UNITED STATES ENVIRONMENTAL PROTECTION AGENCY Office ofAir Quality Planning and Standards Research Triangle Park, North Carolina 27711 

## AUG 23 2010

MEMORANDUM

SUBJECT: Guidance C rning the Implementation ofthe I-hour S02NAAQS for the Prevent n of pzj!ca n 

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# t Office ofA'I Quality Planning and Standards

TO: Regional Air DivisionDirectors

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, eitherthe 1-hour S02 NAAQS or 1-hour S02 standard) of75 ppb, which is attained whenthe 3-year average of the annual 99th-percentile ofI-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 ofpublic 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. Inthe 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.

EPA interprets the Prevention ofSignificantDeterioration (PSD) provisions ofthe 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 ofsource compliance with the new I-hour S02 NAAQS. We anticipate that some new major stationary sources or major modifications, especiallythose involving relatively short stacks, may experience difficulty demonstratingthat emissions from proposed projects will not cause or contribute to a modeled violation ofthe new I-hour S02 NAAQS. We also anticipate problems that sources may have interpreting the modeled I-hour S02 impacts ifthe form ofthe hourly standard is notproperly addressed. To respond to these and otherrelated issues, we are providing the attached guidance, 

### inthe form of two memoranda, for implementing the new I-hour S02NAAQS under the PSD permitprogram.
The first memorandum, titled "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," includes guidance for the preparation and review ofPSD permits with respect to the new I-hour S02 standard. Thatguidance memorandum sets forth a recommended interim I-hour S02 significant impact level (SIL) that states may consider for carrying outthe required PSD air quality analysis for S02, until EPA promulgates a I-hour S02 SIL via rulemaking, and addresses the continued use ofthe existing S02 SignificantEmissions Rate (SER) and SignificantMonitoring 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 AmbientAir 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 oflaw. 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 a consistent approach for estimating S02 air quality impacts from proposed construction or modification of S02emissions sources. Forthe mostpart, the attached guidance focuses onhow existing policy and guidance is relevant to and should be used for implementing the new I-hour S02 NAAQS.

Please reviewthe guidance included inthe two attached memoranda. In the event of questions regarding the general implementation guidance contained inthe first memorandum, please contact Raj Rao (rao.raj@epa.gov). For questions pertaining to the modeling guidance in the second memorandum, please contactTylerFox (fox.tyler@epa

.gov). We are continuing our efforts to address permitting issues related to the implementation ofnew and revised NAAQS, and will issue additional guidance to address the NAAQS as appropriate.

Attachments:

1. Memorandum from AnnaMarie 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 ofSignificant DeteriorationPermits, Including an Interim I-hour S02 Significant ImpactLevel" (August 

### 23,2010). 2. 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" (August 

### 23,2010)

.

cc: AnnaMarie Wood Richard Wayland Lydia Wegman Raj Rao Tyler Fox Dan deRoeck Roger Brode Rich Ossias Elliott Zenick Brian Doster



### 2UNITED STATES ENVIRONMENTAL PROTECTION 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 S02National Ambient Air Quality Standard in Prevention ofSignificantDeteriorationPermits, Including an Interim I-hour S02 Significant Impact Level

FROM: AnnaMarie Wood, Acting Director 

### lsi Air Quality Policy Division

TO: Regional Air DivisionDirectors

INTRODUCTION

We are issuing the following guidance to explain and clarify the procedures that may be followed by applicants for Prevention ofSignificantDeterioration (PSD) permits, and permitting authorities reviewing such applications, to properly demonstrate that proposedprojects to construct and operate will not cause or contribute to a modeled violation ofthe new I-hour sulfur dioxide (S02) National AmbientAir Quality Standard (hereinafter, eitherthe I-hour S02 NAAQS or I-hour S02 standard) that becomes effective on August 23,2010. The EPArevised the primary S02 NAAQS bypromulgating a I-hour S02NAAQS to provide the requisite protection ofpublic health. Under section I65(a)(3) ofthe Clean Air Act (the Act) and sections 52.2I(k) and 5I.I66(k) ofEPA's PSDregulations, to obtain a permit, a source must demonstrate that its proposed emissions increase will not cause or contribute to a violation of"anyNAAQS."

This guidance is intendedto (1) highlightthe importance ofa I-hour averaging period for setting an emissions limitation for S02 inthe PSD permit (2) reduce the modelingburdento implementthe I-hour S02 standard where it canbe 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 ofthe I-hour S02 NAAQS, where appropriate. Accordingly, the techniques described inthis memorandummay be usedbypermit 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 S02NAAQS violations, so thatpermits can be issued in accordance withthe applicable PSD program requirements.This guidance discusses existing provisions in EPA regulations and guidance, and focuses onthe relevancy ofthis infonnation for implementingthe newNAAQS for S02. Importantly, however, this guidance also sets forth a recommended interim I-hour S02 significant impact level (SIL) thatEPAwill use when it evaluates applications and issues permits under the federal PSDprogram, and that states may choose to rely upon to implementtheir PSD programs for S02 ifthey agree that the value represents a reasonable threshold for determining a significant ambient impact, and they incorporate into each pennitrecord a rationale supporting this conclusion. This interim SIL is a useful screening tool that can be used to determine whether or notthe predicted ambient impacts causedby a proposed source's emissions increase will be significant and, ifso whether the source's emissions should be consideredto "cause or contributeto" modeled violations ofthe 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 onthe date a PSD permit is issued. (See, e.g., EPAmemo dated April 1, 20I0, titled "Applicability ofthe Federal Prevention ofSignificant Deterioration PermitRequirements to Newand Revised National Ambient Air Quality Standards.") Due to the promulgation ofthis short-term averaging period (I-hour) for the S02NAAQS, we anticipate that some new major stationary sources or major modifications, especiallythose involving relatively short stacks may experience increased difficulty demonstrating that emissions from proposedproject will not cause or contribute to a modeled violation.

We believethat, in some instances, preliminarypredictions ofviolations couldresult from the use ofmaximum modeled concentrations that do not adequately take into account the form ofthe I-hour standard. To the extent that is the case, ambient S02concentrations inthe form ofthe new I-hourNAAQS should be estimated by applying the recommendedprocedures that account for the statistical form ofthe standard. See EPA Memorandum from TylerFox, 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" (August 23, 20I0) for specific modeling guidance for estimating ambient S02 concentrations consistent with the new I-hour S02NAAQS.

Itis EPA's expectationthat currently available S02guidance, includingthe guidance presented in this memorandum, will assist inresolving some ofthe issues arising from preliminary analyses that showpotential exceedances ofthe new I-hour S02NAAQS that would notbe present under more refined modeling applications. In addition, the techniques described in this memorandum may also help avoid violations ofthe standardthrough design ofthe proposed source orpermit conditions, consistent withexisting regulatory requirements. Moreover, the interim I-hour S02 SIL that is included in this guidance will provide a reasonable screening tool for effectively implementingthe 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 newNAAQS, and for possibly mitigating

2

### modeled violations using any ofthe following: air quality-based permit limits more stringent than what the BestAvailable 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 ofthe existing S02 SignificantEmissions Rate (SER) and SignificantMonitoring Concentration (SMC) to implementthe 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 ofthe 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 ofcompleting S02 ambient air impact analyses. These screening tools include the SILs, as mentioned by the commenter, but also include the SERfor emissions ofS02 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 ofthese useful screening tools for S02 due to the revision ofthe 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 NSRpurposes, new I-hour screening values will be appropriate.

75 FR 35579. EPA intends to conduct an evaluation ofthese issues and submit our findings in the form ofrevised significance levels under notice and commentrulemaking ifany revisions are deemed appropriate. In the interim, for the reasons providedbelow, we recommend the continued use ofthe 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 sectiontitled Introduction, EPA intends to implementthe interim I-hour S02 SIL contained herein under the federal PSD program and offers states the opportunity to use it in their PSD programs ifthey 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 NSRpollutants. When aproposed
new source's potential to emit a pollutant, or a modified source's net emissions increase ofa 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 ofthe significant emissions rates defined in those regulations is specific to an individual pollutant with 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 appliedthe 40 tpy SERfor S02 across all ofthese averaging times, and we are aware ofno reason why it should notbe used for the I-hour averaging period for the presenttime. Therefore, until the evaluation described above and any associated rulemaking are completed, we will use 40 tpy as the SERfor the I-hour standard.

Under existingregulations, an ambient air quality impact analysis is required for "each pollutantthat [a source] would have the potential to emitin 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 "eachpollutant for which [the modification] wouldresult 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 belowthe significant emissions rates inparagraph (b)(23) ofthe regulations. No additional action by EPA orpermitting authorities is necessary at this time to apply the 40 tpy significant emissions rate in existingregulations to the hourly S02 standard.

INTERIM I-HOUR 

### S02 SIGNIFICANT IMPACT LEVEL

Under the PSD program, aproposed newmajor stationary source or major 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 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 notrequired to model anything besides its ownproposed emissions increase to showthatthe proposed source or modification will not cause or contribute to a violation ofthe NAAQS.l
If, on the otherhand, 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 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 ofother sources (existing and newlypermitted), including applicable S02 sources located outside the circular area described above, to account for the cumulative hourly S02 air quality impacts

1 When aproposed source's impact by itselfis not considered to be "significant," EPA has long maintained that any further effort on the part ofthe applicant to complete a cumulative source impact analysis involving other source impacts would only yield information oftrivial or no value with respect to the required evaluation ofthe proposed
source or modification. The concept ofa 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 ofSIL to allow permit applicantto avoid full impact analysis); In re: Prairie State Gen. Co., PSD Appeal No. 05-05, Slip. Op. at 139 (EAB 2006).

4that are predictedto occur. The applicant may also have to gather ambient monitoring data as partofthetotal air quality analysisthat is required for demonstrating compliance withthe NAAQS.2 Accordingly, the source will evaluate its contributionto any modeledviolation ofthe I-hour S02 NAAQS to determine whetherthe source's emissions contributionwill cause or contribute to the modeled violation at any receptor. Notethat inthe accompanying modeling guidance memorandum we are providingrecommended procedures and guidance for completing the modeling analysis to demonstrate compliance withthe new I-hour S02 NAAQS.

Weplanto undertake rulemaking to adopt a I-hour S02 SIL value. However, until such time as a I-hour S02 SIL is defined inthe PSDregulations, we are providing an interim SIL of3 ppb, whichwe intendto use as a screening tool for completingthe required air quality analyses for the new I-hour S02 NAAQS underthe federal PSD program at 40 CFR 52.21. We are also makingthe interim SIL available to States with EPA-approved implementationplans containing
a PSD program to use attheir discretion. To support the application ofthis interim I-hour S02 SIL in each instance, a permitting authority that utilizes it as part ofan ambient air quality analysis should include inthe permitrecord the analysis reflected inthis memorandum and the referenced documents to demonstratethat a modeled air quality impact is de minimis, and thereby would not be consideredto cause or contribute to a modeled violation ofthe NAAQS.3

States may also elect to choose another value that they believe represents a significant air quality impactrelative to the I-hour S02 NAAQS. The EPA-recommended interim I-hour S02 SIL is not intended to supersede any interim SILthat any state chooses to rely uponto implement a state PSD programthat is part ofan approved SIP, orto impose the use ofthe SIL concept on any state that chooses to implementthe PSD program-inparticularthe 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 permittingpurposes inthe state ofconcern, or no SIL at all. The application ofany SIL that is notreflected 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, usingthe interim I-hour S02 SIL, the permit applicant and permitting authority can determine: (1) whether, based onthe proposed increase in S02 emissions, a cumulative air quality analysis is required; (2) the area ofimpactwithinwhich a cumulative air quality analysis should focus; and (3) whether, as part ofa cumulative air quality analysis, the proposed source's S02 emissions will cause or contribute to anymodeledviolation ofthe 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 ofthe 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. 3Where the cumulative air quality analysis identifies a modeled violation ofthe NAAQS or increments, and the proposed source is issued its permitby virtue ofthe 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 permittingprocess to determine the nature ofthe problem and to mitigate it accordingly,



### 5As mentioned above, we are providing an interim I-hour S02 SIL value of3 ppb to implement the federal PSD program. To determine initially whether a proposedproject's emissions increase will have a significant impact (resulting inthe need for a cumulative air quality analysis), this interim SIL shouldbe comparedto either ofthe following:

• The highest ofthe 5-year averages ofthe maximummodeled I-hour S02 concentrations predicted eachyear at eachreceptor, based on 5 years ofNational Weather Service data; or • The highest modeled I-hour S02 concentration predicted across all receptors based
on 1year ofsite-specific meteorological data, orthe highest ofthe multi-year averages ofthe maximummodeled I-hour S02 concentrations predicted each year at eachreceptor, 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 S02 SIL in orderto 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 byusing an impact equal to 4% ofthe I-hour S02NAAQS (which is 75 ppb). OnJune 29, 2010, we issued an interim I-hourN02SIL that used an impact equal to 4% ofthe I-hourN02 standard. As explained inthe June memorandum,
we have chosenthis approach because we believe it is reasonable to base the interim I-hour SIL directly on consideration ofimpacts relative to the corresponding I-hourNAAQS. In 1980, we defined SERfor eachpollutant subjectto PSD. 45 FR 52676 (August 7, 1980) at 52705-52710. For PM and S02, we definedthe SERas the emissions rate thatresulted in an ambient impact equal to 4% ofthe applicable short-term NAAQS. The 1980 analysis focused on levels no higher than 5% oftheprimary standardbecause 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 consideredtwo factors that had an importantinfluence onthe choice ofthe significantimpact levels: (1) cumulative effect onincrement consumption of multiple sources in an area, eachmaking 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 1980 rulemaking and the supporting documentation,4 EPA decided to use 4% ofthe 24-hour primary NAAQS for PM and S02to define the significant emissions rates (SERs) for thosepollutants. See 45 FR 52708. Lookingnow at a I-hour NAAQS for S02, we believe that itis 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 S02 SIL in a future rulemaking that will provide an opportunity for public participation inthe development ofa SIL as part ofthe PSD regulations. AIR-QUALITY BASED EMISSIONS LIMITATIONS

4EPA evaluated de minimis levels for pollutants for which NAAQS had been established in a documenttitled "Impact ofProposed and Alternative De Minimis Levels for Criteria Pollutants"; EPA-450/2-80-0n, June 1980.

6Once a level ofcontrol is determined bythe PSD applicant viathe Best Available Control Technology (BACT) top-downprocess, the applicant must model the proposed source's emissions at the BACT emissions rate(s) to demonstrate thatthose emissions will not cause or contribute to a violation ofany NAAQS orPSD 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-downprocess 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 viathe top-downprocess) as a result ofan adverse impact on the NAAQS orPSD increments. In addition, the use ofcertain dispersiontechniques is permissible for certain proposedprojects for S02 thatmay need to be considered where emissions limitations alone may not enable the source to demonstrate compliance withthe new I-hour S02 NAAQS. This is discussed in greater detail below inthe section addressing GEP stackheightrequirements.

Because compliance with the new S02 NAAQS must be demonstrated onthe basis ofa I-hour averaging period, the reviewing authority should ensure that the source's PSD permit defines a maximum allowable hourly emissions limitationfor S02, regardless ofwhether it is derived from the BACTtop-down approach or it is the result ofan air-quality based emissions rate. Hourly limits are importantbecause they are the foundation ofthe air quality modeling demonstration relative to the I-hour S02NAAQS. Forestimatingthe impacts ofexisting sources, ifnecessary, 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 ofmaximum allowable hourly S02 emissions.

DEMONSTRATING COMPLIANCE WITH THE NAAQS AND INCREMENTS & MITIGATING MODELED VIOLATIONS WITH AIR QUALITY OFFSETS

A 1988 EPAmemorandumprovides 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 ManagementDivision, "Air QualityAnalysis for Prevention ofSignificantDeterioration (PSD)." (July 5, 1988

.)] In cases where the air quality analysis predicts violations ofthe I-hour S02 NAAQS, butthe permit applicant can showthat the S02 emissions increase from the proposed source will not have a significant impact at the pointandtime of any modeledviolation, 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 ofthis analysis. A 2006 decision by the EPA Environmental Appeals Board (EAB) provides detailedreasoning that demonstrates the permissibility ofa finding that a PSD source would not be considered to cause or contribute to a modeled NAAQS violationbecause its estimated air quality impact was

7insignificant at the time and place ofthe 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 areafor the I-hour S02 NAAQS and would cause or contribute to a violation ofthe I-hour S02NAAQS may "reduce the impact ofits 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 byreducing its emissions at the
source (e.g., promoting more efficientproduction methodologies and energy efficiency) orby 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 ofthe NAAQS.

Unlike emissions offsetrequirements in areas designated as nonattainment, in addressing the air quality offset concept, it may notbe 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 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 offsetprogram 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 ofthe modeling analysis for the impacts ofS02 emissions on ambient concentrations ofS02 are addressed in EPA modeling guidance, including the attached August 23,2010 Memorandumtitled "Applicability of Appendix W Modeling Guidance for the I-hour S02 National Ambient Air Quality Standard."

5While 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 appropriatejustification for such values, to use for permitting purposes. In addition, we are recommending an interim SIL as part ofthis guidance for implementing the S02 requirements in the federal PSD program, and in state programs where states chooseto use it.
6 The same provision is contained in EPA's Interpretative Ruling at 40 CFRpart 51 Appendix S, section III.

7 In contrast to Nonattainment New 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 oftheNAAQS. See, In
re Knauf Fiber Glass, GMBH, 8 E.A.D. 121, 168 (EAB 1999).



### 8Although 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 explainedthat those standards will remain in effect for a limited period oftime 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 ofthe initial designations under section 107(d)(1) for the new I-hour S02 NAAQS. Accordingly, the annual and 24-hour S02NAAQS 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 ofthe transition from the existing S02 NAAQS to a revised S02 NAAQS inthat 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 notbe revoked in conjunction with our decisionto revoke the corresponding S02 NAAQS. Instead, the annual and 24-hour S02 increments (Class I, II and III increments) will remain in effectbecause they are defined inthe Clean Air Act at title I, part C, section 163. The annual and 24-hour S02 increments in section 163 are consideredpart ofthe suite ofstatutory increments applicable to sulfur dioxide that Congress expressly included inthe statutoryprovisions for PSD. As such, those increments cannotbe revoked simplybecause we have decided to revoke the annual and 24-hour S02NAAQS, upon which the S02 increments are based. Consequently, sources must continue to demonstrate thattheirproposed emissions increases ofS02 emissions will not cause or contributeto any modeled violation ofthe 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 Actprovides a specific schedule for the promulgation ofadditional regulations, which may include newincrements, following the promulgation ofnew orrevised NAAQS. EPA plans to beginthatrulemaking process inthe
near future to considerthe need for such increments.

"GOOD ENGINEERING PRACTICE" STACKHEIGHT AND DISPERSION TECHNIQUES

Ifapermit applicant is unable to showthat the source's proposed emissions increase will not cause or contribute to a modeled violation ofthe new I-hour S02NAAQS, the problem could bethe result ofplume downwash effects causing high ambient concentrations nearthe
source. In such cases, a source may be able to raise the height ofits existing stacks (or designed stacks ifnot yet constructed) to a "good engineeringpractice" (GEP) stackheight, or at least 65 meters, measured from the ground-level elevation at the base ofthe stack.

While not necessarily eliminatingthe full effect ofdownwash in all cases, raising stacks to GEP height mayprovide substantial air quality benefits in a manner consistent with statutory provisions (section 123 ofthe Act) governing acceptable stackheights to minimize excessive concentrations due to atmospheric downwash, eddies or wakes. Permit applicants should also be
aware ofthe regulatory restrictions on stackheights for the purpose ofmodeling for compliance withNAAQS and increments. Section 52.21(h) ofthe PSD regulations currentlyprohibits the
use ofdispersiontechniques, such as stackheights above GEP, merged gas streams, or intermittent controls for setting S02 emissions limits to meetthe NAAQS and PSD increments.



### 9However, stack heights in existence before December 31, 1970, and dispersion techniques implementedbefore then, are not affected by these limitations. EPA's general stack height regulations are promulgated at 40 CFR 51.1OO(f!), (gg), (hh), (ii), Gj), (kk) and (nn), and 40 CFR 51.118.

a. Stackheights: A source can include only the actual stackheight up to GEP height when modeling to develop the S02 emissions limitations orto determine source compliance with the S02 NAAQS and increments. This is not a limit on the actual height ofany stack constructed by a new source or modification, however, and there 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 stackheight 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 impactmay be modeled using the greater of:
o A de minimis stackheight equal to 65 meters, as measured from the groundlevel elevation at the base ofthe stack, without demonstration or calculation (40 CFR 51.1OO(ii)(1));
o The refined formula height calculatedusing the dimensions ofnearby structures in accordance with the following equation:

GEP

## =H 

### + 1.SL, where H is the height ofthe nearby structure and L is the lesser dimension oftheheight orprojected width ofthe nearby structure (40 CFR 51.100(ii)(2)(ii)).

• A GEP stackheight exceedingthe refined formula height may be approved when it
canbe demonstrated to be necessary to avoid "excessive concentrations" ofS02 caused by atmospheric downwash, wakes, or eddy effects bythe source, nearby structures, or nearby terrain features

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

• Forpurposes ofPSD, "excessive concentrations" means a maximum ground-level concentrationfrom a stack due in whole or in part to downwash, wakes, and eddy effects produced by nearby structures or nearby terrain features whichindividually is at least 40 percent in excess ofthe maximum concentration experienced inthe absence ofsuch effects and (a) which contributes to atotal 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)



### 10Reportedly, for economic and other reasons, many existing source stacks have been constructed at heights less than 65 meters, and source impact analyses may showthatthe 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 itis permissible for themto increase their stack heights up to 65 meters without a GEP demonstration.

b. Other dispersion techniques: The term "dispersiontechnique" includes any practice carried out to increase final plume rise, subject to certain exceptions (40 CFR 51.1OO(hh)(1), (2)(i)

- (v». Beyondthe 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 ofS02. Section 51.100(hh)(2)(v) provides that identified techniques thatincrease final exhaust gas plume rise are not consideredprohibited dispersion techniques pursuantto section 51.1OO(hh)(1)(iii) "where the resulting allowable emissions ofsulfur dioxide from the facility do not exceed 5,000 tons peryear." Thus, proposed modifications that experience difficulty modeling compliance withthe new I-hour S02 NAAQS whenrelying 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"dispersiontechnique" at 40 CFR 51.1

## o O(hh)(1)(iii) describes techniques that are generally prohibited, but which do not apply withrespect 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, orto combine exhaust gases from several existing stacks into one stack, so as to increase the exhaust gas plume rise. Itis important to rememberthat the exemption applies to sources that have facility-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 on the total source emissions ofS02.

The EPA does notrecommend or encourage sources to rely on dispersionto demonstrate compliance with the NAAQS; however, we acknowledge the fact that certain S02sources may legally do so. For example, while increasing stackheight is a method ofdispersion, EPA's rules allow use ofthat approach to the extentthe resulting height meets EPA's requirements defining "good engineering practice (GEP)" stackheight. See 40 CFR 50.1

## o o (hh)(1)(i), 50.1OO(ii)(l)-(3). Nevertheless, EPA encourages PSD applicants to seek otherremedies, including the use ofthe most stringent controls (beyond top-down BACT) feasible orthe acquisition ofemissions reductions (offsets) from other existing sources, to address situations where proposed emissions increases would result in modeled violations ofthe S02 NAAQS.

GENERAL START-UP CONDITIONS

We do not anticipate widespread problems associated withhigh short-term S02 emissions resulting from start-up/shutdown conditions. Many sources are capable ofstarting aunit with natural gas or low-sulfur fuel to avoid significant start-up emissions problems. However, some sources could experience short-term peaks ofS02 during start-up or shutdownthat could adversely affectthe new I-hour S02 NAAQS. The EPA currentlyhas no provisiol;1s for exempting emissions occurring during equipment start-up/shutdown from the BACT

11requirements or for air quality analyses to demonstrate compliance with the S02NAAQS and increments. Therefore, such emissions shouldbe addressed in the required BACT and air quality analyses.

There are approaches to addressing issues related to start-up/shutdown emissions. For example, sources maybe willing to accept enforceable permit conditions limiting equipment start-up/shutdownto certain hours ofthe day when impacts are expectedto be lower than normal. Suchpermitlimitations canbe accounted for inthe modeling ofsuch emissions. Applicants should direct other questions arising concerningprocedures for modeling startup/shutdown emissions to the applicable permitting authority to determine the most current modeling guidance

.
In the event ofquestions regarding the general implementation guidance containedin 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 LydiaWegman, C504-02 Elliott Zenick, OGC BrianDoster, OGC EPA Regional NSR Contacts

12

### UNITED STATESENVIRONMENTAL PROTECTIONAGENCY Office ofAir QualityPlanning and Standards Research TrianglePark, North Carolina 27711 August23,2010

MEMORANDUM
SUBJECT: Applicability ofAppendix 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 DivisionDirectors

INTRODUCTION

On June 2, 2010, EPA announced a new I-hour sulfur dioxide (S02) National Ambient Air Quality Standard (I-hour S02NAAQS or I-hour S02 standard) which is attained when the 3-year average ofthe 99th-percentile ofthe annual distribution ofdaily maximum I-hour concentrations does not exceed 75 ppb at eachmonitor within an area. The final rule for the new I-hour S02 NAAQS was published inthe Federal Register on June 22, 2010 (75 FR 35520- 35(03), and the standard becomes effective onAugust 23,2010 (EPA, 20IOa). This memorandum clarifies the applicability ofcurrent guidance in the Guideline onAir Quality Models 

### (40 CFRPart 51, Appendix W) for modeling S02 impacts in accordance withthe Prevention ofSignificantDeterioration (PSD) permitrequirements to demonstrate compliance withthe new I-hour S02 standard.

SUMMARY OF CURRENT GUIDANCE

Current modeling guidance for estimating ambient impacts ofS02 for comparison with applicable NAAQS is presented in Section 4 ofAppendix W under the 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 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 characterizingthe meteorological influences and dispersion" (Cimorelli, etai., 

### 2004; EPA, 2004; EPA, 2009).
Section 7.2.6 ofAppendix W addresses the issue ofchemical transformation for modeling S02 emissions, stating that:The chemical transformation 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, in urban areas, where synergistic effects among pollutants are ofconsiderable consequence, chemical transformation rates maybe ofconcern. In urban area applications, a half-life of 4 hours may be applied to the analysis ofS02 emissions. Calculations oftransformation 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 documentationis provided. Such conversionfactors for pollutanthalf-life should not be used with screening analyses.

The AERMOD model incorporates the 4 hour half-life for modeling ambient S02 concentrations inurban areas underthe 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 determination ofbackground concentrations for purposes ofa cumulative ambient air quality impact analysis is provided in Section 8.2.
APPLICABILITY OF CURRENT GUIDANCE TO 1-HOUR 

### S02 NAAQS

The current guidance inAppendix W regarding S02 modeling in the context ofthe previous 24-hour and annual primary S02NAAQS and the 3-hour secondary S02NAAQS 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 withthe 3-hour and 24-hour S02 standards should serve as a useful starting point, and maybe adequate in many cases for use in assessing compliance with the new I-hour S02 standard, since issues identified in Table 8-2 ofAppendix 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 ofestimating source impacts
on the I-hour S02 standard. This is especiallytrue ifexisting emission estimates for 3-hour or 24-hour periods are based on averages that include zero (0) orreduced emissions for some ofthe hours.

Given the form ofthe new I-hour S02 standard, we are providing clarificationregarding the appropriate dataperiods for modeling demonstrations ofcompliance withthe NAAQS vs. demonstrations ofattainment ofthe 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) ofAppendix T to 40 CFRPart 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 or at least Iyear ofsite specific data is required." Section 8.3.1.2.b further states that "one year or more (includingpartial 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 S02 standardis defined in terms ofthe 3-year average, this definition does notpreempt or alter the Appendix W requirement for use of5 years ofNWS



### 2meteorological data or at least 1 year ofsite specific data. The 5-year average based onuse 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"rolling 3-year averages," using years 1through 3, years 2 through 4, and years 3 through 5, is notrequired. Furthermore, since modeledresults for S02 are averaged across the number ofyears modeled for comparisonto the new I-hour S02 standard, the meteorological data period should include complete years ofdatato avoid introducing a seasonal bias to the averaged impacts. In order to comply with Appendix W recommendations in
cases where partial 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 on the first complete-year period ofthe available datarecord vs. results based on the last complete-yearperiod ofavailable data may be appropriate, subjectto approval by the appropriate reviewing authority. Such an approach would ensure that all available site specific data are accounted for inthe modeling analysis without imposing an undue burden on the applicant and avoiding arbitrary choices in the selection ofa 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 monitoredbackground 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 PM2

.5NAAQS' (EPA 20l0b), combining the 98th percentile monitoredvalue with the 98th percentile modeled concentrations for a cumulative impact assessment couldresult in a value that is belowthe 98th percentile ofthe combined cumulative distribution and would, therefore, not be protective ofthe NAAQS. However, unlike the recommendations presented for PM2.5, the modeled contributionto the cumulative ambient impact assessment for the I-hour S02 standard should follow the form ofthe standardbased on the 99th percentile ofthe annual distribution ofdaily maximum I-hour concentrations averaged across the number ofyears modeled. A "firsttier" assumption that may be applied without furtherjustification is to add the overall highest hourly background S02 concentration from a representative monitor to the modeled design value, based on the form ofthe standard, for comparisonto the NAAQS. Additional refinements to this "firsttier" approach based on some level oftemporal pairing of modeled and monitored values maybe 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 professionaljudgment by the reviewing authority inthe identification ofnearby and other
sources to be included in the modeled emission inventory, and establishes "a significant concentration gradient inthe vicinity ofthe source" under consideration as the main criterion for this selection. Appendix W also indicates that "the number ofsuch [nearby] sources is expected to be small except in unusual situations." See Section 8.2.3.b.

The representativeness ofavailable ambient air quality data also plays an importantrole in determining which nearby sources should be included in the modeled emission inventory. Key issues to consider inthis regard are the extent to which ambient air impacts ofemissions from nearby sources are reflected in the available ambient measurements, and the degree to

3which emissions from those background sources during the monitoring period are representative ofallowable emission levels under the existingpermits

. The professionaljudgments that are required in developing an appropriate inventory ofbackground sources should strive toward the properbalance between adequately characterizing the potential for cumulative impacts of emission sources withinthe study area to 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 whichbackground sources should be included inthe modeled emission inventory for NAAQS compliance demonstrations, including those described in Chapter C, SectionIV.C.1 ofthe draftNew Source Review Workshop Manual (EPA, 1990), noting again that Appendix W emphasizes the importance ofprofessionaljudgmentinthis process; While the draft workshop manual serves as a useful general reference that provides potential approaches for meetingthe requirements ofNew Source Review (NSR) and PSD programs, it is notthe only source ofEPA modeling guidance. The procedures described in the manual may be appropriate in some circumstances for defining the spatial extent ofsources whose emissions may need to be considered, butnot in others. While the procedures described inthe NSRWorkshop Manual may appear very prescriptive, it should be recognized that "[i]t is not intended to be an official statement ofpolicy and standards and does not establishbinding regulatory requirements." See, Preface.

Given the range ofissues involved inthe determination ofan 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 ofappropriate monitored background concentrations and the selection and appropriate characterization ofmodeled background source emission inventories for use in demonstrating compliance withthe new I-hour S02 standard.


### SUMMARY

To summarize, we emphasize the following points:

1

. Current guidance inAppendix 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 S02NAAQS. 2. While the I-hourNAAQS for S02 is defined interms ofthe 3-year average for monitored design values to determine attainment ofthe NAAQS, this definition does notpreempt or alter the Appendix W requirement for use of5 years ofNWS meteorological data or at least I year ofsite 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 ofModel Formulation with Addendum, EPA-454/R-03-004. U.S. Environmental ProtectionAgency, Research

4Triangle Park, NC.
EPA, 1990. New Source ReviewWorkshop Manual: Prevention ofSignificant Deterioration and Nonattainment Area Permitting

- DRAFT. US. Environmental ProtectionAgency, Research Triangle Park, NC.
EPA, 2004. User's Guide for the 

### AMS/EPARegulatory Model- AERMOD. 

### EPA-454/B-03- 001. US. Environmental Protection Agency, Research Triangle Park, NC.
EPA, 2009. Addendum

- User's Guide for the 

### AMS/EPARegulatory Model- AERMOD. 

### EPA-454/B-03-001. US. Environmental Protection Agency, Research Triangle Park, NC.
EPA, 201Oa. Applicability ofthe Federal Prevention ofSignificant DeteriorationPermit Requirements to New and RevisedNational 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 withPM2

.5 NAAQS. Stephen D. Page Memorandum, dated March 23,2010. US. Environmental Protection Agency, Research Triangle Park, NC.

cc: Richard Wayland, C304-02 AnnaMarie Wood, C504-01 Lydia Wegman, C504-02 Raj Rao, C504-01 Roger Brode, C439-01 James Thurman, C439-01 Dan deRoeck, C504-03 ElliottZenick, OGC Brian Doster, OGC EPA Regional Modeling Contacts

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