South African Journal of Science
On-line version ISSN 1996-7489
RAGHUNANDAN, Atham; MAHUMANE, Gilberto and DIAB, Roseanne. Elevated ozone events over Johannesburg based on analysis of tropospheric ozone partial columns. S. Afr. j. sci. [online]. 2007, vol.103, n.5-6, pp. 248-252. ISSN 1996-7489.
Traditionally, tropospheric column ozone (TCO) is a useful indicator for comparing both temporal and spatial variations in tropospheric ozone. TCO variations over Johannesburg are analysed in this paper with a view to identifying days of enhanced ozone, which could then form the basis of a detailed investigation to determine sources of the elevated ozone. We used ozone data from the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) database for the period 1995 to 1998. A fixed tropopause height of 12 km was employed in this analysis as the upper bound of the troposphere. Seasonal and inter-annual variations in TCO provided a context for this study. A clear seasonal cycle exists, with TCO peaking in September and October. Minimum TCO occurs in autumn, when variability is also least. The lower day-to-day variability in autumn and winter is a reflection of the more settled weather at this time. This period is representative of background tropospheric ozone loadings, on which the dynamic and photochemical influences of other months are superimposed. High-TCO events, defined as exceeding 30 DU (Dobson units), occurred predominantly in spring. Enhancements in the lower troposphere are shown to be generally short-lived (1-2 days) and due to the effects of local surface pollution sources, and arise most likely from biomass burning, which peaks in spring. In contrast, events in the upper troposphere prevailed for a longer period and were due to the penetration of ozone-rich air from the stratosphere, as shown in a case study in September 1998.