Aqueous-Phase Photochemistry of Organic Acids in the Atmosphere

  • Author / Creator
    Lima Amorim, Jéssica
  • The atmospheric processing of biogenic volatile organic compounds (BVOCs) leading to the formation and evolution of secondary organic aerosols (SOAs) is highly complex, involving “multiphase” chemistry occurring in the gas, particle, and aqueous phases (e.g., cloudwater, fog, aerosol liquid water). Among the many compounds formed upon gas-phase oxidation of BVOCs, the water-soluble fraction of the products can partition to cloudwater or aerosol water and undergo aqueous-phase processing (i.e., chemical reactions). Compared to the gas-phase, the aqueous-phase processing of the water-soluble aerosol components is much less understood; this led to our interest in the fundamental understanding of the chemistry taking place within it. In atmospheric aqueous-phases, the effect of the acid−base chemistry and, by extension, aqueous-phase pH, has been a largely ignored factor. Chemical reactions are expected to be affected by cloudwater pH, given that the typical pH values vary between 2 and 7 and that atmospheric aqueous phases are becoming less acidic over the past few decades due to reduced emissions of major precursors of acid rain, i.e., of sulfur dioxide (SO2) and nitrogen oxides (NOx). The overall objective of this work was to understand the effect of solution pH on the kinetics and mechanisms of organic acids, which undergo acid-dissociation in pH ranges relevant to cloudwater. A fundamental component for the simulation of aqueous-phase reactions occurring in cloud and fog waters is the use of a photoreactor. The wavelength and intensity of UV lights used in the photoreactor were determined carefully by a combination of direct measurement and actinometry. Cis-pinonic acid (CPA), a major α-pinene oxidation product, was used as a model compound for the characterization of our photoreactor. Additionally, a kinetic model was developed to assess the OH chemistry within it. The characterized photoreactor was applied for the kinetics investigation of several organic acids under acidic and basic conditions; among which, the water-soluble fraction of α-pinene SOA was targeted. Using a combination of offline and online MS methods, we also investigated the oxidation mechanisms of pinic acid (PA) at pH 2 and pH 8, providing a novel insight into the oxidation products formed. Among those, major α-pinene SOA tracers, such as norpinic acid and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), were identified as primary oxidation products of PA. Overall, this work represents the first systematic investigation of the effect of cloud and fog water pH on the kinetics and mechanisms of atmospherically relevant organic acids. From the results presented in the chapters of this MSc. thesis, we found that the solution pH did not significantly affect the kinetics of larger organic acids, e.g., PA, CPA, limononic acid with OH radicals, while FA kinetics was largely affected by it. In contrast, the variation in cloud and fog water pH can affect the relative importance of different chemical pathways, as shown in the case of PA. These results contribute to the fundamental understanding of the photochemical processing taking place in atmospheric aqueous phases.

  • Subjects / Keywords
  • Graduation date
    Spring 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • 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.