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

Recent studies have suggested that CH4 emissions in Los Angeles and other large cities may be underestimated. We utilized stable isotopes (C-13 and D) and radiocarbon (C-14) to investigate sources of CH4 in Los Angeles, California. First, we made measurements of delta C-13 and delta D of various CH4 sources in urban areas. Fossil fuel CH4 sources (oil refineries, power plants, traffic, and oil drilling fields) had delta C-13 values between -45 and -30 parts per thousand and dD values between -275 and -100 parts per thousand, whereas biological CH4 (cows, biofuels, landfills, sewage treatment plants, and cattle feedlots) had delta C-13 values between -65 and -45 parts per thousand and delta D values between -350 and -275 parts per thousand. We made high-altitude observations of CH4 concentration using continuous tunable laser spectroscopy measurements combined with isotope analyses (C-13, C-14, and D) of discrete samples to constrain urban CH4 sources. Our data indicate that the dominant source of CH4 in Los Angeles has a delta C-13 value of approximately -41.5 parts per thousand and a delta D value between -229 and -208 parts per thousand. Delta C-14 of CH4 in urban air samples ranged from +262 to +344 parts per thousand (127.1 to 134.9 pMC), depleted with respect to average global background CH4. We conclude that the major source of CH4 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 CH4 from natural gas distribution and refining, as this flux may increase with greater reliance on natural gas and biogas for energy needs.