Dual-tracer measurements of concentration profiles in the aqueous mass boundary layer

A combination of two laser-induced fluorescence (LIF) techniques for measurements of vertical concentration profiles of dissolved gases and thus the investigation of micro-turbulence in the aqueous mass boundary layer at a free interface is presented. The first one uses an acid-base reaction of the fluorescence indicator fluorescein at the water surface to visualize the concentration profiles. The second one uses fluorescence quenching of pyrenebutyric acid by oxygen. Both experiments are done simultaneously at the same location. The advantage of a combined experiment is that the two techniques utilized here to visualize the concentration profiles are caused by different independent physical effects, i.e. an acid-base reaction and the fluorescence quenching by oxygen. Simultaneous measurements can help to detect possible systematic errors and offer the chance to learn more about the underlying gas exchange mechanisms. Experiments performed at the circular facility of the University of Heidelberg using a surfactant to suppress waves are presented. For three wind speeds the mean and fluctuating boundary layer properties are investigated