Key comparisons of greenhouse gases and related tracers (CO2, CH4, N2O and CO), coordinated by the BIPM’s CCQM GAWG, have been running for over 20 years and provide a mechanism for demonstrating that independent reference materials are consistent within their SI traceable uncertainties, including those maintained by the WMO’s Central Calibration Laboratory, NOAA. Over the last 10 years, the uncertainty of the key comparison reference value has reduced for several greenhouse gases and is now the same order of magnitude as the data quality objectives (for network compatibility) set by the WMO-GAW. The uncertainties would need to be reduced further by a factor of 3 to 4 in the best cases to be negligible for the highest precision measurements of greenhouse gases that are currently achieved by employing traceability to one specific set of standards, the so-called scale approach. Nevertheless, the current levels of uncertainty achieved have provided the following useful outputs: CCQM-K120 has demonstrated equivalence of CO2 in air standards and provided additional supporting evidence for the scale update to WMO-CO2-X2019; CCQM-K82 has identified minor amount fraction biases in the methane scale and prompted remedial work; CCQM-K68.2019 has demonstrated internal consistency between standards that can be traced to ensembles defining individual nitrous oxide scales (CCQM-K68.2019), with additional work on high accuracy quantification of nitrous oxide in the matrix gas; CCQM-K84 has demonstrated consistency between reference materials of CO in air and outlined the challenges with respect to the stability and further work to improve uncertainties to meet the WMO-GAW data quality objectives. Future requirements and directions for these comparisons include: a) Maintaining at least a decadal repeat frequency, whilst striving to reduce measurement uncertainties, to provide a mechanism to demonstrate that standards used for greenhouse gas scales are consistent with other SI traceable realizations; b) Further development of the comparisons, so that they can be used to monitor and compare greenhouse gas scale relationships and stability, as used by the atmospheric monitoring community. The latter activity will be demonstrated by the development and validation of a manometric facility at the BIPM, for reference measurements of CO2 in air standards, that can be used for on-demand comparisons (BIPM.QM-K2) with other primary reference measurements, and the establishment of local CO2 in air scales that can be the basis of an ongoing key comparison (BIPM.QM-K5) of independently maintained scales including WMO-CO2-X2019. The future challenges for CH4, N2O, and CO in air reference materials and comparisons will also be presented including CO2 isotope ratio, with the CCQM GAWG providing a forum to address these in collaboration with the atmospheric monitoring community and WMO-GAW CCLs.
Topic : Theme 2: Accuracy requirements for atmospheric composition measurements across economic sectors, and temporal and spatial scales.
Reference : T2-A6
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