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Air Quality Modeling

What is Air Modeling?

Air modeling is a way to mathematically simulate atmospheric conditions and behavior. It is usually performed using computer programs.  Using inputs such as meteorology and source emissions, air models can calculate pollutant concentrations in the air or the amount of pollution deposited (deposition) on the ground from the air.  There are many different kinds of air models, and an appropriate air model is selected based on the type of analysis that is needed.

Why is Air Modeling Used?

Air modeling is used for two major reasons.  First, modeling can predict pollutant concentrations or deposition estimates at almost any location.  Air monitors can not be located at all locations due to maintenance and cost considerations.  Air models provide an efficient way to examine air quality over large areas.

Second, air models can predict the impacts of new sources before they are built and also allow an examination of the effects of different types of pollution controls before any actual changes are made to the sources of pollution.  In addition, air modeling is sometimes used to locate air quality monitors in areas where high pollutant concentrations are most likely to occur.

What are the Components of an Air Modeling Analysis?

For a brief summary of the following components, click here.

Source Characterization             Meteorological Conditions
Wind Speed and Direction
Atmospheric Stability
Mixing Height
Terrain Considerations
Receptor Definition 
Urban/Rural Classification
Plume Downwash
Averaging Time Considerations
Ambient Temperature, Relative Humidity, and Pressure

Air Modeling Activities in Region 3.

There are many air modeling activities in Region III.  Air quality modeling is most often used in State Implementation Plans (SIPs) to demonstrate that an area will attain or continue to attain the National Ambient Air Quality Standards for the criteria pollutants: sulfur dioxide, ozone, particulate matter, lead and carbon monoxide.  Air modeling is also used in the permit process related to New Source Review (NSR) and the Prevention of Significant Deterioration (PSD) program.  In addition, some air models are used to evaluate toxic air pollutants as part of the risk assessment process for hazardous waste sites.

More Information

The ultimate source of air modeling information is EPA's Support Center for Regulatory Air Models (SCRAM). This comprehensive site contains all of EPA's regulatory air models, model codes, some National Weather Service (NWS) meteorological data, user's guides, and other air modeling guidance.

The State of Oregon Exit EPA Click for disclaimer Air Quality Dispersion Modeling Page also contains air modeling information as well as useful utilities to facilitate operation of the air models.

Two guidance documents are available for the modeling of hazardous air pollutants:
A Tiered Modeling Approach (54k Zip file) for Assessing the Risks due to Sources of Hazardous Air Pollutants; and
Volume V (246k Zip file) - Procedures for Air Modeling at Superfund Sites.

The United States Geological Service (USGS Exit EPA Click for disclaimer) has terrain elevation data, and the National Climatic Data Center (NCDC Exit EPA Click for disclaimer) has meteorological data.

Visit EPA's Clearinghouse for Inventories and Emissions Factors (CHIEF) for a Compilation of Air Pollutant Emission Factors (AP-42), and other air emission inventories and emission estimation software.

EPA's new sophisticated multi pollutant model for secondary pollutants such as ozone and fine particulate matter is called Models-3.

For NOx SIP Call air modeling data, visit the NOx SIP website.

Detailed information on EPA's air modeling regulations can be found in Guideline on Air Quality Models - Appendix W (pdf file).

For guidance and a computer model to model indoor air releases due to underground contaminated soil and groundwater, check out the Johnson & Ettinger model Subsurface Vapor Intrusion into Buildings.

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