Online shape and density measurement of single aerosol particles

Abstract 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 ( χ ) to quantify particle shape. However, the measurement for χ often requires the knowledge of particle density, which is usually not readily available for atmospheric aerosols. Besides, χ is not a pure geometric descriptor, as it is also dependent on Knudsen number ( K n ). Here, a method, consisting of an aerodynamic aerosol classifier (AAC, aerodynamic particle diameter, D a , measurement), a differential mobility analyzer (DMA, electrical mobility particle diameter, D m , measurement) and a single particle aerosol mass spectrometer (SPAMS, vacuum aerodynamic particle diameter, D v a , measurement), was developed to investigate two geometric descriptors that are solely dependent on particle geometry: the orientationally averaged projected area ( A P ) and the hydrodynamic radius ( R H ). The shape indicator, S, was defined as the square root of the ratio A P / π R H 2 . To test the performance of the AAC-DMA-SPAMS system, the S and χ of 9 types of laboratory-generated particles with known density were measured with this system, and the quantitative relationship between χ (1