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

• Author / Creator

  Al-Jabiri, Mohamad H

• 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.

• Subjects / Keywords

  • Rotational Spectroscopy
  • Physical Chemistry
  • Aerosol Science

• Graduation date

  Fall 2024

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-3ghx-5515

• License

  This thesis is made available by the University of Alberta Library with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.