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

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...