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Abstract 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 oceanpCO₂compare 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 seawaterpCO₂(between 250 and 800μatm) to observe instrument responses over a wide range. Since there is no reference for surface oceanpCO₂measurements, 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 andpCO₂. 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. 

Abstract. The science guiding the EUREC4A campaign and its measurements is presented. EUREC4A comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic – eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC4A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air–sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC4A explored – from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview of EUREC4A's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at https://doi.org/10.25326/165 (Stevens, 2021), and a film documenting the campaign is provided as a video supplement.