Aircraft- and ground-based quantification of coal mine methane emissions in the Hunter Coalfields, Australia

In recent years, credible atmospheric observations of methane emissions suggest that annual methane emission estimates in inventories for some Australian coal mining regions or facilities may be underestimated. A lack of well-constrained, mine-scale studies for open-cut (pit) coal mines continues to hinder discussions on emission estimates and the refinement of estimation methods for Australian open-cut coal mining facilities. Here, we present preliminary results from aircraft- and ground-based atmospheric measurements recorded in November 2024 in the Hunter Coalfield, NSW, Australia.

Australia employs higher-Tier IPCC methodologies – Tier 2 (basin-specific) and Tier 3 (mine-specific, Methods 2 and 3 ) – under its National Greenhouse and Energy Reporting (NGER) Scheme to estimate open-cut coal mine emissions. These methods rely on the use of coal core gas content to estimate methane emissions from open-cut mine complexes. However, Methods 2 and 3 have never been validated using airborne or ground-based time series observations.

While coarse-resolution satellites like TROPOMI can quantify coal mine emissions at regional scales (Sadavarte et al., 2021; Palmer et al., 2021), their limited spatial resolution reduces their effectiveness for verifying annual inventory reported emissions at the scale of individual mines. Additionally, the ability of point-source imaging satellites to quantify emissions from individual open-cut coal mines remains uncertain. Coal seam blasting prior to extraction can be considered a point source; however, open-cut coal mines have various continuous diffuse methane sources that also need to be quantified. The diffuse sources include, among others, emissions from beneath the pit floor, lateral diffusion along coal seams and other rock strata in the mine walls, rock waste piles, and areas of in situ biological production, such as water management ponds. Aircraft- and ground-based technologies have the potential to measure both point and diffuse sources of methane, thus providing a potential pathway for verifying greenhouse gas inventories determined using approved IPCC methodologies.

During this measurement campaign in the Hunter Coalfield, a research aircraft flew instruments to collect in-situ atmospheric measurements of methane and carbon dioxide mole fractions, along with GPS and meteorological data. These data were used to make rate of methane emission estimates downwind of individual coal mine complexes. Aerosol size and particle number concentration measurements and high-resolution airborne  LiDAR imagery were also acquired to aid in source attribution. These measurements were complemented by ground-based EM27/SUN solar absorption spectrometer instruments positioned upwind and downwind of the same coal mining complexes. Comparisons between emissions derived from the aircraft-base, ground station EM27/SUN observations, and operator-reported coal mine methane emissions will be presented.

Palmer, P. I., Feng, L., Lunt, M. F., Parker, R. J., Bösch, H., Lan, X., Lorente, A., and Borsdorff, T.: The added value of satellite observations of methane for understanding the contemporary methane budget, Phil. Trans. R. Soc. A., 379, 20210106, https://doi.org/10.1098/rsta.2021.0106, 2021.

Sadavarte, P., Pandey, S., Maasakkers, J. D., Lorente, A., Borsdorff, T., van der Gon, H. D., Houweling, S., and Aben, I.: Methane emissions from superemitting coal mines in Australia quantified using TROPOMI satellite observations, Environmental Science & Technology, 55, 16573–16580, https://doi.org/10.1021/acs.est.1c03976, 2021.