Stratospheric Chlorine and Nitrogen Chemistry: Observations and Modeling.

In order to predict the effects of anthropogenic perturbations (such as the increase of chlorine) and natural events (such as volcanic eruptions) on the chemical composition of the stratosphere, it is essential to quantitatively test our knowledge of the photochemistry by conducting detailed comparisons between stratospheric observations and models. Through such comparisons, this thesis focuses on explaining sets of simultaneous measurements obtained from balloon (Chapters 3, 4, 5) and ER-2 aircraft (Chapters 2, 6) platforms, with particular emphasis on the basic chemical processes underlying the partitioning of the chlorine and nitrogen families in the stratosphere. Following an introductory first chapter, Chapter 2 tests our knowledge of the photochemical balance between NO_2 and NO. Chapter 3 examines the effects of enhanced heterogeneous chemistry resulting from the eruption of Mt. Pinatubo on the partitioning between NO_2 and HNO_3. Chapter 4 discusses the factors controlling the distribution of chlorine between its main reservoirs (HCl, ClONO_2) and more reactive (HOCl and ClO) forms. Chapter 5 questions the completeness of our understanding of chlorine chemistry and proposes the existence of a new atmospheric species perchloric acid, HClO_4. Finally, Chapter 6 focuses on the heterogeneous processes taking place during the Antarctic winter and their effect on the evolution of reactive chlorine and nitrogen species within and outside of the polar vortex.