Characterizing the volatility and mixing state of ambient fine particles in the summer and winter of urban Beijing

Understanding the volatility of atmospheric aerosols is important for elucidating the formation of fine particles and to help determine their effect on the environment and climate. In this study, the volatility of fine particles (40, 80, 110, 150, 200, and 300 nm) is characterized by the size-dependent volatility shrink factor (VSF) for summer and winter in the urban area of Beijing using measurements of a volatility tandem differential mobility analyzer (VTDMA). We show that there are two persistent aerosol volatility modes (one high-volatility and one less- or non-volatile mode) present both in the summer and winter. On average, the particles are more volatile in the summer (with a mean VSF of 0.3) than in the winter (with a mean VSF of 0.6). Although the new particle formation (NPF) process requires low-volatility vapors to form molecular clusters and nuclei, the significant high-volatility mode around noon on NPF days indicates partitioning of volatile substances into the growing particles during summer. We further retrieve the mixing state of the ambient fine particles from the size-resolved VSF and find that the non-black carbon (BC) particles that formed from nucleation processes accounted for 52 %–69 % of the total number concentration in the summer. On the other hand, particles containing a refractory core that is thought to be BC-containing particles dominate and contribute 67 %–77 % toward the total number concentration in the winter. The diurnal cycles of the retrieved aerosol mixing state for the summer further support the conclusion that the nucleation process is the main contributor to non-BC particles. In addition, the extent of aging of BC particles was characterized as the ratio of the BC diameter before and after heating at 300 ∘C (Dp/Dc), showing that the average ratio of ∼ 2.2 in the winter is higher than the average of ∼ 1.5 in the summer, which indicates that BC aging may be less efficient in summertime. This would result in differences in light absorption enhancement between the cold and warm seasons.