| Summary: | Active thermography has been used for almost 30 years to explore air-sea gas transfer both in laboratory and field experiments. In the early 2000s, some doubt arose whether it was possible to extrapolate heat transfer velocities to gas transfer velocities. Because of the large difference in the molecular diffusivity, different mechanisms may govern the transfer of heat and mass. However, in a recent experimental study at the large annular wind-wave tank in Heidelberg, the Aeolotron, Nagel et al. [2014] could show that heat transfer velocities can be scaled to gas transfer velocities with an accuracy of better than 10%, provided the Schmidt number exponent is known. These measurements were performed using the original active thermographic technique proposed by Jähne et al. [1989], by heating a rather large area at the water surface of up to one square meter. This is required to ensure that water parcels stay longer in the heated patch than the response time of heat transfer across the boundary layer. The setup used by Nagel et al. [2014] still had one disadvantage. The heated patch showed some inhomogeneity in cross-wind direction, caused by the method to expand the beam of the CO2 laser. By using a holographic beam expander, a much more homogeneous irradiation could be achieved. With this improvement it was possible to acquire significantly more accurate heat transfer measurements. In November 2014 this technique was used in air-sea gas exchange measurements using natural seawater from the North Atlantic with various degrees of contamination by natural surface films at the Heidelberg Aeolotron, within the BMBF project SOPRAN (Surface Ocean Processes in the Anthropocene). First results from this experiment will be shown.
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