Air modeling uses U.S. Environmental Protection Agency (U.S. EPA) approved computer programs that use site-specific information including the location and terrain, wind speed and direction, emission rates, and smoke stack information to estimate the concentration of pollutants at various locations.
IDEM’s Office of Air Quality (OAQ) uses AERMOD to model major sources of air toxics for permit information. The U.S. EPA Support Center for Regulatory Atmospheric Modeling (SCRAM) has many resources related to all aspects of air modeling.
Modeling has many advantages over monitoring. Its biggest is savings of time and money. 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.
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 2014. The 2014 NATA looked at 180 pollutants. It used available emissions inventory data and modeled concentrations at the census tract and county levels.
NATA’s findings are reported as raw concentrations, human exposure concentrations, cancer risk (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. NATA is a prioritization tool. 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. The 2002 NATA results indicated northwest Indiana as an area of potential concern. The Lakeshore Air Toxics Study was conducted based on this information.