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Discussion papers | Copyright
https://doi.org/10.5194/amt-2018-42
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 28 Mar 2018

Research article | 28 Mar 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

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 WMO CO2 X2007 scale, which is maintained by NOAA/ESRL (Oceanic and Atmospheric Administration/Earth System Research Laboratory) in Boulder, CO, USA. The international WMO/GAW (World Meteorological Organization/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 we found that during low flow conditions (0.3lmin−1) the tested vertically positioned aluminum cylinders always showed similar CO2 enrichment of 0.090± 0.009μmolmol−1 as the cylinder was emptied from about 140 to 1 bar above atmosphere, following Langmuir’s adsorption/desorption model. 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 as well as 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 case 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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Experiments with CO2 -in-air reference gases in high-pressure aluminum cylinders M. F. Schibig, D. Kitzis, and P. P. Tans https://doi.org/10.15138/G3263N

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Short summary
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, 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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