Observational Evidence of Brown Carbon Photobleaching in Urban Atmosphere at Molecular Level

Brown carbon (BrC) is a crucial light-absorption component in the atmosphere, playing a profound role in the radiation budget. Variations in the light-absorption properties and molecular composition of BrC due to atmospheric photochemical aging process have not been well constrained, leading to high uncertainty in evaluating its global radiative effect. The molecular composition of atmospheric BrC were investigated to stress the BrC photobleaching in this work. In total, 896 organic compounds were identified, which accounted for 2.5%–26.1% of organic aerosol in mass concentration. We found that solar radiation led to the declined mass absorption coefficient at 375 nm (MAC₃₇₅), indicating BrC photobleaching, coinciding with an elevated mass fraction of carboxylic acids (CAs). This phenomenon was more pronounced under higher-energy solar radiation. Specifically, the mass fraction of nitrocarboxylic acids (NCAs) in CAs increased during BrC photobleaching, which was potentially caused by the oxidation of nitrophenols, resulting in an ∼83.3% decrease in MAC₃₇₅. Our findings provide direct observational evidence of BrC photobleaching from a molecular-level perspective and elucidate a potential BrC photobleaching pathway in the urban atmosphere.