Formation mechanism of secondary organic aerosol in aerosol liquid water: A review

Secondary organic aerosol (SOA) is the main component of PM2.5, with great impact on regional air quality and global climate. The traditional view that SOA forms through the partitioning of photochemical processing involving volatile organic compounds (VOCs) cannot fully explain measured SOA concentrations. It has been increasing recognized that SOA can form through aqueous reactions in recent years. Besides cloud/fog aqueous chemistry, aqueous SOA (aqSOA) formation in aerosol liquid water has become one of the frontier scientific problems of atmospheric chemistry. AqSOA precursors enter into aqueous phase through uptake to wet aerosol particles, participate in reactions inside aerosol particles, and then form aqSOA such as organic sulfur compounds and organic nitrogen compounds. This paper provides an overview of the uptake of aqSOA precursors, aqSOA formation mechanism and current research methods of aqSOA. AqSOA precursors include atmospheric oxidants (OH, HO2, O-3, etc.), anthropogenic and biogenic VOCs and related gas-phase oxidation products. Aerosol liquid water can influence the uptake of aqSOA precursors on wet aerosols, but related researches are limited. OH uptake coefficient (gamma(OH)) on different kinds of aerosols varies from 0.02 to 2.41, depending on chemical composition of aerosols and relative humidity. For the uptake of VOCs, take methylglyoxal as an example, measured and theoretical methylglyoxal uptake coefficient (gamma MGLY) differ by 4 to 5 orders of magnitude. Aerosol liquid water may change ionic strength, diffusion limitation and viscosity of wet aerosols, but how these affect the uptake process of aqSOA precursors remain poorly understood. Based on previous analyses of aqueous chemistry, aqueous-phase reactions can be divided into radical reactions and non-radical reactions. Aqueous-phase radical reactions resemble gasphase reactions in general. However, there are also OH radical reactions unique to the aqueous phase: efficient conversion of aldehydes to carboxylic acids, rapid OH oxidation of carboxylate, and radical induced oligomerization. Recent studies also pay increasing attention to the role of other oxidants in the aqueous radical chemistry, like singlet oxygen, peroxyl radicals, peroxides, molecular oxygen (O-1(2)*), and triplet excited states of organic compounds (C-3*). Non-radical reactions include hemiacetal formation, aldol condensation, imine formation and other types of reactions (anhydride formation, organosulfate formation, etc.). Most non-radical reactions lead to the formation of high molecular weight compounds. Although a lot of investigations have been taken to explore aqSOA formation mechanism, the majority are laboratory studies, because of the limit of technology in field measurements. Laboratory simulation includes bulk solution simulation and reaction chamber experiments. Bulk solution cannot simulate typical ambient wet aerosols well, so nowadays reaction chamber is used more widely. There are two types of reaction chambers: smog chamber and flow tube, differing in the volume of reaction chamber and simulated atmospheric oxidation timescale. However, the appropriate application of these laboratory results into field observations and model framework needs further efforts. Moreover, one key factor that has enabled great progress in aqSOA chemistry studies is the development of mass-spectrometric methods, mainly including electrospray ionization-mass spectrometry (ESI-MS), Fourier transform ion cyclotron resonance electrospray ionization mass spectrometry (FTICR-MS), chemical ionization mass spectrometry (CIMS) and extractive electrospray ionization-mass spectrometry (EESI-MS). These techniques can realize accurate molecular level identification of complex compounds. But quantification remains a thorny issue, owing to the absence of available authentic standards. Finally, possible future directions regarding aqSOA chemistry studies are discussed.