Characterising Methane Concentration Variability at Coal Stockpiles in a Surface Coal Mine

The spatiotemporal variability of atmospheric methane concentration at surface coal mines remains insufficiently characterised. The impact of local meteorology and mining activities on methane emission have not yet been fully evaluated. In this study, ground observations of the methane concentration along with meteorology were carried out around coal stockpiles (CSP) in an active surface coal mine in Australia for a continuous 110 days. A network of 16 semiconductor metal oxide sensors (MOS) was deployed to characterise spatiotemporal variability of near surface atmospheric methane. The mean 15-minute maximum and average CH₄ concentrations were approximately 10.40 ppm and 6.09 ppm, respectively which were higher than the mean atmospheric background (~2 ppm). Short-duration methane plumes produced substantial spatial variability across the sensor network, with sensor-mean CH₄ average concentrations ranging from 4.40 ppm to 12.08 ppm and sensor-mean CH₄ maximum concentrations ranging from 5.50 ppm to 28.31 ppm. The maximum CH₄ concentration of 251.8 ppm was recorded during observations, indicating episodic interception of concentrated methane plumes. Clear diurnal variability in methane concentrations was observed, with peaks during the late night and early morning and lower, more consistent concentrations at midday, indicating near-surface methane accumulation. Upwind-downwind analysis showed that mean upwind CH₄ concentrations remained above the atmospheric background, suggesting contributions from sources within the broader surrounding environment. Downwind CH₄ concentrations showed a mean enhancement of 2.39 ppm above upwind levels. The observed variability reflected impact of multiple factors, including coal-handling activity and local meteorological conditions. The highest concentrations occurred when source activity coincided with conditions favouring plume retention or transport towards the sensor network. Elevated mean CH₄ concentrations were most frequently observed with 50–80% relative humidity and temperatures of 5-15 °C. These findings demonstrate that methane concentrations were highly intermittent, temporally episodic and spatially heterogeneous, with high concentrations being rare but recurrent.