UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

RESEARCH TRIANGLE PARK, NC 27711

MAR 23 2010

OFFICE OF

AIR QUALITY PLANNING

AND 8TANDAROS

MEMORANDUM

SUBJECT: Modeling Procedures for Demonstratin

FROM: Stephen D. Page, Direct r

Office of Air Quality PI

TO: See Addressees

This memorandum addresses the need for recommendations regarding appropriate

dispersion modeling procedures which can be used to demonstrate compliance with PM2.5

National Ambient Air Quality Standards (NAAQS). The need for these recommendations arises

from several recent regulatory actions and proposals which increase the likelihood that applicants

for permits under the new source review (NSR) and prevention ofsignificant deterioration (PSD)

programs may be required to demonstrate compliance with PM25 NAAQS rather than relying

upon the PM10 surrogate policy established in 1997. These recommendations are intended to

facilitate appropriate and consistent implementation of current guidance regarding PM25

dispersion modeling contained in the Guideline on Air Quality Models, Appendix W to 50 CFR

Part 51, while acknowledging that such guidance is somewhat limited in detail due to technical

issues associated with PM25 modeling.

This memorandum provides recommendations on two aspects ofthe modeling procedures

for demonstrating compliance with the PM25 NAAQS. First, this memorandum discusses some

ofthe technical issues that must be addressed by any applicant or permitting authority that is

seeking to rely on the PM10 surrogate policy. Second, this memorandum provides additional

information on modeling procedures to demonstrate compliance' with PM25 NAAQS without

relying upon the PMIO smrogate policy.

BACKGROUND

On July 18,1997, EPA revised the NAAQS for particulate matter to add new annual and

24-hour standards for fine particles using PM2.5as the indicator. EPA revised the 24-hour

NAAQS for PM2.5 on September 21,2006, reducing the standard from 65 flg/m3 to 35 flg/m3

EPA also retained the previous 1997 annual standard for PM2.5 and the 24-hour standard for

PMIO, while revoking the previous annual standard for PM1o. For attainment ofthe new 24-hour

PM2.

5NAAQS based on ambient monitoring, the average ofthe 98th percentile 24-hour values

Intemet Address (URL). http://www.epa.gov

Recycled/Recyclable. Printed with Vegetable Oil Based Inks on Recycled Paper (Minimum 25% Postconsumer)
over three years of monitoring must not exceed 35 ftg/m3 The annual PM25 NAAQS is set at 15

ftg/m3 based on the average of the annual mean PM25 concentrations over three years.

Citing significant technical difficulties with respect to PM25 monitoring, emissions

estimation, and modeling, EPA established a policy, known as the PMIO surrogate policy, on

October 23, 1997. This policy allowed permit applicants to use compliance with the applieable

PMIO requirements as a surrogate approach for meeting PM25 NSR requirements until the

technical difficulties were resolved. On May 16, 2008, EPA promulgated final rules governing

the implementation ofthe NSR program for PM25, which included a "grandfathering provision"

allowing applicants for federal PSD permits covered by 40 CFR § 52.21, with complete permit

applications submitted as ofJuly 15,2008, to continue relying on the PM10 surrogate policy. In

response to a petition challenging the continued use ofthe PMIO surrogate policy for issuing PSD

permits, on June 1,2009, EPA issued a 3-month administrative stay ofthe grandfathering

provision for PM25 affecting federal PSD permits to give EPA time to propose repealing the

challenged grandfathering provision. On September 16,2009, the original 3-month stay was

extended to June 22, 2010, to allow additional time for EPA to formally propose repeal ofthe

grandfathering provision from the PM25 NSR implementation rule for federal PSD permits

issues under 40 CFR § 52.21 .. On February I I, 20 I 0, EPA published its proposal to repeal the

grandfathering provision in the Federal Register at 75 FR 6827. These actions cite the fact that

the technical difficulties which necessitated the PMIO surrogate policy have been largely,

although not entirely, resolved.

As part ofthe proposed rulemaking to repeal the grandfathering provision contained in

the federal PSD program, EPA has also proposed to end the use ofthe PM10 surrogate policy for

state PSD programs that EPA has approved as part ofthe state implementation plan (SIP) under

40 CFR § 51.166. Under the PSD programs for PM25 currently in effect for SIP-approved states,

states would be allowed to continue using the PMIO surrogate policy until May 2011, or until

EPA approves the revised SIP for PM25, whichever occurs first. While we continue to allow

states to use the PMIO surrogate policy during their transition to the new PM25 requirements, we

have also made it clear that the policy needs to be implemented by taking into account court

decisions that address the surrogacy concept. Accordingly, an applicant seeking a PSD permit

under a SIP-approved PSD program may still rely upon the PMIO surrogate policy as long as (l)

the appropriateness ofthe PM1o-based assessment for determining PM2.5 compliance has been

adequately demonstrated based on the specifics ofthe project; and (2) the applicant can show

that a PM25 analysis is not technically feasible. Absent such demonstrations, applicants would

be required to submit a PM2.5-based assessment to demonstrate compliance with the PM25

standards, in addition to meeting the other requirements under the NSRlPSD programs.

PMIO SURROGACY DEMONSTRATIONS

Given the need for applicants that continue to rely on the PM10 surrogate policy to

demonstrate the appropriateness ofthe policy based on the specifics ofthe project, we feel that it

is appropriate and timely to address some ofthe tcchnieal issues associatcd with a surrogacy

demonstration. EPA's August 12,2009, Administrative Order in response to petitions regarding

the Title V permit for Louisville Gas and Electric Company (LG&E), Trimble Generating

Station, provides a briefsummary ofthe case law history that bears on the PMIO surrogacy issue

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which suggests that an appropriateness demonstration "would need to address the differences

between PMlO and PM25."J The LG&E order cites two examples in this regard: I) "emission

controls used to capture coarse particles may be less effective in controlling PM2S"; and 2)

"particles that make up PM25 may be transported over long distances while coarse particles

normally only travel short distances." These examples serve to highlight the two main aspects of

PSD permitting for which the appropriateness ofthe surrogate policy should be demonstrated:

I) the Best Available Control Technology (BACT) emission control technology assessment; and

2) the ambient air quality impact assessment to demonstrate compliance with the applicable

NAAQS.

While acknowledging "an evolving understanding ofthe technical and legal issues

associated with the use ofthe PMIO Surrogate Policy," the LG&E order ofIers two steps as

possible approaches for making an appropriateness demonstration, without suggesting that the

"two steps are necessary or sufficient to demonstrate that PMIO is a reasonable surrogate for

PM25" and clearly stating that "these two steps are not intended to be the exclusive list of

possible demonstrations" regarding surrogacy. The two steps offered in the LG&E order are

primarily relevant to the appropriateness demonstration regarding emission controls under

BACT, while the discussion here will be focused on the appropriateness demonstration in

rclation to ambient air impacts.

Given the range of application-specific factors that may need to be addressed for an

appropriateness demonstration in relation to ambient air impacts, it is not practical to provide

detailed guidance regarding how to conduct such demonstrations. However, the following list

identifies some ofthe "differences between PMIO and PM25" in relation to ambient air impacts

that should be addressed in the development of a surrogacy demonstration:

I. While EPA revoked in 2006 the annual PM 10 standard that was in effect when the

surrogate policy, the surrogacy demonstration would still need to address the

appropriateness ofthe PM10 surrogate policy in relation to the annual PM25 standard, and

would likely require a modeling analysis of annual PMIO impacts.

2. The current 24-hour NAAQS of 35 flg/m3 is well below the previous level of 65 flg/m3

that was in effect when the PM10 surrogate policy was established. The background

monitored levels of PM25 are, therefore, likely to account for a more significant ii'action

ofthe cumulative impacts from a modeling analysis relative to the current 24-hour PM25

NAAQS than for PM 10.

3. Secondary formation of PM25 from emissions of NO" SO, and other compounds from

sources across a large domain will often contribute significantly to the total ambient

levels of PM2S, and may be the dominant source of ambient PM25 in some cases. In

contrast, secondarily formed particles are less likely to be signifIcant portion of PM10,

which may result in significant differences in the spatial and temporal patterns of ambient

impacts between PM25 and PM1o.

I A discussion ofthe case law that bears on the PMIO surrogacy issue also appears in the February

11,2010, proposed rule at75 FR 6831-6832.

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4. The probabilistic form of the PM25 NAAQS, based on the multiyear average ofthe 98lh

percentile for the daily standard, differs from the expected exceedance form ofthe PMlo

NAAQS, which allows the standard to be exceeded once per year on average using the

high-sixth-high (1-16]-]) value over 5 years. These differences affect thc temporal and

spatial characteristics ofthe ambient air impacts ofPMIO and PM25 . Differences in the

form ofthe NAAQS also complicate the process of combining modeled impacts with

monitored background levels to estimate cumulative impacts under the NSRJPSD

permitting programs, as well as the determination of whether modeled impacts from the

facility will cause a significant contribution to any modeled violations ofthe NAAQS

that may occur.

These factors complicate the viability of demonstrating the appropriateness ofthe PM10

surrogate policy to comply with the requirement for a PM25 ambient air quality impact

assessment. In light ofthese complications, applicants may elect to use PM25 dispersion

modeling to explicitly meet the requirement of an ambient air quality impact assessment under

the PSI) permitting program, provided that the technical difficulties with respect to pM25

monitoring, emissions estimation, and modeling have been sufficiently resolved in relation to the

specific application.

For surrogacy demonstrations, it is assumed that as an initial step the applicant will have

conducted an appropriate dispersion modeling analysis which demonstrates compliance with the

pM10 NAAQS, including an analysis of annual PM 10 impacts to address item I. A simple

example illustrating when a pMIO modeling analysis might serve as a surrogate for pM25

modeling would be if a clearly conservative assumption is made that all PMIO emissions are

PM25, and the modeled PM10 impacts are taken as a direct surrogate for PM2.5 impacts and

compared to the PM25 NAAQS. If an adcquate accounting for contributions from background

PM25 concentrations to the cumulative impact assessment can be made, and a reasonable

demonstration that the modeled PMIO emission inventory adequately accounted for potential

nearby sources of PM25, then the appropriateness ofsurrogacy could be reasonably found in this

example. An analysis ofsource-specific PM25/ pM10 emission factor ratios may also support the

assun1ption of a more realistic, yet still conservative approach for taking a ratio of modeled PM 10

ambient impacts to provide conservative estimates of pM2.5 impacts.

While additional modeling analyses, short of explicit pM25 modeling, may also be used

to the support the surrogacy demonstration in some cases, it is important to make a clear

distinction between modeling analyscs for purposes ofsurrogacy demonstrations and modeling

analyses that are intended to explicitly demonstrate compliance with the pM25 standards. The

distinction between these two types of modeling analyses may not always be clear, but one

important distinction is whether or not a pM25 emission inventory has been developed as the

basis for the modeling. The distinction between these types of modeling is important because

modeling procedures that may be considered appropriate for one type of analysis may not be

appropriate for the other. The following section elaborates further on this point.

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PM2.5 MODELING ANALYSES

The differences between PMIO and PM2.5 described above in relation to surrogacy

demonstrations, especially items 2 through 4, also have implications on how best to conduct an

explicit PM2.5NAAQS compliance demonstration through dispersion modeling. Due to the

potentially significant contribution from secondary formation of PM2.5, and the more prominent

role of monitored background concentrations of PM25 in the cumulative analysis, certain aspects

ofstandard modeling practices used for PM10 and other criteria pollutants may not be appropriate

for PM25. Our recommendations for addressing these issues in terms of explicit PM25 modeling

analyses are described in more detail below.

Given the issues listed above, and especially the important contribution from secondary

formation of PM25, which is not explicitly accounted for by the dispersion modcl, PSD modeling

of PM25 should currently be vicwed as screening-level analyscs, analogous to the screening

nature ofthe guidance in Section 5.2.4 ofAppendix W regarding dispersion modeling for N02

impacts given the importance of chemistry in the convcrsion ofNO emissions to ambient N02.

The screening recommendations presented below for demonstrating compliance with the PM25

NAAQS through dispersion modeling have been developed with the factors listed above in mind.

As with any modeling analysis conducted under Appendix W, alternative models and methods

may be considered on a case-by-case basis, subject to approval by the Regional Office in

accordance with the recommendations in Section 3.2 on "Use of Alternative Models."

The following sections describe the recommended modeling methods for the two main

stages in a typical PSD ambient air quality analysis: I) preliminary significant impact analysis;

and 2) cumulative impact assessment. The rationale for the recommendations is also provided.

Preliminary Significant Impact Analysis

The initial step in air quality impact assessments under NSRJPSD is typically a

significant impact level analysis to determine whether the proposed emissions increasc from the

proposed new or modified source (i.e., project emissions) would have a "significant" ambient

impact. Thus, the first step ofthe ambient impact analysis is to determine whether those

emissions would result in ambient air concentrations that exceed a de minimis level, referred to

as the Significant Impact Level (SIL). If modeled impacts from the facility do not exceed the

SIL, then the permitting authority may be able to conclude, based on this preliminary analysis,

that the project would not cause or contribute to a violation ofthe NAAQS. Under these

circumstances, EPA would not consider it necessary for the facility to conduct a more

comprehensive cumulative impact assessment that would involve modeling the facility's total

emissions along with emissions from other nearby background sources, and combining impacts

from the modeled emission inventory with representative ambient monitored background

concentrations to estimate the cumulative impact levels for comparison to the NAAQS. The SIL

is also used to establish the significant impact area ofthe facility for purposes of determining the

geographic range ofthe background source emission inventory that would be appropriate should

a cumulative impact assessment be necessary.

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EPA's 2007 proposed rule to establish PSD incrcments, SILs, and a Significant

Monitoring Concentration (SMC) for PM25 included three options for the 1'M2.5 SILs for both

the 24-hour and annual NAAQS. Until the PM25 SILs are finalized, the proposed SILs may not

be presumed to be appropriate de minimis impact levels. However, EPA does not preclude statcs

from adopting interim de minimis impact levels for PM25 to determine whether a cumulative

impact analysis will be necessary, provided that states prepare an appropriate record to support

the value used. Such de minimis levels do not necessarily have to match any ofthe SILs that

have been proposed for PM25, but the levels proposed by EPA and the record supporting EPA's

proposed rule could be considered in the state's determination.

The modeling methods used in this initial significant impact assessment phase ofthe

PM25 analysis, based on either a state's interim de minimis levels or EPA-finalized SILs, are

similar to the methods used for other pollutants, including the use of maximum allowable

emissions. However, due to the probabilistic form ofthe NAAQS, we recommend that the

highest average ofthe modeled annual averages across 5 years for National Weather Service

(NWS) meteorological data or the highest modeled annual average for one year ofsite-specific

meteorological data be compared to the annual screening level (SIL). Similarly, the highest

average ofthe maximum 24-hour averages across 5 years for NWS meteorological data or the

highest modeled 24-hour average for one year ofsite-specific meteorological data should be

compared to the 24-hour screcning level (SIL).

Using the average ofthe highest values across the years modeled preserves one aspect of

the form ofthe NAAQS, while using the average ofthe first highest 24-hour averages rather than

the 98th percentile (8th highest) values from the distribution is consistent with the screening-level

nature ofthe analysis. In addition, since the PM25 NAAQS is based on air quality levels

averaged over time, it is appropriate to use an average modeled impaet for comparison to the SIL

since that willmore accurately characterize the modeled contribution from thc facility in relation

to the NAAQS than use ofthe highest modeled impacts from individual years. At the present

time, the dispersion modeling recommendations presented here are based on modeling only the

primary or direct 1'M25 emissions from the facility.

Cumulative Impact Assessment

Unless modeled ambient air concentrations of PM25 from the project emissions arc

shown to fall below the state's de minimis level or EPA's promulgated SIL (when finalized),

then a cumulative impact assessment would be necessary to account for the combined impact of

facility emissions, emissions from other nearby sources, and representative background levels of

1'M25 within the modeling domain. The cumulative impacts are then compared to the NAAQS

to determine whether the facility emissions will cause or contribute to a violation ofthe NAAQS.

Several aspects ofthe cumulative impact assessment for PM25 will be comparable to

assessments conducted for other criteria pollutants, while other aspects will differ due to the

issues identified above.

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Modeling Inventory

The current guidance on modeling emission inventories contained in Section 8.1 of

Appendix W will generally be applicable for the PM25 modeling inventory, recognizing that

these recommendations only address modeling of primary PM2.5 emissions. The guidance in

Appendix W addresses the appropriate emission level to he modeled, which in most cases is the

maximum allowable emission rate under the proposed permit. Nearby sources that are expected

to cause a significant concentration gradient in the vicinity ofthe facility should generally be

included in the modeled inventory. Since modeling of PM25 emissions has not been a routine

requirement to date, the availability of an adequate PM2.5 emission inventory for background

sources may not exist in all cases. Recommendations for developing PM25 emission inventories

for use in PSD applications will be addressed separately, but existing PMIO inventories may

provide a useful starting point for this effort.

Monitored Background

The determination ofrepresentative background monitored concentrations of PM25 to

include in the PM25 cumulative impact assessment will entail different considerations from those

for other criteria pollutants. An important aspect ofthe monitored background concentration for

PM25 is that the monitored data should account for the contribution ofsecondary PM25

formation representative ofthe modeling domain. As with other criteria pollutants, consideration

should also be given to the potential for some double-counting ofthe impacts from modeled

emissions that may be reflected in the background monitoring, but this should generally he of

less importance for PM2.5 than the representativeness ofthe monitor for secondary contributions.

Also, due to the important role ofsecondary PM2.5, background monitored concentrations of

PM2.5 are likely to be more homogeneous across the modeling domain in most cases, compared

to other pollutants. We plan to address separately more detailed guidance on the determination

of representative background concentrations for PM2.5.

Comparison to NAAQS

Combining the modcled and monitored concentrations of PM25 for comparison to the

PM25 NAAQS also entails considerations that differ from those for other criteria pollutants, due

to the issues identified above. Given the importance ofsecondary contributions for PM25 and

the typically high background levels relative to the NAAQS for PM25, greater emphasis is placed

on the monitored background contribution relative to the modeled inventory. Also, given the

probabilistic form ofthe PM2.5NAAQS, careful consideration must be given to how the

monitored and modeled concentrations are combined to estimate the cumulative impact levels.

The representative monitored PM2.5 design value, rather than the overall maximum

monitored background concentration, should be used as a component ofthe cumulative analysis.

The PM2.5 design value for the annual averaging period is based on the 3-year average ofthe

annual average PM25 concentrations; for the 24-hour averaging period, the design value is bascd

on the 3-year average ofthe 98th percentile 24-hour average PM25 concentrations for the daily

standard. Details regarding the determination ofthe 98th percentile monitored 24-hour value

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based on the number of days sampled during the year are provided in the ambient monitoring

regulations, Appendix N to 40 CFR Part 50.

The modeled annual concentrations of(primary) PM2.5 to be added to the monitored

annual design value should be computed using the same procedure used for the initial significant

impact analysis based on the highest average ofthe modeled annual averages across 5 years for

NWS meteorological data or the highest modeled annual average for one year ofsite-spccific

mctcorological data. The resulting cumulative annual concentration would then be compared to

the annual PM2.

5NAAQS of 15 Ilg/m3.

For the 24-hour NAAQS analysis, the modeled concentrations to be added to the

monitored 24-hour design value should be computed using the same procedure used for the

preliminary analysis based on the highest average ofthe maximum modeled 24-hour averages

across 5 years for NWS meteorological data or the maximum modeled 24-hour average for one

year ofsite-specific meteorological data. As noted above, use ofthe average modeled

concentration across the appropriate time period more accurately characterizes the modeled

contribution from the facility in relation to the NAAQS than use ofthe highest modeled impact

from individual years, while using the average ofthe first highest 24-hour averages rather than

the 98th percentile (8th highest) values is consistent with the screening nature of PM2.5 dispersion

modeling. Furthermore, combining the 98th percentile monitored with the 98th 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.

The recommendations provided above constitute a First Tier modeling analysis for PM25

compliance demonstrations. For applications where impacts from primary PM25 emissions are

not temporally correlated with background PM2.5 levels, combining the modeled and monitored

contributions as described above may bc overly conservative. In these cases, a Second Tier

modeling analysis may be considered that would involve combining the monitored and modeled

PM2.5 concentrations on a seasonal or quarterly basis, and re-sorting the total impacts across the

year to determine the cumulative design value. We plan to provide separately additional details

regarding this Second Tier, including a discussion of circumstances where this approach may be

appropriate.

Determining Significant Contributions to Modeled Violations

If the cumulative impact assessment following these screening recommendations results

in modeled violations ofthe PM2.5NAAQS, then the applicant will need to determine whether

the facility emissions are causing a significant contribution to those modeled violations. A

"significant contribution" determination is based on a comparison ofthe modeled impacts from

the project emissions associated with the modeled violation to the appropriate SIL. Thc

significant contribution determination should be made following the same procedures used

during the initial significant impact analysis, based on a comparison ofthe average ofthe

modeled concentrations at the receptor location showing the violation, across 5 years for NWS

meteorological data and the highest modeled concentration for one year ofsite-specific

meteorological data. For a violation ofthe annual NAAQS, the average ofthc annual values at

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the affected receptor(s) is compared to the SIL, while the average ofthe highest 24-hour average

concentrations at the affected receptor(s) should be used for the 24-hour NAAQS. Use ofthe

average modeled concentration is appropriate in this context since it is consistent with the actual

contribution ofthe facility to the cumulative impacts at the receptor(s) showing violations and

accounts for the fact that modeled violations ofthe 24-hour NAAQS represent average impacts

across the modeling period.

Synopsis

Significant Impact Analysis: Compare the average ofthe highest modeled individual year's

annual averages and the average ofthe first highest individual year's 24-hour average

concentrations fi·om project emissions to their respective screening levels, which may be based

on the state's de minimis levels or EPA-finalized SILs. If modeled impacts exceed the screening

levels, a cumulative impact assessment would need to be performed.

Cumulative Impact Assessment: Develop an emission inventory of background sources to be

included in the modeling analysis using traditional guidance. That would include using the

significant impact area established in the initial significant impact analysis, plus a 50-km annular

ring to determine the geographic extent ofthe background emission inventory. From data

obtained within this combined area, compare the average ofthe highest modeled individual

year's annual averages and the average ofthe first highest individual year's 24-hour averages,

plus representative background monitored concentrations, to their respective NAAQS.

Monitored background concentrations are based on the 3-year average ofthe annual PM2.

5

concentrations, and the 3-year average ofthe 98th percentile 24-hour averages. To determine

whether the proposed project's emissions cause a significant contribution to any modeled

violations ofthe NAAQS, the proposed project's impacts at the affected receptor(s) are

determined based on the average ofthe highest modeled individual years' annual averages and

average ofthe first highest individual years' 24-hour averages from the proposed project's

emissions, and are compared to the state's de minimis levels or EPA-finalized SILs.

Additional Caveats

A few additional caveats should be considered while implementing these

recommendations:

I. The current preferred dispersion model for near-field PM25 modeling, AERMOD, does

not account for secondary formation of PM2.5. Therefore, any secondary contribution of

the facility's or other modeled source's emissions is not explicitly accounted for. While

representative background monitoring data for PM2.

5should adequately account for

secondary contribution from background sources in most cases, ifthe facility emits

significant quantities of PM25 precursors, some assessment oftheir potential contribution

to cumulative impacts as secondary PM25 may be necessary. In determining whether

such contributions may be important, keep in mind that peak impacts due to facility

primary and secondary PM2.5are not likely to be well-correlated in space or time, and

these relationships may vary for different precursors. We plan to issue separately

additional guidance regarding this issue.

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2, While dry and/or wet deposition may be important processes when estimating ambient

concentrations of particulate matter (PM) in general, these factors are expected to be

minor for PM2 5 due to the small particle size, In addition, there may be additional

uncertainty associated with deposition modeling for PM25 due to the variable makeup of

the constituent elements for PM2,5 and the fact that deposition properties may vary

depending on the constituent elements ofPM25 , Therefore, use of deposition algorithms

to account for depletion in estimating ambient PM2,5 eoncentrations should be done with

caution and only when clear documentation and justification ofthe deposition parameters

is provided,

3, While EPA has proposed PSD increments for PM25 , the increments have not been

fInalized yet. Until the increments are finalized, no increment analysis is required for

PM25 , However, it should be noted that some ofthe recommendations presented here in

relation to NAAQS modeling analyses may need to be modified for PM25 increment

analyses due to the differences between the forms ofthe NAAQS and increments, We

plan to provide further clarification ofthese differences separately, once the increments

are finalized,

This memorandum presents EPA's views on these issues concerning modeling procedures

for demonstrating compliance with the PM25 NAAQS, The statements in this memorandum do

not bind State and local governments and the public as a matter of law, If you have any

questions coneerning this memorandum, please contact Tyler Fox, Leader, Air Quality Modeling

Group at (919) 541-5562,

Addressees:

Bill Harnett, C504-0 I

Richard Wayland, C304-02

Scott Mathias, C504-0 I

Tyler Fox, C439-01

Raj Rao, C504-0 I

Roger Brode, C43 9-0 I

Bret Anderson, C43 9-0 I

Dan deRoeck, C504-01

EPA Regional Modeling Contacts

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