Molecular-Level Insights into Salt Particle Surfaces

Abstract

Salt particles are the main aerosol particles in the atmosphere by mass. They can be directly emitted from terrestrial and aquatic sources (primary salt particles) or formed in the atmosphere (secondary salt particles) and are often composed of complex chemical mixtures. Salt particles play a major role in cloud formation, as they facilitate cloud droplet formation by acting as efficient cloud condensation nuclei. In some cases, they also play a role in ice crystal formation by acting as ice nucleating particles. Through the formation of aqueous solutions, they actively contribute to both atmospheric interfacial chemistry (reactions at the solvated surface or gas-solution interface) and bulk-phase chemistry (reactions within the aqueous solution phase). Interfacial and bulk-phase chemistry can differ significantly. This is because surfaces, defined as the topmost atomic or molecular layers, exhibit different properties than bulk phases, such as distinct chemical composition, strong local electric fields and partial solvation of solutes. These properties modify both the thermodynamics and kinetics of chemical reactions, resulting in accelerated and/or inhibited but nevertheless unique chemical pathways. Solvated surface environments are not restricted to aqueous solutions. Solid soluble materials, like salts, begin solvating at relative humidities well below their deliquescence points. Additionally, during the freezing of aqueous salt solutions, salts are preferentially segregated to surfaces, forming salt-rich environments on ice surfaces. However, the reactivity of solvated interfaces remains largely unexplored, as does the influence of specific salts. To assess this reactivity, two key parameters are required: surface chemical composition and surface solvation state. In the atmosphere, where environmental conditions vary widely, both composition and solvation are continuously evolving, yet the molecular-level processes underlying their evolution remain poorly understood. This thesis summarizes studies attempting to elucidate the molecular-level processes governing surface composition, surface solvation, and the associated unique surface reactivity. The investigations have utilized a series of experimental tools, including surface-sensitive techniques, as well as complementary simulations. Both pure and natural salt samples collected from the Arctic Ocean and East Asian deserts were used as the basis for fundamental investigations. Surface chemical composition and solvation of solid salts were evaluated under gas-phase conditions relevant to the atmosphere. The effects of surfactants on freezing efficiency and onset location in immersion mode (surface vs bulk) were investigated to understand factors driving ice crystal freezing morphology. Finally, spontaneous redox chemistry on the surfaces of pure and natural solid salts was observed, with complementary simulations revealing the potential underlying mechanisms. Together, these fundamental investigations shed light on the important role salt particle surfaces can play in atmospheric chemical transformations.