IMPLICATIONS FOR DERIVING REGIONAL FOSSIL FUEL CO2 ESTIMATES FROM ATMOSPHERIC OBSERVATIONS IN A HOT SPOT OF NUCLEAR POWER PLANT 14CO2 EMISSIONS

Using Delta C-14 observations to infer the local concentration excess of CO2 due to the burning of fossil fuels (Delta FFCO2) is a promising technique to monitor anthropogenic CO2 emissions. A recent study showed that (CO2)-C-14 emissions from the nuclear industry can significantly alter the local atmospheric (CO2)-C-14 concentration and thus mask the Delta C-14 depletion due to Delta FFCO2. In this study, we investigate the relevance of this effect for the vicinity of Toronto, Canada, a hot spot of anthropogenic (CO2)-C-14 emissions. Comparing the measured emissions from local power plants to a global emission inventory highlighted significant deviations on interannual timescales. Although the previously assumed emission factor of 1.6 TBq(GWa)(-1) agrees with the observed long-term average for all CANDU reactors of 1.50 +/- 0.18 TBq(GWa)(-1). This power-based parameterization neglects the different emission ratios for individual reactors, which range from 3.4 +/- 0.82 to 0.65 +/- 0.09 TBq(GWa)(-1). This causes a mean difference of -14% in 14CO2 concentrations in our simulations at our observational site in Egbert, Canada. On an annual time basis, this additional (CO2)-C-14 masks the equivalent of 27-82% of the total annual FFCO2 offset. A pseudo-data experiment suggests that the interannual variability in the masked fraction may cause spurious trends in the Delta FFCO2 estimates of the order of 30% from 2006-2010. In addition, a comparison of the modeled Delta C-14 levels with our observational time series from 2008-2010 underlines that incorporating the best available (CO2)-C-14 emissions significantly increases the agreement. There were also short periods with significant observed. C-14 offsets, which were found to be linked with maintenance periods conducted on these nuclear reactors.