Halogen Radical Chemistry at Aqueous Interfaces

Halogens play key roles in the chemical composition of marine boundary layers, the free troposphere and the stratosphere. Atmospheric halogen chemistry is dominated by reactions between gas-phase and aqueous species on the surfaces of the ocean and marine aerosol. The mechanisms of interfacial halogen radical/halide reactions, however, are not fully understood, partly due to the dearth of techniques for in situ monitoring of the products and intermediates of fast interfacial halogen radical reactions. Here, we report the online electrospray mass spectrometric identification of the species produced on the surface of aqueous Br^– and I^– microjets collided by I^•(g) pulses generated from the 266 nm laser photolysis of CH₃I/O₂/N₂ gas mixtures. Mass-specific identification of intermediates and products in D₂O and H₂¹⁸O solutions and their dependences on I^•(g) fluxes let us outline mechanisms of formation. We found that the uptake of I^•(g) on the surface of Br^– and I^– microjets (effective uptake coefficient γ_eff ≥ 2 × 10^–4) yields IBr^•^–/I₂^•^– radical intermediates, which rapidly react with additional I^• to produce trihalides I₂Br^–/IBr₂^–/I₃^– plus I₃O_<i>n</i>^– (<i>n</i> = 1, 2) species within ∼10 μs. Our findings point to a new halogen activation pathway initiated by photogenerated I<sup>•