Critical Role of Iodous Acid in Neutral Iodine Oxoacid Nucleation

[Image: see text] 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.