Photoinitiated Degradation Kinetics of the Organic UV Filter Oxybenzone in Solutions and Aerosols: Impacts of Salt, Photosensitizers, and the Medium

Organic UV filters like oxybenzone (BP3) in sunscreens are seawater pollutants suspected to transfer to the atmosphere via sea spray aerosol (SSA). This study examines the photoinitiated degradation of BP3 in artificial and real seawater compared to SSA mimics containing NaCl and 4-benzoylbenzoic acid (4-BBA). We investigated pure, binary, and ternary mixtures of BP3, NaCl, and 4-BBA using solar-simulated light to isolate the effects of salt and photosensitization on BP3 degradation. Results showed significantly faster degradation in the aerosol phase (<i>J</i> _eff,env ≈ 10^-3-10^-2 s^-1 or <i>t</i> _1/2 < 10 min) compared to bulk solutions (<i>J</i> _eff,env ≈ 10^-6 s^-1 or <i>t</i> _1/2 > 1 day). The photosensitizer enhanced BP3 photodegradation in both phases more than when mixed with salt or all three components in solutions. BP3 photodegradation was most enhanced by salt in the aerosol phase. High-resolution molecular analysis via Orbitrap LC-MS/MS revealed more acutely toxic compounds (benzophenone, benzoic acid, and benzaldehyde) in irradiated aerosols than in solution, supported by electronic structure and toxicity modeling. These findings highlight that seawater may serve as a reservoir for BP3 and other organic UV filters and that upon transfer into SSA, BP3 rapidly transforms, increasing aerosol toxicity.