The direct and indirect interactions of aerosols with solar and terrestrial radiation represent the most significant uncertainty in our understanding of Earth’s climate. Marine aerosols, comprised of freshly emitted or nascent sea spray aerosols (SSA), aged SSA, and secondary marine aerosol (SMA), significantly impact the global radiative budget, cloud and fog formation, and visibility and air quality in coastal marine environments. Biological processes in seawater impact marine aerosol chemical and physical properties. An important but understudied property of marine aerosol is its phase state or viscosity. Viscosity significantly influences gas-to-particle partitioning, liquid and ice cloud nucleation rates, and heterogeneous reaction kinetics, which can affect aerosol chemical composition, wet deposition rates, size, and thus aerosol impacts on climate and air quality. This work investigates the influence of biological activity, particle size, and oxidative aging processes on marine aerosol's molecular composition and phase states. Empirical observations of particle phase state are made through online particle bounce/rebound measurements and offline with atomic force microscopy. Findings indicate nascent SSA particles exhibit increasingly viscous or solid-like characteristics with greater seawater phytoplankton activity and decreasing particle size. SSA aged by OH radical is more viscous than nascent SSA. Increasing organic carbon content and chemical species with higher molecular weights were enriched in SSA during elevated phytoplankton growth in seawater and smaller SSA. We attribute the higher molecular weight and more viscous organic components in SSA to the production of colloidal or gel-like SSA formed through the complexation of polyanionic organic components with divalent cations, specifically Ca2+. In polluted air with high concentrations of marine volatile organic compounds and oxidants equivalent to a week of chemical aging in the atmosphere, SSA becomes more viscous, attributed to the functionalization of particulate organic carbon. Further aging up to the equivalent of weeks in the atmosphere leads to less viscous phase states, attributed to the volatilization and, thus, decreasing molecular weight of the SSA organic carbon. These results have the potential for a better understanding of processes such as ice nucleation and airborne transport of chemical pollutants and toxins in SSA in coastal environments, which are strongly influenced by viscosity.

Understanding the interactions of excitons has been central to the development of technolo-gies such as solar cells and light-emitting diodes. To improve quantum yields and design materials with desired optoelectronic properties, researchers require an understanding of how multiple electron-hole pairs interact. From a fundamental perspective, excitons display rich phase behavior; as composite bosons, they can support both Bose-Einstein condensation as well as quantum Fermi liquids such as Keldysh’s electron-hole liquid. Additionally, since they are analogous to hydrogen atoms, excitons can form molecules in the form of biexcitons and ions in the form of trions. Recent studies on the interactions and phase behavior of excitons have considered indi- rect excitons whose constituents are constrained to reside in distinct planes separated by a distance d. By separating the electrons from the holes, both radiative and non-radiative recombination lifetimes are extended by orders of magnitude, making it easier to study and probe their equilibrium phase behavior. Additionally, indirect excitons possess permanent electric dipole moments, introducing a repulsion between particles. This interaction inhibits the formation of biexcitons and provides more favorable conditions for Bose-Einstein con- densation. Furthermore, there is experimental and theoretical evidence for new emergent phases not previously observed when considering spatially-direct excitons, such as a classical liquid of individual excitons. In this dissertation, we use a broad range of theoretical techniques to study the phase be- havior of indirect excitons. First, we examine the interaction between two indirect excitons using diffusion Monte Carlo, finding long-range dipole-dipole repulsion in addition to short- range attraction for relatively small d. In order to study the effects of introducing a third exciton, we construct basis sets for indirect excitons and apply the full configuration inter- action method, typically used for three-dimensional electronic systems. For very small d, 2 the lowest energy state of the triexciton is a weakly-bound exciton-biexciton van der Waals complex. Because the three-body potential is so weak and not pair-wise decomposable, we find no evidence for a driving force towards the classical condensation of indirect excitons. In order to explain recent observations of a condensed phase of indirect excitons, we consider Keldysh’s electron-hole liquid in a bilayer geometry. Working within the random-phase approximation, we calculate the ground state energy of a dense degenerate phase of unbound carriers, finding good agreement with experimental measurements. Taking the Green’s- function approach to finite temperatures, we evaluate free energies for electrons and holes in a bilayer geometry using the linked-cluster expansion. By solving the law of mass action, we investigate how the exciton Mott transition from bound excitons to free carriers varies with d. Finally, using Maxwell equal-area constructions, we map the quantum liquid-gas phase diagram in the plane of temperature and total carrier density. These diagrams are in close agreement with the aforementioned experimental observations.