From the Pre-critical Nucleus to Atmospheric Aerosol Particles: Insights from Rotational  Spectroscopic, Computational, and Aerosol Characterization Studies
Creators: Al-Jabiri, Mohamad H
Publisher: name: University of Alberta Library
Year: 2024

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