Measurement report: Size-resolved and seasonal variations in aerosol hygroscopicity dominated by organic formation and aging: insights from a year-long observation in Nanjing

Abstract. 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 content (annual average mass fraction: 42.92 % in PM2.5) in fine particles. The mean hygroscopicity parameter (κmean) increases with particle size across all seasons. Particles (40–200 nm) show seasonal κmean variations: winter (0.12–0.24) and spring (0.14–0.25) display relatively higher values attributable to relatively higher secondary inorganic content, while summer (0.12–0.21) and autumn (0.10–0.20) exhibit relatively weaker hygroscopicity due to enhanced contributions from less hygroscopic components. Diurnal patterns are shaped by photochemical aging and aqueous-phase reactions, leading to κmean slight enhancement for larger particles in the afternoon and evening. New particle formation (NPF) events occur most frequently in spring. During spring NPF days, Aitken-mode particles exhibit slightly low hygroscopicity, whereas accumulation-mode particles demonstrate relatively higher hygroscopicity compared to non-NPF days. Regional transport analysis reveals distinct controlling factors: hygroscopicity of 40 nm particles may be mainly controlled by local sources, while 200 nm 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.