Aromatic Nitration Enhances Absorption of Biomass Burning Brown Carbon in an Oxidizing Urban Environment

Brown carbon (BrC) from biomass burning constitutes a significant portion of light-absorbing components in the atmosphere. Although the aging of BrC surrogates from biomass burning has been studied in many laboratory settings, BrC aging behavior in real-world urban environments is not well understood. In this study, through a combination of online dynamic monitoring and offline molecular characterization, the ambient optical aging of BrC was linked to its dynamic changes in molecular composition. Enhanced light absorption by BrC was consistently observed during the periods dominated by oxygenated biomass burning organic aerosol (BBOA), in contrast to periods dominated by primary emissions or secondary formation in aqueous-phase. This enhancement was linked to the formation of nitrogen-containing compounds during the ambient aging of BBOA. Detailed molecular characterization, alongside analysis of environmental parameters, revealed that an increased atmospheric oxidizing capacity, marked by elevated levels of ozone and nighttime NO