Development of a depth resolving boundary layer visualiziation für gas exchange at free water surfaces

In this thesis, a measurement setup is introduced which makes it possible to directly measure two-dimensional, vertical concentration of gases in the water sided boundary layer using Laser-Induced Fluorescence (LIF). While it is impossible to gain knowledge of the physical processes involved in gas exchange using measurements of transfer rates and mass balances, the introduced method makes it possible to directly visualize the physical processes of matter transport in the boundary layer. The measurement method is based on two basic principles: First a fluorescence indicator is used, whose fluorescence intensity is proportional to the local pH-value, thus allowing a spatial resolved measurement of the concentrations of dissolved alkaline or acidic gases can directly be visualized. Second, to create a depth resolution, a second, absorbing dye is added, whose absoprtion maximum lies inside the fluorescence spectrum, so that spectra from different depths show changes in their spectral shape due to the different light path lengths through the absorber. Thus the measured spectrum is the superposition of all depth spectra, which provide the basis of a linear inverse problem. Models for the reconstruction of the depth information will be introduced in the course of this thesis, and the solvability will be analyzed. As the stability of the solution of the inverse problem is almost exclusively determined by the invertibility of the basis function matrix, a confocal microscope was constructed, which allowed the direct measurement of depth spectra. Through this it was made possible to numerically analyze and evaluate the conditioning of the matrix invertibility.