Contrasting the characteristics, sources, and evolution of organic aerosols between summer and winter in a megacity of China.

Organic aerosol (OA) accounts for the largest fraction of fine particles in the ambient atmosphere, however, its formation process remains highly uncertain. Here, with an aim of obtaining insights to OA formation mechanism, we have focused on contrasting the characteristics, sources and evolution of OA of PM (particulate matter with an aerodynamic diameter < 2.5 μm) between summer and winter based on two field campaigns of urban Beijing. The results show that secondary OA (SOA) dominates OA mass in PM both in summer and winter, accounting for 87 % and 74 %, respectively. This is much higher compared to the mass fraction of ~56 % that observed in PM, indicating an important role of SOA in larger size particles. We also show that, the SOA is becoming more dominant in total OA and our observed high SOA proportion is reconciled with its overall upward trends in recent years. The observed mass concentrations of SOA (less oxidized oxygenated OA, LO-OOA; more oxidized OOA, MO-OOA) in winter are about twice higher than those in summer. The Van-Krevelen diagram implies that the addition of carboxylic acid may be a primary oxidation process of OA in summer, while the addition of alcohol/peroxide may play a dominant role in OA evolution in winter. Our observation and analysis illustrate a more efficient conversion from LO-OOA to MO-OOA through aqueous-phase processes in winter of Beijing. While, in summer, cooking OA may be easier to convert to MO-OOA through photochemical process than other OA factors. We further show that the POA is mainly locally emitted, while the origin of SOA is from both regional transport and locally formed. The results of this study may provide policy basis for the precise control of OA pollution and would also help to improve the accuracy of assessing the environmental and climate effects of OA.