Real-Time Measurements of Ocean pH and Sensor Calibration at Depth from a Remotely Operated Vehicle
Edward T. Peltzer, Peter G. Brewer, Rachel M. Dunk, William J. Kirkwood and Peter M. Walz
Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039 USA.

    We have built the essential technology and expertise for conducting a series of small-scale in situ controlled ocean pH and CO2 enrichment experiments for biogeochemical impact studies, using ROVs to carry down, deploy, and manipulate the experimental systems. Essential for this is the ability to accurately measure ocean CO2 system properties at depth. The seawater alkalinity field below about 400 m is well constrained by ocean observations and has been mapped with considerable precision and accuracy, thus by adding in situ pH measurements the CO2 system is in principle well defined.
    In order to make these measurements, we need an accurate and precise pH sensor that is both rugged, stable over the long-term, and calibratable at depth. We have built a PC based interface that allows us to use a SeaBird (model SBE 18) pH sensor with both MBARI ROVs that allows real-time data display in the ROV control room.  While there are some operational difficulties when these sensors are used in a profiling mode due to a pressure and temperature related hysteresis traceable to effects on the liquid junction of the reference electrode, these difficulties can be overcome with the implementation of the appropriate procedures. Once at depth the electrodes typically quickly stabilize, and in the thermally stable and electrically quiet deep ocean remarkably low noise levels (+/- 0.0006) and minimal drift (< 0.0016/hr) are observed. We have extended the ability to calibrate the system at depth by taking down several large volume flexible containers of sea water adjusted to different pH values, and calibrated on the ship by standard buffer techniques. We have developed flow cell procedures whereby the experimental system can be flushed with standards, and any offsets or drift recorded and corrected.
    We present the results of these in situ calibration efforts and give specific examples of how a pH system calibrated at depth can be used to measure in situ the kinetics of the CO2 hydration reaction, provide a precise monitor of CO2 enrichment of seawater as an experimental fluid, and even perform an in situ alkalinity titration. By minimizing cell dimensions and flushing times, and establishing the needed frequency of calibration, it appears to be possible to consider long-term deployments of well-constrained experimental systems for elevated CO2 impact studies.