Phototransformation of 4-phenoxyphenol sensitised by 4-carboxybenzophenone: Evidence of new photochemical pathways in the bulk aqueous phase and on the surface of aerosol deliquescent particles

In addition to direct photolysis, degradation of organic compounds by solar light can also occur by indirect photolysis or photo-sensitised processes. These reactions are important because they are involved in, among others, direct and indirect climate changes, adverse health effects from inhaled particles, effects on cloud chemistry and ozone production. In this work, the importance of atmospheric photo-sensitisation is evaluated in bulk aqueous solution and on the surface of aerosol deliquescent particles. Irradiation experiments in aqueous solution indicate that 4-carboxybenzophenone (CBP) is able to photosensitise the degradation of 4-phenoxyphenol (4 PP). The process takes place via the CBP triplet state (3CBP*), which has an oxidising nature. 4 PP is fluorescent, unlike the photosensitiser CBP, with two emission bands at ∼320 and ∼380 nm. However, addition of CBP to a 4 PP solution considerably decreases the intensity of 4 PP fluorescence bands and causes a very intense new band to appear at ∼420 nm. This behaviour suggests a possible interaction between CBP and 4 PP in solution, which could favour further light-induced processes. Moreover, the new band overlaps with the fluorescence spectrum of atmospheric HULIS (HUmic-LIke Substances), suggesting that supramolecular photosensitiser–substrate interactions may have a role in HULIS fluorescence properties. The interaction between CBP and 4 PP coated on silica particles (gas–solid system) was also investigated under simulated sunlight, and in the presence of variable relative humidity. The water molecules inhibit the degradation of 4 PP, induced by 3CBP* on the surface of aerosol particles, indicating that the process could be even faster on particles than in solution. We demonstrate that phenol substances adsorbed on aerosol surfaces and in bulk solution are substantially altered upon photosensitised processes.