The availability of a bipolar charger using soft X-ray of 9.5 keV to aerosol nanoparticle measurement was evaluated by investigating the characteristics of ion generation and by comparing the ion generation rate and the bipolar charging states of aerosol nanoparticles to those of Am-241 alpha-ray bipolar chargers of 82 and 164 mu Ci radiation. Ion generation of the soft X-ray charger stabilized within 7s after being turned on and off, and remained stable for hours at a sufficient generation rate. The electrical mobilities of bipolar ions in dry nitrogen were close to each other, in contrast to those observed in the alpha-ray chargers. Owing to these electrical mobilities, the electrical mobility distributions of positively and negatively charged particles could be brought to almost identical states, as could the particle size distributions derived individually from the two mobility distributions. This means that inaccuracies can be minimized in measurements using a differential mobility analyzer. The effect of water vapor in ionized gas was also examined to discuss the fundamental applicability of the charger to atmospheric aerosols. For both humid nitrogen and air, the electrical mobilities of bipolar ions were found to be in accordance with those reported in the existing studies, indicating that humidity affected the electrical mobility of, in particular, negative ions. (c) 2004 Elsevier Ltd. All rights reserved.