Special Colloquium: Bridging the gap between lab and field: What we learned from lab measurements about air-sea gas exchange at sea and the influence of convection, wind, waves, and surfactants

durch Prof. Bernd Jähne (Heidelberg University)

Mittwoch 26.11.2025, 14:00 → 15:00 Europe/Berlin

8A-002 - Hörsaal Ostufer / Lecture Hall East (GEOMAR - Standort Ostufer / GEOMAR - East Shore)

Abstract:

The parameterisation of air-sea gas exchange is still governed by empirical approaches because the mechanisms of air-sea gas exchange are difficult to investigate with techniques such as eddy covariance flux measurements and dual-tracer techniques. Therefore, laboratory experiments are needed to identify and quantify these mechanisms, where it is possible to perform systematic studies and use more sophisticated experimental techniques.

However, can this be successful given that laboratory wind and wave conditions differ from those in the ocean? This question will be addressed first, after which the approaches taken to simulate oceanic conditions sufficiently realistically with respect to air-sea gas transfer will be outlined. It will be shown that novel imaging techniques played a key role in measuring gas transfer velocity locally and instantaneously, and in investigating the mechanisms.

This approach has enabled new insights into the mechanisms, and some preliminary results will be presented. At low wind speeds, there is a significant overshoot in gas transfer velocity with short-fetch waves. At medium wind speeds, the gas transfer velocity is remarkably insensitive to the age of the wind waves. Waves enhance gas exchange by frequently disrupting the shear current at the water's surface. This is also why large-scale, even-breaking waves have no significant impact on enhancing gas transfer.

Both insoluble (such as hexadecanol or olive oil) and soluble (such as Triton X-100 or Tergitol 15-S-12) surfactant monolayers can completely suppress wind waves, reducing the gas transfer velocity by up to a factor of five. When waves are completely suppressed, the resulting gas transfer velocity is unaffected by the chemical nature of the surfactant. We could also prove directly that even a compact, dense monolayer does not constitute
direct resistance to gas transfer.

Direct comparison in the Heidelberg Aeolotron also proved that advanced floating chambers cannot measure correct gas transfer velocities when wind-induced gas transfer is dominant, which occurs at wind speeds as low as 2 m/s.

The talk concludes with preliminary thermography measurement results taken during the BASS Baltic Sea Cruise 2025. These results show that it is possible to directly prove that the mechanisms controlling air–sea gas transfer are the same in the laboratory and in the field.