Abstract
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. Nitrate and ammonium are among the main components of biomass burning aerosols and cloud or fog water. Under atmospherically relevant cloud and fog conditions, solutions composed of either VL only or VL with ammonium nitrate were subjected to simulated sunlight irradiation to compare aqSOA formation via the direct photosensitized oxidation of VL in the absence and presence of ammonium nitrate. The reactions were characterized by examining the VL decay kinetics, product compositions, and light absorbance changes. Both conditions generated oligomers, functionalized monomers, and oxygenated ring-opening products, and ammonium nitrate promoted functionalization and nitration, likely due to its photolysis products (⚫OH, ⚫NO2, and NO2- or HONO). Moreover, a potential imidazole derivative observed in the presence of ammonium nitrate suggested that ammonium participated in the reactions. The majority of the most abundant products from both conditions were potential brown carbon (BrC) chromophores. The effects of oxygen (O2), pH, and reactants concentration and molar ratios on the reactions were also explored. Our findings show that O2 plays an essential role in the reactions, and oligomer formation was enhanced at pH <4. Also, functionalization was dominant at low VL concentrations, whereas oligomerization was favored at high VL concentrations. Furthermore, oligomers and hydroxylated products were detected from the oxidation of guaiacol (a non-carbonyl phenol) via VL photosensitized reactions. Last, potential aqSOA formation pathways via the direct photosensitized oxidation of VL in the absence and presence of ammonium nitrate were proposed. This study indicates that the direct photosensitized oxidation of VL may be an important aqSOA source in areas influenced by biomass burning and underscores the importance of nitrate in the aqueous-phase processing of aromatic carbonyls.
Highlights
IntroductionAqueous reactions can be an important source of secondary organic aerosols (SOAs; 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 highly oxygenated 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)
The direct photosensitized oxidation of VL in the absence and presence of ammonium nitrate (AN) under atmospherically relevant cloud and fog conditions have been shown to generate aqueous secondary organic aerosol (aqSOA) composed of oligomers, functionalized monomers, oxygenated ring-opening products, and nitrated compounds
The oligomers from these reaction systems may be rather recalcitrant to fragmentation, based on their high normalized abundance, even at the longest irradiation time used in this study
Summary
Aqueous reactions can be an important source of secondary organic aerosols (SOAs; 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 highly oxygenated 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). Triplet excited states of organic compounds (3C∗) from the irradiation of light-absorbing organics such as non-phenolic aromatic carbonyls (Canonica et al., 1995; Anastasio et al, 1997; Vione et al, 2006; Smith et al., 2an0d14w) ihthavgerebaeteern arqeSpOorAtedyiteoldosxciodmizepaprheedntoolsqOatHh(iSghuenrertaatel.s, 2010; Smith et al, 2014; Yu et al, 2014; Smith et al, 2016). Recent works on triplet-driven oxidation of phenols have mainly focused on changes in physicochemical properties (e.g., light absorption) and aqSOA yield (e.g., Smith et al, 2014, 2015, 2016), with few reports on reaction pathways and products (e.g., Yu et al, 2014; Chen et al, 2020; Jiang et al, 2021)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
