The ocean plays a central role in regulating atmospheric carbon dioxide (CO₂), but resolving fine-scale variations in surface-water pCO₂ and air–sea CO₂ fluxes remains technically challenging. In the framework of Yongyong Hu’s PhD, the DFG group, in collaboration with Patrick Augustin and Marc Fourmentin, developed a compact and highly sensitive optical sensing platform for real-time measurements of dissolved CO₂ and air–sea CO₂ exchange at high temporal and spatial resolution.
Supported by the CNRS-INSU research vessel Sepia II programme, ship-borne campaigns were conducted in the English Channel during summer and winter 2025 under real marine conditions. The measurements revealed clear nearshore-to-offshore gradients in dissolved pCO₂ and a pronounced seasonal increase, in good agreement with SOCAT observations for 2020–2025. Combining pCO₂ measurements with wind-speed observations also enabled the estimation of air–sea CO₂ fluxes. During winter, nearshore waters acted as a net CO₂ source, with representative fluxes of 1.12 and 0.21 mmol m⁻² day⁻¹ at two monitoring sites.
