Department of
Environmental Quality

Air quality dispersion modeling is used to estimate concentrations of pollutants that new (or existing) emissions sources may emit. Examples of emissions sources include stack emissions from industry and areawide emissions from groups of sources such as automobiles or wood stoves. Computer programs are used to make these estimates.

Air modeling is required by the state of Idaho to demonstrate compliance with the National Ambient Air Quality Standards (NAAQS), toxic air pollutant (TAP) standards, and Prevention of Significant Deterioration (PSD) increments.

Modeling and Permitting

Air modeling is required for certain permitting actions to demonstrate that all applicable ambient air quality standards will be met if a proposed construction or modification to a facility is completed or that an existing facility is complying with the standards.

Permitting actions that generally require air modeling include:

• Permits to construct
• Permit to construct exemptions
• Tier II operating permits

The air modeling must demonstrate compliance with all appropriate rules and regulations. If the modeling shows that construction or modification would cause or significantly contribute to a violation of an ambient air quality standard, result in an impact above applicable PSD increment limits, or have a significant impact in a Class I area (certain scenic areas in the country) or a nonattainment area with persistent air quality problems, DEQ cannot issue a permit for the proposed facility. To proceed, the facility's permit application needs to be revised and additional modeling performed.

Idaho's Modeling Guideline

To assist air permit applicants, air quality specialists, and others understand DEQ’s expectations for ambient air impact analysis, DEQ has developed the State of Idaho Air Quality Modeling Guideline. The guideline explains in detail when modeling is required; outlines applicable standards, methodology, and analysis; and provides checklists and templates for conducting modeling and reporting modeling results. Use DEQ's guideline in conjunction with the federal EPA's Permit Modeling Guidance and EPA's 1990 New Source Review Workshop Manual to help expedite DEQ’s review of your permit application.