Radiocarbon levels in annual plants provide a means to map out regional and continental-scale fossil fuel plumes in surface air. We collected corn (Zea mays) across North America during the summer of 2004. Plants from mountain regions of western North America showed the smallest influence of fossil fuel-derived CO2 with a mean Δ14C of 66.3? ¿1.7?. Plants from eastern North America and from the Ohio-Maryland region showed a larger fossil fuel influence with a mean Δ14C of 58.8? ¿ 3.9? and 55.2? ¿ 2.3?, respectively, corresponding to 2.7 ppm ¿ 1.5 ppm and 4.3 ppm ¿ 1.0 ppm of added fossil fuel CO2 relative to the mountain west. A model--data comparison suggests that surveys of annual plant Δ14C can provide a useful test of atmospheric mixing in transport models that are used to estimate the spatial distribution of carbon sources and sinks.