Brown carbon (BrC) have important climate impacts. Phenolic compounds are commonly emitted from biomass burning and are an important class of BrC precursors. Inorganic nitrate is a ubiquitous component of atmospheric aqueous phases. The photolysis of inorganic nitrate in atmospheric aqueous phases generates a variety of reactive oxygen species and reactive nitrogen species that can facilitate the photooxidation of organic compounds. We investigated the formation and evolution of BrC from the aqueous-phase photooxidation of guaiacol and 5-nitroguaiacol initiated by inorganic nitrate photolysis. Upon illumination in the presence of sodium nitrate, guaiacol and 5-nitroguaiacol reacted rapidly to form BrC. The reaction rates and quantities of BrC formed depended on the initial sodium nitrate concentration. The reaction rates of guaiacol were substantially faster than those of 5-nitroguaiacol. The major products produced during the initial stages of photooxidation were formed by the addition of nitro and/or nitroso groups to guaiacol and 5-nitroguaiacol. These products were likely BrC components that 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 guaiacol. 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 highlight how aqueous-phase inorganic nitrate photolysis can drive the formation of BrC by facilitating the photooxidation of phenolic compounds.

Formation and evolution of brown carbon during aqueous-phase nitrate-mediated photooxidation of guaiacol and 5-nitroguaiacol

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.