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Quantifying relationships between independent CO2 scales over decadal time scales
by Mr. Andrew Crotwell, Dr. Joële Viallon, Dr. Paul Brewer, Dr. Tobias Bühlmann, Dr. Armin Jordan, Dr. Mudalo Jozela, Dr. Paul Krummel, Dr. Sangil Lee, Dr. Motoki Sasakawa, Dr. Michela Sega, Dr. Adriaan van der Veen, Dr. Ray Weiss


To quantify CO2 emissions and evaluate future emission reduction strategies there is an urgent need for increased monitoring of atmospheric CO2 at all spatial scales. This increase in monitoring activity must be supported by readily available CO2 in air standards with appropriate characteristics and traceability to ensure that temporal trends and spatial gradients observed in the atmosphere are robust. Within the atmospheric monitoring community this is achieved by realizing calibrations scales, based on defined ensembles of standards, with the amount fraction of CO2 in each standard initially assigned an SI traceable value. Currently there are several CO2 in air scales used for atmospheric monitoring. However only one (the WMO CO2 in air scale maintained by NOAA) is used extensively outside of its host institution. To meet expected increase in demand for such standards, National Metrology Institutes can adapt their facilities for gas standards to create new CO2 in air scales, enabling dissemination of additional standards to meet measurement community needs at national or regional level. In developing such an approach, it is of greatest importance that differences between scales (either existing or proposed new scales) must be understood, quantified, documented and tracked over time. This would allow reporting or conversion to a common scale, and avoid the situation where scale differences could influence the interpretation of atmospheric data The Gas Analysis Working Group of the Consultative Committee for Amount of Substance: Metrology in Chemistry and Biology has formed a Task Group on Greenhouse Gas Scale Comparisons to progress the topic. One primary goal of this task group is to develop a protocol for defining CO2 in air scale relationships which will allow data traceable to independent scales to be combined in scientific studies. To minimize the influence from scale differences, the targeted uncertainty on the scale relationships is 0.02 µmol/mol and this uncertainty should be maintained over decadal time periods. The ongoing nature of these comparisons coupled with the very stringent requirements for the relationships makes this a very challenging endeavour. Here we report on the opportunities and challenges related to developing a protocol to use the BIPM CO2 measurement facilities and system of interlaboratory comparisons as a tool for benchmarking CO2 scales to maintain CO2 in air scale relationships. Issues to be addressed and discussed include the frequency of comparison, the minimum number of standards needed to establish scale relationships, logistical aspects of the comparison, appropriate treatment of the data to ensure robust relationships, data management, and validation of the reported scale relationships and their vulnerabilities. 

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Topic : Theme 2: Accuracy requirements for atmospheric composition measurements across economic sectors, and temporal and spatial scales.
Reference : T2-A5

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