Natural gas seeps and mud volcanoes are widely distributed across terrestrial and shallow submarine sedimentary basins and contribute considerable amounts of fossil methane to the atmosphere. Methane emissions from these systems are commonly interpreted as dominantly thermogenic in origin; however, microbial activity may significantly contribute to, or overprint, these emissions through secondary methanogenesis or methane oxidation during gas migration and storage.Conventional bulk isotope composition (δ¹³C and δD) and hydrocarbon concentration ratios are often insufficient to distinguish secondary microbial contributions from an initial thermogenic source. Independent of bulk isotopic signatures, methane clumped isotopes (Δ¹³CH₃D and Δ¹²CH₂D₂) provide direct constraints on methane formation pathways and post-generation alteration processes. Recent studies have revealed low-temperature near-equilibrium clumped-isotope signatures in mud-volcano systems in Azerbaijan1, indicative of strong microbial overprinting, whereas methane from Japanese mud volcanoes exhibits clumped isotope signatures spanning from far from equilibrium to near equilibrium values2. For the latter, clumped isotope signatures of methane correlate with 13C-position-specific isotope composition of propane, suggesting the biodegradation of higher hydrocarbons is associated with progressive modification of methane clumped isotopes.Here, we investigate methane emissions from mud volcanoes and gas seeps in central and southern Italy (n = 14) and Romania (n = 15). Methane bulk and clumped isotope composition (δ¹³C, δD, Δ¹³CH₃D and Δ¹²CH₂D₂) are analyzed using a quantum cascade laser absorption spectrometer (QCLAS) equipped with a customized gas-inlet system at Empa3. Propane concentrations span from below detection to 0.8%, indicating a wide range of potential microbial influence. Selected samples are further characterized by propane position-specific isotope analyses at Science Tokyo following established protocols by Gilbert et al. 4, providing constraints on the extent of secondary microbial processes affecting higher hydrocarbons.Preliminary clumped-isotope results from Italian mud volcanoes indicate near-equilibrium signatures consistent with strong microbial influence, comparable to patterns reported from Azerbaijan mud-volcano systems. In contrast, Romanian samples exhibit pronounced variability in propane concentrations, providing a critical test case to explore whether methane clumped-isotope systematics transition toward more thermogenic-dominated patterns with secondary microbial influence, similar to those observed in Japanese systems. By integrating new datasets from Italy and Romania with published clumped-isotope and propane intramolecular isotope data, this study explores whether microbial influences on methane emissions follow consistent or system-specific patterns across mud-volcano and gas-seep systems globally. [1] Liu et al., 2023 Geology[2] Gilbert et al., 2025 EGU2025 Abstract[3] Zhang et al., 2025 Anal. Chem.[4] Gilbert et al. 2019 Proc. Natl. Acad. Sci.

Abstract. Nitrous oxide (N₂O) isotopocules provide key insights into microbial nitrogen cycling, but their interpretation requires well-constrained values for both oxygen isotope signatures (δ¹⁸O–N₂O) and intramolecular ¹⁵N site preference (SP) associated with N₂O production pathways. Site preference is widely used to distinguish N₂O formation pathways because bacterial denitrification is generally assumed to yield SP values near 0 ‰ through canonical NorB-mediated NO reduction. However, the extent to which SP remains stable across physiological states and changing NO reduction pathways remains poorly constrained. Likewise, interpretation of δ¹⁸O–N₂O associated with denitrification requires understanding the relative contributions of branching kinetic isotope effects and oxygen atom exchange between nitrite and water during N₂O formation. Here, we investigated N₂O isotopic signatures during denitrification by Pseudomonas aureofaciens (NirK-bearing) and Pseudomonas chlororaphis (NirS-bearing) under active-growth and resuspension conditions using quantum cascade laser absorption spectroscopy (QCLAS) and isotope ratio mass spectrometry (IRMS). SP tracked canonical NorB-mediated NO reduction but transiently increased above +10 ‰ during early N₂O production, indicating temporary activity of alternative NO reductases. These dynamics were only resolved through continuous QCLAS measurements, highlighting the importance of time-resolved isotopic observations. While SP remains a useful indicator of NO reduction mechanisms, these results show that even within denitrification, shifts between NO reduction pathways may lead to variable SP signatures. In parallel, we quantified oxygen atom exchange between nitrite and water using incubations prepared in natural-abundance and ¹⁸O-enriched water. Contrary to expectations from denitrifier-method studies, P. aureofaciens exhibited substantial and highly variable oxygen-atom exchange (38–100 %), far exceeding previously reported values (<9 %). In contrast, P. chlororaphis showed consistently high but less variable exchange (~66 %). Resuspension experiments reproduced the characteristic low- and high-exchange behavior reported for these strains under denitrifier-method conditions, demonstrating that these exchange values are specific to the methodological framework and not representative of actively growing systems. These results show that oxygen atom exchange is not governed solely by nitrite reductase identity (NirS vs. NirK) but is strongly modulated by physiological state and metabolic context. As a result, δ¹⁸O–N₂O cannot be interpreted as a fixed tracer of denitrification pathways outside the constrained conditions of the denitrifier method. Together, these findings suggest that denitrifying bacteria may generate N₂O with a broader range of δ¹⁸O–N₂O and SP than previously assumed. This calls for a reassessment of N₂O isotopocule interpretations and emphasizes the need to integrate isotopic measurements with physiological and biochemical constraints.

Urban areas dominate anthropogenic CO2 emissions and play a key role in climate change mitigation. Efficient emissions reduction requires improved knowledge of the attribution of carbon emissions. Measuring the 14C (radiocarbon) content of atmospheric CO2 is the most direct method to distinguish fossil CO2 emissions (ffCO2) from biogenic and natural fluxes, due to their lack of 14CO2 content. Hence, CO2 produced from the combustion of fossil fuels causes a measurable decrease in the atmospheric 14CO2/CO2 isotope ratio. Results from flask sampling campaigns indicate the potential of high time resolution 14C measurements to attribute flux estimates using urban-scale inversions.We present a novel analytical platform for autonomous and semi-continuous analysis of Δ14C-CO2 in atmospheric air samples with sub-hourly time resolution. The core of the analytics is based on a saturated-absorption cavity ring-down (C14-SCAR) spectrometer (ppqSense). This is coupled to a custom-developed compact quantum cascade laser absorption spectrometer (C13-QCLAS) to provide CO2 purity and δ13CO2. The C14-SCAR and C13-QCLAS share an automated pre-concentration device (NC Technologies), to purify CO2 from air applying a temperature-swing zeolite adsorbent trap and remove N2O interferences. We showcase performance characteristics of the coupled C14-SCAR / C13-QCLAS system for sensitive, i.e. ppm-level detection of fossil CO2 contributions in urban environments, and present first time series data for a monitoring site in the vicinity of Zürich (Dübendorf). Results will be discussed in conjunction with city-wide observations of CO2 and high-resolution simulations of ffCO2 variability.This work is supported by the SNSF project RADIANCE (206021_220392) and part of the projects 24GRD03 MetHIR and 24GRD06 MetCTG.

Advances in laser spectroscopy have significantly simplified the measurement of N2O isotopologues (14N15N16O, 15N14N16O, 14N14N18O), but the raw data require extensive post-processing. This problem arises from the complexity of spectral fitting, which is controlled by an intricate interplay between the physics of vibrational spectroscopy, gas composition, fitting algorithm, and instrumental parameters. Following the general principles of identical treatment, the highest precision and accuracy is achieved when reference gases mimic the sample composition, which underpins our correction and calibration protocol.This study presents a comprehensive and detailed correction and calibration protocol to post-process N2O isotopic data, exemplified by data obtained from three commercial G5131-i cavity ring-down spectroscopy (CRDS) analysers manufactured by Picarro Inc., USA. Experimental correction functions for delta values on changes in N2O, CH4, CO2 and O2 concentrations were determined for individual analysers to derive a mathematical framework, which was verified with spectral simulations. We confirm that the apparent delta-values scale inversely with the N2O concentration, with the slope being analyser-specific and highly variable over short time intervals. Consequently, any G5131-i instrument must be routinely characterised to maintain high-quality data. Furthermore, when CH4 and CO2 concentrations vary simultaneously, their combined spectral interference displays a non-additive interaction. We strongly advise removing CO2 from the sample gas before analysis to ensure optimal data quality unless CH4 / CO2 variations are very small, such as for N2O emissions from upland soils.We provide an end-to-end, stand-alone MATLAB application with a user-friendly interface for standardised data reduction, which was validated by analysis of several known target gases but with different gas compositions. This protocol/MATLAB application aims to support researchers in efficiently obtaining high-quality and reliable N2O isotope data from the tested CRDS analyser model, while also providing a case study for data correction for other analyser models and detection schemes. Therefore, the code can be readily adapted to any isotope system for routine application.

Reducing nitrous oxide (N₂O) emissions from wastewater treatment plants is essential for achieving the sector's net-zero goals by 2050. Yet the complexity of operating full-scale wastewater treatment obscures our understanding of the mechanisms behind N₂O formation and hence, the development of targeted mitigation strategies. Three main pathways are known to contribute to net N₂O emissions, but their relative and absolute importance shifts over time and is controlled by the interplay of multiple parameters. To date, attempts to experimentally disentangle these pathways under conditions representative of full-scale systems have been unsuccessful. This study developed and validated a novel methodology to distinguish major N₂O production pathways. The methodology was implemented over one year of operation in two parallel pilot reactors treating dynamically varying municipal wastewater directly extracted from an actual sewer system, thereby closely replicating full-scale conditions. Online isotope analysis was used to validate pathway separation. Stable isotopes showed that the N₂O production by the hydroxylamine pathway is negligible under all studied conditions. Nitrifier denitrification was successfully isolated from heterotrophic denitrification by controlling the concentration of ammonium, nitrite, and dissolved oxygen and became more active at lower dissolved oxygen concentrations. Lower dissolved oxygen, higher organic carbon availability, and lower pH increased N₂O production by heterotrophic denitrification during aeration. These new insights provide a systematic framework for understanding N2O dynamics and support the development of mitigation strategies at full-scale.

Advances in spectroscopic techniques for measuring radiocarbon (14C) in carbon dioxide (CO2) allow near-real-time analyses of atmospheric CO2 and the characterization of the fossil fuel fraction of CO2 emissions on sub-hourly timescales. Calibration and drift correction of these measurements require high-purity CO2 with near-modern (atmospheric) radiocarbon and stable isotopic (δ13C) signatures. Most commercially available compressed CO2 gases are fossil fuel-derived (14C-dead), while biogenic CO2 remains a niche product with limited purity specifications. Both total gas purity and the presence of ppbv-level impurities of nitrous oxide (N2O) can adversely impact the spectroscopic Δ14C-CO2 measurements.We present results on purity and isotopic characterization (δ13C, Δ14C) of different CO2 gas sources as part of ongoing work to develop CO2 standard gases with modern Δ14C and δ13C signatures. We tested CO2 from distinct biogenic sources, including brewery, ethanol production, and biogas production. We also characterize several high-purity fossil CO2 sources. Gases were tested for Δ14C-CO2 and N2O impurities by saturated-absorption cavity ring-down spectroscopy (SCAR) using a commercial instrument (ppqSense). These measurements were calibrated against spectra from CO2 released from a NIST oxalic acid standard (SRM 4990C). We also characterized Δ14C-CO2 by accelerator mass spectrometry, δ13C-CO2 using isotope ratio mass spectrometry, and the presence of trace impurities using different techniques. Preliminary results show the N2O impurities from three tested biogenic gas sources vary by a factor of 103, the 14C content varies by almost 30%, and the δ13C values vary by over 25 ‰. These results will be presented in the larger context of supporting semi-automated SCAR Δ14C-CO2 measurements in Dübendorf, Switzerland.

RATIONALE:Quantum cascade laser absorption spectroscopy (QCLAS) is a fast and reliable method for analyzing the bulk (𝛿13C-CH4, 𝛿D-CH4) and clumped isotopic (∆13CH3D and ∆12CH2D2) composition of methane. However, precise measurements of ∆12CH2D2 require 0.5 to 1.8 mmol of purified methane (equivalent to 15 to 40 mL at STP). We present a cryogen-aided preparative gas chromatography (GC) method with 1 h cycle time that quantitatively separates methane from complex gas mixtures containing N2, O2, Ar, CO2, H2O, and volatile higher alkanes. METHODS:The method employs two sequential Carboxen 1000 columns, precooled to -10°C and subsequently ramped to 150°C in two heating steps, coupled with cryofocusing on charcoal and cryocollection on silica gel. The method performance was evaluated using a well-characterized in-house methane reference gas, either diluted in air-He matrices or stored in Exetainer vials for up to 3 weeks. Purification tests on thermogenic and biogenic methane samples assessed the repeatability of the purification method across different natural gases. RESULTS:Cooling of the GC columns to -10°C achieves complete chromatographic purification of millimole quantities of methane from contaminant gasses. The processing introduces no measurable bulk (𝛿13C-CH4, 𝛿-CH4) and clumped-isotope (∆13CH3D and ∆12CH2D2) fractionation within the 1σ repeatability of the QCLAS system. The repeatability of methane isotope measurements from natural gas samples is comparable to that obtained in tests using the well-characterized in-house reference gas. CONCLUSIONS:Chromatographic purification of millimole quantities of methane from synthetic and natural gas mixtures requires GC column cooling to at least -10°C to ensure sufficient separation from major air components. The developed method enables rapid and quantitative purification of up to 40 mL of methane without detectable isotope fractionation, demonstrating the applicability and robustness of the method for routine purification prior to QCLAS-based methane clumped-isotope analysis.

The traditional calibration approach for process-based models, such as DayCent, consists of the iterative adjustment of model parameters and comparison of the simulated total N2O flux to measured observations. However, the contributions of individual production pathways, namely nitrification and denitrification, are uncertain. Here, N2O emissions from the soil of sugar beet plots with control (Null) and mineral (NPK) fertilizer treatments were measured by a static chamber technique. The isotopic composition of emitted N2O was analyzed to identify the N2O production pathways. The latter showed that denitrification was the predominant source of N2O emissions at this site. The model’s default settings strongly overestimated the contributions of nitrification. This incorrect allocation of N2O emissions to nitrification could partly be explained by the model’s tendency to underestimate the soil water content during the growing season. DayCent model parameters were also manually adjusted to better represent the observation derived contributions of nitrification and denitrification. Although, this “expert-informed approach”, showed a slightly lower performance concerning the cumulative N2O flux (Null: RMSE = 0.37 kg N ha−1 yr−1, NPK: RMSE = 0.50 kg N ha−1 yr−1) than the traditional calibration (Null: RMSE = 0.15 kg N ha−1 yr−1, NPK: RMSE = 0.10 kg N ha−1 yr−1), it may be considered as more representative because it better reflected the higher contribution from denitrification shown by the isotope data. This study demonstrates that the inclusion of observational methods, such as isotope measurements, can provide important insight into model function and improve pathway-specific estimation of N2O emissions in DayCent.

Nitrous oxide fluxes from urine patches (F(N2O)urine) of grazing livestock are variable over time due to fluctuations in driving parameters, such as soil temperature, water-filled pore space (WFPS), and availability of the source substrates ammonium and nitrate. Therefore, the frequency and timing of flux measurements after urine application are important when determining cumulative F(N2O)urine. In this study, F(N2O)urine was measured in eight experiments at high temporal frequency using an automatic chamber system in a pasture located in Switzerland. A driver analysis using random forest identified the time since urine application as most important predictor for F(N2O)urine. The exponential decay in F(N2O)urine after urine addition was in parallel to decreasing soil ammonium but anticorrelated to nitrate concentration, suggesting that nitrification and nitrifier denitrification are major source processes. Since nitrate showed elevated concentrations up to 122 days after application, bacterial denitrification is most likely responsible for late F(N2O)urine peaks following an increase of WFPS. The isotopic composition of the emitted N2O indicates that nitrification dominates the N2O production immediately after urine application, while bacterial denitrification and nitrifier denitrification become more important with increasing time since urine application. The observed high-frequency emission time series were also used to simulate typical low-frequent manual chamber sampling schedules. They led to considerable deviations (up to ± 30

Combustion-generated nanoparticles with diameters of 10-200 nm are ideal shuttles to transport compounds into the human body. The so-called Trojan Horse effect describes the translocation of persistent nanoparticles covered with adsorbates across the alveolar membrane of the lung to the blood circulation system and further to other organs. The toxicology of such nanoparticles is determined by the chemical nature of their adsorbates. We determined the genotoxic potential of such nanoparticles and quantified levels of carcinogens and mutagens like polycyclic aromatic hydrocarbons (PAHs), nitro-PAHs, polychlorinated dibenzodioxins (PCDDs) and furans (PCDFs) in diesel, gasoline and jet engine exhausts. We studied the impact of catalytic particle filters on these compounds which can bind to the aryl-hydrocarbon receptor (AhR), an important domain of a human transcription factor reaching the cell nucleus, where it interferes with gene transcription and regulation.

There is need to calibrate raw data of N2 and N2O isotopocules due to effects of non-linearity, instability, matrix effects and interference with trace gases. Our objective was thus to supply a variety of suitable standard gases for members of the DASIM research unit (www.DASIM.de) and their partners in sufficient amount for routine use to enable calibration for extended time. In total 23 different mixtures were produced to cover all isotopic approaches to study N2 and N2O production and cycling in soils with stable isotopes and suitable for IRMS and laser spectroscopy.Standards for the 15N gas flux method should mimic mixtures of N2 and N2O emitted from highly 15N enriched nitrate in soil and atmospheric background. These must thus contain unlabelled, single-labelled as well as double-labelled N2 and N2O.N2O standards for natural abundance must cover a range of N2O concentrations and isotopocule values typically found in field flux and laboratory incubation studies to correct for non-linearity and bias.Premixtures were prepared by mixing isotopically enriched or depleted gases which were either commercially available or produced in the lab. Moreover, pure N2O of natural abundance was supplied from a previous project (Mohn et al., 2022, https://doi.org/10.1002/rcm.9296). Premixtures were diluted in artificial atmospheres and compressed in commercial tanks.We will explain the production of mixtures, give an overview of the manufactured mixtures and show first results of analysis in comparison with ideal values.

Livestock facilities and biogas plants pose major challenges for odor assessment due to their spatial extent and the heterogeneity of areal sources. Methods for assessing odor impact in such situations have to overcome points of criticism, such as the lack of reliability and subjectivity in sensory analysis. The aim of this study was to validate an improved procedure for investigating odor plumes by trained assessors. In addition to the widespread approach, which focuses on odor frequency, we combined the odor parameters of intensity and frequency. Due to the relevance of weak and mixed odors, very weak (i.e., perceptible) odors were included, rather than focusing only on recognizable odors (clear, distinct perception). On two farms, a tracer gas approach was implemented to provide an objective measure of dispersion. Comparable spatial patterns in odor parameters (frequency, frequency-weighted odor intensity) and tracer gas concentrations provide a number of key findings to consider in odor assessment of areal sources. Two spatial source configurations were studied—the animal part and the biogas part nested or spatially separated—and discriminated by dosing two different tracer gases. In nested configurations, tracer gases mix homogeneously in the plume and therefore only the combined source can be mapped. By contrast, for spatially separated sources, each position in the plume receives an individual exposure to tracer gases and odor, depending on source arrangement, wind direction, and the adjacent buildings. The presented approach can be extended to reliably track and assign more complex situations. The improved procedures will support objectifying odor impact assessment and pave the way for developing appropriate mitigation strategies.

During oxidation, nitrogenous species in dissolved organic matter (DOM) are critical in the formation of nitrogenous, potentially toxic disinfection byproducts, but their chemical identity remains poorly understood. Here, we developed three complementary approaches to identify and quantify reactive amines in model compounds and DOM, including aliphatic primary and secondary amines, aryl-type primary amines, amino acids, and terminal peptidic amino groups. With the chloramine formation assay, the total reactive amines were quantified for the main subgroups. An assay with continuous ozonation quantified three types of reactive amines based on nitrate formation rate constants (kNO3-): kNO3- < 0.1 M-1 s-1 for secondary and aliphatic primary amines; kNO3- = 0.9-1.9 M-1 s-1 for aryl-type primary amines; kNO3- = 15-110 M-1 s-1 for amino acids and peptidic amino groups. The analysis of 15N/14N ratios of nitrate helped to distinguish reactive amines based on 15N enrichment factors (εN): aliphatic (or aryl-type) primary amines (εN:-9 to -3‰), and amino acids and peptidic amino groups (εN:-28 to -19‰). Overall, 23-27% of the organic nitrogen in DOM isolates comprises oxidant-reactive amines, with 5-6% secondary amines, 10-14% aliphatic primary amines, 4% aryl-type primary amines, 1-4% amino acids, and 0-2% peptidic amino groups. Based on the quantitative characterization of amine moieties in DOM, which are possible precursors of N-DBPs, the formation potential of N-DBPs upon oxidative water treatment was estimated.

Nitrous oxide (N2O), a potent greenhouse gas, primarily stems from oxidative (e.g. nitrification) and reductive (e.g., denitrification) microbial processes in aquatic and terrestrial environments. To better understand spatial and temporal N2O production, the isotopic composition of N2O, specifically 15N/14N and 18O/16O ratios, and the intramolecular distribution of 15N (i.e., site preference, SP), are typically used[1]. Distinguishing multiple concurrent processes with three isotope parameters (δ15N, δ18O, SP) remains, however, a challenge, especially in light of uncertainties regarding the isotope effects for individual processes.Here, we study the isotope effects of N2O production imparted by denitrification, focusing specifically on the intermediate step of nitrite (NO2-) reduction to nitric oxide (NO), which is catalyzed by various nitrite reductases. We study three bacterial denitrifiers: Pseudomonas chlororaphis subsp. aureofaciens, Pseudomonas chlororaphis, and Pseudomonas stutzeri. These bacteria utilize similar nitric oxide reductases (NorB) enzymes, but different nitrite reductase variants (NirS vs. NirK). We anticipate similarities in SP values, mostly controlled by NorB, but differences in δ15N-bulk and δ18O values for generated N2O, given the distinct nitrite reductase enzymes[2]. P. stutzeri strain JM300, expressing both NirS and NirK genes[3], offers a unique opportunity for studying each enzyme's distinct functions and isotopic signatures.Selected strains are incubated in batch experiments of a 0.5 L bioreactor using nitrate as a substrate. The bioreactor's headspace is continuously purged with N2. We monitor bacterial growth, NO2- concentrations, dissolved O2, pH, and temperature throughout the experiment. Simultaneously, we daily collect one sample for nitrite and nitrate N and O isotope analysis. After removing CO2 and water, N2O concentrations are monitored with Fourier-transform infrared spectroscopy. The isotopic composition of N2O is measured online using quantum-cascade-laser spectroscopy, providing real-time analysis with high precision (< 0.1 ‰). This enables real-time tracking of changes in the N and O isotope systematics (i.e., fractionation), in response to changing reaction kinetics.The preliminary data that we present will lay the basis for future investigations into the constraints on systematic heavy-isotope clumping (i.e., relative abundance of doubly substituted N2O isotopologues 15N15N16O, 14N15N18O, 15N14N18O) associated with microbial N2O production. Specifically, we will verify direct and indirect enzymatic controls (i.e., type of Nir; N-O bond equilibration with water) on the clumped-isotope abundance of 14N15N18O. [1] Toyoda, S., et al. (2017). Isotopocule analysis of biologically produced nitrous oxide in various environments. Mass Spectrometry Reviews, 36(2), 135-160. [2] Martin, T. S., et al. (2016). Nitrogen and oxygen isotopic fractionation during microbial nitrite reduction. Limnology and Oceanography, 61(3), 1134-1143. [3] Wittorf, L., et al. (2018). Expression of nirK and nirS genes in two strains of Pseudomonas stutzeri harbouring both types of NO-forming nitrite reductases. Research in microbiology, 169(6), 343-347.

Ammonia emissions produce negative environmental and human health impacts with largest emissions originating from agriculture. Especially in countries with high livestock density, the majority originate from animal housing and application to fields. Measuring total emissions from multiple heterogenous source structures such as farms and waste treatment facilities can be challenging due to losses from transport, deposition, and chemical transformation. Previous studies have shown that quantifying net fluxes at this scale can be achieved by combining concentration measurements up- and downwind of the structures with inverse dispersion modelling to calculate the emissions from a defined source area. However, this method underestimates total emissions, as it does not account for deposition loss, which must be modelled and can introduce large uncertainties (

Methane (CH4) is a greenhouse gas (GHG) with both anthropogenic and natural sources. It also contributes to air quality problems through its role in tropospheric ozone formation. Key source categories of anthropogenic CH4 emissions in Europe are the agricultural sector (~50 %), waste (~22 %), and energy (~15 %), which makes them the focus of intense research for developing mitigation actions. Stable isotope ratio measurement in CH4 provide the information needed to verify emissions by source type. To provide comparable and accurate atmospheric CH4 isotope ratios, there is an increasing need to develop metrological harmonized measurements protocols and procedures. In addition, there is a lack of a metrological infrastructure for source signature information needed to interpret atmospheric isotope ratio measurements, as well as an assessment of uncertainties in atmospheric transport models and inverse estimates of Europe's CH4 emissions.Here, we present the isoMET project that aims to (a) develop a harmonised in situ CH4 isotope dataset of ambient air in Europe to resolve compatibility issues of measurements of δ13C or δ2H in CH4 across multiple laboratories, b) develop a sustainable metrological infrastructure for a dataset for δ13C(CH4) and δ2H(CH4)-emissions source measurements in Europe and to evaluate the potential for source apportionment through clumped isotopes, c) use atmospheric chemistry transport modelling to inform the work in (a) and (b), creating estimates of the minimum measurement requirements for deployed instruments. References[1] isoMET project available at: https://www.npl.co.uk/21grd04-isomet[2] J. A. Nwaboh, J. Mohn, M. Fatima, T. Arnold, V. Ebert, Metrology for European emissions verification on methane isotopes (isoMET), CCQM GAWG-IRWG Workshop on Carbon Dioxide and Methane Stable Isotope Ratio Measurements, LATU (Uruguay), 2023Acknowledgements: The project 21GRD04 isoMET project has received funding from the European Partnership on Metrology, co-financed from the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States. Empa has received funding from the Swiss State Secretaritat for Education, Research and Innovation (SERI).