Twenty-one years of passive sampling monitoring of SO2, NO2 and O-3 at the Cape Point GAW station, South Africa

Sulphur dioxide (SO2), nitrogen dioxide (NO2) and ozone (O3) are considered typical inorganic gaseous pollutants in the atmosphere. The aim of this study was to assess long-term seasonal and inter-annual trends of a 21-year SO2, NO2 and O3 passive sampling (monthly means) dataset collected at the Cape Point Global Atmosphere Watch (CPT GAW) station. Comparison of the concentrations of these gaseous species with other IGAC DEBITS Africa measurement sites indicated that levels of these species were generally similar to other African inland ecosystems, but lower compared to an industrial site in southern Africa. The SO2, NO2 and O3 monthly mean concentrations showed seasonal patterns, which can be attributed to various factors influencing levels of these species at CPT GAW. These factors are generally season specific, which include changes in meteorological conditions and source contributions. Higher NO2 concentrations were also attributed to increased microbial activity in the wet season. The O3 seasonal pattern corresponded to the NO2 seasonality, which was attributed to their related chemistry. SO2 and NO2 concentrations displayed inter-annual variability, while O3 did not indicate significant inter-annual fluctuations. The seasonal and inter-annual variability was explored with a multilinear regression model, in which global, regional and local meteorological factors, as well as population growth were included. Modelling results indicated that variances in SO2 concentrations were predominantly influenced by changes in global forcing factors. Global, regional and local factors played a significant role in NO2 trends, which included the influence of population growth and associated increased anthropogenic activities. It was also established that variances in O3 concentrations were predominantly associated with regional and local factors. Trend analysis indicated that SO2, NO2 and O3 concentrations remained relatively constant over the 21-year sampling period at this background site in the Southern Hemispher