Air quality models use mathematical and numerical methods to simulate the physical and chemical processes that affect air pollutants as they disperse and react in the atmosphere. Based on inputs of meteorological, emissions, and topography data, these models characterize primary pollutants that are emitted directly into the atmosphere and provide an estimate of the downwind concentration of pollutants.
In relation to State Implementation Plans (SIPs), models help evaluate an area’s air quality and determine pollutants attributable to regulated sources. ADEQ’s SIPs Section uses air quality models to create plans that will bring violating areas back into attainment of the National Ambient Air Quality Standards (NAAQS).
Any air quality modeling conducted by the SIPs Section is in accordance with EPA’s guidance and requirements.
Modeling for Non-Attainment Areas
Oftentimes air quality modeling is included in a SIP for a non-attainment area, or an area that is violating the air quality standards. This is part of the SIP’s “attainment demonstration,” or a measured account of how the non-attainment area will attain the NAAQS by a certain date, as mandated by the Clean Air Act. The modeling in the attainment demonstration shows the maximum level of emissions a source can release in order to keep the area from violating the standards and confirms that the controls included in the plan will be sufficient to bring the area back into attainment by a date specified by EPA.
Modeling for Attainment Areas
Air quality modeling can also accompany a maintenance SIP, or a plan for an area that’s already attaining the standards but needs to show it will continue to meet the standards for at least 10 years, as mandated by the Clean Air Act. In this case, the modeling will account for the area’s current sources, emissions, and pollution controls to demonstrate that the area will continue to attain the standards for years to come.