Chemical composition of rain at a regional site on the South African Highveld

Abstract

Relatively limited data have been published on the chemical composition of wet deposition for South Africa, which is considered an important source region for atmospheric pollutants.  Concentrations and wet deposition fluxes of ionic species determined in rain samples collected from December 2014 to April 2018 at a regional site, Welgegund, are presented, and contextualised by wet deposition composition in the north-eastern interior of South Africa.  89% of rain samples collected during the sampling period complied with the data quality objectives of the World Meteorological Organisation.  The total ionic concentration of rainwater at Welgegund was similar to that at two regional sites located within proximity of industrial activities. The pH of rainwater (4.80) was comparable to that determined at two rural background sites, which indicated increased neutralisation.  Similarly to the other sites located in the South African interior, SO₄^2- was the most abundant species in rain, with concentrations thereof in the same order as SO₄^2- levels determined at the two industrially influenced sites.  Lower sulphur and nitrogen fluxes at Welgegund were attributed to lower average annual rainfall.  The anthropogenic (industrial) source group had the largest contribution to wet deposition chemical composition, which signified the influence of major source regions in the South African interior that impact 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.