Ionic clusters in the polar winter stratosphere

A new hybrid model describing the formation of molecular clusters around naturally occurring ions has been applied to study the conditions found in the lower polar stratosphere in the winter season. The key ion species expected under these conditions include positive hydronium water clusters, and aggregates of water and nitric acid on nitrate anions. Our approach combines basic thermodynamic data with the classical Thomson representation of a charged liquid sphere to derive enthalpies and entropies for cluster formation over the entire mass spectrum from the smallest clusters to nanometer-size droplets. We find that, at the lowest temperatures achieved in the polar regions, water and nitric acid ions can readily accumulate 10 or more ligands. The size distributions of several ion families are calculated, uncertainties in these distributions are discussed, and the rates of interaction of cluster ions with aerosols and reactive vapors are estimated.