Modelling atmospheric chemical transformations under South African conditions
Fourie, Gerhardus Dirk
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South Africa is in the process of adapting to new social and economical structures. The government has made it clear that the creation of work and economic growth is of the utmost importance. This policy will put South Africa's limited resources under enormous pressure and will have an adverse impact on the environment. In order to facilitate sound decision making, all aspects of the environmental impacts must be investigated, e.g. the necessary knowledge on atmospheric chemical transformations of air pollutants has to be obtained. This study is aimed at extending the existing knowledge of air pollutants at selected sites and developing a mathematical model that can be used to predict the future impact of air pollutants under southern African conditions. Against this background, it should be noted that volatile organic compounds (VOG's) and their photochemical products are a potential threat to sustainable development due to their role in the formation of tropospheric oxidants. A vac sampling survey was conducted in this study in order to determine the concentration profiles of selected VaG1s in the vicinity of a petrochemical plant. A commercially available passive sampler was used in the study. Benzene, toluene, xylene. acrylonitrile, butanol, propanol, acetone, hexane and pentane, were identified as problem species at the petrochemical plant, and these species were measured In terms of different seasonal variations. The summer survey was successful in obtaining ambient concentrations for the selected VOC's, but during the winter survey none of the selected VOG's were detected in measurable amounts on the samplers. Follow-up surveys must be initiated in order to validate these results, and to explore or develop other sampling techniques for the sampling of ambient VOC's under these 'conditions. In order to test our understanding of atmospheric processes, it is essential to construct models to help interpret their chemistry. A photochemical box model was designed to study the irradiation of a mixture of NOx and non-methane hydrocarbon (NMHC) gases by solar ultraviolet radiation, which leads to the formation of photochemical smog. This photochemical box model was used to model the chemical transformations in biomass and petrochemical diffusive plumes in southern African conditions. Results from the modelled biomass plume show that biomass burning emissions lead to high ozone concentrations, and that this 03 formation in the plume is NO limiting. The results from the modelled petrochemical diffusive plumes showed that the aromatic compounds in the plume are very stable, even in bright sunlight. It also follows from the results that the ozone production in the plumes is NOx limiting. A great deal of research had been undertaken in South Africa on the horizontal and vertical transport of aerosols and trace gases over southern Africa. This research addresses the need to understand the extent of trace gas transport and deposition over South Africa, its neighbours and other African countries in order to assess the extent of transboundary pollution transport. It is therefore important to understand the relationship between the emission of pollutants from major industrial sources and their distribution over the globe. A trajectory transformation model was developed to explore the transport of trace gases and deposition over the subcontinent from the Mpumalanga highveld region, the centre of power generation and chemical industries of South Africa. Five distinctive trajectory patterns were identified and obtained from the Climatological Research Group at the University of the Witwatersrand. The model represents a box-shaped parcel of air, which traverses with a specific trajectory over an emissions grid of southern Africa. The chemistry within this box, and the concentrations of all the species were modelled throughout this navigation process along the trajectory. The values simulated by the trajectory transformation model are slightly higher than those measured along a trajectory. It follows from validation results that realistic 802 residence times and chemical conversion can be obtained from the model. The total sulphur deposition values obtained from the model were also of realistic magnitude. This study succeeded to quantify the transport and deposition of trace gases in such a way that a better description of the transfer of pollutants over international borders was obtained. This study has highlighted the need for the operation of an increase in the number of long term atmospheric monitoring stations which should be spaced in strategic locations throughout the southern African region.
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