Critical Role of Iodous Acid in Neutral Iodine Oxoacid Nucleation.

Nucleation of neutral iodine particles has recently been found to involve both iodic acid (HIO 3 ) and iodous acid (HIO 2 ). However, the precise role of HIO 2 in iodine oxoacid nucleation remains unclear. Herein, we probe such a role by investigating the cluster formation mechanisms and kinetics of (HIO 3 ) m (HIO 2 ) n ( m = 0-4, n = 0-4) clusters with quantum chemical calculations and atmospheric cluster dynamics modeling. When compared with HIO 3 , we find that HIO 2 binds more strongly with HIO 3 and also more strongly with HIO 2 . After accounting for ambient vapor concentrations, the fastest nucleation rate is predicted for mixed HIO 3 -HIO 2 clusters rather than for pure HIO 3 or HIO 2 ones. Our calculations reveal that the strong binding results from HIO 2 exhibiting a base behavior (accepting a proton from HIO 3 ) and forming stronger halogen bonds. Moreover, the binding energies of (HIO 3 ) m (HIO 2 ) n 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 2 could facilitate the nucleation of other acids beyond HIO 3 in regions where base vapors such as ammonia or amines are scarce.