Grant Forster

Atmospheric measurements of greenhouse gases (GHG), including carbon dioxide (CO ₂ ), methane (CH ₄ ) and nitrous oxide (N ₂ O) are essential for assessing climate change and verifying national emission inventories. As global networks rely on high-precision observations of GHGs, 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 consistency and performance 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 CO ₂ , CH ₄ and N ₂ O in whole and synthetic air RMs at six sites using a range of instruments and the results were used to assess consistency 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 Atmosphere Watch (GAW) compatibility goals for CO ₂ and CH ₄ , while N ₂ O remains more difficult as its relatively small atmospheric variability requires measurement biases to be minimised to resolve meaningful atmospheric differences. Synthetic SI-traceable RMs have associated absolute uncertainties on their assigned values for CH ₄ that are comparable to the extended WMO compatibility goals. However, absolute uncertainties for CO ₂ and N ₂ O 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 CH ₄ . However, it is first critical that improved characterisation and compensation of matrix effects arising from instrument-specific responses to differences in RM composition, non-linear instrument response across the relevant mole fraction range, and long-term stability, are achieved for them to become suitable for use in high-precision atmospheric monitoring.

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.

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.

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.

Ice-nucleating particles (INPs) are a subset of aerosol particles that facilitate the freezing of supercooled cloud droplets heterogeneously and influence the radiative properties of supercooled clouds. The role of INPs in the Earth system remains unquantified in part due to poorly constrained representations of their spatial distributions and properties in global and regional models. In this study, we present a quality controlled dataset (https://doi.org/10.5281/zenodo.16745514, Herbert et al., 2026), called GloPINE, comprising 70 000 h of INP concentrations measured using Portable Ice Nucleation Experiment (PINE) instruments that use an expansion chamber to make automated long-term (months to years) and high temporal resolution (3 of ambient air, and performed under conditions relevant for mixed-phase clouds. In the GloPINE dataset, we subset and average the PINE measurements across synoptically relevant time intervals of 6 h and 2 K temperature bins, providing 36 000 INP measurements. Combining PINE expansions over these intervals enhances counting statistics at higher freezing temperatures, decreases the lower limit of measurable INP concentrations, and provides an INP dataset readily applicable to model simulation data and meteorological reanalysis products. Together with complementary INP datasets, GloPINE provides a valuable resource for advancing model evaluation, improving INP source attribution, and informing parameterizations across the full temperature range over which aerosols influence ice formation in clouds.

Methane is emitted from a range of anthropogenic and natural sources, and identifying these sources is important for emissions monitoring and mitigation. Different sources emit methane with different isotopic signatures. However, these signatures can vary spatially and temporally and are often not well understood. Top-down inverse methods can be used with measurements of methane mole fractions to estimate total emissions of methane from all sources. Here, we present an inverse system for concurrently estimating regional fossil-fuel (FF) and non-fossil-fuel (non-FF) emissions, using isotope ratio observations and considering uncertainty in the isotopic signatures. This method is highly adaptable and could be used to estimate emissions from any number of sources. Synthetic data tests with this method show that this inverse system can accurately estimate FF and non-FF methane emissions across the UK, when isotopic source signatures are fixed at known values. However, emissions estimation becomes less accurate when source signature uncertainties are increased to over approximately 50 % of their likely ranges. In a real-world test of this method, we estimated south-east UK FF and non-FF emissions using high-frequency δ ¹³ C-CH ₄ and δ ² H-CH ₄ observations from one UK site, with source signature uncertainties reflecting our current understanding of these values. Results show only a limited impact on emissions uncertainty and magnitude, when compared with a methane-only inversion. This suggests that both an expansion of the UK network of isotope ratio observations and an improved understanding of isotopic signatures is required for this method to be effective in estimating UK FF and non-FF methane emissions with reduced uncertainty compared to traditional inverse methods.

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.

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.

Methane is emitted from a range of anthropogenic and natural sources, and identifying these sources is important for emissions monitoring and mitigation. Different sources emit methane with different isotopic signatures; however these signatures are often uncertain or vary spatially or temporally. Top-down inverse models can be used with measurement of methane mole fractions to estimate total emissions of methane from all sources. We present an inverse model for estimating regional fossil-fuel (FF) and non-fossil-fuel (non-FF) emissions concurrently, using isotope ratio observations and uncertain isotopic signatures. This method is highly adaptable and could be used to estimate emissions from any number of sources. Synthetic data tests with this method show that this model can accurately estimate FF and non-FF methane emissions across the UK, when isotopic source signatures are fixed at known values. However, emissions estimation becomes less accurate when source signature uncertainties rise above approximately 50 % of their likely ranges. In a real-world test of this method, we estimated south-east UK FF and non-FF emissions using high-frequency δ¹³C-CH₄ and δ²H-CH₄ observations from one UK site, with source signature uncertainties reflecting our current understanding of these values. Results show a limited impact on emissions uncertainty or magnitude, when compared with output from an inversion using only mole fraction observations. This suggests that both the ongoing expansion of isotope ratio observations and an improved understanding of isotopic signatures is required for this method to be used to estimate UK FF and non-FF methane emissions, with reduced uncertainty compared to traditional inverse methods.