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. Long-term monitoring of carbon dioxide ( CO2 ) in the atmosphere is keyfor a better understanding of the processes involved in the carbon cycle thathave a major impact on further climate change. Keeping track of large-scaleemissions and removals (sources and sinks) of CO2 requires veryaccurate measurements. They all have to be calibrated very carefully and haveto be traceable to a common scale, the WorldMeteorological Organization (WMO) CO2 X2007 scale, which ismaintained by the National Oceanic and Atmospheric Administration (NOAA) Earth SystemResearch Laboratory (ESRL) in Boulder, CO, USA. The international WMO GAW (Global Atmosphere Watch) program sets ascompatibility goals for the required agreement between different methods andlaboratories ±0.1 µ mol mol −1 for the Northern Hemisphereand ±0.05 µ mol mol −1 for the Southern Hemisphere. Thereference gas mixtures used to pass down and distribute the scale are storedin high-pressure aluminum cylinders. It is crucial that the standards remainstable during their entire time of use. In this study the tested verticallypositioned aluminum cylinders showed similar CO2 enrichment during low-flow conditions (0.3 L min −1) , which are similar to flows often usedfor calibration gases in practical applications. The average CO2 enrichment was 0.090±0.009 µ mol mol −1 as the cylinderwas emptied from about 150 to 1 bar above atmosphere. However, it isimportant to note that the enrichment is not linear but follows Langmuir'sadsorption–desorption model, where the CO2 enrichment is almostnegligible at high pressures but much more pronounced at low pressures. Whendecanted at a higher rate of 5.0 L min −1 the enrichment becomes 0.22±0.05 µ mol mol −1 for the same pressure drop. Thehigher enrichment is related to thermal diffusion and fractionation effectsin the cylinder, which were also dependent on the cylinder's orientation andcould even turn negative. However, the low amount of CO2 adsorbed on thecylinder wall and the fact that the main increase happens at lowpressure lead to the conclusion that aluminum cylinders are suitable to storeambient CO2 -in-dry-air mixtures provided they are not used below 20 bar.In cases where they are used in high-flow experiments that involve significantcylinder temperature changes, special attention has to be paid to possiblefractionation effects.

Experiments with CO&lt;sub&gt;2&lt;/sub&gt;-in-air reference gases in high-pressure aluminum cylinders

Experiments with CO<sub>2</sub>-in-air reference gases in high-pressure aluminum cylinders