Effects of hydration on carbon dioxide exchange across an air-water interface

The rate of dissolved C14O2 depletion from a thermally regulated water tank situated in a controlled-speed wind tunnel was monitored to determine the rate constant for interphase CO2 exchange as a function of air speed and solution pH. The results appear to support the long proposed concept of a layer of laminar (nonturbulent) flow on the surface of bodies of water exposed to wind stress. The CO2 exchange rate for a solution pH in the 6.5 region (where CO2 can react with water and hydroxyl ions to a significant extent) was found to be greater than the rate in the pH <4 region (where CO2 effectively acts as an inert gas) by an amount that closely matched theoretical expectations based on the stable-layer diffusion model. When the surface layer was thicker than 0.20 mm (which corresponded to air speeds under 4 m/sac) and the solution pH was greater than 6.0, there was evidence that a net bicarbonate-CO2 conversion, caused by the molecular CO2 diffusion gradient, took place in the layer and that this conversion was responsible for an increase in the rate of carbon dioxide exchange. Under certain conditions, this unique surface layer reactivity may allow carbon dioxide to be exchanged between the atmosphere and hydrosphere more rapidly than if it were a nonreactive gas.