Evaluating transfer velocity-wind speed relationship using a long-term series of direct eddy correlation CO2 flux measurements

Long-term observations of the marine atmospheric boundary layer were performed by an eddy correlation system, which was set-up on a platform in the Baltic Sea. In this experiment the three-dimensional wind vector and the turbulent fluxes of momentum, sensible and latent heat and CO, were measured for one and a half years. Simultaneously the CO2 partial pressure pCO(2) in surface water was measured by a submersible autonomous moored instrument for CO2 at the platform in 7-m depth. The high-resolution eddy correlation measurements of the atmospheric CO2 flux F-CO, together with the measurements of the CO, partial pressure differences between air and sea Delta p(CO2) led to a long-term data set which provided the possibility to investigate the parameterization of the CO2 transfer velocity k as a function of 10-m wind speed u in a statistical manner. From half-hour mean CO2 fluxes and CO2 partial pressure differences, k was calculated using k=FCO2/(K-0 Delta p(CO2)), with K-0 the CO2 solubility. The half-hour mean data points, used for the determination of the k-u parameterization, show large scatter. However, assuming a linear, quadratic dependency the analysis yields: k(660)=0.365u(2) +0.46u (k at 20 degrees C and salinity 35 psu) with a correlation coefficient of r(2)=0.81. The large scatter indicates that the kinetics of the air-sea CO2 transfer velocity is not only a function of the wind speed alone, but might also be controlled by other environmental parameters and mechanisms, such as sea state and surface coverage with surfactants. (c) 2006 Elsevier B.V All rights reserved.