Comparison of gravimetry and dynamic dilution for the generation of reference gas mixtures with known composition of CO2 at atmospheric amount fraction

Carbon dioxide (CO2) is the most important greenhouse gas from human activities. Its concentration has been growing continuously in the atmosphere reaching a current annual average of 410 micromol·mol-1. Reliable determinations of the atmospheric CO2 concentration are of great importance for both environmental protection and development of models used in climate change predictions. The production of reference mixtures of known concentration and the establishment of sound analytical methods are key activities for the achievement of reliable information on the pollution level and the improvement and maintenance of a worldwide coherent measurement system for the determination of greenhouse gases. The present work deals with the advantages and drawbacks of two methods that assure metrological traceability to SI to measurement results. The first method, more accurate, is based on the gravimetric preparation of gas mixtures in high pressure cylinders, but it requires long preparation procedures. The second one, based on dynamic dilution, represents a more flexible alternative for gas mixture generation. The method requires diluting devices such as mass flow controllers which canalize and portion the gas flows in a mixing chamber. This method enables to generate gas mixtures at the desired amount fraction and can be applied also for the production of mixtures with unstable or reactive components. However, it might lead to larger uncertainties. At INRiM, these methods are used to prepare gas mixtures with specific CO2 amount fractions, which are periodically verified by a Non-Dispersive Infrared analyser to check the stability of the mixture composition over time.