Resolving the overlooked photochemical nitrophenol transformation mechanism induced by nonradical species under visible light.

Nitrophenols present on the surface of particulates are ubiquitous in the atmosphere. However, its atmospheric photochemical transformation pathway remains unknown, for which the crucial effect of visible light is largely overlooked, resulting in an incomplete understanding of the effects of nitrophenols in the atmospheric environment. This study delves into the photolysis mechanism of 4-nitrophenol (4NP), one of the most abundant atmospheric nitrophenol compounds, on the surface of photoactive particulates under visible light irradiation. Unexpectedly, the nonradical species (singlet oxygen, 1 O 2 ) was identified as a dominant factor in driving the visible photolysis of 4NP. The pathways of HONO and p-benzoquinone (C 6 H 4 O 2 ) generation were clarified by acquiring direct evidence of C-N and O-H bond breakage in the nitro (-NO 2 ) and hydroxyl (-OH) groups of 4NP. The further decomposition of HONO results in the generation of NO and hydroxyl radicals, which could directly contribute to atmospheric oxidizing capacity and complicate the PM 2.5 composition. Significantly, the behavior of 1 O 2 -induced visible photolysis of 4NP was universal on the surface of common particulates in the atmosphere, such as A1 dust and Fe 2 O 3 . This work advances the understanding of the photochemical transformation mechanism of particulate-phase atmospheric nitrophenols, which is indispensable in elucidating the role of nitrophenols in atmospheric chemistry.