Quaternary Nucleation of Iodine and Sulfur Oxoacids in the Marine Atmosphere: Unexpected Role of Methanesulfonic Acid

Sulfuric acid (SA), methanesulfonic acid (MSA), iodic acid (HIO₃), and iodous acid (HIO₂) are identified as key nucleation precursors and can coexist in the marine atmosphere. Here, we investigated the potential SA-MSA-HIO₃-HIO₂ quaternary nucleation mechanism by exploring the formation of (SA)_w(MSA)_x(HIO₃)_y(HIO₂)_z (0 ≤ w + x + y ≤ 3, 1 ≤ z ≤ 3) clusters with quantum chemical calculation and kinetics modelling. The results indicate that SA-MSA-HIO₃-HIO₂ can effectively nucleate under marine atmospheric conditions. The nucleation rate is up to 7 orders of magnitude higher than that of SA/MSA-HIO₃-HIO₂, SA-MSA-HIO₂ ternary mechanisms, and SA/MSA/HIO₃-HIO₂ binary mechanisms at some specific conditions. The nucleation is mainly driven by acid-base reaction (HIO₂ as base) and halogen bonds besides hydrogen bonds, with the three acids showing both competitive and cooperative roles. More importantly, it was found that the contribution of MSA to the aerosol nucleation is comparable to SA at equal concentrations. The unexpectedly high contribution of MSA is attributed to its higher halogen-bonding capacity than SA. This study highlights the need to consider the multicomponent nucleation mechanism in the marine atmosphere for accurate aerosol and climate projections, and may serve as important proof that MSA as weak acid can effectively nucleate even coexisting with SA.