On November 20, the United States Environmental Protection Agency (EPA) released updates to Appendix W and the AERMOD modeling system (version 24142). The final rule, published in the Federal Register on November 29, 2024, has an effective date of January 28, 2025. A summary of the changes and links to download the new version of AERMOD are available on EPA’s Support Center for Regulatory Atmospheric Modeling (SCRAM) website. DNR conducted a sensitivity test of the new version of AERMOD. This analysis indicates that results should not change for analyses conducted in Iowa unless one of the new regulatory options are used. Applicants should begin using the new version of AERMOD now, but the old version may continue to be used for projects that are currently underway. A model change that may affect modeling in Iowa is the Generic Reaction Set Method (GRSM), which is a tier-3 NOX-to-NO2 conversion screening option. It should be noted that GRSM requires hourly NOX concentration inputs in addition to hourly ozone data. The DNR has not evaluated the availability of suitable NOX data or tested the GRSM option yet, but will do so when time allows. Applicants can still use the Ozone Limiting Method (OLM) or Plume Volume Molar Ratio Method (PVMRM) when a tier-3 screening analysis is required. There were also changes to AERMOD’s meteorological preprocessor, AERMET. The DNR is already planning to process the next 5-year meteorological dataset (2020-2024) next year, so we will not be reprocessing the existing 2015-2019 datasets. Applicants should continue to use the current meteorological data. The other notable update is EPA’s new guidance for developing appropriate background concentrations for cumulative impact analyses. DNR’s existing background concentration guidance is consistent with this new document, but EPA’s guidance provides additional detail that applicants may find useful. DNR is not planning any changes to our current guidance or the background concentration data itself, but we will update our statewide default background concentrations next year to be consistent with the upcoming 2020-2024 meteorological datasets. Updated site-specific background data will also be provided at that time. Please send any questions to Brad Ashton at Brad.Ashton@dnr.iowa.gov, or by phone at (515) 725-9527.