Nature and characteristics of tropospheric ozone over Johannesburg.
The aim of this thesis is to examine the nature and characteristics of tropospheric ozone over Johannesburg, South Africa. Ozone, water vapour and meteorological profile data, which form part of the MOZAIC (Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft) database for the period 1995 to 2000 were utilized in this study. The thesis is divided into two main parts. The first part deals with the computation of total tropospheric ozone. A clear seasonal cycle, with ozone peaking in September and October is found. It is suggested that the main reason for the spring maximum is biomass burning, combined with prevailing anticyclonic circulation patterns, which facilitate the build-up of ozone over the region. Variability in TTO is greatest in January, September and November and least during autumn and winter (April to July). The lower day-to-day variability in autumn and winter is a reflection of the more settled weather at this time. Interannual variability is least in January and April to June. The autumn and winter ozone values are more consistent and appear to represent background tropospheric ozone loadings on which the dynamic and photochemical influences of other months are superimposed. High TTO events (>30 DU) occurred predominantly during September and October. Enhancements in the lower troposphere occurred mostly in September and seldom lasted for more than 1-2 consecutive days. It is suggested that these events are most likely due to effects of local surface pollution sources, either localised biomass burning or urban-industrial effects. An extended period of enhancement in the 7-12 km layer occurred from 14-17 September 1998 and again on 20 September 1998. The extended duration of this event suggests that it is due to an STE event. Confirmation of this was given in a case study of a particular MOZAIC flight on 16 September 1998 from Johannesburg to Cape Town. The second part of the thesis deals with the classification of ozone profiles and is used to find pattern and order within the profiles. TWINSPAN (Two-Way INdicator SPecies ANalysis), a cluster analysis technique, was used to classify the profiles according to the magnitude and altitude of ozone concentration. Six distinct groups of profiles have been identified and their characteristics described. The HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) trajectory model was used to relate the profiles to the origin of air masses, revealing clearly defined source regions. The mid-tropospheric peak in summer and the low to mid-tropospheric enhancement in spring is attributed to continental areas over central Africa and long-range transport while local sources are responsible for the winter low tropospheric enhancement. Reduced ozone values are due to westerlies bringing in clean maritime air. The classification has highlighted three important findings. Firstly, it has emphasized the pronounced seasonality of ozone profiles. It is evident that seasons are dominated by particular patterns and by inference, the processes and transport patterns that shape individual profiles are seasonally dependent. Secondly, the widely recognized spring maximum in tropospheric ozone has been confirmed in this classification, but a new and equally high summer mid-tropospheric enhancement due to the penetration of tropical air masses from continental regions in central Africa has been identified. Thirdly, it is suggested that the computation of a mean profile and furthermore, extrapolation of trends based on a mean profile is meaningless, particularly for a location on the boundaries of zonally defined meteorological regimes.