Photosensitized Reactions of a Phenolic Carbonyl from Wood Combustion in the Aqueous Phase—Chemical Evolution and Light Absorption Properties of AqSOA

Abstract

Guaiacyl acetone (GA) is a phenolic carbonyl emitted in significant quantities by wood combustion that undergoes rapid aqueous-phase oxidation to produce aqueous secondary organic aerosol (aqSOA). We investigate the photosensitized oxidation of GA by an organic triplet excited state ( 3 C*) and the formation and aging of the resulting aqSOA in wood smokeinfluenced fog/cloud water. The chemical transformations of the aqSOA were characterized in situ using a high-resolution time-offlight aerosol mass spectrometer. Additionally, aqSOA samples collected over different time periods were analyzed using highperformance liquid chromatography coupled with a photodiode array detector and a high-resolution Orbitrap mass spectrometer (HPLC-PDA-HRMS) to provide details on the molecular composition and optical properties of brown carbon (BrC) chromophores. Our results show efficient formation of aqSOA from GA, with an average mass yield around 80%. The composition and BrC properties of the aqSOA changed significantly over the course of reaction. Three generations of aqSOA products were identified via positive matrix factorization analysis of the aerosol mass spectrometry data. Oligomerization and functionalization dominated the production of the first-generation aqSOA, whereas fragmentation and ring-opening reactions controlled the formation of more oxidized second-and third-generation products. Significant formation of BrC was observed in the early stages of the photoreaction, while organic acids were produced throughout the experiment. High-molecular weight molecules (m/z > 180) with high aromaticity were identified via HPLC-PDA-HRMS and were found to account for a majority of the UV−vis absorption of the aqSOA.

Aqueous SOA formation from the photo-oxidation of vanillin: Direct photosensitized reactions and nitrate-mediated reactions

“…The H:C ratios were mostly around 1.0, indicating that the products for experiments A5 to A8 (Table S2) were mainly aromatic species. Compounds with H:C ≤1.0 and O:C ≤0.5 are common for aromatic species, while compounds with H:C ≥1.5 and O:C ≤0.5 are typical for more aliphatic species (Mazzoleni et al, 2012;Kourtchev et al, 2014;Jiang et al, 2021). Moreover, majority of the products for experiments A5 to A8 have double bond equivalent (DBE) values >7, which corresponds to oxidized aromatic compounds (Xie et al, 2020).…”

“…In contrast to the GUA aqSOA reported by Yu et al (2014), the photo-oxidation of GUA in this study yielded nitrated compounds (e.g., C9H14N2O6, C11H14N2O9) from GUA+AN and VL dimers (e.g., C16H12O6) from GUA+VL. However, based on a recent work on the aqueous photo-oxidation of guaiacyl acetone (another aromatic phenolic carbonyl) by DMB triplets, the hydroxylation and dimerization of DMB can also contribute to aqSOA (Jiang et al, 2021). The contributions from DMB-participated reactions were only minor due to the low initial DMB concentration (0.005 mM).…”

“…The 3 C* concentration in typical fog water has been estimated to be >25 times than that of • OH, making 3 C* the primary photooxidant for biomass burning phenolic compounds (Kaur and Anastasio, 2018;Kaur et al, 2019). Recent works on tripletdriven oxidation of phenols have mainly focused on changes of physicochemical properties (e.g., light absorption) and aqSOA yield (e.g., Smith et al, 2014Smith et al, , 2015Smith et al, , 2016, with few reports on reaction mechanisms and characterization of reaction products (e.g., Yu et al, 2014;Chen et al, 2020;Jiang et al, 2021).…”

“…The influences of secondary oxidants from VL triplets, solution pH, the presence of VOCs and inorganic anions, and reactants concentration and molar ratios on these two photo-oxidation pathways were also assessed. The 3 C* of nonphenolic aromatic carbonyls (e.g., 3-4-dimethoxybenzaldehyde, DMB; a non-phenolic aromatic carbonyl) (Smith et al, 2014;Yu et al, 2014;Jiang et al, 2021) and phenolic aromatic carbonyls (e.g., acetosyringone, vanillin) (Smith et al, 2016) have been shown to oxidize phenols, but the reaction products from the latter are unknown. We then examined the photooxidation of guaiacol, another non-carbonyl phenol, in the presence of VL and compared it with nitrate-mediated photooxidation.…”

Aqueous chemical bleaching of 4-nitrophenol brown carbon by hydroxyl radicals; products, mechanism and light absorptivity

“…Formation, chemical processing and decomposition (bleaching) of BrC can occur in air as well as in the atmospheric aqueous particles, which can involve direct photolysis and reactions with hydroxyl radicals (OH) (Hems et al, 2020;Laskin et al, 2015;Jiang et al, 2021;Li et al, 2020a;Forrister et al, 2015;Moise et al, 2015).…”