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EPA Issues New Guidance on NO2 Compliance Demonstrations for the Short-Term NO2 NAAQS


By Larry Kane, Attorney, Bingham Greenebaum Doll LLP

On March 1, 2011, the U.S. EPA’s Office of Air Quality Planning and Standards (OAQPS) issued a technical memorandum providing clarifications on the use of air quality modeling to demonstrate compliance with the 2010 NAAQS for 1-hour nitrogen dioxide (NO2) ambient concentrations (“2011 Guidance”). The new guidance memo supplements earlier guidance provided in a memo dated June 29, 2010 (“2010 Guidance”) on the application of modeling guidance from Appendix W (40 CFR Part 51) for purposes of such compliance demonstrations when required in the context of applications for PSD permits for projects with proposed NO2 emissions.

The new guidance has been characterized by EPA as an effort to “ease compliance demonstrations” for the 1-hour NO2 standard required of PSD applicants by providing some flexibility and exemptions from stricter modeling guidelines in Appendix W that have frequently resulted in projected violations of the new 1-hour standard1. One of the principal difficulties for compliance demonstrations addressed by the new guidance is the modeled impact of a proposed source’s emergency generators whose typical short stacks offer poor dispersion of NO2 emissions.

The new short-term NO2 standard of 100 ppb 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. Although the new guidance memo is specifically directed to modeling demonstrations of compliance with the 1-hour NO2 standard, due to its treatment of options for addressing NOx chemistry relating to conversion to NO2, EPA states that the guidance provided in the memo is otherwise equally applicable to compliance demonstrations for the new 1-hour SO2 standard.

The memo’s introduction summarizes the new guidance as follows:

(1) The memo clarifies procedures for demonstrating compliance with the 1-hour NO2 standard based on the form of the standard, including significant contribution analyses using the interim Significant Impact Level (SIL) established in the 2010 Guidance and describes an update option to the AERMOD model that will support such analyses.

(2) Clarification is provided on the use and acceptance of less conservative Tier 2 and Tier 3 options for NO2, including updated model evaluation results for the OLM and PVMRM options incorporated in the AERMOD model.

(3) The memo recommends that compliance demonstrations for the new 1-hour NO2 standard address emission scenarios that can logically be assumed to be relatively continuous or which occur frequently enough to contribute significantly to the annual distribution of daily maximum 1-hour concentrations based on existing modeling guidelines, which provide sufficient discretion for reviewing authorities to not include intermittent emissions from emergency generators or startup/shutdown operations from compliance demonstrations for the 1-hour NO2 standard under appropriate circumstances.

(4) It also provides additional clarification and a more detailed discussion of the factors to consider in determination of background concentrations as a part of a cumulative impact assessment including identification of nearby sources to be explicitly modeled.

(5) Finally, the memo recommends an appropriate methodology for incorporating background concentrations in the cumulative impact assessment for the 1-hour NO2 standard and details updates to the AERMOD model with an option to include temporally-varying background concentrations within the modeling analysis.


Highlights of the 2011 Guidance follow:

Procedures for Demonstrating Compliance

• Modeling Tools

As mentioned, the 1-hour NO2 standard requires the three-year average of the 98th percentile of the annual distribution of daily maximum 1-hour values to be less than or equal to 100 ppb. An unbiased surrogate for the 98th percentile is the 8th highest of the daily maximum 1-hour values over the year. EPA observes in its 2011 Guidance that the AERMOD dispersion model, which is recommended by Appendix W, has been modified (version 11059) to provide capability to generate the data needed to determine whether the new 1-hour standard is projected to be met, as well as whether a source is projected to cause or contribute to a violation of the new standard based on the interim Significant Impact Level (SIL).

• Use of Interim SIL to Evaluate Project Impacts

One use of the interim SIL for 1-hour NO2 is as a screening tool to determine if the projected increase in NO2 emissions is sufficient to require a cumulative air quality analysis. The 2011 Guidance states that, if the multiyear average of the highest 1-hour NO2 values is below the interim SIL at all receptors, then the project could not contribute significantly to any modeled violation of the 1-hour NO2 standard, with the result that the project would be exempted from the cumulative impact assessment.

• Cumulative Impact Assessments

If a project’s impacts will exceed the SIL at any receptors based on the initial impact analysis, then EPA indicates that a cumulative impact assessment should be completed to determine whether the project’s emissions will cause or contribute to any modeled violations of the 1-hour standard. EPA further indicates that it deems it appropriate and acceptable in most cases to limit the cumulative impact analysis to only those receptors that have been shown to have significant impacts from a proposed new source based on the initial SIL analysis.

If modeled violations of the 1-hour standard are found based on the cumulative impact assessment, then the project’s contribution to all modeled violations should be compared to the interim SIL to determine whether the project causes or contributes to any of the modeled violations.

The significant contribution analysis should examine every multiyear average of daily maximum 1-hour values, beginning with the 8th-highest (i.e., the 98th percentile), continuing down the ranked distribution until the cumulative impact is below the NAAQS. This analysis should be limited to the distribution of daily maximum 1-hour values.

The recent update to the AERMOD model includes an option (the MAXDCONT keyword) to automatically perform this contribution analysis.

Tiered Approach to NO2 Compliance Demonstrations

As a result of the stringency of the new 1-hour NO2 standard and the quite conservative guidance of Appendix W concerning the rate of conversion of various emitted forms of NOx to NO2, the new guidance recognizes that many permit applicants “may find it necessary to use the less conservative Tier 2 or Tier 3 approaches in order to demonstrate compliance with the new NAAQS . . . .” The conservative approach of Tier 1 is to assume that all NOx emitted from a proposed source will be quickly converted to NO2.

To “ease the burden on permit applicants”, EPA specifically recommends2:

• Use of 0.80 as a default ambient ratio of NO2 /NOx for the 1-hour NO2 standard under Tier 2 without additional justification by applicants; and

• General acceptance of 0.50 as a default in-stack ratio of NO2 /NOx for input to the PVMRM and OLM options within AERMOD, in the absence of more appropriate source-specific information on in-stack ratios. However, case-specific approval by the permitting agency is still required.

The Tier 3 approach utilizes either the OLM or the PVMRM option in AERMOD to simulate the basic chemical mechanism of ozone titration, which is the interaction of NO with ambient ozone to form of NO2 and O2. EPA expressly emphasizes that the PVMRM option is not inherently superior to the OLM option in estimating cumulative ambient NO2 concentrations. Each option has certain advantages and disadvantages depending on the site-specific circumstances to be modeled.

Consideration of Intermittent Emissions

EPA readily acknowledges in its memo that modeling of intermittent emissions, such as those from emergency generators or from startup/shutdown operations, has proven to be “one of the main challenges” for permit applicants seeking to demonstrate compliance with the 1-hour standard3. This is largely attributed to poor dispersion of emissions from low stacks characteristic of emergency generators and the dominant effect such emissions would have on modeled ambient impacts under a conservative assumption of continuous operation.

It is recognized by EPA that the intermittent and unpredictable nature of actual emissions from emergency generators, coupled with the probabilistic form of the 1-hour NO2 standard, imply that modeled impacts under conservative guidance of Appendix W could be significantly higher than actual impacts that would be realistically expected. This result is considered inconsistent with the goal of the probabilistic form of the 1-hour standard “to provide a more stable metric for characterizing ambient air levels by mitigating the impact that outliers in the distribution might have on the design value.”4

For various reasons articulated in the memo, EPA states its belief that “the most appropriate data to use for compliance demonstrations for the 1-hour NO2 standard are those based on emissions scenarios that are continuous enough or frequent enough to contribute significantly to the annual distribution of daily maximum 1-hour concentrations.” 5(Emphasis added.) EPA further believes that “existing modeling guidelines provide sufficient discretion for permitting authorities to exclude certain types of intermittent emissions from compliance demonstrations” for the 1-hour standard under such circumstances. EPA further opines that the rationale for its recommended treatment of intermittent emissions applies to “both project emissions and to any nearby or other background sources included in the modeling analysis.”6

EPA cautions that case-specific issues and factors may arise that affect the application of the 2011 Guidance and that not all projects for which compliance demonstrations will be required will fit neatly within a scenario with clearly defined continuous/normal operations as compared to intermittent/infrequent emissions. Consequently, applicants should consider consulting with permitting authorities if any questions arise in the use of the guidance.

Determining Background Concentrations

The 2011 Guidance points out that, unless a facility can demonstrate that ambient impacts produced by its emissions will not exceed the appropriate SIL, a cumulative analysis of ambient impacts will be necessary, and the determination of background concentrations to include in that cumulative impact assessment will be a critical component of the analysis.

According to the 2011 Guidance, the goal of the cumulative impact assessment should be to demonstrate with an adequate degree of confidence in the result that the proposed new or modified emissions will not cause or significantly contribute to violations of the NAAQS.7 Generally speaking, the more conservative the assumptions on which the cumulative analysis is based, the more confidence there will be that the goal has been achieved and the less controversial the review process should be.

• Identifying Nearby Sources to Include in Modeled Inventory

Appendix W stresses the importance of professional judgment by permitting authorities in the identification of nearby and other sources to be included in the modeled emission inventory.8 It also establishes, as a primary criterion for the selection of such sources, the concept of “significant concentration gradient in the vicinity of the source.” Both the 2010 Guidance and the 2011 Guidance caution against the literal and uncritical use of very prescriptive procedures for identifying background sources for NAAQS compliance demonstrations from sources such as the 1990 draft New Source Review Workshop Manual since those procedures can in many cases lead to cumulative impact assessments that are overly conservative. Unsurprisingly, the selection of background sources is said by EPA to involve very case-specific factors. EPA suggests the following:9

1) As a standard practice, contour plots of modeled concentrations should be prepared that clearly depict the impact area of the source, preferably overlaid on a map of the area that identifies key geographic features that could affect dispersion patterns;

2) The controlling meteorological conditions for the project impacts should be identified as clearly as possible. A recent update to AERMOD has been developed that includes new model output options (MAXDAILY and MXDYBYYR keywords) to identify specific time periods on which the modeled design value is based.

3) The location of the meteorological monitoring station whose data is used in the modeling analysis should be provided on the plot of source impacts along with an appropriate wind rose.

EPA states that applicants will need to acquire data on two principal components of the cumulative impact assessment: (i) the location and emissions from nearby background sources that may need to be included in the modeling emission inventory and (ii) the location and magnitude of air quality data from ambient NO2 monitors in the area. A preference is indicated for use of ambient air quality data to account for background conditions when available.

The Guidance alludes to challenges and controversial issues relating to cumulative impact assessments that involve how best to combine a monitored and modeled contribution to account for background concentrations. This is said to require an assessment of both (i) the spatial and temporal representativeness of background monitored concentrations and (ii) the potential for double counting of impacts.10 Appropriate combination of monitored and modeled concentrations is said to depend on a clear understanding of what ambient monitored data represents in relation to the modeled emission inventory. A further caveat is that the increased stringency of the new standard may require more detailed and refined analysis of such issues than has been previously necessary.

• Significant Concentration Gradient Criterion

The 2011 Guidance concedes that little previous guidance is available to explain what “significant” means in the context of “significant concentration gradient.”11 Building on the concept that “gradient” generally refers to “the rate of change of a physical quantity … distance”, the observation is made that the term in the present context refers to ground –level concentration of NO2 and has two dimensions – longitudinal (along wind direction) and lateral (cross-wind direction). While both dimensions may have some importance, the Guidance indicates that, in general, the impact of the longitudinal variability on concentrations at a particular time and point in space will be less significant than the variability associated with the lateral gradient. As a result of various considerations, the 2011 Guidance indicates that “the emphasis on determining which nearby sources to include in the modeling analysis should focus on the area within about 10 kilometers of the project location in most cases.12

It is said that the importance of the lateral gradient relative to the longitudinal gradient will generally increase for sources where the maximum hourly impacts occur under stable conditions due to the narrowness of the plume under such conditions. Another point made is that the pattern of concentration gradients can vary significantly based on the averaging period being assessed. For example, the spatial distribution of peak hourly impacts will typically show several localized concentration peaks with more significant gradients.13

Combining Modeled Results and Monitored Background

EPA states in the 2011 Guidance that the use of the overall highest hourly background concentration as a “first-tier” assumption for a uniform monitored background concentration that may be applied without further justification, as described in the 2010 Guidance, may be overly conservative in many cases but the conservatism is the basis of its designation as a first-tier assumption.

The 2011 Guidance goes on to state that a less conservative “first tier” assumption for a uniform monitored background contribution based on the monitored design value from a representative monitor should be acceptable in most cases. The monitored NO2 design value, i.e., the 98th percentile of the annual distribution of daily maximum 1-hour values averaged across the most recent three years of monitored data, should be used, irrespective of the meteorological data period used in the dispersion modeling.14

EPA provides the following range of examples regarding the issue of what extent of background source monitoring is needed. Use of a first-tier assumption for uniform monitored background concentration may represent a level of conservatism that is sufficient to avoid a need to include any background sources in the modeled inventory if, for example, the number of nearby sources which could contribute to the cumulative impact is relatively small and the available ambient monitor would be expected to reflect their cumulative impacts reasonably well or conservatively. A different extreme is said to be presented where the background source inventory included in the modeling is complete enough and background levels due to mobile sources and/or minor sources that are not explicitly monitored is expected to be small, an analysis based solely on modeled emissions and no monitored background might be considered adequate.

Diurnal patterns of ambient impacts could be a significant factor in determining the most appropriate method for combining modeled and monitored concentrations. An example given is that, if the daily maximum 1-hour impacts from project emissions generally occur at nighttime stable conditions, while maximum monitored concentrations occur during daytime convective conditions, the use of paired modeled and monitored concentrations based on hour of day should provide a more appropriate and less conservative estimate of cumulative impacts compared to a method that does not take diurnal variations into account. Diurnal variability could also be a significant factor in a situation in which contributions from mobile source emissions contribute significantly to modeled or monitored concentrations.

Finally, the 2011 Guidance offers the following conclusions on this topic:15

• The key principle in relation to short-term averaging periods is to determine background concentrations associated with “meteorological conditions accompanying the concentrations of concern.”

• EPA believes that an appropriate methodology for incorporating background concentrations in the cumulative impact assessment for the 1-hour NO2 standard would be to use multiyear averages of the 98th percentile of the available background concentrations by season and hour-of-day, excluding periods when the source in question is expected to impact the monitored concentration.

• The recent update to AERMOD (dated 11059) provides an option (the BACKGRND keyword on the SO pathway) to include temporally-varying background concentrations within the cumulative impact assessment based on these temporal factors. EPA opines that this technique provides a reasonable and efficient method for ensuring that the monitored contribution to the cumulative impact assessment will be representative of the “meteorological conditions accompanying the concentrations of concern.”

1 See, e.g., 2011 Guidance, at 5.

2 Id.

3 2011 Guidance, at 8.

4 Id.

5 Id., at 9.

6 Id., at 10

7 Id., at 12.

8 Appendix W., Section 8.2.3.

9 2011 Guidance, at 13.

10 Id., at 14.

11 Id., at 15.

12 Id., at 16.

13 Id.

14 Id., at 17.

15 Id., at 19-21.


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