Gas transfer experiment on a lake (Kerguelen Islands) using 3 He and SF 6
Résumé
Gas transfer velocities of SF6 and 3He were determined in a Kerguelen Islands lake at wind speeds in the range 0-10 m/s by injecting the two tracers into the water and measuring their concentrations over 40 days. Two methods are investigated for the determination of the relationship linking the gas transfer velocity K to the wind speed W. The first method postulates a power law relationship K=I3W *. This leads to the same exponent ot=1.5+0.2 for both gases. The second method is the classic determination of the gas transfer velocity K• between two tracer measurements at times ti and t; using the well-mixed reservoir assumption. This method proves to be less favorable owing to its nonlinearity bias and also because it induces much scatter in the gas transfer coefficient/wind speed relationship. This dispersion is shown to arise from the experimental scatter of the data and, above all, fi-om the high sensitivity of the method to even small heterogeneities in the tracer vertical distribution. In the present experiment, the Liss and Merlivat correlation [Liss and Merlivat, 1986] is shown to underestimate the actual mean gas exchange rate by about 40%. Our results agree with the recent dual-tracer experiment by I4/hnninkhofet al. [ 1993 ] and are also consistent with CO2 transfer coefficient data derived from the study of •4C oceanic inventories. As expected from gas transfer theories and various experimental observations, the Schmidt number exponent in the comparison of 3He and SF, transfer velocities is found to vary substantially with the transfer regime. However, its variation is found to be greater than that tbrecast by current gas transfer models, with values as high as n=-0.2 for intermediate to strong winds. This again raises the question of the validity of the normalization method for Kco2 calculation d-ore gas transfer experiments, especially in high-wind regimes.
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