Abstract
Experimental data on the precipitation chemistry in the semi-arid savanna of South Africa is presented in this paper. A total of 901 rainwater samples were collected with automatic wet-only samplers at a rural site, Louis Trichardt, and at an industrial site, Amersfoort, from July 1986 to June 1999. The chemical composition of precipitation was analysed for seven inorganic and two organic ions, using ion chromatography. The most abundant ion was SO4 2 − and a large proportion of theprecipitation is acidic, with 98% of samples at Amersfoort and 94% at LouisTrichardt having a pH below 5.6 (average pH of 4.4 and 4.9, respectively). This acidity results from a mixture of mineral and organic acids, with mineral acids being the primary contributors to the precipitation acidity in Amersfoort, while at Louis Trichardt, organic and mineral acids contribute equal amounts of acidity. It was found that the composition of rainwater is controlled by five sources: marine, terrigenous, nitrogenous, biomass burning and anthropogenic sources. The relative contributions of these sources at the two sites were calculated. Anthropogenic sources dominate at Amersfoort and biomass burning at Louis Trichardt. Most ions exhibit a seasonal pattern at Louis Trichardt, with the highest concentrations occurring during the austral spring as a result of agricultural activities and biomass combustion, while at Amersfoort it is less pronounced due to the dominance of relatively constant industrial emissions. The results are compared to observations from other African regions.
Highlights
IntroductionFor example, natural (marine, biogenic, crustal) and anthropogenic emissions influence the chemical composition of trace elements in the atmosphere
Various sources, for example, natural and anthropogenic emissions influence the chemical composition of trace elements in the atmosphere
The main characteristics of the chemical composition of rain collected at Amersfoort and Louis Trichardt, averaged over 13 years, allow emphasising the following points: At Amersfoort, the concentrations of nss-SO24− and NO−3 are as high as those in north-eastern U.S.A. and central Europe, typical of regions affected by acidforming emissions
Summary
For example, natural (marine, biogenic, crustal) and anthropogenic emissions influence the chemical composition of trace elements in the atmosphere. Recent studies in precipitation chemistry have been published for tropical countries (Ayers et al, 2000; Gillett et al, 2000; Williams et al, 1997; Lara et al, 2001). There are relatively few studies on the rainwater chemistry of semi-arid countries (Norman et al, 2001; Satsangi et al, 1998; Herut et al, 2000; Mamane and Gottlieb, 1995), and especially in Africa (Turner et al, 1996; Galy-Lacaux and Modi, 1998). Little information is available for South Africa, which has one of the largest industrialised economies in the southern hemisphere and is the only industrialised regional energy producer on the African continent (Siversten et al, 1995)
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