Robust Sensor for Extended Autonomous Measurements of Surface Ocean Dissolved Inorganic Carbon

Ocean carbon monitoring efforts have increased dramatically in the past few decades in response to the need for better marine carbon cycle characterization. Autonomous pH and carbon dioxide (CO<sub>2</sub>) sensors capable of yearlong deployments are now commercially available; however, due to their strong covariance, this is the least desirable pair of carbonate system parameters to measure for high-quality, in situ, carbon-cycle studies. To expand the number of tools available for autonomous carbonate system observations, we have developed a robust surface ocean dissolved inorganic carbon (DIC) sensor capable of extended (>year) field deployments with a laboratory determined uncertainty of ±5 μmol kg<sup>–1</sup>. Results from the first two field tests of this prototype sensor indicate that measurements of DIC are ∼90% more accurate than estimates of DIC calculated from contemporaneous and collocated measurements of pH and CO<sub>2</sub>. The improved accuracy from directly measuring DIC gives rise to new opportunities for quantitative, autonomous carbon-cycle studies.