Simulating the integrated summertime δ¹⁴CO₂ signature from anthropogenic emissions over Western Europe

Radiocarbon dioxide (¹⁴CO₂, reported in δ¹⁴CO₂) can be used to determine the fossil fuel CO₂ addition to the atmosphere, since fossil fuel CO₂ no longer contains any ¹⁴C. After the release of CO₂ at the source, atmospheric transport causes dilution of strong local signals into the background and detectable gradients of δ¹⁴CO₂ only remain in areas with high fossil fuel emissions. This fossil fuel signal can moreover be partially masked by the enriching effect that anthropogenic emissions of ¹⁴CO₂ from the nuclear industry have on the atmospheric δ¹⁴CO₂ signature. In this paper, we investigate the regional gradients in ¹⁴CO₂ over the European continent and quantify the effect of the emissions from nuclear industry. We simulate the emissions and transport of fossil fuel CO₂ and nuclear ¹⁴CO₂ for Western Europe using the Weather Research and Forecast model (WRF-Chem) for a period covering 6 summer months in 2008. We evaluate the expected CO₂ gradients and the resulting δ¹⁴CO₂ in simulated integrated air samples over this period, as well as in simulated plant samples. We find that the average gradients of fossil fuel CO₂ in the lower 1200 m of the atmosphere are close to 15 ppm at a 12 km × 12 km horizontal resolution. The nuclear influence on δ¹⁴CO₂ signatures varies considerably over the domain and for large areas in France and the UK it can range from 20 to more than 500% of the influence of fossil fuel emissions. Our simulations suggest that the resulting gradients in δ¹⁴CO₂ are well captured in plant samples, but due to their time-varying uptake of CO ₂, their signature can be different with over 3‰ from the atmospheric samples in some regions. We conclude that the framework presented will be well-suited for the interpretation of actual air and plant ¹⁴CO₂ samples.