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Investigations of the Isotopic Composition of Trace Gases in the Stratosphere and of the Quantum Dynamics of Non-adiabatic Atomic Collisions


Measurements and analysis of the mixing ratios and isotopic compositions of carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), and molecular hydrogen (H2) from whole air samples collected during several NASA suborbital missions are presented. In each case, the global annual mean net isotope flux from the stratosphere to the troposphere, estimated empirically using the correlation between measured isotopic composition and N2O mixing ratio, is large enough to influence tropospheric isotopic compositions on hemispheric and potentially regional scales, and hence needs to be considered when making “top-down” inferences about the sources and sinks of the gas, including the impact of human activities, from isotope measurements at the surface. For CO2, in particular, I present and analyze 150 new measurements of Δ14C of CO2 collected from aircraft missions in 1997, 2000, 2004, 2012, and 2013, including the first hemispheric-scale 14CO2 vertical profiles that extend from the lower or middle troposphere into the lower stratosphere over a wide range of latitudes in the Northern Hemisphere for 2013. To within the uncertainties, the global annual mean net 14CO2 fluxes, which are approximately half the global 14C production rates, have not changed between 1997 and 2013, suggesting that stratospheric age spectra are wide enough to damp out variations in the cosmogenic 14C production rate associated with the 11-year solar cycle. This apparent lack of dependence of measured stratospheric 14CO2 on the solar cycle simplifies the application of stratospheric 14CO2 measurements as a model diagnostic to better understand rates and features of the stratospheric circulation and how they may be shifting over time as climate changes; an acceleration of the stratospheric circulation is predicted by climate models but is not, as of yet, confirmed by observations such as mean ages derived from the time lag between tropospheric and stratospheric concentrations of the gases CO2 and SF6 which are increasing in the troposphere. Additionally, these empirical 14CO2 isotope fluxes from the stratosphere to the troposphere are needed for carbon cycle studies. For 14CO2, as well as for the stable isotope compositions of CO2, N2O, CH4 and H2, the global annual mean net isotope flux is large enough that stratospheric chemistry, which enriches each gas in its rare, heavy isotopes, and subsequent transport from the stratosphere to the troposphere can interfere with determining the magnitudes of natural and anthropogenic emissions of these gases from surface observations. Overall, the measurements and net isotope fluxes presented here provide much needed constraints on the isotopic compositions of CO2, N2O, CH4 and H2 and the transport of these gases, necessary for studies that seek to (1) diagnose changes in the stratospheric circulation and residence times over time, (2) quantify sources and sinks in the biogeochemical cycles of these gases, and/or (3) interpret the large number of isotopic composition measurements made at Earth’s surface (e.g., to infer regional and global fossil fuel emissions from Δ14CO2).

In the field of reaction dynamics, the O(1D) + Xe electronic quenching reaction was investigated in a crossed beam experiment. Unexpected large-scale oscillations in the differential cross sections, whose shape and relative phase depend strongly on collision energy, were observed for the inelastic scattering products, O(3P) and Xe. Comparison of the experimental results with time-independent scattering calculations shows qualitatively that this behavior is caused by Stueckelberg interferences, for which the quantum phases of the multiple reaction pathways accessible during electronic quenching constructively and destructively interfere. This investigation into the quantum nature of O(1D) → O(3P) quenching by Xe provides sensitive new experimental constraints on the Xe–O potential energy curves and their couplings, as well as benchmarks for theoretical treatments of quantum scattering in general.

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