On the use of 14 CO 2 as a tracer for fossil fuel CO 2 : Quantifying uncertainties using an atmospheric transport model

Δ 14 CO 2 observations are increasingly used to constrain recently added fossil fuel CO 2 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 CO 2 (CO 2ff ). Our results confirm that Δ 14 CO 2 spatial variability in the Northern Hemisphere troposphere is dominated by the effect of CO 2ff , whereas in the Southern Hemisphere, ocean CO 2 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 CO 2 can be calculated, although spatial variability in free tropospheric Δ 14 CO 2 contributes additional uncertainty to the CO 2ff 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 CO 2 which have Δ 14 C different than that of the atmosphere contribute a bias, which, over the Northern Hemisphere land, is mostly due to the terrestrial biosphere, whereas ocean CO 2 exchange and nuclear industry and natural cosmogenic production of 14 C contribute only weakly. The model indicates that neglecting this bias leads to a consistent underestimation of CO 2ff , typically between 0.2 and 0.5ppm of CO 2 , with a maximum in summer. While our analysis focuses on fossil fuel CO 2 , our conclusions, particularly the choice of background site, can also be applied to other trace gases emitted at the surface.