The Chicago area air toxics monitoring network deployed in 2000 was designed to provide data to meet the following objectives: 1. Determine ambient concentrations of specific compounds of concern; 2. Describe pollutant levels at various locations across the area, assess their geographic variability and perform a comparison of the levels of air toxics found at O'Hare Airport to other sites in the Chicago area; 3. Provide monitoring results consistent with nationally available air toxics information that would allow for a comparison of Chicago area results to data collected for other large U.S. cities; and 4. Determine if the target compound emissions from O'Hare Airport have a measurable impact on air quality in the surrounding communities. To meet the above objectives, the focus was on the compounds identified by USEPA as Urban Air Toxics and on other compounds known to be emitted by mobile sources (e.g., cars, trucks and aircraft). Particular attention was placed on those compounds associated with aircraft operations; e.g., takeoff, landing, refueling and idling.