Online shape and density measurement of single aerosol particles

Aerosols play crucial roles in affecting air quality, climate and human health. Particle morphology and density would provide critical information on the chemical composition, mixing state and aging process of particles. Many investigations used dynamic shape factor (chi) to quantify particle shape. However, the measurement for chi often requires the knowledge of particle density, which is usually not readily available for atmospheric aerosols. Besides, chi is not a pure geometric descriptor, as it is also dependent on Knudsen number (Kn). Here, a method, consisting of an aerodynamic aerosol classifier (AAC, aerodynamic particle diameter, Da, measurement), a differential mobility analyzer (DMA, electrical mobility particle diameter, Dm, measurement) and a single particle aerosol mass spectrometer (SPAMS, vacuum aerodynamic particle diameter, Dva, measurement), was developed to investigate two geometric descriptors that are solely dependent on particle geometry: the orientationally averaged projected area (AP) and the hydrodynamic radius (RH). The shape indicator, S, was defined as the square root of the ratio AP/pi R2H. To test the performance of the AAC-DMA-SPAMS system, the S and chi of 9 types of laboratory-generated particles with known density were measured with this system, and the quantitative relationship between chi (1<1.05) and S (1<1.5) was established at room temperature (T approximate to 300K) and ambient condition. Finally, this method was applied to investigate the S of ambient aerosols in an urban area. A significant fraction of near-spherical aerosols was observed. Thus, their densities can be calculated. Generally, the AAC-DMA-SPAMS system can provide shape, density and the mass spectral information of ambient particle, which can help elucidate their mixing state and aging processes.

Automated summary

Aerosols are crucial in air quality, climate, and human health, with particle morphology and density providing key information on their composition and aging. A new method using AAC-DMA-SPAMS system was developed to investigate geometric descriptors of particles, establishing a quantitative relationship between them. This system can provide valuable insights into the shape, density, and aging processes of ambient particles.