ABSTRACT
By seeding clouds, new particle formation (NPF) has a substantial impact on radiation balance, bio-geochemical cycles and global climate. Over oceans, both methanesulfonic acid (CH3S(O)2OH, MSA) and iodous acid (HIO2) have been reported to be closely associated with NPF events; however, much less is known about whether they can jointly nucleate to form nanoclusters. Hence, quantum chemical calculations and Atmospheric Cluster Dynamics Code (ACDC) simulations were performed to investigate the novel mechanism of MSA-HIO2 binary nucleation. The results indicate that MSA and HIO2 can form stable clusters via multiple interactions including hydrogen bonds, halogen bonds, and electrostatic forces between ion pairs after proton transfer, which are more diverse than those in MSA-iodic acid (HIO3) and MSA-dimethylamine (DMA) clusters. Interestingly, HIO2 can be protonated by MSA exhibiting base-like behavior, but it differs from base nucleation precursors by self-nucleation rather than solely binding to MSA. Due to the greater stability of MSA-HIO2 clusters, the formation rate of MSA-HIO2 clusters can be even higher than that of MSA-DMA clusters, suggesting that MSA-HIO2 nucleation is a non-negligible source of marine NPF. This work proposes a novel mechanism of MSA-HIO2 binary nucleation for marine aerosols and provides deeper insights into the distinctive nucleation characteristics of HIO2, which can help in constructing a more comprehensive sulfur- and iodine-bearing nucleation model for marine NPF.
ABSTRACT
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.
