Source attribution of methane emissions from the Upper Silesian Coal Basin, Poland, using isotopic signatures

Abstract. 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.