Atmospheric methane isotopes identify inventory knowledge gaps in the Surat Basin, Australia, coal seam gas and agricultural regions

In-flight measurements of atmospheric methane (CH4(a)) and mass balance flux quantification studies can assist with verification and improvement in the UNFCCC National Inventory reported CH4 emissions. In the Surat Basin gas fields, Queensland, Australia, coal seam gas (CSG) production and cattle farming are two of the major sources of CH4 emissions into the atmosphere. Because of the rapid mixing of adjacent plumes within the convective boundary layer, spatially attributing CH4(a) mole fraction readings to one or more emission sources is difficult. The primary aims of this study were to use the CH4(a) isotopic composition (13CCH4(a)) of in-flight atmospheric air (IFAA) samples to assess where the bottom-up (BU) inventory developed specifically for the region was well characterised and to identify gaps in the BU inventory (missing sources or over- and underestimated source categories). Secondary aims were to investigate whether IFAA samples collected downwind of predominantly similar inventory sources were useable for characterising the isotopic signature of CH4 sources (13CCH4(s)) and to identify mitigation opportunities. IFAA samples were collected between 100-350m above ground level (ma.g.l.) over a 2-week period in September 2018. For each IFAA sample the 2h back-trajectory footprint area was determined using the NOAA HYSPLIT atmospheric trajectory modelling application. IFAA samples were gathered into sets, where the 2h upwind BU inventory had >50% attributable to a single predominant CH4 source (CSG, grazing cattle, or cattle feedlots). Keeling models were globally fitted to these sets using multiple regression with shared parameters (background-air CH4(b) and 13CCH4(b)). For IFAA samples collected from 250-350ma.g.l. altitude, the best-fit 13CCH4(s) signatures compare well with the ground observation: CSG 13CCH4(s) of -55.4‰ (confidence interval (CI) 95%±13.7‰) versus 13CCH4(s) of -56.7‰ to -45.6‰; grazing cattle 13CCH4(s) of -60.5‰ (CI 95%±15.6‰) versus -61.7‰ to -57.5‰. For cattle feedlots, the derived 13CCH4(s) (-69.6‰, CI 95%±22.6‰), was isotopically lighter than the ground-based study (13CCH4(s) from -65.2‰ to -60.3‰) but within agreement given the large uncertainty for this source. For IFAA samples collected between 100-200ma.g.l. the 13CCH4(s) signature for the CSG set (-65.4‰, CI 95%±13.3‰) was isotopically lighter than expected, suggesting a BU inventory knowledge gap or the need to extend the population statistics for CSG 13CCH4(s) signatures. For the 100-200ma.g.l. set collected over grazing cattle districts the 13CCH4(s) signature (-53.8‰, CI 95%±17.4‰) was heavier than expected from the BU inventory. An isotopically light set had a low 13CCH4(s) signature of -80.2‰ (CI 95%±4.7‰). A CH4 source with this low 13CCH4(s) signature has not been incorporated into existing BU inventories for the region. Possible sources include termites and CSG brine ponds. If the excess emissions are from the brine ponds, they can potentially be mitigated. It is concluded that in-flight atmospheric 13CCH4(a) measurements used in conjunction with endmember mixing modelling of CH4 sources are powerful tools for BU inventory verification.