Aerosol hygroscopicity based on size-resolved chemical compositions in Beijing

Hygroscopicity is an important property of aerosols, which cannot be obtained for a wide range of particle sizes by online observation. A cascade impactor sampler, an essential method for obtaining the size-resolved chemical compositions of ambient aerosols, enables the acquisition of the size-resolved hygroscopicity for particles in multiple modes. A micro-orifice uniform deposit impactor (MOUDI-122, MSP) was used to collect the size-resolved aerosol samples during 2016 and 2017. The water-soluble components in these samples were analysed for different pollution levels in two periods. Then, the hygroscopicity parameter (κ) was calculated. The changing hygroscopicity in different size ranges was directly influenced by variations in the distribution of the water-soluble components. The contribution of sulfate to the κ was much higher in the summer period than that in the winter period due to the higher SO₄^2- concentration in the summer. During the summer period, the contribution of nitrate to the κ of particle sizes above 0.56 μm was significantly higher than that of particles smaller than 0.56 μm, while in the winter period, the contribution of nitrate in finer particles with sizes below 1.8 μm was much higher than that in coarse particles. The contribution of chloride to the κ increased significantly in the winter period due to the influence of heating coal emissions. For particles below 1.0 μm, the contribution and fraction of water-soluble organic compounds (WSOCs) to the aerosol hygroscopicity increased with decreasing particle size. Compared with the aerosol hygroscopicity during the first stage of a pollution episode, the hygroscopicity of particles above 0.18 μm was significantly enhanced during the stages of pollution accumulation and formation. The results in this study were in good agreement with the results of other similar studies or data derived by other methods, indicating that the hygroscopicity based on size-resolved water-soluble components is reliable and can be used in the study of activation, radiation force, and heterogeneous reaction mechanism of particles with multiple sizes.