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Delayed - Revisions to Guideline on Air Quality Models (Appendix W)

  • 3/17/2017 8:20:00 AM
  • View Count 2834
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On March 15, 2017 the United States Environmental Protection Agency (EPA) announced that they extended the temporary delay of the effective date for the revisions to Appendix W.  The final rule, published in the Federal Register on January 17, 2017, has a new effective date of May 22, 2017.  The additional delay is intended to provide incoming EPA officials time to review the rule.

The revisions to Appendix W include several new or updated model options that will help alleviate model over-prediction or allow for more appropriate source characterization.  Of particular importance to applicants in Iowa are the updates to the Nitrogen Dioxide Tier 2 and Tier 3 screening techniques, the optional adjustment of the surface friction velocity (Adjust-u*), and regulatory options to address plume rise for horizontal and capped stacks.

The Iowa DNR is prepared to begin implementing these new model options as soon as the rule becomes effective.  However, given the fact that incoming EPA officials have not completed their review of the rule, it is possible that it may change.  For this reason the Iowa DNR will not begin using the new or updated model options in the revised version of Appendix W until the final effective date. 

In the interim, applicants wishing to utilize model options that would become regulatory defaults under the revised Appendix W, but that are not regulatory defaults under the current version of Appendix W, will need to provide justification.  For Prevention of Significant Deterioration (PSD) projects, applicants will need to follow the requirements in section 3.2.2 of Appendix W, and the DNR will coordinate approval with EPA.  For non-PSD projects the DNR will review each request and determine the appropriateness of the model option on a case-by-case basis. 

EPA also released a new version of the regulatory dispersion model AERMOD (version 16216r).  While this new version of the model contains the options implemented in the delayed version of Appendix W, it also includes several bug fixes.  EPA has indicated that this new version of AERMOD, used without the delayed model options, is the regulatory version of the model during the temporary delay, and that it should be used for all projects.

Visit EPA’s Support Center for Regulatory Atmospheric Modeling (SCRAM) website for additional information about the 2017 Appendix W final rule.

Please send any questions to Brad Ashton at, or by phone at (515) 725-9527.