Some Recent Contributions from the Heidelberg Aeolotron to Understanding Air-Sea Gas Exchange

The lack of knowledge about the parameters controlling the transfer velocity of the exchange of gases and volatile species across the air-sea interface besides the wind speed – such as the sea state (wave age), bubbles, and surfactants - hinders progress of a better estimate of fluxes for all relevant chemical species.

In 2021, a laboratory program was started at the large air-sea interaction facility, the Heidelberg Aeolotron. With four innovative key elements, most disadvantages of previous wind-wave tunnel experiments could be overcome:

• Because of the infinite fetch of the annular facility, wind waves come into equilibrium with the wind that is more similar to the ocean compared to the linear facility.

• The clean environment (walls coated with Teflon foil) facilitates experiments with surface films.

• Two imaging techniques are used to measure transfer velocities locally and instantaneously. In this way, it is also possible to get direct insight into the mechanisms.

• The whole fetch range and non-stationary conditions could be investigated.

An extensive measuring program finished in September 2024. In this talk, the focus is on some of the first results which are regarded to be the most important contributions concerning the conditions in the field:

• The dependence of the transfer velocity on the fetch (wave age) seems to be only significant at lower wind speeds with an overshoot at young wind fields by more than a factor of two. This is an important contribution to the large variability of the gas transfer velocity at low wind speeds.

• Once surface active materials, either soluble or insoluble suppress waves, gas transfer velocities are reduced to the same velocities and are governed by the same mechanisms. The measurements included insoluble films of hexadecanol and olive oil and the soluble surfactants TritonX-100 and Tergitol 15-S-12. At wind speeds larger than 8 m/s, wind waves cannot be suppressed by any films.

• From a statistical analysis of the spatial-temporal patterns gained by both imaging techniques, it is possible to infer the Schmidt number exponent. This means that no longer multi-tracer experiments are required using tracers with a large difference in the diffusion coefficients.

• At high wind speeds, breaking of the dominant waves does not play a dominant role. It is a rather fast surface renewal taking place all over the surface at scales of a few centimeters, which is associated with the smaller-scale wind wave field riding on the dominant wave.

• Simplified forms of the two imaging techniques used in the Aeolotron seem to be suitable also for field measurements. A first experiment is planned for the BASS Baltic Sea cruise in June 2025.

• It was possible to compare gas transfer measured with flux chambers in the Aeolotron with those gained at the free water surface using imaging thermography. The results clearly show that no useful measurements can be performed by flux chambers as soon as wind-induced effects are dominant, which is already the case at wind speeds as low as 2 m/s.