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).
2824
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 <2,5µm in
diameter, while the Beta Gauge’s cut-off point is 10µm, which results in slightly different Fine Particulate Matter
(FPM) concentrations. Meteorological parameters were measured using standard equipment.
Earlier studies of results from short-term monitoring indicated, that the contribution from SO42- only accounts for
less than 10% of the total dry deposition of sulphur on the highveld (Zunckel, 1997). Although the relative
contribution increases with increasing distance from the source region, the actual quantity deposited from SO42drops to almost half (Zunckel, 1997). Since particulates at the two sites were only measured during the short winter
and summer field experiments (Turner et al, 1995; Zunckel et al, 1996), FPM measurements had to be used to
estimate the contribution from SO42-. Considering the mass distribution of anions and cations suspended in the
atmosphere over the highveld (Held et al, 1997), it was decided to conservatively estimate the SO42- contribution to
the FPM as 50%. In view of the small contribution of SO42- to the total sulphur deposition (Zunckel, 1997), any
possible error would not significantly influence the total sulphur deposition results.
The data quality of the ambient air quality parameters, as well as that of wind, temperature and solar radiation
measurements was excellent, with a data recovery of at least 85%. However, problems were encountered with the
quality of humidity and rainfall data as a result of instrumentation problems. This necessitated partial substitution
of humidity data at Palmer from a nearby (Rietfontein) automatic weather station (AWS) of the South African
Weather Bureau (SAWB) after carefully verifying the close relationship of the two sites. At Elandsfontein, all
relative humidity data had to be substituted from the AWS observations of the SAWB at Ermelo airfield, which is
located 55km to the east-south-east at a similar altitude and also well exposed above the surroundings. Rainfall
measurements at three nearby sites were carefully evaluated, but due to the high spatial and temporal variability it
was not possible to substitute them.
The inferential model allows for a ground–cover mix of two vegetation types. Dominant vegetation cover
surrounding both sites was based on field observations (Zunckel, 1998). A mix of 66% grass and 34% maize was
considered to be representative of the Elandsfontein site, while a mix of 85% grass and 15% forest was considered
for Palmer (Zunckel, 1998; Thompson, 1996). For each vegetation combination the model also requires a
maximum leaf area index (LAI) value, which is selected automatically by the model, and a percentage of LAI that
describes the growth state of the vegetation canopy. For the winter periods a dormant canopy was modelled as is
normal on the highveld (LAI = 0,01), whereas in summer a full canopy was assumed (LAI = 1,00) as established
for both sites during the 1994/95 pilot study (Turner et al, 1995; Zunckel et al, 1996).
3.1
Mean Diurnal Variations of the Inferred SO2 Deposition Velocity
The seasonal differences in the mean diurnal variations of the inferred SO2 deposition velocity have been analysed
by Mphepya and Held (1999a) for the full period, but only one example is shown in Figure 3. The diurnal pattern
of Vd reflects the increased SO2 concentrations during the day, as well as the portion of photosynthetically active
vegetation. At both sites a marked diurnal variation is observed in all seasons, but the seasonal trend of Vd during
the day differs significantly between the two sites. Minimum values are close to zero at night and increase steadily
after sunrise to reach a maximum during the midday period after which there is a steady decrease towards sunset.
2828
Convective mixing during the day and dialated leaf stomata are conducive to the daytime maximum, while the
stable night-time boundary layer and closed stomata result in the near-zero minima. At the rural Palmer site, which
has a 15% forest component, highest deposition velocities are inferred during summer (except in 1997 when a
higher Vd was observed in autumn), while at Elandsfontein, which has a 34% maize component, Vd is greatest
during summer (except in 1996 when Vd was higher during the autumn season), with a drastically reduced mean
diurnal variation during winter and autumn. Generally higher deposition velocities occur in summer than in other
months when the atmosphere is more convective, insulation is higher and the vegetation is photosynthetically more
active. All inferred Vd values are higher at Elandsfontein than at Palmer. Since no rainfall data were available for
Elandsfontein, the actual deposition velocities during the rain season would be higher, as indicated in a sensitivity
study of the inferential model to various meteorological parameters (Mphepya and Held, 1999b).
Deposition Velocity (cm/s)
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hours
summer
autumn
winter
spring
Deposition Velocity (cm/s)
0,50
0,45
0,40
0,35
0,30
0,25
0,20
0,15
0,10
0,05
0,00
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Hours
summer
autumn
winter
spring
Figure 3. Mean diurnal variation of the inferred deposition velocity (Vd) of SO2 per season during the period
1 March 1997 to 28 February 1998 (Top: Palmer; Bottom: Elandsfontein).
3.2
Seasonally Averaged Inferred Deposition Velocities of SO2 and SO42-
Averaged inferred deposition velocities (Vd) for SO2 were larger during the summer months and smallest during
winter (Table 2). This can be attributed to the increase in solar radiation, LAI, the photosynthetic activity of
vegetation and variations in meteorological variables such as temperature and surface wetness. During summer
daytime deposition velocities varied from 0,33-0,43cm.s-1, but at night they dropped to 0,13-0,17cm.s-1. The
corresponding figures for winter are 0,14-0,15 and 0,1cm.s-1, respectively. No systematic differences in Vd were
found between Palmer and Elandsfontein. On the other hand, the Vd for sulphate was generally larger during
spring and lowest again during winter. At Palmer, daytime Vd in winter varied from 0,25-0,27cm.s-1, while at
Elandsfontein it was slightly lower (0,19-0,24cm.s-1). Night-time values were extremely low at both sites
(0,02cm.s-1). During spring, the Vd at Palmer varied from 0,30-0,32cm.s-1, but at Elandsfontein it was only
2829
0,28cm.s-1 during 1997 and 1998, but in 1996 it was higher (0,35cm.s-1) than at Palmer. Night-time deposition
velocities were slightly higher at Elandsfontein than at Palmer (0,3 vs 0,2cm.s-1).
Table 2. Seasonally averaged inferred deposition velocities of SO2 and SO42- in cm.s-1 (means 1996 - 1998).
Site
Autumn
PALMER
Day
(Night)
Total
ELANDSFONTEIN
Day
(Night)
Total
SO2
Winter Spring
Summer*
Autumn
SO42Winter Spring
Summer*
0,26
(0,12)
0,38
0,14
(0,10)
0,24
0,24
(0,12)
0,36
0,37
(0,15)
0,52
0,23
(0,02)
0,25
0,26
(0,02)
0,28
0,31
(0,02)
0,33
0,29
(0,03)
0,32
0,26
(0,12)
0,38
0,15
(0,10)
0,25
0,22
(0,11)
0,33
0,35
(0,15)
0,50
0,19
(0,02)
0,21
0,21
(0,02)
0,23
0,30
(0,03)
0,33
0,25
(0,03)
0,28
* December to February
3.3
Seasonally Averaged Dry Deposition Rates (Flux) of SO2 and SO42-
A seasonal pattern in the dry deposition rate of sulphur from SO2 is clearly evident at Elandsfontein, with the
highest deposition rates in summer and lowest in spring or autumn, although SO2 concentrations are generally
greatest during winter and lowest during the summer months. Conversely, no pattern for a seasonal deposition rate
of sulphur from SO2 was observed at Palmer. Sulphur deposition from sulphate is generally highest during spring
(Table 3).
Table 3. Seasonal dry deposition rates (kg.ha-1 per 3-month period) of sulphur from SO2 and SO42- (in brackets).
PALMER
1996
1997
1998
Average
Total
ELANDSFONTEIN
1996
1997
1998
Average
Total
1
2
Autumn
1,48
(0,19)
0,10
(0,14)
0,09
(0,48)
0,56
(0,27)
0,83
Winter
0,10
(0,20)
0,13
(0,31)
0,84
(0,58)
0,36
(0,36)
0,72
Spring
0,06
(0,41)
0,06
(0,44)
0,07
(0,46)
0,06
(0,44)
0,50
Summer1
0,32
(0,19)
0,05
(0,36)
0,082
(0,28)
0,15
(0,28)
0,43
1,55
(0,16)
1,87
(0,14)
1,96
(0,68)
1,79
(0,33)
2,12
1,71
(0,17)
2,05
(0,22)
1,62
(0,48)
1,79
(0,29)
2,08
1,87
(0,35)
1,49
(0,23)
1,49
(0,84)
1,72
(0,47)
2,09
3,34
(0,09)
2,38
(0,48)
2,062
0,84)
2,59
(0,47)
3,06
1 December 1996 to 28 February 1997, etc
Only December 1998
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3.4
Total Annual Sulphur Deposition
The estimated total dry deposition figures from SO2 and SO42- at Elandsfontein and Palmer are compared with the
wet deposition rate at Amersfoort, which is the only monitoring site in the region and situated about 80km southeast of Elandsfontein (Figure 1, just north of Volksrust). It was therefore assumed to be representative for both dry
deposition monitoring sites.
Table 4. Annual deposition rates (kg.ha-1.y-1) of total sulphur.
Dry deposition from SO2 + SO42- = total
Year
1996
1997
1998
Palmer
1,98 + 0,97 = 2,95
0,33 + 1,23 = 1,56
1,07 + 1,79 = 2,86
Elandsfontein
8,46 + 0,77 = 9,23
7,80 + 1,06 = 8,86
7,13 + 2,83 = 9,96
Total wet deposition
from SO42Amersfoort
4,64
6,22
4,21
Relatively large variations of the dry deposition components from one year to the next are observed at Palmer, but
the total dry sulphur deposition only varies between 1,6 and 3,0 kg.ha-1.y-1 (Table 4). This might be attributed to
the fact that Palmer lies on the north-eastern fringe of the source area and is only impacted under certain circulation
patterns, which mostly bring matured air masses. This is confirmed by the sulphur deposition from SO42- being
generally larger than that from SO2. At Elandsfontein, it is the opposite and the total sulphur deposition varies
between 8,9 and 10,0 kg.ha-1.y-1. At Amersfoort, the sulphur deposition from rain varied between 4,2 and 6,2
kg.ha-1.y-1 during the study period. The total sulphur deposition at any site is the total of dry and wet deposition.
Therefore, the estimated total annual sulphur deposition at Elandsfontein is 13,87, 15,08 and 14,17 kg.ha-1.y-1 for
1996, 1997 and 1998, respectively. These figures could even be higher due to the omission of rainfall in the model
calculations for Elandsfontein. The estimated dry deposition exceeds wet deposition by a factor of about 2. At
Palmer, wet deposition exceeds the contribution of dry deposition (assuming that Amersfoort is representative,
which might not be true), but this should be verified with actually monitored wet deposition rates.
4.
CONCLUSION
The degradation of air and rain quality is a well know consequence of wide-spread industrialisation. However, the
control of sulphur emissions from power stations in South Africa would incur great cost and would result in
significant secondary environmental problems involved with both the supply and waste disposal sides of the
operation of such emission control equipment. It was thus considered vital that the need for such equipment be
established on a rational scientific basis before commitments to its installation. This is especially true as, for a
variety of technical reasons, overall sulphur emissions from power stations can eventually be expected to start
decreasing as old technology gradually gives way to new. Great savings could be realised, if no macroscopic
environmental impacts are expected within the life-span of Eskom’s power stations. On the other hand, if negative
impacts were predicted with a reasonable degree of certainty, this would dictate that an appropriate course of
remediation would have to be initiated.
Therefore, a wet-deposition monitoring network was established in 1985, yielding 14 years of data until 1998 from
a total of 32 sites with records of varying lengths and not all operating simultaneously. The project was extended to
include dry deposition processes during 1994. Dry deposition of air pollutants involves the removal from the
atmosphere of gaseous and particulate matter by all processes other than those involving rainfall and other
precipitation. In regions where wet climate prevails, particularly those situated at high latitudes and remote from
major source areas, wet deposition processes predominate. Thus, the measurement of wet-only deposition can
provide a reasonable surrogate for total deposition measurement. However, in arid regions, such as most of the
South African interior during winter, wet-only deposition monitoring does not necessarily provide a reliable
surrogate for total deposition as dry deposition processes can predominate at times.
Isopleths based on selected mean ion concentrations yielded the following results: Hydrogen ions show maximum
concentrations in excess of 90µeq.l -1 centred over the industrial hub of Mpumalanga. As expected, sulphates also
2831
have their maximum there (>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.
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5.
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Agradecimentos: À AM Gomes pela versão em português do resumo.
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