At‐sea intercomparison of a membrane‐based pCO₂ sensor and a traditional showerhead equilibrator system on a Ship‐of‐Opportunity

The seawater partial pressure of carbon dioxide ( p CO 2 ) is an essential ocean variable needed to calculate air‐sea gas exchange and to identify marine carbon sinks and sources. Recent technological developments support autonomous p CO 2 measurements with sensors that are smaller and cheaper. In July 2021, these differences were highlighted during the Integrated Carbon Observation System—Ocean Thematic Centre laboratory intercomparison exercise. A key message from the intercomparison was the need for further field comparisons. Here we present the results from a field test of two generations of ‐4H‐Jena HydroC CO2‐FT membrane‐based sensors alongside a General Oceanics equilibrator system. The intercomparisons were done onboard a ship‐of‐opportunity regularly traveling between Europe and South America. The first phase of the experiment took place in 2021, when the difference between the two instruments was within ± 10 μ atm during 53% of the intercomparison time. For the second phase, improvements were made, including the addition of an automated cleaning routine for the membrane‐based sensor, the installation of a new sensor prototype with the ability to measure a reference gas, and an updated data processing method. These changes improved the performance and, during the last 2023 journey, the mean difference decreased to 2.0 ± 5.0 μ atm, and was within ± 10 μ atm during 97% of the deployment time. This experiment revealed that with a suitable deployment approach considering biofouling and reference gas measurements, membrane‐based sensors can measure seawater p CO 2 within the Global Ocean Acidification Observing Network weather goal of 2.5% relative uncertainty on autonomous installations.