Revised AERMOD Meteorological Data

Febraury 5, 2007

Several users have reported experiencing an error when using the preprocessed meteorological data available on the Air Quality Bureau’s website with the recently released 07026 version of AERMOD. To avoid compatibility issues caused by changes to AERMET, AERMOD will not accept meteorological data created by versions of AERMET that are older than the 06341 release (the Iowa meteorological data was processed using AERMET 04300). Incompatibilities between the two versions of AERMET are caused by several corrections that have been made to the model.

Fortunately, the majority of the corrections that have been made to AERMET were already addressed during the processing of the Iowa meteorological data sets. Specifically, all errors described in the model change bulletin relating to cloud cover, station pressure, elevation, missing value flags and ISH data were accounted for during the processing of the current Iowa meteorological data sets. Therefore, those modifications will not affect the current pre-processed data.

However, several of the other modifications to AERMET were not addressed, so a sensitivity analysis was performed to determine the effect of those changes. The results of the analysis indicated that the model concentrations would change by less than 0.5%. Due to the results of the sensitivity analysis it was decided that the statewide meteorological datasets would not be reprocessed. Instead, the version number in the meteorological data files was changed to match the newest version of AERMET. This allows the files to be used in the newest version of AERMOD. The statewide data will be reprocessed in 2010, once a new five-year period becomes available.

All modeling conducted using the 07026 version of AERMOD should utilize the revised data, and all copies of the previous data should be deleted. The revised data can be found on the Air Quality Bureau’s website here:

www.iowacleanair.com/prof/tech/AERMODMetData.html

Information related to the modifications to the AERMOD modeling system can be found on the EPA’s SCRAM website here:

www.epa.gov/ttn/scram/dispersion_prefrec.htm#aermod

Questions regarding the meteorological data should be directed to Brad Ashton of the Air Quality Bureau. He can be reached by telephone at (515) 242-6532 or by email at Brad.Ashton@dnr.iowa.gov.