Regulatory Roles of Aerosol Liquid Water Content and Aerosol Acidity in Nitrate-Driven Air Pollution

Despite recent improvements in air quality, wintertime PM2.5 pollution remains severe in northern China, with nitrate (NO3−) becoming an increasingly dominant component. However, the roles of aerosol aqueous conditions in regulating nitrate formation and loss are still poorly constrained. Here, 744 hourly PM2.5 samples collected during the winter of 2023–2024 in a typical northern Chinese city were analyzed to investigate the drivers of severe nitrate pollution. The mean PM2.5 concentration reached 106.0 ± 84.4 μg m−3, with a maximum of 377.8 μg m−3. The NO3−/Total Ions (TI) ratio increased steadily to 43% in 2023, reflecting rising NO2 and effective SO2 emission reductions that shifted winter aerosol composition from sulfate-to nitrate-dominated. Four pollution episodes (PE1-PE4) were identified relative to a clean background period. Both PM2.5 and NO3−/TI ratios were strongly enhanced during all episodes. NO3− exhibited a strong dependence on aerosol liquid water content (ALWC) when ALWC <200 μg m−3, but this relationship weakened at higher ALWC, indicating a transition in dominant formation and loss pathways. Nitrate consistently dominated NH4+ neutralization. PE1 showed the highest aerosol pH but the lowest ALWC and nitrogen oxidation ratio, suggesting that more acidic conditions favor NO2 oxidation. At ALWC >200 μg m−3, gas-particle partitioning of nitrate approached quasi-equilibrium, while enhanced deposition indicated accelerated loss. Meanwhile, elevated ALWC increased the particle size through hygroscopic growth, reducing the PM1/PM2.5 ratio. These results demonstrate a threshold-dependent control of ALWC on nitrate production and removal, with ALWC = 200 μg m−3 representing a critical turning point for wintertime nitrate pollution.