Collaborative Research: Improving the accuracy and uncertainty associated with estimated pCO2 from pH sensors on autonomous profiling platforms

Understanding how much anthropogenic carbon dioxide the ocean is taking up from the atmosphere - and how this uptake changes over time - is critical for understanding the habitability of our planet. Uncertainties in the size of the ocean carbon sink remain large, mainly because of sparse measurements of surface water carbon dioxide (pCO2) measurements from ships.

Profiling floats equipped with sensors for biogeochemical (BGC) measurements such as pH – which can be used to estimate pCO2 – have the potential to greatly improve the amount of surface carbon dioxide observations across the world’s ocean, particularly in regions and times of year where shipboard measurements are scarce. The number of BGC floats deployed worldwide is expected to rapidly increase in the coming years through programs like the Global Ocean Biogeochemistry Array (GO-BGC) and the Southern Ocean Carbon and Climate Observations and Modeling program (SOCCOM2).

However, there are still fundamental issues that need to be resolved before we can take full advantage of this emerging, powerful global observational network to improve our estimates of how much anthropogenic carbon the ocean absorbs every year. The overarching aim of this project is to improve the accuracy and precision of pCO2 estimates that are derived from profiling float data. The project will support a postdoctoral researcher and three undergraduate summer interns.

This project seeks to improve the accuracy and uncertainty associated with float-based estimates of pCO2 by addressing the outstanding questions and issues that could lead to systematic biases in these calculations. Specifically, the investigators aim to answer the following questions: 1) What causes the pressure-dependent discrepancy between bottle and float pH? 2) What is the source of the internal inconsistency of the thermodynamic marine inorganic carbon model, which affects the accuracy of pCO2 computed from pH measurements? 3) What are the global spatiotemporal patterns of acidification rate at 1500 m, and how do we model the patterns accurately?

The team will conduct a series of laboratory studies to quantify key thermodynamic constants, test the hypothesis that organic compounds are a substantial contributor for the internal inconsistency, and determine a robust protocol to accurately calculate pCO2 from pH. Furthermore, an open-source, global algorithm will be made available that accurately predicts region-specific anthropogenic carbon estimates, which is necessary to accurately correct float pH sensor drift globally.

Finally, to assess the improvement in accuracy of float based pCO2 estimates based on these efforts, the team will conduct a series of field validation experiments using Spray underwater gliders equipped with the same pH sensor as profiling floats, and a Wave Glider, an autonomous surface vehicle that is equipped with a pCO2 analyzer calibrated with gas standards with an accuracy of plus or minus two microatmospheres. Over the course of the project, the team will test 16 different pH sensors to better constrain the potential systematic biases for the float based pCO2 estimates.

The activities outlined here will lead to more accurate estimates of pCO2 from floats, as well as better constrained uncertainties, and ultimately lead to better estimates for air-sea CO2 flux from the global network of BGC profiling floats.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.