Organic UV filters in sunscreens, such as oxybenzone (BP3), are concentrated and persistent pollutants in seawater because of their broad use and resistance to photolysis due to their efficient vibrational relaxation from excited states. BP3 is hydrophobic and suspected to undergo ocean-to-air transfer via sea spray aerosol (SSA), potentially leading to different reactivity in the aerosol phase compared to bulk water. This study contrasts the photo-initiated degradation reactions and kinetics of BP3 in artificial seawater mixes and real seawater with those in SSA mimics comprised of NaCl and a model photosensitizer, 4-benzoyl benzoic acid (4-BBA). Pure, binary, and ternary mixtures of BP3, NaCl, and 4-BBA were irradiated to isolate the impacts of salt and the photosensitizer on BP3 degradation. Experiments were conducted in a quartz cuvette exposed to solar-simulated light and analyzed via UV-VIS spectroscopy for bulk solutions and a flow tube reactor equipped with broadband UVB lamps for aerosols, analyzed via extractive electrospray ionization time-of-flight mass spectrometry (EESI-TOF). Results indicate significant differences between bulk and aerosol phase photodegradation rates scaled to equivalent solar exposure times, with much faster degradation observed in the aerosol phase (J≈10^-3 s^-1) than in bulk solutions (J≈10^-6 s^-1) in all mixtures. The photosensitizer enhanced BP3 photodegradation in both phases but more so in the bulk phase compared to mixtures with salt or all three components, whereas BP3 photodegradation was enhanced the most in the presence of salt in the aerosol phase. The faster kinetics in the aerosol phase are consistent with previous studies demonstrating increased effective light intensity and decreased solvent-cage effect in spherical particles compared to bulk solutions. High-resolution molecular identification of transformation products via Orbitrap LC-MS/MS showed an enhanced generation of reduced forms of BP3, notably benzophenone, alongside smaller fragmentation products, such as benzoic acid and benzaldehyde, in the aerosol phase compared to the bulk, which enhances the aerosol’s toxicity (i.e., predicted LD₅₀ for rats using EPISTAR). These results demonstrate the stark differences in photodegradation between the bulk and aerosol phases of a prevalent and toxic UV filter and suggest that photo-induced reactions of BP3 are relatively more important in SSA than in seawater concerning its effects on particle toxicity.

Adam Cooper

PhD Student, UC San Diego

Appearances