Atmospheric Bases-Enhanced Iodic Acid Nucleation: Altitude-Dependent Characteristics and Molecular Mechanisms

Iodic acid (IA), the key driver of marine aerosols, is widely detected within the gas and particle phases in the marine boundary layer (MBL) and even the free troposphere (FT). Although atmospheric bases like dimethylamine (DMA) and ammonia (NH₃) can enhance IA particles formation, their different efficiencies and spatial distributions make the dominant base-stabilization mechanisms of forming IA particles unclear. Herein, we investigated the IA-DMA-NH₃ nucleation system through quantum chemical calculations at the DLPNO-CCSD(T)/aug-cc-pVTZ(-PP)//ωB97X-D/6-311++G(3df,3pd) + aug-cc-pVTZ-PP level of theory and cluster dynamics simulations. We provide molecular-level evidence that DMA and NH₃ can jointly stabilize the IA clusters. The formation rates of IA clusters initially decline before rising from the MBL to the FT, owing to variations in mechanism. In the MBL, IA-DMA nucleation predominates, while the contribution of IA-DMA-NH₃ synergistic nucleation cannot be overlooked in polar and NH₃-polluted regions. In the lower FT, IA-DMA-NH₃ nucleation prevails, whereas in the upper FT, IA-NH₃ nucleation dominates. The efficiency of IA-DMA-NH₃ nucleation is comparable to that of IA-iodous acid nucleation in the MBL and sulfuric acid-NH₃ nucleation in the FT. Hence, the IA-DMA-NH₃ mechanism holds promise for revealing the missing sources of tropospheric IA particles.