Technical note: Enhancement of float-pH data quality control methods: A study case in the Subpolar Northwestern Atlantic region

Abstract. Since a pH sensor has become available that is suitable for this demanding autonomous measurement platform, the marine CO₂ system can be observed independently and continuously by BGC-Argo floats. This opens the possibility to detect variability and long-term changes in interior ocean inorganic carbon storage and quantify the ocean sink for atmospheric CO₂. In combination with a second parameter of the marine CO₂ system, pH can be a useful tool to derive the surface ocean CO₂ partial pressure (pCO₂).

The large spatiotemporal variability of the marine CO₂ system requires sustained observations to decipher trends and punctual events (e.g., river discharge, phytoplankton bloom) but also puts a high emphasis on the quality control of float-based pH measurements. In consequence, as the interpretation of changes depends on accurate data, and because sensor offsets or drifts might appear, a consistent and rigorous correction procedure to process and quality-control the data has been established. By applying standardized routines of the Ago data management to pH measurements from a pH/O₂ float pilot array in the subpolar North Atlantic Ocean, we investigate the uncertainties and lack of objective criteria associated with the standardized routines, notably the choice of the reference method for the pH correction (CANYON-B or LIRPH) as well the reference depth for this correction. For the studied float array, significant differences of ca. 0.02 pH units are observed between the two reference methods which can be used to correct float-pH data from water samples. Through comparison against discrete pH data from water samples, an assessment of the adjusted float-pH data quality is presented. The results point out noticeable discrepancies near the surface of > 0.01 pH units. In the context of converting surface ocean pH measurements into pCO₂ data for the purpose to derive air-sea CO₂ fluxes, we conclude that the minimum accuracy requirement of 0.01 pH units (equivalent to the minimum pCO₂ accuracy of 10 µatm for potential future inclusion into the SOCAT database) is not systematically achieved in the upper ocean.

While the limited dataset and regional focus of our study provides only one showcase, it still calls for an additional independent pH reference in the surface ocean. We therefore propose a way forward to enhance the float-pH quality control procedure. In our analysis, the current philosophy of pH data correction against climatological reference data at one single depth in the deep ocean appears insufficient to assure adequate data quality in the surface ocean. Ideally, an additional reference point should be taken at or near the surface where the resulting pCO₂ data are of the highest importance to monitor the air-sea exchange of CO₂ and would have the potential to very significantly augment the impact of the current observation network.