Other Infrastructure of the Exercise

In addition to the common seawater line, a common supply of calibration gases was regarded a key requirement for the exercise, as otherwise systematic errors most likely would have been introduced. We therefore provided a whole suite of calibration gases. Fifteen cylinders with precisely known amounts of CO₂ in natural dry air covering a nominal concentration range from 250 ppmv to 500 ppmv were purchased from the National Oceanic and Atmospheric Administration (NOAA) Climate Monitoring Diagnostics Laboratory (CMDL) in Boulder, Colorado, U.S.A. Before final filling, all aluminum standard cylinders (Scott Specialty Gases Inc., Plumsteadville, Pennsylvania, U.S.A.) undergo a conditioning period of at least one week with clean ambient air. To prepare the standards, the cylinders are filled with ambient air at Niwot Ridge, Colorado. The air is dried using magnesium perchlorate and either scrubbed with Ascarite or spiked with a 10% CO₂-in-air mixture to obtain mixing ratios below or above ambient levels, respectively (Zhao et al. 1997). Six cylinders of this consistent suite of gases were used during the exercise by all groups for calibrating their instruments. Additionally nitrogen (purity 99.999%) was used by some groups for zeroing their gas analyzers.

The mixing ratios of CO₂ in the cylinders were calibrated in the NOAA/CMDL Carbon Cycle Group laboratories on three separate days over a period of 2-3 weeks. The results of these calibrations are summarized in Table 1. The CO₂ mixing ratios are reported as micromoles per mole (µmol/mol = ppmv) of dry air in the World Meteorological Organization (WMO) X85 mole fraction scale, traceable to primary standards at the Scripps Institution of Oceanography (SIO). The NOAA/CMDL calibrations are done by comparison on a nondispersive infrared CO₂ analyzer against four tertiary standards with assigned mixing ratios traceable to SIO (Thoning et al. 1987; Zhao et al. 1997). The uncertainty of the assigned values for the tertiary standards is approximately 0.06 ppmv. The tertiary set of standards used ranges between 250 and 450 ppmv CO₂. The repeatability of the NOAA/CMDL calibrations depends on the stability of the CO₂ mixing ratio in the cylinder and the fit of the analyzer response to the known tertiary standards. For cylinders that are stable and within the range of standards, the repeatability is on the order of 0.01 ppmv. The overall uncertainty associated with precision is therefore about 0.06 ppmv. When calibrating cylinders at the extremes of the tertiary standards or extrapolated outside the range, the reproducibility decreases. For mixing ratios above 450 ppmv, the reproducibility is on the order of (0.3 ppmv and further decreases with the interpolation away from the tertiary standards. The absolute accuracy of the assigned mixing ratios is determined by the accuracy of the SIO standards (Keeling et al. 1986, and references therein).

Table 1. Summary of calibration results for six cylinders with CO₂ in natural dry air The measurements were carried out at the NOAA/CMDL Carbon Cycle Group Laboratory in Boulder, Colorado, U.S.A. These six cylinders constitute the suite of calibration gases used by all participating groups during the exercise.

According to the different power requirements of the analytical systems, the ship provided three different power sources, the standard 220V/50Hz system as well as two additional systems for 110V/50Hz (static transformer) and 110V/60Hz (dynamic transformer).