A new stereoscopic method for collecting particle image velocimetry (PIV) measurements within ~1 mm of a wavy air–water interface with simultaneous measurements of the morphology of the interface is described. The method, termed three-dimensional interfacial particle image velocimetry (3D-IPIV), is tested in a wind wave channel with a wind speed of 5.8 m s−1, water depth of 10 cm and a fetch of ~9 m. Microscale breaking waves populate the interface and their flow patterns are clearly visible in the velocimetry results. The associated capillary waves and surface divergence patterns are observed. Several statistical measurements of the flow are compared with independent measurements and good agreement is found. The method shows great promise for investigating the transfer of momentum, heat and gases across an air–water interface, both in the laboratory and in field settings. Additional methods are described for manufacturing the flow tracers needed for the 3D-IPIV method. These tracers are likely to be useful for other researchers, and have the characteristics of being fluorescent, neutrally buoyant, non-toxic, monodisperse, inexpensive and easy to manufacture.

[1] 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.