Investigating the Early Phases of Aerosol Particle Nucleation: A Broadband Rotational Spectroscopic and Computational Study

Abstract

Atmospheric aerosols consist of solid or liquid particles suspended in air with sizes ranging from a few nanometers to several micrometers. Aerosols have the ability to both scatter and absorb incoming solar radiation, directly influencing Earth’s energy budget and resulting in either cooling or warming effects on our climate. Aerosol formation consists of two phases: nucleation and growth. In the nucleation phase, gaseous molecular species aggregate to form molecular clusters that contain only a few molecules. These clusters are held together by non-covalent interactions such as van der Waals forces and hydrogen bonds. However, these small molecular clusters, and in general, the early stages of nucleation remain poorly understood due to the high degree of compositional and structural variability and the complex network of non-covalent interactions present within the clusters. As a result, these molecular clusters in the early phases of nucleation may adopt many different conformations making it challenging to elucidate their structure. To study the initial stages of nucleation a molecular beam expansion is used to produce and stabilize the clusters. Broadband rotational spectra of the species are recorded using a chirped pulse Fourier transform microwave spectrometer, and several computational techniques are then used to help in the spectroscopic assignments and further characterize the molecular species. In this thesis, I describe my work studying the conformational landscapes of molecules and molecular clusters found in the early phases of nucleation and the non-covalent interactions within them. It is quite challenging to create and analyze molecular complexes containing multiple molecular species simultaneously. To simplify this problem, I focus on oxygenated hydrocarbon monomers and their aggregates with water, one of the most abundant molecular species in the atmosphere. Using a combination of experimental and theoretical techniques, I investigated different isomers of naphthol, an oxygenated polycyclic aromatic hydrocarbon, to gain new III insights into a controversial non-covalent interaction between two hydrogen atoms. The structural and thermodynamic data obtained from the monomer study was then used to investigate the 1-naphthol dimer, which revealed new insights into a complex intermolecular interplay between two molecules. Next, the conformational flexibility of (-)-carveol, a photo-oxidation product of a common biogenic volatile organic compound, limonene, was investigated. The results revealed a complicated conformational landscape which plays a significant role in the aggregation of (-)-carveol with atmospheric species such as water. The α-pinene-water complex was studied to investigate internal dynamics, such as water tunnelling and large amplitude motions, which occur in the molecular clusters found in the early stages of nucleation. Finally, to examine the early phases of aggregation, the 3-methylcatechol monomer and 3-methylcatechol hydrated with up to five water molecules were investigated. The study compared two different aggregation mechanisms and revealed a preference for dispersion assisted aggregation over the more common hydrogen bond dominated mechanism, or droplet aggregation. By studying the fundamental molecular properties of molecular clusters found in the early stages of nucleation, I hope to provide further insights into the initial steps of aerosol particle formation and ultimately bridge the gap between the molecular and macroscopic regimes.