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

摘要

A14CO2 observations are increasingly used to constrain recently added fossil fuel CO2in the atmosphere. We use the LMDZ global atmospheric transport model to examinethe pseudo-Lagrangian framework commonly used to determine recently added fossilfuel CO2 (CO2ff). Our results confirm that z 14CO2 spatial variability in the NorthernHemisphere troposphere is dominated by the effect of CO2ff, whereas in the SouthernHemisphere, ocean CO2 exchange is more important. The model indicates that the freetroposphere, at 3-5 km altitude, is a good choice for "background," relative to which therecently added fossil fuel CO2 can be calculated, although spatial variability in freetropospheric 014CO2 contributes additional uncertainty to the CO2ff calculation.Comparison of model and observations suggests that care must be taken in usinghigh-altitude mountain sites as a proxy for free tropospheric air, since these sites may beoccasionally influenced by (polluted) boundary layer air, especially in summer. Othersources of CO2 which have 0' 4Cdifferent 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 of14C contribute only weakly. The model indicates that neglecting this bias leads to aconsistent underestimation of CO2ff, typically between 0.2 and 0.5ppm of CO2, with amaximum 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 emittedat the surface.