Ionization of acids on the quasi-liquid layer of ice

The ice quasi-liquid layer (QLL) forms on ice surfaces below the bulk ice melting temperature. It is abundant in the atmosphere, and its importance for atmospheric chemistry is recognized. In the present work, we have studied the microscopic mechanisms of acid ionization on the QLL using ab initio molecular dynamics. The model system QLL is established by nanosecond time scale simulations with empirical force fields, while the reactivity of the QLL is studied using ab initio molecular dynamics. Our ab initio simulations reveal that QLL is reactive, exhibiting stable crystalline point defects, which contribute to efficient acid solvation, ionization, and proton transfer. We study in detail deuterated hydrogen iodide (DI) and nitric acid (DNO3). Ionization in both cases benefits from the abundance of weakly bonded hydrogen-bond single-acceptor double-donor water molecular species available on the QLL in high relative concentration. Picosecond time scale ionization is demonstrated for both molecular species. Our results suggest efficient reactivity of acid ionization and proton transfer at temperature ranges appropriate for the upper troposphere and lower stratosphere.