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 …