This article appears as the cover story of the October 2025 issue of EM magazine, a copyrighted publication of the Air & Waste Management Association (A&WMA)

Air quality modeling is a key element of air quality permitting and planning processes. Current regulatory guidance is in the process of being revised and updated to address continued advances in the science and practice of air dispersion modeling. This issue of EM provides an overview of the proposed changes to regulatory guidance and models, as well as information on other developments in regulatory air quality modeling.

Air dispersion modelers in the United States and around the world rely on the guidance contained in the U.S. Environmental Protection Agency’s (EPA) Guideline on Air Quality Models, which is codified at 40 CFR 51, Appendix W. This guidance document, originally published in 1978, has been modified occasionally over the years as advances have been made in dispersion modeling theory and as new standards have been promulgated that have required the use of new and innovative modeling techniques.

The most recent revisions to the Guideline were proposed and published in the Federal Register in October 2023¹. Along with the proposed revisions, EPA released an updated version of the AERMOD Model (and associated pre-processor programs, such as AERMET) that incorporated options that EPA proposed to add as regulatory default options. EPA took comments on the proposed revisions in late 2023 and has been working toward responding to comments and issuing a final revised version since then. A final version of the Guideline on Air Quality Models and new versions of AERMOD and associated programs are expected to be released in fall 2024, as of the time of writing this article.

The articles included in this issue of EM cover the most significant updates that EPA proposed to the Guideline and the AERMOD Model in their October 2023 proposal.

First, an article by Patrick McKean and Cara Keslar provides information on EPA’s “Draft Guidance on Developing Background Concentrations for Use in Modeling demonstrations.”² This guidance expands on the discussion of appropriate background concentrations for modeling found in Section 8.3 of the current Guideline on Air Quality Models. EPA intends this guidance to provide more consistency in the determination of background concentrations for air permit modeling, while continuing to allow case-by-case flexibility. The fact that this guidance is separate from the formal Guideline allows EPA to revise it more frequently as use of and experience with the guidance continues and new questions and answers arise.

Next, we present three articles that examine some of the AERMOD options that EPA has proposed to make regulatory default options. Joseph Sabato and Milena Borissova discuss the proposed incorporation of AERCOARE algorithms into the AERMET pre-processor. This new option within AERMET allows meteorological data representative of over-water environments to be processed in AERMET. Michelle Snyder and co-authors consider the proposed R-LINE source type in AERMOD. This source type is intended to provide a new option to represent emissions from roadways and is expected to be used in roadway “hot-spot” analyses and potentially air permitting modeling analyses as well. Jason Reed provides a summary of the Generic Reaction Set Method (GRSM) option for nitrogen dioxide (NO₂) modeling in AERMOD. This option has the potential to add another option for use in refined, Tier 3 NO₂ modeling and incorporates enhancements to physics and chemistry that are not available in the existing Tier 3 options (OLM and PVMRM).

Finally, Paul Jacobson clarifies the future of the exceptional events rule. The determination of whether certain air quality concentration data are caused by “exceptional events” can be important in calculating design values for the U.S. National Ambient Air Quality Standards (NAAQS) for air quality attainment designations. And, while not specifically related to the proposed revisions to the Guideline on Air Quality Models, the concepts from the exceptional events rule can be useful for determining appropriate monitored background concentrations for use in air permit modeling. The use of the exceptional events rule is expected to be a critical aspect of the upcoming attainment vs. nonattainment designation process with respect to the annual fine particulate matter (PM_2.5) NAAQS, which was reconsidered by EPA and reduced in spring 2024, and the concepts from this rule are expected to be necessary when defining monitored background concentrations for air permit modeling against the annual PM_2.5 NAAQS.

Air dispersion models and modeling guidance have evolved since they came into widespread use in the 1970s and will continue to evolve into the future. To continue to look toward the future of air dispersion modeling, A&WMA will be hosting its 10th Specialty Conference on Air Quality Modeling, November 13–15 in Raleigh, North Carolina. The conference is intended to be held shortly after finalization of EPA’s proposed Guideline revisions, as a way to review the finalized changes and look forward to implementation of the revised guidance and identification of new and evolving challenges. The conference will bring together members of the air dispersion modeling community to share research, discuss challenges, and network to continue to develop the future of air dispersion modeling. The conference will include presentations by EPA’s Air Quality Modeling Group (AQMG) staff, a town hall-style session in which conference attendees can interface directly with AQMG staff and state modelers, and technical papers on a range of air dispersion modeling related topics. More information and registration information for the specialty
conference can be found on A&WMA’s website.

The subtitle of the conference is “A New Chapter,” which reflects the continuing challenges and opportunities faced by the air dispersion modeling community following finalization of the proposed changes to the Guideline on Air Quality Models.

Anthony J. Schroeder is a Principal Consultant with Trinity Consultants in Cleveland, OH. He has been designated a Certified Consulting Meteorologist and Qualified Environmental Professional and is a member of EM’s Editorial Advisory Committee.

References

1. Proposed Revisions to Appendix W to 40 CFR 51, Published October 23, 2023, Federal Register 88, 203
2. Draft Guidance on Developing Background Concentrations for Use in Modeling Demonstrations; EPA-454/P-23-001, October 2023.