Isotopic measurements of atmospheric methane in Los Angeles, California, USA: Influence of “fugitive” fossil fuel emissions

Recent studies have suggested that CH 4 emissions in Los Angeles and other large cities may be underestimated. We utilized stable isotopes ( 13 C and D) and radiocarbon ( 14 C) to investigate sources of CH 4 in Los Angeles, California. First, we made measurements of δ 13 C and δ D of various CH 4 sources in urban areas. Fossil fuel CH 4 sources (oil refineries, power plants, traffic, and oil drilling fields) had δ 13 C values between −45 and −30‰ and dD values between −275 and −100‰, whereas biological CH 4 (cows, biofuels, landfills, sewage treatment plants, and cattle feedlots) had δ 13 C values between −65 and −45‰ and δ D values between −350 and −275‰. We made high‐altitude observations of CH 4 concentration using continuous tunable laser spectroscopy measurements combined with isotope analyses ( 13 C, 14 C, and D) of discrete samples to constrain urban CH 4 sources. Our data indicate that the dominant source of CH 4 in Los Angeles has a δ 13 C value of approximately −41.5‰ and a δ D value between −229 and −208‰. Δ 14 C of CH 4 in urban air samples ranged from +262 to +344‰ (127.1 to 134.9 pMC), depleted with respect to average global background CH 4 . We conclude that the major source of CH 4 in Los Angeles is leakage of fossil fuels, such as from geologic formations, natural gas pipelines, oil refining, and/or power plants. More research is needed to constrain fluxes of CH 4 from natural gas distribution and refining, as this flux may increase with greater reliance on natural gas and biogas for energy needs.