Trends in CH₄ Emission Sources in the Los Angeles Basin from 2010 to 2023 Using Airborne Measurements

California legislation mandates a 40% reduction in CH4 emissions from 2013 levels by 2030. Achieving this requires an accurate inventory of greenhouse gas sources, including urban CH4. This study determined summertime CH4 and C2-C5 alkane emissions from the Los Angeles (L.A.) Basin using airborne field campaign measurements from 2010, 2019, and 2023. Our analysis combined enhancement ratios of CH4 and alkanes versus CO from in situ airborne measurements with the California Air Resources Board's CO emissions inventory. By incorporating known alkane abundances from various sectors, we apportioned the emission sources. We found an annual decline in CH4 emissions from the L.A. Basin at a rate of -7.2 ± 5.8 Gg/year, consistent with literature and ground-site measurements from the Mount Wilson Observatory. The primary CH4 sources were natural gas (52-57% of total emissions) and CH4-dominant sources, like landfills and dairies (41-47%). We also observed an annual increase in ethane emissions of 0.13 ± 0.19 Gg/year, which correlates with increasing ethane abundance in pipeline natural gas and decreasing ethane prices. If this linearly decreasing CH4 emission trend were to continue, the L.A. Basin would be on track to reach the state's 2030 CH4 emission reduction goals.