Distinct particle properties between ultrafine and accumulation modes under clean and polluted urban environments

In this study, we report a phenomenon of fast changing in aerosol hygroscopicity between clean and pollution periods observed frequently in urban Beijing during winter using a hygroscopicity tandem mobility analyzer (H-TDMA). The cause of this phenomenon and the formation process of particles in different modes are discussed. During clean periods, ultrafine mode particles (Nucleation and Aitken modes) mainly stem from nucleation events with subsequent growth. During heavy pollution periods, they originate chiefly from primary emissions. Coarser-mode particles like accumulation mode particles are mainly from primary emission during clean periods and aqueous reactions during pollution periods. This finding based on H-TDMA measurement can make up the deficiency of mass-dependent instruments in analyzing sources and chemical processes of ultrafine mode particles.