Isoprene Oxidation Mechanisms and Secondary Organic Aerosol Formation Under HO2-Dominated Conditions

Citation

Bates, Kelvin Hamilton (2017) Isoprene Oxidation Mechanisms and Secondary Organic Aerosol Formation Under HO2-Dominated Conditions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9930R6T. https://resolver.caltech.edu/CaltechTHESIS:05312017-144547085

Abstract

Isoprene, a volatile hydrocarbon emitted by plants, represents the single most abundant source of non-methane organic carbon to the atmosphere. After its rapid oxidation by OH radicals in the troposphere, isoprene may follow any of a number of complex reaction mechanisms to form more highly functionalized products, depending in large part on the relative abundance of reactive radicals such as HO₂ and NO; some of these products can be sufficiently water-soluble, non-volatile, and/or reactive to partition into atmospheric particles and contribute to the creation of secondary organic aerosol (SOA). In this work, I explore the gas-phase oxidation mechanisms and SOA formation potential of second- and later-generation products formed in the HO₂-dominated reaction cascade, which predominates in remote regions and is estimated to account for over >40% of isoprene oxidation. Pure standards of significant isoprene products, such as isoprene epoxydiols (IEPOX) and C₄ dihydroxycarbonyl compounds, are synthesized, and the rates and product yields of their gas-phase reactions with OH are measured by CF₃O^- chemical ionization mass spectrometry in environmental chamber experiments. Results are compared to field observations from the Southern Oxidant and Aerosol Study in the Southeastern United States, where significant concentrations of these compounds were detected, and are integrated into a global chemical transport model to investigate their effects throughout the atmosphere. Further, the results from these and other gas-phase kinetic and product studies are incorporated into an explicit isoprene oxidation mechanism, designed to simulate the effects of isoprene chemistry on oxidant concentrations and to produce accurate representations of products known to be involved in condensed phase processes, including IEPOX. Finally, additional chamber experiments with synthetic IEPOX and inorganic seed aerosol are performed to derive particle uptake coefficients and examine the effects of particle pH, liquid water content, and chemical composition on IEPOX-SOA formation, using aerosol mass spectrometry and differential mobility analysis. The gas- and particle-phase reaction rates and product yields reported herein, along with the explicit model, provide important constraints on the fate of isoprene-derived carbon in the atmosphere and on the influence the HO₂-dominated isoprene oxidation pathway exerts on SOA and oxidant budgets.

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