UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

Office of Air Quality Planning and Standards 

Research Triangle Park, North Carolina 27711 

AUG 23 2010 

MEMORANDUM 

SUBJECT: Guidance C rning the Implementation of the I-hour S02 NAAQS for the 

Prevent n of pzj!can €t iOI~%prOgram 

FROM: k-stephen D. age!!/:::jtor . 

r t Office of A'I Quality Planning and Standards 

TO: Regional Air Division Directors 

On June 2, 2010, the U.S. Environmental Protection Agency (EPA) announced a new 1-

hour sulfur dioxide (S02) National Ambient Air Quality Standard (hereinafter, either the 1-hour 

S02 NAAQS or 1-hour S02 standard) of75 ppb, which is attained when the 3-year average of 

the annual 99th-percentile of I-hour daily maximum concentrations does not exceed 75 ppb at 

each monitor within an area. EPA revised the primary S02 NAAQS to provide the requisite 

protection of public health. The final rule for the new I-hour S02 NAAQS was published in the 

Federal Register on June 22, 2010 (75 FR 35520), and the standard becomes effective on August 

23,2010. In the same notice, we also announced that we are revoking both the existing 24-hour 

and annual primary S02 standards. However, as explained in this guidance, those S02 standards, 

as well as the 24-hour and annual increments for S02, remain in effect for a while further and 

must continue to be protected. 

EP A interprets the Prevention of Significant Deterioration (PSD) provisions of the Clean 

Air Act and EPA regulations to require that any federal permit issued under 40 CFR 52.21 on or 

after that effective date must contain a demonstration of source compliance with the new I-hour 

S02 NAAQS. We anticipate that some new major stationary sources or major modifications, 

especially those involving relatively short stacks, may experience difficulty demonstrating that 

emissions from proposed projects will not cause or contribute to a modeled violation of the new 

I-hour S02 NAAQS. We also anticipate problems that sources may have interpreting the 

modeled I-hour S02 impacts if the form of the hourly standard is not properly addressed. To 

respond to these and other related issues, we are providing the attached guidance, in the form of 

two memoranda, for implementing the new I-hour S02 NAAQS under the PSD permit program. 

The first memorandum, titled "General Guidance for Implementing the I-hour S02 

National Ambient Air Quality Standard in Prevention of Significant Deterioration Permits, 

Including an Interim I-hour S02 Significant Impact Level," includes guidance for the 

preparation and review of PSD permits with respect to the new I-hour S02 standard. That 
guidance memorandum sets forth a recommended interim I-hour S02 significant impact level 

(SIL) that states may consider for carrying out the required PSD air quality analysis for S02, 

until EPA promulgates a I-hour S02 SIL via rulemaking, and addresses the continued use of the 

existing S02 Significant Emissions Rate (SER) and Significant Monitoring Concentration (SMC) 

to implement the new I-hour S02 standard.. The second memorandum, titled "Applicability of 

Appendix W Modeling Guidance for the I-hour S02 National Ambient Air Quality Standard," 

includes specific modeling guidance for estimating ambient S02 concentrations and determining 

compliance with the new I-hour S02 standard. 

This guidance does not bind state and local governments and permit applicants as a 

matter of law. Nevertheless, we believe that state and local air agencies and industry will find 

this guidance useful for carrying out the PSD permit process and it will provide a consistent 

approach for estimating S02 air quality impacts from proposed construction or modification of 

S02 emissions sources. For the most part, the attached guidance focuses on how existing policy 

and guidance is relevant to and should be used for implementing the new I-hour S02 NAAQS. 

Please review the guidance included in the two attached memoranda. In the event of 

questions regarding the general implementation guidance contained in the first memorandum, 

please contact Raj Rao (rao.raj@epa.gov). For questions pertaining to the modeling guidance in 

the second memorandum, please contact Tyler Fox (fox.tyler@epa.gov). We are continuing our 

efforts to address permitting issues related to the implementation of new and revised NAAQS, 

and will issue additional guidance to address the NAAQS as appropriate. 

Attachments: 

1. Memorandum from Anna Marie Wood, Air Quality Policy Division, to EPA Regional 

Air Division Directors, "General Guidance for Implementing the I-hour S02 National 

Ambient Air Quality Standard in Prevention of Significant Deterioration Permits, 

Including an Interim I-hour S02 Significant Impact Level" (August 23,2010). 

2. Memorandum from Tyler Fox, Air Quality Modeling Group, to EPA Regional Air 

Division Directors, "Applicability of Appendix W Modeling Guidance for the I-hour S02 

National Ambient Air Quality Standard" (August 23,2010). 

cc: Anna Marie Wood 

Richard Wayland 

Lydia Wegman 

Raj Rao 

Tyler Fox 

Dan deRoeck 

Roger Brode 

Rich Ossias 

Elliott Zenick 

Brian Doster 

2 
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

Office of Air Quality Planning and Standards 

Research Triangle Park, North Carolina 27711 

August 23, 2010 

MEMORANDUM 

SUBJECT: General Guidance for Implementing the I-hour S02 National Ambient Air 

Quality Standard in Prevention of Significant Deterioration Permits, Including an 

Interim I-hour S02 Significant Impact Level 

FROM: Anna Marie Wood, Acting Director lsi 

Air Quality Policy Division 

TO: Regional Air Division Directors 

INTRODUCTION 

We are issuing the following guidance to explain and clarify the procedures that may be 

followed by applicants for Prevention of Significant Deterioration (PSD) permits, and permitting 

authorities reviewing such applications, to properly demonstrate that proposed projects to 

construct and operate will not cause or contribute to a modeled violation of the new I-hour sulfur 

dioxide (S02) National Ambient Air Quality Standard (hereinafter, either the I-hour S02 

NAAQS or I-hour S02 standard) that becomes effective on August 23,2010. The EPA revised 

the primary S02 NAAQS by promulgating a I-hour S02 NAAQS to provide the requisite 

protection of public health. Under section I65(a)(3) of the Clean Air Act (the Act) and sections 

52.2I(k) and 5I.I66(k) of EPA's PSD regulations, to obtain a permit, a source must demonstrate 

that its proposed emissions increase will not cause or contribute to a violation of "any NAAQS." 

This guidance is intended to (1) highlight the importance of a I-hour averaging period for 

setting an emissions limitation for S02 in the PSD permit (2) reduce the modeling burden to 

implement the I-hour S02 standard where it can be properly demonstrated that a source will not 

have a significant impact on ambient I-hour S02 concentrations, and (3) identify approaches that 

allow sources and permitting authorities to mitigate, in a manner consistent with existing 

regulatory requirements, potential modeled violations of the I-hour S02 NAAQS, where 

appropriate. Accordingly, the techniques described in this memorandum may be used by permit 

applicants and permitting authorities to perform an acceptable I-hour S02 NAAQS compliance 

modeling assessment andlor properly configure projects and permit conditions in order that a 

proposed source's emissions do not cause or contribute to modeled I-hour S02 NAAQS 

violations, so that permits can be issued in accordance with the applicable PSD program 

requirements. 
This guidance discusses existing provisions in EPA regulations and guidance, and 

focuses on the relevancy of this infonnation for implementing the new NAAQS for S02. 

Importantly, however, this guidance also sets forth a recommended interim I-hour S02 

significant impact level (SIL) that EPA will use when it evaluates applications and issues permits 

under the federal PSD program, and that states may choose to rely upon to implement their PSD 

programs for S02 if they agree that the value represents a reasonable threshold for determining a 

significant ambient impact, and they incorporate into each pennit record a rationale supporting 

this conclusion. This interim SIL is a useful screening tool that can be used to determine 

whether or not the predicted ambient impacts caused by a proposed source's emissions increase 

will be significant and, if so whether the source's emissions should be considered to "cause or 

contribute to" modeled violations of the new I-hour S02 NAAQS. 

BACKGROUND 

On August 23,2010, the new I-hour S02 NAAQS will become effective. Regulations at 

40 CFR 52.21 (the federal PSD program) require permit applicants to demonstrate compliance 

with "any" NAAQS that is in effect on the date a PSD permit is issued. (See, e.g., EPA memo 

dated April 1, 20 I 0, titled "Applicability of the Federal Prevention of Significant Deterioration 

Permit Requirements to New and Revised National Ambient Air Quality Standards.") Due to the 

promulgation of this short-term averaging period (I-hour) for the S02 NAAQS, we anticipate 

that some new major stationary sources or major modifications, especially those involving 

relatively short stacks may experience increased difficulty demonstrating that emissions from 

proposed project will not cause or contribute to a modeled violation. 

We believe that, in some instances, preliminary predictions of violations could result 

from the use of maximum modeled concentrations that do not adequately take into account the 

form of the I-hour standard. To the extent that is the case, ambient S02 concentrations in the 

form of the new I-hour NAAQS should be estimated by applying the recommended procedures 

that account for the statistical form ofthe standard. See EPA Memorandum from Tyler Fox, Air 

Quality Modeling Group, to EPA Regional Air Division Directors, "Applicability of Appendix 

W Modeling Guidance for the I-hour S02 National Ambient Air Quality Standard" (August 23, 

20 I 0) for specific modeling guidance for estimating ambient S02 concentrations consistent with 

the new I-hour S02 NAAQS. 

It is EPA's expectation that currently available S02 guidance, including the guidance 

presented in this memorandum, will assist in resolving some of the issues arising from 

preliminary analyses that show potential exceedances of the new I-hour S02 NAAQS that would 

not be present under more refined modeling applications. In addition, the techniques described 

in this memorandum may also help avoid violations of the standard through design of the 

proposed source or permit conditions, consistent with existing regulatory requirements. 

Moreover, the interim I-hour S02 SIL that is included in this guidance will provide a reasonable 

screening tool for effectively implementing the PSD requirements for an air quality impact 

analysis. 

The following discussion provides guidance for establishing a I-hour emissions 

limitation to demonstrate compliance with the new NAAQS, and for possibly mitigating 

2 
modeled violations using any of the following: air quality-based permit limits more stringent 

than what the Best Available Control Technology provisions may otherwise require, air quality 

offsets, "good engineering practice" (GEP) stack heights, and an interim I-hour S02 SIL. The 

continued use of the existing S02 Significant Emissions Rate (SER) and Significant Monitoring 

Concentration (SMC) to implement the new I-hour S02 standard is also discussed. 

SCREENING VALVES 

In the final rule establishing the I-hour S02 standard, EPA discussed various 

implementation considerations for the PSD permitting program. 75 FR.35520 (June 22, 2010). 

That discussion included the following statements regarding particular screening values that have 

historically been used on a widespread basis to facilitate implementation of the PSD permitting 

program: 

We agree with the commenters that there may be a need for EPA to provide 

additional screening tools or to revise existing screening tools that are frequently used 

under the NSRlPSD program for reducing the burden of completing S02 ambient air 

impact analyses. These screening tools include the SILs, as mentioned by the commenter, 

but also include the SER for emissions of S02 and the SMC for S02. The existing 

screening tools apply to the periods used to define the existing NAAQS for S02, 

including the annual, 24-hour, and 3-hour averaging periods. EPA intends to evaluate the 

need for possible changes or additions to each of these useful screening tools for S02 due 

to the revision of the S02 NAAQS to provide for a I-hour standard. We believe it is 

highly likely that in order to be most effective for implementing the new I-hour 

averaging period for NSR purposes, new I-hour screening values will be appropriate. 

75 FR 35579. EPA intends to conduct an evaluation of these issues and submit our findings in 

the form of revised significance levels under notice and comment rulemaking if any revisions are 

deemed appropriate. In the interim, for the reasons provided below, we recommend the 

continued use of the existing SER for S02 emissions as well as an interim I-hour S02 SIL that 

we are setting forth today for conducting air quality impact analyses for the I-hour S02 NAAQS. 

As described in the section titled Introduction, EPA intends to implement the interim I-hour S02 

SIL contained herein under the federal PSD program and offers states the opportunity to use it in 

their PSD programs if they choose to do so. EPA is not addressing the significant monitoring 

concentration (SMC) for S02 in this memorandum; the existing SMC for S02, at 40 CFR 

52.2I(i)(5)(i) should continue to be used. 

SIGNIFICANT EMISSIONS RATE 

The PSD regulations define SER for various regulated NSR pollutants. When a proposed 

new source's potential to emit a pollutant, or a modified source's net emissions increase of a 

pollutant, would be less than the SER, the source is not required to undergo the requisite PSD 

analyses (BACT and air quality) for that particular emissions increase. Under the terms of 

existing EPA regulations, the applicable SER for S02 is 40 tons per year (tpy). 40 CFR 

52.2I(b)(23); 40 CFR 51.166(b)(23). Each of the significant emissions rates defined in those 

regulations is specific to an individual pollutant with no differentiation by averaging time with 

3 
regard to NAAQS. The NAAQS for S02 have included standards with 3-hour and 24-hour and 

annual averaging times for many years. The EPA has applied the 40 tpy SER for S02 across all 

of these averaging times, and we are aware of no reason why it should not be used for the I-hour 

averaging period for the present time. Therefore, until the evaluation described above and any 

associated rulemaking are completed, we will use 40 tpy as the SER for the I-hour standard. 

Under existing regulations, an ambient air quality impact analysis is required for "each 

pollutant that [a source] would have the potential to emit in significant amounts." [40 CFR 

52.21 (m)(1)(i)(a); 40 CFR. 51. 166(m)(1)(i)(a)]. For modifications, these regulations require this 

analysis for "each pollutant for which [the modification] would result in a significant net 

emissions increase." 40 CFR.52.21 (m)(1 )(i)(b); 40 CFR.51.166(m)(1 )(i)(b ). EPA construes this 

regulation to mean that an ambient impact analysis is not necessary for pollutants with emissions 

rates below the significant emissions rates in paragraph (b)(23) of the regulations. No additional 

action by EPA or permitting authorities is necessary at this time to apply the 40 tpy significant 

emissions rate in existing regulations to the hourly S02 standard. 

INTERIM I-HOUR S02 SIGNIFICANT IMPACT LEVEL 

Under the PSD program, a proposed new major stationary source or major modification 

must, among other things, complete an air quality impact analysis that involves performing an 

analysis of air quality modeling and ambient monitoring data, where appropriate, to demonstrate 

compliance with applicable NAAQS. In order to implement this requirement, EPA traditionally 

has provided a screening tool known as the Significant Impact Level (SIL) to help applicants and 

permitting authorities determine whether a source's modeled ambient impact is significant so as 

to warrant a comprehensive, cumulative air quality analysis to demonstrate compliance with the 

NAAQS. Accordingly, where a proposed source's modeled impact is deemed insignificant, or 

de minimis, using the SIL as a threshold for significance, the applicant is not required to model 

anything besides its own proposed emissions increase to show that the proposed source or 

modification will not cause or contribute to a violation of the NAAQS.l 

If, on the other hand, the source's modeled impact is found to be significant, based on the 

SIL, the applicant will need to complete a comprehensive, cumulative air quality impact analysis 

to demonstrate that the source's emissions will not cause or contribute to a modeled violation of 

any NAAQS. To make this demonstration, EPA has recommended that a cumulative analysis 

cover a circular area measuring out from the source to the maximum distance where the source's 

impact is equal to the SIL. Within this modeling area, the source should also model the impacts 

of other sources (existing and newly permitted), including applicable S02 sources located outside 

the circular area described above, to account for the cumulative hourly S02 air quality impacts 

1 When a proposed source's impact by itself is not considered to be "significant," EPA has long maintained that any 

further effort on the part of the applicant to complete a cumulative source impact analysis involving other source 

impacts would only yield information of trivial or no value with respect to the required evaluation of the proposed 

source or modification. The concept of a SIL is grounded on the de minimis principles described by the court in 

Alabama Power Co. v. Castle, 636 F.2d 323,360 (D.C. Cir. 1980); See also Sur Contra La Contaminacion v. EPA, 

202 F.3d 443,448-49 (1 st Cir. 2000) (upholding EPA's use of SIL to allow permit applicant to avoid full impact 

analysis); In re: Prairie State Gen. Co., PSD Appeal No. 05-05, Slip. Op. at 139 (EAB 2006). 

4 
that are predicted to occur. The applicant may also have to gather ambient monitoring data as 

part of the total air quality analysis that is required for demonstrating compliance with the 

NAAQS.2 Accordingly, the source will evaluate its contribution to any modeled violation ofthe 

I-hour S02 NAAQS to determine whether the source's emissions contribution will cause or 

contribute to the modeled violation at any receptor. Note that in the accompanying modeling 

guidance memorandum we are providing recommended procedures and guidance for completing 

the modeling analysis to demonstrate compliance with the new I-hour S02 NAAQS. 

We plan to undertake rulemaking to adopt a I-hour S02 SIL value. However, until such 

time as a I-hour S02 SIL is defined in the PSD regulations, we are providing an interim SIL of 3 

ppb, which we intend to use as a screening tool for completing the required air quality analyses 

for the new I-hour S02 NAAQS under the federal PSD program at 40 CFR 52.21. We are also 

making the interim SIL available to States with EPA-approved implementation plans containing 

a PSD program to use at their discretion. To support the application of this interim I-hour S02 

SIL in each instance, a permitting authority that utilizes it as part of an ambient air quality 

analysis should include in the permit record the analysis reflected in this memorandum and the 

referenced documents to demonstrate that a modeled air quality impact is de minimis, and 

thereby would not be considered to cause or contribute to a modeled violation of the NAAQS.3 

States may also elect to choose another value that they believe represents a significant air 

quality impact relative to the I-hour S02 NAAQS. The EPA-recommended interim I-hour S02 

SIL is not intended to supersede any interim SIL that any state chooses to rely upon to 

implement a state PSD program that is part of an approved SIP, or to impose the use of the SIL 

concept on any state that chooses to implement the PSD program-in particular the ambient air 

quality analysis-without using a SIL as a screening tool. Accordingly, states that implement 

the PSD program under an EPA-approved SIP may choose to use this interim SIL, another value 

that may be deemed more appropriate for PSD permitting purposes in the state of concern, or no 

SIL at all. The application of any SIL that is not reflected in a promulgated regulation should be 

supported by a record in each instance that shows the value represents a de minimis impact on 

the I-hour S02 standard, as described above. 

As indicated above, using the interim I-hour S02 SIL, the permit applicant and 

permitting authority can determine: (1) whether, based on the proposed increase in S02 

emissions, a cumulative air quality analysis is required; (2) the area of impact within which a 

cumulative air quality analysis should focus; and (3) whether, as part of a cumulative air quality 

analysis, the proposed source's S02 emissions will cause or contribute to any modeled violation 

of the I-hour S02 NAAQS. 

2 A screening tool known as the Significant Monitoring Concentration (SMC) for S02 already exists in the PSD 

regulations. EPA plans to evaluate the existing SMC in light of the new I-hour S02 NAAQS; however, the existing 

value of 13 ~g/m3, 24-hour average, should continue to be used until and unless a revised value is issued through 

rulemaking. 

3 Where the cumulative air quality analysis identifies a modeled violation of the NAAQS or increments, and the 

proposed source is issued its permit by virtue of the fact that its proposed emissions increase is not considered to 

cause or contribute to the modeled violation, it is still the permitting authority'S responsibility to address such 

modeled violations independently from the PSD permitting process to determine the nature of the problem and to 

mitigate it accordingly, 

5 
As mentioned above, we are providing an interim I-hour S02 SIL value of 3 ppb to 

implement the federal PSD program. To determine initially whether a proposed project's 

emissions increase will have a significant impact (resulting in the need for a cumulative air 

quality analysis), this interim SIL should be compared to either of the following: 

• The highest of the 5-year averages of the maximum modeled I-hour S02 

concentrations predicted each year at each receptor, based on 5 years of National 

Weather Service data; or 

• The highest modeled I-hour S02 concentration predicted across all receptors based 

on 1 year of site-specific meteorological data, or the highest of the multi-year 

averages of the maximum modeled I-hour S02 concentrations predicted each year at 

each receptor, based on 2 or more, up to 5 complete years of available site-specific 

meteorological data. 

Additional guidance will be forthcoming for the purpose of comparing a proposed source's 

modeled impacts to the interim I-hour S02 SIL in order to make a determination about whether 

that source's contribution is significant when a cumulative air quality analysis identifies 

violations ofthe I-hour S02 NAAQS (i.e., "causes or contributes to" a modeled violation). 

We derived this interim I-hour S02 SIL by using an impact equal to 4% of the I-hour 

S02 NAAQS (which is 75 ppb). On June 29, 2010, we issued an interim I-hour N02 SIL that 

used an impact equal to 4% of the I-hour N02 standard. As explained in the June memorandum, 

we have chosen this approach because we believe it is reasonable to base the interim I-hour SIL 

directly on consideration of impacts relative to the corresponding I-hour NAAQS. In 1980, we 

defined SER for each pollutant subject to PSD. 45 FR 52676 (August 7, 1980) at 52705-52710. 

For PM and S02, we defined the SER as the emissions rate that resulted in an ambient impact 

equal to 4% of the applicable short-term NAAQS. The 1980 analysis focused on levels no 

higher than 5% ofthe primary standard because of concerns that higher levels were found to 

result in unreasonably large amounts of increment being consumed by a single source. Within 

the range of impacts analyzed, we considered two factors that had an important influence on the 

choice of the significant impact levels: (1) cumulative effect on increment consumption of 

multiple sources in an area, each making the maximum de minimis emissions increase; and (2) 

the projected consequence of a given significant impact level on administrative burden. As 

explained in the preamble to the 1980 rulemaking and the supporting documentation,4 EPA 

decided to use 4% of the 24-hour primary NAAQS for PM and S02 to define the significant 

emissions rates (SERs) for those pollutants. See 45 FR 52708. Looking now at a I-hour 

NAAQS for S02, we believe that it is reasonable as an interim approach to use a SIL value that 

represents 4% of the I-hour S02 NAAQS. EPA will consider other possible alternatives for 

,developing a I-hour S02 SIL in a future rulemaking that will provide an opportunity for public 

participation in the development of a SIL as part of the PSD regulations. 

AIR-QUALITY BASED EMISSIONS LIMITATIONS 

4 EPA evaluated de minimis levels for pollutants for which NAAQS had been established in a document titled 

"Impact of Proposed and Alternative De Minimis Levels for Criteria Pollutants"; EPA-450/2-80-0n, June 1980. 

6 
Once a level of control is determined by the PSD applicant via the Best Available Control 

Technology (BACT) top-down process, the applicant must model the proposed source's 

emissions at the BACT emissions rate(s) to demonstrate that those emissions will not cause or 

contribute to a violation of any NAAQS or PSD increment. However, the EPA 1990 Workshop 

Manual (page B.54) describes circumstances where a proposed source's emissions based on 

levels determined via the top-down process may not be sufficiently controlled to prevent 

modeled violations of an increment or NAAQS. In such cases, it may be appropriate for PSD 

applicants to propose a more stringent control option (that is, beyond the level identified via the 

top-down process) as a result of an adverse impact on the NAAQS or PSD increments. In 

addition, the use of certain dispersion techniques is permissible for certain proposed projects for 

S02 that may need to be considered where emissions limitations alone may not enable the source 

to demonstrate compliance with the new I-hour S02 NAAQS. This is discussed in greater detail 

below in the section addressing GEP stack height requirements. 

Because compliance with the new S02 NAAQS must be demonstrated on the basis of a 

I-hour averaging period, the reviewing authority should ensure that the source's PSD permit 

defines a maximum allowable hourly emissions limitation for S02, regardless of whether it is 

derived from the BACT top-down approach or it is the result of an air-quality based emissions 

rate. Hourly limits are important because they are the foundation of the air quality modeling 

demonstration relative to the I-hour S02 NAAQS. For estimating the impacts of existing 

sources, if necessary, existing S02 emission inventories used to support modeling for compliance 

with the 3-hour and 24-hour S02 standards should serve as a useful starting point, and may be 

adequate in many cases for use in assessing compliance with the new I-hour S02 standard. The 

PSD applicant's coordination with the reviewing authority is important in this matter to obtain 

the most appropriate estimates of maximum allowable hourly S02 emissions. 

DEMONSTRATING COMPLIANCE WITH THE NAAQS AND INCREMENTS & 

MITIGATING MODELED VIOLATIONS WITH AIR QUALITY OFFSETS 

A 1988 EPA memorandum provides procedures to follow when a modeled violation is 

identified during the PSD permitting process. [See Memorandum from Gerald A. Emison , EPA 

OAQPS, to Thomas J. Maslany, EPA Air Management Division, "Air Quality Analysis for 

Prevention of Significant Deterioration (PSD)." (July 5, 1988.)] In cases where the air quality 

analysis predicts violations of the I-hour S02 NAAQS, but the permit applicant can show that 

the S02 emissions increase from the proposed source will not have a significant impact at the 

point and time of any modeled violation, the permitting authority has discretion to conclude that 

the source's emissions will not contribute to the modeled violation. As provided in the July 5, 

1988 guidance memo, because the proposed source only has a de minimis contribution to the 

modeled violation, the source's impact will not be considered to cause or contribute to such 

modeled violations, and the permit could be issued. This concept continues to apply, and the 

significant impact level (described further below) may be used as part of this analysis. A 2006 

decision by the EPA Environmental Appeals Board (EAB) provides detailed reasoning that 

demonstrates the permissibility of a finding that a PSD source would not be considered to cause 

or contribute to a modeled NAAQS violation because its estimated air quality impact was 

7 
insignificant at the time and place of the modeled violations.s [See In re Prairie State Gen. Co., 

13 E.A.D. _, _, PSD Appeal No. 05-05, Slip. Op. at 137-144 (EAB 2006)] 

However, where it is determined that a source's impact does cause or contribute to a 

modeled violation, a permit cannot be issued without some action to mitigate the source's 

impact. In accordance with 40 CFR 51.165(b )6, a major stationary source or major modification 

(as defined at §51.165(a)(1)(iv) and (v)) that locates in a S02 attainment area for the I-hour S02 

NAAQS and would cause or contribute to a violation of the I-hour S02 NAAQS may "reduce 

the impact of its emissions upon air quality by obtaining sufficient emission reductions to, at a 

minimum, compensate for its adverse ambient [S02] impact where the major source or major 

modification would otherwise cause or contribute to a violation .... " An applicant can meet this 

requirement for obtaining additional emissions reductions either by reducing its emissions at the 

source (e.g., promoting more efficient production methodologies and energy efficiency) or by 

obtaining air quality offsets (see below). [See, e.g., In re Interpower a/New York, Inc., 5 E.A.D. 

130, 141 (EAB ,1994)].7 A State may also provide the necessary emissions reductions by 

imposing emissions limitations on other sources through an approved SIP revision. These 

approaches may also be combined as necessary to demonstrate that a source will not cause or 

contribute to a violation of the NAAQS. 

Unlike emissions offset requirements in areas designated as nonattainment, in addressing 

the air quality offset concept, it may not be necessary for a permit applicant to fully offset the 

proposed emissions increase if an emissions reduction of lesser quantity will mitigate the adverse 

air quality impact where the modeled violation was originally identified. ("Although full 

emission offsets are not required, such a source must obtain emission offsets sufficient to 

compensate for its air quality impact where the violation occurs." 44 FR 3274, January 16, 1979, 

at 3278.) To clarify this, the 1988 guidance memo referred to above states that: 

offsets sufficient to compensate for the source's significant impact must be obtained 

pursuant to an approved State offset program consistent with State Implementation Plan 

(SIP) requirements under 40 CFR 51.165(b). Where the source is contributing to an 

existing violation, the required offset may not correct the violation. Such existing 

violations must be addressed [through the SIP]. 

Note that additional guidance for this and other aspects of the modeling analysis for the 

impacts of S02 emissions on ambient concentrations of S02 are addressed in EPA modeling 

guidance, including the attached August 23,2010 Memorandum titled "Applicability of 

Appendix W Modeling Guidance for the I-hour S02 National Ambient Air Quality Standard." 

5 While there is no I-hour S02 significant impact level (SIL) cUlTently defined in the PSD regulations, we believe 

that states may adopt interim values, with the appropriate justification for such values, to use for permitting 

purposes. In addition, we are recommending an interim SIL as part of this guidance for implementing the S02 

requirements in the federal PSD program, and in state programs where states choose to use it. 

6 The same provision is contained in EPA's Interpretative Ruling at 40 CFR part 51 Appendix S, section III. 

7 In contrast to Nonattainment New Source Review permits, offsets are not mandatory requirements in PSD permits 

if it can otherwise be demonstrated that a source will not cause or contribute to a violation ofthe NAAQS. See, In 

re Knauf Fiber Glass, GMBH, 8 E.A.D. 121, 168 (EAB 1999). 

8 
Although EPA announced that it is revoking the annual and 24-hour S02 NAAQS, the 

June 22, 2010 preamble to the final rule announcing the new I-hour S02 NAAQS explained that 

those standards will remain in effect for a limited period of time as follows: for current S02 

nonattainment areas and SIP call areas, until attainment and maintenance SIPs are approved by 

EPA for the new I-hour S02 NAAQS; for all other areas, for one year following the effective 

date of the initial designations under section 107(d)(1) for the new I-hour S02 NAAQS. 

Accordingly, the annual and 24-hour S02 NAAQS must continue to be protected under the PSD 

program for as long as they remain in effect for a PSD area. There is a more detailed discussion 

of the transition from the existing S02 NAAQS to a revised S02 NAAQS in that preamble. Also, 

the same preamble includes a footnote listing the current nonattainment areas and SIP call areas. 

75 FR 35520, at 35580-2. 

In addition, the existing S02 increments (class I, II and III) for the annual and 24-hour 

averaging periods will not be revoked in conjunction with our decision to revoke the 

corresponding S02 NAAQS. Instead, the annual and 24-hour S02 increments (Class I, II and III 

increments) will remain in effect because they are defined in the Clean Air Act at title I, part C, 

section 163. The annual and 24-hour S02 increments in section 163 are considered part of the 

suite of statutory increments applicable to sulfur dioxide that Congress expressly included in the 

statutory provisions for PSD. As such, those increments cannot be revoked simply because we 

have decided to revoke the annual and 24-hour S02 NAAQS, upon which the S02 increments are 

based. Consequently, sources must continue to demonstrate that their proposed emissions 

increases of S02 emissions will not cause or contribute to any modeled violation of the existing 

annual and 24-hour S02 increments for as long as those statutory increments remain in effect. 

Increments for the I-hour averaging period do not yet exist; the Act provides a specific schedule 

for the promulgation of additional regulations, which may include new increments, following the 

promulgation of new or revised NAAQS. EPA plans to begin that rulemaking process in the 

near future to consider the need for such increments. 

"GOOD ENGINEERING PRACTICE" STACK HEIGHT AND DISPERSION 

TECHNIQUES 

If a permit applicant is unable to show that the source's proposed emissions increase will 

not cause or contribute to a modeled violation of the new I-hour S02 NAAQS, the problem 

could be the result of plume downwash effects causing high ambient concentrations near the 

source. In such cases, a source may be able to raise the height of its existing stacks (or designed 

stacks if not yet constructed) to a "good engineering practice" (GEP) stack height, or at least 65 

meters, measured from the ground-level elevation at the base of the stack. 

While not necessarily eliminating the full effect of downwash in all cases, raising stacks 

to GEP height may provide substantial air quality benefits in a manner consistent with statutory 

provisions (section 123 of the Act) governing acceptable stack heights to minimize excessive 

concentrations due to atmospheric downwash, eddies or wakes. Permit applicants should also be 

aware of the regulatory restrictions on stack heights for the purpose of modeling for compliance 

with NAAQS and increments. Section 52.21 (h) of the PSD regulations currently prohibits the 

use of dispersion techniques, such as stack heights above GEP, merged gas streams, or 

intermittent controls for setting S02 emissions limits to meet the NAAQS and PSD increments. 

9 
However, stack heights in existence before December 31, 1970, and dispersion techniques 

implemented before then, are not affected by these limitations. EPA's general stack height 

regulations are promulgated at 40 CFR 51.1 OO(f!), (gg), (hh), (ii), Gj), (kk) and (nn), and 40 CFR 

51.118. 

a. Stack heights: A source can include only the actual stack height up to GEP height 

when modeling to develop the S02 emissions limitations or to determine source compliance with 

the S02 NAAQS and increments. This is not a limit on the actual height of any stack constructed 

by a new source or modification, however, and there may be circumstances where a source 

owner elects to build a stack higher than GEP height. However, such additional height may not 

be considered when determining an emissions limitation or demonstrating compliance with an 

applicable NAAQS or PSD increment. Thus, when modeling, the following limitations apply in 

accordance with §52.21(h): 

• For a stack height less than GEP, the actual stack height must be used in the source 

impact analysis for emissions; 

• For a stack height equal to or greater than 65 meters the impact may be modeled 

using the greater of: 

o A de minimis stack height equal to 65 meters, as measured from the groundlevel elevation at the base of the stack, without demonstration or calculation 

(40 CFR 51.1 OO(ii)(1 )); 

o The refined formula height calculated using the dimensions of nearby 

structures in accordance with the following equation: 

GEP = H + 1.SL, where H is the height of the nearby structure and L is the lesser 

dimension ofthe height or projected width of the nearby structure 

(40 CFR 51.1 00(ii)(2)(ii)). 

• A GEP stack height exceeding the refined formula height may be approved when it 

can be demonstrated to be necessary to avoid "excessive concentrations" of S02 

caused by atmospheric downwash, wakes, or eddy effects by the source, nearby 

structures, or nearby terrain features. 

(40 CFR 51.100(ii)(3), Gj), (kk)); 

• For purposes of PSD, "excessive concentrations" means a maximum ground-level 

concentration from a stack due in whole or in part to downwash, wakes, and eddy 

effects produced by nearby structures or nearby terrain features which individually is 

at least 40 percent in excess of the maximum concentration experienced in the 

absence of such effects and (a) which contributes to a total concentration due to 

emissions from all sources that is greater than the applicable NAAQS or (b) greater 

than the applicable PSD increments. 

(40 CFR 51.100(kk)(1)). 

8 For stacks in existence on January 12, 1979, the GEP equation is GEP = 2.5 H (provided the owner or operator 

produces evidence that this equation was actually relied on in establishing an emission limitation for S02 (40 CFR 

51.100(ii)(2)(i) 

10 
Reportedly, for economic and other reasons, many existing source stacks have been 

constructed at heights less than 65 meters, and source impact analyses may show that the 

source's emissions will cause or contribute to a modeled violation ofthe I-hour S02 NAAQS. 

Where this is the case, sources should be aware that it is permissible for them to increase their 

stack heights up to 65 meters without a GEP demonstration. 

b. Other dispersion techniques: The term "dispersion technique" includes any practice 

carried out to increase final plume rise, subject to certain exceptions (40 CFR 51.1 OO(hh)(1), 

(2)(i) - (v». Beyond the noted exceptions, such techniques are not allowed for getting credit for 

modeling source compliance with the NAAQS and PSD increments. One such exception is for 

sources of S02. Section 51.1 00(hh)(2)(v) provides that identified techniques that increase final 

exhaust gas plume rise are not considered prohibited dispersion techniques pursuant to section 

51.1 OO(hh)(1 )(iii) "where the resulting allowable emissions of sulfur dioxide from the facility do 

not exceed 5,000 tons per year." Thus, proposed modifications that experience difficulty 

modeling compliance with the new I-hour S02 NAAQS when relying on BACT or an air 

quality-based emissions limit alone may permissibly consider techniques to increase their final 

exhaust gas plume rise consistent with these provisions. 

The definition of "dispersion technique" at 40 CFR 51.1 o O(hh) (1 )(iii) describes 

techniques that are generally prohibited, but which do not apply with respect to the exemption 

for S02. Accordingly, it is permissible for eligible S02 sources to make adjustments to source 

process parameters, exhaust gas parameters, stack parameters, or to combine exhaust gases from 

several existing stacks into one stack, so as to increase the exhaust gas plume rise. It is important 

to remember that the exemption applies to sources that have facility-wide allowable S02 

emissions of less than 5,000 tpy resulting from the increase in final exhaust gas plume rise. 

Thus, proposed modifications should not base their eligibility to use dispersion on the amount of 

the proposed net emissions increase, but on the total source emissions of S02. 

The EPA does not recommend or encourage sources to rely on dispersion to demonstrate 

compliance with the NAAQS; however, we acknowledge the fact that certain S02 sources may 

legally do so. For example, while increasing stack height is a method of dispersion, EPA's rules 

allow use of that approach to the extent the resulting height meets EPA's requirements defining 

"good engineering practice (GEP)" stack height. See 40 CFR 50.1 o o (hh) (1 )(i), 50.1 OO(ii)(l )-(3). 

Nevertheless, EPA encourages PSD applicants to seek other remedies, including the use of the 

most stringent controls (beyond top-down BACT) feasible or the acquisition of emissions 

reductions (offsets) from other existing sources, to address situations where proposed emissions 

increases would result in modeled violations of the S02 NAAQS. 

GENERAL START -UP CONDITIONS 

We do not anticipate widespread problems associated with high short-term S02 emissions 

resulting from start-up/shutdown conditions. Many sources are capable of starting a unit with 

natural gas or low-sulfur fuel to avoid significant start-up emissions problems. However, some 

sources could experience short-term peaks of S02 during start-up or shutdown that could 

adversely affect the new I-hour S02 NAAQS. The EPA currently has no provisiol;1s for 

exempting emissions occurring during equipment start-up/shutdown from the BACT 

11 
requirements or for air quality analyses to demonstrate compliance with the S02 NAAQS and 

increments. Therefore, such emissions should be addressed in the required BACT and air quality 

analyses. 

There are approaches to addressing issues related to start-up/shutdown emissions. For 

example, sources may be willing to accept enforceable permit conditions limiting equipment 

start-up/shutdown to certain hours of the day when impacts are expected to be lower than 

normal. Such permit limitations can be accounted for in the modeling of such emissions. 

Applicants should direct other questions arising concerning procedures for modeling startup/shutdown emissions to the applicable permitting authority to determine the most current 

modeling guidance. 

In the event of questions regarding the general implementation guidance contained in this 

memorandum, please contact Raj Rao (rao.raj@epa.gov). 

cc: Raj Rao, C504-01 

Dan deRoeck, C504-03 

Tyler Fox, C439-01 

Roger Brode, C439-01 

Richard Wayland, C304-02 

Lydia Wegman, C504-02 

Elliott Zenick, OGC 

Brian Doster, OGC 

EP A Regional NSR Contacts 

12 
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

Office of Air Quality Planning and Standards 

Research Triangle Park, North Carolina 27711 

August 23,2010 

MEMORANDUM 

SUBJECT: Applicability of Appendix W Modeling Guidance for the I-hour S02 

National Ambient Air Quality Standard 

FROM: Tyler Fox, Leader /s/ 

Air Quality Modeling Group, C439-0I 

TO: Regional Air Division Directors 

INTRODUCTION 

On June 2, 2010, EPA announced a new I-hour sulfur dioxide (S02) National Ambient 

Air Quality Standard (I-hour S02 NAAQS or I-hour S02 standard) which is attained when the 

3-year average of the 99th-percentile of the annual distribution of daily maximum I-hour 

concentrations does not exceed 75 ppb at each monitor within an area. The final rule for the new 

I-hour S02 NAAQS was published in the Federal Register on June 22, 2010 (75 FR 35520-

35(03), and the standard becomes effective on August 23,2010 (EPA, 20IOa). This 

memorandum clarifies the applicability of current guidance in the Guideline on Air Quality 

Models (40 CFR Part 51, Appendix W) for modeling S02 impacts in accordance with the 

Prevention of Significant Deterioration (PSD) permit requirements to demonstrate compliance 

with the new I-hour S02 standard. 

SUMMARY OF CURRENT GUIDANCE 

Current modeling guidance for estimating ambient impacts of S02 for comparison with 

applicable NAAQS is presented in Section 4 of Appendix W under the general heading of 

"Traditional Stationary Source Models." This guidance acknowledges the fact that ambient S02 

impacts are largely a result of emissions from stationary sources. Section 4.2.2 provides specific 

recommendations regarding "Refined Analytical Techniques," stating that "For a wide range of 

regulatory applications in all types of terrain, the recommended model is AERMOD" (see 

Section 4.2.2.b). As described in Section 4.1.d, the AERMOD dispersion model "employs best 

state-of-practice parameterizations for characterizing the meteorological influences and 

dispersion" (Cimorelli, et ai., 2004; EPA, 2004; EPA, 2009). 

Section 7.2.6 of Appendix W addresses the issue of chemical transformation for 

modeling S02 emissions, stating that: 
The chemical transformation of S02 emitted from point sources or single industrial plants 

in rural areas is generally assumed to be relatively unimportant to the estimation of 

maximum concentrations when travel time is limited to a few hours. However, in urban 

areas, where synergistic effects among pollutants are of considerable consequence, 

chemical transformation rates may be of concern. In urban area applications, a half-life of 

4 hours may be applied to the analysis of S02 emissions. Calculations of transformation 

coefficients from site specific studies can be used to define a "half-life" to be used in a 

steady-state Gaussian plume model with any travel time, or in any application, if 

appropriate documentation is provided. Such conversion factors for pollutant half-life 

should not be used with screening analyses. 

The AERMOD model incorporates the 4 hour half-life for modeling ambient S02 concentrations 

in urban areas under the regulatory default option. 

General guidance regarding source emission input data requirements for modeling 

ambient S02 impacts is provided in Section 8.1 of Appendix W and guidance regarding 

determination of background concentrations for purposes of a cumulative ambient air quality 

impact analysis is provided in Section 8.2. 

APPLICABILITY OF CURRENT GUIDANCE TO 1-HOUR S02 NAAQS 

The current guidance in Appendix W regarding S02 modeling in the context of the 

previous 24-hour and annual primary S02 NAAQS and the 3-hour secondary S02 NAAQS is 

generally applicable to the new I-hour S02 standard. Since short-term S02 standards (:S 24 

hours) have been in existence for decades, existing S02 emission inventories used to support 

modeling for compliance with the 3-hour and 24-hour S02 standards should serve as a useful 

starting point, and may be adequate in many cases for use in assessing compliance with the new 

I-hour S02 standard, since issues identified in Table 8-2 of Appendix W related to short-term vs. 

long-term emission estimates may have already been addressed. However, the PSD applicant 

and reviewing authority may need to reassess emission estimates for very short-term emission 

scenarios, such as start-up and shut-down operations, for purposes of estimating source impacts 

on the I-hour S02 standard. This is especially true if existing emission estimates for 3-hour or 

24-hour periods are based on averages that include zero (0) or reduced emissions for some of the 

hours. 

Given the form of the new I-hour S02 standard, we are providing clarification regarding 

the appropriate data periods for modeling demonstrations of compliance with the NAAQS vs. 

demonstrations of attainment of the NAAQS through ambient monitoring. While monitored 

design values for the I-hour S02 standard are based on a 3-year average (in accordance with 

Section l(c) of Appendix T to 40 CFR Part 50), Section 8.3.1.2 of Appendix W addresses the 

length of the meteorological data record for dispersion modeling, stating that "[T]he use of 5 

years of NWS [National Weather Service] meteorological data or at least I year of site specific 

data is required." Section 8.3.1.2.b further states that "one year or more (including partial years), 

up to five years, of site specific data ... are preferred for use in air quality analyses." Although 

the monitored design value for the I-hour S02 standard is defined in terms of the 3-year average, 

this definition does not preempt or alter the Appendix W requirement for use of 5 years ofNWS 

2 
meteorological data or at least 1 year of site specific data. The 5-year average based on use of 

NWS data, or an average across one or more years of available site specific data, serves as an 

unbiased estimate of the 3-year average for purposes of modeling demonstrations of compliance 

with the NAAQS. Modeling of "rolling 3-year averages," using years 1 through 3, years 2 

through 4, and years 3 through 5, is not required. Furthermore, since modeled results for S02 are 

averaged across the number of years modeled for comparison to the new I-hour S02 standard, 

the meteorological data period should include complete years of data to avoid introducing a 

seasonal bias to the averaged impacts. In order to comply with Appendix W recommendations in 

cases where partial years of site specific meteorological data are available, while avoiding any 

seasonal bias in the averaged impacts, an approach that utilizes the most conservative modeling 

result based on the first complete-year period of the available data record vs. results based on the 

last complete-year period of available data may be appropriate, subject to approval by the 

appropriate reviewing authority. Such an approach would ensure that all available site specific 

data are accounted for in the modeling analysis without imposing an undue burden on the 

applicant and avoiding arbitrary choices in the selection of a single complete-year data period. 

The form ofthe new I-hour S02 standard also has implications regarding appropriate 

methods for combining modeled ambient concentrations with monitored background 

concentrations for comparison to the NAAQS in a cumulative modeling analysis. As noted in 

the March 23 2010 memorandum regarding 'Modeling Procedures for Demonstrating 

Compliance with PM2.5 NAAQS' (EPA 20l0b), combining the 98th percentile monitored value 

with the 98th percentile modeled concentrations for a cumulative impact assessment could result 

in a value that is below the 98th percentile of the combined cumulative distribution and would, 

therefore, not be protective of the NAAQS. However, unlike the recommendations presented for 

PM2.5, the modeled contribution to the cumulative ambient impact assessment for the I-hour S02 

standard should follow the form of the standard based on the 99th percentile of the annual 

distribution of daily maximum I-hour concentrations averaged across the number of years 

modeled. A "first tier" assumption that may be applied without further justification is to add the 

overall highest hourly background S02 concentration from a representative monitor to the 

modeled design value, based on the form of the standard, for comparison to the NAAQS. 

Additional refinements to this "first tier" approach based on some level of temporal pairing of 

modeled and monitored values may be considered on a case-by-case basis, subject to approval by 

the reviewing authority, with adequate justification and documentation. 

Section 8.2.3 of Appendix W provides recommendations regarding the determination of 

background concentrations for multi-source areas. That section emphasizes the importance of 

professional judgment by the reviewing authority in the identification of nearby and other 

sources to be included in the modeled emission inventory, and establishes "a significant 

concentration gradient in the vicinity of the source" under consideration as the main criterion for 

this selection. Appendix W also indicates that "the number of such [nearby] sources is expected 

to be small except in unusual situations." See Section 8.2.3.b. 

The representativeness of available ambient air quality data also plays an important role 

in determining which nearby sources should be included in the modeled emission inventory. 

Key issues to consider in this regard are the extent to which ambient air impacts of emissions 

from nearby sources are reflected in the available ambient measurements, and the degree to 

3 
which emissions from those background sources during the monitoring period are representative 

of allowable emission levels under the existing permits. The professional judgments that are 

required in developing an appropriate inventory of background sources should strive toward the 

proper balance between adequately characterizing the potential for cumulative impacts of 

emission sources within the study area to cause or contribute to violations of the NAAQS, while 

minimizing the potential to overestimate impacts by double counting modeled source impacts 

that are also reflected in the ambient monitoring data. 

We would also caution against the literal and uncritical application of very prescriptive 

procedures for identifying which background sources should be included in the modeled 

emission inventory for NAAQS compliance demonstrations, including those described in 

Chapter C, Section IV.C.1 of the draft New Source Review Workshop Manual (EPA, 1990), 

noting again that Appendix W emphasizes the importance of professional judgment in this 

process; While the draft workshop manual serves as a useful general reference that provides 

potential approaches for meeting the requirements of New Source Review (NSR) and PSD 

programs, it is not the only source of EPA modeling guidance. The procedures described in the 

manual may be appropriate in some circumstances for defining the spatial extent of sources 

whose emissions may need to be considered, but not in others. While the procedures described 

in the NSR Workshop Manual may appear very prescriptive, it should be recognized that "[i]t is 

not intended to be an official statement of policy and standards and does not establish binding 

regulatory requirements." See, Preface. 

Given the range of issues involved in the determination of an appropriate inventory of 

emissions to include in a cumulative impact assessment, the PSD applicant should consult with 

the appropriate reviewing authority early in the process regarding the selection and proper 

application of appropriate monitored background concentrations and the selection and 

appropriate characterization of modeled background source emission inventories for use in 

demonstrating compliance with the new I-hour S02 standard. 

SUMMARY 

To summarize, we emphasize the following points: 

1. Current guidance in Appendix W for modeling to demonstrate compliance with the 

previous 24-hour and annual primary S02 standards, and 3-hour secondary S02 standard, 

is generally applicable for the new I-hour S02 NAAQS. 

2. While the I-hour NAAQS for S02 is defined in terms of the 3-year average for monitored 

design values to determine attainment of the NAAQS, this definition does not preempt or 

alter the Appendix W requirement for use of 5 years ofNWS meteorological data or at 

least I year of site specific data. 

REFERENCES 

Cimorelli, A. J., S. G. Perry, A. Venkatram, J. C. Weil, R. 1. Paine, R. B. Wilson, R. F. Lee, W. 

D. Peters, R. W. Brode, and 1. O. Paumier, 2004. AERMOD: Description of Model Formulation 

with Addendum, EPA-454/R-03-004. U.S. Environmental Protection Agency, Research 

4 
Triangle Park, NC. 

EPA, 1990. New Source Review Workshop Manual: Prevention of Significant Deterioration and 

Nonattainment Area Permitting - DRAFT. US. Environmental Protection Agency, Research 

Triangle Park, NC. 

EPA, 2004. User's Guide for the AMS/EPA Regulatory Model- AERMOD. EPA-454/B-03-

001. US. Environmental Protection Agency, Research Triangle Park, NC. 

EPA, 2009. Addendum - User's Guide for the AMS/EPA Regulatory Model- AERMOD. 

EPA-454/B-03-001. US. Environmental Protection Agency, Research Triangle Park, NC. 

EPA, 201 Oa. Applicability of the Federal Prevention of Significant Deterioration Permit 

Requirements to New and Revised National Ambient Air Quality Standards. Stephen D. Page 

Memorandum, dated April 1, 2010. US. Environmental Protection Agency, Research Triangle 

Park, NC. 

EPA,2010b. Modeling Procedures for Demonstrating Compliance with PM2.5 NAAQS. 

Stephen D. Page Memorandum, dated March 23,2010. US. Environmental Protection Agency, 

Research Triangle Park, NC. 

cc: Richard Wayland, C304-02 

Anna Marie Wood, C504-01 

Lydia Wegman, C504-02 

Raj Rao, C504-01 

Roger Brode, C439-01 

James Thurman, C439-01 

Dan deRoeck, C504-03 

Elliott Zenick, OGC 

Brian Doster, OGC 

EP A Regional Modeling Contacts 

5