Seasonal and diurnal variations of surface ozone on the Mpumalanga Highveld
Mokgathle, Bontle Beauty
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Surface ozone concentrations are used to monitor changes and trends observed in the sources of both ozone and precursors, and they are also important indicators of possible health and environmental impacts. Urban and rural air quality can be improved by regular control of major air pollutants such as ozone, NOx, and VOC's. Air quality data (O3, NO and NO2) of two Eskom monitoring sites on the Mpumalanga Highveld, Elandsfontein (26°15'S, 29°25'E, 1742m ASL) and Verkykkop (27°18'S, 29°53'E, 2047m ASL), were analysed and compared, to evaluate the exceedances of air quality threshold values and the annual, seasonal and diurnal variations of air pollutants. The relationship between monitoring sites at higher altitudes and ozone concentrations was also investigated. Vertical profiles of volatile organic compounds were also sampled at Elandsfontein during a field campaign on the 30th and 31st August 2005 using a small aircraft, to investigate their abundance in the atmosphere. The annual and seasonal trends were investigated between January 2000 and December 2004. High ozone concentrations, exceeding the DEAT human health hourly guideline value of 120 ppb were observed at both monitoring sites. High ozone exceedances at night were also observed at Verkykkop. Surface ozone concentrations were higher at Verkykkop compared to Elandsfontein, which were mainly due to transport of formed ozone and its precursors from upwind emission sources with persistent north-westerly winds during the period. Higher NOx concentrations were observed at Elandsfontein than at Verkykkop because Elandsfontein is located near many industries, which contribute to their high emissions. They are also due to motor traffic emissions from nearby traffic highways situated to the West of this monitoring site. They contributed to lower ozone concentrations observed at this site. Elandsfontein is described as a VOC-limited region because it has high concentrations of NOx and low concentrations of ozone, and Verkykkop is a NOx-limited region with low NOx concentrations and high ozone concentrations. Seasonal and diurnal patterns of surface ozone on the Mpumalanga Highveld showed maximum values in spring, while those recorded during summer and autumn showed minimum values. Maximum values of ozone in spring are due to biomass burning which occurs mainly during the dry season. Diurnal cycle of ozone concentrations exhibits maximum values in the afternoon and minimum in the early morning hours and evening. The afternoon maximum is due to photochemical formation of ozone and the evening minimum is partly due to surface deposition, the titration of O3 by NO and no photolysis of ozone precursors causing ozone production at night. Meteorological variables (temperature, wind speed and direction, and solar radiation) were also monitored in order to determine if there is any correlation between ozone concentrations and weather conditions. The correlations that were observed were then statistically (using linear regression analysis or scatter plots) analysed to indicate which meteorological variables and ozone precursors influence the formation of ozone the most. Temperature showed a direct relation with surface ozone at both sites because ozone increased with increasing temperature. Wind direction influences the formation of ozone at Verkykkop, which is downwind the source emission area. High concentrations of NO and NO2 reduced ozone concentrations at Elandsfontein due to titration of O3 by NO and reaction of NO2 and hydroxyl radicals to form HNO3, which is eventually removed from the atmosphere by rain. VOC concentrations were observed to be higher in the morning than in the afternoon and also higher at a lower altitude. Among VOC's that were quantified, toluene was found to be the most abundant VOC with high concentrations, which increase from the morning to the afternoon. Concentration ratios of toluene to benzene increase with the time of the day, which indicate that toluene is emitted from most activities that take place during the day. Alkanes were the most abundant VOC's in the atmosphere among VOC's that couldn't be quantified because they are less reactive towards hydroxyl radicals and thus have a longer atmospheric lifetime.