| Throughout the troposphere and stratosphere, air is continuously ionized by the deposition of energetic galactic cosmic radiation. The initially small ions rapidly evolve into long-lived larger charged molecular aggregates (∼1000 per cc) by attracting a variety of mainly polar species. Masses and compositions of air ions have been broadly characterized through numerous in situ observations. Nevertheless, fundamental knowledge of their characteristics at the molecular scale remains sketchy. Consequently, their role in atmospheric processes is not adequately understood. E.g., variations in galactic cosmic ray fluxes have been correlated with the extent of cloudiness in the lower atmosphere. Ionic clusters may participate since they have been shown to act as efficient sites for vapor condensation and particle nucleation. Charged aggregates also have the potential to affect the rates of heterogeneous chemical reactions in air, and the microphysical properties of the atmospheric aerosol. The work presented here seeks to answer these questions by defining the thermodynamic and kinetic properties of ion clusters in order to predict their evolution for a wide range of environmental conditions. This goal has been pursued via the development of a hybrid approach which synthesizes the thermochemistry information from available laboratory data, quantum mechanical simulations and the macroscopic classical liquid drop model. The methodology has been applied to investigate systematically the thermodynamic properties of hydronium ion-water-nitric acid and nitrate ion-water/nitric acid clusters, leading to the most extensive database for these species to date. Among other results, we have identified the role of cooperative hydrogen bonding in the dissociation of nitric acid within water clusters exceeding specific threshold sizes. The hybrid approach has been generalized to treat other environmentally relevant ion species, including the protonated acetone-water-sulfuric acid system, of which we present the first consistent thermodynamic description. We demonstrate the broader use of hybrid data synthesis in generating fundamental data to study the growth of charged clusters in the atmosphere; and their role in basic chemical and physical processes. This work points to numerous practical applications, including the nucleation of aerosols, phase changes in clouds, surface-catalyzed chemical reactions, and molecular adsorption and dissociation processes at the air-water interface. |
