A RADIOMETRIC METHOD TO MEASURE THE CONCENTRATION BOUNDARY-LAYER THICKNESS AT AN AIR-WATER-INTERFACE

An interferometric technique to measure the thickness of the concentra tion boundary layer at an air-water interface is presented. The techni que uses heat as a proxy tracer for gas transfer by sensing the infrar ed (IR) radiation emitted by the water at wavelengths near 3.8 mu m. T he temperature gradient at the water surface is determined from the IR radiance by using the wavelength variation of the absorption coeffici ent of water in the 3.3- to 4.1-mu m wavelength region. The variations in the absorption coefficient allow the emitted IR radiation to carry information about the surface and subsurface water temperature. Inter ferrometric measurements of the radiance variations as a function of o ptical wavelength can be related to the temperature gradient or the ch ange in water temperature with respect to depth, Previous laboratory m easurements indicate that interfacial temperature gradients can be stu died 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 d iffusion dominates transfer and temperature is constant with depth. Th e thickness of this thermal boundary layer can be estimated directly f rom the depth where the measured gradient becomes constant or can be i nferred by extrapolating the gradient to the known bulk temperature. T he concentration boundary layer thickness can then be related to the t hermal boundary layer depth by appropriately scaling the thermal and m olecular diffusivities. The proposed experiment shows how this informa tion could be used to remotely determine the air-water gas transfer ve locity and investigate the dependence of the concentration boundary la yer thickness oil both molecular diffusivity and physical forcing func tions (i.e., wind speed, near-surface turbulence).