UC San Diego

Aerosols are tiny particles suspended in the atmosphere, which can directly be generated by biogenic and anthropogenic processes or be formed through nucleation of gas-phase species. Aerosols influence the chemical composition of the atmosphere, and have both direct and indirect effects on the Earth's radiative balance. These processes have major implications for climate, ecosystems, and public health. In this dissertation, we first discuss computational studies aimed at providing fundamental insights into the molecular mechanisms of aerosol nucleation through the characterization of structural, thermodynamic, and spectroscopic properties of important nucleation species, with a particular focus on HCl(H₂O)n and HSO₄⁻(HO₂C(CH₂)₂CO₂H)n binary systems. We then describe the development and application of molecular models for characterizing proton transfer and transport in water and ice. In particular, we use computer simulations with our improved multistate empirical valence bond models to characterize the mechanisms responsible for proton mobility in ice Ih as well as on the surface of both ice Ih and amorphous ice. Based on our simulation results, we thus develop a unified picture of proton transfer and transport in and on ice. The last part of this dissertation focuses on the properties of sea spray particles, which represent one of the most important components of biogenic aerosols. Field measurements have demonstrated that sea-spray particles contain a large fraction of organic material, which correlates with the extent of biological activity at the surface microlayer of the ocean. In this context, we perform molecular dynamics simulations to characterize the properties of model sea- spray aerosol surfaces. Specifically, we study the phase behavior and structural properties of three Langmuir monolayers (palmitic acid, dipalmitoyl phosphatidic acid, and Lipid-A) at the air/water interface. Through a detailed analysis of the molecular dynamics trajectories, direct connections between order/disorder transitions of the Langmuir monolayers and water structure/dynamics are determined as a function of surface pressure and structural complexity of the monolayers. Our results provide key molecular-level insights into the physical behavior of organic material at aqueous interfaces which can help understand the reactivity and nucleation properties of sea-spray aerosols