Soutenance

Methane has the second largest contribution to global radiative forcing impact of anthropogenic greenhouse gasses. Since 2007 its atmospheric growth rate, after a period of stability, has again been rising rapidly. Anthropogenic methane emissions hold a large mitigation potential, promoting efforts to curb emissions in accordance with the Paris Agreement. However, the considerable uncertainties regarding methane contributors, drivers and emission estimates even at local scales, hinder the effective implementation of methane mitigation strategies. While many approaches have been established to measure total methane fluxes, the partitioning and characterisation of methane sources by region and processes still need to be better constrained.

This thesis presents practical methods for characterising different CH₄ sources in ambient air measurements at industrial sites, as well as developing more targeted tools. The first chapter focuses on improvements to a CRDS instrument that is commonly deployed for CH₄ and δ¹³CH₄ field measurements. We propose a calibration scheme to correct for C₂H₆ interference on δ¹³CH₄, and enable robust C₂H₆ measurements. The results of this work are then used to explore the added value gained when implemented on data from a natural gas compressor station, a site where high correlation of C₂H₆ and CH₄ is expected. The second chapter continues the investigation of CH₄ sources at the same site; with focus shifted towards the application and comparison of different source apportionment methods from time series analysis based on measurements of multiple species, some co-emitted with CH₄. Here the CH₄ and Volatile Organic Compounds (VOC) source contributions are explored through the use of isotopic analysis, receptor model analysis (PCA and PMF), metrological data and direct samples of natural gas. The third chapter applies a selection of the developed CH₄ source apportionment methods to ambient measurements at biogenic CH₄ sites in the Ile de France region and helps complete the survey of the most relevant anthropogenic CH₄ sources.

This thesis identifies and reports local δ¹³CH₄ source signatures for livestock, wastewater, landfill and natural gas and studies their spatial and temporal variability to aid the constraint of emission inventories. Our findings suggest that source apportionment from δ¹³CH₄ is robust, and adaptable to the majority of sites. Using a combination of tools is ideal for more specific source determination and for an understanding of long and short term variability. The work presented in this thesis offers example applications of these new tools to directed investigations of anthropogenic methane emissions and lays the foundation for future work in this field.