FY 1999

A new automated underway system for making high precision pCO₂ measurements onboard research ships

Feely, R.A., R. Wanninkhof, H.B. Milburn, C.E. Cosca, M. Stapp, and P.P. Murphy

Anal. Chim. Acta, 377, 185–191, doi: 10.1016/S0003-2670(98)00388-2 (1998)

We have developed a new temperature-controlled, automated underway system for making atmospheric and surface ocean pCO₂ measurements onboard research ships equipped with an uncontaminated seawater intake system. Uncontaminated seawater is supplied to a showerhead plexiglass equilibrator. After about 3 min, the air trapped in the equilibrator is equilibrated with seawater. This air is sampled six times per hour. In addition, atmospheric air is sampled three times per hour from the intake on the bow flagstaff through 3/8 in. Dekabon™ tubing to the underway system. The CO₂ measurements are made with a differential, non-dispersive, infrared analyzer Li-Cor™ (model 6252). The underway system operates on an hourly cycle with the first quarter of each hour devoted to calibration with three CO₂ standards, each measured for 5 min. A second-order polynomial calibration curve is calculated from the voltage values of the standards. The remaining time in each hour is used to measure equilibrator air (15 min), bow air (15 min), and equilibrator air once again (15 min). To date, we have successfully used the underway pCO₂ system on 12 cruises of the NOAA Ship Ka'imimoana in the Equatorial Pacific. The analytical precision of the system is approximately 0.3-0.4 ppm CO₂ for seawater and for air.