Spontaneous Production of I₂ at the Surface of Aqueous Iodide Solutions

Several groups have recently reported spontaneous production of atmospherically reactive species, including molecular iodine (I₂) at the air-water interface of droplets. In this study, glancing angle laser-induced fluorescence spectroscopy was used to track the luminol fluorescence at the surface of sodium iodide (NaI) and sodium chloride (NaCl) solutions. Although luminol fluorescence is hardly quenched by halide anions, even up to fairly high concentrations, it is effectively quenched by I₂. We observe luminol fluorescence quenching at the surface of NaI solutions but not at the surface of NaCl solutions, pointing to the formation of I₂ at the surface of NaI solutions. This provides further support for the proposal that the strong electric field or the reduction solvation present at the air-water interface can initiate spontaneous iodide activation and other chemistry there. The spontaneous production of I₂ at the surface of aqueous iodide solutions presents a previously unconsidered source of iodine in the atmosphere.