Factors that influence the temporal variability of atmospheric methane emission from Upper Silesia coal mines: A case study from CoMet mission
Authors: Swolkień Justyna , Fix Andreas , Gałkowski Michał 
Published: June 2022

Abstract. Errors in assumed pollutant emission characteristics can significantly impact the magnitude of the estimated emissions constrained by instantaneous observations obtained with airborne or remote sensing instruments, especially on the local scale. Realistic emissions from individual point sources are a valuable input for numerical models, as by minimizing the errors stemming from inaccurate emissions, they could allow a better characterization of errors caused by transport mechanisms. Here we provide a detailed description of factors influencing the coal-mine methane emission variability, based on high-frequency (up to hourly) temporal data obtained from seven coal mines from the Upper Silesian Coal Basin during CoMet 1.0 (Carbon dioxide and Methane) mission which took place from May 14 to June 13, 2018. The knowledge of these factors for the particular ventilation shaft is essential for linking the observations achieved during the CoMet 1.0 with models, as most of the publicly available data in the bottom-up worldwide inventories provide annual emissions only. The methane concentrations in examined shafts ranged from 0.10 % to 0.55 % during the study period and were subject to a significant variation on a day-to-day basis due to the changing scope of mining works performed underground. The yearly methane average emission rate calculated based on temporal data of the analyzed subset of mines was of the order of 142.68 kt yr-1, an estimate lower by 27 % than the oficially published WUG (State Mining Authority) data and 36 % than reported to E-PRTR (European Pollutant Release and Transfer Register). Additionally, we found that emissions from individual coal mine facilities were over- or underestimated by between 4 % to 60 %, compared to E-PRTR, when short-term records were analysed. We show that the observed discrepancies between annual emissions based on temporal data and public inventories result from, firstly, the incorrect assumption that the methane concentrations in the time-invariant, secondly, from the methodology of measurements, and lastly, from frequency and timing of measurements. From the emission monitoring perspective, we recommend usage of a standardized emission measurement system for all coal mines, similar to the the SMP-NT/A methane fire teletransmission monitoring system (which most coal mines are equipped with). Such a system could, allow for gas flow quantification, necessary for accurate and precise estimations of methane emissions at high temporal resolution. Using this system will also reduce the emission uncertainty due to factors like frequency and timing of measurements. In addition, separating the emissions from individual ventilation shafts and methane drainage stations would be beneficial in closing the gap between bottom-up and top-down approaches for coal mine emissions, as the intermittent releases of unutilized methane from the drainage stations is currently not considered when constructing regional methane budgets.