Towards the realisation of Certified Reference Materials of CO2 in air at ambient amount fraction and known δ13C

In the field of climate change studies, monitoring the increasing levels of CO2 in atmosphere, and discriminating between the natural and anthropogenic sources of CO2, represent two key points to plan and support effective actions for the reduction of the CO2 emissions from human activities in the near future. The efforts ongoing in the metrological community to achieve the comparability of results and to assure accuracy and metrological traceability to the results of CO2 measurements, both in terms of amount fraction and isotopic composition, need to rely on common and stable reference standards. At INRiM, the Italian National Metrology Institute, the realization of gaseous reference materials of CO2 in air at ambient level and known δ13C-CO2 had started in the past years within the European Joint Research Projects (JRPs) 16ENV06 SIRS and 19ENV05 STELLAR. The availability of sound and affordable Certified Reference Materials (CRMs) for the measurement of the isotopic composition of CO2 at ambient amount fraction is foreseen to support the researchers operating in the isotope measurement field by means of spectroscopic techniques. CRMs at known isotopic composition, produced by means of primary methods such as gravimetry, represent a valuable opportunity. CRMs for the CO2 amount fraction at atmospheric level in synthetic air are realised at INRiM by the gravimetric method following the ISO standard 6142-1, in high-pressure cylinders of aluminium alloy, obtaining preparation uncertainties of 0.33 %. These CRMs are prepared from parent mixtures at higher amount fraction, and are realized from different pure CO2 sources. Non-dispersive infrared spectroscopy (NDIR URAS 14 ABB, Switzerland) is used to verify the mixtures for their amount fraction values. Cavity ring-down spectroscopy (CRDS G2131i Picarro, USA) and Fourier transform infrared spectroscopy (FTIR Nicolet iS50 Thermo Scientific, USA) are used to assign the δ13C-CO2 value to the same reference mixtures, in the range from +1.3 ‰ to -42 ‰, with verification uncertainties of 0.2 - 0.3 ‰. The CO2 reference mixtures are periodically verified to evaluate their stability over time, to attain CRMs certified for both CO2 amount fraction and isotopic composition. In this work, the preparation procedure of the reference CO2 mixtures, with a particular focus on the isotopic composition and their analytical verification by CRDS, is described. The evaluation of the measurement uncertainty, for both the amount fraction and the isotopic composition, is presented.