Modelling the long-range transport and transformation of air pollutants over the Southern African region
Fourie, Gerhardus Dirk
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Dispersion modelling of transport, diffusion and chemical transformation of pollutants and trace gases over the Southern African region which spans between 52o South to 1o North, 28o West to 68o East, presents a special challenge due to three major factors. The first factor is associated with the frequent occurrence of a stable anticyclonic environment. This environment inhibits the vertical exchange of air masses and stratifies the troposphere into persistent layers, in which residence times of pollutants are prolonged from several days to weeks over the region. The second factor stems from the different distribution of emission sources in Africa. Biogenic emissions from biomass burning, vegetation and soils are equal to, or substantially bigger than anthropogenic emissions over larger parts of the region. Thirdly, long-range transport is vital for the existence or destruction of many fragile ecosystems that receive nutrients or pollutants mainly from the atmosphere. In addition to these major factors, experimental studies on the tropical meteorological factors affecting the long-range transport and chemical transformation of pollutants are limited, and theoretical understanding of the atmospheric processes in the regions with negligible Coriolis force, is still lacking. Special emphasis should be placed on the identification of key linkages between the physical, chemical and anthropogenic processes governing the functioning of the biogeophysical and biogeochemical systems of Southern Africa that lead to significantly elevated ozone concentrations over considerable sections of the tropics. This thesis describes the development and application of an appropriate dispersion package for studying the peculiarities of the long-range transport, diffusion and chemical transformation of pollutants and trace gases in the Southern Africa region. Special attention is given to the transport of harmful substances from the highly industrialized regions to the predominantly rural areas of the region as well as wet- and dry deposition over sensitive land and water ecosystems. The Lagrangian-Eulerian Diffusion (LED) model developed in this thesis, utilizes in a complimentary way the positive features of the Lagrangian and Eulerian description of hydrodynamic flows. It is well-known that the essence of the Lagrangian method consists of studying the properties and variation of a fixed fluid volume during its motion. Using this idea in the model, any volume of polluted air is identified by the trajectory of its center of mass. The diffusion and transformation processes of pollutants are investigated on the basis of analytical solutions of the appropriate differential equations in Eulerian coordinates with origin at the center of mass of puffs. As part of the basic structural element of the model, the puff allows for approximation of any type of emission source by using proper puff volume and emission time intervals. A unique feature of LED is the use of an appropriate ABL model calculating its dynamics and turbulent characteristics. In the LED model the two-layer parametric ABL model proposed by Yordanov et al. (1983) is included. LED incorporates a linear chemical mechanism for the transformation of sulphur and nitrogen species, as well as modules to calculate dry and wet deposition parameters. LED model results for ambient concentrations, as well as deposition fields, were produced for all months during the year 2000, and compared with the available experimental data at the Deposition of Biogeochemically Important Trace Species (DEBITS) international sites. Data obtained for the evaluation, were in the framework of SAFAFU 2000 research campaign. However, the ground measurements needed for the evaluation of LED were not sufficient due to the small number of observation points (5) , and the geographical location which does not comply with the WMO criteria for baseline regional air quality stations. Another shortcoming stems from the fact that some of the DEBITS stations were not fully operational during the year 2000. The predetermined SAFARI 2000 research plan for the period of integration (2000), did not allow the use of previous years measured data (where the availability of experimental data is somewhat more complete) for the evaluation exercise. Therefore data from before the year 2000, has been used to only evaluate LED capability of producing results which are in the range of the observed inter-annual observations. Despite these inherent difficulties of not having a complete set of experimental data for the evaluation process, the study presents enough evidence that LED is producing reliable results. The annual quantities compare quite accurately. The bigger variation in differences of the monthly quantities is also within the acceptable range. A numerical experiment, carried out will1 au alternative emission data set, clearly shows the importance of revisiting the existing emission data set accepted by the SAFARI 2000 research group. It gives a reasonable explanation for some of the monthly deviations observed at the most northern DEBITS station. LED was also implemented to study the long-range transport from the highly industrialised Highveld region of South Africa, where the versatility of the LED model was clearly demonstrated. Ambient concentration fields, accumulated dry- and wet deposition fields, as well as pH values of precipitation over the modelling region were calculated. These outputs allow the environmental footprint characteristics to be determined. For the specific case study, the environmental impact region is located approximately 500 to 600 kilometers around the industrial region. The comparison of the calculated model results with limited experimental data for the region and lack of outputs from other models and observations, substantiate the use of LED for environmental impact studies, regulatory purposes and decision making. The results in Chapter 7 clearly demonstrate that the phenomenon of long-range transport of air pollutants is a serious, complex and significant problem for the countries in the southern African region. The results also indicate that impacts from highly industrial countries in the region may pose significant risks to developing countries, who relies for example on agriculture as a major contributor to the specific country's gross domestic product (GDP). The LED model supplies objective data, which lays the foundation for the development of holistic regional air quality management plans. The implementation of such a management plan will he obviously beneficial to all countries in the region. The results obtained from this modelling scenario highlights the complexity of transboundary air pollutant transport, as well as its serious developmental consequences. After development, modification and evaluation of the LED, it can be used to assist in identifying potentially high impact areas. Concentration and deposition fields for specific regions allow the study of anthropogenic impacts caused by the transport of major air pollutants such as sulphates and nitrogen oxides. The ability to define the relative importance of each source in the total pollution and deposition fields can be used to determine the most effective strategy for decreasing the emissions of a given region. LED can be applied with confidence as a diagnostic and prognostic tool for air pollution studies at different time and space scales in the southern African region.