Size-resolved hygroscopicity and volatility properties of ambient urban aerosol particles measured by the VH-TDMA system in the autumn of 2023

Abstract. Hygroscopicity, volatility and the hygroscopicity of the non-volatile core of submicron aerosols with diameters of 50 nm, 80 nm, 110 nm and 150 nm were measured using a Volatility Hygroscopicity Tandem Differential Mobility Analyzer (VH-TDMA) system with a relative humidity of 90 % and a thermal denuder temperature of 270 °C from 11 October to 6 November 2023 in Beijing. The mean hygroscopic growth factor (HGF) for particles ranging from 50 nm to 150 nm was 1.15 ± 0.07, 1.24 ± 0.08, 1.30 ± 0.09, and 1.36 ± 0.10, respectively, while the mean volatile shrink factor (VSF) was 0.51 ± 0.05, 0.55 ± 0.04, 0.56 ± 0.05, and 0.56 ± 0.07, respectively. Both the HGF probability density function (HGF-PDF) and the VSF probability density function (VSF-PDF) for all particle sizes exhibited a pronounced bimodal distribution, indicating that the particles were primarily in an external mixing state. Hygroscopicity was observed to increase with particle size in both clean and pollution periods, while volatility decreased slightly with particle size during the clean period, without apparent trend during the pollution period. Additionally, a positive correlation was identified between hygroscopicity and volatility, as well as between the number fraction of nearly hydrophobic (NH) and non-volatile (NV) particles, which was higher correlation during the pollution period. Furthermore, this study measured the HGF of the non-volatile core (HGF_core) of submicron aerosols heated at 270 °C and derived the HGF of the volatile coating (HGF_coating). The mean HGF_coating for particles ranging from 50 nm to 150 nm particles was 1.17 ± 0.08, 1.27 ± 0.10, 1.35 ± 0.10 and 1.41 ± 0.10, respectively, which is 2 % to 7 % higher than the mean HGF for the same particle sizes. The mean HGF_core for particles ranging from 50 nm to 150 nm was 1.08 ± 0.03, 1.07 ± 0.03, 1.07 ± 0.03 and 1.09 ± 0.04, respectively. Notably, the HGF_core values were influenced by marine aerosols during air mass passages through and originating from the Bohai Sea.