MECHANISMS CONTROLLING PHOTOCHEMICAL LOSS OF OZONE IN THE MIDLATITUDE STRATOSPHERE

Abstract

This dissertation evaluates various open-ended issues related to the depletion of stratospheric ozone in the midlatitude Northern Hemisphere (NH) stratosphere and identifies possible sources of uncertainty in global chemistry-climate model predictions of the future recovery of the ozone layer due to the harmful effects of ozone depleting substances. In-situ observations obtained during the Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) mission were analyzed to assess why chlorine activation was not observed, despite the sampling of conditions that several prior studies suggested would lead to rapid heterogeneous chemistry and associated ozone loss in the midlatitude lower stratosphere (LS). Chlorine activation was found to be limited by the availability of inorganic chlorine, adequate partitioning of ClONO2 and HCl to yield appreciable reaction rates, and elevated levels of NOx in the lowermost stratosphere from the lofting of boundary layer pollutants. Our chemical trajectory simulations showed that significant chlorine activation could occur for the conditions observed during DCOTSS if stratospheric sulfate surface area density were enhanced either by major volcanic eruptions, or climate intervention efforts that involve enhancement of stratospheric sulfate aerosols. Next, photochemical steady-state modeling of the DCOTSS dataset revealed that calculations employing the latest kinetic recommendations from the NASA Jet Propulsion Laboratory (JPL) compendium systematically overestimate observed NO2 and ClONO2 by 20-30% and 25-50%, respectively, and provide agreement with observed ClO within ± 5%, in the 18-21 km altitude region. A 500-member Monte Carlo simulation was used to determine that the data could be best fit by a ~23% increase in the formation rate of HNO3 and a ~12% decrease in the formation rate of ClONO2, relative to the latest JPL recommendation. Both reaction rate adjustments were shown to be more consistent with recommendations provided by the International Union of Pure and Applied Chemistry. The kinetic adjustments that allow for the best agreement between modeled and observed NO2, ClONO2, and ClO suggest that global models that use current JPL kinetic recommendations may overestimate NOx-driven ozone loss in the lower stratosphere by ~9%, underestimate halogen-driven loss by ~25%, and underestimate the HOx-driven loss by ~14%. Finally, trends in Cly (HCl + ClONO2) and N2O observed by the space-borne ACE-FTS instrument in the NH and Southern Hemisphere (SH) midlatitudes from 2004 to 2025 were compared to trends in Cly and N2O simulated by eight global chemistry-climate models (CCMs). These comparisons reveal a slower observed decline of stratospheric Cly than is simulated by the CCMs, particularly for the LS at NH midlatitudes. An estimate of the impact on trends in Cly of very short-lived chlorine compounds (VSL-Cl), which are produced by industrial processes and are not yet regulated by the Montreal Protocol and are not considered by the CCMs, led to better agreement between observed and modeled trends in Cly, particularly throughout the SH stratosphere and the NH upper stratosphere. For the NH lower stratosphere, the data revealed a decline in N2O over the past two decades that is indicative of a major change in the stratospheric transport that is not represented in any of the models and is consistent with the observed slower than expected decline in Cly. Together, the results of this dissertation highlight the need to continue to improve our understanding of specific chemical and dynamical processes that regulate ozone in the midlatitude stratosphere, such as the formation rates of HNO3 and ClONO2; the cause of transport changes in the NH lower stratosphere over the past two decades; the role of VSL-Cl species; and, possible harm to the ozone layer by stratospheric sulfate injection.