UNITED STATES ENVIRONMENTAL PROTECTION AGENCY RESEARCH TRIANGLE PARK, NC 27711 

## JUN 29 2010

OFFICE OF AlA QUALITY PLANNING AND STANDARDS MEMORANDUM

SUBJECT: Guidance Concerning the Implementation ofthe I-hourN02 NAAQS for the Prevention ofSignificant Dete ,i'ration**pro~r** FROM: Stephen D, Page, DirectoFy<¥4.'»1<W<./ Office ofAir Quality PljIfUli g d Standard

TO: Regional Air Division Directors

On January 22, 2010, the Environmental Protection Agency (EPA) announced a new 1- hour nitrogen dioxide (N02) National Ambient Air Quality Standard (hereinafter, either the 1- hour N02NAAQS or I-hourN02 standard) of 100 parts per billion (ppb), which is attained when the 3-year average ofthe 98th-percentile ofthe annual distribution ofdaily maximum 1- hour concentrations does not exceed 100 ppb at each monitor within an area. EPA revised the primary N02 NAAQS to provide the requisite protection ofpublic health. The final rule for the
new I-hourN02NAAQS was published in the Federal Register on February 9, 2010 (75 FR 6474), and the standard became effective on April 12,2010, EPA policy provides that any federal Prevention ofSignificant Deterioration (PSD) permit issued under40 CFR 52.21 on or after that effective date must contain a demonstration ofsource compliance with the new I-hour N02 standard,

EPA is aware ofreports from stakeholders indicating that some sources-both existing and proposed-are modeling potential violations ofthe I-hour N02 standard. In many cases, the affected units are emergency electric generators and pump stations, where short stacks and limited property rights exist. However, larger sources, including coal-fired and natural gas-fired power plants, refineries, and paper mills, could also model potential violations ofthe new N02 NAAQS.

To respond to these reports and facilitate the PSD permitting ofnew and modified major stationary sources, we are issuing the attached guidance, in the form oftwo memoranda, for implementing the new I-hourN02NAAQS underthe PSD permit program. The guidance contained in the attached memoranda addresses two areas. The first memorandum, titled, "General Guidance for Implementing the I-hourN02 National Ambient Air Quality Standard in Prevention ofSignificant Deterioration Permits, Including an Interim I-hourN02Significant Impact Level," includes guidance for the preparation and review ofPSD permits with respect to the new I-hourN02standard. This guidance memorandum sets forth a recommended interim 1- hourN02significant impact level (SIL) that states may consider when carrying outthe required

Internet Address (URL). htlp:llwww.epa.gov RecycledIRecyclable • Printed wnh Vegetable Oil Based Inks on Recycled Paper (Minimum 25% Poslconsumer)PSD air quality analysis for

### NOz , until EPA promulgates a I-hour

### NOzSIL viarulemaking. The second memorandum, titled "Applicability ofAppendix W Modeling Guidance for the I-hour 

### NOzNational Ambient Air Quality Standard," includes specific modeling guidance for estimating ambient

### NOz concentrations and determining compliance with the new I-hour

### NOz standard.

This guidance does not bind state and local governments and the public as a matter of law. Nevertheless, we believe that state and local air agencies and industry will find this guidance useful when carrying out the PSD permitprocess. We believe it will provide a consistent approach for estimating 

### NO

### zair quality impacts from proposed construction or modification ofNOx emissions sources. Forthe mostpart, the attached guidance reiterates existing policy and guidance, but focuses on how this information is relevantto implementation ofthe new I-hour

### NOz NAAQS.

Please review the guidance included in the two attached memoranda. Ifyou have questions regarding the general implementation guidance contained in the first memorandum, please contactRaj Rao (rao.raj@epa.gov). Ifyou have questions regarding the modeling guidance in the second memorandum, please contact TylerFox (fox.tyler(flJ,epa.gov). We are continuing our efforts to address permitting issues related to N02 and otherNAAQS including the recently-signed I-hour sulfur dioxide NAAQS. We plan to issue additional guidance to address these new I-hour standards in the nearfuture.

Attachments: I. Memorandum from Anna Marie Wood, Air Quality Policy Division, to EPA Regional Air Division Directors, "General Guidance for Implementing the I-hourN02 National Ambient Air Quality Standard in Prevention ofSignificantDeterioration Permits, Including an Interim I-hour

### NO

### zSignificant Impact Levcl" (June 28, 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 

### NOzNational Ambient Air Quality Standard" (June 28, 2010).

cc: Anna Marie Wood Richard Wayland Raj Rao TylerFox Dan deRoeck Roger Brode Rich Ossias ElliottZenick Brian Doster



### 2UNITED STATES ENVIRONMENTAL PROTECTION AGENCY Office ofAirQuality Planning and Standards Research Triangle Park, North Carolina 27711

June 28, 2010

MEMORANDUM

SUBJECT: General Guidanee for Implementing the I-hourN02National Ambient Air Quality Standard in Prevention ofSignifieantDeterioration Permits, Ineluding an Interim I-hourN02 Signifieant Impaet Level

FROM: Anna Marie Wood, Acting Director/s/ Air Quality Policy Division

TO: Regional Air Division Directors

INTRODUCTION

We are issuing the following guidance to explain and elarify 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 construction will not cause or contribute to a violation ofthe new I-hour nitrogen dioxide (N02) National Ambient Air Quality Standard (hereinafter, either the I-hourN02 NAAQS or I-hourN02 standard) that became effective on April 12,2010. EPA revised the primary N02 NAAQS by promulgating a I-hourN02NAAQS to provide the requisite protection ofpublic health. Under section I65(a)(3) ofthe Clean Air Act (the Act) and sections 52.21(k) and 51.166(k) ofEPA's PSD regulations, to obtain apermit, a souree must demonstrate that its proposed emissions increase will not cause orcontribute to a violation ofany NAAQS.

This guidance is intended to: (I) explain the recommended procedures for stakeholders to follow to properly address concerns over high preliminary modeled estimates ofambient N02 concentrations that suggestpotential violations ofthe new I-hourN02 standard under some modeling and permitting scenarios; (2) help reduce the burden ofmodeling for the hourly N02 standard where it can be properly demonstrated that a source will nothave a significant impact
on ambient I-hourN02 eoncentrations; and (3) identify approaehes that allow sources and permitting authorities to mitigate, in a mannerconsistentwith existing regulatory requirements, potential modeled violations ofthe I-hourN02 NAAQS, where appropriate. Accordingly, the techniques described in this memorandum may be used by permit applicants and permitting authorities to configure projects and permit conditions in orderto reasonably conelude that a proposed source's emissions do not cause or contribute to modeled I-hourN02NAAQS violations so that permits can be issued in accordance with the applicable PSD program requirements.

This guidance discusses existing provisions in EPA regulations and previous guidance for applying those provisions but focuses on the relevancy ofthis information for implementing the



### 3new NAAQS for N02

. Importantly, however, this guidance also sets forth a recommended interim I-hour N02 signifIcant impact level (SIL) that EPA will use for implementing the federal PSD program, and that states may choose to rely upon to implementtheir PSD programs for NOx ifthey agree thatthese values representde minimis impact levels and incorporate into each permitrecord a rationale supporting this conclusion. This interim SIL is a useful screening tool that canbe used to determine whether or notthe emissions from a proposed source will significantly impact hourly N02 concentrations, and, ifsignificant impacts are predicted to occur, whetherthe source's emissions "cause orcontribute to" any modeled violations ofthe
new I-hour N02NAAQS.


### BACKGROUND

On April 12, 2010, the new I-hour N02NAAQS became effective. EPA interprets its regulations at40 CFR 52.2I (the federal PSD program) to require permit applicants to demonstrate 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 of Significant Deterioration Permit Requirements to New and Revised National Ambient Air Quality Standards.") Due to the introduction ofa short-term averaging period for the I-hour N02NAAQS, we anticipate that some stationary sources with relatively short stacks may experience increased difficulty demonstrating that emissions from new construction or modifications will not cause or contribute to aviolation ofthe I-hour N02NAAQS.

We are responding to reports from stakeholders which indicate that some sources, existing and proposed, are modeling high hourly N02concentrations showing violations ofthe 1- hour N02NAAQS-based only on the source's projected emissions ofNOx under some modeling and permitting scenarios. We find that, in many cases, the modeled violations are resulting from emissions at emergency electric generators and pump stations, where short stacks and limited property rights exist. In other cases, the problem may occur during periods ofunit stmlup, particularly where controls may initially notbe in operation. Finally, certain larger sources, including coal-fired and natural gas-fired powerplants, refineries, and papermills could also experience problems in meeting the new I-hour N02NAAQS using particularmodeling assumptions and permit conditions.

We believe that, in some instances, the projected violations result from the use of maximum modeled concentrations that do not adequately take into accountthe form ofthe 1- hour standard, and are based on the conservative assumption of 100% NOx-to-N02conversion in the ambient air. To the extentthatthis is the case, itmay be possible to provide more accurate projections ofambient N02 concentrations by applying currentprocedures which account for the statistical form ofthe I-hourN02 standard, as well as more realistic estimates ofthe rate of conversion ofNOx emissions to ambient N02 concentrations. See EPA Memorandum from Tyler Fox, Air Quality Modeling Group, to EPA Regional Air Division Directors, "Applicability ofAppendix W Modeling Guidance for the I-hourN02 National AmbientAir Quality Standard" (June 28, 2010) for specific modeling guidance for estimating ambient N02 concentrations consistent with the new I-hour N02NAAQS. In addition, where short stacks are currently being used, or are under design, itmay be possible to lessen the source's air quality impacts without improperdispersion by implementing "good engineering practice" (GEP) stack heights to

4increase the height ofexisting or designed stacks to avoid excessive concentrations due to downwash, as described in the guidance below.
It is EPA's expectation that the guidance in this memorandum and available modeling guidance for N02 assist in resolving some ofthe issues arising from preliminary analyses that are reportedly showing potential exceedances ofthe new I-hour N02 NAAQS that would not be present under more refined modeling applications. In addition, the techniques described in this memorandum may also help avoid violations ofthe standard through design ofthe proposed
source orpermit conditions, consistent with existing regulatory requirements, which enable the
source to demonstrate that its proposed emissions increase will not cause orcontribute to a modeled violation ofthe I-hour NOzstandard. Moreover, the interim I-hour N02 SIL that is included in this guidance will provide a reasonable screening tool for efficiently implementing the PSD requirements for an air quality impact analysis.

The following discussion provides guidance concerning demonstrating compliance with the new NAAQS and mitigating modeled violations using air quality-based permit limits more stringentthan whatthe BestAvailable Control Technology provisions may otherwise require, air quality offsets, the use ofGEl' stackheights, possible permitconditions for emergency generators, and an interim I-hour N02 SIL.
AIR-QUALITY BASED EMISSIONS LIMITATIONS

Once a level ofcontrol required by the Best Available Control Technology provisions is proposed by the pSD applicant, the proposed source's emissions mustbe modeled at the BACT emissions rate(s) to demonstrate that those emissions will not cause orcontribute to aviolation ofany NAAQS or pSD increment. EPA's 1990 Workshop Manual (page B.54) describes circumstances where a source's emissions based on levels proposed through the top-down process may notbe sufficiently controlledto preventmodeled violations ofan increment or NAAQS. In such cases, it may be appropriate for pSD applicants to propose a more stringent control option (that is, beyond the level identified via the top-down process) as a result ofan adverse impact on the NAAQS orpSD increments.

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

A 1988 EPA memorandum provides procedures to follow when amodeled violation is identified during the PSD permitting process. See Memorandum from Gerald A. Emison, EPA OAQpS, to Thomas J. Maslany, EPA Air Management Division, "Air Quality Analysis for Prevention ofSignificant Deterioration (pSD)." (July 5,1988). In brief: a reviewing authority may issue a proposed new source or modification a pSD permit only ifit can be shown that the proposed project's emissions will not "cause or contribute to" any modeled violations.

To clarify the above statement, in cases where modeled violations ofthe I-hour N02 NAAQS are predicted, but the permit applicant can show that the NOx emissions increase from the proposed source will not have a significant impactat thepoint andtime of any modeled violation, the permitting authority has discretion to conclude that the source's emissions will not



### 5contribute to the modeled violation. As provided in the July 5, 1988, guidance memo, in such instances, because ofthe proposed source's de minimis contribution to any modeled violation, the source's impact will not be consideredto cause orcontribute to such modeled violations, and the permitcould be issued. This conceptcontinues 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 offinding that a PSD source would not be considered to cause orcontribute to a modeled NAAQS violation bccause its estimated air quality impact was insignificant atthe time and place ofthe modeled violations 1 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 impactdocs cause or contribute to a modeled violation, apermit cannot be issued without some action taken to mitigate the source's impact. In accordance with 40 eFR 51.165(b)2, a major stationary source ormajormodification (as defined at §51.165(a)(l)(iv) and (v» that locates in an N02 attainment area, but would cause or contribute to a violation ofthe I-hourN02 NAAQS anywhere may "reduce the impactofits emissions upon airquality by obtaining sufficientemission reductions to, at a minimum, compensate for its adverse ambient [N02 ] 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 by either reducing its emissions atthe source, e.g., promoting more efficient production methodologies and energy efficiency, or by obtaining air quality offsets (see below). See, e.g., In re Intel]Jower a/New York, Inc., 5 E.A.D. 130, 141 (EAB 1994)3 A State may also provide the necessary emissions reductions by imposing emissions limitations on other sources through an approved State Implementation Plan (SIP) revision. These approaches may also be combined as necessary to demonstrate that a source will not cause or contribute to a violation ofthe NAAQS.

Unlike emissions offsetrequirements in nonattainment areas, in addressing the air quality offset concept, itmay not be necessary for apermit applicantto fully offset the proposed emissions increase ifan emissions reduction oflesserquantity will mitigate the adverse air quality impact on a modeled violation. ("Although full emission offsets are not required, such a source must obtain emission offsets sufficient to compensate for its airquality impact where the violation occurs." 44 FR 3274, January 16,1979, at 3278.) To clarify this, the 1988 guidance
memo referred to above states that:

offsets sufficient to compensate for the source's significant impact must be obtained pursuant to an approved State offset program consistent with State Implementation Plan (SIP) requirements under40 eFR 51.165(b). Where the source is contributing to an

I While there is no I-hourNO, significant impact level (SIlo) currently defined in the PSD regulations, we bclicve that states may adopt interim values, with the appropriatejustification for such values, to use for permitting purposes. In addition, we are recommending an interim SIlo as partofthis guidance for implementing the NO, requirements in the federal PSD program, and in state programs where states choose to use it.
2The same provision is contained in EPA's Interpretative Ruling at40 eFRpart 51 Appendix S, section Ill.
J In contrastto NonattainmentNew Source Review permits, offsets are not mandatory requirements in PSD permits ifit can otherwise be demonstrated that a source will not cause or contribute to a violation ofthe NAAQS. See, In
re Knauf Fiber Gtass, GMBH, 8 E.A.D. 121, 168 (EAB 1999).

6existing violation, the required offset may not correctthe violation. Such existing violations must be addressed [through the SIP].

In addition, in orderto determine the appropriate emissions reductions, the applicant and permitting authority should take into account modeling procedures for the form ofthe I-hour standard and for the appropriate NOx-NOzconversionrate that applies in the area ofconcern. As pati ofthis process, existing ambient ozone concentrations and othermeteorological conditions in the area ofconcern may need to be considered. Note that additional guidance for this and other aspects ofthe modeling analysis for the impacts ofNOx emissions on ambient concentrations ofNOzare addressed in EPA modeling guidance, including the June 28, 2010, Memorandumtitled, "Applicability ofAppendix W Modeling Guidance for the I-hourN02 National AmbientAir Quality Standard."

"GOOD ENGINEERING PRACTICE" STACK HEIGHT & DISPERSION TECHNIQUES

Ifapermit applicant is unable to show thatthe source's proposed emissions increase will not cause or contribute to a modeled violation ofthe new I-hour N02NAAQS, the problem could be the result ofplume downwash effects which may cause high ambient concentrations
nearthe source. In such eases, a source may be able to raise the height ofits existing stacks (or designed stacks ifnot yet constructed) to a GEl' stack height ofat least 65 meters, measured li'om the ground-level elevation atthe base ofthe stack.

While not necessarily totally eliminating the effects ofdownwash in all cases, raising stacks to GEl' heightmay provide substantial air quality benefits in a manner consistentwith statutory provisions (section 123 ofthe Act) governing acceptable stackheights to minimize extensive concentrations due to atmospheric downwash, eddies or wakes. Permit applicants should also be aware ofthe regulatory restrictions on stackheights for the purpose ofmodeling for compliance with NAAQS and increments. Section 52.21(h) ofthe pSD regulations currently prohibits the use ofdispersion techniques, such as stack heights above GEl', merged gas streams,
or intermittent controls for setting NOx emissions limits orto meetthe annual and I-hour NAAQS and annual NOz increments. 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 at40 CFR 51.1 OO(ff), (gg), (hh), (ii), 

### @, (kk) and (nn), and 40 CFR 51.118.

a. Stack heights: A source cannot take credit for thatportion ofa stack height in excess ofthe GEl' height when modeling to develop the NOx emissions limitations orto determine
source compliance with the annual and I-hour N02 NAAQS. It should be noted, however, that this limitation does not limit the actual height ofany stack constructed by a new source or modification.

The following limitations apply in accordance with §52.21 (h):

• For a stack height less than GEl', the actual stack height must be used in the source impact analysis for NOx emissions;

7• For a stack hcight equal to or greaterthan 65 meters, the impact on NOx emission limits may be modeled using the greaterof: o A de minimis stack heightequal to 65 meters, as measured from the groundlevel elevation atthe base ofthe stack, without demonstration orcalculation (40 CFR 51.1 OO(ii)(l»; o The refined formula height calculated using the dimensions ofnearby structures in accordance with the following equation:



### GEl' 

## =H 

### + 1.5L, where H is the height ofthe nearby structure and L is the lesser dimension ofthe height or projected width ofthe nearby structure (40 CFR 51.100(ii)(2)(ii».4

• A GEP stack height exceeding the refined formula height may be approved when it can be demonstrated to be necessary to avoid "excessive concentrations" ofN02 caused by atmospheric downwash, wakes, oreddy effects by the source, nearby structures, ornearby terrain features. (40 CFR 51.100(ii)(3), (jj), (kk»;

• For purposes ofPSD (and NOx/N02), "excessive concentrations" means a maximum ground-level concentration ofN02 due to NOx emissions from a stack due in whole
or in part to downwash, wakes, and eddy effects produced by nearby structures or nearby terrain features which individually is at least 40 percent in excess ofthe maximum N02concentration experienced in the absence ofsuch effects and (a) which contributes to a total N02concentration due to emissions from all sources that is greaterthan the annual or I-hourN02 NAAQS or (b) greaterthan the PSD (annual) increment for NOz. (40 CFR 51.1 OO(kk)(l». Reportedly, for economic and otherreasons, many existing source stacks have been constructed atheights less than 65 meters, and source impact analyses may show thatthe source's emissions will cause or contribute to a modeled violation ofthe annual or I-hourN02 NAAQS. Where this is the case, sources should be aware thatthey can increase their stack heights up to 65 meters without a GEP demonstration.

b. Otherdispersion techniques: The term "dispersiontechnique" includes any practice carried outto increase final plume rise, subjectto certain exceptions (40 CFR 51.1 OO(hh)(l)(iii), (2)(i)

- (v». Beyond the noted exceptions, such techniques are not allowed for getting credit for modeling source compliance with the annual and I-hour N02 NAAQS and annual N02 increment.

4 For stacks in existence on January 12, 1979, the GEP equation is GEP

- 2.5 H (provided the owner or operator produces evidence that this equation was actually relied on in establishing an emission limitation for NOx (40 CFR 5 l.l00(ii)(2)(i)

8OPERATION OF EMERGENCY EQUIPMENT & GENERAL STARTUP CONDITIONS

In determining an emergency generator's potential to emit, existing guidance (EPA
memo titled "Calculating Potential to Emit (PTE) for Emergency Generators," September 6, 1995) allows a default value of500 hours "forestimating the number ofhours that an emergency generator could be expected to operate under worst-case conditions." The guidance also allows for alternative estimates to be made on a case-by-case basis for individual emergency generators. This time period must also consider operating time for both testing/maintenance as well as for emergency utilization. Likewise, existing EPA policy does not allow NOx emissions to be excluded from the source impact analysis (NAAQS and increments) when the emergency equipment is operating during an emergency. EPA provides no exemption from compliance with the NAAQS during periods ofemergency operation. Thus, it is not sufficientto consider only emissions generated during periods oftesting/maintenance in the source impact analysis.

Ifduring an emergency, emergency equipment is never operated simultaneously with other emissions units at the source that the emergency equipment will back up, a worst-case hourly impact analysis may very well occur during periods ofnormal source operation when otheremissions units at the facility are likely to be operating simultaneously with the scheduled testing ofemergency equipment. To avoid such worst-case modeling situations, a permit applicant may commit to schedulingthe testing ofemergency equipment during times when the
source is not otherwise operating, or during known off-peak operating periods. This could provide a basis tojustify not modeling the I-hour impacts ofthe emergency equipment under conditions that would include simultaneous operation with other onsite emissions units. Accordingly, permits for emergency equipment may include enforceable conditions that specifically limitthe testing/maintenance ofemergency equipmentto certain periods oftime (seasons, days ofthe week, hours ofthe day, etc.) as long as these limitations do not constitute dispersion techniques under40 CFR 51.1 (hh)(l)(ii).

We also note that similar problems associated with the modeling ofhigh I-hour N02 concentrations have been reported to occur during startup periods for certain kinds ofemissions units--often because control equipment cannot function during all or aportion ofthe stat1up process. EPA currently has no provisions for exempting emissions occurring during equipment startups fl'om the air quality analysis to demonstrate compliance with the NAAQS. Startup emissions may occur during only a relatively small portion ofthe unit's total annual operating schedule; however, they must be included in the required PSI) air quality analysis for the NAAQS. Sources may be willing to accept enforceable permit conditions limiting equipment startups 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 otherquestions arising concerning procedures for modeling startup emissions to the applicable permitting authority to determine the most current modeling guidance.

9SCREENING VALUES

In the final rule establishing the hourly N02standard, EPA discussed various implementation considerations for the PSD permitting program. 75 FR.6474, 6524 (Feb. 9, 20I0). This 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 also believe that there may be a need to revise the screening tools currently used underthe NSR/PSD program for completing N02analyses. These screening tools include the significant impact levels (SILs), as mentioned by one commenter, but also include the significant emissions rate for emissions ofNOx and the significant monitoring concentration (SMC) for N02

. EPA intends to evaluate thc necd for possible changes or additions to each ofthese important screening tools for NOx/N02 due to the addition ofa I-hourN02NAAQS. Ifchanges or additions are deemed necessary, EPA will propose any such changes for public notice and comment in a separate action. 75 lOR 6525.

EPA intends to conduct an evaluation ofthese issues and submitourfindings in the form ofrevised significance levels under notice and commentrulemaking ifany revisions are deemed appropriate. In the interim, for the reasons provided bclow, we recommend the continued use of the existing significantemissions rates (SER) for NOx emissions as well as an interim I-hour NO

### zSIL that we are setting forth today for conducting air quality impact analyses for the I-hour N02NAAQS. As described in the section titled Introduction, EPA intends to implementthe interim I-hourNO

### zSIL contained herein underthe federal PSD program and offers states the opportunity to use it in theirPSD programs ifthey choose to do so. EPA is not addrcssing the signifIcantmonitoring concentrations in this memorandum.

SIGNIFICANT EMISSIONS RATE

Underthe terms ofexisting EPA regulations, the applicable signifIcantemissions rate for nitrogen oxides is 40 tons per year. 40 CFR 52.21(b)(23); 40 CFR 51.166(b)(23). The signifIcant emissions rates defIned in those regulations are specific to individual pollutants but
are not differentiated by the averaging times ofthe air quality standards applicable to some ofthe listed pollutants. Although EPA has notpreviously promulgated aN02 standard using an averaging time ofless than one year, the NAAQS for S02 have included standards with 3-hour and 24-hour averaging times for many years. EPA has applied the 40 tons peryear significant emissions rate for S02 across all ofthese averaging times. Until the evaluation described above and any associated rulemaking is completed, EPA does not believe it has cause to apply the NO

### z significant emissions rate any differently than EPA has historically applied the S02 signifIcant emissions rate and others that apply to standards with averaging times less than I year.
Under existing regulations, an ambient airquality impact analysis is required for "each pollutantthat [a source] would have the potential to emit in signifIcant amounts." 40 CFR 52.21 (m)(I)(i)(a); 40 CFR. 51.166(m)(I)(i)(a). For modifications, these regulations require this analysis for "each pollutant for which [the modification] would result in a signifIcantnet

10emissions increase." 40 CFR.52.21(m)(l)(i)(b); 40 CFR.5I.I66(m)(l)(i)(b). EPA construes this regulation to mean that an ambient impact analysis is not necessary for pollutants with emissions rates below the significantemissions rates in paragraph (b)(23) ofthe regulations. No additional action by EPA orpermitting authorities is necessary at this time to apply the 40 tpy significant emissions rate in existing regulations to the hourly N02 standard.

INTERIM I-HOUR N02 SIGNIFICANT IMPACT LEVEL

A significant impact level (SIL) serves as auseful screening tool for implementing the PSD requirements for an air quality analysis. The primary purpose ofthe SIL is to serve as a screening tool to identify a level ofambient impactthat is sufficiently low relative to the NAAQS or PSD increments such that the impactcan be considered trivial orde minimis. Hence, the EPA considers a source whose individual impact falls below a SIL to have a de minimis impact on airquality concentrations that already exist. Accordingly, a source that demonstrates thatthe projected ambient impactofits proposed emissions increase does not exceedthe SIL for thatpollutant at a location where a NAAQS or incrementviolation occurs is not considered to
cause orcontribute to that violation. In the same way, a source with aproposed emissions increase ofaparticularpollutantthat will have a significant impact at some locations is not required to model at distances beyond the point where the impactofits proposed emissions is below the SILs for thatpollutant. When aproposed source's impact by itselfis not considered to be "significant," EPA has long maintained that any further effort on the part ofthe applicantto complete acumulative source impact analysis involving other source impacts would only yicld information oftrivial or no value with respectto the required evaluation ofthe proposed source or modification. The conceptofa SIL is grounded on the de minimis principles described by the court inAlabamaPower Co. v. Costle, 636 F.2d 323, 360 (D.C. Cir. 1980); See also Sur Contra La Contaminacion v. EPA, 202 FJd 443, 448-49 (I st Cir. 2000) (upholding EPA's use ofSILto 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)

EPA has codified several SILs into regulations at 40 CFR 5I. I65(b). EPA plans to undertake rulemaking to develop a I-hourN02 SIL for the ncw NAAQS for N02
. However, EPA has recognized that the absence ofan EPA-promulgated SIL does notpreclude permitting authorities from developing interim SILs for use in demonstrating that a cumulative airquality analysis would yield trivial gain. Response to Comments, Implementation ofNew Source Review (NSR) Program for Particulate MatterLess Than 2.5 Micrometers in Diameter (PM25), pg. 82 (March 2008) [EPA-HQ-OAR-2003-0062-0278].

Until such time as a I-hourN02 SIL is defined in the PSD regulations, we are herein providing arecommended interim SIL that we intend to use as a screening tool for completing the required air quality analyses for the new I-hourN02 underthe federal PSD program at40 CFR 52.21. To supportthe application ofthis interim SIL in each instancc, apermitting authority that utilizes this SIL as part ofan ambient air quality analysis should include in the permit record the analysis reflected in this memorandum and the referenced documents to demonstrate that an air quality impact at orbelow the SIL is de minimis in nature and would not
cause a violation ofthe NAAQS.

I IUsing the interim I-hourN02SIL, the permit applicant and permitting authority can determine: (I) whether, based on the proposed increase in NOx emissions, a cumulative air quality analysis is required; (2) the area ofimpact within which a cumulative airquality analysis should focus; and (3) whether, as part ofa cumulative air quality analysis, the proposed source's NOx emissions will cause or contribute to amodeled violation ofthe I-hourN02NAAQS.

In this guidance, EPA recommends an interim I-hourN02 SIL value of4 ppb. To determine initially whether aproposed 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 eitherofthe following:

• The highest ofthe 5-year averages ofthe maximum modeled I-hourN02 conccntrations predicted each year at each receptor, based on 5 years ofNational Weather Service data; or

• The highest modeled I-hourN02 concentration predicted across all receptors based
on I year ofsite-specific meteorological data, orthe highest ofthe multi-year averages ofthe maximum modeled I-hourN02 concentrations predicted each year at eachreceptor, based on 2 ormore, up to 5 complete years ofavailable site-specific meteorological data.

Additional guidance will be forthcoming for the purpose ofcomparing aproposed source's modeled impacts to the interim I-hourN02SIL in orderto make a determination about whether that source's contribution is significant when a cumulative air quality analysis identifies violations ofthe I-hour N02NAAQS (i.e., "causes or contributes to" a modeled violation).

We derived this interim I-hourN02SIL by using an impactequal to 4% ofthe I-hour N02NAAQS (which is 100 ppb). We have chosen this approach because we believe it is reasonable to base the interim I-hourN02SIL directly on consideration ofimpacts relative to the I-hourN02NAAQS. In 1980, we defined SERfor eachpollutant subjectto PSD. 45 FR 52676, August 7,1980 at 52705-52710. For PM and S02, we defined the SER as the emissions rate that resulted in an ambient impact equal to 4% ofthe applicable short-termNAAQS. The 1980 analysis focused on levels no higherthan 5% ofthe primary standard because ofconcerns thathigher 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 ofde minimis emissions levels: (I) cumulative effecton increment consumption ofmultiple sources in an area, each making the maximum de minimis emissions increase; and (2) the projected consequence ofa given de minimis level on administrative burden. As explained in the preamble to the 1980 rulemaking and the supporting documentation,5 EPA decided to use 4% ofthe 24-hourprimary NAAQS for PM and S02 to define the significant emissions rates (SERs) for those pollutants. It was noted that, atthe time, only an annual N02 NAAQS existed. Thus, for reasons explained in the 1980 preamble, to define the SER for NOx emissions we used a design value of2% ofthe annual N02 NAAQS. See 45 FR 52708. Looking now at a short-term NAAQS for N02 , we believe that it is reasonable as an interim approach to use a SIL value that represents 4% ofthe I-hourN02

5 EPA evaluated de minimis leyels for pollutants for which NAAQS had been established in a document titled "Impact ofProposed and Alternative De Minimis Levels for Criteria Pollutants"; EPA-45012-80-0n, June 1980.

12NAAQS. EPA will consider otherpossible alternatives for developing a I-hourN02 SIL in a future rulemaking that will provide an opportunity for public participation in the development of
a SIL as part ofthe PSD regulations.

Several state programs have already adopted interim I-hourN02 SILs that differ (both higher and lower) from the interim value being recommended herein. The EPA-recommended interim I-hourN02 SIL is not intendedto supersede any interim SIL that is now or may be relied upon to implement a state PSD program that is partofan approved SIP, or to impose the use of the SIL concept on any state that chooses to implementthe PSD program-in particularthe ambient airquality 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 dccmcd more appropriate for PSD permitting purposes in the state of concern, orno SIL at all. The application ofany SILthat is notreflected in apromulgated regulation should be supported by arecord in each instance that shows the value represents ade minimis impact on the I-hourN02 standard, as described above.

In the event ofquestions regarding the general implementation guidance contained in this memorandum, please contact Raj Rao (rao.raj(a)epa.gov).

cc: Raj Rao, C504-0I Dan deRocck, C504-03 TylerFox, C439-01 Roger Brode, C439-01 Richard Wayland, C304-02 ElliotZenick, OGC Brian Doster, OGC EPA Regional NSR Contacts

13UNITED STATES ENVIRONMENTAL PROTECTION AGENCY Office ofAir Quality Planning and Standards Research Triangle Park, North Carolina 27711

June 28, 2010

MEMORANDUM

SUBJECT: Applicability ofAppendix W Modeling Guidance for the I-hourN02 National Ambient Air Quality Standard

FROM: Tyler Fox, Leader Air Quality Modeling Group, C439-01

TO: Regional Air Division Directors

INTRODUCTION

On January 22, 2010, EPA announced a new I-hour nitrogen dioxide (N02 ) National Ambient Air Quality Standard (I-hourN02 NAAQS or I-hourN02 standard) which is attained when the 3-year average ofthe 98th-percentile ofthe annual distribution ofdaily maximum I-hourconcentrations does not exceed 100 ppb at each monitor within an area. The final rule for the new I-hourN02NAAQS was published in the Federal Register on February 9, 2010 (75 lOR 6474-6537), and the standard became effective on April 12, 2010 (EPA, 2010a). This memorandum clarifies the applicability ofcurrent guidance in the Guideline onAir Quality Models (40 CFR Part 51, Appendix W) for modeling N02impacts in accordance with the Prevention ofSignificant Deterioration (PSD) permit requirements to demonstrate compliance with the new I-hourN02 standard.

SUMMARY OF CURRENT GUIDANCE

While the new I-hourNAAQS is defined relative to ambient concentrations ofN02 , the majority ofnitrogen oxides (NOx) emissions for stationary and mobile sources are in the form of nitric oxide (NO) rather thanN02

. Appendix W notes thatthe impact ofan individual source on ambient N02depends, in part, "onthe chemical environment into which the source's plume is to be emitted" (see Section 5.l.j). Given the role ofNOx chemistry in determining ambient impact levels ofN02based on modeled NOx emissions, Section 5.2.4 ofAppendix Wrecommends the following three-tiered screening approach for N02modeling for annual averages:

• Tier I

- assume full conversion ofNO to N02 based on application ofan appropriate refined modeling technique under Section 4.2.2 ofAppendix W to estimate ambientNOx concentrations;

• Tier 2

- multiply Tier I result by empirically-derived N02/NOxratio, with 0.75 as the annual national defaultratio (Chu and Meyer, 1991); and

14• Tier 3

- detailed screening methods may be considered on a case-by-case basis, with the Ozone Limiting Method (OLM) identified as a detailed screening technique for point
sources (Cole and Summerhays, 1979).

Tier 2 is often referrcd to as the Ambient Ratio Method, or ARM. Site-specific ambient 

### NOz/NOx ratios derived from appropriate ambient monitoring data may also be considcred as detailed screening methods on a case-by-case basis, with properjustification. Consistcnt with Section 4.2.2, AERMOD is the currcntprcferrcd model for "a widc range ofregulatory applications in all types ofterrain" for purposes ofestimating ambient concentrations of

### NOz , based on NOx emissions, under Tiers I and 2 above. We discuss the role ofAERMOD for Tier 3 applications in more detail below.



### APPLICABILITY OF CURRENT GUIDANCE TO I-HOURN02 

### NAAQS

In general, the Appendix W recommendations regarding the annual N02standard are also applicable to the new I-hourN02 standard, but additional issues may need to be considered in the context ofa I-hour standard, depending on the characteristics ofthe emission sources, and dcpcnding on which tier is used, as summarized below:

• Tier I applies to the I-hour N02 standard without any additionaljustification;

• Tier 2 may also apply to the I-hour N02 standard in many cases, but some additional consideration will be needed in relation to an appropriate ambient ratio for peak hourly impacts since the current default ambientratio is considered to be representative of"area wide quasi-equilibrium conditions"; and

• Tier 3 "detailed screening methods" will continue to be considered on a case-by-case basis for the I-hour

### NOzstandard. However, certain input data requirements and assumptions for Tier 

### 3 applications may be ofgreaterimportance for the I-hour standard than for the annual standard given the more localized nature ofpeak hourly vs. annual impacts. In addition, use ofsite-specific ambient N02/NOX ratios based on ambient monitoring data will generally be more difficult tojustify for the I-hourN02 standard than for the annual standard.

While Appendix W specifically mentions OLM as a detailed screening method under Tier 

### 3, we also considerthe Plume Volume Molar Ratio Method (PVMRM) (Hanrahan, 1999a) discussed under Section 5.1.j ofAppendix W to be in this category at this time. Both ofthese options account for ambient conversion ofNO to N02 in the presence ofozone, based on the following basic chemical mechanism, known as titration, although there are important differcnces between these methods:
(Eq. I)

As noted in Section 5.1.j, EPA is currently testing the PVMRM option to determine its suitability
as arefined method. Limited evaluations ofPVMRM have been completed, which show encouraging results, but the amount ofdata currently available is too limited tojustify a designation ofPVMRM as a refined method for

### NOz (Hanrahan, 1999b; MACTEC, 2005). EPA is currently updating and extending thesc evaluations to cxamine model performance for

ISprcdicting hourly N02concentrations, including both thc OLM and PVMRM options, and results ofthese additional evaluations will be provided at a laterdate. A sensitivity analysis ofthe OLM and PVMRM options in AERMOD has been conducted that compares modeled concentrations based on OLM and PVMRM with Tiers I and 2 for arange ofsource characteristics (MACTEC, 2004). This analysis serves as a useful reference to understand how ambient N02concentrations may be impacted by application ofthis three-tiered screening approach, and includes comparisons for both annual averagc and maximum I-hourN02concentrations.
Kcy modcl inputs for both the OLM and PVMRM options arc the in-stack ratios of N02/NOx emissions and background ozonc concentrations. Whilc thc rcpresentativeness of thesc kcy inputs is important in thc contextofthe annual N02 standard, thcy will gcnerally takc
on cven grcatcr importancc for the new I-hourN02 standard, as cxplaincd in more detail below. Rccognizing thc potential importancc ofthe in-stackN02/NOx ratio for hourly N02compliancc dcmonstrations, we rccommcnd that in-stack ratios used with cithcrthc OLM or PVMRM options bcjustified bascd on thc spccific application, i.e., therc is no "dcfault" in-stackN02INOx ratio for eithcr OLM or PVMRM.

The OLM and PVMRM methods are both available as non-regulatory-defaultoptions within the EPA-preferred AERMOD dispersion model (Cimorelli, et aI., 2004; EPA, 2004; EPA, 2009). As aresult oftheir non-regulatory-dcfault status, pursuantto Sections 3.1.2.c, 3.2.2.a, and A.I.a(2) ofAppendix W, application ofAERMOD with thc OLM or PVMRM option is no longer considered a "prcferred model" and, thcrcfore, requiresjustification and approval by the Regional Office on a case-by-casc basis. While EPA is continuing to cvaluate the PVMRM and OLM options within AERMOD for use in compliance demonstrations for the I-hourN02 standard, as long as they are considered to be non-rcgulatory-defaultoptions, thcir use as altcrnative modeling techniques undcrAppcndix W should bejustified in accordance with Section 3.2.2, paragraph (e), as follows:

"e. Finally, for condition (3) in paragraph (b) ofthis subscction [preferrcd modcl is less appropriatc for thc specific application, orthere is no preferred model], an alternativc refined model may be uscd providcd that:

1. The model has received a scientifIC peerrcview;
11. The model can be dcmonstrated to be applicable to thc problem on a theoretical basis;
111. Thc databases which arc necessary to perform the analysis are available and adequate;
IV. Appropriate pcrformancc evaluations ofthc model have shown that the model is not biased toward underestimates; and
v. A protocol on methods and procedures to be followed has been established."

Since AERMOD is the preferrcd model for dispcrsion for a wide rangc ofapplication, thc focus ofthe alternative model demonstration for usc ofthc OLM and PVMRM options within AERMOD is on the treatment ofNOx chemistry within the model, and does not need to address basic dispersion algorithms within AERMOD. Furthermore, items i and iv ofthc alternative



### 16model demonstration for these options canbe fulfilled in part based on existing documentation (Cole and Summerhays, 1979; Hanrahan, 1999a; Hanrahan, 1999b; MACTEC, 2005), and the remaining items should be routinely addressed as partofthe modeling protocol, irrespective of the regulatory status ofthese options. The issue ofapplicability to the problem on atheoretical basis (item ii) is a case-by-case determination based on an assessment ofthe adequacy ofthe
ozone titration mechanism utilized by these options to account for NOx chemistry within the AERMOD model based on "the chemical environment into which the source's plume is to be emitted" (Appendix W, Section 5.1.j). The adequacy ofavailable data bases needed for application ofOLM and PVMRM (item iii), including in-stackN02/NOx ratios and background
ozone concentrations, is a critical aspect ofthc demonstration which we discuss in more detail below. lt should also be noted that application ofthe OLM or PVMRM methods with other Appendix W models or alternative models, whether as a separate post-processoror integrated within the model, would require additional documentation and demonstration that the methods have been implemented and applied appropriately within that context, including model-specific performance evaluations which satisfy item iv under Section 3.2.2.e.

Given the form ofthe new I-hour N02standard, some clarification is needed regarding the appropriate dataperiods for modeling demonstrations ofcompliance with the NAAQS vs. demonstrations ofattainment ofthe NAAQS through ambient monitoring. While monitored design values for the I-hourN02 standard are based on a 3-yearaverage (in accordance with Section I(c)(2) ofAppendix S to 40 CFR Part 50), Section 8.3.1.2 ofAppendix W addresses the length ofthe meteorological data record for dispersion modeling, stating that "[T]he use of5 years ofNWS [National Weather Service] meteorological data or at least Iyear 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 N02standard is defined in terms ofthe 3-yearaverage, this definition does notpreemptor alter the Appendix W requirement for use of5 years ofNWS meteorological data or at least I year ofsite specific data. The 5-yearaverage based on use of NWS data, or an average across one ormore years ofavailable site specific data, serves as an unbiased estimate ofthe 3-yearaverage for purposes ofmodeling demonstrations ofcompliance with the NAAQS. Modeling of"rolling 3-year averages," using years 1 through 3, years 2 through 4, and years 3 through 5, is notrequired. Furthermore, since modeled results for N02
are averaged across the number ofyears modeled for comparison to the new I-hourN02 standard, the meteorological dataperiod should include complete years ofdata to avoid introducing a seasonal bias to the averaged impacts. In orderto 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-yearperiod ofthe available datarecord
vs. results based on the lastcomplete-yearperiod ofavailable data may be appropriate, subjectto approval by the appropriate reviewing authority. Such an approach would ensure that all available site specific data are accounted for in the modeling analysis without imposing an undue burden on the applicant and avoiding arbitrary choices in the selection ofa single complete-year dataperiod.

The form ofthe new I-hour N02standard also has implications regarding appropriate methods for combining modeled ambient concentrations with monitored background

17concentrations 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 PM25 NAAQS" (EPA, 20IOb), combining the 98th percentile monitored value with the 98th percentile modeled concentrations for a cumulative impact assessment could result in a value that is below thc 98th percentile ofthe combined cumulative distribution and would, thercfore, notbe protective ofthe NAAQS. However, unlike the recommendations presented for PM2S, the modeled contribution to the cumulative ambient impaet assessment for the I-hourN02 standard should follow the form ofthe standard based on the 98th percentile ofthe annual distribution ofdaily maximum I-hour concentrations averaged across the number ofyears modeled. A "first tier" assumption that may be applied without furtherjustifieation is to add the overall highest hourly background N02concentration from a representative monitor to the modeled design value, based on the form ofthe standard, for comparison to the NAAQS. Additional refinements to this "firsttier" approach based on some level oftemporal pairing of modeled and monitored values may be considered on a case-by-case basis, with adequate justification and documentation.

DISCUSSION OF TECHNICAL ISSUES

While many ofthe same technical issues related to application ofAppendix W guidance for an annual N02 standard would also apply in the context ofthe new I-hourN02 standard, there are some important differences that may also need to be considered depending on the specific application. This section diseusses several aspects ofthese teehnical issues related to the
new I-hour N02 NAAQS, including a discussion ofsource emission inventories required for modeling demonstrations ofcompliance with the NAAQS and other issues specifie to eaeh ofthe three tiers identified in Section 5.2.4 ofAppendix W for N02 modeling.

Emission Inventories

The source emissions data are a key input for all modeling analyses and one that may require additional considerations under the new I-hourN02standard is the source emissions data. Section 8.1 ofAppendix W provides guidance regarding source emission input data for dispersion modeling and Table 8-2 summarizes the recommendations for emission input data that should be followed for NAAQS compliance demonstrations. Although existing NOx emission inventories used to support modeling for compliance with the annual N02 standard should serve as auseful starting point, such inventories may not always be adequate for use in assessing compliance with the new I-hourN02standard since some aspeets ofthe guidance in Section 8.1 differs for long-term (annual and qUaJierly) standards vs. short-term (:s 24 hours) standards. In particular, sinee maximum ground-level concentrations may be more sensitive to operating levels and startup/shutdown conditions for an hourly standard than for an annual standard, emission rates and stack parameters associated with the maximum ground-level concentrations for the annual standard may underestimate maximum concentrations for the new I-hour N02standard. Due to the importance ofin-staek N02 /NOx ratios required for application ofthe OLM and PVMRM options within AERMOD discussed above, eonsideration should also be given to the potential variability ofin-stack N02/NOx ratios under different operating conditions when those non-regulatory-default options are applied. We also note that source emission input data recommendations in Table 8-2 ofAppendix W for "nearby sources" and "other sources" that

18may be needed to conduct a cumulative impact assessment include further differences between emission data for long-term vs. short-term standards which could also affect the adequacy of existing annual NOx emission inventories for the new I-hour N02 standard. The terms "nearby sources" and "other sources" used in this context are defined in Section 8.2.3 ofAppendix W. Attachment A provides a more detailed discussion on determining NOx emissions for permit modeling.

While Section 8.2.3 ofAppendix W emphasizes the importance ofprofessionaljudgment by the reviewing authority in the identification ofnearby and other sources to be included in the modeled emission inventory, Appendix W 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 [nearby] sources is expected to be small except in unusual situations." See Section 8.2.3.b. Since concentration gradients will vary somewhat depending on the averaging period being modeled, especially for an annual vs. I-hour standard, the criteria for selection of"nearby" and "other" sources for inclusion in the modeled inventory may need to be reassessed for the I-hour N02 standard.

The representativeness ofavailable 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 ofemissions from nearby sources are reflected in the available ambient measurements, and the degree to which emissions from those background sources during the monitoring period are representative ofallowable emission levels under the existing permits. The professionaljudgments that are required in developing an appropriate inventory ofbackground sources should strive toward the proper balance between adequately characterizing the potential for cumulative impacts of emission sources within the study area to cause or contribute to violations ofthe NAAQS, while minimizing the potential to overestimate impacts by double-counting ofmodeled source impacts that are also reflected in the ambient monitoring data. We would also caution against the literal and uncritical application ofvery prescriptive procedures for identifying which background
sources should be included in the modeled emission inventory for NAAQS compliance demonstrations, such as those described in Chapter C, Section IV.C.1 ofthe draft New Source Review Worhhop Manual (EPA, 1990), noting again that Appendix W emphasizes the importance ofprofessionaljudgment in this process. While the draft workshop manual serves as a useful general reference regarding New Source Review (NSR) and PSD programs, and such procedures may playa useful role in defining the spatial extent ofsources whose emissions may need to be considered, 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 determination ofan appropriate inventory of emissions to include in a cumulative impact assessment, the appropriate reviewing authority should be consulted 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-hourN02 standard. Tier-specific Technical Issues

19This section discusses technical issues related to application ofeach tier in the threetiered screening approach for NO

### zmodeling recommended in Section 5.2.4 Appendix W. A basic understanding ofNOx chemistry and "ofthe chemical environment into which the source's plume is to be emitted" (Appendix W, Section 5.1.j) will be helpful for addressing these issues based on the specific application.

Tier I:

Since the assumption offull conversion ofNO to NO

### zwill provide the most conservative treatment ofNOx chemistry in assessing ambient impacts, there are no technical issues associated with treatment ofNOx chemistry for this tier. However, the general issues related to emission inventories for the I-hourN02 standard discussed above and in Attachment A apply to Tier I.

Tier 2:

As noted above, the 0.75 national defaultratio for ARM is considered to be representative of"areawide quasi-equilibrium conditions" and, therefore, may notbe as appropriate for use with the I-hour N02 standard. The appropriateness ofthis default ambient ratio will depend somewhat on the characteristics ofthe sources, and as such application ofTier 2 for I-hourNO

### zcompliance demonstrations may need to be considered on a source-by-source basis in some cases. The key technical issue to address in relation to this tierrequires an understanding ofthe meteorological conditions that are likely to be associated with peakhourly impacts hom the source(s) being modeled. In general, for low-level releases with limitedplume rise, peak hourly NOx impacts are likely to be associated with nighttime stable/lightwind conditions. Since ambient ozone concentrations are likely to be relatively low for these conditions, and since low wind speeds and stable atmospheric conditions will further limit the conversion ofNO to NO

### zby limiting the rate ofentrainment ofozone into the plume, the 0.75 national defaultratio will likely be conservative for these cases. A similarrationale may apply for elevated sources where plume impaction on nearby complex terrain under stable atmospheric conditions is expected to determine the peak hourly NOx concentrations. By contrast, for elevated sources in relatively Hatterrain, the peak hourly NOx concentrations are likely to occur during daytime convective conditions, when ambient ozone concentrations are likely to be relatively high and entrainment ofozone within the plume is more rapid due to the vigorous vertical mixing during such conditions. Forthese sources, the 0.75 default ratio may notbe conservative, and some caution may be needed in applying Tier 2 for such sources. We also note that the default equilibrium ratio employed within the PVMRM algorithm as an upper bound on an hourly basis is 0.9.

Tier 3:

This tierrepresents a general category of"detailed screening methods" which may be considered on acase-by-case basis. Section 5.2.4(b) ofAppendix W cites two specific examples ofTier 

### 3 methods, namely OLM and the use ofsite-specific ambient N02/NOx ratios supported by ambient measurements. As noted above, we also believe it is appropriate to considerthe

20PVMRM option as aTier 3 detailed sereening method at this time. The diseussion here foeuses primarily on the OLM and PVMRM methods, but we also note that the use ofsite-speeifie ambient N02INOx ratios will be subjeetto the same issues diseussed above in relation to the Tier 

### 2 default ARM, and as a result it will generally be mueh more diffieultto determine an appropriate ambient N02 INOx ratio based on monitoring data for the new I-hour N02standard than for the annual standard.

While OLM and PVMRM are both based on the same simple ehemieal meehanism of titration to aeeount for the eonversion ofNO emissions to N02(see Eg. I) and therefore entail similarteehnieal issues and eonsiderations, there are some important differenees that also need to be eonsidered when assessing the appropriateness ofthese methods for speeifie applieations. While the titration meehanism may eapture the most important aspeets ofNO-to-N02eonversion in many applieations, both methods will sufferfrom the same limitations for applieations in whieh othermeehanisms, sueh as photosynthesis, eontribute signifieantly to the overall proeess ofehemieal transformation.' Sourees loeated in areas with high levels ofVOC emissions may be subjeetto these limitations ofOLM and PVMRM. Titration is generally a mueh faster meehanism for eonverting NO to N02than photosynthesis, and as sueh is likely to be appropriate for eharaeterizing peak I-hourN02 impaets in many eases.
Both OLM and PVMRM rely on the same key inputs ofin-staekN02 /NOx ratios and hourly ambient ozone eoneentrations. Although both methods can be applied within the AERMOD model using a single "representative" background ozone eoneentration, it is likely that use ofa single value would result in very conservative estimates ofpeak hourly ambient concentrations since its use for the I-hour N02standard would be contingent on a demonstration ofconservatism for all hours modeled. Furthermore, hourly monitored ozone concentrations used with the OLM and PVMRM options mustbe eoncurrentwith the meteorologieal data period used in the modeling analysis, and thus the temporal representativeness ofthe ozone data for estimating ambient N02coneentrations could be a factor in determining the appropriateness ofthe meteorological data period for a particularapplication. As noted above, the representativeness ofthese key inputs takes on somewhat greater importance in the contextofa I-hour N02 standard than for an annual standard, for obvious reasons. In the case ofhourly baekground ozone concentrations, methods used to substitute for periods ofmissing data may playamore significantrole in determining the I-hour N02 modeled design value, and should therefore be given greater scrutiny, especially for dataperiods that are likely to be associated with peakhourly concentrations based on meteorological conditions and source characteristics. In other words, ozone data substitution methods that may have been deemed appropriate in prior applieations for the annual standard may not be appropriate to use for the new I-hour standard.

While these technical issues and considerations generally apply to both OLM and PVMRM, the importance ofthe in-stackN02INOx ratios may be more important for PVMRM than for OLM in some cases, due to differences between the two methods. The key difference between the two methods is thatthe amount ofozone available for conversion ofNO to N02 is based simply on the ambient ozone concentration and is independent ofsource characteristics for OLM, whereas the amount ofozone available for conversion in PVMRM is based on the amount ofozone within the volume ofthe plume for an individual source or group ofsources. The plume volume used in PVMRM is calculated on an hourly basis for each source/receptor



### 21combination, taking into account the dispersive properties ofthe atmosphere for that hour. For a low-level release where peak hourly NOx impacts occur close to the source under stable/light wind conditions, the plume volume will be relatively small and the ambient NOzimpact for such
cases will be largely determined by the in-stackNOz/NOx ratio, especially for sources with relatively close fenceline or ambient air boundaries. This example also highlights the fact that the relative importance ofthe in-stackN02/NOx ratios may be greater for some applications than others, depending on the source characteristics and other factors. Assumptions regarding instackN02INOx ratios that may have been deemed appropriate in the context ofthe annual standard may not be appropriate to usc for the new I-hour standard. In particular, it is worth reiterating that the 

### 0.1 in-stack ratio often cited as the "default" ratio for OLM should not be treated as adefault value for hourly NOzcompliance demonstrations.

Another difference between 0 LM and PVMRM that is worth noting here is the treatment ofthe titration mechanism for multiple sources ofNOx. There are two possible modes that can be used for applying OLM to multiple source scenarios within AERMOD: (1) apply OLM to each source separately and assume that each source has all ofthe ambient ozone available for conversion ofNO to N02; and (2) assume that sources whose plumes overlap compete for the available ozone and apply OLM on a combined plume basis. The latter option can be applied selectively to subsets ofsources within the modeled inventory or to all modeled sources using the OLMGROUP keyword within AERMOD, and is likely to result in lower ambient NOz concentrations in most cases since the ambient N02 levels will be more ozone-limited. One of the potential refinements in application ofthe titration method incorporated in PVMRM is a technique for dynamically determining which sources should compete for the available ozone based on the relative locations ofthe plumes from individual sources, both laterally and vertically, on an hourly basis, taking into account wind direction and plume rise. While this approach addresses one ofthe implementation issues associated with OLM by making the decision ofwhich sources should compete for ozone, there is only very limited field study data available to evaluate the methodology.

Given the importance ofthe issue ofwhether to combine plumes for the OLM option, EPA has addressed the issue in the past through the Model Clearinghouse process. The general guidance that has emerged in those cases is that the OLM option should be applied on a sourceby-source basis in most cases and that combining plumes for application ofOLM would require
a cleardemonstration that the plumes will overlap to such a degree that they can be considered as "merged" plumes. However, much ofthat guidance was provided in the context ofapplying the OLM method outside the dispersion model in apost-processing mode on an annual basis. The past guidance on this issue is still appropriate in that context since there is no realistic method to account for the degree ofplume merging on an hourly basis throughout the modeling analysis when applied as apost-processor. However, the implementation ofthe OLM option within the AERMOD model applies the method on a source-by-source, receptor-by-receptor, and hour-byhour basis. As a result, the application ofthe OLMGROUP option within AERMOD is such that the sources only compete for the available ozone to the extent that each source contributes to the cumulative NOx concentration at each receptor for that hour. Sources which contribute significantly to the ambient NOx concentration at the receptor will compete for available ozone in proportion to their contribution, while sources that do not contribute significantly to the ambient NOx concentration will not compete for the ozone. Thus, the OLMGROUP option

22implemented in AERMOD will tend to be "self-correcting" with respect to concerns that combining plumes for OLM will overestimate the degree ofozone limiting potential (and therefore underestimate ambient N02 concentrations). As a result ofthese considerations, we recommend that use ofthe "OLMGROUP ALL" option, which specifies that all sources will potentially compete for the available ozone, be routinely applied and accepted for all approved applications ofthe OLM option in AERMOD. This recommendation is supported by model-tomonitor comparisons ofhourly NOzconccntrations flom the application ofAERMOD for the AtlantaN02risk and exposure assessment (EPA, 2008), and recent re-evaluations ofhourly N02 impacts fl'om the two field studies (New Mexico and Palaau) that were used in the evaluation of PVMRM (MACTEC, 2005). These model-to-monitor comparisons ofhourly N02 concentrations show reasonably good performance using the "OLMGROUP ALL" option within AERMOD, with no indication ofany bias to underestimate hourly NO

### zconcentrations with OLMGROUP ALL. Furthermore, model-to-monitor comparisons based on OLM without the OLMGROUP option do exhibit a bias to overestimate hourly NO

### zconcentrations. We will provide further details regarding these recent hourly NO

### zmodel-to-monitor comparisons at a later date.


### SUMMARY

To summarize, we emphasize the following points:

1. The 3-tiered screening approach recommended in Section 5.2.4 ofAppendix W for annual NO

### zassessments generally applies to the new I-hour NOzstandard. 2. While generally applicable, application ofthe 3-tiered screening approach for assessments ofthe new I-hourN02 standard may entail additional considerations, such
as the importance ofkey input data, including appropriate emission rates for the I-hour standard vs. the annual standard for all tiers, and the representativeness ofin-stack NOz/NOx ratios and hourly background ozone concentrations for Tier 3 detailed screening methods. 3. Since the OLM and PVMRM methods in AERMOD are currently considered nonregulatory-default options, application ofthese options requiresjustification and approval by the Regional Office on a case-by-case basis as alternative modeling techniques, in accordance with Section 3.2.2, paragraph (e), ofAppendix W. 4. Applications ofthe OLM option in AERMOD, subjectto approval under Section 3.2.2.e ofAppendix W, should routinely utilize the "OLMGROUP ALL" option for combining plumes. 5. While the I-hour NAAQS for N02 is defined in terms ofthe 3-year average for monitored design values to determine attainment ofthe NAAQS, this definition does not preempt or alter the Appendix W requirement for use of5 years ofNWS meteorological data or at least 

### 1year ofsite specific data.



### REFERENCES
Cimorelli, A. .I., S. G. Perry, A. Venkatram, J. C. Weil, R. J. Paine, R. B. Wilson, R. F. Lee, W.

23D. Peters, R. W. Brode, and J. O. Paumier, 2004. AERMOD: Description ofModel Formulation with Addendum, 

### EPA-454/R-03-004. U.S. Environmental Protection Agency, Research Triangle Park, NC.
Cole, H.S. and J.E. Summerhays, 1979. A Review ofTechniques Available for Estimation of Short-Term N02 Concentrations. Journal 

## of theAirPollution ControlAssociation, 29(8): 812- 817.
Chu, S.H. and E.L. Meyer, 1991. Use ofAmbient Ratios to Estimate ImpactofNOx Sources on Annual NO

### zConcentrations. Proceedings, 84th Annual Meeting & Exhibition ofthe Air & Waste ManagementAssociation, Vancouver, B.C.; 16-21 June 1991. (l6pp.) (DocketNo. A92-65, II-A-9)
EPA, 1990. New Source Review Workshop Manual: Prevention ofSignificant Deterioration and NonattainmentArea Permitting

- DRAFT. U.S. Environmental Protection Agency, Research Triangle Park, NC.
EPA, 2004. User's Guide for the

### AMS/EPA Regulatory Model- AERMOD. 

### EPA-454/B-03- 001. U.S. Environmental Protection Agency, Research Triangle Park, NC.
EPA, 2008. Risk and Exposure Assessmentto Supportthe Review ofthe NO

### zPrimary National AmbientAir Quality Standard. Office ofAir Quality Planning and Standards, Research Triangle Park, NC.
EPA, 2009. Addendum

- User's Guide for the

### AMS/EPA Regulatory Model- AERMOD. 

### EPA-454/B-03-001. U.S. Environmental Protection Agency, Research Triangle Park, NC.
EPA,2010a. Applicability ofthe Federal Prevention ofSignificant Deterioration Permit Requirements to New and Revised National Ambient Air Quality Standards. Stephen D. Page Memorandum, dated April I, 2010. U.S. Environmental Protection Agency, Research Triangle Park, NC.
EPA,2010b. Modeling Procedures for Demonstrating Compliance with 

### PM

### z .5NAAQS. Stephen D. Page Memorandum, dated March 23,2010. U.S. Environmental Protection Agency, Research Triangle Park, NC.
Hanrahan, P.L., I 999a. The Plume Volume Molar Ratio Method for Determining

### NOzlNOx Ratios in Modeling

- Part I: Methodology

. .! Air & Waste Manage. Assoc., 49, 1324-1331.
Hanrahan, P.L., 1999b. The Plume Volume Molar Ratio Method for Determining

### NOzlNOx Ratios in Modeling

- Part II: Evaluation Studies

. .! Air & Waste Manage. Assoc., 49,1332- 1338.
MACTEC,2004. Sensitivity Analysis ofPVMRM and OLM in AERMOD. Final Report, AlaskaDEC ContractNo. 18-8018-04. MACTEC Federal Programs, Inc., Research Triangle Park, NC.

24MACTEC,2005. Evaluation ofBias in AERMOD-PVMRM. Final Report, AlaskaDEC ContractNo. 18-9010-12. MACTEC Federal Programs, Inc., Research Triangle Park, NC.

cc: Richard Wayland, C304-02 Anna Wood, C504-01 Raj Rao, C504-01 Roger Brode, C439-01 Dan deRoeck, C504-03 Elliot Zenick, OGC Brian Doster, OGC EPA Regional Modeling Contacts

25

### ATTACHMENT A
Background on Hourly NOx Emissions for Permit Modeling for the I-hour NO z

### NAAQS

Introduction

The purpose ofthis attachment is to address questions about availability ofhourly NOx emissions for permitmodeling underthe new N02 NAAQS. It summarizes existing guidance regarding emission inputdatarequirements for NAAQS compliance modeling, and provides background on the historical approach to development ofinventories for N02permitmodeling and computation ofhourly emissions appropriate for assessing the new I-hourN02 standard. Although the NAAQS is defined in terms ofambient N02concentrations, source emission estimates for modeling are based on NOx.

Underthe PSD program, the owneroroperator ofthe source is required to demonstrate that the source does not cause orcontribute to aviolation ofa NAAQS (40 CFR 51.166 (k)(l) and 40 CFR 52.21 (k)(1» and/or PSD increments (40 CFR 51.166 (k)(2) and 52.21 (k)(2». However, estimation ofthe necessary emission input data for NAAQS compliance modeling entails consideration ofnumerous factors, and the appropriate reviewing authority should be consulted early in the process to determine the appropriate emissions data for use in specific modeling applications (see 40 CFR 51, Appendix W, 8.1.1.b and 8.2.3.b) Summary ofCurrent Guidance

Section 8.I ofthe Guideline onAir QualityModels, Appendix Wto 40 CFR Part 51, provides recommendations regarding source emission input data needed to supportdispersion modeling for NAAQS compliance demonstrations. Table 8-2 ofAppendix W provides detailed guidance regarding the specific components ofthe emission input data, including the appropriate emission limits (pounds/MMBtu), operating level (MMBtu/hr), and operating factor (e.g., hr/yr
orhrlday), depending on the averaging time ofthe standard. Table 8-2 also distinguishes between the emission input data needed for the new ormodified sources being assessed, and "nearby" and "other" background sources included in the modeled emission inventory.

Based on Table 8-2, emission input data for new or modified sources for annual and quarterly standards are essentially the same as for short-term standards (:'0 24 hours), based on maximum allowable or federally enforceable emission limits, design capacity or federally enforceable permit conditions, and the assumption ofcontinuous operation. However, there are a few additional considerations cited in Appendix W that could result in different emission input data for the I-hourvs. annual N02 NAAQS. Forexample, while design capacity is listed as the recommended operating level for the emission calculation, peakhourly ground-level concentrations may be more sensitive than annual average concentrations to changes in stack parameters (effluent exit temperature and exit velocity) under different operating capacities. Table 8-2 specifically recommends modeling other operating levels, such as 50 percent or 75 percent ofcapacity, for short-term standards (see footnote 3). Another factor that may affect maximum ground-level concentrations differently between the I-hourvs. annual standard isrestrictions on operating factors based on federally enforceable permit conditions. While federally enforceable operating factors otherthan continuous operation may be accounted for in the emission input data (e.g., ifoperation is limited to 8 am to 4 pm each day), Appendix Walso states that modeled emissions should not be averaged across non-operating time periods (see footnote 2 ofTable 8-2).

While emission input data recommendations for "nearby" and "other" background
sources included in the modeled emission inventory are similarto the new or modified source emission inputs in many respects, there is an important difference in the operating factor between annual and short-term standards. Emission input data for nearby and other sources may reflect actual operating factors (averaged overthe mostrecent 2 years) for the annual standard, while continuous operation should be assumed for short-term standards. This could result in important differences in emission input data for modeled background sources for the I-hour N02 NAAQS relative to emissions used for the annual standard.

Model Emission Inventory for NO z Modeling

Forthe existing annual N02 NAAQS, the permit modeling inventory has generally been compiled from the annual state emission inventory questionnaire (EIQ) or Title V permit applications on file with the relevantpermitting authority (state or local airprogram). Since a state uses the annual EIQ for Title V fee assessment, the state EIQ typically requires reporting of unit capacity, total fuel combusted, andlor hours ofoperation to help verify annual emissions calculations for fee accuracy purposes. Likewise, Title V operating permit applications contain all ofthe same relevant information for calculating emissions. While these emission inventories
are important resources for gathering emission input data on background sources for NAAQS compliance modeling, inventories which are based on actual operations may not be sufficient for short-term standards, such as the new I-hour

### NOzNAAQS. However, appropriate estimates of emissions from background sources for the I-hour

### NOz standard may be derived in many cases from information in these inventories regarding permitted emission limits and operating capacity.
Historically, it has not been a typical practice for an applicant to use the EPA's national emission inventory (NEI) as the primary source for compiling the permit modeling inventory. Since the emission data submitted to the NEI represents annual emission totals, it may not be suitable for use in NAAQS compliance modeling for short-term standards since modeling should be based on continuous operation, even for modeled background sources. Although the NEI may provide emission data for background sources that are more appropriate for the annual 

### NOz standard, the utility ofthe NEI for purposes ofNAAQS compliance modeling is further limited due to the fact that additional information regarding stackparameters and operating rates required for modeling may not be available from the NEI. While records exist in the NEI for reporting stack data necessary for point source modeling (i.e., stackcoordinates, stackheights, exittemperatures, exit velocities), some states do not report such information to the NEI, orthere
are may be errors in the location data submitted to the NEI. Under such conditions, default staek information based upon SIC is substituted and use ofsuch data could invalidate modeling results. Building locations and dimensions, which may be required to accountfor building downwash influences in the modeling analysis, may also be missing or incomplete in many cases.

27A common and relatively straightforward approach for compiling the necessary information to develop an inventory ofemissions from background sources for a permit modeling demonstration is as follows, patterned after the draftNew Source Review Workshop Manual (EPA, 1990). The applicantcompletes initial modeling ofallowable cmission increases associated with the proposcd project and determines the radii ofimpact (ROI) for each pollutant and averaging period, based on the maximum distance at which the modeled ambient concentration exceeds the Significant Impact Level (SIL) for eachpollutant and averaging period. Typically, the largestROI is selected and then a list ofpotential background sources within the ROI plus a screening distance beyond the ROI is compilcd by the permitting authority and supplied to the applicant. The applicanttypically requests permit applications or EIQ submittals from the records department ofthe permitting authority to gather stack data and
sourcc operating data necessary to compute emissions for the modeled inventory. Once the applicant has gathered the relevant data from the permitting authorities, model emission rates are calculated. While this approach is fairly common, it should be noted thatthe draft workshop manual "is not intended to be an official statement ofpolicy and standards and does not establish binding regulatory requirements" (see, Prefacc), and thc appropriate reviewing authority should be consulted early in the process regarding the selection ofappropriate background sourcc emission invcntories for the I-hour N02standard. We also note that Appendix W establishes "a significant concentration gradient in the vicinity ofthe source" under consideration as the main criterion for selection ofnearby sources for inclusion in the modeled inventory, and further indicates that "the number ofsuch [nearby] sources is expected to be small except in unusual situations." See Section 8.2.3.b.

As mentioned previously, modeled emission rates for short-term NAAQS are computed consistent with the recommendations ofSection 8.1 ofAppendix W, summarized in Table 8-2. The maximum allowable (SIP-approved process weightrate limits) or federally enforceable permit limit emissionrates assuming design capacity or federally enforceable capacity limitation
are used to compute hourly cmissions for dispersion modcling against short-term NAAQS such
as the new I-hourN02 NAAQS. Ifa source assumes an enforceable limit on the hourly firing capacity ofa boiler, this is reflected in the calculations. Otherwise, the design capacity ofthe
source is used to compute the model emissionrate. A load analysis is typically necessary to determine the load oroperating condition that causes the maximum ground-level concentrations. In addition to 100 percent load, loads such as 50 percent and 75 percent are commonly assessed. As noted above, the load analysis is generally more important for short-term standards than for annual standards. For an hourly standard, other operating scenarios ofrelatively shortduration such as "startup" and "shutdown" should be assessed since these conditions may result in maximum hourly ground-level concentrations, and the control efficiency ofemission control devices during these operating conditions may also need to be considered in the emission estimation.

Emission Calculation Example

The hourly emissions are most commonly computed from AP-42 emission factors based
on unit design capacity. For a combustion unit, the source typically reports both the unit capacity and the actual total amount offuel combusted annually (gallons, millions ofcubic feet

28ofgas, etc.) to the permitting authority for the EIQ. Likewise, Title V operating permit applications will contain similar information that can be used to compute hourly emissions.

For example, assume you are modeling an uncontrolled natural gas package boiler with a design firing rate 000 

### MMBtu/hr. The AP-42 emission factor for an uncontrolled natural gas external combustion source (AP-42, Section 1.4) for firing rates less than 100 MMBtu/hr is 100 lbs. 

### NOx/1 06 SCI' natural gas combusted. The hourly emission rate is derived by converting the emission factor expressed in terms oflbs. NOxll06 SCI' to lbs. 

### NOx/MMBtu. The conversion is done by dividing the 100 lbs. NOxll06 SCI' by 1,020 to convert the AP-42 factor to lbs. 

### NOx/MMBtu. The new emission factor is now 0.098 lbs. 

### NOx/MMBtu.

For this example, the source has no limit on the hourly firing rate ofthe boiler; therefore, the maximum hourly emissions are computed by multiplying the design firing rate ofthe boiler by the new emission factor.

Eho"dY

### = 0.098 Ibs/MMBtu x 30MMBtu/hr

### =2.94 lbs/hr

Thus 2.94 lbs/hr represents the emission rate that would be input into the dispersion model for modeling againstthe I-hourN02 NAAQS to comport with emission rate recommendations of Section 8.1 ofAppendix W.

It is importantto note that data derived for the annual state emission inventory (El) is based on actual levels offuel combusted for the year, and is therefore different than how allowable emissions are computed for near-field dispersion modeling. Forthe annual El report, a source computes their annual emissions based upon the AP-42 emission factor multiplied by the actual total annual throughput ortotal fuel combusted.

In the 30 MMBtu/hrboilerexample, the annual NOx emissions reported to the NEI is computed by:

Eo",,,,ol = (AP-42 emission factor) x (total annual fuel combusted)

Eo"""ol

### = (100 lbsll06 SCI') x (100 106 

### SCF/yr) 

### = 10,000 lbs. 

### NOx/yr or 5 tons 

### NOx/yr

29