Aerosols and atmospheric circulation characteristics over Durban.

Abstract

The main objective of this study was to investigate the vertical distribution of aerosols over Durban in relation to the vertical stability structure and horizontal transport of air masses. The importance of aerosols in the region is well recognised and recently there have been many international experiments which have focused on aerosol distribution over the subcontinent. Durban is situated at the approximate centre of a giant plume that is known to transport aerosols and trace gases off the east coast of southern Africa and is therefore strategically located for an investigation of the vertical distribution of aerosols. The vertical distribution of aerosols over Durban was measured using a LIDAR (Light Detection And Ranging) system on selected cloud free days in 1997. Backward trajectory modelling was used at selected pressure (standard) levels to determine the origin and transport pathways of aerosols. Six case studies are presented in an attempt to gain insight into the relationship between the vertical distribution of aerosols and absolutely stable layers. The results of the study revealed that the occurrence of absolutely stable layers governs the vertical distribution of aerosols in the troposphere. An absolutely stable layer at ~5km (~500hPa) appears to be the most effective in capping and trapping aerosols in the atmosphere. Below 5km, the atmosphere was charcterised by marked stratification and relatively higher concentration of aerosols. Above 5km, the concentrations were much lower, but generally increased slightly with height. Low aerosol concentrations are observed during post-frontal situations and relatively higher concentrations during anticyclonic conditions. The background to the problem and the objectives of this investigation are elaborated in Chapter 1. A description of the data sets and derived meteorological variables, along with the methodologies applied in this thesis, are given in Chapter 2. A theoretical review of aerosols, including their sources, effects and distribution over the globe and southern Africa, is discussed in Chapter 3. Atmospheric circulation and weather patterns and their relationship to the transport and dispersion of aerosols are described in Chapter 4. The results of the study and an analysis of the major findings are presented in Chapter 5. Finally, Chapter 6 summarises the major findings of this dissertation.