On the use of ¹⁴CO₂ as a tracer for fossil fuel CO₂: Quantifying uncertainties using an atmospheric transport model

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