SI traceability and scales for underpinning atmospheric monitoring of greenhouse gases Metrologia

Metrological traceability is the property of a measurement result whereby it can be related to a stated reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty. The stated reference can be the International System of Units, and more specifically a realisation of a primary standard with its value and uncertainty expressed in SI units, which offers many benefits such as long term stability and the ability to reproduce the standard within its stated uncertainty at any time. Alternatively, measurements can be made traceable to an empirical scale, an approach that has evolved for practical reasons to meet the needs of certain measurement communities. We explore the benefits and drawbacks of these two measurements systems as they have evolved for greenhouse gas measurements at background levels and outline the advantages of using elements from each to secure a more robust measurement infrastructure. In particular, this work examines the importance of compatibility, the accuracy of measurement results and the benefits of comparisons with independent primary standards.