Distinct Ultrafine- and Accumulation-Mode Particle Properties in Clean and Polluted Urban Environments

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