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

Abstract Light absorbing organic aerosols, which are commonly referred to as brown carbon (BrC), have important climate impacts. Phenolic compounds are a class of aromatic compounds that are known BrC precursors. Inorganic nitrate is a ubiquitous component of atmospheric aerosols, clouds, and fog. 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 to form BrC. In this study, we investigated the formation and evolution of BrC from the aqueous-phase photooxidation of guaiacol and 5-nitroguaiacol initiated by inorganic nitrate photolysis. Guaiacol and 5-nitroguaiacol are two water-soluble phenolic compounds that are commonly found in fossil fuel and biomass burning emissions. 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 electron-withdrawing nitro functional group on 5-nitroguaiacol’s aromatic ring likely had a deactivating effect on the ring’s reactivity, which contributed to 5-nitroguaiacol’s lower reactivity compared to guaiacol. 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 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 guaiacol. This indicated that the presence of nitro functional group(s) on the aromatic ring of phenolic compounds will impact the extent of photo-enhancement during BrC formation. Further photooxidation caused the initial products to fragment into smaller molecules that do not absorb near-UV and visible light. This 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 organic compounds.