The ICOS OTC pCO2 instrument intercomparison Limnology and Oceanography: Methods

In 2021, the Ocean Thematic Centre of the European Research Infrastructure “Integrated Carbon Observation System” conducted an international partial pressure of carbon dioxide (pCO2) instrument intercomparison. The goal was to understand how different types of instrumentation for the measurement of ocean pCO2 compare to each other. During the two-week long experiment, we installed various instruments in a tank facility using natural sea water (North Sea). These included direct air–water equilibration systems and membrane-based flow-through instruments along with submersible sensors and instruments that are normally installed on buoys and autonomous surface vehicles. In situ instruments were installed inside the tank and the flow-through instruments were fed the same water using a pumping system. We changed the temperature (between 10°C and 28°C) and the seawater pCO2 (between 250 and 800 μatm) to observe instrument responses over a wide range. Since there is no reference for surface ocean pCO2 measurements, we agreed on a set of instruments serving as intercomparison reference. All data from the different instruments were then compared against the intercomparison reference during periods of stable temperature and pCO2. The study provides important information to enhance future ocean carbon monitoring networks, but makes no direct recommendation for the use of any specific sensor. A major finding is that equilibration through direct air–water contact appears to be more consistent and independent of external factors than equilibration through a membrane or photometric detection. We found several instruments with no temperature measurements at the location of equilibration or with uncalibrated temperature sensors introducing significant uncertainty in the results.