Classification of tropospheric ozone profiles over Johannesburg based on mozaic aircraft data R. D. Diab1, A. Raghunandan1, A. M. Thompson2, and V. Thouret3 1University of Natal, Durban, South Africa 2NASA/GFSC, Greenbelt, Maryland, USA 3CNRS, Toulouse, France
Abstract. Each ozone profile is a unique response to the photochemical and dynamic processes
operating in the troposphere and hence is critical to our understanding of processes
and their relative contributions to the tropospheric ozone budget. Traditionally, mean
profiles, together with some measure of variability, averaged by season or year at a
particular location have been presented as a climatology. However, the mean profile is
difficult to interpret because of the counteracting influences present in the micro-structure.
On the other hand, case study analysis, whilst revealing, only applies to isolated conditions. In a search for pattern and order within ozone profiles, a
classification based on a cluster analysis technique has been applied in this study.
Ozone profiles are grouped according to the magnitude and altitude of ozone
concentration. This technique has been tested with 56 ozone profiles at Johannesburg,
South Africa, recorded by aircraft as part of the MOZAIC (Measurement of Ozone
and Water Vapor aboard Airbus In-service Aircraft) program. Six distinct groups of
ozone profiles have been identified and their characteristics described. The widely
recognized spring maximum in tropospheric ozone is identified through the classification, but a new summertime mid-tropospheric enhancement due to the
penetration of tropical air masses from continental regions in central Africa has been
identified. Back trajectory modeling is used to provide evidence of the different
origins of ozone enhancements in each of the classes. Continental areas over central
Africa are shown to be responsible for the low to mid-tropospheric enhancement in
spring and the mid-tropospheric peak in summer, whereas the winter low-tropospheric
enhancement is attributed to local sources. The dominance of westerly winds through
the troposphere associated with the passage of a mid-latitude cyclone gives rise to
reduced ozone values.
Citation: Diab, R. D., Raghunandan, A., Thompson, A. M., and Thouret, V.: Classification of tropospheric ozone profiles over Johannesburg based on mozaic aircraft data, Atmos. Chem. Phys., 3, 713-723, doi:10.5194/acp-3-713-2003, 2003.