CO₂ Standard Gases

The data collection program for the Indian Ocean survey cruises was set up to record five readings from each of the four calibration gases (the reference and three CO₂ standard gases) every three hours. All of the gases were a mixture of CO₂ in artificial air (oxygen, nitrogen, and argon in atmospheric ratios) prepared by Scott Specialty Gases, Inc. The nominal CO₂ concentrations for the three standard gases were 280, 360, and 450 ppm respectively. A reference gas with a nominal concentration of 200 ppm was used on almost all of the cruises to increase the dynamic range of the detector output (see Appendix A for details). Five tanks of calibrated reference gas were put aboard the R/V Knorr before the first leg of the survey. However, these tanks were exhausted before a resupply container could be sent with additional calibrated gases. After the first two weeks of leg I7N the reference gas was switched to a CO₂-free air tank. Additional gas tanks were delivered to the ship between legs I1 and I10, so the final two legs (legs I10 and I2) were again run with a 200 ppm reference gas. The exact times that the reference tanks were in use as well as the calibrated concentrations are given in Appendix B. The flow rate on the three CO₂ standards used for calibrations was sufficiently low to make one set of tanks last for the entire survey.

All of the CO₂ standards used for this survey were calibrated by R. Van Woy (SIO) using a technique that employs a gas chromatograph (GC)/flame ionization detector (FID) with catalytic conversion to CH₄ (Weiss 1981). The GC system was calibrated against C. D. Keeling-certified standards with concentrations of 213.14, 296.65, 349.97, and 458.06 ppm. The CO₂ standard gases and the initial five reference tanks were calibrated in September 1994, prior to the first cruise. The overall accuracy of the reported final values was estimated to be ±0.3 ppm. After completion of the last leg of the survey, the three standard gases were returned to R. Weiss' laboratory at SIO for post-cruise calibration in June 1996. Table 2 summarizes the initial and final calibrations of these gases. In all cases the post-cruise calibration was within the estimated accuracy of the initial calibration.

Table 2. Calibrated values for CO₂ standards