Aromatic Nitration\nEnhances Absorption of Biomass\nBurning Brown Carbon in an Oxidizing Urban Environment

Brown carbon (BrC) from biomass burning constitutes a\nsignificant\nportion of light-absorbing components in the atmosphere. Although\nthe aging of BrC surrogates from biomass burning has been studied\nin many laboratory settings, BrC aging behavior in real-world urban\nenvironments is not well understood. In this study, through a combination\nof online dynamic monitoring and offline molecular characterization,\nthe ambient optical aging of BrC was linked to its dynamic changes\nin molecular composition. Enhanced light absorption by BrC was consistently\nobserved during the periods dominated by oxygenated biomass burning\norganic aerosol (BBOA), in contrast to periods dominated by primary\nemissions or secondary formation in aqueous-phase. This enhancement\nwas linked to the formation of nitrogen-containing compounds during\nthe ambient aging of BBOA. Detailed molecular characterization, alongside\nanalysis of environmental parameters, revealed that an increased atmospheric\noxidizing capacity, marked by elevated levels of ozone and nighttime\nNO<sub>3</sub> radicals, facilitated the formation of nitrated aromatic\nBrC chromophores. These chromophores were primarily responsible for\nthe enhanced light absorption during the ambient aging of BBOA. This\nstudy elucidates the nitration processes that enhance BrC light absorption\nfor ambient BBOA, and highlights the crucial role of meteorological\nconditions. Furthermore, our findings shed light on the chemical and\noptical aging processes of biomass burning BrC in ambient air, offering\ninsights into its environmental behavior and effects.