A high accuracy reference facility for ongoing comparisons of CO₂ in air standards

The design, performance characteristics and validation of a next generation reference facility for carbon dioxide (CO2) in air standards based on manometry is presented. Novel attributes of the facility include automated operation, avoidance of significant pressure corrections for measurements on extracted CO2, fully characterized trapping efficiencies, and reduced measurement uncertainty. Improvements in system performance have been achieved using specific materials, notably Silconert (R)-treated stainless-steel, providing increased mechanical stability whilst minimizing carbon dioxide adsorption on surfaces, and avoiding use of elastomer O-rings, which would lead to losses of CO2. Full automation of the cryogenic extraction process of CO2 from air has been achieved, avoiding any manual intervention within the temperature stabilized section of the facility, and allowed full characterization and correction for trapping efficiencies and trace water measurement. The facility has been validated across the CO2 in air amount fraction range of (380-800) mu mol mol(-1) using standards with values traceable to the reference value of the CCQM-K120 (2018) comparison. It was demonstrated to operate with a standard measurement uncertainty of 0.09 mu mol mol(-1) at 400 mu mol mol(-1). The automation allows five measurement results per day to be produced with a typical standard deviation of the mean at or below 0.02 mu mol mol(-1). The facility will be used as a stable reference in the future BIPM.QM-K2 ongoing comparison, to compare consistency of amount fraction values in different CO2 in air standards. The CO2 amount fraction in two ensembles of nine BIPM standards covering the same range will also be measured with the facility to provide their SI-traceable values, and further monitored on a longer time scale. Each ensemble will constitute a CO2 in air scale to be compared with other available scales such as WMO.CO2.X2019 through the BIPM.QM-K5 comparison.