Source Apportionment of Ambient Methane Enhancements in Los Angeles, California, To Evaluate Emission Inventory Estimates

Rapid increase in atmospheric methane (CH₄) mixing ratios over the past century is attributable to the intensification of human activities. Information on spatially explicit source contributions is needed to develop efficient and cost-effective CH₄ emission reduction and mitigation strategies to addresses near-term climate change. This study collected long-term ambient CH₄ measurements at Mount Wilson Observatory (MWO) in Los Angeles, California, to estimate the annual CH₄ emissions from the portion of Los Angeles County that is within the South Coast Air Basin (SCLA). The measurement-based CH₄ emission estimates for SCLA ranged from 3.95 to 4.89 million metric tons (MMT) carbon dioxide equivalent (CO₂e) per year between 2012 and 2016. Source apportionment of CH₄, CO, CO₂, and volatile organic compounds (VOCs) measurements were used to evaluate source categories that contributed to ambient CH₄ mixing ratio enhancements (ΔCH₄) at SCLA between 2014 and 2016. Results suggested ΔCH₄ contributions of 56-79% from natural gas sources, 7-31% from landfills, and 4-15% from transportation sources. The SCLA-specific CH₄ emission estimate made using a research grade gridded CH₄ emission inventory suggested contributions of 47% from natural gas sources and 50% from landfills. Subsequent airborne measurements determined that CH₄ emissions from two major CH₄ sources in SCLA were significantly smaller in magnitude than previously thought. This study highlights the importance of studying the variabilities of CH₄ emissions across California for policy makers and stakeholders alike.