The U.S. Environmental Protection Agency (EPA) recently released draft guidance, "Revised Policy on Exclusions from Ambient Air." In this draft revised guidance, EPA is modifying its 1980 policy on the exclusion of certain areas from the scope of "ambient air" under the Clean Air Act.

Sources that are required to do an air quality analysis under the Prevention of Significant Deterioration (PSD) program must conduct a source impact analysis. This analysis involves using air quality dispersion models to predict the impact of a proposed PSD source's emissions on pollutant concentrations in the ambient air. The model results are then used to predict whether the proposed PSD source will cause or significantly contribute to a violation of an ambient air standard, including the National Ambient Air Quality Standards (NAAQS). Previously, EPA defined ambient air as "that portion of the atmosphere, external to buildings, to which the general public has access" in 40 CFR§50.1(e). This definition dictates where the NAAQS apply and where PSD sources must demonstrate compliance with ambient air standards prior to construction.

In the draft revised guidance, EPA changed the way it applies the regulatory definition of ambient air. The change replaces the specific concept of a fence or other physical barrier with the more general concept of "measures."  Measures may include physical barriers that are effective in deterring or precluding access by the general public, however, the goal of this change is to take into account advances in technology and greater experience in a variety of ambient air scenarios since the 1980 policy was published, which can serve as effective deterrents to public access without a traditional physical barrier. Examples given in the guidance include: video surveillance and monitoring, clear signage, routine security patrols, drones, and other future technologies.

The draft revised ambient air policy states that it is appropriate to exclude "the atmosphere over land owned or controlled by the stationary source, where the owner or operator of the source employs measures, which may include physical barriers, that are effective in deterring or precluding access to the land by the general public."

Under the draft guidance, sources will have additional flexibility to determine what areas must be modeled for air quality analyses. However, since EPA';s proposed change is in policy rather than a change in regulation, sources should still consider how their local permitting authorities interpret the meaning of ambient air.

Other issues regarding the definition of ambient air, such as treatment of railways, roads, and bodies of water, were not addressed in the guidance and the previous guidance and definitions from EPA continue to be in effect.

EPA is accepting public comments on the draft guidance through December 21, 2018. To review the draft guidance, visit EPA's website and use this link to submit comments.

For questions regarding how the revised air dispersion modeling guidance may impact your permitting plans, contact Trinity at (800) 229-6655 for more information.