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Proposed Changes to Modeling Guidance Available for Public Comment
Posted: 05/29/2013

The DNR is updating several dispersion modeling guidance documents to provide additional clarification on some aspects of the guidance, and to provide updates in response to recent changes in federal modeling guidance and requirements. These updates include:

  • The clarification that current background concentrations should be used when comparing previous modeling results to the National Ambient Air Quality Standards (NAAQS) in the modeling determination flow chart.
  • Addition of the new 1-hour SO2 NAAQS to the non-PSD modeling guidelines.
  • Additional guidance on when intermittent sources may be excluded from modeling analyses for 1-hour nitrogen dioxide (NO2) and sulfur dioxide (SO2).
  • Clarification of the proper method for using the daily fine particulate matter (PM2.5) paired-sums background data for modeling analyses associated with non-PSD projects.
  • Clarification that the DNR’s guidance on rounding modeled concentrations does not apply to PSD projects.
  • Updates to modeling guidance to reflect recent Court decisions and EPA guidance regarding the use of PM2.5 significant monitoring concentrations and significant impact levels.

It is the DNR’s goal that these revisions will provide improved modeling guidance to applicants submitting projects that include emissions of NO2, SO2, and PM2.5. These updates affect the following documents:

  • Form MD
  • Non-PSD Dispersion Modeling Guidelines
  • PSD Dispersion Modeling Guidelines
  • Rounding of Modeled Concentrations for Comparison with the NAAQS

The draft documents are available for informal public review on the DNR – Air Quality Bureau’s Public Input webpage under “Other Air Quality Items.”

Please send written comments to Brad Ashton by email to, by fax to 515-242-5094, or by mail or hand delivery to DNR Air Quality Bureau, 7900 Hickman Road, Suite 1, Windsor Heights, Iowa, 50324. The DNR must receive all comments by 4:30 p.m., June 3, 2013.