Conversion of Iodide to Hypoiodous Acid and Molecular Iodine at the Air-Water Interface

Sea spray aerosols continuously transfer a significant amount of halides to the marine boundary layer, where they play a major role in the depletion of tropospheric ozone. The reactivity of iodide is of special interest in sea spray aerosols, where this species is enriched relative to chloride and bromide in surface seawater. This work presents laboratory experiments that provide mechanistic information to understand the reactivity of halides in atmospheric aerosols. Pneumatically assisted electrospray is used to aerosolize solutions of sodium iodide (0.01-100 μM), which are rapidly (~3 μs) oxidized by ozone at 25 °C. Reaction products include HIO, IO₂^-, IO₃^-, I₂, HI₂O^-, and I₃^-, all identified by mass spectrometry. The distribution of products varies along two different reaction pathways, one favoring the production of I₂ and HIO for typical tropospheric ozone levels (~50 ppbv), and another one directed to the production of IO₃^- at higher oxidizer concentrations. The formation of products increases exponentially with rising concentrations of initial sodium iodide, [NaI]₀. The process is determined to be pH independent for the pH range 6-8 representative of surface waters. The substitution of aqueous solutions by organic solvents, such as methanol or acetonitrile, causes a decrease in the surface tension and lifetime of the droplets, leading to larger I₂ production. The presence of surface active organic compounds, which alter the structure of the interfacial region, promote the pathway of I₂ formation over IO₃^-. In conclusion, this presentation will show how the oxidation of iodide in aqueous microdroplets can release reactive gas-phase species, such as I₂ and HIO, capable to affect tropospheric ozone globally. Normalized intensity of products observed during the ozonolysis of iodide solutions at 130 ppbv ozone. Cone voltage = 70 V, needle voltage = 2.5 kV.