Ozone Chemistry in the Lower Stratosphere: Drivers, Trends, and Impacts

This thesis seeks to improve our understanding of the connections between and the modulations of anthropogenically driven chemical changes in the stratosphere by natural processes. It probes these questions on timescales that range from daily to decadal and on spatial scales ranging from a few kilometers to the global mean, and presents new results improving our understanding of both the chemistry and physics behind these processes and improvements to our ability to represent them in chemistry-climate models. This work is driven by the belief that improved understanding of the fundamentals of stratospheric chemical and physical processes can simultaneously advance science and help society in evaluating past policy decisions and informing future ones. 
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\nFirst, the impact of large and moderate sized volcanic eruptions on anthropogenically driven ozone depletion and recovery trends is explored. Results confirm previous work showing that large volcanic eruptions increased the rate of ozone depletion prior to the mid-1990s, and for the first time quantify how the combination of an unusually volcanically quiescent period from the mid-1990s to the mid-2000s and a flurry of moderate sized volcanic eruptions after the mid-2000s decreased the rate of ozone recovery until at least 2014. This study is the first to show that the timing of moderate sized volcanic eruptions, not just large ones, can significantly alter ozone decadal trends. 
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\nSecond, the ability of equatorial dynamical flows known as Matsuno–Gill patterns to alter stratospheric chemistry is investigated in the deep tropics for months in the spring and fall for the first time. Results from a chemistry-climate model show these anticyclonic flows both entrain extratropical chlorine and induce cooling, allowing rapid heterogeneous chlorine activation on sulfuric acid aerosols. Perturbations to ClO and NO2 provide a testable signature for the presence of this activation. This study shows a previously unrecognized mechanism in near-equinox months for dynamical influences on the spatial structures of atmospheric composition changes in the deep tropics. 
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\nThird, the success of the Montreal Protocol on Substances that Deplete the Ozone 3 Layer is examined in the context of the record Arctic ozone depletion of Spring 2020. Results indicate that, in a “World Avoided” where ozone depleting substances had continued to increase, the first true Arctic ozone hole would have occurred in Spring 2020 due to the combined effects of higher chlorine and extreme meteorological conditions, and ozone depletion would have begun earlier and lasted longer than seen in the real world. This study also presents an improved parameterization of polar stratospheric cloud impacts on denitrification for the Whole Atmosphere Community Climate Model v.6 (WACCM6), which was necessary to accurately simulate extreme ozone loss. 
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\nFourth, the importance of temperature perturbations from sub-grid scale gravity waves on chemical rates in WACCM6 is explicitly investigated for the first time via a new connection between the orographic gravity wave parameterization and the chemistry module. Results indicate that several key heterogeneous reaction rates proceed faster on average and that this can have a significant effect on monthly chemistry concentrations. The lifetime of PAN also changes in some areas, affecting the distance this tropospheric pollutant can be transported. Verification of temperature perturbations using the COSMIC satellite array is investigated, and preliminary results show generally good agreement when gravity waves’ vertical wavelengths are expected to be small. This study reinforces the importance of accounting for sub-grid scale temperature effects when simulating chemistry. 
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\nFinally, some directions for future work are discussed, including continued improvements to the denitrification parameterization in WACCM and preliminary efforts to observe tropical heterogeneous chlorine activation using satellites.

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