Abstract
Abstract. Gas transfer velocities were measured in two high-speed wind-wave tanks (Kyoto University and the SUSTAIN facility, RSMAS, University of Miami) using fresh water, simulated seawater and seawater for wind speeds between 7 and 85 m s−1. Using a mass balance technique, transfer velocities of a total of 12 trace gases were measured, with dimensionless solubilities ranging from 0.005 to 150 and Schmidt numbers between 149 and 1360. This choice of tracers enabled the separation of gas transfer across the free interface from gas transfer at closed bubble surfaces. The major effect found was a very steep increase of the gas transfer across the free water surface at wind speeds beyond 33 m s−1. The increase is the same for fresh water, simulated seawater and seawater. Bubble-induced gas transfer played no significant role for all tracers in fresh water and for tracers with moderate solubility such as carbon dioxide and dimethyl sulfide (DMS) in seawater, while for low-solubility tracers bubble-induced gas transfer in seawater was found to be about 1.7 times larger than the transfer at the free water surface at the highest wind speed of 85 m s−1. There are indications that the low contributions of bubbles are due to the low wave age/fetch of the wind-wave tank experiments, but further studies on the wave age dependency of gas exchange are required to resolve this issue.
Highlights
The transfer of trace gases across the air–sea interface has been an active field of research for almost 40 years (Jähne, 2019)
This paper reports the results of extensive gas exchange measurements in two different wind-wave tanks with up to 12 tracers covering a wide range of solubilities using fresh water, simulated seawater and seawater
With multi-tracer gas exchange experiments in two highspeed wind-wave tanks, it was possible to separate the mechanisms of air–sea gas transfer into its different components, transfer across the free water surface, transfer across closed bubble surfaces and transfer associated with the bubble volume flux density
Summary
The transfer of trace gases across the air–sea interface has been an active field of research for almost 40 years (Jähne, 2019). Only two gas transfer studies have been performed in hurricane conditions in the Kyoto High-Speed Wind-Wave Tank with 1,4-difluorobenzene and hexafluorobenzene (Krall and Jähne, 2014) and with carbon dioxide (Iwano et al, 2013, 2014) but only in fresh water. Possible candidates are (a) bubbles, which provide an additional surface for the gas transfer, (b) an increased water surface area due to the fragmentation of the water surface at highest wind speeds or (c) a strong increase in near-surface turbulence due to frequent surface renewal and breakup events, or a combination of all three effects. It is not clear whether bubble-induced gas exchange differs between fresh water and seawater
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