Further clarification of this guidance and application of Appendix W for the 1‐hour NO2 standard was

published March 1, 2011 and is available in the Region 7 NSR Policy & Guidance database.

http://www.epa.gov/region07/air/nsr/nsrmemos/appwno2_2.pdf
MEMORANDUM 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

RESEARCH TRIANGLE PARK, NC 27711 

JUN 29 2010 

OFFICE OF 

AlA QUALITY PLANNING 

AND STANDARDS 

SUBJECT: Guidance Concerning the Implementation of the I-hour N02 NAAQS for the 

Prevention of Significan~tete i , ration pro r 

FROM: Stephen D. Page, Directo ~L.lA / Office of Air Quality Plil' ni g ;;tcIsfa~dard 

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 N02 NAAQS or I-hour N02 standard) of 100 parts per billion (Ppb), which is attained 

when the 3-year average of the 98th-percentile of the 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 N02 NAAQS 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 of source compliance with the new I-hour 

N02 standard. 

EPA is aware of reports from stakeholders indicating that some sources- both existing 

and proposed- are modeling potential violations of the 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 of the new N02 

NAAQS. 

To respond to these reports and faci litate the PSD permitting of new and modified major 

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

implementing the new I-hour N02 NAAQS 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 N02 Significant 

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

the new I-hour N02 standard. This guidance memorandum sets forth a recommended interim 1-

hour N02 significant impact level (SIL) that states may consider when carrying out the required 

Internet Address (URL). hHp:/Iwww.epa.gov 

RecycledIRecyclable • Printed wHh Vegetable Oit Based Inks on Recycled Paper (Minimum 25% Poslconsumer) 
PSD air quality analysis for N02, until EPA promulgates a I-hour N02 SIL via rulemaking. The 

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

N02 National Ambient Air Quality Standard," includes specific modeling guidance for 

estimating ambient N02 concentrations and determining compliance with the new I-hour N02 

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 N02 air quality impacts from proposed construction or 

modification of NO x emissions sources. For the most part, the attached guidance reiterates 

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

of the new I-hour N02 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.tylerCfll,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 N02 Significant Impact Level" (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 

N02 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 

2 
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

Office of Air Quality Planning and Standards 

Research Triangle Park, North Carolina 27711 

June 28, 2010 

MEMORANDUM 

SUBJECT: General Guidance for Implementing the I-hour N02 National Ambient Air 

Quality Standard in Prevention of Significant Deterioration Permits, Including an 

Interim I-hour N02 Significant Impact Level 

FROM: Anna Marie Wood, Acting Director /s/ 

Air Quality Policy Division 

TO: Regional Air Division Directors 

INTRO])UCTION 

We are issuing the following guidance to explain and clarify 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 of the 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 I 65(a)(3) of the Clean Air Act (the Act) and sections 52.21(k) and 51.166(k) of EPA's 

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

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

This guidance is intended to: (1) 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 of the 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 concentrations; and (3) identify approaches that allow sources and 

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

potential modeled violations of the 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 conclude that a 

proposed source's emissions do not cause or contribute to modeled I-hour N02 NAAQS 

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

requirements. 

This guidance discusses existing provisions in EPA regulations and previous guidance for 

applying those provisions but focuses on the relevancy of this information for implementing the 

3 
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 if they 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, if significant impacts are predicted to 

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

new I-hour N02 NAAQS. 

BACKGROUND 

On April 12, 2010, the new I-hour N02 NAAQS became effective. EPA interprets its 

regulations at 40 CFR 52.21 (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 of the 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 of the I-hour N02 NAAQS. 

We are responding to reports from stakeholders which indicate that some sources, 

existing and proposed, are modeling high hourly N02 concentrations showing violations of the 1-

hour N02 NAAQS-based only on the source's projected emissions of NO x 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 of the 1-

hour standard, and are based on the conservative assumption of 100% NOx-to-N02 conversion 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 of the I-hour N02 standard, as well as more realistic estimates of the rate of 

conversion of NO x 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 Qnality Standard" 

(June 28, 2010) for specific modeling guidance for estimating ambient N02 concentrations 

consistent with the new I-hour N02 NAAQS. 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 

4 
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 of the issues arising from preliminary analyses that are 

reportedly showing potential exceedances of the 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 of the standard through design of the 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 of the I-hour N02 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 (1'SD)." (July 5,1988). In brief: a reviewing authority 

may issue a proposed new source or modification a 1'SD 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 of the 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 of any 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 of the 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 of this 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 because its estimated air quality impact was insignificant at the time 

and place of the 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 does 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)(1)(iv) and (v» that locates in an N02 attainment area, but would cause 

or contribute to a violation of the 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 of the 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 (SIL) currently defined in the PSD regulations, we believe 

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 SIL as part of this 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 1Il. 

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 of the NAAQS. See, In 

re Knauf Fiber Giass, 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 of the I-hour 

standard and for the appropriate NOx-N02 conversion rate that applies in the area of concern. 

As pati of this 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 of the modeling analysis for the impacts of NOx emissions on ambient 

concentrations ofN02 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 of the 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 cases, a source may be able to raise the height of its existing stacks (or 

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

li'om the ground-level elevation at the base of the stacie 

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

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

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

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

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

for compliance with NAAQS and increments. Section 52.21(h) of the PSI) regulations currently 

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

or intermittent controls for setting NOx emissions limits or to meet the annual and I-hour 

NAAQS and annual N02 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 

of the GEP 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 GEP, the actual stack height must be used in the source 

impact analysis for NOx emissions; 

7 
• For a stack height 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 of the stack, without demonstration or calculation 

(40 eFR 51.1 OO(ii)(1 »; 

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 of the nearby structure and L is the lesser 

dimension of the height or projected width of the nearby structure 

(40 eFR 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 eFR 51.l00(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 of the 

maximum N02 concentration experienced in the absence of such 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 N02. 

(40 eFR 51.100(kk)(I». 

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 of the 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 eFR 

51. I OO(hh)(I )(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 GEl' equation is GEl' - 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 1.1 00(ii)(2)(i) 

8 
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. Likewisc, 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 of testing/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 of the 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 of time 

(seasons, days of the week, hours of the 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 of the stm1up 

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 of the unit's total annual operating 

schedule; however, they must be included in the required PS]) air quality analysis for the 

NAAQS. Sources may be willing to accept enforceable permit conditions limiting equipment 

startups to certain hours of the day when impacts are expected to be lower than normal. Such 

permit limitations can be accounted for in the modeling of such 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 I 0). This discussion included the following statements regarding particular screening values 

that have historically been used on a widespread basis to facilitate implementation of the PSD 

permitting program: 

We also believe that there may be a need to revise the screening tools currently used 

under the NSRlPSD program for completing N02 analyses. These screening tools 

include the significant impact levels (SILs), as mentioned by one commenter, but also 

include the significant emissions rate for emissions of NO x and the significant 

monitoring concentration (SMC) for N02. EPA intends to evaluate the need for possible 

changes or additions to each of these important screening tools for NOx/N02 due to the 

addition of a I-hour N02 NAAQS. If changes or additions are deemed necessary, EPA 

will propose any such changes for public notice and comment in a separate action. 

75 FR 6525. 

EPA intends to conduct an evaluation of these issues and submit our findings in the form 

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

appropriate. In the interim, for the reasons provided below, 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 

N02 NAAQS. 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 if they choose to do so. EPA is not addressing 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 of the air quality standards applicable to some of the 

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 of these 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)(1 )(i)(a); 40 CFR. 51.166(m)(1 )(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.S2.21(m)(l)(i)(b); 40 CFR.SJ.J66(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) of the 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 of the 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 of the applicant to 

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

information of trivial or no value with respect to the required evaluation of the 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. CosrIe, 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. OS-OS, Slip. Op. at 139 (EAB 2006) 

EPA has codified several SILs into regulations at 40 CFR 5 I. I 65(b). EPA plans to 

undertake rulemaking to develop a I -hour N02 SIL for the new 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 of New Source 

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

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

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 of this interim SIL in each instance, 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 of the NAAQS. 

I I 
Using the interim I-hour N02 SIL, 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 of impact 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 of the 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 of the following: 

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

concentrations 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 of site-specific meteorological data, or the highest of the multi-year 

averages of the 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 N02 SIL in order to make a determination about whether 

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

violations of the I-hour N02 NAAQS (i.e., "causes or contributes to" a modeled violation). 

We derived this interim I-hour N02 SIL by using an impact equal to 4% of the I-hour 

N02 NAAQS (which is 100 ppb). We have chosen this approach because we believe it is 

reasonable to base the interim I-hour N02 SIL directly on consideration of impacts relative to 

the I-hour N02 NAAQS. 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% of the applicable short-term NAAQS. The 

1980 analysis focused on levels no higher than 5% of the 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% of the 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% of the 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% of the I-hour N02 

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

"Impact of Proposed and Alternative De Minimis Levels for Criteria Pollutants"; EPA-4S012-80-0n, 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 of the 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 deemed 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 iu this 

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

cc: Raj Rao, C504-0 I 

Dan deRoeck, 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 of the 98th-percentile of the 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 N02 NAAQS was published in the Federal Register on February 9, 2010 (75 FR 

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 N02 depends, in part, "on the chemical environment into which the source's plume is to 

be emitted" (see Section 5.1.j). Given the role of NO x 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 N02 modeling for annual averages: 

• Tier I - assume full conversion of NO 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 referred to as the Ambient Ratio Method, or ARM. Site-specific ambient 

N02INOx ratios derived from appropriate ambient monitoring data may also be considered as 

detailed screening methods on a case-by-case basis, with proper justification. Consistent with 

Section 4.2.2, AERMOD is the current preferred model for "a wide range of regulatory 

applications in all types of terrain" 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 of the emission sources, and 

depending 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 N02 standard. 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 of site-specific ambient N02INOx 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 of these 

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 

differences 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 of PVMRM 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 N02 (Hanrahan, 1999b; MACTEC, 2005). EPA 

is currently updating and extending these evaluations to examine model performance for 

IS 
predicting hourly N02 concentrations, including both the OLM and PVMRM options, and results 

of these additional evaluations will be provided at a later date. A sensitivity analysis of the OLM 

and PVMRM options in AERMOD has been conducted that compares modeled concentrations 

based on OLM and PVMRM with Tiers 1 and 2 for a range of source characteristics (MACTEC, 

2004). This analysis serves as a useful reference to understand how ambient N02 concentrations 

may be impacted by application of this three-tiered screening approach, and includes 

comparisons for both annual average and maximum I-hour N02 concentrations. 

Key model inputs for both the OLM and PVMRM options are the in-stack ratios of 

N02/NOx emissions and background ozone concentrations. While the representativeness of 

these key inputs is important in the context of the annual N02 standard, they will generally take 

on even greater importance for the new I-hour N02 standard, as explained in more detail below. 

Recognizing the potential importance of the in-stack N02/NOx ratio for hourly N02 compliance 

demonstrations, we recommend that in-stack ratios used with either the OLM or PVMRM 

options be justified based on the specific application, i.e., there is no "default" in-stack N02INOx 

ratio for either OLM or PVMRM. 

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

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

2009). As a result of their non-regulatory-default status, pursuant to Sections 3.1.2.c, 3.2.2.a, 

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

longer considered a "preferred model" and, therefore, requires justification and approval by the 

Regional Office on a case-by-case basis. While EPA is continuing to evaluate 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-regulatory-default options, their use as 

alternative modeling techniques under Appendix W should be justified in accordance with 

Section 3.2.2, paragraph (e), as follows: 

"e. Finally, for condition (3) in paragraph (b) of this subsection [preferred model is 

less appropriate for the specific application, or there is no preferred model], an 

alternative refined model may be used provided that: 

I. The model has received a scientifIC peer review; 

11. The model can be demonstrated to be applicable to the problem on a 

theoretical basis; 

111. The data bases which are necessary to perform the analysis are available 

and adequate; 

IV. Appropriate performance evaluations of the 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 preferred model for dispersion for a wide range of application, the focus 

of the alternative model demonstration for use of the OLM and PVMRM options within 

AERMOD is on the treatment of NO x chemistry within the model, and does not need to address 

basic dispersion algorithms within AERMOD. Furthermore, items i and iv of the 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, 200S), and the 

remaining items should be routinely addressed as part of the modeling protocol, irrespective of 

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

basis (item ii) is a case-by-case determination based on an assessment of the adequacy of the 

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 S.I.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 of the dcmonstration which we discuss in more detail 

below. It should also be noted that application of the 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 of the new I-hour N02 standard, some clarification is needed regarding 

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

dcmonstrations of attainment of the 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 1 (c )(2) of Appendix S to 40 CFR Part SO), Section 8.3.1.2 of Appendix W addresses the 

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

years ofNWS [National Weather Service] meteorological data or at least 1 year of site specific 

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

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

the monitored design value for the I-hour N02 standard is defined in terms of the 3-year average, 

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

meteorological data or at least 1 year of site specific data. The S-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 of the 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 of site 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 of the 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 of the 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, 201 Ob), 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 the 98th percentile of the combined cumulative distribution and would, 

therefore, not be protective of the NAAQS. However, unlike the recommendations presented for 

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

standard should follow the form of the standard based on the 98th percentile of the 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 justification is to add the 

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

modeled design value, based on the form of the 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 of the same technical issues related to application of Appendix W guidance 

for an annual N02 standard would also apply in the context of the new I-hour N02 standard, 

there are some important differences that may also need to be considered depending on the 

specific application. This section discusses several aspects of these technical issues related to the 

new I-hour N02 NAAQS, including a discussion of source emission inventories required for 

modeling demonstrations of compliance with the NAAQS and other issues specific to each of the 

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 allmodcling 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 aspects of the guidance in Section 8.1 

differs for long-term (annual and qumierly) standards vs. short-term (:s: 24 hours) standards. In 

particular, since 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-stack N02/NOx ratios required for application of the OLM and 

PVMRM options within AERMOD discussed above, consideration 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 of the source" under consideration as the main criterion for this selection. Appendix W 

also indicates that "the number of such [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 of the 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.l of the draft New Source 

Review WorhhojJ 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 of sources whose emissions may 

need to be considered, it should be recognized that "[iJt 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 N02 modeling recommended in Section 5.2.4 Appendix W. A 

basic understanding of NO x chemistry and "of the 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 of full conversion of NO to N02 will provide the most conservative 

treatment of NO x chemistry in assessing ambient impacts, there are no technical issues 

associated with treatment of NO x 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 of this default ambient 

ratio will depend somewhat on the characteristics of the sources, and as such application of Tier 

2 for I-hour N02 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 of the 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 of NO to N02 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 of site-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 screening method at this time. The discussion here focuses 

primarily on the OLM and PVMRM methods, but we also note that the use of site-specific 

ambient N02INOx ratios will be subject to the same issues discussed above in relation to the Tier 

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

appropriate ambient N02INOx ratio based on monitoring data for the new I-hour N02 standard 

than for the annual standard. 

While OLM and PVMRM are both based on the same simple chemical mechanism of 

titration to account for the conversion of NO emissions to N02 (see Eg. I) and therefore entail 

similar technical issues and considerations, there are some important differences that also need to 

be considered when assessing the appropriateness of these methods for specific applications. 

While the titration mechanism may capture the most important aspects ofNO-to-N02 conversion 

in many applications, both methods will suffer from the same limitations for applications in 

which other mechanisms, such as photosynthesis, contribute significantly to the overall process 

of chemical transformation.' Sources located in areas with high levels ofVOC emissions may be 

subject to these limitations of OLM and PVMRM. Titration is generally a much faster 

mechanism for converting NO to N02 than photosynthesis, and as such is likely to be appropriate 

for characterizing peak I-hour N02 impacts in many cases. 

Both OLM and PVMRM rely on the same key inputs of in-stack N02/NOx ratios and 

hourly ambient ozone concentrations. Although both methods can be applied within the 

AERMOD model using a single "representative" background ozone concentration, 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 N02 standard 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 concurrent with the meteorological data 

period used in the modeling analysis, and thus the temporal representativeness of the ozone data 

for estimating ambient N02 concentrations could be a factor in determining the appropriateness 

of the meteorological data period for a particular application. As noted above, the 

representativeness of these 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 

background 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 

applications 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 of the 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 of NO to N02 is 

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

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

of ozone within the volume of the plume for an individual source or group of sources. The 

plume volume used in PVMRM is calculated on an hourly basis for each source/receptor 

21 
combination, taking into account the dispersive properties of the 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 N02 impact for such 

cases will be largely determined by the in-stack N02/NOx ratio, especially for sources with 

relatively close fenceline or ambient air boundaries. This example also highlights the fact that 

the relative importance of the 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 of the 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 N02 compliance demonstrations. 

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

of the 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: (l) apply OLM to 

each source separately and assume that each source has all of the ambient ozone available for 

conversion of NO 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 of sources within the modeled inventory or to all modeled sources using 

the OLMGROUP keyword within AERMOD, and is likely to result in lower ambient N02 

concentrations in most cases since the ambient N02 levels will be more ozone-limited. One of 

the potential refinements in application of the titration method incorporated in PVMRM is a 

technique for dynamically determining which sources should compete for the available ozone 

based on the relative locations of the 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 of the 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 of that 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 of the 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 of the 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 of these considerations, we 

recommend that use of the "OLMGROUP ALL" option, which specifies that all sources will 

potentially compete for the available ozone, be routinely applied and accepted for all approved 

applications of the OLM option in AERMOD. This recommendation is supported by model-tomonitor comparisons of hourly N02 concentrations ii-om 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 N02 concentrations with 

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

OLMGROUP option do exhibit a bias to overestimate hourly N02 concentrations. We will 

provide further details regarding these recent hourly N02 model-to-monitor comparisons at a 

later date. 

SUMMARY 

To summarize, we emphasize the following points: 

I. The 3-tiered screening approach recommended in Section 5.2.4 of Appendix W for 

annual N02 assessments generally applies to the new I-hour N02 standard. 

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

assessments of the 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 

N02/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 of these 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 of the 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 of the 3-year average for 

monitored design values to determine attainment of the NAAQS, this definition does not 

preempt or alter the Appendix W requirement for use of 5 years of NWS meteorological 

data or at least I year of site specific data. 

REFERENCES 

Cimorelli, A . ./., 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-4S4/R-03-004. U.S. Environmental Protection Agency, Research 

Triangle Park, NC. 

Cole, B.S. and J.E. Summerhays, 1979. A Review of Techniques Available for Estimation of 

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

817. 

Chu, S.B. and E.L. Meyer, 1991. Use of Ambient Ratios to Estimate Impact of NO x Sources on 

Annual N02 Concentrations. Proceedings, 84th Annual Meeting & Exhibition of the 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-4S4/B-03-

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

EPA, 2008. Risk and Exposure Assessment to Support the Review of the N02 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-4S4/B-03-001. U.S. Environmental Protection Agency, Research Triangle Park, NC. 

EPA,2010a. Applicability of the 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 PM2.

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 N02/NOx 

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 N02/NOx 

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 of Bias 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, OGe 

EPA Regional Modeling Contacts 

25 
ATTACHMENT A 

Background on Hourly NOx Emissions for Permit Modeling 

for the I-hour N02 NAAQS 

Introduction 

The purpose of this 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 of the source is required to demonstrate 

that the source does not cause or contribute to a violation of a NAAQS (40 CFR 51.166 (k)(1) 

and 40 CFR 52.21 (k)(1» and/or PSD increments (40 CFR 51.166 (k)(2) and 52.21 (k)(2». 

However, estimation of the 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.l.l.b and 8.2.3.b) 

Summary of Current Guidance 

Section 8. I of the Guideline on Air Quality Models, Appendix W to 40 CFR Part 5 I, 

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 of the 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 of the 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 diffcrent 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 rccommends 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, and/or hours of operation to help verify annual emissions 

calculations for fee accuracy purposes. Likewise, Title V operating permit applications contain 

all of the 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 N02 NAAQS. However, appropriate estimates of 

emissions from background sources for the I-hour N02 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 N02 

standard, the utility of the 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 nccessary for point source modeling (i.e., stack coordinates, stack heights, 

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

are may be errors in the location data submitted to the NEI. Under such conditions, default stack 

information based upon SIC is substituted and use of such data could invalidate modeling results. 

Building locations and dimensions, which may be required to account for building down wash 

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 emission increases 

associated with the proposed 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 compiled by the permitting authority 

and supplied to the applicant. The applicant typically requests permit applications or ElQ 

submittals from the records department of the permitting authority to gather stack data and 

source 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, Preface), and the appropriate reviewing authority should 

be consulted early in the process regarding the selection of appropriate background source 

emission inventories for the I-hour N02 standard. 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 of such [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 emissions for dispersion modeling 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 of the 

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 of relatively 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 of 30 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. NOx/l 06 SCF to lbs. NOx/MMBtu. The conversion is 

done by dividing the 100 lbs. NOx/l 06 SCF 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 of the boiler; therefore, 

the maximum hourly emissions are computed by multiplying the design firing rate of the boiler 

by the new emission factor. 

Ehoudy = 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 (EI) 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 EI 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: 

Eam",al = (AP-42 emission factor) x (total annual fuel combusted) 

Ewl""al = (100 lbs/l 06 SCF) x (100 106 SCF/yr) = 10,000 Ibs. NOx/yr or 5 tons NOx/yr 

29