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Air Dispersion Models: Finally a Little Certainty

The air quality modeling community breathed a collective sigh of relief when a critical U.S. Environmental Protection Agency’s (EPA) modeling guidance finally became effective this year.

BY: MINDA NELSON, P.E, BURNS & MCDONNELL

The air quality modeling community breathed a collective sigh of relief when a critical U.S. Environmental Protection Agency’s (EPA) modeling guidance finally became effective this year. The Guideline on Air Quality Models: Enhancements to the AERMOD Dispersion Modeling System and Incorporation of Approaches to Address Ozone and Fine Particulate Matter (Guideline) is akin to a user’s manual for determining how large power plants affect air quality. The significant revision to the Guideline, sometimes referred to as Appendix W, has been over a year and a half in the making. It has survived two extensions to the effective date because of a change in administration and numerous public comments. At times, the modeling community wondered if the updates to the Guideline would ever become a reality.

Why are the updates a big deal and what is the purpose of the Guideline? The Clean Air Act regulates emissions, but the Guideline details the modeling procedures and is therefore used by the EPA, state agencies, Native American tribes, and industry to prepare and review permits for new sources of air pollution. The Guideline was initially published in 1978, a year after the Clean Air Act was established, to provide direction on model applications and methodology. Since then, new sections and topics have been incorporated into the Guideline over the years, with the last update occurring back in November 2005.

Since science and technology are always evolving, it was time for a significant update to the Guidance to capture the advances in modeling technology and algorithms, to provide more flexibility for regulatory modeling and to incorporate new analytical techniques. Overall, the Guideline updates provide better guidance, increased clarity, and more certainty for the air quality modeling community and industry.

With better guidance for conducting National Ambient Air Quality Standards (NAAQS) and Prevention of Significant Deterioration (PSD) Increment modeling procedures, there is increased certainty for industry when applying for a permit. Procedures outlined in the Air Quality Analysis Chapter of the outdated 1990 Draft New Source Review Manual (NSR Manual) have led to overly conservative and unrealistic modeling practices. For example, when including neighboring sources in a modeling analysis, each of the neighboring source’s maximum potential to emit emissions are modeled along with the ambient background in the area and then compared to the NAAQS threshold, which leads to overly conservative modeled impacts. The Guideline outlines recommended modeling procedures for using actual emissions for neighboring sources. Modeling emission rates that are in-line with the actual operation for neighboring facilities results in modeled impacts that are realistic and not overly conservative.

The codification of the EPA’s Model Clearinghouse, developed in 1988, into the Guideline delivers increased clarity for the interpretation and approval of modeling guidance, as well as case-specific modeling issues. The Model Clearinghouse process helps resolve regulatory modeling issues and techniques, which streamlines procedures across EPA regional offices and simplifies approvals for the regulatory modeling community. The benefit to industry is that the Model Clearinghouse information is public and accessible in a searchable format to obtain the most up-to-date policy and procedures that can be applied to current projects.

To provide more certainty in modeling procedures, the Guideline stresses the appropriate development and vetting of modeling procedures for projects. The EPA has revised the Air Quality Analysis Checklist, which is essentially a 13-page document outlining important aspects of the air quality analysis – such as preconstruction modeling, which model is appropriate for the type of analysis being performed, and meteorological data. The checklist references EPA policies and guidance. When a project requires air dispersion modeling, it should be noted that a pre-application meeting with the state agency should be arranged and a modeling protocol should be prepared that outlines modeling procedures being used for the project. This streamlines the process so there are no surprises when the air permit application is submitted.

Improvements to the air quality models and methodology is a win for the industry since the air dispersion models will more accurately predict emissions from facilities.

In summary, you should be aware of how the Guideline updates will affect your facility or project and keep in mind that compliance is a moving target.

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