Modelling of gas flux through bubbles at the air-water interface

A theoretical approach to gas transfer by bubbles created by breaking waves at the air-water interface is undertaken. Based on a simple model, this study affords a basic understanding of the physical mechanisms. The behaviour of a single bubble is examined. Similarities and differences between the formulation of gas fluxes across a “flat” air-water interface and through bubbles are shown. The gas flux through bubbles is not strictly proportional to the solubility and to the difference of concentrations between the bulk of the water and the surface. Nevertheless, for trace gases, a linear relationship with this difference of concentrations, increasing with solubility is predicted. Over pressure in the bubbles and dissolution favour invasion into the water and imply a water supersaturation at equilibrium. This supersaturation is more important when the gas solubility is low. The rôle of surfactants is studied and it is found that they can reduce the gas exchange by a factor of 5. The consequences of the model for the transfer velocity are briefly presented. A better definition of the bubble lifetime, i.e., a better representation of water dynamics under a breaking wave, is necessary to make more reliable model predictions, mostimportantly for gases with low solubility. Theoretical studies on the bubble source are necessaryin order to avoid, as far as possible, the use of the bubble distribution.