The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Brown carbon (BrC) is a crucial light-absorption component in the atmosphere, playing profound role in radiation budget. Once emitted into atmosphere, BrC undergoes photochemical aging process, which modifies its light-absorption properties significantly. Up to now, variations in the light-absorption properties and molecular composition of BrC due to atmospheric photochemical aging have not been well constrained, leading to highly uncertainty in evaluating its global radiative effect. In this work, we provided the observational evidence of BrC photobleaching in the urban atmosphere from the molecular-level perspective. BrC was extracted from PM_2.5 samples collected in the urban atmosphere and molecular composition was analyzed using a non-target analysis approach. 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 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 the direct observational evidence of BrC photobleaching from molecular-level perspective, and elucidate a potential BrC photobleaching pathway in the urban atmosphere.

brown carbon, photobleaching, photochemical aging