Spatial distribution of Δ¹⁴CO₂ across Eurasia: measurements from the TROICA-8 expedition

Abstract. Because fossil fuel derived CO₂ is the only source of atmospheric CO₂ that is devoid of ¹⁴C, atmospheric measurements of Δ¹⁴CO₂ can be used to constrain fossil fuel emission estimates at local and regional scales. However, at the continental scale, uncertainties in atmospheric transport and other sources of variability in Δ¹⁴CO₂ may influence the fossil fuel detection capability. We present a set of Δ¹⁴CO₂ observations from the train-based TROICA-8 expedition across Eurasia in March–April 2004. Local perturbations in Δ¹⁴CO₂ are caused by easily identifiable sources from nuclear reactors and localized pollution events. The remaining data show an increase in Δ¹⁴CO₂ from Western Russia (40° E) to Eastern Siberia (120° E), consistent with depletion in ¹⁴CO₂ caused by fossil fuel CO₂ emissions in heavily populated Europe, and gradual dispersion of the fossil fuel plume across Northern Asia.

Other trace gas species which may be correlated with fossil fuel CO₂ emissions, including carbon monoxide, sulphur hexafluoride, and perchloroethylene, were also measured and the results compared with the Δ¹⁴CO₂ measurements. The sulphur hexafluoride longitudinal gradient is not significant relative to the measurement uncertainty. Carbon monoxide and perchloroethylene show large-scale trends of enriched values in Western Russia and decreasing values in Eastern Siberia, consistent with fossil fuel emissions, but exhibit significant spatial variability, especially near their primary sources in Western Russia.

The clean air Δ¹⁴CO₂ observations are compared with simulated spatial gradients from the TM5 atmospheric transport model. We show that the change in Δ¹⁴CO₂ across the TROICA transect is due almost entirely to emissions of fossil fuel CO₂, but that the magnitude of this Δ¹⁴CO₂ gradient is relatively insensitive to modest uncertainties in the fossil fuel flux. In contrast, the Δ¹⁴CO₂ gradient is more sensitive to the modeled representation of vertical mixing, suggesting that Δ¹⁴CO₂ may be a useful tracer for training mixing in atmospheric transport models.