Milwaukee, WI, as a source of atmospheric PCBs to Lake Michigan

A field study of atmospheric PCBs in Milwaukee, WI, U.S.A. was conducted on the shore of Lake Michigan. We believe this is the first report of atmospheric PCBs in Milwaukee, although PCBs are well-known to contaminate the sediments of the Milwaukee River and Outer Harbor. Concentrations of PCBs collected during the June 2001 study are similar to concentrations in other urban-industrial areas and higher than PCBs in background air. The average PCB (sum of 88 congener groups) gas-phase concentration in Milwaukee was 1.9 ng m-3 standard deviation 0.78 ng m-3. The average and standard deviation for the particulate-associated PCBs are 0.05 0.02 ng m -3. Particulate-phase PCBs account for less than 5% of the total atmospheric concentration. PCBs in Milwaukee air are a source of PCBs to Lake Michigan. Calculated net gas exchange fluxes predicted for the Milwaukee sampling period ranged from -60 to -400 ng m-2 d-1, where net deposition is indicated by the negative sign. Calculated particle-associated PCB deposition ranged from 80 to 500 ng m-2 d-1. Most of the particle-phase deposition flux is a result of coarse particle deposition and decreases rapidly with distance from shore. Under typical meteorological conditions, particle-associated PCBs depositional flux to the lake surface decreases by 90% within 40 km. For net gas-exchange, the flux reaches zero at about the same distance. At greater distances, particle-phase PCB deposition is negligible, and PCBs are volatilizing at a higher rate than they are being deposited. We calculated that Milwaukee air contributes about 120 kg of PCBs to Lake Michigan each year. This is about 10 times larger than the discharge of PCBs from the Milwaukee River.