable to 

a NIST standard. All volumetric flow rates 

should be corrected to the same reference 

temperature and pressure by using the 

formula below: 

F F c m = ( ) +

298 15

760 273 15

.

.

P

T

m

m

Where: 

Fc = corrected flow rate (L/min at 25 °C and 

760 mm Hg), 

Fm = measured flow rate, (at temperature, Tm

and pressure, Pm), 

Pm = measured pressure in mm Hg, 

(absolute), and 

Tm = measured temperature in degrees 

Celsius. 

4.2.4 Allow the SO2 analyzer under 

calibration to sample zero air until a stable 

response is obtained, then make the proper 

zero adjustment. 

4.2.5 Adjust the airflow to provide an SO2

concentration of approximately 80 percent of 

the upper measurement range limit of the 

SO2 instrument and verify that the total air 

flow of the calibration system exceeds the 

demand of all analyzers sampling from the 

output manifold (with the excess vented). 

4.2.6 Calculate the actual SO2 calibration 

concentration standard as: 


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SO C

F

F

p

t

[ ] 2 =

Where: 

C = the concentration of the SO2 gas standard 

Fp = the flow rate of SO2 gas standard 

Ft = the total air flow rate of pollutant and 

diluent gases 

4.2.7 When the analyzer response has 

stabilized, adjust the SO2 span control to 

obtain the desired response equivalent to the 

calculated standard concentration. If 

substantial adjustment of the span control is 

needed, it may be necessary to re-check the 

zero and span adjustments by repeating steps 

4.2.4 through 4.2.7 until no further 

adjustments are needed. 

4.2.8 Adjust the flow rate(s) to provide 

several other SO2 calibration concentrations 

over the analyzer’s measurement range. At 

least five different concentrations evenly 

spaced throughout the analyzer’s range are 

suggested. 

4.2.9 Plot the analyzer response (vertical 

or Y-axis) versus SO2 concentration 

(horizontal or X-axis). Compute the linear 

regression slope and intercept and plot the 

regression line to verify that no point 

deviates from this line by more than 2 

percent of the maximum concentration 

tested. 

Note: Additional information on 

calibration and pollutant standards is 

provided in Section 12 of Reference 3. 

5.0 Frequency of Calibration 

The frequency of calibration, as well as the 

number of points necessary to establish the 

calibration curve and the frequency of other 

performance checking will vary by analyzer; 

however, the minimum frequency, 

acceptance criteria, and subsequent actions 

are specified in Reference 3, Appendix D: 

Measurement Quality Objectives and 

Validation Template for SO2 (page 9 of 30). 

The user’s quality control program should 

provide guidelines for initial establishment 

of these variables and for subsequent 

alteration as operational experience is 

accumulated. Manufacturers of analyzers 

should include in their instruction/operation 

manuals information and guidance as to 

these variables and on other matters of 

operation, calibration, routine maintenance, 

and quality control. 

6.0 References for SO2 Method 

1. H. Okabe, P. L. Splitstone, and J. J. Ball, 

‘‘Ambient and Source SO2 Detector Based 

on a Fluorescence Method’’, Journal of 

the Air Control Pollution Association, 

vol. 23, p. 514–516 (1973). 

2. F. P. Schwarz, H. Okabe, and J. K. 

Whittaker, ‘‘Fluorescence Detection of 

Sulfur Dioxide in Air at the Parts per 

Billion Level,’’ Analytical Chemistry, vol. 

46, pp. 1024–1028 (1974). 

3. QA Handbook for Air Pollution 

Measurement Systems—Volume II. 

Ambient Air Quality Monitoring 

Programs. U.S. EPA. EPA–454/B–08–003 

(2008). 

BILLING CODE 6560–50–P 


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BILLING CODE 6560–50–C 

■ 6. Appendix A to Part 50 is 

redesignated as Appendix A–2 to Part 

50. 

■ 7. Appendix T to Part 50 is added to 

read as follows: 

Appendix T to Part 50—Interpretation 

of the Primary National Ambient Air 

Quality Standards for Oxides of Sulfur 

(Sulfur Dioxide) 

1. General 

(a) This appendix explains the data 

handling conventions and computations 

necessary for determining when the primary 

national ambient air quality standards for 

Oxides of Sulfur as measured by Sulfur 

Dioxide (‘‘SO2 NAAQS’’) specified in § 50.17 

are met at an ambient air quality monitoring 

site. Sulfur Dioxide (SO2) is measured in the 

ambient air by a Federal reference method 

(FRM) based on appendix A or A–1 to this 

part or by a Federal equivalent method (FEM) 

designated in accordance with part 53 of this 

chapter. Data handling and computation 

procedures to be used in making 

comparisons between reported SO2

concentrations and the levels of the SO2

NAAQS are specified in the following 

sections. 

(b) Decisions to exclude, retain, or make 

adjustments to the data affected by 

exceptional events, including natural events, 

are made according to the requirements and 

process deadlines specified in §§ 50.1, 50.14 

and 51.930 of this chapter. 

(c) The terms used in this appendix are 

defined as follows: 

Daily maximum 1-hour values for SO2

refers to the maximum 1-hour SO2

concentration values measured from 

midnight to midnight (local standard time) 

that are used in NAAQS computations. 

Design values are the metrics (i.e., 

statistics) that are compared to the NAAQS 

levels to determine compliance, calculated as 

specified in section 5 of this appendix. The 

design value for the primary 1-hour NAAQS 

is the 3-year average of annual 99th 

percentile daily maximum 1-hour values for 

a monitoring site (referred to as the ‘‘1-hour 

primary standard design value’’). 

99th percentile daily maximum 1-hour 

value is the value below which nominally 99 

percent of all daily maximum 1-hour 

concentration values fall, using the ranking 

and selection method specified in section 5 

of this appendix. 

Pollutant Occurrence Code (POC) refers to 

a numerical code (1, 2, 3, etc.) used to 

distinguish the data from two or more 

monitors for the same parameter at a single 

monitoring site. 

Quarter refers to a calendar quarter. 

Year refers to a calendar year. 

2. Requirements for Data Used for 

Comparisons With the SO2 NAAQS and Data 

Reporting Considerations 

(a) All valid FRM/FEM SO2 hourly data 

required to be submitted to EPA’s Air Quality 

System (AQS), or otherwise available to EPA, 

meeting the requirements of part 58 of this 

chapter including appendices A, C, and E 

shall be used in design value calculations. 

Multi-hour average concentration values 

collected by wet chemistry methods shall not 

be used. 

(b) Data from two or more monitors from 

the same year at the same site reported to 

EPA under distinct Pollutant Occurrence 

Codes shall not be combined in an attempt 

to meet data completeness requirements. The 

Administrator will combine annual 99th 

percentile daily maximum concentration 

values from different monitors in different 

years, selected as described here, for the 

purpose of developing a valid 1-hour primary 

standard design value. If more than one of 

the monitors meets the completeness 

requirement for all four quarters of a year, the 

steps specified in section 5(a) of this 

appendix shall be applied to the data from 

the monitor with the highest average of the 

four quarterly completeness values to derive 

a valid annual 99th percentile daily 

maximum concentration. If no monitor is 

complete for all four quarters in a year, the 

steps specified in section 3(c) and 5(a) of this 

appendix shall be applied to the data from 

the monitor with the highest average of the 

four quarterly completeness values in an 

attempt to derive a valid annual 99th 

percentile daily maximum concentration. 

This paragraph does not prohibit a 

monitoring agency from making a local 

designation of one physical monitor as the 

primary monitor for a Pollutant Occurrence 

Code and substituting the 1-hour data from 

a second physical monitor whenever a valid 

concentration value is not obtained from the 

primary monitor; if a monitoring agency 

substitutes data in this manner, each 

substituted value must be accompanied by an 


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AQS qualifier code indicating that 

substitution with a value from a second 

physical monitor has taken place. 

(c) Hourly SO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

3. Comparisons With the 1-Hour Primary 

SO2 NAAQS 

(a) The 1-hour primary SO2 NAAQS is met 

at an ambient air quality monitoring site 

when the valid 1-hour primary standard 

design value is less than or equal to 75 parts 

per billion (ppb). 

(b) An SO2 1-hour primary standard design 

value is valid if it encompasses three 

consecutive calendar years of complete data. 

A year meets data completeness requirements 

when all 4 quarters are complete. A quarter 

is complete when at least 75 percent of the 

sampling days for each quarter have 

complete data. A sampling day has complete 

data if 75 percent of the hourly concentration 

values, including State-flagged data affected 

by exceptional events which have been 

approved for exclusion by the Administrator, 

are reported. 

(c) In the case of one, two, or three years 

that do not meet the completeness 

requirements of section 3(b) of this appendix 

and thus would normally not be useable for 

the calculation of a valid 3-year 1-hour 

primary standard design value, the 3-year 1- 

hour primary standard design value shall 

nevertheless be considered valid if one of the 

following conditions is true. 

(i) At least 75 percent of the days in each 

quarter of each of three consecutive years 

have at least one reported hourly value, and 

the design value calculated according to the 

procedures specified in section 5 is above the 

level of the primary 1-hour standard. 

(ii) (A) A 1-hour primary standard design 

value that is equal to or below the level of 

the NAAQS can be validated if the 

substitution test in section 3(c)(ii)(B) results 

in a ‘‘test design value’’ that is below the level 

of the NAAQS. The test substitutes actual 

‘‘high’’ reported daily maximum 1-hour 

values from the same site at about the same 

time of the year (specifically, in the same 

calendar quarter) for unknown values that 

were not successfully measured. Note that 

the test is merely diagnostic in nature, 

intended to confirm that there is a very high 

likelihood that the original design value (the 

one with less than 75 percent data capture of 

hours by day and of days by quarter) reflects 

the true under-NAAQS-level status for that 3- 

year period; the result of this data 

substitution test (the ‘‘test design value’’, as 

defined in section 3(c)(ii)(B)) is not 

considered the actual design value. For this 

test, substitution is permitted only if there 

are at least 200 days across the three 

matching quarters of the three years under 

consideration (which is about 75 percent of 

all possible daily values in those three 

quarters) for which 75 percent of the hours 

in the day, including State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator, have reported concentrations. 

However, maximum 1-hour values from days 

with less than 75 percent of the hours 

reported shall also be considered in 

identifying the high value to be used for 

substitution. 

(B) The substitution test is as follows: Data 

substitution will be performed in all quarter 

periods that have less than 75 percent data 

capture but at least 50 percent data capture, 

including State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator; 

if any quarter has less than 50 percent data 

capture then this substitution test cannot be 

used. Identify for each quarter (e.g., January– 

March) the highest reported daily maximum 

1-hour value for that quarter, excluding Stateflagged data affected by exceptional events 

which have been approved for exclusion by 

the Administrator, looking across those three 

months of all three years under 

consideration. All daily maximum 1-hour 

values from all days in the quarter period 

shall be considered when identifying this 

highest value, including days with less than 

75 percent data capture. If after substituting 

the highest reported daily maximum 1-hour 

value for a quarter for as much of the missing 

daily data in the matching deficient 

quarter(s) as is needed to make them 100 

percent complete, the procedure in section 5 

yields a recalculated 3-year 1-hour standard 

‘‘test design value’’ less than or equal to the 

level of the standard, then the 1-hour primary 

standard design value is deemed to have 

passed the diagnostic test and is valid, and 

the level of the standard is deemed to have 

been met in that 3-year period. As noted in 

section 3(c)(i), in such a case, the 3-year 

design value based on the data actually 

reported, not the ‘‘test design value’’, shall be 

used as the valid design value. 

(iii) (A) A 1-hour primary standard design 

value that is above the level of the NAAQS 

can be validated if the substitution test in 

section 3(c)(iii)(B) results in a ‘‘test design 

value’’ that is above the level of the NAAQS. 

The test substitutes actual ‘‘low’’ reported 

daily maximum 1-hour values from the same 

site at about the same time of the year 

(specifically, in the same three months of the 

calendar) for unknown hourly values that 

were not successfully measured. Note that 

the test is merely diagnostic in nature, 

intended to confirm that there is a very high 

likelihood that the original design value (the 

one with less than 75 percent data capture of 

hours by day and of days by quarter) reflects 

the true above-NAAQS-level status for that 3- 

year period; the result of this data 

substitution test (the ‘‘test design value’’, as 

defined in section 3(c)(iii)(B)) is not 

considered the actual design value. For this 

test, substitution is permitted only if there 

are a minimum number of available daily 

data points from which to identify the low 

quarter-specific daily maximum 1-hour 

values, specifically if there are at least 200 

days across the three matching quarters of the 

three years under consideration (which is 

about 75 percent of all possible daily values 

in those three quarters) for which 75 percent 

of the hours in the day have reported 

concentrations. Only days with at least 75 

percent of the hours reported shall be 

considered in identifying the low value to be 

used for substitution. 

(B) The substitution test is as follows: Data 

substitution will be performed in all quarter 

periods that have less than 75 percent data 

capture. Identify for each quarter (e.g., 

January–March) the lowest reported daily 

maximum 1-hour value for that quarter, 

looking across those three months of all three 

years under consideration. All daily 

maximum 1-hour values from all days with 

at least 75 percent capture in the quarter 

period shall be considered when identifying 

this lowest value. If after substituting the 

lowest reported daily maximum 1-hour value 

for a quarter for as much of the missing daily 

data in the matching deficient quarter(s) as is 

needed to make them 75 percent complete, 

the procedure in section 5 yields a 

recalculated 3-year 1-hour standard ‘‘test 

design value’’ above the level of the standard, 

then the 1-hour primary standard design 

value is deemed to have passed the 

diagnostic test and is valid, and the level of 

the standard is deemed to have been 

exceeded in that 3-year period. As noted in 

section 3(c)(i), in such a case, the 3-year 

design value based on the data actually 

reported, not the ‘‘test design value’’, shall be 

used as the valid design value. 

(d) A 1-hour primary standard design value 

based on data that do not meet the 

completeness criteria stated in 3(b) and also 

do not satisfy section 3(c), may also be 

considered valid with the approval of, or at 

the initiative of, the Administrator, who may 

consider factors such as monitoring site 

closures/moves, monitoring diligence, the 

consistency and levels of the valid 

concentration measurements that are 

available, and nearby concentrations in 

determining whether to use such data. 

(e) The procedures for calculating the 1- 

hour primary standard design values are 

given in section 5 of this appendix. 

4. Rounding Conventions for the 1-Hour 

Primary SO2 NAAQS 

(a) Hourly SO2 measurement data shall be 

reported to AQS in units of parts per billion 

(ppb), to at most one place after the decimal, 

with additional digits to the right being 

truncated with no further rounding. 

(b) Daily maximum 1-hour values and 

therefore the annual 99th percentile of those 

daily values are not rounded. 

(c) The 1-hour primary standard design 

value is calculated pursuant to section 5 and 

then rounded to the nearest whole number or 

1 ppb (decimals 0.5 and greater are rounded 

up to the nearest whole number, and any 

decimal lower than 0.5 is rounded down to 

the nearest whole number). 

5. Calculation Procedures for the 1-Hour 

Primary SO2 NAAQS 

(a) Procedure for identifying annual 99th 

percentile values. When the data for a 

particular ambient air quality monitoring site 

and year meet the data completeness 

requirements in section 3(b), or if one of the 

conditions of section 3(c) is met, or if the 

Administrator exercises the discretionary 

authority in section 3(d), identification of 

annual 99th percentile value is accomplished 

as follows. 

(i) The annual 99th percentile value for a 

year is the higher of the two values resulting 

from the following two procedures. 


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Federal Register / Vol. 75, No. 119 / Tuesday, June 22, 2010 / Rules and Regulations 35597 

(1) Procedure 1. For the year, determine the 

number of days with at least 75 percent of 

the hourly values reported. 

(A) For the year, determine the number of 

days with at least 75 percent of the hourly 

values reported including State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator. 

(B) For the year, from only the days with 

at least 75 percent of the hourly values 

reported, select from each day the maximum 

hourly value excluding State-flagged data 

affected by exceptional events which have 

been approved for exclusion by the 

Administrator. 

(C) Sort all these daily maximum hourly 

values from a particular site and year by 

descending value. (For example: (x[1], x[2], 

x[3], * * *, x[n]). In this case, x[1] is the 

largest number and x[n] is the smallest 

value.) The 99th percentile is determined 

from this sorted series of daily values which 

is ordered from the highest to the lowest 

number. Using the left column of Table 1, 

determine the appropriate range (i.e., row) for 

the annual number of days with valid data 

for year y (cny). The corresponding ‘‘n’’ value 

in the right column identifies the rank of the 

annual 99th percentile value in the 

descending sorted list of daily site values for 

year y. Thus, P0.99, y = the nth largest value. 

(2) Procedure 2. For the year, determine the 

number of days with at least one hourly 

value reported. 

(A) For the year, determine the number of 

days with at least one hourly value reported 

including State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator. 

(B) For the year, from all the days with at 

least one hourly value reported, select from 

each day the maximum hourly value 

excluding State-flagged data affected by 

exceptional events which have been 

approved for exclusion by the Administrator. 

(C) Sort all these daily maximum values 

from a particular site and year by descending 

value. (For example: (x[1], x[2], x[3], * * *, 

x[n]). In this case, x[1] is the largest number 

and x[n] is the smallest value.) The 99th 

percentile is determined from this sorted 

series of daily values which is ordered from 

the highest to the lowest number. Using the 

left column of Table 1, determine the 

appropriate range (i.e., row) for the annual 

number of days with valid data for year y 

(cny). The corresponding ‘‘n’’ value in the 

right column identifies the rank of the annual 

99th percentile value in the descending 

sorted list of daily site values for year y. 

Thus, P0.99,y = the nth largest value. 

(b) The 1-hour primary standard design 

value for an ambient air quality monitoring 

site is mean of the three annual 99th 

percentile values, rounded according to the 

conventions in section 4. 

TABLE 1 

Annual number of 

days with valid data 

for year ‘‘y’’ (cny) 

P0.99,y is the nth 

maximum value of the 

year, where n is the 

listed number 

1–100 ........................ 1 

101–200 .................... 2 

201–300 .................... 3 

301–366 .................... 4 

PART 53–AMBIENT AIR MONITORING 

REFERENCE AND EQUIVALENT 

METHODS 

■ 8. The authority citation for part 53 

continues to read as follows: 

Authority: Sec. 301(a) of the Clean Air Act 

(42 U.S.C. sec. 1857g(a)), as amended by sec. 

15(c)(2) of Pub. L. 91–604, 84 Stat. 1713, 

unless otherwise noted. 

Subpart A—[Amended] 

■ 9. Section 53.2 is amended by revising 

paragraphs (a)(1) and (b) to read as 

follows: 

§ 53.2 General requirements for a 

reference method determination. 

* * * * * 

(a) Manual methods—(1) Sulfur 

dioxide (SO2) and Lead. For measuring 

SO2 and lead, appendixes A–2 and G of 

part 50 of this chapter specify unique 

manual FRM for measuring those 

pollutants. Except as provided in 

§ 53.16, other manual methods for lead 

will not be considered for a reference 

method determination under this part. 

* * * * * 

(b) Automated methods. An 

automated FRM for measuring SO2, CO, 

O3, or NO2 must utilize the 

measurement principle and calibration 

procedure specified in the appropriate 

appendix to part 50 of this chapter 

(appendix A–1 only for SO2 methods) 

and must have been shown in 

accordance with this part to meet the 

requirements specified in this subpart A 

and subpart B of this part. 

■ 10. Section 53.8 is amended by 

revising paragraph (c) to read as follows: 

§ 53.8 Designation of reference and 

equivalent methods. 

* * * * * 

(c) The Administrator will maintain a 

current list of methods designated as 

FRM or FEM in accordance with this 

part and will send a copy of the list to 

any person or group upon request. A 

copy of the list will be available via the 

Internet and may be available from other 

sources. 

■ 11. Table A–1 to Subpart A is revised 

to read as follows: 

TABLE A–1 TO SUBPART A OF PART 53—SUMMARY OF APPLICABLE REQUIREMENTS FOR REFERENCE AND EQUIVALENT

METHODS FOR AIR MONITORING OF CRITERIA POLLUTANTS

Pollutant Reference or 

equivalent Manual or automated Applicable part 50 appendix 

Applicable subparts of part 53 

A B C D E F 

SO2 .......... Reference .................... Manual ......................... A–2 

Automated ................... A–1 ✓ ✓ 

Equivalent .................... Manual ......................... A–1 ✓ ✓ 

Automated ................... A–1 ✓ ✓ ✓ 

CO ............ Reference .................... Automated ................... C ✓ ✓ 

Equivalent .................... Manual ......................... C ✓ ✓ 

Automated ................... C ✓ ✓ ✓ 

O3 ............. Reference .................... Automated ................... D ✓ ✓ 

Equivalent .................... Manual ......................... D ✓ ✓ 

Automated ................... D ✓ ✓ ✓ 

NO2 .......... Reference .................... Automated ................... F ✓ ✓ 

Equivalent .................... Manual ......................... F ✓ ✓ 

Automated ................... F ✓ ✓ ✓ 

Pb ............. Reference .................... Manual ......................... G 

Equivalent .................... Manual ......................... G ✓ ✓ 

Automated ................... G ✓ ✓ 

PM10-Pb ... Reference .................... Manual ......................... Q 

Equivalent .................... Manual ......................... Q ✓ ✓ 


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35598 Federal Register / Vol. 75, No. 119 / Tuesday, June 22, 2010 / Rules and Regulations 

TABLE A–1 TO SUBPART A OF PART 53—SUMMARY OF APPLICABLE REQUIREMENTS FOR REFERENCE AND EQUIVALENT

METHODS FOR AIR MONITORING OF CRITERIA POLLUTANTS—Continued 

Pollutant Reference or 

equivalent Manual or automated Applicable part 50 appendix 

Applicable subparts of part 53 

A B C D E F 

Automated ................... Q ✓ ✓ 

PM10 ......... Reference .................... Manual ......................... J ✓ ✓ 

Equivalent .................... Manual ......................... J ✓ ✓ ✓ 

Automated ................... J ✓ ✓ ✓ 

PM2.5 ........ Reference .................... Manual ......................... L ✓ ✓ 

Equivalent Class I ....... Manual ......................... L ✓ ✓ ✓ 

Equivalent Class II ...... Manual ......................... L1 ✓ ✓2 ✓ ✓1 2

Equivalent Class III ..... Automated ................... L1 ✓ ✓ ✓ ✓1

PM10–2.5 .... Reference .................... Manual ......................... L, O ✓ ✓ 

Equivalent Class I ....... Manual ......................... L, O ✓ ✓ ✓ 

Equivalent Class II ...... Manual ......................... L, O ✓ ✓2 ✓ ✓1 2

Equivalent Class III ..... Automated ................... L1, O1 ✓ ✓ ✓ ✓1

1. Some requirements may apply, based on the nature of each particular candidate method, as determined by the Administrator. 

2. Alternative Class III requirements may be substituted. 

Subpart B—[Amended] 

■ 12. Section 53.20 is amended by 

revising paragraph (b) and Table B–1 in 

paragraph (c) to read as follows: 

§ 53.20 General provisions. 

* * * * * 

(b) For a candidate method having 

more than one selectable measurement 

range, one range must be that specified 

in table B–1 (standard range for SO2), 

and a test analyzer representative of the 

method must pass the tests required by 

this subpart while operated in that 

range. The tests may be repeated for one 

or more broader ranges (i.e., ones 

extending to higher concentrations) than 

the range specified in table B–1, 

provided that the range does not extend 

to concentrations more than four times 

the upper range limit specified in table 

B–1. For broader ranges, only the tests 

for range (calibration), noise at 80% of 

the upper range limit, and lag, rise and 

fall time are required to be repeated. 

The tests may be repeated for one or 

more narrower ranges (ones extending 

to lower concentrations) than that 

specified in table B–1. For SO2 methods, 

table B–1 specifies special performance 

requirements for narrower (lower) 

ranges. For methods other than SO2, 

only the tests for range (calibration), 

noise at 0% of the measurement range, 

and lower detectable limit are required 

to be repeated. If the tests are conducted 

or passed only for the specified range 

(standard range for SO2), any FRM or 

FEM method determination with respect 

to the method will be limited to that 

range. If the tests are passed for both the 

specified range and one or more broader 

ranges, any such determination will 

include the additional range(s) as well 

as the specified range, provided that the 

tests required by subpart C of this part 

(if applicable) are met for the broader 

range(s). If the tests are passed for both 

the specified range and one or more 

narrower ranges, any FRM or FEM 

method determination for the method 

will include the narrower range(s) as 

well as the specified range. Appropriate 

test data shall be submitted for each 

range sought to be included in a FRM 

or FEM method determination under 

this paragraph (b). 

(c) * * * 

TABLE B–1—PERFORMANCE SPECIFICATIONS FOR AUTOMATED METHODS

Performance parameter Units 1

SO 2

O 3 CO NO 2 Definitions and 

test procedures Std. range 3 Lower 

range 2 3

1. Range ......................................... ppm .............. 0–0 .5 <0 .5 0–0 .5 0–50 0–0 .5 Sec. 53.23(a). 

2. Noise .......................................... ppm .............. 0 .001 0 .0005 0 .005 0.5 0 .005 Sec. 53.23(b). 

3. Lower detectable limit ................ ppm .............. 0 .002 0 .001 0 .010 1.0 0 .010 Sec. 53.23(c). 

4. Interference equivalent 

Each interferent ....................... ppm .............. ±0 .005 4±0 .005 ±0 .02 ±1.0 ±0 .02 Sec. 53.23(d). 

Total, all interferents ................ ppm .............. — — 0 .06 1.5 0 .04 Sec. 53.23(d). 

5. Zero drift, 12 and 24 hour .......... ppm .............. ±0 .004 ±0 .002 ±0 .02 ±1.0 ±0 .02 Sec. 53.23(e). 

6. Span drift, 24 hour 

20% of upper range limit ......... Percent ......... — — ±20 .0 ±10.0 ±20 .0 Sec. 53.23(e). 

80% of upper range limit ......... Percent ......... ±3 .0 ±3 .0 ±5 .0 ±2.5 ±5 .0 Sec. 53.23(e). 

7. Lag time ...................................... Minutes ........ 2 2 20 10 20 Sec. 53.23(e). 

8. Rise time .................................... Minutes ........ 2 2 15 5 15 Sec. 53.23(e). 

9. Fall time ...................................... Minutes ........ 2 2 15 5 15 Sec. 53.23(e). 

10. Precision 

20% of upper range limit ......... ppm .............. — — 0 .010 0.5 0 .020 Sec. 53.23(e). 

Percent ......... 2 2 .................. .................... .................. Sec. 53.23(e). 

80% of upper range limit ......... ppm .............. — — 0 .010 0.5 0 .030 Sec. 53.23(e). 

Percent ......... 2 2 — — — Sec. 53.23(e). 

1. To convert from parts per million (ppm) to μg/m3 at 25 °C and 760 mm Hg, multiply by M/0.02447, where M is the molecular weight of the 

gas. Percent means percent of the upper range limit. 

2. Tests for interference equivalent and lag time do not need to be repeated for any lower SO2 range provided the test for the standard range 

shows that the lower range specification is met for each of these test parameters. 


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3. For candidate analyzers having automatic or adaptive time constants or smoothing filters, describe their functional nature, and describe and 

conduct suitable tests to demonstrate their function aspects and verify that performances for calibration, noise, lag, rise, fall times, and precision 

are within specifications under all applicable conditions. For candidate analyzers with operator-selectable time constants or smoothing filters, conduct calibration, noise, lag, rise, fall times, and precision tests at the highest and lowest settings that are to be included in the FRM or FEM designation. 

4. For nitric oxide interference for the SO2 UVF method, interference equivalent is ±0.003 ppm for the lower range. 

* * * * * 

■ 13. Section 53.21 is amended by 

revising paragraph (a) to read as follows: 

§ 53.21 Test conditions. 

(a) Set-up and start-up of the test 

analyzer shall be in strict accordance 

with the operating instructions specified 

in the manual referred to in § 53.4(b)(3). 

Allow adequate warm-up or 

stabilization time as indicated in the 

operating instructions before beginning 

the tests. The test procedures assume 

that the test analyzer has an analog 

measurement signal output that is 

connected to a suitable strip chart 

recorder of the servo, null-balance type. 

This recorder shall have a chart width 

of a least 25 centimeters, chart speeds 

up to 10 cm per hour, a response time 

of 1 second or less, a deadband of not 

more than 0.25 percent of full scale, and 

capability either of reading 

measurements at least 5 percent below 

zero or of offsetting the zero by at least 

5 percent. If the test analyzer does not 

have an analog signal output, or if other 

types of measurement data output are 

used, an alternative measurement data 

recording device (or devices) may be 

used for the tests, provided it is 

reasonably suited to the nature and 

purposes of the tests and an analog 

representation of the analyzer 

measurements for each test can be 

plotted or otherwise generated that is 

reasonably similar to the analog 

measurement recordings that would be 

produced by a conventional chart 

recorder. 

* * * * * 

■ 14. Section 53.22(d) is amended by 

revising Table B–2 to read as follows: 

§ 53.22 Generation of test atmospheres. 

* * * * * 

(d) * * * 

TABLE B–2—TEST ATMOSPHERES

Test gas Generation Verification 

Ammonia .................... Permeation device. Similar to system described in references 1 and 2.Indophenol method, reference 3. 

Carbon dioxide ........... Cylinder of zero air or nitrogen containing CO2 as required 

to obtain the concentration specified in Table B–3.

Use NIST-certified standards whenever possible. If NIST 

standards are not available, obtain 2 standards from 

independent sources which agree within 2 percent, or 

obtain one standard and submit it to an independent 

laboratory for analysis, which must agree within 2 percent of the supplier’s nominal analysis. 

Carbon monoxide ....... Cylinder of zero air or nitrogen containing CO as required 

to obtain the concentration specified in Table B–3.

Use a FRM CO analyzer as described in reference 8. 

Ethane ........................ Cylinder of zero air or nitrogen containing ethane as required to obtain the concentration specified in Table B–3.Gas chromatography, ASTM D2820, reference 10. Use 

NIST-traceable gaseous methane or propane standards 

for calibration. 

Ethylene ..................... Cylinder of pre-purified nitrogen containing ethylene as required to obtain the concentration specified in Table B–3.Do. 

Hydrogen chloride ...... Cylinder1 of pre-purified nitrogen containing approximately 

100 ppm of gaseous HCL. Dilute with zero air to concentration specified in Table B–3.

Collect samples in bubbler containing distilled water and 

analyze by the mercuric thiocyante method, ASTM 

(D612), p. 29, reference 4. 

Hydrogen sulfide ........ Permeation device system described in references 1 and 

2.

Tentative method of analysis for H2S content of the atmosphere, p. 426, reference 5. 

Methane ..................... Cylinder of zero air containing methane as required to obtain the concentration specified in Table B–3.Gas chromatography ASTM D2820, reference 10. Use 

NIST-traceable methane standards for calibration. 

Naphthalene ............... 1. Permeation device as described in references 1 and 2 ..

2. Cylinder of pre-purified nitrogen containing 100 ppm 

naphthalene. Dilute with zero air to concentration specified in Table B–3. 

Use NIST-certified standards whenever possible. If NIST 

standards are not available, obtain 2 standards from 

independent sources which agree within 2 percent, or 

obtain one standard and submit it to an independent 

laboratory for analysis, which must agree within 2 percent of the supplier’s nominal analysis. 

Nitric oxide ................. Cylinder1 of pre-purified nitrogen containing approximately 

100 ppm NO. Dilute with zero air to required concentration.

Use NIST-certified standards whenever possible. If NIST 

standards are not available, obtain 2 standards from 

independent sources which agree within 2 percent, or 

obtain one standard and submit it to an independent 

laboratory for analysis, which must agree within 2 percent of the supplier’s nominal analysis. 

Nitrogen dioxide ......... 1. Gas phase titration as described in reference 6 .............

2. Permeation device, similar to system described in reference 6. 

1. Use an FRM NO2 analyzer calibrated with a gravimetrically calibrated permeation device. 

2. Use an FRM NO2 analyzer calibrated by gas-phase titration as described in reference 6. 

Ozone ......................... Calibrated ozone generator as described in reference 9 .... Use an FEM ozone analyzer calibrated as described in 

reference 9. 

Sulfur dioxide ............. 1. Permeation device as described in references 1 and 2 ..

2. Dynamic dilution of a cylinder containing approximately 

100 ppm SO2 as described in Reference 7. 

Use an SO2 FRM or FEM analyzer as described in reference 7. 


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35600 Federal Register / Vol. 75, No. 119 / Tuesday, June 22, 2010 / Rules and Regulations 

TABLE B–2—TEST ATMOSPHERES—Continued 

Test gas Generation Verification 

Water .......................... Pass zero air through distilled water at a fixed known temperature between 20° and 30° C such that the air 

stream becomes saturated. Dilute with zero air to concentration specified in Table B–3.

Measure relative humidity by means of a dew-point indicator, calibrated electrolytic or piezo electric hygrometer, 

or wet/dry bulb thermometer. 

Xylene ........................ Cylinder of pre-purified nitrogen containing 100 ppm xylene. Dilute with zero air to concentration specified in 

Table B–3.

Use NIST-certified standards whenever possible. If NIST 

standards are not available, obtain 2 standards from 

independent sources which agree within 2 percent, or 

obtain one standard and submit it to an independent 

laboratory for analysis, which must agree within 2 percent of the supplier’s nominal analysis. 

Zero air ....................... 1. Ambient air purified by appropriate scrubbers or other 

devices such that it is free of contaminants likely to 

cause a detectable response on the analyzer. 

2. Cylinder of compressed zero air certified by the supplier 

or an independent laboratory to be free of contaminants 

likely to cause a detectable response on the analyzer. 

1 Use stainless steel pressure regulator dedicated to the pollutant measured. 

Reference 1. O’Keefe, A. E., and Ortaman, G. C. ‘‘Primary Standards for Trace Gas Analysis,’’ Anal. Chem. 38, 760 (1966). 

Reference 2. Scaringelli, F. P., A. E. Rosenberg, E., and Bell, J. P., ‘‘Primary Standards for Trace Gas Analysis.’’ Anal. Chem. 42, 871 (1970). 

Reference 3. ‘‘Tentative Method of Analysis for Ammonia in the Atmosphere (Indophenol Method)’’, Health Lab Sciences, vol. 10, No. 2, 115– 

118, April 1973. 

Reference 4. 1973 Annual Book of ASTM Standards, American Society for Testing and Materials, 1916 Race St., Philadelphia, PA. 

Reference 5. Methods for Air Sampling and Analysis, Intersociety Committee, 1972, American Public Health Association, 1015. 

Reference 6. 40 CFR 50 Appendix F, ‘‘Measurement Principle and Calibration Principle for the Measurement of Nitrogen Dioxide in the Atmosphere (Gas Phase Chemiluminescence).’’ 

Reference 7. 40 CFR 50 Appendix A–1, ‘‘Measurement Principle and Calibration Procedure for the Measurement of Sulfur Dioxide in the Atmosphere (Ultraviolet FIuorscence).’’ 

Reference 8. 40 CFR 50 Appendix C, ‘‘Measurement Principle and Calibration Procedure for the Measurement of Carbon Monoxide in the Atmosphere’’ (Non-Dispersive Infrared Photometry)’’. 

Reference 9. 40 CFR 50 Appendix D, ‘‘Measurement Principle and Calibration Procedure for the Measurement of Ozone in the Atmosphere’’. 

Reference 10. ‘‘Standard Test Method for C, through C5 Hydrocarbons in the Atmosphere by Gas Chromatography’’, D 2820, 1987 Annual 

Book of Aston Standards, vol 11.03, American Society for Testing and Materials, 1916 Race St., Philadelphia, PA 19103. 

■ 15. Section 53.23(d) is amended by 

revising Table B–3 to read as follows: 

§ 53.23 Test procedures. 

* * * * * 

(d) * * * 

TABLE B–3—INTERFERENT TEST CONCENTRATION,1 PARTS PER MILLION

Pollutant Analyzer type 

Hydrochloric 

acid 

Ammonia 

Hydrogen 

sulfide 

Sulfur 

dioxide 

Nitrogen 

dioxide 

Nitric 

oxide 

Carbon 

dioxide 

Ethylene Ozone Mxylene Water 

vapor 

Carbon 

monoxide 

Methane Ethane Naphthalene 

SO2 .............. Ultraviolet fluorescence ............ ............ 5 0.1 4 0.14 0.5 0.5 ............ ............ 0.5 0.2 20,000 ............ ............ ............ 6 0.05 

SO2 .............. Flame photometric ...... ............ ............ 0.01 4 0.14 ............ ............ 750 ............ ............ ............ 3 20,000 50 ............ ............ ............

SO2 .............. Gas chromatography ... ............ ............ 0.1 4 0.14 ............ ............ 750 ............ ............ ............ 3 20,000 50 ............ ............ ............

SO2 .............. Spectrophotometric-wet 

chemical 

(pararosanaline).

0.2 0.1 0.1 4 0.14 0.5 ............ 750 ............ 0.5 ............ .................... ............ ............ ............ ............

SO2 .............. Electrochemical ........... 0.2 0.1 0.1 4 0.14 0.5 0.5 ............ 0.2 0.5 ............ 3 20,000 ............ ............ ............ ............

SO2 .............. Conductivity ................. 0.2 0.1 ............ 4 0.14 0.5 ............ 750 ............ ............ ............ .................... ............ ............ ............ ............

SO2 .............. Spectrophotometricgas phase, including 

DOAS.

............ ............ ............ 4 0.14 0.5 ............ ............ ............ 0.5 0.2 .................... ............ ............ ............ ............

O3 ................ Chemiluminescent ....... ............ ............ 3 0.1 ............ ............ ............ 750 ............ 4 0.08 ............ 3 20,000 ............ ............ ............ ............

O3 ................ Electrochemical ........... ............ 3 0.1 ............ 0.5 0.5 ............ ............ ............ 4 0.08 ............ .................... ............ ............ ............ ............

O3 ................ Spectrophotometric-wet 

chemical (potassium 

iodide).

............ 3 0.1 ............ 0.5 0.5 3 0.5 ............ ............ 4 0.08 ............ .................... ............ ............ ............ ............

O3 ................ Spectrophotometricgas phase, including 

ultraviolet absorption 

and DOAS.

............ ............ ............ 0.5 0.5 0.5 ............ ............ 4 0.08 0.02 20,000 ............ ............ ............ ............

CO ............... Infrared ........................ ............ ............ ............ ............ ............ ............ 750 ............ ............ ............ 20,000 4 10 ............ ............ ............

CO ............... Gas chromatography 

with flame ionization 

detector.

............ ............ ............ ............ ............ ............ ............ ............ ............ ............ 20,000 4 10 ............ 0.5 ............

CO ............... Electrochemical ........... ............ ............ ............ ............ ............ 0.5 ............ 0.2 ............ ............ 20,000 4 10 ............ ............ ............

CO ............... Catalytic combustionthermal detection............. 0.1 ............ ............ ............ ............ 750 0.2 ............ ............ 20,000 4 10 5.0 0.5 ............

CO ............... IR fluorescence ........... ............ ............ ............ ............ ............ ............ 750 ............ ............ ............ 20,000 4 10 ............ 0.5 ............

CO ............... Mercury replacementUV photometric............. ............ ............ ............ ............ ............ ............ 0.2 ............ ............ .................... 4 10 ............ 0.5 ............

NO2 ............. Chemiluminescent ....... ............ 3 0.1 ............ 0.5 4 0.1 0.5 ............ ............ ............ ............ 20,000 ............ ............ ............ ............

NO2 ............. Spectrophotometric-wet 

chemical (azo-dye 

reaction).

............ ............ ............ 0.5 4 0.1 0.5 750 ............ 0.5 ............ .................... ............ ............ ............ ............

NO2 ............. Electrochemical ........... 0.2 3 0.1 ............ 0.5 4 0.1 0.5 750 ............ 0.5 ............ 20,000 50 ............ ............ ............

NO2 ............. Spectrophotometricgas phase............. 3 0.1 ............ 0.5 4 0.1 0.5 ............ ............ 0.5 ............ 20,000 50 ............ ............ ............

1. Concentrations of interferent listed must be prepared and controlled to ±10 percent of the stated value. 

2. Analyzer types not listed will be considered by the Administrator as special cases. 

3. Do not mix with the pollutant. 


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4. Concentration of pollutant used for test. These pollutant concentrations must be prepared to ±10 percent of the stated value. 

5. If candidate method utilizes an elevated-temperature scrubber for removal of aromatic hydrocarbons, perform this interference test. 

6. If naphthalene test concentration cannot be accurately quantified, remove the scrubber, use a test concentration that causes a full scale response, reattach the scrubber, and evaluate response for interference 

* * * * * 

Subpart C [Amended] 

■ 16. Section 53.32 is amended by 

revising paragraph (e)(2) to read as 

follows: 

§ 53.32 Test procedures for methods for 

SO2, CO, O3, and NO2. 

* * * * * 

(e) * * * 

(2) For a candidate method having 

more than one selectable range, one 

range must be that specified in table B– 

1 of subpart B of this part, and a test 

analyzer representative of the method 

must pass the tests required by this 

subpart while operated on that range. 

The tests may be repeated for one or 

more broader ranges (i.e., ones 

extending to higher concentrations) than 

the one specified in table B–1 of subpart 

B of this part, provided that such a 

range does not extend to concentrations 

more than four times the upper range 

limit specified in table B–1 of subpart B 

of this part and that the test analyzer has 

passed the tests required by subpart B 

of this part (if applicable) for the 

broader range. If the tests required by 

this subpart are conducted or passed 

only for the range specified in table B– 

1 of subpart B of this part, any 

equivalent method determination with 

respect to the method will be limited to 

that range. If the tests are passed for 

both the specified range and a broader 

range (or ranges), any such 

determination will include the broader 

range(s) as well as the specified range. 

Appropriate test data shall be submitted 

for each range sought to be included in 

such a determination. 

* * * * * 

■ 17. Table C–1 to Subpart C is revised 

to read as follows: 

TABLE C–1 TO SUBPART C OF PART 53—TEST CONCENTRATION RANGES, NUMBER OF MEASUREMENTS REQUIRED, AND

MAXIMUM DISCREPANCY SPECIFICATIONS

Pollutant Concentration range, parts per million 

(ppm) 

Simultaneous measurements required Maximum 

discrepancy 

specification, 

parts per million 

1-hour 24-hour 

First set Second set First set Second set 

Ozone ...................... Low 0.06 to 0.10 ........................................ 5 6 .................... .................... 0.02 

Med. 0.15 to 0.25 ...................................... 5 6 .................... .................... 0.03 

High 0.35 to 0.46 ....................................... 4 6 .................... .................... 0.04 

Total .................................................... 14 18 .................... .................... ............................

Carbon monoxide .... Low 7 to 11 ................................................ 5 6 .................... .................... 1.5 

Med. 20 to 30 ............................................ 5 6 .................... .................... 2.0 

High 25 to 45 ............................................. 4 6 .................... .................... 3.0 

Total .................................................... 14 18 .................... .................... ............................

Sulfur dioxide ........... Low 0.02 to 0.05 ........................................ 5 6 3 3 0.02 

Med. 0.10 to 0.15 ...................................... 5 6 2 3 0.03 

High 0.30 to 0.50 ....................................... 4 6 2 2 0.04 

Total .................................................... 14 18 7 8 ............................

Nitrogen dioxide ....... Low 0.02 to 0.08 ........................................ .................... .................... 3 3 0.02

Med. 0.10 to 0.20 ...................................... .................... .................... 2 2 0.02 

High 0.25 ................................................... .................... .................... 2 2 0.03 

Total .................................................... .................... .................... 7 8 ............................

PART 58—AMBIENT AIR QUALITY 

SURVEILLANCE 

■ The authority citation for part 58 

continues to read as follows: 

Authority: 42 U.S.C. 7403, 7410, 7601(a), 

7611, and 7619. 

Subpart B [AMENDED] 

■ 19. Section 58.10, is amended by 

adding paragraph (a)(6) to read as 

follows: 

§ 58.10 Annual monitoring network plan 

and periodic network assessment. 

* * * * * 

(a) * * * 

(6) A plan for establishing SO2

monitoring sites in accordance with the 

requirements of appendix D to this part 

shall be submitted to the EPA Regional 

Administrator by July 1, 2011 as part of 

the annual network plan required in 

paragraph (a) (1). The plan shall provide 

for all required SO2 monitoring sites to 

be operational by January 1, 2013. 

* * * * * 

■ 20. Section 58.12 is amended by 

adding paragraph (g) to read as follows: 

§ 58.12 Operating Schedules 

* * * * * 

(g) For continuous SO2 analyzers, the 

maximum 5-minute block average 

concentration of the twelve 5-minute 

blocks in each hour must be collected 

except as noted in § 58.12 (a). 

* * * * * 

■ 21. Section 58.13 is amended by 

adding paragraph (d) to read as follows: 

§ 58.13 Monitoring network completion. 

* * * * * 

(d) The network of SO2 monitors must 

be physically established no later than 

January 1, 2013, and at that time, must 

be operating under all of the 

requirements of this part, including the 


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35602 Federal Register / Vol. 75, No. 119 / Tuesday, June 22, 2010 / Rules and Regulations 

requirements of appendices A, C, D, and 

E to this part. 

■ 22. Section 58.16 is amended by 

adding paragraph (g) to read as follows: 

§ 58.16 Data submittal and archiving 

requirements. 

* * * * * 

(g) Any State or, where applicable, 

local agency operating a continuous SO2

analyzer shall report the maximum 5- 

minute SO2 block average of the twelve 

5-minute block averages in each hour, in 

addition to the hourly SO2 average. 

■ 23. Appendix A to Part 58 is amended 

as by adding paragraph 2.3.1.6 to read 

as follows: 

Appendix A to Part 58—Quality 

Assurance Requirements for SLAMS, 

SPMs and PSD Air Monitoring 

* * * * * 

2.3.1.6 Measurement Uncertainty for SO2. 

The goal for acceptable measurement 

uncertainty for precision is defined as an 

upper 90 percent confidence limit for the 

coefficient of variation (CV) of 10 percent and 

for bias as an upper 95 percent confidence 

limit for the absolute bias of 10 percent. 

* * * * * 

■ 24. Appendix D to Part 58 is amended 

as by revising paragraph 4.4 to read as 

follows: 

Appendix D to Part 58—Network 

Design Criteria for Ambient Air Quality 

Monitoring 

* * * * * 

4.4 Sulfur Dioxide (SO2) Design Criteria. 

4.4.1 General Requirements. (a) State and, 

where appropriate, local agencies must 

operate a minimum number of required SO2

monitoring sites as described below. 

4.4.2 Requirement for Monitoring by the 

Population Weighted Emissions Index. (a) 

The population weighted emissions index 

(PWEI) shall be calculated by States for each 

core based statistical area (CBSA) they 

contain or share with another State or States 

for use in the implementation of or 

adjustment to the SO2 monitoring network. 

The PWEI shall be calculated by multiplying 

the population of each CBSA, using the most 

current census data or estimates, and the 

total amount of SO2 in tons per year emitted 

within the CBSA area, using an aggregate of 

the most recent county level emissions data 

available in the National Emissions Inventory 

for each county in each CBSA. The resulting 

product shall be divided by one million, 

providing a PWEI value, the units of which 

are million persons-tons per year. For any 

CBSA with a calculated PWEI value equal to 

or greater than 1,000,000, a minimum of 

three SO2 monitors are required within that 

CBSA. For any CBSA with a calculated PWEI 

value equal to or greater than 100,000, but 

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

monitors are required within that CBSA. For 

any CBSA with a calculated PWEI value 

equal to or greater than 5,000, but less than 

100,000, a minimum of one SO2 monitor is 

required within that CBSA. 

(1) The SO2 monitoring site(s) required as 

a result of the calculated PWEI in each CBSA 

shall satisfy minimum monitoring 

requirements if the monitor is sited within 

the boundaries of the parent CBSA and is one 

of the following site types (as defined in 

section 1.1.1 of this appendix): population 

exposure, highest concentration, source 

impacts, general background, or regional 

transport. SO2 monitors at NCore stations 

may satisfy minimum monitoring 

requirements if that monitor is located within 

a CBSA with minimally required monitors 

under this part. Any monitor that is sited 

outside of a CBSA with minimum monitoring 

requirements to assess the highest 

concentration resulting from the impact of 

significant sources or source categories 

existing within that CBSA shall be allowed 

to count towards minimum monitoring 

requirements for that CBSA. 

4.4.3 Regional Administrator Required 

Monitoring. (a) The Regional Administrator 

may require additional SO2 monitoring 

stations above the minimum number of 

monitors required in 4.4.2 of this part, where 

the minimum monitoring requirements are 

not sufficient to meet monitoring objectives. 

The Regional Administrator may require, at 

his/her discretion, additional monitors in 

situations where an area has the potential to 

have concentrations that may violate or 

contribute to the violation of the NAAQS, in 

areas impacted by sources which are not 

conducive to modeling, or in locations with 

susceptible and vulnerable populations, 

which are not monitored under the minimum 

monitoring provisions described above. The 

Regional Administrator and the responsible 

State or local air monitoring agency shall 

work together to design and/or maintain the 

most appropriate SO2 network to provide 

sufficient data to meet monitoring objectives. 

4.4.4 SO2 Monitoring Spatial Scales. (a) 

The appropriate spatial scales for SO2

SLAMS monitors are the microscale, middle, 

neighborhood, and urban scales. Monitors 

sited at the microscale, middle, and 

neighborhood scales are suitable for 

determining maximum hourly concentrations 

for SO2. Monitors sited at urban scales are 

useful for identifying SO2 transport, trends, 

and, if sited upwind of local sources, 

background concentrations. 

(1) Microscale—This scale would typify 

areas in close proximity to SO2 point and 

area sources. Emissions from stationary point 

and area sources, and non-road sources may, 

under certain plume conditions, result in 

high ground level concentrations at the 

microscale. The microscale typically 

represents an area impacted by the plume 

with dimensions extending up to 

approximately 100 meters. 

(2) Middle scale—This scale generally 

represents air quality levels in areas up to 

several city blocks in size with dimensions 

on the order of approximately 100 meters to 

500 meters. The middle scale may include 

locations of expected maximum short-term 

concentrations due to proximity to major SO2

point, area, and/or non-road sources. 

(3) Neighborhood scale—The 

neighborhood scale would characterize air 

quality conditions throughout some 

relatively uniform land use areas with 

dimensions in the 0.5 to 4.0 kilometer range. 

Emissions from stationary point and area 

sources may, under certain plume 

conditions, result in high SO2 concentrations 

at the neighborhood scale. Where a 

neighborhood site is located away from 

immediate SO2 sources, the site may be 

useful in representing typical air quality 

values for a larger residential area, and 

therefore suitable for population exposure 

and trends analyses. 

(4) Urban scale—Measurements in this 

scale would be used to estimate 

concentrations over large portions of an 

urban area with dimensions from 4 to 50 

kilometers. Such measurements would be 

useful for assessing trends in area-wide air 

quality, and hence, the effectiveness of large 

scale air pollution control strategies. Urban 

scale sites may also support other monitoring 

objectives of the SO2 monitoring network 

such as identifying trends, and when 

monitors are sited upwind of local sources, 

background concentrations. 

4.4.5 NCore Monitoring. (a) SO2

measurements are included within the NCore 

multipollutant site requirements as described 

in paragraph (3)(b) of this appendix. NCorebased SO2 measurements are primarily used 

to characterize SO2 trends and assist in 

understanding SO2 transport across 

representative areas in urban or rural 

locations and are also used for comparison 

with the SO2 NAAQS. SO2 monitors at NCore 

sites that exist in CBSAs with minimum 

monitoring requirements per section 4.4.2 

above shall be allowed to count towards 

those minimum monitoring requirements. 

* * * * * 

■ 25. Appendix G to Part 58 is amended 

as by revising Table 2 to read as follows: 

Appendix G to Part 58—Uniform Air 

Quality Index (AQI) and Daily 

Reporting 

* * * * * 


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Federal Register / Vol. 75, No. 119 / Tuesday, June 22, 2010 / Rules and Regulations 35603 

TABLE 2—BREAKPOINTS FOR THE AQI 

These breakpoints Equal these AQI’s 

O3 (ppm) 

8-hour 

O3 (ppm) 

1-hour 1

PM2.5

(μg/m 3) 

PM10

(μg/m 3) CO (ppm) SO2 (ppm) 

1-hour 

NO2 (ppm) 

1-hour AQI Category 

0.000–0.059 .. 0.0–15.4 0–54 0.0–4.4 0–0.035 0–0.053 0–50 Good. 

0.060–0.075 .. 15.5–40.4 55–154 4.5–9.4 0.036–0.075 0.054–0.100 51–100 Moderate. 

0.076–0.095 .. 0.125–0.164 40.5–65.4 155–254 9.5–12.4 0.076–0.185 0.101–0.360 101–150 Unhealthy for Sensitive Groups. 

0.096–0.115 .. 0.165–0.204 3 65.5–150.4 255–354 12.5–15.4 4 0.186–0.304 0.361–0.64 151–200 Unhealthy. 

0.116–0.374 .. 0.205–0.404 3 150.5–250.4 355–424 15.5–30.4 4 0.305–0.604 0.65–1.24 201–300 Very Unhealthy. 

(2) .................. 0.405–0.504 3 250.5–350.4 425–504 30.5–40.4 4 0.605–0.804 1.25–1.64 301–400 

(2) .................. 0.505–0.604 3 350.5–500.4 505–604 40.5–50.4 4 0.805–1.004 1.65–2.04 401–500 Hazardous. 

1Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small number of areas where an AQI based on 1-hour ozone 

values would be more precautionary. In these cases, in addition to calculating the 8-hour ozone index value, the 1-hour ozone index value may be calculated, and the 

maximum of the two values reported. 2 8-hour O3 values do not define higher AQI values (≥301). AQI values of 301 or greater are calculated with 1-hour O3 concentrations. 3 If a different SHL for PM2.5 is promulgated, these numbers will change accordingly. 4 1-hr SO2 values do not define higher AQI values (≥200). AQI values of 200 or greater are calculated with 24-hour SO2 concentrations. 

* * * * * 

[FR Doc. 2010–13947 Filed 6–21–10; 8:45 am] 

BILLING CODE 6560–50–P 


srobinson on DSKHWCL6B1PROD with RULES2