Constraining ammonia emissions and deposition through joint NH3-NO2 satellite data assimilation in LOTOS-EUROS

Ammonia (NH₃) and nitrogen dioxide (NO₂) are key components of reactive nitrogen, with strong impacts on air quality, ecosystems, and nitrogen deposition. Long-term constraints on ammonia emissions and deposition remain uncertain due to sparse in situ measurements and limitations of individual satellite products. Here, we jointly assimilate five years (2018-2022) of NH₃ and NO₂ satellite observations over the Netherlands to improve constraints on reactive nitrogen concentrations, emissions, and deposition.

NH₃ retrievals from the Infrared Atmospheric Sounding Interferometer (IASI) and Cross-track Infrared Sounder (CrIS) are assimilated along with NO₂ observations from the TROPOspheric Monitoring Instrument (TROPOMI) within the Long Term Ozone Simulation-EURopean Operational Smog (LOTOS-EUROS) chemical transport model using a Local Ensemble Transform Kalman Filter (LETKF). The co-assimilation produces consistent year-to-year adjustments in modeled NH₃ concentrations, emissions, and deposition, reflecting the chemically linked nature of reduced and oxidized nitrogen. Our model results are evaluated against independent surface observations from the Dutch National Air Quality Monitoring Network (LML), showing reduced surface biases, improved correlations, and a clearer representation of diurnal variability. Sensitivity experiments demonstrate that including TROPOMI NO₂ alongside NH₃ observations leads to the lowest NH₃ surface biases, highlighting the added value of jointly assimilating chemically coupled species. Comparisons with the Dutch Measurements of Ammonia in Nature (MAN) network data show improved temporal correlations but persistent spatial biases related to representativeness differences, while MAN sensors co-located with LML stations exhibit consistent improvements.

In addition, synthetic NH₃ observations from the geostationary Meteosat Third Generation Infrared Sounder (MTG-IRS) are assimilated in a separate experiment to assess the potential of future high-temporal-resolution measurements. These experiments indicate that MTG-IRS will provide substantial added value for constraining ammonia emissions and deposition at diurnal scales. Our results demonstrate that co-assimilation of NH₃ and NO₂ satellite observations provides a robust pathway toward improved monitoring of reactive nitrogen and supports the design and exploitation of next-generation atmospheric composition missions.