Cloud and aerosol formation in the atmosphere are initiated by heterogeneous nucleation and condensation processes on foreign particles. Cloud droplets and aerosols thus formed are subject to further growth by coalescence. Foreign particles in the atmosphere enhance nucleation with varying degrees of efficiency. An understanding of this heterogeneous nucleation requires basic (microphysical and chemical) knowledge, e.g., about the interactions between vapor phase molecules and the surface of the foreign particle, surface diffusion, and gas-to-particle conversion. Heterogeneous nucleation in the atmosphere encompasses numerous particulate systems involving ions, organic and inorganic species, bacteria, cloud condensation nuclei, and other pre- existing aerosols. There is a paucity of basic understanding of the mechanisms leading to gas-to-particle conversion in the atmosphere, particularly heterogeneous nucleation mechanisms. The main objective of proposed research is to study the embryonic stage. While classical approaches use bulk (macroscopic) parameters for the microphysical phenomena of heterogeneous cluster formation and growth, this work will us an approach based on molecular level descriptions. The specific goals of the proposed research are: 1) to find micro- physical and micro-chemical means of computing heterogeneous nucleation rates involving various foreign particles (e.g., ions, cloud condensation nuclei and organic particles), and differences in nucleation rates between different foreign particle species; 2) to reveal microphysical and chemical properties of heterogeneous clusters, in particular their stability, interaction, and energetics at varying temperature and humidity. This work will benefit cloud modeling for meteorology, lead to better understanding of cloud and aerosol formation processes as well as those of smog, dust, and haze, and stimulate experiments associated with droplet formation, sticking coefficients, and free energy changes.
