Formation of secondary organic aerosol from nitrate radical oxidation of phenolic VOCs: Implications for nitration mechanisms and brown carbon formation

Volatile phenolic derivatives are substantially emitted from biomass burning and produced from photochemistry of atmospheric aromatic volatile organic compounds (VOCs). Oxidation of phenolic VOCs at night by nitrate radicals (NO3∙) may represent a significant source of secondary organic aerosols (SOA) and brown carbon (BrC) formation in the atmosphere. In this study, NO3∙ oxidation of five phenolic derivatives, including phenol, catechol, 3-methylcatechol, 4-methylcatechol and guaiacol are investigated in laboratory experiments. The SOA constituents from the NO3∙ oxidation were analyzed using electrospray ionization ion mobility spectrometry time-of-flight mass spectrometry, which allows for characterization and identification of isomers in the oxidation products. Through these analyses, several classes of nitro-containing products in addition to the well-known nitrophenol compounds were observed, including: (1) the nitrophenol type of products with additional hydroxyl functional groups; (2) non-aromatic/ring-opening nitro-products with lower double bond equivalence; (3) phenol and catechol products from the C7 phenolic VOCs with carbon-containing substitutions; and (4) nitrated diphenyl ether dimers. The present work indicates that new products from previously unrecognized pathways are formed during NO3∙ oxidation of phenolic VOCs and may contribute an important portion of the SOA. Some of these products were also observed in ambient aerosols during biomass burning. We suggest that the ubiquity of the nitrophenol type of products in the SOA derived from phenolic VOC + NO3∙ are responsible for the strong light absorption measured in this study. Therefore, elucidation of these pathways will be critical for understanding the nighttime oxidation and BrC formation mechanisms.