Characterizing flow patterns and mixing of fossil fuel-derived CO₂ is important for effectively using atmospheric measurements to constrain emissions inventories. Here we used measurements and a model of atmospheric radiocarbon (¹⁴C) to investigate the distribution and fluxes of atmospheric fossil fuel CO₂ across the state of California. We sampled ¹⁴C in annual C₃ grasses at 128 sites and used these measurements to test a regional model that simulated anthropogenic and ecosystem CO₂ fluxes, transport in the atmosphere, and the resulting Δ¹⁴C of annual grasses (Δ _g ). Average measured Δ _g levels in Los Angeles, San Francisco, the Central Valley, and the North Coast were 27.7 ± 20.0, 44.0 ± 10.9, 48.7 ± 1.9, and 59.9 ± 2.5‰, respectively, during the 2004–2005 growing season. Model predictions reproduced regional patterns reasonably well, with estimates of 27.6 ± 2.4, 39.4 ± 3.9, 46.8 ± 3.0, and 59.3 ± 0.2‰ for these same regions and corresponding to fossil fuel CO₂ mixing ratios (C _f ) of 13.7, 6.1, 4.8, and 0.3 ppm. Δ _g spatial heterogeneity in Los Angeles and San Francisco was higher in the measurements than in the predictions, probably from insufficient spatial resolution in the fossil fuel inventories (e.g., freeways are not explicitly included) and transport (e.g., within valleys). We used the model to predict monthly and annual transport patterns of fossil fuel-derived CO₂ within and out of California. Fossil fuel CO₂ emitted in Los Angeles and San Francisco was predicted to move into the Central Valley, raising C _f above that expected from local emissions alone. Annually, about 21, 39, 35, and 5% of fossil fuel emissions leave the California airspace to the north, east, south, and west, respectively, with large seasonal variations in the proportions. Positive correlations between westward fluxes and Santa Ana wind conditions were observed. The southward fluxes over the Pacific Ocean were maintained in a relatively coherent flow within the marine boundary layer, while the eastward fluxes were more vertically dispersed. Our results indicate that state and continental scale atmospheric inversions need to consider areas where mixing ratio measurements are sparse (e.g., over the ocean to the south and west of California), transport within and across the marine boundary layer, and terrestrial boundary layer dynamics. Radiocarbon measurements can be very useful in constraining these estimates.