[1] We use the nearly ideal tracer 14CO2 to estimate the fossil fuel CO2 enhancement in boundary layer air at two sites in New England and Colorado. Improved Δ14C measurement precision of 1.6–2.6‰ provides fossil fuel CO2 detection capability of 0.8–1.5 ppm. Using the indirect tracers CO and SF6, we obtain two additional independent estimates of the fossil fuel CO2 component, and we assess the biases in these methods with respect to the 14CO2-based estimates. The SF6-based estimates vary considerably from the 14CO2-based estimates, and are at times implausibly large. The CO-based estimates are less variable, but show seasonally coherent biases with respect to the 14CO2 method.