Trends in Methane Emission Sources in the L.A. Basin, California from 2010–2023 using Airborne Measurements

California (CA) legislation currently requires that greenhouse gas emissions be reduced to 40% below 1990 levels by 2030, which will require understanding the sources of urban CH₄ emissions to achieve this goal. Field campaigns in the summers of 2010, 2019, and 2023 collected in situ measurements of methane (CH₄), carbon dioxide (CO₂), carbon monoxide (CO), and C₂–C₅ alkanes throughout the Los Angeles (LA) Basin aboard the NASA DC-8 and NOAA P-3 aircraft. Examining the relative emission ratios of these various alkanes and CH₄ versus CO provides valuable insight into the CH₄ contributions of different sources in the LA Basin. Further, using the CA Air Resources Board CO emissions inventory alongside CH₄/CO enhancement ratios, we determine summertime CH₄ and C₂–C₅ alkanes emissions from the LA Basin. In line with the literature, this analysis shows an annual decline in CH₄ emissions from the basin, with roughly half contributed from natural gas sources and roughly half from CH₄-dominant sources, such as landfills and dairies. Additionally, this analysis provides a unique look at trends in emission sources from C₂–C₅ alkanes during this time frame. For example, ethane emissions were found to be increasing in the basin due to its increasing abundance in pipeline natural gas (NG) even though CH₄ emissions from pipeline NG decreased, and pentane emissions due to evaporated gasoline were found to be decreasing. This analysis shows how CH₄ and other light alkane sources have changed in the past 13 years, providing insight into how CA is performing on their greenhouse gas reduction goals.