Untersuchung von Austauschprozessen an der Wasseroberfläche aus Infrarot-Bildsequenzen mittels frequenzmodulierter Wärmeeinstrahlung

The study presents a new technique for measuring the gas transfer rate at a free water surface with high temporal and spatial resolution, a method suitable for field experiments. The approach is based on the Controlled Flux Technique (CFT) using heat as a proxy tracer for gas exchange. A periodically varying heat flux at water surface is used. The heat flux is generated by a 100 W CO2-laser, while a highly sensitive infrared camera measures the temperature response of the water surface. Heat - and consequently gas transfer rates - can be deduced from the analysis of the spectral amplitude attenuation at water surface. Furthermore, the approach presented allows direct insight into the transport mechanisms of gas exchange processes by an analysis of the phase shift observed at water surface. A first series of measurements at the Heidelberg wind wave facility AEOLOTRON shows that the spectral amplitude attenuation is in keeping with model predictions. The calculated transfer rates for CO2 are also well in accordance with results from other laboratory and field experiments. Contrary to the amplitude attenuation, the calculated phase shifts differ signicantly from model predictions, especially for high wind speeds. This effect, observed for the first time, can be interpreted as being caused by the intermittence of transfer processes. This would indicate that stochastic processes play a larger role in gas transfer than assumed so far.