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-hour N02 NAAQS for the

Prevention of Significant Dete ,i' ration pro~r

FROM: Stephen D, Page, DirectoFy<¥4.'»1<W<./

Office of Air 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-hour N02 standard) of 100 parts per billion (ppb), which is attained

when the 3-year average of the 98th-percentile ofthe annual distribution of daily 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 of public health. The final rule for the

new I-hour N02NAAQS 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 of Significant Deterioration (PSD) permit issued under 40 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 of new and modified major

stationary sources, we are issuing the attached guidance, in the form oftwo memoranda, for

implementing the new I-hour N02NAAQS under the PSD permit program. The guidance

contained in the attached memoranda addresses two areas. The first memorandum, titled,

"General Guidance for Implementing the I-hour N02 National Ambient Air Quality Standard in

Prevention of Significant Deterioration Permits, Including an Interim I-hour N02Significant

Impact Level," includes guidance for the preparation and review of PSD permits with respect to

the new I-hour N02standard. This guidance memorandum sets forth a recommended interim 1-

hour N02significant impact level (SIL) that states may consider when carrying out the 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 NOz SIL via rulemaking. The

second memorandum, titled "Applicability of Appendix W Modeling Guidance for the I-hour

NOz National 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 permit process. We believe it will provide a

consistent approach for estimating NOz air quality impacts from proposed construction or

modification ofNOx emissions sources. For the most part, the attached guidance reiterates

existing policy and guidance, but focuses on how this information is relevant to implementation

ofthe new I-hour NOz NAAQS.

Please review the guidance included in the two attached memoranda. If you have

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

please contact Raj Rao (rao.raj@epa.gov). If you have questions regarding the modeling

guidance in the second memorandum, please contact Tyler Fox (fox.tyler(flJ,epa.gov). We are

continuing our efforts to address permitting issues related to N02 and other NAAQS including

the recently-signed I-hour sulfur dioxide NAAQS. We plan to issue additional guidance to

address these new I-hour standards in the near future.

Attachments:

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

Air Division Directors, "General Guidance for Implementing the I-hour N02 National

Ambient Air Quality Standard in Prevention of Significant Deterioration Permits,

Including an Interim I-hour NOz Significant Impact Levcl" (June 28, 2010).

2. Memorandum from Tyler Fox, Air Quality Modeling Group, to EPA Regional Air

Division Directors, "Applicability of Appendix W Modeling Guidance for the I-hour

NOz National Ambient Air Quality Standard" (June 28, 2010).

cc: Anna Marie Wood

Richard Wayland

Raj Rao

Tyler Fox

Dan deRoeck

Roger Brode

Rich Ossias

Elliott Zenick

Brian Doster

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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

Office of Air Quality Planning and Standards

Research Triangle Park, North Carolina 27711

June 28, 2010

MEMORANDUM

SUBJECT: General Guidanee for Implementing the I-hour N02National Ambient Air

Quality Standard in Prevention of Signifieant Deterioration Permits, Ineluding an

Interim I-hour N02 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 of Significant 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-hour N02 NAAQS or I-hour N02

standard) that became effective on April 12,2010. EPA revised the primary N02 NAAQS by

promulgating a I-hour N02 NAAQS to provide the requisite protection of public health. Under

section I65(a)(3) ofthe Clean Air Act (the Act) and sections 52.21(k) and 51.166(k) of EPA's

PSD regulations, to obtain a permit, a souree must demonstrate that its proposed emissions

increase will not cause or contribute to a violation of any NAAQS.

This guidance is intended to: (I) explain the recommended procedures for stakeholders to

follow to properly address concerns over high preliminary modeled estimates of ambient N02

concentrations that suggest potential violations ofthe new I-hour N02 standard under some

modeling and permitting scenarios; (2) help reduce the burden of modeling for the hourly N02

standard where it can be properly demonstrated that a source will not have a significant impact

on ambient I-hour N02 eoncentrations; and (3) identify approaehes that allow sources and

permitting authorities to mitigate, in a manner consistent with existing regulatory requirements,

potential modeled violations ofthe I-hour N02 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 order to reasonably conelude that a

proposed source's emissions do not cause or contribute to modeled I-hour N02NAAQS

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

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new 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 implement their PSD programs for

NOx ifthey agree that these values represent de minimis impact levels and incorporate into each

permit record a rationale supporting this conclusion. This interim SIL is a useful screening tool

that can be used to determine whether or not the emissions from a proposed source will

significantly impact hourly N02 concentrations, and, ifsignificant impacts are predicted to

occur, whether the source's emissions "cause or contribute to" any modeled violations ofthe

new I-hour N02 NAAQS.

BACKGROUND

On April 12, 2010, the new I-hour N02NAAQS became effective. EPA interprets its

regulations at 40 CFR 52.2 I (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 of a short-term averaging period for the I-hour

N02 NAAQS, 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 a violation 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 N02 NAAQS-based only on the source's projected emissions of NOx 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 of unit

stmlup, particularly where controls may initially not be in operation. Finally, certain larger

sources, including coal-fired and natural gas-fired power plants, refineries, and paper mills could

also experience problems in meeting the new I-hour N02 NAAQS using particular modeling

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 account the form ofthe 1-

hour standard, and are based on the conservative assumption of 100% NOx-to-N02conversion in

the ambient air. To the extent that this is the case, it may be possible to provide more accurate

projections of ambient N02 concentrations by applying current procedures which account for the

statistical form ofthe I-hour N02 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

of Appendix W Modeling Guidance for the I-hour N02 National Ambient Air 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, it may be possible to lessen the source's air quality impacts without

improper dispersion by implementing "good engineering practice" (GEP) stack heights to

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increase the height of existing 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 or permit conditions, consistent with existing regulatory requirements, which enable the

source to demonstrate that its proposed emissions increase will not cause or contribute to a

modeled violation ofthe I-hour NOz standard. 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

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

quality offsets, the use of GEl' stack heights, possible permit conditions for emergency

generators, and an interim I-hour N02 SIL.

AIR-QUALITY BASED EMISSIONS LIMITATIONS

Once a level of control required by the Best Available Control Technology provisions is

proposed by the pSD applicant, the proposed source's emissions must be modeled at the BACT

emissions rate(s) to demonstrate that those emissions will not cause or contribute to a violation

of any NAAQS or pSD increment. 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 not be sufficiently controlled to prevent modeled violations of an increment or

NAAQS. In such cases, it may be appropriate for pSD applicants to propose a more stringent

control option (that is, beyond the level identified via the top-down process) as a result of an

adverse impact on the NAAQS or pSD increments.

DEMONSTRATING COMPLIANCE WITH THE NEW NAAQS & MITIGATING

MODELED VIOLATIONS WITH AIR QUALITY OFFSETS

A 1988 EPA memorandum provides procedures to follow when a modeled violation is

identified during the PSD permitting process. See Memorandum from Gerald A. Emison, EPA

OAQpS, to Thomas J. Maslany, EPA Air Management Division, "Air Quality Analysis for

Prevention of Significant Deterioration (pSD)." (July 5,1988). In brief: a reviewing authority

may issue a proposed new source or modification a pSD permit only if it 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 impact at the point and time ofany modeled

violation, the permitting authority has discretion to conclude that the source's emissions will not

5
contribute 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 considered to cause or contribute to such modeled violations, and

the permit could be issued. This concept continues to apply, and the significant impact level

(described further below) may be used as part ofthis analysis. A 2006 decision by the EPA

Environmental Appeals Board (EAB) provides detailed reasoning that demonstrates the

permissibility of finding that a PSD source would not be considered to cause or contribute to a

modeled NAAQS violation bccause its estimated air quality impact was insignificant at the 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 impact docs cause or contribute to a

modeled violation, a permit 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 or major modification

(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-hour N02 NAAQS anywhere may "reduce the impact of its

emissions upon air quality by obtaining sufficient emission 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 at the source, e.g.,

promoting more efficient production methodologies and energy efficiency, or by obtaining air

quality offsets (see below). See, e.g., In re 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 offset requirements in nonattainment areas, in addressing the air quality

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

emissions increase if an emissions reduction of lesser quantity 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 air quality impact where the

violation occurs." 44 FR 3274, January 16,1979, at 3278.) To clarify this, the 1988 guidance

memo referred to above states that:

offsets sufficient to compensate for the source's significant impact must be obtained

pursuant to an approved State offset program consistent with State Implementation Plan

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

I While there is no I-hour NO, significant impact level (SIlo) currently defined in the PSD regulations, we bclicve

that states may adopt interim values, with the appropriate justification for such values, to use for permitting

purposes. In addition, we are recommending an interim SIlo as part ofthis guidance for implementing the NO,

requirements in the federal PSD program, and in state programs where states choose to use it.

2 The same provision is contained in EPA's Interpretative Ruling at 40 eFR part 51 Appendix S, section Ill.

J In contrast to Nonattainment New Source Review permits, offsets are not mandatory requirements in PSD permits

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

re KnaufFiber Gtass, GMBH, 8 E.A.D. 121, 168 (EAB 1999).

6
existing violation, the required offset may not correct the violation. Such existing

violations must be addressed [through the SIP].

In addition, in order to 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-NOz conversion rate that applies in the area of concern.

As pati ofthis process, existing ambient ozone concentrations and other meteorological

conditions in the area of concern may need to be considered. Note that additional guidance for

this and other aspects ofthe modeling analysis for the impacts of NOx emissions on ambient

concentrations ofNOz are addressed in EPA modeling guidance, including the June 28, 2010,

Memorandum titled, "Applicability of Appendix W Modeling Guidance for the I-hour N02

National Ambient Air Quality Standard."

"GOOD ENGINEERING PRACTICE" STACK HEIGHT & DISPERSION

TECHNIQUES

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

not cause or contribute to a modeled violation ofthe new I-hour N02 NAAQS, the problem

could be the result of plume downwash effects which may cause high ambient concentrations

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

designed stacks if not yet constructed) to a GEl' stack height of at least 65 meters, measured

li'om the ground-level elevation at the base ofthe stack.

While not necessarily totally eliminating the effects of downwash in all cases, raising

stacks to GEl' height may provide substantial air quality benefits in a manner consistent with

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

extensive concentrations due to atmospheric downwash, eddies or wakes. Permit applicants

should also be aware ofthe regulatory restrictions on stack heights for the purpose of modeling

for compliance with NAAQS and increments. Section 52.21(h) ofthe pSD regulations currently

prohibits the use of dispersion techniques, such as stack heights above GEl', merged gas streams,

or intermittent controls for setting NOx emissions limits or to meet the 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 at 40 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 that portion of a stack height in excess

ofthe GEl' height when modeling to develop the NOx emissions limitations or to 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 of any 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;

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• For a stack hcight equal to or greater than 65 meters, the impact on NOx emission

limits may be modeled using the greater of:

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

(40 CFR 51.1 OO(ii)(l »;

o The refined formula height calculated using the dimensions of nearby

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" of N02

caused by atmospheric downwash, wakes, or eddy effects by the source, nearby

structures, or nearby terrain features.

(40 CFR 51.100(ii)(3), (jj), (kk»;

• For 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 N02 concentration due to emissions from all sources that

is greater than the annual or I-hour N02 NAAQS or (b) greater than the PSD (annual)

increment for NOz.

(40 CFR 51.1 OO(kk)(l».

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

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

source's emissions will cause or contribute to a modeled violation ofthe annual or I-hour N02

NAAQS. Where this is the case, sources should be aware that they can increase their stack

heights up to 65 meters without a GEP demonstration.

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

carried out to increase final plume rise, subject to certain exceptions (40 CFR

51.1 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.l 00(ii)(2)(i)

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OPERATION 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 of 500 hours "for estimating the number of hours 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 of emergency operation. Thus, it is not sufficient to consider only

emissions generated during periods oftesting/maintenance in the source impact analysis.

If during 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 of normal source operation when

other emissions units at the facility are likely to be operating simultaneously with the scheduled

testing of emergency equipment. To avoid such worst-case modeling situations, a permit

applicant may commit to scheduling the testing of emergency equipment during times when the

source is not otherwise operating, or during known off-peak operating periods. This could

provide a basis to justify 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 limit the testing/maintenance of emergency equipment to certain periods oftime

(seasons, days ofthe week, hours ofthe day, etc.) as long as these limitations do not constitute

dispersion techniques under 40 CFR 51.1 (hh)(l )(ii).

We also note that similar problems associated with the modeling of high I-hour N02

concentrations have been reported to occur during startup periods for certain kinds of emissions

units--often because control equipment cannot function during all or a portion 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 other questions arising concerning procedures for modeling startup emissions to the

applicable permitting authority to determine the most current modeling guidance.

9
SCREENING VALUES

In the final rule establishing the hourly N02 standard, EPA discussed various

implementation considerations for the PSD permitting program. 75 FR.6474, 6524 (Feb. 9,

20 I0). 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

under the 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 of a I-hour N02NAAQS. If changes 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 submit our findings in the form

ofrevised significance levels under notice and comment rulemaking if any revisions are deemed

appropriate. In the interim, for the reasons provided bclow, we recommend the continued use of

the existing significant emissions rates (SER) for NOx emissions as well as an interim I-hour

NOz SIL 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 implement the

interim I-hour NOz SIL contained herein under the federal PSD program and offers states the

opportunity to use it in their PSD programs ifthey choose to do so. EPA is not addrcssing the

signifIcant monitoring concentrations in this memorandum.

SIGNIFICANT EMISSIONS RATE

Under the terms of existing EPA regulations, the applicable signifIcant emissions 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 not previously promulgated a N02 standard using an

averaging time of less 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 per year 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 NOz

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 air quality impact analysis is required for "each

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

52.21 (m)(I )(i)(a); 40 CFR. 51.166(m)(I)(i)(a). For modifications, these regulations require this

analysis for "each pollutant for which [the modification] would result in a signifIcant net

10
emissions 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 significant emissions rates in paragraph (b)(23) ofthe regulations. No additional

action by EPA or permitting authorities is necessary at this time to apply the 40 tpy significant

emissions rate in existing regulations to the hourly N02 standard.

INTERIM I-HOUR N02 SIGNIFICANT IMPACT LEVEL

A significant impact level (SIL) serves as a useful 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 of ambient impact that is sufficiently low relative to the

NAAQS or PSD increments such that the impact can be considered trivial or de minimis. Hence,

the EPA considers a source whose individual impact falls below a SIL to have a de minimis

impact on air quality concentrations that already exist. Accordingly, a source that demonstrates

that the projected ambient impact of its proposed emissions increase does not exceed the SIL for

that pollutant at a location where a NAAQS or increment violation occurs is not considered to

cause or contribute to that violation. In the same way, a source with a proposed emissions

increase of a particular pollutant that will have a significant impact at some locations is not

required to model at distances beyond the point where the impact of its proposed emissions is

below the SILs for that pollutant. When a proposed source's impact by itself is not considered to

be "significant," EPA has long maintained that any further effort on the part ofthe applicant to

complete a cumulative source impact analysis involving other source impacts would only yicld

information oftrivial or no value with respect to the required evaluation ofthe proposed source

or modification. The concept of a SIL is grounded on the de minimis principles described by the

court in Alabama Power 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 of SIL to

allow permit applicant to avoid full impact analysis); In re: Prairie State Gen. Co., PSD Appeal

No. 05-05, Slip. Op. at 139 (EAB 2006)

EPA has codified several SILs into regulations at 40 CFR 5I. I65(b). EPA plans to

undertake rulemaking to develop a I-hour N02 SIL for the ncw NAAQS for N02. However,

EPA has recognized that the absence of an EPA-promulgated SIL does not preclude permitting

authorities from developing interim SILs for use in demonstrating that a cumulative air quality

analysis would yield trivial gain. Response to Comments, Implementation ofNew Source

Review (NSR) Program for Particulate Matter Less Than 2.5 Micrometers in Diameter (PM25),

pg. 82 (March 2008) [EPA-HQ-OAR-2003-0062-0278].

Until such time as a I-hour N02 SIL is defined in the PSD regulations, we are herein

providing a recommended interim SIL that we intend to use as a screening tool for completing

the required air quality analyses for the new I-hour N02 under the federal PSD program at 40

CFR 52.21. To support the application ofthis interim SIL in each instancc, a permitting

authority that utilizes this SIL as part of an ambient air quality analysis should include in the

permit record the analysis reflected in this memorandum and the referenced documents to

demonstrate that an air quality impact at or below the SIL is de minimis in nature and would not

cause a violation ofthe NAAQS.

I I
Using the interim I-hour N02SIL, 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 air quality analysis

should focus; and (3) whether, as part of a cumulative air quality analysis, the proposed source's

NOx emissions will cause or contribute to a modeled violation ofthe I-hour N02 NAAQS.

In this guidance, EPA recommends an interim I-hour N02 SIL value of 4 ppb. To

determine initially whether a proposed project's emissions increase will have a significant impact

(resulting in the need for a cumulative air quality analysis), this interim SIL should be compared

to either ofthe following:

• The highest ofthe 5-year averages ofthe maximum modeled I-hour N02

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

Weather Service data; or

• The highest modeled I-hour N02 concentration predicted across all receptors based

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

averages ofthe maximum modeled I-hour N02 concentrations predicted each year at

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

meteorological data.

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

modeled impacts to the interim I-hour N02SIL in order to make a determination about whether

that source's contribution is significant when a cumulative air quality analysis identifies

violations ofthe I-hour N02NAAQS (i.e., "causes or contributes to" a modeled violation).

We derived this interim I-hour N02SIL by using an impact equal 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-hour N02SIL directly on consideration of impacts relative to

the I-hour N02NAAQS. In 1980, we defined SER for each pollutant subject to 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-term NAAQS. The

1980 analysis focused on levels no higher than 5% ofthe primary standard because of concerns

that higher levels were found to result in unreasonably large amounts of increment being

consumed by a single source. Within the range of impacts analyzed, we considered two factors

that had an important influence on the choice of de minimis emissions levels: (I) cumulative

effect on increment consumption of multiple sources in an area, each making the maximum de

minimis emissions increase; and (2) the projected consequence of a given 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-hour primary NAAQS for PM and S02 to

define the significant emissions rates (SERs) for those pollutants. It was noted that, at the 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-hour N02

5 EPA evaluated de minimis leyels for pollutants for which NAAQS had been established in a document titled

"Impact of Proposed and Alternative De Minimis Levels for Criteria Pollutants"; EPA-45012-80-0n, June 1980.

12
NAAQS. EPA will consider other possible alternatives for developing a I-hour N02 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-hour N02 SILs that differ (both

higher and lower) from the interim value being recommended herein. The EPA-recommended

interim I-hour N02 SIL is not intended to supersede any interim SIL that is now or may be relied

upon to implement a state PSD program that is part of an approved SIP, or to impose the use of

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

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

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

another value that may be dccmcd more appropriate for PSD permitting purposes in the state of

concern, or no SIL at all. The application of any SIL that is not reflected in a promulgated

regulation should be supported by a record in each instance that shows the value represents a de

minimis impact on the I-hour N02 standard, as described above.

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

memorandum, please contact Raj Rao (rao.raj(a)epa.gov).

cc: Raj Rao, C504-0 I

Dan deRocck, C504-03

Tyler Fox, C439-01

Roger Brode, C439-01

Richard Wayland, C304-02

Elliot Zenick, OGC

Brian Doster, OGC

EPA Regional NSR Contacts

13
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

Office of Air Quality Planning and Standards

Research Triangle Park, North Carolina 27711

June 28, 2010

MEMORANDUM

SUBJECT: Applicability of Appendix W Modeling Guidance for the I-hour N02

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-hour N02 NAAQS or I-hour N02 standard) which is attained

when the 3-year average ofthe 98th-percentile ofthe annual distribution of daily maximum

I-hour concentrations does not exceed 100 ppb at each monitor within an area. The final rule for

the new I-hour N02NAAQS 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 of current guidance in the Guideline on Air Quality

Models (40 CFR Part 51, Appendix W) for modeling N02 impacts in accordance with the

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

with the new I-hour N02 standard.

SUMMARY OF CURRENT GUIDANCE

While the new I-hour NAAQS is defined relative to ambient concentrations ofN02, the

majority of nitrogen oxides (NOx) emissions for stationary and mobile sources are in the form of

nitric oxide (NO) rather than N02. Appendix W notes that the impact of an individual source on

ambient N02depends, in part, "on the chemical environment into which the source's plume is to

be emitted" (see Section 5.l.j). Given the role of NOx chemistry in determining ambient impact

levels ofN02 based on modeled NOx emissions, Section 5.2.4 of Appendix W recommends the

following three-tiered screening approach for N02modeling for annual averages:

• Tier I - assume full conversion ofNO to N02 based on application of an appropriate

refined modeling technique under Section 4.2.2 of Appendix W to estimate ambient NOx

concentrations;

• Tier 2 - multiply Tier I result by empirically-derived N02/NOx ratio, with 0.75 as the

annual national default ratio (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 proper justification. Consistcnt with

Section 4.2.2, AERMOD is the currcnt prcferrcd model for "a widc range ofregulatory

applications in all types ofterrain" for purposes of estimating ambient concentrations of NOz,

based on NOx emissions, under Tiers I and 2 above. We discuss the role of AERMOD for Tier

3 applications in more detail below.

APPLICABILITY OF CURRENT GUIDANCE TO I-HOUR N02 NAAQS

In general, the Appendix W recommendations regarding the annual N02 standard are also

applicable to the new I-hour N02 standard, but additional issues may need to be considered in

the context of a 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 additional justification;

• 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 ambient ratio 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 of greater importance for the I-hour standard

than for the annual standard given the more localized nature of peak hourly vs. annual

impacts. In addition, use ofsite-specific ambient N02/NOX ratios based on ambient

monitoring data will generally be more difficult to justify for the I-hour N02 standard

than for the annual standard.

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

Tier 3, we also consider the Plume Volume Molar Ratio Method (PVMRM) (Hanrahan, 1999a)

discussed under Section 5.1.j of Appendix W to be in this category at this time. Both ofthese

options account for ambient conversion of NO to N02 in the presence of ozone, 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 a refined method. Limited evaluations ofPVMRM have been completed, which show

encouraging results, but the amount of data currently available is too limited to justify a

designation of PVMRM as a refined method for NOz (Hanrahan, 1999b; MACTEC, 2005). EPA

is currently updating and extending thesc evaluations to cxamine model performance for

IS
prcdicting hourly N02concentrations, including both thc OLM and PVMRM options, and results

ofthese additional evaluations will be provided at a later date. 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 a range 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-hour N02concentrations.

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 context ofthe annual N02 standard, thcy will gcnerally takc

on cven grcatcr importancc for the new I-hour N02 standard, as cxplaincd in more detail below.

Rccognizing thc potential importancc ofthe in-stack N02/NOx ratio for hourly N02compliancc

dcmonstrations, we rccommcnd that in-stack ratios used with cithcr thc OLM or PVMRM

options bc justified bascd on thc spccific application, i.e., therc is no "dcfault" in-stack N02INOx

ratio for eithcr OLM or PVMRM.

The OLM and PVMRM methods are both available as non-regulatory-default options

within the EPA-preferred AERMOD dispersion model (Cimorelli, et aI., 2004; EPA, 2004; EPA,

2009). As a result oftheir non-regulatory-dcfault status, pursuant to Sections 3.1.2.c, 3.2.2.a,

and A.I.a(2) of Appendix W, application of AERMOD with thc OLM or PVMRM option is no

longer considered a "prcferred model" and, thcrcfore, requires justification 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-hour N02

standard, as long as they are considered to be non-rcgulatory-default options, thcir use as

altcrnative modeling techniques undcr Appcndix W should be justified 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, or there is no preferred model], an

alternativc refined model may be uscd providcd that:

1. The model has received a scientifIC peer rcview;

11. The model can be dcmonstrated to be applicable to thc problem on a

theoretical basis;

111. Thc data bases 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 of application, thc focus

ofthe alternative model demonstration for usc ofthc OLM and PVMRM options within

AERMOD is on the treatment of NOx chemistry within the model, and does not need to address

basic dispersion algorithms within AERMOD. Furthermore, items i and iv ofthc alternative

16
model demonstration for these options can be 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 part ofthe modeling protocol, irrespective of

the regulatory status ofthese options. The issue of applicability to the problem on a theoretical

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 of available data bases needed for

application of OLM and PVMRM (item iii), including in-stack N02/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-processor or 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 data periods for modeling demonstrations of compliance with the NAAQS vs.

demonstrations of attainment ofthe NAAQS through ambient monitoring. While monitored

design values for the I-hour N02 standard are based on a 3-year average (in accordance with

Section I(c)(2) of Appendix S to 40 CFR Part 50), Section 8.3.1.2 of Appendix W addresses the

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

years ofNWS [National Weather Service] meteorological data or at least I year ofsite specific

data is required." Section 8.3.1.2.b further states that "one year or more (including partial years),

up to five years, ofsite specific data ... are preferred for use in air quality analyses." Although

the monitored design value for the I-hour N02standard is defined in terms ofthe 3-year average,

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

meteorological data or at least I year ofsite specific data. The 5-year average based on use of

NWS data, or an average across one or more years of available site specific data, serves as an

unbiased estimate ofthe 3-year average for purposes of modeling demonstrations of compliance

with the NAAQS. Modeling of "rolling 3-year averages," using years 1 through 3, years 2

through 4, and years 3 through 5, is not required. Furthermore, since modeled results for N02

are averaged across the number of years modeled for comparison to the new I-hour N02

standard, the meteorological data period should include complete years of data to avoid

introducing a seasonal bias to the averaged impacts. In order to comply with Appendix W

recommendations in cases where partial years ofsite specific meteorological data are available,

while avoiding any seasonal bias in the averaged impacts, an approach that utilizes the most

conservative modeling result based on the first complete-year period ofthe available data record

vs. results based on the last complete-year period of available data may be appropriate, subject to

approval by the appropriate reviewing authority. Such an approach would ensure that all

available site specific data are accounted for in the modeling analysis without imposing an undue

burden on the applicant and avoiding arbitrary choices in the selection of a single complete-year

data period.

The form ofthe new I-hour N02 standard also has implications regarding appropriate

methods for combining modeled ambient concentrations with monitored background

17
concentrations for comparison to the NAAQS in a cumulative modeling analysis. As noted in

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

Compliance with PM25 NAAQS" (EPA, 20 IOb), 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, not be protective ofthe NAAQS. However, unlike the recommendations presented for

PM2S, the modeled contribution to the cumulative ambient impaet assessment for the I-hour N02

standard should follow the form ofthe standard based on the 98th percentile ofthe annual

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

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

overall highest hourly background N02 concentration from a representative monitor to the

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

Additional refinements to this "first tier" approach based on some level of temporal 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 of Appendix W guidance

for an annual N02 standard would also apply in the context ofthe new I-hour N02 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 of compliance with the NAAQS and other issues specifie to eaeh ofthe

three tiers identified in Section 5.2.4 of Appendix 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-hour N02 standard is the source emissions

data. Section 8.1 of Appendix 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 a useful starting point, such inventories may not always be adequate for use in assessing

compliance with the new I-hour N02 standard 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 N02 standard.

Due to the importance of in-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 of in-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 of Appendix W for "nearby sources" and "other sources" that

18
may 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 of Appendix W.

Attachment A provides a more detailed discussion on determining NOx emissions for permit

modeling.

While Section 8.2.3 of Appendix W emphasizes the importance of professional judgment

by the reviewing authority in the identification of nearby 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 of available ambient air quality data also plays an important role

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

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

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

which emissions from those background sources during the monitoring period are representative

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

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

proper balance between adequately characterizing the potential for cumulative impacts of

emission sources within the study area to cause or contribute to violations ofthe NAAQS, while

minimizing the potential to overestimate impacts by double-counting of modeled source impacts

that are also reflected in the ambient monitoring data. We would also caution against the literal

and uncritical application of very prescriptive procedures for identifying which background

sources should be included in the modeled emission inventory for NAAQS compliance

demonstrations, 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 of professional judgment 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

of policy and standards and does not establish binding regulatory requirements." See, Preface.

Given the range of issues involved in the determination of an 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 of modeled background source emission inventories for use in demonstrating

compliance with the new I-hour N02 standard.

Tier-specific Technical Issues

19
This section discusses technical issues related to application of each tier in the threetiered screening approach for NOzmodeling recommended in Section 5.2.4 Appendix W. A

basic understanding of NOx 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 of NO to NOz will 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-hour N02 standard discussed above and in Attachment A apply to

Tier I.

Tier 2:

As noted above, the 0.75 national default ratio for ARM is considered to be

representative of "area wide quasi-equilibrium conditions" and, therefore, may not be 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 of Tier

2 for I-hour NOz compliance 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 tier requires an

understanding ofthe meteorological conditions that are likely to be associated with peak hourly

impacts hom the source(s) being modeled. In general, for low-level releases with limited plume

rise, peak hourly NOx impacts are likely to be associated with nighttime stable/light wind

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 NOz by limiting the rate of entrainment of ozone into the plume, the 0.75

national default ratio will likely be conservative for these cases. A similar rationale 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 Hat terrain, 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 of ozone within the plume is more rapid due to the vigorous

vertical mixing during such conditions. For these sources, the 0.75 default ratio may not be

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 tier represents a general category of "detailed screening methods" which may be

considered on a case-by-case basis. Section 5.2.4(b) of Appendix W cites two specific examples

of Tier 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 consider the

20
PVMRM option as a Tier 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 subjeet to the same issues diseussed above in relation to the Tier

2 default ARM, and as a result it will generally be mueh more diffieult to determine an

appropriate ambient N02INOx 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

similar teehnieal 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 suffer from the same limitations for applieations in

whieh other meehanisms, sueh as photosynthesis, eontribute signifieantly to the overall proeess

of ehemieal transformation.' Sourees loeated in areas with high levels ofVOC emissions may be

subjeet to these limitations of OLM and PVMRM. Titration is generally a mueh faster

meehanism for eonverting NO to N02 than photosynthesis, and as sueh is likely to be appropriate

for eharaeterizing peak I-hour N02 impaets in many eases.

Both OLM and PVMRM rely on the same key inputs ofin-staek N02/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 of a single value would result in very conservative estimates of peak hourly ambient

concentrations since its use for the I-hour N02standard would be contingent on a demonstration

of conservatism for all hours modeled. Furthermore, hourly monitored ozone concentrations

used with the OLM and PVMRM options must be eoncurrent with 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 particular application. As noted above, the

representativeness ofthese key inputs takes on somewhat greater importance in the context of a

I-hour N02 standard than for an annual standard, for obvious reasons. In the case of hourly

baekground ozone concentrations, methods used to substitute for periods of missing data may

playa more significant role in determining the I-hour N02 modeled design value, and should

therefore be given greater scrutiny, especially for data periods that are likely to be associated

with peak hourly 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-stack N02INOx 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 that the amount of ozone available for conversion ofNO to N02 is

based simply on the ambient ozone concentration and is independent ofsource characteristics for

OLM, whereas the amount of ozone available for conversion in PVMRM is based on the amount

of ozone 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

21
combination, 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 NOz impact for such

cases will be largely determined by the in-stack NOz/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-stack N02/NOx ratios may be greater for some applications than

others, depending on the source characteristics and other factors. Assumptions regarding instack N02INOx 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 a default value for hourly NOz compliance demonstrations.

Another difference between 0 LM and PVMRM that is worth noting here is the treatment

ofthe titration mechanism for multiple sources of NOx. 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 of which sources should compete for ozone, there is only very limited field study data

available to evaluate the methodology.

Given the importance of the issue of whether 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 of OLM would require

a clear demonstration 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 of applying the

OLM method outside the dispersion model in a post-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 of plume merging on an hourly basis throughout the modeling analysis

when applied as a post-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

22
implemented in AERMOD will tend to be "self-correcting" with respect to concerns that

combining plumes for OLM will overestimate the degree of ozone 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 of hourly NOz conccntrations flom the application of AERMOD for the

Atlanta N02 risk and exposure assessment (EPA, 2008), and recent re-evaluations of hourly 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 of hourly N02

concentrations show reasonably good performance using the "OLMGROUP ALL" option within

AERMOD, with no indication of any bias to underestimate hourly NOz concentrations with

OLMGROUP ALL. Furthermore, model-to-monitor comparisons based on OLM without the

OLMGROUP option do exhibit a bias to overestimate hourly NOz concentrations. We will

provide further details regarding these recent hourly NOzmodel-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 of Appendix W for

annual NOz assessments generally applies to the new I-hour NOz standard.

2. While generally applicable, application ofthe 3-tiered screening approach for

assessments ofthe new I-hour N02 standard may entail additional considerations, such

as the importance of key input data, including appropriate emission rates for the I-hour

standard vs. the annual standard for all tiers, and the representativeness of in-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 requires justification 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), of Appendix W.

4. Applications ofthe OLM option in AERMOD, subject to approval under Section 3.2.2.e

of Appendix 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 of 5 years of NWS 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.

23
D. Peters, R. W. Brode, and J. O. Paumier, 2004. AERMOD: Description of Model 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 of Techniques Available for Estimation of

Short-Term N02 Concentrations. Journal ofthe Air Pollution Control Association, 29(8): 812-

817.

Chu, S.H. and E.L. Meyer, 1991. Use of Ambient Ratios to Estimate Impact ofNOx Sources on

Annual NOz Concentrations. Proceedings, 84th Annual Meeting & Exhibition ofthe Air &

Waste Management Association, Vancouver, B.C.; 16-21 June 1991. (l6pp.) (Docket No. A92-65, II-A-9)

EPA, 1990. New Source Review Workshop Manual: Prevention of Significant Deterioration and

Nonattainment Area Permitting - DRAFT. U.S. Environmental Protection Agency, Research

Triangle Park, NC.

EPA, 2004. User's Guide for the AMS/EPA Regulatory Model- AERMOD. EPA-454/B-03-

001. U.S. Environmental Protection Agency, Research Triangle Park, NC.

EPA, 2008. Risk and Exposure Assessment to Support the Review ofthe NOz Primary National

Ambient Air Quality Standard. Office of Air 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 of Significant 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 PMz.

5 NAAQS.

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,

Alaska DEC Contract No. 18-8018-04. MACTEC Federal Programs, Inc., Research Triangle

Park, NC.

24
MACTEC,2005. Evaluation ofBias in AERMOD-PVMRM. Final Report, Alaska DEC

Contract No. 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 NOz NAAQS

Introduction

The purpose ofthis attachment is to address questions about availability of hourly NOx

emissions for permit modeling under the new N02 NAAQS. It summarizes existing guidance

regarding emission input data requirements for NAAQS compliance modeling, and provides

background on the historical approach to development of inventories for N02 permit modeling

and computation of hourly emissions appropriate for assessing the new I-hour N02 standard.

Although the NAAQS is defined in terms of ambient N02 concentrations, source emission

estimates for modeling are based on NOx.

Under the PSD program, the owner or operator ofthe source is required to demonstrate

that the source does not cause or contribute to a violation of a 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 of numerous 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 of Current Guidance

Section 8. I ofthe Guideline on Air Quality Models, Appendix W to 40 CFR Part 51,

provides recommendations regarding source emission input data needed to support dispersion

modeling for NAAQS compliance demonstrations. Table 8-2 of Appendix 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

or hrlday), depending on the averaging time ofthe standard. Table 8-2 also distinguishes

between the emission input data needed for the new or modified 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 of continuous operation. However, there are a

few additional considerations cited in Appendix W that could result in different emission input

data for the I-hour vs. annual N02 NAAQS. For example, while design capacity is listed as the

recommended operating level for the emission calculation, peak hourly 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 of capacity, for short-term standards (see footnote 3). Another factor that may affect

maximum ground-level concentrations differently between the I-hour vs. annual standard is
restrictions on operating factors based on federally enforceable permit conditions. While

federally enforceable operating factors other than continuous operation may be accounted for in

the emission input data (e.g., if operation 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 of Table 8-2).

While emission input data recommendations for "nearby" and "other" background

sources included in the modeled emission inventory are similar to 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 over the most recent 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 NOz Modeling

For the 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 relevant permitting authority (state or local air program). 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 of operation 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 NOz NAAQS. 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 of NAAQS compliance modeling is further limited

due to the fact that additional information regarding stack parameters 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., stack coordinates, stack heights,

exit temperatures, exit velocities), some states do not report such information to the NEI, or there

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 account for building downwash

influences in the modeling analysis, may also be missing or incomplete in many cases.

27
A common and relatively straightforward approach for compiling the necessary

information to develop an inventory of emissions from background sources for a permit

modeling demonstration is as follows, patterned after the draft New Source Review Workshop

Manual (EPA, 1990). The applicant completes initial modeling of allowable cmission increases

associated with the proposcd project and determines the radii of impact (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 each pollutant and averaging

period. Typically, the largest ROI is selected and then a list of potential background sources

within the ROI plus a screening distance beyond the ROI is compilcd by the permitting authority

and supplied to the applicant. The applicant typically 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 that the draft workshop

manual "is not intended to be an official statement of policy 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 of appropriate background sourcc

emission invcntories for the I-hour N02standard. We also note that Appendix W establishes "a

significant concentration gradient in the vicinity of the source" under consideration as the main

criterion for selection of nearby 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 of Section 8.1 of Appendix W, summarized in Table 8-2.

The maximum allowable (SIP-approved process weight rate limits) or federally enforceable

permit limit emission rates 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-hour N02 NAAQS. If a source assumes an enforceable limit on the hourly firing

capacity of a boiler, this is reflected in the calculations. Otherwise, the design capacity ofthe

source is used to compute the model emission rate. A load analysis is typically necessary to

determine the load or operating 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 short duration

such as "startup" and "shutdown" should be assessed since these conditions may result in

maximum hourly ground-level concentrations, and the control efficiency of emission 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 of fuel combusted annually (gallons, millions of cubic feet

28
of gas, 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 of lbs. NOxll 06 SCI' to lbs. NOx/MMBtu. The conversion is

done by dividing the 100 lbs. NOxll 06 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 against the I-hour N02 NAAQS to comport with emission rate recommendations of

Section 8.1 of Appendix W.

It is important to note that data derived for the annual state emission inventory (El) is

based on actual levels of fuel combusted for the year, and is therefore different than how

allowable emissions are computed for near-field dispersion modeling. For the annual El report, a

source computes their annual emissions based upon the AP-42 emission factor multiplied by the

actual total annual throughput or total fuel combusted.

In the 30 MMBtu/hr boiler example, 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 lbsll 06 SCI') x (100 106 SCF/yr) = 10,000 lbs. NOx/yr or 5 tons NOx/yr

29