Parameterization and micrometeorological measurement of air-sea gas transfer

Because of the combination of small concentrations and/or small fluxes, the determination of air-sea gas fluxes presents unusual measurement difficulties. Direct measurements (i.e., eddy correlation) of the fluxes are rarely attempted. In the last decade, there has been an intense scientific effort to improve measurement techniques and to place bulk parameterizations of gas transfer on firmer theoretical grounds. Oceanic tracer experiments, near-surface mean concentration profiles, eddy accumulation, and direct eddy covariance methods have all been used. Theoretical efforts have focused primarily in the realm of characterizing the transfer properties of the oceanic molecular sublayer. Recent major field efforts organized by the U.S.A. (GASEX-98) and the European Union (ASGAMAGE) have yielded atmospheric-derived results much closer to those from oceanographic methods. In this paper, we review the physical basis of a bulk-to-bulk gas transfer parameterization that is generalized for solubility and Schmidt number. We also discuss various aspects of recent sensor and technique developments used for direct measurements and demonstrate experimental progress with results from ASGAMAGE and GASEX-98. It is clear that sensor noise, sensitivity, and cross talk with other species and even ship motion corrections still need improvement for accurate measurements of trace gas exchange over the ocean. Significant work remains to resolve issues associated with the effects of waves, bubbles, and surface films.