Towards four dimensional visualization of air-water gas exchange

In this work, the transport of a trace gas through the air-water interface at a turbulent wavy water surface has been studied. A novel experimental technique for the visualization of the mass boundary layer has been developed. The technique is based on a system of chemical reactions to feature a binary representation of concentration fields. By penetrating through the air-water interface, the alkaline trace gas ammonia changes the temporal equilibrium of the chemical species and hence the local pH value. The shift in the pH value is visualized by the fluorescent pH indicator Pyranine. The technique to measure a controllable fraction of the water-side mass boundary layer thickness has been verified by simulations and vertical profile measurements with a high temporal and spatial resolution. The experiments were conducted at a linear wind-wave facility at different conditions and concentrations. The suitable concentration of the fluorescent dye and the initial pH value, favorable for maintaining a step-shaped drop of the intensity at a certain depth, have been estimated to be 10^(−5) mol/L and 4, respectively. In additional measurements at the large annular facility Aeolotron, the simplified turbulent structures are observed from below of the facility with a trifocal camera set-up. With the multiple view geometry concept, stereo based reconstruction has been performed by using the images from two cameras. Through the estimated disparity maps the third spatial dimension of the fluorescent turbulent structures is retrieved. Therefore, the processes driving air-water gas exchange can be investigated in the three spatial and the temporal dimensions