Secondary Organic Aerosols from Aqueous Reaction of Aerosol Water

: Liquid water (cloud/fog droplets and aerosols) is ubiquitous in the atmosphere and can provide an important reaction media for aqueous-phase chemical reactions. Gaseous precursors (mainly VOCs) or their gas-phase initial or first-generation oxidation products (including intermediate-volatility and semi-volatile organic compounds; I/SVOCs) can undergo chemical reactions in the atmospheric condensed phase (aqueous phase) to form low-volatility, highly oxidized organic matter[e.g., some key tracer species such as organosulfates (OSs) and organonitrogens (ONs)]. These products largely remain in the particle phase upon water evaporation and are referred to as aqueous secondary organic aerosols (aqSOAs). aqSOAs have been emerging as a research hot topic in atmospheric chemistry, as they can contribute significantly to OAs and thus have important impacts on the environment, climate, and human health. Despite considerable progress, so far, aqSOAs remain poorly understood owing to their complex formation mechanisms. In this review, we focus mainly on the relevant research results on the SOAs formed in aerosol water-aqueous aerosol SOAs (aaSOAs)-including gas-phase precursors, formation mechanisms, laboratory simulations, and field observations, as well as SOA yield and contribution to OAs. Meanwhile, we propose future directions regarding studies of sources and formation mechanisms of aaSOAs, including identification of unknown aaSOA precursors and tracer products, photosensitizer-triggered radical chemistry, formation pathways of OS and ON compounds, field observations and model simulations of aaSOAs.