Abstract View
Brown Carbon Formation and Evolution from Aqueous-Phase Nitrate-Mediated Photooxidation of Phenolic Compounds
THEODORA NAH, Junwei Yang, Wing Chi Au, Haymann Law, Chun Hei Leung, Chun Ho Lam, City University of Hong Kong
Abstract Number: 59
Working Group: Aerosol Chemistry
Abstract
Substantial concentrations of phenolic compounds are emitted during biomass burning. Some of these phenolic compounds are highly water soluble, and can dissolve into atmospheric aqueous phases where they can undergo reactions to form brown carbon (BrC). Inorganic nitrate is a ubiquitous component of atmospheric aerosols, clouds, and fog. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are produced during the photolysis of inorganic nitrate in atmospheric aqueous phases. We report results from our laboratory investigations of the aqueous-phase photooxidation of a series of phenolic compounds (guaiacol, catechol, 5-nitrocatechol, and 4-nitrocatechol) initiated by inorganic nitrate photolysis under different pH conditions. When illuminated with UV light in the presence of inorganic nitrate, these phenolic compounds reacted rapidly to form BrC. Reaction rates of non-nitrated phenolic compounds were substantially faster than those of nitrated phenolic compounds. The -NO2 functional group on the aromatic ring of nitrated phenolic compounds likely had a deactivating effect on the ring’s reactivity, which contributed to their lower reactivity. Dominant products produced during the initial stages of photooxidation were formed by the addition of -NO and/or -NO2 groups to the phenolic compounds. These products likely strongly absorbed near-UV and/or visible light, which led to an observed increase in light absorption (i.e., photo-enhancement) in the near-UV and visible range. Greater photo-enhancement was observed during the nitrate-mediated photooxidation of non-nitrated phenolic compounds. For some of the studied phenolic compounds, the pH of the aqueous phase impacted the reactivity of the phenolic compound and the amount of BrC formed. These results highlight how aqueous-phase inorganic nitrate photolysis can drive BrC formation by facilitating the photooxidation of phenolic compounds.