Δ14CO2 observations are increasingly used to constrain recently added fossil fuel CO2 in the atmosphere. We use the LMDZ global atmospheric transport model to examine the pseudo-Lagrangian framework commonly used to determine recently added fossil fuel CO2 (CO2ff). Our results confirm that Δ14CO2 spatial variability in the Northern Hemisphere troposphere is dominated by the effect of CO2ff, whereas in the Southern Hemisphere, ocean CO2 exchange is more important. The model indicates that the free troposphere, at 3–5 km altitude, is a good choice for “background,” relative to which the recently added fossil fuel CO2 can be calculated, although spatial variability in free tropospheric Δ14CO2 contributes additional uncertainty to the CO2ff calculation. Comparison of model and observations suggests that care must be taken in using high-altitude mountain sites as a proxy for free tropospheric air, since these sites may be occasionally influenced by (polluted) boundary layer air, especially in summer. Other sources of CO2 which have Δ14C different than that of the atmosphere contribute a bias, which, over the Northern Hemisphere land, is mostly due to the terrestrial biosphere, whereas ocean CO2 exchange and nuclear industry and natural cosmogenic production of 14C contribute only weakly. The model indicates that neglecting this bias leads to a consistent underestimation of CO2ff, typically between 0.2 and 0.5ppm of CO2, with a maximum in summer. While our analysis focuses on fossil fuel CO2, our conclusions, particularly the choice of background site, can also be applied to other trace gases emitted at the surface.