Gas transfer velocities in lakes measured with SF₆

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 relationship implies a transition from the “rough surface” to “breaking wave” regime, in broad agreement with previous conclusions. Nevertheless, the data scatter about the fitted lines exceeds that which would be predicted from the associated analytical uncertainties. This implies the observed relationships between k and windspeed are not unique and therefore that additional factors must be important in determining k.