Seasonal influences on
surface ozone variability
in continental
South
Africa
and implications for air quality
Abstract. Although elevated ozone (O3) concentrations are observed in many areas within continental southern Africa, few studies have investigated the regional atmospheric chemistry and dominant atmospheric processes driving surface O3 formation in this region. The aim of this study was to conduct an assessment of comprehensive continuous surface O3 measurements performed at four sites located in continental South Africa. These sites were representative of regional background (Welgegund and Botsalano) and industrial regions (Marikana and Elandsfontein) in the north-eastern interior in South Africa as indicated by comparison with other sites in this region. The regional O3 problem was also shown with O3 concentrations being higher than 40 ppb at many sites in the north-eastern interior, while the South African air quality standard limit for O3 was regularly exceeded at the four sites in this study. O3 levels were generally lower at other background sites in the Southern Hemisphere compared to the South African sites, while similar seasonal patterns were observed. The temporal O3 patterns observed at the four sites resembled typical trends for O3 in continental South Africa, i.e. O3 concentration peaking in late winter and early spring, and daytime O3 peaks associated with increased photochemical production. The seasonal O3 trends observed in continental South Africa were mainly attributed to the seasonal changes in emissions of O3 precursor species and changes in meteorological conditions. Increased O3 concentrations in winter were indicative of increased emissions of O3 precursors from household combustion for space heating and the concentration of low-level pollutants near the surface. A spring maximum was observed at all the sites, which was attributed to increased regional biomass burning during this time. Source area maps of O3 and CO indicated significantly higher O3 and CO concentrations associated with air masses passing over a region where a large number of seasonal open biomass burning occurred in southern Africa, which indicated CO associated with open biomass burning as a major source of O3 in continental South Africa. The relationship between O3, NOx and CO indicated a strong dependence of O3 on CO, while O3 levels remained relatively constant or decreased with increasing NOx. The seasonal changes in the relationship between O3 and precursors species also reflected the seasonal changes in sources of precursors. The instantaneous production rate of O3, P(O3), calculated at Welgegund indicated that at least 40 % of O3 production occurred in the VOC-limited regime. These relationships between O3 concentrations and P(O3) with O3 precursor species revealed that large parts of the regional background in continental South Africa can be considered CO- or VOC-limited, which can be attributed to high anthropogenic emissions of NOx in the interior of South Africa. It was indicated that the appropriate emission control strategy should be CO (and VOC) reduction associated with household combustion and regional open biomass burning to effectively reduce O3 pollution in continental South Africa.
