Global sinks of PCBs: A critical assessment of the vapor-phase hydroxy radical sink emphasizing field diagnostics and model assumptions

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Abstract

[1] Polychlorinated biphenyls (PCBs) are ubiquitous contaminants in the global biogeosphere. Based on extrapolation of reaction rates determined at temperatures in laboratory experiments that were 60–100 degrees above global average tropospheric temperatures, it has been suggested that the gas-phase reaction with the hydroxy radical (HO·) in the troposphere constitutes the strongly dominating sink for PCBs. An assessment of a broad set of field observations relevant to the actual environmental behavior of individual PCB congeners suggests that such an assertion should be treated with some caution. First, if the proposed reaction rates were applied to published tropospheric contents of individual PCB congeners, a continuous order-of-magnitude depletion of the lighter congeners relative to more chlorinated ones would be predicted. However, a large shift in PCB fingerprint is inconsistent with congener-specific sediment archives, where a disproportionate historical depletion of lighter PCBs cannot be found. Alternatively, such HO·-reaction rates would require a continuous source flux of PCBs to today's environment grossly enriched (again by several orders-of-magnitude) in lighter PCB congeners to counterbalance the implied selective congener losses; a scenario that is not supported by available information on emission rates. Further, the existence of a latitudinal trend in tropospheric concentrations, where the highest concentrations are found at lower latitudes, suggests that the fate of PCBs is controlled by temperature-driven partitioning processes rather than a HO·-reaction sink, which would be expected to be more efficient at lower latitudes. Finally, the observed tropospheric variability of individual PCB concentrations conform poorly with both the absolute values and congeneric trends in predicted reaction lifetimes. A reconciliation of these apparent discrepancies is attempted.

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