Measurement report: Size-Resolved and Seasonal Variations in Aerosol Hygroscopicity Dominated by Organic Formation and Aging: Insights from a Year-Long Observation in Nanjing

Aerosol hygroscopicity plays a significant role in atmospheric chemistry, radiation, and climate effects. While previous studies have investigated regional differences in aerosol hygroscopicity, long-term observational studies focusing on seasonal variations in specific regions remain scarce. This study explores size-resolved and seasonal variations in aerosol hygroscopicity in northern Nanjing, using one-year hygroscopicity-tandem differential mobility analyser (H-TDMA) measurements in 2021. Aerosols in the region show relatively low hygroscopicity due to a high organic mass fraction (annual average mass fraction: 42.92 % in PM2.5) in fine particles. The mean hygroscopicity parameter (κmean) increases with particle size across all seasons, with more pronounced size dependence in nucleation-mode particles. Particles (40–200 nm) show seasonal κmean variations: winter (0.12–0.24) and spring (0.14–0.25) display higher values attributable to secondary inorganic aerosols, while summer (0.12–0.21) and autumn (0.10–0.20) exhibit weaker hygroscopicity due to enhanced contributions from less hygroscopic components. Diurnal patterns are shaped by photochemical aging and aqueous-phase reactions, leading to κmean peaks for larger particles in the afternoon and evening. New particle formation events are most frequent in spring, producing initially less hygroscopic particles that become more hygroscopic with aging. Regional transport analysis reveals distinct controlling factors: hygroscopicity of nucleation-mode particles is mainly controlled by local sources, while accumulation-mode particles are more influenced by seasonal air mass transport. These results improve understanding of aerosol–cloud interactions and support regional climate modeling and air quality management in urbanizing areas.