Gas transfer experiment on a lake (Kerguelen Islands) using He-3 and SF6

Gas transfer velocities of SF6 and He-3 were determined in a Kerguelen Isla nds lake at wind speeds in the range 0-10 m/s by injecting the tyro tracers into the water and measuring their concentrations over 40 days. Two method s are investigated for the determination of the relationship linking the ga s transfer velocity K to the wind speed W. The first method postulates a po wer law relationship K=beta W-alpha. This leads to the same exponent alpha= 1.5+/-0.2 for both gases. The second method is the classic determination of the gas transfer velocity K-ij between two tracer measurements at times t( i) and t(j) using the well-mixed reservoir assumption. This method proves t o be less favorable owing to its nonlinearity bias and also because it indu ces much scatter in the gas transfer coefficient/wind speed relationship. T his dispersion is shown to arise from the experimental scatter of the data and, above all, from the high sensitivity of the method to even small heter ogeneities in the tracer vertical distribution. In the present experiment, the Liss and Merlivat correlation [Liss and Merlivat, 1986] is shown to und erestimate the actual mean gas exchange rate by about 40%. Our results agre e with the recent dual-tracer experiment by Wanninkhof et al. [1993] and ar e also consistent with CO2 transfer coefficient data derived from the study of C-14 oceanic inventories. As expected from gas transfer theories and va rious experimental observations, the Schmidt number exponent in the compari son of He-3 and SF6 transfer velocities is found to vary substantially with the transfer regime. However, its variation is found to be greater than th at forecast by current gas transfer models, with values as high as n=-0.2 f or intermediate to strong winds. This again raises the question of the vali dity of the normalization method for K-CO2 calculation from gas transfer ex periments, especially in high-wind regimes.