Critical Role of Iodous Acid in Neutral Iodine Oxoacid Nucleation

Abstract

<p>Nucleation of neutral iodine particles has recently been found to involve both iodic acid (HIO<sub>3</sub>) and iodous acid (HIO<sub>2</sub>). However, the precise role of HIO<sub>2</sub>in iodine oxoacid nucleation remains unclear. Herein, we probe such a role by investigating the cluster formation mechanisms and kinetics of (HIO<sub>3</sub>)<sub>m</sub>(HIO<sub>2</sub>)<sub>n</sub>(m = 0-4, n = 0-4) clusters with quantum chemical calculations and atmospheric cluster dynamics modeling. When compared with HIO<sub>3</sub>, we find that HIO<sub>2</sub>binds more strongly with HIO<sub>3</sub>and also more strongly with HIO<sub>2</sub>. After accounting for ambient vapor concentrations, the fastest nucleation rate is predicted for mixed HIO<sub>3</sub>-HIO<sub>2</sub>clusters rather than for pure HIO<sub>3</sub>or HIO<sub>2</sub>ones. Our calculations reveal that the strong binding results from HIO<sub>2</sub>exhibiting a base behavior (accepting a proton from HIO<sub>3</sub>) and forming stronger halogen bonds. Moreover, the binding energies of (HIO<sub>3</sub>)<sub>m</sub>(HIO<sub>2</sub>)<sub>n</sub>clusters show a far more tolerant choice of growth paths when compared with the strict stoichiometry required for sulfuric acid-base nucleation. Our predicted cluster formation rates and dimer concentrations are acceptably consistent with those measured by the Cosmic Leaving Outdoor Droplets (CLOUD) experiment. This study suggests that HIO<sub>2</sub>could facilitate the nucleation of other acids beyond HIO<sub>3</sub>in regions where base vapors such as ammonia or amines are scarce.</p>