Abstract
Abstract. Methane (CH4) emissions from human activities are a threat to the resilience of our current climate system. The stable isotopic composition of methane (δ13C and δ2H) allows us 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, 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 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 −59.3 ‰ and −37.4 ‰ Vienna Pee Dee Belemnite (V-PDB) and δ2H between −291 ‰ and −137 ‰ 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.
Highlights
Atmospheric emissions of greenhouse gases, defined as gas compounds that absorb and emit thermal infrared radiations from human activities are the main cause of the current warming of our Earth’s climate
The very depleted δ13C values obtained in these previous studies confirm the presence of purely microbial gas reservoirs in the Upper Silesian Coal Basin (USCB) coal deposits, but our results show that thermogenic gas represents a larger part of the fugitive emissions from mining activities in this area than indicated by Kotarba (2001; Fig. 6a)
The source signatures of the pollution events observed in Krakow were compared with signatures from sources sampled around the study area
Summary
Atmospheric emissions of greenhouse gases, defined as gas compounds that absorb and emit thermal infrared radiations from human activities are the main cause of the current warming of our Earth’s climate. 23 % of the additional radiative forcing since 1750 is attributed to CH4, whereas total CH4 anthropogenic emissions represent only 3 % of those of CO2 in term of carbon mass flux (Etminan et al, 2016). Total CH4 emissions have been rising: they increased by 5 % in the period 2008– 2017 (and 9 % in 2017), compared to the period 2000–2006 (Saunois et al, 2020). It is not clear which sources caused these changes, but Saunois et al (2020) estimated anthropogenic emissions to represent 60 % of the total emissions of the past 10 years. It is not clear which sources caused these changes, but Saunois et al (2020) estimated anthropogenic emissions to represent 60 % of the total emissions of the past 10 years. Nisbet et al (2019) showed that the current levels of CH4 emissions are a threat to the adherence of the Paris Agreement goals, but an effective reduction of CH4 emissions requires knowledge of the locations and magnitudes of the different sources
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