A fluorescence technique to measure concentration profiles in the aqueous mass boundary layer

A laser-induced fluorescence (LIF) technique has been developed to measure vertical concentration profiles of dissolved gases in the aqueous mass boundary layer at a wavy interface. An optical wave follower is used to track the surface with a CCD camera. Time series of vertical concentration profiles with a temporal and spatial resolution of 2 ms and 30 μm, respectively have been measured in the circular wind/wave facility at the Institute for Environmental Physics, University of Heidelberg at low and medium wind speeds. From these time series, mean vertical profiles are computed. The mean boundary layer thickness and the transfer velocity are determined. The latter is compared with oxygen transfer velocities obtained by the classical mass balance method. The boundary layer shows substantial variations in thickness. The time series also reveal large eddies sweeping parts of the boundary layer down into the bulk.