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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 (CO(2)e) 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 (Delta CH4) at SCLA between 2014 and 2016. Results suggested Delta CH4 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.