Size-resolved effective density of ambient aerosols measured by an AAC-SMPS tandem system in Beijing

The effective density is an essential physical property of atmospheric aerosols. The study of the effective density helps characterize the aerodynamic behavior of aerosol particles, providing vital information on the chemical composition, mixing state, and aging process of the particles. Size-resolved effective density measurements based on the Aerodynamic Aerosol Classifier-Scanning Mobility Particle Sizer (AAC-SMPS) tandem system were conducted in Beijing from March 10 to 31, 2023. This study analyzed the relationship between the aerodynamic diameter and effective density and the characteristics of the effective density under different pollution conditions. During the entire observation period, the average effective densities for the 150-, 250-, 350-, 450-, and 550-nm particles were 1.48 +/- 0.07, 1.59 +/- 0.08, 1.69 +/- 0.09, 1.73 +/- 0.09, and 1.75 +/- 0.13 g/cm3, respectively, i.e., the effective density was highly particle size-dependent, exhibiting higher values at larger aerodynamic diameters. The size-resolved effective density under different pollution conditions differed and was related to the air mass variations. Under dusty conditions, the effective density of particles was highest under the influence of dust air masses from the northern areas. The effective density of small particles (150-350 nm) of the northern clean air masses was lower than that of the southern polluted air masses; however, the opposite was observed for large particles (450-550 nm). The diurnal effective density variation was generally more pronounced during daytime. Under both clean and dusty conditions, there was a sudden effective density decrease at 150 nm during the evening peak traffic. Under the influence of new particle formation (NPF) events, a significant effective density decrease was observed when the accumulation mode began to grow.