We study the aqueous-phase side of the transfer of carbon dioxide gas across an air-water interface. Quantitative imaging techniques are used to directly visualize the physical processes which determine the average gas transfer rate. The interface is a free surface in the absence of mean shear, with turbulence generated on the water side, well away from the free surface, which then transports itself to the free surface. This turbulence is generated far beneath the free surface by an array of upward-pointing synthetic jets which are each driven according to independent random time series. We show that this method of turbulence generation is superior to the traditional grid-stirred tank in that it exhibits weaker mean secondary flows.

Using Laser Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV) we measure simultaneous concentration and velocity fields, respectively. These are measured in planar fields perpendicular to and intersecting the free surface. From these we calculate turbulent statistics of interest. Namely, the vertical profiles of mean and fluctuating velocity magnitudes, momentum dissipation rate, spatial power spectra for velocity and concentration, and the turbulent mass flux.

Examination of the turbulent mass flux field reveals that downward-traveling fluid, which leaves the concentration boundary layer at the surface and enters the bulk, is responsible for the majority of the gas transfer. This is in contrast to the commonly held view that upward-traveling fluid from the bulk dominates gas transfer. The spectrum of the turbulent mass flux field is nearly flat, showing that motions of all sizes in the inertial subrange contribute equally to the mass transfer. This resolves the longstanding question about which size eddies are responsible for gas transfer.