AERMOD Update

December 1, 2005

The EPA promulgated AERMOD (AMS/EPA Regulatory Model) as a preferred guideline model in Appendix W of 40 Code of Federal Regulations (CFR) Part 51 on November 9, 2005. This promulgation will become effective on December 9, 2005. The department will complete revisions to the Iowa Administrative Code to adopt the revised Appendix W by reference before the middle of 2006. The department will review and update as necessary the Air Dispersion Modeling Guidelines in early 2006 to reflect the use of AERMOD. A workgroup will also be formed to provide recommendations regarding revisions to the modeling exemption portion of the Air Dispersion Modeling Guidelines.

AERMOD replaces the Industrial Source Complex model (ISC3). AERMOD provides better characterization of plume dispersion than ISC3 and incorporates the PRIME downwash algorithms. There will be a one year transition period from ISC3 to AERMOD. The DNR will accept modeling conducted with either ISC3 or AERMOD during this one year transition period. Full implementation of AERMOD will take place one year from promulgation (November 9, 2006); at this time ISC3 will become an alternative model and regulatory use of ISC3 will require prior approval.

The AERMOD system includes three separate components: the meteorological preprocessor AERMET, the terrain preprocessor AERMAP, and the dispersion model AERMOD. The DNR plans to have meteorological data processed for the entire state by the end of 2005. Since the final stage of meteorological processing incorporates site-specific surface parameters, every facility will have a site-specific meteorological data set. Once processing of the meteorological data has been completed, this site-specific meteorological data may be requested from the DNR at no charge. A future listserver item will give additional information on how to request meteorological data from the DNR.

The AERMOD code, model documentation, user's guides, and associated supporting documentation, may be found on the EPA's Support Center for Regulatory Air Models (SCRAM) Web site at www.epa.gov/ttn/scram.

Questions related to AERMOD should be directed to the air quality bureau's modeling group by contacting (515)-242-5100.