Methanesulfonic acid and iodous acid nucleation: a novel mechanism for marine aerosols

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.