Molecular-level study on the role of methanesulfonic acid in iodine oxoacids nucleation

Abstract. Iodic acid (HIO₃) and iodous acid (HIO₂) have been identified to nucleate effectively by the Cosmics Leaving OUtdoor Droplets (CLOUD) experiment at CERN (He et al., 2021, Science), yet it may be hard to explain all HIO₃-induced nucleation. Given the complexity of marine atmosphere, other precursors may be involved. Methanesulfonic acid (MSA), as a widespread precursor over oceans, has been proven to play a vital role in facilitating nucleation. However, its kinetic impacts on synergistic nucleation of iodine oxoacids remain unclear. Hence, we investigated the MSA-involved HIO₃-HIO₂ nucleation process at the molecular level using density functional theory (DFT) and Atmospheric Clusters Dynamic Code (ACDC). The results show that MSA can form stable molecular clusters with HIO₃ and HIO₂ jointly via hydrogen and halogen bonds, as well as electrostatic attraction after proton transfer to HIO₂. Thermodynamically, the MSA-involved clustering can occur nearly without free-energy barrier, following HIO₂-MSA binary and HIO₃-HIO₂-MSA ternary pathway. Furthermore, adding MSA significantly enhance the rate of HIO₃-HIO₂-based cluster formation, even up to 10⁴-fold at cold marine regions with rich MSA and scarce iodine, such as polar Ny-Ålesund and Marambio. Thus, the proposed more efficient HIO₃-HIO₂-MSA nucleation mechanism may provide theoretical evidence for explaining the frequent and intensive burst of marine iodine particles.