In situ evaluation of air-sea gas exchange parameterizations using novel conservative and volatile tracers

Measurements of air-sea gas exchange rates are reported from two deliberate tracer experiments in the southern North Sea during February 1992 and 1993 . A conservative tracer, spores of the bacterium Bacillus globigii var. Nig er, was used for the first time in an in situ air-sea gas exchange experime nt. This nonvolatile tracer is used to correct for dispersive dilution of t he volatile tracers and allows three estimations of the transfer velocity f or the same time period. The first estimation of the power dependence of ga s transfer on molecular diffusivity in the marine environment is reported. This allows the impact of bubbles on estimates of the transfer velocity der ived from changes in the helium/sulphur hexafluoride ratio to be assessed. Data from earlier dual tracer experiments are reinterpreted, and findings s uggest that results from all dual tracer experiments are mutually consisten t. The complete data set is used to test published parameterizations of gas transfer with wind speed. A gas exchange relationship that shows a depende nce on wind speed intermediate between those of Liss and Merlivat [1986] an d Wanninkhof [1992] is found to be optimal. The dual tracer data are shown to be reasonably consistent with global estimates of gas exchange based on the uptake of natural and bomb-derived radiocarbon. The degree of scatter i n the data when plotted against wind speed suggests that parameters not sca ling with wind speed are also influencing gas exchange rates.