Measurement report: Statistical modelling of long-term trends of atmospheric inorganic gaseous species within proximity of the pollution hotspot in South Africa
Authors: Swartz Jan-Stefan , van Zyl Pieter G. , Beukes Johan P. , Galy-Lacaux Corinne , Ramandh Avishkar , Pienaar Jacobus J. 
Published: September 2020

Abstract. 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 SO2, NO2 and
O3 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 SO2, NO2 and O3 levels were
investigated in the model. Average monthly SO2 concentrations at
Amersfoort (AF), Louis Trichardt (LT) and Skukuza (SK) were 9.91, 1.70 and 2.07 µg m−3, respectively,
while respective mean monthly NO2 concentrations at each of these sites
were 6.56, 1.46 and 2.54 µg m−3. Average monthly O3 concentrations were 50.77, 58.44 and 43.36 µg m−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 SO2
variability, with total solar irradiation (TSI) being the most significant
factor at the regional background site LT. Temperature (T) was the most
important factor at SK, located in the Kruger National Park, while
population growth (P) made the most substantial contribution at the
industrially impacted AF site. Air masses passing over the source region
also contributed to SO2 levels at SK and LT. Local and regional factors
made more substantial contributions to modelled NO2 levels, with P
being the most significant factor explaining NO2 variability at all
three sites, while relative humidity (RH) was the most important local and
regional meteorological factor. The important contribution of P on modelled
SO2 and NO2 concentrations was indicative of the impact of
increased anthropogenic activities and energy demand in the north-eastern
interior of South Africa. Higher SO2 concentrations, associated with
lower temperatures, as well as the negative correlation of NO2 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 O3 levels at all three sites,
while the influence of local and regional meteorological factors was also
evident. Trend lines for SO2 and NO2 at AF indicated an increase
in SO2 and NO2 concentrations over the 19-year sampling period,
while an upward trend in NO2 levels at SK signified the influence of
growing rural communities. Marginal trends were observed for SO2 at SK,
as well as SO2 and NO2 at LT, while O3 remained relatively
constant at all three sites. SO2 and NO2 concentrations were
higher at AF, while the regional O3 problem was evident at all three
sites.