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Standards for monitoring atmospheric carbon dioxide

Measurements of the amount fraction of carbon dioxide (CO2) in air standards, performed with cryogenic trapping, have been achieved at the BIPM during the six month visiting scientist secondment of Dr Stephen Maxwell from NIST, USA. This represents an important milestone towards the establishment of the BIPM CO2-PVT measurement system, the name reflecting the fact that the CO2 amount fraction is deduced from accurate measurements of pressure, volume and temperature of whole air and then the CO2 extracted from it, with typical values of 400 µmol mol−1.

The system, shown in the picture, is essentially an ensemble of different glass vessels with different volumes, designed to minimize the adsorption/desorption of CO2 on surfaces. Dr Maxwell developed the system from its status of never having seen CO2 on his arrival, to the stage where we now have an automated system that is capable of making measurements and delivering its first results. He has handed over the development of the facility to Fredrik Arrhén, a visiting guest scientist from SP, Sweden, who is now working on the second version of the system.

The facility will be the basis of ongoing key comparisons for CO2 in air, used to accurately assign the CO2 amount fractions in sets of standards, from clean background air to more polluted urban air (380 µmol mol−1 to 800 µmol mol−1). The BIPM and NIST are coordinating the CCQM-K120 key comparison for CO2 in air in 2016, which will compare gravimetric standards prepared by NMIs as well as standards maintained by the WMO. The target uncertainties for the comparison are the smallest and most challenging ever attempted within the CCQM Working Group on Gas Analysis (GAWG), and designed to be fit for purpose for long-term greenhouse gas and climate change monitoring needs.

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