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Title: Quantifying chemical ozone depletion in the polar stratosphere
Author: MacKenzie, Ian Atholl
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1995
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Chemical depletion of polar stratospheric ozone occurring during periods of enhanced reactive chlorine concentration in the winter and spring is investigated using both models and observations. A computationally-cheap and easily initialised photochemical model utilising ClO measurements from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite is developed. With this model, ozone destruction rates within the polar vortices due to the ClO + ClO, ClO + BrO and ClO + O catalytic cycles are evaluated. The method involves calculating local reactive chlorine concentrations from individual ClO retrievals, and then inferring the diurnal cycle of ClO from a quadratic expression using the relevant kinetic parameters. In test integrations this simple method is shown to give good agreement with more detailed calculations, but its speed of operation and the ease with which the ClO measurements are assimilated make it highly suited to dealing with the large amounts of data generated by MLS. Application of the method to the 1992-1993 Arctic and 1993 Antarctic winters yields maximum vortex-averaged ozone loss rates at 465 K potential temperature of ˜1% per day in both hemispheres. Time-integrated ozone destruction in the Arctic is less mainly because the duration of temperatures sufficiently low to sustain polar stratospheric clouds (PSCs) is shorter, and hence enhanced reactive chlorine concentrations are less persistent. The estimated chemical destruction on isentropic surfaces in the lower stratosphere is broadly similar to the observed change in ozone distribution, implying that the ozone change is dominated by chemical destruction, with dynamics playing a lesser role. An Antarctic winter-vortex is simulated in a chemical general circulation model (GCM) for the months of August and September. Chemical and dynamical impacts on the ozone change in the model are resolved by contrasting the temporal evolution of the 'chemical' ozone field with that of an inert tracer having the same initial distribution. It is found that the model results are consistent with the MLS-based chemistry-only calculations in indicating that there is very little dynamical replenishment of ozone on isentropic surfaces lying below 500 K. At higher altitudes the model implies a somewhat greater role for the transport than does the chemistry-only analysis.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available