Factors influencing the temporal variability of atmospheric methane emissions from Upper Silesia coal mines: a case study from the CoMet mission
Authors: Swolkień Justyna , Fix Andreas , Gałkowski Michał 
Published: December 2022

Abstract. Methane is a powerful greenhouse gas responsible for around 20 % of
radiative forcing (relative to the pre-industrial era) caused by all
long-lived greenhouse gases (WMO, 2021). About 60 % of the global emissions
are from anthropogenic sources, and coal mining is one of the largest
contributors. Emissions are either estimated by bottom-up approaches (based
on inventories) or top-down approaches (based on atmospheric measurements).
Combining those with an accurate error estimation allows us to better
characterise model errors e.g. caused by transport mechanisms. Here we provide a detailed description of factors influencing the coal mine
methane emission variability. We use high-frequency (up to hourly) temporal
data from seven coal mines in the Upper Silesian Coal Basin during the Carbon dioxide and Methane (CoMet 1.0) mission from 14 May to 13 June 2018. Knowledge of these factors for the individual ventilation shaft is essential
for linking the observations achieved during the CoMet 1.0 mission with models, as
most 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 %±0.1 % during the study period. Due to the changing scope of mining
works performed underground, they were subjected to a significant variation
on a day-to-day basis. The yearly methane average emission rate calculated
based on 1 month's set of temporal data of the analysed subset of mines was
of the order of 142.68 kt yr−1 (σ=18.63 kt yr−1), an
estimate 27 % lower than the officially published State Mining
Authority (WUG) data and 36 % lower than reported to the European Pollutant
Release and Transfer Register (E-PRTR). We also found that emissions from individual
coal mine facilities were over- and underestimated by between 4 % to 60 %, compared to the 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 (1) the incorrect assumption that the
methane emissions are time-invariant, (2) the methodology of
measurements, and lastly, (3) the frequency and timing of measurements. From the emission monitoring perspective, we recommend using a standardised
emission measurement system for all coal mines, similar to the
Methane Fire Teletransmission Monitoring System (SMP-NT/A). Legal safety requirements
require all coal mines to implement this system. After an adaptation, the
system could allow for gas flow quantification, necessary for accurate and
precise estimations of methane emissions at a high temporal resolution.
Using this system will also reduce the emission uncertainty due to factors
like frequency and timing of measurements. In addition, it would be
beneficial to separately identify the emissions from individual ventilation
shafts and methane drainage stations. That would bridge the gap between
bottom-up and top-down approaches for coal mine emissions. The intermittent
releases of unutilised methane from the drainage stations are currently not
considered when constructing regional methane budgets.