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

<p>The CO<span class="inline-formula">₂</span> 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 CO<span class="inline-formula">₂</span> observations, because the standard gas mixtures are used to calibrate all measurement systems for precise CO<span class="inline-formula">₂</span> observations. To maintain the consistency of CO<span class="inline-formula">₂</span> values over the long term, a quantitative understanding of the deviations in the CO<span class="inline-formula">₂</span> 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 <span class="inline-formula">±</span> 0.015 and <span class="inline-formula">−</span>0.014 <span class="inline-formula">±</span> 0.011 <span class="inline-formula">µmol</span> mol<span class="inline-formula">⁻¹</span> were observed in the outflowing gas from horizontally and vertically positioned cylinders, respectively, at a flow rate of 0.080 L min<span class="inline-formula">⁻¹</span>. These offsets are attributed to thermal diffusion effects, which diluted and enriched the CO<span class="inline-formula">₂</span> molar fraction by <span class="inline-formula">−</span>0.045 <span class="inline-formula">µmol</span> mol<span class="inline-formula">⁻¹</span> (horizontal cylinder) and 0.048 <span class="inline-formula">µmol</span> mol<span class="inline-formula">⁻¹</span> (vertical cylinder) as the relative pressure dropped to 0.03. In the experiments at same flow rate, the adsorption/desorption effect enriched the CO<span class="inline-formula">₂</span> molar fraction by 0.06 <span class="inline-formula">µmol</span> mol<span class="inline-formula">⁻¹</span> (horizontal cylinder) and 0.10 <span class="inline-formula">µmol</span> mol<span class="inline-formula">⁻¹</span> (vertical cylinder). Therefore, attention should be paid to both thermal diffusion fractionation and adsorption/desorption effects for precise calibration of long-term observations of CO<span class="inline-formula">₂</span> molar fractions, although past studies have ignored the contribution of thermal diffusion fractionation at the low flow rates (<span class="inline-formula">&lt;</span> 0.3 L min<span class="inline-formula">⁻¹</span>) examined in this study. Furthermore, the deviation of the CO<span class="inline-formula">₂</span> 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.</p>