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.