Single-Molecule Catalysis Revealed: Elucidating the Mechanistic Framework for the Formation and Growth of Atmospheric Iodine Oxide Aerosols in Gas-Phase and Aqueous Surface Environments.

Iodine oxide aerosols are ubiquitous in many coastal atmospheric environments. However, the exact mechanism responsible for their homogeneous nucleation and subsequent cluster growth remains to be fully established. Using quantum chemical calculations, we propose a new mechanistic framework for the formation and subsequent growth of iodine oxide aerosols, which takes advantage of noncovalent interactions between iodine oxides (I2 O5 and I2 O4 ) and iodine acids (HIO3 and HIO2 ). Larger iodine oxide clusters are suggested to be formed in a facile manner and with enhanced exothermicity. The newly proposed mechanisms follow both concerted and stepwise pathways. In all these new chemistries, an O:I ratio of 2-2.5 is predicted, which satisfies an experimentally derived criterion recently proposed for identifying iodine oxides involved in atmospheric aerosol formation. Born-Oppenheimer molecular dynamics simulations at the air-water interface suggest that I2 O5 and I4 O10 , which are two of the most common nucleating iodine oxides, react with interfacial water on the picosecond time scale and result in novel nucleating species such as H2 I2 O6 and HI4 O11 - or I3 O8 . An important implication of these simulation results is that aqueous surfaces, which are ubiquitous in the atmosphere, may activate iodine oxides to result in a new class of nucleating compounds, which can form mixed aerosol particles with potent precursors, such as HIO3 or H2 SO4 , in marine air masses via typical acid-based interactions. Overall, these results give a better understanding of iodine-rich aerosols in diverse environments.