### UNITED STATES ENVIRONMENTAL PROTECTION AGENCY**Offi~e** 

### ofAirQuality Planning and Standards

.**Resear~h** 

### TrianglePark, North Carolina 27711


## AUG 23 2010

MEMORANDUM

SUBJECf: Guidance C . g the Implementation ofthe I-hour**S~** 

### NAAQS for the Preven' nof

### ~m~gram FROM: ".?-stephen D.

# ~l**~';J.;;'r·** .


# ,P Office of Qualit;1ianning and Standards

TO: Regional Air Division Directors

On June 2, 2010, the U.S. Environmental ProtectionAgency (EPA) announced a new 1- hour sulfurdioxide**(S~)** 

### National Ambient Air Quality Standard (hereinafter, eitherthe I-hour**S~** 

### NAAQS or I-hour S02 standard) of75 ppb, which is 

### attained whenthe 3-year average of the annual 99th-percentile ofI-hour daily maximumconcentrations does not exceed 75 ppb at each monitor within an area. EPA revised the primary**S~** 

### NAAQS to 

### provide the requisite protectionofpublic health. The final rule for 

### the new I-hour**S~** 

### NAAQS waspublished 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**S~** 

### standards. However, as explained in this guidance, those**S~** 

### standards,

as well as the 24-hour and annual increments for**S~,** 

### remain in effectfor a while further and mustcontinue to 

### be protected.

EPA interprets the Prevention ofSignificantDeterioration (PSD) provisions ofthe Clean AirAct and EPA regulations to require 

### that any federal pennitissued under 40 CFR 52.21 on or after

### that effective date mustcontaina demonstration ofsource compliancewiththe new I-hour S02NAAQS. 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 orcontribute to a modeled violation of

### the new I-hour**S~** 

### NAAQS. We also anticipate problems that sources may have interpreting the modeled I-hour**S~** 

### impacts 

### ifthe form ofthe hourly standard is not properly addressed. To respond to 

### these and otherrelated issues, we are providing the attached guidance, in the form of two memoranda, for implementing the new I-hour**S~** 

### NAAQS under the PSD permitprogram.

The 

### first memorandum, titled "General Guidance for Implementingthe I-hour S02 National AmbientAirQuality Standard in Prevention ofSignificantDeterioration Permits, Including an Interim I-hour**S~** 

### SignificantImpact Level," includes guidance for the preparation and review ofPSD permits with respect to 

### the new I-hour**S~** 

### standard. That

### guidance memorandum sets forth a recommended interim I-hour S02 significant impact level (SIL) that states may considerfor carrying outthe required PSD air quality analysis for**S~,** 

### until EPA promulgates a I-hour**S~** 

### SIL viamlemaking, and addresses the continued use ofthe existing S02 SignificantEmissions Rate (SER) and SignificantMonitoring Concentration (SMC) to implementthe new I-hour S02 standard

.. The second memorandum, titled "Applicability of Appendix WModeling Guidance for the I-hour**S~** 

### National AmbientAir Quality Standard," includes specific modeling guidance for estimating ambient S02 concentrations and determining compliance with the new I-hour**S~** 

### standard.

This guidance does notbind state and local governments and pennit applicants as a matteroflaw. Nevertheless, we believe that state and local air agencies and industry will find this guidance useful for carrying outthe PSD permitprocess and it will provide aconsistent approach for estimating S02 air quality impacts from proposed construction ormodification of S02emissions sources. Forthe mostpart, the attached guidance focuses on how existing policy and guidance is relevant to and should be used for implementing the new I-hour**S~** 

### NAAQS.

Please review the guidance included inthe two attached memoranda. In the event of questions regarding the general implementation guidance contained in the first memorandum, please contactRaj 

### Rao (rao.rai@epa.gov). Forquestions pertaining to the modeling guidance in the second memorandum, please contact TylerFox (fox.tyler@epa.gov). We are continuing our efforts to address pennitting issues related to the implementationofnew and revised NAAQS, and will issue additional guidance to address 

### theNAAQS as appropriate.

Attachments:

1. Memorandum from Anna 

### Marie Wood, 

### Air Quality Policy Division, to EPA Regional AirDivision Directors, "General Guidance for hnplementing the I-hour**S~** 

### National AmbientAir Quality Standard in Prevention ofSignificantDeterioration Permits, Including an Interim I-hour**S~** 

### SignificantImpactLevel" (August 23, 2010). 2. Memorandumfrom TylerFox, Air Quality Modeling Group, to EPA Regional 

### Air Division Directors, "Applicability ofAppendix WModeling Guidance for the I-hour**S~** 

### National AmbientAir Quality Standard" (August 23, 2010).

00: Anna 

### Marie Wood Richard Wayland Lydia Wegman RajRao Tyler Fox DandeRoeck Roger Brode Rich Ossias ElliottZenick Brian Doster

2

### UNITED STATES ENVIRONMENTAL PROTECfION AGENCY Office ofAir 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 ofSignificantDeteriorationPermits, 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



### INTRODUCfION

We are issuing the following guidance to explain and clarifY the procedures that may be followed by applicants for Prevention ofSignificant 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 (SOI) National Ambient Air Quality Standard (hereinafter, either the I-hour S02 NAAQS or I-hour SOz standard) that becomes effective on August 23, 2010. The EPA revised the primary SOz NAAQS bypromulgating a I-hour SOI NAAQS to provide the requisite protection ofpublic health. Under section 165(a)(3) of

### the Clean Air Act (the Act) and sections 52.21 (k) and 5 

### 1.1 66(k) ofEPA's PSD regulations, to obtain a permit, a source mustdemonstrate 

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

This guidance is intendedto (I) highlightthe importance ofa I-hour averaging period for setting an emissions limitation for SOz in the PSD permit (2) reduce the modeling burden to implement the I-hour SOz standard where it can be properly demonstrated that a source will not have a significant impact on ambient I-hour SOI concentrations, and (3) identifY approaches that allow sources and permitting authorities to mitigate, 

### in a manner consistent with existing regulatory requirements, potential modeled violations ofthe I-hour SOz NAAQS, where appropriate. Accordingly, the techniques described inthis memorandum may be 

### used by permit applicants and pennitting authorities to perform an acceptable I-hour S02 NAAQS compliance modeling assessment and/or properly configure projects andpermit conditions 

### in order that a proposed source's emissions do not cause or contributeto modeled I-hour SOz NAAQS violations, so that permits can

### be issued in accordance withthe applicable PSD program requirements.This guidance discusses existing provisions in EPA regulations and guidance, and focuses on the relevancy ofthis infonnation for implementing the new NAAQS for S02. Importantly, however, this guidance also sets forth a recommended interim I-hour SOl 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 ifthey agree that the value represents a reasonable threshold for determining a significant ambient impact, and they incorporate into each permit record a rationale supporting this conclusion. This interim SIL is a useful screening tool that can be used to detennine whether or not the predicted ambient impacts caused by a proposed source's emissions increase will be significant and, ifso whether the source's emissions should be considered to "cause or contribute to" modeled violations ofthe new I-hour SOl NAAQS.


### BACKGROUND

OnAugust 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 demonstmte compliance with "any" NAAQS that is in effect on the date a PSD permit is issued. (See, e.g., EPA memo dated April I, 2010, titled "Applicability ofthe Federal Prevention ofSignificant Deterioration Permit Requirements to New and Revised National Ambient Air Quality Standards.") Due to the promulgation ofthis sbort-tenn averaging period (I-hour) for the S02NAAQS, 

### we anticipate that some new major stationary sources or majormodifications, 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 ofviolations could result from the use ofmaximum modeled concentmtions that do not adequately take into account the form ofthe I-hour 

### standard. To the extent

### that is the case, ambient SOlconcentmtions 

### in the form ofthe 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 ofAppendix W Modeling Guidance for the I-hour S02 National Ambient Air Quality Standard" (August23, 2010) for specific modeling guidance for estimating ambient S02 concentrations consistent with the new I-hour SOl NAAQS.

It is EPA's expectation that currently available S02guidance, including the guidance presented in this memorandum, will assist in resolving some ofthe issues arising from preliminary analyses that show potential exceedances ofthe 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 ofthe standard through design ofthe proposed source orpermit conditions, consistent with existing regulatory requirements. Moreover, the interim I-hour SOl SIL 

### that is included in this guidance will provide a reasonable screeningtool for effectively implementing the PSD requirements for an air quality impact analysis.

The following discussion provides guidance for establishing a I-houremissions limitation to demonstmte compliance with the new NAAQS, and for possibly mitigating

2

### modeled violations using any ofthe following: air quality-based permit

### limits more stringent than whatthe Best Available Control Technology provisions may otherwise require, airquality offsets, "goodengineering practice" (GEP) stackheights, and an interim I-hour S02 SIL. The continueduse ofthe existing S02 SignificantEmissions Rate (SER) and SignificantMonitoring Concentration (SMC) to implementthe new I-hour S02 standard is also discussed.

SCREENING VALUES



### Inthe final rule establishing the I-hour SOI 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 thathave historically been used on a widespreadbasis to facilitate implementation ofthe PSD permitting program:

We agree with the commenters thatthere may be aneed forEPA to provide additional screeningtools orto revise existing screening tools that are frequently used underthe 

### NSRlPSD programfor reducing the burden ofcompleting S0:2 ambient air 

### impactanalyses. These screeningtools include the SILs, as mentioned by the commenter, but also include the SERfor emissions ofSOI and the SMC for S02. The existing screening tools apply to the periods used to define the existingNAAQS for S0:2, including the annual, 24-hour, and 3-houraveraging periods. EPA intends to evaluatethe need forpossible changes oradditions 

### to each ofthese useful screeningtools for S02 due to the revision ofthe S02 NAAQS to provide for a I-hour standard. We believe itis highly likely thatin orderto bemosteffective for implementing the 

### new I-hour averaging periodfor NSR purposes, new I-hour screeningvalues will 

### be appropriate.

75 FR 35579. EPA intends to conduct an evaluationofthese issues and submit ourfindings in the form ofrevised significance levels undernotice and commentrulemaking ifanyrevisions are deemed appropriate. In the interim, for the reasons provided below, we recommend the 

### continued use ofthe existing SERfor S02 emissions as well as an interim I-hour S02 SILthat
we are setting forth today for conducting air

### quality impact analyses for the I-hour SOI NAAQS. As described in the section titled Introduction, EPA intends to implementthe interim I-hour SOI SIL containedherein underthe federal PSD program and offers states the opportunity 

### to use itin theirPSD programs ifthey choose to do so. EPA is not addressing the 

### significant monitoring concentration (SMC) for S0:2 in this memorandum; the 

### existing SMC for S0:2, at40 CFR 52.21 (i)(5)(i) should continue to be used.

SIGNIFICANT EMISSIONS RATE



### The PSD regulations define SERfor various regulatedNSRpollutants. When aproposed
new source's potential to emit apollutant, or amodified source's net emissions increase ofa pollutant, would be less than the SER, the source is notrequired 

### to undergo the 

### requisitePSD analyses (BACT and air

### quality) for that 

### particularemissions increase. Underthe terms 

### of existing EPA regulations, the applicable SERfor SOI is 40 tons per

### year (tpy). 40 CFR 52.2I(b)(23); 40 CFR 51.166(b)(23). Each ofthe significant emissions rates defined in those regulations is specific to an individual pollutantwith no differentiation by averaging time with

3regard 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**S~** across all ofthese averaging times, and we are aware ofno reason why it should not beused 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 airquality impact analysis is required for "each pollutantthat [a source] would have the potential to emit in significant amounts." [40 CFR 52.21 (m)(I)(i)(a); 40 CFR. 51.166(mXI)(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)(I)(i)(b); 40 CFR.51.166(m)(I)(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) ofthe regulations. No additionlil action byEPA 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

Underthe PSD program, a proposed new major stationary source ormajor modification must, among other things, complete an air quality impact analysis that involves performing an analysis ofair quality modeling and ambient monitoring data, where appropriate, to demonstrate compliance with applicable NAAQS. In orderto implementthis requirement, EPA traditionally 

### has provided a screening tool known as the SignificantImpact 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 withthe 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 increaseto show that the proposed source or modification will not cause or contribute to a violation ofthe NAAQS.
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 thatthe source's emissions will notcause or contribute to a modeled violation of any NAAQS. To make this demonstration, EPA 

### has recommended that a cumulative analysis

cover a circular areameasuring 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 ofothersources (existing andnewly permitted), including applicable S02 sources located outside the circular areadescribed above, to account for the cumulative hourly**S~** air quality impacts

I When a proposed source's impact by itselfis nol considered 10be "significant," EPA has long mainlained!hal any further effort on !he part of!he applicant to complete acumulative source impact analysis involving other source impacts wOIlld only yield infonnation oftrivial orno value with respect to lbe required evaluation of!he proposed

source ormodification. The concept ofa SIL is grounded on !he <kminimis principles described by !he court in AlabamaPower Co. v. Costle, 636 F.2d 323, 360 (D.C. Cir. 1980); See also Sur Comra La Conlam/nacion v. EPA, 202 F.3d443,448-49 (I- Cir. 2000) (upholding EPA's use ofSIL to allow permitapplicantto avoidfull impact analysis); In re: PrairieState Gen. Co., PSD Appeal No: 05-05, Slip. Op. at 139 (EAB 2006).



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

### air quality analysis that is required for demonstrating compliance with 

### the NAAQS.2 Accordingly, the source will evaluate its eontribution 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 planto undertake rulemaking to adopt a I-hour S02 SIL value. However, until such time as a I-hour**S~** SIL is defined inthe PSD regulations, 

### we are providing an interim SIL of3 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 ofthis interim I-hour**S~** SIL in eachinstance, a permining authority that utilizes it as part ofan ambient air quality analysis should include in the permitrecord the analysis reflected in this memorandum and the referenced documents to demonstrate that a modeled air quality impact is de minimis, and thereby would notbe considered to cause or contribute to a modeled violation ofthe NAAQS.]

States may also elect to choose another value

### that they believe represents a significant air quality impact relative to the I-hour**S~** NAAQS. The EPA-recommended interim I-hour**S~** 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 ofan approved SIP, or to impose the use ofthe SIL concept on any statethat chooses to implement the PSD program-inparticularthe ambient 

### air quality analysis-without using a SIL as a screening tool. Accordingly, states that implement the PSD program underan EPA-approved SIP may choose to use this interim SIL, another value that may be deemed more appropriate for PSD permittingpurposes in the state ofconcern, or no SIL at aiL The application ofany 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**S~** standard, as described above.

As indicated above, using the interim I-hour S02 SIL, the permit applicant and permining authority can determine: (I) whether, based on the proposed increase in**S~** emissions, a cumulative 

### air quality analysis is required; (2) the area ofimpact within which a cumulative 

### airquality analysis should focus; and (3) whether, as part ofa cumulative 

### airquality analysis, the proposed source's**S~** emissions will cause or contribute to any modeled violation ofthe I-hour**S~** NAAQS.

, A screening tool known as the Significant Monitoring Concentration (SMC) for So, already exists in the PSD regulations. EPA plans to evaluate the existing SMC in light ofthe new I-hour SO, NAAQS; however. the existing value of13 "glm', 24-hour average, should continue to be used until and unless a revised value i. issued through nllemaking.

, Wherethe cumulative airquality analysis identifies a modeled violation ofthe NAAQS or increments, and the proposed source is issued its permit by virtue ofthe fact that its proposed emissions increase is not considered to
cause or contribute to the modeled violation, itis still the permitting authority's responsibility to address such modeled violations independently from the PSD permitting process to determine the nature ofthe problemand to mitigate it accordingly,

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

### 00 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 ofthe following:

• The highest ofthe 5-year averages ofthe maximum modeled I-hour S02 concentrations predicted each year at each receptor, based on 5 years ofNational Weather Service data; or

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

on I year ofsite-specific meteorological data, or the highest ofthe multi-year averages ofthe maximummodeled I-hour S02 concentrations predicted each year at each receptor, based on 2 or more, up to 5 complete years ofavailable site-specific meteorological data.

Additional guidance will be forthcoming for the purpose ofcomparing a proposed source's modeled impacts to the interim I-hour**S~** 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 (Le., "causes or contributes to" a modeled violation).

We derived this interim I-hour S02 SIL by using an impact equal to 4% ofthe I-hour**S~** NAAQS (which is 75 ppb). On June 29, 2010, we issued an interim I-hour**N~** SIL that used an impact equal to 4% ofthe I-hour**N~** 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 ofimpacts relative to the corresponding I-hour NAAQS. 

### In 1980, we defined SERfor eachpollutant subject to PSD. 45 FR 52676 (August 7, 1980) at 52705-52710. ForPM and**S~,** we defined the SER as the emissions rate that resulted in an ambient impact equal to 4% ofthe applicable short-tenn NAAQS. The 1980 analysis focused on levels no higher than 5% ofthe primary standard because ofconcerns 

### that higher levels were found to result in unreasonably large amounts ofincrement being consumed by a single source. Within the range ofimpacts analyzed, we considered two factors 

### that had an important influence onthe choice ofthe significant impact levels: (I) cumulative effect on increment consumption of multiple sources in an area, each making the maximum 

### de minimis emissions increase; and (2) the projected consequence ofa given significant impact level on administrative burden. As explained in the preamble to the 1980rulemaking and the supporting documentation,4 EPA decided to use 4% ofthe 24-hour primary NAAQS for PM and**S~** to define the significant emissions rates (SERs) for those pollutants. See 45 FR 52708. Looking now at a I-hour NAAQS for**S~,** we believe that it is reasonable as an interim approach to use a SIL value 

### that represents 4% ofthe I-hour S02 NAAQS. EPA will consider otherpossible alternatives for developing a I-hour**S~** SIL in a future rulemaking that will provide an opportunity for public participation in the development ofa SIL as part ofthe PSD regulations.

AIR-QUALITY BASED EMISSIONS LIMITATIONS

• EPA evaluated de minimis levels for pollutants for which NAAQS had been estahlished in a document titled "Impact ofProposed and Alternative De Minimis Levels for Criteria Pollulants"; EPA-45012-80-Qn, June 1980.



### 6Once a level ofcontrol 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 ofany 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 ofan 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-downprocess) as a result ofan adverse impact on the NAAQS or PSD increments. 

### In addition, the use ofcertain dispersion techniques is permissible for certain proposed projects for**S~** 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 greaterdetail below in the section addressing GEP stack heightrequirements.

Because compliance with the new S02 NAAQS must be demonstrated on the basis ofa I-hour averaging period, the reviewing authority should ensure that the source's PSD permit defines a maximum allowable hourly emissions limitation for**S~,** regardless ofwhetber it is derived from 

### the BACTtop-down approach or it is the result ofan air-quality based emissions rate. Hourly limits are important 

### because they are the foundation ofthe air quality modeling demonstration relative to the I-hour S02 NAAQS. For estimating the impacts ofexisting
sources, ifnecessary, existing S02 emission inventories used to support modeling for compliance with the 3-hourand 24-hour**S~** 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**S~** standard. The PSD applicant's coordination with the reviewing authority is important in this matterto obtain the most appropriate estimates ofmaximum allowable hourly S02 emissions.



### DEMONSTRATING COMPLIANCEWITII THE NAAQS AND INCREMENTS & 

### MITIGATING MODELED VIOLATIONS WITII 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 ofSignificant Deterioration (PSD)." (July 5, 1988.)] In cases where the air quality analysis predicts violations ofthe 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 

### a/the poin/andrime of any modeledviolarion, 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 contributionto the modeled violation, the source's impactwill 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 ofthis analysis. A 2006 decision by the EPA Environmental Appeals Board (EAB) provides detailed reasoning that demonstrates the permissibility ofa 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



### 7insignificant at the time and place ofthe modeled violations.5 [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 thata source's impact does cause or contribute to a modeled violation, a pennit cannotbe issued without some action to mitigate the source's impact. 

### In accordance with 40 CFR 51.l65(b)6, a major stationary source or major modification (as defined at §51.165(a)(I)(iv) and (v)) that lOcates in a S02 attainment area for the I-hour S<h NAAQS and would cause or contribute to a violation ofthe I-hour S<h NAAQS may "reduce the impact ofits cmissions upon air quality by obtaining sufficient emission reductions to, at a minimum, compensate for its adverse ambient [S<h ] 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 eitherby 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 

### InJerpowerof 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 ofthe NAAQS.

Unlike emissions offsetrequirements in areas designated as nonattainment, in addressing the air quality offset concept, it may not be necessary for a permit applicantto fully offset the proposed emissions increase ifan emissions reduction oflesser quantity will mitigate the adverse air quality impact where the modeled violation was originally identified. ("Although full emission offsets are not required, such a sourcemust obtainemission offsets sufficient to compensate for its airquality 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 [throughthe SIP].

Note that additional guidance for this and other aspects ofthe modeling analysis for the impacts ofS02 emissions on ambientconcentrations ofS<h are addressedin 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."

, While there is no I-hour So, signilicant impact level (SIL) currently defined in the PSD regulations, we believe that states may adopt interim values, with tbe appropriatejustification for such values, to use for permitting purposes. In addition, We are recommending an interim SIL as part ofthis guidance for implementing the SO, requirements in the federal PSD program, and in state programs where states choose to use it

• The same provision is contained in EPA's Interpretative Ruling at40 CFR part 5I Appendix S, section III.

7 In contrast to NonaltainmentNew Source Review permits, offsets are not mandatory requirements in PSD permits ifit can otherwise be demonstrated that a source will not cause or contribute to a violation ofthe NAAQS. See, In

re Krr<mfFiber Glass. GMBH, 8EAD. 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 [mal 

### rule announcing the new I-hour S02 NAAQS explainedthat those standards will remain in effect for a limitedperiod oftime as follows: for current S02 nonattainment areas and SIP call areas, until attainment and maintenance SIPs are approvedby EPA forthe new I-hour S02 NAAQS; for all 

### otherareas, for one year following the effective date ofthe initial designations under section 107(d)(I) forthe new I-hour S02 NAAQS. Accordingly, the annual and 24-hour S02 NAAQS must continue to be protected underthe PSD program for as long as they remain in effectfor a PSD area. There is a more detailed discussion ofthe transition from the existing S02 NAAQS to arevised S02 NAAQS inthatpreamble. 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 notbe revoked in conjUnction with ourdecisionto 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 attitle 1, partC, section 163. The 

### annual and 24-hour S02 increments in section 163 are considered partofthe suite ofstatutory increments applicable to suJfurdioxide that Congress expressly included inthe statutoryprovisions for PSD. As 

### such, those increments cannotberevoked simply because we have decided to revoke the annual and 24-hour S02 NAAQS, uponwhichthe S02 increments are based. Consequently, sources must continue to demonstrate thattheirproposed emissions increases ofS02 emissions will not cause orcontribute to any modcled violation ofthe existing annual and 24-hour S02 increments for as long as those statutory increments remain ineffect Increments forthe I-hour averaging perioddo not yet exist; the Actprovides a specific schedule for the promulgationofadditional regulations, which may include newincrements, following the promulgation ofnew or revisedNAAQS. EPA plans to begin thatrulemaking process inthe

near future to considerthe need for such increments.

"GOOD ENGINEERING PRACTICE" STACKHEIGHT AND DISPERSION TECHNIQUES

Ifapermit applicant is unable to showthatthe source's proposed emissions increase will not cause orcontribute to a modeled violation ofthe new I-hour S02 NAAQS, the problem could be the result ofplume downwasheffects causinghigh ambientconcentrations nearthe

source. In such cases, a source may be able to raise theheight ofits existing stacks (ordesigned stacks ifootyet constructed) to a "goodengineering practice" (GEP) stackheight, or at least 65 meters, measured from the ground-level elevation at

### the base ofthe stack.

While not necessarily eliminatingthe full effectofdownwash in all cases, raising stacks to GEP heightmay provide substantial air

### quality benefits in a mannerconsistentwith statutory provisions (section 123 ofthe Act) governing acceptable stack heights to minimize excessive concentrations due to atmospheric downwash, eddies or wakes. Permit applicants should alsobe
aware ofthe regulatory restrictions on stackheights forthe purpose ofmodeling for compliance withNAAQS and increments. Section 52.2I(b) ofthe PSD regulations currently prohibits the

use ofdispersion techniques, such as stackheights above GEP, merged gas streams, or intermittentcontrols for setting S02 emissions limits to meetthe 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 51.100(11), (gg), (hh), (ii), (jj), (kk) and (nn), and 40 CFR 51.118.

a. Stackheights: A source can include only the actual stack height up to GEPheight when modeling to develop thc S02 emissions limitations orto detennine source compliance with the**S~** NAAQS and increments. This is not a limit on thc actual height ofany stack constructed by a new source or modification, however, andthcre may be circumstances where a source
owner elects to build a stackhigher 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 stackheight 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 ofthc stack, withoutdemonstration or calculation (40 CFR 51.I00(ii)(I»;


### o The refined formulaheight calculated using the dimensions ofnearby structures in accordance with the following equation:



### GEP = 

### H 

### + 1.5L, where H is the height ofthe nearby structure and 

### L is the lesser dimension ofthe heightorrrojected width ofthe nearby structure (40 CFR 51.I00(ii)(2)(ii».

• A GEP stack hcight exceeding the refined formula height may be approved when it
can be demonstrated to be necessary to avoid "excessive concentrations" of**S~** caused by atmospheric downwash, wakes, or eddy effects by the source, nearby structures, or nearby terrain features. (40 CFR 51.100(ii)(3), (jj), (kk»;

• For pwposes ofPSD, "excessive concentrations" means a maximum ground-level concentration from a stackdue in whole or in part to downwash, wakes, and eddy effects produced by nearby structures or ncarby terrain features which individually is at least 40 percent 

### in excess ofthe maximum concentration experienced inthe absence ofsuch 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».

• Forstacks in existence onJanuary 12, 1979, the GEP equation is GEP = 2.5 H(providedthe owner or operator produces evidence thaithis equation was actually relied on in establishing an emission limitation for SO, (40 CFR SI.100(ii)(2Xi)

10

### Reportedly, for economic and otherreasons, many existing source stacks have been constructed atheights 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 SO:! NAAQS. Where this 

### is the case, sources should be awarethat it is permissible for themto increasetheir stackheights up to 65 meters without a GEP demonstration.

b. 

### Otherdispersion techniques: The term "dispersiontechnique" includes any practice carried out to increase final plume rise, subjectto certain exceptions (40 CFR 51.100(hh)(I), (2)(i)

- (v)). Beyond the noted exceptions, such techniques are not allowed for getting credit for modeling source compliance withthe NAAQS and PSD increments. One such exception is for

sources ofS02. Section 51.100(hh)(2)(v) provides that identifiedtechniques thatincrease final exhaust gas plume rise are not eonsideredprohibited dispersion techniques pursuantto section 51.1 00(hh)(1 )(iii)' 

### "where the resulting allowable emissions ofsulfurdioxide from the facility do not exceed 5,000 tons peryear." Thus, proposed modifications that experience difficulty modeling compliance with the new I-hour SO:! NAAQS when relying on BACT or an air quality-based emissions limit alone may permissibly considertechniques to increase theirfinal exhaust gas plume rise consistentwith these provisions.

The definition of"dispersiontechnique" at 40 CFR51.1 OO(hh)(

### 1)(iii) describes techniques that are generallyprohibited, butwhich do not apply with respectto the exemption for SO:!. Accordingly, it 

### is permissible for eligible S02 sources to make 

### adjustments to source process parameters, exhaust gas parameters, stack parameters, orto combine exhaust gases from several existing stacks into one stack, so as to increasethe exhaust gas plume rise. It 

### is important to remember

### thatthe exemption applies to sources thathavefacility-wide allowable S02 emissions ofless than 5,000 tpy resulting from the increase 

### in final exhaust gas plume rise. Thus, proposed modifications should notbase their eligibility to use dispersion on the amount of the proposed net emissions increase, but onthe total source emissions ofSO:!.

The EPAdoes notrecommend orencourage sources to rely on dispersionto demonstrate eompliance with the NAAQS; however, we acknowledge the fact that certain S02sources may legally do so. Forexample, while increasing stackheight is a method ofdispersion, EPA's rnIes allow use ofthat approach to the extent the resulting heightmeets EPA's requirements defining "good engineering practice (GEP)" stack height. See40 CFR 50.1 00(hh)(1)(i), 50.1 oo(ii)(l)-{3). Nevertheless, EPA encourages PSD applicants to seekotherremedies, including the use ofthe most stringentcontrols (beyond top-down BACl) feasible orthe acquisition ofemissions reductions (offsets) from otherexisting sources, to address situations where proposed emissions increases wouldresult inmodeledviolations ofthe SO:! NAAQS.

GENERAL START-UP CONDmONS



### We do not anticipate widespreadproblems associated withhigh short-term SO:! emissions resulting from start-up/shutdown conditions. Many sources are capable ofstarting aunit with natural gas or low-sulfurfuel to avoid significant start-up emissions problems. However, some
sources could experience short-term peaks ofSO:! during start-up or shutdown that could adversely affect the new I-hour SO:! NAAQS. The EPA currently has 

### no**provisio~** 

### for exempting emissions occurring during equipment start-up/shutdownfrom theBACT



### 11requirements 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 permitconditions limiting equipment start-up/shutdown to certain hours ofthe day when impacts are expected to 

### be lower

### than normal. Suchpermit limitations 

### can

### be accounted for in the modeling ofsuch emissions. Applicants should direct other questions arising concerning procedures for modeling startupIshutdown emissions to the applicablepermitting authority 

### to determine the most current modeling guidance.



### In the event ofquestions regarding the general implementation guidance contained inthis memorandum, please contact Raj Rao (raq.raj@epagov).



### cc: Raj 

### Rao, C504-o

### I Dan deRoeck, C504-03 Tyler Fox, C439-0l Roger Brode, C439-0l Richard Wayland, C3Q4-02 L}'dia Wegman, C504-02 Elliott Zenick, OGC Brian Doster, OGC EPA Regional NSR Contacts

12UNITED STATES ENVIRONMENTAL PROTECTION AGENCY Office ofAir Qnality Planning and Standards Research Triangle Park, North Carolina 27711
August 23, 2010

MEMORANDUM

SUBJECT: Applicability ofAppendix W Modeling Guidance for the I-hour S02 National Ambient Air Quality Standard

FROM: Tyler Fox, Leader 

### lsi Air Quality Modeling Group, C439-01

TO: Regional Air Division Directors

INTRODUCTION

On June 2, 20I0, EPA announced a new I-hour sulfur dioxide**(S~)** National Ambient Air Quality Standard (I-hour S02 NAAQS or I-hour**S~** standard) whichis attainedwhen the 3-year average ofthe 99th-percentile ofthe annual distribution ofdaily maximum I-hour concentrations does not exceed 75 ppb 

### at each monitorwithin an area. The final rule for the new I-hour**S~** NAAQS was published inthe Federal Registeron June 22, 2010 (75 FR 35520- 35603), 

### and the standard becomes effective on August23, 2010 (EPA, 2010.). This memorandum clarifies the applicability ofcurrent guidance in the 

### Guideline onAir Quality Models (40 CFR Part 51, Appendix W) for modeling**S~** impacts in accordance with the Prevention ofSignificantDeterioration (PSD) permitrequirements to demonstrate compliance with the new I-hoUr**S~** standard.

SUMMARY OF CURRENT GUIDANCE

Current modeling guidance for estimating ambient impacts ofS02 for comparisonwith applicable NAAQS is presented in Section4 ofAppendix W underthe general heading of "Traditional Stationary Source Models." This guidance acknowledges the fact that ambient S02 impacts are largely a result ofemissions 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 ofterrain, the recommendedmodel is AERMOD" (see Section 4.2.2.b). As described in Section 4.1.d, the AERMOD dispersion model "employs 

### best state-of-practice pararneterizations for characterizingthe meteorological influences and dispersion" (Cimorel1i, 

### etal., 2004; EPA, 2004; EPA, 2009).

Section 7.2.6 ofAppendix W addresses the issue ofchemical transformation for modeling**S~** emissions, stating that:The chemical transfonnation ofS02 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, inurban
areas, where synergistic effects among pollutants are ofconsiderable consequence, chemical transfonnation rates may be ofconcern. In urban area applications, a half-life of 4hours may be applied to the analysis ofS02 emissions. Calculations oftransfonnation coefficients from site specific studies canbe 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 S07 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 ofAppendix W and guidance regarding detennination ofbackground concentrations for purposes ofa cumulative ambient air quality impact analysis is provided in Section 8.2.



### APPLICABILITY OF CURRENT GUIDANCE TO I-HOUR S<hNAAQS

The current guidance in Appendix W regarding S02 modcling in the context ofthe previous 24-hour and annual primary S07 NAAQS and the 3-hour secondary S02 NAAQS is generally applicable to the new I-hour S07 standard. Since short-tenn S02 standards**(~** 24 hours) have been in existence for decades, existing S07 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 withthe new I-hour S02 standard, since issues identified in Table 8-2 ofAppendix W related to short-termvs. 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 ofestimating source impacts

on the I-hour S02 standard. This is especially true ifexisting emission estimates for 3-hour or 24-hourperiods are basedon averages that include zero (0) orreduced emissions for some ofthe hours.

Given the fonn ofthe new I-hour S02 standard, 

### we are providing clarification regarding the appropriate dataperiods for modeling demonstrations ofcompliance withthe NAAQS vs. demonstrations ofattainment ofthe NAAQS through ambientmonitoring. While monitored design values for the I-hour S07 standard are based on a 3-year average (in accordance with Section I(c) ofAppendix T to 40 CFR Part 50), Section 8.3.1.2 ofAppendix W addresses the length ofthe meteorological data record for dispersion modeling, stating that "[T]he use of5 years ofNWS [National Weather Service] meteorological data orat least I year ofsite specific data is required." Section 8.3.1.2.b further states that "one year or more (including partial years), up to five years, ofsite specific data

... are preferred for use in air quality analyses." Although the monitored design value for the I-hour**S~** standard is defined in tenns ofthe 3-yearaverage, this definition does notpreempt or alterthe Appendix W requirement for use of5 years ofNWS



### 2meteorological dataor at least I yearofsite specific data. 

### The 5-year average based on use of NWS data, or an average across one or more years ofavailable site specific data, serves as an unbiased estimate ofthe 3-year average for purposes ofmodeling demonstrations ofcompliance withthe NAAQS. Modeling of"roIling 3-year averages," using years I through 3, years 2 through 4, and years 3 through 5, is not required. Furthermore, since modeled results for S02 are averaged across the numberofyears modeled for comparison to the new I-hour S02 standard, the meteorological dataperiod should include complete years ofdatato avoid introducing a seasonal bias to the averaged impacts. In order to comply with Appendix W recommendations in

cases wherepartial years ofsite specific meteorological data are available, while avoiding any seasonal bias in the averaged impacts, an approach that utilizes the most conservative modeling result based onthe first complete-yearperiod ofthe available datarecord vs. results based onthe lastcomplete-yearperiod ofavailable datamay be appropriate, subject to approval by the appropriate reviewing authority. Such an approach wouldensure 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 inthe selectionofa single complete-year dataperiod.

The form ofthe new I-hour S02 standard also has implications regarding appropriate methods for combining modeled ambient concentrations with monitored background concentrations for comparisonto the NAAQS in a cumulative modeling analysis. As noted in the March 23, 2010 memorandum regarding "Modeling Procedures for Demonstrating Compliance with PMu NAAQS" (EPA, 2010b), combiningthe 98lh percentile monitored value with the 98lh percentile modeled concentrations for a cumulative impact assessment could result in a value that is below the 98th percentile ofthe combined cumulative distribution and would, therefore, notbe protective ofthe NAAQS. However, unlike the recommendations presented for PM2.l, the modeled contribution to the cumulative ambient impact assessment for the I-hour**S~** standard should foIlow the form ofthe standard 

### based onthe 99th percentile ofthe annual distribution ofdaily maximum I-hour concentrations averaged across the numberofyears modeled. A ''firsttier" assumptionthat may be applied without furtherjustification is to addthe overall highest hourly background S02 concentration from arepresentative monitorto the

. modeled design value, 

### based onthe form ofthe standard, for comparisonto the NAAQS. Additional refinements to this "firsttier" approach 

### based on some level oftemporal pairing of modeled and monitored values may be considered on a case-by-case basis, subject to approval by the reviewing authority, with adequatejustification and documentation.

Section 8.2.3 ofAppendix W provides recommendations regarding the determination of background concentrations for multi-source areas. That section emphasizes the importance of professionaljudgmentby the reviewing authority inthe identificationofnearby and other

sources to be included in the modeled emission inventory, and establishes "asignificant concentration gradient in the vicinity ofthe source" under consideration as the main criterion for 

### this selection. Appendix W also indicates 

### that "the number ofsuch [nearby

## 1sources is expected to be small except in lIl)usual situations." 

### See Section 8.2.3.b.

The representativeness ofavailable ambient airquality data also plays an importantrole in determining which nearby sources should be included in the modeledemission inventory. Key issues to consider in this regard are the extent to which ambient airimpacts ofemissions from nearby sources are reflected in the available ambientmeasurements, and the degree to



### 3which emissions from those background sources dwing the monitoring period are representative ofallowable emission levels under the existing pennits. The professionaljudgments that are required in developing an appropriate inventory ofbackground sources should strive toward the proper balance between adequately characterizing the potential for cumulative impacts of emission sources within the study areato cause or contribute to violations ofthe 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 ofvery prescriptive procedures for identifying which background sources should be included in the modeled emission inventory for NAAQS compliance demonstrations, including those described in ChapterC, Section IV.C.1 ofthe draftNew S01Irce Review Workshop Manllal (EPA, 1990), noting again that Appendix W emphasizes the importance ofprofessionaljudgment in this process: While the draft workshop manual serves as a useful general reference 

### that provides potential approaches for meeting the requirements ofNew Source Review (NSR) and PSD programs, it is not the only source ofEPA modeling guidance. The procedures described in the manual may 

### be appropriate in some cireumstances for defming the spatial extent ofsources whose emissions may needto be considered, but not in others. While the procedures described in the NSR Workshop Manual may appearvery prescriptive, it should be recognized that "[i]t is not intended to be an official statement ofpolicy and standards and does not establish binding regulatory requirements." See, Preface.

.
Given the range ofissues involved in the detennination ofan appropriate inventory of emissions to include in a cumulative impact assessment, the PSD applicant should consultwith the appropriate reviewing authority early in the process regarding the selection and proper application ofappropriate monitored backgroundconcentrations and 

### the selection and appropriate characterization ofmodeled background source emission inventories for use in demonstrating compliance with the new I-hour S02 standard.



### SUMMARY

To summarize, we emphasize the following points:

I. 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 ofthe 3-year average for monitored design values to detennine attainment ofthe NAAQS, this definition does not preempt or alter the Appendix W requirement for use of5 years ofNWS meteorological data or at least Iyear ofsite specific data.



### REFERENCES

Cimorelli, A. 

### J., S. G. Perry, A. Venkatram, 

### J. C. Wei!, R. 

### J. Paine, R. B. Wilson, R. F. Lee, W. D. Peters, R. W. Brode, and 

### J. O. Paumier, 2004. AERMOD: Description ofMode1 Fonnulation with Addendum, EPA-454/R-03-004. U.S. Environmental Protection Agency, Research



### 4Triangle Park, NC.

EPA, 1990. New Source Review Workshop Manual: Prevention ofSignificant Deterioration and Nonattainment Area Permitting

- DRAFT. U.S. Environmental Protection Agency, Research Triangle Park, NC.

EPA, 2004. User's Guide for the AMSIEPA Regulatory Model- AERMOD. EPA-454/B·03- 001. U.S. Environmental Protection Agency, Research Triangle Park, NC.

EPA, 2009. Addendum

- User's Guide for the AMSIEPA Regulatory Model- AERMOD. EPA-454/B-03-001. U.S. Environmental Protection Agency, Research Triangle Park, NC.

EPA, 20

### IOa. Applicability ofthe Federal Prevention ofSignificant Deterioration Pennit Requirements to New and RevisedNational Ambient Air Quality Standards. Stephen D. Page Memorandum, dated April 1,2010. U.S. Environmental Protection Agency, Research Triangle Park, NC.

EPA, 2010b. Modeling Procedures for Demonstrating Compliance with PM2.s NAAQS. Stephen D. Page Memorandum, 

### dated March 23, 2010. U.S. Environmental ProtectionAgency, Research Triangle Park, NC.



### cc: Richard Wayland, C304-02 Anna Marie Wood, C504-01 Lydia Wegman, C504-02 Raj Roo, C504-01 Roger Brode, C439-01 James Thurman, C439-01 Dan deRoeck, C504-03 Elliott Zenick, 

### OGC Brian Doster, OOC EPA Regional Modeling Contacts

5