Seasonal Formation, Sources, and Long-Term Evolution of PM2.5-Bound Water-Soluble Inorganic Ions in Nanchang, China: The Increasing Importance of Wintertime Nitrate.

Water-soluble inorganic ions (WSIIs) are major components of PM2.5, yet their seasonal characteristics, formation, sources, and long-term evolution in Nanchang remain poorly resolved. Daytime and nighttime PM2.5 samples collected in summer and winter were analyzed for nine WSIIs using statistical, source-apportionment, trajectory, fire-hotspot, and historical analyses. Mean WSII concentrations were 8.01 ± 3.58 μg m-3 in summer and 23.97 ± 9.42 μg m-3 in winter, accounting for 57.0% and 50.3% of PM2.5, respectively. Sulfate, nitrate, and ammonium comprised 82.0% and 76.4% of WSIIs in summer and winter, with sulfate dominating in summer and nitrate in winter. Ammonium sulfate and ammonium nitrate were the principal secondary forms, and nighttime conditions favored nitrate formation. Source analyses indicated stronger secondary inorganic aerosol and marine influences in summer, whereas nitrate-rich aerosol, biomass burning, mineral dust, and regional transport were more important in winter. Historical observations from 2009 to 2026 showed substantial declines in PM2.5, secondary inorganic ions, and gaseous precursors, with the control challenge shifting after 2019 from sulfate to wintertime nitrate. Coordinated NOx-NH3 controls are needed, and this compositional shift may have implications for health-relevant PM2.5 properties.