The use of receptor models to locate atmospheric pollutant sources: Polychlorinated biphenyls in Chicago

Airborne PCB concentrations are significantly higher in Chicago than in rural areas surrounding the Great Lakes, even though there are few known sources in this region. To locate probable emission sources hybrid receptor models that combine estimates of the motion of the air backward in time from the sampling location with the measured chemical compositions were used. In this study three models, Potential Source Contribution Function Analysis (PSCF), Residence Time Weighted Concentration (RTWC) and Concentration Weighted Trajectory (CWT), were used to locate PCB sources in Chicago. The NOAA Hysplit trajectory model was used to calculate air parcel back-trajectories. Input data were PCB concentrations measured during the Lake Michigan Urban Air Toxics Study (LMUATS), the Atmospheric Exchange Over Lakes and Oceans Study (AEOLOS), part of the Integrated Atmospheric Deposition Network (IADN) and corresponding wind trajectories. Additional samples were obtained during this study in a location west of Chicago to help pinpoint source areas. Results of these models agreed well with each other. These models resolved three PCB source directions. They were (1) a northwest direction pointing toward Madison, WI, (2) a southwest trend toward Joliet, IL and (3) the south side of Chicago in the neighborhood of Lake Calumet.; The modeled results were verified with upwind/downwind field samples at several locations. PCB sources were found to include the sludge drying beds at wastewater treatment facilities, landfills and a large transformer storage yard. It was estimated that approximately 2 to 70 kg of PCBs are entering the air within Chicago each day based on a simple box model and the measured PCB concentrations in the summer. The contributions from these sources to the air PCB concentrations in Chicago ranged from 0.02 kg/day to 0.15 kg/day based on a K-theory dispersion model and an EPA virtual point source dispersion model, respectively. Preliminary results indicate that the Metropolitan Water Reclamation District of Greater Chicago sludge PCB emission is two thirds of the EPA 1990 National Emission Inventory and six times the Region 5 emission inventory.