Seasonal influences on surface ozone variability in continental South Africa and implications for air quality

Although elevated surface ozone (O(3)) concentrations are observed in many areas within southern Africa, few studies have investigated the regional atmospheric chemistry and dominant atmospheric processes driving surface O(3) formation in this region. Therefore, an assessment of comprehensive continuous surface O(3) measurements performed at four sites in continental South Africa was conducted. The regional O(3) problem was evident, with O(3) concentrations regularly exceeding the South African air quality standard limit, while O(3) levels were higher compared to other background sites in the Southern Hemisphere. The temporal O(3) patterns observed at the four sites resembled typical trends for O(3) in continental South Africa, with O(3) concentrations peaking in late winter and early spring. Increased O(3) concentrations in winter were indicative of increased emissions of O(3) precursors from household combustion and other low-level sources, while a spring maximum observed at all the sites was attributed to increased regional biomass burning. Source area maps of O(3) and CO indicated significantly higher O(3) and CO concentrations associated with air masses passing over a region with increased seasonal open biomass burning, which indicated CO associated with open biomass burning as a major source of O(3) in continental South Africa. A strong correlation between O(3) on CO was observed, while O(3) levels remained relatively constant or decreased with increasing NO(x), which supports a VOC-limited regime. The instantaneous production rate of O(3) calculated at Welgegund indicated that ~ 40 % of O(3) production occurred in the VOC-limited regime. The relationship between O(3) and precursor species suggests that continental South Africa can be considered VOC limited, which can be attributed to high anthropogenic emissions of NO(x) in the interior of South Africa. The study indicated that the most effective emission control strategy to reduce O(3) levels in continental South Africa should be CO and VOC reduction, mainly associated with household combustion and regional open biomass burning.