Enhance Ocean Carbon Observations: Successful Implementation of a Novel Autonomous Total Alkalinity Analyzer on a Ship of Opportunity

Abstract

Over recent decades, observations based on merchant vessels (Ships of Opportunity—SOOP) equipped with sensors measuring the CO2 partial pressure (pCO2) in the surface seawater formed the backbone of the global ocean carbon observation system. However, the restriction to pCO2 measurements alone is one severe shortcoming of the current SOOP observatory. Full insight into the marine inorganic carbon system requires the measurement of at least two of the four measurable variables which are pCO2, total alkalinity (TA), dissolved inorganic carbon (DIC), and pH. One workaround is to estimate TA values based on established temperature-salinity parameterizations, but this leads to higher uncertainties and the possibility of regional and/or seasonal biases. Therefore, autonomous SOOP-based TA measurements are of great interest. Our study describes the implementation of a novel autonomous analyzer for seawater TA, the CONTROS HydroFIAⓇ TA system (-4H-JENA engineering GmbH, Germany) for unattended routine TA measurements on a SOOP line operating in the North Atlantic. We present the installation in detail and address major issues encountered with autonomous measurements using this analyzer, e.g., automated cleaning and stabilization routines, and waste handling. Another issue during long-term deployments is the provision of reference seawater in large-volume containers for quality assurance measurements and drift correction. Hence, a stable large-volume seawater storage had to be found. We tested several container types with respect to their suitability to store seawater over a time period of 30 days without significant changes in TA. Only one gas sampling bag made of polyvinylidene fluoride (PVDF) satisfied the high stability requirement. In order to prove the performance of the entire setup, we compared the autonomous TA measurements with TA from discrete samples taken during the first two trans-Atlantic crossings. Although the measurement accuracy in unattended mode (about ± 5 μmol kg^-1) slightly deteriorated compared to our previous system characterization, its overall uncertainty fulfilled requirements for autonomous TA measurements on SOOP lines. A comparison with predicted TA values based on an established and often used parameterization pointed at regional and seasonal limitations of such TA predictions. Consequently, TA observations with better coverage of spatiotemporal variability are needed, which is now possible with the method described here.

A synthesis of ocean total alkalinity and dissolved inorganic carbon measurements from 1993 to 2022: the SNAPO-CO2-v1 dataset

“…As opposed to pCO 2 , surface A T or C T observations are generally obtained from discrete sampling (measured aboard a vessel or onshore). Few cruises offer semicontinuous sea-surface A T or C T observations (e.g., , but new instrumental developments (Seelmann et al, 2020) have now enabled A T measurements on Ship of Opportunity Program (SOOP) lines. In addition to discrete samples analyzed for various projects conducted mainly in the North Atlantic, tropical Atlantic, tropical Pacific, Mediterranean Sea and coastal regions (Table 1), we complemented this synthesis with A T and C T surface observations obtained in the Indian and Southern oceans during the OISO (Océan Indien Service d'Observation) cruises in 1998-2018 (Metzl et al, 2006;Leseurre et al, 2022; data also available from NCEI OCADS: https://www.nodc.noaa.gov/ocads/oceans/VOS_Program/, last access: 22 December 2023) and the recent CLIM-EPARSES cruise conducted in the Mozambique Channel in April 2019 (Lo Monaco et al, 2020.…”

Fabrication and Performance of a Ta2O5 Thin Film pH Sensor Manufactured Using MEMS Processes

“…The continuous dissolution of CO 2 in the atmosphere causes the concentration of hydrogen ions (H + ) in the ocean to eventually increase and the concentration of carbonate (CO 2− 3 ) in the seawater to decrease. Consequently, the pH of seawater ( pH = − lg [H + ) decreases [3][4][5]. This process is called "ocean acidification".…”