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Cycling of PCBs and HCB in the Surface Ocean-Lower Atmosphere of the Open Pacific

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journal contribution
posted on 15.05.2010, 00:00 by Lin Zhang, Rainer Lohmann
Surface ocean and lower atmosphere samples were collected on the R/V Revelle during a scientific cruise from San Diego, CA to New Zealand via Samoa and the South Pacific Gyre (SPG) from 12/2006 to 1/2007. Samples were analyzed for polychlorinated biphenyls (PCBs) and hexachlorobenzene (HCB). ∑ICESPCBs gaseous concentrations (ICES: International Council for the Exploration of the Sea) ranged from 28−103 pg m−3 in the northern hemisphere (NH) and 1.5−36 pg m−3 in the southern hemisphere (SH), whereas dissolved seawater concentrations were between 0.2−15 pg L−1 in the NH and 0.3−7.8 pg L−1 in the SH. Both gas ([PCBs]gas) and dissolved phase concentrations ([PCBs]sw_dis) displayed highest concentrations near North America and lowest in the SPG. In the NH, [HCB]gas ranged from 42−89 pg m−3, higher than the average in the SH (31 pg m−3), while [HCB]sw_dis were comparable in both hemispheres (NH: 0.4−1.6 pg L−1, SH: 0.4−0.8 pg L−1). Fugacity ratio calculations suggest PCBs were volatilizing from surface waters to the overlying atmosphere, and air−water exchange fluxes were ∼0.5 to ∼30.4 ng m−2 d−1. This is the first study reporting the degassing of PCBs from the open ocean into the air. Previous studies deduced net deposition of PCBs into the Atlantic and Arctic Oceans. As has been observed for other oceans, HCB was at/near air−water equilibrium. A mass balance model was used to interpret the short-term variations in [PCBs]gas in the SPG, which was not observed for HCB. It is suggested that hydroxyl radical depletion reaction and air−water gas exchange together controlled the variation in [PCBs]gas.