We designed and constructed a corona-discharge-based unipolar mini-charger and experimentally evaluated its performance. The simple design and compact size make the prototype mini-charger well suited for use with portable aerosol sizing instruments based on particle electrical mobility techniques. In this work, we optimized the extrinsic charging efficiency of the prototype mini-charger for two different aerosol flow rates (i.e., 0.3 and 1.51pm). The optimal settings for the prototype operation are: (i) a corona current of 1 mu A and an ion driving voltage of 40V for the 0.31pm flow rate, and (ii) 2 mu A and 120V for the 1.51pm flowrate. Both intrinsic and extrinsic charging efficiencies of the prototype mini-charger at the optimal operational conditions were then evaluated for particles in diameters ranging from 10 to 200 nm. The intrinsic charging efficiency of the prototype reaches 100% at 20 nm for the 0.31pm flow rate, and 45 nm for the 1.51pm flow rate. The higher intrinsic charging efficiency at the low flowrate is due to the longer residence time of particles in the device. The extrinsic charging efficiency, however, is higher for the 1.51pm flow rate than for the 0.31pm flow rate due to charged particle loss in the prototype. Charge distributions of test monodisperse particles of different sizes were also measured by the Tandem-DMA technique. (C) 2008 Elsevier Ltd. All rights reserved.