Air Quality Dispersion Modeling
Air quality impact modeling is used to estimate concentrations of pollutants in ambient air that result from emissions that occur from new, modified, or existing sources. Computerized atmospheric dispersion models, with input data based on actual meteorological data, geographical data, and emissions estimates, are used to estimate potential air pollutant impacts.
Idaho requires air impact modeling 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 the following:
- Permits to construct
- Permit to construct exemptions
- Tier II operating permits
Air impact modeling shall be performed in accordance with all appropriate rules and regulations. If modeling results show that construction or modification of a facility 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, then DEQ cannot issue a permit for the proposed facility. The facility's permit application shall be revised and additional impact modeling must be performed that demonstrates compliance with applicable standards to DEQ’s satisfaction.
Idaho's Modeling Guideline
DEQ has developed the State of Idaho Guideline for Performing Air Quality Impact Analyses to assist air permit applicants, consultants, and others to understand DEQ’s expectations for ambient air impact analyses. The guideline provides the following: a detailed explanation of when modeling is required; an outline of applicable standards, methodologies, and data that should be used in impact analyses; and 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 the permit application. DEQ requests that the Air Impact Modeling Analysis Report Template Form be used to submit modeling results and a detailed description of the analyses to DEQ as part of the permit application process.