A thorough understanding of methane sources is necessary to accomplish methane reduction targets. Urban environments, where a large variety of methane sources coexist, are one of the most complex areas to investigate. Methane sources are characterised by specific δ¹³C-CH₄ signatures, so high precision stable isotope analysis of atmospheric methane can be used to give a better understanding of urban sources and their partition in a source mix. Diurnal measurements of methane and carbon dioxide mole fraction, and isotopic values at King’s College London, enabled assessment of the isotopic signal of the source mix in central London. Surveys with a mobile measurement system in the London region were also carried out for detection of methane plumes at near ground level, in order to evaluate the spatial allocation of sources suggested by the inventories. The measured isotopic signal in central London (−45.7 ±0.5‰) was more than 2‰ higher than the isotopic value calculated using emission inventories and updated δ¹³C-CH₄ signatures. Besides, during the mobile surveys, many gas leaks were identified that are not included in the inventories. This suggests that a revision of the source distribution given by the emission inventories is needed.

Radiocarbon (¹⁴C) is a powerful tracer of fossil emissions because fossil fuels are entirely depleted in ¹⁴C, but observations of ¹⁴CO₂ and especially ¹⁴CH₄ in urban regions are sparse. We present the first observations of ¹⁴C in both methane (CH₄) and carbon dioxide (CO₂) in an urban area (London) using a recently developed sampling system. We find that the fossil fraction of CH₄ and the atmospheric concentration of fossil CO₂ are consistently higher than simulated values using the atmospheric dispersion model NAME coupled with emission inventories. Observed net biospheric uptake in June–July is not well correlated with simulations using the SMURF model with NAME. The results show the partitioning of fossil and biospheric CO₂ and CH₄ in cities can be evaluated and improved with ¹⁴C observations when the nuclear power plants influence is negligible.

Radiocarbon (¹⁴C) measurements offer a unique investigative tool to study methane emissions by identifying fossil-fuel methane in air. Fossil-fuel methane is devoid of ¹⁴C and, when emitted to the atmosphere, causes a strong decrease in the ratio of radiocarbon to total carbon in methane (Δ¹⁴CH₄). By observing the changes in Δ¹⁴CH₄, the fossil fraction of methane emissions can be quantified. Presently, there are very few published Δ¹⁴CH₄ measurements, mainly because it is challenging to collect and process the large volumes of air needed for radiocarbon measurements. We present a new sampling system that collects enough methane carbon for high precision Δ¹⁴CH₄ measurements without having to transport large volumes of air. The system catalytically combusts CH₄ into CO₂ and adsorbs the combustion-derived CO₂ onto a molecular sieve trap, after first removing CO₂, CO, and H₂O. Tests using reference air show a Δ¹⁴CH₄ measurement repeatability of 5.4‰, similar or better than the precision in the most recent reported measurements. We use the system to produce the first Δ¹⁴CH₄ measurements in central London and show that day-to-day differences in Δ¹⁴CH₄ in these samples can be attributed to fossil methane input. The new system could be deployed in a range of settings to investigate CH₄ sources.

Radiocarbon (¹⁴C) is an optimal tracer of methane emissions, as ¹⁴C measurements enable distinguishing between fossil methane and biogenic methane (CH₄). However, ¹⁴C measurements in atmospheric methane are still rare, mainly because of the technical challenge of collecting enough carbon for ¹⁴C analysis from ambient air samples. In this study, we address this challenge by advancing the system in Zazzeri et al. (2021) into a much more compact and portable sampler and by coupling the sampler with the MICADAS (MIni CArbon DAting System) accelerator mass spectrometry (AMS) system at ETH Zurich, using a gas interface. Here, we present the new sampler setup, the assessment of the system contamination and a first inter-laboratory comparison with LARA (Laboratory for the Analysis of Radiocarbon with AMS) at the University of Bern. With our sampling line, we achieved a very low blank, 0.7 µgC compared to 5.5 µgC in Zazzeri et al. (2021), and a sample precision of 0.9 %, comparable with other measurement techniques for ¹⁴CH₄, while reducing the sample size to 60 L of air. We show that this technique, with further improvements, will enable routine ¹⁴CH₄ measurements in the field for an improved understanding of CH₄ sources.

Northern Italy is one of the most polluted and densely populated areas in Europe. The diversity of land use in the Po basin makes this region an important contributor to greenhouse gas emissions from different sources. Medium and large cities, as well as industrialised areas, contribute significantly to emissions from industrial processes, combustion, waste management and natural gas distribution.As part of the H2020 RI-URBANS project (https://riurbans.eu/) and in synergy with PNRR “ITINERIS” Project, a pilot study has been carried out in the Milan urban area with the aim of supporting the local authorities to evaluate the effectiveness of air quality policies and the effects of pollutants on human health. A one-year long measurement campaign was carried out using a mobile platform in the premises of the CNR facility (AdRMi1, 45°28’47"N 9°13’54"E; 120 m a.s.l.), located in in the urban area of Milan, in the University district. The mobile platform has been equipped with a suite of instruments (ACSM, Vocus Chemical Ionization TOF-MS, Aethalometer, NOx CLD) to provide near real time (NRT) information on aerosol source partitioning and to characterise nanoparticle contributions from urban hot spots. From July 2023 to March 2024, the mobile platform has been equipped with a Cavity Ring Down Analyser for continuous observations of carbon dioxide (CO2) and methane (CH4).In this work we will provide a first characterization of the diel cycles and seasonal variations (from late summer to early spring) of atmospheric CO2 and CH4 in the urban area of Milan. Thanks to the combined analysis of other atmospheric tracers, first insights about the influence by atmospheric processes (i.e. PBL dynamics) and emission sources (fossil fuel and biomass burning combustion, biogenic, waste management) will be discussed. Finally, we provided a preliminary “top-down” estimate of CH4 emission for the Milan urban area by using the interspecies correlation approach: the obtained values were higher (but still within the range of uncertainties) than emissions provided by statistical “bottom-up” inventories (e.g. INEMAR).Acknowledgments: RI-URBANS t has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101036245. This reserach was partially supported by the European Unione- Next Generation EU, Missione 4 Componente 2 - CUP B53C22002150006 - IR0000032 – ITINERIS, Italian Integrated Environmental Research Infrastructures System

Methane mole fractions have been continuously measured since 2018 at Plateau Rosa, a background station in the north-western Italian Alps at 3480 m a.m.s.l., showing a persistent increase in methane over time. Here, we use the methane record to validate the high-resolution transport model FLEXPART coupled with the output of the numerical weather prediction model COSMO for complex alpine terrain and to identify European source regions affecting measurements at the stations. Comparison between simulated methane mole fraction contributions and observed increments indicate that summer advection of methane-rich air masses was not fully captured by the model, and most pollution events were underestimated in magnitude. The Po Valley, just below the Alps and with major urban and industrial activities, is the region affecting most pollution events, mainly from agriculture, while contributions from fuel exploitation increased when air originated from Germany, Poland, Czechia, the Netherlands, and the UK.