Abstract
Atmospheric particulate matter (PM) is a major global concern due to its impacts on air quality and climate and its adverse effects on health. Secondary organic aerosol (SOA), which represents a significant fraction of PM, is generally underestimated by models due to uncertainties arising from knowledge gaps, including formation pathways and chemical composition. Growing evidence indicates that SOA formed through aqueous-phase reactions (aqSOA) can be a significant yet less understood formation pathway. AqSOA can influence aerosol optical properties by contributing to the light-absorbing fraction of organic aerosol referred to as brown carbon (BrC). Biomass burning (BB) emissions play an important role in aqSOA formation as a source of precursors and oxidants. This thesis presents findings from laboratory studies focused on the formation of aqSOA and BrC via dark and photochemical reactions of BB-derived compounds involving organic and inorganic species.We first present comparisons between the heterogenous reactions of droplets containing primary amines with glyoxal (Gly) as a function of relative humidity (RH) and bulk-phase reactions using premixed solutions under dark conditions in terms of reaction kinetics. Kinetic analyses based on in situ Raman/fluorescence and UV-Vis spectroscopy were used to study the particle- and bulk-phase reactions, respectively. The particle-phase rate constants increased as RH decreased and were higher than the bulk-phase values, likely due to the “salting-in” effect at higher salt concentrations. The enhanced BrC formation with decreasing RH can also be explained by the “salting-in” effect. As opposed to the reactions between Gly and ammonium salts (Gen et al., 2018), the promoted dehydration reactions may not be applicable in the enhanced BrC formation due to the higher hygroscopicity of methylaminium salts.
Aside from dark reactions of an aliphatic carbonyl, we examined aqSOA formation via photochemical reactions of aromatic compounds, with particular attention to photosensitization and ammonium nitrate (AN) photolysis. In these studies, the reactants decay kinetics, detected products, and absorbance measurements were used to characterize the reactions. We investigated the direct photosensitized oxidation of vanillin (VL), a phenolic carbonyl, in the absence and presence of AN. Both conditions generated oligomers, functionalized monomers, and oxygenated ring-opening products. AN induced the formation of N-containing compounds and promoted functionalization ascribable to its photolysis products (hydroxyl radical, nitrogen dioxide, and nitrite/nitrous acid; •OH, •NO2, and NO2−/ HONO, i.e., N(III)). The majority of the most abundant products from both systems were potential BrC chromophores. The detection of a potential imidazole derivative in the presence of AN suggested that ammonium participated in the reactions. The influence of dissolved oxygen (O2), pH, and reactants concentration and molar ratios on the reactions were also discussed in detail. We also detected oligomers and hydroxylated products from the oxidation of guaiacol (GUA), a non-carbonyl phenol, via VL photosensitized reactions. Our results indicate that the direct photosensitized oxidation of VL may be an important aqSOA source in BB-impacted areas and highlights the potential importance of AN photolysis in aqSOA formation.
Moreover, we compared aqSOA formation via photosensitization of GUA by non-phenolic (3,4-dimethoxybenzaldehyde, DMB) and phenolic (VL) methoxybenzaldehydes, a subset of aromatic carbonyls, in the absence and presence of AN. GUA oxidation by triplet excited states of DMB, 3DMB*, was faster and exhibited greater light absorption than oxidation by 3VL*, likely due to the stronger photosensitizing ability of DMB and the –OH group of VL making it more susceptible to oxidation and more reactive towards electrophilic aromatic substitution. The detection of N-heterocycles, including imidazoles, supports our earlier findings that ammonium participates in the photosensitized oxidation of phenolic compounds in the presence of AN. Moreover, AN modified the product distributions, albeit in different ways, potentially due to the differences in photosensitizing abilities and structural features of DMB and VL. Our results also imply that photosensitized reactions may promote nitration.
Further, we simulated the direct photosensitized oxidation of mixed aromatic carbonyls (phenolic carbonyls only or VL+2: VL + syringaldehyde or SyrAld + acetosyringone or ActSyr and non-phenolic and phenolic carbonyls or DMB+2: DMB + SyrAld + ActSyr) in the absence and presence of GUA, and compared these to simple systems (direct photosensitized oxidation of VL only and GUA oxidation by 3DMB* or 3VL* only; DMB+GUA or VL+GUA). Compared to simple systems, the presence of more triplet precursors in mixed aromatic carbonyls probably promoted functionalization via increased availability of triplet-derived secondary oxidants (e.g., •OH, 1O2). The faster and more extensive reactions in the presence of DMB than in VL attributable to the stronger photosensitizing ability and higher photostability of DMB than VL were in general agreement with our earlier work on DMB+GUA and VL+GUA. Additionally, GUA oxidation in mixed aromatic carbonyls was estimated to be less efficient than by 3DMB* or 3VL* only likely due to SyrAld and ActSyr being the primary oxidizable substrates that compete with GUA for reactions with 3C*. In the absence and presence of GUA, VL+2 had higher absorbance enhancement than DMB+2, likely due to its higher contribution of dimeric products and the formation of later-generation less absorbing aqSOA via fragmentation and ring-opening reactions in DMB+2. Relative to simple systems, mixed aromatic carbonyls generated more oxygenated and oxidized aqSOA.
Overall, this thesis provides important insights into the aqueous-phase processing of BB-derived compounds, contributing to a better understanding of aqSOA and BrC formation pathways. Recommendations for possible future directions are presented in the concluding remarks.
| Date of Award | 17 Nov 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Chak Keung CHAN (Supervisor) |