Development of Fast Response In-situ Sensors for Simultaneous Measurements of Seawater Carbon Dioxide Parameters

The marine CO₂ (inorganic carbon) system is characterized by four primary parameters – total dissolved inorganic carbon (DIC), total alkalinity (TA), partial pressure of CO₂ (pCO₂), and pH. These parameters are central to the study of the marine carbon cycle and ocean acidification. Simultaneous measurements of two of the four CO₂ parameters are required to fully resolve the seawater CO₂ system, and DIC is one of the preferred parameters. A self-calibrating, in-situ sensor, Channelized Optical System (CHANOS), has recently been developed to provide simultaneous measurements of both DIC and pH, resolving carbonate chemistry with a single system. CHANOS is among the first to achieve simultaneous, in-situ measurements of a desired pair of CO₂ parameters. DIC and pH channels both use flow-through, spectrophotometric methods to detect relative absorbances of the acid and base forms of a pH-sensitive indicator. The precision of CHANOS in laboratory and in-situ tests are ±0.002 and ±3.0 µmol kg^-1 for pH and DIC, respectively. In-situ comparison with bottle sampling and analyses indicate that the accuracies for pH and DIC are ~±0.004 and ~±5.0 µmol kg^-1, respectively. It has been demonstrated that CHANOS can make in-situ, climatology-quality measurements to resolve the CO₂ system in dynamic aquatic environments.

To further improve response time of the sensor, especially for DIC measurements, a new generation of CHANOS-DIC is under development. The new system adapts the recently developed spectrophotometric DIC method to achieve flow-through CO₂ equilibration between an acidified sample and an indicator solution with a response time as fast as ~22s. Continuous measurements are also achievable. Because of the fast response of CHANOS measurements, it is versatile and suitable for deployments on both fixed (e.g. buoys) and mobile (e.g., AUV, ROV, and profilers) platforms.