Investigating Air-Water Gas Transfer via Fluorescence Imaging

This study presents an advanced Fluorescence Imaging technique to make air-water gas exchange dynamics in a wind-wave facility visible up to high wind speeds. Fluorescence is generated by the pH-sensitive dye pyranine, which transitions into its fluorescent alkaline form upon reacting with the invading alkaline trace gas methylamine. A high-intensity laser illumination system combined with a multi-camera setup is implemented at the Heidelberg Aeolotron, enabling high-resolution imaging of fluorescent patterns in gas concentration fields near the water surface. Near-surface turbulent structures such as quasi-streamwise vortices, Langmuir circulations, and microscale wave breaking are identified. A major achievement is the development of a simulation-based method for estimating water-side gas transfer velocities. By modeling one-dimensional mass transport using the Small Eddy Model and comparing simulated fluorescence time series to experimental data, a robust determination of turbulence profiles and gas transfer velocities with uncertainties between 5% and 20% is established. Additionally, an algorithm is developed to detect and track microscalewave breaking events, combining structure tensor-based Oriented FAST and Rotated BRIEF detection, cluster analysis, and optical flow techniques. Based on the detected microscale wave breaking statistics, a first quantitative estimate suggests that microscale wave breaking contributes approximately 35% to 80% of the total water-side gas transfer velocity within a friction velocity range of 0.3 cm/s to 0.8 cm/s.