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The principal volatile sulphur species found in seawater are dimethyl sulphide (DMS), carbonyl sulphide (COS) and carbon disulphide (CS2. Of these, DMS is the most abundant and widespread in its distribution. The predominant oceanic source of DMS is dimethylsulphonioproprionate (DMSP), a compatible solute synthesized by phytoplankton for osmoregulation and/or cryoprotection. Not all species have the same ability to form DMSP; for example, diatoms generally produce little, whereas prymnesiophytes and some dinoflagellates make significantly larger amounts. Much of the release of DMSP and DMS to the water occurs on death or through predation of the plankton. Our recent field data strongly suggest that oxidation of DMS to dimethyl sulphoxide (DMSO) is an important process in the water column, and it is clear that considerable internal cycling in the DMSP/DMS/DMSO system occurs in the euphotic zone. A fraction of the DMS crosses the sea surface and enters the atmosphere where it is oxidized by radicals such OH and NO3to form products such as methanesulphonate (MSA), DMSO and non-sea salt sulphate (NSSS) particles. These particles are the main source of cloud condensation nuclei (CCN) over oceanic areas remote from land.

Knowledge of the uptake of atmospheric carbon dioxide by the North Atlantic is important in understanding the global carbon budget. Obtaining an accurate value for the ocean sinks in the northern hemisphere temperate zone, in particular, would make it possible to be more quantitative about the enigmatic land sink in these latitudes. The global ocean sink is dominated by the North Atlantic, despite its small area in comparison with the North Pacific. Of the possible methods for calculating the uptake, potentially the most informative is that based on the gas exchange equation because information about the seasonal and spatial trends can be obtained. However, there still remain serious questions about which form of the gas exchange coefficient is appropriate to carbon dioxide. Here we summarize current knowledge of the gas exchange coefficient, including recent new evidence which supports lower gas exchange rates and which, therefore, generally supports the Liss-Merlivat prediction for less soluble gases. In view of the uncertainties still surrounding the direct calculation, we investigate methods for calculating the basin-wide uptake by exploiting what is known about the general circulation of the North Atlantic. The uptake can be estimated by dividing it into pre-industrial steady state and anthropogenic contributions. We make a new estimate of the pre-industrial flux based on the heat budget of the North Atlantic, and also consider two earlier calculations of the same quantity. Taking a best value, adding the better-known anthropogenic flux and making small corrections, we find a value of 0.7± 0.15 Gt C a-1for the uptake of the North Atlantic, north of 15° N, in the mid 1980s. Though larger than the gas exchange based estimates of Tanset al.(1990), this value is not enough to obviate the need which they deduced for a sizeable ‘land-based’ northern hemisphere sink for CO2.

The most widety 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 SF6 and 3He 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.