1 

## Responses to Significant Comments on the 2009 Proposed Rule on the Primary National Ambient Air Quality Standards

for Sulfur Dioxide (December 8, 2009; 74 FR 64810)

Docket Number OAR-2007-0352

U.S. Environmental Protection Agency

June 20102 

## Table of Contents

I. Introduction ............................................................................................................... 8

II. Responses to Significant Comments on the Scientific Evidence and Exposure/Risk Information ............................................................................................ 10

A. Comments on the controlle d human exposure evidence ................................. 10

B. Comments on the epidemiology ......................................................................... 22

C. Comments on the air quality, exposure, and risk analyses ............................. 34

III. Responses to Significant Comments on the Adequacy of the Current

Standards ......................................................................................................................... 56

IV. Comments on A New Short-Term SO 2 Primary Standard ............................. 59

A. Indicator ............................................................................................................... 59

B. Averaging time .................................................................................................... 59

C. Form ..................................................................................................................... 61

D. Level ..................................................................................................................... 65

E. Retaining or revoking the current SO 2 NAAQS .............................................. 73

V. Technical Issues with Monitoring Requirements ................................................. 78 A: Comments on Proposed Monitoring Program .................................................... 79 B: Comments on Monitoring Method ....................................................................... 83 C: Comments on deadlines for monitoring plans and deployment ........................ 88 D: Comments on Population Weighted Emissions Index (PWEI) monitors ......... 93

E: Comments on State-Level Emissions Monitors ................................................. 103 F: Comments on siting requirements and requesting waivers ............................. 107 G: Comments on Proposed Alternative Network .................................................. 117 H: Comments on data reporting .............................................................................. 121 I: Comments on Cost of Monitoring Changes ........................................................ 124 J: Comments on the authority and respons ibilities of Regional Administrators (RAs) ........................................................................................................................... 126

VI. Air Quality Index .............................................................................................. 128

VII. Comments on the Process for Reviewing the SO 2 Primary NAAQS ........... 131

VIII. Interpretation of the Clean Air Act ............................................................ 131

IX. Comments on Implementation, Desi gnations, and Exceptional Events ...... 136 A. Comments on designations of non-attainment .................................................. 137

B. Comments on date ................................................................................................ 139

C. Comments on data to be used ............................................................................ 141 D. Future permitting ................................................................................................ 147

X. Responses to Significant Comments on A ppendix T (Interpretation of Primary NAAQS and Exceptional Events Rules) ..................................................................... 153

A. Comments on data completeness and data substitutions .......................... 153

XI. Responses to Significant Comments on the Regulatory Impact Analysis ... 158

XII. References .......................................................................................................... 1633 

## Frequently Cited Documents

The following documents are frequently cited throughout EPA = s response to comments, often by means of the short names listed below:

Integrated Science Assessment (ISA):

EPA. (2008). ISA for Oxides of Sulfur - Health Criteria. National Center for Environmental Assessment, Research Triangle Park, NC. Available at: http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=198843

Preamble to the final rule (final notice): Preamble to the Final Rule on the Review of the Primary National Ambient Air Quality Standards for Sulfur Dioxide; to be published in the Federal Register in

June of 2010.

Proposal notice: Primary National Ambient Air Quality St andard for Sulfur Dioxide: Proposed Rule. 74 FR 64810, December 8, 2009.

Risk and Exposure Assessment (REA): EPA. (2009a). Risk and Exposure Assessment to Support the Review of the SO 2 Primary National Ambient Air Quality St andard. Office of Ai r Quality Planning and Standards, Research Triangle Park, NC. Available at: http://www.epa.gov/ttn/naaqs/st andards/so2/s_so2_cr_rea.html4 

## Acronyms and Abbreviations

A&B Alexander & Baldwin, Inc.

AA The Aluminum Association ABR Association of Battery Recyclers, Inc. ACC American Chemistry Council

AECT Association of El ectric Companies of Texas AEPSC American Electric Power Service Corporation AERR Air Emissions Reporting Requirements AFPA American Forest & Paper Association AirQuality AirQuality Research and Logistics ALA American Lung Association ALA-SC American Lung Association State Chapters Alaska Alaska Department of Environmental Conservation Alexandria City of Alexandria, Virginia ACSBPP Annapolis Center for Science-Based Public Policy

API American Petroleum Institute AQCD Air quality criteria document AQI Air quality index AQS Air quality system ASARCO American Smelting And Refining Company ASC The Amalgamated Sugar Company ATS American Thoracic Society BART Best available retrofit technology

CAA Clean Air Act

CAIR Clean Air Interstate Rule CASAC Clean Air Scientific Advisory Committee

CBSA Core-based statistical area CE Consumers Energy

CEM Continuous emission monitor CFR Code of Federal Regulations

CIBO Council of Industrial Boiler Owners City of New York City of New York Law Department, Environmental Law Division CMAQ Community multi-scale air quality

CO Carbon monoxide

CRA Corn Refiners Association Delaware Delaware Department of Natural Resources and Environmental

Control Dow The Dow Chemical Company DRSI Dominion Resources Services, Inc. Duke Energy Duke Energy Corporation ED visits Emergency department visits EDEC Empire District Electric Company

EDF Environmental Defense Fund

EEI Edison Electric Institute EGU Electric generating unit, or electricity generating utility5 EPA Environmental Protection Agency

EPRI Electric Power Research Institute ExxonMobil ExxonMobil Corporation FEM Federal equivalent method

FEV 1 Forced Expiratory Volume in the first second First Energy FirstEnergy Corporation Florida Florida Department of Environmental Protection FR Federal Register

FRM Federal reference method

GAM Generalized Additive Model GCLC Gulf Coast Lignite Coalition GDI Green Dubuque Inc. Georgia Georgia Department of Natural Resources

Golder Golder Associates Inc. Hamilton Pepper Hamilton Harris County Harris County Public Health and Environmental Services

H-GAC Houston-Galveston Area Council Houston City of Houston, TX

Illinois Illinois Envir onmental Protection Agency Indiana Indiana Department of Environmental Management Iowa Iowa Department of Natural Re sources/Iowa Department of Public

Health ISA Integrated Science Assessment JRSC J.R. Simplot Company KUC Kennecott Utah Copper, LLC KYDEP Kentucky Department for Environmental Protection

LAER Lowest Achievable Emission Rate LEC Lignite Energy Council

Louisiana Chemical Louisiana Chemical A ssociation, Louisana Mid-Continent Oil and Gas Association, and Louisana Pulp and Paper Association Marathon Petroleum Marathon Petroleum Company

MDUC Montana-Dakota Utilities Co. MIDNRE Michigan Department of Natural Resources and Environment Missouri Missouri Department of Natu ral Resources, Air Pollution Control Program MSCC Montana Sulphur and Chemical Company Inc. MWV MeadWestvaco Corporation NAAQS National Ambient Air Quality Standards NACAA National Association of Clean Air Agencies

NAM National Association of Manufacturers

NEI National Emissions Index

NESCAUM Northeast States for C oordinated Air Use Management New Mexico New Mexico Environment Department, Air Quality Bureau NMA National Mining Association

NO 2 Nitrogen dioxide North Carolina North Carolina Department of Environment and Natural Resources6 NO

x Nitrogen oxides

NPRA National Petrochemical & Refiners Association

NRDC Natural Resource Defense Council

NRECA National Rural Electri c Cooperative Association

NSR New Source Review Nucor Nucor Corporation NYSDEC New York State Department of Environmental Conservation NYDOH New York State Department of Health

O 3 Ozone Ohio Ohio Environmental Protection Agency

PCA Portland Cement Association PE Progress Energy Pennsylvania Pennsylvania Departme nt of Environmental Protection Pepper Hamilton Pepper Hamilton

PM Particulate matter

PM 10 Particulate matter with an aer odynamic diameter of less than 10

micrometers

PM 2.5 Particulate matter with an aer odynamic diameter of less than 2.5

micrometers ppb parts per billion PPLC PPL Corporation ppm parts per million PSD Prevention of significant deterioration PWEI Population weighted emissions index RA Regional administrator RACT Reasonable available control technology REA Risk and Exposure Assessment RIA Regulatory Impact Analysis Rio Tinto Rio Tinto Corporation

RMA Rubber Manufacturers Association RRI RRI Energy, Inc.

RTA Rio Tinto Alcan Sebree Works RTC Response to Comment

SC Sierra Club SER Significant emission rate SIDS Sudden infant death syndrome SIL Significant impact level SIP State implementation plan SMC Significant monitoring concentration

SO 2 Sulfur dioxide South Carolina South Carolina Department of Health and Environmental Control South Dakota South Dakota Department of Environment and Natural Resources

SO

x Sulfur oxides Springfield City of Springfield, MO

SAP Ad Hoc Committee of Sulfuric Acid Producers sRaw Specific Airway Resistance7 TAB Texas Association of Business Texas Texas Commission on Environmental Quality

TXOG Texas Oil and Gas Association

TFI The Fertilizer Institute

tpy tons per year UARG Utility Air Regulatory Group URL Upper range limit USDOI United States Department of Interior

Vermont Vermont Air Pollution Control

WEACT WE ACT for Environmental Ju stice (West Harlem Environmental Action, Inc.) WHO World Health Organization Wisconsin Wisconsin Department of Natural Resources8 

## I. Introduction

This document, together with the preamble to the final rule on the review of the primary national ambient air quality standard s (NAAQS) for sulfur dioxide (SO 2 ), presents the responses of the Environmental Protecti on Agency (EPA) to the public comments received on EPA’s proposal of December 8, 2009, proposing revisions to the primary NAAQS for SO 2 , as well as related proposed cha nges to the ambient air monitoring, reporting, and network design requirements for the primary SO 2 NAAQS. 74 FR 64810. All significant issues raised in timely public comments have been addressed. Where

comments were submitted after the close of the public comment period, EPA has responded to the extent feasible.

Due to the number of comments that addresse d similar issues, this response-to-comments document does not generally cross-referen ce each response to the commenter(s) which raised the particular issue involved, although commenters are identified in some cases where they provided particularly detailed comm ents that were used to frame the overall response on an issue.

The responses presented in this document are intended to augment the responses to comments that appear in the preamble to the final rule or to address comments not discussed in the preamble to the final rule. Although portions of the preamble to the final rule are paraphrased in this document where us eful to add clarity to responses, to the extent any (unintended) ambi guity is introduced by this pa raphrasing, the preamble itself

remains the definitive statement of the rationale for the positions adopted in the final rule.

In many instances, particular responses pr esented in this document include cross references to responses on related issues that are located either in the preamble to the SO 2 primary NAAQS final rule, or in this Re sponse to Comments document. In other instances the comment is appropriately addr essed by the Agency’s discussion in other parts of the record. All issues on which the Administra tor is taking final action in the

SO 2 primary NAAQS final rule are addressed in the SO 2 NAAQS rulemaking record.

Accordingly, this Response to Comments document, together with the preamble to the

SO 2 primary NAAQS final rule and the informa tion contained in the Integrated Science Assessment (ISA) (EPA, 2008), the Risk a nd Exposure Assessment (REA) (EPA, 2009a), and the Notice of Proposed Rulemaking shoul d be considered co llectively as EPA’s response to all of the significant comments submitted on EPA’s 2009 SO 2 primary NAAQS proposed rule. This document inco rporates directly or by reference the significant public comments addressed in the preamble to the SO 2 NAAQS final rule as well as other significant public comments that were submitted on the proposed rule.

Consistent with the final decisions presente d in the notice of final rulemaking, comments on the following topics are addressed in this document: the scientific evidence and exposure/risk information (section II) ; the adequacy of the current SO 2 standard to protect public health (section III); revi sions to the current standard in terms of indicator (section IV.A), averaging time (section IV.B), form (section IV.C), leve l (section IV.D) and9 retaining or revoking the current standard s (section IV.E); re visions to the SO 2 monitoring network (section V); the air quality index (VI); th e process for reviewing the standard (section VII); interp retation of the Clean Air Act (section VIII); implementation, designations, and exceptional (section IX); a nd on data handling (section X). Finally, comments related to the Regulatory Impact Analysis are addre ssed (section XI).10 

## II. Responses to Significant Comments on the Scientific Evidence and Exposu re/Risk Information

A. Comments on the controlle d human exposure evidence

(1) Comment: Several industry groups (e.g., AP I, UARG, ACC, ASC, CE, AEPSC, ASARCO, NMA, EEI, RTA) commented that EPA does not provide justification for changing the conclusion in the 1996 re view that adverse effects were based on

SO 2 exposure concentrations that resulted in large decrements in lung function and moderate to severe respiratory sympto ms in exercising asthmatics. That is, industry groups do not believe EPA has provided an adequate rationale for focusing the current review on moderate to severe decrements in lung function and/or respiratory symptoms ranging from mild (perceptible wheeze or chest tightness) to severe (breat hing distress requiring the use of a bronchodilator). For example, UARG comments state:

The genesis of this change is obscure . First, the Proposed Rule seems to postulate that the health risk to asthmatics who experience lung function effects without symptoms might be great er than the health risk to those who experience both symptoms and lung function decrements. See id. at 64816/2. It is simply implausible that a failure to experience symptoms is worse than having them. Second, the Proposed Rule points to advice from CASAC as justification for the focu s on moderate as well as large lung

function decrements. Id. at 64817/3. Bu t the Administrator culls this

advice from comments of individual CA SAC members in the transcript of a CASAC meeting; it is not reflecte d in the letter from the complete CASAC that followed that meeting. Finally, the Administrator simply provides no explanation of why she now considers mild symptoms to be adverse health effects. Clearly, this does not constitute a reasoned analysis in support of the change to the de finition of adversity (see comments provided by UARG, p.10).

Response: The commenters are corr ect that the ISA, and this review, focused on moderate or greater decrements in lung function in the presence or absence of respiratory symptoms in exercising asthma tics. With respect to the ISA, from individual level response data derived from controlled human exposure studies,

the ISA characterized the number a nd percent of exercising asthmatics experiencing a ≥ 100% increase in specific airway resistance (sRaw ) or ≥ 15% decline in forced expiratory volume in the first second (FEV 1 ). The results of this analysis indicated that exposure to SO 2 concentrations as low as 200 -300 ppb for 5 - 10 minutes resulted in approximately 5 - 30% of exercising asthmatics experiencing moderate or greater decrem ents in lung function (ISA, Table 3-1). In addition, at SO 2 concentrations ≥ 400 ppb, 20 - 60% of exercising asthmatics experienced moderate or greater decrem ents in lung function. Notably, at SO 2 concentrations ≥ 400 ppb the ISA noted that moderate or greater decrements in11 lung function are frequently accompanied with reparatory symptoms, while at 200 – 300 ppb there is limited evidence of respiratory symptoms.

The decision in the ISA to focus on m oderate or greater decrements in lung function with or without accompa nying respiratory symptoms was not , as commenters contend, based solely on the advice of individual CASAC panel members. In part, as noted on page 3-4 of the ISA, this judgment reflects recent guidance provided by the American Thoraci c Society (ATS, 2000). That is, theISA states: “In their official statement, the ATS concluded that an air pollutioninduced shift in a population distribution of a given h ealth-related endpoint (e.g., lung function in asthmatic children) should be considered adverse, even if this

shift does not result in the immediate o ccurrence of illness in any one individual in the population (ISA, p.3-4).” Thus, th e ISA concluded that these updated ATS guidelines could reasonably be interprete d to indicate that moderate or greater decrements in lung function, even without respiratory symptoms, could be of public health significance. (See also re sponse #5 below, explaining that EPA would reach the same conclusion without reference to the ATS Guidelines.) Notably, this focus was also in agreem ent with CASAC comments. In their

consensus letter to the Administrato r dated August 8, 2008, CASAC advised EPA to place more of an emphasis on SO 2 concentrations below 0.4 - 0.6 ppm (i.e. below 400 – 600 ppb), stating, “The clinical and epidemiological studies warrant a stronger conclusion about heal th effects at lower levels.” This counsel was based in part on the fraction of asthmatics shown to experience “... moderate or greater decrements in lung function at SO 2 concentrations as low as 0.2 – 0.3 ppm.” (Emphasis added.) Moreover, we note that this is in agreement with previous CASAC recommendations (Henderson 2006) and NAAQS review conclusions (EPA 2006, EPA 2007) indicating that moderate decrements in lung function can be clinically significant in so me asthmatics. See Coalition of Battery Recyclers Association v. EPA , No. 09-1011 (D.C. Cir., May 14, 2010), slip opinion at 9 (reasonable for EP A to conclude that two IQ point loss is an adverse effect based in part on CASAC advice th at such a decrement is significant).

The commenter’s assertion that EPA’s in terpretation of the controlled human exposure studies is grounded in the view that not having symptoms is worse than having them is misplaced. First, neither EPA nor the ISA made that statement. That language does not appear at 74 FR 64816 (the citation given by the commenter). Nonetheless, EPA believe s that focusing on only more severe respiratory symptoms obser ved in controlled human exposure studies would be inappropriate and not protective of public he alth. As discussed in the final ISA (p. 3-4 to 3-5), symptom percep tion is highly variable among asthmatics. This is particularly true in childr en, and may result in an increased risk of respiratory morbidity and mortality in those individua ls who are poor perceivers of changes in pulmonary function, as they will be le ss likely to seek treatment. That is, individuals who experience moderate or greater decrements in lung function, but only mild or perhaps no respir atory symptoms at all are potentially at greater risk of developing hypoxia because they are less likely to seek medical treatment (as12 noted by clinicians on the CASAC panel). Thus, it is possible that in some instances, a failure to have respiratory symptoms can lead to a more serious health

outcomes.

(2) Comment: Several industry groups (e.g., AP I, UARG, ACC, ACSBPP, Nucor, AEPSC, ABR, ASARCO, NMA, Dow, NRECA, NPRA, A&B) generally commented that the effects reported in exercising asthmatics following exposure to 200 - 300 ppb SO 2 for 5-10 minutes should not be considered adverse by the

Administrator. Commenters indicate that at 200 – 300 ppb, decrements in lung function reported in controlled human exposure studies in exercising asthmatics are not statistically significant, and th ey are not frequently accompanied by respiratory symptoms. Many of these groups indicate that EPA relied upon the

advice of select CASAC members rather than consensus CASAC letters in determining adverse effects at 200 - 300 ppb SO 2

.

Response: The commenters are correct that following 5-10 minute SO 2 exposures at 200- 300 ppb, the moderate or greater decrements in lung function experienced by some exercising asthmatics ar e not statistically significant at the group mean level or frequently a ccompanied by respiratory symptoms

. However, we strongly disagree that these effects ar e not adverse to the health of asthmatics. EPA’s rationale for considering health effects associated with 5-10 minute SO 2 exposures from 200- 300 ppb as adverse is based on consensus CASAC advice and recommendations, EPA’s conclusions in previous reviews of other NAAQS, as well as ATS guidelines of what constitute s an adverse effect of air pollution.

These considerations and similar comment s are thoroughly described in sections II.B.1.a, II.E.2.b and II.F.4.b of the preamble to the final rule.

(3) Comment: A number of industry gro ups stated that it is inappropriate to analyze

the effects of SO 2 in terms of percentage of as thmatics affected, and that it is better to conduct analyses on group mean data. Industry groups (e.g., API, MSCC, ACSBPP) also stated that effect s 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. For example API states:

EPA’s conclusions about adverse effects in this range of SO 2 exposure relies on selective consideration of ne gative responses in lung function in only a few individuals from one tail of the tail response distribution while ignoring similar positive and apparen tly beneficial responses in lung function, an approach that is not scientif ically defensible (see comments provide by API; p. 14).

Similarly, ACSBPP commented that:

...day-today variation in pulmonary f unction test results, with a small portion of increases and decreases that are as large as the metric uses for13 moderate decrements. EPA needs to c onsider the day-to-day variation in pulmonary function for asthmatics that may be affected by changes in meteorology, personal activities, exposure to indoor irritant s and allergens, etc. in the analysis (see comm ents provide by ACSBPP; p. 6).

Response: EPA strongly disagrees with the as sertion that the analysis of percentage of asthmatics experi encing moderate or greater SO 2 -induced decrements in lung function reflects on ly normal variation in lung function

unrelated to SO 2 exposure. This analysis clearly shows an increas e in the fraction of asthmatics affected with increasing SO 2 concentrations, an effect which has been consistently demonstrated across studies. EPA further notes that both the ISA and REA have specifically considered and discussed statistical analyses of mean changes (group data) in lung function resulting from controlled exposures to

SO 2 . See ISA at page 3-33, “Statistically significant decrements in lung function accompanied by respiratory symptoms including wheeze and chest tightness have been clearly demonstrated fo llowing exposure to 0.4-0.6 ppm SO 2 . Although studies have not reported statistically significant respiratory effects following exposure to 0.2-0.3 ppm SO 2 , some asthmatic subjects (5-30%) have been shown to experience moderate to large decrem ents in lung function at these exposure concentrations.” In addition, in comments prepared and submitted to the docket for API, William Linn’s analysis shows that, “...it is possible to pick out response

data and statistical results from our st udies that show si gnificant (P < 0.05) pulmonary function changes at 0.2 ppm relati ve to control (fil tered air, 0.0 ppm

SO 2 ),” although Dr. Linn did not consider these effects to be clinically significant.

EPA’s conclusions from cont rolled human exposures to SO 2 have been drawn

from an evaluation of both individual a nd group mean response data. As stated on page 3-9 of the ISA, “Among asthmatics, both the magnitude of SO 2 -induced decrements in lung function and the pe rcent of individuals affected have consistently been shown to incr ease with increasing exposure to SO 2 concentrations between 0.2 and 1.0 ppm.”

The commenters correctly point out that at the lowest concentration tested in freebreathing chamber studies (0.2 ppm), ther e are a similar number of asthmatics experiencing a moderate or greate r decrease in lung function (i.e., ≥ 100 increase

in sRaw or ≥ 15 decrease in FEV 1 ) and experiencing what might be called a moderate improvement in lung function (i.e., ≥ 100 decrease in sRaw or ≥ 15

increase in FEV 1 ). This observation is consistent with data presented in Figures

4-2 and 4-3 of the ISA showing essentially no SO 2 -induced change in lung function at 0.2 ppm when averaged acro ss asthmatics participating in the three cited studies conducted by William Linn

. However, these figures also

demonstrate that asthmatics who are sensitive to SO 2 at a higher concentration (0.6 ppm) experience, on average, a great er decrement in lung function at lower concentrations, including 0.2 ppm, when compared with all subjects combined. Therefore, while some asthmatics are relatively insensitive to SO 2 -induced respiratory effects even at concentrations ≥ 0.6 ppm, there is clear empirical evidence that others experience sign ificant bronchoconstriction following14 exposures to both relatively high (0.6 ppm) and low (0.2 ppm) SO 2 concentrations. Among these SO 2 -sensitive asthmatics, Figures 4-2 and 4-3 show

a clear increase in bronchoc onstriction with increasing SO 2 concentration from 0.2 - 0.4 ppm. Given this clear relationship of exposure and effect at all levels in the sensitive asthmatics (i.e. those who e xperienced significant decrements in lung function at the highest exposure concen tration used (600 ppb)), EPA does not accept the commenter's premise that the results of the study do not demonstrate

adverse effects. It should also be no ted that significant effects have been

observed at concentrations as low as 0.1 ppm (the lowest concentration tested) using mouthpiece exposure systems. Alt hough these results cannot be compared directly with free-breathing chamber st udies, they nonetheless provide valuable information regarding respiratory effects following exposures to low

concentrations of SO 2

. These studies provide evid ence of bronchoconstriction directly attributable to SO 2 at the lowest concentra tions tested among mild and

moderate asthmatics

. We further note that severe asthmatics could experience greater effects. See CASAC’s consensus letter to the Administrator dated August 8, 2008, advising EPA to focus more on c oncentrations < 0.4 ppm, in part because, “...severe asthmatics were not part of these clinical studies, but it is not unreasonable to presume that they woul d have responded to even a greater degree.”

EPA also notes that a variety of factors su ch as variability in baseline conditions, time of day, exercise, and the laboratory exposure setting itself may affect lung function responses in subject s participating in controll ed human exposure studies. Recognizing the potential of these factors to alter pulmonary function, investigators expose study participants to pollutant(s) of inte rest and a control atmosphere, typically filtered air. Exposur es are conducted at the same time of day under the same laboratory conditions . By randomly assigning the order of these exposures, bias in response is di stributed between the exposure days and improves the ability to discern the “true” e ffect of a pollutant exposure. Therefore, the other factors affecting lung function mentioned by the commenter are, to the extent possible, already c ontrolled for by the design of these studies (see ISA, p.1- 4).

(4) Comment: Several industry groups (e.g., UARG, API, ACC, Nucor) and organizations (e.g., EPRI) generally argue that EPA is not applying the 2000 ATS adversity guidelines (ATS 2000) with re spect to population risk appropriately. These groups argue that since the effects of SO 2 are transient and reversible within an hour, there is li ttle chance of an asthmatic affected by these low levels

of SO 2 having diminished reserve lung f unction when confronted by another stimuli (e.g. viral infection). Groups also argue that vari ations in lung function is

a hallmark of asthma and that there is no indication that the most sensitive exercising asthmatics represent a distinct subpopulation that may be affected in

the manner described in the ATS Statement.15 Response: ATS guidelines were not meant to pr ovide strict rules related to the

differentiation between adverse and non-advers e health effects of air pollution. As stated in the guidelines, “...the placement of dividing lines should be a societal judgment and consequently this committee does not propose specific boundaries for separating adverse from nonadverse effects.” Therefore, judgments regarding the adversity of effects following expos ure to specific concentrations of SO 2 represent EPA’s interpretation of the AT S guidelines. These judgments also

reflect CASAC recommendations and conc lusions drawn by EPA in previous NAAQS reviews (as described in final rule preamble sections II.B.1.a, II.E.2.b and II.F.4.b; see particularly final rule preamble section II.E.2.b and the response to Comment 5 in this section)

.

Moreover, although it is correct that th e decrements in lung function caused by exposure to SO 2 in controlled human exposure studies have been shown to be transient, EPA disagrees with the asserti on that there is little chance that an asthmatic would simultaneously be conf ronted by a secondary agent. Such exposure could very well occur, particul arly considering the myriad of other stimuli known to affect lung function in asthmatics (e.g., allergens, cold air, coexposures to other pollutants—not just viral infections). In addition, those asthmatics with diminished reserve lung function caused by other factors such as an existing respiratory infec tion would be at higher ri sk of experiencing adverse respiratory effects if subsequently exposed to SO 2 notwithstanding the transient

nature of SO 2 -induced effects.

(5) Comment: Multiple groups (e.g., ACSBPP) indicated that the ATS guidelines (ATS 2000) regarding adversity due to a population shift for a given health endpoint do not apply to SO 2 effects because the ATS was referring to
permanent decrements in lung function in a population.

Response: EPA disagrees with the commenters’ interpretation of the ATS guidelines at the population level. The exam ple given of an adverse effect at the population level given in the ATS Guidelines is as follows:

A population of children with asthma could have a distribution of lung

function such that no individual ch ild has a level associated with significant impairment. Exposure to air pollution could shift the

distribution toward lower le vels without bringing a ny individual child to a

level that is associated with clinical ly relevant consequences. Individuals within the population w ould, however, have dimini shed 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 consid ered that such a shift in the risk factor distribution, and hence the ri sk profile of the exposed population, should be considered adverse, even in the absence of the immediate occurrence of frank illness (ATS, 2000; p. 668).16 EPA does not believe that the above exampl e was referring to a permanent loss in lung function. The Guidelines refer to permanent loss in lung function specifically when permanent effects are at issue, and further state that any permanent loss of lung function in an indivi dual, or at the population level, is by

definition an adverse effect:

There is also epidemiologic evidence that air pollution may adversely affect lung growth or accelerate th e age-related declin e of lung function. Epidemiologic studies are limited in their power to detect such permanent effects and any evidence of associat ion between air po llution exposure and permanent loss of function is indicat ive of an adverse effect at the population level. Some individuals may sustain clinically relevant, permanent losses of lung function. Th is committee considered that any detectable level of permanent lung function loss attri butable to air pollution exposure should be considered as adverse. (ATS 2000; p. 671).

The example quoted above (p. 668) does not mention permanent effects,

“diminished reserve function” is not an inherently permanent effect, and, since any permanent effect is already deemed adverse under the Guidelines, EPA believes that the example would have no point if it was referring only to permanent effects. Thus, EPA is reas onably interpreting th is part of the

Guidelines to indicate that certain no n-permanent effects at the population level may be considered to be adverse.

Even without considerin g the 2000 ATS guidelines mentioned above, EPA would consider the asymptomatic decrements in lung function associated with 5- 10

minute SO 2 exposures as low as 200 ppb to be adverse. As described in the preamble to the final rule in sections II.B.1.a, II.E.2.b and II.F.4.b, these reasons include the conclusions drawn by EPA a nd CASAC in previous NAAQS reviews regarding the significance of decrements in lung function. That is EPA has stated that similar moderate or greater decrements in lung function (e.g., a ≥ 15% decline

in FEV 1 and/or ≥ 100% increase in sRaw) in peopl e with pre-existing respiratory

disease could result in clinical outco mes such as increased medication usage and/or disruption of normal activitie s (see REA, section 4.3, p. 35-36) which

would also be considered adverse effect s of air pollution under ATS guidelines (ATS 1985). In addition to these conclu sions and CASAC advi ce from previous NAAQS reviews, EPA considered the consensus advice from CASAC in the

current SO 2 NAAQS review (Henderson 2008a ; Henderson 2008b; Samet 2009; and final rule preamble sections II. B.1.a, II.E.2.b and II.F.4.b), as well as

comments from individual clinicians on the CASAC panel during the current review (see final rule preamble secti ons II.B.1.a, II.E.2.b and II.F.4.b). See Coalition of Battery Recyclers Association v. EPA , No. 09-1011 (D.C. Cir., May 14, 2010), slip opinion at 9 (reasonable for EPA to conclude that two IQ point

loss is an adverse effect ba sed in part on CASAC advice that such a decrement is significant).17 EPA finally notes that the current IS A also indicated that asymptomatic decrements in lung function can be clin ically significant. The ISA states:

...it is important to note that sympto m perception is highly variable among asthmatics even during severe episodes of asthmatic bronchoconstriction. An asymptomatic decrease in lung fu nction may pose a significant health

risk to asthmatic individuals as it is less likely that these individuals will seek treatment (ISA, section 3.1.3; p. 3-4).

Taken together, along with the considerat ion that severe asthmatics were not included in the controlled human exposur e studies described in the ISA, the Administrator would have reasonably cons idered moderate or greater decrements in lung function in exercising asthmatics to be adverse, even without considering the 2000 ATS guidelines mentioned above. That being said however, she finds that the 2000 ATS guidelines provide strong support for considering these decrements in lung function, even without accompanying respiratory symptoms, as adverse. In addition, for the reason s outlined above, the Administrator further

notes that she does not believe that only repeated exposures to SO 2 should be

considered adverse to the health of asthmatics.

(6) Comment: Several industry groups (e.g., API, UARG, ACC, CE) commented that EPA is incorrect in assuming that se vere asthmatics would likely have a more pronounced response to SO 2 exposures at a given level, or would respond to even

lower levels of SO 2 . As support for their assertion multiple industry groups cite studies in the ISA stating th at they included “severe as thmatics” as well as a study by Linn et al. (1987) concluding that among asthmatics, responses to SO 2 exposure are not dependant on the clinical severity of asthma and that “the subjects with the highest risk [of tempor ary respiratory disturbances from ambient

SO 2 ] can be identified only be actua lly measuring their responses to SO 2. ”

Response: We disagree with the assertion that severe asthmatics have been

evaluated in 5 - 10 minute controlled huma n exposure studies. These comments and EPA responses are discussed in detail in section II.E.2.b of the preamble to

the final rule.

(7) Comment: EPRI generally commented that EPA is assuming that severe

asthmatics were not included in controll ed human exposure studies. However, EPRI contends that it is unlikely severe asthmatics would be exercising with the

ventilation rates needed to be affected by SO 2

.

Response: EPA strongly disagrees with th e assertion that the most SO 2 -sensitive asthmatics have been included in controlled human exposure studies. EPA recognizes that it is possibl e that the most severe asth matics would be less likely to experience SO 2 -induced bronchoconstriction due to exercise limitations. However, it is also possible that these as thmatics experience respiratory effects of

SO 2 exposure at lower ventilation than is required to produce a response in18 individuals with less severe disease. In addition, there is clear ly a continuum of disease severity in the ge neral population above the leve l of disease among those asthmatic subjects who participated in st udies involving controlled exposures to

SO 2 . It is therefore very possible that a si gnificant fraction of these more severe asthmatics would be capable of experien cing increases in minute volume which

would increase the fraction of SO 2 reaching the tracheobronchial airways. This legitimate inference is supporte d by CASAC’s consensus advice that,

“

... severe asthmatics were not part of thes e clinical studies, but it is not unreasonable to presume that they woul d have responded to even a greater degree.” Henderson (2008b; p. vi).

(8) Comment: Multiple groups (e.g., ACSBPP, MWV, EPRI, Golder, AFPA, EEI, A&B) indicated that the respiratory effects of SO 2 are not adverse because they are transient, reversible a nd/or can be alleviated th rough the use of medication. Moreover, they are similar to the effect s experienced by asthmatics in response to

common stimuli such as cold ai r, or psychological stress.

Response: We agree that the respiratory effects of SO 2 are transient and reversible. However, we str ongly disagree that this indi cates that such effects are

not adverse to the health of asthmatics. EPA further disagrees that effects following SO 2 exposure should not be consider ed adverse because they are

similar to the effects ex perienced by asthmatics in response to common stimuli such as cold air. As noted above, judg ments regarding the adversity of effects following SO 2 exposure are described in detail in sections II.B.1.a, II.E.2.b, and II.F.4.b of the preamble to the final rule . However, additional information is provided below.

Simply because the respiratory effects of SO 2 in exercising asthmatics are transient and similar to those experien ced by asthmatics in response to commonly encountered stimuli (e.g., cold air), it does not mean th at such effects should not

be considered adverse to the health of asthmatics. That is, following SO 2
exposures ≥ 400 ppb, transient moderate or grea ter decrements in lung function are frequently accompanied by respiratory symptoms and this combination of effects would clearly be considered adverse under ATS guidelines (ATS 1985, ATS 2000). The observation that these respir atory effects are similar to those in response to commonly encountered stimu li does not change this conclusion.

In addition, following SO 2 exposures in the range of 200 - 300 ppb, controlled human exposure studies indi cate that exposure to SO 2 can result in an appreciable percentage of exercising asthmatics experi encing moderate or greater decrements in lung function. Notably, CASAC indicat ed that such decrements in lung function can be clinically significant in so me asthmatics. As with exposures at higher levels, the observation that these resp iratory effects are similar to those in response to commonly encountered stim uli does not change the following conclusions: 1) these respiratory effects are the direct resu lt of exposure to SO 2 ; and 2) these effects can be clinically significant in exercising asthmatics. We19 finally note that any respiratory effect experienced by exercising asthmatics in response to SO 2 exposure would likely be in additi on to those respiratory effects resulting from exposure to commonly enc ountered stimuli (that is, if co-exposure occurred). Thus, SO 2 exposure could reasonably be j udged to increase the risk of a more serious respiratory outcome (e.g., an asthma attack) because it could potentially reduce even further, an asthma tic’s lung function that has already been reduced by one or more commonly encountered stimuli.

With respect to the respiratory effects of SO 2 being alleviated through the use of medication, we note as discussed in th e ISA (ISA, section 3.1.3.2), that some

asthma medications have been shown to significantly reduce SO 2 -induced bronchoconstriction. However, the ISA fu rther notes that even among severe asthmatics, the disease is often poorly controlled “...due to inadequate drug therapy or poor compliance among t hose who are on regular medication.” Furthermore, EPA agrees with the statement from the 1985 ATS guidelines that medication usage in potentia lly susceptible populations should not be used as a justification to allow for in creased pollutant concentra tions. In addition, the ATS

has recommended that detectable effects of air pollution on clinical measures (e.g., medication use) be considered adve rse effects of air pollution (ATS, 1985)

(9) Comment: Some health and environmental groups wanted more emphasis placed on mouthpiece studies at 100 ppb. Fo r example ALA et al., stated:

In its analysis of data from chambe r studies in the ISA and in the REA,

EPA focuses on studies of “free brea thing” exposure. In doing so, EPA improperly and arbitrarily downplays important evidence that reported increased airway resistance, a measur e of bronchoconstriction, in subjects with mild asthma at concentrations of 100 ppb. Regrettably, EPA does not rely on the mouthpiece studies in form ulating its proposed standards (see comments provided by ALA et. al ., p. 15).... In downplaying the mouthpiece studies, EPA ignores the larg e segment of people who rely on oral or oronasal breathing some or all of the time (see comments provided by ALA et. al., p. 16).

Moreover, health and environmental groups note that effects at 100 ppb are backed up by studies in laboratory animals.

Response: These comments and EPA’s respon ses were initially discussed in section II.F.4.b of the preamble to the final rule. However, EPA provides

additional details below.

EPA carefully considered the mouthpiece studies and continues to believe that

these studies are not a reas onable proxy for actual exposure. In these studies, SO 2 is delivered directly through the mout h, typically in conj unction with nasal occlusion. This allows a grea ter fraction of the inhaled SO 2 to reach the tracheobronchial airways. Although we agree with commenters that some20 individuals do breathe oronasa lly both while at rest and during exercise, nasal ventilation still constitutes a significan t percentage of total ventilation. The consequence is that in dividuals exposed to SO 2 through a mouthpiece are likely to experience greater respiratory effects from a given SO 2 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 l ung function following exposure to 100 ppb

SO 2 . Therefore, the Administrator reasona bly did not place great weight on these mouthpiece studies when considering th e appropriate level of a 1-hour SO 2 standard. The commenters correctly point out that animal toxicology studies provide some evidence of an SO 2 -induced increase in ai rway hyperresponsiveness at concentrations of 100 ppb. However, thes e effects were observed in only one species, and only following repeat SO 2 exposures.

(10) Comment: MSCC generally commented th at while annual and daily SO 2 concentrations have fallen dramatically over the last 20 years, asthma prevalence has steadily increased. Thus, additional reductions in SO 2 are unlikely to produce additional public health benefits.

Response: EPA acknowledges that the etiology of asthma is complicated and likely involves multiple factors. Howe ver, EPA strongly disagrees with the

assertion that additional reductions in SO 2 will not benefit the health of

asthmatics. Numerous controlled huma n exposure studies have consistently demonstrated that exposure to 5-minute peaks of SO 2 causes decrements in lung function and/or respiratory symptoms in exercising asthmatics. Thus, limiting those 5-minute concentrations is very lik ely to produce public health benefits.

In addition, as noted throughout this NAAQS review, numerous epidemiologic studies have reported associations between SO 2 and markers of respiratory morbidity such as emergency department vi sits for asthma. Thus, it is similarly likely that limiting SO 2 concentrations to below t hose in the locations these studies were conducted will produce public health benefits.

(11) Comment: Rio Tinto generally commented th at the ATS concept of enhancing the risk to a population to an unacceptable degree, without shifting the risks of any one individual to an unaccep table level is impractical. They note that taken to its logical conclusion, an y concentration of SO 2 would decrease reserve lung function and be unacceptable.

Response: As noted in previous responses, ATS guidelines were not meant to provide strict rules related to the diff erentiation between adverse and non-adverse health effects of air pollution. As stated in the guidelines, “...the placement of dividing lines should be a societal judgm ent and consequently this committee does not propose specific boundaries for separa ting adverse from nonadverse effects.” Therefore, EPA’s judgments regarding th e adversity of effects following exposure to specific concentrations of SO 2 represent our interpretation of the ATS21 guidelines along with CASAC recommenda tions and conclusions drawn by EPA in previous NAAQS reviews (as described in final rule sections II.B.1.a, II.E.2.b and II.F.4.b and responses to comments in this document)

. Moreover, the effects

involved in the clinical studies are not minimal. Exposure at the 200 ppb

benchmark can result in moderate or greater decrements in lung function, evidenced by a ≥ 15% decline in FEV 1 and/or ≥ 100% increase in sRaw in an appreciable percentage of exercising asthma tics. As to the asserted inability to draw a line at any lung func tion decrement, the Administ rator has rejected use of a 100 ppb benchmark and has placed minimal reliance on the (equivocal) evidence of lung function decrements associ ated with exposure at that benchmark level. The explanation for doing so (s ee section II.F.4.b of the preamble to the final rule) illustrates that there are re asonable means of differentiating between exposures which raise public hea lth concern and those which do not.22 

## B. Comments on the epidemiology

(1) Comment: A number of commenters (e.g., API, UARG, ACSBPP, MSCC, Springfield, NPRA) indicated that EPA re lies on only 10 epidemiologic studies as a basis for its proposed revisions to the SO 2 NAAQS. Moreover, commenters indicated that the effects reported in these studies are small, sometimes null or negative and largely not statistically si gnificant (ACSBPP specifically identifies studies by Peel et al., 2005, and Tolbert et al., 2007).

Response: First

, EPA disagrees with the commenters that the Agency relied on only ten epidemiologic studies as a basis for the proposed revisions to the primary

SO 2 NAAQS. Notably, the ISA’s conclu sion that there was sufficient evidence to infer a causal relationship between respiratory morbidity and short-term (5- minutes to 24-hours) exposure to SO 2 (ISA, section 5.2) is based on the consistency, coherence, and plausibility of findings observed in controlled human exposure studies of 5 - 10 minutes, epid emiologic studies mostly using 1-hour daily maximum and 24-hour average SO 2 concentrations, and animal toxicological studies using expos ures of minutes to hours (id. ).

With respect to the epidemiologic evidence, EPA notes that sinc e the last review

of the SO 2 NAAQS, there have been more than 50 peer-reviewed studies examining the relationship between SO 2 concentrations and emergency department visits and hospital admissi ons for respiratory causes published worldwide. Taken together, the ISA concl uded that these studies provide evidence to support an association between ambient SO 2 concentrations and respiratory morbidity heath outcomes. (ISA, secti on 5-2, p. 5-5; see al so ISA at p. 3-24) [“studies generally observed small, positive associations between ambient SO 2 concentrations and ED visits and hospita lizations, particularly among children and older adults”]; ISA, p. 3-27 [“In summary, small, positive associations were

observed between ambient SO 2 concentrations and ED visits and asthma hospitalizations”]. The evidence from these studies is supported by additional panel studies of respiratory symptoms and medication use that demonstrate

“consistent evidence of an a ssociation between ambient SO 2 exposure and increased respiratory symptoms in childre n, particularly those with asthma or chronic respiratory symptoms” (ISA p. 5- 11). The ISA concluded that these epidemiologic studies were consistent and coherent. This evidence was consistent in that associations were reported in st udies 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 resu lts from epidemiologic studies of shortterm (predominantly 1-hour daily maximum or 24-hour average) SO 2 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 respirator y effects observed in controlled human exposure studies of 5 - 10 minutes provided a basis for a progression of respiratory morbidity that could lead to the emergency department visits and hospital admissions observed in epidemiol ogic studies (ISA, section 5.2; p. 5-5).23 In addition, the ISA found that wh en evaluated as a whole, SO 2 effect estimates in multi-pollutant models generally remained positive and relatively unchanged when co-pollutants were included. Id. Therefore, a lthough recognizing the

uncertainties associated with separating the effects of SO 2 from those of cooccurring pollutants (e.g., PM 2.5 ), the ISA concluded that the limited available

evidence indicates that the effect of SO 2 on respiratory health outcomes appears to be generally robust and inde pendent of the effects of gaseous co-pollutants, including NO 2 and O 3 , as well as particulate co-p ollutants, particularly PM 2.5 (ISA, section 5.3; p. 5-9; see also id. at p. 5-5).

Given the conclusions discussed above, the Administrator reasonably considered relevant air quality information available from locations where epidemiologic studies were conducted in determining th e level of a revised standard. More specifically, in assessing the extent to wh ich these studies and their associated air quality information could inform the level of a new 99 th percentile 1-hour daily maximum standard for the U.S., air quality information from the U.S. and Canada

was considered most relevant since th ese areas have similar monitor network designs and patterns of air quality. Howe ver, as described in proposal section II.F.4.a, SO 2 concentrations reported for Cana dian studies were not directly comparable to those reported for U.S. studies due to the use of different monitoring protocols in those studies. Thus, the Administrator focused on 99 th
percentile air quality information ava ilable from locations where ten U.S. emergency department/hospitalization st udies were conducted. However, we disagree with these commenters that th e air quality information mentioned above was the sole basis for the range of le vels proposed. The Administrator also

considered the extent to which the le vel of a new 1-hour standard limited 5- minute concentrations/exposures of con cern identified from controlled human exposure studies. Thus, in considering a reasonable range of levels to propose and in adopting the ultimate 75 ppm 1-hour standard, the Administrator considered the epidemiologic and controll ed human exposure evidence, as well as the air quality, exposure and risk information.

We also generally disagree that epidemiol ogic effect estimates have to be large and/or statistically significan t in order to be considere d. When examining a large body of epidemiologic evidence, such as is available for the association of

ambient SO 2 concentrations and respiratory morbidity, it is appropriate to

evaluate the trend of the di rection and magnitude of eff ect estimates and evaluate the consistency of the dir ections and magnitudes of these effect estimates when determining evidence for causality, rather th an focus on the statistical significance

of each individual effect estimate. As noted above, the ISA concluded that these epidemiologic studies were consistent a nd coherent (including the results from Peel et al., 2005 and Tolbert et al., 2007). Additionally, it is not necessary for the effect estimates to be large in magnitude to be considered to have a relevant public health impact. For example, a 1% increase in daily ED visits or hospital admissions in a single urban area due to increased ambient SO 2 concentrations

translates into millions of annual excess ED visits and hospital admissions when24 applied to the entire US popul ation. Such increases are clearly relevant to public

health. See Coalition of Batte ry Recyclers Association v. EPA , No. 09-1011 (D.C. Cir., May 14, 2010), slip opinion at 8 (“This assertion conf uses the ‘critical distinction between population and individual risk’, wher ein a small change ... at

the level of an individual is a substantia l change at the leve l of a population”).

(2) Comment: A number of commenters (e.g. AP I, UARG, ACC, ACSBPP, ABR, GCLC, CIBO, EEI) generally indicated that the results of epidemiologic studies are confounded by co-pollutants, and t hus EPA cannot confidently identify SO 2 as the causative agent of the resulting h ealth outcome (e.g., ED visits).

Response: EPA notes the response to this comment in section II.E.2.a of the preamble to the final rule. However, EP A provides additional information below.

EPA consistently recognizes that other pollutants are also associated with health outcomes, as is reflected in the fact th at EPA has established regulations to limit emissions of the particulate criteria pollu tants as well as other gaseous criteria pollutants. In its assessment of the health evidence regarding SO 2 , EPA has carefully evaluated the potential for c onfounding, effect measure modification or

other interacti ons between SO 2 and other criteria pollutants, and concluded that

the results attributable to SO 2 are robust. See, e.g. ISA at pages 5-5 and 5-9.

To evaluate the potential for confoundi ng in the epidemiologic evidence, EPA focused especially on studies whic h used two-pollutant models (SO 2 plus one other pollutant) as the incl usion of each additional po llutant in the model can decrease model stability and affect the precision of the effect estimate. This decrease in model stability is often re flected in wider confidence intervals, making it less likely for a statistically si gnificant result to be observed. Thus, when a statistically significant effect estimate observed in a single pollutant model is no longer statistically significan t in a copollutant model (even though the magnitude and direction of the effect estim ate has not changed substantially), this may be an artifact of model instability.

Although the presence of othe r pollutants in the ambien t air mixture complicates efforts to quantify specific SO 2 -related health effects, a number of epidemiologic

studies have evaluated associations with SO 2 in models that also include cooccurring pollutants such as PM, O 3 , CO, and/or NO 2 . The evidence summarized

in the ISA indicates that SO 2 associations generally remain robust in these multipollutant models and supports a direct effect of short-term SO 2 exposure on respiratory morbidity (see ISA Figure 3-8) . This evidence supports the effects observed in experimental (i.e., controlle d human exposure) studies that have evaluated respiratory symptoms and l ung function (see ISA section 3.1.6), which provide plausibility and c oherence of these effects. As noted, the ISA (section 5.2) concluded that the robustness of epid emiologic findings to adjustment for copollutants, coupled with data from hu man experimental and animal studies,25 support a determination that the relationship between SO 2 and respiratory morbidity is causal.

(3) Comment: Industry commenters indicate that ma ny epidemiologic studies relied upon by EPA did not employ multi-polluta nt models to address potential confounding. Several groups indicated th at lack of potential confounding by PM was especially problematic. Some co mmenters (e.g., ASARCO) also indicated out of the over 50 peer reviewed epid emiologic studies evaluated by EPA, only two found a statistically signi ficant association between SO 2 and emergency department visits when other pollutants were considered.

Response: Results in ISA Figure 3-8 indica te that the association of SO 2 with respiratory morbidity is robust to the a ddition of copollutants, including PM. While some individual studies may re port specific findings that are more influenced by copollutants, Figure 3-8 cl early demonstrates the reasonableness of

EPA’s conclusion of robustness. See Schw artz (1995), Burnett et al., (1997), NY DOH (2006), Lin et al., ((2003), Sunyer et al., (1997), Anderson et al., (1998), Ito et al., (2007), Hajat et al., (1999), Tsai et al., (2006) , all of which are studies showed results which remained statistically significant for SO 2 in two-pollutant models, many of which included PM.

(4) Comment: A number of industry commenters (e.g., API, UARG) indicate that EPA failed to discuss the only epidemio logic study looking at associations between hospital admissions for asthma, wheeze or shortness of breath and 5-

minute SO 2 peaks (Donoghue and Thomas; 1999) . Moreover, UARG indicates that EPA failed to account for a study by Erbas and Hyndman finding that “the effects of...sulfur dioxide were highly sensitive to model specification for both COPD and asthma [HAs].” UARG further states:

The fact that EPA has failed to examine, or even acknowledge the existence of these studies after they were pointed out in public comments, is a striking indication that EPA ha s not fulfilled its duty under the CAA (see comments provided by UARG; p. 20).

Response: EPA has reviewed both of the stud ies identified by commenters. The first, by Donoghue and Thomas (1999) is ri ddled with a number of limitations that

in EPA’s view made it too unreliable to include in the ISA and REA. The limitations of this study include: 1) the statistical analysis included Poisson regression, and used GAM models and was conducted with S-Plus software. The study was published before Dominici et al. (2002) identified default GAM convergence issue and never reanalyzed, making it likely to be GAM impacted and hence unreliable (see EPA (2004), Air Quality Criteria for Particulate Matter, section 8.1.3.1 and 8.4.2 (EPA/600/P-99/002bF)) ; 2) The failure to present average, median, minimum or maximum SO 2 concentrations. Instead only the percent of days when the maximum 5-minute SO 2 concentration at any of the 10

SO 2 monitors exceeded 3 benchmark levels (800, 2145, and 5434 μg/m 3 ) is26 presented. 3) The failure to present the number of hospital visits or hospital admissions included in analyses. 4) The te mporal scale of the exposure data is 5- minute concentration, but temporal scale of outcome data is a 24 hour period. Thus, the precision of the 5-minute SO 2 concentrations is lo st on the less precise 24-hour outcome data. For example, it isn’ t clear if the hospita l visit or hospital

admission occurred before or after the 5-minute SO 2 concentration was measured. 5) A seasonal effect (by month) was obs erved, but the study authors did not adjust for influenza hospital visits to account for this potential confounder/effect

modifier.

Similarly, the study by Erbas and Hyndman (2005) was viewed by EPA to be too

unreliable to include in the ISA and REA. The authors provide no information on the number of hospital admissions observ ed for COPD or asthma during the 3.5 year study period. Additionally, there is no information that characterizes the

concentration of SO 2 during the study period.

(5) Comment: A number of commenters (e.g., AP I) indicate that EPA did not account for publication bias in the review of the SO 2 NAAQS

Response: EPA recognizes the possibility of publication bias, which can potentially occur in any field of study. In the discussion of the evaluation of epidemiologic studies, EPA observed that one of the advantages of multi-city studies is that “by their ve ry nature can reduce uncert ainty related to publication bias” (ISA, p. 3-1).

EPA does not agree that reported associations between SO 2 and health effects are an artifact of publication bias. EPA acknow ledges that publication bias can result in potential overestimation of the estimate d risk in a body of literature. However, for an individual study, factors such as expos ure error or selecti on of results from an individual lag period from among seve ral positive associations can result in underestimation of an effect estimate (i .e. a more pronounced bias in the other direction).

(6) Comment: A number of industr y commenters indicated that EPA selectively presented results of epidemiologic studies. For example, API indicated that in the NYDOH (2006) analysis EPA se lectively considered the positive associations in the Bronx NY, while not considering the negative associations in Manhattan (see comments by API; p. 17).

Response: EPA disagrees that the results of the NYDOH (2006) from the Bronx and Manhattan were presented selectively. Effect estimates from both locations are presented in Figures 3-7 (p. 3-26) and 3-8 (p. 3-29) of the ISA. Additionally, on page 3-25, the ISA states, “Anothe r study conducted in New York City (NYDOH, 2006) found a 10% (95% CI 5, 15) excess risk in asthma hospital admissions per 10 ppb increase in 24-hour average SO 2 for Bronx residents, but a

null association for the residents of Ma nhattan (-1% [95% CI: -11, 11]).” EPA27 fails to see how presenting effect estima tes from both locations in the text and figures of the ISA constitutes a se lective portrayal of the results. In addition, EPA

notes that the REA and proposal in cluded figures (see RE A figures 5-2 and 5-3 and proposal figures 2 and 3) with resu lts from both the Bronx and Manhattan study areas.

(7) Comment: A number of commenters (e .g., ACSBPP) argue that it is inappropriate for EPA to use the same epidemiologic analyses to suggest causal associations in reviews for multiple criteria pollutants. In general, these groups argue that the same epidemiologic analyses have been used to attribute health

effects to O 3 , PM, NO 2 and now, SO 2 . This is likely leading to double, triple, or quadruple counting of health effects.

Response: EPA strongly disagrees that the agency is “double or triple counting” by attributing effects to SO 2 in the current review that are attributed to different pollutants in other NAAQS reviews. EP A consistently recognizes that other pollutants are also associated with health ou tcomes, as is reflected in the fact that EPA has established regulations to limit emissions of the particulate criteria pollutants as well as other gaseous criteria pollutants. In it s assessment of the health evidence regarding SO 2 , EPA has carefully evaluated the potential for confounding, effect measure modification or other interactions between SO 2 and other criteria pollutants, and conclude d that the results attributable to SO 2 are robust (See Figure 3-8 on page 3-29 of ISA). The combination of this epidemiologic evidence w ith evidence from controlled human exposure and animal toxicological studies provides clear and convincing evidence of the specificity of these adverse respiratory effects to SO 2 exposure. Together, these

lines of evidence are consistent and cohe rent, explain the temporal and biologic gradients observed, and provi de biological plausibility for the observed effects. See also Air Quality Criteria for Pa rticulate Matter (EPA, 2004) at 8-254, indicating how EPA distingui shes effects attributable to PM from those

attributable to SO 2 : “In many of these studies, PM with and without added components of gases thusly appears to be a key putative agent. However, care must be exercised in interpreting such results, taking into account what is known about the toxicology and clinical studies of the gases. It is often clear that these gases, at concentrations present of give n the nature of the effects, do not carry sufficient biologic plausibility to substa ntially affect the results seen. For example, SO 2 is mostly absorbed in upper airways under normal breathing conditions and, although it might affect airw ay neural reflexes to contribute to asthma exacerbation at typical U.S. ambi ent levels, it is not likely to exert

sufficient effects on COPDS or CVD to contribute to excess morbidity and mortality.”

(8) Comment: Some industry groups (e.g., AP I, UARG, ExxonMobil) commented

that reliance on central mon itors in epidemiologic studies leads to a high degree of exposure misclassification. For exampl e, UARG states: “when all of the copollutants are measured with error, robustn ess of the association of one pollutant28 in multi-pollutant models is not sufficient to infer an absence of confounding (see comments provided by UARG; p. 18)

Response: As noted in Section 2.6.4 of the ISA, although EPA agrees that SO 2 measurements from central monitors is subject to potentially large measurement error when used to reflect population e xposures in epidemiologic studies, the Agency disagrees with the suggestion that statistically significant associations between centrally monitored air quality c oncentrations and adve rse health effects

measured in these studies are invalid as a result. This section concluded that such measurement errors in community time-ser ies and panel epidemiologic studies are more likely to underestimate the strength and the significance of any association

between SO 2 and any adverse health effect s observed in the study, thereby decreasing the likelihood of an associati on reaching statistical significance and likewise decreasing the possibility of a false identification of an association (Section 2.6.4.4, p. 2-63). Thus, the measur ement error makes it more difficult to detect a positive and statistic ally significant effect estimate, even when such an association exists. Moreover, the signal that drives statistical associations

between ambient concentrations and health effects in time-series studies is the day-to-day changes in concen tration, not the absolute da ily values. Appropriately

located central SO 2 monitors can adequately ch aracterize such day-to-day changes. Exposure studies have found st atistically significant regression slopes between personal exposure to SO 2 and ambient SO 2 concentrations at fixed-site monitors (see ISA section 2.6.3.2, p. 2-56), in dicating that fluctuations in ambient concentration are an important driver of fluctuations in SO 2 exposure.

Time-series epidemiologic studies evalua te associations between day-to-day changes in air pollution and health outcom es. Therefore, the measurement error argument cannot be used to nullify an eff ect that has been observed. If anything, as noted, it is likely that the re al effects are likely to be larger than those that were estimated. Therefore, for the purposes of determining whether public health protection is warranted in li ght of the available evidence, EPA believes that it has interpreted the evidence from these epidemiologic studies reasonably.

(9) Comment: Some industry groups (e.g., UARG) indicate that there is no evidence from controlled human exposure studies to suggest that brief exposures to low

concentrations of SO 2 could lead to hospital visits. For example, UARG states:

Although no one would suggest that clin ical studies could or should be conducted with the intent of sending peopl e to the hospital, there is also no

indication from the clinical studies of any population that would be susceptible to SO 2 such that a brief exposure to a low concentration could lead to hospital visits (see comments provided by UARG; p. 16)

Response: As described in Section 5.2 of the ISA (p. 5-2), the evidence from controlled human exposure, toxicological and epidemiologic studies for the respiratory health effects of SO 2 are consistent with the mode of action of SO 2 as29 it is currently understood. The immediate effect of SO 2 on the respiratory system is bronchoconstriction. This response is me diated by chemosensitive receptors in the tracheobronchial tree. These receptors trigger reflexes at the central nervous system level resulting in bronchocons triction, mucus secretion, mucosal vasodilation, cough and apnea followed by rapid shallow breathing. Asthmatics

are more sensitive to the effects of SO 2 likely resulting from preexisting

inflammation associated with this dis ease. This inflammation may lead to enhanced release of mediators, alterations in the autonomic nervous system and/or

sensitization of the chemosensitive re ceptors. These biological processes are likely to underlie decreased lung func tion and increased hyperresponsiveness observed in response to SO 2 exposure.

The asthmatics participating in studies involving controlled exposures to SO 2 must be relatively healthy, and do not likely represent the most sensitive individuals in the populati on. Controlled human exposure studies include very small numbers of subjects, particularly when compared to the large populations considered in epidemiologic investigations. These populations do include the

most sensitive individuals who are not eligible or willing to participate in controlled human exposure studies either due to study-specific inclusion or exclusion criteria, or self-selection. In a ddition, as presented in the ISA (p. 5-5) it is quite possible that effect s observed in epidemiologic studies could be driven in large part by higher peak e xposures within a 24-hour peri od. Thus, as noted in the ISA (id.), the effects of SO 2 on respiratory symptoms, lung function, and airway

inflammation observed in the controlled human exposure studies provide a basis for the progression of respiratory morbidity resulting in increas ed ED visits and hospital admissions, especially among susceptible populations (i.e., asthmatics).

(10) Comment: Several industry commenters (e.g., UARG) indicated that there is a disconnect between effects seen at higher SO 2 concentrations in controlled human exposure studies and those observed at lo wer concentrations in epidemiologic analyses. For example, UARG states:

EPA recognizes that human clinical st udies ‘provide dir ectly applicable information for determining causality’” while for epidemiological studies “the degree of uncertainty introdu ced by confounding variables (e.g., other pollutants) affects the level of conf idence that the health effects being investigated are attributable to SO 2 exposures.” 74 Fed. Reg. at 64815/3 - 64816/1. Thus, the discrepancy between the nature of effects attributed to

SO 2 in epidemiologic studies at ambient levels and those in clinical studies at much higher SO 2 concentrations must call into question the reliability of the epidemiological studies as evidence of effects from SO 2 exposure (see comments provided by UARG; p.17)

Response: Please see the response to the previous comment that characterizes the progression from the respiratory effects observed in controlled human exposure studies to those observed in epidemiol ogic studies. In addition, although the30 concentrations investigated in controll ed human exposure studies are higher than the mean concentrations reported in epidem iologic studies, it is important to note that epidemiologic studies conducted in the United States have reported

associations between ambient SO 2 concentrations measured at area-wide monitors

in the current network and increase d respiratory symptoms, emergency department visits, and hospital admissions.

As noted in the ISA (Tab le 2-8, p. 2-33), 1-hour SO 2 concentrations measured at

area-wide monitors in the United States ha ve been observed in the range of 50 to 100 ppb, with maximum concentrations between 600 and 700 ppb. In addition, a very limited number of U.S. monitors report 5-minute SO 2 concentrations, and upper-percentile 5 minute concentrations at these monitors can be 200-300 ppb or higher (ISA, Figure 2-31, p. 2-44). Base d on these considerations, EPA believes

that the SO 2 concentrations that occurred in th e locations of epidemiologic studies may have included concentrations that overlapped with those reported to cause respiratory effects in contro lled human exposure studies.

(11) Comment: Several industry commenters (e.g. ACSBPP) indicated that EPA downplayed major findings re garding uncertainty due to model selection. For example, ACSBPP states:

Model selection uncertainty relate s to confounding of air pollutant associations by temporal trends, weathe r and co-pollutants. During the last ozone review, EPA acknowledged that the uncertainties in the estimates of pollutant effects are understated by consideration of the statistical uncertainty of the fitted model alone. Much more uncertainty arises from the lack of information regarding th e choice of appropriate models for adjusting confounding by other covariat es, and the choice of appropriate lag structures (see comments provided by ACSBPP; p.8).

Response: EPA has not ignored issues rela ted to model specification for epidemiologic studies, such as selection of models and approa ches to adjust for meteorological and temporal variables. As observed in the ISA (p. 3-1), extensive discussions of the issues surrounding m odel selection and model specification have been presented in the PM AQCD (EPA, 2004 ) and the O 3 AQCD (EPA, 2006) and are thus not reiterated at length in the ISA. The SO X ISA makes clear, however, that these issues were carefully considered in selecting studies for

inclusion in the ISA and interpreting the results of the body of epidemiologic evidence. Indeed, EPA went to great le ngths to do so, plac ing special emphasis on the epidemiologic studies which remain ed statistically significant in twopollutant models which included PM. See section II.F.4.e in the preamble to the proposed rule and section II.F.4.c in the preamble to the final rule.

(12) Comment: Several commenters (e.g. ACSBPP) indicate that EPA chose the best lag times from studies with multiple lag tim es to support it’s assertion of a causal relationship between short-term SO 2 exposure and adverse respiratory effects.31 Response: EPA reiterates that it believes in the importance of a comprehensive evaluation that considers and weighs a variety of evidence, including biological plausibility of associations between th e various pollutants and health outcomes, and focuses on the stability of the size of the effect estimates in time-series studies considering both single- and multi-pollutant models, rather than just looking at statistical significance in a large number of alternative models as a basis to delineate between real and suspect associ ations. EPA finds that the approach of simply counting the number of statistically significant results across all models does not give adequate wei ght to important statistica lly significant results as a consequence. This has the effect of we ighting all models e qually, regardless of plausibility or statistical power, and it allows a lack of statistically significant results for one lag structure to essentially cancel out statistically significant results based on another lag structure. That is, EPA does not agree, for example, that a statistically significant association betw een emergency department visits for asthma and same day exposure to SO 2 should be completely discounted by a finding in the same study that an asso ciation between emergency department

visits for asthma and SO 2 exposure several days prio r to the ED visit is not statistically significant. Hea lth effects associated with relatively more immediate exposures could well be the consequence of a biological mechanism that would not reasonably be expected to result in the same health effect several days after exposure. Thus, EPA does not believe that it is appropriate to simply average out statistically significant a nd nonsignificant results deri ved from models with different lags.

EPA has an established hierarchy for c hoosing which effect estimates will be presented in the text and figures of the ISA when effect estimates from multiple lag days are presented: (1 ) effect estimates from distributed lags; (2) effect estimates for the average of multiple days (e.g., 0-2); (3) a priori lag days selected by the study authors; (4) if only individua l lag days were presented, the estimate with the highest magnitude and level of statistical significance is selected from among the biologically plausible lag days available.

(13) Comment: Commenters (e.g. ACSBPP) indicated that EPA did not discuss all of

the results and/or all of the authors conclusions pr esented in the original publication (with regard to Sheppard 2003, Jaffe et al., 2003, Schwartz et al., 1996, Wilson et al. 2005, Anderson et al. 1998)

Response: EPA may not discuss the author ’s conclusions presented in the original publications because the interpreta tion of the results of a single study may differ greatly from the interpretation of a large body of evidence, which is what is

evaluated and serves as the basis for cau sal determinations in the ISA. As mentioned in the response to epidemio logy Comment 1, EPA evaluates the trend of the direction and magnitude of effect estimates and evaluates the consistency of the directions and magnitudes of thes e effect estimates when determining32 evidence for causality, rather than focusi ng on the statistical significance of each

individual effect estimate.

(14) Comment: ACSBPP indicated that there is a bi ologically implausible wide range of effect estimates in epidemiologic studies

Response : In responding to similar comment s made with regard to the 2004 PM AQCD, EPA has recognized that in the e xpansive body of literatu re, especially in the multi-city studies, there appears to be spatial heterogeneity in city-specific

excess risk estimates for relationshi ps between short-term ambient SO 2 concentrations and acute health effects. The reasons for variation in effect

estimates are not well understood. Factor s likely contributing to the apparent heterogeneity include geographic differe nces in air pollution mixtures, and personal and sociodemographic fa ctors potentially affecting SO 2 exposure.

The commenters focused on multi-city resu lts (e.g., Sunyer et al., 2003; Barnett et al., 2005; Ballester et al., 2006) , and raised a series of technical issues with the analytical approaches used, including the use of a sta ndard analytical approach that did not allow for city-specific modeli ng of factors such as meteorology that likely vary from city to city, and the fact that many multi-city studies have

combined results from communities with fe wer available data that would likely not have been considered adequate for use in single-city analyses. Beyond the

fact that these studies we re conducted in Europe (S unyer et al. 2003), Australia (Barnett et al. 2005), or focused on cardia c hospital admissions (Ballester et al. 2006), and thus did not play a large role in the current regula tory process, the EPA does not agree that the issues raised diminish the value of these analyses.

Another key factor that cont ributes to heterogeneity or variation between areas is a study’s statistical power. EPA observes th e importance of st atistical power for interpreting these results, and EPA agrees that it is reasonable to focus on results from studies with greater power. Despite there being some evidence for greater variation in magnitude and precision of SO 2 -health associations between geographic areas, EPA concluded in the ISA that the extensive body of epidemiology evidence demonstrated “an association between ambient SO 2 concentrations and ED visits and hospita lizations for all respiratory causes, in particular among children and older adults (65+ years), and for asthma.” (EPA, 2008, p. 3-28)

(15) Comment: Some groups generally remarked th at the term “robust” in the ISA is not used consistently or systematically across NAAQS reviews. For example,

ACSBPP stated:

In one instance, the ISA indicates that the term robust is used to indicate that there was little change in th e magnitude of the central estimate, though statistical signific ance may have been lost. Since EPA is using similar methods to evaluate the epidem iology for other criteria pollutants,33 it is appropriate to consid er the criterion for robustn ess used in other ISAs. In the draft CO ISA, EPA uses the fact that the association remains positive in copollutant models to define robustness. (see comments provided by ACSBPP; p. 19)

Response: The term “robust” is used cons istently throughout NAAQS reviews to indicate that results were essentially th e same regardless of the manner in which the statistical models were specified. St atistical significance of the results is not considered in the application of the term “robust”.

(16) Comment: EPRI generally commented that EP A should consider that all of the epidemiologic studies employed linear mode ls that do not examine the possibility

of a threshold at which h ealth responses may occur.

Response: EPA evaluated the concentration- response function and potential thresholds in Section 4.1 (p. 4-1 to 4-7) of the ISA. The ISA concluded that a clear increase in the magnitude of respiratory effects was observed with increasing exposure concentrations be tween 0.2 and 1.0 ppm during 5-10 minute

SO 2 exposures in controlled human exposure studies. The ISA also noted that the concentration-response function be tween short-term exposure to SO 2 and respiratory morbidity could not be dis tinguished from linear across the entire concentration range. Population-leve l studies did not indicate a possible threshold, though due to limitations asso ciated with observing a possible

threshold in these studies the limited evidence was determined to be inconclusive regarding the presence of an effect threshold at current ambient levels.

(17) Comment: With respect to the epidemiologic evidence, MSCC commented:

There are other factors that may positively associate with increased levels

of admissions and with increased levels of SO 2 (if SO 2 is present) – gusty, windy days, particulate loads, pollen, ambient temperature effects, patient activities, even days of the week, to name a few. Further, for such studies, the monitored levels, do not likely indi cate the concentrations of actual

SO 2 exposure for the population, but at best a subset of any constrained very near the monitor(s). An association that is sometimes negative, sometimes positive, and rarely signific ant, is not compelling to use to conclude a direct cause/effect relationship (see comments provided by MSCC; p.5)

Response: EPA evaluates each of the epidemiologic studies to ensure that potential confounding variables, including meteorological variables and day of week, are included in time-series studies of ED visits and hospital admissions to

determine whether observed health effect s are associated with air pollutant concentrations and not with these poten tial confounders. A time-series study that does not account for these potential confounders would have a difficult time34 getting accepted for publication in the peer-reviewed literature, and if it did,

would not be included as evidence in the ISA.

EPA disagrees that exposure measur ement error is cause to ignore the observational evidence from epidemiologi c evidence, and again reiterates (see response to epidemiology Comment 1) that the ISA’s conclusion that there was

sufficient evidence to infer a causal rela tionship between respiratory morbidity and short-term (5-minutes to 24-hours) exposure to SO 2 (ISA, section 5.2) is based on the consistency, coherence, and plausibility of findings observed in controlled human exposure studies of 5 - 10 minutes, epid emiologic studies mostly using 1-hour daily maximum and 24-hour average SO 2 concentrations, and animal toxicological studies using exposures of minutes to hours (id. ).

(18) Comment: Some groups indicate that there is no evidence to suggest that children or older adults may be at in creased risk of experiencing adverse respiratory effects following SO 2 exposure.

Response: The ISA concludes that there is “l imited epidemiologic evidence to suggest that children and ol der adults (65+ years) ar e more susceptible to the adverse respiratory effects associated with ambient SO 2 concentrations when compared to the general population” (ISA, pg. 4-14). Specifically, on page 4-12,

the ISA states:

“A number of studies, investigatin g the association between ambient SO 2 levels and ED visits or hospital ad missions for all respiratory causes or asthma, stratified their analyses by age group. Figure 4-6 summarizes the evidence of age-specific associations between SO 2 and acute respiratory ED visits and hospitalizations. Seve ral studies demonstrated that excess risk of ED visits or hospitalizations for all respiratory causes was higher for children (e.g., Atkinson et al., 1999a; 1999b; Petroeschevsky et al. 2001) and older adults (e.g., Petroesc hevsky et al., 2001; Wilson et al., 2005; Wong et al., 1999) when compared to the risk for adults or all ages together. This is more clearly depict ed in the summary density curves in Figure 4-7 and Figure 4-8, created using the effect estimates presented in Figure 4-6. As shown in these two figures, the effect estimates for children and older adults are slightly larg er than that for adults or all ages for both all respiratory diseases and asthma ED visits and hospitalizations.”



## C. Comments on the air quality, exposure, and risk analyses

(1) Comment: Several commenters (e.g., API, UARG, ACSBPP, ACC, JSRC, ABR, ASARCO, GCLC) indicated that it was inappropriate for EPA to ‘degrade” current air quality to just meet the curre nt 24-hour and/or an nual standards. Industry groups argue that there is no plau sible scenario that could lead to current35 air quality, which is well below the curre nt standards, regressing back to “just meeting” the current standards.

Response: EPA disagrees with the commenters . In determining whether the current primary standards are requisite to protect public health with an adequate margin of safety, EPA finds that it is a ppropriate to consider exposures and risk associated with air quality that has been adjusted to just meet the current

standards. Similar comments and EPA’s responses are discussed in detail in section II.E.2.c of the final rule preamble.

(2) Comment: Several commenters (e.g., API, UARG, ACSBPP, ACC, CE, TXOG, JRSC, ABR, ASARCO, NRECA) indicated that it was inappropriate for EPA to compare exposure and risks associated w ith just meeting alternative 1-hour standards to those just meeting the current SO 2 standards. These groups argue the more relevant comparison to just mee ting alternative 1-hour standards (or the current standards) is to the exposure and risk associated with “as is” air quality,

which is well below the current SO 2 standards. Moreover, these groups argue that if this comparison is done, there is little to no benefit to public health by adopting a 1-hour standard in the proposed range.

Response: The response to this comment follows the same logic as that outlined in the preceding comment. Thus, EPA disa grees with the commenters that it is inappropriate to consider exposures and ri sk associated with air quality that has been adjusted to just meet the potential alternative standards in assessing what potential NAAQS could protect public health with an adequate margin of safety. Similar comments and EPA’s responses are discussed in detail in section II.E.2.c of the final rule preamble.

(3) Comment: Several commenters (e.g., AP I, UARG, FE, MSCC, AECT, NAM) indicated that the proposed alternative standards are not necessary because SO 2 emissions and/or ambient levels have fallen dramatically over the years and will continue to do so. Many of these groups also point out this will result in

diminished risk estimates associated with ambient SO 2 concentrations as well (which industry groups point out are already small).

Response: As described in detail in final rule section II.E.2.c, EP A is required to review whether the present standards – not present air quality – are requisite to protect public health with an adequate ma rgin of safety. Section 109 (b) (1). Similarly, EPA may not consider projected future air quality and its associated risks when reviewing the adequacy of th e current standards to protect public health with an adequate margin of safety.

(4) Comment: Several commenters (e.g., ACSBPP) generally indicated that the only relevant approach to assessing risks associated with SO 2 exposure are those in the risk assessment chapter of the REA (Chapter 9). These groups argue that the air quality and exposure analyses presente d in Chapters 8 and 9 of the REA36 substantially overestimate risks associated with SO 2 exposure because they do not account for the fact that not all exposed asthmatics at elevated ventilation rates will experience a lung function response. Mo reover, if only the results of the risk assessment are taken into account, one would se e that there is very little risk of a lung function response in exposed asth matic children at 150 ppb and above.

Response: EPA disagrees with the commente rs that primary emphasis should be placed on the results of the quantitative risk assessment when assessing the extent to which the current and potential altern ative standards limit 5-minute benchmark exposures of potential concern and their associated health effects. As described in the REA (Chapters 7-9) and the proposal (section II.C), EPA used three different approaches (i.e., air quality analysis, e xposure analysis, and quantitative risk assessment) for assessing exposures and risks associated with these 5-minute

benchmarks identified from controlled human exposure studies. Rather than relying on just one of these approach es, EPA recognized that each of these analyses has strengths and limitations (s ee proposal preamble section II.C) and thus, reasonably considered th e results of all of these an alyses (albeit to different extents, see below) to evaluate the ex tent to which the current, and potential alternative 1-hour standards limit exposure and risks associated with these 5- minute benchmark concentrations (see pr oposal preamble sections II.E.1.b and Tables 2 – 4).

As noted in the final rule’s preamble (s ection II.E.2.c) however, the Administrator did place relatively more weight on the re sults of the St. Louis exposure analysis

than she did on the results of the St. L ouis quantitative risk assessment. EPA notes the results of the quantitative risk assessment likely have greater uncertainty than the results of the exposure analysis considered alone. That is, because the results of the exposure analysis are used as inputs into th e quantitative risk assessment, the risk assessment will have all the uncertainties of the exposure analysis (see proposal preamble section II.C; 74 FR at 64823 and REA, section 8.11), as well as the additional uncertainti es inherent in the quantitative risk assessment (see proposal preamble section II.C; 74 FR at 64823 and REA, section 9.4). EPA finally notes that the Administ rator’s proposed and final decisions with regard to the adequacy of the current st andards and the elements of a new shortterm standard appropriately placed subs tantial weight on the epidemiologic and controlled human exposure evidence presented in the ISA.

As discussed above, EPA believes that it would be inappropriat e to consider only the results of the quantitative risk assessm ent. Thus, we believe the commenter’s contention of relatively little risk of an adverse lung function response given a 1- hour standard level of 150 ppb is not compe lling in isolation. We believe that this is especially true given that the St. Loui s exposure analysis estimates that a 1-hour standard at 150 ppb would only protect about 88 percent of asthmatic children at moderate or greater exertion in that city from experiencing at least one exposure per year greater than or equal to the 200 ppb 5-minute benchmark exposure of

concern.37 (5) Comment: Several commenters (e.g. ACSBPP) generally indicated that the risk estimates presented in Chapter 9 of the REA and in the proposal are substantially overestimated because they only cons ider decrements in lung function, independent of whether there were accompanying respiratory symptoms.

Response: EPA disagrees that the quantita tive risk estimates presented in Chapter 9 are substantially overestimated because they only consider decrements in lung function, independent of whether they were accompanied by respiratory symptoms. As noted above (see section II.A) , and in detail in fi nal rule preamble section II.E.2.b, EPA considered moderate or greater decrements in lung function, even without accompanying respiratory symptoms to be adverse effects of SO 2 exposure. Thus, EPA believes that it wa s reasonable to consider the decrements in lung function health endpoint in its quantitative risk assessment.

(6) Comment: Some commenters gene rally indicated (e.g., AC SBPP, TAB) that the

scenarios under which an asthmatic at elev ated ventilation (e.g., while exercising) is likely to come into contact with an SO 2 benchmark concentration of concern is exceedingly rare and therefor e not a public health issue appropriate for a NAAQS. In addition, some of these groups indicat ed that there were few monitored 5-

minute SO 2 levels above 5-minute benchmark concentrations of concern. For example, ACSBPP indicates that Figure 7-19 of the REA shows that only a portion of the available mon itors with 5-minute concentrations have exceedences of the various benchmarks, and at monito rs with these exceedences, values above 400 ppb are very rare (comments by ACSBPP; p. 27).

Response: EPA disagrees with these comments . EPA first notes the results of the St. Louis exposure analysis indicati ng that numerous asthmatic children at moderate or greater exertion (e.g., while ex ercising) in St. Louis are estimated to experience at least one exposure per year above the 5-minute benchmark

concentrations of concern identified from controlled human exposure studies (see REA Figure 8-19 and proposal Table 3 at 74 FR 64841). For example, if air quality is simulated to just meet the curre nt standards in St. Louis, approximately

24% and 73% of asthmatic ch ildren at moderate or greater exertion are estimated to experience at least one exposure per year above the 400 and 200 ppb benchmark exposures of concern, respectively. Id.

In addition, the figure referenced by th e commenter describes the number of days/year measured 5-minute SO 2 concentrations from 98 monitors in 13 states

from 1997 -2007 exceeded 5-minute benchmark concentrations identified from controlled human exposure studie s. We first note that this subset of monitors that reported measured 5-minute SO 2 concentrations was relatively small. That is, over the same time frame, there were 933 m onitors in 49 states, the District of Columbia, Puerto Rico and the Vi rgin Islands measuring 1-hour SO 2 concentrations. In additi on, most of the monitors that did measure 5-minute concentrations only did so for a limited span of time, ranging from only several38 hundred, to at most, a few thousand hours in a year. REA Appendix A, Table A.5-1. Furthermore, it is important to consider that those monitors reporting 5- minute concentrations do not represen t data from a dedicated 5- minute monitoring network, but rather a voluntar y submission of 5-minute values from existing monitors placed for the purpose of evaluating attain ment of the 24-hour and annual average SO 2 NAAQS, which does not requir e monitoring at locations of expected maximum concentrations (much less maximum 5-minute concentrations). Thus, there is consider able uncertainty that the relatively few monitors reporting 5-minute SO 2 concentrations were sited in the proper locations to record 5-minute peaks of SO 2 above benchmark values.

Notwithstanding these decidedly no n-optimized monitoring conditions, approximately 2% of the monitored days (approximately one week in a year) exceeded the 200 ppb benchmark. REA p. 120. EPA regards this amount of potential exposure to be significant. See also American Lung Ass’n v. EPA , 134 F. 3d 388, 392 (D.C. Cir. 1998) (noting facts in previous review that “’repeated’ exposure is ‘significant’ a nd that thousands of asthmatics are exposed more than once a year”). Indeed, this comment app ears to raise the same issue as that presented in the previous review: are repeated SO 2 exposures resulting in adverse effects to a susceptible population a public health problem, even if the exposures

are confined to relative ly localized areas. American Lung Ass’n v. EPA , 134 F. 3d at 392. The D.C. Circuit rejected the argument that such exposures were not a public health problem by virtue of bei ng localized and infrequent, noting that

hundreds of thousands of asthmatics were exposed to these events annually, and

remanded the standard to EPA for want of a satisfactory explanation. The facts here, as set out in the REA, indicate that the current standard allows exposures exceeding health benchmarks (the benchmarks of 200 ppb and 400 ppb), in multiple areas and occurring on multiple days. See, e.g. REA Figure 7-19; see also CASAC letter of May 18, 2009 at p. 15 (“The panel agrees that the current 24-hour and annual standards are not adequate to protect public health, especially in relation to short term exposures to SO 2 (5-10 minutes) by exercising asthmatics.”) This risk-based information is one of the reasons EPA is determining that the presen t standards are not requisit e to protect public health with an adequate margin of safety

(7) Comment: UARG commented on the documentati on and replicability of the data set used for the air quality data analys es. Concern was expressed regarding the

number of monitors available and numb er of 5-minute and hourly measurements used in their analyses ve rsus those used by EPA.

Response: We have responded to this comment on the 1 st draft REA by improving the documentation of the raw data sets used and the screening procedure to generate the final data se t used for developing the PMR statistical model. See REA section 7.2.1, pages 73-766. In this section, there are descriptions regarding the t ypes of data used for the ai r quality analyses including: 1) monitors reporting 5-minute maximum SO 2 , 2) monitors reporting continuous39 5-minute SO 2 concentrations and, 3) m onitors reporting 1-hour SO 2 concentrations. Briefly, all ambient m onitoring data were downloaded from EPA’s Air Quality System (AQS). The processing of each data set is then adequately described given criteria for in clusion or exclusion from the final data set used for analysis. A final combin ed data set was constructed by EPA to

maximize the number of 1-hour measurem ents that also had corresponding 5-

minute maximum concentrations . It was this final data set that was used to

construct a statistical model used to predict 5-minute maximum concentrations

from 1-hour concentrations.

(8) Comment: A few commenters (ACC, UARG, API) requested improved documentation of and justification for the approach used to adjust ambient air quality data to simulate just meeting the current and alte rnative standards.

Response: EPA has expanded the discussi on regarding the proportional method used to adjust the air qu ality data (REA, section 7.2.4) and benchmark levels (REA, section 8.8.3), each ju stified by analyses conducted by Rizzo (2009). A comparison between historical, high concen tration monitor data and recent, low

concentration monitor data indicated th at changes have occurred in a mostly proportional manner, giving support for the proportional method used to simulate alternative air quality scenarios.

(9) Comment: ABR and ASARCO commented on th e approach used to estimate 5-

minute maximum concentrati ons from 1-hour average SO 2 concentrations. More specifically, concern was e xpressed regarding the peak-to-mean (PMR) ratios developed from a subset of ambient monito rs and their application to the broader ambient monitoring network. These commen ters claim that “if the characteristics

and micro-scale conditions of the sm all subset of 5-minute concentration monitors, which significantly influence m easured concentrations, differ from the broader monitoring network, then the estimated exposures are not accurately represented.” Differences in two identif ied ambient monitor characteristics (i.e., monitor objective and surrounding emi ssion sources) are described by the commenters as either “increasing the uncer tainty” in applying calculated PMRs to the broader monitoring network and leading “to an overestimation of exposures to

SO 2 .”

Response : EPA evaluated the attributes of both the ambient monitors that report

5-minute SO 2 concentrations and those monito rs that report only 1-hour average concentrations (REA, section 7.2.2) and did so using several physical characteristics including monitor objective, scale, land-use, and setting; types of

emission sources within 20 km of a monitor; and the population residing with 5, 10, 15, or 20 km of a monitor. EPA acknowledged any differences where present, though in general, the composition of mon itors from the two data sets was very similar regarding each of these physical attr ibutes. EPA feels that the qualitative evaluation performed was a reasonable and informative comparison.40 EPA is well aware of the greater percenta ge of monitors identified as having a source-oriented objective among the m onitors that reported 5-minute concentrations. EPA also recognizes that there can be high concentrations and significant concentration variability measur ed at these source-oriented monitors. This is the primary reason for developing a statistical model based on an ambient monitor’s concentration variability a nd the 1-hour concentr ation level (REA, section 7.2.3). The statistical model was designed to use information and relationships derived from the 5-minute measurements and appropriately assign

PMRs to 1-hour concentration measuremen ts. EPA calculated the coefficient of variation (COV) at each ambient monitors and used that to categorize every

monitor into one of three COV categories ( ≤ 100%, <100- ≤ 200%, and >200%). Then, distributions of PMRs were ca lculated from the available 5-minute

measurement data to also reflect the differe nces in concentration levels as well as the concentration variability expected to occur at each of the monitors REA, section 7.2.3.2). Hence, our method is in agreement with the ABR/ASARCO comments made regarding the potential link between source characteristics, concentration variability, and 1-hour average concentration.

(10) Comment: UARG commented that the coeffici ent of variation (COV) is an inappropriate summary statistic for the m onitor data. They argue that the “COV measure is generally reserved for normally distributed data, which SO 2 concentrations certainly are not.”

Response: The COV statistic was not used by EPA as a descriptor to define a

monitor’s ambient concentration distribu tion. It was used to categorize the

monitor into one of three variability groups (low – ≤ 100% COV; medium <100-

≤ 200% COV, and high >200% COV). EPA acknowledges that measured ambient concentrations at a monitor may not necessarily reflect that of a normal distribution, and in fact cons idered this in the developm ent of the PMR statistical model. An alternative approach was used that incorporated the geometric standard deviation (GSD) in categorizing the ambient monitors (i.e., a statistic more appropriate in describing the va riability associated with a lognormal distribution). Five-minute maximum SO 2 concentrations were estimating using a PMR statistical model that either used COV or GSD as the categorical variable. Results demonstrated that the PMR model using COV to define a monitor’s variability performed be tter than that using GSD (REA, section 7.2.3.4).

(11) Comment: A few commenters (e.g., API, UAR G, ASARCO) generally indicated that exposures and risk estimates pres ented in the REA and proposal are over estimates because they rely on emissions estimates from 2002 and that

information on the stack emission characteri stics (i.e., stack lo cation, stack height, stack diameter, stack flow rate and st ack temperature) are uncertain. For example, API points out that: “SO 2 emissions fell from 111,057 thousand short tons nationally in 2002 to 77,685 thousand short tons nationally in 2008 (see comments provided by API; p.22)41 Response: We first note that values API cited are the 2002 and 2008 emission

estimates for CO rather than SO 2 . Total SO 2 emissions fell from 14,774 thousand short tons nationally in 2002 to 11,429 t housand short tons nationally in 2008, a

national reduction of about 23%. 1

EPA acknowledges there is uncertainty in using the 2002 NEI emissions data. EPA has appropriately characterized both th e exposure and risk results associated with the “as is” modeling scenario as re flective of year 2002. No adjustment was made to reflect any apparent emission reductions (i.e., post-2002) in either study area. Therefore, the “as is” 2002 exposure and risk results should not necessarily be interpreted as t hose that might result from using current air quality conditions (e.g., year 2008). If there have b een reductions to the specific SO 2 emission sources modeled in these study areas rela tive to the 2002 emissions used, then the estimated exposure and risk results would likely be an overestimate of exposure and risk associated with recent y ear (e.g., 2008) emissions.

However, as discussed above in the re sponse to comments C(1) through C(3), EPA is interested in evaluating the risk of adverse health effects associated with a variety of air quality conditions including just meeting the current and alternative standards. Just as adjustments were made to 2001-2006 ambient monitor concentrations (REA, sec tion 7.2.4), adjustments were made to the exposure benchmark level (REA, section 8.8.3) to reflect air quality just meeting the

current or alternative standards. These hypothetical scenario-based investigations are independent of what current air quali ty conditions or emissions levels might actually be, assuming that the variability in measured ambient concentrations (and hence their emission sources) remain relatively stable. To justify this assumption as appropriate, EPA evaluated historical trends in ambien t air quality (REA, section 7.4.2.5).

EPA judged the 2002 NEI to be the most accurate, comprehensive SO 2 emissions data base available for use in the exposu re and risk analyses. As noted in the REA, the 2002 NEI emissions data were not used alone but were supplemented

with Clean Air Markets Di vision (CAMD) Unit Level Emissions Database of continuous emissions monitoring (CEM) emissions data where available and Emissions Modeling System for Hazardous Pollutants model (EMS-HAP) data to specify temporal profiles. Because of the importance of acc urate locations of emission sources relative to population receptors for exposure assessment, the locations of all emitting stacks were ve rified and corrected where necessary, based on GIS analysis, as described in the REA (section 8.4.3.1). In addition all port-related emission sources, which we re characterized as nonpoint area emissions, had boundaries specified based on GIS analysis of aerial photographic images.

1 1970 - 2008 Average annual emissions, all criteria pollutants in MS Excel - June 2009. http://www.epa.gov/ttn/chief/trends/

.42 (12) Comment: The REA included exposure analys es that were performed in the greater St. Louis metropolitan area a nd Green County, MO using U.S. EPA’s promulgated air quality dispersion model AERMOD (AERMIC MODEL) to

estimate 1-hour ambient SO 2 concentrations at the census block level. ABR and ASARCO commented that the model evaluation performed when AERMOD was originally developed showed that for short-term averages the ratio of the predicted

concentration to the observed concentra tion ranged up to 1.35, and speculated that the discrepancies were due to the “co nservativeness” of AERMOD in certain

situations.

Response: AERMOD has been evaluated agains t several data sets of observed

concentrations. The results of th e AERMOD development model evaluation showed, for short-term average concentra tions, an overall average predicted-toobserved ratio of 1.03 with a range among sites from 0.76 to 1.35. 2 Thus, this

evaluation does not provide evidence that AERMOD is inherently “conservative”, but rather suggests somewhat variab le but unbiased model performance.

A more relevant assessment of how AERMOD performed is provided by the comparison of model predicted to ambi ent monitor measured concentrations (REA, section 8.4.5). Two types of pred icted-to-measured comparisons were made: (a) predicted and measured concen tration distributions for a particular location were matched by percentile and co mpared, and (b) diurnal patterns of predicted and measured concentrations were compared by averaging concentrations for each hour of the day, e.g., the concentrations for the first hour of the day were averaged across all simulated days. Recognizing that uncertainties in model inputs can result in uncertainties in model predictions, particularly with respect to location, fo r both types of comparisons the predicted concentrations were processed for all modeling receptors within 4 km of the

monitor site so that the observed values could be compared to both the predictions at the exact location of the monitor, as well as the range of predictions in the vicinity. The AERMOD 1-hour co ncentration predictions showed good agreement with measured concentrations, pa rticularly for those at the upper end of the concentration distribution, concentrations most likely to be associated with 5-

minute benchmark level exceedances.

(13) Comment: ASARCO commented that a substantial fraction of the SO 2 emissions in Greene County were characterized as nonpoint sources, and generally questioned the approach and data used by EPA to model these nonpoint emission sources (e.g., the accuracy of the emissions estimates, assuming uniform emission densities across a census tract, and sel ection of emission release heights).

Response: First of all, EPA has adopted th e term “nonpoint” to refer to all stationary emission sources th at are not inventoried at th e facility-level, and hence these sources are not incorporated into the point source component of the NEI.

2 See AERMOD: Latest Features and Evaluation Results . EPA-454/R-03-003. http://www.epa.gov/ttn/scram/7th conf/aermod/aermod_mep.pdf43 Rather these emissions are specified at the county-level. For the Greene County study area, to improve the spatial resolu tion of the nonpoint source emissions, the county level aggregate emissions data pr ovided in the NEI were allocated to

census tracts based on land use. For example, the county total of nonpoint

industrial emissions was allocated to trac ts in proportion the amount of industrial land use in each tract. Given the time, resources, and data available to conduct this exposure and risk assessment, it was not feasible to estima te the locations of these facilities any more precisely. Therefore, it was assumed for modeling purposes, that the emissions were rel eased uniformly over the tract.

When considering SO 2 emissions, the majority of these nonpoint sources are comprised of small industrial and comme rcial/institutional facilities combusting fuels. For the fuel combustion categories , NEI estimates were derived from state level fuel consumption data, and allocated to counties based on employment data for appropriate sectors. In the case of industrial nonpoint sources, the fuel combustion data was first corrected for an estimate of fuel consumption by industrial point sources to avoid double-counting. 3

As noted above, given the lack of loca tion data for nonpoint sources, it was not feasible to specify the locations any more precisely than census tr acts. If there are only a small number of nonpoint sources in a tract, this approach would tend to reduce the spatial variance of the con centration predictions compared to specifying precise locations for each small f acility. The result could be either an

over- or under-estimate of the number of exceedances at the population receptors. However, if there are numerous such fac ilities dispersed throu ghout the tract, this approach will provide a reasonable characte rization of the spa tial pattern of the

emissions.

EPA selected release heights for non-point area sources as 10.0 m for rural tracts and 20.0 m for urban tracts. Although no da ta were identified by EPA to estimate release heights for these nonpoint sources, the release heights were not selected “arbitrarily” as mentioned by the comment er. First, engineering judgment was used to estimate the release height for a small industrial or commercial/institutional facility of 2 to 3 st ories. Then an estimate of plume rise was added, since AERMOD does not apply a plume rise algorithm to area sources. Finally a series of sensitivity si mulations were conducted to characterize model performance at the ambient monitor locations in order to make final estimates for release parameters.

Again, the model evaluation discussed in REA section 8.4.5 suggests that the approaches to modeling the nonpoint sour ces in both Greene County and the St. Louis metropolitan study areas resulted in acceptable model performance,

3 See Documentation for the Final 2002 Nonpoint Sector (Feb 06 Version) National Emission Inventory for
Criteria and Hazardous Air Pollutants. ftp://ftp.epa.gov/EmisInventory/2002finalnei/documentation/nonpoint/2002nei_final_nonpoint_documentat ion0206version.pdf

.44 especially in the upper end of the concentration dist ribution, where exceedances

of concentration thresholds of c oncern are most likely to occur.

(14) Comment: UARG suggested that some of th e exposure analysis could not be replicated with the data pr ovided in the first draft REA. In particular they questioned the specification of block receptors within 20 km of major point sources, given a discrepancy between the num ber of block receptors stated in the draft report for the Southwest and Jame s River Power Plants in Greene Co MO and the number they calculated based on lo cation coordinates they derived from visual inspection of Google Earth. They suggest that the discrepancy may be due to a discrepancy in the location coordi nates used for the REA and those they

derived.

Response: The UTM coordinates of each st ack modeled for the final REA are listed in Appendix B, Table B.3-1. The discrepancy in the number of block receptors stated in the draft report and that found by the commenter is likely due to a discrepancy in the number of rel ease points used to characterize the James River Power Plant. For this study we ch aracterized the James River Power Plant with 10 emission release points, each w ith unique location parameters, while the commenter used only two. Hence, we found slightly more Census blocks within 20 km of the plant than reported by the commenter.

(15) Comment: Several commenters (e.g., API, UARG, Dominion) indicated EPA did not properly consider the exposure and ri sk results for Greene County rather, EPA focused on the larger risk estimates presented for St. Louis.

Response: EPA acknowledges that more weight was placed on the exposure and risk results in St. Louis compared to Greene County. EPA believes this to be a reasonable approach: the St. Louis in formation provided more probative information as to the adequacy or inadequ acy of the current standards. As stated

in the REA: Exposures and risks have been estim ated for two study areas in Missouri (i.e., Greene County and several coun ties representing the St. Louis urban area) which have significan t emission sources of SO 2 . As noted in section 8.10, there were differences in the nu mber of exposures above benchmark

values when the results of the Greene County and St. Louis exposure assessments were compared. Moreov er, given that th e results of the exposure assessment were used as inputs into the quantitative risk assessment, it was not surprising that there were also far fewer asthmatics

at elevated ventilation rates estimate d to have a moderate or greater lung function response in Greene county wh en compared to St. Louis. The

difference in the St. Louis and Green e County exposure and quantitative risk results are likely indicative of the different types of locations they represent (see section 8.10). Greene County is a rural county with much lower population and emission densities, compared to the St. Louis study area which has population and emissions density similar to other urban45 areas in the U.S. It therefore fo llows that there would be greater exposures, and hence greater numbers and percentages of asthmatics at elevated ventilation rates experienci ng moderate or greater lung function responses in the St. Loui s study area. Thus, when considering the risk and exposure results as they relate to the adequacy of the current standards (as well as the need for considering poten tial alternative standards), the St.

Louis results are more informative in that they suggest that the current standards may not adequately protec t public health. Moreover, staff judges that the exposure and risk es timates for the St. Louis study area provide useful insights into exposures and risks for other urban areas in the U.S. with similar population and SO 2 emissions densities (REA, p. 364).

(16) Comment: JRSC referenced prior comments by UARG stating that the individual data from controlled human exposure st udies used in the quantitative risk assessment had significant errors associat ed with it, and that it was unclear if

these errors had been corrected (comments by JRSC, p. 2).

Response: EPA conducted a full quality assura nce review of the controlled human exposure data, which included personal communication and technical guidance from the principal investigator of the original studies, William Linn. This review, presented in Johns and Simmons (2009), found a very limited number of minor errors in the original individual subject lung function data presented in the ISA. However, the resu lts of the review did not change the conclusions presented in the ISA. In fact , the quality assuranc e review resulted in

an increase in the percentage of responders re ported in Table 3-1 of the ISA and

9-3 of the REA.

(17) Comment: UARG generally concluded that under all air quality scenarios EPA’s quantitative risk assessment substantially overestimated risk because EPA did not use proper methods to estimate the paramete rs of the exposure-response functions used in its analyses. UARG contends this is because many of the subjects in the controlled human exposure studies fr om which EPA’s exposure-response functions were derived (see REA, Table 9- 3) were exposed to more than one SO 2 concentration, yet EPA treated each exposur e event as being independent (e.g., if the same subject was exposed to 200 and 300 ppb SO 2 , EPA considered these as representing two independent exposur e events). UARG contends that observations from the same subject exposed to different SO 2 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 SO 2 concentrations are not independent of each other, they estim ated appreciably less risk than that estimated by EPA.

Response: EPA notes that it is true that observations fo r the same subject across

the different SO 2 concentrations to which th e subject was exposed are not46 completely independent observa tions – although for any given SO 2 concentration, the observations are (mostly) independent. 4 The important question is what effect not considering the dependence of in tra-subject observations across SO 2 concentrations has on the estimated exposure-response relationship.

To address this perceived problem of dependent observati ons, UARG estimates subject-specific exposure-response functions. 5 Based on these subject-specific exposure-response functions, UARG estim ates the two parameters in the (population-level) logistic and probit e xposure-response functions necessary for use in the risk assessment. EPA plotted the logistic and probit exposure-response functions estimated by UARG along with the logistic and probit exposureresponse functions estimated by EPA, as well as the underlying data. 6,7 These are shown in Figure 1.

As Figure 1 shows, the logistic and pr obit exposure-response functions estimated by UARG (the blue curves) do not fit th e underlying data (t he proportions of subjects who responded at each exposure leve l) nearly as well as the functions estimated by EPA (the black curves). This suggests that, even though it deals with the “dependency problem,” the UARG approach has its own problems.

4 There were a few instances where a subject was exposed to the same SO 2 concentration more than once. EPA noted this in the REA, stating the following: “Some subjects in the controlled human exposure studies participated in more than one study and were exposed to a given SO 2 concentration more than once. However, because there were insu fficient data to estimate subject-specific response probabilities, we assumed a single response probability (for a given definition of response) for all individuals and treated the repeated exposures for a single subject as independent exposures in the binomial distribution.”

5 UARG first claims that “the co rrect likelihood for such a case [in which there are several observations per subject] would be multinomial” and they set up a multinomial likelihood function. However, UARG appears not to use it.

6 The UARG curves are based on the estimated valu es given for the two parameters in each function presented in Table 4 of “Technical Comments on the Primary National Ambient Air Quality Standard for Sulfur Dioxide. Proposed Rule Docket no. EPA-HQ-OAR-2007-0352, by Edmund A.C. Crouch, Ph.D. and Laura C. Green, Ph.D., D.A.B.T. Cambridge Environmental Inc. Cambridge, Massachusetts. February 8, 2010 (Docket No. EPA-HQ-OAR-2007-0352-1061.1) Corrected April 6, 2010.

7 The functional forms for the logistic and probit functions are given in (equation 9-3) and (equation 9-4), respectively, on p. 327 of the REA. The EPA-estim ated curves and the underl ying data are shown in Figure 9-2 on p. 330 of the REA.47 Figure 1. EPA- and UARG -Estimated Logistic and Probit Exposure-Response Functions (for Lung Function Response Defined as an Increase in sRaw of ≥ 100%) and Underlying

Data

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

0 0.2 0.4 0.6 0.8 1

SO 2 Concentration (ppm)Response Rate The original data

EPA probit

EPA 2-parameter logistic

UARG 2-parameter logistic

UARG probit

UARG is not specific about the nature of the problems that will result from using

a likelihood function that does not consider intra-subject dependencies. They note only that “the consequence of using this incorrect likelihood for common subjects was to decrease (in the REA analysis) the estimated slope of the population E 100 distribution function (the magnitude of the γ parameter in both [the logistic and probit exposure-response] models), and in crease the estimated population fraction responding at low exposures (in the extrapolated region below 0.2 ppm SO 2 ).” 8
They note in a footnote that “this cons equence was evaluated by performing the calculations both ways [i.e ., the way EPA estimated the parameters and the way UARG estimated them].” That is, they concluded that EPA’s estimated curves are “wrong” because they differ markedly from their own curves. This presumes, however, that UARG’s curves are indeed “correct.” The comparison of how each of the curves reflects the underlying da ta (Figure 1) suggests otherwise.

8 From: Comments of the Utility Air Regulatory Group on the Risk and Exposure Assessment to Support

the Review of the SO 2 Primary National Ambient Air Quality Sta ndards: Second Draft (March 2009). Docket ID No. EPA-HQ-OAR-2007-0352. June 11, 2009. Attachment 1 (Technical Comments by Edmund A. C. Crouch, Ph.D., Laura C. Green, Ph.D., D.A.B.T. and Joshua M. Galanter, M.D. on the SO 2 REA (June 10, 2009), p. 12.48 While their approach of estimating subj ect-specific exposure-response functions addresses the issue of intra-subject depe ndencies, EPA notes that it creates other serious problems. In particular, EPA notes that there do not appear to be sufficient data for any of the subjects to estimate a subject-specific exposureresponse function. 9 The form of the individual-specific exposure-response function they chose is a quadratic functi on with three parameters. Thus for each subject they presumably had to estimate the subject-specific values of these three parameters. However, none of the s ubjects in the seven controlled human exposure studies on which EPA relied were exposed to more than three non-zero

SO 2 levels: over half were exposed to only one or two non-zero SO 2 levels.

Thus, EPA notes that it app ears that the data for estimating these subject-specific exposure-response functions were severely limited, especially given that a large percentage of the total number of subj ects had fewer exposures than the number of parameters UARG was attempting to es timate. UARG attempted to estimate three parameters in its exposure-respons e functions, but over fifty percent of subjects had only one or two exposures. It appears, moreover, that UARG’s population-level exposure-response functi on estimates depended on these subjectspecific exposure-response function estimates, and this could explain why UARG’s estimated population-level exposur e-response functions do not fit the underlying controlled human exposure data nearly as well as the EPA-estimated functions. In attempting to address the problem of intra-subject dependencies, UARG seems to have created different prob lems. It is not surprising, then, that neither their estimated logistic function nor their estimated probit function fit the underlying data very well.

There are other approaches to dealing with the intra-subject dependency issue thatavoid the problem associated with the UARG approach. Not considering intrasubject dependencies is effectively omitting an individual-level effect from the model. This would be a problem if the omitted individual-level effect were correlated with the assignment of exposur es (for example, if the hyper-responsive subjects were disproportiona tely exposed to the lowest levels), but there is no reason to suppose this is th e case. Moreover, any possi ble bias in the estimated logit or probit model parameters is likely to be minor, since the majority of the subjects had only one or two exposur es, so the degree of intra-subject dependencies is small; indeed, visual inspection of the EPA fitted curves in Figure 1 does not suggest any substantial biases.

EPA further notes that the approach we used to estimate the parameters of the exposure-response functions was not first introduced in this SO 2 risk assessment; it was previously used in the O 3 risk assessment completed in 2008, which used data from controlled human exposure stud ies much like the data used in the SO 2 risk assessment. This approach was s uggested to EPA by an applied statistician

9 This was noted in the Risk and Exposure Assessment, see http://www.epa.gov/ttn/naaqs/standards/s o2/data/200908SO2REAFinalReport.pdf , p. 328.49 serving on the CASAC which at the time was overseeing the O 3 NAAQS review. Notably, this approach allowed EPA to us e all of the individual subject data. Consequently, as noted in section II.E.2.c of the preamble to the final rule, EPA does not accept UARG’s view that the methodology used in EPA’s quantitative risk assessment was inappropriate.

(18) Comment: UARG generally concluded that th e risk estimates presented in Chapter 9 of the REA and in the proposal are substantially overestimated because of the use of 50 ppb bins by EPA.

More specifically, in their original comments submitted on June 11, 2009, UARG noted that EPA’s approach:

...serves to overestimate the number of annual ‘events’ (defined as an

increase in corrected relative sRaw of ≥ 100% among asthmatics in St. Louis). This is primarily because the REA analysis bins SO 2 exposures into 50 pbb-wide bins. An accurate cal culation must take into account the distribution of values, particularly in the lowest bin — wherein (i) ambient air concentrations range from zero to 50 ppb, and (ii) the REA predicts

95% of the sRaw events. 10

That is, 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 exposu res from 0 - 50 ppb were assigned the probability of an adverse lung functi on response 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 concludes that this is because the vast majority of e xposures 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 SO 2 exposures the higher probability of a lung function response associated with the midpoint of the 0 -50 ppb exposure bin (i.e., the great majority of exposures were < 25 ppb but were assigned the probability of an adverse lung function response at 25 ppb).

In follow-on comments submitted in February 2010, 11 UARG takes advantage of a technique EPA used to streamline the estimation of exposures under the

10 “Comments of the Utility Air Regulatory Group on the Risk and Exposure Assessment to Support the

Review of the SO 2 Primary National Ambient Air Quality Standards: Second Draft (March 2009).” Docket ID No. EPA-HQ-OAR-2007-0352. June 11, 2009. Attachment 1: “Technical Comments by Edmund A. C. Crouch, Ph.D., Laura C. Green, Ph.D., D.A.B.T. and Joshua M. Galanter, M.D. on the SO 2 REA (June 10, 2009), p. 31.

11 “Technical Comments on the Primary National Ambi ent Air Quality Standard for Sulfur Dioxide. Proposed Rule Docket No. EPA-HQ-OAR-2007-0352, by Edmund A.C. Crouch, Ph.D. and Laura C. Green, Ph.D., D.A.B.T. Cambridge Environmental Inc. Cambridge, Massachusetts. February 8, 2010 (Docket No. EPA-HQ-OAR-2007-0352-1061.1) Corrected April 6, 2010.50 different air quality scenar ios considered. As descri bed in Section 8.8.3 of the July 2009 Final SO 2 REA,

... instead of proportionally adjusting the ambient concentrations, [EPA] proportionally adjusted the health effect benchmark levels used in each exposure modeling domain. The benchmar k levels were adjusted rather than the air quality to reduce the processing time associated with the modeling of several thousands of rece ptors in each of the large exposure modeling domains. A proportional adju stment of the selected benchmark level (i.e., division by the adjustment f actor) is mathematically equivalent to a proportional adjustment of th e air quality concentration (i.e., multiplication by the adjustment factor). Therefore, the end effect of adjusting the exposure model input concentrations upward versus adjusting exposure model benchmark levels downward is identical.

This “walkway” between SO 2 concentrations under each alternative SO 2 standard

and SO 2 concentrations under recent conditions allowed EPA to carry out the exposure analysis for all the different ai r quality scenarios in a single APEX run, resulting in an APEX ou tput table with many SO 2 exposure levels under year 2002 “as is” air quality conditions. UARG used these many exposure levels

under “as is” conditions and EPA’s factors for converting from recent conditions to each of the alternative air quality s cenarios to create much smaller “bins.” Applying EPA’s approach of calculating the expected number of lung function responses at the midpoint of each “bin” to these much smaller “bins,” UARG showed that use of 50-ppb “bins” does indeed overstate the total number of occurrences of lung function response unde r each air quality scenario. UARG further notes that this methodological con cern was raised in its comments on the second draft REA, but EPA failed to addre ss this issue and relied heavily on this metric in the proposal with respect to the adequacy of the current and potential

alternative standards.

Response: EPA generally agrees with UARG’ s technical comments that there is

a substantial overestimation of the total occu rrences of lung f unction 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 Administ rator’s rationale with respect to the adequacy of the current and potential a lternative 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 2 nd draft SO 2 REA, the assignment of response probability to the midpoint of the exposure bin combined with the lack of more finely divided in tervals in this range can lead to significant overestimation of risks base d on total occurrences of a defined lung function response. This is be cause the distribution of population51 exposures for occurrences is not ev enly distributed across the bin, but rather is more heavily weighted towa rd 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 occurren ces 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 th e policy assessment chapter of the REA (REA, chapter 10) or in the Administrato r’s rationale sections of the proposal preamble. Rather, the policy assess ment chapter of the REA and the Administrator’s rationale at the propos al 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 in cidence 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.

This is confirmed by using the same t echnique used by UARG, described above, but applying it to this metric. The results , for asthmatic children in St. Louis, are shown in Table 1 below using the origin al 50-ppb bin approach and the more disaggregated bins identified by UARG.52 Table 1. Estimates of the Number of Asthma tic Children in St. Louis with at Least One Lung Function Response (Defined as an Increase in sRaw>=100%) Under Different Air Quality Scenarios Using 50 ppb Bins Compared with Estimates Using More Disaggregated

Bins

Using 50 ppb Bins Using More Disaggregated

Bins Using 50 ppb Bins Using More Disaggregated

Bins

Recent conditions 588 591 100.5% 1.41% 1.42%

Current Standard 8021 8090 100.9% 19.23% 19.39%

Alternative Std: 99/50 396 406 102.5% 0.95% 0.97%

Alternative Std: 99/100 1221 1204 98.6% 2.93% 2.89%

Alternative Std: 99/150 2243 2214 98.7% 5.38% 5.31%

Alternative Std: 99/200 3369 3346 99.3% 8.08% 8.02%

Alternative Std: 99/250 4555 4541 99.7% 10.92% 10.89%
*There were 41714 asthmatic children in St. Louis in this analysis. Number of Asthmatic Children Who Respond at

Least Once Percent of Asthmatic Children Who Respond at

Least Once* Air Quality Scenario New Result

as a Percent

of Original

Result

As Table 1 shows, the use of 50 ppb bins does not result in ov erestimates of the number or percent of asthmatic children in St. Louis with at least one lung function response across the air quality s cenarios. While in some air quality scenarios the estimate based on the 50-ppb bi n approach is slightly greater than the estimate based on more disaggregated bi ns, in others the estimate based on the 50-ppb bin approach is slightly less than the estimate based on more disaggregated bins. The differences a ppear to be random and are judged not significant from a policy perspective.

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 wei ght that she placed on the scientific evidence as described in the ISA, the Admi nistrator placed relatively more weight on the results of the St. Louis exposure analysis (see secti on II.E.2.c of the preamble to the final rule for more information).

(19) Comment: In their comments submitted on April 6, 2010, UARG notes that “the

Final REA also omitted to take account of the additional information available on the experiments of Linn et al. (1983) that became av ailable between the Draft REA and Final REA, another 69 samples with 14 positive results.” 12

12 “Technical Comments on the Primary National Ambi ent Air Quality Standard for Sulfur Dioxide. Proposed Rule Docket No. EPA-HQ-OAR-2007-0352, by Edmund A.C. Crouch, Ph.D. and Laura C. Green, Ph.D., D.A.B.T. Cambridge Environmental Inc. Cambridge, Massachusetts. February 8, 2010 (Docket No. EPA-HQ-OAR-2007-0352-1061.1) Corrected April 6, 2010, p. 15.53 Response: While it was not possible to re -estimate the exposure-response functions incorporating this additional data mentione d by UARG, EPA did create a new graph showing the original and adju sted data as well as the EPA-estimated and the UARG-estimated logistic and prob it exposure-response functions (based on the original set of data) for lung function response de fined as an increase in

sRaw of ≥ 100%. Shown below are all the original data points (black triangles) as well as the adjusted data points (red cr osses) incorporating the additional data from Linn et al. (1983) for the three SO 2 exposures for which that study provided data (0.2, 0.4, and 0.6 ppm). As shown in Figure 2 below, the three adjusted data points are all quite close to the original data points used to develop the exposureresponse functions; it is therefore highly unlikely that the in corporation of the Linn et al. (1983) data would alter th e estimated exposure-response functions appreciably.

Figure 2. EPA- and UARG (Crouch/Green)-Estimated Logistic and Probit ExposureResponse Functions (for Lung Function Response Defined as an Increase in sRaw of

≥ 100%), Underlying Original Data, and Data Incorporating Linn et al. (1983)

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

0 0.2 0.4 0.6 0.8 1

SO 2 Concentration (ppm)Response Rate The original data

probit

2-parameter logistic

UARG 2-parameter logistic

UARG probit

Adjusted data - including Linn et al. 1983 (Computed Differences)

(20) Comment: Springfield pointed out that Table 10 -5 of the REA contained an error.

The 1-hour SO 2 concentrations reported for Greene County were not from the monitor that recorded the highest SO 2 concentrations in the county. Springfield was concerned that the exposure and risk estimates presented in the REA for54 Greene County were based on erroneous data, and thus did not accurately represent the impacts of SO 2

.

Response: Springfield is correct that REA Table 10-5 identified the incorrect monitor as that recording the highest 1-hour daily maximum SO 2 concentrations in Greene County. We note that Table 10-5 was only used to demonstrate that in most counties, the 99 th percentile 1-hour daily maximum SO 2 concentration corresponded to the 4 th highest 1-hour daily maximum SO 2 concentration and that

the 98 th percentile 1-hour daily maximum SO 2 concentration corresponded to the

7 th -8 th highest 1-hour daily maximum SO 2 concentration. Thus, the results of the air quality, exposure and risk assessment for Greene County were not affected by this error. In addition, we note that in general, as noted above in response 8 of this section, the results of the Greene County exposure and risk assessments were

not considered as informative as those in St. Louis.

(21) Comment: Springfield commented:

Springfield is the seat of Greene County, Missouri, a locale mentioned several times in the proposal a nd in the supporting Risk Exposure Analysis. Understandably, we noted with considerable interest the discussion of EPA’s exposure modeling fo r this area. We find it especially telling that the model returned exposur e results that apparently were too inconsequential for further cons ideration (see, for example, p.64827, “(t)hus, when considering the risk an d exposure results as they relate to the adequacy of the current standard s, the REA concluded that the St.

Louis results were more informativ e [than Greene County]...”). Despite this lack of any cogent exposure fi ndings for our “r ural” (population 270,000) county, the proposed standard could very well place Greene County in nonattainment (see comment s provided by Springfield, section II.d.ii)

Response: Springfield is correct that EPA found the expos ure and risk results for

St. Louis more informative than thos e in Greene County for informing the adequacy of the current and potential alternative standard s. As stated in the REA (and is quoted in th e earlier response):

Exposures and risks have been estim ated for two study areas in Missouri (i.e., Greene County and several coun ties representing the St. Louis urban area) which have significan t emission sources of SO 2 . As noted in section 8.10, there were differences in the nu mber of exposures above benchmark

values when the results of the Greene County and St. Louis exposure assessments were compared. Moreov er, given that th e results of the exposure assessment were used as inputs into the quantitative risk assessment, it was not surprising that there were also far fewer asthmatics

at elevated ventilation rates estimate d to have a moderate or greater lung function response in Greene county wh en compared to St. Louis. The55 difference in the St. Louis and Green e County exposure and quantitative risk results are likely indicative of the different types of locations they represent (see section 8.10). Greene County is a rural county with much lower population and emission densities, compared to the St. Louis study area which has population and emissions density similar to other urban

areas in the U.S. It therefore fo llows that there would be greater exposures, and hence greater numbers and percentages of asthmatics at elevated ventilation rates experienci ng moderate or greater lung function responses in the St. Loui s study area. Thus, when considering the risk and exposure results as they relate to the adequacy of the current standards (as well as the need for considering poten tial alternative standards), the St.

Louis results are more informative in that they suggest that the current standards may not adequately protec t public health. Moreover, staff judges that the exposure and risk es timates for the St. Louis study area provide useful insights into exposures and risks for other urban areas in the U.S. with similar population and SO 2 emissions densities (REA, p. 364).

That being said, it is important to note se veral points. First, these analyses only estimated exposure and risks associat ed with modeled 5-minute benchmark

concentrations of concern identified from the controlled human exposure

literature: thus these considerations alone do not take in to account 1-hour SO 2 levels in U.S. locations epidemiologic studies reported posi tive effects between

ambient SO 2 and emergency department visits and/or hospitalizations (i.e., epidemiologic considerations taken into account by the Administrator in setting a new 1-hour standard at 75 ppb). In a ddition, Tables 7-12 to 7-14 of the REA indicate that when air quality is adjusted to just meet the current standards, Greene County had an appreciable number of days per year on average when 5-

minute SO 2 concentrations exceeded 5-minute be nchmark levels. This indicates

5-minute concentrations of concern coul d be present in Greene County under the current standards, and it is thus possibl e with increased population growth and/or shifts in activity patterns, a greater number of SO 2 exposures above 5-minute benchmark levels could result (if air quali ty just met the current standards). Finally, EPA notes that the NAAQS are national standards meant to provide protection in both rura l and urban areas.

(22) Comment: UARG generally commented that th e risk and exposure assessment

did not take into account the 3-hour s econdary standard when estimating the

extent to which the current standard s limit 5-minute peaks above benchmark

levels of concern.

Response: 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 safe ty, are requisite to protect the public health.” Section 109(b)(1). In contrast, a secondary standard, must “specify a level of air quality the atta inment and maintenance of which, in the56 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.” 13 Thus, EPA found it reasonable when considering the ad equacy of the current standards to protect public health, to only consider the primary SO 2 standards in its air quality, exposure and risk analyses.



## III. Responses to Significant Comments on the Adequacy of

the Current Standards

(1) Comment : Many industry groups (e.g., API, UARG, ACC, DRSI, ExxonMobil, PE, CIBO, TFI, EEI, Dow, MWV, NMA, Duke Energy, GCLC) commented that, given the presence of numerous co-pollutant s in the air, the epidemiologic studies do not support the contention that SO 2 itself is causing health effects. For example, UARG stated:

The epidemiological evidence cannot determine that SO 2 is a cause of or a contributor to hospital admissions (“ HA”), emergency department (“ED”) visits or respiratory symptoms, the ef fects cited in the Proposed Rule (see comments provided by UARG; p.3)

Thus, these groups generally conclude that the epidemiologic evidence should not be considered when evaluating the adequacy of the current standards

Response: These comments and EPA’s response s were presented in detail in section II.E.2.a of the preamble to the final rule. EPA also provided additional

information on similar comments above in section II.B

(2) Comment: Many industry groups (e.g., API, ACC, PE, EEI, CIBO) commented

that adverse health effects do not occur following 5 - 10 minute SO 2 exposures < 400 ppb. In addition, some groups (e.g., UAR G, Duke Energy) commented that adverse respiratory effects do not occur in exercising asthmatics following SO 2 exposures below 600 ppb. The disagreemen t is not whether effects occur in exercising asthmatics at these exposure leve ls 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

SO 2 exposure levels below 600 or 400 ppb is inappropriately based on an unsound interpretation of ATS guidelines. Mo re specifically, th ese groups generally contend that decrements in lung f unction without accompanying respiratory symptoms are not adverse effects of SO 2 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

13 EPA is currently conducting a separate review of the secondary SO 2 NAAQS jointly with a review of the secondary NO 2 NAAQS (see http://www.epa.gov/ttn/naaqs/st andards/no2so2sec/index.html for more information).57 followed the advice of individual CASA C members, rather than consensus CASAC written comments on the ISA and REA when concluding respiratory

effects associated with SO 2 exposures below 600 or 400 ppb are adverse. Thus, these groups generally conclude that 5- minute benchmark levels less than 600 or 400 ppb should not be considered when ev aluating the adequacy of the current

standards.

In addition to disagreeing with EPA’s pr oposed finding of adverse health effects following 5 - 10 minute SO 2 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 SO 2 exposures at a given level, or would respond to even lower levels of SO 2 and that this should be taken into account when judging the adequacy of the current standards.

Response : EPA disagrees with these commen ts, and believes (as does CASAC)

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. These comments and EPA’s responses were presented in detail in section II.E.2.b

of the final rule. Similar comments and EPA responses were also discussed above

in section II.A of this document.

(3) Comment: Several commenters disc ussed the analyses of SO 2 -associated exposures and health risks presented in th e REA. As in past reviews (e.g., EPA 2005, EPA 2007), EPA has estimated risks asso ciated with the current standards to inform judgments on the public health risks that could exist under different standard options. Some industry comme nters (e.g., API, UARG, LEC, ASARCO, NRECA) concluded that when considering the adequacy of the current standards,

the Administrator should c onsider exposures and risks associated with actual SO 2 air quality rather than air quality allowed by the current NAAQS. These groups also note adjusting air quality to just m eet the current and potential alternative standards is highly uncertain. They c onsequently challenged the relevance and appropriateness of EPA’s use of SO 2 concentrations that have been simulated to just meet the current standards in assessing the adequacy of the current standards.

Response : EPA disagrees with these comments. These comments and EPA’s responses were presented in detail in section II. E.2.c of the preamble to the final rule. Similar comments and EPA responses were also discussed above in section

II.C.

(4) Comment: UARG generally concluded that under all air quality scenarios, the results of EPA’s quantitative risk assessmen t (the third of the analyses conducted in the REA (chapter 9) are substantiall y overestimated because EPA did not use proper methods to estimate the parameters of the exposure-response functions used in its analyses. UARG contends this is because many of the subjects in the58 controlled human exposure studies fr om which EPA’s exposure-response functions were derived (see REA, Table 9- 3) were exposed to more than one SO 2 concentration, yet EPA treated each exposur e event as being independent (e.g., if the same subject was exposed to 200 and 300 ppb SO 2 , EPA considered these as representing two independent exposur e events). UARG contends that observations from the same subject exposed to different SO 2 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 SO 2 concentrations are not independent of each other, they estim ated appreciably less risk than that estimated by EPA.

Response : EPA presented this comment a nd provided an initial qualitative response in section II.E.2.c of the preamble to the fina l rule. A more detailed technical response to this comment is discussed above in section II.C.

(5) Comment: UARG concluded that EPA further overestimates the total number of occurrences of an adverse lung func tion response (i.e., total number of

occurrences of increases in sRaw ≥ 100 or 200% and/or declines in FEV 1 ≥ 15 or 20%) in its quantitative risk assessmen t. More specifica lly, 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 as signed the probability of an adverse lung function response occurring at 25 ppb), resulted in a subs tantial overestimate

of the total number of o ccurrences of lung function re sponses in asthmatics at moderate or greater exertion. UARG genera lly concludes that this is because the vast majority of exposures of asthmatic s 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 SO 2 exposures the higher probability of a lung function response associat ed with the midpoint of the 0 -50 ppb exposure bin. UARG contends that this results in a substantial

overestimation of the total number of o ccurrences of lung fu nction response in

asthmatics and asthmatic child ren at moderate or great er exertion. UARG further notes that this methodological concern was raised in its comments on the second draft REA, but EPA failed to address this i ssue and relied heavily on this metric in the proposal with respect to the adequacy of the current and potential alternative

standards.

Response : EPA presented this comment a nd provided an initial qualitative response in section II.E.2.c of the preamble to the fina l rule. A more detailed technical response to the binning issue discussed in this comment can be found

above in section II.C.59 

## IV. Comments on A New Short-Term SO 2 

## Primary Standard

This section discusses comme nts received on EPA’s proposed 1-hour standard. Some commenters provided comments on the cost or economic impact of monitoring, implementation, or compliance a ssociated with the proposed SO 2 NAAQS. As noted in section I.B of the preamble, the Clean Air Act bars consideration of costs in setting the NAAQS, and accordingly EPA has not considered costs, including the costs or economic impact of monitoring, implementation or compliance, in revising the primary SO 2 NAAQS.



## A. Indicator

Few public commenters directly addressed the issue of the indicator for the standard. These commenters generally endorsed th e proposal to continue to use SO 2 as the

indicator for ambient SO

x .



## B. Averaging time

(1) Comment: As discussed above, industry comme nters who disagreed with setting a new 1-hour standard generally based th is conclusion on their interpretation of

the scientific evidence and their conclusi on that this evidence does not support the proposed revisions to the current SO 2 NAAQS.

Response: These comments and EPA’s respon ses were presented in detail in sections II.E.2.a and II.E.2.b of the preamble to the final rule.

(2) Comment: Some industry commenters (e.g. ASARCO, RTA, ABR, Nucor, KUC) and South Dakota expr essed that EPA should ha ve 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 exampl e, in its comments, South Dakota 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 averag ing period would smooth out the variability of the upper range measurements and provide a mo re stabile standard” (see comments provided by South Dakota; p. 1) Simila rly, RTA stated in its comments: “the short-term averaging period defined by EP A (i.e., 5-minutes 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 peri ods to limit short-term peaks of SO 2 ...a longer term average could provide additional stability to the standard, while at the60 same time effectively protecting public health” (see comments provided by RTA; p. 3)

Response: These comments and EPA’s respon ses were presented in detail in

section II.F.2.b of the final rule

(3) Comment: Although health and environmenta l groups were supportive of setting a new 1-hour standard to protect ag ainst short-term exposures to SO 2 (depending

on the level of the 1-hour standard sel ected), these groups generally commented that a 5-minute standard to protect agains t health effects associated with 5-minute peaks would be optimal (e.g., ALA, Sier ra Club, EDF). For example, in its comments ALA et al., stated: “We need a short-term SO 2 standard, optimally a 5- minute standard, to protect against bursts of pollution that can result from start-up, shutdown, upset, malfunction, downwash, co mplex terrain, atmospheric inversion conditions, and other situations” and that “EPA has over emphasized a concern about the stability of a 5-minute standar d... 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 (see comments provided by ALA et al., p. 11-12)

Response: These comments and EPA’s respon ses were presented in detail in section II.F.2.b of the preamble to the final rule. We also again emphasize

CASAC’s conclusion that “that a one-hour standard is the preferred averaging time”. Samet (2009) at p. 15.

(4) Comment: ALA generally commented that EPA was not setting a 5-minute

standard because of concerns over the number of monitors needed. ALA notes

that this concern is no t lawful under the CAA.

Response: As noted in final rule preamble sections II.F.2.b and II.F.2.c, EPA is not setting a 5-minute standard because of concerns over the stability of a 5- minute standard, and the deleterious eff ects an unstable standard could have on public health protection. The number of monitors needed should a 5-minute standard be adopted was not a considerat ion in the Administrator’s rationale for not setting a 5-minute standard.

In addition



## , as noted in final preamble sect ion II.F.2.b, 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 th at 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 cont rol 5 - 10 minute peak exposures and whether there shoul d be a 5-minute averaging time, CASAC stated that the REA had presented a “convincing rationale” (Samet 2009, p. 16)61 for a 1-hour standard, and th at “a one-hour standard is the preferred averaging time” (Samet 2009, p. 15).

(5) Comment: ALA favored a rolling average rather than a block average for determining compliance with the NAAQS. That is, ALA stated:

EPA has proposed a block average to determine compliance with the onehour daily maximum standard. That is, compliance would be measured against concentrations recorded in each hour of the day. However, this method of measuring compliance misses high concentrations that span the top of the hour. Our lungs do not diffe rentiate between high concentrations that occur during hours beginning at the top of the hour, and all other hours in the day. To provide requisite health protection from short term exposures, EPA must base complian ce on a rolling average for the onehour standard. Such a rolling average w ould be consistent with a not to be exceeded form of the standard (see comments provided by ALA et. al., p. 14)

Response: EPA acknowledges that there ar e multiple ways of potentially determining compliance with the SO 2 NAAQS. However, we disagree that a rolling average is needed to provide re quisite protection against short-term SO 2 exposures. The form of the sta ndard being finalized is the 99 th percentile of the distribution of 1-hour daily maximum SO 2 concentrations averaged over 3 years. Thus, since EPA is already considering the 99 th percentile of the distribution of the highest 1-hour SO 2 concentrations in each day of the year, we believe that using a rolling average as opposed to a bloc k average would result in negligible, if any, additional public health benefits. Moreover, we note that a rolling average for the 1-hour standard was not proposed



## C. Form

(1) Comment: A number of industry gro ups (e.g., NAM, ASARCO) and South Dakota preferred a 98 th percentile form. In genera l, their preference for a 98 th
percentile form was based on their co nclusion that a form based on the 98 th
percentile would be more stable than a form based on the 99 th percentile, and that

a 98 th percentile form is consistent with the forms selected in recent NAAQS reviews (i.e. PM 2.5 and NO 2 ). For example AirQuality stated:

The Administrator should reconsider her proposal and choose instead the

98 th percentile (or equivale nt nth highest value) fo rm of the standard for the added reliability and stability it offers in determining compliance or progress towards attainment. This a pproach has been promulgated for

recent revisions of the PM 2.5 and NO 2 standards and this consistency

should be maintained with SO 2 (see comments provided by AirQuality; p. 1).62 Some groups also maintained that a 98 th percentile form should be selected since a

98 th percentile form is consistent w ith the forms selected in recent NAAQS reviews (i.e. PM 2.5 and NO 2 ).

Response: These comments and EPA’s respon ses were presented in detail in section II.F.3.b of the preamble to the final rule

(2) Comment: Dow commented that “using the 99 th percentile could still cause an

area to be considered a n on-attainment area for an unusual event in one year, using the 98th percentile would, with the 3 year rolling average reduce this possibility and allow better identification of real issues where resources need to be deployed.”

Response: EPA disagrees with th is comment. The form of the standard is the 3- year average of the 99 th percentile of the annual distribution of 1-hour daily

maximum SO 2 concentrations. Moreover, analys es in the REA indicate that in most locations analyzed, the 99 th percentile corre sponds to the 4 th highest SO 2 concentration. Thus, an unusual event in one year would have to occur over at least four days, or at least on four di fferent occasions (and each occasion would have to be on a different day). Moreove r, as noted by the commenter, the use of the 3-year average will also mitigate this possibility. Finally, it is possible an

unusual natural event co uld be considered an excep tional event and not result in an area being designated as being in nonatta inment (see preamble to the final rule section VII.B).

(3) Comment: MSCC commented that:

In place of the 99th percentile , 3 year/1hour average proposed, consideration should reasonably be give n instead to a standard that is reasonably protective of the 400 ppb threshold above, more directly related to it, and which recognizes that not every measured exceedence of a threshold concentration represents the failure of a feasible control strategy. This could take the form of a standard th at, for example, identified 400 ppb as the excessive concentration for a 5-10 minute period in given hour, and required hourly compliance at the 97th, 98th or 99th percentile level for a given year, in conjunction with a requirement for reasonable data completeness in each calendar quarter. Replication should be required to eliminate flukes that could and do arise from measurement errors, random events such as fires, accidents, natural events, and other circumstances beyond reasonable human control, as well as predictable spikes arising from deliberate cultural events such as fireworks displays (see comments provided by MSCC; p. 6).

Response: EPA disagrees with the comm enter. As noted throughout the preamble to the final rule (e.g. see sec tion II.E.2.b) and this RTC document (e.g.,63 see section II.A), EPA considers the resp iratory effects associated with 5- 10

minute SO 2 exposures as low as 200 ppb to be adve rse to the health of asthmatics. Thus, we reject the idea that the form of the standard should be targeted to limit 5- minute peaks above the 400 ppb threshold. Mo reover, the form of the standard is the 3-year average of the 99 th percentile of the annual di stribution of 1-hour daily

maximum SO 2 concentrations. Thus, it is extr emely unlikely that areas would be designated as nonattainment due to cultural events such as fireworks displays.

(4) Comment: A number of health and envi ronmental groups supported a 99 th
percentile form, but expressed that they w ould prefer a more restrictive form, such

as a no-exceedence based form. In a ddition, 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 occurr ence of 5-minute peaks of SO 2 above benchmark levels.

Response: These comments and EPA’s respon ses were presented in detail in section II.F.3.b of the preamble to the final rule

(5) Comment: ALA generally commented that EPA favors a percentile form because it provides more stability than a mo re restrictive exceedence based form. Moreover, ALA indicates that EPA’s concerns over stability have no basis in fact

and no relevance under the CAA.

Response: ALA is incorrect in their sugges tion that the primary reason EPA is setting a percentile based form is because of standard stability. The rationale for a percentile based form is discussed in deta il in section II.F.3.b and II.F.3.c of the preamble to the final rule. In brief, this decision was based on multiple considerations including sele cting a form of a new 1-hour standard that reflected

that health evidence in the ISA indica ting that the percentage of asthmatics affected and the severity of the re sponse increases with increasing SO 2 concentrations. Thus, a percentile form is appropriate in order to give due weight to years when 1-hour SO 2 concentrations are well above the level of the standard, than to years when 1-hour SO 2 concentrations are just above the level of the standard. Moreover, we note that a c oncentration 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).

(6) Comment: The CBD commented that:

As a logical matter, it seems odd thatEPA would reject an exceedancebased form due to its tendency to give less weight to years when

concentrations are well above the st andard, but would propose a three-year averaging period that could do the same thing, i.e., give diminished weight64 to years with high concentrations (see comments provided by the CBD p. 6; note: footnotes have been omitted).

CBD also commented that:

EPA does not explain its th ree-year averaging proposal except to state that it “increases the stability of the st andard.” Under the Clean Air Act, however, a NAAQS must be requisite to protect public health with an adequate margin of safety, regardless of cost and implementation issues. 42 U.S.C. § 7409(b)(1); see also Whitm an, 531 U.S. at 464-71. Therefore,

if EPA in fact believes that a stable regulatory standard is requisite to protect public health, EPA should expl ain in its responses to comments why this is so, and further should explain why a three-year averaging period would be required to produce a stable regulatory standard (see comments provided by the CBD p. 7; note: footnotes have been omitted).

Response: The commenter is correct that in part, EPA selected a concentration based form in order to give due weight to years when 1-hour SO 2 concentrations

are well above the level of the sta ndard than to years when 1-hour SO 2 concentrations are just above the level of the standard. However, EPA also

considered that a concentration based form, averaged over three years, would likely be appreciably more stable than an exceedence based form. Selecting a form that is relatively stable is import ant to prevent locations from frequently shifting in and out of attainment ther eby reducing public h ealth protection by disrupting an area’s ongoing implemen tation plans and associated control programs. Thus, EPA notes that this de cision is based on more stable standards providing increased public he alth protection and not on implementation or cost issues. See American Trucking Assn’s v. EPA, 283 F. 3d at 374-75.

(7) Comment: Some groups preferred the 4 th highest form to the 99 th percentile. For example, AirQuality stated that if a 99 th percentile or 4th highest form is chosen

over a 98 th percentile or 7-8 th highest form, than a 4 th highest form is preferable. AirQuality states: The “4th” highest daily maximum concen tration is preferable to the “99 th ” form in that it can provide an una mbiguous determination of a “design value” in the face of missing data if concentrations are high. The 99 th
percentile form does not provide that certainty if data are missing (see comments provided by AirQuality, p.2).

Response: Appendix T Section 3.c describes two ways that a design value can still be considered valid even though it is incomplete. Section 3.c.i explains that even if the days are incomplete but th e quarters are complete then the design

value that is above the standard is va lid. Also Section 3.c.ii provides for two diagnostic data substitution tests for use w ith data sets that do not meet the 75% requirement. These tests are complicated and may not be perceived as transparent to all the public, but EPA believes that this complexity is necessary and65 appropriate given the usefulness of the test s in allowing at least some areas to be clearly determined to be meeting or not meeting the NAAQS despite the data incompleteness.

In addition, EPA notes that the 99 th percentile was favored over the 4 th highest form since it results in a sampling from th e same part of the annual distribution of 1-hour daily maximum SO 2 concentrations regardless of the number of 1-hour daily maximum concentrations reported in a given year for a particular location.



## D. Level

(1) Comment: Many industry groups (e.g., NAM , Golder, AFPA, AA, Marathon Petroleum) indicated that other enviro nmental/health agencies (e.g., California Air Resource Board, World Health Orga nization (WHO)) have used the same

health information from controlled huma n exposure and epidemiologic studies to

set a 1-hour SO 2 standard at a level higher than the proposed range of 50 - 100 ppb. Thus, these groups generally contend that EPA is interpreting the health information too stringently.

Response: EPA disagrees with these comments. The California Air Resource Board adopted its 1-hour standard in 1984 a nd retained this standard in 1995 (see http://www.arb.ca.gov/research/aaqs/caaq s/so2-1/so2-1.htm), while the WHO guidelines were last updated in 2005 (see http://www.euro.who.int/Document/E90038.pdf ). Thus, especially with respect to the California 1-hour standard, it is highly unlikely that all the health information

available in the current SO 2 NAAQS review was available in these previous

reviews.

We also note that there are significant di fferences in the 1-hour standard being finalized by EPA and the existing W HO 10-minute recommendation. For example, the WHO recommendation is a ten minute recommendation equal to approximately 190 ppb (see http://www.euro.who.int/Document/E90038.pdf ). Because of the difference in averaging tim es, in order for EPA to protect against

similar 5-10 minute SO 2 concentrations of concern w ith a 1-hour standard, that necessarily means that a 1- hour standard would have to be lower than the 190 ppb WHO guidelines. The WHO also has a 24-hour SO 2 recommendation while EPA is revoking its 24-hour standa rd and using a 1-hour sta ndard to protect against health effects associated with averagi ng times from 5-minutes to 24-hours.

In addition, we note that the form of the standard EPA is finalizing differs from

that of the California 1- hour standard. While th e 1-hour standard EPA is finalizing is the 99 th percentile of the distribut ion of 1-hour daily maximum SO 2 concentrations averaged over 3 years, the Ca lifornia standard is more restrictive. That is, the form of the Ca lifornia standard indicates th at the level is not to be exceeded (see http://www.arb. ca.gov/research/aaqs/aaqs 2.pdf). Moreover, in66 addition to a 1-hour standard, California al so has a 24-hour standard at a level of about 40 ppb. Thus, comparing only the levels of different short-term standards is often misleading.

Finally, we note that the CAA entrusts th e EPA Administrator to set standards for criteria pollutants that are in her judgment, adequate to protect public health with an adequate margin of safety, and that these standards are to be based on air quality criteria, not on standards established by othe r entities.

(2) Comment: With regard to the controlled human exposure evidence, health and environmental groups generall y concluded that a 1-hour SO 2 standard no higher than 50 ppb is needed to protect against 5-minute SO 2 benchmark exposures as low as 100 ppb identified from mouthpiece exposure studies, rather than the 200 ppb 5-minute SO 2 benchmark identified from “fr ee breathing” controlled human exposure studies. More specifically, in their combined comments, ALA et. al.,

stated: In its analysis of data from chambe r studies in the ISA and in the REA,

EPA focuses on studies of free breat hing exposure. In doing so, EPA improperly and arbitrarily downplays important evidence that reported increased airway resistance, a measur e of bronchoconstriction, in subjects with mild asthma at concentrations of 100 ppb. Regrettably, EPA does not rely on the mouthpiece studies in form ulating its proposed standards (see comments provided by ALA et. al ., p. 15)...In downplaying the mouthpiece studies, EPA ignores the larg e segment of people who rely on oral or oronasal breathing some or all of the time (see comments provided by ALA et. al., p.16)

Response: These comments and EPA’s respon ses were presented in detail in section II.F.4.b of the preamble to the fi nal rule. Similar comments and EPA responses were also discu ssed above in section II.A.

(3) Comment : Health and environmental groups (e.g., ALA, ATS, EDF, NRDC, Sierra Club, CBD) and the Alexandria Department of Transportation and Environmental Services generally conc luded 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 signif icant, relationships between ambient

SO 2 concentrations and hospital admissi ons where 99th percentile 1-hour concentrations ranged from 50 - 460 ppb. Of these studies, two showed positive and sometimes statistically significant relationships in singlepollutant models at 50 ppb, and th ree 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 SO 2 effects are generally independent” of those of co-67 pollutants like particulate matter. Gi ving 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 th e lowest concentration at which positive, adverse relationships have been demonstrated: 50 ppb (note that footnotes were omitted; see comments provided by CBD p.5).

Response: These comments and EPA’s respon ses were presented in detail in section II.F.4.b of the preamble to the final rule. We also again emphasize

CASAC’s view that a 50 ppb 1-hour standard (99 th percentile form) is by no means compelled, that no level lower than 50 should be considered, and that EPA could legitimately consider standards with higher levels. Samet (2009) at p. 16.

EPA’s final determination as to level is consistent with this advice

(4) Comment: A number of industry groups di d not support setting a new 1-hour

SO 2 standard. However, several of these groups (e.g., UARG, API, ASARCO)

and the SC Chamber of Commerce concl uded that, if EPA does choose to set a new 1-hour standard, the level of that standard should be ≥ 150 ppb. In addition,

South Dakota and Ohio recommended standa rd levels at 150 ppb. As a basis for this recommendation, these groups genera lly emphasized uncertainties in the scientific evidence. As noted above, th ese commenters typica lly concluded that the available epidemiologic studies do not support the conclusion that SO 2 causes the reported health effects. This was ba sed on their assertion that the presence of co-pollutants in the ambient air precl udes the identification of a specific SO 2 contribution to reported effects.

Response: These comments and EPA’s respon ses were presented in detail in section II.F.4.b of the preamble to the final rule.

(5) Comment: Industry groups (e.g., API, ABR, ASARCO) had several comments with respect to the epidemiologi c study conducted by the NYDOH (NYDOH, 2006). First, these groups generally concl uded that the results of this study are mixed. That is, while SO 2 effect estimates were positive and statistically significant even in multipollutant models with PM 2.5 or NO 2 in the Bronx, SO 2 effect estimates were actually ne gative in Manhattan in both single and multipollutant models. These groups also contend that this report was not peerreviewed and that the aut hors 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 th ese reasons, industry groups generally concluded that this study should not be relied upon by the Administrator in considering the appropriate leve l of anew 1-hour standard.

Response: These comments and EPA’s respon ses were presented in detail in section II.F.4.b of the preamble to the final rule. Other comments and EPA responses with respect to this study are presented below.68 (6) Comment: Some groups (e.g., ASARCO, ABR) also commented that the NYDOH (NYDOH, 2006) results are uncer tain because it was noted by the

authors that the SO 2 concentrations used to estimate relative risk associated with

SO 2 exposure were arbitrary (see ASARCO comments, p.6). Others commented (e.g., ACSBPP) that the lack of association in Manhattan is not consistent with a

causal SO 2 impact. Furthermore, others commen ted that the discrepant results in

the Bronx and Manhattan could be e xplained by residual confounding by other

factors such as correlatio n with other pollutants or smoking prevalence, which was not controlled for in the analysis.

Response: These commenters misconstrued or misunderstood the text when they

claimed the SO 2 concentrations used to estimate the relative risk associated with

SO 2 exposure were arbitrary. On Pa ge 21 of the NYDOH study (2006), the authors state, “It should be noted that the choice of c oncentration increments used to compute the RRs is an arbitrary one.” The commenters failed to include the

word “increments” in their comments. Wh at the study authors are referring to is that they present their results per some incremental change in exposure

concentration. The ISA standardizes all of the studies so that effect estimates describing 24-hour average SO 2 concentrations all have an incremental change of 10 ppb and effect estimates describing 1-hour maximum SO 2 concentrations all have an incremental change of 40 ppb. It is also common in the peer-reviewed literature to see investigat ors use the mean concentra tion, the interquartile range of concentrations, or the standard deviat ion as the incremental change. In this case, the authors use the mean SO 2 concentration averaged over the two study sites (11 ppb; See Table 3, page 50 of NYDOH, 2006). What’s more, they present a comparison of relative risks co mputed using altern ative concentration increments in Table 4b (page 52 of NYD OH, 2006). No where do the authors

claim that the SO 2 concentrations themse lves were arbitrary.

With respect to the lack of association in Manhattan, EPA notes that there were 29,987 asthma visits in the Bronx during the study period, and only 5,014 asthma visits in Manhattan during this same period (See Table 1, page 38 of NYDOH, 2006). The much smaller sample size in Manhattan makes it more difficult to detect a statistically significant associa tion when compared to the sample size available in the Bronx. Additionally, the demographics of the population in the Bronx and Manhattan differ by age, race a nd socioeconomic status, all of which may be susceptibility factors that would explain why an association was observed

in the Bronx and not Manhattan.

In addition, we note that the authors ac knowledge that "Varia tion in effects of unmeasured co-pollutants, such as indoor allergens, environmental tobacco smoke

or local traffic and industrial emissi ons, might also influence the apparent differences in acute asthma ED responses to ambient air pollution observed in the two communities. Increased exposure to such local measured pollutants could directly increase baseline asthma morbidit y and might also indirectly increase the response to changes in ambient air pollu tants by increasing airway inflammation69 and hyper-responsiveness to acu te airway irritants. Data were not available to address these possible effects in this report." In fact , by the nature of the study design, these potential confounde rs are not able to be measured on an individual level in any time-series studies of ED vi sits or hospital admissions. Thus this limitation applies to all time-series studi es and is not specific to NYDOH, 2006. In light of this limitation, th e authors still conclude that "The results suggest that the criteria pollutants PM 2.5 , SO 2 , O 3 and NO 2 had a statistically detectable impact on acute asthma ED visits in a community with a relatively hi gh baseline rate of acute asthma exacerbations. In two-po llutant and three-pollutant regression models, O 3 and SO 2 , and to a lesser extent maximum one-hour PM 2.5 , were the most robust pollutants. In other words, these pollutants exhi bited less change in their effect estimates as additional pollutants were added to the models."

Additional discussion with respect to the apparent discrepancy in results in the Bronx and Manhattan study areas can be f ound in section II.F.4.b of the preamble

to the final rule.

(7) Comment: Some groups (e.g., ASARCO, ABR) al so commented with respect to the NYDOH (NYDOH, 2006) analysis that th e Administrator failed to note that the positive association in the Bronx was observed at biologica lly implausible lag times of 3 or 4 days but not at 0-2 days.

Response: The commenters are incorrect in sta ting that the associations observed in the Bronx were observed at "biologically implausible lag times of 3 or 4 days, but not at 0-2 days". In Table 4a (page 51, NYDOH, 2006) the authors present the relative risk for the "5-day mean" which is the average of days 0, 1, 2, 3, and 4. As noted in the response to Commen t 12 in section II.B, EPA relies heavily on the effect estimates for the average of multiple days. Perhaps the commenters were referring to Figure 8 (page 71, NYDOH, 2006) which presents the relative risk for each of the single day lags from 0 to 4. Here, there are elevated relative risks on lag day 0, 2, 3, and 4 with the one s on days 2, 3, and 4 appearing to be statistically significant. Single lag day 2 is statistic ally significant and has the relative risk of greatest magn itude. This response is not biologically implausible.

(8) Comment: With respect to Ito et al., (2007), industry groups generally

commented that since the SO 2 effect estimate did not remain statistically significant in multipollutant models with NO 2 , this study does not indicate an independent effect of SO 2 on emergency department visits in the NYC study area. API specifically commented:

The RR for an increase of 6 ppb SO 2 was statistically significant (1.20; 95% CI: 1.13, 1.28) and remained so when PM 2.5 , O 3 , or CO was included in the model, but became nonsignificant when NO 2 was included in the model (RR not provided, 95% CI: 0.9, 1.1). Because associations with SO 2 could be attributable to NO 2 , this study cannot be used to assess the effects70 of SO 2 on health effects with small incr emental increases in exposure (see comments provided by API; p. B-4).

Response: These comments and EPA’s respon ses were presented in detail in section II.F.4.b of the preamble to the final rule

(9) Comment: With respect to Ito et al., (2007) , ABR commented that the authors

state: “

SO 2 and CO showed lower monitor to monitor correlation and low withincity precision of the mean levels, indicating that these pollutants’ risk

estimates could be biased in short term health effects models (see comments by ABR; p.5)

ABR also notes that the authors concl uded the study with a precaution against using multipollutant models in health analyses.

Response: EPA agrees with Ito et al. (2007) that the lower monitor correlations and within-city precision c ould bias short-term health effects models. But as addressed in the response to other co mments (see section II.B), EPA maintains that measurement errors in community time-series and panel epidemiologic studies are more likely to underestimate the strength and the significance of any

association between SO 2 and adverse health effects, thereby decreasing the likelihood of an association reaching statistical signi ficance and likewise decreasing the possibility of a false iden tification of an association (Section 2.6.4.4, p. 2-63). Thus, the measurement erro r makes it more difficult to detect a positive and statistically significant effect estimate, even when such an association exists. Moreover, the signa l that drives statistical associations

between ambient concentrations and health effects in time-series studies is the day-to-day changes in concen tration, not the absolute da ily values. Appropriately

located central SO 2 monitors can adequately ch aracterize such day-to-day changes. Exposure studies have found st atistically significant regression slopes between personal exposure to SO 2 and ambient SO 2 concentrations at fixed-site monitors (Section 2.6.3.2, p. 2-56), indicat ing that fluctuations in ambient concentration are an important driver of fluctuations in SO 2 exposure.

EPA agrees that multipollutant models should be used and interpreted cautiously in health analyses. Currently, these mu ltipollutant models are the most common tools used by investigators to try to dise ntangle the effects of individual criteria pollutants. When examined across the body of evidence, EPA believes that the results of the multipollutant models can be informative about the robustness of associations observed in epidemiologic studies.

(10) Comment: With respect to Schwartz (1995), industry groups generally

commented that the results of this st udy are mixed, and therefore should not be71 considered by the Administrator. More specifically, these commenters noted that although the results in New Haven remain ed statistically significant in the presence of PM 10 , the SO 2 effect estimate in Tacoma was reduced and no longer statistically significant in the presence of PM 10 . Commenters also noted that in both cities, the SO 2 effect estimate was reduced and no longer statistically significant in the presence of O 3

.

Response: These comments and EPA’s respon ses were presented in detail in section II.F.4.b of the preamble to the final rule

(11) Comment: Some commenters i ndicated that because EPA did not find the epidemiologic evidence adequate enough to support a quantitative risk assessment based on epidemiology, then these studies should not be relied upon for informing

the level of a standard.

Response: EPA disagrees with this comment. Using epidemiologic studies for the purpose of a quantitative risk assessmen t is a separate issue from using air quality information from locations in which epidemiologic studies were conducted to help inform the level of a new 1-hour standard. EPA judged that a quantitative risk assessment using concentration-response functions derived from any one epidemiologic study would not produ ce quantitative results that would be particularly useful in the decision ma king process. However, although no one epidemiologic study was appropriate for us e in a quantitative risk assessment, EPA concluded that looking at SO 2 air quality in the ten locations U.S. epidemiologic studies reported positiv e associations between ambient SO 2 and emergency department visits or hospitaliz ations was informative with respect to setting the level of a new 1-hour standard.

(12) Comment: Dow commented that “if EPA opts to set a 1-hour NAAQS for SO 2 , the expected difference between "monitor locations of expected maximum shortterm concentration" and area-wide concentrations should be considered in establishing the standard (see comments provided by Dow; p.2).

Response: EPA set the level of a new 99 th percentile 1-hour daily maximum SO 2 standard at 75 ppb. In doing so, EPA c onsidered that the monitors in three locations where epidemiologic studies (i.e., NYDOH (2006), Ito et al., (2007), Schwartz (1995)) reported statistically si gnificant associations in multipollutant models with PM may not ha ve recorded the highest 99 th percentile 1-hour daily

maximum SO 2 concentration across a given stud y area. This consideration is

described in section II.F.4.c of th e preamble to the final rule.

(13) Comment: In general, EPRI commented that using the epidemiology to set a 1- hour standard is problematic because few of these studies considered a 1-hour averaging time72 Response: EPA acknowledges that many of the ep idemiologic studies considered in the ISA and REA used longer averag ing times than 1-hour (e.g., 24-hour, 3- hour). However, we disagree with that using 1-hour air quality information from the areas these epidemiologic studies were conducted to inform a new 1-hour standard is problematic. EP A notes that just because a particular epidemiologic study used a longer averaging time, it does not rule out the po ssibility that the effects reported in those studies may be associated with shorter-term SO 2 concentrations. In fact, when describing epidemiologic studies observing positive

associations between ambient SO 2 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). EPA notes that

this assertion is based on both the large body of controlled human exposure studies reporting respiratory effects following 5 - 10 minute SO 2 exposures, as well as a body of epidemiologic studies reporting positive associations between <

24-hour ambient SO 2 concentrations and respir atory morbidity endpoints.

(14) Comment: In addition to generally concludi ng that the epidemiology is too

uncertain to demonstrate that SO 2 has an independent effect on the respiratory effects reported in those studies, many industry groups (e.g., API, ACC, PE, EEI, CIBO, CRA, Pepper Hamilton, RMA, Louisiana Chemical, A&B, TFI) also generally commented that adverse hea lth effects do not occur following 5-10

minute SO 2 exposures < 400 ppb in controlled human exposure studies (an issue

also discussed above in final rule sec tion II.E.2). Thus, these groups generally

maintained that the level of a 1-hour st andard should not take into account limiting 5-minute peaks ≤ 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.

Response: These comments and EPA’s respon ses were presented in detail in section II.F.4.b of the preamble to the final rule.

(15) Comment: Both industry (e.g., Rio Tinto) and environmental/health groups (e.g., ALA et al.,) referenced the 5-minute peak to 1-hour mean ratios in REA Table 10- 1 as a rationale for selecting a specific level for a 1-hour SO 2 standard to adequately protect against 5-minute SO 2 concentrations of concern (notably, industry was generally opposed to a new 1- hour standard, but they still suggested

standard levels in the event the Administ rator chose to adopt such a standard).

Response: EPA did not use the 5-minute peak to 1-hour mean ratios from REA

Table 10-1 in its consideration for the level of a new 1-hour SO 2 standard. Rather, EPA used these ratios as an in itial consideration for an appropriate averaging time for a new short-term SO 2 standard. As noted in the proposal, this information suggested that a 1-hour averag ing time was likely to be more efficient at limiting 5-minute concentrations of con cern than a 24-hour averaging time. As stated in footnote 19 of the preamble to the proposal:73 The analysis of peak to mean ratios was used as an initial screen to evaluate which averaging times could be suited to control 5-minute peaks

of SO 2 . The more sophisticated analysis for ultimately determining that a one-hour averaging time set at an appr opriate level could effectively limit these 5-minute peaks was the air qual ity, exposure, and risk analyses discussed in section II.F.4. (74 FR 64831)

That is, EPA reasonably based its decisi on on the appropriate level for limiting 5-

minute concentrations of concern iden tified from controlled human exposure studies on the sophisticated air quality, exposure, and ri sk analyses presented in detail in chapters 7 – 9 of the REA and the summary of the estimates from those analyses presented in sections II.F.4.b a nd II.F.4.c of the preamble to the proposal (see especially Tables 2 - 4). However, as previously noted EPA did not place the same weight on all of these analyses. For example, EPA placed relatively more weight on the St. Louis exposure analysis than it did on the St. Louis quantitative risk assessment (see final ru le preamble section II.E.2.c).

We also note that the ratios presented in Table 10-1 are from a small number of

monitors in areas volunt arily reporting 5-minute SO 2 concentrations from the years 2004 -2006. Moreover, not all the monitors in Table 10-1 reported 5- minute concentrations during the entire th ree year period (see REA, Table 10-1). Thus, while EPA finds this data was reasona ble to use as an initial screen for an appropriate averaging time, we do not think it is a sufficient basis for selecting the level of a new 1-hour standard to prot ect against 5-minute concentrations of concern. Again, EPA relied on the more s ophisticated air quality, exposure, and risk analyses as they indicate the exte nt to which a given level of a 1-hour

standard limited 5-minute concentrations of concern.



## E. Retaining or revoking the current SO 2 

## NAAQS

(1) Comment: Public health (e.g., ALA, ATS) and environmental organizations (e.g., CBD, WEACT) were generally opposed to revoking the cu rrent 24-hour and annual standards. These groups generall y concluded that the 24-hour standard

should be revised while the annual standard s hould be retained. In support of this position, ALA et al., cited air quality inform ation from the REA indicating that if air quality was simulated to just meet a 99 th percentile 1-hour daily maximum standard in the proposed range of 50 – 100 ppb, then in some locations analyzed,

99 th percentile 24-hour average SO 2 concentrations would be above concentrations (i.e., above 99 th percentile 24-hour aver age concentrations) in cities where U.S. emergency departme nt visit and hospital admission studies reported positive associations with SO 2 . In addition, many of these groups were opposed to revoking the current annual st andard. In general, these groups concluded that given the uncer tainties associated with SO 2 exposure and longterm health effects, EPA should err on the side of being health protective and retain the existing annual standard.74 Response: These comments and EPA responses are discussed in detail in final

rule section II.F.5.b.

(2) Comment: NAACA, NESCAUM, and Vermont, while supportive of revoking the existing standards, also suggested EP A explore setting a new 24-hour standard to minimize the potential that multiple hours within a day would exceed a 1-hour

standard

Response: EPA generally agrees with the co mmenters that given the form of the standard selected, a 99 th percentile daily maximum, th ere is potential for multiple hours within a day that would exceed the 1- hour level selected for that standard. However, given typical patterns in air quality data, the frequency of days having multiple hours above a given 1-hour level is likely rare. We also again emphasize

CASAC’s conclusion that “that a one-hour standard is the preferred averaging time.” Samet (2009) at p. 15.

(3) Comment: The CBD commented that:

In proposing to revoke the 24-hour sta ndard, EPA appears to assume that health effects found in epidemiologi c studies based on 24-hour exposures are driven entirely or primarily by very short-term (five- to ten-minute) peak exposures. This assumption, however, is not borne out by the evidence (see comments provided by the CBD p. 7).

Response: EPA provided a response to similar comments in final rule preamble

section II.F.5.b. We further note that EPA’ s rationale does not assume that all 24- hour associations in epidemiologic studi es are primarily being driven by 5- 10 minute peak SO 2 concentrations. Rather, it reason ably considers that the overall body of health evidence suggests that respiratory effects following SO 2 exposure may be most related to averaging times ≤ 1-hour (see preamble section II.F.5.b).

(4) Comment: The CBD commented that EPA shoul d better explain why it proposed

to revoke the current annual standard. The CBD also noted that:

CASAC points out that EP A has not focused on causality in the same way in reviewing the health effects of other pollutants. EPA’s proposal also may run afoul of a long line of cas es holding that a NAAQS must not only protect against known hazards, but also must protect against uncertain and incompletely understood hazards. See Am. Trucking Ass’ns, 283 F.3d at 369; Am. Lung Ass’n, 134 F.3d at 389; Lead Indus. Ass’n, 647 F.2d at 1152-55. (see comments provided by the CBD p. 8; note: footnotes have been omitted)

Response: EPA provided its rationale for revoki ng the annual standard in final rule preamble section II.F.5.b. We note fu rther, that although the commenter is75 correct that the adequate margin of safety standard affords authority to EPA to act to protect against uncerta in or incompletely understood hazards, EPA is not compelled to act when there is evidence th at no hazard exists. The ISA finds that there is inadequate evidence to infer a causal relationship between morbidity and mortality from long-term SO 2 exposure. ISA p. 5-6; see also ISA p. 5-13 explaining the lack of evidence of causality between long-term SO 2 exposure and particular health end points. See Am erican Farm Bureau Federation v. EPA, 559 F. 3d 512, 538-39 (D.C. Cir. 2009) (“The EP A reasonably decided that an annual coarse particle standard is not necessary because, as the Criteria document and the Staff paper make clear, the latest scientif ic data do not indica te that long-term exposure to coarse particle s poses a health risk”).

With respect to CASAC, the CBD provided the following information in a

footnote:

See Samet 2009 at 9 (“Some discussion is needed that indicates why the risk assessment for this pollutant, in c ontrast to others, is limited only to

health effects that are classified as sufficient to infer causality.”).

EPA notes that we placed primary empha sis on those health endpoints for which the ISA concluded the strongest evidence. More specifically, the preamble to the final rule states (see in troduction to section II):

In reaching this decision, EPA has dr awn upon an integrative synthesis of the entire body of evidence on human health effects associated with the presence of SO 2 in the ambient air, and upon the results of quantitative exposure and risk assessments reflec ting this evidence. As discussed below, this body of evidence addresse s a broad range of health endpoints associated with exposure to SO 2 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 SO 2 (see section II.B below). Thus, the rationale for this

final decision on the SO 2 NAAQS focused primarily on respiratory morbidity following short-term (5-mi nutes to 24-hours) exposure to SO 2 , for which the ISA found a causal relationship.

(5) Comment: Amanda L. Matthews urged EPA to retain the annual standard in order to protect against long-term SO 2 effects on prenatal/neonatal health outcomes. She contends that EPA ignored several key long-term SO 2 exposure studies with regard to these health outcomes. In particular: Hoppenbrouwers (1981) with regard to Sudden Infant D eath Syndrome (SIDS); Liu et al., (2003) with regard to increases in pre-term de livery and low birth weight; and Dales et al., (2004) with regard to SIDS

Response: As noted by the commenter, EPA found the association between

ambient SO 2 and long-term health effects to be inadequate to infer the presence or76 absence of a causal relationship. That is, the ISA found the long-term health evidence to be of insufficient quantity, qua lity, consistency, or statistical power to

make a determination as to whether SO 2 was truly associated with these health outcomes (ISA, Table 1-2). Notably, this included an evaluation of the possible associations between long-term (weeks to year) exposure to SO 2 and adverse prenatal and neonatal outcomes. EPA furt her notes that the general scientific

conclusions of the ISA were reviewed and accepted by CASAC. With regard specifically to the studies mentioned above , EPA did consider Liu et al., (2003) as well as Dales et al., (2004), but still f ound the overall breadth of the information relating long-term exposure to SO 2 and adverse prenatal and neonatal outcomes inadequate to infer the presence or abse nce of a causal relati onship. With respect to the study by Hoppenbrouwers et al. (1981) , this study was not included in the final ISA because the focus of the document was on recent scientific studies, and re-evaluation of only some key older st udies (primarily key older controlled human exposure studies). Additionally, the lag time used in the Hoppenbrouwers et al. (1981) study was 7 weeks, whic h is not consistent with any known mechanism by which SO 2 may cause adverse health ou tcomes. That being said, EPA can confidently state that inclusion of this study in the ISA would not have changed the final conclusion with respect to prenatal and neonatal outcomes.

(6) Comment: Amanda L. Matthews urged EPA to retain the annual standard generally commenting that a 1-hour standa rd in the proposed range will likely not prevent annual SO 2 concentrations from exceeding the level of the current annual

standard.

Response: EPA disagrees with this comment . Table 10-4 of the REA gives a

clear indication that 99 th percentile 1-hour daily maxi mum standards in the range of 50 -100 ppb (and indeed up to 150 ppb) will likely prevent annual SO 2 concentrations from exceeding the level of the current annual standard (i.e. approximately 30 ppb). In fact, Tabl e 10-4 estimates that given a 100 ppb standard, the highest annual SO 2 concentration in any c ounty analyzed in this table would only be 15.5 ppb. Thus, a standard level of 75 ppb would be expected to keep annual SO 2 concentrations even lower. EPA notes that REA Appendix Tables D-5 and D-6 show similar information.

(7) Comment: Health and environmental groups urged EPA to maintain or lower the existing 24-hour standard. As evidence th at both a standard designed to limit 5- 10 minute peaks of SO 2 and a standard to limit 24-hour average concentrations of

SO 2 is needed, some of these groups cite SO 2 WHO guidelines and standards in

SO 2 standards in California. For example, ALA states:

...both World Health Organization and the state of California recommend or maintain both a short-term ( hourly or 10-minutes) and a 24-hour standard. The World Health Orga nization has recommended both a 10

minute standard and a 24-hour standard. Their 24- hour standard

recommendation is for a standard of 7 ppb, far more stringent than the77 U.S. standard, and their recommendation for a 10-minute standard is 175 ppb. The California Air Resources Bo ard has established a 24-hour standard of 40 ppb, not to be exceeded. Th is standard is in addition to the

state‘s one-hour standard for SO 2 (note that footnotes have been omitted; see comments provided by ALA et. al., p. 9).

Response: As noted above, the CAA entrus ts the EPA Administrator to set standards for criteria pollutants that ar e in her judgment, adequate to protect public health with an adequate margin of safety, and that these standards are to be based on air quality criteria, not on standards establis hed by other entities.

That being said, EPA notes that a 99 th percentile 1-hour da ily maximum standard set at a level of 75 ppb is expected to limit 5-minute to 24-hour SO 2 concentrations associated with respirator y morbidity effects in both epidemiologic and controlled human exposure studies. See sections II.F.2.b, II.F.2.c, II.F.4.b and II.F.4.c of the final rule preamble for more detail.78 

## V. Technical Issues with Monitoring Requirements

(1) Comment : LEC expressed concern that EP A’s proposed two-pronged monitoring network based on the Population Weighted Emissions Index (PWEI) metric and a

state’s contribution to the national SO 2 emissions inventory w ould likely result in monitors near major sources where SO 2 “hot spots” are likely and fail to provide

sufficient monitors in non-urban and ru ral areas to appropriately characterize background concentrations for permitting purposes. LEC suggests that EPA encourage states to site area-wide mon itors to establish ba ckground concentration for use in permitting and to track air quality trends. When a state does not have an appropriate background monitor, LEC included suggestion on using sourceoriented monitoring data to estimate background.

Response: As explained in sections III and IV of the preamble to the final rule, EPA intends to use a hybrid approach that combines monitoring and modeling, using each of these analytic tools where th ey are most appropriate and effective. As a result, EPA has revised the scope of the monitoring network is revised so that it is no longer required to be sour ce-oriented in nature (i.e., “hot-spot” monitoring only). Instead the final monitoring network design now has the flexibility to address multiple m onitoring objectives including source characterization, highest concentratio n, population exposure, general background, or regional transport. As a result of this flexibility, the commenter’s concern should be alleviated because, as part of the required monitoring network, states may site monitors to address the need for measuring background concentrations.

(2) Comment : RRI recommended EPA change its monitoring guidance in order to

“allow the use of emission distributions in a probabilistic implementation of compliance modeling for short-term proba bilistic standards such as the 1-hour

SO 2 NAAQS.”

Response: Under a hybrid analytical approa ch, EPA anticipates using both ambient monitoring and the modeling of SO 2 sources in the implementation of the revised NAAQS. EPA anticipates that dispersion modeling will be used to assess impacts of large and medium sized sources. Therefore, as indicated in the final rule, EPA intends to develop guidance for the use of refined air quality dispersion modeling in implementing the new SO 2 1-hour NAAQS and the issue of how to appropriately characterize emissions will be addressed in that guidance, as

needed.

(3) Comment : NACAA provided comments urging EPA to provide greater flexibility in the siting requirements for new SO 2 monitors in order to avoid requiring the

installation of redundant SO 2 monitors and allow for states to use a broader range

of criteria to determine the best locations for SO 2 monitors. NACAA also included suggestions on when duplicative monito rs could be removed. NYSDEC also noted that the two prong approach co uld result in duplicative monitoring.79 Response: Under a hybrid analytical approac h, introduced in section III of the preamble to the final rule, the monitoring network is no longer required to be source-oriented in nature. As further expl ained in section IV.B of the preamble, the required monitoring network has the flexibility to have monitoring sites characterizing one or more of the follo wing objectives: source characterization, highest concentration, population exposu re, general background, or regional transport (discussed in s ection IV.B.3 of the preamb le). EPA believes this flexibility alleviates much of the con cern raised over redundant or unnecessary monitoring and, therefore, it is unnecessary to address the specific suggestions on reducing such duplicative monitoring.



### IV.A: Comments on Propos ed Monitoring Program

(1) Comment : NYSDEC, noting the significant shift in SO 2 emissions over the last three decades away from small and mobile emitters of SO 2 to large, stationary sources, expressed concern in its comments that the “proposed SO 2 monitoring network design does not adequately addre ss this source transition and will not ensure that the largest point sources are adequately characterized.” To address

this source transition effectivel y, NYSDEC recommended using a CBSA emissions threshold (50,000 tpy) and a point source emissions threshold (20, 000 tpy) instead of a PWEI or state’s contribution to SO 2 emissions approach for siting monitors. NESCAUM made similar threshold recommendations.

Response: Under a hybrid analytical approac h, introduced in section III of the preamble to the final rule, and further e xplained in section IV,V, and VI of the preamble, EPA anticipates using both am bient monitoring and the modeling of

SO 2 sources to assess compliance with the new 1-hour SO 2 NAAQS. We believe

that for a short-term 1-hour standard it is more technica lly appropriate, efficient, and effective to use modeling as the prin cipal 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. As a result, EPA anticipates

that SO 2 sources will be adequately charac terized by either, or both of, ambient monitoring and source modeling.

(2) Comment : Indiana urged EPA to emphasize the placement of SO 2 monitors in “areas that record higher va lues and away from those found to be below the level of the proposed NAAQS.”

Response: Under EPA’s anticipated use of a hybrid analytical approach, as

introduced in section III of the preamble to the final rule and the monitoring network design described in section IV, the final monitoring network design is more flexible than the proposed networ k design. The monitor objectives of the final network design are broadened in th e final rule to include assessment of population exposure, general b ackground concentrations, SO 2 transport, long-term trends, and source oriented and/or highest concentration data. This increased80 flexibility gives states the ability to select areas where they believe ambient monitoring is most warranted. As a resu lt, we believe that the final network design allows states to emphasize placement of SO 2 monitors in “areas that record higher values and away from those found to be below the level of the proposed NAAQS” as the commenter suggested.

(3) Comment : The City of Alexandria expresse d concern in its comments that, while an increased focus on monitoring SO 2 concentrations in high impact areas is a positive change by EPA, Alexandria’s own experience in air quality monitoring “shows that local monitoring should only be us ed as a backstop, in order to assist in compliance verification, and only after air quality model-based limitations are developed and implemented by the large stat ionary sources that have the potential to contravene NAAQS protec tions.” Under the proposed network design, while

new SO 2 monitors could be placed to identi fy areas of localized nonattainment caused by discrete stationary sources, th e commenter believes that sole reliance on these locally-sited monitors is mispl aced. The commenter contends this is because monitoring methods cannot fully and comprehensively identify all areas of non-compliance caused by a single sour ce’s maximum potential emissions, for all of the possible local meteorological conditions. Instead, the commenter suggests using modeling major sources to demonstrate compliance with the SO 2 NAAQS.

Response: Partly in response to these and other comments, and a re-examination of our historical approach to SO 2 NAAQS implementation, we now anticipate using a hybrid analytic appr oach combining the use of monitoring and available modeling to assess compliance with the new 1-hour SO 2 NAAQS. EPA discusses its revised intended approach in chapters III, IV, V, and VI of the preamble to the final rule, EPA believes that under a hybrid analytical approach, the commenter’s

concerns should be addressed.

(4) Comment : NACAA stated that “EPA and NACAA jointly developed a National Monitoring Strategy that provided a fram ework for operation and maintenance of the national air monitoring networks. The primary purpose of the National Monitoring Strategy was to gain a bette r understanding of the relationships and potential reaction in the atmosphere of various pollutants, develop a strategy to apply the lessons learned and use the in formation developed for a variety of purposes, not just for determining attainment/non-attainment.” NACAA

concluded that it is “...c oncerned that EPA appears to be changing the focus of the monitoring program as currently described in the National Monitoring Strategy from recording ambient measur ements to measuring emissions from sources, based on the more sour ce-oriented monitoring lead, NO 2 and SO 2 proposals.”

Response: Under a hybrid analytical approac h, introduced in section III of the preamble to the final rule, EPA anticipat es using both ambient monitoring and source modeling in implementing the SO 2 NAAQS (fully discussed in sections81 III, IV, V and VI of the preamble). As a result, the required monitoring network is no longer required to be source-oriented in nature, as explained in section IV.B of the preamble. The required monitori ng network has the flexibility to have monitoring sites serve multiple monitoring objectives. EPA believes that under this approach, there is no l onger cause for concern that SO 2 monitoring sites can not be used in the multi-pollutant paradi gm that the monitoring strategy promotes, and that the NCore network exemplifies. Further, NCore monitoring sites that measure SO2 are allowed to satisfy mi nimum monitoring requirements resulting from this rulemaking.

(5) Comment : Several commenters (ALA/EDF /NRDC/SC, CBD, and Missouri) urged EPA to revise its approach to determining how many SO 2 monitors are required in each state to require more monitors. These commenters asked that

more SO 2 monitors be required in order to more directly monitor air quality around the nation’s many large SO 2 emission sources. For example, ALA/EDF/NRDC/SC argues that “the proposed 348 monitors are grossly inadequate to detect peak concentrations from the 4,400 stationary sources, and an unknown number of area sources, which emit mo re than 50 tons of sulfur dioxide per year.”

Response: To address these and other comm ents, EPA re-examined its proposed monitoring-focused approach and its historical approach to SO2 NAAQS implementation. As a result, EPA intends to use a hybrid analytical approach, as

introduced in section III of the preamble to the final rule. Under this approach, EPA anticipates using both ambient monitoring and source modeling thus allowing more individual sources to be addressed, while maintaining a stable monitoring network, which can serve mu ltiple monitoring objectives. EPA believes a hybrid analytical approach as described in sections III, IV, V, and VI

addresses the concern rais ed by these commenters that the proposed monitoring network would be inadequate to detect peak concentrations from a large number of stationary sources.

(6) Comment : NESCAUM commented that it doe s “not support EPA’s proposed requirement that any PWEI- or CBSA-tri ggered monitors ‘sha ll not count toward satisfying any required mon itors resulting from the state emissions triggered requirements’ (74 FR 64880). Additional SO 2 monitors should onl y be required if the CBSA-triggered monitor count is le ss than the state emissions triggered

count.” NESCAUM also recommended ex empting states from the emissionstriggered monitoring requirement if they had SO 2 inventory of less than 0.1% of the national inventory.

Response: In this comment, NESCAUM refers to the second prong of the proposed monitoring network which requi red minimum number of monitors

based on the state’s contribution to the national SO 2 emissions inventory. In the final monitoring network design, explained in section IV.B.4 of the preamble to82 the final rule, EPA did not re tain this second prong to the network design. As a result, the concerns raised by this commenter should no longer exist.

(7) Comment : South Carolina submitted comments noting that the agency “believes the number of monitors, distributed as propos ed, is greater than needed to provide a network sufficient to provide report ing to the general public and support compliance with the standards in areas wh ere there is a reasonable potential to exceed the proposed standard...The number and allocation of monitors in the proposal using the PWEI appears to mi ss some high emission density areas and require monitoring in many locations unlik ely to experience significant impact.”

Response: The statement that “...the number of monitors, distributed as proposed, is greater than needed...” implies that the proposed network was viewed to be redundant and would have required unnecessa ry monitoring in certain areas. Under a hybrid analytical approach, in troduced in section III and further explained in section IV.B of the preamble to the final rule, the number of required monitors is lessened, the m onitoring network is no longe r required to be sourceoriented in nature, and the network object ives of the final ne twork design are now

broadened to include assessment of population exposure, general background concentrations, SO 2 transport, long-term trends , and source oriented and/or highest concentration data. As a result, the required monitoring network has the flexibility to have monitoring sites characte rizing one or more of these objectives. EPA believes this flexibility alleviates the concern raised over redundant or unnecessary monitoring. The commenter also suggested that the proposed allocation of monitors based on the use of the PWEI may “...miss some high emission density areas and require monitoring in many locations unlikely to experience significant impact.” As explained in the preamble, EPA believes where high emissions density occurs due to large sources, the modeling component of our anticipated hybrid approach would be more technically appropriate and effective in assessing ambient SO 2 concentrations and compliance with the SO 2 NAAQS. In addition, states always have the prerogative to monitor where th ey believe monitoring is warranted above minimum monitoring requirements. Further, as discussed in section IV.B.6 of the preamble to the final rule, EPA Regional Administrators (working with states) may require additional monitoring above the minimum requirements if they believe monitoring objectives remain unf ulfilled in an area even though a state may be satisfying minimum monitoring re quirements. The commenter also suggested that the proposed monitoring a pproach could lead to “...monitoring in many locations unlikely to experience sign ificant impact.” The final network design, discussed in section IV.B.4 in the preamble to the final rule, utilizes the PWEI, but with some changes in the br eakpoints used to require monitors, ultimately requiring lesser number of monito rs in the final network than by the proposed use of the PWEI. EPA believes th at the anticipated use of modeling, the states and RA’s authority to require m onitoring above the minimum requirements, and the changes to the PWEI breakpoint s addresses the commenters concern.83 (8) Comment : City of New York commented that New York City presently has three

SO 2 monitors located in its metropolitan ar ea and urges EPA to ensure that the

final SO 2 monitoring plan does not reduce the nu mber of required monitors in the New York City area.

Response: Under the final network design, di scussed in section IV.B.4 in the preamble to the final rule, and based on the 2005 NEI and 2008 Census data, EPA estimates that the New York City CBSA ha s a PWEI value that will require it to operate at least 3 monitors within the CBSA.



### IV.B: Comments on Monitoring Method

(1) Comment : Two industry groups provided co mments noting that, while EPA is proposing a significant change in the perm itted total interferent limit for the new FRM, the interferent limits need to be further tightened in order to reduce the potential for false positive exceedances of the SO 2 NAAQS. Given the stringent

SO 2 standards being proposed, AirQuali ty recommends “a more realistic specification would limit each interferen t to no more than 3 ppb and total interference to no more than 12 ppb.” UARG recommends EPA “further limit the

total of all interferents” without providing a specific number. NESCAUM also submitted a comment requesting EPA “tighten the NO interference metric for the

new SO 2 Federal Reference Method from 100 to 300 to 1. The proposed value of

100 results in substantial NO inte rference at sites with low SO 2 levels in urban areas, such as NCORE sites.”

Response: As discussed in secti on IV.A.2.b of the preamble to the final rule, EPA

reconsidered what is reasona bly feasible with current technology to determine the final interferent equivalent limit requirements. In evaluating the limits, we must keep in mind that the test concentrations specified for most of the interferents (in Table B-3) are substantially higher than the concentrations normally observed in

ambient air. A review of test data submitted for numerous SO 2 FEM applications

showed that it is not feasible to fu rther lower the limit requirement of each interferent. However, in respons e to the NESCAUM comment, EPA has concluded that that the limit requi rement for NO interferent for SO 2 analyzers can be lowered to ±3 ppb (167:1) for the lowe r measurement range to reduce possible

NO interference at sites in urban areas with low SO 2 levels. In reconsidering the requirement for the total of all interferents, we also reassessed the high probability that many of the test interferents would not coexist in ambient air at these test concentrations. Therefore, we concl uded that the limit requirement for total interference equivalent for SO 2 analyzers can be eliminated entirely.

(2) Comment : UARG requested that EPA provide additional information to prove that existing FEMs qualify as FRM and FEM under the proposed new definitions as EPA stated without provi ding supporting information.84 Response: As explained in secti on IV.A.1.b of the preamble to the final rule, there is no legal impediment to continued use of SO 2 FEMs without redesignation pursuant to the new automated SO 2 FRM. With respect to the technical issue raised by the commenter, these automate d continuous monitoring methods have been tested against the test and perf ormance requirements of 40 CFR Part 53, which are designed specifically to test such continuous methods. For decades, these FEMs have been gathering and reporting hourly averages (almost exclusively) for monitoring agencies a nd so are technically sound to provide monitoring data for determining compliance with the new one-hour NAAQS. EPA has clarified the proposed regulato ry text so that the rules state unambiguously that both FRMs apply to the new one-hour standard (as well as to the 24-hour and annual standard so long as they are retained), as do all presently designated FEMs.

(3) Comment : AirQuality requested EPA to reeval uate its decision to not change the span drift requirement at the 80% URL wh ich EPA left at +5.0%. “Ambient air

monitors in the 21 st century should be able to hold span drift to no more than +2.5% under the conditions specified in EPA testing and EPA should adjust the span drift specification (at 80% URL) accordingly.”

Response: In response to this comment, EPA has reevaluated the span drift limits. Based on information from FEM testing and manufacturers’ data (EPA, 2009c), EPA has concluded that the span drift re quirement at 80% URL can be lowered to ±3%, as stated in section IV.A.2.b of the preamble to the final rule.

(4) Comment : ExxonMobil expressed support fo r EPA “allowing monitoring agencies to choose mobile monitoring that meets monitoring quali ty requirements. This would, over time, expand geographic c overage and be responsive to changes in roadway emissions and community co-l ocation relationships, while facilitating optimal deployment of monitoring resources.”

Response: EPA appreciates the corroboration expressed in this comment and agrees with ExxonMobil that the revised pe rformance specifications of automated

SO 2 methods included in Table B-1 of 40 CFR Part 53 for noise, lower detection limit, interference equivalent, zero drift, span drift, lag time, rise time, fall time, and precision (discussed in section IV.A.2.a and A.2.c of the preamble of the final rule) will allow for monitoring that meets various quality requirements and will be responsive to changing needs.

(5) Comment : Missouri provides recommendations fo r several technical changes to EPA’s proposed monitoring method. These changes are:

o Page 64870 Appendix A-1, section 2.4, Sampling considerations – “While the use of a particulate filter can be a good practice it can also cause problems. Whether or not a sample line filter is re quired has normally been spelled out in the reference and equivalency descriptions; some require the filter and some don't. Th e wording here seems to make it a85 universal requirement. We suggest ch anging the wording or removing this section and referencing the List of Designated Reference and Equivalent from EPA's National Exposure Research Laboratory.”

o Page 64871 Appendix A-1, sections 4.1 .1 and 4.1 .2. – “These two

sections cause confusion. Since the goal is to control the flow outputs of the dilution system to within 2% of a NIST traceable standard, we propose combining the two sections and making this concept clearer. For example: ‘The air and standard gas combined dilution system shall measure and control both the air and standard gas flows. Each of these flows will be

calibrated to a NIST traceable standard to deliver flows accurate to within

+/- 2% of the NIST traceable standard.’”

o Page 64871 Appendix A-1, section 4.1 .6.1 – “We have not seen problems with using cylinders with concentrations below 50 parts per million (ppm). A problem with using cylinders with concentrations of 50 ppm and greater is that, in order to dilute to a concentration approximately equal to the standard, e.g., 0.055 ppm, requires usi ng the normal 10 liters per minute (lpm) air flow controller and 100 c ubic centimeters per minute (cc/min) gas flow controller at less accurate li mits of their design range, 9 lpm and

10 cc/m. Unless this section is based on recent study, we recommend consulting with manufacturers.”

Response: 1 ) The use of a particulate filter on the sample inlet line may or may not be required for existing specifi c FRM or FEM analyzers, depending on manufacturer requirements, which are refl ected in EPA/ORD’s List of Designated Reference and Equivalent Methods. Miss ouri’s suggestion is a reasonable one. However, for new SO 2 FRM analyzers, EPA believes that the benefits of an inlet line particulate filter and the unifo rmity provided by a mandatory filter requirement outweigh possible disadvantages of using such a filter. Therefore, the particulate filter requirement of secti on 2.4 of the FRM has not been changed.

2) Sections 4.1.1 and 4.1.2 of the FRM desc ribed the separate required functions of (1) flow regulation and (2) flow m easurement. These separate functions correspond to the separate schematic boxes for each of these two functions shown in Figure 2 of the FRM. However, both functions are often co mbined in a single device, such as a mass flow controller. In response to this comment to reduce possible confusion regarding flow control and measurement, the language of these

sections has been revised to describe both the flow rate regulation and flow rate measurement functions together for the SO 2 standard gas (4.1.1) and for the dilution air (4.1.2). This arrangement is appropriate because the flow rate measurement devices may need to be ca librated differently and specifically for nitrogen (standard gas) and for air.

3) In response to this comment, whic h we believe is very appropriate, the minimum concentration requirement for the SO 2 calibration standard, specified in section 4.1.6.1 of the FRM, was re-evaluated. EPA has concluded that this requirement can be changed to 10 ppm. The language of section 4.1.6.1 has been86 revised to incorporate this change. These changes to the language of the FRM are discussed in Section IV.A.1.b of the preamble.

(6) Comment : A number of organizations e xpressed support for EPA’s proposed FRM using UVF as a means of both impr oving the accuracy of monitoring data and reducing the operational cost of FR Ms. (AirQuality, CE, H-GAC, Houston, KYDEP, MSCC, North Carolina, Pennsylva nia, South Carolina, South Dakota, UARG, and Wisconsin)

Response: EPA appreciates the corroboration ex pressed in these comments. As

discussed in section IV.A.1.c of the preamble to the final rule, we are finalizing

the new automated SO 2 FRM based on UVF technology as Appendix A-1 to 40 CFR Part 50, titled “Reference Measuremen t Principle and Calibration Procedure for the Measurement of Sulfur Dioxide in the Atmosphere (Ultraviolet Fluorescence Method).”

(7) Comment : Both North Carolina and South Caro lina urge EPA to retain existing

FRM and FEM standards so that mon itoring agencies can avoid additional monitoring expenditures and continue to ut ilize existing equipment as long as the data produced from the instruments can meet the monitoring and data quality objectives. Similarly, NYS DEC recommended EPA perm it “existing FEMs to be retained while superseding the existing FRM” rather than keep the existing FRM for a number of years. NYSDEC also commented that the existing FEMs should be retained because “the existing standa rd range FEMs typically used by State and Local monitoring agencies are acceptable for SO 2 monitoring for comparison to the range of the proposed standard.” In addition, AirQuality also expressed support for utilizing “existi ng FEM monitors for initia l attainment designation (before they are phased out)” even if they “may create some difficulties” because

“there is no real alternative.”

Response: EPA concurs with these comments. Section IV.A.1.b of the preamble to the final rule explains that there are sound reasons for not withdrawing the existing FRM at this time, as doing so w ould result inappropriately in cancellation of existing FEM designations as well. Th e consequent costs and disruptions to State and Local monitoring networks are clearly unwarranted given that existing FEMs (and the existing FRM as well) ac curately measure ambient one-hour SO 2 concentrations. Because supersession of an FRM, as defined in §53.16, requires cancellation of FEMs based on the superseded FRM, EPA is exercising its discretion not to supersede the existing FRM but rather to simply add a second

FRM for SO 2 . As described in section IV.A.1.c of the preamble, EPA plans to rescind the original manual FRM at a fu ture time when the new FRM analyzers have permeated the network.

(8) Comment : UARG’s comments included the claim that EPA is violating the CAA by proposing to have multiple FRM standards effective at the same time.87 According to UARG’s interpretation of the CAA, EPA is only allowed to have one operational FRM standard at a time for a NAAQS.

Response: As discussed in section IV.A.1.b of the preamble to the final rule, there is nothing in the Act which mandates a single FRM for each NAAQS. There are sound policy reasons for not withdrawing th e existing FRM at this time, as this would result in cancellation of all existing SO 2 FEM designations as well. See § 53.16 (b). Cancellation of the FE M designations of all these SO 2 analyzers now would be potentially very disruptive to State, Local, and other monitoring networks, and is technically unwarrant ed given that existing FEMs and the existing FRM accurately measure ambient one-hour SO 2 concentrations. Therefore, EPA is exercising its disc retion not to supersede the existing SO 2 FRM

and add a second FRM.

(9) Comment : Center for Biological Diversity recommends in its comments that “EPA should move more expeditiously in phasing out the existing FRM and implementing new performance standards.”

Response: In section IV.A.1.b of the preamble to the final rule, we describe how withdrawing the existing FRM at this time would result in cancellation of existing FEM designations as well and so would be potentially very di sruptive to State, local, and other monitoring networks. We have identified no negative impact from retaining the existing FRM for some period of time to support the approval of existing FEMs. However, as stated in section IV.A.1.c of the preamble, EPA plans to rescind this manual method at a future time when the new FRM analyzers have permeated the network.

(10) Comment : AirQuality submitted comments supportive of EPA for “seeking to upgrade the performance specifica tions for both FRM and FEM SO 2 monitors.

Response: EPA appreciates the corroboration expressed in these comments. As

discussed in section IV.A.2.a and A.2.c of the preamble of the final rule, EPA has included revised performance specifications for automated SO 2 methods in Table B-1 of 40 CFR Part 53 for noise, lower de tection limit, interference equivalent, zero drift, span drift, lag time, rise time, fall time, and precision. In addition, to address the need for more sensitive lower measurement ranges for SO 2 analyzers, EPA has added a separate set of perf ormance requirements that would apply specifically to narrower measurement ranges, i.e. ranges extending from zero to concentrations less than 0.5 ppm.

(11) Comment : Wisconsin recommends EPA revise its monitoring requirements “to specifically recognize digital recorders rather than main taining the tie to analog technology” in Section 53.21 in order to increase the time resolution of SO 2 monitoring data.88 Response: In section IV.A.2.b of the preamble of the final rule, EPA acknowledges that industry has moved away from strip chart recording technology to digital data recording. The language of §53.21 calls for a graphic representation of analyzer re sponses to test concentrati ons to facilitate visual

examination of test results. It 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 secti on provides a clear and appropriate model to help define the type of graphic repres entation needed for the Part 53 tests. EPA believes that the language of §53.21 is adequa tely broad to permit digital or other types of data recording devices. With regard to acquisition of field monitoring data, there is no requirement for use of ch art recorders in lieu of digital or other types of data recorders as long as hou rly (or shorter time) averages can be accurately recorded.



### IV.C: Comments on deadlines for monitoring plans and deployment

(1) Comment : Louisiana Chemical requested EPA provide 6 additional months “for states to submit a monitoring plan (i.e. until January 1, 2012).”

Response: The final network design, discusse d in section IV.B.4 of the preamble to the final rule, requires fewer monitors than proposed. Further, states have the flexibility to use required monitors to serve multiple monitoring objectives, as discussed in section IV.B.3 of the preamble . As a result, EPA believes that much of the existing monitoring network will be able to satisfy minimum monitoring requirement promulgated in this rule. EPA roughly estimates that 41 CBSAs will have to establish a new site, or insert an SO 2 analyzer into existing sites within those CBSAs that currently do not have an SO 2 analyzer operating. As a result,

and as discussed in section IV.B.7 of the preamble, EPA does not believe extra time for network plan development or deployment is necessary.

(2) Comment : South Carolina requested revisi ons to the expectations for the proposed monitoring plan such that “t he Annual Monitoring Plan due July 1, 2011, should only be expected to contain a general description of the expected

SO 2 monitoring network unless the monitori ng organization plans to implement

some SO 2 monitoring as part of the calendar year 2012 Annual Monitoring Plan. The 2013 Annual Monitoring Plan (due to EP A by July 1, 2012) is the appropriate document for review and comment of any a dditions or modifications required for the January 1, 2013 implementation of the SO 2 required monitoring.”

Response: The final network design, discussed in section IV.B.4 of the preamble to the final rule, requires fewer monitors than proposed. Further, states have the flexibility to use required monitors to serve multiple monitoring objectives, as discussed in section IV.B.3 of the preamble . As a result, EPA believes that much of the existing monitoring network will be able to satisfy minimum monitoring requirements promulgated in this rule . EPA roughly estimates that only 4189 CBSAs will have to establish a new site, or insert an SO 2 analyzer at existing sites, within those CBSAs that currently do not have an SO 2 analyzer operating. Therefore, as discussed in section IV .B.7 of the preamble, EPA believes that

states will have sufficient time to deve lop annual monitoring plans that include the requirements of this rulemaking by July 1, 2011.

(3) Comment : Ohio provided comments on the proposed monitoring plan that expressed significant skepticism that large SO 2 emitter states would be able to complete all the necessary work before the July 1, 2011 deadline for state monitoring plans. Ohio point s out that a state-wide an alysis that distributes 18

PWEI monitors in 10 counties and 9 emi ssions triggered monitors (under the proposed network design) within the st ate is a large undertaking requiring significant analysis and resources.

Response: EPA believes that states will have sufficient time to develop annual monitoring plans that incl ude the requirements of this rulemaking by July 1, 2011 because the final network design, discussed in section IV.B.4 of the preamble to the final rule, requires fewer monitors th an proposed. Further, states have the flexibility to use required monitors to serve multiple monitoring objectives, as discussed in section IV.B.3 of the preamble . As a result, EPA believes that much of the existing monitoring network will be able to satisfy minimum monitoring requirements promulgated in this rule.

(4) Comment : ALA/EDF/NRDC/SC recommended EPA accelerate the deployment

schedule for SO 2 monitors because “the sooner mon itors are in place, the sooner the public will experience the health benefits of the new standard.”

Response: Rather than a monitoring-focu sed approach to assess ambient SO 2 concentrations and compliance with the SO 2 NAAQS, EPA anticipates using a hybrid analytic approach that combines monitoring and modeling, using each of these analytic tools where they are most appropriate and effective. We anticipate placing greater emphasis on modeling than did the proposed rule as the most technically appropriate, efficient, and readily available method for assessing

short-term ambient SO 2 concentrations in areas with 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 sites than did the proposed rule. Therefore, the minimum requirements for the SO 2 monitoring network is of a smaller scale than propos ed, and we do not expect monitoring to become the primary method by which ambient concentrations are compared to the

new 1-hour SO 2 NAAQS. States have the flexibili ty to use required monitors to serve multiple monitoring objectives, as discussed in section IV.B.3 of the preamble. As a result, EPA believes that much of the existing monitoring network will be able to satisfy minimu m monitoring requirement promulgated in this rule. However, EPA received comm ent from some states urging us to consider their burden (bot h financially and with regard to personnel) of implementing multiple rulemakings with overlapping schedules. With this90 consideration in mind and the anticipate d use of modeling de scribed above, EPA

does not believe there is sufficient reas on to accelerate th e deployment of SO 2 monitors sooner than the proposed schedule. As discussed in section IV.B.7 of the preamble, the schedule requires s ubmission of annual monitoring plans that will reflect the requirements of this ru lemaking by July 1, 2011, and is further requiring that the network deployment be complete by January 1, 2013.

(5) Comment : Louisiana Chemical requested EPA provide a one-year extension to the proposed SO 2 monitor deployment deadline so that state agencies would have until January 1, 2014 to install the required SO 2 monitors.

Response: The final network design, discusse d in section IV.B.4 of the preamble to the final rule, requires fewer monitors than proposed. Further, states have the flexibility to use required monitors to serve multiple monitoring objectives, as discussed in section IV.B.3 of the preamble . As a result, EPA believes that much of the existing monitoring network will be able to satisfy minimum monitoring requirement promulgated in this rule. EPA roughly estimates that 41 CBSAs will have to establish a new site, or insert an SO 2 monitors into existing sites within those CBSAs that currently do not have an SO 2 analyzer operating. As a result,

and as discussed in section IV.B.7 of the preamble, EPA does not believe extra time for network plan development or deployment is necessary.

(6) Comment : Three state agencies requested EP A allow state agencies to deploy

SO 2 monitors on a phased schedule. Wiscons in’s comments requested that state agencies be allowed to inst all half of the requisite SO 2 monitors by the proposed deadline of January 1, 2013 with th e second half of the required SO 2 monitors being installed by January 1, 2014. According to Wisconsin, the phased

installation schedule is nece ssary because “state and local agencies will need more time to establish the network because establishing new sites near point sources is time-consuming, and expensive.” South Carolina provided comments requesting that, if EPA was going to move forward with the proposed SO 2 monitoring plan, it allow state agenci es phase in the implementation of SO 2 monitors “with the largest source/highest probability population exposure areas designated for implementation in 2013 (some proportion of the highest PWEI monitors) and establishment of the remain ing PWEI and the state level emissions triggered monitoring required by the followi ng year.” Finally, Iowa also requests a phased implementation schedule for all stat es required to install more than four

SO 2 monitors that would extend the final deadline for all SO 2 monitors to be deployed by two years (January 1, 2015).

Response: The final network design, discusse d in section IV.B.4 of the preamble to the final rule, requires fewer monitors than proposed. Further, states have the flexibility to use required monitors to serve multiple monitoring objectives, as discussed in section IV.B.3 of the preamble . As a result, EPA believes that much of the existing monitoring network w ill be able to satisfy the minimum91 monitoring requirement promulgated in th is rule. EPA roughly estimates that 41 CBSAs will have to establish a new site, or insert an SO 2 monitor into existing sites within those CBSAs that currently do not have an SO 2 monitor operating. As a result, and as discussed in sec tion IV.B.7 of the preamble, EPA does not believe extra time for network plan development or deployment is necessary.

(7) Comment : Both NYSDEC and MIDNRE provided comments arguing that EPA’s implementation deadline for SO 2 monitors was not realistic given the scope of the work, limited state funding, and the impleme ntation of other monitoring programs during the same time period.

Response: The final network design, discusse d in section IV.B.4 of the preamble to the final rule, requires fewer monitors than proposed. Further, states have the flexibility to use required monitors to serve multiple monitoring objectives, as discussed in section IV.B.3 of the preamble . As a result, EPA believes that much of the existing monitoring network will be able to satisfy minimum monitoring requirement promulgated in this rule. EPA roughly estimates that 41 CBSAs will have to establish a new site, or insert an SO 2 analyzer into existing sites within those CBSAs that currently do not have an SO 2 analyzer operating. Given the fewer number of required monitors and th e flexibility of monitoring objectives discussed in section IV.B.7 of the prea mble, EPA does not believe extra time for network plan development or deployment is necessary.

(8) Comment : Four state agencies expressed suppo rt for the proposed 5-year network review process being linked with SO 2 network adjustments (Indiana, South Carolina, South Dakota, and Wisconsin). South Carolina and South Dakota also recommended that network adjustments also be allowed on an annual basis as part of the annual monitoring plan.

Response: In section IV.B.7 of the preamble to the final rule, EPA has clarified that existing requirements for both annua l and 5-year reviews are appropriate

tools to ensure that the SO 2 network is sited and op erated appropriately. In response to the comments, adjustments to the network may be made on annual basis as part of annual network plan s required in 40 CFR §58.10. However EPA does not intend for required monitors to move from spot to spot on a frequent (i.e. annual) basis. In particular, annual monitoring plans would be expected to identify, for example, if an area has minimum monitoring requirement where it previously did not as a result of updated PWEI calculations.

(9) Comment : North Carolina recommended that “if the EPA is going to base the monitoring requirements on em ission inventories, then it would make sense to make adjustments to the monitoring networ k on the same cycle that the statewide emissions inventories are update d, i.e., either every 3 year s or 6 years to coincide with AERR inventory requirements.”92 Response: In section IV.B.7 of the preamble to the final rule, EPA has clarified that existing requirements for both annua l and 5-year reviews are appropriate

tools to ensure that the SO 2 network is sited and operated appropriately. Regarding network adjustments, annual ad justments to the network may be made, and documented in annual monitoring plans. However EPA does not intend for required monitors to move from spot to spot on a frequent (i .e. annual) basis. Annual monitoring plans would be expected to identify, for example, if an area now has minimum monitoring requirement wh ere it previously did not as a result of updated PWEI calculations. EPA not es that the Air Emissions Reporting Requirement (AERR) requires full inventor y updates every three years. However, as discussed in section IV.B.5 of the pr eamble, EPA believes that the NEI is an appropriate tool by which to calculate PW EI values that are used to determine minimum monitoring requirements for SO 2 monitoring. The process by which the NEI will be updated is being adjusted (by use of the Emissions Inventory System [EIS]) in a manner that will allow for more frequent insertion of state-supplied emissions data, resulting in a more up- to-date inventory. Any introduction of actual reductions that may be occurring (suc h as federally enforceable reductions, like MACT rules) will therefore be reflect ed more quickly in the inventory. EPA believes that the NEI is limited only if states choose not to submit data for inclusion, or in a timely manner, and thus, strongly encourages states to submit all

available emissions data as fr equent as feasible to ensure the NEI is as accurate as possible.

(10) Comment : Five organizations opposed EPA’s proposed five-year update schedule

for SO 2 monitors. NYSDEC saw no need for an every five-year update if state agencies were “permitted to use the best available data to determine where and how many monitors are needed.” Alaska opposed the proposal on the grounds that the update process was not necessary if EPA followed Alaska’s other recommendation to allow state agencies to remove SO 2 monitors that no longer

demonstrate concentrations of concern.” Ohio opposed the proposal, because as the proposal is currently being interpreted, states will be required to duplicate “the same analysis every five years under the network assessment. Such a requirement would be duplicative, unnecessary, and overly burdensome.” NACAA and Illinois both considered the five-year siting upda te process to be duplicative given the annual and 5-year network reviews that are alread y in place.

Response: As discussed in secti on IV.B.7 of the preamble to the final rule, EPA has clarified that existing requirement s for both annual and 5-year reviews are appropriate tools to ensure that the SO 2 monitoring network will be sited and operated appropriately, per 40 CFR §58.10. EPA did not intend for states to execute a parallel, and duplicate revi ew, every 5-years solely for the SO 2 network. Annual adjustments to the network may be made, and documented in annual monitoring plans, however EPA does not intend for required monitors to move from spot to spot on a frequent (i.e. annual) basis. Annual monitoring plans would be expected to identify, for ex ample, if an area now has minimum93 monitoring requirement where it previously did not as a result of updated PWEI

calculations. Regarding Alaska’s recommendation to allow state agencies to remove SO 2 monitors that no longer demonstrate concen trations of concern, EPA notes that existing rules (40 CFR § 58.14 (c)) allow st ates to remove any monitors above minimum monitoring requirements if certa in conditions are met, and provided Regional Administrator approval.



### IV.D: Comments on Population We ighted Emissions Index (PWEI)

monitors

(1) Comment : Four state agencies provided comments criticizing EPA’s PWEI metric as too simplistic and/or flawed to be the only tool for determining where prong 1 SO 2 monitors are to be located (Iowa, NESCAUM, NYSDEC, and Wisconsin). Wisconsin “believes that m odeling, population dist ribution, and local

conditions must be considered in determ ining whether monitoring is required.”

Response: Under a hybrid analytical approac h, introduced in section III of the preamble to the final rule, EPA anticipat es using both ambient monitoring and source modeling in implementing the SO 2 NAAQS (discussed in sections III, IV, V and VI of the preamble). As a resu lt, the required monitoring network is no longer required to be source-oriented in natu re, as explained in section IV.B of the preamble. The required monitoring network has the flexibility to have monitoring sites characterizing one or more of the following objectives: source characterization, highest concentratio n, population exposure, general background, or regional transport. EP A believes that the flexibility of how monitors may serve varied monitor objectives in the final ne twork design is balanced by the continued use of the PWEI to require them. The use of the PWEI provides a mechanism to focus required monitoring in areas where there is an increased coincidence of population and SO 2 emissions.

(2) Comment : Four commenters expressed conc ern with the use of CBSAs for determining the number and location of SO 2 monitors. Delaware does not favor EPA’s proposed CBSA/PWEI metric for SO 2 monitors because it does not properly take into account interstate CBSAs that require special consideration “for determining the need for additional mon itors beyond the current PWEI calculation

method.”

Response: There is precedent for usi ng CBSAs (which encompasses both Metropolitan Statistical Areas [MSAs] and Micropolitan Statistical Areas) as a means to identify urban areas (e.g. the requirement of NO 2 monitors in CBSAs based on population [40 CFR Part 58 Appe ndix D, section 4.3]). EPA believes

that in situations where a CBSA covers more than one state, or monitoring agency, those entities are in a position to negotiate with each other (and with EPA Regional assistance) how best to sati sfy minimum monitoring requirements for94 that CBSA. In all cases, states always have the prerogative to conduct additional monitoring above the minimum requirements for an area as they see fit.

(3) Comment : NESCAUM provided comments seeki ng EPA clarification on how monitoring issues will be resolved for CBSAs that span across EPA regions and consequently have separate Regional Administrators.

Response: EPA believes that in situations wh ere a CBSA covers more than one state, or monitoring agency, those entities are in a position to negotiate with each other (and with EPA Regional assistan ce) how best to satisfy minimum monitoring requirements for that CBSA. Similarly, EPA expects that the Regional Administrators, and possibly EPA Headquarters, would also work together as necessary in d ealing with any cross boundary issues that may arise.

(4) Comment : South Carolina and Missouri reques ted in their comments that EPA relax the requirement that SO 2 monitors be located with in the boundaries of the assigned CBSA in order “to allow monitori ng organizations the ability to best address sources and potential maximum imp acts associated with an area without unnecessary restrictions.”

Response: EPA intends to use a hybrid analytic al approach, introduced in section III of the preamble to the final rule, that combines monitoring and modeling, using each of these analytic tools where th ey are most appropriate and effective. In addition, the scope of the monitoring netw ork is revised so that it is no longer required to be source-oriented in nature (i.e., “hot-spot” monitoring only). Instead the final monitoring network design now has the flexibility to address multiple monitoring objectives incl uding source characterizati on, highest concentration, population exposure, general background, or regional transport. Thus the final monitoring network design is more flexib le than the proposed network design as

discussed in section IV.B of the preamble. This increased flexibility gives states the ability to select areas where they believe ambient monitoring is most warranted within a CBSA with required m onitoring. However, EPA believes that requiring monitors in CBSAs by using th e PWEI metric appropriately focuses

monitors into areas with increased population and SO 2 emissions. EPA has also included a provision to allow a required monitor to be sited outside of its parent CBSA. As explained in section IV.B.3 of the preamble, a source-oriented/high concentration monitor may be placed outside of its parent CBSA if it is sited to characterize a source inside that pare nt CBSA whose PWEI value triggered minimum monitoring requirements. Furthe r, as discussed in section IV.B.6, EPA Regional Administrators (working with states) may require additional monitoring above the minimum requirements if they believe monitoring objectives remain unfulfilled in an area (including areas out side of CBSAs) even though a state may be satisfying minimum monitoring requireme nts. Finally, states always have the prerogative to conduct additional monito ring above the minimum requirements for an area as they see fit.95 (5) Comment : South Carolina commented that EPA “needs to develop a better

definition for Core-Based Statistical Areas (CBSA) which is not explicitly defined by the U.S. Census Bureau.” In addi tion, South Carolina “strongly encourages EPA to specifically seek public comment on the definition and use of CBSA, to

include an annual list of counties comprising each CBS A in the context of the standard implementation and monitoring requirements.”

Response: EPA believes that the term CBSA is adequately defined by the U.S. Office of Management and Budget (OMB ). OMB defines metropolitan and micropolitan statistical areas according to published standards that are applied to Census Bureau data. The general con cept of a metropolitan or micropolitan

statistical area is that of a core area containing a subs tantial population nucleus, together with adjacent communities having a high degree of economic and social integration with that core. Currently defined metropolitan and micropolitan

statistical areas are based on application of 2000 standards (which appeared in the Federal Register on December 27, 2000) to 2000 d ecennial census data. Current metropolitan and micropolitan statistical area definitions were announced by OMB effective June 6, 2003 (http://www.census.gov/population/ www/metroareas/aboutmetro.html).

(6) Comment : Both South Carolina and LEC provi ded comments noting that the use of PWEI as the metric for locating prong 1 SO 2 monitors will emphasize urban areas at the expense of non-urban and rura l areas which will be left with few if any SO 2 monitors. Without an area-wide m onitoring network, state agencies will

find it difficult conduct SO 2 permitting in these areas (LEC) and will “lose the

broader context needed to evaluate” SO 2 concentration data (South Carolina).

Response: : Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is more flexible than the proposed network design, as disc ussed in section IV.B of the preamble. The scope of the monitoring network is revi sed so that it is no longer required to be source-oriented in nature (i.e., “hot-s pot” monitoring only). Instead, the final monitoring network design now has the flex ibility to address multiple monitoring objectives including source characterization, highest concentration, population exposure, general background, or regional tr ansport. This increased flexibility gives states the ability to select areas where they believe ambient monitoring is most warranted within a CBSA with requi red monitoring. As a result, states may

site monitors to address the need to measure background concentrations or to provide data for a broader c ontext to evaluate ambient SO 2 concentrations. EPA notes that it currently has no mini mum monitoring requirements (except for NCore) for SO 2 , but as of the 3 rd quarter of 2009, EPA estimated that approximately 433 SO 2 sites were operating nationwi de, based on data states have submitted to AQS. Under the final ne twork design, and including monitors required at NCore sites, EPA estimates that 191 monitors would be required to be96 operational by January 1, 2013. That numb er is well short of the 433 analyzers operating in late 2009. EPA believes that states have and likely will continue to place SO 2 monitors in various locations (including non- urban areas) to produce information upon which PSD permitting determinations may be based.

(7) Comment : Wisconsin commented that “if EPA is intent on placing monitors in areas of higher population, it may be useful to establish a population threshold for evaluating exposure potential w ithin a CBSA to en sure that monitoring is directed toward the higher priority locations.” Wisconsin’s recommendation is that EPA should “consider allowing monitoring agen cies the flexibility, under the first prong, to determine whether SO 2 monitors would be su itable in metropolitan

statistical areas with populations between 50,000–350,000” or “in rural areas, if necessary, and that such monitors would fulfill prong 1 requirements.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is more flexible than the proposed network design, as discu ssed in section IV.B of the preamble. This increased flexibility gives states the ability to select areas where they believe ambient monitoring is most warranted within a CBSA with required monitoring. EPA’s contemplated use of the hybrid anal ytical approach, results in a lesser emphasis on the monitoring network desi gn to assess compliance with the SO 2 NAAQS than did the proposed rule. Th erefore, the minimum requirements for

the SO 2 monitoring network are smaller th an those proposed. EPA does not believe that the minimum monitoring requi rement should be further relaxed than the final network design, in order to have a monitoring network coupled with source modeling under a hybrid analytical a pproach, which is focused to provide information to support protecting public health, in this case by focusing monitoring in areas where there is an increased coinciden ce of population and

SO 2 emissions. In regard to monitors bei ng necessary in more rural areas, states always have the prerogative to conduct additional monitoring above the minimum requirements for an area as they see fit. Further, as discusse d in section IV.B.6 of the preamble, EPA Regional Administrato rs (working with states) may require additional monitoring above the minimum requirements if they believe monitoring objectives remain unfulfilled in an area (including areas outside of CBSAs) even though a state may be satisfying minimum monitoring requirements.

(8) Comment : Iowa requested that EPA consider supplementing the “2005 NEI with the most recent and representative point source emissions inventory data available, using both CEMs data and em issions reports from Type A major emitting facilities. Additionally, recent federally enforceable reductions not reflected in current emissions inventories, such as may be expected from Best Available Retrofit Technology (BART) requi rements, should be considered.”

Response: EPA believes that the NEI is an ap propriate tool by which to calculate PWEI values that are used to determine minimum monitoring requirements for97 SO 2 monitoring. As discussed in section IV .B.5 in the preamble to the final rule, the process by which the NEI will be update d is being adjusted in a manner that will allow for more frequent insertion of state supplied emissions data, resulting in a more up-to-date inventory. Any intr oduction of actual reduc tions that may be occurring will therefore be reflected more quickly in the inventory, a result that the commenter appears to desire. EPA be lieves that the NEI is limited only if

states chose not to submit available inform ation timely for inclusion, and thus, we strongly encourage states to submit all av ailable emissions data as frequent as

feasible to ensure the NEI is as accurate as possible.

(9) Comment : Pennsylvania provided comments recommending that thresholds be included within the PWEI calculation method to focus monitoring in high population areas that could potentially violate the proposed new one-hour SO 2 standards. Pennsylvania recommended th at prong one monitors be limited to “urbanized areas with a population of at least 50,000 people or greater where

there is a sulfur dioxide source or combination of sources within 50 miles emitting a total of at least 20,000 tons of SO 2 per year.”

NYSDEC comments included remarks that “the PWEI calculation method should be changed to remove or reduce the infl uence of population in the calculation,” and “because the PWEI approach does not appear to be more predictive, the Department does not recommend a PWEI breakpoint. Instead, the Department recommends a CBSA emissions value of 50,000 tpy as a threshold where monitoring would be required. In addition, if the site has a 3 year design value

that is less than 1⁄2 of the standard, that monitor shoul d no longer be required.” (.0145) Similarly, NESCAUM recomme nded EPA “adopt an emissions-only approach, resulting in fewer CBSA monitors ” including the use of a “threshold of 50,000 tpy CBSA SO 2 emissions to trigger the firs t CBSA monitor, and a second CBSA monitor required when emi ssions exceed 200,000 tpy. CBSAs with

emissions under these thresholds could be monitored under the state emissions triggered prong.”

Response: EPA discusses this issue and the examples for alternative thresholds in section IV.B.5 of the preamble to the fina l rule. As explained there, EPA believes

that these thresholds neither provide an adequate number of monitors nor do they include an adequate number of CBSAs that should have monitors. Using the

commenters’ thresholds would result in too sparse a network to meet the appropriate monitoring objectives. EPA be lieves that the broa der scope of the monitoring objectives of the network is be st served by using the PWEI metric to focus ambient monitoring into areas wher e there is an increased coincidence of population and SO 2 emissions.

(10) Comment : Center for Biological Diversity complained that EPA’s proposed PWEI calculation method is too weighted towards monitoring high population

areas and “could effectivel y deny the health benefits of a short-term SO 2 standard

to residents of areas with some of the nation’s highest total SO 2 emissions” because they live in rural areas.98 Response: Partly in response to these and other comments and an examination of our historical approach to SO 2 NAAQS implementation, we now anticipate using a hybrid analytic approach combining the use of monitoring and available modeling to assess compliance with the new 1-hour SO 2 NAAQS. Rather than a monitoring-focused approach EPA anticipates using each of these analytic tools where they are most appropriate and e ffective. We anticipate placing greater emphasis on modeling than did the propos ed rule as the most technically appropriate, efficient, and readily ava ilable method for assessing short-term

ambient SO 2 concentrations in areas with point sources. This projected change in approach would necessarily result in a lesser emphasis on the monitoring network than did the proposed rule. Therefor e, the minimum requirements for the SO 2 monitoring network is of a smaller scal e than proposed, and we do not expect monitoring to become the primary met hod by which ambient concentrations are compared to the new 1-hour SO 2 NAAQS. EPA discusses its intended approach in sections III, IV, V, and VI of the preamble to the final rule. EPA believes that under a hybrid analytical a pproach, the commenter’s po int about the lack of monitoring in CBSAs with low populations but with high emissions, (e.g., greater than 100, 000 tpy) is no longer a concern. EPA notes that states always have the prerogative to conduct additional monitori ng above the minimum requirements in any area they see fit.

(11) Comment : Missouri’s comments included th e recommendation that EPA adopt the alternative PWEI equation PWEI = [Population + Emissions (TPY) * 100] / 100 because this equation “allows for highly populated areas with lower emissions to have necessary monitori ng while emphasizing areas with lower populations that have higher emissions. Th e thresholds for this proposal would be PWEI over 225,000 - three monitors, PW EI over 75,000 - two monitors, and PWEI over 15,000 - one monitor. The new a pproach would require any area with over 15,000 tons per year of S0 2 to have at least one monitor.” According to Missouri’s calculations, “the total number of required monitors using the revised PWEI would be 250 including 187 CBSAs as compared to 231 monitors in 131 CBSAs under EPA's proposal.”

Response: EPA anticipates using a hybrid anal ytic approach, as introduced in section III of the preamble to the final ru le, combining the use of monitoring and available modeling to assess co mpliance with the new 1-hour SO 2 NAAQS. Rather than a monitoring-focused approach, EPA anticipates using each of these analytic tools where they are most appr opriate and effective. This projected change in approach would necessarily re sult in a lesser emphasis on monitoring than did the proposed rule. EPA has adju sted the final network design, discussed in section IV.B.4 in the preamble to the final rule, which uses the PWEI metric, but with some changes in the breakpoi nts used to require monitors, which ultimately results in requiring fewe r monitors than the number proposed (discussed in section IV.B.5 of the pream ble). EPA believes that fewer monitors are needed to serve monitoring objectives under a hybrid analytical approach, as99 compared to either the proposed numbe r or in the example provided by the

commenter

(12) Comment : Three organizations recommended EPA revise the PWEI calculation

method in order to reduce the number of SO 2 monitors that would be placed in areas unlikely to have SO 2 concentrations violating EPA’s proposed new SO 2 standards (Illinois, NACAA, and North Carolina). According to NACAA, “EPA

should limit the total number of monitors required in CBSAs based on additional metrics, such as total number of monitors , historical data, ar ea, trends analysis and/or modeling, and allow for removal of monitors” if SO 2 concentrations prove to not be close to violating the SO 2 NAAQS. Similarly, South Carolina finds fault with the proposed PWEI equation because it doesn’t take into consideration “the area of the CBSA, relative distribution of population or emissions within or

outside the boundaries” which can “lead to inconsistency and possibly inefficient

use of resources across the CBSAs.” S outh Carolina’s recommended solution to this problem is for EPA to “develop a re fined Index incorporating the proposed PWEI population and emissions elements, but truly normalizing the result to

better account for the differences in CBS As. CBSA area, population distribution and source categories in a nd near the CBSAs are among the possible factors that

should be considered”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is more flexible than the proposed network, as discussed in section IV.B of the preamble. The final network design, discussed in section IV .B.4 in the preamble to the final rule, utilizes the PWEI metric, but with so me changes in the breakpoints used to require monitors, resulting in fewer requi red monitors than proposed. Under our current approach, states are free to use such metrics suggested above to aid in siting their required monitors within a gi ven CBSA. One commenter states that

the use of PWEI can “...lead to incons istency and possibly inefficient use of resources across the CBSAs.” On a national scale, EPA believes that no nationally applied network will always perfectly fit every area of the country. EPA has finalized a minimally required netw ork which we believe is necessary to ensure that a network of adequate size and focus will be operated. The number of minimally required monitors in this rule is much less than the number estimated to be in operation at the time of this rulemaking. EPA does not believe that the application of the PWEI will lead to an inefficient use of resources. Further, the final network design allows states more flexibility in monitor siting than that proposed (as discussed in section IV.B.3 of the preamble), which should further prevent states from operating a minimally required monitor in a fashion that they

would consider to be inefficient.

(13) Comment : MIDNRE’s comments included a re commendation that EPA modify

the PWEI calculation method to take a dvantage of natural break points between the PWEI values of CBSAs when determining how many SO 2 monitors are required in an area so that CBSAs with almost identical PWEI scores are not100 arbitrarily required to have different numbers of SO 2 monitors deployed. Similarly, South Carolina commented th at the “break points in the PWEI approach for the number of monitors (1,000,000, 10,000 and 5,000) are arbitrary. Any index developed to determine pot ential for population exposure and the number of required monitors should have a logical and documented basis for the breakpoints used to determine the mini mum monitoring requirements.” South

Carolina also recommended “the PWEI be truly normalized to better address the differences in area, population and source type and distribution between CBSAs.”

Response: Under a hybrid analytical approach, introduced in section III of the preamble to the final rule, the final networ k design (discussed in section IV.B.4) still uses the PWEI as a means by which to require monitoring. However, the application of the PWEI has been adjusted, as discusse d in section IV.B.5 of the preamble, where EPA did consider the MIDNRE suggestion in the final rule.

(14) Comment : Both Florida and Louisiana Chemi cal recommended that EPA revise its PWEI calculation method to take into account the impact of stack heights on

SO 2 concentrations when calculating the number of SO 2 monitors required for a

CBSA and their location.

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is more flexible than the proposed network design, as discu ssed in section IV.B of the preamble. The required monitoring network no longe r must be wholly source-oriented in nature, but rather can serve multiple mon itoring objectives, as discussed in section

IV.B.3. As a result EPA does not believe it necessary for states to consider

individual source characteristics when requiring a given number of monitors for an area. Further, states now have increas ed flexibility in de ciding the location of required monitors, which should address the commenter’s concerns.

(15) Comment : Iowa commented that EPA shoul d “abandon use of the PWEI in designing the monitoring network. The PWEI is scientifically unsound as it fails to address: 1) the characteristics of the dominant SO 2 source type, 2) population distributions, 3) recent and representa tive emissions data, 4) the number of SO 2 sources in a state, and 5) th e role of atmospheric dispersal in relating emissions to ambient impacts.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is more flexible than the proposed network design, as discu ssed in section IV.B of the preamble. EPA expects the hybrid analytical appro ach to combine modeling and monitoring, using each where they are most appropr iate and effective. The required monitoring network is no longer wholly source-oriented in nature, and can now serve multiple monitoring objectives, as discussed in section IV.B.3. However, EPA is retaining the use of the PWEI me tric to appropriately focus monitoring

resources into areas with increa sed coincidence of population and SO 2 emissions.101 As states decided where to place th eir required monitoring, under the final network design requirements, they can consider the characteristics of the

dominant SO 2 source type in that area, populati on distributions, and the role of atmospheric dispersal in relating emissions to ambient impacts. For some of these, modeling may be a more appropriate a nd effective tool. If a state feels that more monitors are required, above the mi nimum requirements, to address any one of a myriad of issues, EPA notes that states always have the prerogative to conduct additional monitoring in any locati ons they see fit consistent with the overall monitoring objectives. With regard to ‘recent’ emissions data bei ng utilized in the PWEI, as discussed in section IV.B.5 of the preamble, EPA believe s that the NEI is an appropriate tool by which to calculate PWEI values. The process by which the NEI will be updated is being adjusted in a manner that will allow for more frequent insertion of state-supplied emissions data, resulting in a more up-to-date inventory. Any introduction of actual reductions that may be occurring will therefore be reflected more quickly in the inventory. EPA believes that the NEI is limited only if states choose not to submit data for inclusion, or in a timely manner, and thus, strongly encourages states to submit all available em issions data as frequent as feasible to ensure the NEI is as accurate as possible.

(16) Comment: Wisconsin remarked in its comments that, “in states such as Wisconsin, with large CBSA containing re latively small population centers and large, dispersed sources, placing mon itors at the maximum downwind location does not necessarily result in effective protection of public health. Some CBSAs may have excess monitors and others ma y have inadequate coverage based on local conditions. The maximum hourly dow nwind locations may not correlate well with population centers.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, EPA anticip ates using both modeling and monitoring where each of these is most appropriate and effective. In the example that Wisconsin gives of small population cente rs with large sources, Wisconsin’s concern could be addressed by using mode ling rather than monitoring as it may be more conducive in assessing ambient SO 2 concentrations and compliance with

the SO 2 NAAQS. In addition, the final monitoring network design is more flexible than the proposed network design, as discussed in section IV.B of the preamble. The required monitoring networ k is no longer wholly source-oriented in nature, and can now serve multiple monitoring objectives, as discussed in section IV.B.3, including monitors sited to specifically characterize population

exposure.

(17) Comment : Many commenters expressed concerns about inaccuracies in the 2005 NEI data and recommended that states be permitted to utilize the “best available emissions data when selecting the sources where monitors will be established” rather than being limited to the NEI inventory. (DSRI, MIDNRE, NESCAUM,102 NYSDEC, North Carolina, South Ca rolina, South Dakota, UARG, and Wisconsin)

Response: In response to the comments, EPA recognizes that states do receive and submit emissions information for some sources on an annual basis, but PWEI calculations must be made using the mo st recent NEI data. As discussed in section IV.B.5 of the preamble, EPA believe s that the NEI is an appropriate tool

to calculate PWEI values that are us ed to determine minimum monitoring requirements for SO 2 monitoring. The process by which the NEI will be updated is being adjusted in a manner that will al low for more frequent insertion of state supplied emissions data, resulting in a more up-to-date inventory. Any

introduction of actual reductions that may be occurring (such as federally enforceable reductions, like MACT rules) will therefore be reflected more quickly in the inventory. EPA believes that the NEI is limited only if states choose not to submit data for inclusion, or in a time ly manner, and thus, strongly encourages states to submit all available emissions data as frequent as feasible to ensure the NEI is as accurate as possible.

(18) Comment : Delaware expressed concern that basing monitor allocation and siting on emissions inventories “may not be the most relevant method for sourceoriented monitoring network design. Faci lities with large emissions at high

elevations often have a much smaller impact on 1-hour concentrations than

smaller facilities with lo w level emissions. Unless the site allocation method can incorporate critical information such as stack height, this method is unlikely to produce the most meaningful results.”

Response: Rather than the proposed monitori ng-focused approach to assess compliance with the SO 2 NAAQS, EPA anticipates using a hybrid analytical approach, introduced in section III of the preamble to the final rule, which combines monitoring and modeling using each of these where they are most appropriate and effective which should address the commenters concerns. As further explained in sections IV, V, and VI of the preamble, EPA anticipates that both ambient monitoring and the modeling of SO 2 sources would play a role in the implementation of the revised SO 2 primary NAAQS. Since it is anticipated that dispersion modeling will be used to assess impacts of individual SO 2 sources, the required monitoring network is no longer required to be wholly sourceoriented in nature, and now can serve multiple monitoring objectives as discussed in section IV.B.3 of the preamble.

(19) Comment : Pennsylvania, concerned about EPA’s over-reliance on the NEI for determining SO 2 monitor locations, proposed “that if the emission inventory is to be used, that a source threshold be de veloped, similar to lead, where modeling would be used to determine if the expected maximum one-hour SO 2 concentration is at least 50 percent of the NAAQS for those fac ilities emitting 50,000 tons per year based on the most recent inventory available at the time of final rule.... The

determination of facilities should also take into account the installation of control103 devices (scrubbers) that would reduce SO 2 emissions to less than 50,000 tons per year if the installation of the devices would occur prior to January 1, 2013.... This approach would allow states and regions to analyze the effect of stack height on

local ambient concentrations.”

Response: Pennsylvania’s suggestion is similar in some regards to the alternative network design, described which was provide d in the proposed rule for comment. In the proposed rule, EPA took comment on alternative network design; however it is not adopting that alternative netw ork design which should alleviate the concerns expressed by the commenter. Instead, EPA anticipates using a hybrid analytical approach, as introduced in secti on III of the preamble to the final rule. The final monitoring network design is mo re flexible than the proposed network design, as discussed in sec tion IV.B of the preamble. The required monitoring network is no longer wholly source-oriented in nature, as modeling of source may in effect provide source-oriented concen tration data. We now anticipate using a hybrid analytic approach combining the use of monitoring an d available modeling to assess compliance with the new 1-hour SO 2 NAAQS. We anticipate placing greater emphasis on modeling than did the proposed rule as it is technically appropriate, efficient, and readily ava ilable method for assessing short-term

ambient SO 2 concentrations in areas with larg e point sources. Thus the alternative network design or the approach suggested by Pennsylvania, where modeling informs where monitoring is to occur is not necessary and the projected change in our approach should address the commenters’ concerns. .



### IV.E: Comments on State-Level Emissions Monitors

(1) Comment : H-GAC commented that it suppor ts EPA’s proposed state-level emissions monitoring plan of locating monitors in areas where maximum SO 2 concentrations are expected. NESCAUM also supports source-oriented SO 2 monitoring but with the caveat that state agencies ought to have the authority to

remove SO 2 monitors after three-y ears of deployment if the design value is less

than 50% of the standard. NESCAUM also supports “a minimum number of sites within a state, based on the state emissi ons triggered monitor count as proposed by EPA.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is more flexible than the proposed network design, as discu ssed in section IV.B of the preamble. The final network design does not retain the proposed requirement for monitors based solely on state level emissions (t he second prong of the proposed network design), as discussed in section IV.B.4 of the preamble. The required monitoring network is no longer wholly source-or iented in nature, and can now serve multiple monitoring objectives, as discu ssed in section IV.B.3. EPA believes state and local air agencies should cons ider monitoring, as appropriate, those

sources which are not as conducive to di spersion modeling. Such sources include (1) sources classified as non-point so urces.(“area-sources”) such as shipping104 ports, (2) a source situated in area of comp lex terrain and/or situated in a complex meteorological regime, (3) locations that have multiple, relatively small sources with overlapping plumes. EPA also notes that existing rules (40 CFR § 58.14 (c )) allow states to remove any monitors above minimum monitoring requirements

if certain conditions are met, and provided Regional Administrator approval.

(2) Comment : Pennsylvania commented that it oppos es the proposed design of the state-level emissions monitoring progra m and requests that EPA remove the program in favor of allowing Regional Ad ministrators and state and local air pollution control agencies “to determ ine the number and location of any

additional SO 2 monitors.” Similarly, South Caroli na commented that state-level

emissions monitors are unn ecessary and redundant if EP A allows state agencies more flexibility in siting PW EI monitors such as by allowing the monitors to be

located outside the boundaries of the CBSA.

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is more flexible than the proposed network design, as disc ussed in section IV.B of the preamble. The final monitoring network design does not retain the proposed requirement for monitors based solely on state level emissions (the second prong of the proposed network design), as discussed in section IV.B.4 of the preamble.

(3) Comment : Several organizations provided comments in opposition to EPA’s proposed requirement that every state ha ve at least one state-level emissions

monitor. South Carolina recommended th at “EPA should not require one monitor in every state unless the NCore site can serve as the sole monitor.” AirQuality recommended EPA not require state-level em issions monitors for any states that

contribute less than 0.5% to the nation’s total SO 2 emissions. In addition, Vermont

recommended that state’s that emit less than 0.1% of the nation’s total SO 2 emissions should be permitted to site th eir one required state-level emissions

monitor at the state’s discretion rather than at an in-state SO 2 emissions source

since no emissions sources in such a st ate would be likely to generate an SO 2 concentration of concern.

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final m onitoring network design no longer requires monitors based solely on state level emissions (the second prong of the proposed network design), as discussed in section IV.B.4 of the preamble. The removal of the second prong of the monitoring networ k in the final network design should

address the commenters’ concerns.

(4) Comment : Both Delaware and South Dakota provided comments in support of EPA’s “requirement that each state have at least one monitor” in the state-level emission monitor prong of the network design. (Delaware and South Dakota)105 Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is more flexible than the proposed network design, as disc ussed in section IV.B of the preamble, and no longer will require monitors base d solely on state level emissions (the second prong of the proposed network design ), as discussed in section IV.B.4 of the preamble. However, if a state feels that more monitors are required, above the minimum requirements, to address any one of a myriad of issues, EPA notes that states always have the prerogative to conduct additional monitoring in any locations they see fit consistent with overall monitoring objectives.

(5) Comment : Iowa expressed concern that “it will be difficult to obtain an accurate picture of the attainment status of th e ambient air in any state based on the number of monitors allocated in the proposed rule.”

Response: Rather than the proposed monito ring-focused approach to assess compliance with the SO 2 NAAQS, EPA anticipates using a hybrid analytical approach, introduced in section III of the preamble to the final rule, which combines monitoring and modeling using each of these where they are most appropriate and effective. As further explained in sections IV, V, and VI of the preamble, EPA anticipates that both am bient monitoring and the modeling of SO 2 sources would play a role in th e implementation of the revised SO 2 primary NAAQS and provide more accurate informa tion on what areas may or may not be in attainment than the proposed monitoring-focused approach.

(6) Comment : NYSDEC recommended that state-leve l emissions monitors be used exclusively for source-oriented monito ring but also be permitted “to replace

PWEI monitors if the source is within a CBSA.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is more flexible than the proposed network design, as disc ussed in section IV.B of the preamble, and no longer requires monito rs based solely on state level emissions (the second prong of the proposed network design), as discussed in section IV.B.4 of the preamble.

(7) Comment : NYSDEC recommended “that monitori ng be considered for any point source with greater than 20,000 tpy SO 2 emissions.”

Response: Under a hybrid analytical approac h, introduced in section III of the preamble to the final rule, and further expl ained in sections IV, V, and VI of the preamble, EPA anticipates using both am bient monitoring and the modeling of

SO 2 sources to play a role in the implemen tation of the revised NAAQS. If a state believes that monitoring is necessary to evaluate source impacts (whether sources have been modeled or not), the state ma y focus its required monitors for that purpose (for sources inside CBSAs which have minimum monitoring requirements). If a state believes that a source outside of a CBSA with minimum106 monitoring requirements warrants ambient monitoring resources to characterize source impacts, EPA notes that states al ways have the prerogative to conduct monitoring above the minimum requirements as they see fit consistent with the over all monitoring objectives. Further, Regional Administrators also have the authority (as discussed in section IV.B.6 of the preamble) to require additional monitors above the minimum requirements, if monitoring object ives are not being met in an area although the state ma y be fulfilling minimum monitoring objectives. EPA expects Regi onal Administrators to work with states to address

in such circumstances.

(8) Comment : NACAA and North Carolina commented that for siting state-level emissions monitors, “EPA must allo w agencies to determine the most scientifically defensible location, while taking into account potential exposures and access to locations with adequate siting.” (NACAA an d North Carolina) Similarly, Dow commented “that EPA's final rule should provide additional flexibility with respect to locating monito rs within state bounda ries solely based

on their contribution to annual SO 2 emissions.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is more flexible than the proposed network design, as discu ssed in section IV.B of the preamble. The required monitoring network is no l onger wholly source-o riented in nature, and can serve multiple monitoring objectives , as discussed in section IV.B.3. The greater flexibility to site monitors to meet different and varied monitoring objectives should address the concerns e xpressed by these commenters. If a state believes that monitoring resources outsid e of a CBSA with minimum monitoring requirements are warranted, EPA notes that states always have the prerogative to conduct monitoring above the minimum requirements as they see fit consistent with the overall monitoring objectives.

(9) Comment : RRI and API cautioned that “if a state’s SO 2 monitoring network does not include suitably sited background mon itors, but instead only includes sourceoriented monitors, specific EPA guideline methods for using this source oriented monitoring data to estimate backgro und for compliance modeling need to be developed.” (API and RRI)

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is more flexible than the proposed network design, as discu ssed in section IV.B of the preamble. The required monitoring network is no l onger wholly source-o riented in nature, and now can serve multiple monitoring obj ectives, discussed in section IV.B.3, including non-source oriented monitors such as those char acterizing general background. However, EPA does not inte nd for the required network to be shifted to only provide ‘background’ values for modeling purposes, as we believe

that such an action would be at the expense of other monitoring objectives. EPA encourages states to consider monitoring near sources which are not conducive to107 modeling, such as (1) sources classified as non-point sources (. “area-sources”) such as shipping ports, (2) a source situ ated in area of complex terrain and/or situated in a complex meteorological re gime, (3) locations that have multiple, relatively small sources with overlapping plumes, and also consider monitoring for population exposures. Because monitori ng objectives can be broader now and include monitoring for background, EPA be lieves the commenters concern is addressed and specific guideline methods to estimate background from sourceoriented monitoring data are not needed at present.

(10) Comment : Ohio criticized EPA’s proposed mo nitoring plan for being too stateoriented in its monitor allocation proce ss. Ohio recommended that “allocation of

monitors based on emissions should not be based upon state borders but rather areas of highest concentration regard less of boundaries.... An analysis, based upon the final level of the standard, shoul d be conducted to determine what level of emissions have an impact on achievi ng the standard followed by an analysis nationally to determine which areas of emissions necessitate monitors (again, regardless of boundaries). Stat es are capable of worki ng with our neighbors to

determine which state would be in the best position to site and operate a monitor.” Dow expressed a similar view in noting that “assigning monitors based on which state has the most emissions may be in consistent with the ‘area of highest concentration’ criteria” given th at the emissions from major SO 2 sources “are more likely to impact distant areas than local areas and the resultant ground level

concentrations are often minimal.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is more flexible than the proposed network design, as disc ussed in section IV.B of the preamble, and no longer requires monito rs based solely on state level emissions (the second prong of the proposed network design), as discussed in section IV.B.4 of the preamble. The final network design is intended to focus the minimally required

monitors into areas that have an in creased coincidence of population and SO 2 emissions through the use of the PWEI, as discussed in sections IV.B.4 and IV.B.5 of the preamble. As states decide where to place their required monitoring, under the final network design requirements, they can consider a number of monitoring objectives, including identifying the highest concentrations. States may find modeling more useful and appropriate to meet certain objectives such as identifying peak concentrations near large sources. EPA also notes that states always have the prerogative to conduct additional monitoring in any locations they see fit consistent with the overall monitoring objectives.



### IV.F: Comments on siting requir ements and requesting waivers

(1) Comment : City of New York supports EPA’s position that SO 2 monitors should be focused on measuring “maximum ground-le vel concentrations in areas of both higher population and higher emissions.”108 Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is more flexible than the proposed network design, as discu ssed in section IV.B of the preamble. The required monitoring network is no longe r wholly source-orien ted in nature, as modeling of source may be more appropriate , efficient, and effective in providing

information on source-oriented concentrati on data. However, EPA notes that as explained in section IV.B.3, states should consider monitoring near sources which are not as conducive to modeling, such as (1) sources classified as non-point sources (“area-sources”) such as shipping ports, (2) a source situated in area of complex terrain and/or situated in a comp lex meteorological regime, (3) locations that have multiple, relatively small sour ces with overlapping plumes, and also to consider monitoring for population exposures.

(2) Comment : Multiple organizations provided comments in support of EPA focusing the SO 2 monitoring program on measuring maximum concentrations of

SO 2 in the ambient air near major sources of SO 2 emissions. (ATS, Center for Biological Diversity, Harris Count y, Houston, and NYSDEC). As ALA/EDF/NRDC/SC notes, “People in rural areas have long been neglected by a monitoring network strategy th at places most monitors in large cities. Large SO 2 sources such as smelters and power plan ts have created pollution hotspots in less populated areas. The people who live, work, and breathe in the vicinity of these sources are those most in need of protection from SO 2 air pollution.” In addition, NYSDEC argued that “by adequately monitoring significant sources of SO 2 we expect that we will be prot ecting more populated areas.”

Response: In response to these and other co mments and an examination of our historical approach to SO 2 NAAQS implementation, we anticipate using a hybrid analytical approach, as introduced in sect ion III of the preamble to the final rule which combines the use of monitoring and available modeling to assess compliance with the new 1-hour SO 2 NAAQS. The final monitoring network design is focused, through use of the PWEI , into areas where there is increased coincidence of population and SO 2 emissions (section IV.B.4 and IV.B.5 of the preamble). Under this approach, EPA expects many SO 2 sources in more rural

areas to be modeled to characterize th eir ground-level impacts which should address the concerns expressed by the commenters. In addition, there are provisions that allow monitoring outs ide of urban areas where minimum monitoring requirements are focused. First, states always have the prerogative to conduct monitoring above the minimum requirements as they see fit consistent with the overall monitoring objectives (see 40 CFR Part 58, Appendix D, section 1.1). Second, as discussed in sectio n IV.B.6, EPA Regiona l Administrators (working with states) may require additional monitoring above the minimum requirements if they believe monitoring objectives remain unfulfilled in an area (urban or rural) even though a stat e may be satisfying minimum monitoring requirements.109 (3) Comment : A number of organiza tions expressed a preference for EPA to use population exposure to high ambient air SO 2 concentrations as the primary criterion for determining where SO 2 monitors would be located. Many organizations provided comments similar to NPRA’s that for the “purposes of designing a monitoring system to support implementation of a NAAQS, monitoring plans should not include locations where there is litt le or no prospect of public exposure, but must be broadly related to ambient air conditions faced by the public at large.” (ABR, Exxon Mobil, Golder, NPRA, PCA, Rio Tinto, RTA, UARG, Delaware, Indiana, North Caroli na, South Carolina, South Dakota, and, Wisconsin)

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is focused, through use of the PWEI, into areas where there is increase d coincidence of population

and SO 2 emissions (section IV.B.4 and IV.B.5 of the preamble). We now anticipate using an approach combining the use of monitoring and available modeling to assess compliance with the new 1-hour SO 2 NAAQS. Therefore, the required monitoring network is no longer wholly source-oriented in nature, and can now serve multiple monitoring objectiv es, as discussed in section IV.B.3, including population exposure. Further, EPA encourages states to consider monitoring near sources which are not as conducive to modeling, such as (1) sources classified as non-point source s (i.e., “area-sources”) such as shipping ports, (2) a source situated in area of comp lex terrain and/or situated in a complex meteorological regime, (3) locations that have multiple, relatively small sources with overlapping plumes, and also to consider monitoring for population

exposures.

. (4) Comment : Five organizations reque sted that state and local agencies be provided the authority to remove SO 2 monitors: “based on an assessment of local conditions and risks (Wisconsin), ”if the am bient data is not likely to violate the NAAQS” (MIDNRE), “if measured design valu es at the site are less than 75% of

the selected standard leve l” (AirQuality), or if “the data show that the

concentrations in the area are <75% of the NAAQS” in a CBSA with only one

monitor or “concentrations are <80% of the NAAQS” for a CBSA with more than one monitor (Alaska and NACAA). AirQuality also remarked that the decision to

remove SO 2 monitors should “include relevant stakeholder input that includes consideration of continuing monitoring of a ‘clean air’ area because the monitor provides the community confidence the operations of nearby sources are well within health protective limits.”

Response: Under existing regulation (40 CFR 58.14( c)), there is a process states may utilize to receive EPA Regional Admi nistrator approval to shut down an existing SO 2 monitoring site. However, EPA notes that this applies to monitors in

excess of the established minimum monitoring require ments in 40 CFR Part 58 Appendix D.110 (5) Comment : Several organizations requested EP A provide greater flexibility for state agencies in the siting and operati on of monitoring stations and conducting network reviews in order to protect the public health as well as possible (Iowa, MIDNRE, and South Carolina).

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final monitoring network design has greater flexibility than the propose d network design, as discusse d in section IV.B of the preamble. The final monitoring network de sign now has the flexibility to address multiple monitoring objectives includ ing source characterization, highest concentration, population exposure, gene ral background, or regional transport (discussed in section IV.B.3 of the preamble).

(6) Comment : South Carolina commented that EPA’s “monitoring requirement must include the ability for states to addres s the needs for area and regional background concentration measurements” by allowing fo r at least some area-wide monitoring stations to stay in th ere current locations.

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final monitoring network design has greater flexibility than the propose d network design, as discusse d in section IV.B of the preamble. The required monitoring networ k is no longer wholly source-oriented in nature, and now can serve multiple monitoring objectives, as discussed in section IV.B.3, including siting of non-s ource oriented monitors such as those characterizing background concentrations (within a CBSA where monitoring is required). If a state beli eves that monitoring resour ces outside of a CBSA with minimum monitoring requirements are warr anted, EPA notes that states always have the prerogative to conduct monito ring above the minimum requirements as they see fit consistent with the overall monitoring objectives.

(7) Comment : MIDNRE requested that state agen cies “have the flexibility to substitute a nearby pre-existing monitori ng station that may be located at a population-oriented location if its locati on is within a pres cribed distance (e.g., three kilometers).”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final monitoring network design has greater flexibility than the propose d network design, as discusse d in section IV.B of the preamble. The required monitoring networ k is no longer wholly source-oriented in nature, and now can serve multiple monitoring objectives, as discussed in section IV.B.3. Regarding the commente r’s specific suggestion of allowing existing monitors to be used, this comme nt was directed at the proposed network design’s requirement that all minimally re quired monitors be source-oriented.

Under the more flexible final networ k design, EPA believes the commenter’s

concern should be alleviated.111 (8) Comment : RRI commented that EPA “should encourage states to provide a stakeholder process so that affected em ission sources can be consulted in the state’s planning process for the monitor placement, which may include a dispersion modeling analysis conducted by the state.”

Response: Under a hybrid analytical approac h, a state’s monitoring network plan, including any source-oriented monitoring that might be conducted under the plan will go through a public notice process. As required in 40 CFR 58.10, states develop an annual monitoring network pla n, which would describe its monitoring network including any proposed or fort hcoming changes to the monitoring network, which is available for public in spection prior to submission to EPA. RRI’s comment regarding the use of a disp ersion modeling analysis to determine monitor placement is directed at the proposed and alternative network design on

which EPA took comment but is no t adopting in the final rule.

(9) Comment : City of New York requested EPA “require that st ates provide an opportunity for affected municipalities to provide relevant information and suggestions about monitor locations, a nd that states shall consider such information in their proposals to the EPA” because oftentimes local agencies have information useful to the siting of air qua lity monitors such as the “location of existing sensitive populations, future de mographic shifts, and the scope of planned developments, and othe r governmental initiatives.”

Response: Monitoring network plans that states develop for the SO 2 monitoring network will go through a public notice pr ocess. As required in 40 CFR §58.10, states develop an annual monitoring network plan, which would describe its monitoring network including any proposed or forthcoming changes to the monitoring network, which is available fo r public inspection prior to submission

to EPA.

(10) Comment : MIDNRE recommended EPA conduct a feasibility study to justify “the need for multiple SO 2 monitors within a single CBSA and the need for new monitoring stations.” In the feas ibility study, “the maximum modeled concentrations should be compared with the design values generated by urban areas that already operate a rich density of SO 2 monitors. By comparing the ambient data and isopleths created fro m it with dispersion modeling outputs, relationships between monitoring and modeli ng data can be estimated for CBSAs. This could provide insights into how am bient stations can be substituted for maximum concentration sites, leveraging the infrastructure. The level of the NAAQS may need to be adjusted downward to retain the same level of protection

as if a maximum concentra tion site were used.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final m onitoring network design is focused on, through use of the PWEI, into areas wher e there is increased coincidence of population and SO 2 emissions (section IV.B.4 a nd IV.B.5 of the preamble).112 Under this approach we an ticipate combining the use of monitoring and available modeling to assess compliance with the new 1-hour SO 2 NAAQS. Therefore, the required monitoring network no longer must be wholly source-oriented in nature, and now can serve multiple monitoring objectives, as discussed in section IV.B.3, while the modeling of sources in effect provide source-oriented concentration

data. EPA believes the more flexible fi nal network design addresses the concerns raised by the commenter, significantly relieving what some considered a redundant or unnecessary monitoring burden on states, reducing the need for new monitoring sites that would have been required under the proposal, and only requiring multiple monitors in urban areas where the relatively highest coincidences of population and SO 2 emissions are occurring.

(11) Comment : API and RRI commented that “EPA should explicitly consider the needs of compliance modeling in siting monitors,” making sure to “avoid doublecounting regional background c oncentrations to be obt ained from appropriate monitors, which are added to modeled impact s from a specific facility” (API and RRI).

Response: We recognize the need for and di stinct role that representative background monitored concentrations play in support of modeling demonstrations for compliance with the standards. Howe ver, we feel the monitoring language in this final rule as written appropriate ly addresses the needs of compliance modeling by including the measurement of background concentrations as one of the monitoring objectives. Further, we believe the monitoring rule is not an appropriate regulatory forum to address the needs identified in the comment

further. As indicated in the final rule , EPA intends to develop guidance for the use of refined air quality dispersion modeling in implementing the new SO2 1- hour NAAQS and the issue of how to appropriately define background

concentrations will be addressed in that guidance, as needed.

(12) Comment : RRI and LEC suggested EPA de velop a more robust method for calculating background c oncentrations of SO 2 by “assigning background on an hour-by-hour basis from one or more mon itors, accounting for wind direction and excluding hours when a monito r is directly downwind of the its existing source. Another approach could be the development of SO 2 background climatology according to wind direction and season, based upwind monitors that are not influenced by local sources.” RRI con tinued by stating that “refined modeling approaches that consider realistic comb inations of source impacts and regional background concentrations are critical to avoid false identifications of nonexistent non-attainment areas.”

Response: EPA recognizes the technical challeng es associated with conducting a cumulative ambient air quality impact an alysis given the 1-hour averaging time

and the statistical form of the new sta ndard. Existing guidance for demonstrating compliance with NAAQS through dispersion m odeling is expected to be generally applicable and adequate for the new hourly SO2 standard, and that guidance113 provides an appropriate amount of flexibility to address specific issues that may arise on a case-by-case basis. Howeve r, EPA expects to develop additional modeling guidance to support the proper implementation of the new standard. Further, we believe the monitoring rule is not an appropriate regulatory forum to

address the needs identified in the comment further.

(13) Comment : RRI urged EPA not to allow “state s to use ‘hot spot’ monitors to characterize regional ba ckground concentrations that are needed for comprehensive modeling analyses.” In addition, RRI requested EPA allow states to permit “affected facilities to conduct refined modeling to determine whether there is a need for monitoring near thes e facilities, and where any monitor should be placed.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is focused, through use of the PWEI, into areas where there is increase d coincidence of population

and SO 2 emissions (section IV.B.4 and IV.B.5 of the preamble). Under this approach we anticipate combining the us e of monitoring and available modeling to assess compliance with the new 1-hour SO 2 NAAQS. Therefore, the required monitoring network no longer must be who lly source-oriented in nature, and now can serve multiple monitoring objectives, as discussed in section IV.B.3, while the modeling of sources in effect provid e source-oriented concentration data. Because of the greater flexibility of the final monitoring network to address multiple monitoring objectives including general background (discussed in section IV.B.3 of the preamble), EPA exp ects that non-source oriented monitoring

data will be available for use as b ackground concentration input for modeling analysis. Regarding the “n eed for monitoring” near f acilities, states should consider monitoring near sources which ar e not as conducive to modeling, such as (1) sources classified as non-point sources (i.e., “area-sources” ) such as shipping ports, (2) a source situated in area of comp lex terrain and/or situated in a complex meteorological regime, (3) locations that have multiple, relatively small sources with overlapping plumes.

(14) Comment : Louisiana Chemical recomme nds that the placement of SO 2 monitors “be based on projected emissions rather th an historical data” since “significant

reductions in levels of sulfur dioxide emissions have occurred since 2005 and

these will continue to decr ease. Decisions on monitor locations should be based on enforceable agreements, permits a nd regulations in the process of implementation.”

Response: Rather than the proposed monito ring-focused approach to assess compliance with the SO 2 NAAQS, EPA anticipates using a hybrid analytical approach, introduced in section III of the preamble to the final rule, which combines monitoring and modeling using each of these where they are most appropriate and effective which should a ddress the commenters concerns. Since it is anticipated that dispersion modeli ng will be used to assess impacts of114 individual SO 2 sources, the required monitoring network is no longer wholly source-oriented in nature, and now can serve multiple monitoring objectives, as discussed in section IV.B.3, while the m odeling of sources in effect provide

source-oriented concentration data. This approach should address the commenter’s concerns regarding the appropriate location of monitors.

(15) Comment : MSCC requests “greater considera tion...be given so that the limited monitoring resources are placed in the area s judged most likely to benefit from the measurements–most likely to a) have populations at risk, b) have

concentrations above the threshold of the standard, and c) most likely to have a

feasible means of correct ing any problem found.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is focused, through use of the PWEI, into areas where there is increase d coincidence of population

and SO 2 emissions (section IV.B.4 and IV.B.5 of the preamble). The required monitoring network no longer must be who lly source-oriented in nature, and now can serve multiple monitoring objectives, as discussed in section IV.B.3, while the modeling of sources in effect provid e source-oriented concentration data. EPA believes the final network design, in combination with the projected use of modeling, alleviates the concer ns raised by the commenter.

(16) Comment : Dow recommended EPA relocate “only a portion of the current monitors during the first few years of short term data collection” in order that data from the new monitoring stations ca n be compared with the pre-existing monitoring stations and community monitors to gain a better understanding of the strengths and weaknesses of the new m onitoring system. Dow also expressed concern that “the new monitoring network could result in some monitoring results that are variable and responsive to small changes in placement.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is focused, through use of the PWEI, into areas where there is increase d coincidence of population

and SO 2 emissions (section IV.B.4 and IV.B.5 of the preamble). Under this approach we anticipate combining the us e of monitoring and available modeling to assess compliance with the new 1-hour SO 2 NAAQS. Thus, the required monitoring network is no longer wholly source-oriented in nature, and now can serve multiple monitoring objectives, di scussed in section IV.B.3, while the modeling of sources may be more effici ent and effective in providing sourceoriented concentration data. EPA be lieves the final network design, in combination with the projected use of modeling, should alleviate the concerns raised by the commenter.

(17) Comment : City of Alexandria commented that, based on its own monitoring experiences, “local monitoring should onl y be used as a backstop, in order to assist in compliance verification, an d only after air quality model-based115 limitations are developed and implemented by the large stationary sources that have the potential to contravene NAAQS protections.”

Response: Partly in response to these and ot her comments and an examination of our historical approach to SO2 NAAQS implementation, we now anticipate using a hybrid analytic approach, as introduced in section III of the preamble to the final rule, combining the use of monitori ng and available modeling to assess compliance with the new 1-hour SO 2 NAAQS. Rather than a monitoring-focused approach EPA anticipates using each of these analytic tools where they are most appropriate and effective. We anticip ate placing greater emphasis on modeling than did the proposed rule as the most technically appropriate, efficient, and readily available method for assessing shor t-term ambient SO2 concentrations in areas with large point sources. Under th is approach, the required monitoring network no longer must be wholly source- oriented in nature, and now can serve multiple monitoring objectives, as disc ussed in section IV.B.3, while the modeling of sources in eff ect provide source-oriented concentration data. EPA believes states should consider monitoring in areas or near so urces that are not conducive to modeling, such as (1) sources classified as non-point sources (“areasources”) such as shipping ports, (2) a sour ce situated in area of complex terrain and/or situated in a complex meteorol ogical regime, (3) locations that have multiple, relatively small sources with overlapping plumes. This approach should

address the commenter’s concern.

(18) Comment : AirQuality recommended EPA “cons ider adding simple particle

detectors to SO 2 sites to further characterize the natu re of the plume. If other such parameters are to be required at SO 2 sites EPA should be prepared to fund the capital cost of the equipment involved.”

Response: While EPA does not disagree with the reasoning of the commenter’s suggestion, EPA is only addressing the SO 2 monitoring network design here. Particulate matter is a separate NAAQS po llutant which is on another time-line for review. However, EPA encourages and applauds state efforts to make monitoring sites multi-pollutant.

(19) Comment : Wisconsin requested “a clarifi cation on the minimum monitoring requirements presented on page 64850. When EPA discusses minimum SO 2 monitoring requirements, the thir d requirement is ‘any ongoing SO 2 monitoring

must have a least one monitor sited to measure the maximum concentration of

SO 2 in that area.’ Further clarification is requested for the term ‘area’. What is the special extent that one existing monitor must measure?”

Response: Due to the change of approach fr om proposal, this comment is now

moot.

(20) Comment : Several organizations provided co mments in support of granting SO 2 monitor waivers when violation of the SO 2 NAAQS has been proven to be116 unlikely. Iowa recommended that “existi ng monitors that record low values” continue to be considered “subject to removal in accordance with existing regulations contained in 40 CFR 58 Subpart B paragraph 58.14” as well as being

considered for a waiver when “there are no monitoring locations in the prescribed area where site specific ambient impact analyses predict a NAAQS violation, there are no monitoring locations available in the modeled “hot spot” that meet EPA siting requirements, or the modeled “h ot spot” occurs in an area that is inaccessible or unpopulated.” MIDNRE r ecommended EPA permit a monitoring requirement to be waived “if modeling shows maximum concentrations are less than 50 percent of the NAAQS” so that st ate agencies can efficiently use their limited budgets. (Iowa, MIDNRE, and South Carolina)

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the required monitoring network no longer must be wholly source-oriented in nature, and now can serve multiple monitoring objectives, as discussed in section IV .B.3, while the modeling of sources we

believe would be more efficient and effective in providi ng source-oriented concentration data. Further, fewer mon itors are required in the final network design versus that number which was propos ed. Due to the flexibility in the monitoring network design and the adjustme nts made to the PWEI thresholds by which monitoring is required (discussed in section IV.B.4 and IV.B.5), EPA does not believe that a waiver provision is necessary. Regarding site shutdown, 40 CFR 58.14(c) explains how a state ca n seek EPA Regional Administrator approval to shut down an existing SO 2 monitoring site. However, EPA notes that this applies to monitors in excess of the established minimum monitoring requirements in 40 CFR Part 58 Appendix D.

(21) Comment : Three commenters expressed concer n that the “two pronged approach in the proposed regulation will lead to duplicative monitoring in some areas and require monitors in areas where monito rs are not needed” and recommend EPA allow monitors to be removed “(1) wher e there is only one monitor for a given

source or Core Based Statistical Ar ea (CBSA) and data show that the

concentrations in the area are less than 75 percent of the National Ambient Air Quality Standards (NAAQS); or (2) where there is more than one monitor in a given CBSA or for a given source and data show that concentrations are less than 80 percent of the NAAQS.” (North Carolina, NACAA, and Ohio)

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the required monitoring network no longer must be wholly source-oriented in nature, and now can serve multiple monitoring objectives, discussed in section IV.B.3, wh ile the modeling of sources would be more efficient and effective in providing source-oriented concen tration data. EPA believes an approach that combines use of monitoring and modeling and not requiring monitors under a second prong as proposed alleviates the commenters’ concerns regarding duplicative monitoring. In addition, due to the flexibility in the monitoring network design and the adju stments made to the PWEI thresholds117 by which monitoring is required (discusse d in section IV.B.4 and IV.B.5), EPA does not believe that a waiver provision is necessary.

(22) Comment : Wisconsin recommends that th e “proposed rule should provide flexibility for states to eliminate monito rs from CBSAs based on an assessment of local conditions and risks and EPA Regional Directors should have the ability to approve waivers or exceptions to the monitoring requirements in the rule.” Wisconsin also recommends “EPA allow flexibility for monitor siting in rural areas, if necessary, and that such monitors would fulfill prong 1 requirements.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the required monitoring network no longer must be wholly source-oriented in nature, and now can serve multiple monitoring objectives including siting monitors in ru ral areas consistent with the monitoring objectives, as discussed in section IV.B.3, while the modeling of sources in effect provide source-oriented concentration da ta. Due to the flexibility in the monitoring network design and the adjustme nts made to the PWEI thresholds by which monitoring is required (discussed in section IV.B.4 and IV.B.5), EPA does not believe that a waiver provision is necessary. Regarding the request for “...EPA Regional Directors should have the ability to approve waivers or exceptions to the monitoring requirements in the rule”, EPA disagrees with such a concept. The minimum monitoring require ments are a ‘minimum’ to ensure that a network of sufficient size and focus is maintained to support the NAAQS.



### IV.G: Comments on Proposed Alternative Network

(1) Comment : Five commenters expressed suppor t for the proposed alternative monitoring network and the use of disp ersion modeling to determine where SO 2 monitors should be located. For instan ce Iowa recommended “that the final rule contains provisions that require monitors to be si ted only at locations where dispersion modeling indicates that the NAAQS is violated” (Alexandria, Delaware, South Dakota, a nd Center for Biological Diversity). Delaware’s support for the alternative monitoring netw ork did come with the recommendation that EPA revise its approach to “include the impact of modeled concentrations on populated areas, and not rely on modele d ambient concentrations alone to

determine the number or locat ion of monitoring sites.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the final mon itoring network design is focused, through use of the PWEI, into areas where there is increase d coincidence of population

and SO 2 emissions (section IV.B.4 and IV.B.5 of the preamble). Under this approach, we anticipate combining the use of monitoring an d available modeling to assess compliance with the new 1-hour SO 2 NAAQS. Thus, the required monitoring network no longer must be who lly source-oriented in nature, and now can serve multiple monitoring objectives, discussed in section IV.B.3, while the modeling of sources we believe would be more efficient and effective in118 providing source-oriented c oncentration data. EPA believes that at hybrid analytical approach is superior to both the proposed network design and the alternative network design because redunda nt and/or unnecessary monitoring is reduced, required monitoring can serv e multiple monitoring objectives, and modeling can provide data that would ha ve otherwise only been collected by a relative fewer number of source-oriented monitors compared to the number of

sources that can be modeled. EPA’s reasons for believing that modeling SO 2 sources is more effective than just source-oriented SO 2 monitoring is discussed in detail in section IV.B.2 of the preamble.

(2) Comment : Five commenters expressed opposi tion to EPA’s proposed alternative monitoring network with three of th e organizations opposing the proposed alternative network design because it “w ould not distinctly use population as a factor in deciding where monitors sh ould be placed.” (AirQuality, South Carolina, and UARG). In addition, NPRA opposed EPA’s proposed alternative network because it would “effectively make the SO 2 NAAQS a “source oriented” NAAQS” which is “not supported by the clear language of the CAA.” Finally, Indiana opposed the alternative network because the state agency preferred “to determine placement of the monitors” rath er than have the monitor locations determined strictly from dispersion modeling.

Response: In the final rule, EPA is not adopting the alternative monitoring network. Instead, under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the fi nal monitoring network design is focused, through use of the PWEI, into areas wher e there is increased coincidence of population and SO 2 emissions (section IV.B.4 and IV.B.5 of the preamble). Under this approach, we anticipate combining the use of monitori ng and available modeling to assess compliance with the new 1-hour SO 2 NAAQS. Thus, the required monitoring network no longer must be wholly source-oriented in nature, and now can serve multiple monitoring objectives, as discussed in section IV.B.3, while the modeling of sources we believe could be more appropriate, efficient, and effective in providing source-oriented concentration data. EPA believes that a hybrid analytical approach is superior to both the proposed network design and the alternative network design because redundant and/or unnecessary monitoring is reduced, required monitoring can serve multiple monitoring objectives, and modeling can provide data that would ha ve otherwise only been collected by a relative fewer number of source-oriented monitors compared to the number of

sources that can be modeled. EPA’s reasons for believing that modeling SO 2 sources is more effective than just source-oriented SO 2 monitoring is discussed in detail in section IV.B.2 of the preamble.

(3) Comment : API provided comments recommendi ng that if dispersion modeling was used in the monitor location process that “EPA should encourage states to provide a stakeholder proce ss so that affected emission sources can be consulted in the state’s planning process for the m onitor placement...If an affected source does not agree with the state’s analysis on the placement of one or more SO 2119 monitors, they should have the right to provide their own stud y for state review and consideration if they choose to do so.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the required monitoring network no longer must be wholly source-oriented in nature, and now can serve multiple monitoring objectives, as discussed in section IV .B.3, while the modeling of sources we

believe would be more efficient and effective in providi ng source-oriented concentration data. EPA is encouraging st ates to consider monitoring in areas or

near sources that are not conducive to mode ling, such as (1) sources classified as non-point sources (i.e., “area-sources”) such as shipping ports, (2) a source situated in area of complex terrain and/or situated in a complex meteorological regime, (3) locations that have multiple, relatively small sources with overlapping plumes. The decision on the appropriate locations for the required monitors falls on the states, who will detail such decisi ons in their annual monitoring network plan. States must develop annual monitoring network plans under 40 CFR §58.10, and the plan must be available to the public prior to submission to EPA.

(4) Comment : UARG opposes the proposed use of AERMOD for any requisite dispersion modeling because it would “s imply not be appropriate for siting a monitor at the likely site of the expe cted maximum 1-hour concentration–the model has been validated by comparing it with observed high ambient concentrations, but its performance for ma tching concentrations at a specific time

or location has not been evaluated.”

Response: The revised approach for monitori ng shifts the focus of monitoring toward characterizing ambient concentra tions that are best achieved through ambient monitoring, such as areas with a large number of smaller SO 2 sources or areas with complex meteorol ogical influences, such as coastlines, that may be more difficult to characterize throug h modeling. Under a hybrid analytical approach (as introduced in s ection III of the preamble to the final rule), modeling will typically not be relied upon to site a single source-oriented ambient monitor. The limitation of a single monitor to adequately account for peak hourly SO 2 ambient concentrations in the vicinity of SO 2 emission sources, even if dispersion modeling was conducted to site the monitor, is part of the rationale for revising the focus and objectives for th e ambient monitoring network.

(5) Comment : NYSDEC expressed concern in its comments that “using AERSCREEN or SCREEN3 may not be the most practical approach for determining the number and location of required monitors” because “the data required to run the models are only read ily available for permitted facilities,” which would result in incomplete and inaccurate dispersion models.

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the required monitoring network no longer must be wholly source-oriented in nature, and now can serve multiple monitoring120 objectives, as discussed in section IV .B.3, while the reliance on appropriate refined dispersion modeling would be used to provide source-oriented

concentration data.

(6) Comment : RRI and LEC suggested EPA de velop a more robust method for calculating background c oncentrations of SO 2 by “assigning background on an hour-by-hour basis from one or more mon itors, accounting for wind direction and excluding hours when a monito r is directly downwind of the its existing source. Another approach could be the development of SO 2 background climatology according to wind direction and season, based upwind monitors that are not influenced by local sources.” ( RRI (.0388), LEC (.0257)) RRI continued by stating that “refined modeling approaches that consider realistic combinations of source impacts and regional background con centrations are critical to avoid false

identifications of non-existent non-at tainment areas.” (RRI (.0389))

Response: EPA recognizes the technical challeng es associated with conducting a cumulative ambient air quality impact an alysis given the 1-hour averaging time

and the statistical form of the new sta ndard. Existing guidance for demonstrating compliance with NAAQS through dispersion m odeling is expected to be generally applicable and adequate for the new hourly SO2 standard, and that guidance provides an appropriate amount of flexibility to address specific issues that may arise on a case-by-case basis. Howeve r, EPA expects to develop additional modeling guidance to suppor t the proper implementati on of the new standard.

(7) Comment : NYSDEC recommended that if the alternative monitoring network approach is pursued, potential monitoring should be limited “to the source types of highest concern (e.g. smelters, co al burning plants, >20,000 tpy emission sources). In addition, modeling can be used in areas of uncertainty, as an auxiliary or complementary tool to othe r methods for assessing the likelihood of NAAQS exceedance.”

Response: EPA has not pursued the altern ative network design. The final network design is discussed in section IV .B of the preamble to the final rule.

(8) Comment : North Carolina made several reco mmendations for how the alternative monitoring network proposal could be im proved. First, North Carolina suggested that any “facility whose smokestack SO 2 emissions are above a determined emission rate would be required to perf orm modeling to demonstrate that the SO 2 concentration at the fence-line does not exceed whatever ambient standard EPA establishes. If modeling doe s not demonstrate compliance, the facility could then be required to reduce emissions from th e stack, install cont inuous emissions monitoring (CEM) in the stack itself, or re quire a fence-line monitor at the target facility. This will allow flexibility to work with facilities to determine if monitoring is required and will reduce the cost and consequences of nonattainment.” Second, EPA could create a monitoring approach similar “to the lead network where modeling can be used to demonstrate wh ether the facility121 impacts the NAAQS. Facilities where th e model indicates the ambient air is a certain amount below the NAAQS” would not have to monitor. “This approach

would also reward states that have taken actions to reduce SO 2 emissions from their facilities and place the monitori ng burden on states that have more significant SO 2 emissions.”

Response: EPA has not pursued the alternative network design, but instead has identified a hybrid analytical approac h, as introduced in section III of the preamble to the final rule, where the re quired monitoring network no longer must be wholly source-oriented in nature , and now can serve multiple monitoring objectives, as discussed in section IV .B.3, while the modeling of sources we

believe would be more efficient and effective in providi ng source-oriented

concentration data. This combination of monitoring and modeling is anticipated to provide the data for implementation of and designations under the revised primary SO 2 NAAQS. EPA believes that a hybri d analytical approach embodies much of the suggestions made by North Caro lina, thus addressing their concerns.

(9) Comment : Missouri recommended “EPA shoul d not require states to conduct refined modeling in every case that the screening tools show a violation. Many times, the screening tools ar e overly conservative and might lead to analysis in

areas that is not warranted. Many states will have a bett er indication of areas that might violate the new NAAQS without this type of screening approach due to previous permit modeling exercises or additional ambient monitoring data collected by third parties.”

Response: This comment is no longer rele vant under a hybrid analytical approach, as discussed in sect ions III, IV, V, and VI in the preamble to the final

rule.



### IV.H: Comments on data reporting

(1) Comment : Iowa commented that EPA will n eed to emphasize that “long term monitoring commitments are required, as computation of design values at a monitoring site requires at le ast three years of data.”

Response: EPA concurs with Iowa that monitoring sites are long term commitments. EPA expects that many of the existing sites in urban area with required minimum monitors will be used to meet minimum monitoring requirements of this final rule, thus con tinuing operation of some sites that could be already considered ‘long-term’ sites.

(2) Comment : NESCAUM recommended if a “source-or iented site has or is expected

to have SO 2 levels at 75% or more of the NAAQS... that five-minute wind data

also be collected at the site.”122 Response: EPA did not propose, and is not fina lizing a requirement for wind data at source oriented monitoring sites. EP A encourages states, as appropriate, or possible, to collect meteorologi cal data at such sites in or der to better characterize source impacts on the monitoring site.

(3) Comment : Both Alaska and NACAA oppose re quiring states to measure and report data at increments below one hour. In Alaska’s comments, the Department notes that upgrading its “cu rrent data collection infrastructure to capture, store

and review data on a smaller time scal e than one hour would impose substantial costs” because Alaska does not currently possess monitors capable of reporting

five-minute SO 2 concentrations. Similarly, NAC AA commented that while report

five-minute SO 2 concentrations “may be easily achievable for some agencies with the appropriate data collection infrastruc ture, requiring all agencies to perform these duties impose significant costs.”

Response: EPA notes that all continuous UVF analyzers produce readings that can be aggregated into time intervals as short as 1-minute. Such units should easily be capable of reporting 5-minut e averages. EPA believes that the prevalence of data acquisition systems that can acquire monitor output signals and formulate averages as short as 1-minute is nearly ubiquitous among state, local, and tribal agencies. Such data systems can typically be configured to store and report 1-minute averages to a central database which subsequently can be configured to aggregate data into the desired averaging period (e.g., 5-minute, 15- minute, and 60-minute). Although agencies may be most familiar with reporting averages over 60-minute periods, the capab ility to create alternative average periods and report such averages in AQS compatible formats likely resides within most current data systems. EPA acknow ledges that some investment may be necessary for those few agencies whose data systems cannot currently meet the required reporting requirements for SO 2 ; however, EPA also believes that the investment in such systems will provide efficiencies in data handling across all

continuous measurements that wi ll outweigh the initial cost.

(4) Comment : Missouri recomme nds EPA require SO 2 concentrations to be reported in ppm rather than ppb in order to be c onsistent with other the reporting of other pollutants and the Air Quality Index.

Response: Since the standard itself is expressed in parts per billion (rather than in fractional parts per million), EPA believes it appropriate for SO 2 concentrations to be reported the same way.

(5) Comment: Seven organizations submitted comments in opposition to EPA proposal to require state and local agencies to report the maximum 5-minute block average because the reporting requirement would represent too great a burden on

state finances and labor and because th e additional reporting requirements would not be necessary in order to demonstrate compliance with the proposed new SO 2 standard. (Iowa, NYSDEC, LEC, RRI, South Carolina, and KYDEP)123 Response: EPA has finalized the requirement to report the maximum 5-minute data block occurring in each hour. EPA’s reasons for this requirement are

discussed in detail in section IV.C of the preamble to the final rule.

(6) Comment : AirQuality recommended “hourly me t data should be reported to EPA while the maximum 5-minute block average met data should be reported at the State’s option” so that EPA will be able “to accurately determine the source of the

measured SO 2 .”

Response: EPA did not propose, and is not fina lizing a requirement for wind data at source oriented monitoring sites. EP A encourages states, as appropriate, or possible, to collect meteorologi cal data at such sites in or der to better characterize source impacts on the monitoring site.

(7) Comment : Six organizations provided commen ts supporting the requirement that state agencies submit all 12 5-minute block SO 2 concentrations to EPA along with the 1-hour average either because the or ganization considered reporting all 12 5-

minute concentrations “less resource inte nsive than selecting the 5-minute max for each hour” (NYSDEC) or because it “will provide higher quality data for future NAAQS reviews” (AirQuality) . Center for Biological Diversity, Wisconsin, ATS, and ALA/EDF/NRDC/SC)

Response: EPA has finalized the requiremen t to report the maximum 5-minute data block occurring in each hour. EPA’s reasons for this requirement are

discussed in detail in section IV.C of the preamble to the final rule.

(8) Comment : Three organizations submitted co mments opposing the requirement that state agencies provide all 12 5-minute block SO 2 concentrations because the requirement would create too great a re porting burden for financially strapped state agencies. (Missouri, South Carolina, and South Dakota)

Response: EPA has finalized the requiremen t to report the maximum 5-minute data block occurring in each hour, as disc ussed in detail in section IV.C of the preamble to the final rule.

(9) Comment : Both ALA/EDF/NRDC/SC and S outh Dakota provided comments supporting the requirement that state agen cies report the highest 5-minute rolling average SO 2 concentration from each hour. In its justification for supporting a highest 5-minute rolling average repo rting requirement, ALA/EDF/NRDC/SC noted “the rolling average will yield data more appropriate to the evaluation of health concerns than block averaging wh ich is arbitrary and does not actually report the highest 5-minute con centration.” (South Dakota and ALA/EDF/NRDC/SC) In addition, Center for Biological Diversity expressed support for EPA requiring states to repor t the highest 5-minute rolling average

SO 2 concentration for each hour at SO 2 monitors where possible.124 Response: EPA has finalized the requiremen t to report the maximum 5-minute data block occurring in each hour. See di scussion in section IV.C of the preamble

to the final rule.

(10) Comment : Three organizations provided co mments opposing the requirement that state agencies report highest 5-minute rolling average SO 2 concentrations because the use of rolling averages would be burdensome to financially strapped state agencies in charge of the SO 2 monitoring network. (North Carolina, Indiana, and South Carolina)

Response: EPA has finalized the requiremen t to report the maximum 5-minute data block occurring in each hour, as disc ussed in detail in section IV.C of the preamble to the final rule.



### IV.I: Comments on Cost of Monitoring Changes

(1) Comment : Numerous commenters expressed se rious concern about large the cost of the ambitious monitoring network EPA is proposing and how it will be funded. These commenters said that state gove rnments are already struggling with budget

cuts and the addition of a new SO 2 monitoring program on top of other monitoring program expansions is likely to result in cuts to state air quality monitoring programs, regardless of federal requirem ents, without federal funding assistance. Many comments called on the federal governme nt to significantly increase federal funding for the development and operation of the proposed SO 2 monitoring network in general terms. (NYSDEC, Delaware, Indiana, AEPSC, NESCAUM, Illinois, South Carolina, Texas, Wisconsin, CE, Ohio). Others explicitly requested

that EPA fund the new SO 2 monitoring program with Section 103 funds rather than leave the monitoring network unf unded or funded with Section 105 funds (which require matching state and local funds). (Florida, Iowa, Pennsylvania , MIDNRE , North Carolina , Alaska , NACAA, KYDEP, and Vermont)

Response: EPA notes that $15 million in new STAG funding has been proposed in the President's budget for fiscal ye ar 2011 for the purpose of supporting new monitoring equipment purchases related to the NAAQS. EPA also notes that additional STAG funding was proposed to support "core" programs which can

include activities such as ambient mon itoring. EPA will work with the Regional Offices, NACAA, and monitoring agencies to develop a plan fo r allocating these funds to the new monitoring needs related to recent NAAQS revisions.

(2) Comment : Several commenters expressed concern that EPA significantly underestimated the deployment and operation costs for new SO 2 monitors. For example Vermont estimated that the associated costs for a source-oriented monitor would “be at least 30 to 40 pe rcent higher than EPA's estimates.” (NYSDEC, Delaware, Alas ka, NACAA, and Vermont).125 Response: EPA uses its best available estimati on in determining costs for network deployment and operations, as required fo r inclusion the Information Collection Review (ICR) that accompanies each rulemaking.

(3) Comment : NACAA and Vermont suggest ed in their comments that EPA consider “promulgating regulations that make the sources pay directly for the monitoring efforts carried out by state and local agencies” as a means of addressing the funding crisis for SO 2 monitoring and other NAAQS monitoring. (NACAA and Vermont).

Response: EPA is not in a position to address or execute such an action as part of this rulemaking. EPA notes that $15 m illion in new STAG funding has been proposed in the President's budget for fiscal year 2011 for the purpose of supporting new monitoring equipment purch ases related to the NAAQS. EPA

also notes that additiona l STAG funding was propos ed to support "core" programs which can include activities such as ambient monitoring. EPA will work with the Regional Offices, NACAA, and monitoring agencies to develop a plan for allocating these f unds to the new monitoring needs related to recent NAAQS revisions.

(4) Comment : Georgia requested in its comment s that EPA withdraw its proposed monitoring program proposal “until a holis tic analysis of th e total costs is conducted and adequate new fundi ng is identified and assured.”

Response: EPA understands that each individual NAAQS review is not occurring in a vacuum. However, EPA will not suspend the promulgation of monitoring rules that directly support revised NAAQS . EPA notes that $15 million in new STAG funding has been proposed in the President's budget for fiscal year 2011 for the purpose of supporting new monitori ng equipment purchases related to the NAAQS. EPA also notes that additi onal STAG funding was proposed to support "core" programs which can include activiti es such as ambient monitoring. EPA will work with the Regional Offices, NACAA, and monitoring agencies to develop a plan for allocating these funds to the new monitoring needs related to recent NAAQS revisions.

(5) Comment : Several commenters suggested th at EPA should not implement any new FRM or FEM changes until sufficient new funding is procured to fund the deployment of the entire new SO 2 monitoring network because the funding challenge for the SO 2 monitoring network will only be made worse by the forced

retirement of functional SO 2 monitors that no longer meet the new FRM and FEM standards. (MIDNRE, AEPSC).

Response: Under this rulemaking, there will not be an entirely new monitoring network. The promulgation of a new FRM, which coexists with the existing FRM and FEMs, will not force the retirement of functional and currently operating

FEMs.126 (6) Comment : Several commenters recommended that, because of the limited funds for deploying a new SO 2 monitoring network, dispersion modeling should be utilized along with other criteria (i.e. population expos ure) to determine which

SO 2 monitors are the highest priorities to de ploy so that a smaller, but still health protective, SO 2 monitoring network can be deploy ed. (North Carolina, South Carolina, Vermont)

Response: As discussed in section IV.B.3 and IV.B.4 of the preamble to the final rule, the final monitoring network desi gn calls for less monitoring sites and is more flexible than the proposed network de sign. As a result, EP A anticipates that only 41 new monitors will need to be deployed to satisfy minimum monitoring requirements. Therefore, we believe that the final network de sign alleviates the

commenters concerns.



### IV.J: Comments on the authority and responsibilities of Regional Administrators (RAs)

(1) Comment : Several commenters expressed supp ort for requiring RAs be in “close consultation with the [appropriate] state agency” when determining when to require additional SO 2 monitors (North Carolina, Louisiana Chemical, South Carolina, and AirQuality). Similarly, AirQuality recommended EPA include provisions requiring RAs to provide a public comment period for proposed

additional SO 2 monitors before a final decisi on is made. Finally, Louisiana Chemical suggested EPA require RAs to “consider certain objective factors when determining whether to require any additional ambient SO 2 monitors to the

network.”

Response: EPA agrees with the commenters ’ suggestions, and expects that Regional Administrators would work with state and local air agencies when evaluating and possibly requiring add itional monitors above the minimum requirements. Regarding public input, any additional monitoring that a Regional Administrator might require of a state w ould be included in that state’s annual monitoring plan. States must develo p annual monitoring network plans as required by 40 CFR 58.10, those plans must be available to the public prior to

submission to EPA.

(2) Comment : Several organizations re commended that an RA’s authority to require

additional SO 2 monitors be contingent upon the RA/EPA providing all the additional funding necessary to deploy and operate the additional monitoring stations (NYSDEC, Delaware, North Caro lina, South Carolina, and AirQuality).

recommended that additional SO 2 monitors required by the RA but dispute by state, local, or tribal monitoring aut horities should be funded by the federal government “for three years or until enough data is collected to reach an attainment designation and a ddress the concern.” (.0843)127 Response: In the event that an RA does requi re additional monitors above the minimum required, EPA will work with the monitoring agencies to develop a plan on how funds might be allocated for any new monitoring.

(3) Comment : ALA/EDF/NRDC/SC recommended EPA consider the likelihood of RAs requiring additional SO 2 monitors when creating the federal budget for the monitoring program.

Response: EPA always tries to create as accura te a budget as possible, and if any information regarding RA required monito ring is available as budgets are being developed, such information would be cons idered in the budget creation process.

(4) Comment : Florida recommended EPA grant RAs the authority to approve reductions in the number of required SO 2 monitors if it can be shown that marginal SO 2 monitors will not significantl y benefit the public health.

Response: EPA disagrees with the concep t that allows “RAs...to approve reductions in the number of required SO 2 monitors if it can be shown that marginal SO 2 monitors will not significantly be nefit the public health.” EPA

notes that this comment was directed at the proposed monitoring network design which required a greater number of mon itors compared to the final network design which is part of a hybrid analytical approach (introduced in section III of the preamble of the final rule). Th e minimum monitoring requirements are a

‘minimum’ to ensure that a network of sufficient size and focus is maintained to support the NAAQS.

(5) Comment : Florida recommended EPA “allow for the regional administrator to approve adjustment of the monitoring requi rements to provide for flexibility for monitors to address multiple purposes.”

Response: Under a hybrid analytical approach, as introduced in section III of the preamble to the final rule, the required monitoring network no longer must be wholly source-oriented in nature, and now can serve multiple monitoring objectives, as discussed in section IV.B .3, while the modeling of sources would be more efficient and effective in prov iding source-oriented SO2 concentration data. Due to the flexibility in the mo nitoring network design EPA believes the

commenter’s concerns are alleviated.

(6) Comment : AirQuality recommended “EPA s hould consider adding guidance to its monitoring network proposal that focu ses on school-oriented sites in section d. ‘Monitoring required by the regional admi nistrator’ [64854] to clarify if schools may be an example of ‘a case where a source having modest emissions still has high potential to cause a violation of the NAAQS in a community or neighborhood.’”128 Response: Under the final rule, Regional Admi nistrators may consider requiring monitoring wherever monitoring objectives are not being met. EPA anticipates

that in situations such as the exam ple given by the commenter, the Regional

Administrators would consider on a case- by-case basis the need for additional

monitors in such situations.



## VI. Air Quality Index

(1) Comment: Pennsylvania commented that EPA must provide timely implementation rules and guidance for issues such as SHLs, emergency action levels, and the updated AQI in order to ad equately protect public health and the

environment.

Response: EPA agrees with this comment wh ich is why we have revised the Air Quality Index (AQI). EPA did not propos e to change breakpoints at the higher end of the AQI scale (from 200 to 500), wh ich would apply to state contingency plans or the Significant Harm Level ( 40 CFR 51.16), because the information

from this review does not inform deci sions about breakpoin ts at those higher

levels.

(2) Comment: NESCAUM commended EPA’s pr oposal to revise the AQI concurrently with the NAAQS. In a February 8, 2007, letter to EPA, NESCAUM indicated that the current AQI is not well designed for its current uses (e.g., addressing real-time exposures with additional messaging at lower levels approaching the standard). They stated that EPA should revisit and overhaul the AQI to address the multiple purposes it now serves and better serve public health protection (e.g., adjust the AQI to reflect shorter averaging times, account for multi-pollutant cumulative impacts, and consider additional contaminants).

Response: Structural changes to the AQI, called an overhaul by this commenter, would have to be made through a separate rulemaking.

(3) Comment: Letters from a mass comment ca mpaign sponsored by the Sierra Club, and comments from NACAA, South Caro lina, and Vermont support revisions to the AQI to conform with the new SO 2 primary standard, if it is promulgated.

Response: EPA agrees with these commenters.

(4) Comment: North Carolina strongly recomm ended that EPA establish the AQI breakpoints for SO 2 when they propose the final SO 2 NAAQS rule. North Carolina recommended that a 100 AQI correspond with the short-term SO 2 NAAQS, consistent with other NAAQS pollutants. The 50 and 150 AQI breakpoints should also be consistent with the other NAAQS pollutants. North129 Carolina is concerned that failing to adequately define the AQI breakpoints might lead to problems such as those with the PM 2.5 AQI.

Response: EPA has identified all of the ch anged breakpoints in the AQI, and has made revisions to the 50 and 150 AQI breakpoints that are consistent with recent decisions made for other NAAQS.

(5) Comment: Air quality commented that the range for the AQI 50 breakpoint is acceptable because the approach EPA used to ar rive at it is consistent with that of previous standards.

Response: EPA agrees with this commenter.

(6) Comment: NYSDEC recommended that th e requirement for hourly AQI forecasts be delayed at least one year so that the data being collected and

forecasted is shown to be reliable. In the meantime, EPA may want to make the

SO 2 AQI based on a longer interval (e.g., 24-hour) that can be more accurately forecasted (NYSDEC) EPA should provide guidance for forecasting 1-hour SO 2 data and for communicating these forecasts to the public in a timely manner.

Response: 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 cr eating and communicating an SO 2 forecast. We will work with State agencies that want to develop an SO 2 forecasting program on issues including, but not limited to, forecasting for short term periods.

(7) Comment: While Wisconsin supports changing the AQI concurrent with the new NAAQS, they believe it is impractical to incorporate SO 2 into its forecasting and public health notification program because SO 2 does not behave like a regional pollutant. Exceedances may occur with li ttle or no warning and for two hours or less. They requested EPA consider the resources necessary for public

communications at the state and local levels , particularly in areas where other air quality exceedances ar e relatively rare.

Response: 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 cr eating and communicating an SO 2 forecast. We will work with State agencies that want to develop an SO 2 forecasting program on issues including, but not limited to, forecasting for short term periods.

(8) Comment: NESCAUM commends EPA on its propos al to revise the Air Quality Index (AQI) concurrently with the SO 2 NAAQS. NESCAUM has some questions regarding how EPA envisions reporting and forecasting SO 2 concentrations for the AQI using source-oriented monitors. For example, how does EPA expect geographic regions to be delineated, given the more localized nature of a sourceoriented network?130 Response: This final rule departs from the pr oposed rule in that it allows for a combined monitoring and modeling appro ach. Because of this, the monitoring network is not required to be wholly s ource-oriented in nature. States have flexibility to allow required monito ring sites to serve multiple monitoring objectives including characterizing sour ce 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.

(9) Comment: Additionally, NACAA requests that EPA work with state and local agencies to develop a forecast model and appropriate guidance for incorporating

SO 2 into their daily air quality forecasts.

Response: We agree that there will be ne w challenges associated with creating and communicating an SO 2 forecast. We will work with State agencies that want to develop an SO 2 forecasting program on issues including, but not limited to, forecasting for short term periods.

(10) Comment: The State of Vermont APCD suppor ts the EPA proposal to revise the

SO 2 AQI concurrently with the primary SO 2 NAAQS.

The ability to provide timely and geographically relevant air quality reporting, alerts and forecasts are im portant components of public health protection. However, we have some questions regarding how the EPA envisions reporting and forecasting SO 2 concentrations for the AQI using source oriented monitors. For example, how does EPA expect geographic regions to be delineated, given the mo re localized nature of a sourceoriented network? Additionally, we al so urge the EPA to work with the states to revisit and overhaul the AQI in light of the multiple purposes it now serves, including: adjusting th e AQI to reflect shorter averaging times; accounting for multi-pollutant (i.e., cumulative) impacts; and considering additional contaminants. By so doing, public health protection

would be better served.

Response: This final rule departs from the proposed rule in that it allows for a combined monitoring and modeling appro ach. Because of this, the monitoring network is not required to be wholly s ource-oriented in nature. States have flexibility to allow required monito ring sites to serve multiple monitoring objectives including characterizing sour ce impacts, highest concentrations, population exposure, background, and regional transport. Further, EPA expects131 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.



## VII. Comments on the Process for Reviewing the SO 2 

## Primary NAAQS

(1) Comment: SAP generally commented that th ere was insufficient monitoring data for establishing a new 1-hour SO 2 standard. This comment er contends that EPA should propose to place additional monito rs by 2013 and then review whether a new 1-hour standard is required.

Response: EPA disagrees with the commente r. Using the existing monitoring network numerous epidemiologic studies have reported posi tive associations between mostly 24-hour averag e and 1-hour daily maximum SO 2 concentrations

and serious health outcomes such as hos pital admissions. We further note that CASAC has approved: 1) the science underl ying the determination in the ISA that there is a causal relationship between respiratory morbidity and short-term (5- minutes to 24-hour) exposure to SO 2 ; and 2) analyses in the REA estimating the

extent to which a 1-hour standard w ill protect asthmatics from 5-minute SO 2 concentrations of concern identified fr om controlled human exposure studies. Finally, we note EPA is legally obligated to make a decision by June 2, 2010.



## VIII. Interpretation of the Clean Air Act

(1) Comment: EPA cannot take into account costs of compliance on industries and state governments in establishing NAAQS; however it can and should take into account the effects of its po licies on the economic health of the people affected, and particularly on economically Februa ry 8, 2010 Page 9 disadvantaged groups who appear to experience asthma dispropor tionately. It should not fail to consider

the effects of that economic health a nd of economic stress (and distress) on the people’s health in the affected areas. Put another way, the transfer of production jobs available to less educated persons from the US to China, from nonattainment areas to non-classified areas, and from less educated US laborers and craftsmen to well-paid engineers may be good for world productivity or good for a working man in China or in anothe r less regulated area – outside the nonattainment area, but the economic stress is not without stress or health impact on the persons displaced or le ft behind or their children.132 Response: However deeply felt the comment er’s argument is, it has no legal merit and in fact has been rejected rep eatedly. In Whitman v.American Trucking

Associations . 531 U.S. 457, 466 (2001), the United States Supreme Court considered the argument that "the economi c cost of implementing a very stringent standard might produce health losses sufficien t to offset the health gains achieved in cleaning the air -- for example, by cl osing down whole industries and thereby impoverishing the workers and consumers dependent upon those industries." The Court continued, "That is unquestionably true, and Congress was unquestionably aware of it." The Court went on the reje ct the argument based on the statutory text: "requisite to protect the public health with an adequate margin of safety" makes no mention of cost, unlike the plethora of ot her CAA provisions which do.

The Circuit Court of Appeals for the Dist rict of Columbia Circuit has likewise rejected the argument. "AIS I next contends that EPA e rred in refusing to consider the health consequences of unemploymen t in determining the primary standards for particulate matter. This claim is enti rely without merit. In three previous cases this court has emphatic ally stated that section 109 does not permit EPA to consider such costs in promulgating national ambient air quality standards. It is only health effects relating to pollutants in the air that EPA may consider. See [section 108 (a) (2) ("Air qua lity criteria for an air pollutant shall accurately

reflect the latest scientific knowledge useful in indicat ing the kind and extent of all identifiable effects on public health or welfare which may be expected from the presence of such pollutant in the am bient air, in varyi ng quantities.")

Consideration of costs associated with alleged health risks from unemployment would be flatly inconsistent with the statute, legisl ative history and case law on this point." NRDC v. EPA , 902 F. 2d 962, 972-73 (D.C. Cir. 1990).

(2) Comment: The Clean Air Act requires that h ealth effects be projected with a reasonable degree of scientific certainty. The commenter believes that setting a 1- hour standard in the range of 50-100 ppb is not supported by the scientific evidence. The Agency’s review and util ization of the information contained in the ISA and the REA do not meet the requirements of the CAA, and EPA’s promulgation of a final standard with in the range proposed would be more stringent than necessary . (Progress Energy)

Response: EPA firmly believes that the sta ndards adopted in the final rule are well supported by the air quality criteria as set out in the ISA and REA, and has explained why in the preambles to the pr oposed and final rules, as well as this Response to Comment Document. Nor ar e the standards more stringent than necessary, given that they are establis hed at a level slightly below the 99 th
percentile 1-hour daily maximum SO 2 concentrations in the epidemiologic studies EPA reasonably considered to be especially probative.

The commenter’s statement that “health effects must be projected with a reasonable degree of scientific certai nty” under the CAA (unsupported by legal citation), is also misleading. For example, the requirement in the Act that primary133 NAAQS provide an “adequate margin of safety” specifically directs EPA to act

even if a risk is uncertain in nature or degree, or not even identified. See S. Rep. No. 91-1196, at 10 (1970); see also Amer ican Farm Bureau et al. v. EPA , 559 F. 3d 512, 532-33 (D.C. Cir. 2009) (“Although the evidence of danger from coarse PM is, as the EPA recognizes, ‘inconcl usive’, the agency need not wait for conclusive findings before regulating a pollutant it reasonably believes may pose a significant risk to public healt h....’The Clean Air Act requires EPA to promulgate protective primary NAAQS even where, as here, the pollutant’s risks cannot be quantified or preci sely identified as to natu re or degree’” (internal citations omitted)). In the situati on here, where the ISA finds a causal relationship between respiratory morbidity and short-term exposure to SO2, and support in the epidemiologic evidence and ri sk analysis for all of EPA’s decisions as to the elements of a revised standard, there is certainly sufficient certainty for

EPA to revise the standard.

(3) Comment: UARG commented that when re viewing an existing NAAQS (which previously was determined to be at the level requisite to protect human health according to the CAA), the Administrato r must provide a reasoned explanation for any conclusion that the existing NAAQS is no longer at the level requisite to protect public health with an adequate ma rgin of safety. The commenter cites in

support Motor Vehicle Mfrs. Ass’n v. St ate Farm Mut. Auto. Ins. Co. , 463 U.S. 29,42 (1983) (“an agency changing course by rescinding a rule is obligated to supply a reasoned analysis for the change beyond that which may be required when an agency does not act in the first instance”); Atchison, Topeka and Santa Fe Ry. Co. v. Wichita Bd. of Trade , 412 U.S. 800, 808 (1973) (an agency must “explain its departure from prior norms”); and AT&T Corp. v. FCC , 263 F.3d 729, 736 (D.C. Cir. 2001) (reasoned decision maki ng standard requires explanation of departure from prior decision) . The commenter continue s, “Rather, the changed

conclusions are the result of changes in EPA’s interpretation of the evidence. Only if she can supply a “reasoned analysis” for these changes can the Administrator rely on them for a change to the 1996 conclusion that effects of public health significance begin at 600 ppb, or for a reversal of the 1996 decision that no new short-term NAAQS was needed to protect public health with an adequate margin of safety. She has failed to present such an analysis.”

Response: EPA must fully explain the ba sis for its decision in this NAAQS review (including explanation of polic ies and judgments involved in those decisions, and explanations of how any changes in the science since the last review were evaluated in making a decisi on. CAA section 307 (d) (3), (d) (6) (A) and (B). EPA has done so here. EPA, however, is not bound by previous NAAQS determinations, and carries no special bur den of explanation (beyond providing a reasoned explanation) if it deviates from prior determinations. The cases the commenter cites are inapposite. The pl urality opinion in Atchison, Topeka & Santa Fe Ry. Co. involved a situation wher e the Interstate Commerce Commission, in an adjudicative proceeding, issued a rate order which deviated from prior settled adjudicative precedent of the Commission (412 U.S. at 805-06).134 The Commission had a settle principle (referr ed to in the opinion as “settled rule”) addressing the general issue involved in th e case, and attempted to distinguish the pending rate determination from that principle. The adequacy of the Commission’s explanation for a departure fr om the settled principle was the basic issue on judicial review.

The Court addressed this issue with an ove rall focus on the need for an agency to explain its decision, as the fundamental precondition for adequate judicial review. An agency needs to explain its reasoning in order for a court to determine if the decision was with Congress’ delegation of st atutory authority. In that context, an adjudicatory agency could, through a course of agency action, establish a settled rule or principle that embodies the agency’s informed judgment on the appropriate way to carry out the policies committed to it by Congress. In those circumstances a court reviewing an agen cy decision should properly presume that the agency’s settled principle embodies the agency’s view on what will best carry out the policies of Congress, calling for an adequate explanation by the agency if it departs from its own norm. The basic purpose of both the presumption and the need to explain a deviation from the settle norm derives from the obligation that an agency explain how its decision conforms to the policies committed to it by Congress. An agency may establish such a settled norm, and if it does then that norm provides an explanation for agency action in future cases. Once such a norm has been established, however, an ag ency must explain a deviation from it so the reviewing court can determine whether the agency’s decision is still consistent with the policies committed to it by Congress.

EPA does not see that this situation ha s any relevance to the periodic NAAQS review process, or that EP A established any settled rule for interpretation of SO 2 clinical studies in the la st review (much less estab lish one through a trial-type adjudicatory process) as in Atch ison, Topeka & Santa Fe Ry. Co.

To the contrary, section 109 mandates a periodic review based on an updated review of the science. Prior NAAQS de terminations, based on the then current science, do not have status as adjudicativ e precedent, “settled rule”, or settled interpretation. The required review of the air quality criteria is to ensure that NAAQS decision are based on “the latest scientific knowledge” regarding effects of the pollutant in question, and this on-going dynamic process extends to interpretations of scientific evidence.

Nor does the statement from State Farm that agencies changing an existing rule are “obligated to supply a reasoned anal ysis beyond that which may be required when an agency does not act in the firs t instance” (463 U.S. at 42) support the proposition that there is some special burden over and beyond the requirements of section 109 (b) and (d) necessary to justify a change. The action reviewed in that case was a rescission of an existing rule adopted pursuant to a statutory scheme which required that standards be practicable . That is unlike the statutory scheme of the NAAQS, which not only requires pe riodic review, but requires that those135 reviews be based on air quality criteria refl ecting “the latest sc ientific knowledge” regarding the effects of th e pollutant in question.

(4) Comment: API commented that although the Proposed Rule acknowledges that

the SO 2 REA estimated “short-term exposures and potential human health risks”

associated with recent ambient SO 2 levels, 74 Fed. Reg. at 64,821, the Administrator completely ignores those es timates in considering whether a new 1- hour NAAQS is necessary for public health protection. Those analyses, however, are very relevant for addressing the risk s in a real world context. See Whitman

, 531 U.S. at 494-95 (Breyer, J., concurring in part and concurring judgment). They indicate that exposures and risks associated with just attaining the 1-hour NAAQS that the Administrator has proposed (and th erefore apparently deems “tolerable”) 22 are comparable to those associated with recent air quality.

Response: The issue here is whether, in determining for purposes of section 109 (d) (1) whether it is appropriate to revise the current primary NAAQS for SO 2 , EPA should consider the air quality allowe d by that current st andard or existing air quality. First, it is apparent that th e provision requires review of the current standard, not current air quali ty. Second, EPA is to consider the test in section 109 (b) (1) in determining if it is “appropriate” for EPA to revise the standard (see section 109 (d)(1)). Section 109 (b) (1) requires that a primary NAAQS – not air quality at the time of a NAAQS – be requisi te to protect the public health with an adequate margin of safety. Again, it is the standard itself which is at issue. Third, section 109 (d) (1) requires EPA to consider any revise d air quality criteria in determining if it is appropriate to re vise the NAAQS. Those criteria require

consideration of “the latest scientific knowledge useful in indicating the kind and extent of all identifiable effects on public health ... which may be expected from the presence of such polluta nt in the ambient air, in varying quantities” and thus certainly authorize consideration of the effects the pollutant has at quantities allowed by the current standard. Conseque ntly, EPA is at the least allowed to consider conditions allowed by the current standard in determining if it is appropriate to revise it.

The commenters cite part of Justice Br eyer’s concurring opinion in American Trucking Assn’s, but that opinion is not applicable here. It does not discuss the question of whether EPA may consider actu al or allowable air quality in deciding whether or not to revise a primary NAAQS.

(5) Comment: NPRA, which supports comments submitted by API, said that the CAA neither requires nor supports the proposed 1-hour SO 2 NAAQS because the proposed standard is four times lower th an the lowest tested concentration in human exposure clinical tria ls and is eight times lowe r than the level at which statistically significant huma n exposure results occur. Without further substantial evidence from clinical human exposure trials, the proposed and alternative SO 2 NAAQS are “arbitrary and capricious.”136 Response: This comment ignores the differen ce in averaging times, so that the commenter’s comparison is apples to or anges. Moreover, it is entirely permissible for EPA to base determin ations as to a standard’s level on epidemiologic studies, and indeed is required to consider such studies in developing standards since they “reflect th e latest scientific knowledge useful in indicating the kind and extent of all identifiable effect s on public health ... which may be expected from the presence of [SO 2 } in the ambient air”, i.e. part of the air quality criteria. CAA section 108 (a) (2).

(6) Comment: LEC, API, and RREIE commented that the definition of ambient air quality is interpreted too br oadly to include areas that are hypothetically but not practically (or legally or safely) accessi ble to the public and that dispersion modeling is often applied to th ese areas. (LEC, API, RRI, RRI)

Response: EPA is now contemplating a hybrid approach to implementation

which includes increased utilization of dispersion modeling. See sections III-VI of the preamble to the final rule.

(7) Comment: PE, supporting the comments of UARG and EEI, commented that

EPA review and utilization of the info rmation in the ISA and the REA do not meet the requirements of the CAA. EPA’s promulgation of a final standard within the proposed range would be more stringent than necessary.

Response: EPA disagrees. The primary standard adopted in this rule is requisite to protect public health with an adequate margin of safety for the reasons given in section II.F of the pream ble to the final rule.



## IX. Comments on Implementation, Designations, and Exceptional Events

(1) Comment: Missouri believes that the Reas onable Future Progress requirement under CAA Section 171 should not be interp reted to mean a generally linear progression towards attainment. Missouri’ s previous analyses showed that many

of the SO 2 sources are coal combustion and other large single-point emitters, and EPA acknowledges that these types of sour ces will likely be responsible for most of the impact in non-attainment areas. Step-down functions are not possible with these types of sources, Missouri claims. Missouri suggested EPA consider an allin-one approach that would allow for a single control process (e.g., scrubber) on a large source to bring an area into atta inment and meet the requirements of Reasonable Future Progress, reasonable available control technology, reasonable available control measures, and conti ngency measures for this type of nonattainment area. Such an approach would only be allowed with EPA Regional Office approval upon verification that it meets statutory requirements.

Response: This final rule is not taking fina l action regarding any implementation issues that may become the subject of future SIP rulemaking actions. Missouri’s137 comments would be more appropriately ra ised in the context of a specific SIP action, and are beyond the scope of today’s final rule. Therefore, EPA is not taking a position here regarding whether it agrees with the approach stated by the

commenter above.

(2) Comment: A commenter stated that the annual SO 2 standard should not be revoked until EPA approves SIPs in a ttainment areas under the future SO 2 secondary standard, which may also be based on an annual averaging time.

Response: EPA disagrees with the comment. This rulemaking concerns only the primary standards for SO 2 . 74 Fed. Reg. at 64812 n. 2. The annual SO 2 standard is a primary standard, not a secondary standard. See 40 CFR section 50.4 (a). The exclusive secondary standard for SO 2 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 rule making, as just note d. The commenter’s request to retain the annual primary standard until SIPs reflecting a new secondary standard are approved is eff ectively a request to amend the present secondary standard, and is therefore inappr opriate 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 SO 2 concentrations well below the level of the

current annual standard. 74 FR at 64845. Thus, there would be no loss of protection to public welfar e due to revocation of the annual primary standard.



## A. Comments on designat ions of non-attainment

General comments on designa tions of non-attainment

(1) Comment: NESCAUM, NACAA, and ALA/ EDF/NRDC/SC support EPA’s proposal to keep non-attainment designa tions and subsequent requirements under

the current SO 2 NAAQS in effect until the non-a ttainment area submits, and EPA approves, a SIP for the new SO 2 NAAQS. The commenters believe this is needed to maintain the needed public health protection and regulatory coverage until a

new and workable SO 2 reduction plan is in place. ALA/EDF/NRDC/SC suggested keeping the current standards in place for at least a year during the

transition to the new standards in order to comply with CAA provisions and to avoid arbitrarily and unjus tifiably relaxing the health protections. (NESCAUM

Response: EPA appreciates the support of the commenters on the transition strategy for SO2.

(2) Comment: Pennsylvania commented that EPA must provide timely implementation rules and guidance for issues such as SILs, emergency action levels, and the updated AQI in order to ad equately protect public health and the

environment.138 Response: See preamble section V.D.2. related to NSR and PSD requirements and section VII related to the AQI which addresses this comment.

(3) Comment: Sulfuric Acid Producers commented that EPA should examine if the proper regulatory infrastructure at the state and local leve ls is ready to address an

increase in SO 2 non-attainment. EPA should al so provide guidance for LAER

and RACT for SO 2 . EPA should also address if these programs will have a material effect on reducing the negative health impacts that are claimed in the

REA.

Response: As stated in the implementation sect ion of the preamble to the final NAAQS review notice, EPA believes th at generally there are appropriate guidance and regulations currently in place to address the revised SO 2 NAAQS. However, EPA agrees that providing a dditional guidance on modeling for the new 1-hour standard is needed, and will be providing that guidance in time to help states and sources implement the new NAAQS. EPA also believes that the anticipated approach discussed in th e preamble for designating areas for SO 2 also would provide states with sufficient time to address any control plans necessary to address areas that may be violating th e revised NAAQS and to provide public health protection by helping areas that are violating the NAAQS attain the standard as expediti ously as practicable.

(4) Comment: Though EPA does not specifically indi cate an approach for setting

non-attainment area boundaries for the new SO 2 NAAQS, MIDNRE is concerned that EPA is considering using CBSAs, since EPA repeatedly mentions usingCBSAs in determining PWEI and since EPA has often designated large, multicounty areas as non-attainment for othe r pollutants. MIDNRE believes CBSAs may be too large for SO 2 designations, since EPA is treating SO 2 as sourcespecific and heterogeneous in a give n area. MIDNRE strongly recommends flexibility in setting non- attainment area boundaries, many of which should be smaller than CBSAs and maybe even smaller than counties.

Response: As stated in the designati ons section of the final SO 2 NAAQS preamble, the EPA intends to initially c onsider the county line as the presumptive nonattainment boundary for SO 2 unless the State provides justification in its recommendation as to why a different boundary would be more appropriate. Today’s final rule does not bind states or EPA to treating the county line as the nonattainment boundary, but only explains the starting point for area-specific analyses. This approach would provide States with a clear starting point yet would allow the flexibility to show that a smaller or larger boundary is more appropriate. It is also co nsistent with the approach that EPA has taken for other NAAQS.

(5) Comment: ALA/EDF/NRDC/SC commented th at EPA has historically and incorrectly made designations only when new standards are adopted. The new monitoring network is likely to identify many areas with high SO 2 levels. As139 such, in the final rule, ALA/EDF/NRDC/S C urged EPA to include a timetable for the post-2012 evaluation of th e attainment status of a ll monitored counties.

Response: T he commenters essentially claim that SO 2 areas initially designated as “unclassifiable” and which subsequently violate the revised SO 2 NAAQS need to be redesignated to “nonattainment” to ensu re that such areas take action to bring them into attainment. Section 107(d)(3) of the CAA provides EPA with discretion to redesignate areas at any time following initial designations. As discussed in the preamble to the final ru le, we believe our expected approach to implementation of the new 1-hour SO 2 NAAQS would fully address the

commenters’ concern. U nder CAA section 110(a)(1) all areas designated as

“unclassifiable” would need to submit a SI P revision that demonstrates attainment with both monitoring and modeling data, as we ll as include measures to attain the

SO 2 revised NAAQS if necessary. Under th e CAA, that SIP revision will be due three years after promulgation of the NAAQS or, June 2013. By utilizing section 110(a)(1) of the CAA, the EPA’s expected approach would result in States with unclassifiable areas implementing the revised SO 2 NAAQS sooner than if EPA were to rely exclusively upon redesignati on under section 107( d)(3) to trigger a SIP demonstrating attainment. In any even t, if States are unable to submit SIPs showing attainment by 2013, EPA has the au thority to redesignate the area as “nonattainment,” in addition to disapp roving the SIP and pr omulgating a FIP.

We anticipate that under the expected a pproach discussed in the final rule, the delay in implementation reflected in the commenters’ concerns should not occur, and that many areas in the country would be able to demonstrate attainment in

their section 110(a)(1) SIPs--the effect of wh ich would be more areas attaining the revised NAAQS as expeditiously as practicable.



## B. Comments on date

(1) Comment: UARG is concerned that the followi ng proposal is an error, with regards to how 2011 data is to be included for SO 2 designations: “2011 data must be flagged and detailed event documentation submitted 60 days after the end of a calendar quarter in which the event occurred or by March 31, 2011.” According to UARG, this March, 2011 deadline seems odd for a number of reasons: (a) it seems arbitrary, and (b) it would not allow for consid eration of a full calendar year of 2011 data (74 FR 64873/1 define s a year as a calendar year, so considering any 2011 data requires that all 2011 data be considered). UARG

recommended the deadline for detailed 2011 documentation submittal be March 31, 2012, or as an alternative, 60 days af ter the end of each calendar quarter; the deadline should be March 31, 2013, if necessary to qualify FRMs and FEMs under new specifications.

Response: The commenter notes that the inco rrect March 31, 2011 date as shown in the proposal for flagging and documen ting 2011 data should be corrected to March 31, 2012, and will be corrected in the Final Rule Preamble, Regulatory140 Text and Tables. Noting th is date correction, the Agency stated in the proposal,

“If a state intends 2011 data to be consid ered 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 even t occurred or by March 31, 2012, whichever date occurs first. Again, EPA believes th ese 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.” A deadline of March 31, 2012 is the midpoint of the first 6 months of 2012, and provides equally 3 months for states and 3 months for EPA to process exceptional events claims associated with the last qua rter of calendar year 2011. The Agency

continues to believe that three months is adequate time for a state to review and document any SO2-related exceptional even t that may occur in the last three

months of 2011. Since the initial area designations must be finalized by EPA by July 2012, the Agency does not believe it is reasonable to establish a deadline later than March 31, 2012. EPA needs sufficient time to review any statesubmitted documentation during April through June of 2012 and make final decisions regarding exceptional events claims from 2011 prior to finalizing the designations in July 2012.

(2) Comment: ALA/EDF/NRDC/SC commented that this proposal suggests that SO 2 Part D SIPs be due 18 months from the e ffective date of an area’s non-attainment designation, and that the attainment da te will be determined from the nonattainment effective date. CAA Secti on 191 requires submission of the Part D SIP within 18 months “of the designation,” and Section 192 requires attainment as quickly as possible but no later than 5 years from the non-attainment designation

date. ALA/EDF/NRDC/SC stated that in neither of these cases does the statue

allow measurement of the relevant time from the effective date of designation. Section 107 requires promulgation of desi gnations within specified timeframes, and it does not provide for delayed effective dates.

Response: The statute uses different language to establish deadlines for issuing designations under section 107 and submitting SIPs under section 192. Under section 107(d)(1)(A), states must subm it designations recommendations no later than 1 year “after promulgation of a ne w or revised” NAAQS, and under section 107(d)(1)(B)(i) EPA must promulgate designa tions no later than 2 years “from the date of promulgation of the new or revi sed” NAAQS. The courts have interpreted “promulgation” as meaning the date of signature of a final rule and its broad dissemination, so the time period under which states are to recommend designation and EPA is to issue designations does not need to await the effective date of the rule, which is later than it s promulgation. In contrast, under sections 191 and 192, the dates for starting the re quirements for nonattainment area SIP submittal and attainment are the dates “of the designation” and “of the nonattainment designation,” respectively. These provisions do not refer to the date of “promulgation,” as under section 107, but rather to the action taken, which EPA believes is reasonable to interpret as meaning the date that the state or area141 actually becomes a nonattainment area (i .e., the effective date of EPA’s designation action).



## C. Comments on data to be used

(1) Comment: NPRA, supporting API comments, noted that EPA is proposing to conclusively designate an area with a violating monitor as being in nonattainment, while exercising undefined di scretion when the opposite occurs. NPRA thinks it is unjustified to treat da ta differently when the information is valid and comes from the same type of monitors that are sited according to existing EPA regulations and guidance.

Response: First, in this rulemaking, EPA is not taking conclusive action on any area’s designation, nor did EPA propose to take such conclusive action. Second, as we explain in sections III, IV, V a nd VI of the final rule preamble, for SO 2 EPA has long taken the position that the us e of monitoring alone cannot always be relied upon to identify sources that cause or contribute to NAAQS violations.

Sulfur dioxide is emitted from sources which vary in size and stack height, causing a “patchwork” of violations--notall of which can be measured by a single, or even several, monitors. Theref ore, in order to get a true picture of ambient air conditions in an area, EPA has traditionally depended upon both monitoring and modeling data, as appropria te, to accurately c onfirm that an area is in compliance with the NAAQS. This approach to SO 2 designations is consistent with that taken in previous actions, including those referred to in the final rule preamble. In addition, the C AA itself does not prescribe the type of data by which EPA’s designation decisions may be informed, or what relative weight to give different type s of data when they lead to different conclusions. For example, CAA section 107(d)(3)(A) refers to “air quality data, planning and control considerations, or any other air quality-related considerations the Administrator deems appropriate” as fo rming the basis for a redesignation decision. This certainly w ill allow EPA to take reasonable action in response to situations where, for example, modeling da ta indicate an area is not attaining the standard, while monitoring data might not reveal a violation.

(2) Comment: In order to obviate the need for states to instal l monitors at great cost

where concentrations of SO 2 are a small fraction of the NAAQS, and to ensure that areas lacking robust monitoring data can be designated as non-attainment,

NESCAUM and VTPAC recommended that EPA allow modeling be used in conjunction with monitoring data to bette r determine non-attainment areas.

Response: See sections III and IV of the pr eamble for the discussion on EPA’s anticipated approach to the use of monitoring and modeling for the revised SO 2 standard.142 (3) Comment: In modeling compliance with a statistically-based standard, API recommended that EPA consider applying realistic emissions profiles that are based on the expected frequency distribution of emissions.

Response: EPA currently plans to develop further guidance regarding modeling demonstrations of compliance with the NAAQS. EPA is taking API’s recommendation under advisement, but today’ s final rule is not taking final action in response to API’s request.

(4) Comment: RRI and API recommend that EP A develop refined modeling tools and guidance that will not penalize s ources by being overly conservative, especially when considering combined impacts of the primary sources and background sources. It woul d be burdensome to states if highly conservative modeling results created false new non-attainment designations.

Response: EPA understands the importance of appropriately characterizing contributions from background sources , whether through representative monitoring data or through explicit m odeling of background source emissions, as part of dispersion modeling analyses fo r compliance demonstrations recognizing the new form of the NAAQS. EPA expect s to provide guidance on procedures for determining background concentrations , combining monitored background concentrations and modeled impacts to estimate the total air quality concentration to support the implementa tion phase of the NAAQS.

(5) Comment: By the 2012 deadline for determining attainment/non-attainment for

the new SO 2 NAAQS, PCA is concerned that less than three years of short-term

data will be available from the existi ng monitoring network, and even less data will be available from the proposed new monitoring network. PCA stated that it seems premature to determine attainme nt/non-attainment based on the existing monitoring network (which EPA admits is not representative of areas of maximum impact) and to then require states to prepare and submit SIPs to address non-attainment designations. It seems im prudent to go through this burdensome process while knowing that different, deeper , and more widespread emissions cuts

will be needed to address non-attain ment identified by the proposed new monitoring network.

Response: As we explain in sections III, IV, V and VI of the preamble to the final rule, we expect to follow an approach that would designate most areas in the country as “unclassifiable” in June 2012, pending receipt of additional data from

SO 2 monitors and modeling conducted following our development of further modeling guidance under the 1-hour SO 2 standard. Areas that present both modeling and monitoring da ta that support a designation of “attainment” by June

2012 and that EPA finds demonstrate attain ment would be designated as such. Areas for which monitoring or refined modeling data show sources cause or contribute to NAAQS violati ons would be designated as “nonattainment.” We believe following such an approach woul d avoid the problem identified by the143 commenter, as it would be far less depe ndent upon receipt of monitoring data

under the new network.

(6) Comment: UARG commented that non-attainme nt decisions should be based on monitors that qualify as federal reference monitors under EPA’s new specifications.

Response: EPA is not taking final action on a ny designation in this rulemaking. We expect, based on our discussed anticip ated approach, to initially designate most areas in the country as “unclass ifiable” in June 2012, pending receipt of additional data. Under section 110(a)(1) SIPs we expect states to rely upon appropriate modeling and monitoring da ta to support demonstrations of

attainment and maintena nce of the revised SO 2 NAAQS--demonstrations that will be assisted greatly by new rules that will reduce SO 2 levels nationwide.

(7) Comment: UARG, CE (supporting comment s by UARG and EE), and NRECA commented that EPA has two options for making initial attainment/nonattainment designations with regard to whatever 1-hour NAAQS it adopts. These commenters believe that first, EPA could designate all areas as unclassifiable until adequate data have been collected with the new FRM. Second, if EPA believes that many existing monitors might qualify as new FRMs or FEMs, EPA could

extend the deadline fo r attainment/non-attainment de signations by a year. These commenters argued that EPA recently used CAA § 107(d)(1)(B) to defer designations for the 2008 ozone NAAQS by a y ear while it reconsiders that rule (75 FR 2936, Jan. 19, 2010), and UARG, CE, and NRECA believe it would be just as appropriate to use C AA § 107(d)(1)(B) to extend the SO 2 designation owing to an absence of FRM or FEM data. EPA could use that year to determine which monitors would qualify under th e new FRM and FEM definitions. For those monitors that qualify, EPA could then appropriately rely on existing data to make designations.

Response: As the commenters suggest, under th e anticipated approach discussed in sections III, IV, V and VI of the preamble the EPA expects to designate most areas as “unclassifiable” pending receipt of additional air quality data and refined

SO 2 modeling from the states confirming th eir status as either meeting or not meeting the new SO 2 NAAQS. The commenters are correct that under section 107(d)(1)(B)(i), the CAA provides the EPA an additional third year to complete initial designations in the event that th ere is insufficient data on which to base designations. EPA considered this op tion, but expects that it would not be necessary to resolve the timing problem the commenters raised. Instead, we believe that an approach like that discu ssed in the preamble would allow States to develop the necessary data to support section 110(a)(1 ) SIP demonstrations of

attainment and maintenance of the new 1-hour SO 2 NAAQS that rely upon expected SO 2 reductions to be achieved by the attainment date.144 Furthermore, by designating most areas as “unclassifiable” pending receipt of

additional data under the new SO 2 monitoring rules and refined modeling, we would be enabling states to account for new national and regional rules that we anticipate will have significant SO 2 benefits.

Regarding monitoring methods, as discussed in section IV.A of the preamble to the final rule, there may be more than one FRM and associated FEMs that may provide data for comparison to the NAAQS. As a result, the current FEMs in use in the SO2 network are appropriate for continued use to provide SO2 concentrations for comparison to the NAAQS.

(8) Comment: ACC recommended that EPA ma ke attainment/non-attainment designations in 2012 based on current mon itoring data, not as unclassified in certain cases as proposed.

Response: For most areas, EPA does not expect to receive either monitoring or refined modeling data in order to issue attainment or nonattainment designations by 2012. In such situations where areas cannot be classified on the basis of available information as meeting or not meeting the NAAQS, CAA section 107(d)(1)(A)(iii) requires th at EPA issue an “unclassi fiable” designation. We expect initial design ations in June 2012 would be based on monitoring data plus refined modeling data that States choose to include to demonstrate that an area is or is not meeting the new SO2 NAAQS. Ab sent definitive data from necessary monitoring and refined modeling, the EPA expects to designate most areas as “unclassifiable.” This appr oach would still allow Stat es to request an initial designation of “attainment,” provided that in their recommendations to EPA due June 2011, they included necessary clean monitoring data, as well as refined modeling data to support a de signation of “attainment.”

(9) Comment: MSCC commented that if one three-year period at one monitor is good enough for non-attainment designati on, it should also be good enough for attainment designation, rebuttable by further failure in the future.

Response: This comment essentially makes the same claim as NPRA’s comment below. As we explained above, due to the unique characteristics of SO 2 , EPA has not traditionally relied exclusively u pon monitoring to support designations and

attainment determinations for the SO 2 NAAQS. While this final rulemaking is not taking final action on any specific ar ea’s designation, and while all future designation actions under the new 1-hour SO 2 NAAQS will be based on their own facts, as we explained in s ections III, IV, V and VI of the preamble, we anticipate using a hybrid approach in issuing designations that w ould continue to rely upon appropriate modeling and monitoring da ta to ensure that our conclusions regarding specific areas are accurate.

Furthermore, by designating most areas as “unclassifiable” pending receipt of

additional data under the new SO 2 monitoring rules and refined modeling, we145 would be enabling States to account for new and regional national rules that we anticipate will have significant SO 2 benefits.

(10) Comment: NPRA, which supports comments by AP I, said that EPA’s proposal to discriminate against monitors demonstra ting attainment is contrary to the CAA

and cannot be finalized. NPRA states that the CAA requires EPA to utilize the monitoring information that is availa ble within the statutory period for designations, and that, follo wing the promulgation of a new or revised NAAQS, the governor of each state must submit to EPA a list of all areas of attainment, non-attainment, and unclassifiable in the state, and EPA can modify the submission so long as the state is notifi ed and has an opportunity to demonstrate that the modification is inappropriate.

Response: First, in this final rulemaki ng promulgating the new 1-hour SO 2 NAAQS and associated mon itoring requirements, EPA is not taking final action on any approach toward issuing designations. EPA will issue future designations

under CAA section 107 based on the information that is before the agency at that time. We therefore disagr ee with the commenter's asse rtion that the proposed NAAQS represented a "proposal to discriminate agains t monitors demonstrating attainment," as the NAAQS proposal did not in itself propose to either accept or reject any such monitoring data for any sp ecific area. While we explain in today's notice that we expect to follow EPA's longstanding approach of relying upon both monitoring and modeling to inform future designations actions, only in such future actions will the agency's approach to this issue become final.

Second, we note that the Clean Air Act does not specify the kind of data upon which a future designation action must be based. Regarding initial designations, section 107(d)(1) makes no reference to the type of information that may inform a designation. Rather, secti on 107(d)(1) repeated ly states that EPA may issue a designation that the agency "deems a ppropriate," indica ting that EPA has substantial discretion in issuing designations. See

, e.g. , NACAA v. EPA , 489 F.3d 1221, 1229 (D.C. Cir. 2007) (characterizi ng EPA's authority to promulgate aircraft engine emission standard s under CAA section 231(a)(3) " as [it] deems appropriate " as "both explicit and extraordinaril y broad" and as leaving "a gap for the agency to fill" (emphasis in original)). In fact, in its only reference to the type of data that may inform initial design ations, section 107(d) (1)(A)(iii) uses the phrase “on the basis of available inform ation,” without restriction. Moreover, CAA section 107(d)(3)(A) provides that EPA may issue re-designations "on the basis of air quality data, planning and c ontrol considerations, or any other air quality -related considerations the Ad ministrator deems appropriate," without narrowing the type of information EPA ma y consider or specifying in what way EPA must treat any information. Thus, we do not agree that the Act supports the commenter's objection.

(11) Comment: ASC requested a detailed analysis of how predictive air dispersion modeling results will be used for atta inment/non-attainment designations under the new proposed NAAQS. For example, “how will modeling be used to146 determine impacts to at-risk people? The proposed rule needs to include a discussion of the accuracy and precision of the predicted modeling results compared to actual ambient monitored concentrations.”

Response: AERMOD, EPA’s preferred model for NSR/PSD permitting under Appendix W, 40 CFR Part 51, has been ex tensively evaluated based on 17 field study databases, several of which included hourly SO 2 monitored concentrations from operating facilities. Although AERM OD results were not meant to be compared against monitors on a "paired in time and space" basis, past AERMOD simulations, including those used for SO 2 Risk and Exposure Assessment, have shown good agreement with monitoring da ta. EPA feels that with proper emissions data, source characterizati on, and meteorological data, AERMOD results will generally compare well against ambient measurements.

(12) Comment: One commenter stated that the proposed NAAQS presents a host of complications related to nona ttainment area attainment de monstrations that will need to be fully addressed in the “Gui deline on Air Qualit y Modeling” (Appendix W, 40 CFR part 51) before implementation.

Response: EPA expects to provide technical guidance on modeling and analyses as part of SIP demonstrations. We expect that the models cu rrently required for NSR/PSD permitting under our Guideline on Air Quality Modeling (40 CFR Part 51, Appendix W) will be suitable for dem onstrating compliance with the revised

SO 2 NAAQS.

(13) Comment: The commenter states that the pr oposed rule states that AERMOD, CALPUFF, and all other guideline m odels, such as Offshore and Coastal Dispersion Model (OCD) will need to be modified to provide for the 99th percentile (4th highest), or possibly the 98th percentile (8th highest) daily 1-hour concentration per year at each model receptor.

Response: At this time, EPA plans to issue further guidance on use of the AERMOD dispersion model to accommodate the new 1-hour standard. EPA is also developing a generic AERMOD post- processor to aid in calculating the design values for criteria pollutants for new and proposed NAAQS, including

SO 2 . As with the recently published NO2 NAAQS, EPA will publish a notice on the SCRAM website regarding modeli ng for the new 1-hour SO2 NAAQS.

(14) Comment: The commenter further st ates that the proposed SO 2 standard will present much greater compliance hurdles in comparison to the present standard.

The commenter states that the manne r that background concentrations are incorporated into the modeling results w ill be critical for realistic and not overly conservative compliance demonstrations. (RRI Energy)

Response: EPA understands the importance of background concentrations and combining them with modeling resu lts for compliance demonstrations147 recognizing the new form of the NAAQS. EPA expects to provide guidance on procedures for determining backgr ound concentrations, and combining background concentrations and modeled imp acts to estimate the total air quality concentration to support the implementation phase of the NAAQS.

(15) Comment: One commenter states that the existing EPA models which are used for the implementation of the permitting program are not reliable for a short term

1-hr standard for SO 2 . The commenter further states that the current models are not capable of predicting 1-hr SO 2 impacts. (South Dakota)

Response: EPA’s regulatory model for NSR/PSD permitting under Appendix W, 40 CFR Part 51 has been extensively eval uated based on 17 field study databases, several of which included hourly SO 2 monitored concentrations from operating facilities. Since all applications of the model are based on hourly predictions, there is no information to support the comm enter’s claims and no basis to expect that model performance of AERMOD w ill not be appropriate for this 1-hour

standard.



## D. Future permitting

(1) Comment: CIBO commented on the nexus between the proposed rule and the NSR and PSD permitting programs. “A new SO 2 1-hour standard of 50-100 ppb would make it nearly impossible for indus tries to obtain perm its for any changes

that involve increased use of sulfur-bear ing fuels or raw materials. Industries require workable permitting systems in order to expand, modernize equipment, and meet future energy demands.” CIBO is disappointed th at the regulatory impact analysis does not re alistically assess this econom ic impact on the nation’s manufacturing base and jobs , and believes non-EGU point sources will bear the brunt of the proposed rule. “Overall, the new hourly SO 2 standard will be impractical when applied to the major New Source Review program.

Response: The commenter states that a new 1-hour SO 2 standard of 50-100 ppb would make it nearly impossible for industr ies to obtain permits for increased use of sulfur bearing fuels and therefore non-EGUs will bear the brunt of the revised standard. The commenter did not provide any specific examples or analysis to support these claims. Furthermore, EPA does not construe the CAA to allow the Agency to take such implementation considerations into account when establishing NAAQS to protect public health with an adequate margin of safety. The CAA and EPA’s implementing rules ge nerally require compliance with or minimal impact on the NAAQS as a condition of PSD permitting. For nonattainment NSR permitting the CAA provide s that, as a prerequisite to being permitted to construct and operate a new or modified stationary source, increased emissions from such source be offset by reductions in existing emissions so as to ensure reasonable progress toward attainment of the NAAQS.148 (2) Comment: ASARCO, supporting comments by API, UARG, and NMA, commented that three years of on-site me teorological data would be required to demonstrate compliance with the three- year average NAAQS, which, in their opinion, is unreasonable for typical industr ial projects that are time sensitive.

Response: The definition of the form of the 1-hour SO 2 NAAQS, based on a three-year average, is in relation to ambient monitoring data, and has no implication with regard to the length of meteorological data period required for dispersion modeling. The recommenda tions in Section 8.3.1.2 of EPA's Guideline on Air Quality Models (40 CFR Part 51, Appendix W) regarding the length of meteorological data record for dispersion modeling would apply for the

new 1-hour SO 2 NAAQS. That provision states that “[t]he use of 5 years of NWS meteorological data or at least l year of site specific data is required."

(3) Comment: ASARCO, supporting comment s by API, UARG, and NMA, commented that in order to implement the PSD program, EPA should adopt SILs such that facilities can determine if a pr oject is “significant” from an air quality perspective. “The development of SILs and other appropriate thresholds has often lagged far behind the implementation of the new NAAQS, resulting in significant implementation difficulties in obtaining PSD permits.”

Response: EPA agrees that the SIL is an us eful screening tool for implementing the PSD requirements for SO 2 and all other regulated NSR pollutants. EPA plans to undertake rulemaking to establish a 1- hour SO2 SIL as a screening tool to facilitate implementing the 1-hour SO 2 NAAQS in the PSD permitting program. In addition, we are considering provid ing guidance which could include an

interim SIL value for SO 2 that could be used by state permitting authorities while the final rule for a SIL is being completed, provided that the states establish an appropriate record for individual pe rmitting actions based on the supporting technical information provided by EPA show ing that a particular screening value reflects a de minimis impact level.

(4) Comment: ASARCO, supporting comment s by API, UARG, and NMA, commented that if the 1-hour NAAQS is less than the 3-hour or 24-hour NAAQS,

these increment values will potentially exceed the 1-hour NAAQS. This contradicts CAA Section 163(b)(2) as it rela tes to PSD increments – that section

lists allowable increases in SO 2 concentrations above th e baseline concentration.

Response: EPA acknowledges that, because of the stringency of the new onehour SO 2 NAAQS, the 3-hour and 24-hour SO 2 increments allow greater

increases in ambient concentrations of SO 2 than does the new 1-hour SO 2 NAAQS level. The CAA clearly provide s that the amount of deterioration allowed by any increment cannot cause th e NAAQS to be exceeded. Therefore,

the new 1-hour SO 2 NAAQS would govern the amount of air quality deterioration

that would be allowed in an area over the 3-hour and 24-hour averaging periods.149 It should also be pointed out that because the existing 3-hour and 24-hour

increments for SO 2 are defined by the CAA at section 163(b), EPA cannot alter those increments without prior congression al action authorizing EPA to make any necessary changes. EPA does not interpret section 163(b) as establishing a right to degrade air quality to the level of the increments that would preclude EPA from establishing the 1-hour SO2 NAAQS level established in this action. See,

National Corn Growers Ass’n v. EPA , 291 F.3d 1, 12 (D.C. Cir. 2002).

(5) Comment: CRA commented that a stringent short-term SO 2 standard greatly increases the difficulty of modeling to de monstrate compliance, since the standard that these plants are held to may not nece ssarily reflect the annual or daily average emission rates of affected facilities. ) CRA said that the current SO 2 increment standards are appropriate for disper sion modeling requirements under the new source review program, and that it would be unnecessary to promulgate a 1-hour

PSD increment for SO 2

.

Response: Section 166(a) of the CAA authorizes EPA to promulgate “regulations for the prevention of significant deteri oration” for pollutants for which NAAQS are promulgated after 1977. Thus, we be lieve that it would be appropriate to

consider the need for a short-term increment for SO 2 consistent with the 1-hour averaging period for the new SO 2 NAAQS. Historically, EPA has developed increments for each applicable averaging period for which a NAAQS has been promulgated. However, the court determined in earlier NO 2 increment litigation that increments for a particular polluta nt do not necessarily need to match the averaging periods that have been estab lished for NAAQS for the same pollutant. Environmental Defense Fund, Inc. v. EPA , 898 F.2d 183, 189-190 (D.C. Cir. 1990)(“... the ‘goals and purposes’ of the PSD program, set forth in § 160, are not

identical to the criteria on which the am bient standards are based.”) We intend to

evaluate the need for a 1-hour SO 2 increment in a subsequent rulemaking.

(6) Comment: DRSI, supporting the comments by UARG, commented that if EPA intends to specify new PSDs, SILs, SE Rs, and SMC requirements specific to a new 1-hour standard, then it should c onduct a separate notice-and-comment rulemaking to allow adequate opportun ity for public review and input.

Response: EPA does intend to use a separate rulemaking to propose screening

tools for the new 1-hour SO 2 NAAQS that would be esta blished in regulations. As described in our response to an earl ier comment, we are also considering the development of guidance that suggests an interim SIL for SO 2 that state permitting authorities may use implement the new 1-hour SO 2 NAAQS if they establish an appropriate record. We recognize the importance of public participation in the ultimate development of any SIL that is used in permitting. An interim SIL reflected in any EPA guidance should be subject to public comment whenever it is applied by a pe rmitting authority in permitting action. The public would also have an opportuni ty to comment on any permanent SIL EPA proposed to establish by rulemaking.150 (7) Comment: Alexandria commented that it is fa lse to say that the SIP and NSR programs are effective means of addressi ng the public health impacts of existing large stationary sources that violate th e NAAQS. First, many of the “oldest, dirtiest pollution sources” are essentially perpetually grandfathered from the requirement to operate with the controls required to ensure compliance with the NAAQS (while new sources must adhere to strict procedures and controls). Second, recent changes in the federal NSR program have allowed large stationary sources significant flexibility in determining what time periods constitute their baseline. The result of the above two poi nts is that the reliance on the SIP-based monitoring method of NAAQS implementati on practically guarantees that many thousands to millions of people throughout the country will continue for many years to suffer the health effects of fr equent and chronic exposures in excess of the short-term NAAQS. This may fo llow many years, possibly decades of exposure from the emissions of proximate single sources at levels exceeding those corresponding to adverse health effects. Third, when emissions from existing major sources are shown by themselves to violate health standards, vast resources must be committed. For example, the NSR program (both major and minor, and its requisite ambient air quality standard s compliance demonstration) is triggered only when a physical modification at a s ource leads to annual emission increases. However, many large electricity-generating units and other stationary sources use physical modifications to modify thei r operating conditions, often to improve financial performance. These physical m odifications can increase plant output, though annual emissions decrease or do not change from historical baseline.

Response: EPA generally agrees with the comme nt that the NSR program applies to existing sources when they modify the existing source (e.g, modify existing units or add additional units) and that the NSR program does not specifically address existing violations of the NAAQ S by un-modified existing sources. However, another provision of the CAA, section 110(a)(1) an d (a)(2)(A) requires that the state implementation plans dem onstrate compliance (including schedules and timetables) with the NAAQS for exis ting sources. Such provisions are not limited to the regulation of only new sour ces or sources that propose to modify their existing facili ties. In addition, areas that ar e found to be in violation of the NAAQS may be required to control those sources that are determined to be contributing to violations of the NA AQS through designation of the area as nonattainment, or through the implementation of a SIP call for the affected area. In these cases, states are required to a dopt appropriate controls, or emissions limits, on the affected sources in order to bring about attainment in the area.

(8) Comment: Alexandria commented that EPA should not rely solely on any number of locally-cited monitors. This is because (a) the SIP process, and its categorical source control measures, is designed for assuring that regional pollutant levels comply with NAAQS, and (b) monitoring methods cannot fully and comprehensively identify all area s of non-compliance caused by a single source’s maximum potential emissions fo r all possible loca l meteorological151 conditions. A SIP remedy to a NAAQS c ontravention by a single source may take at least eight years, given the following timeline for an area for which conditions indicate a potential violati on of the new NAAQS by a particular source: (1) regulators might assign a m onitor(s) to the area, (2) public money would need to be expended for an air qua lity monitoring analysis, ensuring that those monitors be correctly located at the discrete points where emissions and meteorology together produce impact s exceeding the NAAQS, (3) after three years of modeling, a non-attainment area is defined, and (4) after another year,

control measures commensurate with th e NAAQS would be stipulated through the SIP (Alexandria) Instead, Alexandria requests that EPA propose and implement a streamlined procedure by which local ai r quality agencies can petition a single major source to demonstrate NAAQS comp liance using standardized modeling techniques prescribed by the “Guideline.” These results would then be used to determine an emissions control regime for a source that is NAAQS compliant.

Alexandria stated that EPA has invest ed significant resources in developing standard modeling tools, preprocessors, and guidelines, such that an existing major source should now readily be ab le to calculate how their maximum potential emissions affect near by ambient pollutant levels.

Response : In sections III, IV V and VI of the preamble to the final rule, we discuss our anticipated a pproach to implementing the new 1-hour NAAQS. We expect that many, if not a ll of Alexandria’s concerns , would be satisfactorily addressed by such an approach.

(9) Comment: ALA/EDF/NRDC/SC commented that it is unlawful to use SILs and/or SMCs to waive or limit the CAA requirements for air quality impact analyses. For SILs: The CAA unequi vocally mandates that a proposed PSD

source or modification must demonstrate that its emissions “will not cause or

contribute” to a violation of increments or of any NAAQS. “No major emitting facility” may be constructed unless it m eets this requirement, and the emissions from a facility may not cause or contri bute to an exceedance of “any” increment “for any pollutant in any area” or to an exceedance of “any” NAAQS. The statute requires that each permit applicant must conduct an air quality analysis, including obtaining continuous air quality monitori ng data, which is “gathered for the purposes of determining whether emissions from such facility will exceed the

maximum allowable increases or the ma ximum allowable concentration permitted under this part.” For SMCs: Section 165( e)(2) mandates a full year of continuous air quality monitoring for each major sour ce subject to the PSD program, without exception, other than a limited provision that allows for less than a year’s worth of monitoring based on a determination “that a complete analysis for such purposes may be accomplished in a shorter period.” ALA stated that exception “hardly amounts to the aut hority to waive monitoring entirely,” which, in their opinion, is what SMCs do.

Response: The EPA disagrees with this commente r’s claim that it is unlawful to use SILs and SMC as screening tools to provide a reasonable implementation152 approach for PSD/NSR purposes. We believe that the concept of a SIL, or SMC, is grounded on the de minimis principles described by the court in Alabama Power v. Costle, 636 F.2d 323, 362 (D.C. Cir. 1979) (Alabama Power) . In that case reviewing EPA’s 1978 PSD regulations, the court recognized that “there is likely a basis for an implication of de minimis authority to provide exemption when the burdens of regulation yield a gain of trivial or no value.” Alabama

Power at 360. Additional discussion of EPA’s legal authority to promulgate SILs and SMCs are provided in 72 Fed. Reg. 54112, 54139-41 (Sept. 21, 2007).

(10) Comment: Missouri commented that the revi sed 3-hour, 24-hour, and annual SO 2 increment standards could be addresse d in a way similar to how Congress

resolved the PM 10 issue, though the issue is not quite the same since the PM 10 issue involved a new size distinction. Based on the PM 10 issue, though, Congress’ intent is clearly only to require increment standards for pollutants and averaging times where the public or environment n eeds protection. It does not make sense

to continue to include an increment eval uation for an averaging time that does not exist. It is very difficult to believe that a standard that is prop osed to be tightened

to this extent could be allowed to dete rmine the issuing of permits to sources using the same principles that were adopt ed under the current standard. The more difficult question is, according to Missouri, "What is the correct course of action for areas that have already triggered a minor source ba seline and have violated one of the existing increment standards or are near an increment violation?" Another critical issue, in their opinion, is the implem entation of new significant impact levels, significant monitoring concentrations, and significant emission

rates for the new standard.

Response : As described in an earlier response, the CAA explicitly defines

increments for SO 2 that EPA cannot alter withou t prior authorization from Congress. EPA recognizes that there may be little need for annual and 24-hour

SO 2 increments following the revocation of the annual and 24-hour NAAQS. Nevertheless, these increments must remain intact until and unless Congress

decides to amend the CAA to itself ma ke, or authorize EPA to make, any necessary changes.

(11) Comment: Dow, supporting ACC and API co mments, recommended that EPA clarify that compliance with the new standard should be determined only by the ambient monitoring network. Dow clarified that state permitting agencies do not have to require regulated entities to conduct air dispersion modeling unless required by a review of the PSD air permit application.

Response: Under the anticipated implementation approach discussed in sections III, IV, V and VI of the preamble to the final rule, EPA expects to rely upon both monitoring and refined dispersion m odeling to inform designations and determinations of whether implementati on plans show attainment of the new 1- hour NAAQS. We note that section 110(a )(2)(K) of the CAA provides that the153 Administrator may prescribe the use of ambient modeling for the purpose of predicting the effect on am bient air quality of any (emphasis added) emissions for which there is a NAAQS. Therefore, the use of ambient modeling need not be limited to only sources that must meet th e PSD requirements. A prerequisite to the issuance of any construction permit fo r a stationary source is that it

demonstrates that its emissions do not cause or contribute to a violation of any NAAQS. See, e.g., 40 CFR 51.160(a), (b), (f). M eeting this requirement may not in all cases require a modeling demonstra tion; however, we have in the past concluded that modeling is sometim es necessary to demonstrate NAAQS compliance as part of the issuance of even minor source permits, and we expect

this to continue to be true. EPA’ s implementing rules for applying ambient modeling [see 40 CFR part 51, Appendix W, Guideline on Air Quality Models] contain the specific modeling requirements for existing and new sources. States

are authorized under the SIP to use ambient modeling for these purposes. Nothing in the CAA suggests that only ambient monitoring, instead of modeling, must be used to determine source imp acts and compliance with the NAAQS. The rules at 40 CFR 51.160 (f) requiring state pl ans to contain legally enforceable procedures also specify that the modeli ng meet the requirements of Appendix W. See 51.160 (f).



## X. Responses to Signific ant Comments on Appendix T (Interpretation of Primary NA AQS and Exceptional Events Rules)

A. Comments on data complete ness and data substitutions

(1) Comment : Iowa commented that they woul d prefer to couple a lower NAAQS level with simple design value calculati ons that are transparent to the public, rather than couple a higher NAAQS level with a complicated design value. As

EPA moves towards one-hour standards fo r many gaseous pollutants, it is desirable to standardize the design value calculations across the pollutants. Iowa recommended using a quarterly data captu re of at least 75% to produce a valid

annual 98 th or 99 th percentile for design value calculat ions. The final form of the

standard must be robust so as to more effectively control air pollution. According to Iowa, regulating the tails of the SO 2 distribution will cause great year-to-year variability in attainment, and the mo deling hot spots that dictate monitor placement may also be diffuse and poorly defined when regulating the SO 2 distribution tails.

Response: EPA agrees with the comment on data completeness. The final rule incorporates this completeness requireme nt, but also provides for two diagnostic

data substitution tests for use with data se ts that do not meet the 75% requirement. These tests are complicated and may not be perceived as transparent to all the public, but EPA believes that this complex ity is necessary and appropriate given154 the usefulness of the tests in allowing at least some areas to be clearly determined to be meeting or not meeting the NAAQS despite the data incompleteness.

(2) Comment: NYSDEC does not believe that data substitution should be employed to make attainment/non-attainment designations.

Response: This issue is discussed in th e NFR Preamble text, Section VII, Appendix T--Interpretation of the Primar y NAAQS for Oxides of Sulfur and Revisions to the Exceptional Events Rule, Interpretation of the NAAQS for Oxides of Sulfur (Section VII.A). See also the response to the above comment.

(3) Comment : UARG and DSRI agreed that there should be completeness criteria for compliance data, but disagree with EPA’ s suggested approach. UARG and DSRI argued that the data completeness rule should include a system for determining when incomplete data should be used, rather than giving discretion to the Administrator (such discretion is in conflict with the current data completeness rule and opens the possibility of seemingl y arbitrary decisions). Since only 75% of the expected data are required for completeness, when hypothetical data are substituted for missing data, enough substitute d data should be used to bring the expected data to 75% rather than incl uding enough substituted data to bring the expected data to 100%. The goal of the monitoring program should be a high degree of accuracy in ambient SO 2 measurements, and this accuracy will increase not just with an increasing pe rcentage of expected data but also with an increasing percentage of actual monitoring data.

Response: The issue of substituting data up to 75% versus 100% completeness as part of the diagnostic data substitution tests is discussed in the NFR Preamble text, Section VII, Appendix T--Interpreta tion of the Primary NAAQS for Oxides of Sulfur and Revisions to the Excepti onal Events Rule. The commenter also disagrees with a proposed provision allowi ng the Administrator to use incomplete

data even in a situation in which the data substitution te st does not produce a validated design value. The final rule retains this provision, which also is a feature of the data interp retation appendices for the PM 2.5 and the 1-hour NO 2 NAAQS. EPA believes that situations may arise in which using incomplete data is appropriate but which cannot be anticipated at this time. EPA notes that the public will have opportunity to comment on an y such use of incomplete data that has regulatory consequences for a state, because EPA provides public notice prior to any final designation, clean data dete rmination, or SIP approval/disapproval

action.

(4) Comment: Wisconsin commented that EPA shoul d consider allowing data to be qualified rather than invalidated for certain quality control failures like baseline

drift and some calibration verification fa ilures. Wisconsin agreed that some situations may call for using data substitution, but there may be many situations where the data gatherer/analyst notices a data bias that can be adjusted for, leading to a better estimat e of actual concentrations versus using a data155 substitution procedure. For example, a verification standard may exceed limits by a couple percent when the sample concentr ations are near baseline–that is, the magnitude seems reliable but the absolute co ncentration is questionable. In this case, under the current data handling procedures, this important qualifying information would be lost. Wisconsin pr oposed that the data analyst be allowed to build in any necessary ma rgin of error to the data.

Response: Section 2.a of Appendix T says that all data required to be submitted to AQS meeting requirements of part 58 shall be used in design value calculations.

The situations and alternative courses of action described in the comment arise prior to submission of data to AQS, and are not addressed in Appendix T. Such issues are addressed by EPA monitoring gui dance, which is outside the scope of this rulemaking.

(5) Comment: Several commenters expressed support for EPA’s proposed 75% completeness requirement for daily and quarterly data.

Response: EPA agrees with the comment on data completeness. The NFR does what the comment suggests.

(6) Comment: North Carolina and AirQuality e xpressed support for EPA’s proposed substitution criteria, although AirQuality onl y explicitly approves of the criteria if

the form of the standard is the 4 th highest concentration.

Response: This issue is discussed in th e NFR Preamble text, Section VII, Appendix T--Interpretation of the Primar y NAAQS for Oxides of Sulfur and Revisions to the Exceptional Events Rule.

(7) Comment: Iowa “do not favor the provisions in the proposed rule that disallow substitution of secondary monitor data fo r primary monitor data except in cases when the primary monitor is permanently removed from the site” because it will decrease data quality and consequently recommended that the “provision be

removed from the final rule.”

Response: This issue is discussed in th e NFR Preamble text, Section VII, Appendix T--Interpretation of the Primar y NAAQS for Oxides of Sulfur and Revisions to the Exceptional Events Rule.

(8) Comment: Iowa recommended EPA allow “sta tes to upload hourly data from multiple monitors to the AQS database” in order to “construct a composite data set in AQS for which each hourly value repr esents the average of the valid hourly data collected for each instrument at the monitoring site. The data from the composite data set could then be used for computation of design values.”156 Response: This issue is discussed in the NFR Preamble text, Section VII, Appendix T--Interpretation of the Primar y NAAQS for Oxides of Sulfur and Revisions to the Exceptional Events Rule.

(9) Comment: MIDNRE, concerned about the inherent instability of a short-term SO 2 standard (1, 3, or 8-hou r), provided comments r ecommending EPA modify the methodology for siting SO 2 monitors so that state agen cies can “average monitors within a specified area and compare the averaged level to the standard for attainment purposes or to classify a s ubset of data as not applicable for comparison of the NAAQS, in the same wa y that the original fine particulate network was set up.” MIDNRE also expre ssed concern that “both the PWEI and

contribution to national SO 2 emissions are emission based, and thus, redundant. The redundancy creates an added, unn ecessary financial burden by requiring

additional monitors. For states with a high contribution to the national inventory as well as many large urban areas, the number of monitoring sites and the short implementation time line would create an excessive burden.”

Response: The commenter makes two suggestions said to be tied to a concern that the form and level of the proposed standard may cause some sites to flip in and out of attainment. The commenter provi ded a graph showing the variation of the three-year averages of various 1-hour SO 2 metrics from 1993 to 2007 for a monitoring site in Detroit, MI. It is not clear whether the "1st high" and other metrics shown in the graph are based on all hours of the year, or on daily

maximum 1-hour values.

EPA's first response is that the graph provided by the commenter does not show a great deal of instability of the 4th hi gh metric, which (assuming the graph is presenting the 4th high daily maximum valu e) is essentially the same as the proposed 99th percentile form of the standard. The value of this metric ranged between 40 and 80 ppb during this time pe riod, and was appreciably outside the 40 to 60 ppb range only in the three 3-year periods that included data from 1999. EPA recognizes that while using a 3-year average reduces the instances of areas flipping in and out of attainment, there like ly will always be some such cases in areas whose long term average air quality is close to the standard such that a single particularly bad year causes the design value to exceed the standard for three successive years.

The commenter's first suggestion is that st ates be allowed to "average monitors" within a specified area and compare the averaged levels to the standard for attainment purposes. It is not clear wh ether the suggestion envisions averaging

same-hour concentrations across sites befo re identifying the 99t h percentile (for example) value of the daily maximum 1-hour concentration, or averaging the three-year average of annual 99th percen tile values across sites. The first approach has the potential to greatly reduce the level of protection afforded by a NAAQS set at a given level, because in a ny single hour the extent of the area of high SO 2 concentration may be quite limited such that averaging across sites157 (even sites not widely separated in monitoring network terms) could result in an average value substantially below the actual concentration to which some individuals may be exposed. The second approach could have a lesser tendency to reduce the level of pr otection, if the sites being averaged have similar transport/dilution relationships to the same dominant SO 2 emission source(s) so that they experience a similar distributi on of 1-hour concentr ations during the year although not matched across sites hour-by-hour. However, the second approach could still tend to result in a reported concentration below the highest concentration to which some people may be exposed. EPA sees no advantage to such an approach. EPA notes that if sp atial averaging were applied to monitoring data, the Administrator would have to c onsider this feature when selecting the level of the NAAQS. EPA also notes that in any case a spatial averaging approach could only be appropriate and re levant if multiple monitors are used near single sources, which EPA in genera l believes would often be a poor use of monitoring resources given the number of sources with potential to cause violations of the NAAQS.

The second suggestion is that a subset of m onitors be classified as not applicable for comparison to the NAAQS, and the co mmenter makes reference to the fine

PM network. The 1997 final rule for the PM 2.5 NAAQS established two new

standards: an annual standard based on PM 2.5 concentrations from single or multiple community-oriented monitors; a nd a 24-hour standard based on 24-hour PM2.5 concentrations at population-oriented monitors within an area. 71 FR 61144, 61146, 61164-65 (October 17, 2006). Also see 40 CFR 58.30. This form

of the annual standard was intended to characterize area-wide PM2.5 concentrations, in conjunction with a 24-hour standard designed to provide adequate protection against localized peak or seasonal PM 2.5 levels. The 24-hr standard was deigned to provi de protection for people residi ng in or near localized

areas of elevated concentrations. Id. These provisions were based largely on the specific evidence before EPA in that NAA QS review, which was focused largely on epidemiologic evidence with only limited evidence from clinical studies. EPA made various public health policy judgme nts concerning the a ppropriate level, averaging time, and form of the PM 2.5 standards in light of that evidence.

The body of evidence in this SO 2 review is quite different, including important clinical evidence as well as epidemiologic evidence. This body of evidence did not lead EPA to propose provisions con cerning community-oriented monitors or population oriented monitors as a way to provide appropria te public health protection from short-term exposure to SO 2 . The commenter raises concerns over the stability of a 1-hour SO 2 standard; however the monitoring provisions for the

PM 2.5 NAAQS described above were not base d on concerns over the stability of the standard. The issue of the stability of the standard is important, and it is

addressed in this review thr ough the use of a percentile fo rm and a 3 year average. Commenter presents no evidence or ar gument supporting use of a different approach to address the issue of stability of the standard.158 

## XI. Responses to Significant Comments on the Regulatory Impact Analysis

(1) Comment : Benefits from direct exposure to SO 2 are small; most of the benefits comes from emissions of particulate matter. (CIBO, AECT, South Carolina Chamber of Commerce, PE, EEI, UARG).

Response: EPA concurs that the mone tized benefits of reduced SO 2 exposure appear small when compared to th e monetized benefits of reduced PM 2.5 exposure. This result is consistent with other recent RIAs, where the PM 2.5 cobenefits represent a large proportion of total monetized benefits. This result is amplified in this RIA by th e decision not to quantify SO 2 -related premature mortality and other morbidity endpoints due to the uncertaintie s associated with estimating those endpoints. Studies have shown that there is a relationship

between SO 2 exposure and premature mortality, but that relationship is limited by potential confounding. Because prematur e mortality generally comprises over 90% of the total monetized benefits, this decision may substa ntially underestimate

the monetized health benefits of reduced SO 2 exposure.

EPA does not concur with the characterizati on of the benefits be ing restricted to only the monetized SO 2 -related health benefits. As there are several other categories of benefits associated with the SO 2 reductions, EPA believes it is appropriate to compare the total monetized benefits with the costs. It is inappropriate to consider only one category of monetized benefits in isolation of the other benefits anticipated from the rule.

(2) Comment : At the low levels of exposure be ing studied, co-pollutant impact may be improperly attributed to SO 2 . (AEPSC, UARG)

Response : EPA concurs that co-pollutants present in the ambient air may

contribute to the health effects attributed to SO 2 in single pollutant models, which could lead to overestimating the SO 2 risks if those co-pol lutants are highly

correlated with SO 2 . Where available, we have selected multipollutant effect estimates to control for the potential c onfounding effects of co -pollutants; these include NYDOH (2006), Schwartz et al. ( 1994) and O’Conner et al. (2007). Because the majority of the m onetized benefits are from PM 2.5 -related premature mortality and we did not estimate SO 2 -related mortality, it is unlikely that potential confounding would have a substa ntial effect on the total monetized

benefits.

(3) Comment : Very few benefits come from reducing exposure to SO 2 ; virtually all of the benefits come from reduced exposur e to particulate matter. PM co-benefits159 should not be addressed in this RIA. (CIBO, PE, South Carolina Chamber of Commerce, EEI. UARG).

Response : EPA does not concur that the PM 2.5 co-benefits should excluded from

the total monetized benefits. Because SO 2 is a precursor to PM 2.5, reducing SO 2 emissions in the projected non-attainment areas will also reduce PM 2.5 formation, human exposure and the incidence of PM 2.5 -related health effects. The PM cobenefits are directly rela ted to the reduction of SO 2 emissions, not ancillary

controls installed to reduce PM.

Reducing SO 2 emissions is associated with a variety of benefit categories, only some of which we were able to quantify and monetize in this analysis. In this analysis, we monetized the bene fits associated with some SO 2 -related morbidity endpoints and PM-related mortality and morbidity endpoints, but we did not

monetize the benefits associated with SO 2 -related mortality, acid deposition, mercury methylation from su lfate, or visibility.

The goal of an RIA is to provide a comp rehensive estimate of all the anticipated

costs and benefits associated with a re gulatory action, regardless of the underlying justification for the action. Co-benefits that occur as a result of a regulatory action are appropriate to include in the RI A, and it is appropriate to compare the

all of the monetized benefits with the cost s. It is inappropria te to consider only one category of monetized benefits in isol ation of the other benefits anticipated

from the rule.

(4) Comment : Health benefits from PM contro ls are “double-counted” with the PM NAAQS and other rules. (UARG, Pepper Hamilton, AA).

Response : EPA does not concur that the PM co-benefits estimated for this rule are “double-counted” with the PM NAAQS. The emission reductions estimated for NAAQS analyses are incremental to those estimated for previous NAAQS analyses. Reducing SO 2 also reduces PM 2.5 formation, human exposure and the

incidence of PM 2.5 -related health effects, regardless of the rule that requires the reduction. Because the control strategies to attain the NAAQS are illustrative, there may be unavoidable degree of overlap with rules that will be promulgated subsequent to this rulemaking. However, both the costs and benefits would be

double-counted in that scenario.

Note also that EPA models PM 2.5 -related premature mortality using a nothreshold, log-linear model, which is consistent with the conclusion in the Integrated Science Assessment for Pa rticulate Matter (EPA, 2009c). The

emission reductions from this rule occur in areas with varied concentrations of

PM 2.5 , including both regions that are in attain ment with fine particle standard and would not be required to develop control st rategies for the fine particulates, and

those that would not be in attainment.160 (5) Comment : There is a NAAQS underway for fine particulates; if virtually all the benefits are from PM, then control PM and not SO 2 . Claiming PM mortality as a

benefit in the SO 2 NAAQS is misleading and misplaced. (RMA, PCA).

Response : The level of the SO 2 NAAQS is determined on the basis of the health

effects associated with SO 2 exposure. Emission reductions from other rules such as the PM NAAQS may help an area ach ieve attainment, but those emission

reductions do not affect the health basis for the SO 2 NAAQS.

(6) Comment : Consideration of PM co-benefits may be appropriate for longer-term standards with longer averag ing periods; it is inappropr iate to consider those

benefits in a short-term standard based on short-term effects. (CRA )

Response : EPA does not concur that it is in appropriate to consider PM cobenefits for a short-term SO 2 standard. Although the averaging period of the

standard could affect the emission reduc tions needed to meet the standard, the averaging period of the sta ndard is irrelevant to PM 2.5 formation from those SO 2 emissions.

(7) Comment : EPA’s assertion that there woul d be high PM mortality is among the least justified of all EPA’s impact assumptions. (PCA).

Response : EPA does not concur with the characterization of PM-related premature mortality as the “least justif ied” assumption. After evaluating the body of scientific literature, the Integrated Science Assessment for Particulate Matter concluded that the relationship between both short-term and long-term exposure

to PM 2.5 is causally associated with premature morality (EPA, 2009c).

(8) Comment : The total monetized costs exceed the benefits from reducing SO 2 emissions by a significant amount. ( Pepper Hamilton, NPRA, AECT, TAB, EEI, UARG)

Response : Total monetized benefits of the SO 2 standard exceed the costs by a substantial margin. EPA believes it is in appropriate to consider only one category

of monetized bene fits (direct SO 2 health impacts) in isola tion of the other benefits (particulate matter health impacts) anticipated from compliance with the standard.

(9) Comment : The RIA may substantially undere stimate the health benefits of reducing exposure to SO 2 in cities like New York by relying on national data to

estimate base incidence rates

. (City of New York)

Response : EPA concurs that the PM 2.5 co-benefits are based on national averages,

and thus do not reflect local variab ility in population density, meteorology, exposure, baseline health incidence rates, or other local factors. This methodology might lead to underestimates in some locations such as New York City and overestimates in other locati ons, but EPA believes that the overall161 national estimate is not biased. (Based on projections, EPA estimates that New York City would attain the alternate st andards without additional controls.)

(10) Comment : Any revision to the SO 2 NAAQS that makes those standards more stringent will result in in creased cost of electricity to cooperative consumerowners. Because cooperatives have a disp roportionate amount of fossil-fuel fired generation when compared to the electri c utility industry as a whole, these negative impacts could be disp roportionately higher for electric co-ops and their consumer owners – particularly in rural areas. (NRECA).

Response : In the RIA for the proposal, EPA included an economic impact analysis of the impact of the illustrative control strategies for areas to meet the 50 ppb alternative standard. That economic im pact analysis shows that the electric power industry will only experience annuali zed costs of less than 0.4 percent of total industry revenues in 2020. While we do not provide impacts for individual power plants and their owners, the impact estimates do suggest that, at a broad scale, the electric power indus try, as affected by illustrativ e control strategies such as those included in this RIA, will not experience substantial impacts from compliance with this NAAQS.

(11) Comment : This RIA does not realistically assess the economic impact on the nation’s manufacturing base and jobs. Non- EGU point sources will bear the brunt of the proposed rule and the economic im pact on existing industrial facilities, including potential job losses, has not been accounted for in the RIA. (CIBO).

Response : In the RIA for the proposal, EPA included an economic impact analysis of the impact of the illustrative control strategies for areas to meet the 50 ppb alternative standard. That economic im pact analysis shows that industries with non-EGU point sources affected by th e illustrative contro l strategies will only experience annualized costs of less than 0.01 percent of total industry revenues in 2020. While we do not provide impacts for individual facilities and firms, the impact estimates do suggest that , at a broad scale, industries potentially affected by illustrative control strategies such as those included in this RIA will not experience substantial impacts fr om compliance with this NAAQS.

(12) Comment : Neither the Proposed Rule nor its accompanying Regulatory Impact Analysis (RIA) discuss the Proposed Rule’s potentially significant economic impacts on sulfuric acid plants. Neither document attempts to compare those economic impacts to the relatively sma ll contribution of the sulfuric acid manufacturing industry to ambient SO 2 . (Ad Hoc Committee of Sulfuric Acid

Producers ).

Response : In the RIA for the proposal, EPA included an economic impact analysis of the impact of the illustrative control strategies for areas to meet the 50 ppb alternative standard. That economic im pact analysis shows that industries with non-EGU point sources such as the chemical manufacturing industry (which162 includes sulfuric acid plants) that may be affected by this NAAQS will only experience annualized costs of less than 0.01 percent of total industry revenues in 2020. While we do not provide impacts fo r individual facilities and firms, the impact estimates do suggest that, at a broad scale, sulfuric acid plants that may be potentially affected by illustr ative control strategies such as those included in this RIA will not experience substantial impacts from compliance with this NAAQS.

(13) Comment : The RIA states that EPA was unable to identify SO 2 emissions control measures that would be capable of bringi ng some counties into attainment of such

a SO 2 standard. (Ad Hoc Committee of Sulfuric Acid Producers ).

Response : In this analysis we were not able to find controls that would bring all

areas into attainment with the alternative standards in all areas. It is uncertain what controls States would put in place to attain a tighter standard. We should

also note that because of data and res ource limitations for this broad national analysis, we are not able to adequately re present in this analysis the impacts of some local emission control programs.

(14) Comment : The RIA also does not make clear how or if retrofits and replacements of boilers burning residual oil and switching to natural ga s or distillate heating oil

is accounted for in the estim ated costs of reducing SO 2 emissions. (City of New York)

Response : The RIA does not include any retr ofits or replacements of boilers burning residual oil and switchi ng to natural gas. The c ontrol measure considered for these sources was flue gas desulfurization. We should also note that because

of data and resource limitations for this br oad national analysis, we are not able to adequately represent in this analysis th e impacts of some local emission control programs, or specific strategies that may be available to some sources in some

areas.163 

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