Measurement report: Statistical modelling of long-term trends of atmospheric inorganic gaseous species within proximity of the pollution hotspot in South Africa

South Africa is considered an important source region of atmospheric pollutants, which is compounded by high population and industrial growth. However, this region is understudied, especially with regard to evaluating long-term trends of atmospheric pollutants. The aim of this study was to perform statistical modelling of inline-formulaSO₂, inline-formulaNO₂ and inline-formulaO₃ long-term trends based on 21-, 19- and 16-year passive sampling datasets available for three South African INDAAF (International Network to study Deposition and Atmospheric Chemistry in Africa) sites located within proximity of the pollution hotspot in the industrialized north-eastern interior in South Africa. The interdependencies between local, regional and global parameters on variances in inline-formulaSO₂, inline-formulaNO₂ and inline-formulaO₃ levels were investigated in the model. Average monthly inline-formulaSO₂ concentrations at Amersfoort (AF), Louis Trichardt (LT) and Skukuza (SK) were 9.91, 1.70 and 2.07 inline-formulaµg minline-formula⁻³, respectively, while respective mean monthly inline-formulaNO₂ concentrations at each of these sites were 6.56, 1.46 and 2.54 inline-formulaµg minline-formula⁻³. Average monthly inline-formulaO₃ concentrations were 50.77, 58.44 and 43.36 inline-formulaµg minline-formula⁻³ at AF, LT and SK, respectively. Long-term temporal trends indicated seasonal and inter-annual variability at all three sites, which could be ascribed to changes in meteorological conditions and/or variances in source contribution. Local, regional and global parameters contributed to inline-formulaSO₂ variability, with total solar irradiation (TSI) being the most significant factor at the regional background site LT. Temperature (inline-formulaT) was the most important factor at SK, located in the Kruger National Park, while population growth (inline-formulaP) made the most substantial contribution at the industrially impacted AF site. Air masses passing over the source region also contributed to inline-formulaSO₂ levels at SK and LT. Local and regional factors made more substantial contributions to modelled inline-formulaNO₂ levels, with inline-formulaP being the most significant factor explaining inline-formulaNO₂ variability at all three sites, while relative humidity (RH) was the most important local and regional meteorological factor. The important contribution of inline-formulaP on modelled inline-formulaSO₂ and inline-formulaNO₂ concentrations was indicative of the impact of increased anthropogenic activities and energy demand in the north-eastern interior of South Africa. Higher inline-formulaSO₂ concentrations, associated with lower temperatures, as well as the negative correlation of inline-formulaNO₂ levels to RH, reflected the influence of pollution build-up and increased household combustion during winter. The El Niño–Southern Oscillation (ENSO) made a significant contribution to modelled inline-formulaO₃ levels at all three sites, while the influence of local and regional meteorological factors was also evident. Trend lines for inline-formulaSO₂ and inline-formulaNO₂ at AF indicated an increase in inline-formulaSO₂ and inline-formulaNO₂ concentrations over the 19-year sampling period, while an upward trend in inline-formulaNO₂ levels at SK signified the influence of growing rural communities. Marginal trends were observed for inline-formulaSO₂ at SK, as well as inline-formulaSO₂ and inline-formulaNO₂ at LT, while inline-formulaO₃ remained relatively constant at all three sites. inline-formulaSO₂ and inline-formulaNO₂ concentrations were higher at AF, while the regional inline-formulaO₃ problem was evident at all three sites.