Role of ammonia, aerosol pH and liquid water content in secondary inorganic aerosols formation in suburban Nanjing, China during the winter-spring transition.

Fine particulate matter (PM2.5) pollution, dominated by secondary inorganic aerosols (SIA), remains a critical environmental issue in China's Yangtze River Delta. In this study, we utilized hourly high-resolution measurements of water-soluble ions and precursor gases collected in suburban Nanjing from January to March 2021. By coupling these observations with the ISORROPIA-II and WRF-Chem models, we elucidated how aerosol liquid water content (ALWC) and pH govern SIA formation in ammonia (NH3)-rich environments, and further established quantitative policy thresholds for emission mitigation. Observations revealed that SIA accounted for nearly half (49.55%) of the PM2.5 mass (mean: 34.4 μg/m3), with nitrate being the dominant species. Continuous ammonia-excess conditions provided sufficient NH3 to generate ammonium (NH4+) for complete anion neutralization, driving continuous SIA accumulation. Mechanistically, rising ALWC significantly enhanced aqueous-phase oxidation and promoted the partitioning of nitrate and ammonium into the particle phase. Meanwhile, aerosol pH strictly regulated both the gas-particle partitioning equilibrium of semi-volatile species and the sulfate production pathways, with transition metal ion (TMI)-catalyzed oxidation acting as the dominant pathway. Thermodynamic sensitivity analysis indicated that local conditions were strictly total nitrate (TNO3)-limited. Simulations from both the ISORROPIA-II and WRF-Chem models revealed that due to the strong buffering effect of the substantial gaseous NH3 pool, PM2.5 mass responds minimally to moderate NH3 reductions, requiring emission cuts of over 60% to drive significant PM2.5 mitigation. In contrast, reducing nitric acid (HNO3) alone yielded a mitigation efficiency comparable to joint HNO3 and NH3 controls. Therefore, mitigating PM2.5 pollution in such NH3-rich regions requires prioritizing nitrogen oxide (NOx) emission controls, supplemented by ammonia regulations tailored to local aerosol thermodynamics.