Sunlight-Driven Diel Variation of Reactive Bromine Species in Coastal Surface Seawater

Reactive bromine species (RBS) play a critical role in global halogen cycling and atmospheric oxidative processes, yet their temporal dynamics and formation mechanisms in marine environments are not fully understood. Here, we report the first continuous observation of diel variations of RBS in coastal surface seawater using a newly developed osmotic pump-based sampler coupled with in situ derivatization. Hour-resolved time-series monitoring was conducted at a subtropical coastal wetland during summer and winter, revealing pronounced diel patterns characterized by strong daytime enhancement and nighttime depletion of RBS, closely tracking solar irradiance. Peak RBS concentrations occurred around midday, reaching up to 32.6 nmol/L in summer and 24.0 nmol/L in winter. Concurrent measurements of Fe(II) and H₂O₂, together with on-site perturbation experiments and laboratory simulated photo-Fenton reaction, demonstrate that photo-Fenton reactions are the dominant pathway driving RBS formation in coastal surface seawater. The presence of dissolved organic matter (DOM) significantly enhances photo-Fenton efficiency through ligand-to-metal charge transfer-mediated Fe(III) photoreduction and Fe(II) regeneration, amplifying RBS production by approximately 5.6-fold. In contrast, DOM photosensitization alone generated negligible RBS under comparable conditions. These results reveal sunlight-driven photo-Fenton, coupled with DOM-mediated iron cycling, is a major contributor to RBS dynamics in coastal environments. This study advances our understanding of diel bromine activation in coastal waters and highlights the importance of sunlight-driven photo-Fenton chemistry in marine halogen cycling.