983205 - Increasing Role of Dispersion Modeling in Facility Design

The Clean Air Act implemented air quality modeling as an integral part of the air quality permitting process for new and modified facilities. Since then, dispersion modelers have been analyzing impacts on ambient air quality by solving dispersion equations, using lookup tables, evaluating single sources with computerized screening models, and modeling site-wide, regional, and global air quality impacts with refined dispersion tools that include Gaussian, Lagrangian and Eulerian models.

Originally, dispersion modeling requirements were relatively easy to meet, making them just one more part of an air permit application. However, the introduction of new and more stringent National Ambient Air Quality Standard (NAAQS) standards has made dispersion modeling a major bottleneck for many projects. That is the case because of the introduction of 1-hour NO2, 1-hour SO2 and 24-hour PM2.5 NAAQS which have become a major challenge for many applicants.

Historically, modelers received readily available emission parameters and emission rates to use in the dispersion modeling evaluations. The results from the modeling analysis were then presented to the permit applicant or permitting authorities. When the model results were not favorable, the permit applicant typically provided another set of input parameters to be used in a new modeling iteration. The process may include several modeling iterations before a favorable scenario that meets the applicable air quality standards can be found. Commonly it was not the modeler’s decision to make adjustments to the design of any stack parameters.

As NAAQS standards have become more stringent, more modeling iterations are needed to find favorable results. The trend is that when modeling the most stringent NAAQS (e.g., 1-hour NO2 and 24-hour PM2.5) the design team has started to rely more heavily on dispersion modeling in ascertaining the mitigation alternatives that can produce a favorable run. With this direction modelers create multiple scenarios, evaluate refinements, develop new techniques, and play a much larger role in the design process of new and modified facilities subject to modeling requirements.

This panel will discuss the ever evolving role of modelers including real-world examples where dispersion modeling heavily dictates the design of a proposed site. The opportunity, challenge, risk and liability that come along with the changing mindsets of modelers will also be discussed.