MEMORANDUM 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

RESEARCH TR IANGLE PARK, NC 27711 

MAR 23 2010 

OFFICE OF 

AIR QUALITY PLANNING 

AND STANDARDS 

SUBJECT: Modeling Procedures for Demonstratin Ol~jnCe with PM2 5 NAAQS 

Office 

Stephen 

of 

D, 

Air 

Page

Quality 

, Direct 

PI 

r 

nd Standa2.J 

rd~ 

See Addressees 

FROM: 

TO: 

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 of significant deterioration (PSD) 

programs may be required to demonstrate compliance with PM2.5 NAAQS rather than relying 

upon the PM 10 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 eFR 

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

issues associated with PM2.5 modeling. 

This memorandum provides recommendations on two aspects of the modeling procedures 

for demonstrating compliance with the PM2.5 NAAQS. First, this memorandum discusses some 

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

seeking to rely on the PM] o surrogate policy. Second, this memorandum provides additional 

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

relying upon the PM] 0 surrogate 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 PM2.5 as the indicator. EPA revised the 24-hour 

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

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

PM] o, while revoking the previous annual standard for PM]o. For attainment of the new 24-hour 

PM2.

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

Intemet Address (U RL) • hHp:/Iwww.epa.gov 

RecycledIRecyclable • 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 PM2.5 NAAQS is set at 15 

ftg/m3 based on the average of the annual mean PM2.5 concentrations over three years. 

Citing significant technical difficulties with respect to PM25 monitoring, emissions 

cstimation, and modeling, EPA established a policy, known as the PMlo surrogate policy, on 

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

PMlo 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 of July 15,2008, to continue relying on thc PMlo surrogate policy. In 

response to a petition challenging the continued use of the PMlo surrogate policy for issuing PSD 

permits, on June 1,2009, EPA issued a 3-month administrative stay of the 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 of the 

grand fathering provision from the PM2.5 NSR implementation rule for federal PSD permits 

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

grandfathering provision in the Federal Regis!er 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 of the proposed rulemaking to repeal the grandfathering provision contained in 

the federal PSD program, EPA has also proposed to end the use of the PM 10 surrogate policy for 

state PSD programs that EPA has approved as part of the 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 PMlo 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 PMlo surrogate policy as long as (I) 

the appropriateness of the PMlo-based assessment for determining PM2.5 compliance has been 

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

that a PM2.5 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 PM 10 surrogate policy to 

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

is appropriate and timely to address some of the technical issues associated 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 brief summary of the case law history thai bears on the PMIO surrogacy issue 

2 
which suggests that an appropriateness demonstration "would need to address the differences 

between PMlO and PM25."] 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 of the surrogate policy should be demonstrated: 

1) 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 of the technical and legal issues 

associated with the use of the 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 

PM2.5" 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 

relation 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 of the "differences between PMIO and PM25" in relation to ambient air impacts 

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

l. While EPA revoked in 2006 the annual PM[o standard that was in effect when the 

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

appropriateness of the PM[o 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 flgim3 is well below the previous level of 65 flgim3 

that was in effect when the PM[o surrogate policy was established. The background 

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

of the cumulative impacts from a modeling analysis relative to the current 24-hour PM2.5 

NAAQS than for PM 10. 

3. Secondary formation of PM25 from emissions of NO" SOx 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 ofPM[o, 

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

impacts between PM25 and PM [0. 

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

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

3 
4. The probabilistic form of the PM2.5 NAAQS, based on the multiyear average of the 981h 

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

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

high-sixth-high (I-16H) value over 5 years. These differences affect the temporal and 

spatial characteristics of the ambient air impacts of PM 10 and PM25 . Differences in the 

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

monitored background levels to estimate cumulative impacts under the NSRJPSI) 

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

facility will cause a significant contribution to any modeled violations of the NAAQS 

that may occur. 

These factors complicate the viability of demonstrating the appropriateness of the PMlo 

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

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

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

thc PSI) permitting program, provided that the technical difficulties with respect to PM2.5 

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 

PMlo 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 PMlo impacts are taken as a direct surrogate for PM2.5 impacts and 

compared to the PM2.5 NAAQS. If an adequate accounting for contributions from background 

PM2.5 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 of surrogacy could be reasonably found in this 

example. An analysis of source-specific PM2.5/ PMlo 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 analyses for purposes of surrogacy demonstrations and modeling 

analyses that are intended to explicitly demonstrate compliance with the PM2.5 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. 

4 
PM2.5 MODELING ANALYSES 

The differences between PMJO 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 PM2.5 NAAQS 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 PM2.

5 in the cumulative analysis, certain aspects 

of standard 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 model, PSD modeling 

of PM25 should currently be viewed as screening-level analyses, analogous to the screening 

nature of the guidance in Section 5.2.4 of Appendix W regarding dispersion modeling for N02 

impacts given the importance of chemistry in the conversion of NO 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 increase from the 

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

impact. Thus, the first step of the 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 of the 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 of the facility for purposes of determining the 

geographic range of the background source emission inventory that would be appropriate should 

a cumulative impact assessment be necessary. 

5 
EPA's 2007 proposed rule to establish PSD increments, 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 PM2.5 SILs are finalized, the proposed SILs may not 

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

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 of the 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 of the 

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 of the NAAQS, we recommend that the 

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

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

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

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

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

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

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

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

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

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

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

since that will more accurately characterize the modeled contribution from the facility in relation 

to the NAAQS than use of the 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 are 

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 of the NAAQS. 

Several aspects of the 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. 

6 
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 of the facility should generally be 

included in the modeled inventory. Since modeling of PM2.5 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 PMJO inventories may 

provide a useful starting point for this effort. 

Monitored Background 

The determination of representative 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 of the monitored background concentration for 

PM25 is that the monitored data should account for the contribution of secondary PM2.5 

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

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

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

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

Also, due to the important role of secondary 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 modeled and monitored concentrations of PM25 for comparison to the 

PM2.5 NAAQS also entails considerations that differ from those for other criteria pollutants, due 

to the issues identified above. Given the importance of secondary 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 of the PM2.5 NAAQS, 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 of the cumulative analysis. 

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

annual average PM2.5 concentrations; for the 24-hour averaging period, the design value is based 

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

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

7 
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 of the modeled annual averages across 5 years for 

NWS meteorological data or the highest modeled annual average for one year of site-specific 

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

the annual PM2.

5 NAAQS 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 of the maximum modeled 24-hour averages 

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

year of site-specific meteorological data. As noted above, use of the average modeled 

concentration across the appropriate time period more accurately characterizes the modeled 

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

from individual years, while using the average of the 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 of the combined cumulative distribution and would, therefore, not be 

protective of the 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 be 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 of the PM2.5 NAAQS, 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 of the modeled impacts from 

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

significant contribution determination should be made following the same procedures used 

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

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

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

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

8 
the affected receptor(s) is compared to the SIL, while the average of the highest 24-hour average 

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

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

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

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

across the modeling period. 

Synopsis 

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

annual averages and the average of the 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 of the background emission inventory. From data 

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

year's annual averages and the average of the 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 of the annual PM2.

5 

concentrations, and the 3-year average of the 981h percentile 24-hour averages. To determine 

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

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

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

average of the 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: 

1. 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.

5 should adequately account for 

secondary contribution from background sources in most cases, if the facility emits 

significant quantities of PM25 precursors, some assessment of their 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 PM2.5 are 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. 

9 
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, therc 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 propcrties may vary 

depending on the constitucnt clements ofPM25 , Therefore, use of deposition algorithms 

to account for depletion in estimating ambient PM2.5 concentrations should be done with 

caution and only when clear documentation and justification of the deposition parameters 

is provided, 

3, While EPA has proposed PSD incremcnts 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 of the recommendations presented here in 

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

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

plan to provide further clarification of these differences separately, once the increments 

are finalized, 

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

for demonstrating compliance with the PM2.5 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 concerning this mcmorandum, please contact Tyler Fox, Leader, Air Quality Modeling 

Group at (919) 541-5562, 

Addressees: 

Bill Harnett, C504-0 1 

Richard Wayland, C304-02 

Scott Mathias, C504-0 1 

Tyler Fox, C439-01 

Raj Rao, C504-0 I 

Roger Brode, C43 9-0 1 

Bret Anderson, C43 9-01 

Dan deRoeck, C504-01 

EPA Regional Modeling Contacts 

10