At high concentrations of ambient bipolar ions, charged aerosol particles show a stationary charging state as a function of particle size. When the particle diameter exceeds 0.1 mum, a considerable fraction of multiply charged particles is present, which hinders size classification of particles, when a differential mobility analyzer (DMA) is used. However, at low ion concentrations, particle charging by the ions can be suppressed, and in principle, the production of multiply charged particles can be reduced even when the particle size is larger than 0.1 mum.In this study, a use of a power-adjustable soft X-ray emitter was investigated for the first time as the ionization source of an aerosol charger. The bipolar ion concentration was found to be adjustable between 10(10) and 10(13) ions/m(3). Through proper control of the bipolar ion concentration in the soft X-ray aerosol charger, multiply charged particles were reduced to less than 5% of the singly charged particles, while the singly charged particles remained at a relatively high concentration with most of the particles in size range of 0.1-1.0 mum. Therefore, highly monodisperse aerosol particles in the 0.1-1.0-mum-diameter range could be generated using this technique with a commercialized differential mobility analyzer. (C) 2003 Elsevier B.V. All fights reserved.