Air monitoring refers to measuring pollutants in the air we breathe, and it is a widely used method of obtaining quality assured air quality data. However, air monitoring is not the only way of obtaining air quality data. Air quality modeling is also a common and reliable data source to estimate air quality impacts from emission sources.
Air quality modeling refers to the use of mathematics and computer programs to estimate concentrations of pollutants in the air. Air quality modeling is a United States Environmental Protection Agency (U.S. EPA) approved method for evaluating air quality impacts from air emission sources such as factories and roads. Modelers utilize specialized computer programs to obtain air quality data for permitting, planning and air quality studies using inputs including emissions data, stack information, and meteorological data from nearby weather stations such as wind speed and direction. An “air dispersion model” simulates how air pollutants, emitted by a source, disperse and react in the atmosphere to affect ambient air quality. In addition to evaluating impacts from existing sources, air quality modeling is utilized to predict future impacts from proposed emission sources as well as anticipated growth in urban areas.
Air modeling may be preferred over air monitoring for several reasons. For example, air monitoring has certain limitations including the expense of installing and maintaining samplers and instruments. Collecting monitoring data for an analysis takes time, from months to years, and results are limited to the specific location of the monitor. An advantage of using an air dispersion model is that it can enable the evaluation of multiple sources and locations within short timeframes such as hours or days.
Modeling for Air Permits and Air Quality Standards
Air quality modeling data is utilized for air permitting and National Ambient Air Quality Standards (NAAQS) implementation. The Office of Air Quality (OAQ) requires air dispersion modeling to be conducted to demonstrate that a source will not cause or significantly contribute to the violation of the health-based NAAQS. Modeled air quality data must comply with the Prevention of Significant Deterioration (PSD) increments, which are allowable increases that prevent the air quality in clean (attainment) areas from deteriorating to the level set by the NAAQS and toxic thresholds in U.S. EPA regulations. IDEM follows all air quality modeling procedures set forth in the U.S. EPA Guideline on Air Quality Models for PSD, nonattainment New Source Review (NSR), and State Implementation Plan (SIP) revisions. Resources include:
- Meteorological Data
- Download meteorological data specific to regions of the state.
- Terrain Data [ZIP]
- Instructions to obtain AERMAP ready GeoTIFF terrain data for 1-degree sections of any state. You will find instructions on page 4 of the PDF.
- National Ambient Air Quality Standards (NAAQS) Inventory [XLS]
- Statewide inventory of NAAQS sources for refined modeling.
- PSD Inventory [XLS]
- Statewide inventory for refined modeling.
- Background Concentrations [XLSX]
- Background concentrations from monitors for the most recent three years of complete data.
- U.S. EPA Support Center for Regulatory Air Models (SCRAM)
- Contains guidance, user manuals and information for models.
- IDEM Air Quality Modeling Policies [PDF]
- Contains links for dispersion models and other related tools and information.
- Hazardous Air Pollutants (HAPS) Table [XLS]
- Contains thresholds for acute and chronic risk for HAPS.
- Lookup Screening Tables [XLSX]
- Tables can be used to screen NAAQS sources.