Maria Elena Popa

The ongoing debate over the origin of rising atmospheric methane levels highlights gaps in our understanding of its global budget. To our knowledge, we present the first reconstructed clumped isotopic composition of atmospheric methane dating back to the early 1990s, based on large-volume firn air samples from the Greenland ice cap. Our measurements indicate that atmospheric ∆¹²CH₂D₂ around 1993 was 10 ± 2 per mil lower than in the mid-2020s, likely marking the lowest level over the last millennium. We attribute this shift to the unprecedented anthropogenic perturbation of global methane emissions during the industrial period. The temporal lag in the atmospheric isotopic signature reflects the disequilibrium effect caused by the longer atmospheric lifetime of ¹²CH₂D₂ relative to methane (CH₄). Moreover, our results suggest that the kinetic isotope effect in the removal of ¹²CH₂D₂ by the hydroxyl radical needs to be much smaller than previously estimated. Future changes in atmospheric ∆¹²CH₂D₂ will continue to reflect evolving CH₄ emission and removal pathways.

Uncertainties regarding the origin of rising atmospheric methane levels underscore gaps in our understanding of the global methane cycle. The clumped isotopic composition (Δ13CH3D and Δ12CH2D2) of methane has recently been developed as an additional tracer to constrain atmospheric methanesources and sinks (1, 2). We present a novel perspective by reconstructing the clumped isotopic composition of atmospheric methane dating back to the early 1990s, based on large-volume firn air samples from the Greenland ice cap. Our measurements indicate that atmospheric ∆12CH2D2 around 1993 was 10 ± 2 ‰ lower than in the mid-2020s.We used a two-box atmospheric model to investigate the drivers of the observed ∆12CH2D2 evolution. Emission fluxes for 1980–2024 were taken from an inversion constrained by atmospheric CH4, δ13C, and δD observations (3). However, sensitivity experiments show that this large ∆12CH2D2 increase cannot be explained by recent changes in methane source composition, as the globally averaged source ∆12CH2D2 varies by less than 1 ‰ over recent decades. Instead, the signal reflects the long-term source-sink disequilibrium effects (4). Owing to the longer atmospheric lifetime of 12CH2D2 relative to other methane isotopologues, Δ12CH2D2 responds slowly to perturbations and records changes in the methane budget over multi-decadal to centennial timescales, leading to a temporal lag relative to changes in methane concentration and bulk isotopic ratios.Extending the simulations back to 1200 CE shows that accelerating methane emissions during industrialisation progressively drove atmospheric Δ12CH2D2 to lower values, reaching a minimum of ~40 ‰ in the late 20th century. The subsequent slowdown in methane growth after 1990 allowed partial re-equilibration, leading to the observed increase in Δ12CH2D2.Our results demonstrate that Δ12CH2D2 uniquely records the anthropogenic perturbation of the global methane cycle and suggest that the firn air samples measured in this study capture the lowest Δ12CH2D2 values of the past millennium. M. Sivan, T. Röckmann, C. van der Veen, M. E. Popa, Extraction, purification, and clumped isotope analysis of methane (Δ13CDH3 and Δ12CD2H2) from sources and the atmosphere. Atmos. Meas. Tech. 17, 2687-2705 (2024).M. A. Haghnegahdar et al., Tracing sources of atmospheric methane using clumped isotopes. Proceedings of the National Academy of Sciences 120, e2305574120 (2023).B. Dasgupta et al., Global Methane Emission Estimates from a Dual-Isotope Inversion: New Constraints from δD-CH₄. EGUsphere 2025, 1-21 (2025).P. P. Tans, A note on isotopic ratios and the global atmospheric methane budget. Gl. Biogeochem. Cycles 11, 77-81 (1997).

Many halogenated trace gases are important greenhouse gases and/or contribute to stratospheric ozone depletion, yet their spatial distribution and temporal evolution in the stratosphere remain poorly constrained. We here present a new high-altitude dataset of a large range of these gases. The results are based on a large balloon flight in the Arctic in summer 2021. Air samples were collected using a passive (AirCore) as well as an active (cryogenic) technique; the former being the largest AirCore flown to date, thus enabling the quantification of an expanded variety of halogenated gases. The evaluation of the results demonstrates good comparability in most cases, but also revealed strengths and weaknesses for both sampler types. In addition, we show examples of the scientific value of this data, including the identification of air masses likely originating from the Asian Monsoon region, and the derivation of the average stratospheric transit times (i.e., the mean ages of air) from multiple tracers.

Many halogenated trace gases are important greenhouse gases and/or contribute to stratospheric ozone depletion, yet their spatial distribution and temporal evolution in the stratosphere remain poorly constrained. We here present a new high-altitude dataset of a large range of these gases. The results are based on a large balloon flight in the Arctic in summer 2021. Air samples were collected using a passive (AirCore) as well as an active (cryogenic) technique; the former being the largest AirCore flown to date, thus enabling the quantification of an expanded variety of halogenated gases. The evaluation of the results demonstrates good comparability in most cases, but also revealed strengths and weaknesses for both sampler types. In addition, we show examples of the scientific value of this data, including the identification of air masses likely originating from the Asian Monsoon region, and the derivation of the average stratospheric transit times (i.e., the mean ages of air) from multiple tracers.

Many halogenated trace gases are important greenhouse gases and/or contribute to stratospheric ozone depletion, yet their spatial distribution and temporal evolution in the stratosphere remain poorly constrained. We here present a new high-altitude dataset of a large range of these gases. The results are based on a large balloon flight in the Arctic in summer 2021. Air samples were collected using a passive (AirCore) as well as an active (cryogenic) technique; the former being the largest AirCore flown to date, thus enabling the quantification of an expanded variety of halogenated gases. The evaluation of the results demonstrates good comparability in most cases, but also revealed strengths and weaknesses for both sampler types. In addition, we show examples of the scientific value of this data, including the identification of air masses likely originating from the Asian Monsoon region, and the derivation of the average stratospheric transit times (i.e., the mean ages of air) from multiple tracers.

Many halogenated trace gases are important greenhouse gases and/or contribute to stratospheric ozone depletion, yet their spatial distribution and temporal evolution in the stratosphere remain poorly constrained. We here present a new high-altitude dataset of a large range of these gases. The results are based on a large balloon flight in the Arctic in summer 2021. Air samples were collected using a passive (AirCore) as well as an active (cryogenic) technique; the former being the largest AirCore flown to date, thus enabling the quantification of an expanded variety of halogenated gases. The evaluation of the results demonstrates good comparability in most cases, but also revealed strengths and weaknesses for both sampler types. In addition, we show examples of the scientific value of this data, including the identification of air masses likely originating from the Asian Monsoon region, and the derivation of the average stratospheric transit times (i.e., the mean ages of air) from multiple tracers.

Within the HEMERA balloon infrastructure project, a stratospheric balloon carrying a multi-instrument payload to a maximum altitude of 31.2 km was launched on 12 August 2021. On board the openly constructed gondola, several types of instruments were used for simultaneous air sampling and in-flight measurements to characterize climate-relevant trace gases in the stratosphere and troposphere, as well as to compare and evaluate different instrumental approaches and sampling techniques. For observations of the main greenhouse gases carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and sulfur hexafluoride (SF₆), flask with AirCore sampling and in-flight spectrometry were deployed. Overall, results from different methods agree well. While better precision was achieved for the post-flight measurements of AirCore devices and flask sampling, in situ spectrometry provided a higher degree of detail on the vertical structure of the CH₄ profile. Age of air was derived from mixing ratios of CO₂ and SF₆. As seen in previous studies, higher values were obtained from SF₆ than from CO₂. Correcting for chemical losses, maximum values of 4.4–5.1 years were derived from SF₆ mixing ratios at altitudes above 20 km compared to 4.2–5.0 years from CO₂ mixing ratios. The resulting dataset should be well suited for multi-tracer approaches to derive age of air, particularly in combination with a large suite of halocarbons measured from flask and AirCore sampling and one more AirCore sample which was reported in a companion publication (Laube et al., 2025).

Carbonyl sulfide (COS) has been proposed as a proxy for gross primary production (GPP), as it is taken up by plants through a comparable pathway as CO₂. COS diffuses into the leaf and undergoes an essentially one-way reaction in the mesophyll cells, catalyzed by the enzyme carbonic anhydrase (CA), and does not exit the leaf again. In order to use COS as a proxy for GPP, however, the mechanisms of COS uptake and its coupling to CO₂ uptake need to be well understood. Characterizing the isotopic discrimination of COS during plant uptake can provide useful information on the COS uptake process and can help to constrain the COS budget. This study presents joint measurements of isotope discrimination during plant uptake for COS (CO³⁴S) and CO₂ (¹³CO₂ and C¹⁸O¹⁶O). A C₃ plant, sunflower (Helianthus annuus), and a C₄ plant, papyrus (Cyperus papyrus), were enclosed in a flow-through plant chamber and exposed to varying light levels. The incoming and outgoing gas compositions were measured online, and discrete air samples were taken for isotope analysis. The COS uptake flux was around 75 pmol mol^–1 for sunflower and between 99 and 110 pmol mol^–1 for papyrus. The corresponding ³⁴Δ for COS was 3.4 ± 0.8 ‰ for sunflower and 2.6 ± 0.3 ‰ for papyrus. For CO₂, a negative relationship was observed between the uptake flux and the isotopic discriminations ¹³Δ and ¹⁸Δ. The CO₂ uptake and Δ values indicate that our sunflower behaved as expected for a C₃ plant, while the papyrus was not displaying typical C₄ behavior, perhaps due to the relative low light conditions during our experiments.

Carbonyl sulfide (COS) has been proposed as a proxy for gross primary production (GPP), as it is taken up by plants through a comparable pathway as CO₂. COS diffuses into the leaf and undergoes an essentially one-way reaction in the mesophyll cells, catalyzed by the enzyme carbonic anhydrase (CA), and does not exit the leaf again. In order to use COS as a proxy for GPP, however, the mechanisms of COS uptake and its coupling to CO₂ uptake need to be well understood. Characterizing the isotopic discrimination of COS during plant uptake can provide useful information on the COS uptake process and can help to constrain the COS budget. This study presents joint measurements of isotope discrimination during plant uptake for COS (CO³⁴S) and CO₂ (¹³CO₂ and C¹⁸O¹⁶O). A C₃ plant, sunflower (Helianthus annuus), and a C₄ plant, papyrus (Cyperus papyrus), were enclosed in a flow-through plant chamber and exposed to varying light levels. The incoming and outgoing gas compositions were measured online, and discrete air samples were taken for isotope analysis. The COS uptake flux was around 75 pmol mol^–1 for sunflower and between 99 and 110 pmol mol^–1 for papyrus. The corresponding ³⁴Δ for COS was 3.4 ± 0.8 ‰ for sunflower and 2.6 ± 0.3 ‰ for papyrus. For CO₂, a negative relationship was observed between the uptake flux and the isotopic discriminations ¹³Δ and ¹⁸Δ. The CO₂ uptake and Δ values indicate that our sunflower behaved as expected for a C₃ plant, while the papyrus was not displaying typical C₄ behavior, perhaps due to the relative low light conditions during our experiments.