Mechanisms of air-sea CO ₂ exchange in the central Baltic Sea

Abstract. Air-sea gas exchange regulates the cycling of climate-relevant gases such as carbon dioxide (CO2), yet large uncertainties remain in its quantification. The gas transfer velocity (K), a key parameter for estimating CO2 flux, is usually expressed as a function of wind speed (U10N). This approach overlooks the role of fetch and surfactants, which can substantially affect K. However, no field study has systematically quantified their combined effects under fetch-limited and surfactant-abundant ocean conditions. To fill this research gap, we conducted air-sea gas exchange studies during a cruise in the central Baltic Sea, a system with high surfactant levels and a short fetch. We report independent determinations of K using eddy covariance (EC) and dual-tracer techniques, together with direct measurements of natural surfactants and modelled wave parameters. Both methods yield consistent results; however, EC-based CO2 transfer velocities are on average 33 % lower than those reported in the open ocean in previous EC studies. Sea-state-dependent parameterisations indicate that limited fetch reduces K by 8 %, while elevated surfactant concentrations may have contributed to the additional 25 % reduction. We developed an updated parameterization that includes wind stress, sea state, and surfactants. When applied to climatological forcing, it yields a 40 % stronger seasonal cycle of CO2 flux in the Baltic Sea than obtained with the conventional U10N-based parameterization. These findings highlight the need to move beyond U10N in the parameterization of K and estimation of regional fluxes, especially when evaluating the potential of marine carbon dioxide removal (mCDR) in coastal seas.