Trace gases and air-sea exchanges

The most widely used approach for calculating the flux of gases across the sea surface is from the product of the concentration difference across the interface and a kinetic parameter, often called the transfer velocity. During the NERC North Sea Community Research Project (CRP) a considerable effort was made to improve our knowledge of both of these terms. Concentration measurements were made on nine survey cruises (February to October 1989) for dimethyl sulphide (DMS) (and its precursor dimethylsulphonioproprionate DMSP, both dissolved and particulate), as well as for a variety of natural and man-made low molecular mass halocarbons. To better define the relationship between transfer velocity and wind speed a novel double tracer technique was used on two of the process cruises in the North Sea CRP. The tracers added to the water were SF$_{6}$ and $^{3}$He and from the measured change in their concentration ratio over time, four estimates of the transfer velocity were made, one at a rather high wind speed (ca. 17 m s$^{-1}$). The results are in general agreement with the relationship of Liss & Merlivat (1986) based on laboratory and lake studies and theoretical considerations, and constitute their first real test at sea. Combining the above results for the transfer velocity with the detailed concentration fields measured in the CRP has enabled us to calculate fluxes across the sea surface for the measured gases with a much finer time and space resolution than was possible hitherto. Some implications of the calculated fluxes for atmospheric chemistry in Europe are discussed.