LIF measurements of concentration profiles in the aqueous mass boundary layer

A laser-induced fluorescence (LIF) technique is described to measure 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. The technique is capable of measuring two-dimensional vertical concentration profiles at a rate of 200 frames/s and a spatial resolution of 16 mu m. The mass boundary layer at a free surface is characterized by significant fluctuations. Direct surface renewal is observed. The mean profiles also support rather surface renewal models than turbulent diffusion models.