Emmal Safi

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.

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.

The stable isotope ratios of carbon (δ¹³C­(CH₄)) and hydrogen (δ²H­(CH₄)) in methane (CH₄) from atmospheric air samples provide a tracer that can help distinguish the relative contribution of emission sources. These can be continuously measured at atmospheric monitoring stations by optical isotope ratio spectrometer (OIRS) instruments, providing data that is complementary to isotope ratio mass spectrometry (IRMS) measurements. OIRS instruments directly measure the amount fraction of the ¹²CH₄, ¹³CH₄, and ¹²CH₃ ²H isotopologues, in contrast to the IRMS method of conversion to CO₂ and H₂, so they have different calibration needs related to reference materials (RMs) and analysis protocol. We use a single high-purity source of CH₄, which has been isotopically characterized by IRMS, to produce two calibration RMs that bracket the sample in amount fraction. The isotope ratio measurement is calibrated via the isotopologue amount fraction, which is used to derive analytical expressions for the combined uncertainty. We test this approach using a 550 μmol mol^–1 sample (representative of the amount fraction produced from air sampled by the NPL preconcentrator) prepared from a separate CH₄ source. The combined standard uncertainty for the isotope ratio of this sample is 0.19 ‰ for δ¹³C­(CH₄) and 1.1 ‰ for δ²H­(CH₄), including uncertainty contributions from the isotopic assignment of the CH₄ used for the RMs, their gravimetric preparation, and the spectrometer noise. The dominant contribution to this is from the uncertainty in isotopic assignment of the CH₄ used in RM preparation. The next largest contribution to the uncertainty budget is the measurement noise, estimated from the Allan-Werle deviation, and the smallest contribution is from the preparation uncertainty in the total CH₄ amount fraction in the RMs. We demonstrate that preparing the bracketing RMs from a common CH₄ source results in correlations between isotopologue amount fractions and that neglecting this leads to an overestimation of the isotope ratio uncertainty.

Preconcentration of methane (CH₄) from air is a critical sampling step in the measurement of singly and doubly substituted isotopologue ratios. We demonstrate the potential for isotope fractionation during preconcentration onto and elution from the common trapping material HayeSep-D and investigate its significance in laser spectroscopy measurements. By altering the trapping temperature for adsorption, the flow direction of CH₄ through the trap and the time at which CH₄ is eluted during a desorption temperature ramp, we explain the mechanisms behind fractionation affecting δ¹³C(CH₄) and δ²H(CH₄). The results highlight that carbon isotope fractionation is driven by advection and diffusion, while hydrogen isotope fractionation is driven by the interaction of CH₄ with the adsorbing material (tending to smaller isotopic effects at higher temperatures). We have compared the difference between the measured isotope ratio of sample gases (compressed whole air and a synthetic mixture of CH₄ at ambient amount fraction in an N₂ matrix) and their known isotopic composition. An open-system Rayleigh model is used to quantify the magnitude of isotopic fractionation affecting measured δ¹³C(CH₄) and δ²H(CH₄), which can be used to calculate the possible magnitude of isotopic fractionation given the recovery percentage. These results provide a quantitative understanding of isotopic fractionation during the sample preparation of CH₄ from ambient air. The results also provide valuable insights applicable to other cryogenic preconcentration systems, such as those for measurements that probe the distribution of rarer isotopologues.

The precipitation of hydrated phases from a chondrite-like Na–Mg–Ca–SO₄–Cl solution is studied usingin situsynchrotron X-ray powder diffraction, under rapid- (360 K h⁻¹,T= 250–80 K,t= 3 h) and ultra-slow-freezing (0.3 K day⁻¹,T = 273–245 K,t= 242 days) conditions. The precipitation sequence under slow cooling initially follows the predictions of equilibrium thermodynamics models. However, after ∼50 days at 245 K, the formation of the highly hydrated sulfate phase Na₂Mg(SO₄)₂·16H₂O, a relatively recent discovery in the Na₂Mg(SO₄)₂–H₂O system, was observed. Rapid freezing, on the other hand, produced an assemblage of multiple phases which formed within a very short timescale (≤4 min, ΔT= 2 K) and, although remaining present throughout, varied in their relative proportions with decreasing temperature. Mirabilite and meridianiite were the major phases, with pentahydrite, epsomite, hydrohalite, gypsum, blödite, konyaite and loweite also observed. Na₂Mg(SO₄)₂·16H₂O was again found to be present and increased in proportion relative to other phases as the temperature decreased. The results are discussed in relation to possible implications for life on Europa and application to other icy ocean worlds.