Inverse modelling of N2O fluxes over Europe: An EYE-CLIMA initiative

Nitrous Oxide (N₂O) is a long-lived and highly potent greenhouse gas, recognized as the third most significant contributor to radiative forcing, with a substantial proportion of its emissions originating from a large area source, agricultural soils, due to the application of mineral fertilizer and livestock manure. As part of the Horizon Europe project EYE-CLIMA, we performed atmospheric inversions to improve the estimates of N₂O fluxes across Europe at two spatial resolution scales. The first inversion, spanning the period from 2005 to 2023, was performed at a resolution of 0.5° × 0.5°. The second inversion, covering the period from 2018 to 2023, was carried out at a higher resolution of 0.2° × 0.2°. The method integrates the Community Inversion Framework (CIF) with the Lagrangian particle dispersion model, FLEXPART v11 (CIF-FLEXPART), to estimate N₂O emissions using ground-based measurements of atmospheric N₂O concentrations. Comprehensive prior N₂O flux estimates were generated by incorporating monthly data from key source categories, including agriculture, other anthropogenic activities such as combustion, industry or waste treatment, biomass burning, natural soils, and ocean fluxes. For consistency, observed atmospheric concentrations of N₂O were sourced from a newly harmonized dataset for Europe, compiled collaboratively by EYE-CLIMA and the Horizon Europe projects AVENGERS and PARIS.

Following the inversion, the modelled concentrations showed improved agreement with observations, capturing the seasonal cycle and increasing trend from 2005 onward. Statistical analyses revealed high correlations between modelled and observed concentrations at most stations. The N₂O emissions from the inversion differ from the prior estimates in intensity and spatial distribution with increased emissions in regions of specifically high agricultural activity and reductions in other areas. Monthly flux variations exhibited a consistent seasonal cycle, with peak emissions occurring in early summer (May–June) and lower emissions during winter months. Across all years, total posterior emissions were lower than the prior estimates. While the phase of the seasonal cycle remained consistent from year to year, interannual variability in the amplitude was observed.