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Critical Role of Iodous Acid in Neutral Iodine Oxoacid Nucleation.

Authors :: Zhang R
Xie HB
Ma F
Chen J
Iyer S
Simon M
Heinritzi M
Shen J
Tham YJ
Kurtén T
Worsnop DR
Kirkby J
Curtius J
Sipilä M
Kulmala M
He XC
Source :: Environmental science & technology [Environ Sci Technol] 2022 Oct 04; Vol. 56 (19), pp. 14166-14177. Date of Electronic Publication: 2022 Sep 20.
Publication Year :: 2022
Abstract: Nucleation of neutral iodine particles has recently been found to involve both iodic acid (HIO <subscript>3</subscript> ) and iodous acid (HIO <subscript>2</subscript> ). However, the precise role of HIO <subscript>2</subscript> in iodine oxoacid nucleation remains unclear. Herein, we probe such a role by investigating the cluster formation mechanisms and kinetics of (HIO <subscript>3</subscript> ) <subscript> m </subscript> (HIO <subscript>2</subscript> ) <subscript> n </subscript> ( m = 0-4, n = 0-4) clusters with quantum chemical calculations and atmospheric cluster dynamics modeling. When compared with HIO <subscript>3</subscript> , we find that HIO <subscript>2</subscript> binds more strongly with HIO <subscript>3</subscript> and also more strongly with HIO <subscript>2</subscript> . After accounting for ambient vapor concentrations, the fastest nucleation rate is predicted for mixed HIO <subscript>3</subscript> -HIO <subscript>2</subscript> clusters rather than for pure HIO <subscript>3</subscript> or HIO <subscript>2</subscript> ones. Our calculations reveal that the strong binding results from HIO <subscript>2</subscript> exhibiting a base behavior (accepting a proton from HIO <subscript>3</subscript> ) and forming stronger halogen bonds. Moreover, the binding energies of (HIO <subscript>3</subscript> ) <subscript> m </subscript> (HIO <subscript>2</subscript> ) <subscript> n </subscript> 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 <subscript>2</subscript> could facilitate the nucleation of other acids beyond HIO <subscript>3</subscript> in regions where base vapors such as ammonia or amines are scarce.