Influence of Baltimore's urban atmosphere on organic contaminants over the northern Chesapeake Bay

Air and precipitation samples were collected along an urban to over-water to rural transect across the northern Chesapeake Bay as a preliminary investigation into the spatial extent of elevated atmospheric concentrations of urban-derived persistent organic pollutants. Air samples were collected daily from June 3-9, 1996, along the transect as part of the Atmospheric Exchange over Lakes and Oceans project. Total (gas + particle bound) atmospheric polycyclic aromatic hydrocarbon concentrations [C-PAK] ranged from 0.4 to 114 ng/m(3), and gas phase polychlorinated biphenyl concentrations [Sigma-PCB] ranged from 0.02 to 3.4 ng/m(3). Strong concentration gradients were found for both PAHs and PCBs, with the highest concentrations in the city and the lowest at the downwind rural site. Gas and particle bound PAHs varied independently in the city possibly due to strong but geographically separated emission sources. A precipitation event collected during westerly winds contained fourfold higher Sigma-PAH and twelvefold higher Sigma-PCB concentrations at the over-water site than at the rural background location, further indicating that the urban plume extends from Baltimore, MD, over the northern Chesapeake Bay over a spatial scale of approximately 30 km.