Department or Program
Primary Wellesley Thesis Advisor
Rachel H. R. Stanley
Gas exchange at high wind speed is not well understood—few studies have been conducted at wind speeds above 15 m s-1, and significant disagreement exists between gas exchange models at high wind speeds. In particular, the flux due to bubbles is not explicitly included in many gas exchange models, despite the fact that bubble-mediated gas exchange becomes increasingly important at higher wind speeds. The goal of my thesis project is to quantify air-sea gas exchange under high wind speeds and to examine the relationship between noble gas measurements, bubble spectra, wave-type, and water temperature. Noble gases serve as excellent tracers for this purpose, as they are biologically and chemically inert, and have a wide range of solubility and diffusivity that responds differently to physical forcing. Over the course of five days, we conducted 35 experiments at the SUrge STructure Atmospheric InteractioN (SUSTAIN) wind-wave tank with wind speeds at 20 - 50 m s-1, water temperatures at 20°C, 26°C, and 32°C, and wave conditions including uniform (regularly breaking) waves and JONSWAP (random, real ocean-like) waves. Continuous Ne, Ar, Kr, and Xe ratio measurements were obtained by a Gas Equilibration Mass Spectrometer (GEMS). Additionally, discrete noble gas measurements were collected at the beginning of select experiments and at the end of all experiments for He, Ne, Ar, Kr, and Xe. Bubble size and volume spectra were obtained using an underwater shadowgraph imaging device. Other physical measurements such as continuous salinity, water temperature, wind/wave velocities, and atmospheric pressure were also obtained. Our result from the conditions with the highest saturation anomalies suggests that steady state saturation anomalies of gases level off as wind speed increases. Additionally, both the temperature dependence of noble gas saturation anomalies and the coherence between bubble surface area spectra and saturation anomalies suggest that partially dissolving bubbles may have an important flux contribution at higher wind speeds. Since the SUSTAIN wind-wave tank is much shallower than the real ocean, we cannot directly apply our results to the ocean to make predictions. Nonetheless, the relationship between gas flux and bubble size spectra, wind, and wave conditions learned from this work provide us with important insights to improve gas exchange models.