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Gas Transfer Velocities for 3He in a Lake at High Wind Speeds

A gas exchange experiment was performed in a lake of the Kerguelen Islands by using helium-3 as a tracer. The objective was to study the rate of gas exchange at air-water interface for high wind speeds, up to 20m/s. After injection and initial mixing of the tracer, the escape rate was monitored by collecting water samples for mass spectrometry analysis. After 20 days, the lake had almost returned to the atmospheric equilibrium and a new tracer injection was done, allowing the acquisition of a second 3 He time-series. Assuming a well-mixed flat bottom reservoir, the 3 He transfer coefficient - wind speed relationship, determined using a power law fit, leads to k 3He = 1.1 W 1.5 , where k 3He is the 3 He transfer velocity in cm/h at the temperature of the lake (T=1°C, Sc=310) and W, the wind speed in m/s at 10 m height. Taking into account the possible vertical heterogeneity of the lake (assuming a vertical diffusion coefficient K v =50cm/s 2 ), and a more realistic bathymetry, leads to a 32% increase in the transfer velocity, thus giving k 3He = 1.45 W 1.5 cm/h. This result is consistent within the uncertainties with our recent measurements in a larger lake using 3 He and SF 6 [Jean-Baptiste and Poisson, 2000]. It is significantly above the Liss and Merlivat [1986] parametrization.