On October 23, 2023, the United States Environmental Protection Agency (EPA)
published notice of a proposed rule that includes revisions to EPA’s
Guideline on Air Quality Models found at 40 CFR 51 Appendix W. The
Guideline contains EPA’s requirements and recommendations for regulatory air dispersion modeling, such as for air permitting or State Implementation Plan (SIP) revisions, in the United States. Along with the proposed revisions to the
Guideline, EPA released revised versions of the AERMOD Model and the AERMET meteorological data pre-preprocessor that incorporate model options that correspond with some of the proposals in the
Guideline. EPA will hold a public hearing on the proposed rulemaking in conjunction with the
13th Conference on Air Quality Modeling on November 14-15, 2023 and will take comments on the proposed rule until December 22, 2023.
The primary updates to the AERMOD and AERMET programs incorporate proposed Beta options that EPA has proposed for regulatory non-default status in the Guideline revisions. Beta options within AERMOD can be used on a case-by-case basis with formal approval from EPA. The proposed Beta options may become regulatory default options in a later release of AERMOD and AERMET depending on the comments EPA receives on the proposed rule and EPA’s response to comments. A brief description of the proposed Beta options follows.
- AERMET-COARE for Overwater Marine Boundary Layer Environments. EPA has incorporated algorithms that account for different atmospheric behavior over water or near large bodies of water into AERMOD. These algorithms, derived from the Coupled Ocean-Atmosphere Response Experiment (COARE), have been included in AERMET. The use of meteorological data prepared using the COARE algorithms has been approved by EPA for several permitting projects in recent years, particularly for permitting construction of offshore wind farms.
- GRSM Tier 3 Option for NO2 Modeling. The General Reaction Set Method (GRSM) option was a Beta option within the previous AERMOD release (22112). GRSM is a Tier 3 option within AERMOD to account for atmospheric reactions between NO, NO2, and ozone when modeling concentrations of NO2. GRSM will be a third Tier 3 option, should the proposal be finalized, added to the existing regulatory default Ozone Limiting Method (OLM) and Plume Volume Molar Ratio Method (PVMRM) options. GRSM includes consideration of the travel distance and time that it takes for atmospheric chemical reactions to occur, which is not a consideration for OLM and PVMRM.
- RLINE Mobile Source Type. The RLINE source type is an addition to the AREA, LINE, and VOLUME source types that are currently available for mobile source modeling. The RLINE source type has been developed in collaboration with the Federal Highway Administration and is intended for near-surface releases to simulate mobile source dispersion in the near-road environment. Thus, this source type is expected to be used in Transportation Conformity modeling analyses and other modeling of tailpipe emissions on public roads and highways.
EPA has also proposed updates to the existing guidance within Section 8 of the
Guideline relating to the development of representative background concentrations for National Ambient Air Quality Standards (NAAQS) modeling analyses. EPA has proposed to update Section 8 of the
Guideline to “provide a more robust framework for characterizing background concentrations for cumulative modeling with particular attention to identifying and modeling nearby sources in multi-source areas”. Concurrent with the
Guideline proposal, EPA has released a draft guidance entitled “
Draft Guidance on Developing Background Concentrations for Use in Modeling Demonstrations“. It “details the EPA-recommended framework with illustrative examples to assist permit applicants in characterizing a credible and appropriately representative background concentration for cumulative impact analyses including the contributions from nearby sources in multi-source areas”. As part of this revision, EPA is proposing to remove the term “significant concentration gradient” and its related content from Section 8.3.1 due to the “ambiguity and lack of definition of this term in the context of modeling multi-source areas”.
The preamble to the proposed rule also provides a summary of ongoing AERMOD model development. The following options continue to be under development as Alpha (i.e., experimental, non-regulatory) options within AERMOD. More details on each of these Alpha options is available in
Section V of the preamble to the proposed rule.
- Refined Low Wind Options
- PRIME Building Downwash Updates
- Downwash from Offshore Drilling Platforms
- Extended RLINE Source Type Including Barriers and Depressed Roadways
- Travel Time Reaction Method (TTRM) for Consideration of Travel Distance/Time in NOX to NO2 Conversion
- Plume Rise and Dispersion for Highly Buoyant Plumes
- Aircraft Emissions Plume Rise
For more information on the proposed Guideline on Air Quality Models and AERMOD/AERMET revisions, contact your local Trinity Office by calling 866.830.0796.
