Eddy covariance CO2 air-sea fluxes under variable surfactant conditions in the Baltic Sea

The gas transfer velocity k for the air-sea exchange of CO₂ is often parameterized as a function of wind speed alone, as wind speed fundamentally controls turbulence at the air-sea interface and thus the flux across it. However, numerous other processes affect the air-sea gas exchange, such as the presence of surface-active substances (surfactants) directly at the interface, the so-called sea surface microlayer (SML). These processes are not explicitly accounted for in the wind speed-only parameterizations. Surfactants in the SML likely reduce k, potentially due to two effects. Firstly, the surfactants represent a physicochemical barrier at the interface, and secondly, they dampen the turbulence at the interface. Consequently, the presence of surfactants leads to lower gas transfer velocities than estimated from the wind speed-only parameterizations of k, especially since the SML can be stable up to medium high wind speeds. The mechanisms that control how exactly surfactants in the SML affect the air-sea gas exchange are, however, not yet fully understood. Therefore, it is important to measure air-sea gas exchange under various surfactant conditions to potentially include the SML effects in future parameterizations of k. During a research cruise to the Gotland Basin in the early summer of 2025, the direct air-sea flux of CO₂ was measured using the eddy covariance method. This method is particularly well-suited to study the influence of surface processes on gas exchange, as it can determine k on timescales of 10 minutes and is therefore likely to resolve the variability in different surfactant states. In addition to the direct CO₂ flux measurements, a range of other parameters influencing air-sea flux were also measured. In particular, the surfactants in the SML were sampled and analysed during the research cruise. Consequently, we investigate the behaviour of k under not only varying wind speeds, but now also under various surfactant states, including the presence of a surface slick.