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 (NH 3 ) 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-NH 3 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 NH 3 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-NH 3 synergistic nucleation cannot be overlooked in polar and NH 3 -polluted regions. In the lower FT, IA-DMA-NH 3 nucleation prevails, whereas in the upper FT, IA-NH 3 nucleation dominates. The efficiency of IA-DMA-NH 3 nucleation is comparable to that of IA-iodous acid nucleation in the MBL and sulfuric acid-NH 3 nucleation in the FT. Hence, the IA-DMA-NH 3 mechanism holds promise for revealing the missing sources of tropospheric IA particles.