UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

RESEARCH TRIANGLE PARK, NC 2771 1 

OFFICE OF 

AIR QUALITY PLANNlFlG 

AMlmANOdRDS 

MEMORANDUM 

SUBJECT: Applicability of Appendix W Modeling Guidance for the 1 -hour NO2 

National Ambient Air Quality Standard 

FROM: 

TO: 

Tyler Fox, ~eaderyq. 

Air Quality Modeling Group, 439-0 1 

Regional Air Division Directors;, 

INTRODUCTION 

On January 22,20 1 0, EPA announced a new 1 -how nitrogen dioxide (Na) National 

Ambient Air Quality Standard ( l -hour NOz NAAQS or 1 -hour NOz 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 1 -hour N& NAAQS was published in the Federal Register on February 9,20 1 0 (75 FR 

6474-65 3 71, and the standard became effective on April 1 2,20 1 0 (EPA, 20 1 Oa). This 

memorandum clarifies the applicability of current guidance in the Guideline on Air Quality 

Models (40 CFR Part 5 1, Appendix W) for modeling NOi impacts in accordance with the 

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

with the new 1-hour NO;? standard. 

SUMMARY OF CURRENT GUIDANCE 

While the new 1 -hour NAAQS is defined relative to arnbient concentrations of Na, the 

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

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

ambient NO;! 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 NOx chemistry in determining ambient impact 

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

following three-tiered screening approach for NOz modeling for annual averages: 

Tier 1 - assume full conversion of NO to NO2 based on application of an appropriate 

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

concentrations; 
2

• Tier 2 - multiply Tier 1 result by empirically-derived NO2/NOx ratio, with 0.75 as the 

annual national default ratio (Chu and Meyer, 1991); and 

• 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 

NO2/NOx 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 NO2, 

based on NOx emissions, under Tiers 1 and 2 above. We discuss the role of AERMOD for Tier 

3 applications in more detail below. 

APPLICABILITY OF CURRENT GUIDANCE TO 1-HOUR NO2 NAAQS 

In general, the Appendix W recommendations regarding the annual NO2 standard are also 

applicable to the new 1-hour NO2 standard, but additional issues may need to be considered in 

the context of a 1-hour standard, depending on the characteristics of the emission sources, and 

depending on which tier is used, as summarized below: 

• Tier 1 applies to the 1-hour NO2 standard without any additional justification; 

• Tier 2 may also apply to the 1-hour NO2 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 1-hour NO2 standard. However, certain input data requirements and 

assumptions for Tier 3 applications may be of greater importance for the 1-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 NO2/NOx ratios based on ambient 

monitoring data will generally be more difficult to justify for the 1-hour NO2 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 NO2 in the presence of ozone, based on the 

following basic chemical mechanism, known as titration, although there are important 

differences between these methods: 

 NO + O3 → NO2 + O2 (Eq. 1) 

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 
3

designation of PVMRM as a refined method for NO2 (Hanrahan, 1999b; MACTEC, 2005). EPA 

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

predicting hourly NO2 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 NO2 concentrations 

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

comparisons for both annual average and maximum 1-hour NO2 concentrations. 

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

NO2/NOx emissions and background ozone concentrations. While the representativeness of 

these key inputs is important in the context of the annual NO2 standard, they will generally take 

on even greater importance for the new 1-hour NO2 standard, as explained in more detail below. 

Recognizing the potential importance of the in-stack NO2/NOx ratio for hourly NO2 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 NO2/NOx 

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, et al., 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.1.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 1-hour NO2

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; 

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

theoretical basis; 

iii. 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 
4

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 of the alternative 

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 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 5.1.j). The adequacy of available data bases needed for 

application of OLM and PVMRM (item iii), including in-stack NO2/NOx ratios and background 

ozone concentrations, is a critical aspect of the demonstration 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 1-hour NO2 standard, some clarification is needed regarding 

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

demonstrations of attainment of the NAAQS through ambient monitoring. While monitored 

design values for the 1-hour NO2 standard are based on a 3-year average (in accordance with 

Section 1(c)(2) of Appendix S to 40 CFR Part 50), 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 5 

years of NWS [National Weather Service] meteorological data or at least l 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 1-hour NO2 standard is defined in terms of the 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 1 year of site 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 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 NO2

are averaged across the number of years modeled for comparison to the new 1-hour NO2

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. 
5

The form of the new 1-hour NO2 standard also has implications regarding appropriate 

methods for combining modeled ambient concentrations with monitored background 

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 PM2.5 NAAQS” (EPA, 2010b), 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 

PM2.5, the modeled contribution to the cumulative ambient impact assessment for the 1-hour NO2

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

distribution of daily maximum 1-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 NO2 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 NO2 standard would also apply in the context of the new 1-hour NO2 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 1-hour NO2 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 NO2 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 1-hour NO2 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 NO2 standard should serve 

as a useful starting point, such inventories may not always be adequate for use in assessing 

compliance with the new 1-hour NO2 standard since some aspects of the guidance in Section 8.1 

differs for long-term (annual and quarterly) standards vs. short-term (≤ 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 1-hour NO2 standard. 

Due to the importance of in-stack NO2/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 NO2/NOx ratios under different operating conditions when those 
6

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 

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 1-hour NO2 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. 1-hour standard, 

the criteria for selection of “nearby” and “other” sources for inclusion in the modeled inventory 

may need to be reassessed for the 1-hour NO2 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.1 of the draft New Source 

Review Workshop 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 play a useful role in defining the spatial extent of sources 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 1-hour NO2 standard. 
7

Tier-specific Technical Issues

This section discusses technical issues related to application of each tier in the threetiered screening approach for NO2 modeling recommended in Section 5.2.4 Appendix W. A 

basic understanding of NOx 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 1: 

Since the assumption of full conversion of NO to NO2 will provide the most conservative 

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

associated with treatment of NOx chemistry for this tier. However, the general issues related to 

emission inventories for the 1-hour NO2 standard discussed above and in Attachment A apply to 

Tier 1. 

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 1-hour NO2 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 1-hour NO2 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 from 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 NO2 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 flat 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 
8

of Tier 3 methods, namely OLM and the use of site-specific ambient NO2/NOx ratios supported 

by ambient measurements. As noted above, we also believe it is appropriate to consider the 

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 NO2/NOx 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 NO2/NOx ratio based on monitoring data for the new 1-hour NO2 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 NO2 (see Eq. 1) 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 of NO-to-NO2 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 of VOC emissions may be 

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

mechanism for converting NO to NO2 than photosynthesis, and as such is likely to be appropriate 

for characterizing peak 1-hour NO2 impacts in many cases. 

Both OLM and PVMRM rely on the same key inputs of in-stack NO2/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 1-hour NO2 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 NO2 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 

1-hour NO2 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 

play a more significant role in determining the 1-hour NO2 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 1-hour standard. 

While these technical issues and considerations generally apply to both OLM and 

PVMRM, the importance of the in-stack NO2/NOx 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 NO2 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 
9

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 

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 NO2 impact for such 

cases will be largely determined by the in-stack NO2/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 NO2/NOx ratios may be greater for some applications than 

others, depending on the source characteristics and other factors. Assumptions regarding instack NO2/NOx ratios that may have been deemed appropriate in the context of the annual 

standard may not be appropriate to use for the new 1-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 NO2 compliance demonstrations. 

Another difference between OLM 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: (1) apply OLM to 

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

conversion of NO to NO2; 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 NO2

concentrations in most cases since the ambient NO2 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 
10

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 

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 NO2 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 NO2 concentrations from the application of AERMOD for the 

Atlanta NO2 risk and exposure assessment (EPA, 2008), and recent re-evaluations of hourly NO2

impacts from 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 NO2

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

AERMOD, with no indication of any bias to underestimate hourly NO2 concentrations with 

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

OLMGROUP option do exhibit a bias to overestimate hourly NO2 concentrations. We will 

provide further details regarding these recent hourly NO2 model-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 NO2 assessments generally applies to the new 1-hour NO2 standard. 

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

assessments of the new 1-hour NO2 standard may entail additional considerations, such 

as the importance of key input data, including appropriate emission rates for the 1-hour 

standard vs. the annual standard for all tiers, and the representativeness of in-stack 

NO2/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 1-hour NAAQS for NO2 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 l year of site specific data. 
11

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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 of the Federal Prevention of Significant Deterioration Permit 

Requirements to New and Revised National Ambient Air Quality Standards. Stephen D. Page 

Memorandum, dated April 1, 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., 1999a. The Plume Volume Molar Ratio Method for Determining NO2/NOx 

Ratios in Modeling – Part I: Methodology. J. Air & Waste Manage. Assoc., 49, 1324–1331. 

Hanrahan, P.L., 1999b. The Plume Volume Molar Ratio Method for Determining NO2/NOx 

Ratios in Modeling – Part II: Evaluation Studies. J. Air & Waste Manage. Assoc., 49, 1332-

1338. 
12

MACTEC, 2004. Sensitivity Analysis of PVMRM and OLM in AERMOD. Final Report, 

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

Park, NC. 

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, OGC 

EPA Regional Modeling Contacts 
A-1 

ATTACHMENT A 

Background on Hourly NOx Emissions for Permit Modeling 

for the 1-hour NO2 NAAQS 

Introduction

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

emissions for permit modeling under the new NO2 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 NO2 permit modeling 

and computation of hourly emissions appropriate for assessing the new 1-hour NO2 standard. 

Although the NAAQS is defined in terms of ambient NO2 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, 8.1.1.b and 8.2.3.b) 

Summary of Current Guidance

Section 8.1 of the 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 of the emission input data, including the appropriate 

emission limits (pounds/MMBtu), operating level (MMBtu/hr), and operating factor (e.g., hr/yr 

or hr/day), 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 (≤ 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 1-hour vs. annual NO2 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 
A-2

percent of capacity, for short-term standards (see footnote 3). Another factor that may affect 

maximum ground-level concentrations differently between the 1-hour vs. annual standard are 

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 W also 

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 1-hour NO2 NAAQS 

relative to emissions used for the annual standard. 

Model Emission Inventory for NO2 Modeling

For the existing annual NO2 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 1-hour NO2 NAAQS. However, appropriate estimates of 

emissions from background sources for the 1-hour NO2 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 NO2

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 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 stack 

information based upon SIC is substituted and use of such data could invalidate modeling results. 
A-3

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. 

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 EIQ 

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 1-hour NO2 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 1-hour NO2 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. 
A-4

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 

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/106 SCF natural gas combusted. The hourly emission rate is derived by converting the 

emission factor expressed in terms of lbs. NOx/106 SCF to lbs. NOx/MMBtu. The conversion is 

done by dividing the 100 lbs. NOx/106

 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. 

Ehourly = 0.098 lbs/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 1-hour NO2 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: 

Eannual = (AP-42 emission factor) x (total annual fuel combusted) 

Eannual = (100 lbs/106

 SCF) x (100 106 SCF/yr) = 10,000 lbs. NOx/yr or 5 tons NOx/yr