In late January 2025, two regulations become effective that have implications for air permitting projects requiring air dispersion modeling analyses. Revisions to the Secondary National Ambient Air Quality Standard (NAAQS) for SO2 and a revision to the Unites State Environmental Protection Agency’s (EPA’s) Guideline on Air Quality Models were finalized in late 2024 and are set to become effective.
Secondary SO2 NAAQS Revision
On December 27, 2024, the EPA published notice of a final rule that revises the existing secondary NAAQS for SO2. The secondary NAAQS for SO2 is being revised to be an annual average, averaged over three consecutive years, with a level of 10 parts per billion (ppb). In the same rulemaking, EPA retained the existing secondary NAAQS for nitrogen oxides and particulate matter. The rule becomes effective on January 27, 2025.
The previous secondary NAAQS for SO2 was a 3-hour average standard of 0.5 parts per million (ppm) or 500 ppb, which was not to be exceeded more than once per year. Upon the effective date of the regulation, this standard will be replaced with the annual average standard; therefore, the 3-hour average standard will no longer be in effect. One implication of this change will be that air dispersion modeling analyses for SO2 will no longer need to show compliance with the 3-hour average SO2 NAAQS. Note however that the 3-hour average PSD increment standard for SO2 will remain in effect and where PSD increment compliance is required, an analysis against this standard will still be necessary.
For air permitting projects requiring air dispersion modeling for SO2, EPA indicated in the preamble to the final rule that it was developing technical justification for how a demonstration of compliance with the 1-hour primary SO2 NAAQS could suffice to demonstrate compliance with the new secondary SO2 standard. A memo containing this demonstration was released by EPA on December 10, 2024. In this memo, EPA states:
Based on the attached technical analysis, the EPA has determined that a demonstration that increased SO2 emissions will not cause or contribute to a violation of the primary 1-hour SO2 standard can suffice to demonstrate that SO2 emissions will also not cause or contribute to a violation of the secondary annual SO2 standard. Thus, permit applicants and reviewing authorities may rely on the demonstration for the primary 1-hour SO2 NAAQS to also satisfy the demonstration requirement for the secondary annual SO2 NAAQS. Permit applicants and reviewing authorities are not required to follow this alternative PSD demonstration approach but may choose to do so based on this memorandum and the attached technical analysis.
For air permitting projects triggering PSD for SO2 or otherwise requiring air dispersion modeling for SO2, citation to this memo may relieve applicants of the requirement to complete a separate analysis to demonstrate compliance with the new annual average secondary NAAQS if compliance with the 1-hour SO2 NAAQS is demonstrated.
Guideline on Air Quality Models Revisions
On October 23, 2023, the 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. These revisions were finalized in late 2024 with notice on the finalization being published in the Federal Register on November 29, 2024. The effective date for the final rule was initially set at January 28, 2025. However, on January 28, 2025, a Federal Register notice was published that delayed the effective date until March 21, 2025.
Along with the revisions to the Guideline, EPA released revised versions of the AERMOD Model and several of its associated pre-processor programs (e.g., AERMET, AERMAP, AERSURFACE) that incorporate model options that correspond with some of the proposals in the Guideline and make other bug fixes and enhancements.
AERMOD Option Updates
The primary updates to the AERMOD and AERMET programs elevate previous Beta options to regulatory non-default status in the Guideline. Beta options within AERMOD can be used on a case-by-case basis with formal approval from EPA whereas regulatory options can be used without added justification beyond a typical modeling protocol. The newly finalized regulatory options were not changed as compared with the options proposed in 2023. A brief description of the finalized regulatory 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 starting with the 2022 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 is a third Tier 3 option, 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 Background Guidance
EPA has also finalized 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 updated 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 finalization, EPA has finalized guidance entitled “Guidance on Developing Background Concentrations for Use in Modeling Demonstrations”. The guidance “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”. A draft of this guidance was released by EPA in October 2023 and made available for public comment. Based on comments received, EPA made a few minor clarifications in the final guidance and also added hypothetical examples that are intended to show the use of the guidance in different scenarios. As part of this revision, EPA also removed 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”.
For more information on the finalized Guideline on Air Quality Models, AERMOD program revisions, and SO2 NAAQS revisions, contact Tony Schroeder, CCM, QEP or your Trinity Office by calling 866.830.0796
