Using an inferential model to estimate dry deposition of SO2 and NOX (as NO2) in Lephalale in the Waterberg-Bojanala priority area

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science June 2015Lephalale is the home of Matimba, one of Eskom’s coal-fired power stations. Matimba is the biggest power station with a dry cooling system in the world. There are other industries (including coal mines) currently in operation in close proximity to the station. This industrial area is expected to grow as more industrial activities are planned for the following years. These activities will aggravate the levels of air pollution and possibly result in it being a “hot spot” for air pollution. The impact of air quality on health is covered by the National Ambient Air Quality Standards (NAAQS), but the impact of air quality on the terrestrial and aquatic ecosystem is not known. Therefore, this study focuses on the deposition of nitrogen oxides (NOx) (as nitrogen dioxide (NO2)) and sulphur dioxide (SO2) within Lephalale in the Waterberg-Bojanala Priority Area. Additionally, inter-annual variability of NOx and SO2 ambient concentrations and back trajectories of air masses were analysed. The study obtained ambient air quality data and meteorological data from Eskom for the period 2008–2012, while additional meteorological data were obtained from the Agricultural Research Council (ARC) and the South African Weather Service (SAWS). An inferential model was used to estimate the dry deposition flux of SO2 and NOx (as NO2), and the Hybrid Single Particle Langrangian Integrated Trajectory (Hysplit) Model was used to cluster back trajectories of air masses. The results of the seasonal dry deposition velocities of SO2 (0.17 to 0.23 cm/s) and NOx (0.10 to 0.15 cm/s) (as NO2) were higher in summer and lower in winter. They were also within the magnitude of the deposition velocities found in previous studies in the Highveld. The high deposition velocities in summer were attributed to photosynthetically active vegetation, turbulence and solar radiation. However, seasonal dry deposition fluxes of SO2 and NOx were higher in winter across the years. The higher flux values in winter were attributed to higher ambient concentrations of the trace gases. Additionally, the annual dry deposition flux of SO2 ranged between 0.43 and 0.67 kg S ha-1 yr-1, while NOx (as NO2) ranged between 0.84 and 1.05 kg N ha-1 yr-1 over the period studied. The annual deposition flux values found in the current study are lower than those found in previous studies in the Highveld. This difference could be because of the lower ambient concentrations of SO2 and NOx observed in this study. There is an inter-annual variability of the ambient concentrations of SO2 and NOx during the period 2008–2012. However, the difference is not large or statistically significant. The dominant direction of the back trajectories of air masses is east and southeast across all seasons for the entire period of 2008–2012. This lack of seasonal pattern in back trajectories and source regions cannot explain the seasonal changes in ambient concentrations (SO2 and NOx). Hence, climatic factors (e.g. change in weather) or seasonal changes in combustion source intensity must be responsible