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Formation of light absorbing secondary organic aerosols from the aqueous-phase nitrate-mediated photooxidation of phenolic compounds

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A large mass fraction of atmospheric aerosols is comprised of secondary organic aerosols (SOA). Atmospheric aqueous phases (e.g., cloudwater, aerosol water) are important reaction media for SOA formation. Water-soluble organic gases formed from gas-phase reactions of volatile organic compounds (VOCs) partition into atmospheric aqueous phases where they react to form low-volatility organic matter that remain after water evaporation. Some of the SOA formed from aqueous-phase reactions can absorb light in the near-UV and visible ranges. Light absorbing organic aerosols are commonly referred to as brown carbon (BrC). Phenolic compounds are a class of water-soluble aromatic compounds that are emitted from combustion processes. One class of phenolic compounds that have strong light absorptivity in the near-UV and visible ranges are nitrophenols. Inorganic nitrate is a ubiquitous component of aerosols, clouds and fog. Inorganic nitrate photolysis in aqueous phases forms hydroxyl (OH), nitroso (NO), and nitrite (NO2) radicals that can participate in aqueous-phase reactions. Recent studies showed that inorganic nitrate photolysis can promote SOA formation in aqueous phases by facilitating the photooxidation of organic compounds. However, many
questions regarding the composition and light absorption properties of SOA formed during nitrate-mediated photooxidation remain.

We present studies aimed at understanding BrC formation from the aqueous-phase nitrate-mediated photooxidation of a series of phenolic compounds (guaiacol, catechol, 5-nitroguaiacol, and 4-nitrocatechol). Upon illumination in the presence of inorganic nitrate, the phenolic compounds reacted rapidly to form BrC. The reaction rates and quantities of BrC formed depended on the initial inorganic nitrate concentration. Reaction rates of non-nitrated phenolic compounds were substantially faster than those of nitrophenolic compounds. The electron-withdrawing nitro functional group on the aromatic ring likely had a deactivating effect on the ring’s reactivity, which contributed to the lower reactivity of nitrophenolic compounds. Major products produced during the initial stages of photooxidation were formed by the addition of nitro and/or nitroso groups to the aromatic ring. These products were BrC components that contributed to an increase in light absorption (i.e., photo-enhancement) in the near-UV and visible range. Greater photo-enhancement was observed during the photooxidation of non-nitrated phenolic compounds. Further photooxidation caused the initial
products to fragment into smaller molecules that do not absorb near-UV and visible light, which resulted in an observed decrease in light absorption (i.e., photo-bleaching). These results highlighted how BrC formation in atmospheric aqueous phases can be driven by inorganic nitrate photolysis.

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Title#EnvChem2021: Chemistry of the Whole Environment Research
PlaceUnited Kingdom
Degree of recognitionInternational event