WET AND DRY DEPOSITION IN SOUTH AFRICA

WET AND DRY DEPOSITION IN SOUTH AFRICA Gerhard Held* and Jonas Mphepya Eskom Technology Services International, Johannesburg, South Africa E-mail: [email protected] * Current affiliation: Instituto Pesquisas Meteorológicas, Universidade Estadual Paulista, Bauru S.P., Brazil E-mail: [email protected] RESUMO A África do Sul tem em abundância reservas naturais que incluem as de carvão mineral, cuja extração é realizada a céu aberto na província de Mpumalanga. Isso resultou num rápido desenvolvimento de indústrias pesadas na região, incluindo as de geração de eletricidade, emitindo anualmente 2 milhões de toneladas de SO2 numa área relativamente pequena do platô sul-africano. Com isso tornou-se extremamente importante determinar-se o destino da emissão desses poluentes e estabelecer-se os possíveis impactos ao meio ambiente. Isso motivou a implementação de uma extensa rede de monitoramento ambiental da qualidade do ar ao final dos anos 70, incluindo medidas de deposição úmida (desde 1985) e deposição seca (desde 1996). Neste artigo os resultados obtidos através dessa rede de monitoramento da qualidade da água da chuva são apresentados na forma de isopletas para íons selecionados (hidrogênio, sulfato, nitrato, cloreto, potássio e totais orgânicos). A deposição seca de enxofre foi determinada em dois locais, usando-se o modelo de inferência (‘Inferential Model’) da NOAA para calcular a velocidade de deposição e após isso a deposição de enxofre total. Os resultados, para três anos de observação, mostraram que o total anual de deposição de enxofre variou entre 9,2-10,0 kg por hectare no centro da área industrial e de 1,6-3,0 kg por hectare na área remota.

1.

INTRODUCTION

South Africa is blessed with an abundance of mineral resources, which has led to the development of mining and industry in the central region of the country. Accompanying the industrial development are environmental impacts, and of particular concern in this case are the emissions of pollutants into the atmosphere. Approximately 72% of South Africa’s primary energy is sourced from coal (Wells et al, 1996). However, South Africa has lower sulphur coal (≤1%) compared with Northern Hemisphere coals, which limits the total sulphur dioxide emissions. The highly industrialised region of Mpumalanga on the South African plateau (‘highveld’, 1400-1700m above sea level) accounts for approximately 90% of South Africa’s scheduled emissions, while household combustion only contributes about 2,5% (Wells et al, 1996). The total emissions by scheduled industries in South Africa in metric tonnes per year are as follows (the percentage contribution from the industrial region in Gauteng and Mpumalanga is shown in brackets; Wells et al, 1996): Sulphur dioxide (SO2) Nitrogen oxides (NOx) Fine Particulate Matter (FPM)

2 120 452 t/year (94%) 1 004 716 t/year (91%) 331 399 t/year (86%)

Anthropogenic and natural air pollutants are deposited to the earth’s surface through wet and dry processes. Deposition rates of these pollutants need to be determined in order to estimate their impact on ecological systems. While the measurement of wet deposition is relatively straight forward, involving the collection and analysis of rainfall samples, direct measurements of dry deposition are complex and expensive. As a consequence, rain quality on the South African plateau has been monitored since 1984/85 (Turner, 1993; Turner and De Beer, 1996; Held et al, 1996), while the dry deposition fluxes were only calculated for the first time in 1994/95 for a two-week winter and summer period (Turner et al, 1995; Zunckel et al, 1996), respectively, using gaseous, particulate and meteorological measurements as input into the Inferential Model which was developed by the National Oceanic and Atmospheric Administration’s (NOAA) Atmospheric Turbulence and Diffusion Division (ATDD) in Oak Ridge, Tennessee (Hicks et al, 1991; Meyers et al, 1991). In 1993/94 the Kiepersol Joint Venture (KJV) project was initiated by the Department of National Health (now under the auspices of the Department of Environmental Affairs and Tourism), Eskom and the CSIR to establish a National Network for Acid Rain Research with added financial or in-kind support from other industries and organisations. Currently, it comprises 14 sampling sites (Held et al, 1999).

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A comprehensive statistical analysis of rain quality data for all sites operating since 1985 has been undertaken by Galpin (1999).

2.

REGIONAL WET DEPOSITION FROM 1985/86 TO 1998/99

By the mid 1980s Eskom, the South African Power Utility, had initiated research into the ‘acid rain’ phenomenon to establish to what extent this was a problem in South Africa and whether strategies would have to be devised to combat sulphur emissions from power station stacks, either in new plant or on existing facilities. After seven and ten years of rain quality monitoring, respectively, detailed analyses of the data were undertaken. It was found that in southern Africa, rain acidity appeared to be controlled by the wide-spread occurrence of biomass fires in the region (Turner, 1993;Turner and De Beer, 1996). However, the effects of fossil fuel emissions, superimposed on the biomass burning cycle, could be clearly seen, dominating the industrial part of the Mpumalanga highveld and subsequently downwind regions. Ion concentrations in rain water similar to those found in other industrialised regions of the world were measured, but total wet-deposition values were relatively low due to marginal rainfall quantities in this region, as well as the seasonality of rainfall (no rain during the winter months). A total of 32 sites had been operating for periods varying from one rain year up to 14 rain years. A rain year is defined as the period from 1 July to 30 June in the following year. Figure 1 shows the geographic location of these sites. However, only four sites have records for the full 14-year period. Rain quality data from all sites since 1985 have now been analysed, irrespective of the period or length of observations. Regional wet-deposition maps were produced in isopleth form for selected ion species, based on sites with reasonably long records to warrant averaging of the concentrations.

Figure 1. Wet deposition monitoring sites operating during the 1985/86 to 1998/99 rain years. KJV sites are those currently monitoring under the auspices of the KJV Project. 2825

2.1

Long-Term Averages of Ion Concentrations

Annual averages of the ion concentrations were calculated for all sites, but only 13 sites were selected for “longterm” averages, which could range from a couple of year’s of records to 14 years. The main criteria for selection were regional representativeness and data quality rather than the length of records. Isopleths for selected ions (hydrogen, sulphate, nitrate, chloride, potassium and total organics) were then constructed to represent regional concentrations based on these 13 sites (Figure 2). Their geographic position is indicated by a dot in Figure 2. However, it should be noted that these records are not necessarily concurrent. a)

d)

b)

e)

c)

f)

Figure 2. Volume-weighted mean ion concentrations (µeq.l -1): The dots indicate sites for which reasonable means were available. a) Hydrogen; b) Sulphate; c) Nitrate; d) Chloride; e) Potassium; f) Total organics. 2826

Hydrogen ions show maximum concentrations in excess of 90 µeq.l -1 centred over the industrial hub of Mpumalanga (Figure 2a). As expected, sulphates also have their maximum there, but the isopleths indicate a downwind trend towards the east-south-east, which is due to the oxidation of SO2 to SO42- along its pathway (Figure 2b). A similar pattern emerges for the nitrates, but their downwind dispersion is less (lower concentrations), due to a faster oxidation rate than that for sulphates (Figure 2c). Chlorides indicate highest concentrations over the interior, but the maritime component, also highlighted by the statistical analysis (Galpin, 1999), is clearly manifested by an inland low just upwind of the escarpment and again an increase of concentrations towards the coast (Figure 2d). Noteworthy is an increase of chloride towards the north. Potassium, shows a similar pattern to that of chloride, but its mean concentration decreases towards the north (Louis Trichardt), which is thought to have a smaller industrial signature, but more influence of biomass burning products (Figure 2e; Turner, 1993). Airborne sampling in a veld fire over the Mpumalanga highveld also indicated lower than expected concentrations of potassium in the plume (Snyman et al, 1997), possibly indicating a different composition of the biomass fuel on the highveld compared to other savannah regions (Helas and Pienaar, 1996). Finally, the total organics (calculated as the sum of acetates and formates) indicate a maximum over the central interior, extending from Mpumalanga into the Free State (Figure 2f), with a sharp decreasing gradient towards the coastal region, but relatively high concentrations in the northern region, which certainly confirms the influence of biomass combustion products at Louis Trichardt. 2.2

Comparison Between Dry and Wet Rain Years

It had already been found by Turner (1993) that the variability of acidity in South African rain is driven by the availability of biomass fuel for combustion, while the industrial component is relatively constant from one year to another. Therefore, from the 14 years of rain quality data, the wettest and most dry season were selected on the basis of rainfall totals over the whole monitoring area. These were 1987/88 as “wettest” and 1991/92 as the “most dry” season, entirely based on rainfall measured at the monitoring sites, which may not necessarily conform with records of the South African Weather Bureau. The isopleths plotted for these two rain years obviously highlighted a lot of details at the 13 selected sites, which could lead to confusion about the main issue here. However, a simple comparison was attempted between biomass and industrial signatures. The ranges of the observed concentrations are shown in Table 1. To summarise it, during a wet year, concentrations of total organics are about one magnitude higher and potassium one magnitude lower than during a dry year. Sulphate concentrations are about twice as high and nitrate concentrations even one magnitude higher during a wet than a dry year.

Table 1. Comparison of selected ion concentration ranges during a wet and dry rain year. Ion species SO42NO3K+ Total organics

3.

Concentration range (µeq.l -1 ) 1987/88 (wet) 1991/92 (dry) 45 - 76 27 – 41 22 - 35 0,8 – 3,2 4 - 11 27 – 52 21 - 40 4 – 12

DRY DEPOSITION OF SULPHUR

The NOAA inferential model (Hicks et al, 1991; Meyers et al, 1991) was chosen as an alternative for direct measurements of dry deposition as it showed best estimated results of deposition velocities for SO2 and other pollutants. The dry deposition flux is calculated as a product of a modelled deposition velocity and measured ambient concentration. The calculation of deposition velocity is based on an understanding of the chemical and physical processes of dry deposition as described through site variables and meteorological measurements. The dry deposition process is represented in the inferential model by the sum of the three resistances in the lowest 50m of the atmosphere and represents the total resistance to transfer of a pollutant from the atmosphere to the vegetation canopy. These three are the aerodynamic resistance which is determined by atmospheric properties such as turbulent exchange, the boundary-layer resistance which is associated with the transfer of material across the thin layer of air in contact with the vegetation and the surface resistance which is determined by the plant physiological 2827

properties. As a result of these controlling factors, deposition velocity is plant specific and it has strong diurnal and seasonal cycles (Meyers et al, 1991; Matt and Meyers, 1993). The meteorological parameters used for the prediction of deposition velocity are the standard deviation of wind direction (sigma theta), ambient temperature, relative humidity, solar radiation, surface wetness and rainfall. The wetness parameter was calculated from the ambient temperature and relative humidity, being equal to 1 when the dew point depression is less than 1,5°C and 0 in all other cases (Turner et al, 1995; Zunckel et al, 1996). All other parameters together with SO2 and particulates are routinely measured at Elandsfontein and Palmer and recorded as hourly averages. Elandsfontein is situated on a hill almost in the centre of the industrial region, some 150m above the surrounding area, while Palmer is located about 100km north-east of it, near the escarpment, in an area which is thought to be representative of rural conditions, as the air flow there rarely comes from the industrial region (Held et al, 1996). A set of almost three year’s data was used to calculate the dry deposition at Elandsfontein (just north of Bethal) and Palmer (between Middelburg and Lydenburg) on the Mpumalanga highveld (Figure 1). These sites are part of the Eskom Ambient Air Quality Monitoring Network. SO2 was monitored continuously by means of an UVfluorescent analyser. Particulate concentration was measured by means of a nephelometer (1996-1997), which was later replaced by a Beta Gauge. It is important to note, that the nephelometer measures aerosols of 80µeq.l -1), but the isopleths indicate a downwind trend towards the east-south-east, which is due to the oxidation of SO2 to SO42- along its pathway, following prevailing winds. A similar pattern emerges for the nitrates, but their downwind dispersion is less (lower concentrations), due to a faster oxidation rate than that for sulphates. Chlorides indicate highest concentrations over the interior (20-25µeq.l -1), but the maritime component, also highlighted by the statistical analysis, is clearly manifested by an inland low just upwind of the escarpment and again an increase of concentrations towards the coast (>20µeq.l -1). Noteworthy is an increase of chloride towards the north. Potassium, shows a similar pattern to that of chloride, but its mean concentration decreases towards the north (Louis Trichardt), which is thought to have a smaller industrial signature, but more influence of biomass burning products. Airborne sampling in a veld fire over the Mpumalanga highveld during 1997 also indicated lower than expected concentrations of potassium in the plume, possibly indicating a different composition of the biomass fuel on the highveld compared to other savannah regions. Finally, the total organics (calculated as the sum of acetates and formates) indicate a maximum over the central interior (>30µeq.l -1), extending from Mpumalanga into the Free State, with a sharp decreasing gradient towards the coastal region, but relatively high concentrations in the northern region, which certainly confirms the influence of biomass combustion products at Louis Trichardt. A simple comparison was attempted between biomass and industrial signatures during a wet and dry rain year. During a wet year, concentrations of total organics are about one magnitude higher and potassium one magnitude lower than during a dry year. Sulphate concentrations are about twice as high and nitrate concentrations one magnitude higher during a wet than a dry year. The NOAA inferential model was chosen as an alternative for direct measurements of dry deposition as it showed best estimated results of deposition velocities for SO2 and other pollutants. The dry deposition flux is calculated as a product of a modelled deposition velocity and measured ambient concentration. The seasonally averaged diurnal variation of the inferred deposition velocities of SO2 showed, that highest daytime figures were observed in the centre of the industrial region (Elandsfontein) during summer (0,47cm.s-1) and lowest during spring and winter (0,15-0,17cm.s-1). At the rural Palmer site, the daytime deposition velocities in winter were similar to those at Elandsfontein, but lower at night. The highest deposition velocities, however, occurred during autumn (0,37cm.s-1). The highest deposition fluxes of sulphur were observed at Elandsfontein from SO2 during the summer of 1996/97 (3,34 kg.ha-1 over 3-months) and from sulphate during spring and summer of 1998 (0,84 kg.ha-1 per 3-month period). During the period March 1996 to December 1998, the mean dry deposition rates for total sulphur on the highveld varied from 9,2-10,0 kg S ha-1.y-1 at the central Elandsfontein site, and from 1,6-3,0 kg S ha-1.y-1 at Palmer, which is located near the escarpment on the north-eastern side of the industrial region. Contrary to the siting criteria for the United States Environmental Protection Agency CASTNet sites, which ensure that all monitoring sites are relatively free from direct influence from nearby sources (Clarke et al, 1997), Elandsfontein is located in close proximity to a number of major point sources and impacted by frequent fumigation. Therefore, the annual sulphur dry deposition rates at Elandsfontein, which include the contribution from sulphur dioxide and sulphate, are higher than anywhere recorded in the CASTNet (Clarke et al, 1997). Total dry deposition rates of sulphur during the threeyear period (1996-1998) are slightly lower than those found for shorter observation periods (Zunckel et al, 1996; Clarke et al, 1997) which could be attributed to different assumptions regarding the contribution from sulphate, as well as the different choice of seasons contributing to annual totals. Dry deposition certainly contributes a significant portion of the total deposition loads in South Africa, especially on the highveld. Previous knowledge of dry deposition processes in this country was based largely on theoretical and empirical considerations. The current study has extended this knowledge considerably. Although much still needs to be done in proving the methodology in the local context, particularly with respect to aerosol components and the relevant plant physiology data for dominant south African vegetation needed for the model, the methods developed by ATDD in the United States appear to offer by far the best option for dry deposition in the highveld. In this regard, existing ambient air quality monitoring stations can be readily adapted for this work at little additional expense. This strongly suggests a practical basis for a national dry deposition monitoring strategy. Further, the model sensitivity study has clearly demonstrated that it could be possible to re-analyse existing SO2 and FPM records to estimate total dry deposition. 2832

5.

REFERENCES

Clarke JF, Edgerton ES and Martin BE, 1997. Dry deposition calculations for Clean Air Status and Trends Network, Atm. Environ., 31(21), 3667-3678. Galpin JS, 1999. Statistical analysis of rain quality data from the KJV network. Eskom Report TRR/T99/055, Johannesburg, 55pp. Helas G and Pienaar JJ, 1996. Biomass burning emissions. (Chapter 3 in: Air pollution and its impacts on the South African highveld, G Held, BJ Gore, AD Surridge, GR Tosen, CR Turner and RD Walmsley, eds), 12-15, Environmental Scientific Association, Cleveland, 144 pp. Held G, Scheifinger H, Snyman GM, Tosen GR and Zunckel M, 1996. The climatology and meteorology of the highveld (Chapter 9 in: Air pollution and its impacts on the South African highveld, G Held, BJ Gore, AD Surridge, GR Tosen, CR Turner and RD Walmsley, eds), 60-71, Environmental Scientific Association, Cleveland, 144 pp. Held G, Pienaar JJ, Snyman GM and Lachman G, 1997. Vertical distribution of pollutants over the Mpumalanga highveld (Summary 1994 - 1996). Eskom Report TRR/T97/043, Johannesburg, 33pp. Held G, Galpin JS, De Beer GH and Mphepya J, 1999. The Kiepersol Project: A National Network for Acid Rain Research (July 1998 - June 1999). Confidential Report to the Department of Environmental Affairs & Tourism, Eskom Report TRR/T99/056, Johannesburg, 76pp. Hicks BB, Hosker RP, Meyers TP and Womack JD, 1991. Dry deposition inferential measurement technique - I. Design and test of a prototype meteorlogical and chemical system for determining dry deposition. Atm. Environ., 25A(10), 2345-2359. Matt DR and Meyers TP, 1993. The use of inferential technique to estimate dry deposition of SO2. Atm. Environ., 27A(4),2345-2359. Meyers TP, Hicks BB, Hosker RP, Womack JD and Satterfield LC, 1991. Dry deposition inferential measurement technique, II. Seasonal and annual deposition rates of sulphur and nitrate. Atm. Environ., 25A(10), 2361-2370. Mphepya J and Held G, 1999a. Deposition studies in 1999. Eskom Report RES/RE/00/10399, Johannesburg, 50 pp. Mphepya J and Held G, 1999b. Dry deposition of sulphur on the Mpumalanga highveld, 1996-1998. Proceedings, of the1999 Annual Conference of NACA, Cape Town, 7-8 October 1999, 10pp. Snyman GM, Held G and Pienaar JJ, 1997. Vertical distribution of pollutants observed during the 1997 field experiments. Eskom Report TRR/T97/017, Johannesburg, 37pp. Thompson M, 1996. A standard land-cover classification scheme for remote sensing in South Africa. SAJ Sci, 92, 34-42. Turner CR, 1993. A seven year study of rainfall chemistry in South Africa, Proceedings of the 1993 NACA Conference: Clean Air Challenges in a Changing South Africa, Dikhololo, Brits, 11-12 November 1993. Turner CR and De Beer GH, 1996. A review of ten year’s rain quality data for the South African interior. Eskom Report TRR/S96/131, 18pp. Turner CR, Wells RB, Zunckel M, 1995. Dry deposition monitoring methodologies for the highveld region. Proceedings of the 26th Annual Clean Air Conference of NACA, Durban, 22-24 November 1995. Wells RB, Lloyd SM and Turner CR, 1996. National Air Pollution Source Inventory (Chapter 1 in: Air pollution and its impacts on the South African highveld, G Held, BJ Gore, AD Surridge, GR Tosen, CR Turner and RD Walmsley, eds), 60-71, Environmental Scientific Association, Cleveland, 144 pp. Zunckel M, Turner CR and Wells RB, 1996. Dry deposition of sulphur on the Mpumalanga highveld: A pilot study using the inferential method. SA J Sci, 92, 485-491. Zunckel M, 1997. Dry deposition of sulphur over Mpumalanga. Preprints, Fifth International Conference on Southern Hemisphere Meteorology and Oceanography, Pretoria, 7-11 April 1997, American Meteorological Society, 252-253. Zunckel M, 1998. Dry deposition of sulphur in South Africa. Papers of the 11th World Clean Air and Environment Congress, Volume 6, Paper 17A-3, IUAPPA/NACA, Durban, 13-18 September 1998, 6pp. Agradecimentos: À AM Gomes pela versão em português do resumo. 2833