Diffusive Exchange of Gaseous Polycyclic Aromatic Hydrocarbons and Polychlorinated Biphenyls Across the Air−Water Interface of the Chesapeake Bay

Abstract

Dissolved and gas-phase concentrations of nine polycyclic aromatic hydrocarbons and 46 polychlorinated biphenyl congeners were measured at eight sites on the Chesapeake Bay at four different times of the year to estimate net diffusive air−water gas exchange rates. Gaseous PAHs are absorbed into the bay's surface waters during the spring, and lighter compounds revolatilize in the late summer and early fall due to seasonal changes in surface water temperature and atmospheric PAH levels. On an annual basis, the atmosphere is a net source of volatile PAHs to the Chesapeake Bay, and gas absorption may be the largest external source of fluorene and phenanthrene, providing up to three times the combined loadings from wet and dry aerosol deposition and from tributaries. Largest PAH absorptive fluxes occurred in the northern Chesapeake when prevailing winds carried PAH-enriched air from the Baltimore-Washington urban area over the bay. In contrast to PAHs, PCBs volatilize from the Chesapeake Bay throughout the year, with the largest fluxes occurring in September and the smallest fluxes in June. However, higher chlorinated (6−8) homologues are absorbed by bay waters during most of the year. Highest PCB volatilization rates were observed in the northern Chesapeake Bay and near the James River in the southern bay, indicating volatilization offsets PCB loading from the bay's tributaries. Volatilization is the dominant removal process for PCBs from the Chesapeake Bay, removing an estimated 400 kg/year. This value is larger than current external PCB loadings, suggesting that release of PCBs from historically contaminated sediments supports volatilization from the bay.