Chemical enhancement of the CO2 gas exchange at a smooth seawater surface

Tank experiments were conducted to quantify the chemical enhancement of the CO2 gas exchange and its dependency on turbulence and pH. Positive and negative CO2 partial pressure differences were generated between the headspace and the seawater sample. The gas exchange transfer velocities (k) were determined by recording the CO2 concentrations in the headspace using an IR CO2 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 CO2 invasion (α=1.34) and CO2 evasion (α=1.08). These differences were attributed to different pH (8.6 and 7.9, respectively) originating from different CO2 partial pressures in water during uptake and release experiments. An exponential fit was used to describe α as a function of pH: α=1.6×10−8e1.98pH+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 CO2 budgets whereas the effect on the global CO2 balance is negligible.