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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 11, issue 10 | Copyright
Atmos. Meas. Tech., 11, 5565-5586, 2018
https://doi.org/10.5194/amt-11-5565-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Oct 2018

Research article | 12 Oct 2018

Experiments with CO2-in-air reference gases in high-pressure aluminum cylinders

Michael F. Schibig1, Duane Kitzis1,2, and Pieter P. Tans1 Michael F. Schibig et al.
  • 1Global Monitoring Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80305, USA
  • 2Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309, USA

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 ±0.1µmolmol−1 for the Northern Hemisphere and ±0.05µmolmol−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 (0.3Lmin−1), which are similar to flows often used for calibration gases in practical applications. The average CO2 enrichment was 0.090±0.009µmolmol−1 as the cylinder was emptied from about 150 to 1bar 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 5.0Lmin−1 the enrichment becomes 0.22±0.05µmolmol−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 20bar. 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.

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For long-term monitoring of CO2 in the atmosphere it is crucial that standard reference gas mixtures stored in high-pressure aluminum cylinders, used to calibrate global measurements, remain stable over their whole time of use. This study finds a reproducible small enrichment as a cylinder is emptied at a low-flow rate, as well as somewhat larger and variable effects when a cylinder is emptied using a high-flow rate.
For long-term monitoring of CO2 in the atmosphere it is crucial that standard reference gas...
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