Assessment of Autonomous pH Measurements for Determining Surface Seawater Partial Pressure of CO₂

The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program currently operates >80 profiling floats equipped with pH sensors in the Southern Ocean. Theoretically, these floats have the potential to provide unique year-around estimates of pCO₂ derived from pH measurements. Here, we evaluate this approach in the field by comparing pCO₂ estimates from pH sensors to directly measured pCO₂. We first discuss data from a ship's underway system which covered a large range in temperature (2–30°C) and salinity (33.6–36.5) over 43 days. This pH sensor utilizes the same sensing technology but with different packaging than those on SOCCOM floats. The mean residual varied between −4.6 ± 4.1 and 8.6 ± 4.0 (1σ) μatm, depending on how the sensor was calibrated. However, the standard deviation of the residual, interpreted as the ability to track spatiotemporal variability, was consistently < 5 μatm and was independent of the calibration method. Second, we assessed the temporal stability of this approach by comparing pCO₂ estimated from four floats over 3 years to the Hawaii Ocean Time-series. Good agreement of −2.1 ± 10.4 (1σ) µatm was observed, with coherent seasonal cycles. These results demonstrate that pCO₂ estimates derived from profiling float pH measurements appear capable of reproducing spatiotemporal variations in surface pCO₂ measurements and should provide a powerful observational tool to complement current efforts to understand the seasonal to interannual variability of surface pCO₂ in underobserved regions of the open ocean.