FINALLY!! Guidance and Updated Programs Catch Up With New 1-Hour NAAQS
Posted: January 9th, 2012Author: All4 Staff
There has recently been a flurry of activity on U.S. EPA’s Support Center for Regulatory Atmospheric Modeling (SCRAM) website. U.S. EPA’s Air Quality Modeling Group (AQMG) has been busy updating the AERMOD air dispersion model and its supporting pre-processors to finally catch up with the new 1-hour nitrogen dioxide (NO2) and sulfur dioxide (SO2) National Ambient Air Quality Standards (NAAQS) promulgated on April 12, 2010 and June 22, 2010, respectively. AQMG has also developed memoranda to provide guidance to permit applicants on how to incorporate the new NAAQS into air quality modeling demonstrations in support of Prevention of Significant Deterioration (PSD) permitting projects. Since March 1, 2010, AQMG has released the following programs and guidance on the SCRAM website:
- March 1, 2011 – a clarification memo entitled “Additional Clarification Regarding Applicability of Appendix W Modeling Guidance for the 1-hour NO2 NAAQS’ was issued to assist in PSD permit modeling for NO2. Certain aspects of the memo apply to the new 1-hour SO2 NAAQS as well.
- March 4, 2011 – U.S. EPA released draft user’s guide addenda for the updates to AERMOD and AERMET, as well as a draft user’s guide for the new AERMINUTE program.
- March 17, 2011 – an updated version of AERSCREEN (Version 11076) became available.
- March 25, 2011 – a memo entitled “Area Source Designations for the 2010 Revised Primary Sulfur Dioxide National Ambient Air Quality Standards” was issued.
- April 7, 2011 – he AERMOD model version 11059 was released.
- April 8, 2011 – the AERMET processors version 11059 was released.
So what does all of this mean to potential air permit applicants? The best news to come from this recent flurry of activity is that the AERMOD air dispersion model is now capable of outputting results in the form of the new standards and can now provide other useful output options for evaluating air quality modeling results. The documents also provide guidance on how to incorporate the new 1-hour NO2 and SO2 NAAQS into a PSD air quality modeling analysis. The guidance should allow for less review time for state agencies and U.S. EPA, and could potentially lead to faster turnaround time on permit applications (provided that there are no NAAQS issues). The updated model and guidance will also be incorporated into the 1-hour SO2 NAAQS implementation process where dispersion modeling is being used for the designation process (see the SO2 NAAQS Modeling Blurb in this edition of 4 The Record for more information on the 1-hour SO2 NAAQS implementation process).
What doesn’t this mean to potential air permit applicants? Above all, the new 1-hour NO2 and SO2 NAAQS remain extremely restrictive. Although guidance has been provided on less conservative methods to use in assessing compliance with the 1-hour NAAQS, permit applicants may still need to be prepared to develop approaches for reducing NO2 and SO2 emissions in order to show compliance with the new standards.
For those interested in more technical detail, read on for a discussion on the updated models and modeling guidance.
The “Additional Clarification Regarding Applicability of Appendix W Modeling Guidance for the 1-hour NO2 NAAQS” memorandum supplements a June 29, 2010 guidance memo and provides further clarification and guidance on the application of 40 CFR Part 51, Appendix W modeling guidance for the 1-hour NO2 NAAQS. The memorandum:
- Clarifies procedures for demonstrating compliance with the 1-hour NO2 NAAQS based on the form of the standard.
- Provides clarification on the use and acceptance of Tier 2 and Tier 3 (as described in the Application of the Tiering Approach for NO2 section) options for NO2.
- Provides clarification regarding treatment of intermittent emissions compliance demonstrations for the 1-hour NAAQS.
- Recommends an appropriate methodology for incorporating ambient background concentrations into the cumulative impact assessment for the 1-hour NO2 and SO2 NAAQS.
Demonstrating Compliance with the 1-hour NO2 and SO2 NAAQS
The first step in completing an air quality modeling evaluation as part of a PSD permit application is to model the project-related emissions and compare the resulting modeled concentrations to the pollutant specific Significant Impact Levels (SILs). If the modeled concentrations are below the SILs (the interim 1-hour SILs are 7.5 mg/m3 for NO2 and 7.9 mg/m3 for SO2), no further analysis is required. If the modeled concentrations are above the SILs, a cumulative air quality modeling analysis that accounts for all sources at the facility being modeled in addition to other local facilities is required (NAAQS and PSD Increment). Given the more complex analysis procedures associated with the form of the new 1-hour NO2 and SO2 NAAQS, the memorandum states that, in most cases, it is acceptable to only include the receptors in the cumulative air quality modeling analysis at which the SILs were exceeded. A typical air quality modeling analysis has between 4,000 and 6,000 receptors. Only including the receptors in the cumulative air quality modeling analysis which exceed the SILs could potentially cut the number of receptors by 50% or more. This in turn will lead to faster run times for the AERMOD air dispersion model and less data to review when analyzing the results.
The memorandum also provides guidance on demonstrating compliance with the new 1-hour NO2 and SO2 NAAQS when conducting a NAAQS modeling analysis. Although the form of the new 1-hour NAAQS is the 3-year average of the 98th percentile (for NO2) or 99th percentile (for SO2) of the daily maximum 1-hour concentrations, the guidance document states that the new NAAQS does not supersede 40 CFR Part 51, Appendix W guidance, which requires five (5) years of National Weather Service (NWS) meteorological data or one (1) year of site-specific meteorological data for air quality modeling evaluations. Therefore, to demonstrate compliance with the new 1-hour NAAQS when using NWS data, the 5-year average of the 98th percentile of the daily maximum 1-hour NO2 concentrations must not exceed 100 parts per billion (ppb), while the 5-year average of the 99th percentile of the daily maximum 1-hour SO2 concentrations must not exceed 75 ppb. Similarly, when using one (1) year of site-specific meteorological data, the 98th percentile of the daily maximum 1-hour NO2 concentrations must not exceed 100 ppb, while the 99th percentile of the daily maximum 1-hour SO2 concentrations must not exceed 75 ppb. The guidance document further states that since the form of the new 1-hour NO2 and SO2 NAAQS is based on the annual distribution of daily maximum 1-hour values, only the first highest values from each day need to be used in the analysis.
In U.S. EPA’s newly released version of AERMOD (version 11059), options have been added to output results in the form of the new standards (i.e., multi-year averages of the 98th and 99th percentile of the daily maximum 1-hour concentrations). In addition, options have been added to help analyze the results in the form of the new standards. Before this new version of AERMOD was released, programmers were having a field day trying to develop post-processors that could handle the form of the new standards. All of the user-developed post-processors had to be reviewed and approved by reviewing agencies leading to longer review time for permit applications.
Application of the Tiering Approach for NO2
U.S. EPA has given further guidance on the application of the tiering approach for modeling 1-hour NO2 impacts. The three (3) tiers for modeling NO2 concentrations are as follows:
- The Tier 1 approach assumes full conversion of nitrogen oxide (NO) to NO2 and assumes that all oxides of nitrogen (NOX) that are emitted are converted to NO2 in the atmosphere.
- The Tier 2 approach assumes a default ambient ratio for the conversion of NO to NO2 in the atmosphere.
- The Tier 3 approach utilizes the Plume Volume Molar Ratio Method (PVMRM) or the Ozone Limiting Method (OLM) options in AERMOD.
U.S. EPA has recommended the use of 0.80 as a default ambient ratio for the 1-hour NO2 standard under Tier 2 without additional justification by applicants. Similarly, U.S. EPA has recommended the general acceptance of 0.50 as a default in-stack ratio of NO2/NOX for input to the PVMRM and OLM options within AERMOD under Tier 3, in the absence of more appropriate source-specific information of in-stack ratios. Since the Tier 2 and Tier 3 approaches are less conservative than the Tier 1 approach, it is anticipated that this will ease the burden on permit applicants in demonstrating compliance with the 1-hour NO2 NAAQS, as well as easing the burden on permitting authorities in reviewing such applications.
Clarification Regarding Treatment of Intermittent Emissions
U.S. EPA believes that including intermittent emission sources such as emergency generators and/or intermittent emission scenarios such as startup/shutdown operations would effectively impose an additional level of stringency beyond that intended by the level of the standard itself. U.S. EPA concluded that because the form of the 1-hour NAAQS is the 98th and 99th percentile of the annual distribution of daily maximum 1-hour values that intermittent sources which do not operate continuously or frequently enough to contribute significantly to the annual distribution of daily maximum 1-hour concentrations do not need to be included in 1-hour NAAQS analyses.
Incorporating Background Concentrations
As part of a NAAQS air quality modeling analysis, ambient background concentrations must be added to the cumulative modeled concentrations resulting from a permit applicant’s facility and other local sources. In U.S EPA’s memorandum, a 3-tiered approach for incorporating background ambient air concentrations has been outlined. The three (3) tiers for incorporating ambient background concentrations are as follows:
- The Tier 1 approach to including background ambient NO2 and SO2 concentrations in a NAAQS analysis requires that the maximum 1-hour concentration from the most recent three (3) years from a representative monitor be added to the modeled design concentration (98th percentile of the daily maximum 1-hour NO2 concentrations and 99th percentile of the daily maximum 1-hour SO2 concentrations).
- The Tier 2 approach incorporates background concentrations by hour-of-day. Specifically, the 8th highest monitored concentration for each hour (1 to 24) from each day from the most recent three (3) years is calculated and the appropriate hourly value is added to the modeled concentration.
- The Tier 3 approach incorporates background concentrations by season and hour-of-day. Specifically, the 3rd highest monitored concentration for each hour (1 to 24) from each day over one (1) season from the most recent three (3) years is calculated and the appropriate value is added to the modeled concentration.
The 3-tiered approach for incorporating background concentrations will allow permit applicants a less conservative approach for incorporating background concentrations into a NAAQS analysis. It is also pointed out in the memorandum that U.S. EPA does not recommend the approach of combining monitored and modeled concentrations on an hour-by-hour basis using hourly monitored background data collected concurrently with the meteorological data period being processed by the model. U.S. EPA has indicated that combining monitored and modeled concentrations on an hour-by-hour basis will only be accepted in rare cases of relatively isolated sources. A single ambient monitor, or even a few monitors, will not be adequately representative of hourly concentrations across the modeled domain to preclude the need to include emissions from nearby background sources in the modeled inventory. U.S. EPA does not believe that background monitored levels for each hour are spatially uniform. U.S. EPA also does not believe that the monitored values are fully representative of background levels at each receptor for each hour. In U.S. EPA’s newly released version of AERMOD (version 11059), options have been added for incorporating background concentrations using the Tier 2 and Tier 3 methods.
Updated and New Programs
In addition to the updates to AERMOD previously mentioned, AQMG has released updated versions of AERMET and AERSCREEN, as well as a new program, AERMINUTE. The updated version of AERMET includes modifications that address issues associated with processing National Weather Service (NWS) Integrated Surface Hourly Data (TD-3505) and includes updates for incorporating data from the newly released AERMINUTE program. The updated version of AERSCREEN fixes a number of bugs that were identified in the beta version, and also adds enhancements that help make the program more user-friendly. The AERMINUTE program was developed to average 2-minute NWS Automated Surface Observing System (ASOS) wind speed data, which the National Climatic Data Center (NCDC) has recently begun archiving, into 60-minute average wind speed data for use in AERMET. The program was developed to alleviate the issue of high occurrences of calm and variable wind conditions reported at NWS ASOS stations. This has the potential to provide permit applicants with a greater number of complete datasets for use in air quality modeling evaluations.
SO2 NAAQS Implementation
Following the release of the guidance memorandums and updated programs by AQMG, U.S. EPA’s Office of Air Quality Planning and Standards (OAQPS) was able to incorporate all of the guidance and updated programs into a memorandum on “Area Designations for the 2010 Revised Primary Sulfur Dioxide Ambient Air Quality Standards.” The memo provides information on the schedule and process for designating areas for the purpose of implementing the 1-hour SO2 NAAQS. The memo includes three (3) attachments:
- Timeline for the 2010 Primary SO2 NAAQS Designation Process.
- Determining Designations and Appropriate Boundaries for the 1-hour, 75 ppb SO2 NAAQS.
- Modeling Guidance for the SO2 NAAQS Designations.
U.S. EPA has laid out the following timeline for the designation process:
|U.S. EPA promulgates SO2 NAAQS||June 3, 2010|
|States and tribes flag exception event-influenced SO2 monitoring data from 2008-2009||October 1, 2010|
|States and tribes flag exceptional event-influenced SO2 monitoring data from 2010; provide detailed documentation to support all 2008-2010 claims||No later than June 1, 2011|
|States and tribes submit recommendations for area designations to U.S. EPA||No later than June 3, 2011|
|U.S. EPA notifies states and tribes concerning any intended modifications to their recommendations (120-day letters)||February 3, 2012 (not later than 120 days prior to final designation)|
|U.S. EPA publishes public notice for state tribal recommendations and U.S. EPAâ€™s intended modifications and initiates 30-day public comment period||February 20, 2012|
|End of 30-day public comment period||March 20, 2012|
|States and tribes submit additional information to demonstrate why a U.S. EPA modification is inappropriate||April 3, 2012|
|U.S. EPA promulgates final SO2 designations||No later than June 3, 2012|
As was outlined in the preamble to the final 1-hour SO2 NAAQS rule, given the limited network of ambient SO2 monitors, an analytic approach that uses both air quality monitoring and modeling has been recommended for the attainment designation process. The memo indicates that U.S. EPA does not generally expect states to provide refined dispersion modeling information along with their initial designations. Therefore, U.S. EPA expects most areas to be designated unclassifiable for the June 3, 2011 deadline. U.S. EPA has indicated in the memo that states can submit additional designation information within 60 days of receiving U.S. EPA’s 120-day letter.
The modeling guidance contained in the memo outlines procedures for implementing the following evaluations:
- Developing non-attainment boundaries for areas with a violating ambient SO2 monitor if the presumptive county boundaries are not used.
- Giving further evidence for an attainment designation in an area without a violating ambient monitor.
As was also mentioned in the preamble to the final 1-hour SO2 NAAQS rule, the memo also indicates that U.S. EPA still intends to issue additional guidance for areas designated as unclassifiable. The guidance will include technical direction on how to conduct refined dispersion modeling to demonstrate future NAAQS attainment.