Estimating regional fossil fuel CO 2 concentrations from 14 CO 2 observations: challenges and uncertainties.

The direct way to estimate the regional fossil fuel CO 2 surplus (ΔffCO 2 ) at a station is by measuring the Δ 14 CO 2 depletion compared with a respective background. However, this approach has several challenges, which are (i) the choice of an appropriate Δ 14 CO 2 background, (ii) potential contaminations through nuclear 14 CO 2 emissions and (iii) masking of ΔffCO 2 by 14 C-enriched biosphere respiration. Here we evaluate these challenges and estimate potential biases and typical uncertainties of 14 C-based ΔffCO 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 ΔffCO 2 representativeness bias when using the MHD Δ 14 CO 2 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 ΔffCO 2 estimates if not corrected for. The ΔffCO 2 masking due to 14 C-enriched heterotrophic CO 2 respiration can lead to similar ΔffCO 2 biases as the nuclear contaminations, especially in summer. Our evaluation of all components contributing to the uncertainty of ΔffCO 2 estimates reveals that, due to the small ffCO 2 signals at ICOS stations, almost half of the 14 C-based ΔffCO 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'.