Sustained Reductions of Bay Area CO Emissions 2018-2022.

Cities represent a significant and growing portion of global carbon dioxide (CO) emissions. Quantifying urban emissions and trends over time is needed to evaluate the efficacy of policy targeting emission reductions as well as to understand more fundamental questions about the urban biosphere. A number of approaches have been proposed to measure, report, and verify (MRV) changes in urban CO emissions. Here we show that a modest capital cost, spatially dense network of sensors, the Berkeley Environmental Air Quality and CO Network (BEACON), in combination with Bayesian inversions, result in a synthesis of measured CO concentrations and meteorology to yield an improved estimate of CO emissions and provide a cost-effective and accurate assessment of CO emissions trends over time. We describe nearly 5 years of continuous CO observations (2018-2022) in a midsized urban region (the San Francisco Bay Area). These observed concentrations constrain a Bayesian inversion that indicates the interannual trend in urban CO emissions in the region has been a modest decrease at a rate of 1.8 ± 0.3%/year. We interpret this decrease as primarily due to passenger vehicle electrification, reducing on-road emissions at a rate of 2.6 ± 0.7%/year.