The impact of urban areas on the deposition of air toxics to adjacent surface waters: A mass budget of PCBs in Lake Michigan in 1994

The impact of urban air toxics on the proximate water bodies was investigated as part of the AEOLOS Project (Atmospheric Exchange Over Lakes and Oceans). The hypothesis of this project was that emissions of hazardous air pollutants into the coastal urban atmosphere increased atmospheric depositional fluxes to proximate Great Waters. Areas of major field and modeling campaigns were located in southern Lake Michigan near Chicago, IL and northern Chesapeake Bay near Baltimore. The impact on Lake Michigan of PCB emissions from the urban area of Chicago was evaluated through field experiments to determine atmospheric concentrations, surface water concentrations, wet and dry deposition and bi-directional gas exchange. These values were used to construct a PCB mass budget for the Lake Michigan ecosystem, which includes the urban influence. PCB emissions in the Chicago atmosphere lead to dramatically increased atmospheric concentrations of PCBs off shore and significantly increased wet, dry particle and air-water exchange fluxes in southern Lake Michigan compared to the regional signal. High PCB concentrations occur over the lake only when the wind is from the direction of the urban/industrial complex inclusive of the shoreline from Gary, IN to Evanston, IL toward the lake. Although atmospheric loading of PCBs was much higher in the southern basin of Lake Michigan, water column concentrations have significantly decreased from 1980 to 1994 through the rapid uptake by settling particles, and have continued to decrease at a rate of 0.17 yr^-1. In contrast to earlier PCB mass budgets that missed significant inputs, outputs and/or process terms, inclusion of the urban-influenced air-water exchange processes and a re-evaluation of new lake data effectively closed the PCB budget for Lake Michigan. Atmospheric exchange processes dominated the whole lake budget. The Chicago area, with high industrial density, emits large quantities of hazardous organic pollutants and has important impacts on the down wind coastal atmosphere and proximate surface waters.