Some measurements of the mixing state of soot-containing particles at urban and non-urban sites

Number fraction and size of soot-containing particles were investigated at urban and non-urban sites using the dialysis of water-soluble components and electron microscopy. The samples were collected with a low-pressure impactor and the particles of 0.08–1.6 μm diameter were examined. At the urban site, Sapporo, Japan, soot-containing particles were present in high number fractions (42–49%), whereas low number fractions (3–11%) were measured at the non-urban sites of Fukue Island, Japan, and Poker Flat, Alaska. The mass fractions of elemental carbon, obtained by chemical bulk analysis, showed similar results. On the other hand, the number fractions of internally mixed soot particles in soot-containing particles were 19–72% at Fukue Island and 56% at Poker Flat, whereas only 5–7% were observed in Sapporo. The high variability in the measured number fractions in Fukue Island is discussed from the viewpoint of the air mass trajectories. The size of internally mixed soot particles tended to be larger than externally mixed soot particles. The fraction of internally mixed soot particles in soot-containing particles increased with particle size.