BUBBLES AND THE AIR-SEA EXCHANGE OF GASES IN NEAR-SATURATION CONDITIONS

The net air-sea transfer of gases is not simply proportional to the surface gradient of gas concentration as is widely assumed but, when proper account of bubbles is taken, is asymmetrical with a net invasion of gas at 100% saturation. The nitrogen and oxygen saturation levels and the variation of the composition of a bubble during its lifetime affect the transfer of all gases. The net transfer of gases is not a function of the size and depth of the bubbles and the concentration of dissolved gases alone, but is dependent on the composition of the bubbles and thus the time history of each bubble. A numerical Monte-Carlo model incorporating the basic equations of bubble dynamics and a parameterization of upper ocean mixing is constructed. The development of the distribution and composition of bubbles, after the initial injection of air bubbles by breaking waves, in the upper ocean layer is modelled, and the transfer of nitrogen, oxygen, carbon dioxide and argon thereby determined. The variation of the flux of these gases with the supersaturation of each gas and with the strength of Langmuir circulation is investigated. An extension of the model elucidates the importance of bubbles at various wind speeds for each of the four gases. Bubble-mediated gas transfer will support supersaturations of typically 1-2% for nitrogen, oxygen, argon and other very poorly soluble gases, but will not support a globally significant supersaturation of carbon dioxide.