Relationships between wind speed and gas transfer, combined with knowledge of the partial pressure difference of CO2 across the air-sea interface are frequently used to determine the CO2 flux between the ocean and the atmosphere. Little attention has been paid to the influence of variability in wind speed on the calculated gas transfer velocities and the possibility of chemical enhancement of CO2 exchange at low wind speeds over the ocean. The effect of these parameters is illustrated using a quadratic dependence of gas exchange on wind speed which is fit through gas transfer velocities over the ocean determined by the natural-14C disequilibrium and the bomb-14C inventory methods. Some of the variability between different data sets can be accounted for by the suggested mechanisms, but much of the variation appears due to other causes. Possible causes for the large difference between two frequently used relationships between gas transfer and wind speed are discussed. To determine fluxes of gases other than CO2 across the air-water interface, the relevant expressions for gas transfer, and the temperature and salinity dependence of the Schmidt number and solubility of several gases of environmental interest are included in an appendix.

Relationship between wind speed and gas exchange over the ocean

In this chapter we attempt to present a brief introduction to the subject of air-sea gas exchange First the basic equations governing such exchange are given, then a review of some models proposed to describe the gas transfer process Following this, experimental approaches through both laboratory (principally using wind/water tunnels) and field measurements are summarised Finally, we present what seems to us to be the best current synthesis of the wind tunnel and field results for the prediction of gas exchange rates across the sea surface

Air-Sea Gas Exchange Rates: Introduction and Synthesis

Measurements of air-sea gas exchange rates are reported from two deliberate tracer experiments in the southern North Sea during February 1992 and 1993. A conservative tracer, spores of the bacterium Bacillus globigii var. Niger, was used for the first time in an in situ air-sea gas exchange experiment. This nonvolatile tracer is used to correct for dispersive dilution of the volatile tracers and allows three estimations of the transfer velocity for the same time period. The first estimation of the power dependence of gas transfer on molecular diffusivity in the marine environment is reported. This allows the impact of bubbles on estimates of the transfer velocity derived from changes in the helium/sulphur hexafluoride ratio to be assessed. Data from earlier dual tracer experiments are reinterpreted, and findings suggest that results from all dual tracer experiments are mutually consistent. The complete data set is used to test published parameterizations of gas transfer with wind speed. A gas ex- change relationship that shows a dependence on wind speed intermediate between those ofLiss and Merlivat [1986] and Wanninkhof [1992] is found to be optimal. The dual tracer data are shown to be reasonably consistent with global estimates of gas exchange based on the uptake of natural and bomb-derived radiocarbon. The degree of scatter in the data when plotted against wind speed suggests that parameters not scaling with wind speed are also influencing gas exchange rates.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999GB900091

In situ evaluation of air-sea gas exchange parameterizations using novel conservative and volatile tracers

[ 1] Lake sediments are "hot spots'' of methane production in the landscape. However, regional and global lake methane emissions, contributing to the greenhouse effect, are poorly known. We develop ...

/pdf/methane-emissions-from-lakes-dependence-of-lake-259drtee2b.pdf

Methane emissions from lakes: Dependence of lake characteristics, two regional assessments, and a global estimate

The relationship between gas exchange and wind speed is used extensively for estimating bulk fluxes of atmospheric gases across the air-sea interface. Here, I provide an update on the frequently used method of Wanninkhof (1992). The update of the methodology reflects advances that have occurred over the past two decades in quantifying the input parameters. The general principle of obtaining a relationship constrained by the globally integrated bomb-14CO2 flux into the ocean remains unchanged. The improved relationship is created using revised global ocean 14C inventories and improved wind speed products. Empirical relationships of the Schmidt number, which are necessary to determine the fluxes, are extended to 40°C to facilitate their use in the models. The focus is on the gas exchange of carbon dioxide, but the suggested functionality can be extended to other gases at intermediate winds (≈4−15 m s−1). The updated relationship, expressed as k = 0.251 (Sc/660)−0.5 where k is the gas transfer velocity, is the average squared wind speed, and Sc is the Schmidt number, has a 20% uncertainty. The relationship is in close agreement with recent parameterizations based on results from gas exchange process studies over the ocean.

https://aslopubs.onlinelibrary.wiley.com/doi/epdf/10.4319/lom.2014.12.351

Relationship between wind speed and gas exchange over the ocean revisited

Detailed gas exchange measurements from two circular and one linear wind/wave tunnels are presented. Heat, He, CH4, CO2, Kr, and Xe have been used as tracers. The experiments show the central importance of waves for the water-side transfer process. With the onset of waves the Schmidt number dependence of the transfer velocity k changes from k ∝ Sc−⅔ to k ∝ Sc−½indicating a change in the boundary conditions at the surface. Moreover, energy put into the wave field by wind is transferred to near-surface turbulence enhancing gas transfer. The data show that the mean square slope of the waves is the best parameter to characterize the free wavy surface with respect to water-side transfer processes.

On the parameters influencing air-water gas exchange

Gas exchange experiments have been carried out in the Heidelberg circular wind tunnels and the large linear I.M.S.T. wind/wave facility in order to study the wave field and Schmidt number dependence of gas exchange. With a smooth water surface the experimental results agree well with the model treating the water surface as a rigid wall both in the Sc−2/3 dependence and the absolute rates. Gas exchange rates with a rough wavy surface are 3–5 times higher than in the smooth case and the Schmidt number dependence is lower (Sc−n, n = 0.55 ± 0.05). The total square wave slope seems to be a more appropriate parameter to describe the enhanced rates than the capillary wave slope.

Wind/Wave-Tunnel Experiment on the Schmidt Number — and Wave Field Dependence of Air/Water Gas Exchange

https://zenodo.org/record/14544/files/Huber_Diplom1984a.pdf

Aufbau eines gaschromatographischen Messsystems für Gasaustauschmessungen; Windkanalmessungen zur Schmidtzahl- und Wellenbildabhängigkeit des Gasaustausches

Werner Huber

Papers

On the parameters influencing air-water gas exchange

Wind/Wave-Tunnel Experiment on the Schmidt Number — and Wave Field Dependence of Air/Water Gas Exchange

Aufbau eines gaschromatographischen Messsystems für Gasaustauschmessungen; Windkanalmessungen zur Schmidtzahl- und Wellenbildabhängigkeit des Gasaustausches