The Australian methane budget: Interpreting surface and train‐borne measurements using a chemistry transport model

We investigate the Australian methane budget from 2005–2008 using the GEOS‐Chem 3D chemistry transport model, focusing on the relative contribution of emissions from different sectors and the influence of long‐range transport. To evaluate the model, we use in situ surface measurements of methane, methane dry air column average (XCH 4 ) from ground‐based Fourier transform spectrometers (FTSs), and train‐borne surface concentration measurements from an in situ FTS along the north–south continental transect. We use gravity anomaly data from Gravity Recovery and Climate Experiment to describe the spatial and temporal distribution of wetland emissions and scale it to a prior emission estimate, which better describes observed atmospheric methane variability at tropical latitudes. The clean air sites of Cape Ferguson and Cape Grim are the least affected by local emissions, while Wollongong, located in the populated southeast with regional coal mining, samples the most locally polluted air masses (2.5% of the total air mass versus <1% at other sites). Averaged annually, the largest single source above background of methane at Darwin is long‐range transport, mainly from Southeast Asia, accounting for ∼25% of the change in surface concentration above background. At Cape Ferguson and Cape Grim, emissions from ruminant animals are the largest source of methane above background, at approximately 20% and 30%, respectively, of the surface concentration. At Wollongong, emissions from coal mining are the largest source above background representing 60% of the surface concentration. The train data provide an effective way of observing transitions between urban, desert, and tropical landscapes.