AbstractThe 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.