Seasonal and Annual Air−Water Exchange of Polychlorinated Biphenyls across Baltimore Harbor and the Northern Chesapeake Bay

As part of the Atmospheric Exchange Over Lakes and Oceans Study (AEOLOS), air−water exchange fluxes of polychlorinated biphenyls (PCBs) were determined along a transect in the Baltimore Harbor from the Inner Harbor of Baltimore, MD, to the mainstem of the northern Chesapeake Bay to assess the overall contributions of urban source PCBs through air−water exchange and their impacts on coastal waters. Six sites were sampled during three intensive sampling periods in June 1996, February 1997, and July 1997 and at two sites (urban and rural) every ninth day between March 1997 and March 1998 to measure spatial and temporal variability in the PCB air−water exchange fluxes. During the intensive sampling campaigns, net total PCB (t-PCBs) volatilization fluxes ranged between 5 and 2120 ng m-2 day-1, varying on both temporal and spatial scales. Volatilization fluxes were highest in February, driven by high winds and elevated dissolved PCB concentrations. Throughout the annual sampling period, t-PCB volatilization fluxes were similar between urban (130 μg m-2 yr-1) and rural (120 μg m-2 yr-1) sites. Approximately 10% of the dissolved t-PCB inventory in the water column in Baltimore Harbor exchanges with the atmosphere each day. From air−water exchange alone, the estimated residence time of dissolved t-PCBs in the harbor is approximately 10 days (estimated hydraulic residence time of water in the harbor is between 7 and 14 days), indicating that air−water exchange is an efficient removal mechanism of t-PCBs from urban coastal waters. Furthermore, the annual flux of t-PCBs volatilizing from Baltimore Harbor is approximately 12% of the gas-phase t-PCB inventory over the water, and at the northern Chesapeake Bay site, the t-PCB flux is approximately 40% of the gas-phase t-PCB inventory. This suggests that air−water exchange of t-PCBs has the potential to be a significant source of PCBs to the rural atmosphere.