Polychlorinated biphenyls in Chicago precipitation: Enhanced wet deposition to near-shore Lake Michigan

To determine if elevated levels of atmospheric contaminants in urban areas enhance atmospheric deposition to adjacent surface waters, precipitation was sampled at three stations along a transect from Chicago, IL, across southern Lake Michigan. Rainwater was collected during several storms in July 1994 and January 1995 in southern Chicago aboard the R/V Lake Guardian positioned 15 km east of downtown Chicago and at a rural site along the southeastern shore of the lake as part of the study Atmospheric Exchange Over Cakes and Oceans (AEOLOS). Total PCB concentrations in Chicago precipitation ranged from 4.1 ng/L (January 19, 1995) to 189 ng/L (July 21, 1994) and were 2-3 orders of magnitude higher than the measured regional background concentrations. Concentrations of PCBs in urban precipitation were dominated by particle-bound congeners, implying PCB enrichment in rainwater due to efficient scavenging of highly contaminated particulate matter from the urban atmosphere. PCB levels in precipitation falling over southern Lake Michigan were from two to as much as 400 times greater than the measured regional background concentration, indicating that the ''urban plume'' of Chicago increases atmospheric deposition of contaminants to Lake Michigan over spatial scales of tens of kilometers. Enriched urban precipitation extending out over Lake Michigan provides 50-400% greater PCB wet deposition loadings than background precipitation.