Assessing site-to-site greenhouse gas measurement consistency in the UK and Ireland atmospheric monitoring network using whole and synthetic air reference materials

Abstract. Atmospheric greenhouse gas (GHG) measurements are essential for assessing climate change and verifying national emission inventories. As global networks rely on high-precision observations of potent GHGs such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), ensuring accuracy between observation stations is essential. Maintaining accuracy within a network requires the use of whole air reference materials (RMs) with well-defined calibration scales, such as those maintained by the National Oceanic and Atmospheric Administration (NOAA) for the World Meteorological Organisation (WMO). With the global expansion of atmospheric monitoring networks there is a growing need for readily available RMs, stimulating the development of complementary alternatives such as synthetic air RMs traceable to the International System of Units (SI).

In the United Kingdom (UK) and Ireland, the atmospheric GHG monitoring network, comprising long-term atmospheric monitoring stations, is equipped with high precision in situ GHG analysers. These observations are used to verify bottom-up assessments of national emissions. Here, a case study is presented which provides the first direct, network‑wide evaluation of whole air versus synthetic air RMs under routine field conditions, allowing an assessment of the implications of SI‑traceable RMs within an active national atmospheric monitoring system. We assess the UK and Ireland’s network accuracy using WMO-NOAA scale-traceable whole air RMs and SI-traceable synthetic air RMs, through a blind multi-site round-robin intercomparison. Between 2021 and 2022, measurements were made of CO₂, CH₄ and N₂O in whole and synthetic air RMs at six sites using a range of instruments and the results were used to assess biases between sites.  The measured mole fractions for each cylinder, at each site were then compared with the assigned value on the WMO-NOAA scale and the SI-traceable values for the whole air and synthetic air RMs, respectively. Weighted residual analysis reveals that whole air RMs generally meet the WMO/Global Atmospheric Watch (GAW) compatibility goals for CO₂ and CH₄, while N₂O remains more difficult due to its small atmospheric variability and instrument performance. Synthetic SI-traceable RMs have associated absolute uncertainties on their assigned values for CH₄ that are comparable to the extended WMO compatibility goals. However, absolute uncertainties for CO₂ and N₂O remain significantly larger than both the compatibility and extended compatibility goals, which is consistent with previous studies. Overall, these results indicate that while scale-traceable whole air RMs remain essential for achieving the highest level of network compatibility required for atmospheric monitoring, SI-traceable synthetic RMs could play a valuable complementary role, particularly for CH₄. However, it is first critical that improved characterisation of matrix effects, mole fraction dependant instrument responses, and long-term stability, are achieved for them to become suitable for high-precision atmospheric monitoring.