Estimating regional fossil fuel CO2 concentrations from 14CO2 observations: challenges and uncertainties

The direct way to estimate the regional fossil fuel CO2 surplus (Delta ffCO(2)) at a station is by measuring the Delta(CO2)-C-14 depletion compared with a respective background. However, this approach has several challenges, which are (i) the choice of an appropriate Delta(CO2)-C-14 background, (ii) potential contaminations through nuclear (CO2)-C-14 emissions and (iii) masking of Delta ffCO(2) by C-14-enriched biosphere respiration. Here we evaluate these challenges and estimate potential biases and typical uncertainties of C-14-based Delta ffCO(2) estimates in Europe. We show that Mace Head (MHD), Ireland, is a representative background station for the Integrated Carbon Observation System (ICOS) atmosphere station network. The mean Delta ffCO(2) representativeness bias when using the MHD Delta(CO2)-C-14 background for the whole observation network is of order 0.1 +/- 0.3 ppm. At ICOS sites, the median nuclear contamination leads to 25% low-biased Delta ffCO(2) estimates if not corrected for. The Delta ffCO(2) masking due to C-14-enriched heterotrophic CO2 respiration can lead to similar Delta ffCO(2) biases as the nuclear contaminations, especially in summer. Our evaluation of all components contributing to the uncertainty of Delta ffCO(2) estimates reveals that, due to the small ffCO(2) signals at ICOS stations, almost half of the C-14-based Delta ffCO(2) estimates from integrated samples have an uncertainty that is larger than 50%. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.