Gas transfer velocities in lakes measured with SF6

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Abstract
The experimentally-determined relationships between air-water gas transfer velocity and windspeed are presented for two small, rapidly wind mixed lakes in upland SW England. High-precision estimates of the gas transfer velocity, k, with daily resolution, were derived by monitoring the rate of evasion from the lakes of added sulphur hexafluoride, SF6, an inert, sparingly soluble, man-made gaseous tracer. Corresponding data on in situ wind speeds and directions, and surface water temperatures were automatically logged as a time series of 4 min averages, using a battery-powered device. The results significantly extend the existing field database and show a strong dependence of k, normalized to CO2 at 20 °C, on windspeed in the range ˜ 2—13 ms-1, corrected to a height of 10 m. No correlation was found between k and wind direction. The data are fitted with two least-squares straight lines which intersect at a windspeed of 9.5 ± 3 ms-1 (at z= 10 m), beyond which significant steepening of the k vs. windspeed rel...