AERMOD Meteorological Data Update

On January 9, 2008, the EPA released revised guidance for determining the surface characteristics that are applied to the meteorological data used to run the AERMOD air dispersion model. The guidance is contained in the AERMOD Implementation Guide, which can be found on the SCRAM website here:

www.epa.gov/scram001/

The revised guidance reduces the upwind fetch used to determine the Surface Roughness Length from three kilometers to one kilometer, and changes the method used to calculate the average from the simple geometric mean to an inverse distance-weighted geometric mean. These modifications give more weight to the surface characteristics in the immediate vicinity of the meteorological tower.

Additionally, the Albedo and Bowen Ratio are no longer sector-dependant. Instead, a regional average is calculated over a 10 km by 10 km domain centered on the meteorological tower.

These modifications have been implemented in the recently released AERSURFACE program, along with multiple updates to the surface characteristics tables (refer to the AERSURFACE user guide for details, which is also available on the SCRAM website).

These modifications have the potential to significantly alter the results calculated by AERMOD. As such, the statewide meteorological data sets have been reprocessed using the new AERSURFACE program. All previous corrections to the meteorological data sets have been carried forward to the new data.

Preliminary sensitivity tests indicate that the updated meteorological data will cause predicted concentrations to increase by approximately 10% on average. Low release sources will be most significantly impacted by this change due to the decreased Surface Roughness Lengths produced by the new guidance.

All modeling analyses submitted as of Friday, March 7, 2008, must utilize the updated data. Projects with modeling protocols approved by the DNR prior to this date can use either the meteorological data available at the time the protocol was approved or the current data. However, all future modifications must utilize the updated meteorological data. All copies of the old data should be deleted.

The updated data can be found on the Air Quality Bureau’s website here:

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

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.