Secondary organic aerosol (SOA) is the major species in the atmospheric particles and has an important impact on the atmospheric environment, climate and human health. In recent years, the formation of aqueous SOA has been becoming a research hotspot. This is due to its comparable contribution to SOA budget to that from the traditional reaction of gas phase, and the important roles in explaining the discrepancies between field observation and model results, field observation and chamber simulation results in the laboratory on size, distribution, concentration and aging degree of SOA, which cannt be understood by the traditional SOA formation through gas phase. In this article, we introduce current major ways to form aqueous SOA in laboratory studies, including non-radical reactions in the dark (related to hydration, acetal/hemiacetal, aldol condensation and catalytic reactions) and radical reactions under the light. Meanwhile, the prospects for the study direction of the formation of aqueous SOA are given.

Contents
1 Introduction
2 The major physical and chemical processes of the formation of aqueous SOA
3 Formation of aqueous SOA
3.1 Reactions under the dark
3.2 Photochemical reactions
4 Conclusions