Mechanisms of air-sea CO₂ 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 (U_10N). 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 U_10N-based parameterization. These findings highlight the need to move beyond U_10N in the parameterization of K and estimation of regional fluxes, especially when evaluating the potential of marine carbon dioxide removal (mCDR) in coastal seas.