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 secondary oxidants from 3VL*, pH, the presence of volatile organic compounds (VOCs) and inorganic anions, and reactants concentration and molar ratios on VL photo-oxidation were also explored. Our findings show that the secondary oxidants (1O2, O2•-/•HO2, •OH) from the reactions of 3VL* and O2 play an essential role in VL photo-oxidation. Enhanced oligomer formation was noted at pH < 4 and in the presence of VOCs and inorganic anions, probably due to additional generation of radicals (•HO2 and CO3•-). Also, functionalization was dominant at low VL concentration, whereas oligomerization was favored at high VL concentration. Furthermore, guaiacol oxidation by photosensitized reactions of VL was observed to be more efficient relative to nitrate-mediated photo-oxidation. Lastly, potential VL photo-oxidation pathways under different reaction conditions were proposed. This study indicates that the direct photosensitized oxidation of VL, which nitrate photolysis products can further enhance, may be an important aqSOA source in areas influenced by biomass burning emissions.

Highlights

  • Aqueous reactions can be an important source of secondary organic aerosols (SOA) (Blando and Turpin, 2000; Volkamer et al, 2009; Lim et al, 2010; Ervens et al, 2011; Huang et al, 2011; Lee et al, 2011; Smith et al, 2014) such as highlyoxygenated and low-volatility organics (Hoffmann et al, 2018; Liu et al, 2019) which may affect aerosol optical properties due to contributions to Brown Carbon (BrC) (Gilardoni et al, 2016)

  • This study indicates that the direct photosensitized oxidation of VL, which nitrate photolysis products can further enhance, may be an important aqueous secondary organic aerosol (aqSOA) source in areas influenced by biomass burning emissions

  • This study shows that the photo-oxidation of VL via its direct photosensitized reactions and in the presence of nitrate can generate aqSOA composed of oligomers, functionalized monomers, oxygenated ring-opening products, and nitrated compounds

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Summary

Introduction

Aqueous reactions can be an important source of secondary organic aerosols (SOA) (Blando and Turpin, 2000; Volkamer et al, 2009; Lim et al, 2010; Ervens et al, 2011; Huang et al, 2011; Lee et al, 2011; Smith et al, 2014) such as highlyoxygenated and low-volatility organics (Hoffmann et al, 2018; Liu et al, 2019) which may affect aerosol optical properties due to contributions to Brown Carbon (BrC) (Gilardoni et al, 2016).

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