Mechanistic insights into nitric acid-enhanced iodic acid particle nucleation in the upper troposphere and lower stratosphere

Abstract. In the upper troposphere and lower stratosphere (UTLS), new particles frequently form to seed cloud condensation nuclei (CCN), thereby affecting radiative forcing and global climate. Iodic acid (IA) particles have been widely detected in the UTLS; however, how they form is still largely unknown. Given the abundance of nitric acid (NA) and ammonia (NH₃) in the UTLS and their nucleation potential, we explore the influence of NA and NH₃ on IA nucleation by quantum chemical calculations and cluster dynamics simulations. The structural analysis indicates that NA and NH₃ can cluster with IA via hydrogen bonds, halogen bonds, and electrostatic attractions between ions. The small-sized IA–NA–NH₃ clusters have lower free energies than typical sulfuric acid (SA)–NA–NH₃ clusters in the upper troposphere, exhibiting greater stability and higher nucleation efficiency. Moreover, the NA-enhanced effect on the established efficient IA–NH₃ nucleation is more evident at lower temperatures, especially with richer NA and NH₃. In the extremely low-temperature UTLS, the proposed IA–NA–NH₃ ternary pathway dominates nucleation, while in the mid troposphere with higher temperatures, the role of NA is minor due to its rapid evaporation. These findings underscore the important role of NA in iodine particle formation in the UTLS, offering mechanistic insights into the missing secondary particle sources.