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EPA Delays 30 Rules - Including Appendix W Modeling Update

On January 23, 2017, Catherine McCabe, Acting Administrator of the EPA, released a final rule, "Delay of Effective Date for 30 Final Regulations Published by the Environmental Protection Agency between October 28, 2016 and January 17, 2017," which delays the effective date of 30 EPA rules until March 21, 2017. Of particular importance to NSR followers, one of the delayed rules is "Revisions to the Guideline on Air Quality Models: Enhancements to the AERMOD Dispersion Modeling System and Incorporation of Approaches to Address Ozone and Fine Particulate Matter," (the Appendix W Update Rule) published January 17, 2017.


EPA's action is in accordance with the memorandum issued by the Assistant to the President and Chief of Staff, "Regulatory Freeze Pending Review," discussed earlier in this blog. EPA took the action under the "good cause" exemption at 5 U.S.C. section 553(b)(B) and 553(d)(3).

The EPA will "review and reconsider" each of the thirty regulations. Additional delays beyond the March 21, 2017 date will go through public notice and comment.

The Appendix W Update Rule, published at 82 Fed. Reg. 5182, includes a number of changes to the modeling guidelines. Some of the more important changes, now deferred, are as follows:

  • Clarifying which parts of Appendix W are mandatory versus guidelines;

  • Addition of a "LOWWIND3" option to address a tendency to overpredict concentrations at low velocities;

  • Enhancements to address plume rise for horizontal and capped stacks;

  • Updates to the NO2 Tier 2 and Tier 3 screening techniques;

  • Removal of SCREEN3 as a screening model;

  • Replacing CALINE3 as the preferred model for refined modeling for mobile source applications;

  • Promulgating a two-tiered demonstration approach maintaining a preference for photochemical grid models;

  • Removed CALPUFF as a preferred model, while allowing other Lagrangian models to be considered where appropriate; and

  • Finalized a more formal role for the Model Clearinghouse in reviewing modeling issues.

As noted above, all of these enhancements and changes are now deferred until March 21, 2017 (excluding, perhaps, as a practical matter, those changes merely codifying existing practice).


We hope to have a more definitive review of the Appendix W Update Rule in a future blog post.

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