Underway pCO₂ Measurements

Underway pCO₂ was measured by the method of Schneider et al. (1992). Surface seawater was continuously pumped at a rate of 200-300 mL/min into a glass equilibrator with a volume of approximately 300 mL. The seawater was equilibrated with continuously circulating air entering the bottom of the equilibrator through a frit from a closed loop system. The latter included a heat exchanger to keep the air at the sample temperature, a filter and water trap, and an infrared analyzer (Siemens, Ultramat 5F) to determine the CO₂ content of the equilibrated air. The IR analyzer and the equilibrator temperature sensor were connected to a PC and to an analog recorder for data display and preservation. The time constant for the equilibration was about 3 min, which corresponded to a spatial resolution of 0.5 mi with the ship speed at 10 kn. Atmospheric air was periodically measured, and the system was calibrated every 12 h using calibration gases with CO₂ mixing ratios of 252.5 and 412.8 ppm. Pressure corrections were made for the effect of water vapor and the pressure at the inlet of the IR analyzer, while the correction for the small difference between in situ and measuring temperature (<10°C) was made according to Gordon and Jones (1973). Fig. 7 in report shows the plot of underway measurements of temperature, salinity, sea surface pCO₂, and air pCO₂ during the R/V Meteor Cruise 22/5 in South Atlantic Ocean.