Abstract

LOURENS, Alexandra S.M. et al. Investigating atmospheric photochemistry in the Johannesburg-Pretoria megacity using a box model. S. Afr. j. sci. [online]. 2016, vol.112, n.1-2, pp.01-11. ISSN 1996-7489.  http://dx.doi.org/10.17159/sajs.2016/2015-0169.

Urban air pollution has become a major concern over the past decades. One of the largest conurbations in Sub-Saharan Africa is developed around the cities of Johannesburg and Pretoria (Jhb-Pta megacity). In this study, a photochemical box model with a detailed representation of ozone (O₃) formation chemistry was used to investigate the state of current air quality and photochemical processes in the Jhb-Pta megacity, as well as scenarios that could possibly mitigate air pollution. Results indicated that the Jhb-Pta megacity is within a VOC-limited (or NOx-saturated) regime. Major sources of NOx include transport from the Mpumalanga Highveld and local traffic emissions. O₃ levels in the Jhb-Pta megacity will be more effectively reduced if VOC (volatile organic compound) emissions are decreased. A reduction of NOx emissions leads to an increase in O₃ because of a decrease in titration through the reaction with NO. The same effect was observed in various cities worldwide where O₃ levels increased when NOx emissions were reduced during emission control strategies. The effect of reducing vehicular emissions in the Jhb-Pta megacity on the production of O₃ was also investigated. A significant increase of approximately 23 ppb O₃ was observed when emissions of VOCs, NOx and CO were reduced by changing from Euro-0 to Euro-3 vehicles. It is therefore recommended that VOC emissions are decreased together with the implementation of Euro-3 and cleaner vehicles in the future.

Keywords : megacities; air pollution; photochemical box model; ozone; NOx.

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