Abstract: A system and method to measure a size distribution of particles based on their electrical mobility. The method includes: introducing, via a sheath flow inlet, a particle free sheath flow into a chamber formed by two parallel walls which are separated by a gap, the chamber having a width and a length, the sheath flow having a direction along the length of the chamber and flowing a laminar manner; introducing an aerosol sample flow into the chamber downstream of the sheath inlet such that the aerosol sample flow joins the particle free sheath flow in a laminar manner; applying an electric field between the two parallel walls of the chamber, the field having a strength which varies across the width of the chamber; extracting an output aerosol flow through a first outlet downstream of the sample inlet; and outputting an excess flow equal to a sum of the sheath flow and aerosol sample flow minus the output aerosol flow.