Measurement report: size-resolved particle effective density measured by the AAC-SMPS and implications for chemical composition

Abstract. The effective density (ρ_eff) is closely associated with the aging process and can serve as a tracer for chemical composition. Recently, studies investigating the effect of particle size on density have been limited. In this study, size-resolved ρ_eff was characterized with a tandem of an aerodynamic aerosol classifier (AAC) and a scanning mobility particle sizer (SMPS) system during one month of observation in Hangzhou. The results indicate that the ρ_eff values of the particles exhibit a unimodal distribution, with average values ranging from 1.47 g/cm³ to 1.63 g/cm³, increasing as the particle diameter increases. The diurnal variation is more pronounced for small particles (d_ae <350 nm), which generally exhibit lower density during the day and higher density at night. The relationship between ρ_eff and particle diameter varies under different pollution conditions due to differences in the chemical composition of the particles. additive explanations (SHAPs) revealed good relationships between ρ_eff and the bulk composition of particles with diameters smaller than 350 nm. Since the size-resolved chemical composition of particles is still challenging, a new method to investigate the size-resolved chemical composition was proposed, in which the size-resolved composition can be derived from the ρ_eff and fixed material density of secondary inorganic aerosols (SIAs), organic aerosols (OAs) and black carbon (BC).