UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

JAN 4 2012 


OFFICE OF 

AIR AND RADIATION 

Mr. Robert Ukeiley 

435R Chestnut Street, Suite 1 

Berea, Kentucky 40403 

Dear Mr. Ukeiley: 

The Environmental Protection Agency has reviewed the July 28, 2010 petition (Petition) you submitted 

on behalf of the Sierra Club requesting that the EPA initiate rulemaking to designate air quality models 

for ozone and fme particles (PM2.5) for use by all major sources applying for a prevention of significant 

deterioration (PSD) permit. The Administrator asked that I respond on her behalf. In light of our review, 

the EPA believes it is appropriate to engage in a rulemaking process to consider whether updates to 

EPA's Guideline on Air Quality Models as published as Appendix W of 40 CFR Part 51 (Appendix W) 

are warranted. Therefore, the EPA is granting the Petition to engage in rule making to evaluate updates to 

Appendix Wand, as appropriate, incorporate new analytical techniques or models for ozone and 

secondary PM2.5. This is not to suggest that the EPA agrees with the specific contentions and allegations 

in the Petition; however, the EPA will take the information in the Petition under advisement in the 

rulemaking process. 

The EPA has initiated actions that will support rulemaking to address modeling and technical analysis of 

ozone and PM2.5 within Appendix W, including scheduling the 10th Conference on Air Quality 

Modeling in March 2012 where we intend to discuss methods for addressing ozone and secondary 

PM2.5 impacts; and beginning work on a draft guidance for PM2.5 permit modeling, both of which are 

described in more detail below. These guidelines are periodically revised to ensure that new model 

developments or regulatory requirements are incorporated. 

The EPA will use the existing process and procedures under Section 320 of the Clean Air Act (CAA) to 

complete the appropriate rulemaking process to update Appendix W in response to this Petition. Section 

320 requires the EPA to conduct a conference on air quality modeling at least every three years and give 

special attention to appropriate modeling necessary for carrying out the provisions in Part C of Title I of 

the CAA relating to PSD. The EPA has used these conferences to develop standardized air quality 

modeling procedures particularly in connection with PSD permitting. The EPA's Guideline was 

originally published in Apri11978. Section 165(e)(3)(D) ofthe CAA requires the Administrator to adopt 

regulations specifying with "reasonable particularity each air quality model or models to be used under 

specified sets of conditions for purposes ofthis part." To carry out these requirements, in June 1978, the 

Guideline on Air Quality Models was incorporated by reference in regulations promulgated for PSD [40 

CFR 51.24].43 FR 26388,26398 (June 19, 1978); 51 FR 32176 (Sept. 1986); 53 FR 392 (Jan. 6, 1988). 

The 1990 CAA Amendments provided for continued authority to conduct air quality modeling 

Internet Address (URL) • http.//wwwepa.gov 

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conferences and associated revisions to Appendix W that form the basis for the EPA's continuing efforts 

to prescribe with "reasonable particularity" air quality models, and meteorological and emission data 

bases, suitable for modeling National Ambient Air Quality Standards (NAAQS) and increments. See 

CAA § 320,42 U.S.C. §7620. 

The EPA most recently used this process to update Appendix W with provisions that incorporated two 

new dispersion models. The 7th Conference on Air Quality Modeling was held in June 2000 with the 

purpose ofreceiving comments on the EPA's proposed rule from April 2000 to add several new 

modeling techniques to Appendix W. Based on public comments from this conference, Appendix W was 

substantially revised through notice and comment rulemaking with a final rule in April 2003 that 

adopted the CALPUFF model for long range transport to address impacts on federal Class 1 areas and a 

subsequent final rule in November 2005 that replaced the Industrial Source Complex (ISC3) model with 

the AMS/EP A Regulatory MODel (AERMOD) for near-field regulatory assessments. 

The complex chemistry of ozone and secondary formation ofPM2.5 are well-documented and have 

historically presented significant challenges to the designation ofparticular models for assessing the 

impacts ofindividual stationary sources on the formation ofthese air pollutants (NARSTO, 2000; 

NARSTO, 2004; Seinfeld andPandis, 1998; Cohan and Napelenok, 2011). Because ofthese 

considerations, the EP Ns judgment in the past has been that it was not technically sound to designate 

with particularity specific models that must be used to assess the impacts of a single source on ozone 

concentrations.! Instead, the EPA has chosen to satisfy the requirements of Section 165(e) (3) (D) ofthe 

CAA through a process of determining particular models or other analytical techniques that should be 

used on a case-by-case basis. As stated in Section 5.2.I.c ofAppendix W, the "[c]hoice ofmethods used 

to assess the impact of an individual source depends on the nature ofthe source and its emissions." 

Under this guideline, the appropriate methods are determined in consultation with the EPA Regional 

Office on a case-by-case basis. 40 C.F.R. Part 51, Appendix W, section 5.2.l.c. A modeling protocol 

should be developed and approved by the EPA Regional Office, the state/local agency, and the applicant 

to ensure that the analysis conducted will conform to the recommendations, requirements, and principles 

ofAppendix W. 40 C.F.R. Part 51, Appendix W, section 3.2.2. Given the complexities of ozone 

formation, this case-byMcase consultation process has been as reasonably particular as the EPA 

considered justified in the past. Appendix W section 5.2.2 provides similar direction for such a case-bycase consultation process in addressing secondary PM2.5 and encourages control agencies with 

jurisdiction over areas having potential air quality issues related to secondary PM2.5 to use models which 

integrate chemical and physical processes important to the formation, transport, and decay ofsulfates 

and nitrates, such as Models-3/CMAQ. 

However, recent advances in photochemical modeling science suggest that it may now be reasonable for 

the EPA to provide more specific, generally-applicable guidelines that identify particular analytical 

techniques or models that may be used under specific circumstances for assessing the impacts of an 

individual source on ozone concentrations and on the secondary formation ofPM2.s. These advances 

have resulted in some methods that may allow for tracking the formation and transport of ozone and 

secondary PM2.5 impacts from specific emissions sources and calculating the contribution of sources and 

1 We note that this technical judgment has no effect on the obligation ofsources subject to PSD to 

conduct a source impact analysis and demonstrate that a proposed source or modification will not cause 

or contribute to a violation of any NAAQS or applicable increment. 40 CFR 5I.166(k); 52.21(k). That 

is, the inclusion of a process rather than a specific preferred model in Appendix W does not relieve the 

source ofthe requirement to make this demonstration, which necessarily involves an analysis. 

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precursors to ozone and PM2.S at individual receptor locations. In fact, the EPA engaged the modeling 

community on these developments at the 9th Conference on Air Quality Modeling in October 2008 by 

devoting a session to photochemical modeling with instrumented techniques such as "source 

apportionment" to promote understanding oftheir emerging capabilities by the regulatory modeling 

community. 

The EPA has scheduled the 10th Conference on Air Quality Modeling for March 13-15, 2012, in 

Research Triangle Park, North Carolina. We intend to issue the Federal Register notice announcing the 

conference in early 2012 which will outline the agenda with the topic of chemistry models and related 

techniques for addressing impacts of ozone and secondary PM2.S being a major session and focus ofthe 

EPA's efforts to engage the regulatory modeling community. This modeling conference will serve as the 

initial venue for gaining public input to begin the rulemaking process of updating Appendix W. As was 

the case in promulgating new dispersion models (CALPUFF and AERMOD) in 2003 and 2005, the EPA 

expects to form a similar workgroup to conduct the necessary evaluations and inter-comparisons of 

technical approaches and models to inform the rulemaking process and provide sufficient technical 

justification for those technical approaches and/or models that are ultimately determined to be 

appropriate for incorporation into Appendix W. This workgroup will be critical in informing the 

rulemaking process, and the reports and other findings will be made publicly available and be central to 

discussion at future modeling conferences. Consistent with the past practice described earlier, the EPA 

expects such discussion to occur at the 11 th Conference on Air Quality Modeling with consideration of 

the specifics ofthe EPA's proposed rule to update Appendix W. 

Furthermore, as this complex rulemaking process proceeds, the EPA will be taking additional steps in 

the interim to enhance understanding of acceptable techniques for evaluating impacts of individual 

source emissions on ozone concentrations and secondary PM2.S formation. The EPA is preparing and 

will be seeking input on draft guidance for PM2.Spermitmodeling that we intend to release for public 

comment in mid-January 2012. This guidance is necessary in the interim to inform case-by-case 

determination ofappropriate methods as sources should now conduct a PM2.s-based analysis under NSR 

and PSD to demonstrate compliance with the PM2.SNAAQS rather than relying upon a PMIO-based 

analysis in accordance with past EPA guidance. 76 FR 28646, 28648, 28659 (May 18, 2011); 

Memorandum from Director of Office ofAir Quality Planning and Standards, "Modeling Procedures for 

Demonstrating Compliance with PM2.S NAAQS" (March 23, 2010). In addition, with respect to both 

ozone and PM2.S, the EPA will also be taking steps to emphasize the importance of the existing 

consultation process and to provide greater clarity to"states and the regulated community regarding our 

expectations for a complete and successful process of consulting with the EPA's Regional Offices 

pursuant to Appendix W sections 5 .2.1.c and 5 .2.2.1.c to identifY the most appropriate analytical 

techniques to be used on a case.,.by-case basis for addressing the impacts of individual sources on ozone 

concentrations and secondary PM2.S formation. 

The Petition also suggests that the EPA should take the position that it neither has nor will use a 

"significance" test as part ofthe PSD compliance demonstration for the ozone NAAQS. The Petition 

further implies that the Petitioner interprets significance levels to be inconsistent with EPA's PSD 

regulations at 40 CFR 52.21(k) because that regulation refers to proposed sources or modifications that 

cause or contribute to a violation, but does not include the word "significantly" as a modifier for "cause 

or contribute to a violation." Petition at 13. The EPA has not established a significant impact level (SIL) 

for ozone in its regulations (40 CFR 51.l65(b), 51.l66(k) (2), 52.21 (k) (2)) or identified a specific SIL 

for ozone in any guidance. However, footnote 1 to sections 51.166(1) (5) (1) ofthe EPA's regulations 

says the following: "No de minimis air quality level is provided for ozone. However, any net emission 

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increase of 100 tons per year or more of volatile organic compounds or nitrogen oxides subject to PSD 

would be required to perform an ambient impact analysis, including the gathering of air quality data" 

The EPA included a similar note in a guidance listing Significant Impact Levels. The 1990 NSR 

Workshop Manual, page C.28 footnote b says the following with respect to the then-applicable one-hour 

ozone NAAQS: "No significant ambient impact concentration has been established. Instead, any net 

emissions increase of 100 tons per year of VOC subject to PSD would be required to perform an 

ambient impact analysis." Based on these statements, this 100 tons per year (TPy) value has been used 

by some permitting authorities in a manner similar to a SIL to assess whether a detailed air quality 

analysis should be conducted for ozone. While these statements suggest a less rigorous analysis may be 

appropriate for sources emitting less than 100 TPY ofthese precursors, they have not been revisited by 

the EPA since the promulgation ofthe 8-hour ozone NAAQS and do not reflect a categorical conclusion 

by the EPA that every source emitting less than 100 TPY ofNOx or VOCs will not cause or contribute 

to a violation ofthe current ozone NAAQS. The EPA believes it unlikely a source emitting below these 

levels would contribute to such a violation ofthe 8-hour ozone NAAQS, but consultation with an EPA 

Regional Office should still be conducted in accordance with section 5.2.1.c. of Appendix W when 

reviewing an application for sources with emissions ofthese ozone precursors below 100 TPY. 

The EPA will take the Petition's recommendation regarding the appropriateness of a significance level 

for the NAAQS compliance demonstration for ozone under advisement. Nevertheless, EPA believes it 

generally has authority to use significance levels and that use ofsuch levels can be consistent with the 

PSD requirements. For example, ifmodeling becomes applied more routinely as part ofthe required 

NAAQS compliance demonstration for ozone, we believe that it may become appropriate to define a 

SIL for ozone to help facilitate that demonstration. The EPA recently promulgated a SIL for PM2.

s and 

explained in that action why we believe the CAA does not preclude the EPA from establishing 

significance values to effectively implement the PSD requirement to demonstrate that a source's 

emissions will not cause or contribute to a violation of a NAAQS or PSD increment. 75 FR 65864, 

October 20,2010. The Petition also states that "if the EPA insists on setting a significance level, it 

should be no more than 0.3 parts per billion." The EPA will take this recommendation under advisement 

as we consider whether to establish a SIL or similar value for ozone. 

Finally, the Petition requested that the EPA finalize the proposed PM2.5 increments (72 FR 54112, 

September 21,2007). The EPA promulgated the final rule in 2010 (75 FR 65864, October 20,2010) to 

establish several components for making PSD permitting determinations for PM2.5 including increments, 

SILs, and a significant monitoring concentration. This final rule became effective on December 20, 

2010. 

Thus, for the reasons set forth above, the EPA grants the Petition for rulemaking as described in this 

response. We appreciate your interest in these matters and look forward to your contributions to the 

upcoming 10th modeling conference and subsequent rulemaking process. 

Sincerely, 

ina McCarthy 

Assistant Administrator 

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