Detection and isotopic characterisation of methane emissions:An integrated approach developing and applying mobile methods and stationary measurements

Methane (CH4) is a powerful greenhouse gas which is formed through both microbial and geological processes. Emissions of CH4 originate from natural sources, such as wetlands, and from anthropogenic sources, such as agriculture, fossil fuel infrastructure, and landfills. Due to anthropogenic activity, atmospheric concentrations of CH4 have increased by 160% since preindustrial times and are responsible for around 20% of total radiative forcing. Considerable uncertainties over the contribution of different sources remain, due to large spatial and temporal variability and because emission sources are often co-located in the landscape, hindering accurate attribution of emissions. Thus, understanding regional CH4 sources is important for reducing fugitive emissions and to better constrain atmospheric CH4 budgets. Stable isotope analysis is a powerful method for constraining methane budgets as source categories differ in their isotopic signatures. My aim in this thesis was to develop and apply isotope-based methods to characterise, and attribute CH4 emissions. I developed a system for mobile isotopic CH4 measurements and used instrument comparisons and model simulations to evaluate system performance. My findings have implications for the interpretation and comparability of data and provide a framework for optimising sampling strategies (Chapter 2). I used dual-isotope sampling and mobile measurements to characterise and identify emission sources in North West England prior to the start of shale gas exploration. My results show that dual isotope analysis can distinguish between microbial emission sources in the region and provide evidence for offshore emissions. Mobile measurements identified fugitive emissions from landfills and gas pipelines (Chapter 3). To investigate seasonal variations in wetland isotopic signatures, I performed a 2.5-year monitoring study at an ombrotrophic peat bog. Despite large changes in emission fluxes over time and, I found constant isotopic signatures throughout the sampling period (Chapter 4). The work presented in this thesis provides insights for evaluating novel methods for CH4 emission measurements and contribute to the understanding of emission sources needed to effectively constrain CH4 budgets and reduce emissions