Changes in carbon dioxide concentration in the atmosphere are of high environmental and political importance, with increasing levels of emissions and background monitoring. Performance criteria for CO2 monitoring networks are being set, with the most stringent of these being for background ambient levels where data compatibility from different monitoring sites should be at the 0.1 µmol/mol level for mole fractions at the 400 µmol/mol. This in turn requires accurate calibration standards, which historically have been traceable to one ensemble of primary gas standards, which establish a scale for atmospheric background CO2 measurements, and were value assigned by a manometric method. The consistency of manometric and gravimetric methods for gas standard value assignment is being investigated at the BIPM, which has developed its CO2-PVT measurement system. The name PVT reflects that the CO2 amount fraction in air is deduced from accurate measurements of pressure, volume and temperature of whole air and then of the CO2 extracted from it by cryogenic trapping. The whole air and pure CO2 samples parameters are measured in two distinct volumes, the ratio of which is predetermined using a well-known method based on series of expansions of air, introducing three auxiliary volumes to reduce the uncertainty. Performances of two versions of the system, one made of glass, the other one of treated stainless steel will be presented, by comparison with CO2 in air standards prepared gravimetrically by National Metrology Institutes. Sources of uncertainties and potential biases will be reviewed, discussing how a target operating standard uncertainty approaching 0.1 µmol/mol at CO2 in air nominal mole fractions of 400 µmol/mol can be achieved.
Joële Viallon is Principal Chemist at the BIPM, responsible for the management of the Gas Metrology work programme. She led or took part in the organization of international comparisons organized by the BIPM on air quality relevant gases (NO, NO2, HCHO, O3) and Greenhouse gases (CH4, CO2). She is also a BIPM expert representative at the GAWG (Gas Analysis Working Group) of the CCQM (Consultative Committee for Amount of Substance), at the World Meteorology Organization (WMO) expert group on Volatile Organic Compounds and at the WMO expert group on ozone absorption cross-sections.
She holds a PhD in Physics from the University Claude Bernard (Lyon, France), obtained in 2000 after a thesis on the structure of metallic clusters studied by spectroscopy associated with time-of-flight mass spectrometry.