LIF Investigation of the Mechanisms Controlling Air-Water Mass Transfer at a Free Interface

A laser-induced fluorescence (LIF) technique is described to measure two-dimensional vertical concentration profiles of gases in the aqueous mass boundary layer at a free water surface. The technique uses an acid-base reaction of the fluorescence indicator fluorescein at the water surface to visualize the concentration profiles. It is shown that this technique is capable of measuring the
two-dimensional vertical concentration profiles at a rate of 200 frames per second with a spatial resolution of 20 µm. The results show that the decrease of the measured mean concentration profiles into the bulk is faster than predicted by the small eddy model. The measurements agree with the predictions of the surface renewal model. Combined experiments with a wave slope imaging technique reveal that there is no local correlation between wave slope and boundary layer thickness. The image sequences indicate that there is only a weak correlation between the occurrence of surface renewal phenomena and steep wave trains. Surface renewal effects for a fully developed wave field renew the fluid in the aqueous mass boundary layer up to the water surface. This is not true for the case of a flat interface. Here, the changes of the boundary layer thickness after a surface renewal event are significantly smaller.