Long-term trends of ambient gaseous concentrations at South African DEBITS sites and wet deposition at Cape Point

Abstract

Although South Africa is considered an important source region for atmospheric pollutants, this region is considered to be understudied with regard to atmospheric composition, especially in terms of long-term assessments of atmospheric pollutant concentrations. The Deposition of Biogeochemically Important Trace Species (DEBITS) task of the International Global Atmospheric Chemistry (IGAC) programme was initiated in 1990 in collaboration with the Global Atmosphere Watch (GAW) network of the World Meteorological Organisation (WMO) to investigate long-term concentrations and deposition of biogeochemical species in the atmosphere for regions in the tropics for which limited long-term datasets exist. Four DEBITS sites, representative of semi-arid savannah, are located in the north-eastern interior of South Africa, i.e. Amersfoort (AF), Louis Trichardt (LT), Skukuza (SK) and the Vaal Triangle (VT), while one South African coastal site is also included, i.e. the Cape Point Global Atmosphere Watch (CPT GAW) station. The general aim of this study was to assess the long-term trends of SO2, NO2 and O3 concentrations measured with passive samplers at the South African DEBITS sites located in the interior, and the marine background site, as well as to evaluate long-term wet deposition at CPT GAW. Since measurements were only conducted from 2009 to 2014 at VT, this site was not considered in this study, while comprehensive assessments of precipitation chemistry were previously reported for AF, LT and SK. A 21-year (1995 to 2015) SO2, NO2 and O3 passive sampling dataset was available for CPT GAW, while 19- (1997 to 2015), 21- (1995 to 2015) and 16-year (2000 to 2015) SO2, NO2 and O3 passive sampling datasets were available for AF, LT and SK, respectively. The first part of the study entailed an evaluation of the long-term temporal trends at the marine CPT GAW site, as well as development of a multiple linear regression model in order to assess the influence of variances in source contribution, as well as local, regional and global meteorology on SO2, NO2 and O3 concentrations. Thereafter, the statistical model developed for CPT GAW was employed in an assessment of long-term SO2, NO2 and O3 concentration measurements at the sites located in the north-eastern interior of South Africa (AF, LT and SK) for which the influence of local, regional and global factors was also considered in the model. Finally, the chemical composition of rain water samples collected from 2004 to 2012 at CPT GAW during the wet season (May to October) was determined. The SO2, NO2 and O3 monthly mean concentrations determined at CPT GAW showed seasonal variability, 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 SO2 and NO2 concentrations during winter could be attributed to pollution build-up, as well as being more frequently impacted by air masses passing over the Cape Town metropole. 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 CPT GAW. A comparison between the SO2, NO2 and O3 concentrations measured at CPT GAW with other African DEBITS sites indicated that levels of these species were generally similar to other African inland ecosystems, but lower compared to the industrially impacted AF site. Long-term temporal trends indicated seasonal and inter-annual variability at AF, LT and SK, 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, Louis Trichardt (LT). Temperature (T) was the most important factor at Skukuza (SK), located in the Kruger National Park, while population growth (P) made the most substantial contribution at the industrially impacted Amersfoort (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 to 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. 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 upwards trend in NO2 levels at SK signified the influence of growing rural communities. Marginal trends were observed for SO2 at SK, as well as for 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 in the South African interior. The chemical composition of rain samples collected at CPT GAW indicated that the VWM concentrations of Na+ and Cl- were significantly higher compared to the VWM concentrations of other ionic species, as well as VWM concentrations thereof at the sites in the South African interior. The average pH of rainwater was slightly lower than the pH of unpolluted rainwater, mainly due to NO3- associated with the occasional influence of the Cape Town metropole. In contrast to the sites situated in the north-eastern South African interior, where anthropogenic SO42- was the major constituent in rainwater, SO42- at CPT GAW was entirely associated with marine air with no anthropogenic contribution. Sulphur and nitrogen depositions at CPT GAW were two orders of magnitude lower than sulphur and nitrogen depositions in the South African interior. It was also indicated that 94% of the chemical content at CPT GAW can be attributed to the marine source