Factors governing rain chemistry and wet deposition at a regional background site in South Africa
Wet deposition is an atmospheric sink that is essential to sustain the Earth-atmosphere biogeochemical balance. Deposited species can either be introduced as nutrients or toxins into a surface environment, which can be either beneficial or detrimental to the health of an ecosystem. Anthropogenic emissions of sulfur- and nitrogen oxides, for example, have been shown to acidify rainwater, which leads to acidification of surface water bodies, leaching of essential nutrients and mobilisation of heavy metals. Rainwater chemistry is generally considered to reflect the impacts of global and regional atmospheric pollution on the environment. The removal of atmospheric species through precipitation is influenced by various factors, which include emission source strengths, ecosystem-specific factors, atmospheric transport, chemical reactivity and physical processes. In-cloud scavenging of gaseous species and particulates in the atmosphere occurs during cloud nucleation and other in-cloud processes, while below-cloud scavenging of atmospheric species takes place during a rain event. The deposition measurements have been identified as a key priority in future atmospheric chemistry research. South Africa is an important source region of atmospheric pollutants. As the largest industrialised economy in Africa, South Africa contributes 2.5% to global coal consumption, is the 9th largest S-emitting country, has a large NO2 hotspot over the Mpumalanga Highveld, and is characterised by wide-spread open biomass burning. A few studies have been conducted on the chemical composition of wet deposition in South Africa through the Deposition of Biogeochemically Important Trace Species (DEBITS) project endorsed by the International Global Atmospheric Chemistry (IGAC) project of the Global Atmospheric Watch (GAW) programme of the World Meteorological Organisation (WMO). In this study, rain samples were collected at the Welgegund atmospheric research station, a regional background site in the North-West Province of South Africa. Welgegund was recently included into the renamed African component of DEBITS, i.e. the International Network to study Deposition and Atmospheric chemistry in Africa (INDAAF). Welgegund is situated on a commercial farm on the Highveld of South Africa, approximately 100 km west of the Johannesburg-Pretoria megacity. The site is impacted by air masses passing over the major source regions in the South African interior, as well as a relatively clean region to the west where no large point sources are located. The aim of this study was to identify and determine the major factors governing the chemical composition of rainwater and wet deposition at this regional background site in South Africa, while a novel technique for relating rain chemistry to air mass history was explored. In addition, rain chemistry at Welgegund was also contextualised with the four other DEBITS sites located in the north-eastern interior of South Africa. A custom-made automated wet-only rain sampler was used to collect rain samples on an event basis. Rain sampling at Welgegund complied with the field protocols of the WMO for precipitation chemistry measurements. Collected rain samples were analysed with a Dionex ICS 3000 suppressed ion chromatograph system. Data quality was ensured by complying with the WMO Data Quality Objectives for precipitation chemistry. All analytical techniques were also verified through participation in the bi-annual inter-laboratory comparison study managed by the WMO. Cloud base height (CBH) during the onset of a rain event was measured with a ceilometer, which was related to air mass history by performing back trajectory analyses. In addition, rain intensity was also monitored, while other ancillary measurements continuously performed at Welgegund were also used to assist in elucidating factors influencing the chemical composition of rain. In total, 119 rain samples collected from December 2014 to April 2018 at Welgegund complied with the data quality objectives of the WMO, which represented 89% of all rain events occurring during the sampling period. Rainwater chemistry and wet deposition fluxes of ionic species determined in rain samples collected at Welgegund indicated that the total ionic concentration of rainwater was similar to two background sites located within proximity of industrial activities. However, the pH of rainwater (4.80) indicated increased neutralisation and was comparable to that determined at two rural background sites. Lower S- and N fluxes at Welgegund compared to the industrially influenced sites were attributed to lower annual average rainfall (573.3 mm). Similar to the four other South African sites, SO42- was the most abundant species in rain, with concentrations thereof in the same order determined at the two industrially influenced sites The major source groups influencing rainwater ionic content identified with empirical calculations and explorative statistical analyses included anthropogenic- (industrial), crustal-, marine-, agricultural- and biomass burning sources. A large anthropogenic (industrial) source group contribution to wet deposition chemical composition signified the influence of major source regions in the South African interior impacting Welgegund. Relatively large contributions were also calculated from marine and crustal sources. The influence of agricultural activities was also evident, while biomass burning had the lowest contribution due to open biomass burning occurring mainly during the dry season. An advanced assessment on large-scale factors influencing the chemical composition of rain in the South African interior was conducted by relating rain events at Welgegund to air mass history at CBH and at arrival heights below clouds (100 m a.g.l.). Air mass histories at CBH reflected the influence of the region where cloud formation occurred on rain chemistry, while 100 m a.g.l. air mass histories indicated the influence of below-cloud scavenging. Hierarchical clustering analyses (HCA) were performed during which two different approaches were followed, i.e. (1) clustering according to the chemical composition of rain that was related to air mass histories at the two air mass arrival heights, and (2) grouping based on air mass histories at 100 m a.g.l. and CBH arrival heights that was associated with chemical composition. In each of these approaches, the optimum solutions yielded three clusters. Clustering according to the ionic composition of rain events grouped the events together in relation to their total VWM concentrations, i.e. from high to low VWM concentrations. Correlation of air mass histories to the three clusters did indicate, to an extent, that higher VWM concentrations of ionic species in rain were associated with air masses at 100 m a.g.l. passing over anthropogenic source regions. Clustering of air mass histories grouped air masses passing predominantly over pre-defined source regions together, i.e. air masses passing over anthropogenic source regions, a clean western background region and oceans. The rain chemistry associated with clusters determined for air masses at 100 m a.g.l. arrival heights could be related to the influence of different source regions, with, especially, the influence of large point sources, the clean western background sector and oceans evident. Clustering according to the chemical composition of rain and air masses at an arrival height of 100 m a.g.l. did reflect the regional impact of anthropogenic activities in the north-eastern part of South Africa on rain chemistry, while the influence of household combustion was also evident. In addition, these two clustering approaches also indicated higher VWM concentrations of ionic species associated with increased rain intensity. Although air masses arriving at CBH could partially be related to rain chemistry, no significant correlations between air mass histories at CBH and ionic composition of rain were evident. Therefore, it seemed that below-cloud scavenging was more important to the chemical composition of rain samples in this part of South Africa. Although clustering analysis revealed some correlations between air mass history and chemical composition, it emphasised the complexity associated with correlating rain chemistry to sources of atmospheric species. Rain events were also categorised according to three main synoptic patterns, namely tropical-temperate surface troughs, surface troughs with coastal low pressures, and surface troughs with temperate westerly wave disturbances, in order to investigate the mesoscale influence of the type of convection on rainwater chemistry. Although some association between the type of uplift and the rainwater chemistry was evident, surface flow patterns varied significantly in the specific groups identified. Case studies were conducted in order to further explore the advantages associated with the availability of CBH height measurements in conjunction with rain sampling. These case studies were chosen in order to support the clustering analyses conducted and illustrate other factors influencing the chemical composition of rain in this part of South Africa that were not clearly indicated by the clustering analyses. These case studies included exploring the influences of anthropogenic source regions, below-cloud scavenging and rain intensity as revealed by statistical analysis, as well as the impacts of pollution build-up and open biomass burning on rain chemistry. The influence of anthropogenic activities in the north-eastern interior of South Africa on the rainwater chemistry was evident and substantiated in these case studies. The influence of below-cloud scavenging on rain chemistry at Welgegund was also signified by investigating the chemical composition of two successive rain events associated with similar air mass histories. In addition, higher VWM concentrations of ionic species in rain events associated with increased rain intensity, suggested by statistical analysis, were also supported by case studies comparing the rain events with the maximum and highest average rain intensities to the rain event with the lowest average rain intensity. The impact of pollution build-up during winter was also illustrated, with rainfall associated with this period corresponding to higher loading of ionic species. The impact of open biomass burning was also indicated, although the peak open biomass burning and wet seasons in South Africa do not coincide. In addition, it was also shown that long-range transport of species associated with open biomass burning could influence rain chemistry in the South African interior.