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Proposed Revisions to EPA’s Guideline on Air Quality Models

On 07/29/2015, EPA’s proposed revision to the air quality modeling guideline (Appendix W to 40 CFR Part 51) was published.

Appendix W Factsheet:


  • Proposed enhancements to the scientific formulation of the preferred nearfield dispersion model, AERMOD, including modeling techniques to address the secondary chemical formation of fine particle and ozone pollution from direct, single source emissions of pollutants that form them such as sulfur dioxide, oxides of nitrogen, volatile organic compounds. (See details below)
  • Proposed streamlining of resources necessary to conduct regulatory modeling with AERMOD by incorporating model algorithms from the Buoyant Line and Point Source (BLP) model and replacing the CALINE3 model used for mobile source applications including fine particle pollution (PM2.5, PM10), and carbon monoxide (CO) hot-spot analyses.
  • Remove CALPUFF as a preferred model for long range air quality assessments and recommending its use as a screening technique along with other Lagrangian models for addressing PSD increment beyond 50 km from a new or modifying source. This proposed change does not affect the EPA’s recommendation in the 2005 BART Guidelines to use CALPUFF in the BART determination process.

Addressing Single-Source Impacts on Ozone and Secondary PM2.5:

  • EPA has determined that advances in photochemical modeling science indicate it is now reasonable to provide more specific, generally applicable guidance that identifies particular models or analytical techniques that may be used under specific circumstances for assessing the impacts of an individual source on ozone and secondary PM2.5.
  • EPA believes photochemical grid models (CAMx and CMAQ) are generally most appropriate for addressing ozone and secondary PM2.5 because they provide a spatially and temporally dynamic realistic chemical and physical environment for plume growth and chemical transformation.
  • EPA is proposing a two tiered demonstration approach for addressing single-source impacts on ozone and secondary PM2.5. The appropriate tier for a given application should be selected in consultation with the appropriate reviewing authority and be consistent with EPA guidance.
  • The 1st tier involves use of technically credible relationships between precursor emissions and a source’s impacts that may be published in the peer-reviewed literature; developed from modeling that was previously conducted for an area by a source, a governmental agency, or some other entity and that is deemed sufficient; or generated by a peer reviewed reduced form model.
  • The 2nd tier involves application of more sophisticated case-specific chemical transport models (e.g., photochemical grid models) to be determined in consultation with the EPA Regional Office and conducted consistent with new EPA single-source modeling guidance.
  • To fully implement these proposed changes to the Guideline related to addressing ozone and secondary PM2.5 impacts, the EPA intends to pursue a separate rulemaking to establish a technical basis and new values for PM2.5 Significant Impact Levels (SILs) and to introduce a new demonstration tool for ozone and PM2.5 precursors referred to as Model Emissions Rates for Precursors (MERP).
  • A MERP would neither replace the existing Significant Emissions Rates (SERs) for these pollutants nor serve as the basis for the applicability of PSD requirements to sources with emissions above the SER. However, a MERP would represent a level of emissions of precursors that is not expected to contribute significantly to concentrations of ozone or secondarily formed PM2.5.

Any comments on the proposed guideline must be sent to EPA’s OAQPS office by October 27, 2015.

Please contact your EPS consultant for more information.

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