Characterisation of methane sources in Australia using isotopic analyses

Abstract

Methane (CH4) is the second most abundant anthropogenic greenhouse gas (GHG) (after carbon dioxide), accounting for around 20 per cent of global emissions. At the global scale, the primary driver for the increasing CH4 mixing ratio in the atmosphere remains under debate. Resolving the debate is challenging because many CH4 sources are co-located resulting in source attribution errors. This thesis demonstrates that CH4 stable isotope analysis can be used to constrain source attribution in rural, industrial and urban settings. There is a paucity of isotopic measurements for all CH4 sources in Australia. CH4 plumes were mapped using laser-based CH4 analysers transported in a car or aircraft. Discrete air samples were collected as part of these surveys and analysed for both their CH4 mole fraction and stable isotopes composition. This enabled the characterisation of the isotopic signatures of major sources of CH4. Two settings of high CH4 emissions in Australia were studied: the coal seam gas (CSG) fields and adjacent agricultural districts in the Surat Basin, Queensland, and the city of Melbourne. The Surat Basin is a major CSG producing basin where numerous CH4 sources are co-located and poorly characterised. The results presented in this thesis (Chapters 2 and 3) demonstrate that CSG sources and ruminants can be distinguished by using dual isotope tracers (carbon and hydrogen) and that the isotopic insights can be used to verify the regional bottom-up emission inventory. A vehicle-mounted GHG analyser was used to map the CH4 mole fraction in the ground-level atmosphere throughout Melbourne. Major CH4 sources detected included plumes from natural gas distribution network leaks, landfills, wastewater treatment plants and domestic fires. The isotopic signatures of these plumes were characterised and used to interpret a one-year time series dataset of CH4, and stable isotope composition collected at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Aspendale monitoring site. The seasonal and diel trends of the observed CH4 mole fraction and stable isotope composition coupled with meteorology data show that CH4 mole fraction enhancements were observed under low wind speed conditions when there was limited mixing and dilution of each plume, suggesting the measurements captured were recording emissions from local sources. Isotopic evidence demonstrated that microbial sources such as landfills and wastewater treatment plants are major contributors to the elevated CH4 recorded at the monitoring site.