Investigating Dual Character of Atmospheric Ammonia on Particulate NH₄NO₃: Reducing Evaporation Versus Promoting Formation

Ammonium nitrate (NH4NO3) is a major constituent of fine particulate matter 13 (PM2.5), playing a critical role in air quality and atmospheric chemistry. However, the dual 14 regulatory role of ammonia (NH3) in both the formation and volatilization of NH4NO3 15 under ambient atmospheric conditions remains inadequately understood. To address this 16 gap, we conducted high-resolution field measurements at a clean tropical coastal site in 17 China using an integrated system of Aerosol Ion Monitor-Ion Chromatography (AIM-IC), 18 Scanning Mobility Particle Sizer (SMPS), and online OC/EC analyzers. These observations 19 were complemented by thermodynamic modeling (E-AIM) and source apportionment via 20 Positive Matrix Factorization (PMF) model. The E-AIM simulations revealed persistent 21 thermodynamic disequilibrium, with particulate NO3- tending to volatilize even under 22 NH3gas-rich conditions during the northeast monsoon. This suggests that NH4NO3 in PM2.5 23 forms rapidly within fresh combustion plumes and/or those modified by non-precipita-24 tion clouds, and then undergoes substantial evaporation as it disperses through the at-25 mosphere. Under the southeast monsoon conditions, reactions constrained by sea salt aer-26 osols became dominant, promoting the formation of particulate NO3- while suppressing 27 NH4NO3 formation despite ongoing plume influence. In scenarios of regional accumula-28 tion, elevated NH3 concentrations suppressed NH4NO3 volatilization, thereby enhancing 29 the stability of particulate NO3- in PM2.5. PMF analysis identified five source factors, with 30 NO3- in PM2.5 primarily associated with emissions from local power plants and the large-31 scale regional background, showing marked seasonal variability. These findings highlight 32 the complex and dynamic interplay between the formation and evaporation of NH4NO3 33 in NH3gas-rich coastal atmospheres.