Aqueous CO 2 gradients for air–sea flux estimates

Abstract

The concentration of gaseous carbon dioxide (CO 2) in surface seawater is a fundamental control on the CO 2 flux between the ocean and atmosphere. However, the concentration gradient in the aqueous mass boundary layer determines the magnitude and direction of the flux. The gradients of CO 2 in the aqueous mass boundary layer cannot be measured directly and are usually inferred from partial pressures or fugacities of CO 2 ( f CO 2 ) in the air and water. In addition to the f CO 2 , the temperatures at the top and bottom of the aqueous mass boundary layer must be known to determine the thermodynamic driving force of CO 2 gas transfer. Expressing the gradient in terms of the aqueous CO 2 concentration, [CO 2aq], also avoids some conceptual ambiguities. In particular, expressing the CO 2 as a fugacity, which is defined relative to the gas phase, when the gas exchange rate is controlled in the aqueous mass boundary layer often leads to errors in interpretation with respect to changes in boundary layer temperature. As a result the enhanced CO 2 flux caused by the cool skin effect appears to be overestimated. Apart from the difficulties estimating the temperature at the top and bottom of the aqueous mass boundary layer, the temperature dependence of solubility and fugacity of CO 2 is uncertain to the degree that it can bias air–sea CO 2 flux estimates. The CO 2aq at the surface, [CO 2aq0], is at equilibrium with the atmospheric CO 2 level. As [CO 2aq0] is strongly temperature dependent, it will be significantly higher at high latitude compared to low latitude, while atmospheric CO 2 levels show much less of a gradient.