An inter-comparison of autonomous in situ instruments for ocean CO2 measurements under laboratory-controlled conditions


  • A suite of autonomous CO2 instruments was systematically tested in a laboratory-controlled tank.
  • A unique inter-comparison study of autonomous pH, pCO2 and alkalinity instruments.
  • Internal consistency of CO2 system was evaluated using sensor measurements.


Observations of the marine CO2 system are important for understanding ocean acidification, air-sea CO2 fluxes and the marine carbon cycle in general. A variety of autonomous in situ instruments have been developed and used towards this end. There is, however, a lack of inter-comparison studies of currently available sensor technologies. In this study, a total of 10 instruments including commercially available pH, partial pressure of CO2 (pCO2), and total alkalinity (AT) sensors were tested and compared in a 5000 L seawater tank located at Scripps Institution of Oceanography (SIO), California, USA. The test took place over ~12 days (August 16 to 28, 2016) where conditions in the tank were artificially varied to encompass a wide range of AT, pH and pCO2 as well as temperature and salinity. To assess accuracy, independent measurements of pH, pCO2dissolved inorganic carbon (DIC) and AT were made using benchtop instrumentation. We also evaluated internal consistency, comparing the measured parameter from the sensors with the calculated parameter, e.g. pCO2 calculated from AT and pH compared with directly measured pCO2. DIC can be precisely derived (within ±5 μmol/kg) over a wide range of conditions from measured pH or pCO2 paired with AT. Sensor accuracy established by discrete samples is sufficient for short-term and seasonal dynamical studies, but their ability in determining long-term (e.g. climate) variability could not be evaluated because of the brevity of the study. These results provide insights into sensor performance and strategies for data quality control for future studies of ocean acidification and carbon cycling.

Shangguan Q., Prody A., Wirth T. S., Briggs E. M., Martz T. R. & DeGrandpre M. D., 2022. An inter-comparison of autonomous in situ instruments for ocean CO2 measurements under laboratory-controlled conditions. Marine Chemistry 240: 104085. doi: 10.1016/j.marchem.2022.104085. Article.

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