Long-term high-frequency measurements of dibromomethane in the atmosphere at algae-rich and algae-poor coastal sites

Dibromomethane (CH 2 Br 2 ), a natural stratospheric ozone depleting substance, is mostly emitted from the ocean, but the relative importance of coastal (or macroalgae) and open ocean emissions is unknown. We made long-term high-frequency measurements of CH 2 Br 2 concentrations at two remote coastal sites in Japan, on the subtropical Hateruma Island (poor in macroalgae) and at Cape Ochiishi (rich in macroalgae). CH 2 Br 2 concentrations at Hateruma showed prominent seasonal variation, being lower in summer (around 0.94 ppt) than in winter (around 1.23 ppt). In contrast, CH 2 Br 2 concentrations at Ochiishi were highly variable, often exceeding 2 ppt in the summer but with minimum baseline concentrations close to those from Hateruma; in the winter the concentrations were almost constant at about 1.3 ppt. Analysis of the data suggested that (1) emissions from macroalgae were not likely to extend offshore, but instead were localized near the shore, (2) strong macroalgal emissions of CH 2 Br 2 were almost limited to the summer, but it was not reflected in the seasonality of the baseline concentrations of CH 2 Br 2 in the atmosphere, and therefore (3) macroalgal or coastal emissions of CH 2 Br 2 in the temperate zone might have a rather limited contribution to the global CH 2 Br 2 sources. These findings are especially important for the understanding of the tropospheric and stratospheric bromine budget.