Evaluation of coal mine methane inventory methods using aircraft-based approaches in the Bowen Basin, Australia

Australia uses a blend of IPCC Tier 2 and 3 bottom-up approaches to estimate and report fugitive coal mine methane (CH$_4$) emissions. To date, Tier 3 reporting for underground coal mines, which predominantly relies on direct measurements of ventilated air, has not been systematically assessed against top-down atmospheric measurements. Tier 2 coal core-based emission factors and Tier 3 model guidelines for estimating surface (open-cut) mine emissions similarly lack verification. Here, an in-situ and a remote sensing aircraft-based approach were used to quantify CH$_4$ emission rates from 17 coal mines in the Bowen Basin, Australia. When compared to bottom-up mean annual reported estimates, airborne estimates from underground mines showed a non-significant mean positive bias of 0.28 t h$^{−1}$ (p = 0.28, n = 8 estimates) and good agreement (normalised root mean squared error (NRMSE) = 0.20). When aggregated, top-down measured emissions from all underground mines were within 8 % of bottom-up reported totals. In contrast, aircraft-based estimates from surface mines showed a significant mean positive bias of 3.7 t h$^{−1}$ CH$ _4$ (p = 0.001, n = 10 estimates) and poor agreement (NRMSE = 0.86). In aggregate, top-down emissions from all surface mines were 3.6 times the bottom-up totals. Alignment between airborne top-down and measured vented underground coal mine emissions increases confidence in Australia’s Tier 3 direct monitoring methods, while the difference between top-down and bottom-up surface mine emission estimates suggests core-based methods require review. Given that surface mines in the Basin alone account for ∼ 38 % of national production, the contribution of coal mining to Australia’s CH$_4$ emissions may exceed the reported ∼ 19 %.