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Aqueous SOA Formation from the Photo-oxidation of Vanillin: Direct Photosensitized Reactions and Nitrate-Mediated Reactions
BEATRIX ROSETTE GO MABATO, Yan Lyu, Yan Ji, Yong Jie Li, Dan Dan Huang, Xue Li, Theodora Nah, Chun Ho Lam, Chak K. Chan, City University of Hong Kong, China
Abstract Number: 124
Working Group: Aerosol Chemistry
Abstract
Vanillin (VL), a phenolic aromatic carbonyl abundant in biomass burning emissions, forms triplet excited states (3VL*) under simulated sunlight leading to aqueous secondary organic aerosol (aqSOA) formation. This direct photosensitized oxidation of VL was compared with nitrate-mediated VL photo-oxidation under atmospherically relevant cloud and fog conditions through examining the VL decay kinetics, product compositions, and light absorbance changes. The majority of the most abundant products from both VL photo-oxidation pathways were potential Brown carbon (BrC) chromophores. In addition, both pathways generated oligomers, functionalized monomers, and oxygenated ring-opening products, but nitrate promoted functionalization and nitration, which can be ascribed to its photolysis products (•OH, •NO2, and N(III), NO2- or HONO). Moreover, a potential imidazole derivative observed from nitrate-mediated VL photo-oxidation suggested that ammonium may be involved in the reactions. The effects of oxygen (O2), pH, and reactants concentration and molar ratios on VL photo-oxidation were also explored. Our findings show that O2 plays an essential role in VL photo-oxidation, and oligomer formation was enhanced at pH < 4. Also, functionalization was dominant at low VL concentration, whereas oligomerization was favored at high VL concentration. Furthermore, comparisons of the apparent quantum efficiency of guaiacol photodegradation indicate that in this study, guaiacol oxidation by photosensitized reactions of VL is less efficient relative to nitrate-mediated photo-oxidation. Lastly, potential aqSOA formation pathways via VL photo-oxidation were proposed. This study indicates that the direct photosensitized oxidation of VL and nitrate-mediated VL photo-oxidation may be important aqSOA sources in areas influenced by biomass burning emissions.