%0 Generic
%A Kräuter, Christine
%C Heidelberg
%D 2015
%F heidok:18209
%R 10.11588/heidok.00018209
%T Visualization of air-water gas exchange
%U https://archiv.ub.uni-heidelberg.de/volltextserver/18209/
%X The mechanisms of air-sea gas transfer are studied in a large annular air-sea interaction facility, the Heidelberg Aeolotron. A novel visualization technique is used, in which an alkaline gas (ammonia) in ppm concentration levels is absorbed into slightly acid water (pH = 4). The concentration gradient in the aqueous mass boundary layer is transformed into a sharp pH gradient, which is made visible by a fluorescent pH indicator (Pyranine). Regulating the gas flux into the water allows controlling the thickness of the fluorescent layer. By observing the fluorescence induced by high power LEDs on a 2D surface patch ((21 x 25) cm²) with a camera from above at 100 fps and a spatial resolution of 150 μm, the processes creating near-surface turbulence can be studied even at a wavy water surface. Results from an extensive set of experiments are presented, covering wind speeds from 0.4 - 8.6 m/s, different concentrations of a soluble surfactant, and waves with limited and unlimited fetch. The fluorescence footprints of different processes known to contribute  to gas exchange, e.g. Langmuir circulations and microscale breaking, are identified. It is found that the transition of the Schmidt number exponent, which is essential for  the scaling of transfer velocities of different gases, from 2/3 to 1/2 is correlated with the frequency of microscale breaking. Surfactants shift the onset of surface renewal  events to higher wind speeds. Direct comparison to simultaneously captured thermal imagery shows that despite the large difference in diffusion coefficients between ammonia and heat by a factor of 100, the transport of both tracers is governed by the same mechanisms.