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Mechanisms of Heterogeneous Oxidations at Model Aerosol Interfaces by Ozone and Hydroxyl Radicals

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Mechanisms of Heterogeneous Oxidations at Model Aerosol Interfaces by Ozone and Hydroxyl Radicals University of Kentucky UKnowledge Theses and Dissertations--Chemistry Chemistry 2017 MECHANISMS OF HETEROGENEOUS OXIDATIONS AT MODEL AEROSOL INTERFACES BY OZONE AND HYDROXYL RADICALS Elizabeth A. Pillar-Little University of Kentucky, [email protected] Author ORCID Identifier: https://orcid.org/0000-0003-0444-2300 Digital Object Identifier: https://doi.org/10.13023/ETD.2017.269 Right click to open a feedback form in a new tab to let us know how this document benefits ou.y Recommended Citation Pillar-Little, Elizabeth A., "MECHANISMS OF HETEROGENEOUS OXIDATIONS AT MODEL AEROSOL INTERFACES BY OZONE AND HYDROXYL RADICALS" (2017). Theses and Dissertations--Chemistry. 80. https://uknowledge.uky.edu/chemistry_etds/80 This Doctoral Dissertation is brought to you for free and open access by the Chemistry at UKnowledge. It has been accepted for inclusion in Theses and Dissertations--Chemistry by an authorized administrator of UKnowledge. For more information, please contact [email protected]. STUDENT AGREEMENT: I represent that my thesis or dissertation and abstract are my original work. Proper attribution has been given to all outside sources. I understand that I am solely responsible for obtaining any needed copyright permissions. I have obtained needed written permission statement(s) from the owner(s) of each third-party copyrighted matter to be included in my work, allowing electronic distribution (if such use is not permitted by the fair use doctrine) which will be submitted to UKnowledge as Additional File. I hereby grant to The University of Kentucky and its agents the irrevocable, non-exclusive, and royalty-free license to archive and make accessible my work in whole or in part in all forms of media, now or hereafter known. I agree that the document mentioned above may be made available immediately for worldwide access unless an embargo applies. I retain all other ownership rights to the copyright of my work. I also retain the right to use in future works (such as articles or books) all or part of my work. I understand that I am free to register the copyright to my work. REVIEW, APPROVAL AND ACCEPTANCE The document mentioned above has been reviewed and accepted by the student’s advisor, on behalf of the advisory committee, and by the Director of Graduate Studies (DGS), on behalf of the program; we verify that this is the final, approved version of the student’s thesis including all changes required by the advisory committee. The undersigned agree to abide by the statements above. Elizabeth A. Pillar-Little, Student Dr. Marcelo I. Guzman, Major Professor Dr. Mark A. Lovell, Director of Graduate Studies MECHANISMS OF HETEROGENEOUS OXIDATIONS AT MODEL AEROSOL INTERFACES BY OZONE AND HYDROXYL RADICALS DISSERTATION A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the College of Arts and Sciences at the University of Kentucky By Elizabeth Ann Pillar-Little Lexington, Kentucky Director: Dr. Marcelo I. Guzman Lexington, Kentucky 2017 Copyright © Elizabeth Ann Pillar-Little 2017 ABSTRACT OF DISSERTATION MECHANISMS OF HETEROGENEOUS OXIDATIONS AT MODEL AEROSOL INTERFACES BY OZONE AND HYDROXYL RADICALS Atmospheric aerosols play an important role in climate by scattering and absorbing radiation and by serving as cloud condensation nuclei. An aerosol’s optical or nucleation properties are driven by its chemical composition. Chemical aging of aerosols by atmospheric oxidants, such as ozone, alters the physiochemical properties of aerosol to become more hygroscopic, light absorbing, and viscous during transport. However the mechanism of these transformations is poorly understood. While ozone is a protective and beneficial atmospheric gas in the stratosphere, it is a potent greenhouse gas in the troposphere that traps heat near the Earth’s surface. It also impacts human heath by irritating the respiratory tract and exacerbating cardiovascular diseases. Additionally, ozone can alter the ecosystem through oxidizing plant foliage which can lead to deforestation and crop losses as well. Both gases and aerosols in the troposphere can react with ozone directly and indirectly with hydroxyl radicals. While daytime aging is thought to be primarily driven by photochemical processes and hydroxyl radicals, ozone is thought to be a key player in nighttime or dark aging processes that can alter the physicochemical properties of aerosols. Measured concentrations of trace gases and aged aerosol components in the field are higher than values predicted based on laboratory studies and computer simulations. Consequently, new experimental approaches are needed to narrow the gaps between observations and mechanistic understandings. In this dissertation, a plume of microdroplets was generated by pneumatically assisted aerosolization and then exposed to a flow of ozone before entering a mass spectrometer. This surface-specific technique allowed for the real-time analysis of reaction products and intermediates at the air-water interface. This work explores the in situ oxidation of iodide, a component of sea spray aerosols, by 0.05 – 13.00 ppmv ozone to explore how heterogeneous oxidation could enhance the production of reactive iodide species. Methods to study the reaction channels and intermediates were also established to not only determine a mechanism of iodide oxidation by ozone, but to enable the study of more complex systems. The developed approach was then applied to examine the oxidation of catechol and its substituted cousins, a family of compounds selected to model biomass burning and combustion emissions, at the air-water interface. While literature suggested that the primary mechanism of catechol oxidation by ozone would be the cleavage of the C1-C2 bond, it was determined that this was only a minor pathway. An indirect oxidation channel dominated heterogeneous processes at the air-water interface, giving rise to hydroxyl and semiquinone radicals that recombine to produce polyhydroxylated aromatics and quinones. This new mechanism of aging represents an overlooked channel by which brown, light-absorbing carbon aerosols are produced in the atmosphere. In addition, the work investigates how reactions on solid particulate aerosols proceed under variable relative humidity. Thin films were developed alongside a novel flow- through reactor to study of how aerosols are transformed by ozone and hydroxyl radicals when exposed to 50 ppbv - 800 ppmv of ozone. This system was employed to probe how catechol reacts with ozone under variable relative humidity. Further work was undertaken to model the adsorption process at the air-solid interface under variable humidity, permitting the estimation of the reactive uptake of ozone by the film at concentrations (50-200 ppbv) seen in rural and urban areas. Together, these results provide an increased understanding of how heterogeneous oxidation of aerosols contributes to aerosol aging processes as well as free radical production in the troposphere. KEYWORDS: Aerosols, Atmospheric Chemistry, Hydroxyl Radical, Mass Spectrometry, Ozone. Elizabeth A. Pillar-Little 07/02/17 MECHANISMS OF HETEROGENEOUS OXIDATIONS AT MODEL AEROSOL INTERFACES BY OZONE AND HYDROXYL RADICALS By Elizabeth Ann Pillar-Little Dr. Marcelo I. Guzman Director of Dissertation Dr. Mark A. Lovell Director of Graduate Studies 07/02/17 For my parents Michael and Nancy Pillar and husband Timothy Pillar-Little None of this would been possible without your unending love and support. ACKNOWLEDGEMENTS It is somewhat overwhelming to look back over the last six years at the immeasurable number of people who have supported me during my time in graduate school. I was originally very intimidated about moving to Lexington, but I quickly found an entire community of people who helped me a lot. These kind gestures of this community of people, their warm words, and acts of generosity made the long road to the Ph.D. a pleasurable and enriching experience. During my time at the University of Kentucky, my advisor, Dr. Marcelo Guzman, has been a source of immense support, encouragement, and inspiration during my entire career. He has placed a lot of trust and faith in me over these six years by placing me in charge of several hundreds of thousands of dollars of equipment as we pursued mechanisms of aerosol processing. When research would get discouraging or overwhelming, he would be there with uplifting words or a few well-placed jokes to help me examine experimental difficulties from a new perspective. He always knew exactly what to say to get me headed off in the right direction, even if I was a bit slow moving at times. He constantly pushed me outside of my comfort zone to promote my growth as a scientist and as a person. I consider myself extremely lucky to have worked for such a wonderful mentor. I feel great pride in having worked for you; I hope I have done you proud as well. Getting to know and work alongside members the Guzman group, past and present, has been one of the best aspects of my time in graduate school. No matter how long or short of a time I shared an office with Richa Althaye, Shasha Xia, Evie Zhou, Alexis Eugene, Elizabeth Bollinger, Nick English, Robert Camm, Samah Saad, Chris Ward, Sohel Rana, iii Masoumeh Doranni, Michele Kuceki, Jimmy Kaindu, Dr. Matias Aguirre, Travis Schuyler, Nandini Gundaram Suresh, Ariful Hoque, and Kayvon Ghayoumi, the laboratory felt like a home away from home because of the strong sense of comradery we had from discourse about current events, conference adventures, and sharing in each other’s joys and sorrows. I am extremely grateful for Richa’s help in showing me the ropes when I joined the laboratory. Without her detailed instructions on everything from laboratory basics, poster making, and mass spectrometry, I would not have hit the ground running like I did. I could not have asked for better companions during my grad school journey to have shared the last five years with than Alexis and Evie. They have always been there to offer help and opinions when I get stuck on something, and listen to my venting about research, teaching, or my sore joints.
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