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Regulatory Update   |   Jul 31st, 2015 The New Path Forward: EPA’s Proposed Revision of the Guideline on Air Quality Models

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EPA’s proposed revision of 40 CFR 51, Appendix W Guideline on Air Quality Models (generally referred to as Appendix W), was published in the Federal Register on July 29, 2015. The proposed revisions outline a new path forward for the air quality modeling community. They provide relief on many technical issues addressed since the last revision ten years ago and present some new regulatory modeling challenges.

The proposed revisions include enhancements and updates to the AERMOD modeling system. AERMOD is EPA’s preferred model for most applications requiring air quality assessments within 50 kilometers of a project site. A new version of the AERMOD model, Version 15181, was released in conjunction with the proposed Appendix W revisions. This new version:

  • Addresses and corrects well-documented over-prediction of certain modeling scenarios (e.g., low wind speeds conditions and tall stacks near small urban areas).
  • Promotes previously non-default nitrogen dioxide screening approaches coded in the model (e.g., ambient ratio methods, ozone limiting methods, plume volume molar ratio method) to default options which will eliminate the need to justify their use every time.
  • Eliminates the need to use separate models for assessing impacts from buoyant line sources and mobile sources.

Understanding and properly applying these enhancements and updates will result in a more accurate air quality modeling compliance demonstration.

The proposed revision poses a challenge by removing the only preferred long range transport (LRT) model in Appendix W without recommending a replacement. Projects with potential significant impacts beyond 50 kilometers will face additional technical and administrative challenges to develop a modeling plan or protocol that will need to be negotiated on a case-by-case basis with the permitting agency and EPA.

The proposed Appendix W contains a new chapter dealing with models for ozone and secondarily formed particulate matter. Under the proposed guideline, projects will need to model precursor emissions to determine secondary pollutant impacts to ozone and fine particulate matter concentrations. In the past, basic assumptions and calculations were sufficient to estimate secondary pollutant impacts. EPA is proposing a two-tiered approach to assess these impacts: a qualitative approach and an advanced modeling approach.

In Tier 1, a “technically credible” relationship between existing model data and the expected project emissions is evaluated to assess impacts on ozone and fine particulate matter. Tier 2 would require the application of a complex photochemical grid model. A project requiring a Tier 2 assessment of secondary pollutant impacts will require more time and effort to develop a modeling protocol, get concurrence and approval before running the analyses.

A public hearing on the proposed revisions is scheduled for August 12 – 13, 2015 at EPA’s offices in Research Triangle Park, NC. TRC staff will be among the many commenters who will provide constructive criticism and recommendations for EPA to consider in developing the final revisions. We have been involved in meetings, workshops, conferences and discussions leading to the proposed revisions and will continue to participate to help develop final revisions that are scientifically-based without being excessively burdensome.

EPA’s proposed revisions provide a new path forward for the application of regulatory models. It is likely that much of the proposed guidance and new requirements for compliance demonstrations and permit support modeling will find their way into the final rule revision expected in mid-2016. TRC’s broad modeling and permitting capabilities and more than 40 years of consulting experience can provide the support needed to develop an appropriate strategy for your project.


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