Air Quality Dispersion Modeling

Requirement for Dispersion Modeling

The Clean Air Act (CAA) mandated that air quality modeling be used as a tool to demonstrate compliance with attainment and maintenance of the National Ambient Air Quality Standards (NAAQS) and Prevention of Significant Deterioration (PSD) Increments. As part of the CAA, the U.S. Congress mandated consistency and encouraged the standardization of model applications, thus the Environmental Protection Agency (EPA) published the first Guideline on Air Quality Models in April 1978 and most recently as Appendix Win May 2017.

In Connecticut, Section 22a-174-3a of the Regulations of Connecticut State Agencies (RCSA) requires the owner or operator of certain stationary sources of air pollution to apply for and obtain a permit prior to construction and operation or modification of a stationary source. Under subsection (i)(2), "the commissioner may request any owner or operator to submit an ambient air quality impact analysis using applicable air quality models and modeling protocols approved by the commissioner." Air quality dispersion modeling is the commissioner's accepted method for estimating air quality impacts.

Conducting Ambient Air Quality Impact Analyses

CT DEEP has written an Ambient Impact Analysis Guideline (AIAG)to detail the conditions in which the commissioner will request an ambient impact analysis and offers guidance on how to perform these analyses. The current air quality dispersion modeling software preferred by both the EPA and CT DEEP for New Source Review (NSR) and PSD programs is the AERMOD Modeling System. For more information on AERMOD and dispersion modeling, please refer to the EPA's Support Center for Regulatory Atmospheric Modeling (SCRAM) webpage on Air Quality Dispersion Modeling.

Meteorological Data

CT DEEP provides processed AERMET files that can be directly used modeling with AERMOD. The following map is a visual interface to download the latest meteorological files. Previous years of meteorological data can be downloaded here.

It is strongly encouraged to consult with the AQME team on which meteorological data sets is most appropriate for your permit modeling needs.

Note: Meteorological data sets from AWOS sites (OXC & SNC), smaller municipal airports (HFD & MMK), and New York (HPN) are provided for informational purposes and should not be used for modeling unless approved of by CT DEEP.

Multi-Source Modeling

In some cases, other nearby sources must be included in refined modeling analyses. The latest version of the Air Inventory Radius Search Tool allows a user to enter a geographic location (“centroid”) and a radius to retrieve a list of inventoried point sources within that radius.

Content Last Updated: March 29, 2022