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Modeling Guidance, Meteorological Data, and Background Value Updates
Posted: 01/22/2013
The Iowa DNR has made updates to several modeling guidance documents, including the “Air Dispersion Modeling Guidelines For Non-PSD, Pre-Construction Permit Applications,” “Air Dispersion Modeling Checklist,” “Air Dispersion Modeling Guidelines For PSD Projects,” and the “Modeling Protocol Template.” These documents replace previous document versions. The documents are used to assist in the completion of dispersion modeling analyses conducted by applicants and by the DNR that are associated with construction permitting projects. 

The DNR has also added a new guidance document called “Rounding Conventions.” This document provides guidance on truncation criteria and rounding conventions that can be applied to modeled concentrations when comparing values with the National Ambient Air Quality Standards (NAAQS). This guidance is intended to improve consistency in how the rounding conventions are applied to modeled concentrations.

The DNR has reprocessed all of the 2005-2009 meteorological datasets using the new version of AERMET released by U.S. Environmental Protection Agency (EPA) December 17, 2012. Consistent with EPA guidance a minimum wind speed threshold of 0.5 meters per second (m/s) was used during the processing of this revised dataset. The new meteorological data can be differentiated from the previous versions by the letter “C” appended to the end of the file names. Applicants should use the new meteorological data for all modeled projects submitted as of January 22, 2013. The data can be found on the DNR's meteorological data webpage. The revised guidance documents include updated default background values for all pollutants. In addition, daily PM2.5 background data has been created that compliments the 2005-2009 meteorological datasets. This data can be used in non-PSD analyses and is available on the DNR’s background data webpage.

Purpose of the Modeling Guidance Updates

The DNR updated the modeling guidance documents to incorporate a new streamlined modeling determination process for non-Prevention of Significant Deterioration (non-PSD) projects. The new modeling determination process has been incorporated into the Form MD. Updates to the modeling guidance documents also provide for the implementation of the 2006 fine particulate (PM2.5) and 2010 1-hour nitrogen dioxide (NO2) and sulfur dioxide (SO2) NAAQS as applicable. 

Need for Guidance Changes

The proposed changes to the modeling guidance documents are needed to incorporate the revised modeling determination process for non-PSD projects and to provide guidance for modeling required in conjunction with the DNR’s implementation of the PM2.5 and 1-hour NO2 NAAQS. Review of minor projects for the PM2.5 NAAQS and the 1-hour NO2 NAAQS begins January 22, 2013. Review of major (PSD) projects for the PM2.5 NAAQS and the 1-hour NO2 and SO2 NAAQS is already required by EPA. 

Summary of Guidance Changes

For non-PSD projects, the modeling guidance has been updated to reflect the new streamlined modeling determination process used on the Form MD. The modeling guidance documents have been updated to include significant impact levels for the new NAAQS to be evaluated, the new NAAQS, and specific guidance related to the new NAAQS including the three-tiered screening approach for NO2 and evaluation of the 1-hour NAAQS for intermittent use sources. The modeling guidance documents also reflect updated default background concentrations. 

Need for Meteorological Data Changes

On December 17, 2012 the EPA released new versions of both AERMOD and its meteorological pre-processor AERMET. These new versions incorporate several important changes that can affect the concentrations predicted by the model. A summary of the changes to both AERMOD and AERMET can be found in the model change bulletins available on EPA’s Support Center for Regulatory Air Quality Models (SCRAM) website.

Effect of AERMOD and AERMET Changes on Concentrations

Sensitivity tests were conducted with both the new model and the new meteorological data. The results of these tests indicate the maximum model concentrations will range from one percent lower for 1-hour averages to six percent higher for annual averages. Closer inspection of the results shows that the new version of AERMOD produces the increased concentrations while the new meteorological data causes the net decreased concentrations for one-hour averages. 

Questions regarding the updates to the modeling guidance documents may be directed to Lori Hanson at (515) 281-8911 or Questions regarding the meteorological data may be directed to Brad Ashton at (515) 242-6532 or

This list serve article is intended for consultants, industries, environmental groups, general public, and other entities regulated under the Clean Air Act. It was written by the DNR Air Quality Bureau to share regulatory news, program updates and technical guidance.