Chemical enhancement of the CO 2 gas exchange at a smooth seawater surface

Abstract

Tank experiments were conducted to quantify the chemical enhancement of the CO 2 gas exchange and its dependency on turbulence and pH. Positive and negative CO 2 partial pressure differences were generated between the headspace and the seawater sample. The gas exchange transfer velocities ( k) were determined by recording the CO 2 concentrations in the headspace using an IR CO 2 analyser. Acidified seawater samples were used to determine non-enhanced transfer velocities and provided the reference to calculate enhancement factors, the ratio of the enhanced k and the non-enhanced k. Different surface turbulence was generated by circulating the headspace at different flow rates. Chemical enhancement was only observed at low turbulence, which corresponded to k values of 2.5–4 cm h −1. Different enhancement factors α were observed during CO 2 invasion ( α=1.34) and CO 2 evasion ( α=1.08). These differences were attributed to different pH (8.6 and 7.9, respectively) originating from different CO 2 partial pressures in water during uptake and release experiments. An exponential fit was used to describe α as a function of pH: α=1.6×10 −8e 1.98pH+1 ( k<4 cm h −1). The resulting enhancement factors, which are only partly consistent with previous experimental and model studies, indicate that chemical enhancement may be important for regional and temporal CO 2 budgets whereas the effect on the global CO 2 balance is negligible.