Atmospheric reactive nitrogen and its dry deposition regimes under anthropogenic influence: Insights from intensive and long-term monitoring in Switzerland

Excessive nitrogen deposition from anthropogenic activities poses significant challenges to ecosystems and air quality.1 The atmospheric deposition of ammonium and nitrate plays a critical role in regulating ecosystem productivity and driving particulate matter formation, with impacts that vary across spatial and temporal scales.In this study, high time-resolution measurements of gas-phase nitric acid (HNO3) and ammonia (NH3), as well as particulate nitrate and ammonium were conducted at an agricultural site in Switzerland. These measurements were complemented by 15 years of long-term monitoring data at the same site, providing a comprehensive record of changes in atmospheric gas and aerosol species over time. Aerosol pH was estimated using the ISORROPIA thermodynamic model2 and evaluated using a well-established approach based on the agreement between observed and predicted partition ratios of nitrogen species. The intensive measurement shows that the diurnal cycles of HNO3 and NH3 partitioning exhibited distinct patterns. HNO3 tended to partition into the particle phase during the night, driven by cooler temperatures, while NH3 remained predominantly in the gas phase throughout the day and night, regulated by high aerosol pH characteristics at the sampling site.The dry deposition regimes of HNO3 and NH3 were investigated in relation to aerosol liquid water content and acidity following the approach of Nenes et al. (2021).3 The findings indicate that NH3 deposition is rapid, meaning it tends to deposit near its sources, raising concerns about its localized ecological impacts. Aerosol mass formation was found to be primarily sensitive to HNO3 concentrations. Long-term monitoring data spanning 15 years revealed that reduction in SO2 emissions did not lead to increases in aerosol pH owing to the buffering effect of NH3 in the NH3-rich environment. The decline in sulfate concentration has driven a clear shift in aerosol mass sensitivity, transitioning from NH3-sensitive to NH3 -insensitive regime. Comparative measurements at forested sites in Switzerland provide further insight into the diurnal cycle of aerosol pH and reactive nitrogen deposition, highlighting the influence of anthropogenic activities on nitrogen dynamics across different ecosystems. These findings show the complex interplay between rapidly fluctuating diurnal aerosol acidity and reactive nitrogen deposition, offering important reference for designing effective pollutant mitigation strategies.References:(1) Wim de Vries.: Impacts of nitrogen emissions on ecosystems and human health: A mini review, Current Opinion in Environmental Science & Health, 2021, 21:100249, DOI: 10.1016/j.coesh.2021.100249. (2) Fountoukis, C. and Nenes, A.: ISORROPIA II: a computationally efficient thermodynamic equilibrium model for K+–Ca2+–Mg2+–NH4+–Na+–SO42−–NO3−–Cl−–H2O aerosols, Atmospheric Chemistry and Physics, 7, 4639&#821