Interactive comment on “ Seasonal influences on surface ozone variability in continental South Africa and implications for air quality ” by Tracey

“. emissions of NOx in the interior of South Africa. The study indicated that the most effective emission control strategy to reduce O3 levels in continental South Africa should be CO and VOC reduction mainly associated with household combustion and regional open biomass burning. ” Section 3.3. 1 (now Section 3.4.1): “. characterise the dispersion of biomass burning emissions over southern Africa (Mafusire et al., 2016). Therefore the regional transport of CO and VOCs (and NOx to a lesser extent) associated with biomass burning occurring from June to September in southern Africa can be considered an important source of surface O3 in continental South Africa (Fig. A4). ” Section 3.4. 1 (now Section 3.5.1): “In Fig. 8 the correlations between O3, NOx and CO concentrations at Welgegund, Botsalano and Marikana are presented, which clearly indicates higher O3 concentrations associated with increased CO levels, while O3 levels remain relatively constant (or decrease) with increasing NOx. The highest O3 concentrations occur for NOx levels below 10 ppb, since the equilibrium between photochemical production of O3 and chemical removal of O3 shifts towards the former i.e. greater O3 formation. In general there seems to exist a marginal negative correlation between O3 and NOx (Fig. A6) at all four sites, which is a reflection of the photochemical production of O3 from NO2 and the destruction of O3 through NOx titration. These correlations between NOx, CO and O3 indicate that O3 production in continental South Africa is limited by CO (and VOCs) concentrations, i.e. VOC-limited. ” Section 3.4. 2 (now Section 3.5.2): “. concentrations, which is most pronounced (highest CO/NOx ratios) during winter and spring. This indicates that the winter and spring O3 maximum is primarily driven by increased peroxy radical production from CO and VOCs. The seasonal maximum in O3 concentration coincides with the maximum CO concentration at the background sites, whilst the O3 peak occurs just after June/July when CO peaked at the polluted site Marikana (Fig. A5). This observed seasonality in O3 production signifies the importance of precursor species emissions from open biomass burning during winter and spring in this region, while household combustion for space heating and cooking is also an important source of O3 precursors as previously discussed. ” Section 3.5. 2 (now Section 3.6.2): “As indicated above (Section 3.4 and 3.5), O3 formation in the regions where Welgegund, Botsalano and Marikana are located can be considered VOC-limited, while the highly industrialised region with high NOx emissions where Elandsfontein is located could also be considered VOC-limited. Rural remote regions are generally considered to be NOx-limited due to the availability of NOx and the impact of BVOCs (Sillman, 1999). However, Jaars et al. (2016) indicated that BVOC concentrations at a savannah-grassland were at least an order of magnitude lower compared to other regions in the world. Therefore very low BVOC concentrations together with high anthropogenic emissions of NOx in the interior of South Africa result in VOC-limited conditions at background sites in continental South Africa. It is evident that reducing CO and VOC concentrations associated with anthropogenic emissions e.g. household combustion, vehicular emissions and industries, would be the most efficient control strategy to reduce peak O3 concentrations in the interior of South Africa. It is also imperative to consider the seasonal variation in the CO and VOC source strength in managing O3 pollution in continental southern Africa. This study also revealed the significant contribution of biomass burning to O3 precursors in this region, which should also be considered when implementing O3 control strategies. However, since open biomass burning in southern Africa is of anthropogenic and natural origin, while O3 concentrations in continental South Africa are also influenced by trans boundary transport of O3 precursors from open biomass burning occurring in other countries in southern Africa (as indicated above), it is more difficult to control. Nevertheless, open biomass burning caused by anthropogenic practices (e.g. crop residue, pasture maintenance fires, opening burning of garbage) can be addressed. ”