METHANE SOURCE APPORTIONMENT USING CLUMPED ISOTOPOLOGUES

Abstract

The global atmospheric methane concentration continues to rise, exacerbating the greenhouse effect. This increase results from the rapid growth of methane source emissions that reflects an imbalance between sources and sinks. The development of methane clumped isotopologue (doubly-substituted methane molecules, 13CH3D and 12CH2D2) measurements provides new opportunities for methane emission source apportionment and for distinguishing methane sink intensities and pathways. However, significant uncertainties remain regarding the intensity and isotopic characteristics of various methane sources and sinks. This dissertation provides a detailed introduction to various methane sources—categorized either by activity (fossil fuel-related, agriculture, waste, biomass burning, and wetlands) or by formation mechanisms (thermogenic, pyrogenic, and microbial)—and methane sinks (including OH, Cl, O(1D), and soil), along with their subcategories and spatial and temporal flux variations. It further discusses the bulk carbon and hydrogen isotope characteristics of these sources and the isotope effects associated with different methane formation and oxidation pathways. The discussion then extends from bulk isotopes to clumped isotopologues. My research aims to enhance our understanding of clumped isotopologue signatures for specific methane sources and sinks and to develop a global atmospheric methane budget model incorporating clumped isotopologues. Chapter 2 introduces this novel measurement technique. Chapter 3 presents vehicle exhaust methane clumped isotopologue measurements, and provides representative signatures for abiotic and biomass burning methane. The isotope effects of abiotic catalyzed methane oxidation are also discussed. Chapter 4 presents the first clumped isotopologue measurements of atmospheric methane and demonstrates how these data refine previous estimates of the global total methane source compositions. Chapter 5 explores an attempt to use air measurements to infer methane sources and track source variations. Chapter 6 reports measurements from Arctic firn-trapped air samples, allowing the reconstruction of an atmospheric methane clumped isotopologue signal profile for the past 30 years. These time-resolved clumped isotopologue data are incorporated into a global model, providing strong constraints on the source-sink imbalance. Finally, Chapter 7 summarizes the findings and discusses future research directions.