From the Pre-critical Nucleus to Atmospheric Aerosol Particles: Insights from Rotational Spectroscopic, Computational, and Aerosol Characterization Studies

Abstract

Over the past several years in Alberta, the frequency and severity of wildfires has increased and resulted in the loss of lives, destruction of housing, and devastation of large forest areas. A byproduct of these fires are toxic emissions that contain primary aerosols and volatile organic compounds that can lead to the formation of secondary organic aerosols. Atmospheric aerosols are colloidal suspensions of liquid or solid particles in the atmosphere, which can have severe impacts on both human health and climate. The process of secondary organic aerosol formation involves the initial release of volatile organic compounds and their subsequent oxidation by atmospheric species, such as, for example, OH radical, to produce more oxygen-rich compounds of lower volatility. These semi-volatile compounds can the agglomerate, i.e., partition from the gas to the particle phase, and form aerosol particles. General aspects of this nucleation process, such as the participation of organic acids, water, sulfuric acid, and ammonia, the formation of a critical nucleus after which particle growth is spontaneous, and the involvement of hydrogenbonding and dispersion interactions, are known. Specific information about the formation of the initial clusters and complexes, such as the effect of conformational flexibility on the strength and the number of intermolecular interactions, is still not complete. To study the early stages of nucleation of semi-volatile organic compounds with water or themselves, I utilize jet-cooled Fourier transform microwave spectroscopy and quantum chemical calculations. This combination allows for a detailed analysis of the intra- and intermolecular noncovalent interactions that stabilize specific conformers. My studies have shown that the cis-conformation of the carboxylic acid in para-aminobenzoic acid, paranitrobenzoic acid, para-chlorobenzoic acid, para-hydroxybenzoic acid, and vanillic acid is the lowest energy conformer. Additionally, I found that functional groups in the para-position of benzoic acid can affect the structure of the acid group, depending on their ability to donate or withdraw electron density. ortho-functional groups may play an indirect role in cluster formation through intramolecular interactions that limit their ability to act as inter-molecular hydrogen bond donor and acceptor, as demonstrated for the case of vanillic acid monohydrate. As a proxy for large semi-volatile organic compounds, benzyl benzoate, an ester with terminal benzyl and phenol groups, was studied. In this molecule, CH···O hydrogen bonding plays a significant role in determining its structural preference in the monomer, while in the dimer, π-π stacking structural motifs are central. Polyvinylchloride, PVC, is an important building material and co-combusts with biomass, such as wood, in house fires. PVC combustion leads to emission of hydrogen chloride and atmospheric aerosol particles, a mixture that becomes more complex in biomass cocombustion scenarios. To gain a better understanding of the influence of PVC on emissions from wildfires, I conducted laboratory based PVC pyrolysis experiments. The resulting hydrogen chloride emissions, aerosol particle size distribution, and aerosol particle optical properties were studied using a range of aerosol characterization instruments and analytical tools. A 61% mass loss was observed after 40 minutes of PVC pyrolysis at 350°C, with an average of 25% of those emissions being gaseous hydrogen chloride. The emitted aerosol particles are predominantly composed of organic compounds that form concentration-dependent agglomerates. These particles have a maximum in the size distribution of approximately 150 nm, falling within the accumulation mode of biomass burning-derived aerosol particles. Additionally, I found that the aerosol particles emitted from PVC pyrolysis have a wavelength-dependent absorption with an absorption Ångstrom coefficient (AAC) between 5.8 and 6.1. That means that these particles that contain semi-volatile organic compounds preferentially absorb at lower wavelengths compared to soot particles.