Experiments with CO 2 -in-air reference gases in high-pressure aluminum cylinders
Abstract. Long-term monitoring of carbon dioxide ( CO2 ) in the atmosphere is key
for a better understanding of the processes involved in the carbon cycle that
have a major impact on further climate change. Keeping track of large-scale
emissions and removals (sources and sinks) of CO2 requires very
accurate measurements. They all have to be calibrated very carefully and have
to be traceable to a common scale, the World
Meteorological Organization (WMO) CO2 X2007 scale, which is
maintained by the National Oceanic and Atmospheric Administration (NOAA) Earth System
Research Laboratory (ESRL) in Boulder, CO, USA. The international WMO GAW (Global Atmosphere Watch) program sets as
compatibility goals for the required agreement between different methods and
laboratories ±0.1 µ mol mol −1 for the Northern Hemisphere
and ±0.05 µ mol mol −1 for the Southern Hemisphere. The
reference gas mixtures used to pass down and distribute the scale are stored
in high-pressure aluminum cylinders. It is crucial that the standards remain
stable during their entire time of use. In this study the tested vertically
positioned aluminum cylinders showed similar CO2 enrichment during low-flow conditions (0.3 L min −1) , which are similar to flows often used
for calibration gases in practical applications. The average CO2 
enrichment was 0.090±0.009 µ mol mol −1 as the cylinder
was emptied from about 150 to 1 bar above atmosphere. However, it is
important to note that the enrichment is not linear but follows Langmuir's
adsorption–desorption model, where the CO2 enrichment is almost
negligible at high pressures but much more pronounced at low pressures. When
decanted 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. The
higher enrichment is related to thermal diffusion and fractionation effects
in the cylinder, which were also dependent on the cylinder's orientation and
could even turn negative. However, the low amount of CO2 adsorbed on the
cylinder wall and the fact that the main increase happens at low
pressure lead to the conclusion that aluminum cylinders are suitable to store
ambient 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 significant
cylinder temperature changes, special attention has to be paid to possible
fractionation effects.