Phase‐Doppler anemometer measurement of bubble concentrations in laboratory‐simulated breaking waves

Abstract

Breaking waves and the bubble plumes they generate are thought to be an important air‐sea gas transfer mechanism. In order to compare bubble populations generated by laboratory‐simulated breaking waves and model bubble‐mediated airwater gas transfer in a whitecap simulation tank (WST), it was necessary to measure bubble sizes and water velocities. The phase‐Doppler anemometer (PDA) is well suited for this research because it provides simultaneous measurement of the size and velocity of a bubble. Bubble concentrations can be estimated from PDA data records with an accuracy of 15% (±1σ) for bubbles with radii in the range of 50 to 160 μm and 40% (±1σ) for larger bubbles. However, estimation of bubble concentrations requires knowledge of the instrument‐scattering cross section as a function of bubble radius. A procedure for calibrating the scattering cross section of the PDA was developed and tested. The PDA was then used to measured size‐segregated bubble concentrations in the WST as a function of depth, water temperature, total concentration of dissolved gas, and surfactant concentration. These measurements show that increases in dissolved gas concentration or decreases in water temperature increase bubble concentrations. Surfactants increased the concentration of small bubbles. Comparison of the WST bubble populations to measurements of oceanic bubbles showed that the two agree within the experimental uncertainty of the PDA.