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

Abstract. Iodic acid (HIO3) and iodous acid (HIO2) have been identified as nucleating effectively by the Cosmics Leaving Outdoor Droplets (CLOUD) experiment at CERN, 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 the 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 the Atmospheric Cluster Dynamics Code (ACDC). Our results show that MSA can form stable molecular clusters with HIO3 and HIO2 jointed via hydrogen bonds, halogen bonds, and electrostatic attraction after proton transfer to HIO2. Thermodynamically, the MSA-involved clustering can occur nearly without a free-energy barrier, following the HIO2–MSA binary and HIO3–HIO2–MSA ternary pathway. Furthermore, our results show that considering MSA will significantly enhance the calculated rate of HIO3–HIO2-based cluster formation, by up to 104-fold in cold marine regions containing rich MSA and scarce iodine, such as the polar regions Ny-Ålesund and Marambio. Thus, the proposed more efficient HIO3–HIO2–MSA nucleation mechanism may provide theoretical evidence for explaining the frequent and intensive bursts of marine iodine particles.