Studies of Intermolecular Interactions in Atmospheric Aggregates: From Molecular Clusters to Aerosols

• Author / Creator

  Schnitzler, Elijah G.

• Intermolecular interactions dictate the fate and impact of atmospheric oxygenated organic compounds that form from the photo-oxidation of hydrocarbons. Here, intermolecular interactions in aggregates involving photo-oxidation products of aromatic precursors are examined using an interdisciplinary approach. Molecular-scale aggregates – in particular, complexes of water and carboxylic acids – were investigated using rotational spectroscopy. The lowest-energy structures of the monohydrates of two aromatic carboxylic acids, benzoic acid and o-toluic acid, were determined experimentally; the water and acid moieties hydrogen bond to give six-membered intermolecular rings. The percentage of each acid hydrated in the gas phase is predicted to be low, but these interactions are important for aromatic acids at the air-water interface of aqueous particles. The two lowest-energy isomers of the oxalic acid–water complex, a probable nucleation precursor, were also observed. In the lowest-energy isomer, water bridges the two acid groups and lowers the barrier to decarboxylation. In the other isomer, water hydrogen bonds to only one group and increases the barrier. For larger dicarboxylic acids, the latter topology is predicted to be the most stable, so water-catalysed enhancement of overtone-induced decarboxylation is plausible for only oxalic acid. The spectra, structures, and internal dynamics of four methyl- and dimethylnaphthalenes, small polycyclic aromatic hydrocarbons present in the atmosphere, were also investigated. Nano-scale aerosols – in particular, internally-mixed particles of black carbon and secondary organic aerosol – were investigated using smog chamber experiments. The morphological evolution of black carbon due to coatings of secondary organic aerosol is the same for a series of four aromatic precursors. The evolution of black carbon is found to be dependent on the coating surface tension; higher surface tensions lead to more compact black carbon particles, as intermolecular interactions between molecules in the coating become stronger. Based on the observed surface tension-dependence, the surface tension of secondary organic aerosol derived from m-xylene was determined, providing an essential constraint on global climate models. Cumulatively, these investigations provide insights into the fate and impact of oxidation products in the gas phase, at the air-water interface, and in the bulk aerosol phase.

• Subjects / Keywords

  • Smog chamber experiments
  • Intermolecular interactions
  • Secondary organic aerosol
  • Hydrogen-bonded clusters
  • Carboxylic acid hydrates
  • Soot aggregates
  • Rotational spectroscopy

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R38S4JX4F

• License

  This thesis is made available by the University of Alberta Libraries 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.