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Analysis systems incorporating atmospheric observations could provide a powerful tool for validating fossil fuel CO _2 (ffCO _2 ) emissions reported for individual regions, provided that fossil fuel sources can be separated from other CO _2 sources or sinks and atmospheric transport can be accurately accounted for. We quantified ffCO _2 by measuring radiocarbon ( ^14 C) in CO _2 , an accurate fossil-carbon tracer, at nine observation sites in California for three months in 2014–15. There is strong agreement between the measurements and ffCO _2 simulated using a high-resolution atmospheric model and a spatiotemporally-resolved fossil fuel flux estimate. Inverse estimates of total in-state ffCO _2 emissions are consistent with the California Air Resources Board’s reported ffCO _2 emissions, providing tentative validation of California’s reported ffCO _2 emissions in 2014–15. Continuing this prototype analysis system could provide critical independent evaluation of reported ffCO _2 emissions and emissions reductions in California, and the system could be expanded to other, more data-poor regions.