Photochemical and Dynamics Studies of Oxygen Isotope Exchange Reactions of CO₂

Citation

Yeung, Laurence Yip-Lun (2010) Photochemical and Dynamics Studies of Oxygen Isotope Exchange Reactions of CO₂. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/TQKB-3T76. https://resolver.caltech.edu/CaltechTHESIS:12222009-120450275

Abstract

This dissertation describes laboratory studies of three oxygen isotope exchange reactions — O(¹D) + CO₂, O₂ + CO₂, and O(³P) + CO₂ — and their importance to oxygen cycling in the upper atmosphere. First, we studied the isotope exchange reaction O(¹D) + CO₂, which is believed to govern the oxygen-isotope budget of CO₂ in the stratosphere. Our combined field, laboratory, and modeling study of the exceptionally rare ¹⁶O¹³C¹⁸O isotopologue revealed that O(¹D) + CO₂ explains only part of the stratospheric CO₂ isotopologue budget, not all of it as previously thought: O(¹D) + CO₂ could not explain the large enrichments of ¹⁶O¹³C¹⁸O (i.e., of the "Δ₄₇" tracer) at high Northern latitudes. Mesospheric and heterogeneous chemistry of CO₂ were proposed as possible sources of this meridional variation in ¹⁶O¹³C¹⁸O. Therefore, we performed crossed-molecular-beam experiments to investigate the chemistry of CO₂ at hyperthermal collision energies; this class of reaction could be important in the upper atmosphere, where low gas densities and high rates of photochemistry increase the relative probability of hyperthermal reactions. Our experimental and theoretical study of the O₂ + CO₂ isotope exchange reaction showed that the reaction can occur through a short-lived CO₄ reaction complex, which leads to O₂ + CO₂ products that are highly internally excited, but possibly still in their ground electronic state. Our study of O(³P) + CO₂ collisions at hyperthermal energies showed that O(³P) + CO₂ isotope exchange can occur in the upper atmosphere, proceeding through a short-lived CO₃ reaction complex. The O(³P) + CO₂ → O₂ + CO reaction was also observed, and our data suggest that it can proceed through a 'stripping' mechanism or a CO₃ complex. These reactions demonstrate new ways in which oxygen can be cycled through CO₂ in the atmosphere; their isotope effects, manifest in the isotopic composition of atmospheric CO₂, may impose independent constraints on atmospheric transport and biosphere-atmosphere interactions.

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