Atomic and Molecular Ions in Atmospheric Chemistry: Interfacial Reactivity, Emerging Mechanisms, and Future Perspectives

Abstract Atomic and molecular ions (AIs and MIs) play distinct yet complementary roles in atmospheric chemistry, influencing aerosol reactivity, cloud formation, and climate. AIs, with high charge density and structural simplicity, generate intense interfacial fields that accelerate reactions such as halogen activation and sulfate formation. In contrast, MIs, with complex bonding and hydration, enable redox transformations, proton transfer, and molecular rearrangement, particularly at interfaces where reactivity deviates from bulk behavior. Specific MIs such as ammonium and sulfate can also generate strong interfacial fields, showing this behavior is not exclusive to AIs. We distinguish between the local field near individual ions or pairs and the interfacial field, the mesoscale field, from organized ion–water structures at surfaces. Traditionally treated as one class, new evidence highlights fundamental mechanistic differences between AIs and MIs in interfacial dynamics, ion pairing, and ability to structure fields. This study introduces the AIMI framework, which explicitly differentiates between atomic and molecular ion‐driven processes and emphasizes the need for atomic‐level insight into interfacial reactivity. Advances in APXPS, SFG, THz spectroscopy, and cryogenic ion‐trapping now resolve ion stratification, hydration asymmetry, and field‐driven reactivity at unprecedented resolution. By re‐evaluating transformations through the lens of ion‐specific structure and behavior, the AIMI approach reveals new mechanistic pathways and improves predictive capability for multiphase reactions. Integrating atomic‐ and molecular‐level perspectives is critical for resolving persistent challenges in atmospheric modeling, including unexplained reactive species, SOA variability, and ice nucleation efficiency.