Advancing calibration practices for total dissolved inorganic carbon measurements in seawater

Accurate and precise measurements of total dissolved inorganic carbon (DIC) in seawater are essential for evaluating key ocean carbon cycle processes such as ocean acidification, carbon storage, air-sea fluxes, and carbon dioxide removal (CDR) monitoring, reporting, and verification (MRV). Although calibration procedures for instrumentation used to measure seawater DIC are available, their adoption by many oceanographic laboratories has been limited due to perceived complexity or lack of technical support. As a result, single-point calibration with CO₂-in-seawater Reference Material (RMs) from Andrew Dickson’s laboratory at Scripps Institution of Oceanography has become a prevailing practice. While the use of these RMs has substantially improved the repeatability and reproducibility of DIC measurements, reliance on single-point calibration can significantly increase measurement uncertainty, especially for samples with DIC values far from the typical RM values of surface Pacific water (1950–2100 μmol kg^-1; https://www.ncei.noaa.gov/access/ocean-carbon-acidification-data-system/oceans/Dickson_CRM/batches.html). This practice leaves no way to assess measurement accuracy over the typical DIC measurement range (1800-2300 µmol kg^-1).

To address these challenges, an International Association for the Physical Sciences of the Oceans (IAPSO) Best Practice Study Group was established in late 2023. The study group aims to improve DIC measurement accuracy by facilitating and promoting the adoption of instrument calibration procedures. Enhancing the accuracy of DIC measurements will improve the reliability of ocean carbon cycle assessments and contribute to better-informed climate change mitigation strategies.

We will present the state-of-the-art calibration techniques for seawater DIC measurements employed in oceanographic laboratories worldwide, based on findings from a public survey conducted by our study group. We will also outline our planned activities, which include: (1) the preparation and dissemination of updated Standard Operating Procedures (SOPs) for DIC instrumentation calibration, (2) the development of practical calibration solutions and exploration of potential commercial opportunities, and (3) the evaluation of the broader impact of adopting the updated SOPs for calibration.