Regulatory Update   |   Aug 7th, 2015 Is Your Project “In the Blue?" EPA Proposes New Class I Modeling Guidelines

C1map 2015-07-14 300-only question

In July 2015, the EPA released a proposed revision to their guideline for performing air quality modeling. It includes several changes that will significantly affect the budget and scope of air permitting projects if codified as proposed. Among those is a big change to the methodology of modeling air quality impacts at all Mandatory Federal Class I areas around the country.

A Mandatory Federal Class I area is any of 156 protected areas established by the Clean Air Act and managed by the U.S. Fish and Wildlife Service, National Park Service, or Forest Service.

In order to protect these areas from significant air quality reduction, if a project is within 300 kilometers of a Class I area, air quality modeling analysis is necessary. The EPA is now proposing modifying the screening approach to determine if a significant impact from a project will occur at any Class I area between 50 and 300 kilometer buffer from a proposed project (“in the blue”).

The new screening approach calls for an arc of model receptor points to be defined 50-km away from a project, between the project and all Class I areas within 300-km of the project. A dispersion model is then run to calculate pollutant concentration levels at those 50-km receptors. If this first-level approach indicates concentration levels below the EPA-defined Significant Impact Levels for Class I areas for each pollutant, the project is considered to have an insignificant impact on the air quality of the associated Class I area for that pollutant.

However, if the proposed screening approach shows values above the Class I Significant Impact Levels, even at only a 50-km receptor arc, the project will be required to perform more refined modeling analyses to determine the impact of the project on the air quality of the Class I area at receptors within the Class I area of concern.

That refined modeling was previously done using an EPA-approved and recommended long range transport model with specific settings and configurations explicitly given by EPA guidance. The process was relatively straight-forward and defined. Now, with the newly proposed guidelines, EPA is explicitly not listing a preferred model for far-field Class I impacts and is leaving the modeling methodology up to the applicant and permitting agency to determine on a case-by-case basis what the best model and model parameters should be. The proposed guideline states quite clearly that:

“For assessment of Class I ambient impacts, under the proposed Guideline, there will not be a preferred model for distances beyond 50 km.”


“Applicants should establish [Class I area modeling] approaches (models and modeling parameters) on a case-by-case basis in consultation with the appropriate reviewing authority, Regional Office, and the affected Federal Land Managers.”

While on the surface, this proposed approach may appear to increase the potential scientific representation of a project’s far-field impacts, in practice it increases the uncertainty, technical challenges, and agency interaction of each affected project.

The proposed approach will require significant modeling expertise, a keen understanding of the nuances of Class I area modeling, and strong agency communication skills. With TRC as a partner, your project will have that expertise, understanding, and communication skills to solve the technical challenge of having a project “in the blue.”

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