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Summary of 2021 RSL Dispersion Modeler's Workshop

On the afternoons of June 21 through June 23, EPA held the 2021 Virtual Regional, State, and Local (RSL) Dispersion Modeler’s Workshop. The first afternoon was “stakeholder day” where non-regulatory stakeholders (essentially consultants and industry) were allowed to attend, with the remainder of the RSL Workshop being for the regulators only. Read on for a summary of what happened on the first afternoon.

Most of “stakeholder day” was focused on the latest news concerning AERMOD. EPA stated that their plan is to propose a regulatory update to the AERMOD modeling system in the fall of 2022, with the plan being for the final rule to come in the spring of 2023. The goal is to time this with the 13th Conference on Air Quality Models which would serve as the public hearing for the regulatory update. (Recall that Section 320 of the Clean Air Act mandates that EPA conduct this conference at least every three years, and the last one was held in October 2019). As part of this regulatory update to AERMOD there will be some small edits to the Guideline on Air Quality Models, but not the massive changes that happened back in 2017.

After providing an overview of the recent release of Version 21112 of AERMOD, including the noted bugs and recommended workarounds (for more on that see, the remainder of the afternoon consisted of two sessions focused on AERMOD development priorities.

EPA’s first presentation regarding AERMOD development was an overview of its process for improving the model. EPA talked about how the first step is for somebody in the modeling community and EPA to have a discussion concerning a modeling issue, and if there’s a basis for improving the model then EPA will request a White Paper. After EPA and the stakeholder work collaboratively, the White Paper is posted to SCRAM, and eventually the enhancement is added to AERMOD either as an ALPHA option (developmental options that are currently for research and are not available for regulatory use) or BETA option (peer-reviewed options that are ready for consideration as an alternative model).

These White Papers ( are a good indication of where EPA’s focus currently is concerning improvements to AERMOD. At this time there are White Papers posted for the following topics:

  • Low wind speed options

  • Saturated plumes

  • Structure downwash

  • NO2 modeling

  • Mobile source modeling

  • Overwater modeling

In its presentation EPA also mentioned several other topics that, while they’re not at the top of their list, they are on EPA’s proverbial radar for AERMOD enhancements. These topics are as follows:

  • Buoyant plume rise (buoyant line and point sources)

  • Deposition (a White Paper is coming soon)


  • AERSURFACE (EPA is currently reviewing a White Paper received on this topic)

The next presentation was on NO2 conversion, specifically trying to improve NO2 predictions for a wide variety of sources. The results of the model performance with two new ALPHA options (the General Reaction Set Method and the Travel Time Reaction Method) were presented against two new datasets available, one from Oklahoma and one from Colorado. Also during that presentation EPA mentioned that the NO2/NOx in-stack ratio database has been updated with new data courtesy of the Electric Power Research Institute and Pipeline Research Council International.

Following that a presentation concerning downwash was given, which addressed the six ALPHA downwash options from Version 19191 and the two new ALPHA options in Version 21112 of AERMOD—AWMAUTURBHX and AWMAENTRAIN.

After that it was on to the topic of overwater/offshore modeling. EPA announced that their goal is to replace the Offshore and Coastal Dispersion (OCD) model (around since the 1980s) with AERMOD. To do this, three topics need to be incorporated into AERMOD: the treatment of overwater boundary layer meteorology, structure downwash associated with platforms (i.e., a structure that is not solid), and shoreline/coastal fumigation. Work is ongoing in all three of these areas.

It was then back on to land for an update on RLINE and mobile source modeling and development concerning terrain and the impact of various noise barriers on dispersion. The AERMOD development sessions concluded with EPA providing an update on its work regarding plume rise and low winds.

The final session kicked off with EPA talking about its upcoming overhaul of AERMET. There won’t be any formulation changes so no regulatory action will be required, but the code itself is going to be “modernized.” One of the most noticeable outcomes of this will be that the new version of AERMET will only have two stages (Stage 2, part of AERMET since its inception, will be eliminated). EPA is currently updating the user’s guide and hopes to release the new AERMET in the fall of this year.

Sticking with the subject of meteorological data EPA also gave an update on prognostic meteorological data and the planned updates associated with MMIF data.

All in all, there’s a lot going on in the world of AERMOD, but there wasn’t any discussion of any other models. EPA anticipates that development will need to stop around August of next year to meet the deadline of the 13th Conference on Air Quality Models being by 2023.

Finally, the hope is that this was the last of the “virtual” workshops, with face-to-face meetings resuming as early as later this year.

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