Air-Sea CO2-Exchange in a Large Annular Wind-Wave Tank and the Effects of Surfactants

Abstract

Wind, chemical enhancement, phytoplankton activity, and surfactants are potential factors driving the air-sea gas exchange of carbon dioxide (CO 2 ). We investigated their effects on the gas transfer velocity of CO 2 in a large annular wind-wave tank filled with natural seawater from the North Atlantic Ocean. Experiments were run under 11 different wind speed conditions (ranging from 1.5 ms -1 to 22.8 ms -1 ), and we increased the water pCO 2 concentration twice by more than 950 atm for two of the seven experimental days. We develop a conceptual box model that incorporated the thermodynamics of the marine CO 2 system. Surfactant concentrations in the sea surface microlayer (SML) ranged from 301 to 1015 gL -1 (as Triton X-100 equivalents) with enrichments ranged from 1.0 to 5.7 in comparison to the samples from the underlying bulk water. With wind speeds up to 8.5 ms -1 , surfactants in the SML can reduce the gas transfer velocity by 54%. Wind-wave tank experiments in combination with modeling are useful tools for obtaining a better understanding of the gas transfer velocities of CO 2 across the air-sea boundary. The tank allowed for measuring the gas exchange velocity under extreme low and high wind speeds; in contrast, most previous parametrizations have fallen short because measurements of gas exchange velocities in the field are challenging, especially at low wind conditions. High variability in the CO 2 transfer velocities suggests that gas exchange is a complex process not solely controlled by wind forces, especially in low wind conditions.