Aerosol particles present in the atmosphere influence climate through their interactions with light and their effect on cloud formation, and negatively impact air quality with significant health implications. In the atmosphere, aerosol particles can undergo transformations through an array of processes, including evaporation and condensation of semi-volatile compounds, oxidation initiated by heterogeneous reactions, and photochemistry initiated by solar illumination. The evolution of particle composition influences its physical and optical properties, which need to be understood to fully assess their role in the environment. To study the physical and optical properties of aerosol particles, a linear quadrupole electrodynamic balance (LQ-EDB) was developed and used to levitate single aerosol particles using electric fields. This method allows for contactless probing of particle properties in controlled conditions that mimic the natural environment. To probe the size and optical properties of the particle, a broadband light source was integrated with the LQ-EDB to illuminate the particle, giving rise to morphology dependent resonances (MDRs) that were sampled in a back-scattered geometry with a spectrometer. The wavelength of the MDR’s was used to determine the droplet size, refractive index, and wavelength-dispersion parameters via Mie theory. These methods were initially benchmarked in measurements of the evaporation rates of semi-volatile organic particles and the refractive index of aqueous salt particles. These measurements identified the facility of the methods in exploring the optical properties of aerosol that regulate their interactions with incoming solar radiation. In general, aerosol particles in the atmosphere have a cooling effect due to the scattering of incoming solar radiation. However, particles containing light absorbing species contribute to a warming effect. Using the LQ-EDB, we explored the physical and optical properties of aerosol particles containing light absorbing brown carbon (BrC) chromophores, such as those generated by wildfires and biomass burning. We report the optical properties, hygroscopic growth, and phase behavior as a function of relative humidity for a surrogate BrC particle containing 4-nitrocatechol and ammonium sulfate, revealing complex phase behavior that has implications for the optical properties and chemical reactivity of BrC aerosol. Subsequently, a new LQ-EDB was developed that incorporated heating elements to study the vapor pressures and influence of phase state on the rate of evaporation of a series of linear dicarboxylic acids from 293 to 350 K. These measurements provide clear evidence that phase state, either crystalline or amorphous, is a controlling factor in the rate of evaporation and contribute to our broader understanding of liquid-vapor partitioning in organic aerosol, organic solvents, fuel mixtures, and semi-volatile household products.

Aerosol particles are ubiquitous in the Earth’s atmosphere and play an important role in climate, air quality and health. The chemical composition of aerosol particles is complex and primarily associated with their source of origin. Once in the atmosphere, the composition is continuously evolving as a result of photochemical and heterogeneous chemical reactions. Consequently, composition-dependent properties that define the physical state and optical properties are also continuously changing, with important consequences for the dynamics and impacts of aerosols in the atmosphere. Laboratory-based methods have allowed an extensive exploration of the microphysical properties and chemical transformations of atmospheric aerosols. Single particle levitation methods are used to measure the microphysical properties of particles and connect with their evolving chemical composition. Properties such as size, hygroscopicity, phase morphology, and composition can be precisely measured by coupling levitation methods with analytical tools, such as mass spectrometry.

This dissertation describes the development of a new experimental platform connecting a linear quadrupole electrodynamic balance (LQ-EDB) with mass spectrometry (MS) that is capable of interrogating single levitated particles undergoing atmospherically relevant transformations. Two sampling methods, paper spray (PS) and an open port sampling interface (OPSI), were developed to transfer and ionize chemical analytes from levitated particles to the MS for compositional analysis. Laboratory studies were carried out to understand both non-reactive transformations and heterogeneous oxidation of levitated particles by precisely measuring their physical and optical properties as a function of evolving chemical composition. Non-reactive transformations include the particle-to-gas partitioning of semi-volatile species, while heterogeneous oxidation includes ozonolysis measurements on oleic and elaidic acid particles to understand the influence of particle phases on transformation kinetics and product distribution.

A key objective of this dissertation is to gain molecular-level understanding of how aerosol particles evolve due to chemical transformations and provide size-resolved detailed knowledge of their characteristics. It is essential to link the microphysical properties, composition, and reaction timescales for particles to quantify these evolving properties. Ultimately, the developments and measurements reported in this dissertation provide a foundation for exploring the chemistry of aerosol particles of complex composition and morphology to better understand their role in the environment.