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

Abstract

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 O3 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 NOx, 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 NOx 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.