Advancements in Methane in Air Standards for ground-based concentration measurement and emissions quantification

Methane (CH₄) emission quantification that is based on ground level measurements of enhancements in CH₄ in air concentrations requires instruments that are accurately calibrated with fit for purpose gas standards. Developments in the availability, accuracy and internal consistency of CH₄ in air gas standards will be described, with standards now available with standard uncertainties below 1 nmol/mol over the range of below ambient background concentrations to over 3000 nmol/mol. When greater levels of precision are required for the measurement of enhancements in CH₄ concentrations, the scale approach for standards can be adopted, as is done for WMO mole fraction scale standards for CH₄ in air, with recent comparative measurements demonstrating that internal consistencies between standards of 0.1 nmol/mol can be reached.

According to WMO, the annual increase in atmospheric CH₄ was 16 nmol/mol in 2022 and 11 nmol/mol in 2023, both of which exceed the average growth rate observed over the past decade. In addition, enhancements in atmospheric CH₄ concentrations at short temporal scales due to localized emissions can be measured at ground-level. When selecting gas calibration standards to be used for such measurements, the achievable uncertainty and internal consistency of the standards used needs to be considered. The aim is to ensure that measured changes can be undoubtedly attributed to atmospheric concentration enhancements, and not to differences in standards used at different sites. Additionally, a consistent system in which all measurement results are traceable to the same references allows different datasets to be combined without the introduction of biases.

To enhance the accuracy, reliability, and robustness of global CH₄ measurements over the past two decades, the BIPM, National Metrology Institutes (NMIs) and the WMO’s Central Calibration Laboratory have worked on improving the system of CH₄ standards traceable to the International System of Units (SI), and demonstrating equivalence with standards developed for the WMO scale, the latter used principally for background CH₄ trend measurements .

Improvements in the compatibility of CH₄ in air standards from 2003 to 2023 will be described. Current standards now have accuracies of better than 1 nmol/mol, and pairwise comparisons of standards have demonstrated internal consistencies of 0.1 nmol/mol in sets of standards from National Metrology Institutes and the WMO’s CCL. This builds upon  preliminary comparisons of primary CH₄-in-air gas standards conducted in 2003 (CCQM-P41), showing a standard deviation of approximately 30 nmol/mol and 10 nmol/mol for a more limited set of standards. Whereas in 2013, the CCQM-K82 comparison studied CH4-in-air primary reference mixtures in the range of 1800 nmol/mol to 2200 nmol/mol, with a demonstrated tenfold improvement in compatibility, with uncertainties of reference values for standards ranging from 0.68 nmol/mol to 0.71 nmol/mol and a standard deviation of 1.70 nmol/mol across the standards. In 2023, a new comparison (CCQM-K82.2023) was conducted to further monitor the compatibility of CH₄-in-air primary reference mixtures within the 1800–2200 nmol/mol range. Preliminary  results will be discussed, as well as progress in extending the availability of  CH₄ in air scale standards, when the very highest levels of internal consistency between standards is required.