Size-resolved effective density of submicron particles during autumn in the North China Plain

Effective density is one of the most important physical property of atmospheric aerosols, which is link to particle formation and aging process. Combined characterization of density, chemical composition and source evolution of aerosol is crucial for understanding their interactions and effects on environment and climate. The effective density of sub-micrometer aerosol particles was investigate at a heavily polluted rural site in the North China Plain from 16 October to 1 November 2019. A tandem technique coupling a Centrifugal Particle Mass Analyzer (CPMA) with a differential mobility analyzer (DMA) and a Condensation Particle Counter (CPC) were used to determine the effective density of ambient aerosol particles with selected diameters of 50, 100, 150, 220 and 300 nm. The measured effective density is higher during clean period than pollution period, with average values ranged from 1.13 to 1.36 g/cm³, which is lower than the reported values in Shanghai and Beijing. Similar diurnal cycles of effective density are observed for the five diameters, that is, started to increase in the morning and reached a peak in the afternoon around 13:00-16:00, then decreased and remained at a relative low value during the night. Two valleys are found during morning and evening rush hours for particle diameter smaller than 150 nm, which is likely to stem from the higher fresh emissions such as BC, BBOA and HOA. In most cases, measured particle effective density shows a single-modal distribution. But during clean days, bimodal distribution was observed with an extra low-density mode peaking at around 0.5 -1.0 g/cm³, which may be attributed to freshly emitted soot particles.