An instrument intercomparison exercise in the Skagerrak allows extending the FerryBox pCO2 observational coverage across the Central and Southern North Sea

Abstract

The partial pressure of carbon dioxide (pCO2) in surface seawater is an important biogeochemical variable, influencing the direction of air–sea carbon dioxide exchange. Large-scale observations of pCO2 are facilitated by Ships-of-Opportunity (SOOP-CO2) 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-CO2 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 pCO2 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-CO2 methods. Validating membrane-based sensors and using the expanded coverage and higher frequency measurements they provide can enable a thorough characterization of pCO2 variability in both open oceans and dynamic coastal seas.