Seasonal and spatial variations of chloroform, trichloroethylene, tetrachloroethylene, chlorodibromomethane and bromoform in the Northern Yellow Sea and Bohai Sea

Environmental context Volatile halocarbons play significant roles as halogen carriers in atmospheric chemistry where they contribute to global warming. We studied temporal and spatial variations of volatile halocarbons in the Northern Yellow Sea and Bohai Sea, and showed that their concentrations were governed by terrestrial input, hydrographic features and biological activity. The emission of volatile halocarbons from coastal regions could have important impacts on the budgets of atmospheric reactive halogen. Abstract Concentrations of five volatile halocarbons (VHCs), namely, chloroform (CHCl3), trichloroethylene (C2HCl3), tetrachloroethylene (C2Cl4), chlorodibromomethane (CHBr2Cl) and bromoform (CHBr3), in the Northern Yellow Sea and Bohai Sea were determined during the spring of 2013 and autumn of 2012. Strong seasonality in the concentrations of VHCs (except for CHCl3) were observed. Concentrations of CHCl3 were markedly higher (1.5 fold) to coincide with the higher concentration of chlorophyll a during the spring. The elevated concentrations of C2HCl3, C2Cl4, CHBr2Cl and CHBr3 were found to match with the higher inputs of land runoff during autumn. The VHCs distributions evidently decreased along the freshwater plume from the mouth of rivers, such as Yellow and Yalu Rivers, to the open sea. The elevated concentrations of the selected halocarbons arise from terrestrial input, complicated hydrographic features, such as the Yellow River effluent, Yalu River, Yellow Sea Coastal Current and Yellow Sea Cold Water, and biological release by phytoplankton. Correlation analyses were conducted among Chl-a, salinity and the concentrations of these gases to investigate possible controls for the concentrations of these gases. Significant correlation was only observed between the concentrations of CHBr2Cl and Chl-a in the surface layer during spring. We made the tentative conclusion that phytoplankton biomass might not be the main limiting factor of sources of VHCs in the surface water. The sea-to-air fluxes indicated that the Northern Yellow Sea and Bohai Sea act as sources of gas in the atmosphere during spring and autumn.