An instrument intercomparison exercise in the Skagerrak allows extending the FerryBox pCO 2 observational coverage across the Central and Southern North Sea

International audience; The partial pressure of carbon dioxide (pCO 2) in surface seawater is an important biogeochemical variable, influencing the direction of air-sea carbon dioxide exchange. Large-scale observations of pCO 2 are facilitated by Ships-of-Opportunity (SOOP-CO 2) equipped with underway measuring instruments that are becoming more autonomous. Here we performed a comparison between a FerryBox-integrated membrane-based sensor and a showerhead equilibration sensor installed on two SOOP-CO 2 between 2013 and 2018. We identified time-and space-adequate crossovers in the Skagerrak Strait, where the two ship routes often crossed. We found a mean total difference of 1.5 ± 10.6 μatm and a root mean square error of 11 μatm. The pCO 2 values recorded by the two instruments showed a strong linear correlation with a coefficient of 0.91 and a slope of 1.07 (± 0.14), despite the dynamic nature of the environment and the difficulty of comparing measurements from two different vessels. We showed the strength of having a sensor-based network with a high spatial coverage that can be cross-checked against conventional SOOP-CO 2 methods. Validating membrane-based sensors and using the expanded coverage and higher frequency measurements they provide can enable a thorough characterization of pCO 2 variability in both open oceans and dynamic coastal seas.