CO₂ deviation in a cylinder due to consumption of a standard gas mixture

Abstract. The CO₂ molar fraction in standard gas mixtures is known to deviate as a result of adsorption/desorption to/from the inner surface of a high-pressure cylinder and thermal diffusion fractionation caused by the temperature distribution in the cylinder. This deviation reduces the consistency of atmospheric CO₂ observations, because the standard gas mixtures are used to calibrate all measurement systems for precise CO₂ observations. To maintain the consistency of CO₂ values over the long term, a quantitative understanding of the deviations in the CO₂ molar fraction in a standard gas mixture is needed. Thus far, this understanding has not been achieved sufficiently well, because the contribution of thermal diffusion fractionation is less well understood than that of adsorption/desorption. In this study, offsets of 0.013 ± 0.015 μmol mol⁻¹ and −0.014 ± 0.011 μmol mol⁻¹ were observed in the outflowing gas from horizontally and vertically positioned cylinders, respectively, at a flow rate of 0.080 L min⁻¹. These offsets are attributed to thermal diffusion effects, which diluted and enriched the CO₂ mole fraction by −0.045 μmol mol⁻¹ (horizontal cylinder) and 0.048 μmol mol⁻¹ (vertical cylinder) as the relative pressure dropped to 0.03. In the experiments at same flow rate, the adsorption/desorption effect enriched the CO₂ mole fraction by 0.06 μmol mol⁻¹ (horizontal cylinder) and 0.10 μmol mol⁻¹ (vertical cylinder). Therefore, attention should be paid to both thermal diffusion fractionation and adsorption/desorption effects for precise calibration of long-term observations of CO₂ molar fractions, although past studies have ignored the contribution of thermal diffusion fractionation at the low flow rates (<0.3 L min⁻¹) examined in this study. Furthermore, the deviation of the CO₂ molar fraction depends only on the pressure relative to the initial pressure of the cylinder. This result suggests that the recommendation by the World Meteorological Organization (WMO) to replace the standard gas mixture once the cylinder pressure drops to 2 MPa needs to be revised.