Factors influencing surface ozone variability over continental South Africa and implications for air quality and agriculture
Extensive scientific research has been conducted on surface ozone (O3) concentrations over several decades in North America and Europe. Through these efforts, significant information is available on the role of chemistry, meteorology and transport in the formation and local accumulation of O3, as well as the adverse effects of O3 on human health and vegetation in these regions. However, only a limited number of studies on surface O3 measurements are available for southern Africa. The Highveld, located in the high-lying plateau in the interior of South Africa, is considered to be exposed to the highest O3 concentrations. This region is the most densely populated part of South Africa, and the industrial and economic heartland of the country. High O3 concentrations are observed in many urban and rural areas within the interior of South Africa, where concentrations exceed the South African National Ambient Air Quality Standard for O3. Continuous, long-term measurements of surface O3 concentrations are valuable indicators of possible health and environmental impacts, which can be used to inform efficient and effective regulatory standards. Within the southern African region, the only continuous, long-term record of surface O3 concentrations is from the Cape Point Global Atmosphere Watch station, which is far removed from the Highveld region of South Africa and the meteorological patterns that dominate the interior of South Africa. Together with understanding the factors affecting surface O3 concentrations, more research is needed on the impacts of O3 pollution on ecosystems. In terms of the effects on vegetation, although local O3 concentrations in southern Africa are cumulatively above the European critical levels for crop damages, no vegetation damage has been reported. Scepticism remains that the reason that damages are not identified is due to a lack of local research attention on this topic. The impact of O3 on agriculture in southern Africa is important from an economic and food security perspective. An atmospheric measurement-based study using continuous, long-term O3 measurements from four sites representing regional background and anthropogenically polluted regions in the north-eastern interior of South Africa, covering different time periods between 2006 and 2015, was conducted to explore regional and seasonal O3 pollution variability over continental South Africa, as well as the most important sources contributing to O3 concentrations in this region. Previous studies have suggested that the formation of surface O3 over southern Africa is attributed to the combined contribution of precursors from anthropogenic and biogenic sources. The four different environments, i.e. clean savannah at Botsalano, polluted savannah at Marikana, semi-clean grassland at Welgegund, and polluted grassland at Elandsfontein showed a similar seasonal pattern, i.e. late winter and early spring peaks, which were ascribed to increased open biomass burning endemic to this region. Back trajectory analysis was performed from which source maps were compiled at the two regional background sites, which indicated that higher O3 concentrations corresponded with increased CO concentrations in air masses passing over a region in southern Africa, where a large number of open biomass fires occurred from June to September. The regional transport of CO associated with open biomass burning in southern Africa was therefore considered a significant source of surface O3 in continental South Africa. The spring peak in O3 in southern Africa occurs a little earlier than the spring peak in biogenic VOCs, suggesting that it is more likely biomass burning that is contributing to maximum O3 than biogenic VOCs. In addition, biogenic VOC concentrations were significantly lower compared to biogenic VOC concentrations measured in other ecosystems in the world. Furthermore, to the extent that emissions of CO are proportional to those of reactive VOCs, the findings suggest that continental South Africa is VOC-limited rather than NOx-limited. Therefore, the appropriate emission control strategy should be CO and VOC reduction, with the sources being mostly regional open biomass burning and household combustion, to effectively reduce peak O3 in continental South Africa. The measurement data from the four sites was also examined by multivariate statistical methods, i.e. multiple linear regression (MLR), principal component analysis (PCA) and a generalised additive model (GAM) to identify and quantify the influence of the chemical and meteorological factors driving O3 variability over continental South Africa. The common finding with these statistical models was that the most important parameters explaining daily maximum O3 variation in continental South Africa were relative humidity, temperature and CO concentrations, while NO levels explained O3 variability to some extent. PCA indicated that these factors are not collinear after addressing multicollinearity in the data. Inter-comparison of the three statistical methods in the prediction of daily maximum O3 indicated that GAM offered a slight improvement over MLR. Furthermore, all of the methods highlighted that relative humidity is one of the most important variables influencing O3 levels in semi-arid South Africa, with increases in O3 associated with decreases in relative humidity. Possible causes for the relationship, as suggested by literature, mainly involve loss of O3 or precursor species in the atmosphere in the aqueous phase or lower relative humidity being associated with meteorological conditions not conducive to O3 formation. The statistical models confirmed that regional-scale O3 precursors coupled with meteorological conditions play a critical role in the daily variation of O3 levels in continental South Africa. An eight-month trial was conducted to assess the sensitivity of two sugarcane cultivars (Saccharum spp.) commonly farmed in South Africa to chronic exposure to elevated O3 levels, as well as combined elevated O3 and CO2 concentrations in open-top chambers during the growth season. As far as the candidate can assess, this was the first attempt to present quantitative exposure-response data on sugarcane under controlled conditions. Two indicators of stress could be monitored, namely plant growth and plant physiology related to photosynthetic performance. The results indicated that the growth parameters of the NCo376 cultivar showed no significant response to increased O3. Although the growth of the N31 variety indicated some response to increased O3, it also showed the ability to adapt to increased O3 levels. Some evidence of elevated CO2 countering the effects of elevated O3 on growth, especially for the N31 cultivar, was observed. The physiological function (photosynthesis) of the NCo376 sugarcane variety was more susceptible to O3 exposure compared to the N31 cultivar. The combined effects of elevated O3 and elevated CO2 improved photosynthetic efficiency and chlorophyll content relative to the O3-treated plants to a certain extent, with the effects more pronounced for N31 than NCo376. It was indicated that the N31 variety is probably more tolerant towards high O3 levels compared to NCo376. In general, this study indicated that the effects of O3 chronic exposure were not as severe as expected in the sugarcane, while it was also indicated that these plant species are capable of evolving in order to tolerate and adapt to elevated O3 levels.