Theoretical Prediction of Ion Clusters Relevant to the Atmosphere: Size and Mobility

The clustering of water vapor about ions is important because of its relevance to atmospheric electrical processes. For this reason, the authors emphasize particularly the description of the size distribution (concentrations) and mobilities of the small ion clusters at various humidities. From present theoretical study, it is found that most of the hydronium ions H sub(3) O super(+) tend to associate with a small number of water molecules to form a hydrated ion cluster, even at extremely low humidities, in the range of 5 x 10 super(-) super(3) to 1%. At atmospherically more realistic humidities and at the room temperature, the computed number of water molecules in the hydrated ion clusters is predicted to be relatively small. It is then conjectured that ion-induced nucleation process (if it occurs) begins rather from the small hydrated ion clusters that initially existed even at extremely low humidities in the atmosphere. In addition, it is found that, in general, the hydrated ion clusters of small sizes corresponding to the mass range of 2-5 water molecules are responsible for the ion mobility range of 2-2.5 cm super(-) super(2) (V sec) super(-) super(1) . For reduced mobility below 2.0 cm super(2) (V sec) super(-) super(1) , the mass of the hydrated ion cluster is predicted to be greater than that of approximately five water molecules. The simultaneous estimation of size distribution and mobility aids in better understanding of the observed mobility spectra and the nature of atmospherically important prenucleation clusters, including the information of their electric conductivities in the atmosphere.