Measurement report: Methane (CH<sub>4</sub>) sources in Krakow, Poland: insights from isotope analysis

Abstract

Abstract. 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.

Boreas: A Sample Preparation-Coupled Laser Spectrometer System for Simultaneous High-Precision In Situ Analysis of δ13C and δ2H from Ambient Air Methane

“…The measurements of the BT are also shown to illustrate the repeatability of Boreas relative to the magnitude of changes in ambient air. The standard uncertainty of δ 2 H, estimated from the rolling standard deviation of four measurements, relative to the magnitude of changes seen in the atmospheric samples is particularly small as compared with the same dataset for δ 13 C. Simultaneous analysis of both isotope systems using a single instrument will prove highly valuable for interpretation of atmospheric CH 4 (e.g., as illustrated by Menoud et al 11 for identifying fossil source CH 4 in Krakow, Poland). The most significant pollution event (occurring at the end of 2020) shows a particularly heavy δ 13 C signature, yet a light δ 2 H signature is maintained, indicating the significant potential for these measurements for emission source apportionment.…”

“…In populated regions, such as Europe, different types of emission sources are in proximity, making it difficult to usefully verify emissions (i.e., quantitative separation of different sources) using atmospheric measurements. However, in bringing together the source-specific information gained from isotopic observables and the benefits of coupling high-frequency measurements with high-resolution atmospheric chemistry transport model (ACTM) outputs, we will be able to make a significant improvement in our quantitative understanding of sector-specific fluxes. , To this end, impressive attempts have been made to take IRMS systems to atmospheric observatories for high-precision, in situ, frequent analysis of both δ 13 C­(CH 4 ) and δ 2 H­(CH 4 ). − These studies generated thousands of measurements over many months, providing data that could be assimilated into ACTMs for interpretation.…”

“… 8 , 9 To this end, impressive attempts have been made to take IRMS systems to atmospheric observatories for high-precision, in situ, frequent analysis of both δ 13 C(CH 4 ) and δ 2 H(CH 4 ). 9 − 11 These studies generated thousands of measurements over many months, providing data that could be assimilated into ACTMs for interpretation.…”

Atmospheric methane isotopes identify inventory knowledge gaps in the Surat Basin, Australia, coal seam gas and agricultural regions

“…For CH4 emission studies both carbon (d 13 C) and hydrogen (dD) isotopic composition can help with determining CH4 sources and the extent of mixing of various sources (Lowry et al, 2020;Menoud et al, 2020;Menoud et al, 2021;Röckmann et al, 110 2016;Townsend-Small et al, 2015), but in this study only d 13 C is used. Due to the population range in d 13 CCH4 values for each source, d 13 CCH4 may or may not be useful for source attribution (Lan et al, 2021;Milkov and Etiope, 2018;Menoud et al 2022a;Quay et al, 1999;Sherwood et al, 2017Sherwood et al, , 2020.…”