USEPA Finalizes Changes to Air Quality Modeling Guidance

On January 17, 2017, the U.S. EPA published a final rule that revises the Guideline on Air Quality Models (40 CFR Part 51, Appendix W).  This publish date had established February 16 as the rule's effective date; however, the Presidential directive in the January 20 "Regulatory Freeze Pending Review" memorandum and associated rule filed on January 24 establishes a new effective date of March 21, 2017.  The Guideline, last updated in 2005, provides direction to EPA, states, tribes, and industry on how to conduct air dispersion modeling, how to perform assessments of air quality, and how to revise their plans for improving air quality.  Specifically, changes to the Guideline mean changes to non-negotiable requirements for air quality modeling conducted for State Implementation Plan (SIP) revisions and for New Source Review (NSR) and Prevention of Significant Deterioration (PSD) programs.  EPA has issued the new guidance in the form of a regulation that is legally binding.

EPA's finalized changes include several important updates, including:

  • ARM replaced by ARM2 as the regulatory Tier 2 NO2 screening method;
  • OLM and a revised version of PVMRM made regulatory default Tier 3 NO2 screening;
  • Low wind speed turbulence adjustment (ADJ_U*) algorithm in AERMET made regulatory default option;
  • Proposed LOWWIND3 option, intended to address issues with AERMOD overprediction under low wind speed conditions, not finalized as a regulatory default option in AERMOD;
  • Use of prognostic meteorological data, processed through EPA's MMIF pre-processor, finalized as an option for regulatory modeling applications;
  • Guidance finalized on methodologies to address the secondary chemical formation of ozone and PM2.5 associated with precursor emissions (i.e., NOx and VOC for ozone and NOx and SO2 for PM2.5) from single sources;
  • CALPUFF removed from list of EPA "preferred" models for long-range transport assessments, but still available for use in screening analyses along with other Lagrangian models, such as SCICHEM; and
  • Updates finalized to recommended modeling procedures for cumulative impact analyses, including use of actual emissions for nearby sources and definition of background concentrations.

In the state of Washington, complex terrain makes the use of prognostic data with the MMIF pre-processor in lieu of onsite meteorological data particularly exciting for remote areas far from airports or other approved weather stations.  This development could dramatically improve timelines for greenfield projects.  While 12 km mesoscale datasets are expected to be available nationally, the University of Washington generates 1.3 km-resolution data that cover the entire state of Washington, which now span over one year.

For more information, please join us for our complimentary webinar on February 7, 2017, EPA's Final Changes To The Guideline On Air Models.  View the Final Rule in the Federal Register here.

For questions on the recent changes or assistance with dispersion modeling, contact Anna Henolson at or (253) 867-5600.