Fig. 5a also indicates that the particles number concentration first increases and then decreases with the particle size. And there is a highest level of the particles number in the particle size range of about 0.1–1.0 lm. Besides the formation of aerosol particles, it is also related to the removal mechanism of particles in the WFGD system. This is caused either by inertia or diffusional force depending on the size of the particles to be separated. For particles with a size below 0.1 lm, the only mechanism of particle removal is diffusional forces, which increase with decreasing particle size and hence the enhanced particles number decreases. For particles larger than 1 lm, the removal by inertial forces dominates. With increasing particle size the inertial force of the particles increases and hence the particles number decreases. Therefore, the particle number concentration for particle size below 0.1 lm and larger than 1 lm is lower than that of particle size range of 0.1–1.0 lm at the outlet of the WFGD system. For the case of steam addition in the particle growth region, the improvement of grade removal efficiency first increases and then tends to stable with the particle size.