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Air modeling refers to the use of complicated computer programs that use site-specific information like wind speed and direction, emission rates, smoke stack height, smoke stack exit speed, etc. to estimate the concentration of pollutants at various locations.
Modeling has many advantages over monitoring. Its biggest is savings of time and money. While complicated models can take days to run and often require powerful computers with large amounts of memory, these costs pale in comparison to the costs of monitoring. Modeling allows you to estimate the concentration of pollutants at thousands of points, over multiple years, in a matter of hours or days. Monitoring requires expensive equipment and analysis and only gives information about the points where monitors were placed and for the times that a sample was collected.
U.S. EPA’s Support Center for Regulatory Atmospheric Modeling (SCRAM) has many resources related to all aspects of air modeling. IDEM’s Office of Air Quality (OAQ) uses AERMOD to model major sources of air toxics for permit information.
IDEM’s Office of Air Quality (OAQ) has used a separate modeling program developed by U.S. EPA Region 6, the Regional Air Impact Modeling Initiative (RAIMI), for larger areas. In U.S. EPA’s words:
The U.S. Environmental Protection Agency (U.S. EPA), Region 6, established the Regional Air Impact Modeling Initiative (RAIMI) to evaluate the potential for health impacts as a result of exposure to multiple contaminants from multiple sources, at a community level of resolution.
Often when regulatory agencies perform air modeling, they are attempting to ascertain the impact that a specific facility is having or will have on an area. As such, most modeling programs are geared towards this type of analysis. RAIMI allows the user to input information about multiple sources and get an idea of how all those sources combined will affect the surrounding community.
The National Air Toxics Assessment (NATA) is a national-scale modeling program that began in 1996 and is performed by U.S. EPA every three years. The most recent published results are for 2005. The 2005 NATA looked at 180 pollutants. It used available emissions inventory data and modeled concentrations at the census block and county levels.
NATA’s findings are reported as raw concentrations, human exposure concentrations, cancer risk (when applicable), neurological hazard (when applicable), and respiratory hazard (when applicable). It is important to remember when looking at NATA data that this is a very “broad brush” look at air toxics and is only meant to help indicate where further analysis is necessary rather than to indicate that there is an actual problem. For example, the 1996 and 1999 NATAs indicated that the southwest quadrant of Indianapolis had potential air toxics issues. IDEM conducted a more refined analysis, in the form of the Southwest Indianapolis Air Toxics Study, and found, for the most part, concentrations to be much lower than those predicted in NATA.