On January 17, 2017, EPA published a final rule updating its Guideline on Air Quality Models (Guideline) codified at 40 CFR Part 51 Appendix W. The effective date of the final rule was delayed until March 21, 2017 in accordance with an executive order of the President and has been further delayed until May 22, 2017. 82 Fed. Reg. 14324 (March 20, 2017). EPA, states, air quality control agencies and industry use the Guideline when conducting modeling to estimate ambient concentrations of pollutants for purposes of permitting new or modified sources under the New Source Review (NSR) program, including permit actions under the Presentation of Significant Deterioration (PSD) program, as part of State Implementation Plan (SIP) submittals and revisions and conformity or other assessments required by regulation. The Guideline was first incorporated into the PSD program regulations in 1978 and has been periodically updated with the last update in 2005.

Although effective in 2017, reviewing agencies are given one year to integrate changes to EPA’s preferred models and revisions to the requirements and recommendations of the Guideline. Transportation conformity changes have to be integrated within three years. In the interim one year period, modeling protocols based on the 2005 Guideline may be approved at the discretion of the reviewing agency. Any refined analyses started before the end of the interim three-year period with a preferred model based on the 2005 Guideline version can be completed after the end of the transition period.

The 2017 Guideline includes enhancements to the EPA preferred model, which continues to be the American Meteorological Society/EPA Regulatory Model (AERMOD) and incorporation of a tiered approach for the secondary chemical formation of ozone and fine particulate matter (PM2.5) associated with precursor emissions from single sources. Several models have been removed from the Guidelines and EPA changed the preferred status of some models. Included is EPA’s final action to codify a screening approach to address long-range transport for assessing NAAQS and PSD increments, removal of the California Puff model (CALPUFF) as a preferred model and confirmation of the recommendation to consider CALPUFF as a screening technique.

One enhancement to AERMOD is the adoption of an option (ADJU*) into the AERMET meteorological processor for AERMOD to address issues with model over prediction of ambient concentrations from some sources during light wind, stable conditions.

In addition to other enhancements to AERMOD, EPA incorporated a program, AERSCREEN, as the recommended screening model for AERMOD, which may be used in applications across all types of terrains and for applications involving building downwash.

Other changes to the Guideline include certification of EPA’s practice of consulting and coordinating with EPA’s Model Clearinghouse prior to approval of alternate models; updated procedures to clarify the factors to be considered in conducting both a single source and cumulative impact analysis; updated use of meteorological input data for regulatory dispersion modeling; and editorial changes to update and reorganize information in the Guideline. The 2017 Guideline can be found at https://www.gpo.gov/fdsys/pkg/FR-2017-01-17/pdf/2016-31747.pdf