NSFGEO-NERC: Seawater pH: Bridging the Gap Between Free and Total pH Scales From Estuaries to the Deep Ocean, and Application to Current Sensors

About This Grant

The goal of this project is to improve the ability of oceanographers to study the marine carbon cycle. This collaboration between institutions in the U.S. and the United Kingdom (UK) combines state of the art modelling and new laboratory measurements of pressure effects on solutions, with expertise in sensor development and ocean pH measurement. The project will make key advances towards (i) quantifying changes in the global carbon cycle from the surface to the interior ocean, (ii) supporting the development of new sensors, and (iii) enabling better integration of autonomous pH measurements into models of the ocean carbon cycle. Beneficiaries will be those of the global carbon observing community requiring accurate calibrations for a wider range of salinities and to deep ocean pressures, and scientists using pH measured on both scales to assemble global datasets, model biogeochemical processes, and ultimately improve decision making. This is a project jointly funded by the U.S. National Science Foundation’s Directorate for Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the UK via the NSF/GEO-NERC Lead Agency Opportunity. This Lead Agency Opportunity allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award recommendation, each Agency funds the proportion of the budget that supports scientists at institutions in their respective countries. Traditionally, pH measurements were made from research vessels and reported on the total pH scale. Recent technological advancements have enabled an array of autonomous robotic floats and gliders to measure pH, which vastly increases spatiotemporal coverage of pH measurements. However, most of these autonomous pH measurements are made on the free pH scale. Combining these datasets requires conversion between the total and free pH scales, but current knowledge gaps lead to high and poorly-constrained uncertainties. This project aims to address this knowledge gap and provide a model that will allow conversion between the total and free pH scales with well-defined uncertainties. This will be accomplished by developing a thermodynamic model of the solutions used to calibrate the total pH scale. It builds on a previous thermodynamic model, but extends its conditions to full ocean depth, as well as to low salinity, estuarine environments. The model development leverages a new thermodynamic dataset collected over the last five years. The model results will be validated in a controlled laboratory setting, as well as on a research cruise where spectrophotometric pH measurements made on the total scale will be compared to sensor pH measurements made on the free pH scale. 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.