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

Abstract

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.

Observing Changes in Ocean Carbonate Chemistry: Our Autonomous Future

“…For example, a careful bottom-up uncertainty analysis yielded an estimate of ± 2.86% uncertainty in the SOCCOM float-based pCO 2 estimates, equating to approximately ± 11.4 μatm at 400 μatm pCO 2 [98]. This represents the absolute accuracy of a single float, but the ability of pH-derived pCO 2 to track spatiotemporal patterns over, e.g., a seasonal cycle is significantly better and is estimated to be ± 5.5 μatm [140]. However, the seasonal to multi-annual long-term stability of surface pCO 2 estimates has not been characterized.…”

Autonomous in situ calibration of ion‐sensitive field effect transistor pH sensors

“…Following Johnson et al (2016) and Takeshita et al (2020 a ), we transition from the nomenclature above and that which was previously published by this group (Martz et al 2010; Bresnahan et al 2014; Takeshita et al 2014, 2018) to engineering nomenclature which facilitates utilization of a temperature‐independent calibration value named k 0 . E* is thus redefined here as k 0 + k 2 × T C , where T C is temperature in °C, and k 2 is ∂E*/∂T , the calibration coefficient's temperature sensitivity.…”

Technical Note: Stability of tris pH buffer in artificial seawater stored in bags

“…It is atypical for other pH sensors, including coastal moored sensors, to have an automated or remote validation. Therefore, on such deployments, validation has largely relied on discrete samples taken alongside the sensor (Bresnahan et al, 2014;McLaughlin et al, 2017;Takeshita et al, 2018), which presents unique challenges; primarily that spatiotemporal discrepancy can lead to errors of > 0.1, especially in highly dynamic systems (Bresnahan et al, 2014).…”