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

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

Many freshwater lakes are supersaturated in CO 2 with respect to the atmosphere. This concentration gradient implies a net flux of CO 2 from the water to the air. The actual rate of gas exchange is governed by both this concentration gradient and the gas transfer coefficient, k, To directly measure k, we added the chemically and biologically inert gas, sulfur hexaflouride (SF 6 ), to the epilimnion of Mirror Lake, New Hampshire, a small (15 ha), low-wind softwater lake. k was independent of wind speed over the 50-d summer stratification period and averaged 2.65 ± 0.12 cm h -1 (95% CI; normalized to a Schmidt number of 600); k 800 was better correlated to precipitation events than it was to wind speed. Our data support the idea that gas exchange across the air-water interface is largely independent of wind at low wind speeds. The surface water of Mirror Lake was persistently supersaturated in CO 2 with respect to the atmosphere. During a 3.5-year period the partial pressure of CO 2 in the surface waters of the lake averaged 726 ± 39 μatm (95% CI) and showed substantial seasonal variation (360-2,000 μatm). Diel and day-to-day variation in CO 2 were very small compared to the CO 2 pool. We combined our estimates of k with weekly measurements of the partial pressure of CO 2 to estimate CO 2 gas exchange in the lake. Mirror Lake released from 26 to 50 g C m -2 to the atmosphere each year, depending on the method of calculating k. Atmospheric CO 2 exchange is a large term in the C economy of the lake-the most conservative gas flux estimate is about four times as large as outflow plus seepage of total dissolved inorganic carbon and 1.5 times as large as the export of dissolved organic C from the lake.

/pdf/atmospheric-exchange-of-carbon-dioxide-in-a-low-wind-57b939gt1s.pdf

Atmospheric exchange of carbon dioxide in a low-wind oligotrophic lake measured by the addition of SF6

The diffusion coefficients D of important gas tracers dissolved in water and seawater were measured with a modified Barrer method. The measurements include the gases He, Ne, Kr, Xe, H2, CH4, and CO2 dissolved in distilled water in the temperature range from 5 to 35°C, and He and H2 dissolved in seawater in the same temperature range. The maximum systematic error is estimated to be well below 5%. The isotopic fractionation in the diffusion coefficient, ∈D, was determined to be (−0.87 ± 0.05)‰ for 13CO2/12CO2 and (15 ± 3)% for 3He/4He.

Measurement of the diffusion coefficients of sparingly soluble gases in water

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

Theory and experimental results of a new method are described directly investigating the transfer processes across the aqueous viscous boundary layer. The method is based on a known and controllable flux density being applied at the interface. Then the local transfer velocity can be determined by monitoring the tracer concentration at the water surface within minutes. Moreover, the time constant for the transport across the boundary layer (“surface renewal time”) can be measured directly. Comparison of the theoretical and measured frequency response of the boundary layer yields significant deviations. The technique is put into operation for heat transfer measurements. Direct comparisons with gas exchange measurements in several wind/wave facilities verify that the gas transfer velocity can be accurately extrapolated from the heat transfer measurements. A new way is opened both for detailed studies of the transfer processes in wind/wave facilities and the urgently needed direct parameterization of the transfer velocity as a function of windshear, wave parameters, and water turbulence in natural systems as rivers, lakes and the ocean. This paper includes (as a first example) measurements on the fetch dependency of the transfer process. DOI: 10.1111/j.1600-0889.1989.tb00135.x

Investigating the transfer processes across the free aqueous viscous boundary layer by the controlled flux method

Investigating the transfer process across the free aqueous boundary layer by the controlled flux method

Es wird ein Messystem vorgestellt, das lokal und momentan Austauschkoeffizienten an der Phasengrenze zwischen den Gewassern und der Atmosphare bestimmen kann. Die bisher nicht gekannte raumliche und zeitliche Auflosung wird durch das neue Konstantflus-Mesverfahren erreicht. Als Tracer dient Warme. Mit dem neuen feldgangigen Verfahren kann erstmals die Wiederbeluftungsrate in situ bestimmt und parametrisiert werden.

/pdf/eine-neue-methode-zur-lokalen-und-momentanen-bestimmung-der-18yrv5g9rj.pdf

Peter Libner

Papers

On the parameters influencing air-water gas exchange

Investigating the transfer processes across the free aqueous viscous boundary layer by the controlled flux method

Investigating the transfer process across the free aqueous boundary layer by the controlled flux method

Eine neue Methode zur lokalen und momentanen Bestimmung der Wiederbelüftungsraten von Gewässern