Formation Pathways and Composition of Iodine Oxide Ultra-Fine Particles

Environmental Context. Bursts of ultra-fine particles (diameter < 10 nm) in the daytime coastal marine boundary layer at low tide coincide with the observation of iodine oxide radicals. The detection of iodine in the particles suggests a direct link between the biogenic emission of iodine-containing vapours and subsequent particle nucleation and growth. These coastal aerosols are therefore most likely iodine oxide polymers. However, the reaction pathways leading to the homogeneous nucleation of these particles are currently an area of uncertainty, as is their final composition. These ultra-fine particles are potentially important as a source of cloud condensation nuclei, and as a major pathway for enriching iodine in marine aerosol.

Abstract. Iodine oxide nanoparticles were generated photochemically from I₂ in the presence of O₃, and their morphology and composition analyzed by transmission electron microscopy (TEM). The particles exhibit fractal morphologies consistent with agglomerative coagulation, and have an O/I ratio of 2.45 ± 0.08, indicating that they are composed of I₂O₅. Quantum calculations show that gas-phase I₂O₅ could be formed by a series of exothermic reactions involving the oxidation of I₂O₂, I₂O₃ and I₂O₄ by O₃. In order to form pure I₂O₅ particles, modelling calculations indicate that the rate coefficients for these reactions probably need to be faster than 6 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ at 295 K. Applying this model to the atmosphere shows that ultra-fine iodine oxide particles formed in the coastal marine boundary layer would then consist of I₂O₅.