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Coarse atmospheric aerosol: size distributions of trace elements
Atmospheric Environment 35 (2001) 5321–5330
Coarse atmospheric aerosol: size distributions of trace elements K. Eleftheriadisa, I. Colbeckb,* b
a NCSR ‘‘Demokritos’’, 15310 Ag Paraskevi, Attiki, Greece Department of Biological Sciences, University of Essex, Colchester, Essex CO4 3SQ, UK
Received 26 September 2000; received in revised form 20 May 2001; accepted 29 May 2001
Abstract A sampler, employing nine single stage impactors placed in parallel within a portable wind tunnel, has been used to determine the metal content of coarse atmospheric aerosol. The wind tunnel maintains a constant flow environment for the collectors housed inside it, so that representative sampling conditions are achieved compared to the varied ambient wind conditions. At a flow rate of 8 m s 1 the 50% cut-off diameters of the impactors ranged from 7.8 to 38.8 mm. Measurements were conducted at a rural and urban site near Colchester in south east England. The samplers were analysed by PIXE for P, K, Ca, Fe, Ti, Mn, Cu, V, Co, Cr, Br, Zn, Ni, Sc and Pb. It is found that the sampler can be employed to quantitatively characterise the elemental mass size distribution for aerosol larger than 10 mm. The results indicate that a small fraction of the above earth and trace elements’ metal mass is present in particles greater than 10 mm. This fraction for earth metals (Ca, K, Ti) is comparatively greater in the rural site than the urban site, while for trace metals (Mn, V, Cu, Cr) this fraction constitutes a more significant part of the coarse mass at the urban site. Trace element concentrations were of a similar order of magnitude to earlier literature reports. Although the number of measurements was limited it can be concluded that the size distributions obtained were characteristic of an unpolluted area. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Coarse aerosol; Trace elements; Tunnel sampler; Single stage impactor; Size distribution
1. Introduction A balance between sources, chemical transformations in the atmosphere, long-range transport effects and removal processes influences the composition of atmospheric particles. Elements associated with natural sources are typically found within the coarse mode (aerodynamic diameter >2.5 mm), whilst elements emitted from anthropogenic sources are associated with the fine mode (aerodynamic diameter o2.5 mm). Metals are present from both natural and anthropogenic sources. By measuring metal concentrations as a function of particle size, information may be obtained *Corresponding author. Tel: +44-1206-872203; Fax: +441206-872592. E-mail addresses: [email protected] (K. Eleftheriadis), [email protected] (I. Colbeck).
concerning their source. There has been a number of trace metal sampling programmes in the UK ranging from small scale research projects to national sampling campaigns (QUARG, 1996). The latter have the advantage of covering a number of sites with a standardised analytical method and strict quality control procedures. Salmon et al. (1978) analysed data on trace element concentrations in atmospheric aerosol, collected between 1957 and 1974, at a rural site in central southern England. Measurements of atmospheric concentrations of trace metals have been made in several urban areas of the UK since 1974 (Lee et al., 1994; Cawse et al., 1994) whilst a multi-element survey has been in operation since 1976. Urban concentrations were typically between 3 and 10 times higher than those at rural sites. The elements, which showed significant seasonal variability, were generally from anthropogenic and marine sources i.e. Br, Pb, Zn, V, Cl and Na. None
1352-2310/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 3 0 4 - 1
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of these measurements provided information on the size distribution. It was assumed that since they were made in urban areas most of the trace metals would be present in the fine mode. Additionally the sampling design was such that under typical varying wind conditions, only particles of less than a few mirometres diameter were likely to be collected with high efficiency (Lee et al., 1994). Previous studies of the size distribution of individual elements in the atmospheric aerosol have typically been limited to particle sizes up to 20 mm (Davidson and Osborne, 1987). These studies show a sharp decline in the metal mass concentration for sizes larger than 10 mm. There are limited data about the chemical speciation of large particles (Noll et al., 1990). This is not only due to the relative unimportance of these particles to long range aerosol transport studies and human lung deposition, but also to the great difficulties involved in their representative sampling from the atmosphere. Their contribution to atmospheric deposition and the overall chemical balance in the ambient aerosol should be investigated. For instance atmospheric deposition is an important pathway for the transfer of pollutants from the atmosphere to land and water. Deposition of toxic materials such as heavy metals, PCBs and PAHs can result in significant ecological damage (Shaw, 1987). Several workers have shown that large particles are responsible for a large proportion of the deposition flux (Shahin et al., 2000; Paode et al., 1998, 1999; Tai et al., 1999; Zufall et al., 1998). Tai et al. (1999) found that particles larger than 10 mm contributed up to 90% of the aerosol dry deposited mass even at non-urban locations. The purpose of this study is to demonstrate the ability of novel instrumentation to yield size resolved elemental concentrations in coarse and larger ambient aerosol.
2. Instrumentation Recently we have described a wind tunnel sampler, employing single stage impactors, for the collection and size fractionation of ambient aerosols (Eleftheriadis and Colbeck, 2000). The tunnel sampler satisfies the criteria for representative sampling (aspiration efficiency 10075%, Vincent, 1989) for particles up to 60 mm in aerodynamic diameter and for ambient wind speeds in the range 0.5–10 m s 1. Nine impactor strips each with a different cut-off diameter and made of heavy duty stainless steel foil tensioned between two support rods were installed in parallel within the tunnel (Fig. 1). A fan at the tunnel outlet provided a constant air flow within the tunnel, with the flow velocity set at 8 m s 1, while isoaxial sampling was achieved by a wind vane mounted on the tunnel. A thermistor anemometer is placed in front of the strips to monitor the flow velocity within the sampler. Depending on ambient wind speed the voltage supplied to the fan is regulated, so that the flow velocity is maintained close to 8 m s 1. The size fractionation characteristics of the impaction strips (widths equal to 1, 1.5, 2, 3, 5, 7, 9, 12 and 25 mm) were determined experimentally (Eleftheriadis and Colbeck, 1992). At a flow rate of 8 m s 1 the 50% cut-off diameter of the strips ranged from 7.8 to 38.8 mm. Although well defined and reliable in terms of aerodynamic diameter the efficiency curves did not match the sharp cut-off behaviour of conventional cascade impactors. However samples collected on impaction strips offer the advantages of easy handling, excellent means of particle examination by optical or electron microscopy and quantitative chemical speciation. The main disadvantage arises from overloading which limits the sampling time and consequently the aerosol mass collected. The true size distribution can be
Fig. 1. Schematic diagram of the tunnel sampler.
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extracted from a set of strip samples when an inversion method is applied on the results where the strip collection efficiency curves are used as Kernel functions (Ramachandran and Kandlikar, 1996; Kandlikar and Ramachandran, 1999).
3. Field experiments and analysis The ambient aerosol measurements obtained by the Wind Tunnel Sampler were conducted around the town of Colchester (population 157,000) in the South East of England. The area is characterised by flat land with no major industrial activity involving stack emissions, except agriculture. Colchester is a relatively small town and with the exception of motor vehicle traffic it is not expected to contribute high aerosol emissions to the surrounding area (Colchester Borough Council, 2000). Other aerosol inputs to this area may result from the London Metropolitan area and the North Sea, when the prevailing winds favour transport from these two areas. In view of the above topography and environment, two sites thought to reflect some specific conditions were selected for sampling. A map of the region including the two sites is shown in Fig. 2. The first sampling location
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was considered as one directly affected by a local aerosol source. This was a Civic Amenity site (refuse landfill) situated at the edge of Colchester. The sampler was operating at a distance of around 500 m away from an open tipping area but only during the early evening hours when the site was not active. The above conditions were thought to provide moderate aerosol generation by natural processes such as resuspension by the ambient wind. A small tarmac works area, 1 km to the south, may also contribute some aerosol. The second sampling site was located in a rural area 12 km due west of the first site. It was surrounded by open cultivated land with no major agriculture activities taking place during the sampling period. Four measurements were obtained on each site. Sampling took place at almost weekly intervals during a two month summer period. The sampling time was around 4 h. The sampler was placed on a platform so that the tunnel mouth was 2 m above ground. Atmospheric conditions were characterised by dry warm weather with very light winds at the refuse site, while moderate winds were predominant at the rural site. One measurement taken at the latter location involved very strong winds (around 10 m s 1). The measurements described above were performed in a manner suitable for PIXE analysis. Polycarbonate
Fig. 2. Location of the sampling sites (1) rural site, (2) civic amenity site.
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filter membranes were cut to the size of the strips and attached on them after they had been immersed in a solution of L-Apiezon grease. The latter was used in order to reduce particle bounce from the strips. Both the filter and grease materials are relatively X-ray emission free. After sampling the filters together with control blanks were kept in airtight containers and later analysed in the PIXE facilities of the University of Birmingham. Due to limited analysis time only six strips per measurement were analysed. These were the 2, 3, 5, 7, 12 and 25 mm samples, thought to be adequate to produce the elemental size distributions. Analysis was performed for a wide range of elements such as P, Si, Cl, S, K, Ca, Fe, Ti, Mn, Cu, V, Co, Cr, Br, Zn, Ni, Sc and Pb. The results showed consistent concentrations for most earth elements and some trace elements. The masses detected for Br, Zn, Ni, Sc and Pb were frequently lower than the analytical detection limits or the values from the respective blank samples. Moreover, these elements did not appear consistently in all measurements and therefore they were excluded from any further study. Silicon was also excluded due to the erratic nature of the respective measurements probably due to the arbitrary collection of giant sand particles on the samples.
4. Size distributions of elemental concentrations The aerosol mass concentrations were calculated for the remaining earth elements and trace metals. The arithmetic mean of the concentration for every element at each site was determined. One measurement from the rural site involving very high ambient wind conditions was excluded from the average as it was found to have elemental concentrations not compatible with the predominant aerosol mass found in the other three measurements. The extreme value estimation (EVE) deconvolution method (Hopke and Paatero, 1994; Aalto et al., 1990; Tapper and Paatero, 1990; Paatero et al., 1988) was applied on the two sites elemental concentration averages in order to produce the individual size distributions for each element. The EVE calculations are described in detail by Eleftheriadis (1993) and Aalto et al. (1990). The error estimates required in the calculations were derived from flow rate fluctuations recorded during sampling and ranged between 15% and 20%. The EVE method was applied over a size range between 1 and 100 mm. The standard deviation of the model guassians selected were between 1.4 and 2. The strips used for sampling here have their size collection characteristics separated by very narrow size intervals. Small standard deviations around 1.4 enhance the fine mass differences between the strip stages and give
solutions, which often reveal important features of the individual distributions. However, these narrow model gaussians may also produce unnaturally shaped distributions with sharp peaks. On the other hand when a large standard deviation was employed, any fine structure was removed from the resulting distributions. The final solution selected for each elemental distribution was a compromise between the above extreme situations with the chi-square value also used as a criterion. Each solution of the inverted size distributions by EVE is given as a family of solutions with a minimum and maximum concentration with respect to any size. The solutions presented here correspond to the arithmetic mean of the family solutions. The size distribution of the atmospheric concentration for each element measured at the two sites are displayed in Fig. 3. The part of the distribution lying to the left of the main peak towards the small sizes is characteristic of an ideal situation where the concentration approaches zero at around 1 mm. This is due to the characteristic collection efficiency function of the strips rather than the true size distribution of the elemental mass. The results have been normalised with respect to the total concentration calculated from the EVE results. Due to the uncertainty involved with the EVE calculations between 1–2 mm, where the collection efficiency function approaches zero, results are presented for the range between 2–100 mm. The lower size also coincides with the generally accepted dividing line between coarse and fine size fractions of the atmospheric aerosol (Whitby, 1978). However, this is a characteristic of the total aerosol mass and does not always describe individual elements discussed here. It is well known (Milford and Davidson, 1985) that most elemental distributions shown here, like those of some trace elements, have a large part of their mass distributed over the accumulation size mode (0.1–1 mm). The size distributions presented here for the two sites show that most crustal elements (Fe, Ca, K and Ti) display a peak in their mass concentration between 3 and 7 mm. Despite the quantitative difference in the total mass concentration of the above elements at the two sites the shape of their respective size distributions is generally similar. The only exception is potassium, which displays a clear second mode at around 10 mm at the rural location. P was detected only at the civic amenity site and shows a maximum at around 6 mm. These results rather contradict the general idea that coarse aerosol is accumulated in a mode with a peak at around 10 mm (Whitby, 1978). Instead, the general picture emerging shows that the above elements, although characteristic of the soil dusts which are the dominant source of coarse aerosol, are not necessarily the greatest portion of its mass at sizes greater than 10 mm. A better picture could be formed if the concentrations of Al and Si together with that of biogenic aerosol were also determined.
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Fig. 3. (a) Normalised size distributions of K, Mn, Fe, P, Ca and Ti in ambient aerosol (b) Normalised size distributions of V, Co, Cu and Cr in ambient aerosol.
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A greater variation on the characteristics of their size distribution can be observed for the trace metals studied here. A clear distinction between the two sites is evident. All these elements (Co, V, Mn and Cu), excluding Cr, appear to have their ambient concentrations maximising between 2 and 3 mm at the rural site and between 4 and 6 mm at the refuse site. Chromium displays a surprising bimodal distribution with a first peak similar to those of the other metals and a second peak at around 80 mm. The phenomenon is consistent for both sites, although the magnitude of the second peak is greater at the civic amenity site. The total concentration above 2 mm calculated by EVE for each element is displayed in Table 1. The maximum degree of uncertainty on these values is similar to the respective error estimates used in the EVE calculations and equal to around 20%. The measure of the uncertainty is assumed to be the maximum residual value resulting from the deviation between the strip measurements and the best fit calculated by EVE. The concentration of the large particles (>10 mm) is also reported in Table 1 for both sites. An interesting picture emerges by comparing the average concentrations of the two size fractions at the two sampling sites. First, the earth elements (Ca, Fe, Ti and K) are present at the refuse site with concentrations a great deal higher than those at the rural site. However, the large particle fraction (>10 mm) of these elements does not share the same features. Only iron displays a lower concentration at the rural site while the remaining elements have similar or higher mass concentrations compared to those at the civic amenity site. The total coarse fraction concentrations of the trace metals are higher at the rural site with the exception of Co. Several questions can be raised about the nature of the elemental distributions at the two sites. The atmospheric conditions during sampling and the area surrounding
Table 1 Mean ambient concentrations (ng/m3) Element
Fe Ca K P Ti Mn V Cu Cr Co
Civic amenity site
Rural site
>2 mm
>10 mm
>2 mm
>10 mm
2530 3900 1550 108 317 38 3 10 5 10
123 432 40 15 5 6 1 3 3 1
833 2022 1083 F 76 45 9 18 5 6
26 414 98 F 14 1 2 2 1 *
the sites play an important role. Dry warm weather during the measurements at the civic amenity site indicate heavy aerosol load in the area. The low wind speeds (o2 m s 1) recorded coupled with the time of the day (early evening) that sampling was performed, indicate that the aerosol produced mechanically during the day at the site or the nearby town had its small size fraction still suspended over the area, while the larger particles had settled. This may be the reason for the absence of large particles among the earth elements and the fact that the concentrations of trace metals is lower than those found at the rural site. Refuse is not known to be a substantial source of such metals (Pacyna, 1985). The type of soil in the area and vegetation cover are also critical of the resulting size distributions. Higher wind speeds (4–7 m s 1) prevailed during sampling at the rural site, favouring increased resuspension of soil dust. This may explain the higher ratio of larger (>10 mm) to total coarse (>2 mm) concentration observed for the soil dust derived earth metals (Ca, K, Ti) at the rural site, compared to that at the civic amenity site. Some explanation for the decline of metal concentrations in the large atmospheric aerosol can be found in studies dealing with erodible soils, the major source of coarse aerosol. It is understood that metals exist in rocks and soils in a form of various species like SiO2, TiO2, Al2O3, Fe2O3, FeO, CaO, K2O, P2O5 and MnO. (Mason, 1966; Stelson and Seinfeld, 1981). Schutz and Rahn (1982) studied the variation of elemental concentration with particle size in dust and soils at remote areas. It was found that the relative concentration of most earth metals in the 2–400 mm particle size range, normalised with respect to the concentration of larger soil grains (160–400 mm), show a sharp decrease of one to two orders of magnitude for particle sizes greater than 20 mm, while Si was the only element that displayed increasing concentrations with increasing particle size. This phenomenon has been attributed to the lack of mineral species like clay minerals in very large particles. Although soil composition in the above studies is not of the same composition as the one found in Southeast England, the size distributions for the earth metals presented here display a similar trend. Vegetative cover in the area where the measurements were conducted may also play an important role in resuspension of dust from the ground. The two sites discussed here, were located at areas with different vegetation characteristics with the rural site richer in vegetation. Wu et al. (1992) found that leaves reduce resuspension rates of particles from their surface. Field studies though, (Sehmel, 1980) did not reveal a clear decrease between for example grass and asphalt surfaces. It is clear though that the Civic Amenity site was adjacent to larger areas covered with loose soil.
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5. Comparison with other studies Measurements of the chemical speciation of very large atmospheric aerosol are sparse. The only studies indicating the presence of most of the above elements in large aerosol are those by Noll et al. (1985, 1990). However, these measurements are based on the 50% cut off sizes of impaction strips similar to the ones used in this study. This method for calculating the size distribution has been found to be misleading. Most information about the size distribution of metal species in the atmospheric aerosol is available from studies employing cascade impactors for sampling. Most investigations have been on urban areas (Horvath et al., 1996; Infante and Acosta, 1991; Anderson et al., 1988; Orsini et al., 1986; Spengler and Thurston, 1983; Zoller et al., 1974), whilst those in rural areas are lacking (Horvath et al., 1996; Injuk et al., 1992; Adams et al., 1983; ElShobokshy, 1984). Other studies focusing on the importance of large particles were conducted during
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the Lake Michigan Mass Balance Study (Shahin et al., 2000), where the deposition flux of earth and trace metals was investigated. Short term measurements were conducted with the Noll Rotary Impactor (Paode et al., 1999). It was found that very large particles, despite their small mass concentration contribute greatly to deposition fluxes due to their high deposition velocity. The aim here is to identify any universal characteristics of the elemental distributions in the measurements conducted during this study. The size distributions from measurements at the rural site were considered more suitable for this exercise. This was due to the clear influence of specific conditions on the results at the civic amenity site. It is clear that comparisons with data from studies conducted in Britain would be more suitable for credible conclusions due to common aerosol sources, topography, vegetation cover and climate. Elemental size distributions from two such studies (Pattenden, 1974; Cawse, 1974) are also included in Fig. 4. These measurements were performed with Andersen Cascade
Fig. 4. Comparison between normalised size distributions for Fe, Mn, Cu, Co, V and Cr, measured at the rural site and from studies in the UK (Cawse, 1974; Pattenden, 1974).
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impactors at Chilton (England) and Trebanos (Wales). Chilton is a rural site, in central southern England, whilst Trebanos is just on the outer limit of an industrial zone, which includes oil refining, steelworks and a nickel smelter. The elemental size distributions for Fe, Mn and Cr measured here, show very good agreement with the respective results from Chilton and partial agreement with those at Trebanos. There is also partial agreement for Co with both reference distributions. The distribution determined here for V seems to follow the trend observed at Trebanos. Data for Cu were available only at Trebanos. There is also general similarities in trend here. The agreement with the rural site at Chilton confirms expected similarities in elemental size distributions between areas with common characteristics. The size distributions at Trebanos are often bimodal. Taking into account. the industrial sources present in the vicinity of this site the large particle mode absent from the other two sites can be attributed to such sources. It has to be noted that the characteristics of the metal size distributions determined here are also consistent with findings in other regional studies around the world (Keronen et al., 1991; Kasahara et al., 1992). Finally, direct quantitative comparisons between the mass concentration of metals measured here and in other studies in the U.K is difficult due to the lack of measurements for the coarse fraction alone as it was determined in the present study. However, an attempt was made here to extract information from the data given in Table 2. The results from two long term studies of the total mass concentration of certain metals are displayed. The values are averages over five and ten years sampling periods from measurements at rural and urban sites throughout Britain. At first the concentrations determined here for many of these metals seem excessively high compared to the rural and even urban values in the other studies. It is evident that the greatest difference exists for the earth elements (Fe and Ti) which have most of their mass distributed across the coarse particle size range and the comparison between the Table 2 Ambient concentrations (ng m 3) from studies in the UK Element
Urban (total)a 1985–89
Rural (total)a 1972–81
Rural (>2 mm) this study
Fe Ti Mn V Cu Cr Co
1000 4.8b 34 25 30 11 1.5b
390 28 34 11 24 7 0.4
833 76 45 9 18 5 6
a b
Data quoted in QUARG (1993). Concentrations at or below detection limits.
results is more meaningful. The concentrations of the trace elements with more than 50% of their mass present at small sizes (Cawse, 1974) are more difficult to compare. It has to be noted that credible conclusions from these comparisons are hampered by several conflicting factors. First these averages incorporate various atmospheric conditions with widely different aerosol masses, while the measurements in these study were performed during a specific situation of heavy atmospheric aerosol load. The measurements used for reference span over many years during which a decline in metal concentrations was observed across Britain (Lee et al., 1994). The sampling methods used include filter holders sampling at 901 with respect to the wind (Pattenden, 1974). The collection efficiency of such devices for large particles is far from ideal (Vrins et al., 1984) and leads to underestimation of the aerosol mass in the coarse size-range. It can be concluded from Table 2 that the concentrations are of the same magnitude. The distributions found for the trace elements V, Co and Cu at the rural site display maximum concentrations at around 2 mm. Taking into account the limitations of this study at the above size range it can be concluded that the majority of the mass is found in much smaller sizes. This is expected for these elements which are usually released in the atmosphere by anthropogenic industrial sources (Pacyna, 1985). Their concentration, though, does not drop as sharply with size as it is thought. The above characteristic is not visible in the distributions of the same elements measured at the industrial site, probably disguised by aerosol released in the air by the materials originating from the site.
6. Conclusions The tunnel sampler employing single stage impactors is found to be a useful instrument for representative sampling of coarse aerosol. Elemental analysis of size fractionated samples can be performed and the size distribution in the range of 2–100 mm can be calculated for each element. The high volume intake of the sampler allows the collection of adequate mass for analysis over sampling periods of a few hours (3–5) in suburban areas of the U.K. Results from measurements in two U.K. sites for a number of common earth and trace metals show that their concentration in the coarse aerosol fraction peaks at around 3–7 mm. The mass concentration of the fraction >10 mm is found to be between 10–15% of the total coarse mass for most elements.
Acknowledgements We would like to thank Dr Lakhumal Luhana for performing the PIXE analysis.
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Vincent, J.H., 1989. Aerosol Sampling; Science and Practice. Wiley, Chichester, UK. Vrins, E., Hofschreuder, P., ter Kuile, W.M., van Nieuwland, R., Oeseburg, F., 1984. Sampling efficiency of aerosol samplers for large windborne particles. In: Liu, B.Y.H., Pui, D.Y.H., Fissan, H.J. (Eds.), Aerosols: Science, Technology and Industrial Applications of Airborne Particles. Elsevier, New York. Whitby, K.T., 1978. The physical characteristics of sulfur aerosol. Atmospheric Environment 12, 135–159. Wu, Y.L., Davidson, C.I., Russell, A.G., 1992. Controlled wind-tunnel experiments for particle bounceoff and resuspension. Aerosol Science and Technology 17, 245–262. Zoller, W.H., Glatney, E.S., Duce, R.A., 1974. Atmospheric concentration and sources of trace metals in the South Pole. Science 183, 198–200. Zufall, M.J., Davidson, C.I., Caffrey, P.F., Ondov, J.M., 1998. Airborne concentrations and dry deposition fluxes of particulate species to surrogate surfaces deployed in Southern lake Michigan. Environmental Science and Technology 32, 1623–1628.
Coarse atmospheric aerosol: size distributions of trace elements K. Eleftheriadisa, I. Colbeckb,* b
a NCSR ‘‘Demokritos’’, 15310 Ag Paraskevi, Attiki, Greece Department of Biological Sciences, University of Essex, Colchester, Essex CO4 3SQ, UK
Received 26 September 2000; received in revised form 20 May 2001; accepted 29 May 2001
Abstract A sampler, employing nine single stage impactors placed in parallel within a portable wind tunnel, has been used to determine the metal content of coarse atmospheric aerosol. The wind tunnel maintains a constant flow environment for the collectors housed inside it, so that representative sampling conditions are achieved compared to the varied ambient wind conditions. At a flow rate of 8 m s 1 the 50% cut-off diameters of the impactors ranged from 7.8 to 38.8 mm. Measurements were conducted at a rural and urban site near Colchester in south east England. The samplers were analysed by PIXE for P, K, Ca, Fe, Ti, Mn, Cu, V, Co, Cr, Br, Zn, Ni, Sc and Pb. It is found that the sampler can be employed to quantitatively characterise the elemental mass size distribution for aerosol larger than 10 mm. The results indicate that a small fraction of the above earth and trace elements’ metal mass is present in particles greater than 10 mm. This fraction for earth metals (Ca, K, Ti) is comparatively greater in the rural site than the urban site, while for trace metals (Mn, V, Cu, Cr) this fraction constitutes a more significant part of the coarse mass at the urban site. Trace element concentrations were of a similar order of magnitude to earlier literature reports. Although the number of measurements was limited it can be concluded that the size distributions obtained were characteristic of an unpolluted area. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Coarse aerosol; Trace elements; Tunnel sampler; Single stage impactor; Size distribution
1. Introduction A balance between sources, chemical transformations in the atmosphere, long-range transport effects and removal processes influences the composition of atmospheric particles. Elements associated with natural sources are typically found within the coarse mode (aerodynamic diameter >2.5 mm), whilst elements emitted from anthropogenic sources are associated with the fine mode (aerodynamic diameter o2.5 mm). Metals are present from both natural and anthropogenic sources. By measuring metal concentrations as a function of particle size, information may be obtained *Corresponding author. Tel: +44-1206-872203; Fax: +441206-872592. E-mail addresses: [email protected] (K. Eleftheriadis), [email protected] (I. Colbeck).
concerning their source. There has been a number of trace metal sampling programmes in the UK ranging from small scale research projects to national sampling campaigns (QUARG, 1996). The latter have the advantage of covering a number of sites with a standardised analytical method and strict quality control procedures. Salmon et al. (1978) analysed data on trace element concentrations in atmospheric aerosol, collected between 1957 and 1974, at a rural site in central southern England. Measurements of atmospheric concentrations of trace metals have been made in several urban areas of the UK since 1974 (Lee et al., 1994; Cawse et al., 1994) whilst a multi-element survey has been in operation since 1976. Urban concentrations were typically between 3 and 10 times higher than those at rural sites. The elements, which showed significant seasonal variability, were generally from anthropogenic and marine sources i.e. Br, Pb, Zn, V, Cl and Na. None
1352-2310/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 3 0 4 - 1
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of these measurements provided information on the size distribution. It was assumed that since they were made in urban areas most of the trace metals would be present in the fine mode. Additionally the sampling design was such that under typical varying wind conditions, only particles of less than a few mirometres diameter were likely to be collected with high efficiency (Lee et al., 1994). Previous studies of the size distribution of individual elements in the atmospheric aerosol have typically been limited to particle sizes up to 20 mm (Davidson and Osborne, 1987). These studies show a sharp decline in the metal mass concentration for sizes larger than 10 mm. There are limited data about the chemical speciation of large particles (Noll et al., 1990). This is not only due to the relative unimportance of these particles to long range aerosol transport studies and human lung deposition, but also to the great difficulties involved in their representative sampling from the atmosphere. Their contribution to atmospheric deposition and the overall chemical balance in the ambient aerosol should be investigated. For instance atmospheric deposition is an important pathway for the transfer of pollutants from the atmosphere to land and water. Deposition of toxic materials such as heavy metals, PCBs and PAHs can result in significant ecological damage (Shaw, 1987). Several workers have shown that large particles are responsible for a large proportion of the deposition flux (Shahin et al., 2000; Paode et al., 1998, 1999; Tai et al., 1999; Zufall et al., 1998). Tai et al. (1999) found that particles larger than 10 mm contributed up to 90% of the aerosol dry deposited mass even at non-urban locations. The purpose of this study is to demonstrate the ability of novel instrumentation to yield size resolved elemental concentrations in coarse and larger ambient aerosol.
2. Instrumentation Recently we have described a wind tunnel sampler, employing single stage impactors, for the collection and size fractionation of ambient aerosols (Eleftheriadis and Colbeck, 2000). The tunnel sampler satisfies the criteria for representative sampling (aspiration efficiency 10075%, Vincent, 1989) for particles up to 60 mm in aerodynamic diameter and for ambient wind speeds in the range 0.5–10 m s 1. Nine impactor strips each with a different cut-off diameter and made of heavy duty stainless steel foil tensioned between two support rods were installed in parallel within the tunnel (Fig. 1). A fan at the tunnel outlet provided a constant air flow within the tunnel, with the flow velocity set at 8 m s 1, while isoaxial sampling was achieved by a wind vane mounted on the tunnel. A thermistor anemometer is placed in front of the strips to monitor the flow velocity within the sampler. Depending on ambient wind speed the voltage supplied to the fan is regulated, so that the flow velocity is maintained close to 8 m s 1. The size fractionation characteristics of the impaction strips (widths equal to 1, 1.5, 2, 3, 5, 7, 9, 12 and 25 mm) were determined experimentally (Eleftheriadis and Colbeck, 1992). At a flow rate of 8 m s 1 the 50% cut-off diameter of the strips ranged from 7.8 to 38.8 mm. Although well defined and reliable in terms of aerodynamic diameter the efficiency curves did not match the sharp cut-off behaviour of conventional cascade impactors. However samples collected on impaction strips offer the advantages of easy handling, excellent means of particle examination by optical or electron microscopy and quantitative chemical speciation. The main disadvantage arises from overloading which limits the sampling time and consequently the aerosol mass collected. The true size distribution can be
Fig. 1. Schematic diagram of the tunnel sampler.
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extracted from a set of strip samples when an inversion method is applied on the results where the strip collection efficiency curves are used as Kernel functions (Ramachandran and Kandlikar, 1996; Kandlikar and Ramachandran, 1999).
3. Field experiments and analysis The ambient aerosol measurements obtained by the Wind Tunnel Sampler were conducted around the town of Colchester (population 157,000) in the South East of England. The area is characterised by flat land with no major industrial activity involving stack emissions, except agriculture. Colchester is a relatively small town and with the exception of motor vehicle traffic it is not expected to contribute high aerosol emissions to the surrounding area (Colchester Borough Council, 2000). Other aerosol inputs to this area may result from the London Metropolitan area and the North Sea, when the prevailing winds favour transport from these two areas. In view of the above topography and environment, two sites thought to reflect some specific conditions were selected for sampling. A map of the region including the two sites is shown in Fig. 2. The first sampling location
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was considered as one directly affected by a local aerosol source. This was a Civic Amenity site (refuse landfill) situated at the edge of Colchester. The sampler was operating at a distance of around 500 m away from an open tipping area but only during the early evening hours when the site was not active. The above conditions were thought to provide moderate aerosol generation by natural processes such as resuspension by the ambient wind. A small tarmac works area, 1 km to the south, may also contribute some aerosol. The second sampling site was located in a rural area 12 km due west of the first site. It was surrounded by open cultivated land with no major agriculture activities taking place during the sampling period. Four measurements were obtained on each site. Sampling took place at almost weekly intervals during a two month summer period. The sampling time was around 4 h. The sampler was placed on a platform so that the tunnel mouth was 2 m above ground. Atmospheric conditions were characterised by dry warm weather with very light winds at the refuse site, while moderate winds were predominant at the rural site. One measurement taken at the latter location involved very strong winds (around 10 m s 1). The measurements described above were performed in a manner suitable for PIXE analysis. Polycarbonate
Fig. 2. Location of the sampling sites (1) rural site, (2) civic amenity site.
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filter membranes were cut to the size of the strips and attached on them after they had been immersed in a solution of L-Apiezon grease. The latter was used in order to reduce particle bounce from the strips. Both the filter and grease materials are relatively X-ray emission free. After sampling the filters together with control blanks were kept in airtight containers and later analysed in the PIXE facilities of the University of Birmingham. Due to limited analysis time only six strips per measurement were analysed. These were the 2, 3, 5, 7, 12 and 25 mm samples, thought to be adequate to produce the elemental size distributions. Analysis was performed for a wide range of elements such as P, Si, Cl, S, K, Ca, Fe, Ti, Mn, Cu, V, Co, Cr, Br, Zn, Ni, Sc and Pb. The results showed consistent concentrations for most earth elements and some trace elements. The masses detected for Br, Zn, Ni, Sc and Pb were frequently lower than the analytical detection limits or the values from the respective blank samples. Moreover, these elements did not appear consistently in all measurements and therefore they were excluded from any further study. Silicon was also excluded due to the erratic nature of the respective measurements probably due to the arbitrary collection of giant sand particles on the samples.
4. Size distributions of elemental concentrations The aerosol mass concentrations were calculated for the remaining earth elements and trace metals. The arithmetic mean of the concentration for every element at each site was determined. One measurement from the rural site involving very high ambient wind conditions was excluded from the average as it was found to have elemental concentrations not compatible with the predominant aerosol mass found in the other three measurements. The extreme value estimation (EVE) deconvolution method (Hopke and Paatero, 1994; Aalto et al., 1990; Tapper and Paatero, 1990; Paatero et al., 1988) was applied on the two sites elemental concentration averages in order to produce the individual size distributions for each element. The EVE calculations are described in detail by Eleftheriadis (1993) and Aalto et al. (1990). The error estimates required in the calculations were derived from flow rate fluctuations recorded during sampling and ranged between 15% and 20%. The EVE method was applied over a size range between 1 and 100 mm. The standard deviation of the model guassians selected were between 1.4 and 2. The strips used for sampling here have their size collection characteristics separated by very narrow size intervals. Small standard deviations around 1.4 enhance the fine mass differences between the strip stages and give
solutions, which often reveal important features of the individual distributions. However, these narrow model gaussians may also produce unnaturally shaped distributions with sharp peaks. On the other hand when a large standard deviation was employed, any fine structure was removed from the resulting distributions. The final solution selected for each elemental distribution was a compromise between the above extreme situations with the chi-square value also used as a criterion. Each solution of the inverted size distributions by EVE is given as a family of solutions with a minimum and maximum concentration with respect to any size. The solutions presented here correspond to the arithmetic mean of the family solutions. The size distribution of the atmospheric concentration for each element measured at the two sites are displayed in Fig. 3. The part of the distribution lying to the left of the main peak towards the small sizes is characteristic of an ideal situation where the concentration approaches zero at around 1 mm. This is due to the characteristic collection efficiency function of the strips rather than the true size distribution of the elemental mass. The results have been normalised with respect to the total concentration calculated from the EVE results. Due to the uncertainty involved with the EVE calculations between 1–2 mm, where the collection efficiency function approaches zero, results are presented for the range between 2–100 mm. The lower size also coincides with the generally accepted dividing line between coarse and fine size fractions of the atmospheric aerosol (Whitby, 1978). However, this is a characteristic of the total aerosol mass and does not always describe individual elements discussed here. It is well known (Milford and Davidson, 1985) that most elemental distributions shown here, like those of some trace elements, have a large part of their mass distributed over the accumulation size mode (0.1–1 mm). The size distributions presented here for the two sites show that most crustal elements (Fe, Ca, K and Ti) display a peak in their mass concentration between 3 and 7 mm. Despite the quantitative difference in the total mass concentration of the above elements at the two sites the shape of their respective size distributions is generally similar. The only exception is potassium, which displays a clear second mode at around 10 mm at the rural location. P was detected only at the civic amenity site and shows a maximum at around 6 mm. These results rather contradict the general idea that coarse aerosol is accumulated in a mode with a peak at around 10 mm (Whitby, 1978). Instead, the general picture emerging shows that the above elements, although characteristic of the soil dusts which are the dominant source of coarse aerosol, are not necessarily the greatest portion of its mass at sizes greater than 10 mm. A better picture could be formed if the concentrations of Al and Si together with that of biogenic aerosol were also determined.
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Fig. 3. (a) Normalised size distributions of K, Mn, Fe, P, Ca and Ti in ambient aerosol (b) Normalised size distributions of V, Co, Cu and Cr in ambient aerosol.
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A greater variation on the characteristics of their size distribution can be observed for the trace metals studied here. A clear distinction between the two sites is evident. All these elements (Co, V, Mn and Cu), excluding Cr, appear to have their ambient concentrations maximising between 2 and 3 mm at the rural site and between 4 and 6 mm at the refuse site. Chromium displays a surprising bimodal distribution with a first peak similar to those of the other metals and a second peak at around 80 mm. The phenomenon is consistent for both sites, although the magnitude of the second peak is greater at the civic amenity site. The total concentration above 2 mm calculated by EVE for each element is displayed in Table 1. The maximum degree of uncertainty on these values is similar to the respective error estimates used in the EVE calculations and equal to around 20%. The measure of the uncertainty is assumed to be the maximum residual value resulting from the deviation between the strip measurements and the best fit calculated by EVE. The concentration of the large particles (>10 mm) is also reported in Table 1 for both sites. An interesting picture emerges by comparing the average concentrations of the two size fractions at the two sampling sites. First, the earth elements (Ca, Fe, Ti and K) are present at the refuse site with concentrations a great deal higher than those at the rural site. However, the large particle fraction (>10 mm) of these elements does not share the same features. Only iron displays a lower concentration at the rural site while the remaining elements have similar or higher mass concentrations compared to those at the civic amenity site. The total coarse fraction concentrations of the trace metals are higher at the rural site with the exception of Co. Several questions can be raised about the nature of the elemental distributions at the two sites. The atmospheric conditions during sampling and the area surrounding
Table 1 Mean ambient concentrations (ng/m3) Element
Fe Ca K P Ti Mn V Cu Cr Co
Civic amenity site
Rural site
>2 mm
>10 mm
>2 mm
>10 mm
2530 3900 1550 108 317 38 3 10 5 10
123 432 40 15 5 6 1 3 3 1
833 2022 1083 F 76 45 9 18 5 6
26 414 98 F 14 1 2 2 1 *
the sites play an important role. Dry warm weather during the measurements at the civic amenity site indicate heavy aerosol load in the area. The low wind speeds (o2 m s 1) recorded coupled with the time of the day (early evening) that sampling was performed, indicate that the aerosol produced mechanically during the day at the site or the nearby town had its small size fraction still suspended over the area, while the larger particles had settled. This may be the reason for the absence of large particles among the earth elements and the fact that the concentrations of trace metals is lower than those found at the rural site. Refuse is not known to be a substantial source of such metals (Pacyna, 1985). The type of soil in the area and vegetation cover are also critical of the resulting size distributions. Higher wind speeds (4–7 m s 1) prevailed during sampling at the rural site, favouring increased resuspension of soil dust. This may explain the higher ratio of larger (>10 mm) to total coarse (>2 mm) concentration observed for the soil dust derived earth metals (Ca, K, Ti) at the rural site, compared to that at the civic amenity site. Some explanation for the decline of metal concentrations in the large atmospheric aerosol can be found in studies dealing with erodible soils, the major source of coarse aerosol. It is understood that metals exist in rocks and soils in a form of various species like SiO2, TiO2, Al2O3, Fe2O3, FeO, CaO, K2O, P2O5 and MnO. (Mason, 1966; Stelson and Seinfeld, 1981). Schutz and Rahn (1982) studied the variation of elemental concentration with particle size in dust and soils at remote areas. It was found that the relative concentration of most earth metals in the 2–400 mm particle size range, normalised with respect to the concentration of larger soil grains (160–400 mm), show a sharp decrease of one to two orders of magnitude for particle sizes greater than 20 mm, while Si was the only element that displayed increasing concentrations with increasing particle size. This phenomenon has been attributed to the lack of mineral species like clay minerals in very large particles. Although soil composition in the above studies is not of the same composition as the one found in Southeast England, the size distributions for the earth metals presented here display a similar trend. Vegetative cover in the area where the measurements were conducted may also play an important role in resuspension of dust from the ground. The two sites discussed here, were located at areas with different vegetation characteristics with the rural site richer in vegetation. Wu et al. (1992) found that leaves reduce resuspension rates of particles from their surface. Field studies though, (Sehmel, 1980) did not reveal a clear decrease between for example grass and asphalt surfaces. It is clear though that the Civic Amenity site was adjacent to larger areas covered with loose soil.
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5. Comparison with other studies Measurements of the chemical speciation of very large atmospheric aerosol are sparse. The only studies indicating the presence of most of the above elements in large aerosol are those by Noll et al. (1985, 1990). However, these measurements are based on the 50% cut off sizes of impaction strips similar to the ones used in this study. This method for calculating the size distribution has been found to be misleading. Most information about the size distribution of metal species in the atmospheric aerosol is available from studies employing cascade impactors for sampling. Most investigations have been on urban areas (Horvath et al., 1996; Infante and Acosta, 1991; Anderson et al., 1988; Orsini et al., 1986; Spengler and Thurston, 1983; Zoller et al., 1974), whilst those in rural areas are lacking (Horvath et al., 1996; Injuk et al., 1992; Adams et al., 1983; ElShobokshy, 1984). Other studies focusing on the importance of large particles were conducted during
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the Lake Michigan Mass Balance Study (Shahin et al., 2000), where the deposition flux of earth and trace metals was investigated. Short term measurements were conducted with the Noll Rotary Impactor (Paode et al., 1999). It was found that very large particles, despite their small mass concentration contribute greatly to deposition fluxes due to their high deposition velocity. The aim here is to identify any universal characteristics of the elemental distributions in the measurements conducted during this study. The size distributions from measurements at the rural site were considered more suitable for this exercise. This was due to the clear influence of specific conditions on the results at the civic amenity site. It is clear that comparisons with data from studies conducted in Britain would be more suitable for credible conclusions due to common aerosol sources, topography, vegetation cover and climate. Elemental size distributions from two such studies (Pattenden, 1974; Cawse, 1974) are also included in Fig. 4. These measurements were performed with Andersen Cascade
Fig. 4. Comparison between normalised size distributions for Fe, Mn, Cu, Co, V and Cr, measured at the rural site and from studies in the UK (Cawse, 1974; Pattenden, 1974).
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impactors at Chilton (England) and Trebanos (Wales). Chilton is a rural site, in central southern England, whilst Trebanos is just on the outer limit of an industrial zone, which includes oil refining, steelworks and a nickel smelter. The elemental size distributions for Fe, Mn and Cr measured here, show very good agreement with the respective results from Chilton and partial agreement with those at Trebanos. There is also partial agreement for Co with both reference distributions. The distribution determined here for V seems to follow the trend observed at Trebanos. Data for Cu were available only at Trebanos. There is also general similarities in trend here. The agreement with the rural site at Chilton confirms expected similarities in elemental size distributions between areas with common characteristics. The size distributions at Trebanos are often bimodal. Taking into account. the industrial sources present in the vicinity of this site the large particle mode absent from the other two sites can be attributed to such sources. It has to be noted that the characteristics of the metal size distributions determined here are also consistent with findings in other regional studies around the world (Keronen et al., 1991; Kasahara et al., 1992). Finally, direct quantitative comparisons between the mass concentration of metals measured here and in other studies in the U.K is difficult due to the lack of measurements for the coarse fraction alone as it was determined in the present study. However, an attempt was made here to extract information from the data given in Table 2. The results from two long term studies of the total mass concentration of certain metals are displayed. The values are averages over five and ten years sampling periods from measurements at rural and urban sites throughout Britain. At first the concentrations determined here for many of these metals seem excessively high compared to the rural and even urban values in the other studies. It is evident that the greatest difference exists for the earth elements (Fe and Ti) which have most of their mass distributed across the coarse particle size range and the comparison between the Table 2 Ambient concentrations (ng m 3) from studies in the UK Element
Urban (total)a 1985–89
Rural (total)a 1972–81
Rural (>2 mm) this study
Fe Ti Mn V Cu Cr Co
1000 4.8b 34 25 30 11 1.5b
390 28 34 11 24 7 0.4
833 76 45 9 18 5 6
a b
Data quoted in QUARG (1993). Concentrations at or below detection limits.
results is more meaningful. The concentrations of the trace elements with more than 50% of their mass present at small sizes (Cawse, 1974) are more difficult to compare. It has to be noted that credible conclusions from these comparisons are hampered by several conflicting factors. First these averages incorporate various atmospheric conditions with widely different aerosol masses, while the measurements in these study were performed during a specific situation of heavy atmospheric aerosol load. The measurements used for reference span over many years during which a decline in metal concentrations was observed across Britain (Lee et al., 1994). The sampling methods used include filter holders sampling at 901 with respect to the wind (Pattenden, 1974). The collection efficiency of such devices for large particles is far from ideal (Vrins et al., 1984) and leads to underestimation of the aerosol mass in the coarse size-range. It can be concluded from Table 2 that the concentrations are of the same magnitude. The distributions found for the trace elements V, Co and Cu at the rural site display maximum concentrations at around 2 mm. Taking into account the limitations of this study at the above size range it can be concluded that the majority of the mass is found in much smaller sizes. This is expected for these elements which are usually released in the atmosphere by anthropogenic industrial sources (Pacyna, 1985). Their concentration, though, does not drop as sharply with size as it is thought. The above characteristic is not visible in the distributions of the same elements measured at the industrial site, probably disguised by aerosol released in the air by the materials originating from the site.
6. Conclusions The tunnel sampler employing single stage impactors is found to be a useful instrument for representative sampling of coarse aerosol. Elemental analysis of size fractionated samples can be performed and the size distribution in the range of 2–100 mm can be calculated for each element. The high volume intake of the sampler allows the collection of adequate mass for analysis over sampling periods of a few hours (3–5) in suburban areas of the U.K. Results from measurements in two U.K. sites for a number of common earth and trace metals show that their concentration in the coarse aerosol fraction peaks at around 3–7 mm. The mass concentration of the fraction >10 mm is found to be between 10–15% of the total coarse mass for most elements.
Acknowledgements We would like to thank Dr Lakhumal Luhana for performing the PIXE analysis.
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