Size-resolved hygroscopicity of submicrometer urban aerosols in Shanghai during wintertime

Information about the hygroscopic growth factors and mixing state is important for understanding the haze formation mechanism in the highly polluted Yangtze River Delta. Herein, the size-resolved hygroscopicity of ambient aerosols in Shanghai was studied during wintertime. The hygroscopic growth factors were measured using a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA) for dry particles with diameters of 30–250 nm on the campus of Fudan University. A modal external mixture containing two hygroscopic groups was observed. The mean hygroscopic growth factors of the less-hygroscopic group at 85% relative humidity were lower than 1.10, and the average number fraction of the less-hygroscopic group varied in the range of 0.33–0.17, decreasing slightly with the increase of dry particle size. The mean hygroscopic growth factors of the more-hygroscopic group showed there was a significant difference between Aitken and accumulation mode particles, nearly 1.3 for Aitken mode particles and above 1.4 for accumulation mode particles. On the basis of the ammonium sulfate model, the average hygroscopic volume fraction of the more-hygroscopic particles was estimated to be in the range of 0.47–0.70, and there was a large gap between the hygroscopic volume fractions of Aitken and accumulation mode particles. The hygroscopic growth against relative humidity test not only showed that deliquescence relative humidity depended on particle size, but also revealed that the addition of nitrate on the particles was initially promoted by sulfate condensation. The results also suggested that most accumulation mode particles were deliquesced under haze conditions.

Research Highlights
► There was a significant difference between Aitken and accumulation mode particles in terms of hygroscopic behavior.
► The hygroscopic growth factors and deliquescence relative humidity were a function of particle size.
► Most of the accumulation mode particles were deliquesced under haze conditions.