Simulating the integrated summertime Delta(CO2)-C-14 signature from anthropogenic emissions over Western Europe

Radiocarbon dioxide ((CO2)-C-14, reported in Delta(CO2)-C-14) can be used to determine the fossil fuel CO2 addition to the atmosphere, since fossil fuel CO2 no longer contains any C-14. After the release of CO2 at the source, atmospheric transport causes dilution of strong local signals into the background and detectable gradients of Delta(CO2)-C-14 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 (CO2)-C-14 from the nuclear industry have on the atmospheric Delta(CO2)-C-14 signature. In this paper, we investigate the regional gradients in (CO2)-C-14 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 (CO2)-C-14 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 1 (CO2)-C-14 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 x 12 km horizontal resolution. The nuclear influence on Delta(CO2)-C-14 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 Delta(CO2)-C-14 are well captured in plant samples, but due to their time-varying uptake of CO2, 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 (CO2)-C-14 samples.