A corona ionizer for unipolar charging of manometer-sized aerosol particles has been constructed and evaluated. Charging efficiency and particle deposition loss (electrostatic and diffusional) have been measured for particle mobility-equivalent diameters below 10 nm, at different aerosol flow rates, different corona polarity and voltages, and also at two different positions of the corona electrode tip. Even though electrostatic deposition of charged particles within the ionizer was relatively large at high corona voltages, charging efficiencies as high as about 30% for 10 nm particles could be achieved for positive and negative corona. This figure is almost one order of magnitude higher than that attainable with radioactive bipolar chargers, and similar to that recently reported for other types of unipolar chargers developed by several researchers. (C) 2003 Elsevier Science Ltd. All rights reserved.