Abstract. Methane (CH4) is a potent greenhouse gas and a key
precursor of tropospheric ozone, itself a powerful greenhouse gas and air
pollutant. Methane emissions across Queensland's Surat Basin, Australia,
result from a mix of activities, including the production and processing of
coal seam gas (CSG). We measured methane concentrations over 1.5 years from
two monitoring stations established 80 km apart on either side of the main
CSG belt located within a study area of 350 km × 350 km. Using an inverse modelling approach coupled with a bottom-up inventory, we quantify methane emissions from this area. The inventory suggests that the
total emission is 173.2 × 106 kg CH4 yr−1, with
grazing cattle contributing about half of that, cattle feedlots ∼ 25 %, and CSG processing ∼ 8 %. Using the inventory emissions in a forward regional transport model indicates that the above sources are
significant contributors to methane at both monitors. However, the model
underestimates approximately the highest 15 % of the observed methane
concentrations, suggesting underestimated or missing emissions. An efficient
regional Bayesian inverse model is developed, incorporating an hourly
source–receptor relationship based on a backward-in-time configuration of the forward regional transport model, a posterior sampling scheme, and the
hourly methane observations and a derived methane background. The
inferred emissions obtained from one of the inverse model setups that uses a
Gaussian prior whose averages are identical to the gridded bottom-up
inventory emissions across the domain with an uncertainty of 3 % of the
averages best describes the observed methane. Having only two stations is
not adequate at sampling distant source areas of the study domain, and this
necessitates a small prior uncertainty. This inverse setup yields a total
emission of (165.8 ± 8.5) × 106 kg CH4 yr−1,
slightly smaller than the inventory total. However, in a subdomain covering
the CSG development areas, the inferred emissions are (63.6 ± 4.7) × 106 kg CH4 yr−1, 33 % larger than those from the
inventory. We also infer seasonal variation of methane emissions and examine
its correlation with climatological rainfall in the area.
Supplementary material to "Quantifying methane emissions from Queensland's coal seam gas producing Surat Basin using inventory data and an efficient regional Bayesian inversion"
