Air’sea fluxes and transfer velocity of CO2 over the North Sea: results from ASGAMAGE

Abstract

The paper presents an analysis, performed at the Royal Netherlands Meteorological Institute (KNMI), of data obtained at a research platform 9 km off the Dutch coast within the framework of the air’sea gas exchange program ASGAMAGE†. The air’sea transfer velocity of CO2 was determined directly, that is, by observing CO2 fluxes and air’sea concentration differences simultaneously. CO2 fluxes were determined by means of the eddy correlation technique. Special care was taken to avoid the effects water vapour on the CO2 flux measurements. The air and water near the air’sea interface were treated as well-mixed with respect to CO2. The combination of flux and concentration data allowed the computation of the transfer velocity for CO2 without recourse to other gases. Results for two observation periods, one with downward CO2fluxes (May) and one with upward CO2 fluxes (October), are consistent. A relation with UN,10, the wind speed adjusted to a height of 10 m and neutral stratification, was determined for the pooled data from the two experimental phases. The relation found was: k660 = 0.54U2N,102 cm h-1, with k660 the CO2 transfer velocity normalized to salt water (35‰) at a temperature of 20 °C, and UN,10 in m s-1. The 95% confidence interval of the coefficient extends from 0.46 to 0.63. No relations with other geophysical parameters could be found from the present data set.ASGAMAGE is a contraction of ASGASEX (for Air Sea Gas Exchange), an earlier experiment in this field, and MAGE, the Marine Aerosol and Gas Exchange activity of the International Global Atmospheric Chemistry program (IGAC).