A New Optical Bubble Measuring Device; A Simple Model for Bubble Contribution to Gas Exchange

Abstract

We describe the set up and performance of a new bubble gauge, based on light scattering by bubbles. Bubble spectra obtained simultaneously with He- and Rn- gas exchange rates in the large wind/wave facility of the I.M.S.T., Marseille, show lower bubble densities at the same wind speed than in the wind/wave facility, Hamburg University, but the similar radius dependencies (r⁻³) and bubble penetration depths. Comparison of He- and Rn- gas exchange rates show no significant influence of bubbles in our experiments. A simple model is developed, showing, that all bubbles equally contribute to gas exchange, down to a certain cut off radius, mainly dependent on the solubility of the tracer. He is transferred 5 times better by bubbles than CO₂, where half of the difference is caused by the lower solubility, the other half by the higher diffusion coefficient. The model agrees with our data, where no bubble effect was found, and with the Hamburg results, where a considerable contribution for oxygen is obtained.