Measurement report: Hygroscopicity and mixing state of submicron aerosols in the lower free troposphere over central China: Local, regional and long-range transport influences

Abstract

Understanding the hygroscopicity and mixing state of atmospheric aerosol particles is crucial for accurately assessing their role in cloud formation and subsequent climate impacts. However, measurements in the lower free troposphere - a representative atmospheric layer characterizing regional background conditions in aerosol transport and atmospheric evolution - remain sparse, especially in regions influenced by both anthropogenic emissions and long-range transported air masses. This study adds further data on size-resolved hygroscopicity and mixing state measurements of aerosols at Mt. Hua (2060 m a.s.l., central China) during October-November 2021 using a Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA). Results reveal a clear size-dependence of aerosol hygroscopicity, with the mean hygroscopicity parameter (κmean) increased from 0.20 for 30 nm particles to 0.30 for 200 nm particles. The ambient submicron aerosols were primarily externally mixed, dominated by more-hygroscopic (MH) particles, with no significant diurnal variation, indicating minimal influence from boundary layer dynamics. Starting from 6 November, the combined influence of mineral dust and regional heating activities led to a notable reduction in aerosol hygroscopicity, especially in larger particles. This decline was driven by an increase in weakly hygroscopic components such as mineral dust, black carbon, and organic compounds, highlighting the impact of both long-range transport and regional emissions on aerosol composition. Notably, during episodes of striking high relative humidity (RH > 80 %) in Cluster 5, atmospheric aerosols containing mineral dust showed unexpected hygroscopic enhancement, suggesting in situ RH-driven chemical processing that increased aerosol hygroscopicity. Atmospheric aerosols at Mt. Hua displayed distinct hygroscopic properties compared to other high-altitude sites, underscoring regional differences in aerosol sources and free tropospheric processing. These findings advance our understanding of aerosol aging and processes in the lower free troposphere over central China, and offer crucial observational constraints for modeling aerosol-cloud interaction and regional climate impacts.