Unexpectedly significant stabilizing mechanism of iodous acid on iodic acid nucleation under different atmospheric conditions

Iodous acid (HIO₂) has been shown to play a stabilizing role in the nucleation of iodic acid (HIO₃) (He et al., 2021). However, the stabilization effect and specific stabilizing mechanism of HIO₂ on HIO₃ nucleation under different atmospheric conditions remain unclear. Therefore, we studied these two issues under different temperatures and nucleation precursor concentrations using density functional theory combined with the Atmospheric Cluster Dynamics Code. We found that HIO₂ can form clusters with HIO₃ via strong hydrogen bonds, halogen bonds, and proton-transfer, substantially enhancing the stability of HIO₃ clusters and decreasing the energy barrier of HIO₃-based cluster formation at different temperatures and nucleation precursor concentrations. The particle formation rate and cluster concentrations of HIO₃–HIO₂ nucleation were negatively correlated with temperature and positively correlated with HIO₂ concentration. The enhancements by HIO₂ on the particle formation rate and cluster concentration of HIO₃ nucleation were positively correlated with temperature and HIO₂ concentration. Interestingly, even at a low HIO₂ concentration (1.0 × 10⁵ molecules cm⁻³), the enhancement on the particle formation rate and cluster concentration of HIO₃ nucleation by HIO₂ were both unexpectedly up to 4.1 × 10⁴-fold at 283 K. Therefore, HIO₃–HIO₂ nucleation can be extremely rapid in cold regions, and the enhancement by HIO₂ can be significant, especially in warm regions even at relatively high HIO₂ concentrations.