Abstract
Abstract. In this study we present gas-exchange measurements conducted in a large-scale wind–wave tank. Fourteen chemical species spanning a wide range of solubility (dimensionless solubility, α = 0.4 to 5470) and diffusivity (Schmidt number in water, Scw = 594 to 1194) were examined under various turbulent (u10 = 0.73 to 13.2 m s−1) conditions. Additional experiments were performed under different surfactant modulated (two different concentration levels of Triton X-100) surface states. This paper details the complete methodology, experimental procedure and instrumentation used to derive the total transfer velocity for all examined tracers. The results presented here demonstrate the efficacy of the proposed method, and the derived gas-exchange velocities are shown to be comparable to previous investigations. The gas transfer behaviour is exemplified by contrasting two species at the two solubility extremes, namely nitrous oxide (N2O) and methanol (CH3OH). Interestingly, a strong transfer velocity reduction (up to a factor of 3) was observed for the relatively insoluble N2O under a surfactant covered water surface. In contrast, the surfactant effect for CH3OH, the high solubility tracer, was significantly weaker.
Highlights
The world’s oceans are key sources and sinks in the global budgets of numerous atmospherically important trace gases, in particular CO2, N2O and volatile organic compounds (VOCs) (Field et al, 1998; Williams et al, 2004; Millet et al, 2008, 2010; Carpenter et al, 2012)
In this study we present gas-exchange measurements conducted in a large-scale wind–wave tank
The wind–wave tank is interpreted in terms of a box model
Summary
The world’s oceans are key sources and sinks in the global budgets of numerous atmospherically important trace gases, in particular CO2, N2O and volatile organic compounds (VOCs) (Field et al, 1998; Williams et al, 2004; Millet et al, 2008, 2010; Carpenter et al, 2012). Gas exchange between the ocean and the atmosphere is a significant conduit within global biogeochemical cycles. The model assumes that close to the interface turbulent motion is suppressed and that the transfer of gases is controlled by molecular motion (expressed by the diffusion coefficient D). This leads to the formation of two mass boundary layers on both sides of the interface. In the upper part of the airside mass boundary layer, turbulent transport becomes significant. Depending on the solubility of the gas in question, its transfer could be restricted by one or both sides of the interface (i.e air-side and water-side controlled)
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