Formation Pathways and Composition of Iodine Oxide Ultra-Fine Particles

Abstract

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.