# Precipitation chemistry in semi-arid areas of Southern Africa: a case study of a rural and an industrial site

J.-N. Mphepya, Jacobus J Pienaar, Corinne Galy-Lacaux, G. Held, C.-R. Turner

► To cite this version:

J.-N. Mphepya, Jacobus J Pienaar, Corinne Galy-Lacaux, G. Held, C.-R. Turner. Precipitation chemistry in semi-arid areas of Southern Africa: a case study of a rural and an industrial site. Journal of Atmospheric Chemistry, 2004, 47 (1), pp.1-24. 10.1023/B:JOCH.0000012240.09119.c4. hal-00136459

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# Precipitation Chemistry in Semi-Arid Areas of Southern Africa: A Case Study of a Rural and an Industrial Site

J. N. MPHEPYA<fnref n="1" />,<fnref n="4" />, J. J. PIENAAR<fnref n="1" />, C. GALY-LACAUX<fnref n="2" />, G. HELD<fnref n="3" /> and C. R. TURNER<fnref n="4" />

**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 $SO_4^{2-}$ and a large proportion of the precipitation is acidic, with 98% of samples at Amersfoort and 94% at Louis Trichardt 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.

**Key words:** acid rain, chemical composition, precipitation chemistry, semi-arid savanna, wet deposition.

## 1. Introduction

Various sources, for example, natural (marine, biogenic, crustal) and anthropogenic emissions influence the chemical composition of trace elements in the atmosphere. The gases and aerosols from these sources are incorporated into raindrops through various processes during rain events, hence by analysing the chemical composition of rain samples, the chemical species present in the atmosphere can be

<footnote n="1">¹School of Chemistry and Biochemistry, Potchefstroom University for Christian Higher Education, Potchefstroom 2520, South Africa, e-mail: CHEJJP@PUKNET.PUK.AC.ZA</footnote>

<footnote n="2">²Laboratoire d'Aérologie, Observatoire Midi-Pyrénées, Toulouse, France</footnote>

<footnote n="3">³Instituto de Pesquisas Meteorológicas, Universidade Estadual Paulista, Bauru, S.P., Brazil</footnote>

<footnote n="4">⁴Eskom Technology Services International, Private Bag X40175, Cleveland 2022, South Africa</footnote>
identified and quantified to obtain information about the atmospheric constituents. Knowledge of the chemical composition of precipitation is therefore critical in understanding regional and local air pollution and its effects on the eco-systems.

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). In contrast, 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). Relatively 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).

Eskom, the major power utility in South Africa, has been monitoring rain chemistry on a continuous basis since 1985 (Turner *et al.*, 1996) and, as of 1995, has supported the International Global Atmospheric Chemistry (IGAC) programme DEBITS (Deposition of Biogeochemically Important Trace Species) for Africa IDAF (IGAC DEBITS AFRICA) with wet deposition and passive sampling results from two of its monitoring sites. The main focus of the IDAF programme is to identify the chemical cycles of important nutrient species in the atmosphere, including the emissions from biomass burning, ecosystems and man-induced activities, such as change of land use and industrial emissions. To achieve this, the IDAF project set up 10 monitoring stations in various African ecosystems. The precipitation composition differs significantly in various tropical regions, depending on the dominant sources within the region considered.

In this paper the precipitation chemistry data from two sites on the South African plateau ('Highveld'), characterising industrial (Amersfoort) and rural (Louis Trichardt) environments, respectively, are presented for the period 1986 to 1999, within the context of the IDAF network. The main objectives are to:

* determine the mean chemical composition of rainwater in a rural and an industrial area within the semi-arid savanna of South Africa, based on a large set of quality-checked data;

* identify the potential anthropogenic and natural sources of the chemical content of precipitation;

* determine the seasonal variability and temporal trends of the rainwater composition;

* present a comparative study of precipitation characteristics between the semi-arid regions of Africa, viz., Niger and South Africa.

## 2. Regional Climate and Sampling Sites

Most of the rainfall over the central and northern parts of South Africa occurs during the summer months from October to March, mostly in the form of convective precipitation, while the southern part (Cape Province) is characterised by
winter rainfall. The predominant circulation type in southern Africa is governed by
subtropical recirculation of air over the subcontinent with the average frequency
of recirculated air being estimated to be 64% (Tyson *et al.*, 1996). Over the central
interior the airflow responsible for the dispersion of pollutants is mostly from north-
westerly directions. Details of the climate characteristics and their relevance to the
transport of pollutants are summarised in Held *et al*. (1996), while the most com-
mon pathways, based on a five-year trajectory climatology, have been identified by
Piketh and Prangley (1998).

Figure 1 shows the location of the 10 IDAF stations operating in 2002. The Amersfoort site (No. 9) is located at 27°08′ south and 29°49′ east in the south-eastern part of the Mpumalanga Province, 1646 m above mean sea level (amsl). This is roughly 200 km south-east of Johannesburg and 50–100 km south-east to south-south-east of the major industrial activities, which also include the main centre of electric power generation in the Mpumalanga Highveld area. Amersfoort falls into a climatological transition region between the Highveld and the nearby escarpment. The nearest representative long-term climatological observations are from Ermelo, about 90 km north-north-east of Amersfoort (SAWB, 1986). The mean annual precipitation there is 755 mm, with January (126 mm) and November (131 mm) having the highest mean monthly rainfall. Rain is recorded on 96.7 days/year (≥0.1 mm), of which on average 24 days receive ≥10 mm/day. However, from May to August the average monthly rainfall totals are ≤20 mm. Surface winds on the South African plateau are mostly driven by orographic features at night (very stable boundary layer), while during daytime the overlying gradient winds will break through (convective boundary layer), resulting in large diurnal and seasonal variations (Held *et al.*, 1996). For the purpose of rain quality one should therefore rather look at winds within the mixing layer (2,500 m amsl; SAWB, 1950), which would be responsible for the dispersion of pollutants. During winter and spring, most winds in the Amersfoort area blow from westerly directions (south-west to north-west, 78%), being slightly more south-westerly in autumn, while during summer only 47% of winds come from the west and 28% from south-easterly directions. Thus, pprevailing winds can be expected to frequently transport pollutants from the metropolitan and industrial regions (Gauteng and Mpumalanga) towards this site. This station is representative of a site strongly influenced by industry in a semi-arid area in southern Africa.

The Louis Trichardt site (No. 8) is located at 23°00′ south and 30°02′ east in the Northern Province, 961 m amsl, roughly 400 km to the north-north-east of Johannesburg. Although the mean annual precipitation is very similar (748 mm) to that at Amersfoort, the distribution of rain differs, viz. only 83.8 days/year with ≥0.1 mm are recorded on average, of which 20 days/year receive ≥10 mm/day. January is the month with the highest mean total (164 mm), while only 8 mm fall during August. From May to September ≤20 mm/month are recorded on average (SAWB, 1986). Winds over the Northern Province (Pietersburg wind rose for 2,500 m amsl; SAWB, 1950) are more uniform in frequency than in the Amersfoort area, but show
<figure><img src="image_2.png" /><figcaption>**Figure 1.** (a) Vegetation and location map of the 10 measurement stations of the IDAF network in 2002 (1. Banizoumbou, Niger; 2. Senpou, Mali; 3. Lamto, Ivory Coast; 4. Ngola, RCA; 5. Bomassa, Congo; 6. Zoétélé, Cameroon; 7. Mount Kenya, Kenya; 8. Louis Trichardt, RSA; 9. Amersfoort, RSA; 10. Cape Point, RSA). (b) The Louis Trichardt and Amersfoort sampling sites, as well as major power stations are shown in the map below (after Galy-Lacaux and Modi, 1998).</figcaption></figure>
a distinct seasonal variation with a more dominant easterly component during summer, veering through south-east in autumn, to predominantly southerly in winter, followed by slightly more frequent northerly-component winds in spring. Winds rarely blow directly from the industrialised region towards this site. However, the site is ideally located to detect any pollution that has been recirculated within the frequently occurring sub-continental anticyclone system. There are no major cities or towns within a 100 km radius. The area is predominantly used for agricultural purposes, such as maize and cattle farming, and therefore it can be assumed that Louis Trichardt represents a rural site in a semi-arid region in southern Africa.

# 3. Collection Procedure, Chemical Analysis and Data Quality

## 3.1. COLLECTION PROCEDURE

Rainwater samples were collected on an event basis with an automated wet-only sampler (AeroChemetric) that exposes the collector, a high-density polyethylene (HDPE) bucket, only when precipitation is falling. The device is equipped with a sensor that detects the onset of precipitation and activates a mechanism that opens the lid with the first raindrop and closes it just after the rain event. The advantages of automated wet-only sampling are: contamination of rainwater by dust is minimal before and after the events, and the delay in the execution of manual operation is reduced to a minimum. All samples were removed from the collectors as soon as possible, but at least within 24 hours after an event ended. Thereafter, the collected precipitation was divided into two samples in polyethylene bottles. The pH of one of the samples was immediately measured and the other sample was stored in frozen condition until being transported to the laboratory for chemical analysis. Upon arrival at the laboratory, 25 ml of aliquots were taken for immediate conductivity and pH measurements. The remaining sample was filtered through a 0.2 µm filter and the filtrates were chemically analysed.

## 3.2. CHEMICAL ANALYSIS

The chemical content of rainwater samples was determined by ion chromatography at the chemical laboratory of Eskom TSI. The ion chromatography systems used in this study are presented in Table I, including the main characteristics of the analytical parameters for the determination of mineral ions (sodium, ammonium, potassium, magnesium, calcium, chloride, nitrate and sulphate) and organic ions in rainwater samples. The highest-grade salts commercially available from suppliers were used to prepare stock standards from which appropriate dilutions were made. The analytical precision of organic ion measurements was 10%, while for other ions the precision was 5% or better. $H^+$ was calculated from the pH measurements using a Radiometer instrument with a combined electrode (ROSS) filled with KCl (3 M) and saturated with AgCl. For pH calibration, two buffer solutions at pH 4.01
and 7.00 were used. After the pH calibration, a buffer solution at pH 6.01 was used as control sample.

### 3.3. DATA QUALITY

In performing data quality control in the field, an attempt is made to identify and exclude samples, which were obviously contaminated or are questionable. Several methods were used to determine the suitability of a particular sample for inclusion in the database. Firstly, a sample could be visibly contaminated, i.e., if the sample was discoloured or contained insects or other foreign material. The site operator screens the sample to exclude such contaminated samples.

Secondly, the chemical laboratory of Eskom TSI is participating in the World Meteorological Organization (WMO) inter-laboratory comparisons since 1995 (WMO, 1997, 1998, 1999). This program annually evaluated all ion chromatography chemical analysis, pH and conductivity measurements performed in the laboratory and results compared to the reference values. Figure 2 shows the results of the WMO inter-calibration in 1999 for all mineral ions included in the synthetic samples (WMO, 1999). Ion concentrations analysed by the Eskom Laboratory are well correlated with the linear regression coefficient *r* being close to 1. According to the Eskom WMO intercomparison results, the analytical precision for all the ions species presented in this study is estimated to be better than 5%. Further quality control of the chemical analyses of field samples is performed by calculating either the ionic balance and/or the conductivity test. The latter is specifically used to detect analytical errors, as well as to ensure that major ions had been determined accurately. The calculations are based on the percent ion balance criteria, as described by WMO (EPA, 1994). The criteria has been applied to all precipitation samples in the database and only data that passed the WMO specifications was used in this study.

The total number of precipitation samples collected and chemically analysed during the study period is given in Table II. From July 1986 to June 1999, 655 and 314 rain events were collected at Amersfoort and Louis Trichardt, respectively, with distinct wet (October through March) and dry (April to September) seasons. In total, 588 and 262 samples for Amersfoort and Louis Trichardt, respectively, were send to the Eskom Laboratory for chemical analysis, while 67 and 52 rain events, respectively, had only sufficient volume for measurements of pH at the field site. Of these samples, 437 and 223, respectively, passed the WMO criteria and were used in this study. The mean annual precipitation during the study period was 755 and 745 mm, respectively at Amersfoort and Louis Trichardt. The total rainfall depth of 5,888 and 4,678 mm, collected at Amersfoort and Louis Trichardt, respectively, can be considered as representative of the entire rainy season (July–June) during which a total of 7,321 and 6,012 mm of precipitation, respectively, was recorded.
<table><caption>Table I. Ion Chromatography analytical parameters and sequence for chemical analysis of the precipitation samples</caption><thead><tr><th>Apparatus</th><th>Analysis</th><th>Columns</th><th>Regeneration</th><th>Eluant/rate<br>(analysis time)</th><th>Calibration</th><th>Control standard</th></tr></thead><tbody><tr><td>Dionex 4000<br>series</td><td>Anions +<br>organic acids</td><td>AG4A-SC +<br>AS4A-SC</td><td>Chemical<br>suppression<br>(regeneration<br>by)</td><td>NaOH gradient<br>0.5 mM<br>↓<br>0.0125 M<br>H<sub>2</sub>SO<sub>4</sub>)</td><td>Initial solution:<br>1000 ppm Cl<sup>-</sup>,<br>CH<sub>3</sub>COO<sup>-</sup>, HCOO<sup>-</sup><br>2500 ppm SO<sub>4</sub><sup>2-</sup>,<br>NO<sub>3</sub><sup>-</sup></td><td>Prepared in-house:<br>CH<sub>3</sub>COO<sup>-</sup> = 3.00 ppm<br>HCOO<sup>-</sup> = 1.50 ppm<br>Cl<sup>-</sup> = 2.00 ppm<br>NO<sub>3</sub><sup>-</sup> = 1.00 ppm<br>SO<sub>4</sub><sup>2-</sup> = 4.00 ppm</td></tr><tr><td>gradient<br>mode +<br>automated<br>sampler</td><td>HCOOH<br>CH<sub>3</sub>COOH<br>Cl<sup>-</sup><br>NO<sub>3</sub><sup>-</sup><br>SO<sub>4</sub><sup>2-</sup></td><td></td><td></td><td>40 mM<br>1.5 ml/min<br>(26 min)</td><td>Dilution factors:<br>1/500, 1/1000,<br>1/2000</td><td></td></tr><tr><td>Dionex 4000<br>series</td><td>Cations</td><td>CG 12 +<br>CS 12</td><td>Electric<br>suppression<br>(SRS)</td><td>20 mM M.S.A.<br>1 ml/min<br>(12 min)</td><td>Initial solution:<br>Na<sup>+</sup>, NH<sub>4</sub><sup>+</sup>, Ca<sup>2+</sup> =<br>1000 ppm<br>K<sup>+</sup>, Mg<sup>2+</sup> = 500 ppm</td><td>Prepared in-house:<br>Na<sup>+</sup>, NH<sub>4</sub><sup>+</sup> = 0.50<br>ppm<br>K<sup>+</sup>, Mg<sup>2+</sup> = 0.20 ppm<br>Ca<sup>2+</sup> = 0.40 ppm</td></tr><tr><td>manual<br>(loop)<br>injection</td><td>Na<sup>+</sup><br>NH<sub>4</sub><sup>+</sup><br>K<sup>+</sup><br>Mg<sup>2+</sup><br>Ca<sup>2+</sup></td><td></td><td></td><td></td><td>Dilution factors:<br>1/500, 1/1000,<br>1/2000</td><td></td></tr></tbody></table>

The following steps describe the analysis sequence: 1 – standards (high first, low last), 2 – demineralised water, 3 – control standard, 4–5 samples and control standard, 5 – repeat step 4 for up to 20 samples. (For more than 20 samples new standards are prepared and run).
<figure><img src="image_3.png" /><figcaption>Figure 2. Results of the 21st inter-laboratory comparison of the World Meteorological Organisation (WMO, 1999) for the IC analysis of mineral ions (Na⁺, NH₄⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, NO₃⁻, SO₄²⁻) at the Eskom Laboratory in 1999.</figcaption></figure>

<table><caption>Table II. Summary of rain samples collected at Amersfoort and Louis Trichardt between July 1986 and June 1999</caption><thead><tr><th rowspan="2"></th><th colspan="2">Number of events</th><th colspan="2">Precipitation amount (mm)</th></tr><tr><th>Amersfoort</th><th>Louis Trichardt</th><th>Amersfoort</th><th>Louis Trichardt</th></tr></thead><tbody><tr><td>Total number of samples collected</td><td>655</td><td>314</td><td>7321</td><td>6012</td></tr><tr><td>Samples measured for field pH only</td><td>67</td><td>52</td><td>519</td><td>521</td></tr><tr><td>Samples sent for chemical analysis</td><td>588</td><td>262</td><td>6803</td><td>5492</td></tr><tr><td>Samples which passed the WMO criteria</td><td>437</td><td>223</td><td>5888</td><td>4678</td></tr></tbody></table>

## 4. Chemical Composition of Precipitation

Table III presents the volume-weighted mean chemical composition, the annual wet deposition and the averaged pH calculated from the volume weighted mean concentration of H⁺ for Amersfoort and Louis Trichardt.
<table><caption>Table III. Volume-weighted mean (VWM) chemical composition of precipitation and yearly wet deposition at Amersfoort and Louis Trichardt during 1986–1999. The measurements from Banizoumbou only represent the 1996 rainy season</caption><thead><tr><th rowspan="2">Ions</th><th colspan="2">Amersfoort</th><th colspan="2">Louis Trichardt</th><th colspan="2">Banizoumbou<sup>c</sup></th></tr><tr><th>VWM<br>μeq.l<sup>-1</sup></th><th>Wet deposition<br>mmol.m<sup>-2</sup>.yr<sup>-1</sup></th><th>VWM<br>μeq.l<sup>-1</sup></th><th>Wet deposition<br>mmol.m<sup>-2</sup>.yr<sup>-1</sup></th><th>VWM<br>μeq.l<sup>-1</sup></th><th>Wet deposition<br>mmol.m<sup>-2</sup>.yr<sup>-1</sup></th></tr></thead><tbody><tr><td>ph</td><td>4.35</td><td></td><td>4.91</td><td></td><td>5.7</td><td></td></tr><tr><td>H<sup>+</sup></td><td>44.9</td><td>329.0</td><td>12.2</td><td>73.1</td><td>2.1</td><td>1.0</td></tr><tr><td>Na<sup>+</sup></td><td>9.3</td><td>68.2</td><td>9.3</td><td>56.1</td><td>7.7</td><td>3.7</td></tr><tr><td>NH<sub>4</sub><sup>+</sup></td><td>22.3</td><td>163.4</td><td>9.7</td><td>58.0</td><td>12.9</td><td>6.3</td></tr><tr><td>K<sup>+</sup></td><td>4.7</td><td>34.1</td><td>3.8</td><td>22.8</td><td>4.7</td><td>2.3</td></tr><tr><td>Ca<sup>2+</sup></td><td>18.7</td><td>68.6</td><td>12.0</td><td>36.0</td><td>31.2</td><td>7.6</td></tr><tr><td>Mg<sup>2+</sup></td><td>6.7</td><td>24.5</td><td>4.1</td><td>12.2</td><td>5.6</td><td>1.4</td></tr><tr><td>NO<sub>3</sub><sup>-</sup></td><td>25.0</td><td>183.0</td><td>8.0</td><td>48.2</td><td>12.3</td><td>6.0</td></tr><tr><td>Cl<sup>-</sup></td><td>9.8</td><td>71.6</td><td>10.0</td><td>60.1</td><td>7.4</td><td>3.6</td></tr><tr><td>SO<sub>4</sub><sup>2-</sup></td><td>59.1</td><td>216.0</td><td>14.5</td><td>43.6</td><td>8.6</td><td>2.1</td></tr><tr><td>HCOO<sup>-</sup></td><td>7.5 (6.0)<sup>a</sup></td><td>55.3</td><td>12.9 (11.5)<sup>a</sup></td><td>77.6</td><td>4.7 (4.5)<sup>a</sup></td><td>2.3</td></tr><tr><td>CH<sub>3</sub>COO<sup>-</sup></td><td>6.1 (2.0)<sup>a</sup></td><td>44.8</td><td>8.2 (4.3)<sup>a</sup></td><td>49.3</td><td>3.1 (2.7)<sup>a</sup></td><td>1.5</td></tr><tr><td>No. of events</td><td>437.0</td><td></td><td>223.0</td><td></td><td>29</td><td></td></tr><tr><td>Total (mm): total</td><td>7321.3</td><td></td><td>6012.2</td><td></td><td>489.5</td><td></td></tr><tr><td>Annual mean</td><td>563.2</td><td></td><td>462.5</td><td></td><td>489.5</td><td></td></tr></tbody></table>

a Dissociated fraction.

b Wet deposition is calculated by using the annual mean rainfall amount for the period 1986–1999.

c Galy-Lacaux and Modi (1998).
The volume-weighted composition in µeq.l⁻¹ of any ion was computed from the ionic concentration Cₙ and the depth of the rainfall Hₙ (mm) of the events collected:

$$ \text{Volume weighted mean} = \sum C_n H_n / \sum H_n . $$

The annual wet deposition, expressed in mmol.m⁻².yr⁻¹, is calculated from the total rainfall amount. The period mean pH value at Amersfoort, calculated from the volume-weighted mean concentration of H⁺, is 4.35 (Table III), which indicates an acidic value. However, the pH of individual events showed a very large fluctuation, ranging from 3.5 to 6.2 (Figure 3). The overwhelming majority of precipitation events, viz. 98%, had a pH value below 5.6, which is the pH of unpolluted water equilibrated with atmospheric CO₂, 8% were below 4.0, while only 1% of the samples showed pH values of above 6.0.

At Louis Trichardt, the annual mean pH was 4.91, which is only a slightly acidic value. As can be seen from Figure 3, the distribution of the pH range is shifted towards higher pH values, with individual events ranging from 4.1 to 6.2. About 94% showed a pH of less than 5.6, but no samples were below 4.0, while about 6% of precipitation events had a pH above 6.0, suggesting a strong presence of alkaline species in these rain samples. The IDAF station of Niger, Banizoumbou, also presents a weak acidity with an average pH = 5.7 (Galy-Lacaux and Modi, 1998). Similar results were reported for Haifa in Israel by Mamane and Gottlieb (1995).

The precipitation at Amersfoort is dominated by the SO₄²⁻ ion (59.1 µeq.l⁻¹), with the next most abundant ions being H⁺, NO₃⁻, NH₄⁺, and Ca²⁺ with 44.9, 25.0, 22.3, and 18.7 µeq.l⁻¹, respectively (Table III). These five ions represent 79% of the ionic content of the rainwater samples. At Louis Trichardt, the precipitation is also dominated by SO₄²⁻, but to a lesser degree than at Amersfoort, with only 14.5 µeq.l⁻¹. The other abundant ions are H⁺, Ca²⁺, NH₄⁺, and NO₃⁻ with concentrations of 12.2, 12.0, 9.7, and 8.0 µeq.l⁻¹, respectively (Table III). Unlike Amersfoort and Louis Trichardt, Banizoumbou shows a high Ca²⁺ concentration, followed by NH₄⁺ and NO₃⁻ (Galy-Lacaux and Modi, 1998).

The chemical composition of precipitation was further studied by calculating correlation coefficients. The ‘Spearman’ method was selected to compute the correlation coefficients at the 95% confidence interval. The correlation coefficients between the different species are examined in order to determine the potential sources that influence the rainwater composition and/or the potential origin of some of the species that are transported in the same air masses.

## 4.1. MARINE AND TERRIGENOUS CONTRIBUTIONS

The most common method of evaluating the contribution of sea salts to ion concentrations in precipitation is to compare the Cl⁻/Na⁺ ratio in the precipitation to that in seawater. The sea is considered to be a major source of both ions. At both sites the annual ratio of Cl⁻ and Na⁺ are close to the seawater ratio (Table V). The
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>Figure 3. Frequency distribution of rainfall pH values for Amersfoort (top) and Louis Trichardt (bottom) for the 1986–1999 rainy seasons.</figcaption></figure>

relatively strong correlation between the two ions (correlation coefficients of 0.78 and 0.86 for Amersfoort and Louis Trichardt, respectively; Table IV) shows that the marine constituents of the Indian Ocean influence the chemical content of the precipitation. The mean annual ratios of chlorine to sodium ions for Amersfoort and Louis Trichardt (1.054 and 1.075, respectively) were very close to that of seawater (1.161). According to the $Cl^-$/$Na^+$ ratio measured, one can calculate the contribution of sea salt particles in the dissolved phase using the following equation:

$$[X]_{\text{marine}} = ([X]/[Na^+]_{\text{seawater}} \times [Na^+]_{\text{rain}},$$
<table><caption>Table IV. Correlation coefficients between some ions in rainwater for the dry season, wet season and annually, 1986–1999. The Spearman' method was used to compute the 95% confidence intervals of the correlation coefficients. Only pairs with correlation above 0.5 were included</caption><thead><tr><th rowspan="2">Ion pairs</th><th colspan="3">Amersfoort</th><th colspan="3">Louis Trichardt</th></tr><tr><th>Dry season</th><th>Wet season</th><th>Annual</th><th>Dry season</th><th>Wet season</th><th>Annual</th></tr></thead><tbody><tr><td>H<sup>+</sup> vs SO<sub>4</sub><sup>2-</sup></td><td>0.87</td><td>0.86</td><td>0.86</td><td>0.75</td><td>0.68</td><td>0.68</td></tr><tr><td>H<sup>+</sup> vs NO<sub>3</sub><sup>-</sup></td><td>0.75</td><td>0.80</td><td>0.79</td><td>0.63</td><td>0.64</td><td>0.63</td></tr><tr><td>H<sup>+</sup> vs HCOO<sup>-</sup></td><td></td><td></td><td></td><td></td><td>0.65</td><td>0.62</td></tr><tr><td>H<sup>+</sup> vs CH<sub>3</sub>COO<sup>-</sup></td><td></td><td></td><td></td><td></td><td>0.57</td><td>0.53</td></tr><tr><td>NH<sub>4</sub><sup>+</sup> vs SO<sub>4</sub><sup>2-</sup></td><td>0.72</td><td>0.78</td><td>0.77</td><td>0.83</td><td>0.63</td><td>0.63</td></tr><tr><td>NH<sub>4</sub><sup>+</sup> vs NO<sub>3</sub><sup>-</sup></td><td>0.74</td><td>0.75</td><td>0.74</td><td>0.85</td><td>0.75</td><td>0.76</td></tr><tr><td>Ca<sup>2+</sup> vs SO<sub>4</sub><sup>2-</sup></td><td>0.68</td><td></td><td></td><td>0.54</td><td></td><td></td></tr><tr><td>Ca<sup>2+</sup> vs NO<sub>3</sub><sup>-</sup></td><td>0.67</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mg<sup>2+</sup> vs SO<sub>4</sub><sup>2-</sup></td><td>0.64</td><td>0.55</td><td>0.52</td><td>0.53</td><td>0.51</td><td>0.51</td></tr><tr><td>Mg<sup>2+</sup> vs NO<sub>3</sub><sup>-</sup></td><td>0.74</td><td>0.50</td><td>0.52</td><td></td><td></td><td></td></tr><tr><td>Na<sup>+</sup> vs Cl<sup>-</sup></td><td>0.66</td><td>0.87</td><td>0.78</td><td>0.98</td><td>0.71</td><td>0.86</td></tr><tr><td>Mg<sup>2+</sup> vs Cl<sup>-</sup></td><td>0.69</td><td></td><td>0.52</td><td>0.69</td><td></td><td>0.52</td></tr><tr><td>K<sup>+</sup> vs Cl<sup>-</sup></td><td>0.73</td><td>0.55</td><td>0.59</td><td>0.73</td><td>0.55</td><td>0.59</td></tr><tr><td>Ca<sup>2+</sup> vs Mg<sup>2+</sup></td><td>0.71</td><td>0.60</td><td>0.57</td><td>0.59</td><td>0.60</td><td>0.59</td></tr><tr><td>Mg<sup>2+</sup> vs Na<sup>+</sup></td><td>0.65</td><td>0.50</td><td></td><td>0.90</td><td>0.65</td><td>0.68</td></tr><tr><td>SO<sub>4</sub><sup>2-</sup> vs NO<sub>3</sub><sup>-</sup></td><td>0.80</td><td>0.86</td><td>0.85</td><td>0.71</td><td>0.65</td><td>0.59</td></tr><tr><td>CH<sub>3</sub>COO<sup>-</sup> vs HCOO<sup>-</sup></td><td>0.67</td><td>0.71</td><td>0.70</td><td>0.64</td><td>0.84</td><td>0.83</td></tr><tr><td>NH<sub>4</sub><sup>+</sup> vs CH<sub>3</sub>COO<sup>-</sup></td><td></td><td></td><td></td><td></td><td>0.62</td><td>0.59</td></tr><tr><td>NH<sub>4</sub><sup>+</sup> vs HCOO<sup>-</sup></td><td></td><td>0.60</td><td>0.55</td><td></td><td>0.72</td><td>0.69</td></tr></tbody></table>

where $[X]_{\text{marine}}$ is the marine contribution for element X in $\mu\text{eq. l}^{-1}$, $[\text{Na}^{+}]_{\text{rain}}$ is the concentration of sodium ions in rain ($\mu\text{eq. l}^{-1}$), and $[X]/[\text{Na}^{+}]_{\text{seawater}}$ is the concentration ratio in seawater (Keene et al., 1986). The sum of marine contributions ($\text{Mg}^{2+}$, $\text{Ca}^{2+}$, $\text{Cl}^{-}$, $\text{SO}_{4}^{2-}$ and $\text{K}^{+}$ ions) to the total ionic content of Amersfoort and Louis Trichardt was 11 and 23%, respectively (calculated from Table III). As shown in Table IV, $\text{Mg}^{2+}$ was correlated with $\text{Ca}^{2+}$ (0.57 at Amersfoort and 0.59 at Louis Trichardt), as well as with $\text{SO}_{4}^{2-}$ (0.52 at Amersfoort and 0.51 at Louis Trichardt). This positive relationship suggests the importance of the particles eroded from semi-arid regions to the chemical content of precipitation. Soil dust is the major constituent of the regional aerosol loading of the atmosphere over South Africa (Piketh et al., 1999).

Taking into account the marine contribution determined above and adjusting for the contribution of $\text{SO}_{4}^{2-}$ from terrigenous sources, the sum of the potential
<table><caption>Table V. Seasonal and annual ionic ratio in rain</caption><thead><tr><th rowspan="2">Ions</th><th rowspan="2">Sea-water <sup>a</sup></th><th colspan="3">Amersfoort</th><th colspan="3">Louis Trichardt</th></tr><tr><th>Dry season</th><th>Wet season</th><th>Annual</th><th>Dry season</th><th>Wet season</th><th>Annual</th></tr></thead><tbody><tr><td>K<sup>+</sup>/Na<sup>+</sup></td><td>0.022</td><td>1.108</td><td>0.424</td><td>0.505</td><td>0.324</td><td>0.474</td><td>0.409</td></tr><tr><td>Ca<sup>2+</sup>/Na<sup>+</sup></td><td>0.044</td><td>2.598</td><td>1.935</td><td>2.011</td><td>1.080</td><td>1.460</td><td>1.290</td></tr><tr><td>Mg<sup>2+</sup>/Na<sup>+</sup></td><td>0.227</td><td>1.108</td><td>0.663</td><td>0.720</td><td>0.408</td><td>0.474</td><td>0.441</td></tr><tr><td>Cl<sup>-</sup>/Na<sup>+</sup></td><td>1.161</td><td>1.853</td><td>0.946</td><td>1.054</td><td>1.080</td><td>1.066</td><td>1.075</td></tr><tr><td>SO<sub>4</sub><sup>2-</sup>/Na<sup>+</sup></td><td>0.121</td><td>6.461</td><td>6.337</td><td>6.355</td><td>1.746</td><td>1.553</td><td>1.559</td></tr></tbody></table>

terrigenous ions (Ca²⁺ + Na⁺ + SO₄²⁻ + Mg²⁺) can be estimated for Amersfoort and Louis Trichardt to be 19 and 27% of the total ionic precipitation content, respectively. In contrast, at Banizoumbou, potential terrigenous ions contribute 57% of the total ionic content of which 96% is from Ca²⁺. This high terrigenous contribution could emphasise the direct influence of soil dust on precipitation chemistry in this region, therefore highlighting the importance of the heterogeneous chemistry between gas and soil particles on precipitation chemistry and the associated indirect terrigenous contributions (Galy-Lacaux and Modi, 1998; Galy-Lacaux et al., 2001). The wet deposition of SO₄²⁻ at Amersfoort and Louis Trichardt is 216.3 and 43.6 mmol.m⁻².yr⁻¹, respectively (Table III). The SO₄²⁻ concentration in wet deposition measured at Amersfoort is of the same magnitude to that of industrial sites in North American (Dovland and Pederson, 1996). The SO₄²⁻ wet deposition at Louis Trichardt is high in comparison with rural semi-arid savanna eco-system sites in Africa, e.g., Banizoumbou (2.1 mmol.m⁻².yr⁻¹; Table III).

## 4.2. NITROGENOUS CONTRIBUTIONS

NO₃⁻ and NH₄⁺ concentrations of 25.0 and 22.3 µeq.l⁻¹, respectively, in the precipitation at Amersfoort are relatively high compared to Louis Trichardt (8.0 and 9.7 µeq.l⁻¹, respectively; Table III). NO₃⁻ in precipitation tends to be highest where air is most polluted with nitrogen oxides. Both Amersfoort and Louis Trichardt sites do not have a high population density, nor numerous vehicles. However, Amersfoort is downwind of the industrial region where NOₓ emissions are high. At both sites, NH₄⁺, NO₃⁻ and SO₄²⁻, all of which can be attributed to anthropogenic pollution, are highly correlated to each other (Table IV), although the correlations at Louis Trichardt appear to be higher in the dry season than the wet season, while at Amersfoort they are slightly higher during the wet season. The NO₃⁻ and NH₄⁺ concentrations at Louis Trichardt are more or less comparable to those of Banizoumbou (12.3 and 12.9 µeq.l⁻¹, respectively; Table III). On the African ecosystem scale, the amount of inorganic nitrogen contained in precipitation (Ta-
ble III) at Louis Trichardt, which is representative of a dry savanna region, is comparable to those of the wet savanna of Lamto ($NO_3^-$ = 8.9 and $NH_4^+$ = 16.8 $\mu$eq.l$^{-1}$; Yoboué, 1991) and the equatorial forest of Dimonika (8.6 and 6.4 $\mu$eq.l$^{-1}$, respectively; Lacaux *et al.*, 1992). Higher $NO_3^-$ and $NH_4^+$ concentrations were measured in Gopalpura, India (42.6 and 43.4 $\mu$eq.l$^{-1}$, respectively; Satsangi *et al.*, 1998).

$NH_4^+$ is another form in which nitrogen occurs in rainwater. The $NH_4^+$ component of precipitation results from the incorporation of gaseous $NH_3$ and particles containing $NH_4^+$ in clouds and rainwater. Major sources of $NH_3$ include bacterial decomposition of urea in animal excreta and emissions from natural or fertilised soils (Schlesinger and Hartley, 1992), savanna fires and domestic fuel wood burning (Delmas *et al.*, 1991; Brocard *et al.*, 1996). $NH_4^+$ wet deposition at Amersfoort and Louis Trichardt is 163.4 and 58.0 mmol.m$^{-2}$.yr$^{-1}$, respectively. These values are higher than at Banizoumbou (6.3 mmol.m$^{-2}$.yr$^{-1}$) and illustrate the intensity of the $NH_4^+$ sources in the semi-arid savanna (Table III), possibly due to agricultural activities and the petrochemical plants on the Mpumalanga Highveld, which have been found to emit $NH_3$ (Van der Walt *et al.*, 1998).

$NO_3^-$ in rainfall is the final result of multiphase reaction processes of nitrogenous gases, atmospheric particles and cloud water. Nitric oxide is the major oxide of nitrogen released from biomass burning. Biogenic NO emissions from South African savannas are equal to the amount of NO generated by biomass burning, but slightly less than those from industry (Otter *et al.*, 2001). Soil texture, soil water content, soil nitrogen status and burning present a high spatial and temporal variability of NO emissions (Matson *et al.*, 1989; Ambus and Christensen, 1994; Levine *et al.*, 1996). The nitrogen compound flux necessary to equilibrate the Amersfoort and Louis Trichardt wet deposition of 163.4 and 58.0 mmol.m$^{-2}$.yr$^{-1}$ (Table III), respectively, is 74 and 25 ngN(NO) m$^{-2}$.s$^{-1}$, respectively. Parsons *et al.* (1996) measured emissions of 3.5 ngN(NO) m$^{-2}$.s$^{-1}$ on the infertile soils in the semi-arid savanna of South Africa. The regional emissions from savanna soils are therefore not sufficient to equilibrate the wet deposition of nitrate measured at Amersfoort and Louis Trichardt. This is contrary to the situation at the Banizoumbou site (Galy-Lacaux and Modi, 1998).

### 4.3. ACIDIC CONTRIBUTIONS

Like in other parts of the world, acidity is a key aspect of atmospheric chemistry in southern Africa. As can be seen in Table IV, $H^+$ is highly correlated with $SO_4^{2-}$ and $NO_3^-$ at both sites, although the correlation between $H^+$ and $SO_4^{2-}$ was slightly higher in the dry season than in the wet season. In general, $CH_3COO^-$ and $HCOO^-$ were highly correlated (0.70 at Amersfoort and 0.83 at Louis Trichardt) at both sites. However, $CH_3COO^-$ and $HCOO^-$ were correlated with $H^+$ only at Louis Trichardt (Table IV). For this reason it can be concluded, that these species contribute a substantial amount to the acidity of Louis Trichardt's rainwater.
<table><caption>Table VI. Estimation of SO₄²⁻ sources in µeq.l⁻¹</caption><thead><tr><th>Site</th><th>Total</th><th>Excess</th><th>Marine</th><th>Terri-<br>genous</th><th>Anthropo-<br>genic</th></tr></thead><tbody><tr><td>Amersfoort</td><td>59.1</td><td>58.0</td><td>1.1</td><td>8.8</td><td>49.2</td></tr><tr><td>Louis Trichardt</td><td>14.5</td><td>13.4</td><td>1.1</td><td>5.6</td><td>7.8</td></tr></tbody></table>

<table><caption>Table VII. Contribution of mineral and organic acids to the total acidity</caption><thead><tr><th rowspan="2"></th><th colspan="2">Amersfoort</th><th colspan="2">Louis Trichardt</th></tr><tr><th>µeq.l<sup>-1</sup></th><th>%</th><th>µeq.l<sup>-1</sup></th><th>%</th></tr></thead><tbody><tr><td>Sulphuric acid</td><td>49.2</td><td>60</td><td>7.8</td><td>25</td></tr><tr><td>Nitric acid</td><td>25.0</td><td>30</td><td>8.0</td><td>25</td></tr><tr><td>Organic acids</td><td>8.0</td><td>10</td><td>15.8</td><td>50</td></tr><tr><td>Total H<sup>+</sup>, estimated</td><td>82.2</td><td></td><td>31.6</td><td></td></tr><tr><td>H<sup>+</sup>, measured</td><td>44.9</td><td></td><td>12.2</td><td></td></tr></tbody></table>

The excess SO₄²⁻ can be attributed to either terrigenous sources or anthropogenic pollution. Assuming that the SO₄²⁻ contribution from terrigenous sources is the concentration in excess of that supplied by gypsum (Delmas, 1981)

$$[SO_4^{2-}]_{\text{anthropogenic}} = [SO_4^{2-}]_{\text{rainfall}} - 0.47[Ca^{2+}]$$

and adjusting for the contributions of sulphate from marine and terrigenous sources, one can infer the contribution of the anthropogenic sources (Table VI).

The average acidity at Amersfoort (44.9 µeq.l⁻¹) is high compared to that at Louis Trichardt (12.2 µeq.l⁻¹; Table III). Assuming that acidity is due to excess SO₄²⁻, organic acids (HCOO⁻ + CH₃COO⁻) and NO₃⁻, one can deduce the acidity derived from a mixture of mineral acids (H₂SO₄, HNO₃) and organic acids (HCOOH, CH₃COOH), as shown in Table VII.

The H⁺ ion concentration (44.9 µeq.l⁻¹ and 12.2 µeq.l⁻¹, at Amersfoort and Louis Trichardt, respectively; Table III), measured during the study period, is considerably less than the total contribution estimated from organic and mineral acids. This would imply that only some portion of these ions might have contributed to rain acidity. The mineral acids at Amersfoort represent the greater part of free acidity compared to the contribution from the organic acids. This is contrasted by the fact, that the total organic acid concentration at Louis Trichardt is high compared to that at Amersfoort. This suggests that organic acids dominate the sources of acidity in Louis Trichardt, while mineral acids dominate at Amersfoort,
<table><caption>Table VIII. Source group contribution to precipitation in Amersfoort and Louis Trichardt</caption><thead><tr><th></th><th>Amersfoort<br>(%)</th><th>Louis Trichardt<br>(%)</th></tr></thead><tbody><tr><td>Marine</td><td>11</td><td>23</td></tr><tr><td>Terrigenous</td><td>19</td><td>27</td></tr><tr><td>Fossil fuel</td><td>35</td><td>15</td></tr><tr><td>Biomass</td><td>9</td><td>24</td></tr><tr><td>Other (H<sup>+</sup> and NH<sub>4</sub><sup>+</sup>)</td><td>26</td><td>11</td></tr></tbody></table>

which is in agreement with previously published data (Turner *et al.*, 1996). The total acidic contribution to the overall volume-weighted mean ion concentrations at Amersfoort and Louis Trichardt is 38 and 30%, respectively (Table III). If all these ions ($SO_4^{2-}$, $NO_3^-$ and organic) existed in the form of free acids, the precipitation pH would be 4.09 (Amersfoort) and 4.50 (Louis Trichardt), which is lower by 0.3 pH and 0.4 pH units, respectively, than actually measured (4.35 and 4.91). The relatively large difference indicates that the acids present in the rainwater samples have gone through a significant process of neutralisation.

## 4.4. SOURCE GROUP ESTIMATION

In order to infer the contribution of each source to the total precipitation content, it was assumed that all the nitrogen oxide originated from fossil fuel combustion and no correction for terrigenous and biomass burning contributions was made, hence the contribution of this source was overestimated. Table VIII shows the estimates of the relative contribution from the different sources to the chemical composition of precipitation at Louis Trichardt and Amersfoort.

These estimates suggest that the fossil fuel sources were the highest contributor to the total precipitation content at Amersfoort. However, at Louis Trichardt, marine, terrigenous and biomass sources contribute more or less equal amounts to the total precipitation content. This is in good agreement with previous estimates (Turner *et al.*, 1996), showing that, at Amersfoort the fossil fuel sources were the biggest contributor to potential acidity, while at Louis Trichardt biomass products predominated.

# 5. Seasonal Variability and Temporal Trends of Annual Concentrations

Chemical data for precipitation at Amersfoort and Louis Trichardt were analysed in greater detail for possible seasonal patterns. $H^+$, nss-$SO_4^{2-}$, $NO_3^-$ and $NH_4^+$ show a strong seasonal variability at Louis Trichardt, while at Amersfoort the seasonal
variation of precipitation chemistry data is less pronounced, especially for H⁺ and nss-SO₄²⁻ (Figure 4). This clearly illustrates the influence of fossil-fuel, as Amersfoort is downwind of the major industrial complexes. At Louis Trichardt, acidity in terms of the H⁺ ion concentration generally reaches a maximum in spring (September–November) and a minimum in winter (June–August). The magnitude and ranges of acidity differed considerably between the two sites (Figure 4). Amersfoort had the highest H⁺ ion concentration throughout the year, with average monthly values ranging from 27 µeq.l⁻¹ to 55 µeq.l⁻¹, while at Louis Trichardt the concentrations were lower, ranging from a monthly average of 5 µeq.l⁻¹ in July to over 34 µeq.l⁻¹ in September. The difference in magnitudes and ranges between the stations may be partly due to location, since Louis Trichardt is in a rural environment, while Amersfoort is downwind of the Mpumalanga Highveld industrial source region. During the dry season (May to September) the concentrations of H⁺, nss-SO₄²⁻, NO₃⁻ and NH₄⁺ are relatively high, especially in August and September, when the influence of local fires and of extensive savanna fires in the southern hemisphere is most pronounced. Thereafter, the concentrations decrease more or less evenly until they reach low values from December to February. The concentrations generally do not fluctuate much during the wet season (October to March). The NH₄⁺ concentration shows an interesting seasonal behaviour, with the volume-weighted monthly mean values reaching a sharp peak in September, especially at Louis Trichardt. Since major sources of NH₃ emissions are known to be animal wastes and fertilisers (Schlesinger and Hartley, 1992), the maximum in NH₄⁺ concentration may be due to early spring fertilisation at local agricultural sites.

In the dry season, the seasonal ionic ratio is higher at Amersfoort, than during the wet season (Table V), while the wet season ionic ratios are higher at Louis Trichardt than the dry season ionic ratios, except for the Cl⁻/Na⁺ and SO₄²⁻/Na⁺ ionic ratios, for which the difference is 0.01 and 0.20, respectively. This indicates the contribution of particles from additional sources like particles emitted by biomass burning of the savanna (Cachier and Ducret, 1991), which influence the content of chloride in the air masses affecting the two sites. Other possible sources could include industry, but this would have to be related to seasonal changes in the transport patterns, as the industrial emissions are more or less constant throughout the year.

The increased concentrations observed during spring are likely the results of a combination of large-scale circulation and source strength (transport and emissions). The strength and the seasonal shift of the Indian Ocean Anticyclone have important effects on the seasonal variation of transport patterns to and from the Highveld, which obviously impact on the removal mechanisms, causing them to also be seasonally dependent (Piketh and Prangley, 1998).

Annual mean concentrations of major ions during 1986–1999 show that during 1986/1987, the H⁺ concentration was 87 and 18 µeq.l⁻¹, at Amersfoort and Louis Trichardt, respectively, whereas the concentration in 1998/1999 had decreased to
<figure><img src="image_6.png" /><figcaption>Figure 4. Monthly mean concentration of major ions during 1986–1999, for Amersfoort (black), and Louis Trichardt (white) for the 1986–1999 rainy seasons.</figcaption></figure>

67 and 8 $\mu$eq.l$^{-1}$, at the two sites, respectively (Figure 5). Overall, the acidity has clearly decreased during the 13-year period, although with some fluctuations. This might be attributable to the removal/extinguishing of the major burning colliery discard dumps and shut-downs of some of Eskom's Power Stations, as well as more efficient combustion processes and improved removal of particulate emissions at Eskom's coal-fired Power Stations.

At Amersfoort, the nss-SO$_4^{2-}$ concentration was higher than that of any other ion by a factor of two during the 13-year period, whereas at Louis Trichardt, the nss-SO$_4^{2-}$ concentration was of the same order of magnitude as that of other major ions. At Louis Trichardt, the nss-SO$_4^{2-}$ concentration fluctuated between 45 and about 65 $\mu$eq.l$^{-1}$.

The concentration of nitrate was highest in 1998/1999 at both sites. At Louis Trichardt, the $NH_4^+$ concentration fluctuated greatly. However, the highest concentration of $NH_4^+$ was recorded at Amersfoort in 1998/1999, with nss-SO$_4^{2-}$ and $NO_3^-$ also being high. The $NH_4^+$ ion is biologically converted to nitric acid after deposition, which ultimately makes $NH_4^+$ an acidifying substance. This implies that the
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>Figure 5. Annual mean concentration of major ions and annual precipitation totals (rain years July–June during the period 1986–1999), for Amersfoort (black), and Louis Trichardt (white).</figcaption></figure>

ecosystem in the Amersfoort area has been acidified during the study period. The ionic concentration in precipitation is largely determined by air concentrations and precipitation amounts. Above average precipitation totals were observed during 1987/1988, 1988/1989 and 1995/1996, which could have resulted in a decrease of concentrations. Despite the increase in precipitation amounts, concentrations of major ions were generally high. Thus, the precipitation amount should not directly be related to the temporal trends in concentrations that subsequently emerge.
# 6. Discussion

The study of the chemical composition of rainwater and wet deposition in regions representative of rural and industrial areas in South Africa was undertaken for the first time to determine the chemical species present in precipitation during a 13-year period and to examine the relative contributions from various sources. The nature of the chemical species found in precipitation depends on numerous interacting mechanisms, such as the intensity of the sources of the species or of their precursors, the transportation and dynamic transfer in the atmosphere, atmospheric chemistry and the physical characteristics of 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-SO<sub>4</sub><sup>2-</sup> and NO<sub>3</sub><sup>-</sup> are as high as those in north-eastern U.S.A. and central Europe, typical of regions affected by acid-forming emissions. However, most of the acidity is neutralised by the base cations, Mg<sup>2+</sup>, Ca<sup>2+</sup> and NH<sub>4</sub><sup>+</sup>. Therefore, the average pH of precipitation is around 4.35, which is an acid value. At Louis Trichardt, the concentrations of nss-SO<sub>4</sub><sup>2-</sup> and NO<sub>3</sub><sup>-</sup> are comparable with western African sites (Lacaux et al., 1992; Galy-Lacaux and Modi, 1998). As there are no major anthropogenic sources directly affecting Louis Trichardt, this could be attributed to aged aerosol, subject to sub-continental recirculation (Held et al., 1994). Marine, terrigenous, fossil-fuel and biomass sources influence the precipitation composition at Amersfoort and Louis Trichardt. The marine and terrigenous sources are well established by the ratio of Na<sup>+</sup> to Cl<sup>-</sup>, being closer to the seawater ratio, and the high Ca<sup>2+</sup>, SO<sub>4</sub><sup>2-</sup>, and Mg<sup>2+</sup> ion concentrations coming from dust dispersed as a result of soil erosion and entrainment of the particles in the atmosphere. It is difficult to discriminate between mineral dust originating from soil erosion and dust emitted by coal-fired power plants, which are very dominant sources for SO<sub>4</sub><sup>2-</sup> in South Africa (Puxbaum et al., 2000). However, based on the relationship of trace elements, Maenhaut et al. (1996) showed that mineral dust at Palmer, a site on the Highveld, about 200 km east of Johannesburg, was mainly soil-derived. Coal combustion could be an additional potential source for the terrigenous compounds, especially Ca<sup>2+</sup>, which is a major component of fly ash. One can therefore expect, that at Amersfoort high concentrations of Ca<sup>2+</sup> from fly ash are observed, since it is located downwind of the majority of coal-fired power plants. The NO<sub>3</sub><sup>-</sup> and SO<sub>4</sub><sup>2-</sup> ions are correlated, due to the similarity in their behaviour in precipitation and the co-emission of their precursors SO<sub>2</sub> and NO<sub>x</sub> (Saxena et al., 1996), which clearly indicates fossil-fuel source influences.

The other signatures of atmospheric sources are highlighted by the relative high organic (HCOOH and CH<sub>3</sub>COOH) and nitrogenous ion contents in the precipitation. The atmospheric sources of HCOOH and CH<sub>3</sub>COOH in rain have not yet been properly identified. Likely sources include anthropogenic sources, i.e., vehicular emissions (Talbot et al., 1988) and natural sources e.g., ants (Graedel and Eisner, 1988), plants (Talbot et al., 1990) and soil (Sanhueza and Andreae, 1991), as well
as direct emissions from biomass burning (Talbot *et al.*, 1988) and also oxidation of precursor compounds in the gas phase (Madronich and Calvert, 1990). Particulates and gaseous acids are scavenged by clouds or rain and in-cloud oxidation of aldehydes. Relatively large concentrations of gaseous HCOOH (0.7 ppbv) and CH<sub>3</sub>COOH (0.8 ppbv) are present in the atmosphere of the Mpumalanga Highveld (Snyman and Held, 1996) and scavenging of these acids during rainfalls could be a significant source of the HCOO<sup>-</sup> and CH<sub>3</sub>COO<sup>-</sup> ions in the rain samples at Amersfoort and Louis Trichardt. Furthermore, a statistical correlation between HCOO<sup>-</sup> and CH<sub>3</sub>COO<sup>-</sup> concentrations was found in both Amersfoort and Louis Trichardt rain samples.

The NH<sub>4</sub><sup>+</sup> content (22.3 µeq.l<sup>-1</sup> for Amersfoort and 9.7 µeq.l<sup>-1</sup> for Louis Trichardt) can be attributed to the NH<sub>3</sub> sources associated with agricultural and industrial activities. For instance, NH<sub>4</sub><sup>+</sup> is a component of fertilisers, but also a product of animal wastes. Regarding the industrial sources, NH<sub>3</sub> can be present in emissions from coal gasification. The NO<sub>3</sub><sup>-</sup> signatures in precipitation at Louis Trichardt and Amersfoort are not of the same order of magnitude as the natural emissions from semi-arid savanna soils (Parsons *et al.*, 1996; Otter *et al.*, 2001). At Amersfoort, nitrate ion concentrations and deposition values are comparable to levels in Japan (Lee *et al.*, 2000) and European countries (Dovland and Pederson, 1996).

The acidity of precipitation during the study period had an average concentra-
tion of 44.9 µeq.l⁻¹ (pH of 4.35) and 12.2 µeq.l⁻¹ (pH of 4.91), for Amersfoort and
Louis Trichardt, respectively. It was estimated from the average chemical compo-
sition, that this acidity originates from a mixture of inorganic and organic acids. At
Amersfoort, 90% of the acidity originates from inorganic sources, of which 60% is
in the form of sulphuric acids, while at Louis Trichardt 50% is derived from organic
acids, with nitric and sulphuric acids contributing equal amounts (Table VII). The
relative contributions of organic acids at these South African sites are higher than in
Banizoumbou, but lower than in the tropical and equatorial areas. For example, the
organic contribution is of the order of 60–80% of that in the Brazilian Amazon
(Andreae *et al.*, 1988; Andreae *et al.*, 1990; Williams *et al.*, 1997), Venezuela
(Galloway *et al.*, 1982) and in Australia (Likens *et al.*, 1987).

**7. Conclusion**

Rain samples from Amersfoort and Louis Trichardt have been analysed for the
period 1986–1999 to characterise the chemical composition of precipitation and
identify possible sources. The major conclusions from the present study are:

* Mineral acids (H<sub>2</sub>SO<sub>4</sub> and HNO<sub>3</sub>) seem to be the primary contributors to precipitation acidity in Amersfoort, while at Louis Trichardt both organic (HCOOH and CH<sub>3</sub>COOH) and mineral acids contribute equal amounts of acidity.
* Marine, terrigenous, nitrogenous and anthropogenic sources influence precipitation in southern Africa. Fossil-fuel sources are larger contributors to the total precipitation ion content at Amersfoort, while at Louis Trichardt, marine terrigenous and biomass sources contribute equal amounts to the total precipitation ion content.

* Alkaline species, mainly related to terrigenous sources, have been identified as a significant contributor to acid rain neutralisation, especially at Louis Trichardt.

* $H^+$, nss-SO$_4^{2-}$, NO$_3^-$ and $NH_4^+$ ions exhibit a seasonal pattern in Louis Trichardt with the highest precipitation concentrations occurring during the austral spring, possibly as a result of increases in local agricultural activities.

## Acknowledgements

The authors gratefully acknowledge the financial support of Eskom Research and NRF for this study. Mr. G. de Beer, the field technician, the site operators and the chemical analysts at the Eskom Laboratory are acknowledged for their technical support. The scientific advice and collaboration of J. P. Lacaux of MEDIAS, France, and coordinator of the DEBITS/IGAC programme, is highly appreciated.

## References

Ambus, P. and Christensen, S., 1994: Measurements of N$_2$O emission from fertilized grassland: An analysis of spatial variability, *J. Geophys. Res.* **99**, 16549–16555.

Andreae, M. O., Talbot, R. W., Andreae, T. W., and Hariss, R. C., 1988: Formic and acetic acid over the central Amazon region, Brazil, 1, dry season, *J. Geophys. Res.* **93**, 1616–1624.

Andreae, M. O., Talbot, R. W., Berresheim, H., and Beecher, K. M., 1990: Precipitation chemistry in central Amazonia, *J. Geophys. Res.*, **95**, 16987–17000.

Ayers, G. P., Peng, L. C., Fook, L. S., Kong, C. W., Gillett, R. W., and Manins, P. C., 2000: Atmospheric concentrations and deposition of oxidised sulfur and N species at Petalinga, Malaysia, *Tellus* **52B**, 60–73.

Brocard, D., Lacaux, C., Lacaux, J. P., Kouadio, G., and Yoboué, V., 1996: Emissions from the combustion of biofuels in western Africa, in J. S. Levine (ed.), *Global Biomass Burning: Atmospheric, Climatic and Biospheric Implications*, MIT Press, Cambridge, Massachusetts, pp. 350–360.

Cachier, H. and Ducret, J., 1991: Influence of biomass burning on African rains, *Nature* **352**, 228–230.

Delmas, R., 1981: Contribution à l'étude des forêts équatoriales comme sources naturelles de dérivés soufrés atmosphériques, PhD Thesis, Université Paul Sabatier de Toulouse, Toulouse, France.

Delmas, R., Loudjani, P., Podaire, A., and Menaut, J. C., 1991: Biomass burning in Africa: An assessment of annually burnt biomass, in J. S. Levine (ed.), *Global Biomass Burning: Atmospheric, Climatic and Biospheric Implications*, MIT Press, Cambridge, Massachusetts, pp. 126–133.

Dovland, H. and Pederson, U., 1996: Acid deposition in Europe, in D. M. Whelpdale and M. S. Kaiser (eds), *Global Acid Deposition Assessment*, World Meteorological Organization, pp. 53–67.

EPA, 1994: *Quality Assurance Handbook for Air Pollution Measurement Systems*, EPA/600R-94/038e, United States Environmental Protection Agency, Office of Research and Development, Washington.
Galloway, J. N., Likens, G. E., Keene, W. C., and Miller, J. M., 1982: Precipitation in remote areas of the world, *J. Geophys. Res.* **87**, 8771–8786.

Galy-Lacaux, C. and Modi, A. I., 1998: Precipitation chemistry in the Sahelian Savanna, *J. Atmos. Chem.* **30**, 319–343.

Galy-Lacaux, C., Carmichaël, G. R., Song, C. H., Lacaux, J. P., and Modi, I., 2001: Heterogenous processes involving nitrogenous compounds and Saharan dust inferred from measurements and model calculations Region, *J. Geophys. Res.* **106**, 12559–12578.

Gillett, R. W., Ayers, G. P., Mhwe, T., Selleck, P. W., and Harjanto, H., 2000: Concentration of N and sulfur species in gas and rainwater from several sites in Indonesia, *Water, Air and Soil Pollution* **120**, 205–215.

Graedel, T. E. and Eisner, T., 1988: Atmospheric formic acid from formicine ants: A preliminary assessment, *Tellus* **40**, 335–339.

Held, G., Scheifinger, H., and Snyman, G. M., 1994: Recirculation of pollutants in the atmosphere, *SA J. Sci.* **90**, 91–97.

Held, G., Scheifinger, H., Snyman, G. M., Tosen, G. R., and Zunckel, M., 1996: The climatology and meteorology of the highveld, in G. Held, B. J. Gore, A. D. Surridge, G. R. Tosen, C. R. Turner, and R. Walmsley (eds), *Air Pollution and its Impacts on the South African Highveld*, Environmental Scientific Association, Cleveland, pp. 60–71.

Herut, B., Starinsky, A., Katz, A., and Rosenfeld, D., 2000: Relationship between the acidity and chemical composition of rainwater and climatological conditions along a transition zone between large deserts and Mediterranean climate, Israel, *Atmos. Environ.* **34**, 1281–1292.

Keene, W. C., Pszenny, A. A. P., Galloway, J. N., and Hartley, M. E., 1986: Sea-salt corrections and interpretation of constituents ratios in marine precipitation, *J. Geophys. Res.* **91**, 6647–6658.

Lacaux, J. P., Delmas, R., Kouadio, G., Cros, B., and Andreae, M. O., 1992: Precipitation chemistry in the Moyombe forest of equatorial Africa, *J. Geophys. Res.* **97**, 6195–6206.

Lara, L. B. L. S., Artaxo, P., Martinelli, R. L., Victoria, R. L., Camargo, P. B., Krusche, A., Ayers, G. P., Ferraz, E. S. B., and Ballester, M. V., 2001: Chemical composition of rainwater and anthropogenic influences in the Piracicaba River Basin, Southeast Brazil, *Atmos. Environ.* **35**, 4937–4945.

Lee, B. K., Hong, S. H., and Lee, D. S., 2000: Chemical composition of precipitation and wet deposition of major ions on the Korean peninsula, *Atmos. Environ.* **34**, 563–575.

Levine, J. S., Winstead, L. E., Parsons, D. A. B., Scholes, M. C., Scholes, R. J., Cofers III, W. R., Cahoon Jr., D. R., and Sebacher, D. I., 1996: Biogenic soil emissions of nitric oxide (NO) and nitrous oxide (N2O) from savannas in South Africa, *J. Geophys. Res.* **101**, 23089–23697.

Likens, G. E., Keene, W. C., Miller, J. M., and Galloway, J. N., 1987: Chemistry of precipitation from a remote terrestrial site in Australia, *J. Geophys. Res.* **92**, 13299–13314.

Madronich, S. and Calvert, J. G., 1990: Permutation reactions of organic peroxy radical in the troposphere, *J. Geophys. Res.* **95**, 5697–5717.

Maenhaut, W., Salma, I., Annegarn, H. J., Andreae, M. O., and Cafmeyer, J., 1996: Regional atmospheric aerosol composition and sources in the eastern Transvaal, South Africa, impacts of biomass burning, *J. Geophys. Res.* **101**, 23631–23650.

Mamane, Y. and Gottlieb, J., 1995: Ten years of precipitation chemistry in Haifa, Israel, *Water, Air and Soil Pollution* **82**, 549–558.

Matson, P. A., Vitousek, P. M., and Schimel, D. S., 1989: Regional extrapolation of trace gas flux based on soils and ecosystems, in D. S. Schimel and M. O. Andreae (eds), *Exchange of Trace Gases between Terrestrial Ecosystems and the Atmosphere*, John Wiley and Sons, New York, pp. 97–108.

Norman, M., Das, S. N., Pillai, A. G., Granat, L., and Rodhe, H., 2001: Influence of air mass trajetories on the chemical composition of precipitation in India, *Atmos. Environ.* **35**, 4223–4235.

Otter, L. B., Marufu, L., and Scholes, M. C., 2001: Biogenic, biomass and biofuel sources of trace gases in southern Africa, *SA J. Sci.* **97**, 131–138.
Parsons, D. A. B., Scholes, M. C., Scholes, R. J., and Levine, J. S., 1996: Biogenic NO emissions from savanna soils as a function of fire regime, soil type, soil nitrogen and water status, *J. Geophys. Res.* **101**, 23683–23688.

Piketh, S. J. and Prangley, A. R., 1998: Trajectory climatology of transboundary transport from the highveld, *Eskom Report TRR/T98/038*, Cleveland, 24 pp.

Piketh, S., Annegarn, H. J., and Tyson, P. D., 1999: Lower-tropospheric loading over South Africa: The relative contributions of aeolian dust, industrial emissions and biomass burning, *J. Geophys. Res.* **104**, 1597–1607.

Puxbaum, H., Rendl, J., Allabashi, R., Otter, L. B., and Scholes, M. C., 2000: Mass balance of the atmospheric aerosol in a South Africa subtropical savanna (Nylsvley, May 1997), *J. Geophys. Res.* **105** (D16), 20697–20706.

Sanhueza, E. and Andreae, M. O., 1991: Emission of formic and acetic acids from tropical savanna soils, *Geophys. Res. Lett.* **18**, 1707–1710.

Satsangi, G. S., Lakhani, A., Khare, P., Singh, S. P., Kumari, K. M., and Srivastava, S. S., 1998: Composition of rain water at a semi-arid rural site in India, *Atmos. Environ.* **32**, 3783–3793.

SAWB, 1950: *Upper Winds in Southern Africa*, WB 13, South African Weather Bureau, Dept. of Transport, Pretoria, 281 pp.

SAWB, 1986: *Climate of South Africa. Climate statistics up to 1984*, WB 40, South African Weather Bureau, Department of Environment Affairs, Pretoria, 136 pp.

Saxena, A., Kulshrestha, U. C., Kumar, N., Kumari, K. M., and Srivastava, S. S., 1996: Characterization of precipitation in AGRA, *Atmos. Environ.* **30** (20), 3405–3412.

Schlesinger, W. H. and Hartley, A. E., 1992: A global budget for atmospheric NH₃, *Biogeochemistry* **15**, 191–211.

Siversten, B., Matal, C., and Pereira, L. M. R., 1995: *Sulphur Emissions and Transfronteir Air Pollution in Southern Africa*, Report 35, SADC ELMS, Lesotho, 112 pp.

Snyman, G. M. and Held, G., 1996: *Vertical Distribution of Atmospheric Particulates and Meteorological Conditions during the 1996 Field Experiments*, TRR/S96/206, Eskom, Johannesburg, 37 pp.

Talbot, R. W., Andreae, M. O., Berresheim, H., Jacobs, D. J., and Beecher, K. M., 1990: Sources and sinks of formic, acetic and pyruvic acids over central Amazonia, 2, Wet season, *J. Geophys. Res.* **95**, 16799–16811.

Talbot, R. W., Beecher, K. M., Hariss, R. C., and Cofers III, W. R., 1988: Atmospheric geochemistry of formic and acetic acids at a mid-latitude temperate site, *J. Geophys. Res.* **93**, 1638–1652.

Turner, C. R., Wells, R. B., and Olbrich, K. A., 1996: Deposition chemistry in South Africa, in G. Held, B. J. Gore, A. D. Surridge, G. R. Tosen, C. R. Turner, and R. D. Walmsley (eds), *Air Pollution and its Impacts on the South African Highveld*, Environmental Scientific Association, Cleveland, pp. 80–85.

Tyson, P. D., Garstang, M., Swap, R. J., Edwards, M., and Kållberg, P., 1996: An air transport climatology for subtropical southern Africa, *International Journal of Climatology* **16**, 151–163.

Van der Walt, H., du Toit, P., Pienaar, J. J., and Julies, A., 1998: Integration of passive sampler monitoring in non-electrified residential areas into a modern ambient air quality monitoring network, *Proceedings, 11th World Clean Air and Environment Congress*, Durban, 13–18 September 1998, 5 pp.

Williams, M. R., Fischer, T. M., and Melack, J. M., 1997: Chemical composition and deposition of rain in Central Amazon, Brazil, *Atmos. Environ.* **31**, 207–217.

WMO, 1997 (1998, 1999): Nineteenth (twentieth and twentyfirst) acid rain performance survey, World Precipitation Chemistry Center of the WMO QA/SAC, State University of New York, Albany.

Yoboué, V., 1991: Caractéristiques physiques et chimiques des aérosols et des pluies collectés dans la savanne humide de Côte d'Ivoire, PhD No. 914 Thesis, Université Paul Sabatier de Toulouse, Toulouse, France. 103 pp.