Trace elements in daily collected aerosols at a site in southeast England

Atmospheric Environment Vol. 25A, No. 5/6, pp. 985-996° 1991. Printed in Great Britain.

0004-6981/91 $3.00+0.00 © 1991 Pergamon Press plc

TRACE ELEMENTS IN DAILY COLLECTED AEROSOLS AT A SITE IN SOUTHEAST ENGLAND RAAD RASHAD YAAQUB,*~" T. D. DAVIES,* T. D. JICKELLS*~ a n d J. M. MILLER§ *School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K., t Marine Science Centre, Bashah University, Bashah, lraq and §Air Resources Laboratory, Silver Spring, Maryland, U.S.A. (First received 16 April 1990 and in final form 23 August 1990) Abstract--Daily collections of aerosols have been made at a site in southeast England over a 1 year period and the samples analyzed for 13 components including soot carbon, major anions, some major cations and five trace metals (Pb, Cd, Zn, Mn and Ni). The results are consistent with other more limited studies in the southern North Sea and can be used to tentatively identify primary and secondary sources for the components of the aerosol. Elemental ratios for some components derived from anthropogenic sources vary slightly depending on the source region over which the air has recently travelled. Classification of the samples by back trajectory analysis shows that for all components, except those derived from seawater, concentrations are highest in air arriving from the east. This observation is discussed in terms of meteorological control on the transport and deposition of aerosols. Specifically, high pressure over Europe allows the accumulation of pollutants through reduced atmospheric dispersion and limited removal by precipitation. Subsequent long-range transport results in high concentrations of aerosols at the sampling site. By contrast, transport from the west is associated with efficient dispersal and removal processes. Key word index: Aerosols, trace metals, North Sea.

INTRODUCTION

The atmosphere is being increasingly recognized as an important source of trace metals and nutrients to marine ecosystems (GESAMP, 1990). Recent reports suggest that this is true even for areas such as the North Sea, despite large river inputs. For example, more than 25% of the nitrate and 33-72% of the cadmium entering the area are estimated to arrive via the atmosphere (DOE, 1987). However, such atmospheric inputs are more difficult to quantify than other inputs to coastal waters, and the estimates of atmospheric inputs to the North Sea are substantially more uncertain that river or direct discharges (DOE, 1987). There is considerable concern throughout northeast Europe over the possible pollution of the North Sea and in order to formulate effective pollution control strategies for the North Sea it is important that our knowledge of atmospheric inputs be improved. To predict future patterns of inputs, we must have some knowledge of pollutant concentrations in coastal marine air, an understanding of the governing processes and of the mechanisms of deposition to the sea surface. Here we report the results of 1 year of daily aerosol collections at a North Sea Coastal site in southeast England. The samples collected were analysed for a range of inorganic species. The results of these analyses are discussed in terms of the atmospheric processes controlling the aerosol concentrations. This is the first time to our knowledge that a data set with such intensive sampling over such a long period has

:l:To whom correspondence should be addressed. 985

been developed for the southern North Sea for so wide a range of components. Such data have been reported for the Scandinavian/Baltic region (e.g. Lannesforse et at., 1983; Oblad and Selin, 1986), the G e r m a n coast (Stoessel, 1987) and the southern Bight of the North Sea (Debeurwaerder et al., 1983). In addition earlier data from Cambray et al. (1975) are available from this area. The importance of our data lie only partly in the improved data base of concentrations of components in aerosols but, perhaps, more importantly it allows for the first time a clear evaluation of the meteorological control of the aerosol composition regime to be considered.

METHODS

Samples were collected at the U.K. Meteorological Office Upper Air Station at Hemsby (Fig. 1), a rural site 10 km from any significant conurbation and 50 km from the small city of Norwich. The site is about 1 km from the coast and 10 m above sea level. Filters were changed daily at about 0900 h, the exact time being noted. Filters were loaded and unloaded indoors and the filter head then installed on the pump and shelter. Used filters were stored in plastic bags and frozen. Blanks of filters installed for 24 h without air sampling, as well as filters that were not used at all were routinely analyzed and sample concentrations corrected for these blanks (Yaaqub, 1989). Samples were collected using a standard high volume air sampler (US EPA, 1971; WHO, 1976) with a flow rate of about 0.8 m 3 min-t, the exact flow rates being routinely measured. The samples were collected on Whatman 41 filter papers. Although these filters have been the subject of some recent debate (Lodge, 1986; Lodge and Moore, 1989) they were selected for use in this study for several reasons. Firstly, they are inexpensive, widely available and robust. Secondly,

986

RAAD RASHADYAAQUBet al. analyses. m

(1) Acid digestion for trace metal analyses. (2) Water extraction for major anions. (3) Reflectance measurements for soot carbon. Acid digestion

Norwich ~

/

Fig. l. Location of sampling site.

they have rather low blanks compared to the main alternative, glass fibre filters (GFA) (Yaaqub, 1989). We have argued previously (Watts et al., 1987b) that, under high volume sampling conditions, these filters are relatively efficient. Our own studies support this conclusion (Yaaqub, 1989). We inserted a back-up filter behind the main filter on some occasions and analyzed this separately. Based on these measurements Pb, Mn, Cd, Ni and Cu were collected by the first filter with -~80% efficiency (full details in Yaaqub (1989)). This confirms our earlier conclusion (Watts et al., 1987b) and that of Lowenthal and Rahn (1987) and Prospero (1989) that, while Whatman 41 filters are less efficient than some alternative filters, this effect is small compared to other uncertainties in sampling. These filters still represent an acceptable and cost-effective filter medium for many air sampling studies. Another complication with many filters is the formation of artefacts with gaseous species: for example the conversion of sulphur dioxide or nitrous oxides to particulate sulphate or nitrate species (e.g. Harrison and Pio, 1983). Savoie and Prospero (1982) and Coutant (1977) have argued that nitrate and sulphate artefacts are small for Whatman 41 filters, whilst Appel et al. (1979) have argued that such artefacts are large. We have not evaluated the possibility of artefacts in detail and our analyses of nitrate and sulphate aerosol may include some derived from gaseous species by reaction with the filter. Disproportionation reactions could also affect the concentrations of nitrate, chloride and sulphate (Harrison and Allen, 1990). Of particular concern would be losses of sea salt chloride by the formation of HC1 after reaction with acid species. However, the consistency of the Mg/chloride relationships discussed below suggest that this effect is minor. For simplicity, we subsequently refer only to chloride, nitrate and sulphate, though these chemical entities as measured may not be the only ones collected by the filters. In the laboratory, all filter handling was carried out in a laminar flow clean air cupboard and plastic gloves were worn. The unexposed edge of the filter was removed and the filter then divided into four equal portions. Homogeneity experiments suggested that the uncertainty introduced by this procedure was ~<5% for Pb, Mn, Mg, Fe, Zn and K. Uncertainties were higher for Ca (7%), Cd (7.5%), Ni (11%) and Cu (17%). The latter two elements are special cases because of instrumentation problems and contamination from the sampler, respectively (see later). Three separate quarters of the filter were then used for three separate

This was done by slowly boiling the quarter filter to dryness with 10 ml of concentrated high purity nitric acid over more than 12 h, subsequently redissolving the residue in 15 ml of 1% nitric acid overnight and then making up to 25 ml in 1% acid. The solution was then centrifuged and analyzed by flame or graphite furnace atomic absorption spectroscopy on a Pye Unicam SPG atomic absorption spectrophotometer for Pb, Cd, Cu, Mn, Mg, Fe, Zn, K and Ca. About half the samples were also analyzed for Ni, though sensitivity for this element was poor. Nitric acid digestion is not capable of totally solubilizing all components of the filter and aerosol but comparisons with quarters of the filter digested in a mixture of hydrofluoric and nitric acids (which we assume to totally digest the aerosol) indicated f> 80% of the Mg, Fe, Ca, Cd, Cu and Pb was dissolved by the nitric acid digestion procedure. The figure was only 35% for Al. We thus conclude that the nitric acid digest is at least 80% efficient for the elements considered here and this is acceptable for our purposes given the reduced hazard compared with that associated with a large number of HF digestions. The effect of inefficiencies in acid digestion procedures is to introduce a systematic error of ~<20% into our final results. Such uncertainties must be considered in the context of the large variability in sample concentrations (1-2 orders of magnitude) and also in comparison with systematic errors that can arise during sampling as discussed above. These systematic errors must be considered when absolute concentrations are discussed, but they are of less importance when comparing the results of aerosols associated with different source regions which have been collected and analysed by similar methods, as is done here. W a t e r extraction

A 7 cm 2 disc was punched out of a second quarter filter and this was then ultrasonically extracted into 25 ml of high purity water for 30 min. The solution was then filtered and analyzed for chloride, nitrate and sulphate on a Dionex System 12 Ion chromatograph (Watts et al., 1987a). Reflectance

Reflectance of a further quarter of the filter was measured using an EIL reflectometer. Such optical methods are widely used to provide rapid determinations of soot carbon in aerosols and give results comparable to time consuming chemical procedures (Hansen et al., 1984) with reproducibilities of about + 15% (Hansen and Novakov, 1988). Calibration equations (WSL, 1966) were used to determine concentration estimates of soot (black) carbon. Subsequent calibrations based on direct measurements of carbon indicate that results are accurate to + 15% (Davies et al., 1991).

UNCERTAINTIES Interferences in the analyses were evaluated using s t a n d a r d addition techniques a n d found to be minor. Precision, based o n replicate analyses of samples was better t h a n 10% for Ni, Ca, Mg, Cu, Cd, K a n d Pb; 13% for Fe a n d Zn; a n d 15% for Mn. Accuracy is affected by r a n d o m a n d systematic errors. Systematic errors may arise from sampling, filter artefacts a n d digestion procedures. Such errors c a n n o t readily be

Trace elements in daily collected aerosols quantified, but we later assess the comparability of our data with those of other studies conducted in the southern North Sea region. Laboratory blanks (i.e. unused filters) were ~<5% of sample concentrations for Pb, Cd, Cu, Mn, Mg, K and Ca and ~<20% for Zn, Ni and Fe. Field blanks (i.e. filters exposed in the field) were within 20% oflaboratory blanks for Mn, Pb, Cd, Ni, Cu and Ca. Significant field blanks were noted only for Mg, K, Fe and Zn. During the analyses of the samples it became apparent that samples were affected by contamination with copper from the brushes of the air sampling motor, a problem noted previously by others (e.g. Hoffman and Duce, 1971; Patterson, 1980). Attempts to divert the air emitted from the motor away from the sampler were only partially successful. The problem appears to be unique to copper; we will therefore not discuss the copper results further.

DISCUSSION Average concentrations In Table 1 the mean and standard deviations of all the data are presented. The values for Mn, Fe and Ni are similar to those reported by Cambray et al. (1975) for aerosols collected at rural sites within 50 km of our site. Zinc values reported here are about half those reported by Cambray et al. (1975) while lead values are a third of those reported by Cambray et al. (1975). The lower lead concentrations probably reflect, at least in part, the decline in the use of lead in petrol over this period as has been recorded by other sampling programmes in the U.K. (e.g. Pattenden and Branson, 1987). The lower zinc concentrations may simply reflect changes in air concentrations due to reduced emissions or improved emission control (though this has not been documented) or it may reflect different sampling protocols. Concentrations of Pb, Mn, Fe and Zn reported here

987

are very similar to those reported by Stoessel (1987) for a site in the F.R.G. at a similar latitude to Hemsby. Thus the results reported here appear to be consistent with the limited published data from northwest Europe. A review of other published data from around the North Sea (Yaaqub, 1989) suggests that there is a marked gradient in concentrations with higher concentrations of Zn, Fe, Mn and probably Pb in the southern area compared to further north. Lannefors et al. (1983) detected a gradient of decreasing concentrations from Scandinavia through to Greenland. The distribution of zinc based on published measurements is shown in Fig. 2.

Enrichment factors Enrichment factors (El) have been used to try to determine the sources of the different components in the aerosols (Wiersma and Davidson, 1986). In this work we have used Mg as an indicator of seawater sources (Keene et al., 1986). Seawater ratios of elements are taken from Riley and Chester (1971) for major elements, and Jones and Jeffries (1983) for trace metals. Since the ratios of major ions one to another in seawater are very constant, we consider E f seawater values which differ from unity by a factor of 2 or less to be indicative of seawater sources for those elements. We have chosen Fe as an indicator of crustal sources (Wiersma and Davidson, 1986). Our choice of reference elements is limited by the digestion procedure, since aluminium is not effectively extracted. At a site in northeast Europe closer to, and more heavily influenced by, anthropogenic sources (Kiel Bight, 55°N, 10.5°E), Schneider (1987) has suggested that 40% of the iron is of anthropogenic origin. On the other hand, in southern Sweden, Lannefors et al. (1983) found Fe to have an Efvalue of 3 (with respect to Ti) but could not conclude if this resulted from natural effects, such as variations in crustal ratios, or whether there was an anthropogenic source. If there is

Table 1 Mean of daily aerosol concentrations at Hemsby (ngm 3) March 1987March 1988 Variable

Mean concentration

Standard deviation

Soot C Pb Cd Mn Mg Fe Zn K Ca Ni CINO 3 ExSO 2-

104 34 1.1 10 247 216 41 234 329 2.7 4581 6820 5223

71 31 1.0 11 162 203 37 129 218 2.9 3534 6824 4655

Ex excess with respect to sea salt, see text. n=302 except for Ni where n= 173.

Range 2 466 1.3-196 0.05 5.4 1.1-86 19-964 8 1201 1.4-237 54-796 57-1405 0.12-17 124-27,805 13-50,000 261-28,033

RAADRASHADYAAQUBet al.

988

Zn concentration distribution around and over the North Sea (units are ng m"3)

Fig. 2. Zinc concentrations around the North Sea. Data sources: Cambray et al., 1975; Dedeurwaerder et al., 1982; Krezham and Cosemans, 1977; Lanneforse et al., 1983; Pacyna et al., 1984; Stoessel, 1987, this work.

Table 2. Enrichment factors (a) Ef shale Element

Mean Ef

Standard deviation

Range

Pb Cd Mn Mg Fe Zn K Ca Ni C1-

504 1436 3.15 7.9 1 118 3.5 4.3 10.3 5561

433 2051 3.1 11.7

36-3055 122-14,881 0.3-32 0.3-109

115 6.4 4.2 10.3 --

10-1275 0.3-95 0.2-38 1-89 --

(b) Ef seawater Element

Mean Ef

Pb Cd Mn Mg Zn K Ca Ni C1-

6,080,000 117,030 53,680 1 218,490 3.1 4.1 59,270 1.2

a small anthropogenic component to the Fe at the Hemsby site, this would result in an underestimation of the Ef shale values. Crustal elemental composition has been estimated from the compilations of Turekian and Wedepohl (1961) for global average shale. In view of the variability of different shale compositions

(Wiersma and Davidson, 1986; Arimoto et al., 1990), we consider E f shale values that differ from unity by less than a factor of 5 (Arimoto and Duce, 1987) to suggest a crustal sources for that element. In Table 2 the mean enrichment factors with respect to shale and seawater are listed. The standard devi-

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ations and ranges of Efshales are also listed and these emphasize the skewed nature of the distribution and the great variability of large Efvalues. Because of this, the mean Ef values should be treated cautiously for elements with large Ef values. This applies to almost all Ef seawater values and consequently only the means are reported. Based on enrichment factors we can attempt to apportion the elements by sources, based on annual average enrichment factors. Mn probably has a predominantly crustal source while chloride appears to have a predominantly marine source. K and Ca have small Ef values with respect to both shale and seawater. In view of the constancy of seawater composition for major ions, it is evident that, on average, 30% of K and 25% of Ca come from this source. Such calculations are more difficult for crustal sources because of the variability of ratios in shale and crust. The possible contributions of soil and limestone are also confounding factors but, clearly, crustal sources are of importance for these elements. For K, Efvalues for some samples are > 10 with respect to shale and seawater, suggesting a third source which is possibly anthropogenic. Furthermore, K and soot C are unique among the elements in having a concentration maximum in autumn (Yaaqub, 1989). This may suggest that the third source of K is from agricultural stubble burning (Andreae et al., 1983), which occurs in late summer/autumn in the U.K. For Mn it is evident that occasionally much higher Ef values are encountered, particularly when crustal contributions (as measured by Fe) are low (Fig. 3a). This implies a secondary source which cannot be seawater (because of the large Ef seawater values for Mn), and is therefore likely to be anthropogenic. Lannefors et al. (1983) and Schneider (1987) also suggested a modest anthropogenic contribution to the aerosol concentrations of Mn and K. The other elements (Pb, Cd, Zn and Ni) show large AE(A)

25-5/6-K

enrichments with respect to both sources which cannot be explained by the uncertainties in the Ef values. We conclude that they originate from a third, probably anthropogenic, source. Consideration of reported enrichments of trace metals in bursting bubbles compared to bulk seawater (Piotrowicz et al., 1978; Weisel et al., 1984) does not change this conclusion (Yaaqub, 1989). Such a conclusion appears to be consistent with emission inventories for these elements (Pacyna, 1986). Neither marine nor crustal sources are significant for nitrate and this component is considered to be dominantly anthropogenic. For sulphate, both anthropogenic and seawater sources are important, but for simplicity we have calculated the sulphate that is present in excess of the seawater source (exSO4), and consider only this subsequentl~ Since crustal sources are a trivial source of sulphate, exSO 4 is also presumed to be anthropogenic, though biogenic sources may make a contribution (Turner et al., 1988). In the case of Mg, there is evidence that, at high Fe concentrations the Mg Ef shale tends toward unity (Fig. 3b). ~rhus, there is a secondary crustal source of Mg besides the main seawater source. This, to some extent, compromises the value of Mg as a seawater reference element, and Na may be preferable. However, in practice, this is not a serious problem since, for 75% of samples, Ef shale values of Mg are >2. The effect of corrections for the crustal Mg component would be to increase the Ef seawater values slightly, but since only chloride is demonstrated to have a seawater source this is the only species affected. The very small Ef value for chloride with respect to seawater (1.2) argues that, in fact, artefacts from crustal Mg or loss of sea salt chloride (see earlier), are small. Detailed corrections for the crustal component in Mg could, in theory be made, but in practice this is not appropriate because of the uncertainties in Mg/Fe ratios in shale. By considering only certain periods, or specific

RAADRASHADYAAQUBet al.

990

samples, it is also possible to identify secondary sources. During periods of low Mg concentrations in winter (i.e. reduced sea salt input), chloride/Mg ratios occassionally become substantially greater than seawater values (Fig. 4a). Several of the highest of these values appear to be associated with high nitrate concentrations (Fig. 4b) suggesting a secondary anthropogenic source of chloride, though most of the time this source is overwhelmed by the marine source. The presence of hydrogen chloride in the atmosphere of southeast England, derived from U.K. coal burning sources has been noted by Harrison and Allen (1991). Presumably the effect is seen during winter in our samples because of maximum coal consumption at this time and a dominance of trajectories from U.K. sources (79% of trajectories at this time, see later) allowing rapid transport from U.K. sources and reducing the marine influence. Inter-element relationships

In order to identify elements whose concentrations vary in similar fashions, we have subjected the total data set to three different statistical manipulations. (1) Linear regressions using both parametric and non-parametric techniques. (2) Principal component analysis after standardization and in one case also after log transformations. (3) Cluster analysis. None of these statistical techniques are ideal for the classification of aerosols. Linear regressions offer the advantage of simplicity and are essentially objective, but with large numbers of variables they can yield complex results because of co-correlations. The principal component and cluster methods attempt to group together elements with the greatest similarities but require subjective decisions about the eigenvalue cutoff, or the most appropriate clustering method, re-

spectively. By using all three techniques we have been able to test the robustness of the conclusion from any one technique by comparison with the alternatives. Full details of the results are presented in Yaaqub (1989). The three techniques yielded similar results although there were some differences both between techniques, and within techniques, when different assumptions were made. The five-component principal component analysis for all the elements run on the 302 samples is presented in Table 3, since we ultimately found the principal component method to be preferable in terms of the balance between objectivity and information. The five-component solution explains 65% of the variance although the communality for individual elements is rather low. This could reflect the fact that we have not measured all components of the aerosol, analytical uncertainties, or the absence of unambiguous crustal or seawater indicator elements. However, the communalities are highest for elements with clear sources, such as Cu, Mg and chloride. This would not necessarily be the case if any of the above problems were of major significance. Rather, we suggest the low communality reflects a common meteorological control on aerosol elemental concentrations making clear differentiation of sources for aerosol components difficult except where this effect is overwhelmed as in the case of seawater or sampler motor sources. The issue of meteorological control is discussed further later. Despite the low communality of the five components, three appear to be consistent with the enrichment factor results discussed earlier. Firstly, Cu stands out as a separate component because of its source from the motor of the air sampler. Secondly, there is a seawater component of Mg and chloride. There is a component that is anthropogenic containing soot carbon, lead and cadmium. However the two components that

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Trace elements in daily collected aerosols

991

Table 3. Principal component analysis with eigenvalue />0.92 Element Soot C Pb Cd Cu Mn Mg Fe Zn K Ca C1NO 3 ExSO 2Eigenvalues % Variance

Communality 0.51 0.62 0.47 0.91 0.63 0.82 0.72 0.63 0.57 0.75 0.8 0.85 0.57 67.9

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0.87 3.75 28.8

0.9 0.7 1.61 12.6

1.39 10.7

1.1 8.4

0.92 7.4

For clarity only values >0.3 are shown. n = 302. Principal component analysis utilized orthogonal varimax rotation with data standardized to zero mean and unit variance (SPSS-X User Guide, 1988).The five-component solution presented was selected because the addition of further components did not significantly improve the percentage of the total variance explained. between them explain over 40% of the variance are not so simply characterized. The first consists of Mn, Fe, Zn, K and Ca and is thus predominantly crustal, though the Efshale for zinc is 118 and this anomalous behaviour of zinc is discussed later. The second component consists of nitrate and excess sulphate and rather less strongly Fe. The association of nitrate, excess sulphate and iron in Table 3 is not predicted by the enrichment factor classification of sources. It is not surprising that nitrate and excess sulphate should behave differently to other anthropogenically derived elements given their gas phase chemistry, but the association with iron, although rather weak, is unexpected and cannot be explained on the basis of this data set. In general, we have not considered principal component analyses of subsets of the data because, as the sample sizes become smaller, the reliability of the technique is reduced. However, two particular cases are worthy of note. Firstly, considering only the winter period (December, January and February), Mg and chloride occur in separate factors, consistent with the earlier suggestions of a secondary chloride source evident in winter. Secondly, Ni cannot be incorporated in the main principal component analysis because it was measured only on about half of the samples. A separate analysis utilizing only samples with Ni data shows this element in two components, associated with both crustal elements (Fe, Mn, Ca) and anthropogenic components (nitrate, excess sulphate, Cd, Pb). This assignment appears to be consistent with the observed rather small Ef shale value for Ni compared to Pb, Cd and Zn (Table 2).

loading=

Back trajectories Another approach to source apportionment is to classify the samples by the area of origin of the air parcel. This can be done using a variety of meteorological classifications such as surface winds, weather types and back trajectories. We have used all three techniques with the data set here and a detailed comparison of the results are presented elsewhere (Yaaqub, 1989). In general, the three techniques give similar results and here we shall consider only trajectory analysis. For this study the 5-day back trajectories were derived from the ARL gridded trajectory model (Harris, 1982) for 0000 and 1200 h daily at 850 mb and 700 mb. The back trajectories were classified into four sectors as shown in Fig. 4. The classification is assumed to approximately identify the following area sources. Sector Sector Sector Sector

1. 2. 3. 4.

"U.K. sources". "Remote or marine sources". "East European sources". "West European sources".

Of course these classifications are not mutually exclusive; for example the ultimate origin of air passing over the U.K. (sector 1) is likely to be marine (Atlantic Ocean). The following cases were reported as unclassified: (1) where a trajectory crossed a sector boundary; (2) where the two back trajectories in a 24 h period were placed in different sectors and (3) where back trajectories at the two different heights were in different sectors. Transport at this site is dominated by trajectories from the west (Table 4) and this sector dominates the overall loadings (Table 5) defined as: sector mean concentration x number of days total mean concentration x total number of days

× 100.

992

RAAD RASHADYAAQUBet al. Table 4. Mean and standard deviations of concentrations (ng m-3) by sector

Element Soot C Pb Cd Mn Mg Fe Zn K Ca CINO~ ExSO~Frequency of transport

U.K.

Western Europe

100+61 31 + 25 1.1 +0.9 8.5+7.7 212__+133 186_+ 164 33 -+24 211 -+ 118 287 5-148 4198 + 3262 6164-+6207 4230 -+ 3608 63%

138+61 62 + 46 1.3_+0.9 20+ 17 271 + 188 351 + 255 84 -+ 58 297-+ 126 480 + 322 4177 -+2742 94255-6023 8259 + 5996 9%

Sector Eastern Europe 140+78 34 +__23 1.5+0.9 19+ 14 382-+ 154 391 + 249 68 __+46 361 + 185 581 -+ 361 5641 -+ 2641 11,253+7217 11,208 + 4859 4%

North

Unclassified

87+93 20 + 25 0.75+0.8 7+9 322 ___230 133 ___127 28 -+_3 !.5 197+78 300 5-144 6455 5- 5584 3818+3982 4037 5-4168 10%

108+92 39 + 43 1.2+ 1.3 12.5+ 14 306+ 187 263 _ 279 49 -+46 282 + 140 367 _ 305 5174 + 3782 8504+9153 7195 + 5878 14%

Table 5. Percentage loading from each sector

Element

U.K.

Western Europe

Soot C Pb Cd Mn Mg Fe Zn K Ca CINO~ ExSO~-

60 58 61 52 54 53 49 57 55 57 57 50

12 16 10 17 10 14 18 11 13 8 12 14

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North

Unclassified

8 6 7 7 13 7 7 9 9 14 6 8

15 16 16 17 17 17 19 17 16 16 18 19

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Fig. 5. Sectors used for classification of back trajectories, see text.

Trace elements in daily collected aerosols This analysis is based on only 1 year of data and longterm climatologies always reveal inter-annual variability (e.g. Miller and Harris, 1985; Davies et al., 1986). However, the dominance of westerly trajectories is a consistent feature in this area (Lamb, 1950; Barry and Chorley, 1982), although there is a seasonal variation (Davies et al., 1990) which can confound interpretations because of the annual cycle in strengths of some sources and other meteorological variables (e.g. precipitation). Highest concentrations of all constituents measured, except Mg and chloride, are associated with transport from the east (Table 4). Yaaqub (1989), using both, parametric and non-parametric tests, noted that the differences in concentrations for most elements, and for most assigned sector pairings were significantly different. The exception was the sector 3/sector 4 pairing, which produced a statistically significant difference (p = 0.05) only for Pb, using a parametric test. The importance of the northern sector (sector 2) as a source of chloride and magnesium is clearly the result of this sector offering the largest exposure to the sea at this site. However the association of the highest concentrations of the other elements with easterly trajectories is not consistent with either the distribution or proximity of sources of these various elements (Pacyna, 1984), since there are important sources of most elements within the U.K., which are probably closer than at least some of the eastern European sources. Harrison and Allen (1990) noted higher concentrations of nitrate, nitric acid, sulphate and ammonium in air masses arriving in southeast England from Europe. We suggest that this pattern of high concentrations in air arriving from the European continent is controlled by meteorology rather than source strengths. Transport from the east is typically associated with outflow from a long-lived anticyclone moving to a location over northern Europe. Under these circumstances the regional pollution plume which is outflowing along the southwesterly flank of the anticyclone may be experiencing inhibited dispersion because of the rela-

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tively stable thermal structure of the lowest 1-2 km of the atmosphere. Moreover, precipitation amounts along these easterly trajectories are frequently small or non-existent (Yaaqub, 1989). A more detailed description of the synoptic and meteorological conditions associated with regional pollution transport from eastern Europe to the U.K. is given in Davies et al. (1991). Transport from the west is frequently associated with more effective dispersion (greater wind speeds) and the presence of frontal or shower precipitation, an effective removal mechanism for the aerosol. Detailed analyses of individual events of particularly high or low concentrations confirm this picture (discussed in detail elsewhere (Yaaqub, 1989)). Yaaqub (1989) concluded that > 70% of "high pollution" events (at least four elements having concentrations more than two standard deviations greater than the mean) were associated with high pressure weather types, whereas 'low pollution' events were not associated with any single weather type but frequently occur in conjunction with precipitation. The overwhelming importance of meteorological conditions may also explain the relative ineffectiveness of the principal component analysis to distinguish between crustal and anthropogenic components, since both will be similarly influenced. Elemental tracers

The use of elemental tracers to identify source regions has been developed by Rahn and co-workers (e.g. Rahn and Lowenthal, 1984). The technique utilizes the distinctive emission patterns of anthropogenic trace elements in different regions and assumes that this distinction is maintained during transport. We have attempted to use this approach for five anthropogenic components of the aerosol as measured in this study; Pb, Zn, Cd, soot C and nitrate. Following the approach of Schneider (1987), we have calculated the ratios of these elements to zinc, as a characteristic anthropogenic element (Table 6) using the data in Table 4. The resulting statistics must be interpreted

Table 6. Selected elemental ratios by sector Sector Element Pb/Zn Cd/Zn SootC/Zn NO3/Zn n

U.K.

Western Europe

Eastern Europe

North

0.94 _+0.99 0.032+0.036 3.00 + 2.79 184+227 183

0.73 __+0.73 0.015+0.015 1.63 __+1.30 112 + 104 26

0.50 __+0.47 0.021+0.019 2.04 __+1.76 164__+153 12

0.71 ___1.08 0.027+0.042 2.97 __+4.60 136+208 30

Note errors in ratios are propogated from standard deviations in Table 5 using the method of Miller and Miller (1988). The following are significantly different at 80% significance level or greater based on t-tests. Pb/Zn UK/WE, UK/EE. Cd/Zn UK/WE, UK/EE. SootC/Zn UK/WE, UK/EE, N/WE. NO•/Zn UK/WE.

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cautiously because of the different sample numbers in the sectors. However, there do appear to be differences between the trace element ratios in the three land based sectors though the significance is low. The northern sector is characterized by large uncertainties in the ratios, perhaps because of mixing of a variety of remote sources over long transit times. The data in Table 6 suggest the following characteristics of the regional aerosols. U.K. sector: enriched in Pb, Cd, soot C and nitrate relative to Zn. West European sector: enriched in Pb but deficient in Cd, soot C and nitrate relative to Zn. East European sector: deficient in Pb and Cd and enriched in soot C and nitrate relative to Zn. This pattern is broadly consistent with emission inventories for Cd, Pb and Zn in 1985 (Pacyna, 1987). The results of Schneider (1987) and Lannefors et al. (1983) can also be used to derive Pb/Zn ratios for aerosols, though the decline in lead use throughout Europe, at rates that vary from country to country, make such comparisons difficult. However, the results of these earlier studies also suggest a low Pb/Zn ratio in eastern Europe compared to western Europe. Because of the much greater number of samples collected from the U.K. sector, the aerosol pattern from this sector will dominate the overall data set. Thus the similar behaviour of Pb, Cd and C in this sector may explain why the principal component analysis groups these three elements together and zinc behaves differently.

CONCLUSIONS

Aerosol sampling at a rural coastal site in southeast England has allowed the identification of three sources of aerosols based on enrichment factors, with results similar to those of previous studies. One component is derived from marine sources and is dominated by Mg and chloride, a second is crustal and dominated by Fe and Mn. The third is anthropogenie and contains the Cd, Pb, Ni, Zn, soot carbon, excess sulphate and nitrate. Ca and K have significant crustal and marine sources. This classifation is crude and secondary anthropogenic sources of Mn, K and chloride has been identified. Principal component analysis identifies a marine component (Mg, chloride) and an anthropogenic one (soot C, Pb, Cd) but the major component are mixed ones containing predominantly crustal elements (Fe, Mn, K, Ca) and anthropogenic components (Zn, nitrate, excess sulphate). Classification of the collected samples by back-trajectories indicate that highest concentrations of all but the marine components are associated with transport from the east, though such transport occurs relatively infrequently. However, while all elements increase in concentrations in air

arriving from the east, they do not all increase by the same proportion and thus it appears possible to find small differences in the chemistry of aerosols from different source regions. This difference may explain the rather different behaviour of Cd, Pb and soot C as identified by the principal component analysis, since these three are relatively enriched in air arriving from the west compared to zinc. The association of high concentrations with transport from the east, incomplete resolution of crustal and anthropogenic components in principal components analyses, and the low significance of differences in elemental ratios by air mass source regions are all consistent with a strong meteorological control on aerosol concentrations. Thus the development of high pressure systems over Europe allows the accumulation of aerosol because of reduced vertical mixing. As the polluted air is transported to the west, the continuing relative stability of the air and the relative absence of wet removal processes can lead to high aerosol concentrations over the southern North Sea region. By contrast, transport from the west (which dominates transport to this site in terms of relative frequency and weighted concentrations) is generally associated with greater wind speeds and often with frontal systems and rainfall which efficiently removes contaminants resulting in lower concentrations of aerosols at this site. What may be of considerable significance, however, is that although precipitation amounts are relatively low with easterly trajectories, the precipitation that falls may be very heavily contaminated. Davies et al. (1991) have determined that wet pollutant deposition in northeast Scotland is dominated by the relatively infrequent precipitation associated with easterly trajectories, because of the very high pollutant loading in the atmosphere. The conclusions of this study concerning the major sources of aerosol material are similar to those reported in many other studies, though the relative importance of the sources change from place to place. However, the very large data set does allow us to identify secondary sources and also to attempt to characterize ratios for a few components by backtrajectory sector grouping. However, the unique feature that can be revealed by this data set concerns the interaction of meteorological factors with emission source distributions and strengths to produce the observed concentrations. An understanding of these interactions is critical for the accurate prediction of atmospheric inputs to this, or any other coastal area, where rapidly changing meteorological transport regimes prevail. Work is now underway to extend aerosol measurements over the North Sea and to directly measure atmospheric deposition. Acknowledgements--Our thanks go to the staff of the U.K.

Meteorological Office's upper air station at Hemsby; without their assistance with filter changing this study would not have been completed. The work described here was carried out while one of us (RRY) was in receipt of a scholarship from the

Trace elements in daily collected aerosols government of Iraq. We wish to thank two anonymous reviewers for their extensive comments which significantly improved this manuscript.

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