Congeneric iodine oxoacids and iodine oxides, key nucleating vapours in the marine atmosphere, have been reported to nucleate individually. However, whether they can nucleate together remains unknown. Here, we provide molecular-level evidence that I2O4, the iodine oxide with the highest nucleation potential towards iodine oxoacids, can synergistically nucleate with HIO3-HIO2. The nucleation rate of HIO3-HIO2-I2O4 is 1.5 to 6.8 times higher than that of the known most efficient iodine-associated two-component (HIO3-HIO2) nucleation at 278.15 K, enhancing the role of iodine-containing species in marine atmospheric particle formation. Microscopic analysis of the three-component cluster configurations revealed that an unexpected acid‒base reaction between I2O4 and HIO2/HIO3 is a key driver of this efficient synergistic nucleation, besides hydrogen bonds and halogen bonds. We identified halogen bond-induced basicity enhancement as the chemical nature of I2O4 behaving as a base in the nucleation process with HIO2/HIO3. Such a basicity enhancement effect can be extended to other iodine-containing species, e.g., HIO2 and the more acidic HIO3, suggesting a common feature in interactions between iodine-containing species. Our findings clarify the synergistic nucleation of iodine oxoacids and iodine oxides and highlight the necessity of considering the effect of halogen bond-induced basicity enhancement on the formation of iodine-containing particles.
