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

The direct way to estimate the regional fossil fuel CO₂ surplus (ΔffCO₂) at a station is by measuring the Δ¹⁴CO₂ depletion compared with a respective background. However, this approach has several challenges, which are (i) the choice of an appropriate Δ¹⁴CO₂ background, (ii) potential contaminations through nuclear ¹⁴CO₂ emissions and (iii) masking of ΔffCO₂ by ¹⁴C-enriched biosphere respiration. Here we evaluate these challenges and estimate potential biases and typical uncertainties of ¹⁴C-based ΔffCO₂ 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 ΔffCO₂ representativeness bias when using the MHD Δ¹⁴CO₂ 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 ΔffCO₂ estimates if not corrected for. The ΔffCO₂ masking due to ¹⁴C-enriched heterotrophic CO₂ respiration can lead to similar ΔffCO₂ biases as the nuclear contaminations, especially in summer. Our evaluation of all components contributing to the uncertainty of ΔffCO₂ estimates reveals that, due to the small ffCO₂ signals at ICOS stations, almost half of the ¹⁴C-based ΔffCO₂ 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'.