Particles can be charged by two mechanisms in the presence of free ions: (1) field charging and (2) diffusion charging. Introducing a particle to the electric field alters the field, resulting in free ions being directed to and adsorbed by the particle as shown in the inset of Fig. 2; this is referred to as field charging. Diffusion charging is induced by collisions between ions and particles due to the random motion of the ions in the gas. Diffusion charging tends to occur in the absence of an electric field and usually with very small particle sizes (less than 0.2mm)