Estimating methane emissions from biological and fossil-fuel sources in the San Francisco Bay Area

We present the first sector-specific analysis of methane (CH₄) emissions from the San Francisco Bay Area (SFBA) using CH₄ and volatile organic compound (VOC) measurements from six sites during September – December 2015. We apply a hierarchical Bayesian inversion to separate the biological from fossil-fuel (natural gas and petroleum) sources using the measurements of CH₄ and selected VOCs, a source-specific 1 km CH₄ emission model, and an atmospheric transport model. We estimate that SFBA CH₄ emissions are 166–289 Gg CH₄/yr (at 95% confidence), 1.3–2.3 times higher than a recent inventory with much of the underestimation from landfill. Including the VOCs, 82
27% of total posterior median CH4 emissions are biological and 17
3% fossil fuel, where landfill and natural gas dominate the biological and fossil-fuel CH₄ of prior emissions, respectively.