A Radiometric Method to Measure the Concentration Boundary Layer Thickness at an Air–Water Interface*

An interferometric technique to measure the thickness of the concentration boundary layer at an air–water interface is presented. The technique uses heat as a proxy tracer for gas transfer by sensing the infrared (IR) radiation emitted by the water at wavelengths near 3.8 μm. The temperature gradient at the water surface is determined from the IR radiance by using the wavelength variation of the absorption coefficient of water in the 3.3- to 4.1-μm wavelength region. The variations in the absorption coefficient allow the emitted IR radiation to carry information about the surface and subsurface water temperature. Interferometric measurements of the radiance variations as a function of optical wavelength can be related to the temperature gradient or the change in water temperature with respect to depth. Previous laboratory measurements indicate that interfacial temperature gradients can be studied in great detail using this approach. Very near the water surface, the temperature gradient is linear, and heat transport occurs mainly by molecular conduction. Beyond this molecular conduction zone, eddy diffusion dominates transfer and temperature is constant with depth. The thickness of this thermal boundary layer can be estimated directly from the depth where the measured gradient becomes constant or can be inferred by extrapolating the gradient to the known bulk temperature. The concentration boundary layer thickness can then be related to the thermal boundary layer depth by appropriately scaling the thermal and molecular diffusivities. The proposed experiment shows how this information could be used to remotely determine the air–water gas transfer velocity and investigate the dependence of the concentration boundary layer thickness on both molecular diffusivity and physical forcing functions (i.e., wind speed, near-surface turbulence).