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Modeling

IDEM’s Air Toxics Program utilizes different types of data to evaluate how air toxics may be affecting the quality of the ambient outdoor air. In addition to air quality data acquired from IDEM’s air toxics monitoring network, air risk staff also utilizes air quality data acquired from air models. An air model is a mathematical simulation of air quality that scientists produce using approved computer programs and known inputs including air quality monitoring data and emissions data from contributing sources.

Models also use inputs including but not limited to information about terrain, weather conditions such as wind speed and direction, the rate of emissions, and height of stacks at sources to produce estimates of pollutant concentrations at specific locations.

Models have certain advantages over monitoring and are widely used by agencies that are responsible for controlling air pollution. Although ambient air quality monitoring provides actual measurements of substances in the air, monitoring has certain limitations including 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. Modeling is an alternative that saves money and time because it does not require the installation of expensive equipment, and it enables the estimation of pollutants at thousands of points, over multiple years, in a matter of hours or days. The U.S. EPA Support Center for Regulatory Atmospheric Modeling (SCRAM) provides detailed information and many resources related to all aspects of air modeling. IDEM uses air modeling data for regulatory purposes, including the development of state implementation plans and strategies for the reduction of harmful air pollutants. AERMOD is a U.S. EPA-approved system that is widely used for air permitting.

U.S. EPA utilizes air toxics modeling to issue the National Air Toxics Assessment (NATA) every three years, using inputs from state agencies and local governments across the nation. 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. As such, NATA is a prioritization tool for U.S. EPA and states. IDEM’s Air Toxics Program utilizes the NATA data in its routine work to screen for potential air quality problems from air toxics in Indiana. Should NATA data indicate the potential for increased public health risks from air toxics in a particular neighborhood or community, IDEM may conduct an air risk assessment.

Air toxics studies conducted by IDEM include the Southwest Indianapolis Air Toxics Study, which was initiated in response to 1996 and 1999 NATA data indicating a potential concern in the southwest quadrant of Indianapolis. Following a detailed assessment of air monitoring data, modeling data, toxicity information and emissions sources, IDEM found, for the most part, concentrations in southwest quadrant of Indianapolis to be much lower than those predicted by the NATA data. IDEM conducted the Indiana Lakeshore Air Toxics Study in response to U.S. EPA’s 2002 NATA data indicating northwest Indiana as an area of potential concern. The study findings indicated that air toxics concentrations and risk within the lakeshore area are not as high as reported by previous national scale screening analyses. Air toxics concentrations are comparable to concentrations seen in other cities. There is some increased risk associated with emissions from some industrial permitted sources and IDEM is working with those sources to identify pollution prevention opportunities. The results also support the efforts being made to reduce emissions from mobile sources nationwide.

The Air Toxics Program provides complete details of its studies, including air risk assessments and periodic ToxWatch Data Analysis reports, online.