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Summary of 13th Conference on Air Quality Models


Last week I attended the 13th Conference on Air Quality Models at EPA’s campus at Research Triangle Park, NC. I have to say, selfishly I’m glad EPA holds these meetings at RTP—I grew up in the Raleigh area so it always gives me a good excuse to go home and see family!


Anyway, the focus of this conference was on the updates to AERMOD that EPA put out last month as well as refined guidance pertaining to the determination of appropriate background concentrations in NAAQS and SIP modeling. All of the presentations from the conference are now up online: https://gaftp.epa.gov/Air/aqmg/SCRAM/conferences/2023_13th_Conference_On_Air_Quality_Modeling/Thirteenth_Modeling_Conference-Final_Agenda.pdf


As far as Conference on Air Quality Models go, this one was pretty straightforward. The three main updates to AERMOD—adding COARE in AERMET (useful for offshore modeling), GRSM as a Tier 3 NO2 screening option (beta now but likely will be a regulatory option sometime in 2024), and RLINE as a source type (useful for modeling mobile sources)—have been in the works for a while. GRSM likely will be most useful to the majority of the modeling community as it will be nice to have another arrow in our proverbial NO2 modeling quiver.


The new guidance on background concentrations in NAAQS modeling was perhaps the most interesting part of the meeting. The impetus for this new guidance was EPA doing away with the “significant concentration gradient” language in Section 8.3 of the Guideline on Air Quality Models; EPA has decided that language was unclear and I have to say I agree—I never really figured out what it meant.


So that concept is now gone, and to make things clearer EPA has published a 47-page document entitled “Draft Guidance on Developing Background Concentrations for Use in Modeling Demonstrations.” My take on this (shared by others) is that while the goal of clarifying how background concentrations in modeling are developed is a worthy one (in this context a “background” concentration isn’t simply a monitored number, but rather the combination of the modeling of what are commonly called “offsite” sources plus a concentration value from an ambient monitor), I don’t feel like this document really does that. It definitely presents several different approaches for “sharpening the pencil” when it comes to deciding how to determine the background concentration in a cumulative modeling analysis, but it still remains pretty subjective. Because of that subjectivity, the more “refined” one gets with the approach the more scrutiny it will get from regulators, which of course means more time in review and delays in permitting. That said, I do like that some of these more refined approaches are now documented because if you follow this guidance it should make the regulatory review process more straightforward.


I’ll conclude with a somewhat philosophical note, which I touched on briefly in my quick LinkedIn post from the conference last week. I’m proud as an American that my government does something like this—not everywhere in the world does the government allow people to not only attend these kinds of meetings but to voice dissenting opinions. In these EPA meetings you’ll find government officials, people from industries, consultants, and environmental groups. All have different perspectives (and yes, agendas), and I think it’s important that all those points of view are represented. Of course, not everybody agrees all the time, but that’s a good thing. I truly believe that the collective interplay between industry and air quality in the United States is much better off because of this transparent and participatory process than if the government simply sat behind closed doors and imposed its will on its citizens.

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