Preparing for an extensive ∆¹⁴CO₂ flask sample monitoring campaign over Europe to constrain fossil CO₂ emissions

Abstract. During 2024, an intensive Δ¹⁴CO₂ flask sampling campaign is being conducted at 12 stations across Europe as part of the CO2MVS Research on Supplementary Observations (CORSO) project. These Δ¹⁴CO₂ samples, combined with CO₂ atmospheric measurements, aim to improve fossil CO₂ emission estimates across Europe through inverse modeling. In this study, we perform a series of Observing System Simulation Experiments (OSSEs) to assess the added value of this intensive campaign and explore different sampling strategies for optimizing fossil fuel emission estimates. The strategies focus on selecting samples for inversions based on their fossil CO₂ and nuclear ¹⁴C composition.

We evaluate three sampling approaches: (1) a base scenario using uniform sampling without specific selection criteria, comparing current methods with the addition of flask samples; (2) a strategy that selects samples with high fossil CO₂ content; and (3) a combined approach that accounts for nuclear ¹⁴C contamination to reduce potential biases from nuclear facilities. Our results suggest that higher sampling density improves the accuracy of fossil CO₂ estimates, especially during low-emission periods, such as summer. Increasing the number of samples reduces uncertainty, enhancing the robustness of inverse modeling outcomes. Selecting samples based on fossil CO₂ contamination further refines the estimates, but the most significant uncertainty reduction occurs when nuclear contamination is also considered. This combined strategy effectively mitigates biases in regions with high nuclear activity, like France and the UK. These findings highlight the importance of increasing sampling frequency and strategically selecting samples to improve fossil fuel emission estimates across Europe.