In the stagnant film (thin-film) model of air-sea gas exchange, the piston velocity (mass transfer velocity) k is given by k = D/z, where D is the molecular diffusion coefficient of the gas and z is the thickness of the stagnant surface film through which the gas must diffuse.
Using k = D/z with D = 1x10^-5 cm^2/s and z = 20 micrometers = 20x10^-4 cm = 2x10^-3 cm: k = (1x10^-5)/(2x10^-3) = 5x10^-3...
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