An investigation into the effect of the mass concentration of fine particles on the equivalent ionic mobilities of the system composed of small ions and large ions in the atmosphere is presented. A measurement apparatus is designed using direct current corona discharge at room temperature. Positive and negative ion mobilities are extracted from the electric field strength and ion current density measured in the apparatus. With the presence of fine particles, equivalent ionic mobilities are found to decrease exponentially with the mass concentration of the particles. The distributions of electric field and space charge density, the contributions to the charge density from ions and fine particles are also calculated.