Modeling support for an extensive Δ14CO2 flask sample monitoring campaign over Europe to constrain fossil CO2 emissions

Abstract. During 2024, an intensive Δ14CO2 flask sampling campaign was conducted at 12 sampling stations across Europe as part of the CO2MVS Research on Supplementary Observations (CORSO) project. These Δ14CO2 samples, combined with CO2 atmospheric measurements, are intended to enhance the estimation of fossil CO2 emissions over Europe through inverse modeling. In this study, we perform a series of Observing System Simulation Experiments (OSSEs) to evaluate the added value of such an intensive campaign as well as the different flask sample selection strategies on estimating fossil fuel emissions. We explore three main selection strategies and compare them against the currently more widely used method of two-week integrated samples: (1) collecting flask samples every 3 d according to a uniform schedule, without applying specific selection criteria; (2) selecting flask samples with high fossil CO2 content to better isolate anthropogenic signals; and (3) combining fossil CO2 selection with consideration of nuclear 14CO2 contamination to reduce potential biases from nuclear emissions. The results suggest that higher sampling density improves the estimation of fossil CO2 emissions, particularly during periods of high fossil fuel activity, such as in winter, while integrated sampling remains more effective during summer months when emissions are lower. Increasing the number of flask samples significantly reduces uncertainty and enhances the robustness of inverse modeling results. In addition, selecting samples with a high fossil CO2 content shows potential for improving the accuracy of emission estimates. The largest reduction in uncertainty is achieved when sample selection actively avoids periods of potential high nuclear 14CO2 contamination. This approach helps minimize potential biases, particularly in regions with significant nuclear activity such as France and the UK. These findings highlight the importance of not only increasing sampling frequency but also carefully selecting samples based on their fossil CO2 and nuclear 14CO2 composition to improve the reliability of fossil fuel emission estimates across Europe.