Updates to AERMOD and Guideline on Air Quality Models

Hear ye, hear ye! There’s a lot happening in the air dispersion modeling world these days.

For starters, this coming November 14th and 15th EPA is going to be holding the 13th Conference on Air Quality Models at its offices in Research Triangle Park, North Carolina. The public can attend (Blue Sky Modeling will be there), and the topics of discussion will be updates to AERMOD and new guidance on developing background concentrations for air quality modeling.

AERMOD version 23132 was released today (October 23, 2023 EPA), replacing version 22112. As with most new versions there are several bug fixes and a few substantive changes. With this update the change that probably affects the most people is the promotion of the Generic Reaction Set Method (GRSM) to the list of approved Tier 3 methods for modeling NO2, which now gives users a third Tier 3 approach to NO2 modeling (along with OLM and PVMRM). Like PVMRM and OLM, GRSM requires NO2/NOx in-stack ratios and ambient hourly ozone concentrations; in addition, the use of GRSM will require hourly ambient NOx concentrations.

So from now on if you’re doing NO2 modeling and need to refine your impacts beyond the Tier 2 approach of ARM2, be sure to consider GRSM as an option.

EPA is also proposing to update its guidance on developing background concentrations for use in cumulative modeling (as presented in Section 8.3 of the Guideline on Air Quality Models), with the primary change being to delete the mention of a “significant concentration gradient” as that concept has proved to be confusing and unclear. EPA is also publishing new draft guidance on developing background concentrations.

Stay tuned for more details after the 13th Conference next month.