[en] Analysis systems incorporating atmospheric observations could provide a powerful tool for validating fossil fuel CO₂ (ffCO₂) emissions reported for individual regions, provided that fossil fuel sources can be separated from other CO₂ sources or sinks and atmospheric transport can be accurately accounted for. We quantified ffCO₂ by measuring radiocarbon (¹⁴C) in CO₂, 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 ffCO₂ simulated using a high-resolution atmospheric model and a spatiotemporally-resolved fossil fuel flux estimate. Inverse estimates of total in-state ffCO₂ emissions are consistent with the California Air Resources Board’s reported ffCO₂ emissions, providing tentative validation of California’s reported ffCO₂ emissions in 2014–15. Continuing this prototype analysis system could provide critical independent evaluation of reported ffCO₂ emissions and emissions reductions in California, and the system could be expanded to other, more data-poor regions. (letter)