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Anthropogenic emissions are the primary source of atmospheric methane (CH4) growth. However, estimates of anthropogenic CH4 emissions still show large uncertainties on global and regional scales. Differences in CH4 isotopic source signatures δ13C and δ2H can help to constrain different source contributions (e.g. fossil, waste, agriculture, etc.). The Upper Silesian Coal Basin (USCB) represents one of the largest European CH4 emission regions, with more than 500 Gg CH4 yr-1 released from more than 50 coal mine ventilation shafts and other anthropogenic sources. During the CoMet (Carbon Dioxide and Methane Mission) campaign in June 2018 methane observations were conducted from a variety of platforms including aircraft and cars to quantify these emissions. Beside the continuous sampling of atmospheric methane concentration, numerous air samples were taken from inside and around the ventilation shafts (1–2 km distance) and aboard the High Altitude and Long Range Research Aircraft (HALO) and DLR Cessna Caravan aircraft, and analyzed in the laboratory for the isotopic composition of CH4. The airborne samples downwind of the USCB contained methane from all sources in the region and thus enabled determining the mean signature of the USCB accurately. This mean isotopic signature of methane emissions was -50.9 ± 0.7 ‰ for δ13C and -226 ± 9 ‰ for δ2H. This is in the range of previous USCB studies based on samples taken within the mines for δ13C, but more depleted in δ2H than reported before. Signatures of methane enhancements sampled upwind of the mines and in the free troposphere clearly showed the influence of biogenic sources (e.g. wetlands, waste, ruminants). The ground-based samples taken during CoMet allowed determining the source signatures of individual coal mine ventilation shafts. These signatures displayed a considerable range between different shafts and also varied for individual shafts from day to day. Mean shaft signatures range from -60 ‰ to -42 ‰ for δ13C and from -200 ‰ to 160 ‰ for δ2H. A gradient in the signatures of sub-regions of the USCB is reflected both in the aircraft data as well as in the ground samples with emissions from the southwest being most depleted in δ2H and emissions from the south most depleted in δ13C. The average signature of -49.8 ± 5.7 ‰ in δ13C and -184 ± 32 ‰ in δ2H from the ventilation shafts fits with values from previous studies, but clearly differs from the USCB regional signature in δ2H. We assume that the USCB plume mainly contains fossil coal mine methane and biogenic methane from waste treatment, because the USCB is a highly industrialized region with few other possible methane sources. Assuming a biogenic methane signature between and -320 ‰ and -280 ‰ for δ2H, the biogenic methane emissions from the USCB account for 15–50 % of total emissions. The share of anthropogenic-biogenic emissions from this densely populated industrial region is underestimated in commonly used emission inventories. Generally, this study demonstrates the importance and usefulness of δ2H-CH4 observations for methane source attribution, but highlights the need of comprehensive and extensive sampling from all possible source sectors.

Recent climate change mitigation strategies rely on the reduction of methane (CH4) emissions. Carbon and hydrogen isotope ratio (δ13CCH4 and δ2HCH4) measurements can be used to distinguish sources and thus to understand the CH4 budget better. The CH4 emission estimates by models are sensitive to the isotopic signatures assigned to each source category, so it is important to provide representative estimates of the different CH4 source isotopic signatures worldwide. We present new measurements of isotope signatures of various, mainly anthropogenic, CH4 sources in Europe, which represent a substantial contribution to the global dataset of source isotopic measurements from the literature, especially for δ2HCH4. They improve the definition of δ13CCH4 from waste sources, and demonstrate the use of δ2HCH4 for fossil fuel source attribution. We combined our new measurements with the last published database of CH4 isotopic signatures and with additional literature, and present a new global database. We found that microbial sources are generally well characterised. The large variability in fossil fuel isotopic compositions requires particular care in the choice of weighting criteria for the calculation of a representative global value. The global dataset could be further improved by measurements from African, South American, and Asian countries, and more measurements from pyrogenic sources. We improved the source characterisation of CH4 emissions using stable isotopes and associated uncertainty, to be used in top-down studies. We emphasise that an appropriate use of the database requires the analysis of specific parameters in relation to source type and the region of interest. The final version of the European CH4 isotope database coupled with a global inventory of fossil and non-fossil δ13CCH4 and δ2HCH4 source signature measurements is available at https://doi.org/10.24416/UU01-YP43IN (Menoud et al., 2022a).

Reducing methane (CH4) emissions has a great potential for climate change mitigation. However, uncertainties remain in the CH4 budget: the emissions calculated from atmospheric observations do not match the ones reported in inventories. The stable isotopic composition of methane, which is the proportion of heavier molecules compare to the most abundant form, provide information on the formation pathway. Therefore, measurements data on two isotopes of CH4, carbon 13 (d13C-CH4) and deuterium (d2H-CH4), allowed us to evaluate the emission inventories and investigate spatial variations in Europe. We performed continuous measurements of ambient air over several months at fixed locations, in order to evaluate the influence of specific methane sources over a region. The CH4 isotopic composition data from in the North coast of the Netherlands and in the city of Krakow, Poland, were dramatically different. Biogenic sources, mostly of anthropogenic origin, such as from ruminant farming or waste management, are prominant in the Netherlands, as suggested by the emissison inventories. In Krakow, we detected large emissions from coal mines, but also from the city. The latter are not always reported in inventories, and appear to come from the use of fossil fuels, for exemple the use of coal for residential heating. The uses of d2H-CH4 as a tracer was particularly useful in the case of Poland, as there is an overlap in the d13C-CH4 from biogenic and fossil fuel sources. We showed that time series of CH4 isotopic composition in ambient air can help to assess emission inventories. A large measurement campaign was organised in Romania, focusing on methane emissions from the oil and gas industry. The exploitation of fossil fuels is causing direct emissions of CH4 through leaks. We took samples close to oil and gas installations, but also while flying a scientific aicraft over the regions of extraction. The isotopic composition of CH4 from Romania was hardly studied until now, and we created an unprecedented dataset from our samples. We attributed most pollution of the regions we surveyed to the oil and gas industry. We identified microbial pathways in the subsurface formations that would help interpreting atmospheric data on larger scales. We gathered all measurements in a publicly available database, and combined with the previous literature. Over Europe, we found strong regional variations in the isotopic composition from fossil fuel sources but not from microbial sources. Our dataset brings new tools for the interpretation of methane stable isotope data, especially by providing a significant amount of new d2H-CH4 observations. Future studies can use it for source attribution, and as input to atmospheric models.

Methane (CH4) emissions from human activities are a threat to the resilience of our current climate system. The stable isotopic composition of methane (I13C and I2H) allows us to distinguish between the different CH4 origins. A significant part of the European CH4 emissions, 3.6ĝ€¯% in 2018, comes from coal extraction in Poland, the Upper Silesian Coal Basin (USCB) being the main hotspot. Measurements of CH4 mole fraction (χ(CH4)), I13C, and I2H in CH4 in ambient air were performed continuously during 6 months in 2018 and 2019 at Krakow, Poland, in the east of the USCB. In addition, air samples were collected during parallel mobile campaigns, from multiple CH4 sources in the footprint area of the continuous measurements. The resulting isotopic signatures from sampled plumes allowed us to distinguish between natural gas leaks, coal mine fugitive emissions, landfill and sewage, and ruminants. The use of I2H in CH4 is crucial to distinguish the fossil fuel emissions in the case of Krakow because their relatively depleted I13C values overlap with the ones of microbial sources. The observed χ(CH4) time series showed regular daily night-Time accumulations, sometimes combined with irregular pollution events during the day. The isotopic signatures of each peak were obtained using the Keeling plot method and generally fall in the range of thermogenic CH4 formation-with I13C between-59.3g and-37.4g Vienna Pee Dee Belemnite (V-PDB) and I2H between-291g and-137g Vienna Standard Mean Ocean Water (V-SMOW). They compare well with the signatures measured for gas leaks in Krakow and USCB mines. The CHIMERE transport model was used to compute the CH4 and isotopic composition time series in Krakow, based on two emission inventories. The magnitude of the pollution events is generally underestimated in the model, which suggests that emission rates in the inventories are too low. The simulated isotopic source signatures, obtained with Keeling plots on each simulated peak, indicate that a higher contribution from fuel combustion sources in the EDGAR v5.0 inventory would lead to a better agreement than when using CAMS-REG-GHG v4.2 (Copernicus Atmosphere Monitoring Service REGional inventory for Air Pollutants and GreenHouse Gases). The isotopic mismatches between model and observations are mainly caused by uncertainties in the assigned isotopic signatures for each source category and the way they are classified in the inventory. These uncertainties are larger for emissions close to the study site, which are more heterogenous than the ones advected from the USCB coal mines. Our isotope approach proves to be very sensitive in this region, thus helping to evaluate emission estimates.

Methane (CH4) emissions from human activities are a threat to the resilience of our current climate system, and to the adherence of the Paris Agreement goals. The stable isotopic composition of methane (δ13C and δ2H) allows to distinguish between the different CH4 origins. A significant part of the European CH4 emissions, 3.6 % in 2018, comes from coal extraction in Poland; the Upper Silesian Coal Basin (USCB) being the main hotspot. Measurements of CH4 mole fraction (χ(CH4)), δ13C and δ2H in CH4 in ambient air were performed continuously during 6 months in 2018 and 2019 at Krakow, Poland, 50 km east of the USCB. In addition, air samples were collected during parallel mobile campaigns, from multiple CH4 sources in the footprint area of the continuous measurements. The resulting isotopic signatures from sampled plumes allowed us to distinguish between natural gas leaks, coal mine fugitive emissions, landfill and sewage, and ruminants. The use of δ2H in CH4 is crucial to distinguish the fossil fuel emissions in the case of Krakow, because their relatively depleted δ13C values overlap with the ones of microbial sources. The observed χ(CH4) time series showed regular daily night-time accumulations, sometimes combined with irregular pollution events during the day. The isotopic signatures of each peak were obtained using the Keeling plot method, and generally fall in the range of thermogenic CH4 formation – with δ13C between −55.3 and −39.4 ‰ V-PDB, and δ2H between −285 and −124 ‰ V-SMOW. They compare well with the signatures measured for gas leaks in Krakow and USCB mines. The CHIMERE transport model was used to compute the CH4 and isotopic composition time series in Krakow, based on two emission inventories. The χ(CH4) are generally under-estimated in the model. The simulated isotopic source signatures, obtained with Keeling plots on each simulated peak using the EDGAR v5.0 inventory, indicate that a higher contribution from fuel combustion sources in EDGAR would lead to a better agreement. The isotopic mismatches between model and observations are mainly caused by uncertainties in the assigned isotopic signatures for each source category, and the way they are classified in the inventory. These uncertainties are larger for emissions close to the study site, which are more heterogenous than the ones advected from the USCB coal mines. Our isotope approach proves to be very sensitive in this region, thus helping to evaluate emission estimates.