- Isotopologues of carbon dioxide (CO2), water (H2O), methane (CH4), carbon monoxide (CO), oxygen (O2), carbonyl sulfide (COS), and nitrous oxide (N2O)
- Novel tracers and biological analogues
- Polyisotopocules including "clumped isotopes"
- Non-mass-dependent isotopic fractionation and related isotope anomalies
- Intramolecular stable isotope distributions ("isotopomer abundances")
- Quantification of isotope effects
- Analytical, methodological, and modelling developments
- Flux measurements
Orals: Tue, 5 May, 16:15–17:55 | Room 1.31/32
Subtropical forest soils are global hotspots of nitrous oxide (N2O) emissions and are increasingly exposed to extreme meteorological variation. Episodic drying-rewetting events can strongly alter N cycling, yet the mechanisms by which event-scale precipitation frequency at constant precipitation amount regulate N2O production and reduction in N-saturated forests remain poorly constrained. Here, we conducted an in-situ precipitation simulation after a summer drought at the Tieshanping forest site in Southwest China under three treatments: Control without adding water, single heavy precipitation event (30 mm on the first day), and multiple precipitation event (10 mm d−1 over the first three days; same total amount as the single precipitation event).
We measured N2O fluxes together with the natural abundance N2O isotopes, δ15Nbulk and δ15NSP (i.e. 15Nα − 15Nβ), as well as soil moisture, KCl-extractable mineral N and water-extractable organic carbon. Isotopocule-based mapping and end-member mixing were used to partition production pathways and quantify the N2O reduction to N2. The single precipitation event rapidly increased water-filled pore space (WFPS) to more than 90% and triggered a pronounced N2O emission peak (more than 200 μg N m−2 h−1) which was dominated by denitrification, while N2O reduction remained limited. Under multiple precipitation events, the peak N2O flux was delayed and followed by strong negative fluxes (−130 μg N m−2 h−1), accompanied by a marked increase in δ15NSP (~40‰), indicating enhanced N2O reduction. Notably, cumulative N2O emissions during this 5-day simulation were highest under single precipitation events (5 mg N m−2), followed by control treatment and multiple precipitation events (3.7 and 0.8 mg N m−2, respectively).
Across all treatments, soil moisture together with availability of soil nitrate and labile carbon controlled the shifts in N2O sources and sinks we observed. Our findings provide process-level constraints on how event-scale precipitation frequency reshape N2O source-sink dynamics in N-saturated subtropical forests, and highlight the importance of incorporating precipitation frequency and intensity into predictions of forest N2O responses under future extreme climate events.
How to cite: Zhang, B., Yu, L., Wang, Y., and Homyak, P.: Precipitation frequency constrains N2O source-sink dynamics in an N-saturated subtropical forest: Insights from natural abundance N2O isotopes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-407, https://doi.org/10.5194/egusphere-egu26-407, 2026.
Nitrous oxide (N2O) emissions from biological nitrogen removal dominate the carbon footprint of the wastewater treatment (WWT) sector. Understanding both the major N2O production pathways and its reduction to dinitrogen (N2) is essential for effective mitigation. In WWT, N2O is mainly produced via three microbial pathways: (i) hydroxylamine oxidation, (ii) nitrifier denitrification, and (iii) heterotrophic denitrification; only the latter can also reduce N2O to N2. Analysis of the isotopologues, 14N15N16O, 15N14N16O, and 14N14N18O, relative to 14N14N16O, expressed in the δ notation, enables pathway identification by comparing measured signatures to reported endmember values. However, those endmember values are derived from limited pure culture or lab incubations and may not represent complex ecosystems such as those in activated sludge of WWT plants sufficiently, necessitating system-specific source signatures. This study combines isotopic analysis of produced N2O with dedicated process control to disentangle microbial pathways and quantify N2O reduction under realistic operating conditions. Online N2O isotopic measurements were performed over a one-year period using off-axis integrated cavity output spectroscopy (LGR-ABB) at two 8 m3 pilot-scale sequencing batch reactors during aeration phases treating municipal wastewater (Eawag, Dübendorf, Switzerland). Results indicate no significant contribution of hydroxylamine oxidation to N2O production, while both nitrifier and heterotrophic denitrification emit N2O under specific process conditions and are characterized by system specific isotopic endmembers. The availability of NH4+ as an electron donor is a prerequisite for nitrifier denitrification, while heterotrophic denitrification needs low dissolved oxygen (DO) concentration. Process-specific isotopic fingerprints were applied to disentangle active pathways and assess their response to pH, carbon availability, and DO. In parallel, we calculated the fraction of reduced N2O by applying the Rayleigh equation coupled with published fractionation factors to our data. N2O reduction decreased within individual aeration phases and varied between cycles. Preliminary results from explainable machine learning identified pH, temperature, NO3-, DO, and peak NO2- as key drivers of N2O reduction. This work provides a comprehensive isotopic framework for simultaneously resolving N2O production pathways and N2O reduction dynamics in WWT, advancing process understanding and informing operational strategies to mitigate greenhouse gas emissions in the wastewater sector.
How to cite: Keck, H., Strubbe, L., Magyar, P. M., Froemelt, A., Joss, A., and Mohn, J.: Isotopic analysis to identify N2O production pathways and to quantify its reduction in wastewater treatment , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11478, https://doi.org/10.5194/egusphere-egu26-11478, 2026.
Rapid urbanization and industrialization in Southeast Asia have substantially increased emissions of reactive nitrogen (N), raising concerns over atmospheric nitrogen deposition and its environmental impacts in the region. Here, we combine nitrogen stable isotopes measurements with precipitation chemistry and flux observations to investigate the magnitude and sources of wet nitrogen deposition in two major cities, Singapore (SG) and Ho Chi Minh City (HCMC). Both SG and HCMC exhibit high annual wet nitrogen deposition fluxes of 31.3 and 30.4 kg N ha-1 yr-1, respectively, with ammonium (NH4+) and nitrate (NO3-) as dominant components. Isotope mixing models indicate comparable contributions from fossil and non-fossil fuel sources to NOx emissions in both cities. In SG, combustion-related NH3 sources account for ~66% of NH4+ deposition, whereas in HCMC, volatilized sources such as agriculture and waste play a more significant role. Dissolved organic nitrogen was primarily attributed to biogenic emissions, including plant debris. Seasonal variations in deposition are associated with monsoon-driven transboundary transport, reflecting the regional coupling of nitrogen emissions and deposition. Our findings demonstrate the strong influence of anthropogenic activities and cross-border pollutant transport on nitrogen deposition in tropical megacities, with implications for targeted emission control strategies in rapidly urbanizing regions.
How to cite: He, S., Su, T.-H., Zhang, X.-Y., Zhang, K., To, T. H., Wang, X., and Liu, X.: Tracing anthropogenic nitrogen deposition in Southeast Asian megacities: Isotopic evidence from Singapore and Ho Chi Minh City , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2292, https://doi.org/10.5194/egusphere-egu26-2292, 2026.
The oxygen-17 isotope anomaly (Δ17O) serves as a powerful tool to elucidate the chemical transformation mechanisms of atmospheric reactive nitrogen species such as NO2 and HONO. Current studies employ the denuder collection methods to convert atmospheric NO2 and HONO into nitrite for isotopic analysis. However, accurate Δ17O measurement of atmospheric NO2 and HONO is hampered by the lack of internationally recognized nitrite isotope reference materials with applicable Δ17O signals. In this study, we prepared new nitrite isotope standards with nonzero Δ17O signals through oxygen isotope exchange between high-purity nitrite reagents and 17O-enriched water. Using a developed ozone oxidation calibration method, the Δ17O values of a newly prepared nitrite standard (i.e., N-Δ17O-1) and the international nitrite reference material RSIL-N10219 were determined as (69.7 ± 1.0) ‰ (n = 10, 1σ) and (-8.7 ± 0.3) ‰ (n = 11, 1σ), respectively. The two additional O-17 enriched nitrite standards were then measured and calibrated against RSIL-N10219 and N-Δ17O-1, yielding Δ17O values of (34.5 ± 0.3) ‰ (n = 6, 1σ) and (6.4 ± 0.1) ‰ (n = 8, 1σ), respectively. The δ15N and δ18O values of the three home-made nitrite isotope standards were also calibrated against international nitrite reference materials. This study introduces a new and reliable method to obtain the Δ17O values of nitrite, and the establishment of Δ17O values of nitrite standards provides a foundation for accurately assessing Δ17O variations atmospheric NO2 and HONO. The latter will facilitate the application of the Δ17O tracer in investigating atmospheric cycling of reactive nitrogen and radicals.
How to cite: Zhou, T., Albertin, S., Jiang, Z., Savarino, J., and Geng, L.: Preparation and calibration of O-17 enriched nitrite isotope standards, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19689, https://doi.org/10.5194/egusphere-egu26-19689, 2026.
Earth atmospheric O2 is mainly produced by biosphere photosynthesis, and biosphere respiration is also one of the main consumers of this gas. The evolution of atmospheric O2 is thus linked to global biosphere productivity, and in particular the isotopic composition of O2 (δ18O and δ 17O). Quantitative interpretation of the isotopic composition of O2 in the past relies on robust estimate of oxygen fractionation coefficients associated with the relevant biological processes: photosynthesis and respiration. In the past decades, some determinations of these biological fractionation coefficients were performed in uncontrolled large-scale environments or at the scale of the micro-organisms in conditions very different from the natural environment. There are thus uncertainties in the applicability of the previous determinations of the O2 fractionation for the interpretation of δ18O and δ17O of atmospheric O2.
In order to come up with coherent estimates of oxygen biological fractionation coefficients applicable to the scale of plants or ecosystems, we developed closed biological chambers as a biosphere replica, with controlled environment parameters, and measured the dynamics of O2 concentration and of its isotopic composition.
Our set-up is based on round-bottom stainless steel tube of 10 cm in diameter and 88 cm in height to simulate a water column, on top of which we place a structure equipped with sensors (temperature, CO2 concentration, O2 elemental and isotopic measurements) to obtain a closed system. The multiplexing system that we developed can allow to use 6 tubes simultaneously to run replicate studies in parallel with the same environmental conditions.
We present here 3 measurement series, lasting between 2 and 9 months, run with the freshwater species, chlorella vulgaris. These measurement series permit to optimize the use of our newly developed system for aquatic closed biological chambers. We also determined the isotopic discrimination associated with 18O/16O of O2 during respiration as -30 permil which is higher than most of the previously published values. We will also compare these results with new values measured with our setup for oceanic species (the diatoms Phaeodactylum). Finally, we will use the newly determined fractionation coefficients to improve interpretation of the δ18O of O2 record in air bubbles from ice cores
How to cite: Bienville, N., Landais, A., Fiorini, S., Piel, C., Sauze, J., Lemaire, B., Geyskens, N., Prie, F., Jossoud, O., Paul, C., Chaillot, J., Chollet, S., Abiven, S., and Dapoigny, A.: An intermediary scale setup to measure O2 fractionation factors of aquatic biosphere and application to the interpretation of the δ18O of O2 records found in deep ice cores., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8150, https://doi.org/10.5194/egusphere-egu26-8150, 2026.
Methane (CH4) is a strong greenhouse gas, yet its global budget remains incompletely constrained. The uncertainties in its sources and sinks limit the implementation of successful mitigation. Stable isotope analysis (δ13C-CH4 and δ2H-CH4) offers powerful constraint for methane source attribution, but the accuracy of these constraints depends on accurate values of the kinetic isotope effects (KIEs) associated with its primary removal process, reaction with the OH radical.
Here, we present new laboratory measurements of both carbon and hydrogen isotope fractionation during the CH4 + OH reaction. Our experimental design included extensive control runs to eliminate potential interferences from secondary radical species. In addition, we used kinetic chemical model and a reaction - transport model to verify that the observed fractionation results are exclusively driven by the OH oxidation.
We determined the fractionation across a wide temperature range to cover various atmospheric condition. Our data reveal a moderate but clear temperature dependence for both δ13C-CH4 and δ2H-CH4 fractionation, which is evaluated against theoretical estimates to assess its implications. These findings resolve previous literature discrepancies and provide a refined benchmark for inverse modeling applications.
How to cite: Chen, C.-C., Adnew, G., Gilbert, A., van der Veen, C., Mikkelsen, M., Johnson, M., Li, J., Krol, M., and Röckmann, T.: High-precision determination of the temperature-dependent kinetic isotope effect for the CH4 + OH reaction, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12644, https://doi.org/10.5194/egusphere-egu26-12644, 2026.
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).
How to cite: Sivan, M., Sun, J., Martinerie, P., Popa, M. E., Farquhar, J., Krol, M., van der Veen, C., Dasgputa, B., Haghnegahdar, M. A., Jensen, C. M., Yang, J.-W., Freitag, J., Fourteau, K., Blunier, T., and Röckmann, T.: Anthropogenic perturbations to atmospheric methane reflected in Greenland firn air clumped isotope measurements, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10863, https://doi.org/10.5194/egusphere-egu26-10863, 2026.
Stable isotope measurements of atmospheric CO2 are a powerful tool for partitioning contributions of different CO2 sources and sinks. In addition to the conventional tracers δ13C and δ18O, the “clumped isotope” tracer Δ47 can improve the distinction between high- and low-temperature sources of atmospheric CO₂ in urban studies. Despite its potential, Δ₄₇ measurements of atmospheric CO₂ remain sparse, particularly from long-term observations. One reason for this may be the high effort of manually processing samples for the measurement of Δ47 in atmospheric CO₂, with high precision analysis typically require several hours per sample. Here, we present an automated preparation line coupled with a dual inlet isotope ratio mass spectrometer (MAT253+). This setup enables automated extraction and purification of atmospheric CO2 and measurement of approximately five atmospheric CO₂ samples per day with sample preparation time of about 90 minutes. Over a 10-month period, the system achieved a reproducibility of ∼ 0.005 ‰ for δ13C, ∼ 0.01 ‰ for δ18O, and ∼ 0.011 ‰ for Δ47.
Regular measurements using this setup provided insight into the temporal change in atmospheric Δ47 in the semi-urban area of Heidelberg (Germany). In addition to the technical challenges, also the scientific interpretation of atmospheric Δ47 data is not straightforward, because this “clumped isotope” tracer exhibits nonlinear behavior during air-mass mixing. Consequently linear extrapolation approaches such as the traditional Keeling plots can yield biased source signature estimates. We therefore present a thorough correction procedure applicable to cases where CO₂ enhancements are too small to allow a direct nonlinear fit.
How to cite: Eckhardt, H., Schmidt, M., Röckmann, T., and Frank, N.: Semi-continuous automated Δ47 measurements of atmospheric CO2, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22195, https://doi.org/10.5194/egusphere-egu26-22195, 2026.
Carbon monoxide (CO) is an indirect short-lived climate forcer with uncertainties both in sources and its role in atmospheric oxidation. Based on nine winter-long dual-isotope campaigns at two South Asian receptor sites intercepting the continental outflow, we quantified CO source contributions and emission–sink dynamics. Combustion accounts for 68–74% of South Asia regional CO (including 34–37% from biomass burning) with secondary atmospheric oxidation contributing 26–32% (dominated by oxidation of non-methane volatile organic compounds NMVOCs at 21–26% with methane oxidation contributing 5.5–6.4%). These isotope-observational constraints suggest a twice higher role for atmospheric oxidation than in model estimates. Spatially, the absolute contributions of both primary and secondary CO decrease from the Indo-Gangetic Plain (IGP) to the northern Indian Ocean, indicating enhanced oxidation near source regions, while the relative contribution of secondary CO increases. Observation-model comparison suggests that continental transport dominates CO over adjacent oceanic regions, while local production is minor. During the COVID-19 pandemic, combustion-derived CO fell sharply, NMVOC-derived CO rose, and CH4-derived CO remained stable, suggesting enhanced oxidation from reduced competition among precursors. Our results reveal a far greater contribution of CO from atmospheric oxidation in South Asia than in current model estimates, highlighting the need for sustained emission controls to deliver concurrent climate and health benefits.
How to cite: Yao, P., Holmstrand, H., van der Veen, C., Elena Popa, M., A. Brashear, C., Budhavant, K., Remani Manoj, M., Romson, J., Salam, A., Röckmann, T., and Gustafsson, Ö.: Multi-year dual-isotope fingerprinting at South Asian receptor sites constrain carbon monoxide sources and enhanced oxidation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12027, https://doi.org/10.5194/egusphere-egu26-12027, 2026.
Nitrogen (N) oxides (NOx= NO + NO2) are central to both climate and air quality, acting as short-lived climate forcers while driving the formation of harmful pollutants such as tropospheric ozone (O3) and particulate matter. Although fossil fuel combustion dominates global NO emissions, natural sources—including terrestrial and aquatic ecosystems—contribute an estimated 20–30% to the atmospheric NO budget.1 These natural sources remain poorly constrained, limiting confidence in surface ozone projections under future climate scenarios. Global and regional models are thought to under-estimate terrestrial contributions, particularly from soils, due to poor representation of these sources in emission inventories. Freshwater ecosystems have been considered negligible and are entirely absent from the IPCC AR6 assessment. Improved characterisation of natural NOx emissions requires new measurement approaches capable of distinguishing between different emission sources.
Isotopic analysis of N species provides a powerful tool for source attribution and process identification, as different emission pathways can imprint distinct isotopic composition. We present the development of a laser-induced fluorescence (LIF) instrument for highly sensitive, isotopologue-resolved measurement of atmospheric nitric oxide. The instrument uses a Cavity PQS Nd:YAG Cr⁴⁺:YAG microlaser with precise wavelength control near 214.8 nm to selectively excite ¹⁴N¹⁶O, ¹⁵N¹⁶O, and ¹⁴N¹⁸O by probing distinct rovibrational transitions. Through off-resonance fluorescence detection, the instrument is designed to enable differentiation of isotopologues without signal overlap, providing near-simultaneous, real-time quantification. We expect to achieve sub-pptv sensitivity, enabling measurements of natural NOx emissions in remote environments. Future perspectives include the isotopic fingerprinting of biotic and abiotic NO emissions, with broad applicability in aquatic and terrestrial biogeochemistry studies.
[1] Jaeglé et al. (2005) Faraday Discussions DOI: 10.1039/B502128F
How to cite: Alden, J., Perrette, Y., Berthome, Q., Faure, B., Souhaité, G., Boustead, G., and Bourgeois, I.: Development of a Highly Sensitive Laser-Induced Fluorescence Instrument for Isotopologue-resolved Measurements of Atmospheric Nitric Oxide. , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7145, https://doi.org/10.5194/egusphere-egu26-7145, 2026.
Gross primary production (GPP) is a key driver of the terrestrial carbon cycle, yet it cannot be directly measured at the ecosystem scale. Carbonyl sulfide (COS) has emerged as a promising tracer for GPP because it shares the same leaf-level diffusion pathway as CO₂ and is, with negligible re-emission, irreversibly taken up by plants through carbonic anhydrase. As a result, ecosystem-scale COS fluxes provide an independent constraint on photosynthetic CO2 uptake and offer new opportunities to evaluate and refine GPP estimates derived from eddy covariance and modeling approaches. However, uncertainties remain regarding how vegetation type, canopy structure, and environmental conditions modulate COS uptake across ecosystems. In particular, the leaf relative uptake rate (LRU) of COS and CO2 deposition velocities, commonly used to infer GPP from COS fluxes, varies across species and depends on factors such as photosynthetic active radiation and vapor pressure deficit.
Here, we present a multi-site synthesis of ecosystem-scale COS exchange measurements spanning a broad range of vegetation types and climatic conditions, including savanna (Quercus ilex), temperate mountain grassland, agricultural cropland (Glycine max.), temperate deciduous forest (Fagus sylvatica), and temperate coniferous forest (Pinus sylvestris). These sites differ markedly in plant functional types, leaf morphology, phenology, canopy structure, and typical environmental forcing, providing an ideal framework to investigate controls on COS uptake across ecosystems.
All COS fluxes are derived using eddy covariance measurements and processed with a unified workflow using the EddyUH software, ensuring methodological consistency across sites and enabling robust cross-ecosystem comparisons without confounding effects from data processing choices. This harmonized approach allows us to focus on ecosystem-specific drivers rather than methodological artifacts.
Our analysis explores how differences in plant species influence COS uptake dynamics, and how these interact with environmental drivers such as photosynthetically active radiation, vapor pressure deficit, air temperature and soil moisture.
The results presented will provide new insights into ecosystem-specific COS exchange behavior and its implications for using COS as a tracer for GPP across heterogeneous landscapes. Ultimately, this work aims to improve our understanding of how vegetation and climate jointly regulate COS fluxes and to support the broader application of COS-based approaches for constraining ecosystem-scale photosynthesis.
How to cite: Spielmann, F., Hammerle, A., De-Vries, A., Sakowska, K., and Wohlfahrt, G.: Cross-Ecosystem Patterns in Carbonyl Sulfide Exchange, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9266, https://doi.org/10.5194/egusphere-egu26-9266, 2026.
Recent theoretical and laboratory studies [1, 2] provide new and updated estimates of carbon and hydrogen kinetic isotope effects in the gas-phase reactions removing atmospheric methane (CH4). In particular, new temperature dependence of kinetic fractionation in reactions with hydroxyl (OH) and chlorine (Cl) radicals is determined for a range of isotopomers, including the multiply substituted (clumped) ones. In this study, we use the ECHAM/MESSy Atmospheric Chemistry (EMAC) model in a comprehensive chemistry-inclusive hind-cast setup [3] to obtain projections of the nominal and effective sink fractionation in atmospheric CH4 throughout the 1960–2020 period, based on the new isotope kinetic data. Use of EMAC allows to obtain realistic spatiotemporal distribution of fractionation magnitudes resulting from convolution of temperature and sink rate distributions, further modified by atmospheric transport and mixing.
Our simulations yield a range of significantly different projections for most of the isotopomers, compared either to the literature values or to theoretical approach/laboratory data considered. The laboratory and more advanced theoretical approaches yield larger fractionations for both 13CH4 and CH3D in reactions with OH. The opposite is obtained for the reactions with Cl, however with more advanced theory being closer to the laboratory data-based estimates. For clumped isotopomers, comparison to available literature data yields no systematic relationships.
The obtained time series witness a significant (up to 1.5‰) increase in the total nominal 13CH4 sink fractionation during 1960–1990 due to the changes in the stratospheric Cl sink distribution, following the onset of anthropogenic chlorofluorocarbons (CFC) emissions. After the global ban on CFCs, a reverse gradual decrease on the order of 0.1‰/decade is projected. A similar, though much smaller in relative magnitude, evolution is estimated for CH3D. Whilst the mean OH the sink rate-weighted atmospheric temperature exhibits a slight positive trend, the Cl and O(1D) sink rates-weighted temperatures witness larger decreases, in line with tropospheric warming and stratospheric cooling occurring in the last decades. We discuss the implications and uncertainties of our findings for isotope-inclusive efforts to improve past and present CH4 atmospheric budget estimates.
References
1. M. K. Mikkelsen, et al., Kinetic isotope effects in methane oxidation reactions: temperature dependence of the OH and Cl KIEs for 13CH4, CDH3, 13CDH3, CD2H2, CD3H, and CD4 from 100 to 500 K, AGU Fall Meeting, B13N-1736, 2025. https://agu.confex.com/agu/agu25/meetingapp.cgi/Paper/1911885
2. C.-C. Chen, C. van der Veen, G. Adnew, T. Röckmann, Comparative analysis of 13CKIE and DKIE in CH4-OH reaction, AGU Fall Meeting, A21M-2178, 2025. https://agu.confex.com/agu/agu25/meetingapp.cgi/Paper/1944364
3. P. Jöckel, et al., RD1SD: EMAC CCMI-2022 hindcast simulations with specified dynamics, ERA-5, 1979-2019. World Data Center for Climate (WDCC) at DKRZ. Deposited 18 June 2024. https://doi.org/10.26050/WDCC/ESCiMo2_RD1SD
How to cite: Gromov, S., Chen, C.-C., Johnson, M. S., Mikkelsen, M. K., Pozzer, A., and Röckmann, T.: Atmospheric methane sink isotope fractionation throughout last six decades: Projections using new kinetic data and implications for CH4 budget, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18527, https://doi.org/10.5194/egusphere-egu26-18527, 2026.
Although our ability to reconstruct precise atmospheric carbon dioxide (CO2) levels is currently limited to the last 800,000 years, CO2 has played a fundamental role in regulating Earth's climate and biosphere evolution since the Precambrian.
A significant rise in airborne CO2 began with the industrial revolution, driven largely by the byproduct release of hydrocarbon combustion. The resulting increase in tropospheric CO2 concentrations has led to global warming and associated climate change impacts, including rising sea levels, extreme weather events, and biodiversity loss. Urban areas, as major consumers of fossil fuels, are key contributors to rising emissions. However, in geologically active regions, natural volcanic (geogenic) emissions also contribute significantly to the local carbon budget. In such mixed environments, measurements of bulk CO2 concentration alone cannot resolve the source apportionment between geogenic and anthropogenic origins. Consequently, a combination of concentration measurements and stable isotope analysis is required to distinguish these sources effectively.
This study monitors the stable isotope composition and concentration of CO2 in Sicily (Italy) and Canary Islands (Spain). Laser-based isotope analyzers were deployed onsite to detect various CO2 isotopologues. Each instrument measured the various isotopologues of CO2 (e.g., COO, 13COO, and C18OO), and total CO2 concentration. Measurements were conducted at Palermo, in the Madonie Mountains, Puerto Naos (La Palma), and Puerto de la Cruz (Tenerife). Data were referenced hourly and calibrated daily using standard reference materials, achieving an accuracy of ± 0.25‰ for isotope compositions and ±1 ppmv for concentration.
We present a comparison of CO2 isotope compositions across diverse environmental settings. The results demonstrate that volcanic and anthropogenic emissions can be successfully distinguished based on the carbon isotope signature (δ13C-CO2) of atmospheric CO2. Furthermore, variations in both concentration and isotope composition related to latitude (sub-tropical to mid-latitude) and altitude (sea level to approximately one-hundred meters above sea level) were investigated. These findings highlight the necessity of dual-tracer monitoring (concentration and isotopes) in volcanic and urban regions to evaluate greenhouse gas emission dynamics, inform climate mitigation strategies, and assess environmental health risks.
How to cite: Di Martino, R. M. R., Gurrieri, S., Liotta, M., Pérez, N. M., Asensio-Ramos, M., Padrón, E., Melián, G. V., Hernández, P. A., Méndez-Pérez, C., Padilla, G. D., and Coldwell, B. C.: Differentiating anthropogenic and geogenic carbon dioxide (CO2) sources in urban and volcanic environments. Case studies from Sicily (Italy), La Palma and Tenerife (Canary Islands, Spain), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6762, https://doi.org/10.5194/egusphere-egu26-6762, 2026.
Changes in atmospheric sinks could explain the renewed increase of CH4 and the simultaneous decrease in δ13C(CH4) since 2007. In this work, we present comprehensive numerical sensitivity simulations to explore how atmospheric methane oxidation (both by OH and Cl), uptake by soil, and uncertainties in the kinetic isotope effect (KIE) influence the simulated global atmospheric δ13C(CH4) trend. Furthermore, we examine the sensitivity of the latter to reduced emissions from biomass burning, which have relatively high isotopic source signatures. We use the state-of-the-art global chemistry-climate model EMAC with a simplified approach to simulate CH4 loss. The model considers all four CH4 isotopologues and the (partly temperature-dependent) isotope fractionation during physical and chemical loss of CH4. Our setup uses recent CH4 emission inventories and accounts for regional differences in the corresponding isotopic signatures depending on source material and type of formation. Our results emphasize the importance of atmospheric sinks when interpreting the global CH4 budget with respect to δ13C(CH4).
How to cite: Nickl, A.-L., Jöckel, P., Winterstein, F., and Schmidt, A.: Modelling the impact of atmospheric sink variability and CH4 biomass burning emissions on the global mean δ13C(CH4) trend with the comprehensive chemistry-climate model EMAC., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9958, https://doi.org/10.5194/egusphere-egu26-9958, 2026.
Ammonia (NH3), the most abundant atmospheric alkaline trace gas, plays a crucial role in fine particulate matter formation and nitrogen cycling. While agriculture is the primary source, recent studies highlight significant contributions from non-agricultural urban sources. However, NH3 spatiotemporal variability is complex and not fully understood, necessitating reliable and high time-resolution data. For more comprehensive source apportionment, combining such data with stable nitrogen isotope ratio (δ15N) analysis serves as a powerful approach.
To enable high time-resolution measurements, we previously developed a continuous measurement system using a counter-current flow tube (CCFT) technique (Huy et al., J. Atmos. Chem. 73, 223-240, 2016). Initially designed for ambient levels, its performance at elevated concentrations typical of emission sources has not yet been evaluated. In this study, we modified the CCFT measurement system to collect NH3 as ammonium (NH4+) in an aqueous solution for δ15N analysis. We evaluated its absorption efficiency and δ15N measurement accuracy across a wide range of NH3 concentrations.
Gaseous NH3 was captured in pure water using the modified CCFT sampling system (Huy et al., 2016). Sample air was drawn at 1.0 L min-1 into a vertical tube, while pure water was introduced from the top at 0.12 mL min-1. NH3 was absorbed by diffusion and dissolution into the counter flowing solution. NH4+ concentrations were determined by ion chromatography; δ15N was measured using the denitrifier method and a GasBench II system coupled to an isotope ratio mass spectrometer. Detailed procedures are provided in Kawashima et al. (Rapid Commun. Mass Spectrom. 35, e9027, 2021). To evaluate the collection efficiency and isotopic accuracy, the modified CCFT system was operated in parallel with a conventional boric acid (BA) trap system as a reference.
For concentrations measured by a BA trap ([NH3]BA) exceeding 600 μg m-3, CCFT absorption efficiencies were clearly below 1.0, whereas efficiencies nearly reached 1.0 below 300 μg m-3. The difference in δ15N of NH3 between the CCFT and BA systems increased with [NH3]BA, reaching 12.17 ‰ at 1307.1 μg m-3. This suggests that within the CCFT sampler, lighter 14NH3 is less efficiently collected than heavier 15NH3. The isotopic difference was particularly pronounced above 400 μg m-3.
How to cite: Fujii, Y., Tachibana, A., Sawabe, M., Kawashima, H., and Takenaka, N.: Performance of a Counter-Current Flow Tube Method for Gaseous NH3 Collection and Isotope Analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11530, https://doi.org/10.5194/egusphere-egu26-11530, 2026.
Atmospheric nitrate, comprising particulate NO3- and gas-phase nitric acid (HNO3), is a highly significant product of the oxidation of NOx (= NO + NO2). Its isotopic composition (Δ17O, δ18O and δ15N) provides valuable information on NOx sources and atmospheric oxidation pathways, making nitrate preserved in ice cores a proxy for past atmospheric chemical reactivity. However, the interpretation of these ice core records is currently limited by an incomplete understanding of both isotope fractionation and isotope clumping effects associated with different nitrate formation pathways.
To better constrain these effects, we conducted a series of atmospheric chamber experiments in CESAM to investigate atmospheric nitrate production via two major nocturnal formation pathways: N2O5 heterogeneous hydrolysis on aerosol particles and the oxidation of volatile organic compounds by the NO3 radical. Reactant concentrations, temperature, and humidity were monitored and controlled throughout each chamber experiment, and the resulting nitrate was collected on filters for isotope analysis. In addition, particulate NO3- formation and aerosol chemical composition were quantified simultaneously using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), providing process-level constraints on nitrate production for each formation pathway.
This study aims to investigate whether distinct isotopic signatures arise between nitrate produced via N2O5 hydrolysis and NO3-VOC reactions. The isotopic composition of the produced nitrate (Δ17O, δ18O, and δ15N) will be analysed to quantify pathway-dependent isotope effects. In addition, a newly developed methodology using the ESI-Orbitrap mass spectrometer will be applied to measure clumped isotopes (i.e. Δ15N18O and Δ18O18O) in the produced nitrate, to evaluate whether clumped isotope signatures provide an additional constraint on nitrate formation mechanisms.
How to cite: Alden, K. A., Saville, J., Witwicky, J., Caillon, N., Cazaunau, M., Acharja, P., Pangui, E., Lopez, D., Dewald, P., Beltran, L., Cirtog, M., Picquet-Varrault, B., and Savarino, J.: Investigating Nitrate Formation Pathways Using Isotope Analysis in Controlled Chamber Experiments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17312, https://doi.org/10.5194/egusphere-egu26-17312, 2026.
In the face of natural and anthropogenic emissions, the habitability of Earth’s atmosphere is maintained thanks to atmospheric oxidants – photochemically produced reactive species which destroy toxic pollutants, remove greenhouse gases and maintain chemical stability. Understanding the chemical dynamics of the atmosphere is hence crucial for accurate predictions of air quality and radiative forcing in a changing climate, as identified in IPCC AR6. Because the species involved are often highly reactive, studying historic atmopsheric chemistry is challenging: consequently, there is no consensus on the magnitude or the sign of the relationship between atmospheric oxidative capacity and global climate.
One avenue for past atmospheric chemistry reconstructions is isotopic analysis of nitrate – an oxidation product of atmospheric NOx – archived in non-polar ice cores. The N and O isotope compositions of ice core nitrate depend on past oxidation reactions and past NOx sources, while newly-accessible nitrate clumped isotopes may provide complementary information on nitrate formation pathways. However, useful signals can be obscured by isotopic fractionation during nitrate transport, deposition and burial, while seasonal variations in atmospheric chemistry or snow accumulation can bias ice core records. These difficulties often make ice core nitrate isotope interpretations non-unique, limiting their utility as investigative tools for past atmospheric chemistry.
To investigate the processes controlling nitrate isotopes archived in non-polar ice cores, we collected firn cores and weekly high-volume atmospheric samples at high altitude sites in the Mont-Blanc massif (France/Italy) and the Cordillera Oriental (Bolivia). Using the newly-adapted Electrospray-Orbitrap mass spectrometer, we investigated the seasonality of atmospheric nitrate isotope ratios δ15N, δ18O and Δ17O, and clumped isotopes Δ15N18O and Δ18O18O, and compared atmospheric isotopic signals to those in contemporaneously-deposited firn over an accumulation season. We find substantial seasonal isotopic variability in atmospheric nitrate, which is partially preserved in firn core records. However, several isotopic disagreements could reflect syn- or post-depositional isotopic fractionation processes, and the isotopic seasonality should be carefully considered when intepreting ice core records where accumulation is seasonal.
How to cite: Saville, J., Witwicky, J., Attonde, D., Zheng, J., Gautier, E., Ginot, P., Caillon, N., and Savarino, J.: How Well Do Nitrate Isotopes in Alpine Ice Cores Preserve Atmospheric Signals?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11953, https://doi.org/10.5194/egusphere-egu26-11953, 2026.
Atmospheric molecular hydrogen (H2) is increasingly being considered as an important energy carrier in future energy systems. Due to leakages during production, storage, transport, and use of H2, a rise in atmospheric H2 levelsis expected. Such an increase may lead to a prolonged lifetime of methane, enhanced tropospheric ozone concentrations, and increased stratospheric water vapor. Although the major sources and sinks of atmospheric hydrogen are relatively well known, large uncertainties remain in the global hydrogen budget due to limited observational constraints and an incomplete understanding of the underlying processes.
Measurements of isotopic signatures of H2 provide a powerful tool to distinguish between different source and sink processes and to better constrain the hydrogen budget, for example by providing improved observational input for atmospheric models. However, recent observations of the hydrogen stable isotope (δD) remain scarce.
Here we present new measurements of atmospheric H2 and its stable isotopologue HD, carried out at the Institute for Marine and Atmospheric research Utrecht (IMAU). The system separates the H2 from the air matrix and determines its isotopic composition using isotope-ratio mass spectrometry (IRMS). The dataset includes atmospheric samples from globally distributed sampling networks, including station data and (Atlantic) ship transects, and local sources.
These new observations contribute to a better observational basis for understanding the regional and global hydrogen cycle and provide valuable input for future studies of atmospheric hydrogen.
How to cite: van Rijswijk, C., Woolley Maisch, C., Röckmann, T., and van der Veen, C.: Constraining the regional and global hydrogen cycle using stable isotope measurements, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14935, https://doi.org/10.5194/egusphere-egu26-14935, 2026.
Eddy covariance measurements are a standard practice when measuring fluxes of CO2 between the surface and the atmosphere. However, they may be limited in their use for disentangling and quantifying the different contributors to these fluxes (i.e. sources and sinks). Measuring the stable isotopologue composition of the CO2 flux, using ¹³C and ¹⁸O signatures, allows for a more direct flux partitioning, based on differences in the isotopologue composition of CO2 sources in ecosystems. Yet, studies investigating isotopologue fluxes remain scarce and mostly limited to short time periods on weekly or monthly scales.
Here we present a new setup for continuous, long-term stable isotopologue eddy covariance measurements of CO2 using a quantum cascade laser absorption spectrometer and evaluate ongoing data collection over a two-year period (2025-2027) in a managed beech forest in central Germany. Furthermore, we discuss the calibration strategy and performance requirements necessary to conduct high-frequency isotopologue measurements suitable for eddy covariance applications and present first flux calculation results.
First results show that frequent instrument calibration of the isotope raw reading is critical and must be performed regularly. We identify pressure and temperature fluctuations as major sources of instrumental drift. To address this, we developed an automated calibration system that performs hourly drift corrections and daily concentration-dependence corrections to reach the precision needed for eddy covariance measurements and resolve the subtle differences in the environmental signal.
Our results highlight important methodological requirements, for continuous, long-term, isotopologue eddy covariance measurements. This work can act as a stepping stone toward the implementation of similar measurements into existing flux observation networks such as ICOS or FLUXNET. Furthermore, this represents an important step toward using stable isotopologues to better understand ecosystem-atmosphere exchange processes by characterizing greenhouse gas sources and sinks in ecosystems.
How to cite: Boersma, O. J., Brüggemann, N., Claß, M., Emad, A., Markwitz, C., Rothfuss, Y., Tunsch, E., and Knohl, A.: Towards continuous, long-term eddy covariance measurements of CO2 isotopologues, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12183, https://doi.org/10.5194/egusphere-egu26-12183, 2026.
Atmospheric reactive nitrogen oxides including NOₓ and nitrate acquire oxygen isotope anomalies (Δ17O = δ17O - 0.52 × δ18O) through ozone-driven chemistry. Prior chemical transport modeling studies have investigated the spatiotemporal patterns of Δ17O of atmospheric nitrate, yet these approaches remain fundamentally localized as they neglect inter-grid transport effects (Alexander et al., 2020; Walters et al., 2024). Transport process can influence Δ17O both directly through mixing and indirectly by altering precursor concentrations, thereby modulating isotopic transfer during chemical reactions. However, integrating transport into Δ17O modeling has been hindered by the requirement to track multiple isotopologues per species, which would substantially increase the complexity in chemical mechanism and computational cost.
This study introduces a novel, computational efficient Δ17O modeling framework with the transport effect incorporated, in which Δ17O is treated directly as prognostic variable. The contributions of chemical and transport processes to Δ17O evolution are separated using operator splitting. The Δ17O transfer during chemistry is computed explicitly following the method of Morin et al. (2011). The Δ17O transport equations are solved using a similar numerical scheme for the Eulerian transport equation. We apply this framework within an adapted version of the PACT-1D model (Tuite et al 2021) to examine how boundary layer dynamics impact the Δ17O variability in reactive nitrogen oxides. In particular, modeled Δ17O values of atmospheric nitrate are evaluated against recent vertical profile observations. This comparison aims to improve our understanding of the controlling factors on nitrate Δ17O and to assess its utility as a proxy for atmospheric oxidation capacity.
How to cite: Jiang, Z. and Geng, L.: Exploring the impact of boundary layer dynamics on the Δ17O of reactive nitrogen oxides with the PACT-1D model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18848, https://doi.org/10.5194/egusphere-egu26-18848, 2026.
Posters virtual: Tue, 5 May, 14:00–18:00 | vPoster spot 2
EGU26-644 | ECS | Posters virtual | VPS5
Insights into global carbon cycling using bi-monthly measurements of triple oxygen isotopes in CO₂ from Cape PointTue, 05 May, 14:06–14:09 (CEST) vPoster spot 2
In recent years, the triple-oxygen isotopic composition (Δ′¹⁷O) of CO₂ has emerged as a powerful tracer of atmospheric carbon cycling. The Δ′¹⁷O signature of tropospheric CO₂ is controlled by key processes, including global biosphere-atmosphere CO₂ exchange, tropospheric residence times, and stratosphere-troposphere mixing, each of which modifies CO₂ composition through dynamic isotopic fractionation. High-precision measurement of Δ′¹⁷O is essential for constraining models that predict future changes in atmospheric CO₂, yet current datasets remain limited owing to extreme low abundance of ¹⁷O and the considerable analytical challenges involved for accurate and precise isotopic measurement. A further obstacle is the absence of a well-constrained global background Δ′¹⁷O value for atmospheric CO₂ that restricts proper evaluation of deviations arising from diverse source contributions. As a result, model predictions of tropospheric Δ′¹⁷O(CO₂) often diverge substantially from observational constraints.
To address this gap, we have initiated high-precision measurements of δ¹³C, δ¹⁸O, and Δ′¹⁷O in atmospheric CO₂ using TILDAS (Tunable Infrared Direct Laser Absorption Spectroscopy) at the Stable Light Isotope Laboratory in University of Cape Town, South Africa. Bi-monthly air samples have been collected at the Global Atmospheric Watch (GAW) Cape Point station, South Africa, since December 2024. Given the station’s location, sampling is preferentially conducted under south-easterly wind conditions to minimize local anthropogenic influence. CO₂ is extracted, purified, and analysed with a precision of ±10 ppm (1σ). We will present the Δ′¹⁷O record and evaluate its correspondence with existing predictive models. We will also discuss perturbation of local Δ′¹⁷O values by regional fluxes, such as anthropogenic inputs or seasonal biospheric exchange. This initiative aims to provide the first annual Δ′¹⁷O (CO₂) baseline from the Southern Hemisphere and improve the accuracy of predictive models of the carbon cycle.
How to cite: Ghoshmaulik, S., Labuschagne, C., and Hare, V.: Insights into global carbon cycling using bi-monthly measurements of triple oxygen isotopes in CO₂ from Cape Point, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-644, https://doi.org/10.5194/egusphere-egu26-644, 2026.
