Atmospheric Peroxy Radical Chemistry Studied by Infrared Kinetic Spectroscopy

Citation

Hui, Aileen Oyama Ah Yee (2019) Atmospheric Peroxy Radical Chemistry Studied by Infrared Kinetic Spectroscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9H0-1H20. https://resolver.caltech.edu/CaltechTHESIS:01172019-135430092

Abstract

Peroxy radical (RO₂) chemistry plays a central role in atmospheric science. RO₂ radicals control the oxidative capacity of the atmosphere and are key intermediates in the cycling of ozone in the troposphere in regions where levels of nitrogen oxides (NO_x ≡ NO + NO₂) are high. In more remote regions of the Earth where NO_x concentrations are low, RO₂ loss is dominated by reaction with hydroperoxy (HO₂) and other peroxy radicals, leading to removal of reactive radicals from the troposphere. However, for certain species of RO₂, reaction with HO₂ can also propagate radical chemistry by recycling hydroxyl (OH) radicals, a primary oxidant in the atmosphere. Both HO₂ and OH radicals play important roles in atmospheric oxidation reactions. Accurate characterization of various RO₂ reactions that contribute to consuming and recycling HO_x (HO_x ≡ HO₂ + OH) are instrumental for understanding the effects of atmospheric composition on climate forcing, air quality, and the global ozone budget.

My thesis work used laboratory studies to characterize gas phase organic peroxy radical (RO₂) reactions that are relevant to atmospheric chemistry. The main body of my work focused on studying the self reactions of HO₂ and RO₂ as well as their cross reactions (HO₂ + RO₂) using a time-resolved experimental technique called Infrared Kinetic Spectroscopy (IRKS), which combines infrared (IR) and ultraviolet (UV) absorption spectroscopy. Perfect isolation of one specific reaction is generally difficult or impossible due to radical recycling and secondary chemistry, which leads to numerous reactions occurring simultaneously. Thus, although laboratory studies provide controlled environments to study these reactions, the accuracy of the results are highly dependent on the experiment's selectivity and sensitivity to specific transient species. In this work, a new mid-IR (MIR) laser was implemented into the existing apparatus for measuring OH radicals. In addition to UV absorption spectroscopy, the IRKS apparatus was equipped for selective detection of HO₂ and OH radicals in the near-IR (NIR) and MIR, respectively.

The kinetics and product yields of a reaction are two important factors for assessing the impact of these reactions on the atmosphere. IRKS was used to study the kinetics of the self reactions of acetylperoxy (CH₃C(O)O₂) and acetonylperoxy (CH₃C(O)CH₂O₂), as well as their reactions with HO₂. The OH and O₃ yields from the reaction of HO₂ + CH₃C(O)O₂ were also measured, providing the first parametrization of the temperature dependence of the OH product channel. The temperature dependence of the HO₂ self reaction kinetics was re-investigated to resolve discrepancies in the results previously reported in the literature. Furthermore, this work identified and characterized rate enhancement effects on the HO₂ self reaction by the adducts of HO₂ formed from the reaction of HO₂ with methanol (CH₃OH), acetaldehyde (CH₃CHO), and acetone (CH₃C(O)CH₃), which were used as radical precursors for HO₂, CH₃C(O)O2, and CH₃C(O)CH₂O₂, respectively.

Finally, the improved sensitivity of the IRKS apparatus enabled the investigation of fast reaction kinetics. The rate constants of Cl atoms with CH₃OH and CH₃CHO were measured over a wide range of temperatures. The results were consistent with previously reported values in the literature and additionally validated assumptions that had been made regarding the temperature independence of these reactions. The temperature and pressure dependences of the OH yield from the reaction of CH₃CO with O₂ were also studied to resolve current discrepancies in the literature. The yield of HO₂ from this reaction was also directly measured for the first time.

Thesis Files