Air-water gas transfer impacts many important biogeochemical processes, yet current understandings involve large uncertainty. This arises because the process depends on a complex interaction between molecular diffusion and fluid motions that has not been adequately characterized. Here we show the first experimental support for a mechanistic model that relates near-surface motions to gas transfer coefficients over a range of wind conditions, including those leading to breaking wavelets. We find that the square root of the root mean square surface-velocity divergence varies linearly with both the gas transfer coefficient, as predicted by theory, as well as with mean square surface slope. Besides advancing the understanding of the mechanisms governing air-water gas transfer, these results suggest easy-to-measure parameters that could, with further investigation, provide gas transfer coefficients in field settings.