We designed and evaluated a chip-type unipolar charger based on a micro-electromechanical systems (MEMS) process for a compact, portable, inexpensive, and easy-to-deploy instrument to measure real-time aerosol particle size distribution with reasonable accuracy. The charger was evaluated using two parameters: particle loss and charging characteristics which were the product of penetration (p) and average number of charges (n(p)). Particle losses were below 16.6%, and p.n(p) was approximately proportional to d(p)(2). After evaluation, the number concentrations and the geometric mean diameters of NaCl aerosol particles were estimated using a method presented in Park, An, and Hwang (2007a) and were compared to scanning mobility particle sizer (SMPS) data. To simulate the charger, a commercial computational fluid dynamics (CFD) software, FLUENT (6.3 version), with external user defined function (UDF) code, was used to solve the electric field, flow field, average particle charge, and particle trajectories. The measured data of particle loss, average particle charge, number concentration, and geometric mean diameter were in good agreement with the results calculated by FLUENT. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.