UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

Offi~e of Air Quality Planning and Standards

.Resear~h Triangle Park, 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 Protection Agency (EPA) announced a new 1-

hour sulfur dioxide (S~) National Ambient Air Quality Standard (hereinafter, either the I-hour

S~ NAAQS or I-hour S02 standard) of 75 ppb, which is attained when the 3-year average of

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

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

protection ofpublic health. The final rule for the new I-hour S~ NAAQS was published in the

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

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

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

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

must continue to be protected.

EPA interprets the Prevention ofSignificant Deterioration (PSD) provisions ofthe Clean

Air Act and EPA regulations to require that any federal pennit 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,

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

emissions from proposed projects will not cause or contribute to a modeled violation ofthe 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 other related issues, we are providing the attached guidance, in the form of

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

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

National Ambient Air Quality Standard in Prevention ofSignificant Deterioration Permits,

Including an Interim I-hour S~ Significant Impact 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 consider for carrying out the required PSD air quality analysis for S~,

until EPA promulgates a I-hour S~ SIL via mlemaking, and addresses the continued use ofthe

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

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

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

includes specific modeling guidance for estimating ambient S02 concentrations and determining

compliance with the new I-hour S~ standard.

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

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

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

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

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

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

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

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

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

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

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

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

Attachments:

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

Air Division Directors, "General Guidance for hnplementing the I-hour S~ National

Ambient Air Quality Standard in Prevention ofSignificant Deterioration Permits,

Including an Interim I-hour S~ Significant Impact Level" (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 S~

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

00: Anna Marie Wood

Richard Wayland

Lydia Wegman

RajRao

Tyler Fox

DandeRoeck

Roger Brode

Rich Ossias

Elliott Zenick

Brian Doster

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UNITED STATES ENVIRONMENTAL PROTECfION AGENCY

Office of Air Quality Planning and Standards

Research Triangle Park, North Carolina 27711

August 23, 2010

MEMORANDUM

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

Quality Standard in Prevention ofSignificant Deterioration Permits, Including an

Interim I-hour S02 Significant Impact Level

FROM: Anna Marie Wood, Acting Director lsi

Air Quality Policy Division

TO: Regional Air Division Directors

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 ofthe 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 by promulgating a I-hour SOI NAAQS to provide the requisite

protection of public health. Under section 165(a)(3) ofthe Clean Air Act (the Act) and sections

52.21 (k) and 51.166(k) ofEPA's PSD regulations, to obtain a permit, a source must demonstrate

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

This guidance is intended to (I) highlight the importance of a 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 in this 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 and permit conditions in order that a

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

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

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

focuses on the relevancy 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

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 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 S02 NAAQS, we anticipate

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

relatively short stacks may experience increased difficulty demonstrating that emissions from

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

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

from the use ofmaximum modeled concentmtions that do not adequately take into account the

form ofthe I-hour standard. To the extent that is the case, ambient SOl concentmtions 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" (August 23,

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 or permit conditions, consistent with existing regulatory requirements.

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

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

analysis.

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

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

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modeled violations using any ofthe following: air quality-based permit limits more stringent

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

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

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

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

SCREENING VALUES

In the 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 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 S0:2 ambient air

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

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

screening tools apply to the periods used to define the existing NAAQS for S0:2,

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 NSR purposes, 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 comment rulemaking ifany revisions are

deemed appropriate. In the interim, for the reasons provided below, 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 SOI NAAQS.

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

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 S0:2 in this memorandum; the existing SMC for S0:2, at 40 CFR

52.21(i)(5)(i) should continue to be used.

SIGNIFICANT EMISSIONS RATE

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

new source's potential to emit a pollutant, or a modified source's net emissions increase 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 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 pollutant with no differentiation by averaging time with

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

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

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

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

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

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

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

52.21 (m)(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 by EPA or permitting authorities is necessary at this time to apply the 40 tpy significant

emissions rate in existing regulations to the hourly S02 standard.

INTERIM I-HOUR S02 SIGNIFICANT IMPACT LEVEL

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

I When a proposed source's impact by itselfis nol considered 10 be "significant," EPA has long mainlained!hal any

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

impacts wOIlld only yield infonnation oftrivial or no value with respect to lbe required evaluation of!he proposed

source or modification. The concept ofa SIL is grounded on !he <k minimis principles described by !he court in

Alabama Power Co. v. Costle, 636 F.2d 323, 360 (D.C. Cir. 1980); See also Sur Comra La Conlam/nacion v. EPA,

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

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

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

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

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

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

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

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

a PSD program to use at their discretion. To support the application ofthis interim I-hour S~

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

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

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

thereby would not be 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 of an approved SIP, or to impose the use ofthe SIL

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

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

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

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

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

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

the I-hour 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 air quality analysis should focus; and (3) whether, as part ofa cumulative air quality

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 of 13 "glm', 24-hour average, should continue to be used until and unless a revised value i. issued through

nllemaking.

, Where the cumulative air quality 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, it is still the permitting authority's responsibility to address such

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

mitigate it accordingly,

5
As 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 of the following:

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

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

Weather Service data; or

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

on I year ofsite-specific meteorological data, or the highest ofthe multi-year

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

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

meteorological data.

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

modeled impacts to the interim I-hour 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 SER for each pollutant subject to PSD. 45 FR 52676 (August 7, 1980) at 52705-52710.

For PM 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 of concerns 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 on the

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 of a given significant impact level on administrative burden. As

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

decided to use 4% 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 other possible alternatives for

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

participation in the development of a 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 of Proposed and Alternative De Minimis Levels for Criteria Pollulants"; EPA-45012-80-Qn, June 1980.

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

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

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

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

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

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

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

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

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

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

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 greater detail

below in the section addressing GEP stack height requirements.

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

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

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

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

with the 3-hour and 24-hour 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 matter to obtain

the most appropriate estimates ofmaximum allowable hourly S02 emissions.

DEMONSTRATING COMPLIANCE WITII 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 of Significant 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/and rime ofany modeled violarion, 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 detailed reasoning that

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

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

7
insignificant at the time and place 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 that a source's impact does cause or contribute to a

modeled violation, a pennit cannot be 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 either by reducing its emissions at the

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

obtaining air quality offsets (see below). [See, e.g., In re InJerpower ofNew 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 offset requirements in areas designated as nonattainment, in addressing

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

proposed emissions increase if an emissions reduction 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 offset program consistent with State Implementation Plan

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

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

violations must be addressed [through the SIP].

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

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

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

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

, 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 appropriate justification 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 at 40 CFR part 5I Appendix S, section III.

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

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

re Krr<mfFiber Glass. GMBH, 8 EAD. 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 explained that

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)(I) for the new I-hour S02 NAAQS.

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

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

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

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

75 FR 35520, at 35580-2.

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

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

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

increments) will remain in effect because they are defined in the Clean Air Act at title 1, part C,

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

suite ofstatutory increments applicable to suJfur dioxide that Congress expressly included in the

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

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

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

increases ofS02 emissions will not cause or contribute to any modcled 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 Act provides a specific schedule

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

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

near future to consider the need for such increments.

"GOOD ENGINEERING PRACTICE" STACK HEIGHT AND DISPERSION

TECHNIQUES

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

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

could be the result ofplume downwash effects causing high ambient concentrations near the

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

stacks ifoot yet constructed) to a "good engineering practice" (GEP) stack height, or at least 65

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

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

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

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

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

aware ofthe regulatory restrictions on stack heights for the purpose ofmodeling for compliance

with NAAQS and increments. Section 52.2I(b) ofthe PSD regulations currently prohibits the

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

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

9
\

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

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

regulations are promulgated at 40 CFR 51.100(11), (gg), (hh), (ii), (jj), (kk) and (nn), and 40 CFR

51.118.

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

when modeling to develop thc S02 emissions limitations or to detennine source compliance with

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

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

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

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

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

accordance with §52.21 (h):

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

impact analysis for emissions;

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

using the greater of:

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

(40 CFR 51.I00(ii)(I»;

o The refined formula height calculated using the dimensions of nearby

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 height or rrojected 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 stack due 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 in the

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 on January 12, 1979, the GEP equation is GEP = 2.5 H (provided the owner or operator

produces evidence thai this equation was actually relied on in establishing an emission limitation for SO, (40 CFR

SI.100(ii)(2Xi)

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

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

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

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

stack heights up to 65 meters without a GEP demonstration.

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

carried out to increase final plume rise, subject to certain exceptions (40 CFR 51.100(hh)(I),

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

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

sources of S02. Section 51.100(hh)(2)(v) provides that identified techniques that increase final

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

51.1 00(hh)(1)(iii)' "where the resulting allowable emissions ofsulfur dioxide from the facility do

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

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

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

exhaust gas plume rise consistent with these provisions.

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

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

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

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

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

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

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

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

the proposed net emissions increase, but on the total source emissions of SO:!.

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

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

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

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

"good engineering practice (GEP)" stack height. See 40 CFR 50.1 00(hh)(1)(i), 50.1 oo(ii)(l)-{3).

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

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

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

increases would result in modeled violations ofthe SO:! NAAQS.

GENERAL START-UP CONDmONS

We do not anticipate widespread problems associated with high short-term SO:! emissions

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

natural gas or low-sulfur fuel 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/shutdown from the BACT

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

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

analyses.

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

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

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

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

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

modeling guidance.

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

memorandum, please contact Raj Rao (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

12
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

Office of Air Qnality Planning and Standards

Research Triangle Park, North Carolina 27711

August 23, 2010

MEMORANDUM

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

National Ambient Air Quality Standard

FROM: Tyler Fox, Leader lsi

Air Quality Modeling Group, C439-01

TO: Regional Air Division Directors

INTRODUCTION

On June 2, 20 I0, EPA announced a new I-hour sulfur dioxide (S~) National Ambient

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

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

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

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

35603), and the standard becomes effective on August 23, 2010 (EPA, 2010.). This

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

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

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

with the new I-hoUr S~ standard.

SUMMARY OF CURRENT GUIDANCE

Current modeling guidance for estimating ambient impacts of S02 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 of emissions from stationary sources. Section 4.2.2 provides specific

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

regulatory applications in all types 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 pararneterizations for characterizing the meteorological influences and

dispersion" (Cimorel1i, et al., 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 of S02 emitted from point sources or single industrial plants

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

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

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

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

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

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

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

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

should not be used with screening analyses.

The AERMOD model incorporates the 4 hour half-life for modeling ambient 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 of Appendix W and guidance regarding

detennination of background concentrations for purposes ofa cumulative ambient air quality

impact analysis is provided in Section 8.2.

APPLICABILITY OF CURRENT GUIDANCE TO I-HOUR S<h NAAQS

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 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 of estimating source impacts

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

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

hours.

Given the fonn ofthe new I-hour S02 standard, we are providing clarification regarding

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

demonstrations of attainment ofthe NAAQS through ambient monitoring. While monitored

design values for the I-hour S07 standard are based on a 3-year average (in accordance with

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

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

years ofNWS [National Weather Service] meteorological data or at 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-year average,

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

2
meteorological data or at least I year ofsite 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 of compliance

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

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

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

cases where partial years 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 data record vs. results based on the

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

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

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

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

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

methods for combining modeled ambient concentrations with monitored background

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

the March 23, 2010 memorandum regarding "Modeling Procedures for Demonstrating

Compliance with PMu NAAQS" (EPA, 2010b), combining the 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, not be 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 on the 99th percentile ofthe annual

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

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

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

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

Additional refinements to this "first tier" 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 adequate justification 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

professional judgment by the reviewing authority in the identification ofnearby and other

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

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

this selection. Appendix W also indicates that "the number ofsuch [nearby1sources is expected

to be small except in lIl)usual situations." See Section 8.2.3.b.

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

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

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

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

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

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

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

proper balance between adequately characterizing the potential for cumulative impacts of

emission sources within the study area to cause or contribute to violations 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 of very prescriptive

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

emission inventory for NAAQS compliance demonstrations, including those described in

Chapter C, Section IV.C.1 ofthe draft New S01Irce Review Workshop Manllal (EPA, 1990),

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

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

potential approaches for meeting the requirements 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 need to be considered, but not in others. While the procedures described

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

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

regulatory requirements." See, Preface. .

Given the range ofissues involved in the detennination of an appropriate inventory of

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

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

application of appropriate monitored background concentrations and the selection and

appropriate characterization 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 of 5 years of NWS meteorological data or at

least I year 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

4
Triangle Park, NC.

EPA, 1990. New Source Review Workshop Manual: Prevention of Significant 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, 20IOa. Applicability ofthe Federal Prevention of Significant Deterioration Pennit

Requirements to New and Revised National 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 Protection Agency,

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