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.
Competing Interests: Competing interests. The authors declare that they have no conflict of interest.