Investigating atmospheric photochemistry in the Johannesburg-Pretoria megacity using a box model

Abstract

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 (O 3 ) 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. O 3 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 O 3 because of a decrease in titration through the reaction with NO. The same effect was observed in various cities worldwide where O 3 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 O 3 was also investigated. A significant increase of approximately 23 ppb O 3 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.

Spatial, temporal and source contribution assessments of black carbon over the northern interior of South Africa

“…The area in which Elandsfontein is situated is a well-known international NO 2 hotspot, with tropospheric column densities similar to what is observed over southeast Asia (Lourens et al, 2012(Lourens et al, , 2016. It is widely accepted that NO 2 in this hotspot mainly originates from NO x emission from coal-fired power stations.…”

Seasonal influences on surface ozone variability in continental South Africa and implications for air quality

“…Anthropogenic sources of VOCs are largely due to industrial and vehicular emissions (Jaars et al, 2014), while biogenic VOCs are also naturally emitted (Jaars et al, 2016). Regional biomass burning, which includes household combustion for space heating and cooking, agricultural waste burning, and open biomass burning (wild fires), is a significant source of CO, NO x , and VOCs (Macdonald et al, 2011;Crutzen and Andreae, 1990;Galanter et al, 2000;Simpson et al, 2011) in southern Africa. In addition, stratospheric intrusions of O 3 -rich air to the free troposphere can also lead to elevated tropospheric O 3 concentrations (Diab et al, 1996(Diab et al, , 2004.…”

“…Under VOC-limited conditions, O 3 concentrations increase with increasing VOCs, while a region is considered NO x limited when O 3 production increases with increasing NO x concentrations. Results from a photochemical box model study in South Africa, for instance, revealed that the Johannesburg-Pretoria megacity is within a VOC-limited regime ( Lourens et al, 2016 ). VOC reductions would, therefore, be most effective in reducing O 3 , while NO x controls without VOC controls may lead to O 3 increases.…”

Seasonal influences on surface ozone variability in continental South Africa and implications for air quality

“…In order for South Africa to develop an effective national/provincial management plan to reduce O3 concentrations through controlling NOx and VOC emissions, it is important to determine whether a region is NOx-or VOC-limited. Results from a photochemical box model study in South Africa, for instance, revealed that the Johannesburg-Pretoria megacity is within a VOC-limited regime (Lourens et al, 2016). 5…”