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Air Permitting- Air Technical Support

Air Technical Support
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Air Quality Modeling
Introduction and General Information
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 Increments (PSD). As part of the CAA, Congress also mandated consistency and encouraged the standardization of model applications by publishing the first Guideline on Air Quality Models (Guideline) in April 1978.  The Guideline provides consistency for estimating air quality impacts of criteria pollutants used in assessing control strategies and development of emission limits.


What is the purpose of Air Quality Modeling
The purpose of Air Quality Modeling is to assess the impact of air pollution from stationary sources.  Section 22a-174-3a of the Regulations of Connecticut State Agencies (RCSA) requires the owner of certain stationary sources of air pollution to apply for and obtain a permit prior to construction, modification and operation of the source. The regulation requires the owner of any source to demonstrate that the operation of a source will not cause or contribute significantly to a violation of any federal or state air quality standard.  The regulation also requires this demonstration to include estimates of air quality impacts approved by the DEEP Commissioner via the use of dispersion modeling.
The links below contain the latest DEEP and EPA guidance documents, and clarification policy memos for conducting modeling analyses in Connecticut.  Also note: EPA released the latest version of Appendix W: "Revisions to the Guideline on Air Quality Models" and most recent updates to the AERMOD modeling system and pre-processors, supporting technical documentation and latest guidance memos on SCRAM.  Please review and follow DEEP's and EPA's modeling guidance and documentation when conducting New Source Review Permit Modeling.  If you have any questions, please call the DEEP's modeling staff and we will be more than happy to answer questions, review and or resolve any potential modeling issues in the permitting process.


Appendix W Revisions to the Guideline on Air Quality Models: Enhancements to the AERMOD Dispersion Modeling System and Incorporation of Approaches to Address Ozone and Fine
Particulate Matter (EPA, May 22, 2017)


Model Clearinghouse Operational Plan (EPA, December 2, 2016)


Clarification on the Use of AERMOD Dispersion Modeling for Demonstrating Compliance with the NO2 National Ambient Air Quality Standard (EPA, September 30, 2014)


Guidance for PM2.5 Permit Modeling (EPA, May 20, 2014)



Additional Clarification Regarding Application of Appendix W Modeling Guidance for the 1-hour
(EPA, March 1, 2011)

Data Sources Required for Refined Modeling

The following data files are required for inputs into AERMOD for refined dispersion modeling analyses to determine compliance with the NAAQS.

Meteorological Data

The link above contain a short discussion of meteorological data required for modeling and the latest available five (5) years of data required for refined modeling.  These files are compressed and must be downloaded onto a computer and expanded into ASCII files that AERMOD can read.

Background Air Quality for Modeling

Background Air Quality data is required as part of NSR modeling analyses to demonstrate compliance with the latest NAAQS for SO2, NO2, PM2.5, PM10 and CO.  This link contains a discussion of the background air quality, and how the data is calculated and used in modeling; to determine the latest design values based on the form of the standards and averaging times for each pollutant.

Ozone Data used for the 1-hour NO2 NAAQS Compliance Demonstration
Hourly ozone data may be used for compliance demonstration with the 1-hour NO2 NAAQS.  Latest EPA modeling guidance concerning the 1-hour NO2 NAAQS, allows the use of hourly ozone data invoking the PVMRM or OLM methods. The hourly ozone files consist of the most recently available five (5) years of data that are matched concurrently with the most recently available five (5) years of meteorological data.  Each of these files are space delimited that contain year, month, day and hour followed by the hourly ozone concentration (ppb) from each monitoring site that are compressed and named according to site location.

2014-18 Hourly Background Ozone Data

DEEP Air Inventory Radius Search Tool

The latest version DEEP Air Inventory Radius Search Tool allows you to enter a geographic location (“centroid”) in UTM format, and a radius in kilometers, and retrieve a list of inventoried point sources within that radius to aid in the preparation of cumulative-source modeling analyses for an application.
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Content Last Updated February 27, 2020