The accurate and reliable determination of carbon dioxide (CO2) levels in atmosphere is important for the role of this gas in the greenhouse effect. In addition, the rising level of CO2 absorbed by the oceans from the atmosphere causes an acidification of the marine waters, with fallouts on the global ecosystem. In this framework, the application of the metrological concepts is necessary to assure the reliability of the measurement results. At INRIM, activities for the preparation and verification of CO2 primary reference mixtures are carried out. A facility for the production of primary CO2 standards by gravimetry has been used since 2004. Due to the huge amount of gas mixtures that are needed to cover different ranges of molar fractions during the calibration of instrumentation, a new system for dynamic dilution was recently set up. An overview of the above-mentioned activities, with a focus on some crucial issues related to mixture verification and stability studies, is presented in this work.