Regulatory Update   |   Aug 31st, 2015 The Secondary Frontier: Proposed EPA Secondary Pollutant Modeling Guideline Creates a New Challenge


EPA’s proposed revision to the Guideline on Air Quality Models (Guideline) was published in the Federal Register on July 29, 2015. The Guideline specifies the methods and preferred models that should be used to predict ambient air concentrations needed for permitting and other regulatory applications. One of the significant changes in the proposed revisions responds to a petition submitted on behalf of the Sierra Club five years ago seeking the creation of rules to model ozone and fine particulate matter. At the time, EPA committed to developing explicit guidance for applicants to model so called “secondary” ozone and fine particulate matter impacts. With the newly proposed revision, EPA is looking to follow-through with their commitment.

A New Challenge

Pollutants can either be directly emitted into the atmosphere (“primary” pollutants like Sulfur Dioxide), or they can form in the atmosphere itself through chemical reactions (“secondary” pollutants like Ozone). The formation of secondary pollutants, and specifically, the attribution of the secondary pollutants to a specific project for permitting purposes is a complicated and highly nonlinear process.

In years past, EPA recognized that air quality modeling technology was not mature enough to determine a project’s secondary pollutant impacts. However, with the proposed rule, EPA now has concluded that modeling technology has advanced to the point where guidance can be developed. The current technology that EPA believes is most appropriate for the task is photochemical grid modeling.

Photochemical grid modeling is quite complex, and requires significant computing resources, the development of expensive inputs (e.g. weather model data and regional emissions inventories) and a good understanding of community-developed analysis methods to estimate a project’s secondary pollutant impacts.

What Tier is your Facility in?

These technical and practical challenges led EPA to propose a tiered approach to determining secondary pollutant impacts from a project.

  • In the first tier, an applicant will compare their primary pollutant emissions to levels as-of-now undefined by EPA. If their primary emissions are below those levels, EPA will consider there to be no secondary pollutant issues, and no estimation of secondary pollutants would be needed.
  • In the next tier, the applicant will search peer-reviewed literature and modeling data sets to find a credible relationship between their primary emissions and potential secondary pollutant impacts to estimate their project’s secondary pollutant impacts.
  • In the final tier, modeling will need to be done, and complex photochemical grid modeling is the only suggested avenue in the proposed guidelines. An applicant would work with the regulatory agency to develop a modeling protocol on a case-by-case basis.

The tiered approach would require significant analytical and modeling expertise and strong working relationships with the review and permitting agencies as each case will be treated on an individual basis. While the proposed rule would not be final until mid-2016, your plans for future major projects should develop a strategy for estimating secondary pollutant impacts that is appropriate for your project and schedule.

TRC Can Help

Contact us if you have questions or need assistance in estimating your pollutant impacts. We have the expertise, understanding, and communication skills to solve the challenge of estimating secondary pollutant impacts in the new guideline frontier.

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Air, Permitting and Licensing

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