Chemical enhancement of CO2 exchange in natural waters

Exchange of CO2 under low turbulence conditions and high pH can be enhanced by hydration reactions of CO2 with hydroxide ions and water molecules in the boundary layer. A series of field experiments was performed on several lakes, including alkaline closed‐basin lakes, using enclosures (helmets) to study the enhancement process in nature. In addition, the enhancement of CO2 exchange was studied in laboratory experiments with freshwater and seawater. The results of the experiments are compared with published theoretical calculations. Within the experimental uncertainties and shortcomings of the chemical enhancement models, reasonable agreement was observed between experimental and theoretical results for seawater. The experiments indicate, in accordance with theory, that chemical enhancement has a minor effect on air‐sea gas exchange of CO2 under average oceanic turbulence conditions. However, for the equatorial CO2 source regions, with high temperatures and low winds, the calculated CO2 enhancement amounts to 4–8% of the total exchange. The observations on lakes show poorer agreement with models, which is attributed to experimental uncertainty and poor characterization of the chemistry of the lake waters. The experiments show that chemical enhancement of CO2 is ubiquitous for the alkaline closed‐basin lakes with enhancements of up to a factor of three.