Ionization of small atmospheric acid–base clusters and its prospective role in seeding the growth of aqueous clusters

In the present study, we have examined the ionization of a set of small atmospheric acid–base clusters. The calculated ionization energy (IE) ranges from ∼660 kJ mol⁻¹ to ∼1120 kJ mol⁻¹, with clusters consisting of just the bases yielding the smallest IEs, while those of the acids yield the largest. For neutral clusters that form most readily, the IEs range from ∼790 kJ mol⁻¹ to ∼880 kJ mol⁻¹. These values are generally smaller than those of other chemicals with significant atmospheric presence. Thus, the radical cations of the acid–base clusters are not likely to ionize other atmospheric species. Charged atmospheric species have been hypothesized to facilitate the growth of large clusters. We have carried out a case study of the sequential hydration of an acid–base cluster and compared our results with previous studies of forming pure water clusters. The neutral acid–base cluster binds water more strongly than pure water clusters. Further strengthening of the binding can be seen for the radical cationic acid–base cluster. The resulting acid–base–water clusters may further represent a source of H₃O⁺, which may proceed to facilitate cloud formation.