Aerosol particles were collected in Tsukuba, Japan with an impactor in ambient air downstream of a differential mobility analyzer (DMA) at three electrical mobility diameters of 200, 300 and 400 nm in dry conditions (relative humidity of < 3%). The particles collected on a silicone-oil coated film were examined by electron microscopy in order to obtain the diameter, shape and volatility properties by electron-beam bombardment. Measurements with an optical counter (OPC) were also carried out parallel to the particle collection. Most of the number frequency distributions peaked at the DMA diameters. Ambient particles showed wider size frequency distributions as compared with particles generated from a hydrosol of ammonium sulfate with Monarch 71 channel soot. Circularity factors of pure soot particles decreased with increasing particle diameter and their irregular shape produced broad size frequency distributions. Comparison of soot particles in OPC and electron microscopy indicated the agreement between two methods. (C) 2003 Elsevier Ltd. All rights reserved.