Observational Evidence of Brown Carbon Photobleaching\nin Urban Atmosphere at Molecular Level

Brown carbon (BrC) is a crucial light-absorption\ncomponent in the\natmosphere, playing a profound role in the radiation budget. Variations\nin the light-absorption properties and molecular composition of BrC\ndue to atmospheric photochemical aging process have not been well\nconstrained, leading to high uncertainty in evaluating its global\nradiative effect. The molecular composition of atmospheric BrC were\ninvestigated to stress the BrC photobleaching in this work. In total,\n896 organic compounds were identified, which accounted for 2.5%–26.1%\nof organic aerosol in mass concentration. We found that solar radiation\nled to the declined mass absorption coefficient at 375 nm (MAC<sub>375</sub>), indicating BrC photobleaching, coinciding with an elevated\nmass fraction of carboxylic acids (CAs). This phenomenon was more\npronounced under higher-energy solar radiation. Specifically, the\nmass fraction of nitrocarboxylic acids (NCAs) in CAs increased during\nBrC photobleaching, which was potentially caused by the oxidation\nof nitrophenols, resulting in an ∼83.3% decrease in MAC<sub>375</sub>. Our findings provide direct observational evidence of\nBrC photobleaching from a molecular-level perspective and elucidate\na potential BrC photobleaching pathway in the urban atmosphere.