Factors governing the chemical composition of rain at a regional site in South Africa

Abstract

Precipitation chemistry is influenced by a number of complex processes, which include the temporal and spatial evolution of air masses. Previous wet deposition studies in South Africa could not distinctly relate the influence of air mass history on rain chemistry in order to substantiate the influence of different sources. The Welgegund atmospheric monitoring station in South Africa measures several atmospheric parameters, which include vertical profiling of the atmosphere that could assist in relating rain chemistry to air mass history. Therefore, the aim of this study was to conduct an advanced assessment on factors influencing chemical composition of rain in the South African interior by relating individual rain events at Welgegund to air mass history at cloud base height (CBH) and arrival heights below clouds, as well as supplementary in situ measurements conducted at the site. Hierarchical cluster analysis was performed through two approaches, i.e. clustering based on the chemical composition of rain, as well as grouping based on air masses arriving at CBH and 100 m above ground level (a.g. l.). Although statistical analyses highlighted the complexity associated with correlating rain chemistry to sources of chemical species, it proved useful to determine some correlations between rain chemistry and air mass history. Clustering according to the chemical composition of rain events grouped rain events from high to low volume weighted mean (VWM) concentrations. Correlation of air mass histories to these clusters indicated that higher VWM concentrations were associated with air masses at 100 m a.g.l. passing over anthropogenic source regions. Air mass history clustering grouped air masses passing predominantly over predefined source regions. The rain chemistry of clusters determined for air masses at 100 m a.g.l. arrival heights could be related to the influence of different source regions, with the impact of large point sources, the clean western background region and oceans especially evident. No significant correlations between air mass histories at CBH and ionic composition were evident. It was concluded that below-cloud atmospheric chemical composition was more significant to the chemical composition of rain in this part of South Africa. A few case studies were also conducted in order to further explore factors influencing the chemical composition of rain