Arctic mercury depletion and its quantitative link with halogens

Gas phase elemental mercury (HgA degrees) was measured aboard the NASA DC-8 research aircraft during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign conducted in spring 2008 primarily over the North American Arctic. We examined the vertical distributions of HgA degrees and ozone (O-3) together with tetrachloroethylene (C2Cl4), ethyne (C2H2), and alkanes when HgA degrees- and O-3-depleted air masses were sampled near the surface (< 1 km). This study suggests that HgA degrees and O-3 depletions commonly occur over linear distances of similar to 20-200 km. Horizontally there was a sharp decreasing gradient of similar to 100 ppqv in HgA degrees over < 10 km in going from the bay near Ellesmere Island to the frozen open ocean. There was a distinct land-ocean difference in the vertical thickness of the HgA degrees-depleted layer - being variable but around a few 100 meters over the ocean whereas occurring only very near the surface over land. Data support that atmospheric mercury depletion events are driven by HgA degrees reactions with halogen atoms. Derived from data collected aboard the DC-8, we present mathematical expressions giving the rates of HgA degrees and O-3 depletions as a function of the radical concentrations. These relationships can be a useful metric to evaluate models that attempt to reproduce springtime Arctic HgA degrees and/or O-3 depletion events, and they can also be employed to obtain order-of-magnitude estimates of radical concentrations and the ratio [Br]/[Cl].