Realisation of gas reference materials of CO2 in air at known isotopic composition

The accurate and reliable determination of carbon dioxide (CO2) levels in atmosphere, together with its isotopic composition, is fundamental to monitor the trends of this gas in atmosphere and to support the prediction of its future increase on a global scale. The determination of the isotopic composition of CO2 can support the assessment of the uptake of CO2 in the environmental compartments, and can help to distinguish natural from anthropogenic carbon in the atmosphere. In this regard, INRIM started an activity for the development of new reference gas mixtures of CO2 at ambient amount fraction in high pressure cylinders with different isotopic compositions, to cover the range -48 ‰ to -8 ‰ vs VPDB for δ13C and -27 ‰ to +2 ‰ vs VPDB for δ18O, with uncertainty targets of 0.1 ‰ for δ13C-CO2 and 0.5 ‰ for δ18O-CO2. The activity was carried out in the framework of the EMPIR project 16ENV06 “Metrology for stable isotope reference standards - SIRS”, and it is now continuing with the project 19ENV05 “Stable isotope metrology to enable climate action and regulation – STELLAR”. Selected commercial pure CO2 were used to prepare different CO2 reference materials in high pressure cylinders by using the gravimetric method, adapting the INRIM procedure applied to the preparation of CO2 in N2/synthetic air mixtures. Starting from these pure materials, CO2 mixtures in synthetic air at atmospheric amount fraction were prepared. The prepared mixtures were then sent to the Max Planck Institute for Biogeochemistry (MPI-BGC), for the validation of their isotopic values. A sampling system was set up for the preparation of the CO2 samples sent for validation, in order to minimise fractionation effects during the sampling. An appropriate uncertainty budget was developed for the reference mixtures, by taking into account all the relevant uncertainty contributions.