Estimating regional fossil fuel CO2concentrations from14CO2observations: challenges and uncertainties

Abstract

The direct way to estimate the regional fossil fuel CO2surplus (ΔffCO2) at a station is by measuring the Δ14CO2depletion compared with a respective background. However, this approach has several challenges, which are (i) the choice of an appropriate Δ14CO2background, (ii) potential contaminations through nuclear14CO2emissions and (iii) masking of ΔffCO2by14C-enriched biosphere respiration. Here we evaluate these challenges and estimate potential biases and typical uncertainties of14C-based ΔffCO2estimates 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 ΔffCO2representativeness bias when using the MHD Δ14CO2background 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 ΔffCO2estimates if not corrected for. The ΔffCO2masking due to14C-enriched heterotrophic CO2respiration can lead to similar ΔffCO2biases as the nuclear contaminations, especially in summer. Our evaluation of all components contributing to the uncertainty of ΔffCO2estimates reveals that, due to the small ffCO2signals at ICOS stations, almost half of the14C-based ΔffCO2estimates 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'.