Abstract Description: Dispersion modeling of a new or modified source of pollutants subject to the National Ambient Air Quality Standards (NAAQS) has been an integral part of air permitting for over five decades. For a major source, one with potential emissions greater than a specific emission threshold, modeling has been required to determine air quality impacts to determine if such impacts are below certain NAAQS as well as Prevention of Significant Deterioration (PSD) incremental levels. Even for minor sources below such emission thresholds, some states have required modeling. In all cases the goal to ensure that these new emissions did not cause or contribute to an exceedance and possible violation of the NAAQS. Since the 1970’s through the end of the 21st century and through the last 22 years of the 22nd century, the magnitude and form of the NAAQS have changed as tighter standards and new forms of the standards have evolved. Meanwhile, dispersion models have changed as a function of faster computing speeds, new science, more robust meteorological data sets, and added features and options to the models. The Guideline on Air Quality Models, first in its infant EPA publication format (1978) and later in two Federal Register versions (2005 and 2017) has been consistent in proving guidelines for model applications especially with regard to air permitting. This paper will compare a relatively simple new major source facility subject to PSD with a few source types in a 1980’s modeling setting versus a 2020’s setting. Models, model options, meteorological data sets, and guidance of the times will be used to perform the modeling as well as applicable standards. This analysis will then compare the attributes and shortcomings of each model analysis and the effect it has on air quality impacts and emission limits. Qualitative discussion will be offered comparing these two eras of permit modeling.
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