The SO3 removal properties were investigated in the wet flue gas desulfurization (WFGD) system. The relationship of the aerosol emission properties and the SO3 concentration in the flue gas was explored experimentally based on the ammonia desulfurization system. The impacts of the operating parameters, such as the liquid-to-gas ratio (L/G), the flue gas temperature, the superficial velocity, and the particle concentration in the flue gas on the SO3 removal efficiency were studied. The results showed that when the SO3 concentration in the flue gas increased, the aerosol emission concentration was higher although the aerosol sizes became smaller within the measurement range of the electrical low pressure impactor (ELPI). When the SO3 concentration was 12 mg/Nm(3) and 47 mg/Nm(3), respectively, the aerosols emitted from the WFGD system showed the similar morphologies. Under the same operating conditions, the SO3 removal efficiencies in the ammonia-based WFGD system were higher than those in the limestone-gypsum WFGD system, both of which rose with the increase of SO3 concentrations. In the ammonia-based WFGD, the SO3 removal efficiency would be improved when the L/G and the particle concentration in the flue gas were raised and the flue gas temperature and the superficial velocity were reduced. Two SO3 removal mechanisms were proposed in the ammonia-based WFGD processes. One was the collision trap mechanism and the other was the heterogeneous reaction mechanism. The sulfuric acid aerosols formed though the quenching process of hot flue gas were measured to be mainly submicron particles. For the removal of these particles, the Brownian diffusion was proved to be the predominant removal mechanism in the collision trap processes, rather than the inertial impaction. (C) 2016 Elsevier B.V. All rights reserved.