A dilution sampler has been designed and manufactured to simulate the effects of dilution processes on particulate matter emissions from coal-fired power plants and other combustion systems. The sampler allows independent control of the dilution ratio and residence time. Experiments were performed to examine the effects of these parameters on the particulate emissions of a pilot-scale pulverized coal combustor burning a low sulfur bituminous coal. Measurements included the particle size distribution in the range from 0.003 to 20 mum and the PM2.5 mass emission rate. The residence time and dilution ratio do not influence the particle mass emission rate, but have a significant effect on the size distribution and the total number of particles emitted. Increasing the residence time dramatically decreases the total particle number concentration, and shifts the particle mass to larger sizes. Increasing the dilution ratio increases the concentration of ultrafine particles. The effects of residence time can be explained quantitatively by the coagulation of the emitted particles; however, the effects of dilution ratio are more complex because the dilution ratio influences both the coagulation rate and gas-to-particle conversion.