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.