Comment on egusphere-2025-5926

<strong class="journal-contentHeaderColor">Abstract.</strong> Ammonia (NH₃) and nitrogen dioxide (NO₂) are key components of reactive nitrogen, strongly affecting air quality and ecosystem health. However, long-term constraints on ammonia emissions and deposition remain uncertain due to sparse in situ measurements and limitations of individual satellite products. We jointly assimilate five years (2018&ndash;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 the Cross-Track Infrared Sounder (CrIS) are combined with NO₂ observations from the TROPOspheric Monitoring Instrument (TROPOMI) within the LOTOS-EUROS chemical transport model using a Local Ensemble Transform Kalman Filter. The co-assimilation produces coherent year-to-year adjustments in modeled NH₃ concentration, emission, and deposition fields. Validation against measurements from the Dutch National Air Quality Monitoring Network (LML) shows reduced biases, clearer diurnal cycles, and improved correlations. Sensitivity experiments demonstrate that including TROPOMI NO₂ alongside IASI and CrIS NH₃ yields the lowest NH₃ surface bias versus LML, highlighting the added value of coupling chemically related satellite observations. Comparisons with monthly Measurements of Ammonia in Nature (MAN) observations showed improved correlations but persistent spatial biases due to representativeness differences, while MAN sensors co-located with LML stations exhibited consistent improvements. These results demonstrate that co-assimilating complementary satellite observations can substantially improve constraints on ammonia emissions and deposition, with direct relevance for air-quality assessment and nitrogen policy applications.