Methane (CH₄), carbon dioxide (CO₂), carbon monoxide (CO), and C₂–C₅ alkanes were measured throughout the Los Angeles (L.A.) basin in May and June 2010. We use these data to show that the emission ratios of CH₄/CO and CH₄/CO₂in the L.A. basin are larger than expected from population-apportioned bottom-up state inventories, consistent with previously published work. We use experimentally determined CH₄/CO and CH₄/CO₂ emission ratios in combination with annual State of California CO and CO₂ inventories to derive a yearly emission rate of CH₄ to the L.A. basin. We further use the airborne measurements to directly derive CH₄ emission rates from dairy operations in Chino, and from the two largest landfills in the L.A. basin, and show these sources are accurately represented in the California Air Resources Board greenhouse gas inventory for CH₄. We then use measurements of C₂–C₅ alkanes to quantify the relative contribution of other CH₄ sources in the L.A. basin, with results differing from those of previous studies. The atmospheric data are consistent with the majority of CH₄ emissions in the region coming from fugitive losses from natural gas in pipelines and urban distribution systems and/or geologic seeps, as well as landfills and dairies. The local oil and gas industry also provides a significant source of CH₄ in the area. The addition of CH₄ emissions from natural gas pipelines and urban distribution systems and/or geologic seeps and from the local oil and gas industry is sufficient to account for the differences between the top-down and bottom-up CH₄ inventories identified in previously published work.