The ocean’s pH is falling by about 0.017 per decade near the surface, but the two main ways scientists measure it give different numbers—and no one has yet reconciled them. This mismatch matters because ocean acidification alters the balance of CO₂, bicarbonate, and carbonate ions that marine life depends on, and it complicates efforts to monitor the chemistry of the whole water column. One scale (total pH) is used for high-precision shipboard measurements; the other (free pH) comes from thousands of autonomous sensors on floats and gliders. Converting between them is unreliable, especially at depth and in low-salinity estuaries, so datasets cannot be combined into a consistent global picture. The researchers will build a chemical model that calculates both pH scales in calibration buffers for any salinity, temperature, and pressure down to the deep ocean. They will test it with laboratory measurements and a real-world deployment of both sensor types at sea. If successful, the model will allow oceanographers to merge decades of shipboard pH data with modern float records, producing the high-resolution global pH inventories needed to track acidification and assess geoengineering schemes that deliberately alter ocean chemistry.
View original technical description
Ocean acidification (OA) from the dissolution of excess atmospheric CO2 has potentially profound effects on the ocean food chain, its biogeochemistry, and society. Central to our understanding of OA, and efforts to respond to it, are the quantification of pH – decreasing by about 0.017 per decade near the surface – and the speciation of total dissolved carbonate (the balance between CO2, HCO3-, and CO32-) throughout the ocean. This has become especially important given current interest in geo-engineering which can involve the large-scale manipulation of ocean chemistry including pH. Two forms of pH are in common use by oceanographers: total pH (pHT), a measure of the H+ + HSO4- content in moles per kg of seawater, and free pH (pHF), a measure of H+ only. The total pH scale is used for ‘climate quality’ (±0.003 in pH) spectrophotometric measurements of open ocean seawater, mostly under surface conditions. Because there are relatively such few measurements, pHT is also estimated indirectly from measured total inorganic carbon and alkalinity. Efforts to efficiently monitor ocean pH at all depths have led to the use of in-situ solid state ISFET sensors which instead measure free pH. Deployed globally, these are becoming the principal source of pH data from the open ocean (e.g., the BGC-Argo programme). However, ISFET sensors are calibrated differently from those for pHT, and conversion between the two scales is uncertain even at surface pressures. Accuracy and self-consistency in the quantitative relationships between pHT and pHF for calibration and measurement, and with carbonate acid-base equilibria in seawater, are essential but are not yet achieved. Present and future global OA monitoring and remediation efforts urgently need consistent and interconvertible datasets between measured pHT (mainly from shipboard measurements) and pHF (from floats and other automated platforms). This requires accurately quantifying the relationship between free and total pH, in the pH buffers used for instrument calibration and in seawater, for a range of salinities, temperatures, and pressures. Extensions to low salinities and high pressures, for use with novel sensors, are also needed. We address these challenges by developing a model of chemical speciation in the pH buffer solutions used to calibrate the pHT scale to calculate directly both their total and free pH. Earlier models have already yielded insights into how the total pH scale yields H+ + HSO4- concentrations that differ from the conventional thermodynamic values essential for biogeochemical calculations, and they are recommended for further development by international standards organisations. We build on this, bringing to the project extensive new data for buffer solution thermodynamic properties, essential to the work, and make new measurements to determine pressure effects. The resulting model, intended to replace purely laboratory-based approaches, will be able to calculate both forms of pH in the buffer solutions to deep ocean pressures, with quantified uncertainties, for the calibration of pH instrumentation and conversion of measurements between the two scales. It will be tested by laboratory pHF measurements to high pressures, and then by a deployment of both ISFET pHF and miniaturised pHT sensors at sea. Applications include the measurement of seawater pH based upon accurate calibrations for a wider range of salinities (including estuarine environments) and to the deep ocean; and development of global pH inventories with the high temporal and spatial resolution needed to understand changing oceanic carbonate and OA processes.
Plain English summaries and category classifications on this site are generated by AI and may not perfectly reflect the original research.
Is something wrong? Let us know