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.