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Carbon is essential to the Earth’s system functioning, playing a major role in physical and biogeochemical processes in the atmosphere, the terrestrial biosphere, and the oceans. The concentration of carbon-based greenhouse gases in the atmosphere, such as carbon dioxide (CO₂) and methane (CH₄), has been increasing since the industrial era. Therefore, assessing the redistribution of these greenhouse gases between the Earth’s reservoirs has become essential for understanding the current climate system and modelling future climate scenarios.

The oceans are a component of the global carbon cycle, and their role as sinks and sources of greenhouse gases has significant implications for the Earth’s climate. The gas exchange between the atmosphere and the ocean is driven by the concentration difference in these two reservoirs. However, the turbulent processes in the layers adjacent to the ocean surface control the efficiency of the transport.

This thesis investigates mechanisms controlling the air–sea gas exchange using direct measurements of CO₂ and CH₄ fluxes from the Östergarnsholm station in the Central Baltic Sea. The gas exchange of both gases is found to have a strong variability at time scales from sub-hourly to inter-annual. The region is found to be a net source of CH₄, with both the concentration gradient and wind as controlling mechanisms. In the case of the CO₂ fluxes, the variability is strongly modulated by local processes such as sea spray and water-side convection, as well as precipitation. Interestingly, an asymmetric effect is observed, with these processes enhancing the upward transport of CO₂ but not the downward flux. Furthermore, a model-based sensitivity analysis of the gas transfer velocity is performed to evaluate the effect of the forcing mechanisms on the air-sea gas exchange at a regional scale. The results show that water-side convection, precipitation, and surfactants strongly modulate the spatio-temporal variability of the CO₂ fluxes in the Baltic Sea.