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Organic aerosol is formed and transformed in atmospheric aqueous phases(e.g., cloud and fog droplets and deliquesced airborne particles containingsmall amounts of water) through a multitude of chemical reactions.Understanding these reactions is important for a predictive understanding ofatmospheric aging of aerosols and their impacts on climate, air quality, andhuman health. In this study, we investigate the chemical evolution ofaqueous secondary organic aerosol (aqSOA) formed during reactions ofphenolic compounds with two oxidants – the triplet excited state of anaromatic carbonyl (3C∗) and hydroxyl radical (<mo>•</mo>OH).Changes in the molecular composition of aqSOA as a function of aging timeare characterized using an offline nanospray desorption electrosprayionization mass spectrometer (nano-DESI MS) whereas the real-time evolutionof SOA mass, elemental ratios, and average carbon oxidation state (OSC)are monitored using an online aerosol mass spectrometer (AMS). Our resultsindicate that oligomerization is an important aqueous reaction pathway forphenols, especially during the initial stage of photooxidation equivalent to ∼  2 h irradiation under midday winter solstice sunlightin Northern California. At later reaction times functionalization (i.e.,adding polar oxygenated functional groups to the molecule) and fragmentation(i.e., breaking of covalent bonds) become more important processes, forminga large variety of functionalized aromatic and open-ring products withhigher OSC values. Fragmentation reactions eventually dominate thephotochemical evolution of phenolic aqSOA, forming a large number of highlyoxygenated ring-opening molecules with carbon numbers (nC) below 6. Theaverage nC of phenolic aqSOA decreases while average OSC increasesover the course of photochemical aging. In addition, the saturation vaporpressures (C∗) of dozens of the most abundant phenolic aqSOAmolecules are estimated. A wide range of C∗ values is observed,varying from < 10−20 µg m−3 for functionalizedphenolic oligomers to > 10 µg m−3 for small open-ringspecies. The detection of abundant extremely low-volatile organic compounds(ELVOC) indicates that aqueous reactions of phenolic compounds are likely animportant source of ELVOC in the atmosphere

atmospheric chemistry and physics

Molecular transformations of phenolic SOA during photochemical aging in the aqueous phase: competition among oligomerization, functionalization, and fragmentation