Mixing properties of submicrometer aerosol particles in the urban atmosphere—with regard to soot particles

Individual aerosol particles were collected on three days with different meteorological conditions in June 2000 in the urban atmosphere of Tsukuba, Japan. The samples collected with an electrostatic aerosol sampler (EAS) were examined by electron microscopy. The mixing properties of submicrometer aerosol particles of 0.02–0.2 μm radius were studied using the dialysis (extraction) of water-soluble material. Atmospheric aerosol particles were classified into four types with respect to the mixtures of water-soluble and water-insoluble material. The proportions of particles with water-soluble material (hygroscopic particles) ranged from 20% to 80% in the whole radius range and tended to increase with increasing radius. Moreover, by the morphological appearance, soot-containing particles were classified into two types, i.e., externally mixed soot-particles and internally mixed soot-particles. The number fractions of internally mixed soot-particles increased with increasing radius. It is found that the volume fraction of water-soluble material ( ) for the internally mixed soot-particles increased with increasing radius. In a “polluted” case, the sample showed a dominant number fraction (75%) of internally mixed soot-particles in the larger radius range of 0.1–0.2 μm.