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.