Springtime depletion of tropospheric ozone, gaseous elemental mercury and non-methane hydrocarbons in the European Arctic, and its relation to atmospheric transport

Using a trajectory climatology for the period 1992–2001 we have examined how seasonal changes in transport cause changes in the concentrations of tropospheric ozone (O3), gaseous elemental mercury (GEM) and non-methane hydrocarbons (NMHCs) observed at the Mt. Zeppelin station, Ny-Ålesund (78.9°N, 11.9°E). During April–June O3 depletion events were frequently observed in connection with air transport across the Arctic Basin. The O3 loss was most pronounced in air masses advected close to the surface. This result supports the idea that the O3 depletion reactions take place in the lowermost part of the atmosphere in the central Arctic Basin. A strong positive correlation between springtime O3 depletion events and the oxidation of GEM to divalent mercury was found. During air mass advection from Siberia, the Barents Sea and the Norwegian Sea the strongest correlation was observed during April–May, whereas air masses originating from the Canadian Arctic and the central Arctic areas showed the highest O3–GEM correlation in May–June. We suggest that this 1-month lag could either be due to the position of the marginal ice zone or temperature differences between the northwestern and northeastern air masses. In connection with springtime O3 depletion events low concentrations of some NMHCs, especially ethane and ethyne, were observed, indicating that both bromine (ethyne oxidant) and chlorine radicals (ethane oxidant) are present in the Arctic atmosphere during spring. In winter, negative correlations between O3 and NMHCs were found in connection with air transport from Europe and Siberia, which we interpret as O3 destruction taking place in industrially contaminated plumes.