Air‐Sea Gas Exchange in the Coastal Baltic Sea: Implications for Marine Carbon Dioxide Removal

Abstract Air‐sea gas exchange affects the biogeochemical cycling of trace gases such as CO 2 and dimethyl sulfide (DMS) on a global scale, thereby influencing Earth's climate. In nearshore regions, differences in wind fetch and surfactants are expected to have an impact on gas transfer velocity ( k ). Accurate determination of air‐sea gas exchange in nearshore regions is crucial for assessing the efficacy of carbon dioxide removal (CDR) techniques, as many CDR methods are expected to be deployed in these regions. In this study, we used the 3 He/SF 6 dual tracer technique to determine k and investigate factors that control air‐sea gas exchange in a nearshore inland sea ecosystem, the coastal Baltic Sea. We found that k was, on average, about 39% lower than in other coastal and offshore regions at the same wind speed, with a more pronounced reduction at higher wind speeds and during developing wave conditions. Most of the wind speed/gas exchange parameterizations proposed for the Baltic Sea were found to overestimate k . The lower k was likely due to wind fetch limitation, wind‐wave interactions, and the presence of surfactants.