Quantifying N₂O Flux over the EU27+3 Region Using CIF-CHIMERE Model for 2005–2023

Nitrous oxide (N₂O) is the third most important long-lived greenhouse gas after CO₂ and CH₄, yet large uncertainties remain in its regional emission estimates. In this study, we apply the regional inverse modeling system CIF-CHIMERE to quantify N₂O surface fluxes over the EU27+3 region (European Union, United Kingdom, Norway, and Switzerland) for the period 2005–2023, providing a long-term and high spatiotemporal resolution assessment of N₂O fluxes. The inversion is primarily constrained by in situ atmospheric N₂O measurements from the ICOS (Integrated Carbon Observation System) ground-based station network across Europe, and uses the CIF-CHIMERE transport model coupled with a four-dimensional variational (4D-Var) data assimilation framework to estimate posterior N₂O fluxes. For 2005–2023, inversions are conducted at a spatial resolution of 0.5° × 0.5°, while for 2018–2023 the resolution is refined to 0.2° × 0.2°. In both configurations, hourly surface fluxes are estimated, enabling analysis of diurnal, seasonal, and interannual variability. The inversions significantly improve the representation of localized emission patterns and short-term flux dynamics. Overall, the results provide a top-down dataset for evaluating bottom-up inventories and for improving the understanding of regional and temporal variability in N₂O emissions across EU27+3.