Metrological traceability paths for stable isotope measurements of CO2 in air

Measurements of carbon stable isotopes 13C and 12C and of their ratio 13C/12C in atmospheric carbon dioxide (δ13C-CO2) are indicators of the anthropogenic contribution to CO2 emissions. At present, δ13C-CO2 falls within the CIPM MRA traceability exception. Gaseous Certified Reference Materials (CRMs) are needed as proper references to support the scientific community dealing with δ13C-CO2 in atmospheric measurements. The paper presents an approach to assign δ13C-CO2 values to CO2 gas mixtures at atmospheric amount fraction following two independent traceability paths, leading to candidate CRMs with established metrological traceability to the Vienna Pee Dee Belemnite scale, which represents an international recognized primary scale in this framework. INRiM is working towards the attainment of internationally recognized Calibration and Measurement Capabilities in the CIPM MRA framework for the production of such CRMs, which represents a fundamental step to meet the increasing demand of proper references for stable carbon isotopes measurements in atmosphere.