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Modeling Guidance Updates and Implementation of One-hour SO2 NAAQS
Posted: 06/11/2013

The DNR has updated 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 incorporate comments received during the recent informal public comment period.

These revisions provide improved modeling guidance to applicants submitting projects that include emissions of NO2, SO2, and PM2.5. The affected documents include:

  •  Form MD
  • Non-PSD Dispersion Modeling Guidelines
  • PSD Dispersion Modeling Guidelines
  • Rounding of Modeled Concentrations for Comparison with the National Ambient Air Quality Standards (NAAQS)

The revised guidance is available on the DNR – Air Quality Bureau’s Dispersion Modeling webpage.

Specific updates include:
The clarification that current background concentrations should be used when comparing previous modeling results to the NAAQS in the modeling determination flow chart.

  • Clarification that the modeling determination flowchart is pollutant-specific.
  • 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.

Concurrent with these guidance updates the DNR is implementing the 1-hour SO2 NAAQS for minor source projects. Effective June 3, 2013 any project that is required to be modeled for SO2 will need to also consider the 1-hour SO2 NAAQS. Implementation of the 1-hour SO2 NAAQS for Prevention of Significant Deterioration (PSD) projects started in June 2010.

Questions 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.