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Comparative studies of trace elements in air particulate in northern Nigeria
The Science of the Total Environment, 10 (1978) 239--244 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
COMPARATIVE STUDIES OF TRACE ELEMENTS IN AIR PARTICULATE IN NORTHERN NIGERIA
F. BEAVINGTON Bayero University, Kano (Nigeria)
P. A. CAWSE Environmental & Medical Sciences Division, AERE HarweU, Oxon (England)
(Received March 18th, 1978)
ABSTRACT
Atmospheric measurements were made throughout 1976 at a rural site in the Nigerian savanna. Concentrations of trace and major elements in air were an order of magnitude higher in the dry season, when air is of Saharan origin, than in the rainy season when maritime tropical air dominates. The annual average concentrations of elements originating from soil dust, Al, Ce, Mg, Sc and Ti were more than an order of magnitude higher compared with southern England, whereas levels of heavy metals were generally lower. The enrichment factors (ratio of elements to Sc in the air relative to ratios of the same elements in average soil) found in northern Nigeria for As, Br, Cl, Pb, Sb and Zn were two orders of magnitude below those in rural England: this is attributed to less industrial development at equatorial relative to northerly latitudes. Enrichment factors in Nigeria are also compared with data from south Africa and urban regions in England.
INTRODUCTION This p a p e r r e p o r t s i n f o r m a t i o n o n t h e c o n c e n t r a t i o n s , seasonal v a r i a t i o n a n d origin o f t r a c e a n d m a j o r e l e m e n t s in air p a r t i c u l a t e in n o r t h e r n Nigeria. T h e s e e l e m e n t s include b o t h p o t e n t i a l l y t o x i c p o l l u t a n t s a n d t h o s e t h a t are essential f o r p l a n t a n d a n i m a l n u t r i t i o n . In p a r t i c u l a r , m e a s u r e m e n t s w e r e n e e d e d t o establish t h e ' n a t u r a l ' or b a c k g r o u n d c o n c e n t r a t i o n s o f e l e m e n t s in t h e air a t this stage in t h e industrial d e v e l o p m e n t o f West A f r i c a a n d in a global c o n t e x t , t o c o m p a r e t h e s e levels w i t h results o f a t m o s p h e r i c a n a l y s e s at o t h e r l a t i t u d e s . C o l l e c t i o n a n d analysis o f d e p o s i t e d d u s t a n d r a i n w a t e r was m a d e s i m u l t a n e o u s l y w i t h air m e a s u r e m e n t s a n d will b e r e p o r t e d separately. Climate A t t h e end o f April t h e w i n d d i r e c t i o n o v e r n o r t h e r n Nigeria c h a n g e s f r o m m a i n l y n o r t h e r l y t o s o u t h e r l y as m a r i t i m e t r o p i c a l air f r o m t h e S o u t h A t l a n t i c b e c o m e s d o m i n a n t . T h e a s s o c i a t e d r a i n y s e a s o n w h i c h begins w i t h an o c c a s i o n a l s t o r m in April a n d M a y , intensifies t o give 76% o f t h e m e a n
240 a n n u a l rainfall o f 8 4 6 m m ( K a n o ) in J u l y , A u g u s t a n d S e p t e m b e r . T h e rains cease in O c t o b e r a n d t h e f o l l o w i n g 6 - - 7 m o n t h s are s u b j e c t t o a d r y air m a s s o f S a h a r a n origin, w i t h a s s o c i a t e d n o r t h e r l y H a r m a t t a n w i n d . F r o m D e c e m b e r t o F e b r u a r y a n d s o m e t i m e s later, t h e a t m o s p h e r e is v e r y d u s t y so t h a t visibility is r e d u c e d t o 3 0 0 - - 4 0 0 m . I t s h o u l d b e n o t e d t h a t in 1 9 7 6 t h e H a r m a t t a n i n f l u e n c e w a s b e l o w average.
MATERIALS
AND METHODS
A p p a r a t u s f o r collecting s a m p l e s w a s installed in t h e s a v a n n a at B a g a u d a (lat. 1 1 ° 3 6 ' N , 56 k m SSE o f K a n o ) a n d m e a s u r e m e n t s b e g a n o n J a n u a r y 1st, TABLE 1 ELEMENTAL CONCENTRATIONS IN AIR AT BAGAUDA, NIGERIA, AND COMPARISON WITH NON-URBAN SITES AT OTHER LATITUDES (Nano g kg -1 air) a Results for all sites are mean concentrations from Jam--Dec. 1976. Figures in parenthesis denote % element in dried air particulate material, if present at >0.1% concentration. Element
Al As Br Ca C.cl Ce Cl Co Cr Cu Fe Hg I In
K Mn Mg Na Ni Pb Sb Sc Se Ti V Zn
Bagauda, Nigeria 11-N
5600 0.49 6.5 5900 <1 9.1 250 1.4 7.0 39 4200 <0.4
(5.3%) (0.007%) (5.0%) (0.25%)
(3.7%)
/--.2 <0.02
1250 60 1900 500 3.0 12 0.25 1.2 <0.4 390 7.9 15
Chilton, Oxon England b 51-N~
260 3.4 36 750 2.7 0.41 1800 0.33 2.4 18 320 <0.05
(0.97%) (0.17%) (3.3%) (7.9%)
(1.4%)
2.1 0.071
(1.2%) (1.4%) (0.48%) (0.013%)
510 13 140 630 4.7 120 2.5 0.081 1.9
<26 11 (0.014%) 88
(0.42%)
anano g kg-l x 1.226 = nano g m-S.
bCawse (1977). CCawse, P. A., unpublished data.
Pelindaba near Pretoria S. Africac 25°S
1400 1.4 28 1700 <7 3.5 820 0.60 10 49 1000 <0.3 <3 0.025
(2.0%)
(0.65%) (2.8%)
440 56 330 190 <6
(0.52%)
(<0.1%)
63 0.8 0.32 0.43 96
(0.40%)
3.6 55
241 1976. Air particulate was collected continuously at 1.5 m above ground according to procedures developed in previous surveys (Cawse and Peirson, 1972) using pre-weighed cellulose filters (Whatman 40 grade). Air filters were changed every m o n t h , the volume of air filtered being about 300m 3 (Cawse, 1977). The preferred technique for analysis was instrumental neutron activation supported by X-ray fluorescence for determination of Ni and Pb. The results for 25 elements are reported here (Table 1 ) b u t analysis was also made for Cs, Eu, La, Mo, Rb, Sm, Th and W. Accuracy was validated by analysis of standard reference material (National Bureau of Standards orchard leaves) and by an analytical inter-comparison using air filters (Cawse, 1976).
RESULTS AND DISCUSSION
At Bagauda, the concentrations of elements found in air were relatively steady from January to April 1976, as shown in Fig. 1 for selected elements. However, over the next six months there was an order of magnitude decrease in concentrations until November and December, when the levels increased to approach the relatively high values observed in January. Such extreme differences have never been observed in Europe and are attributed to the weather pattern. The rainfall pattern and change in weight of total suspended particulate
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TOTAL SUSPENDED PARTICULATE RAINFALL 0
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174 161
184 136 I 57
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TSP(/.~gKg-~air I RAINFALL Irnm)
Fig. 1. Elemental concentration in air near ground level at Bagauda, Nigeria, 1976.
242 (TSP) demonstrates the seasonal contrast in weather (Fig. 1): the dry season had an average TSP of 143 pg kg -1 air compared with 28 pg kg -1 during the rainy season from May to October. In non-urban regions of the U.K. the TSP ranged from 18--37 pg kg -~ during 1976 (Cawse, 1977). The World Health Organisation have recommended (WHO, 1972) an annual mean TSP not >33 pg kg -l (>40 #g m-3 ), with 98% of observations <98 ~g kg-I (or <120/~g m-3 ). Annual average concentrations of elements measured in air in northern Nigeria are compared in Table 1 with levels recorded at non-urban locations in Europe (Chilton, Oxfordshire) and in southern Africa (Pelindaba, 25 km west of Pretoria) during 1976. At Bagauda, concentrations of A1, Ce, Mg, Sc and Ti are more than an order of magnitude greater than at Chilton, and large increases in levels of Ca, Co and Mn are also found. In contrast, elements that are frequently associated with industrial emissions in England and Europe generally show lower concentrations in Nigeria, e.g., As, Br, C1, Ni, Pb, Sb, Se, V and Zn. Pretoria has intermediate air concentrations for most elements, but gives levels higher than Chilton for Cr and Cu and lower than Bagauda for K, Na and V. The percentage composition of the air particulate at Bagauda (derived from TSP loadings and element concentrations in air) shows that 18% of the dry weight is accounted for by eleven of the elements listed in Table 1. Enrichment factors were calculated from the ratios of elements to Sc in air relative to the ratios of the same elements in average soil. These enrichments in air particulate at the non-urban stations at Bagauda, Chilton and Pretoria are compared with corresponding data from urban sites in London and Swansea, south Wales (Table 2). At Bagauda the element enrichment factors are generally low and typical of re-suspended soil dust: they reflect the relative absence of heavy industry and vehicle emissions. At Chilton however, the industrial and maritime influences at this latitude result in enrichment factors which are two orders of magnitude greater than at Bagauda for As, Br, C1, Pb, Sb and Zn, and one order of magnitude higher for I, In, Na, Ni, Se and V. This distance of Bagauda and Pretoria from the sea is reflected by relatively low enrichment factors for Br, C1 and Na. The influence of urbanisation on enrichment factors in air particulate is appreciated by comparison of Bagauda results with Fleet Street, London, and central Swansea (Table 2). The urban sites show increases in element enrichments that are greatest for Pb, Br, Sb, Zn, Ni and CI: this is attributed to automobile exhaust, combustion of fossil fuels, incineration and metal working. In the case of Pb and Br, an increase of up to 1000-fold is observed. Also, analysis of rainwater around the smelter at Wollongong, Australia (Beavington, 1977), and unpublished data from this author on air particulate in the same vicinity, confirm very high enrichments of heavy metals in such environments. Clearly, the very rural nature of northern Nigeria and of this latitude generally, results in low enrichment factors for elements that are found at high enrichment both in non-urban and urban regions in higher, more industrial latitudes (Peirson et al., 1974). The enrichments found near Pretoria, in the southern hemisphere, are closer to Bagauda values than those at Chilton, England.
243 TABLE 2 COMPARISON OF AIR P A R T I C U L A T E ENRICHMENT F A C T O R S (SELECTED ELEMENTS) Element Element Enrichment factor = ~ conch, in air/ concn, in soil using average soil Sc Sc data from Bowen (1966). Non-urban measurements were made from January--December 1976. Urban measurements were made as follows: Fleet St., July 1972; Central Swansea, December 1972--May 1973. The measurements for these further stations other than Bagauda, were made: by Harwell as part of separate research programmes. Element
Non-urban
Latitude
Bagauda, Nigeria 11°N
Urban Chilton, Oxon, England 51°N
Pelindaba, Pretoria, S. Africa 25°S
Fleet Street, London a 5 l°N
Central Swansea~ S. Wales 51°N
90 km
500 km
60 km
1.5 km
0.3 49 630 1580 3.7 76 0.7 195 8.6 10 1030 31 790 9.6 155
0.4 5.0 120 185 1.7 53 0.6 18 0.7 ~3 140 2.6 46 0.8 24
2.2 -6260 730 2.7 83 0.9 -2.3 11 7680 45 145 26 130
0.2 130 2900 2100 27 130 1.1
Distance from sea 700 km Al As Br Cl Co Cu Fe In Na Ni Pb Sb Se V Zn
0.5 0.5 7.9 16 1.1 12 0.7 <:4 0.5 0.5 7.2 0.2 ~11 0.5 1.8
14 75 2300 30 610 95 290
aCambray, R. S., private communication. bpattenden (1974).
Monthly results for concentrations of elements in air were examined for correlations between pairs of elements. At Bagauda, there were 86 highly significant ( P ~ 0 . 0 0 1 ) correlations compared with only 47 at Chilton, almost certainly the result of the defined climatic regime in Nigeria. For example, some of the highest correlations at Bagauda occurred between A1--Fe, A1--Sc, Ce--Fe, Ce--Sc and Fe--Sc indicating that climatic features regulate inputs to the atmosphere and distribution from the predominant natural source which is presumed to be soft dust: enrichment factors indicate that industrial emissions are minimal. Similar correlations were reported from work in the Ivory Coast (Crozat et al., 1974). Measurements of the dry deposition velocity of elements, that is related to particle size (Chamberlain, 1966), were derived from the basic analyses of air particulate and dry deposition at Bagauda, and indicate a very uniform aerosol with small
244 particle size f o r all e l e m e n t s o v e r t h e H a r m a t t a n p e r i o d , c o n s i s t e n t w i t h well-mixed material originating f r o m t h e Sahara desert. A l t h o u g h c o n t r i b u tion f r o m locally raised soil d u s t is n o t e n t i r e l y e x c l u d e d , t h e h o m o g e n e i t y o f t h e aerosol in t h e d r y season suggests t h a t this is u n i m p o r t a n t .
ACKNOWLEDGEMENT This w o r k was s u p p o r t e d b y the Ministry o f Overseas D e v e l o p m e n t , L o n d o n , and we wish t o a c k n o w l e d g e t h e i r interest and assistance.
REFERENCES
Beavington, F., Environ. Pollut., 13 (1977) 127. Bowen, H. J. M., Trace Elements in Biochemistry, Academic Press, New York, 1966. Cawse, P. A. and D. H. Peirson, An Analytical Study of Trace Elements in the Atmospheric Environment, AERE Rep. R 7131, HMSO, London, 1972. Cawse, P. A., A Survey of Atmospheric Trace Elements in the U.K.: Results for 1976, AERE Rep. R 8869, HMSO, London, 1977. Cawse, P.A., Intercomparison of Air Filter Deposits for Trace Elements, AERE Rep. R 8191, HMSO, London, 1976. Chamberlain, A. C., Proc. R. Soc. A296 (1966) 45. Crozat, G., J. L. Domergue, V. Bogui and J. Fontan, Atmos. Environ., 7 (1974) 1103. Pattenden, N. J., Atmospheric Concentrations of Some Trace Elements in the Swansea/ Neath/Port Talbot area, AERE Rep. R 7729, HMSO, London, 1974. Peirson, D. H., P. A. Cawse and R. S. Cambray, Nature, 251 (1974) 675. W.H.O. Health Hazards of the Human Environment, WHO, Geneva, 1972.
COMPARATIVE STUDIES OF TRACE ELEMENTS IN AIR PARTICULATE IN NORTHERN NIGERIA
F. BEAVINGTON Bayero University, Kano (Nigeria)
P. A. CAWSE Environmental & Medical Sciences Division, AERE HarweU, Oxon (England)
(Received March 18th, 1978)
ABSTRACT
Atmospheric measurements were made throughout 1976 at a rural site in the Nigerian savanna. Concentrations of trace and major elements in air were an order of magnitude higher in the dry season, when air is of Saharan origin, than in the rainy season when maritime tropical air dominates. The annual average concentrations of elements originating from soil dust, Al, Ce, Mg, Sc and Ti were more than an order of magnitude higher compared with southern England, whereas levels of heavy metals were generally lower. The enrichment factors (ratio of elements to Sc in the air relative to ratios of the same elements in average soil) found in northern Nigeria for As, Br, Cl, Pb, Sb and Zn were two orders of magnitude below those in rural England: this is attributed to less industrial development at equatorial relative to northerly latitudes. Enrichment factors in Nigeria are also compared with data from south Africa and urban regions in England.
INTRODUCTION This p a p e r r e p o r t s i n f o r m a t i o n o n t h e c o n c e n t r a t i o n s , seasonal v a r i a t i o n a n d origin o f t r a c e a n d m a j o r e l e m e n t s in air p a r t i c u l a t e in n o r t h e r n Nigeria. T h e s e e l e m e n t s include b o t h p o t e n t i a l l y t o x i c p o l l u t a n t s a n d t h o s e t h a t are essential f o r p l a n t a n d a n i m a l n u t r i t i o n . In p a r t i c u l a r , m e a s u r e m e n t s w e r e n e e d e d t o establish t h e ' n a t u r a l ' or b a c k g r o u n d c o n c e n t r a t i o n s o f e l e m e n t s in t h e air a t this stage in t h e industrial d e v e l o p m e n t o f West A f r i c a a n d in a global c o n t e x t , t o c o m p a r e t h e s e levels w i t h results o f a t m o s p h e r i c a n a l y s e s at o t h e r l a t i t u d e s . C o l l e c t i o n a n d analysis o f d e p o s i t e d d u s t a n d r a i n w a t e r was m a d e s i m u l t a n e o u s l y w i t h air m e a s u r e m e n t s a n d will b e r e p o r t e d separately. Climate A t t h e end o f April t h e w i n d d i r e c t i o n o v e r n o r t h e r n Nigeria c h a n g e s f r o m m a i n l y n o r t h e r l y t o s o u t h e r l y as m a r i t i m e t r o p i c a l air f r o m t h e S o u t h A t l a n t i c b e c o m e s d o m i n a n t . T h e a s s o c i a t e d r a i n y s e a s o n w h i c h begins w i t h an o c c a s i o n a l s t o r m in April a n d M a y , intensifies t o give 76% o f t h e m e a n
240 a n n u a l rainfall o f 8 4 6 m m ( K a n o ) in J u l y , A u g u s t a n d S e p t e m b e r . T h e rains cease in O c t o b e r a n d t h e f o l l o w i n g 6 - - 7 m o n t h s are s u b j e c t t o a d r y air m a s s o f S a h a r a n origin, w i t h a s s o c i a t e d n o r t h e r l y H a r m a t t a n w i n d . F r o m D e c e m b e r t o F e b r u a r y a n d s o m e t i m e s later, t h e a t m o s p h e r e is v e r y d u s t y so t h a t visibility is r e d u c e d t o 3 0 0 - - 4 0 0 m . I t s h o u l d b e n o t e d t h a t in 1 9 7 6 t h e H a r m a t t a n i n f l u e n c e w a s b e l o w average.
MATERIALS
AND METHODS
A p p a r a t u s f o r collecting s a m p l e s w a s installed in t h e s a v a n n a at B a g a u d a (lat. 1 1 ° 3 6 ' N , 56 k m SSE o f K a n o ) a n d m e a s u r e m e n t s b e g a n o n J a n u a r y 1st, TABLE 1 ELEMENTAL CONCENTRATIONS IN AIR AT BAGAUDA, NIGERIA, AND COMPARISON WITH NON-URBAN SITES AT OTHER LATITUDES (Nano g kg -1 air) a Results for all sites are mean concentrations from Jam--Dec. 1976. Figures in parenthesis denote % element in dried air particulate material, if present at >0.1% concentration. Element
Al As Br Ca C.cl Ce Cl Co Cr Cu Fe Hg I In
K Mn Mg Na Ni Pb Sb Sc Se Ti V Zn
Bagauda, Nigeria 11-N
5600 0.49 6.5 5900 <1 9.1 250 1.4 7.0 39 4200 <0.4
(5.3%) (0.007%) (5.0%) (0.25%)
(3.7%)
/--.2 <0.02
1250 60 1900 500 3.0 12 0.25 1.2 <0.4 390 7.9 15
Chilton, Oxon England b 51-N~
260 3.4 36 750 2.7 0.41 1800 0.33 2.4 18 320 <0.05
(0.97%) (0.17%) (3.3%) (7.9%)
(1.4%)
2.1 0.071
(1.2%) (1.4%) (0.48%) (0.013%)
510 13 140 630 4.7 120 2.5 0.081 1.9
<26 11 (0.014%) 88
(0.42%)
anano g kg-l x 1.226 = nano g m-S.
bCawse (1977). CCawse, P. A., unpublished data.
Pelindaba near Pretoria S. Africac 25°S
1400 1.4 28 1700 <7 3.5 820 0.60 10 49 1000 <0.3 <3 0.025
(2.0%)
(0.65%) (2.8%)
440 56 330 190 <6
(0.52%)
(<0.1%)
63 0.8 0.32 0.43 96
(0.40%)
3.6 55
241 1976. Air particulate was collected continuously at 1.5 m above ground according to procedures developed in previous surveys (Cawse and Peirson, 1972) using pre-weighed cellulose filters (Whatman 40 grade). Air filters were changed every m o n t h , the volume of air filtered being about 300m 3 (Cawse, 1977). The preferred technique for analysis was instrumental neutron activation supported by X-ray fluorescence for determination of Ni and Pb. The results for 25 elements are reported here (Table 1 ) b u t analysis was also made for Cs, Eu, La, Mo, Rb, Sm, Th and W. Accuracy was validated by analysis of standard reference material (National Bureau of Standards orchard leaves) and by an analytical inter-comparison using air filters (Cawse, 1976).
RESULTS AND DISCUSSION
At Bagauda, the concentrations of elements found in air were relatively steady from January to April 1976, as shown in Fig. 1 for selected elements. However, over the next six months there was an order of magnitude decrease in concentrations until November and December, when the levels increased to approach the relatively high values observed in January. Such extreme differences have never been observed in Europe and are attributed to the weather pattern. The rainfall pattern and change in weight of total suspended particulate
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TOTAL SUSPENDED PARTICULATE RAINFALL 0
2.3
9.1
47
174 161
184 136 I 57
0
TSP(/.~gKg-~air I RAINFALL Irnm)
Fig. 1. Elemental concentration in air near ground level at Bagauda, Nigeria, 1976.
242 (TSP) demonstrates the seasonal contrast in weather (Fig. 1): the dry season had an average TSP of 143 pg kg -1 air compared with 28 pg kg -1 during the rainy season from May to October. In non-urban regions of the U.K. the TSP ranged from 18--37 pg kg -~ during 1976 (Cawse, 1977). The World Health Organisation have recommended (WHO, 1972) an annual mean TSP not >33 pg kg -l (>40 #g m-3 ), with 98% of observations <98 ~g kg-I (or <120/~g m-3 ). Annual average concentrations of elements measured in air in northern Nigeria are compared in Table 1 with levels recorded at non-urban locations in Europe (Chilton, Oxfordshire) and in southern Africa (Pelindaba, 25 km west of Pretoria) during 1976. At Bagauda, concentrations of A1, Ce, Mg, Sc and Ti are more than an order of magnitude greater than at Chilton, and large increases in levels of Ca, Co and Mn are also found. In contrast, elements that are frequently associated with industrial emissions in England and Europe generally show lower concentrations in Nigeria, e.g., As, Br, C1, Ni, Pb, Sb, Se, V and Zn. Pretoria has intermediate air concentrations for most elements, but gives levels higher than Chilton for Cr and Cu and lower than Bagauda for K, Na and V. The percentage composition of the air particulate at Bagauda (derived from TSP loadings and element concentrations in air) shows that 18% of the dry weight is accounted for by eleven of the elements listed in Table 1. Enrichment factors were calculated from the ratios of elements to Sc in air relative to the ratios of the same elements in average soil. These enrichments in air particulate at the non-urban stations at Bagauda, Chilton and Pretoria are compared with corresponding data from urban sites in London and Swansea, south Wales (Table 2). At Bagauda the element enrichment factors are generally low and typical of re-suspended soil dust: they reflect the relative absence of heavy industry and vehicle emissions. At Chilton however, the industrial and maritime influences at this latitude result in enrichment factors which are two orders of magnitude greater than at Bagauda for As, Br, C1, Pb, Sb and Zn, and one order of magnitude higher for I, In, Na, Ni, Se and V. This distance of Bagauda and Pretoria from the sea is reflected by relatively low enrichment factors for Br, C1 and Na. The influence of urbanisation on enrichment factors in air particulate is appreciated by comparison of Bagauda results with Fleet Street, London, and central Swansea (Table 2). The urban sites show increases in element enrichments that are greatest for Pb, Br, Sb, Zn, Ni and CI: this is attributed to automobile exhaust, combustion of fossil fuels, incineration and metal working. In the case of Pb and Br, an increase of up to 1000-fold is observed. Also, analysis of rainwater around the smelter at Wollongong, Australia (Beavington, 1977), and unpublished data from this author on air particulate in the same vicinity, confirm very high enrichments of heavy metals in such environments. Clearly, the very rural nature of northern Nigeria and of this latitude generally, results in low enrichment factors for elements that are found at high enrichment both in non-urban and urban regions in higher, more industrial latitudes (Peirson et al., 1974). The enrichments found near Pretoria, in the southern hemisphere, are closer to Bagauda values than those at Chilton, England.
243 TABLE 2 COMPARISON OF AIR P A R T I C U L A T E ENRICHMENT F A C T O R S (SELECTED ELEMENTS) Element Element Enrichment factor = ~ conch, in air/ concn, in soil using average soil Sc Sc data from Bowen (1966). Non-urban measurements were made from January--December 1976. Urban measurements were made as follows: Fleet St., July 1972; Central Swansea, December 1972--May 1973. The measurements for these further stations other than Bagauda, were made: by Harwell as part of separate research programmes. Element
Non-urban
Latitude
Bagauda, Nigeria 11°N
Urban Chilton, Oxon, England 51°N
Pelindaba, Pretoria, S. Africa 25°S
Fleet Street, London a 5 l°N
Central Swansea~ S. Wales 51°N
90 km
500 km
60 km
1.5 km
0.3 49 630 1580 3.7 76 0.7 195 8.6 10 1030 31 790 9.6 155
0.4 5.0 120 185 1.7 53 0.6 18 0.7 ~3 140 2.6 46 0.8 24
2.2 -6260 730 2.7 83 0.9 -2.3 11 7680 45 145 26 130
0.2 130 2900 2100 27 130 1.1
Distance from sea 700 km Al As Br Cl Co Cu Fe In Na Ni Pb Sb Se V Zn
0.5 0.5 7.9 16 1.1 12 0.7 <:4 0.5 0.5 7.2 0.2 ~11 0.5 1.8
14 75 2300 30 610 95 290
aCambray, R. S., private communication. bpattenden (1974).
Monthly results for concentrations of elements in air were examined for correlations between pairs of elements. At Bagauda, there were 86 highly significant ( P ~ 0 . 0 0 1 ) correlations compared with only 47 at Chilton, almost certainly the result of the defined climatic regime in Nigeria. For example, some of the highest correlations at Bagauda occurred between A1--Fe, A1--Sc, Ce--Fe, Ce--Sc and Fe--Sc indicating that climatic features regulate inputs to the atmosphere and distribution from the predominant natural source which is presumed to be soft dust: enrichment factors indicate that industrial emissions are minimal. Similar correlations were reported from work in the Ivory Coast (Crozat et al., 1974). Measurements of the dry deposition velocity of elements, that is related to particle size (Chamberlain, 1966), were derived from the basic analyses of air particulate and dry deposition at Bagauda, and indicate a very uniform aerosol with small
244 particle size f o r all e l e m e n t s o v e r t h e H a r m a t t a n p e r i o d , c o n s i s t e n t w i t h well-mixed material originating f r o m t h e Sahara desert. A l t h o u g h c o n t r i b u tion f r o m locally raised soil d u s t is n o t e n t i r e l y e x c l u d e d , t h e h o m o g e n e i t y o f t h e aerosol in t h e d r y season suggests t h a t this is u n i m p o r t a n t .
ACKNOWLEDGEMENT This w o r k was s u p p o r t e d b y the Ministry o f Overseas D e v e l o p m e n t , L o n d o n , and we wish t o a c k n o w l e d g e t h e i r interest and assistance.
REFERENCES
Beavington, F., Environ. Pollut., 13 (1977) 127. Bowen, H. J. M., Trace Elements in Biochemistry, Academic Press, New York, 1966. Cawse, P. A. and D. H. Peirson, An Analytical Study of Trace Elements in the Atmospheric Environment, AERE Rep. R 7131, HMSO, London, 1972. Cawse, P. A., A Survey of Atmospheric Trace Elements in the U.K.: Results for 1976, AERE Rep. R 8869, HMSO, London, 1977. Cawse, P.A., Intercomparison of Air Filter Deposits for Trace Elements, AERE Rep. R 8191, HMSO, London, 1976. Chamberlain, A. C., Proc. R. Soc. A296 (1966) 45. Crozat, G., J. L. Domergue, V. Bogui and J. Fontan, Atmos. Environ., 7 (1974) 1103. Pattenden, N. J., Atmospheric Concentrations of Some Trace Elements in the Swansea/ Neath/Port Talbot area, AERE Rep. R 7729, HMSO, London, 1974. Peirson, D. H., P. A. Cawse and R. S. Cambray, Nature, 251 (1974) 675. W.H.O. Health Hazards of the Human Environment, WHO, Geneva, 1972.