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 (pCO₂) 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 pCO₂ 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 μatm 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 μatm, and was within ± 10 μatm 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 pCO₂ within the Global Ocean Acidification Observing Network weather goal of 2.5% relative uncertainty on autonomous installations.