The important process of air-sea gas exchange still lacks sufficient understanding. Field measurements are available only in a rather narrow range of wind speeds from 4–20 m/s and still show conflicting results. No reliable estimates are possible for lower wind speeds, because all available techniques (dual-tracer, eddy covariance and active thermography) are not suitable to be used at such low wind speeds. In wind-wave tunnels the conditions deviate significantly from those at the open ocean. The common linear facilities have a short interaction length between wind and waves and thus generate only young wind seas far away from a wind sea in equilibrium with the wind (“fetch gap”). Even in an annular facility with an infinite fetch as the Heidelberg Aeolotron, the waves are different from the ocean: because of the limited water depth, waves cannot travel fast enough (“wave age gap”).
In this talk a new approach is introduced to overcome these limitations and to perform gas exchange measurements in the laboratory, which sufficiently realistic simulate all relevant oceanic conditions at low and medium wind speeds. New imaging techniques are applied to measure the gas transfer velocity locally and instantaneously in the Heidelberg Aeolotron under non-stationary conditions. In this way the whole fetch range can be covered including decaying wind seas, when the wind speed is lowered. The wave age gap can be overcome by using heavier gases in the atmosphere of the Aeolotron. The imaging techniques include active thermography to measure the heat transfer across the aqueous viscous boundary layer and a novel opto-chemical technique to image the mass boundary layer and to measure the gas transfer velocity. Also the influence of surfactants, which is very important for lower wind speeds, will be studied in detail.