Air Quality Modeling

Dispersion Modeling Basics

One of the many “regulatory activities” which is part of the permitting process involves the computer simulation of air pollutant emissions from a source to predict resultant air pollution concentrations in the area downwind from the source’s location.  This process is often referred to as dispersion modeling or permit modeling, and predicted concentrations are usually referred to as “impacts”.  Tennessee's Division of Air Pollution Control employs personnel who are trained to conduct dispersion modeling analysis to simulate the impacts of proposed sources on air quality in the state.  These personnel also review dispersion modeling analyses for larger sources, which are submitted by companies or their environmental consultants.After the permit staff determines that the source’s actual and potential emissions are within the calculated emission limit (a.k.a. allowable emissions) for the source, the Division’s modelers may be tasked with assessing the impact of the source’s emissions in the surrounding community. Owners of larger proposed sources referred to as “major” sources, are often required to provide dispersion modeling analysis which predict surrounding air pollution impacts when they submit construction permit applications to the Division.  Modeling analysis for smaller non-major sources is often provided by the Division’s modelers. 

Dispersion modelers use USEPA approved modeling techniques to predict “ambient impacts” (pollutant concentrations in the area surrounding the facility where the source is located).  After ambient impacts are predicted, the Division’s modelers assess whether the impacts may cause or contribute significantly to any expected exceedance of an air quality standard for criteria pollutants or acceptable ambient level for HAPs. Ambient air quality standards and acceptable ambient levels are established pollutant concentration thresholds, which should not be exceeded by predicted or measured ambient concentration levels to avoid adverse health effects to the citizens of the community, or avoid adverse environmental effects to the health of the community’s natural resources.  National Ambient Air Quality Standards (NAAQS) are established by USEPA for the criteria pollutants and recognized by state and local air quality agencies as the maximum air pollutant concentrations which can be measured or predicted in a community before the community is designated a Non-Attainment Area (NAA).  NAAQS are listed by USEPA at the following web-site: . Tennessee's ambient air quality standards can be found here:

Assessed pollutant concentrations for a community, are the sum of predicted pollutant impacts from a proposed source, and actual measured “background” pollutant concentrations in the community, when background values are available from local ambient monitors.  This sum (measured concentrations plus proposed predicted impacts) for a pollutant must remain below the corresponding NAAQS or AAL for that pollutant.  If this sum exceeds one of the NAAQS levels or an AAL, then the predicted impacts may cause non-attainment for the pollutant, or may cause exceedances of an AAL if the source is constructed as proposed. Similarly, the subtracted difference between the NAAQS (or an AAL) and a measured “background” pollutant concentration for an area is sometimes referred to as the “remaining air quality” (RAQ) for the area.  Hence, all predicted air pollutant impacts in the ambient area surrounding a proposed source should remain at or below the assessed RAQ for the community, before the state will issue an air pollution control permit for the source. 

Dispersion Modeling Process

During the permitting process, the permit writer and manager decide whether or not the source should be modeled to determine whether a NAAQS or AAL may be threatened by emissions from the new source.  For larger sources proposed in an attainment area covered by the USEPA’s Prevention of Significant Deterioration program (i.e. PSD sources), or for large sources proposed in a non-attainment area (NAA) covered by the USEPA’s Non-attainment New Source Review program (i.e. NNSR sources), the source owner must often provide a dispersion modeling analysis with the permit application.  In such cases, a permitting meeting is normally scheduled to discuss any modeling required of the owner. 

If a source is comparatively small and considered to be a “true minor” or “conditional major” source, the Division may opt to have the Division’s modeling staff provide modeling to determine whether the proposed new or modified source may threaten air quality in the surrounding area.  If the state provides modeling support, the modeling staff often contacts the owner for additional information required for modeling the source.  If the state’s modeling analysis of the source indicates that the source may threaten a NAAQS or AAL, the Division’s modeling staff will contact the owner for additional information required to refine the modeling analysis. 

If a modeling analysis provided by the state (or by a consultant) for a proposed source indicates that the source will likely threaten a NAAQS or AAL, then additional, more refined modeling may be provided by the company.  If additional, refined modeling demonstrates that the source no longer poses a likely threat to the associated NAAQS or AAL, and if a review by the Division’s modeling staff also shows that the modeling conforms to established state modeling guidance, then a construction permit may be issued for the proposed source.