Next generation carbon dioxide isotope ratio gas reference materials The aim of this work package is to considerably improve gas reference materials of carbon dioxide that were developed as part of the EMPIR funded SIRS project (16ENV06). These reference materials have target uncertainties of 0.1 ‰ for d13C-CO2 and 0.5 ‰ for d18O-CO2 measurements and must be strengthened to make them meet requirements for underpinning global monitoring. For example, WMO-GAW stipulates target uncertainties of 0.01 ‰ and 0.05 ‰ for d13C-CO2 and d18O-CO2 measurements of atmospheric carbon dioxide, respectively. Furthermore, the GAW-WMO “network compatibility goal for measurements of both isotopes of carbon dioxide is 0.1 ‰. The proposed work aims to reduce the uncertainties of available atmospheric carbon dioxide standards and to meet the stringent and ambitious GAW-WMO uncertainty goals. Initially this work package will aim to verify and improve the link between the carbon dioxide gas reference materials to the internationally accepted carbon and oxygen stable isotope VPDB scale (task 1.1). For oxygen, ties will also be established with the international water-based VSMOW/SLAP -scale. In task 1.2, the carbon dioxide gases that have been calibrated on the VPDB scale will be used to produce mixtures of carbon dioxide in CO2-free air matrix at ambient amount fractions. These mixtures will be calibrated on the “Jena Reference Air Set” scale (JRAS-06), the WMO-GAW recommended scale for isotopic measurements of atmospheric carbon dioxide. This work package will also contribute to the effort to produce SI traceable values for isotope ratios of carbon dioxide gas (task 1.3). This is particularly pertinent now as the Commission of Isotopic Abundances and Atomic Weights (CIAAW) has recently amended the best measurement for the 13C/12C ratio of the 0 -point on the VPDB scale.