Semivolatile organic compounds in urban and over-water atmospheres

Concentrations of semi-volatile organic contaminants were measured both in air and precipitation in and downwind of Chicago, IL and Baltimore, MD as part of the A&barbelow;tmospheric E&barbelow;xchange O&barbelow;ver L&barbelow;akes and O&barbelow;ceans&barbelow; (AEOLOS) project. Precipitation events were collected simultaneously in the city and over the water to measure increased wet depositional fluxes of polychlorinated biphenyls to Lake Michigan during May and July 1994 and January 1995. Elevated atmospheric concentrations in Chicago, IL increase atmospheric loadings of PCBs to Lake Michigan by at least a factor of two over regional background levels. Precipitation loadings, bidirectional gas exchange and dry deposition combine to increase measured surface water concentrations of PCBs in Lake Michigan during periods of southwesterly winds which transport the urban air mass across the lake. PCB concentrations in surface waters were higher during winter than in spring or summer, but PAH concentrations did not vary significantly with season. However, when placed in historical context, Lake Michigan PCB concentrations have declined ten fold over fourteen years from 1980 to 1994.; Size segregated airborne particulate samples collected around and over southern Lake Michigan show geometric mean diameters of polycyclic aromatic hydrocarbons that are correlated with the compound's sub-cooled liquid vapor pressures. More volatile compounds were found on larger particles. The slope of the relationship between GMD and vapor pressure depends on the transit time from the shoreline, suggesting that higher wind speeds induce faster dry deposition of large particles. Measured gas/particle partitioning of these compounds is modeled according to a three dimensional multiple linear regression that includes the influences of vapor pressure, particle size and measured aerosol fractional organic carbon content. Each of these terms is significant in the full model but, addition of the latter two terms appears to be practically unimportant in describing partitioning of these compounds between the gas and particle bound phases. Finally, spatial trends in PAH and PCB concentrations in the Baltimore/northern Chesapeake Bay atmosphere during June 1996 are similar to those observed in Chicago. These observations further support the hypothesis that urban centers increase loadings to adjacent surface waters.