Variability of the gaseous elemental mercury sea-air flux of the Baltic Sea.

The importance of the sea as a sink for atmospheric mercury has been established quantitatively through models based on wet and dry deposition data, but little is known about the release of mercury from sea areas. The concentration of elemental mercury (Hg0) in sea surface water and in the marine atmosphere of the Baltic Sea was measured at high spatial resolution in February, April, July, and November 2006. Wind-speed records and the gas-exchange transfer velocity were then used to calculate Hg0 sea-air fluxes on the basis of Hg0 sea-air concentration differences. Our results show that the spatial resolution of the surface water Hg0 data can be significantly improved by continuous measurements of Hg0 in air equilibrated with water instead of quantitative extraction of Hg0 from seawater samples. A spatial and highly seasonal variability of the Hg0 sea-air flux was thus determined. In winter, the flux was low and changed in direction. In summer, a strong emission flux of up to 150 ng m(-2) day(-1) in the central Baltic Sea was recorded. The total emission of Hg0 from the studied area (235000 km2) was 4300 +/- 1600 kg in 2006 and exceeded deposition estimates.