Use of both δ¹³C and δ²H measurements can be used to constrain methane sources. δ¹³C isotopes have been used to help identify the reasons for the continued growth in atmospheric methane, which since 2007 has coincided with a decline in δ¹³C. δ²H could offer a third dimension to help constrain the global methane budget, but its use has been limited because less data are available. There is a need for better identification of δ²H isotopic source signatures, and more long-term atmospheric data records.

We present results of field campaigns carried out in a variety of source regions to characterise isotopic signatures and consider complexities in constraining source signatures for some categories. We also consider use of methane isotopic measurements at different scales for source partitioning.

The isotopic signatures of urban emissions of methane have been characterised in London, Bucharest and Ho Chi Minh City. Methane sources in these cities are very different, with emissions being mostly from gas leaks in London, from wastewater and gas leaks in Bucharest, and from waste and traffic in Ho Chi Minh City.

Measurements of cattle methane emissions in Jersey and Kenya show different isotopic signatures in methane from manure and eructation. Cattle diet, the age of manure and waste management practices cause variability in the isotopic signature of emitted methane.

Wetland methane emissions from sites across Finland and Canada were collected in summer 2022. The Finnish boreal wetland methane isotopic signatures were δ²H -326 ± 19 ‰ and δ¹³C -68 ± 4 ‰, comparable with the results from Canada. Both δ²H and δ¹³C  in methane from boreal wetlands tends to be more depleted in the heavier isotope than in tropical wetland methane emissions.

Both δ¹³C and δ²H can be used in the UM-UKCA chemistry climate model which includes multiple methane tracers tagged by isotopic composition and source type. It is hoped that better characterisation of the regional variability in isotopic signatures of some sources will help improve the ability to model the global methane budget.

How to cite: Fisher, R., Woolley Maisch, C., Lowry, D., France, J., Fernandez, J., Warwick, N., and Nisbet, E.: Source apportionment of methane using δ13C and δ2H , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19065, https://doi.org/10.5194/egusphere-egu24-19065, 2024.

Estimating methane emissions using atmospheric inversion methods that assimilate only methane observations presents challenges in accurately attributing methane sources and sinks to specific emission sectors.

We explore the potential of co-assimilating observations of co-emitted species alongside methane observations to address this challenge and provide improved sectoral distribution of methane emissions.

Ethane is a promising candidate for this purpose. It is primarily co-emitted with methane in the fossil fuel emission sector, particularly through fugitive emissions from natural gas extraction, and is also co-emitted in the biomass and biofuel burning emission sectors, with negligible emissions in other sectors.

To assess the potential of co-assimilating ethane observations with methane observations, we use a global chemistry-transport model, LMDZ-SACS, with the Community Inversion Framework (CIF) to perform response functions analysis on methane and ethane emissions over a multi-year period.

We utilize the response functions results to meaningfully construct full source-receptor relationship matrices at available observation site, as well as, error covariance matrices for a control vector that includes both species emissions and initial conditions, and perform a large set of analytical inversions that assimilate in-situ and flask observations of both methane and ethane, as well as methane-only observations.

This methodology can not only provide improved estimates of the sectoral distribution of methane sources and sinks, but also extends the scope of the analysis to include ethane emissions.

How to cite: Martinez, A., Saunois, M., Berchet, A., Pison, I., and Bousquet, P.: Enhancing Methane Source Attribution through Co-Assimilation of Ethane Observations in Atmospheric Inversion Methods, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6317, https://doi.org/10.5194/egusphere-egu24-6317, 2024.

Greenhouse gas (GHG) emissions require accurate quantification for the development of effective mitigation strategies. Top-down methods to estimate GHG emissions combine ambient GHG measurements, atmospheric chemical transport models (ACTMs), and prior independent information on what is understood of fluxes (including isotopic source signatures where applicable). High frequency stable isotope ratio measurements of δ¹³C-CH₄ and δH-CH₄ have potential to help differentiate changes in the sources of CH₄ emissions at regional scales. While independent efforts to make in situ, high frequency observations are being made at multiple locations, currently there is no network of harmonised measurements across Europe (with each site using different calibration and traceability strategies tailored to their specific analytical setup).

In this work we study CH₄ isotope ratio measurements made at independently managed sites (Heathfield, UK; Heidelberg, Germany; and Zeppelin, Norway) using three different combinations of ACTMs and associated meteorology: NAME with the UK Met Office Unified Model; FLEXPART with ECMWF IFS analysis and short-term forecasts; and STILT with ECMWF IFS analysis and short-term forecasts. The use of multiple models aims to investigate the magnitude of simulated differences to evaluate model uncertainty in this system. We will demonstrate the extent to which model simulations can be used to investigate analytical problems (e.g. measurement offsets between sites) as well as provide initial results on the potential for a network of high-frequency in situ isotope ratio measurements to understand changes in European CH₄ emissions over the coming decades.

How to cite: Chung, E., Arnold, T., Rennick, C., Safi, E., Kikaj, D., Thornton, B., Manning, A., Henne, S., Ganesan, A., and Karstens, U.: Use of multiple atmospheric chemistry transport models to interpret high frequency methane isotope ratio measurements at independently managed sites, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19775, https://doi.org/10.5194/egusphere-egu24-19775, 2024.

Methane (CH₄) is the second most important direct anthropogenic greenhouse gas after carbon dioxide (CO₂). Its lifetime is 10 times shorter than that of CO₂, and its warming potential 80 times greater over a 20-year period. Reducing methane emissions therefore represents a lever for rapid action on global warming. The Sud-PACA region (south-eastern France), classified by IPCC as a climate "hotspot", is part of these efforts to reduce CH₄ emissions, with the aim of achieving carbon neutrality by 2050. To achieve this, it is essential to reduce the uncertainties of regional CH₄ emissions. Although over 50% of regional methane is estimated to be emitted in the south-western part of the region, there are few CH₄ measurements in this highly urbanized and industrialized area. With a view to filling this gap and better characterizing anthropogenic sources of CH₄, a PICARRO G2401 CRDS analyser and meteorological station were set up in May 2021 as part of the ANR COol-AMmetropolis project at Port-de-Bouc (43°24'7.056''N; 4°58'55.459''E), surrounded by numerous petrochemical industries. This station continuously measures CH₄, CO₂ and CO concentrations to study the variability of these species for the different wind sectors.  The spatio-temporal variability of CH₄ concentration and the identification of its sources using co-emitted species will be presented. All these data represent the first measurements of CH₄ in this industrial area and will also be used to independently verify regional inventories. 

How to cite: Bosio, P., Xueref-Remy, I., Blanc, P.-É., Riandet, A., Gille, G., Armengaud, A., and Oppo, S.: Continuous in-situ measurements of atmospheric CH4 at the Port-de-Bouc urban-industrial station (south-east of France): a two-year analysis of CH4 spatio-temporal variability and source identification using co-emitted species., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10170, https://doi.org/10.5194/egusphere-egu24-10170, 2024.

The global impact of COVID-19 on communities and economies has led to questions about decreasing environmental risks and pollution due to the decreased industrial and transport activity. One of the key concerns revolves about the atmospheric rise in CO₂ levels and the associated arising fossil carbon load, constituting the global climate change. The quantification of fossil-origin atmospheric carbon load is addressed through the use of natural radiocarbon (¹⁴C), a unique scientific tool. Fossil sources lack ¹⁴C activity, while recent biogenic carbon contains radiocarbon. This study centers on revealing long-term trends in atmospheric ¹⁴C levels, particularly during the year of the pandemic, in comparison to the preceding five years in Hungary. Atmospheric CO₂ and tree rings from the studied six years were subjected to ¹⁴C analysis from three distinct locations.

One of the examined cities, Budapest - Hungary's capital - is a highly urbanized land with a reported 1.7 million population. Despite the city's extensive vehicular and human activity, a "state of danger" was in effect in Hungary from March to June 2020 due to the first wave of COVID-19. The sampling sites had been characterized by a busy urban environment, with a mix of vehicular activities contributing to the local atmosphere.

The second urban sampling site is Debrecen, a smaller but evolving city that can be found in the eastern part of Hungary. It’s the second largest Hungarian city - around 200 thousand citizens – and it is currently experiencing an industrial revolution by the construction of major factories. Significant contribution to pollution in this area come from urban vehicular traffic and the surrounding agricultural regions.

The background ¹⁴C signal used in the study is from the easternmost Integrated Carbon Observation System(ICOS) atmospheric regional background station (HUN) and NOAA background site, at Hegyhátsál. Mole fraction has been continuously monitored at four elevations at HUN station since September 1994. For this research integrated atmospheric ¹⁴CO₂ samples, supplemented with CO₂ mole fraction measurements, were used from October 2014 to December 2020. The data was studied from the aspect of temporal variation and altitudinal differences. CO₂ mole fraction data of the free tropospheric background ICOS station at Jungfraujoch (Switzerland) were used. The outcomes of the trend analysis reveal the fluctuations in atmospheric fossil carbon load throughout the pandemic, which offers valuable insights into the environmental effects of reduced human activities in Hungary.

Prepared with the professional support of the Doctoral Student Scholarship Program of the Co-operative Doctoral Program of the Ministry of Innovation and Technology financed from the National Research, Development and Innovation Fund and supported by the PARIS project (Grant Agreement No. 820846), which is funded by the European Commission through the Horizon 2020 research programme.

How to cite: Baráth, B. Á., Varga, T., Major, I., Haszpra, L., Vargas, D., and Molnár, M.: Investigating the impact of COVID-19 on the atmospheric 14C trend and fossil carbon load at urban and background sites in Hungary, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13083, https://doi.org/10.5194/egusphere-egu24-13083, 2024.

The increasing level of atmospheric greenhouse gases and the effect of this trend, climate change, is one of the greatest environmental issues of the anthropogenic era. The increasing trend of greenhouse gas levels after industrialization is related to urban environments, where industrial and traffic-related activity and emissions are concentrated. In response to this, the European system, the ICOS (Integrated Carbon Observation System) was established and started the ICOS cities program, where coordinated greenhouse gas observations are carried out besides the regional background measurements and samplings. Similarly to this program, atmospheric air samples were collected at the Institute for Nuclear Research, Debrecen. During the sampling campaigns in three different seasons (winter, spring and summer), a minimum of 23 samples were collected in the morning and afternoon during weekdays and weekends as well. The samples are processed within a collaboration between Utrecht University, where the stable isotope composition of CO₂ and CH₄ were measured, and the Institute for Nuclear Research, Hungary, where the mole fraction of CO₂ and CH₄ and radiocarbon ratio of CO₂ were measured. Based on the isotope composition results and stable isotope fingerprint of carbon dioxide and methane sources, the differentiation of the possible emission sources of these gases can be made. Using the radiocarbon, we can estimate the fossil CO₂ contribution in urban areas. The preliminary results show that there is a great fossil contribution to the CO₂ fraction, on the other hand, a great local biological contribution was observed in the CH₄ fraction in every season. Based on measurements and literature, the source of the massive biological discharge could be the sewage pipeline system, even in winter. Our dataset shows that this kind of CH₄ emitter can exceed fossil sources in Debrecen, Hungary.

How to cite: Varga, T., Major, I., Bán, S., Baráth, B. Á., Röckmann, T., van Es, J., van der Veen, C., and Molnár, M.: Methane and carbon dioxide observations at Debrecen, Hungary: mole fraction and isotope ratio measurements in three different seasons, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7845, https://doi.org/10.5194/egusphere-egu24-7845, 2024.

The atmospheric concentration of CO₂ is crucial in urban areas due to its connection with air quality, pollution, and climate change. Monitoring airborne CO₂ concentrations is vital for environmental management and public safety.  In the lower 50 meters of the atmosphere, CO₂ emissions impact human health and ecosystems, making data at this level essential for addressing carbon-cycle and public-health questions. In volcanic zones, CO₂ variations may correlate with volcanic activity, impacting local ecosystems and human health. In certain regions with high natural CO₂ emissions, geogenic CO₂ profoundly affects the environment. Despite volcanoes' local impact may be important, hydrocarbon combustion is the primary driver of increased atmospheric CO₂ and global warming climate.

This study presents survey results on stable isotope composition of carbon and oxygen of CO₂ and airborne CO₂ concentration in the Campi Flegrei caldera, a high volcanic risk area threatening the Naples metropolitan area. In the last 50 years, two major volcanic unrests (1969–72 and 1982–84) were monitored using seismic, deformation, and geochemical data. The current unrest started in 2005, involving pressurization of the underlying hydrothermal system as a causal factor of the current uplift in the Pozzuoli area.

This research illustrates the use of a mobile laboratory to better understand emissions dynamics and quantify volcanic-origin emissions. Results shows that CO₂ levels in Napoli's urban area exceed background atmosphere levels, indicating an anthropogenic origin from fossil fuel combustion. Conversely, in Pozzuoli's urban area, the stable isotope composition reveals a volcanic origin of the airborne CO₂. This study demonstrates how a spatial survey of stable isotope composition of airborne CO₂ is crucial for understanding emission dynamics. Distinguishing geogenic from anthropogenic emissions is challenging, especially through air CO₂ concentration measurements alone. The findings emphasize the importance of monitoring atmospheric CO₂, especially in areas with volcanic risks, contributing valuable insights for environmental and public health management.

How to cite: Di Martino, R., Gurrieri, S., Paonita, A., Caliro, S., and Santi, A.: Unveiling urban atmospheres: a comprehensive study on CO2 dynamics, air quality, and volcanic impacts in Napoli and Pozzuoli, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3765, https://doi.org/10.5194/egusphere-egu24-3765, 2024.

Monitoring the long-term changes in the stable carbon isotope ratio of carbon dioxide (expressed as δ¹³C-CO₂) is a useful tool to track the fate of atmospheric CO₂ and variations in the global carbon cycle. Due to the small but significant change in global mean atmospheric δ¹³C-CO₂ of approximately -0.75‰ over the last three decades, a robust and traceable method is required to track the long-term change in δ¹³C-CO₂ globally. The Stable Isotope Lab at the INSTAAR of the University of Colorado Boulder has been partnering with NOAA’s Global Monitoring Laboratory since 1990 to measure CO₂ stable isotopes within the Global Greenhouse Gas Reference Network. Here, we present our latest data product, globally distributed observations of δ¹³C-CO₂ in the past 32 years. We have improved our traceability by moving our data onto the CO₂ -in-air JRAS-06 isotopic scale (a representation of V-PDB). We also have established robust quality management systems and have improved our methods of quantifying uncertainty.  Our data demonstrate how the interactions among the atmosphere, the biosphere and anthropogenic activities had been recorded in δ¹³C-CO₂ from more than 70 sites worldwide. The data reveal distinct seasonal variations of δ¹³C-CO₂ that change with latitude, highlighting spatial differences in the influence of anthropogenic activities, net photosynthesis, and ocean-atmosphere CO₂ exchange. Long-term observations also show that the spatiotemporal patterns of δ¹³C-CO₂ vary interannually, which is mainly related to the impact of climate variability on the terrestrial biosphere. We are actively engaged in using these data in complex modeling frameworks to better understand the inter-relationships between climate and the global carbon cycle.

How to cite: Michel, S. E., Tans, P., Miller, J., Ortega, J., Braun, K., Leon, T., Vaughn, B., Clark, R., Li, J., and White, J.: Thirty-two years of high precision data on the stable isotopes of carbon dioxide from a successful collaboration between NOAA and INSTAAR , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14041, https://doi.org/10.5194/egusphere-egu24-14041, 2024.

The multiply-substituted isotopologues of methane are of significant interest due to their increased ability to distinguish between methane formation and destruction processes, in comparison to singly-substituted isotopologues, as shown previously by Thiagarajan et al.(2022) and Sivan et al.(2022). Methane isotopologues, unlike the isotopologues of carbon dioxide, do not easily transition towards thermodynamic equilibrium in the atmosphere and therefore, ambient air methane isotopologues, offer constraints on the atmospheric methane sources and sinks (Chung and Arnold, 2021). These processes potentially offer new insight into the causes of variation in methane concentrations in the last two decades.

We present developments in the separation of the components of air, using a helium-cooled cryostat. Working on both pre-concentrated air mixtures and laboratory created gas mixtures, we extract methane from the contaminants and other atmospheric gases using the cryostat, applicable to a minimum methane concentration of ~1% (Stolper et al., 2015), then we analyse using a TFS Ultra HR-IRMS. We demonstrate that our cryostat separations successfully extract methane and krypton from laboratory gas mixtures containing the components of atmospheric air, without causing methane fractionation.

We also present further developments in measuring and calibrating the isotopologues of methane by high-resolution mass spectrometry. We successfully created thermodynamically equilibrated samples of methane in the 250-500^oC range using a nickel catalyst and are working on the 1-250^oC range using a γ-Al₂O₃ catalyst (Eldridge et al., 2019). It is essential to have an extensive calibration curve to best constrain the effects of scale compression on the calculated deltas and therefore reduce sources of further error, hence the extension of this calibration range.

Further work will add additional reference and sample points to the absolute reference frame created by the equilibrated samples, optimise the cryogenic/gas chromatographic purification methods for more complex gas mixtures, and optimise the IRMS workflow to reduce the necessary air sample sizes.

References:

• Chung, E. and Arnold, T., 2021. Potential of clumped isotopes in constraining the global atmospheric methane budget. Global Biogeochemical Cycles, 35(10), p.e2020GB006883.
• Eldridge, D.L., Korol, R., Lloyd, M.K., Turner, A.C., Webb, M.A., Miller III, T.F. and Stolper, D.A., 2019. Comparison of Experimental vs Theoretical Abundances of 13CH3D and 12CH2D2 for Isotopically Equilibrated Systems from 1 to 500 C. ACS Earth and Space Chemistry, 3(12), pp.2747-2764.
• Liu, Q., Li, J., Jiang, W., Li, Y., Lin, M., Liu, W., Shuai, Y., Zhang, H., Peng, P. and Xiong, Y., 2024. Application of an absolute reference frame for methane clumped-isotope analyses. Chemical Geology, p.121922.
• Sivan, M., Röckmann, T., Slomp, C.P., van der Veen, C. and Popa, M.E., 2022, May. Isotopic characterization of methane: insights from clumped isotope (13CDH3 and CD2H2) measurements. In EGU General Assembly Conference Abstracts (pp. EGU22-4029).
• Stolper, D.A., Martini, A.M., Clog, M., Douglas, P.M., Shusta, S.S., Valentine, D.L., Sessions, A.L. and Eiler, J.M., 2015. Distinguishing and understanding thermogenic and biogenic sources of methane using multiply substituted isotopologues. Geochimica et Cosmochimica Acta, 161, pp.219-247.
• Thiagarajan, N., Pedersen, J.H., Brunstad, H., Rinna, J., Lepland, A. and Eiler, J., 2022. Clumped isotope constraints on the origins of reservoir methane from the Barents Sea. Petroleum Geoscience, 28(2), pp.petgeo2021-037.

How to cite: Houston, A., Gazetas, O., Defratyka, S., Arnold, T., and Clog, M.: Further Cryogenic Separation and Mass Spectrometry Developments: Towards Ambient Air Methane Clumped Isotopes Measurements , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11447, https://doi.org/10.5194/egusphere-egu24-11447, 2024.

The measurement of the stable carbon and oxygen isotope ratio of (atmospheric) carbon dioxide (CO₂) is a useful technique for the investigation and identification of the sources and sinks of the most abundant greenhouse gas by far. For this reason, we are presenting a measuring system here that enables a wide range of users to carry out stable isotope analysis of atmospheric CO₂ using off the bench hard- and software.

The fully automated system uses cryogenic and gas chromatographic separation to analyse CO₂ from 12 mL whole air samples and consists of an autosampler, a Gasbench II, a downstream cryo trap and a continuous flow gas interface feeding into a sector field mass spectrometer (GC Pal/Gasbench II/Cold Trap/Conflo IV/DeltaV Plus). The evaluation of the system performance was based on the analysis of samples prepared from eight CO₂ sources (four CO₂ reference gases and four artificial air tanks).

The overall measurement uncertainty (averaged single standard deviation (1σ) of measurement replicates from each CO₂ source) in the determination of the carbon and oxygen isotope ratio was 0.04 ‰ and 0.09 ‰ (n=24). Furthermore, we were able to show that the measurement data also allowed for the quantification of the CO₂ mole fraction, with a precision of 1.2 µmol mol^-1 in the analysis range of 400 to 500 µmol mol^-1.

The method to be presented was summarized and published in the form of a protocol (DOI: 10.1002/rcm.9647) providing a detailed description of the measurement setup and the analysis procedure, how raw data should be evaluated and gives recommendations for sample preparation and sampling to enable a fully automated whole air sample analysis. We look forward to further discussion with interested users to elaborate on potential improvements/extensions/application options.

How to cite: Leitner, S., Meeran, K., and Watzinger, A.: Stable isotope analysis of atmospheric CO2 using a Gasbench II – Cold Trap – IRMS setting, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8275, https://doi.org/10.5194/egusphere-egu24-8275, 2024.

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.

How to cite: Chénier, N., Magyar, P. M., Emmenegger, L., Lehmann, M. F., and Mohn, J.: Isotopic fractionation of N and O in N2O from denitrification: insights from the comparative analysis of Pseudomonas strains with distinct nitrite reductase enzymes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3351, https://doi.org/10.5194/egusphere-egu24-3351, 2024.

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.

How to cite: Well, R., Buchen-Tschiskale, C., Burbank, J., Dannenmann, M., Lewicka-Szczebak, D., Mohn, J., Rohe, L., Scheer, C., Tuzzeo, S., and Wolf, B.: Production of standard gases for routine calibration of stable isotope ratios of N2 and N2O , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11996, https://doi.org/10.5194/egusphere-egu24-11996, 2024.

Nitrous oxide (N₂O), a greenhouse gas and ozone-depleting molecule with a 116-year atmospheric lifetime, is accumulating in the atmosphere at an accelerating pace. While it is known that the conversion of anthropogenic nitrogen pollutants to N₂O by the environmental nitrogen cycle predominately drives this accumulation, essential questions remain regarding the spatial and temporal balance of N₂O production and consumption. Stable isotope measurements of δ¹⁵N, δ¹⁸O, and ¹⁵N site preference (SP) in N₂O provide valuable constraints on its sources and sinks. Given the complex array of nitrogen cycle processes and their overlapping isotopic signatures, they are often not sufficient to deconvolve them completely. The ‘clumped’, or multiply-substituted, isotopologues ¹⁴N¹⁵N¹⁸O, ¹⁵N¹⁴N¹⁸O, and ¹⁵N¹⁵N¹⁶O provide three additional independent constraints on N₂O sources and processing, with potential to provide insight into source partitioning and reaction mechanisms.

Spectroscopic approaches have emerged as central tools for quantification of N₂O isotopes in atmospheric and environmental samples due to their sensitivity, suitability for continuous or on-site measurement, and isotopologue-specificity. We present an updated approach to the measurement of the eight most abundant isotopic variants of nitrous oxide, including these rare clumped isotopologues, using a quantum cascade laser absorption spectrometer with a 36 m multipass cell. A dual-laser system offers the opportunity to choose a pair of spectral windows that contains strong ro-vibrational absorption lines of the rarest isotopologues, enabling precise clumped isotope ratio measurements on relatively small (<10 µmol) sample amounts. Samples are introduced to the spectrometer and compared to reference materials through a customized gas inlet system, which enables fast switching between samples and references, thereby maximizing reproducibility and sample throughput. Nitrous oxide heated in the presence of γ-alumina at 200 ºC has previously been found to approach equilibrium compositions of ¹⁵N¹⁴N¹⁸O, ¹⁴N¹⁵N¹⁸O, and SP. We constrain the controls on this catalytic reaction by varying temperature, pressure, substrate and catalyst concentrations, and catalyst activation conditions; and we confirm the equilibrium nature of this reaction under various conditions by heating reference gases prepared gravimetrically to have ¹⁵N¹⁴N¹⁸O, ¹⁴N¹⁵N¹⁸O, ¹⁵N¹⁵N¹⁶O, and ¹⁴N¹⁴N¹⁸O elevated above natural abundances. Finally, equilibrating N₂O at several distinct temperatures gives multiple anchor points to an absolute reference scale for clumped and position-specific isotope measurements, enabling future measurement of samples from natural sources.

How to cite: Magyar, P., Prokhorov, I., Brunamonti, S., Chénier, N., Emmenegger, L., Tuzson, B., and Mohn, J.: Advances in clumped isotope measurements of nitrous oxide by laser spectroscopy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15126, https://doi.org/10.5194/egusphere-egu24-15126, 2024.

Methane (CH₄) is the second most significant greenhouse gas after carbon dioxide. The concentration of methane in the atmosphere has been continuously increasing for several years and is currently at almost 2000 ppb. Methane emissions arise from a variable mix of thermogenic sources, such as oil, natural gas, and coal mining, or biogenic sources, such as wetlands and agriculture. Co-emitted gases such as ethane (C₂H₆) and other hydrocarbons can be utilised as a tracer to discern thermogenic (co-emission of ethane) and biogenic (no ethane is emitted) methane sources (Commane et al., 2023, https://doi.org/10.5194/amt-16-1431-2023).

Ethane concentrations in the Northern hemisphere were decreasing between 1970 and 2010, which was attributed to better emission controls from oil and gas production, storage, and distribution, as well as exhaust emissions from cars and trucks. However, emissions are currently on the rise once again, linked to additional emissions from oil and gas production for example from the Eastern USA, (D Helmig et al., 2016, https://doi.org/10.1038/ngeo2721).

In November 2023 we started CH₄ / C₂H₆ measurements in ambient air from a rooftop air inlet (approx. 15 m above ground) at the Empa research campus in Dübendorf, Switzerland using a MIRA Ultra analyser (AERIS Technologies, USA). To correct for drift effects and to calibrate the analyser, two cylinders of compressed air were analysed in regular time intervals. To minimize interferences of water vapour, sample and calibration gases were dehumidified. Concentration measurements were compared to results of a CRDS analyser (model 2401, Picarro, USA) for CH₄ and to a GC-MS system (model 5975C, Agilent Technologies, USA) for C₂H₆.

Compressed air measurements demonstrate that the MIRA Ultra gas analyser meets the manufacturer's specifications of sensitivity for CH₄ and C₂H₆ of < 1 ppb/s and < 500 ppt/s, respectively. We will present a comparison of C₂H₆/CH₄ data from the MIRA ULTRA analyser and GC-MS / CRDS. We foresee to complement CH₄/C₂H₆ measurements with CH₄ isotope analysis (δ¹³C, δD-CH₄) by TREX-QCLAS and relate temporal variations to differences in CH₄ source contributions by means of FLEXPART simulations.

How to cite: Mohn, J., Zeyer, K., Müller, M. J., Schlauri, P., Vollmer, M. K., Brito Melo, D., and Henne, S.: Measurement of atmospheric methane and ethane at a suburban site using mid-IR absorption spectroscopy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16736, https://doi.org/10.5194/egusphere-egu24-16736, 2024.

Nitrous oxide (N₂O) has a significant global warming potential of about 300 times that of CO₂ and a steadily rising atmospheric concentration. Therefore, understanding N₂O production and consumption pathways in major source ecosystems, such as agricultural soils, coupled with accurate quantification of associated N₂O emissions, is critically important in the context of climate change.

The relative abundance of ¹⁵N and ¹⁸O substituted N₂O isotopocules (e.g.,¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O, ¹⁴N¹⁴N¹⁸O) to the predominant isotopic form (¹⁴N¹⁴N¹⁶O) serves as valuable tracers for the distinction between important biogeochemical soil processes, such as nitrification and denitrification, which in turn enhances our understanding of N₂O emissions. In this regard, advances in cavity-ring-down-spectroscopy (CRDS) have enabled precise measurement of isotopic species in ambient N₂O, which holds key advantages over isotope ratio mass spectrometry in its ability to measure online, on-site and site-specific with respect to ¹⁵N substitution in the N₂O-molecule.

Despite the CRDS technique's ease in measuring the isotopic composition of N₂O-isotopocules, the apparent isotope data requires significant post-processing, since spectral fitting is controlled by a complex interplay between fundamental physical parameters, instrumental parameters, gas matrix composition, instrumental drift, and fitting algorithms, some of which also depend on the absolute N₂O concentration. Therefore, to retrieve accurate and comparable results, it is necessary to apply appropriate reference gases with minimal differences in gas composition to the sample in the measurement sequence. Remaining deviations between sample and reference have to be post-corrected using predefined, analyser-specific correction functions.

This work provides a comprehensive and detailed correction and calibration protocol, exemplified by the reduction of N₂O isotopic data obtained from a Picarro G5131-i isotopic and gas concentration analyser. This protocol outlines the theoretical and mathematical framework for the necessary corrections and suggests a logical order for applying these corrections. Moreover, the protocol provides a standalone MATLAB code for streamlined and automatic data reduction that can be employed once the required analyser-specific correction functions are established. The developed algorithms were validated on a suite of target gases, which accounts for concentration-based interferences from various species.

With this protocol, we aim to enable researchers to accurately and efficiently acquire high-quality N₂O isotope data from CRDS instruments and similar devices and contribute to standardized community guidelines for post-processing N₂O isotope data. In a prototype application, we analysed N₂O from automated flux chambers to track biogeochemical processes in agricultural soils. Subsequently, these insights will be integrated into soil biogeochemical models, to enable upscaling of emission data.

How to cite: Havsteen, J., Fatima, M., Brunamonti, S., Hausmaninger, T., Pogány, A., Well, R., and Mohn, J.: Challenges in N2O isotope measurements using CRDS analysers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17480, https://doi.org/10.5194/egusphere-egu24-17480, 2024.

The stable isotopic signatures of atmospheric methane (CH₄) – carbon δ ¹³C(CH₄) and hydrogen δ ²H(CH₄) – are tracers that can help distinguish the relative contributions from different emissions sources. Optical isotope ratio spectrometers (OIRS) deployed at atmospheric monitoring stations have the capability for continuous measurements, providing time series data that can complement sampling campaigns using isotope ratio mass spectrometry (IRMS). OIRS instruments, however, require larger volumes of calibration gases than IRMS and the measurement is of the isotopologues directly (¹²CH₄, ¹³CH₄ and ¹²CH₃D) rather than conversion to CO₂ and H₂. Here, we demonstrate the calibration method for Boreas, a preconcentrator-OIRS system deployed at an atmospheric monitoring station in the South of England and show that these measurements are compatible with those made by IRMS. Measurements with Boreas are referenced to a whole air working standard that is sampled in sequence with air, following the principle of identical treatment. We show the results of a field comparison to IRMS measurements of bag samples taken from the same air inlet simultaneously with the preconcentrator.

The calibration method uses mixtures prepared gravimetrically at a range of amount fractions from a single high-purity CH₄ parent that has been characterised for δ ¹³C and δ ²H by IRMS. This method is capable of calibration over a wide range of amount fraction and isotopic composition. A rigorously derived uncertainty budget shows that the major contributions are from the uncertainty in the assignment of δ ¹³C and δ ²H of the methane parent and the spectrometer, with minimal contribution from uncertainty in the amount fraction of the standards.

How to cite: Rennick, C., Yeo, C., Wilson, F., Safi, E., Hopkinson, E., Hillier, A., Pearce, R., France, J., Lanoiselle, M., Lowry, D., and Arnold, T.: Calibration uncertainty of optical isotope ratio spectroscopy measurements of methane and field comparison with mass spectrometry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17623, https://doi.org/10.5194/egusphere-egu24-17623, 2024.

Accurate measurements of amount fractions and isotopic compositions of greenhouse gas such as carbon dioxide (CO₂) and methane (CH₄) provide valuable insights on their atmospheric composition and origin. Commonly used field deployable commercial laser spectrometers that measure amount fractions and isotopic ratios are often calibrated with reference gases with certified amount fractions and/or isotopic composition. Reference gases, also known as calibration reference materials (CRMs), can be for example synthetic mixtures of e.g. CO₂ in N₂, where the gas matrix N₂ does not match that of the sample (e.g. ambient air) to be measured. A mismatch in the composition of the gas matrix of a CRM and sample can lead to a considerable bias in the amount fraction or isotopic ratio results of the sample due to changes in the measured spectra which e.g. are not perfectly captured by the analysers’ fitting routine.

In this work, we demonstrate the quantification of matrix effects for two commercial CRDS analysers measuring CO₂ and CH₄ amount fractions and isotope ratios. In our experiments with synthetic air gas matrix where the O₂ concentration was varied, we measured (for a 1 % change in the O₂ concentration in the gas matrix) a relative change of 0.15 % for the amount fractions of two major CO₂ isotopologues and 0.07 % for the amount fractions of two major CH₄ isotopologues. Similarly, in terms of isotopic δ¹³C values, we found matrix effects larger than 0.2 for both CO₂ and CH₄. We present options for correcting the gas matrix effects and discuss the underlying assumptions made during the analysis. Amount fraction results for CO₂ and CH₄ are reported including δ¹³C isotope ratio results. Our work concludes that a matrix mismatch when using a commercial laser spectrometer can lead to considerable biases in amount fraction and isotope ratio results, and appropriate correction approaches have to be applied in order to achieve accurate and reliable results.

Acknowledgements: This work has received partial funding from the EMPIR programme (19ENV05 STELLAR project) co-financed by the Participating States and from the European Union's Horizon 2020 research and innovation programme. Part of this work has also received funding from the European Partnership on Metrology (21GRD04 isoMET project), co-financed from the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States. 

How to cite: Nwaboh, J. A., Braden-Behrens, J., Emad, A., Bohlius, H., and Ebert, V.: Quantifying and empirically correcting apparent gas matrix effects:Example measurements for two CRDS analyzers for CO2 and CH4 amount fractions and 13/12C isotope ratios , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18802, https://doi.org/10.5194/egusphere-egu24-18802, 2024.

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), 2023

Acknowledgements: 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).

How to cite: Fatima, M., Nwaboh, J., Mohn, J., Arnold, T., and Ebert, V.: The isoMET project on ambient CH4 monitoring, source signature information and modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20560, https://doi.org/10.5194/egusphere-egu24-20560, 2024.