Photochemical and Dynamics Studies of Oxygen Isotope Exchange Reactions of CO₂

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