Aqueous secondary organic aerosol formation from the direct photosensitized oxidation of vanillin in the absence and presence of ammonium nitrate

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.

Particulate nitrate photolysis in the atmosphere

“…The increased NO 2 formation results from the reaction of nitrite with superoxide and OH radicals produced from photosensitizing reactions of vanillin. 135…”

Single-particle Raman spectroscopy for studying physical and chemical processes of atmospheric particles

“…Nitrate photolysis proceeds via three channels: one produces NO 2 and OH radicals, another produces NO − 2 and O( 3 P), and the last generates ONOO − (Scharko et al, 2014;Benedict et al, 2017). The third channel's quantum yield is considered negligible compared with the former two since ONOO − is unstable (Mabato et al, 2022). Particulate nitrate photolysis in the atmospheric has been reviewed by a recent paper (Gen et al, 2022).…”

Aqueous SOA formation from photosensitized guaiacol oxidation: Comparison between non-phenolic and phenolic methoxybenzaldehydes as photosensitizers in the absence and presence of ammonium nitrate

“…Photosensitized reactions involving triplet excited states of organic compounds ( 3 C*) are efficient pathways for the formation of secondary organic aerosol in the aqueous phase (aqSOA; Smith et al, , 2016Yu et al, 2014Yu et al, , 2016Chen et al, 2020;Jiang et al, 2021;Mabato et al, 2022). Upon irradiation by solar radiation, photosensitizers form an excited triplet state that directly reacts with substrates (e.g., phenols), and can generate singlet oxygen ( 1 O 2 ), superoxide (O 2 •-) or hydroperoxyl ( • HO 2 ) radicals, and hydroxyl radicals ( • OH) upon reactions with O 2 and substrates (George et al, 2018;Chen et al, 2020), thereby facilitating the oxidation of rather volatile species and contributing to aqSOA formation.…”

“…By contrast, phenolic carbonyls have been mainly studied as aqSOA precursors via • OH-, nitrate-, nitrite-, and 3 DMB*-mediated oxidation (Li et al, 2014;Huang et al, 2018;Pang et al, 2019;Jiang et al, 2021;. However, strongly light-absorbing phenolic carbonyls (e.g., molar absorptivity above 300 nm ≥7 × 10 3 M -1 cm -1 ) can also serve as photosensitizers to promote aqSOA formation (Smith et al, 2016;Mabato et al, 2022). For instance, the direct photosensitized oxidation of phenolic carbonyls (i.e., oxidation of phenolic carbonyls by their 3 C* or 3 C*-derived oxidants) such as vanillin (VL; another methoxybenzaldehyde) efficiently form low-volatility products, with aqSOA mass yields of up to 140% (Smith et al, 2016).…”

“…Nitrate and ammonium facilitate the formation of aqSOA and brown carbon (BrC) via a number of pathways. Nitrate photolysis can produce • OH and nitrating agents (e.g., • NO 2 ; Minero et al, 2007;Huang et al, 2018;Mabato et al, 2022;Wang et al, 2022;Yang et al, 2022), and ammonium reacts with carbonyls to yield N-containing heterocycles (e.g., imidazoles) and oligomers capable of UV-Vis light absorption (De Haan et al, 2009, 2011Nozière et al, 2009Nozière et al, , 2010Nozière et al, , 2018Shapiro et al, 2009;Yu et al, 2011;Lee et al, 2013;Powelson et al, 2014;Gen et al, 2018;Grace et al, 2019;Mabato et al, 2019). Furthermore, nitrate photolysis may be an important process for SO 2 oxidation and SOA formation in the particle phase (Gen et al, 2019a(Gen et al, , 2019bZhang et al, 2020Zhang et al, , 2021Zhang et al, , 2022, and it can potentially modify the morphology of atmospheric viscous particles (Liang et al, 2021).…”