Processes of gas exchange across an air-water interface

Abstract

Exchange constants (k) for the transfer of oxygen, carbon dioxide and water vapour across an air-water interface have been compared simultaneously using both a laboratory tank filled with water and a wind-water tunnel. The results indicate that the exchange of water vapour is controlled by resistance (1/k) in the air, but that transfer of oxygen or carbon dioxide is largely dependent on resistance in the liquid phase. The wind tunnel experiments demonstrate that the exchange constants for both oxygen and carbon dioxide increase approximately as the square of the wind velocity, whereas the exchange constant for water vapour increases linearly with wind velocity. Breaking of the water surface from below has considerably more effect in increasing the exchange of oxygen than it has on the transfer of water vapour. Under calm conditions and with the water pH > 5 there is a measurable enhancement of the exchange of carbon dioxide relative to oxygen. This enhancement is thought to be due to the ionic species gradient present at the liquid surface under these conditions. There is reasonable agreement between the experimentally determined carbon dioxide exchange enhancements and those predicted from theoretical relationships. The possibility of this enhancement of carbon dioxide exchange occurring at the sea surface seems unlikely, except under very calm conditions or where the rate of hydration of CO2 is considerably increased by enzyme catalysis. © 1973.