Field measurements of air-sea CO2 exchange

The direction and magnitude of air‐sea exchanges depend on the difference of gas partial pressure (ΔP) between phases and on a gas exchange coefficient (K). In this work, field air‐sea CO2 exchange and ΔPco2 have been measured simultaneously to study the in situ relations between parameters. Experiments have been carried out at three sites: the Bay of Calvi (Corsica), the Ligurian Sea, and the North Sea. Flux measurements were made with a direct chamber method to detect invasion or evasion of gas. Due to its bed of seagrass (P(Posidonia), the Bay of Calvi is the site of regular, daily, and yearly variations of surface CO2 content. Daily variations are often too small to overcome the variation of K and to find a correlation between hourly measured fluxes and daily changes of ΔP. The yearly variation of Pco2 in surface water (from 300 ppm in winter to 900 ppm in summer) has however permitted derivation of a linear relationship between mean fluxes and ΔP that fits well with fluxes measured at the two other sites under similar meteorological conditions and thus with nearly the same K. Calculated coefficients range between 10−5 and 4 × 10−5 m s−1 and so agree well with those determined with radiotracers or in wind tunnels. Measurements in the North Sea under different meteorological conditions show the influence of wind speed and sea state on exchanges measured by the in situ methodology used and allow the results to be compared with those obtained by others in wind tunnels.