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Atmospheric Pb isotopic composition and trace metal concentration as revealed by epiphytic lichens:
Atmospheric Environment 35 (2001) 3681}3690
Atmospheric Pb isotopic composition and trace metal concentration as revealed by epiphytic lichens: an investigation related to two altitudinal sections in Eastern France F.J. Doucet, J. Carignan* CNRS - CRPG, 15 rue Notre-Dame Les Pauvres, BP20, 54501 Vandoeuvre-les-Nancy, France Received 27 July 1999; received in revised form 21 September 2000; accepted 7 November 2000
Abstract During Fall 1996, epiphytic lichens were collected along altitudinal sections in two areas of France (the Vosges mountains in the North-East, and the Alps, in Haute-Savoie) in order to verify any geographic distribution of atmospheric metals on a small scale. These lichens have various Pb isotopic compositions (Pb/Pb"1.126}1.147) which are correlated with the altitude of sampling. Lichens sampled near valleys display isotopic ratios signi"cantly less radiogenic than those sampled at several hundred to thousand meters of altitude. In the Vosges sections, Pb concentrations and isotopic compositions of lichens may be used to de"ne three zones: (1) valley: Pb-rich and non-radiogenic ratios, (2) transition: low-Pb and intermediate isotopic compositions, (3) mountain: heterogeneous Pb concentrations but more radiogenic and homogeneous Pb isotopic composition. Other metals (Zn, Cu, Cd, As), when normalised one to another, are not fractionated between these zones and display homogeneous relative abundance along the altitudinal sections of both sites. Variation of Pb/Pb ratios with altitude is interpreted in terms of mixing of at least two pollution sources: one being the petrol (leaded and/or unleaded) combustion, and the other being of industrial origin. The latter is characterised by a more radiogenic isotopic composition. The Pb isotopic composition of #ue gas residues from di!erent municipal solid waste combustors in the Rhine valley and in other areas of France would suggest that these plants might be an important source of industrial Pb in the atmosphere. If the average industrial Pb in France has a Pb/Pb close to 1.15, between 60 and 80% of the total Pb in lichens from the Rhine valley would come from gasoline combustion, whereas 85}90% of the Pb would have an industrial origin in lichens from higher altitude in the Vosges mountains. Although lichens from the Alps were collected at higher altitude, the percentage of industrial Pb for these lichens would be slightly lower (65%). Major winds and convection winds in the di!erent valleys must then play an important role in term of distribution of atmospheric Pb in function of altitude. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Atmosphere; Lead; Metals; Lichens; Pollution
1. Introduction * Corresponding author. Fax: #33-03-8351-1798. E-mail addresses: [email protected] (F.J. Doucet), [email protected] (J. Carignan). Present address: Department of Chemistry, Birchall Centre for Inorganic Chemistry and Materials Science, Keele University, Sta!ordshire ST5 5BG, UK.
Since the middle of the 20th century, industrial production (e.g. metal re"ning, waste incineration burning of coal and wood) and vehicular tra$c (leaded petrol combustion) have released large amounts of heavy metals (Pb, Cu, Zn, Cd, Ni) into the atmosphere (e.g. Patterson and Settle, 1987; Nriagu and Pacyna, 1988).
1352-2310/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 5 1 0 - 0
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F.J. Doucet, J. Carignan / Atmospheric Environment 35 (2001) 3681}3690
Two approaches have been used to identify sources of metals and other pollutant elements in the atmospheric environment. One involves identifying di!erences in the multi-elemental composition of atmospheric aerosols (e.g. Narita et al., 1999) and precipitation (e.g. Fujita et al., 2000). The other approach exploits di!erences in the lead isotopic composition (e.g. Hopper et al., 1991; Monna et al., 1997; Aberg et al., 1999; VeH ron et al., 1999; Chiaradia and Cupelin, 2000). However, it is believed that the two approaches together are necessary for a more precise identi"cation of atmospheric pollution sources (e.g. Sturges and Barrie, 1989; Halstead et al., 2000; Simonetti et al., 2000). The serious increase in world-wide environmental contamination caused by trace metals is well documented (e.g. Nriagu and Pacyna, 1988; Nriagu, 1989). Over the past two decades, the progressive restriction on lead addition in gasoline has resulted in a signi"cant decrease of the global atmospheric lead burden, as recorded in ice from Greenland (Murozumi et al., 1969; Boutron et al., 1991), in corals from Bermuda (Shen and Boyle, 1987), in Mississippi deltaic surface sediments (Trefry et al., 1985), lake and estuary sediments (Lucotte et al., 1995; Gobeil et al., 1995), herbarium specimens (Weiss et al., 1999) and in the Sargasso Sea waters (Sherrel et al., 1992). There has been a general variation in aerosol lead isotopic signatures and lead concentration in France during the last 15 yr (VeH ron et al., 1987; Grousset et al., 1994), interpreted as resulting from the decrease of atmospheric pollution by Pb. According to Grousset et al. (1994), the Pb isotopic signatures of aerosols over southern Europe reveal the presence of two di!erent sources: one fraction would be the natural lead contained in crust-derived particles, mostly from the Sahara Desert; the other fraction would be anthropogenic European derived mostly from gasoline consumption. A feature of this previous study is that the analysed aerosols were sampled in both urban and mountainous sites. However, data available in the literature on the composition of aerosols prior to ca. 1985 concern mostly samples collected in urban area, and most samples collected after 1985 are from remote areas (see the review in Grousset et al., 1994). Although VeH ron et al. (1999) have documented such evolution of Pb isotopic ratios for aerosols collected in Northern France between 1983 and 1994, some studies showed that chemical and isotopic gradients of atmospheric Pb may exist between urban and rural areas of North America (Sturges and Barrie, 1989; Carignan and Doucet, 1997). The present study aims to verify any variation in the distribution of atmospheric metals on a small geographic scale, in particular in relation to the altitude of the sampling site. For this, epiphytic lichens collected on tree branches were used as natural "lters of atmospheric matter, since epiphytic lichens derive moisture and nutrients exclusively from air. Indeed, the use of epiphytic
lichens as bio-monitors for trace element air pollution is widely acknowledged (e.g. Lawrey and Hale, 1988; Rope and Pearson, 1990; Sloof and Wolterbeek, 1991; Lawrey, 1993; Carignan and GarieH py, 1995). Therefore, it is assumed that the chemical composition of epiphytic lichens can be used to estimate that of the atmosphere. The principal accumulation mechanisms are via extracellular complexing of metals by ion-exchange processes, by particle entrapment and by direct uptake of gases (Deruelle and Lallemant, 1983).
2. Samples and methods During Fall 1996, epiphytic lichen samples were collected on tree branches along altitudinal sections from two areas of France, the Vosges mountains in the NorthEast (Haut-Rhin) and the Alps in the Centre}East (HauteSavoie). The "rst area (the Vosges) is located West of the Valley of the Rhine River. Lichens have been collected between altitudes of 200 and 1300 m. Samples were collected along a small road called `la Route des Cre( tesa, from the valley (200 m in altitude) to the `col de la Schluchta and the `col du Bonhommea. The second area (the Alps) is located close to the Annecy Lake. Samples were collected along a small local road on the East #ank of the Semnoze mountains between altitudes of 1700 and 1000 m. A sample was also collected in the valley, south of the Annecy Lake in a recreation park (450 m in altitude). With the exception of the valleys themselves, these two areas are mostly uninhabited, and remote from industries and highways or national roads. The epiphytic lichens used for analysis were fruticulose specimens of Evernia sp. and/or Usnea sp. The lichens were collected from trees near the tip of tree branches less than a few millimeters in diameter, whenever possible. This was done to obtain lichens that were exposed to the atmospheric signal for a time period as short as possible (i.e., less than a few years). Samples were taken with pre-cleaned plastic tweezers, and transferred to sealedplastic bags. In the laboratory, the lichen samples were immediately separated from their substratum, warmdried at 1053C for 4 h, and stored in hermetic vessels. Grinding and homogenisation of lichen samples is di$cult and need a lot of manipulation. Because of possible contamination during the preparation, the samples were not powdered. Between 50 and 100 mg of an aliquot of small lichen branches were directly transferred to Te#on威 capsules and digested in a mixture of HNO , H O and HF. Results obtained on two aliquots of three di!erent lichen samples yielded sometimes small di!erences in concentrations (5}25% relative di!erences) but very similar metal/metal ratios. Heavy metal concentrations were measured using a Perkin-Elmer ICP-MS SCIEX ELAN 5000. Reagent blanks were carried through all procedures and were negligible in all cases.
F.J. Doucet, J. Carignan / Atmospheric Environment 35 (2001) 3681}3690
using anion-exchange chromatography (Manhe`s et al., 1980). All lead isotope ratios were measured using a TIMS Finnigan MAT 262, operated in multi-collection mode. A mass fractionation correction of 0.12% amu\ was applied to all ratios. Repeated measurements of the NBS 981 Pb standard material yielded uncertainties (2) lower than 0.1, 0.2, 0.3, and 0.4% for the Pb/Pb, Pb/Pb, Pb/Pb, and Pb/Pb ratios, respectively.
Table 1 Certi"ed values and measured concentrations of the BCR-CRM 482 lichens standard Elements
Certi"ed values (ppm)
Measured values (ppm)
Al Cd Cu Ni Pb Zn
1103$24 0.56$0.02 7.03$0.19 2.47$0.07 40.9$1.4 100.6$2.2
1187$15 0.46$0.04 6.83$0.19 2.33$0.17 39.6$0.6 102.5$1.7
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3. Results and discussion
Each value is the mean of four replicates. Concentrations are reported on a dry weight base. The $ values are S.Ds.
All the results on trace element concentrations and Pb isotopic compositions measured in lichens are reported in Table 2.
The accuracy of the method was veri"ed by applying it to a lichen standard reference material (BCR-CRM 482: Community Bureau of Reference CRM 482; Table 1). Lead separation for isotopic measurements was done
3.1. Pb isotope and trace element variations with altitude Fig. 1 shows the evolution of the Pb/Pb ratios measured in lichens in relation to their altitude of
Table 2 Trace element concentrations (ppm) and Pb isotopic composition in lichens from the studied areas Altitude (m)
Lichen
Vosges area 900 1100 900 510 1100 800 560 510 800 200 855 1050 630 430 900 510 700 1300 900 320
Evernia sp. Evernia sp. Usnea sp. Evernia sp. Evernia sp. fruticuleux Evernia sp. tapis (chou) fruticuleux Evernia sp. Evernia sp. Evernia sp. Folioses Evernia sp. Evernia sp. Usnea sp. Evernia sp. Evernia sp. Evernia sp. Mixed
Alps area Valley 1075 1300 1400 1400 1468 1468 1700 1700
Evernia sp. Evernia sp. Evernia sp. ? Evernia sp. Evernia sp. Usnea sp. Usnea sp. Evernia sp.
n.d.: not determined.
Pb/Pb
Pb/Pb
1.145
2.113
1.140 1.147
2.119 2.112
1.141 1.147 1.127 1.147
2.118 2.113 2.131 2.113
1.141 1.144 1.136 1.147 1.146 1.146 1.145
2.118 2.116 2.123 2.111 2.112 2.113 2.114
1.128 1.140
2.132 2.118
1.136
2.123
1.136 1.133
2.123 2.125
Al
As
Cd
Cr
Cu
Ni
Pb
Zn
46 131 38 90 296 271 213 333 221 1625 238 298 175 258 208 69 237 751 208 563
0.37 0.23 0.29 0.14 0.47 0.41 0.37 0.26 0.70 1.11 0.56 0.58 0.49 0.39 0.26 0.13 0.51 1.48 0.39 0.50
0.23 0.24 0.19 0.25 0.43 0.33 0.13 0.31 0.46 0.38 0.55 0.87 0.16 0.18 0.02 0.12 0.24 0.91 0.15 0.44
1.31 0.90 0.31 0.60 1.27 0.67 0.85 1.01 0.95 3.73 0.53 1.31 0.65 0.42 0.10 n.d. 0.72 1.65 1.40 1.65
4.22 2.44 2.76 1.59 4.82 4.27 3.41 3.29 4.80 11.97 3.57 4.02 5.34 2.67 1.85 1.53 3.55 6.69 2.26 5.15
0.89 0.68 0.48 0.32 1.21 0.53 0.69 0.48 0.92 3.00 2.97 1.25 0.86 0.84 0.81 0.32 0.66 2.41 0.89 2.58
8.6 10.7 5.5 2.6 13.4 5.5 4.5 5.0 10.9 69.8 13.2 15.9 9.4 3.7 2.9 4.7 5.1 23.5 3.6 8.2
61.6 21.6 15.8 29.2 63.2 60.8 30.3 21.7 40.0 72.5 32.6 59.6 36.6 21.4 16.1 12.1 21.1 67.4 25.3 30.3
460 311 259 260 270 472 219 87 341
0.80 0.33 0.19 0.81 0.68 0.32 0.45 0.78 0.76
0.72 0.27 0.24 0.44 0.40 0.22 0.36 0.26 0.44
6.01 1.47 0.58 1.29 0.48 1.70 0.92 2.58 2.51
7.58 3.51 2.67 5.35 5.46 3.70 4.21 3.68 8.36
7.56 5.42 4.84 22.6 5.31 5.23 5.34 4.40 6.44
17.8 5.6 5.1 8.7 13.3 8.0 6.8 5.9 28.3
72.0 49.6 32.8 55.0 74.3 56.1 51.4 43.5 112
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Fig. 1. The Pb/Pb ratios measured in lichens against the altitude of the sampling site. (a) cross section in the Vosges Mountains; (b) cross section in the Alps (Haute Savoie).
sampling. Lichens sampled in the valleys of both studied areas are characterised by the lowest Pb/Pb ratio, having a composition close to that of leaded (Pb/ Pb"1.109$0.004; average for six di!erent distributors) and unleaded (Pb/Pb"1.124) gasoline used in France (Roy, 1996). On the other hand, samples collected at altitude are characterised by higher Pb/Pb ratios, which suggest the contribution of more radiogenic sources for atmospheric Pb. The isotopic composition of lichens from the Haute Savoie area do not vary systematically with altitude from 1000 to 1700 m; they rather yield relatively homogeneous Pb/Pb ratios (1.136$0.003). Lichens from the Vosges area display an evident correlation between their isotopic composition and the altitude at which they were sampled. Lichens sampled between 700 and 1300 m have a very homogeneous composition with an average Pb/Pb ratio of 1.146$0.001. Lichens located at mid-altitude, from 350 to 500 m, are less homogeneous in composition (Pb/Pb ratio of 1.141$0.003), and may represent a transition zone between the valley signal and that of higher altitude. The variation of the Pb isotopic composition of lichens with altitude may then
result from a mixing of at least two sources of Pb. The least radiogenic one, at the valley bottoms, is probably Pb emitted from petrol combustion, and the more radiogenic one, at altitude, is of an unknown origin. Either the radiogenic source is not homogeneous in composition at the regional scale (Pb/Pb of 1.146$ 0.001 and 1.136$0.003 for the Vosges and the Alps, respectively), or the proportions of the sources in altitude are di!erent at the two localities. One way to discriminate the contribution of anthropogenic sources with respect to natural sources for heavy metals in the environment is to normalize metal concentrations to Al concentration and to compare the ratio obtained to that of upper crustal rocks. The contribution of anthropogenic sources is expressed as the enrichment factor (EF"[metal/Al] /[metal/Al] ) of a given metal. According to the metal EF's (Table 3), between 98.5 and 99.9% of the total Pb measured in lichens would be in excess relative to crustal Pb. This means that the isotopic composition of lichens may directly be taken as that of atmospheric pollution Pb and that variations of Pb/Pb ratios re#ect di!erent proportions of two or more pollution sources with distinctive lead isotopic composition. The Pb and Al concentrations measured in lichens (Vosges) in function of their sampling altitude are reported in Fig. 2(a). From the valley (200 m) to an altitude of about 400}500 m, both Pb and Al concentrations decrease systematically. From 400 to 1300 m, Al concentrations stay relatively constant (mean$S.E: 240$50 ppm) whereas Pb concentrations, on average, increase systematically from (3}5) to 15}20 ppm. Unfortunately, the Pb/Al ratios in lichens are not correlated with altitude, probably because of the scatter of the data in the di!erent trends. This means that the shift in isotopic composition do not simply represent the fading out of one of the source (Pb from the combustion of gasoline in the valley for example). Other metals also having high EF's, such as Cd and Zn, have a concentration distribution similar to that of Pb in Fig. 2(a), whereas As and Cu display a behaviour rather similar to Al (see Table 3 for metal EF's). Cr and Ni are metals having the lowest EF's and no particular correlation is observed between their concentrations and the altitude. Fig. 2(b) presents Pb concentrations in correlation with the Pb isotopic composition for lichens sampled in the Vosges area. The data were divided according to their observed distribution in the Pb/Pb vs. altitude diagram of Fig. 1: valley, transition zone and mountains. Lichens from the transition zone have a large range of isotopic composition but fairly uniform and low Pb concentrations (3$1.5 ppm). On the other hand, lichens from the mountain zone have homogeneous isotopic ratios and present a wide range of Pb concentrations (3.5}23.5 ppm). Assuming that all the lichens have accumulated the atmospheric signal for about the same period of time, and that they have similar EF's, their total Pb concentration
F.J. Doucet, J. Carignan / Atmospheric Environment 35 (2001) 3681}3690
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Table 3 Enrichment factors measured in lichens from the studied areas Altitude (m)
Lichen
EF-As
EF-Cd
EF-Cr
EF-Cu
Vosges area 900 1100 900 510 1100 800 560 510 800 200 855 1050 630 430 900 510 700 1300 900 320
Evernia sp. Evernia sp. Usnea sp. Evernia sp. Evernia sp. fruticuleux Evernia sp. tapis (chou) fruticuleux Evernia sp. Evernia sp. Evernia sp. folioses Evernia sp. Evernia sp. Usnea sp. Evernia sp. Evernia sp. Evernia sp. mixed
428 95 410 86 86 81 93 42 169 37 125 105 149 80 68 103 115 106 100 48
4173 1484 3994 2288 1192 987 493 766 1704 194 1899 2401 750 579 91 1427 831 994 592 634
65 16 18 15 10 6 9 7 10 5 5 10 9 4 1 n.d. 7 5 16 7
295 60 234 57 52 51 51 32 70 24 48 43 98 33 29 71 48 29 35 29
Alps area Valley 1075 1300 1400 1400 1468 1468 1700 1700
Evernia sp. Evernia sp. Evernia sp. ? Evernia sp. Evernia sp. Usnea sp. Usnea sp. Evernia sp.
94 56 39 168 135 36 109 481 120
1291 698 749 1396 1201 377 1357 2484 1067
30 11 5 11 4 8 10 68 17
53 36 33 66 65 25 62 136 79
EF-Ni
EF-Pb
EF-Zn
78 21 51 14 16 8 13 6 17 7 50 17 20 13 16 19 11 13 17 18
747 329 582 117 181 81 84 61 199 173 223 214 216 57 55 273 86 126 69 59
1516 187 471 367 242 254 161 74 205 51 155 226 237 94 88 199 101 102 138 61
66 70 75 350 79 45 98 203 76
156 72 79 135 198 68 125 270 334
177 180 143 240 311 134 266 565 373
n.d.: not determined.
would then re#ect the total atmospheric #ux. Hence, the altitudinal section of the Vosges Mountains may be divided in three di!erent domains, not only by the isotopic composition of lichens but also in terms of global atmospheric #uxes. These domains are: (i) the valley, having the highest #uxes for all the studied metals; (ii) a transition zone, having the lowest #uxes for many studied metals; (iii) the mountain, having intermediate metal #uxes but homogeneous Pb isotopic composition suggesting a single source. The distribution of di!erent metal/metal ratios measured in lichens is presented in relation with the altitude at which they were sampled in Figs. 3(a)}(c). At both localities, Vosges and Alps, Cu/As and Zn/Cd ratios yield similar average values and present no systematic in function of altitude (Figs. 3(b) and (c)). On the other hand, Zn and Cu normalised to Pb concentrations form a convex pattern when plotted against altitude. This pattern is
particularly visible for the Vosges samples, for which a second-degree regression "t is shown in Fig. 3(a). This distribution is somewhat the inverse of the one obtained in Fig. 2(a) for Pb concentrations against altitude, and is probably controlled by the largest variation of the Pb #ux in comparison to other metals or to the decoupling of sources. In any case, the di!erent sources enlightened by Pb isotopes do not seem to be discriminated by metal/metal ratios. 3.2. Sources of metals in lichens Relative metal abundance could be used to trace sources of emissions. However, there are a lot of processes between the locus of emission and the precipitation and accumulation of metals that may lead to fractionation of elements. For example, these processes may be (1) various atmospheric residence times, (2) di!erent solubility of
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Fig. 2. (a) Pb and Al concentrations (ppm) in lichens from the Vosges Mountains in function of altitude. (b) Pb concentration vs. Pb/Pb with di!erent domains as outlined from the Pb/Pb vs. altitude of Fig. 1.
elements, and (3) fractionation during up-take and bioaccumulation. One way to estimate if bio-accumulation processes fractionate metals from each other between the atmosphere and the lichens would be to compare their chemical composition to that of rainwater. For example, Zn/Pb, Cu/Pb, and Zn/Cd ratios measured in lichens (Fig. 3) yield average and variation of ratios very similar to those measured in rain samples from Paris (Roy, 1996) and Nancy in NE France (Kazpard, 1997). On the other hand, although the Cu/As ratio measured in lichens for both studied areas is homogeneous, this ratio is highly heterogeneous in rain samples (2}400; Kazpard, 1997) and average at more than 50. However, the median value yield Cu/As of 9 in rain from Nancy, which is much closer to the Cu/As ratio measured in lichens (9}9.5; Fig. 3(b)). This suggests that the lichen's chemical composition represents that of the rain and that no signi"cant bio-accumulation fractionation occurs within the variation observed. Except for Zn, metal/metal ratios (Pb, Cd, Cu) measured in lichens are also in the range of ratios calculated from total atmospheric #uxes estimated from Pacyna (1984) for Europe, Nriagu and Pacyna (1988) and Nriagu
Fig. 3. Relative abundance of metals measured in lichens in function of altitude of sampling. Lines through data points in Fig. 3(a) are second degree "ts showing variations in the Zn/Pb and Cu/Pb ratios observed for the Vosges samples. These variations are mainly attributed to the larger variation of Pb concentrations along the sections.
(1989) for the global #uxes and Simonetti et al. (2000) for NE North America. The Zn/Pb ratio calculated from estimated global emissions (0.4$0.27; Nriagu and Pacyna, 1988), is much lower than Zn/Pb ratios measured in lichens from France and precipitation samples from France and North America (5}10 on average; Simonetti et al., 2000; Kazpard, 1997). Furthermore, total #uxes of Zn and Pb calculated recently for NE North America would correspond to Zn/Pb ratios of 80 to more
F.J. Doucet, J. Carignan / Atmospheric Environment 35 (2001) 3681}3690
than 500 (Simonetti et al., 2000). High depositional budgets of Zn in the late 1990s may re#ect either an increase in atmospheric industrial emissions (since 1980s), a lower residence time in the atmosphere, or particular chemical speciation which a!ects the deposition of Zn from the lower troposphere (Simonetti et al., 2000). Anyhow, if the di!erence between the Zn/Pb ratio measured in precipitation and the one calculated using total #uxes is simply due to the importance of dry deposition of Zn, this would imply that the uptake of elements by lichens fractionate the dissolved and particulate fractions from `weta and `drya deposition. This may have implications for the interpretation of Pb isotopes measured in lichens if dissolved and particulate Pb in rainwater are not in isotopic equilibrium. Roy (1996) have shown that dissolved and total (dissolved#particulate) Pb phases in a given rain sample from Paris yield the same isotopic composition, the dissolved phase representing between 4 and 65% of the total Pb. We will then make the assumption that dissolved and particulate Pb in rainwater are e!ectively in isotopic equilibrium and that the composition of lichens represent that of total atmospheric Pb for a given area. In Fig. 4, the Pb isotopic composition of lichens from this study is compared with data reported in the literature: (i) wet and dry aerosols from di!erent areas of western Europe (Grousset et al., 1994); (ii) dry aerosols (1982}1995) from northern France (VeH ron et al., 1999); (iii) lichens sampled close to main roads in France (Innocent et al., 1996); (iv) rainwater (1993) sampled in Paris (Roy, 1996). The isotopic composition of lead ores from Broken Hill, Australia, used as anti-knock additives in French gasoline (80% of the total Pb added in 1994}1995; VeH ron et al., 1999), measured Pb isotopic ratios in French gasoline (Roy, 1996), the isotopic composition of Saharan dust (Grousset et al., 1994) and pre-industrial marine sediments (Sun, 1980) and preliminary results on #y ashes from di!erent waste combustors in France have also been reported in Fig. 2. As stated in the introduction, Grousset et al. (1994) have documented a general variation in lead isotopic compositions measured in aerosols over the last 15 yr in France re#ecting a decrease of the atmospheric pollution over this time scale. A detailed study of VeH ron et al. (1999) reported similar variations for aerosols sampled in northern France between 1982 and 1995. These authors attributed this shift in isotopic composition to the more and more importance of industrial sources relative to the burning of leaded gasoline. The isotopic ratios measured in lichens appears signi"cantly less radiogenic than those obtained by Grousset et al. (1994) on aerosols sampled in remote areas (French Alps) for the 1990s period, but in the range of aerosols collected in remote areas between 1985 and 1990 (Grousset et al., 1994), of aerosols collected in the more industrialised area of Northern France in 1994 (VeH ron et al., 1999) and of rainwater collected in
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Fig. 4. Pb}Pb isotope diagram comparing lichens data with rainwater and aerosols data from the literature (references are indicated in the legend). Fields for the Broken Hill ore (Doe and Stacey, 1974; Pb additive in gasoline), for Saharan Holocene loess (Grousset et al., 1994) and pre-industrial sediments (Sun, 1980) and for #y ashes from waste combustors in France (our preliminary unpublished data) were reported for comparison.
Paris in 1993 (Roy, 1996). Lichens collected near valleys in the Vosges and Alps areas have a Pb isotopic composition towards that of lichens sampled near main roads elsewhere in France (Innocent et al., 1996) and towards that of unleaded and leaded gasoline used in France (Roy, 1996). Lichens sampled in altitude, in particular those from the Vosges mountains, have isotopic compositions very similar to that of #ue-gas residues (#y ashes) from di!erent municipal solid waste combustors in the Rhine valley and in other areas of France (Fig. 4). Lead, and other metals, emitted from these waste combustors is then a possible major end-member to account for the Pb isotopic variation observed in Fig. 4, not only for the Vosges area but also in the Paris and Northern France regions. As di!erent possible sources have Pb isotopic compositions falling on the single trend de"ned in Fig. 4, it is di$cult to estimate the various proportions of one and the others. Although VeH ron et al. (1999) have shown deviation from this trend for aerosols sampled near some industrial plants in the Nord-Pas de Calais region (France), it might be useful to put in correlation the isotopic composition and the Pb concentration of the di!erent samples. Fig. 5 presents the lichen's data (this study and Innocent et al., 1996 and unpublished data) and aerosol's data from Grousset et al. (1994) in a Pb/Pb vs. 1/Pb diagram. Fields for upper crust, Broken Hill ore, leaded gasoline in France and #y ashes from waste combustors are reported for comparison. In such a diagram, mixing between two sources having di!erent Pb concentrations and Pb isotopic compositions should de"ne a straight line. Lichens do not de"ne a mixing but rather form a sub-horizontal array
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F.J. Doucet, J. Carignan / Atmospheric Environment 35 (2001) 3681}3690
then record an average `industriala Pb-signal, independently of their concentrations. This industrial signal (with Pb/Pb"1.15) is probably the non-radiogenic end-member recorded by aerosols (Grousset et al., 1994). This value of 1.15 is compatible with the isotopic ratios measured in recently collected (1992) sphagnum mosses in Switzerland (Weiss et al., 1999) and in #y ashes collected from an incinerator in Geneva (Switzerland; Chiaradia and Cupelin, 2000). Supposing that the variation in Pb isotopic compositions observed for lichens collected in the Vosges and the Alps areas is the result of mixing between two major sources of Pb: (1) gasoline combustion and (2) industrial Pb emitted from waste combustors (or other plants), it would then be possible to calculate relative contributions of these sources in relation to the altitude of sampling. If the average industrial Pb in France has a Pb/Pb close to 1.15, between 60 and 80% of the total Pb in lichens from the Rhine valley would come from gasoline combustion, whereas 85}90% of the Pb would have an industrial origin in lichens from higher altitude in the Vosges mountains. Although lichens from the Alps were collected at higher altitude, the percentage of industrial Pb for these lichens would be slightly lower (65%). Major winds and convection winds in the di!erent valleys must then play an important role in terms of distribution of atmospheric metals in function of altitude. Fig. 5. Data for lichens from France (this study and Innocent et al., 1996 and unpublished data) and for aerosols from France (Grousset et al., 1994) in a Pb/Pb vs. 1/Pb diagram. Fields for upper crust (Taylor and McLennan, 1995; Sun, 1980), Broken Hill ore (Doe and Stacey, 1974), Saharan Holocene loess (Grousset et al., 1994), leaded gasoline in France (Roy, 1996) and #y ashes from waste combustors in France (our preliminary unpublished data) are also reported in the diagram. Linear arrays obtained with aerosols data and lichens data both intersect the #y ashes "eld at Pb/Pb"1.15.
suggesting a single source independently of the concentration. Only the most Pb-rich lichens ('10 ppm) have compositions approaching the leaded gasoline "eld. Although aerosols (1987}1992) data de"ne a linear array, it is evident that their composition does not result from a mixing between Pb from gasoline combustion (or Pb from Broken Hill ore) and natural crustal Pb. Only an aerosol from Paris collected in 1982 and one from the NW Mediterranean Sea collected in 1979 have Pb isotopic composition within the leaded gasoline "eld. An interesting fact is that the trends obtained with data for the 1987}1992 aerosols and for lichens both intersect the Pb "eld for waste combustors (Pb/Pb"1.15). On the contrary to aerosols collected in remote areas in France between 1987 and 1992, there is no evidence for any crustal component in lichens collected in di!erent regions of France between 1995 and 1997. Most lichens
4. Conclusions Trace metal concentrations and lead isotopic compositions have been measured in epiphytic lichens sampled along altitudinal sections in the Vosges mountains and the Alps, in the area of Annecy (Haute Savoie), France. The Pb isotopic ratios measured are correlated to the altitude of sampling particularly for lichens from the Vosges mountains. For most metals, excess relative to crustal abundance is measured in lichens suggesting that these are derived from anthropogenic activities. Based on Pb isotopes and Pb concentrations, the altitudinal section in the Vosges may be divided into three di!erent zones: (1) valley: Pb-rich and non-radiogenic ratios, (2) transition: low-Pb and intermediate isotopic compositions, (3) mountain: heterogeneous Pb concentrations but more radiogenic and homogeneous Pb isotopic composition. However, when normalised to each other, the other metals (Zn, Cd, Cu, As) have relatively homogeneous ratios along the sections and do not discriminate the di!erent zones. Except for Zn, the relative abundance of metals in lichens is very similar to that measured in rainwater and that calculated from estimated total atmospheric #uxes. The variation of Pb isotopic composition of lichens with altitude is interpreted as the result of mixing between Pb from gasoline combustion, (60}80% of the total
F.J. Doucet, J. Carignan / Atmospheric Environment 35 (2001) 3681}3690
Pb in valleys), and Pb from industrial activities, possibly emitted by solid waste combustors. At altitude, industrial Pb may account for 85}90% of the total Pb in lichens from the Vosges but only 65% for lichens from the Alps section. Acknowledgements Luc Marin, Guy Mevelle and Laurie Reisberg are acknowledged for their help in the laboratory. Financial support was provided by the CRPG and the CNRSService d'Analyse des Roches et des MineH raux. This is a CRPG contribution No. 1497. References Aberg, G., Pacyna, J.M., Stray, H., Skjelkvale, B.L., 1999. The origin of atmospheric lead in Oslo, Norway, studied with the use of isotopic ratios. Atmospheric Environment 33, 3335}3344. Boutron, C.F., GoK rlach, U., Candelone, J., Bolskov, M.A., Delmas, R.J., 1991. Decrease in anthropogenic lead, cadmium and zinc in Greenland snows since the late 1960s. Nature 353, 153}156. Carignan, J., Doucet, F., 1997. Isotopic composition of atmospheric lead in California as inferred by epiphytic lichens. EOS, Transactions, American Geophysical Union Abstracts 98 (46), 769. Carignan, J., GarieH py, C., 1995. Isotopic composition of epiphytic lichens as a tracer of the sources of atmospheric lead emissions in southern Quebec, Canada. Geochimica et Cosmochimica Acta 59 (21), 4427}4433. Chiaradia, M., Cupelin, F., 2000. Behaviour of airborn Pb and temporal variations of its source e!ects in Geneva (Switzerland): comparison of anthropogenic versus natural processes. Atmospheric Environment 34, 959}971. Deruelle, S., Lallemant, R., 1983. Les lichens teH moins de la pollution. Librairie Vuibert, Paris, 108. Doe, B.R., Stacey, J.S., 1974. The application of lead isotopes to the problems of ore genesis and ore prospect evaluation: a review. Economic Geology 69, 757}776. Fujita, S., Takahashi, A., Weng, J.H., Huang, L.F., Kim, H.K., Li, C.K., Huang, F.T.C., Jeng, F.T., 2000. Precipitation chemistry in East Asia. Atmospheric Environment 34, 525}537. Gobeil, C., Johnson, W.K., Macdonald, R.W., Wong, C.S., 1995. Source and burden of lead in St. Lawrence estuary sediments: isotopic evidence. Environmental Science and Technology 28, 193}201. Grousset, F.E., Quetel, C.R., Thomas, B., Buat-Menard, P., Donard, O.F.X., Bucher, A., 1994. Transient Pb isotopic signatures in the western European atmosphere. Environmental Science and Technology 28, 1605}1609. Halstead, J.R., Cunninghame, R.G., Hunter, K.A., 2000. Wet deposition of trace metals to a remote site in Fiordland, New Zealand. Atmospheric Environment 34, 655}676. Hopper, J.F., Ross, H.B., Sturges, W.T., Barrie, L.A., 1991. Regional source discrimination of atmospheric aerosols in Europe using the isotopic composition of lead. Tellus 43B, 45}60.
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Innocent, C., GarieH py, C., Carignan, J., 1996. Cd and Pb concentrations and Pb isotopic compositions of lichens from Western Europe: constraints on the sources of atmospheric heavy metals pollution. EOS, Transactions, American Geophysical Union, Fall Meeting 77, 46. Kazpard, V., 1997. GeH ochimie des preH cipitations collecteH es a` Nancy et des sources eH chantillonneH es en Lorraine. MeH moire de DEA } Physique et Chimie de la Terre de l'INPL, Institut National Polytechnique de Lorraine, p. 65. Lawrey, J.D., 1993. Lichens as monitors of pollutant elements at permanent sites in Maryland and Virginia. Bryologist 96, 339}341. Lawrey, J.D., Hale, M.E., 1988. Lichen evidence for changes in atmospheric pollution in Shenandoah National Park, Virginia. Bryologist 91, 21}23. Lucotte, M., Mucci, A., Hillaire-Marcel, C., Pichet, P., Grondin, A., 1995. Global anthropogenic mercury contamination in remote lakes of Northern QueH bec (Canada). Water, Air and Soil Pollution 80, 467}476. Manhe`s, G., Alle`gre, C.J., DupreH , B., Hamelin, B., 1980. Lead isotope study of basic-ultrabasic layered complexes: speculations about the age of the Earth and primitive mantle characteristics. Earth and Planetary Science Letters 47, 370}382. Monna, F., Lancelot, J., Croudace, I.W., Cundy, A.B., Lewis, J.T., 1997. Pb isotopic composition of airborne particulate material from France and Southern United Kingdom: implications for Pb pollution sources in urban areas. Environmental Science and Technology 31, 2277}2286. Murozumi, M., Chow, T.J., Patterson, C., 1969. Chemical concentrations of pollutant lead aerosols, terrestrial dusts and sea salts in Greenland and Antarctic snow strata. Geochimica et Cosmochimica Acta 33, 1247}1294. Narita, Y., Tanaka, S., Santosa, S.R., 1999. A study on the concentration, the distribution, and the behavior of metals in atmospheric particulate matter over the North Paci"c Ocean by using inductively coupled plasma mass spectrometry equipped with laser ablation. Journal of Geophysical Research 104, No. D21, 26859}26866. Nriagu, J.O., 1989. A global assessment of natural sources of atmospheric trace metals. Nature 338, 47}49. Nriagu, J.O., Pacyna, J.M., 1988. Quantitative assessment of worldwide contamination of air, water and soils by trace metals. Nature 333, 134}139. Pacyna, J.M., 1984. Estimation of the atmospheric emissions of trace elements from anthropogenic sources in Europe. Atmospheric Environment 18 (1), 41}50. Patterson, C.C., Settle, D.M., 1987. Review of data on eolian #uxes of industrial and natural lead to the lands and seas in remote region on a global scale. Marine Chemistry 22, 137}162. Rope, S.K., Pearson, L.C., 1990. Lichens as air pollution biomonitors in a semiarid environment in Idaho. Bryologist 93, 50}61. Roy, S., 1996. Utilisation des isotopes du Pb et du Sr comme traceurs des apports anthropiques et naturels dans les preH cipitations et les rivie`res du bassin de Paris. Doctorat de l'UniversiteH Denis Diderot } Paris 7 et de l'IPGP, Institut de Physique du Globe de Paris, p. 320. Shen, G.T., Boyle, E.A., 1987. Lead in corals: reconstruction of historical industrial #uxes to the surface ocean. Earth and Planetary Science Letters 82, 289}304.
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Sherrel, R.M., Boyle, E.A., Hamelin, B.J., 1992. Isotopic equilibration between dissolved and suspended particulate lead in the Atlantic Ocean: evidence from Pb-210 and stable Pb isotopes. Journal of Geophysical Research 97, 11257}11268. Simonetti, A., GarieH py, C., Carignan, J., 2000. Pb and Sr isotopic evidence for sources of atmospheric heavy metals and their deposition budgets in Northeastern North America. Geochimica et Cosmochimica Acta 64(20), 3439}3452. Sloof, J.E., Wolterbeek, H.T., 1991. National trace element air pollution monitoring survey using epiphytic lichens. Lichenologist 23, 139}165. Sturges, W.T., Barrie, L.A., 1989. The use of stable lead 206/207 isotope ratios and elemental composition to discriminate the origins of lead in aerosols at a rural site in Eastern Canada. Atmospheric Environment 28, 1645}1657. Sun, S.S., 1980. Lead isotopic study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs.
Philosophical Transactions of the Royal Society of London 297, 409}445. Taylor, S.R., McLennan, S.M., 1995. The geochemical evolution of the continental crust. Reviews of Geophysics 33 (2), 241}265. Trefry, J.H., Metz, S., Trocine, R.P., Nelsen, T.A., 1985. A decline in lead transport by the Mississipi river. Science 230, 439}441. VeH ron, A., Flament, P., Bertho, M.L., Alleman, L., Flegal, R., Hamelin, B., 1999. Isotopic evidence of pollutant lead sources in Northwestern France. Atmospheric Environment 33, 3377}3388. VeH ron, A., Lambert, C.E., Isley, A., Linet, P., Grousset, F., 1987. Evidence of recent lead pollution in deep north-east Atlantic sediments. Nature 326, 278}281. Weiss, D., Shotyk, W., Kramers, J.D., Gloor, M., 1999. Sphagnum mosses as archives of recent and past atmospheric lead deposition in Switzerland. Atmospheric Environment 33, 3751}3763.
Atmospheric Pb isotopic composition and trace metal concentration as revealed by epiphytic lichens: an investigation related to two altitudinal sections in Eastern France F.J. Doucet, J. Carignan* CNRS - CRPG, 15 rue Notre-Dame Les Pauvres, BP20, 54501 Vandoeuvre-les-Nancy, France Received 27 July 1999; received in revised form 21 September 2000; accepted 7 November 2000
Abstract During Fall 1996, epiphytic lichens were collected along altitudinal sections in two areas of France (the Vosges mountains in the North-East, and the Alps, in Haute-Savoie) in order to verify any geographic distribution of atmospheric metals on a small scale. These lichens have various Pb isotopic compositions (Pb/Pb"1.126}1.147) which are correlated with the altitude of sampling. Lichens sampled near valleys display isotopic ratios signi"cantly less radiogenic than those sampled at several hundred to thousand meters of altitude. In the Vosges sections, Pb concentrations and isotopic compositions of lichens may be used to de"ne three zones: (1) valley: Pb-rich and non-radiogenic ratios, (2) transition: low-Pb and intermediate isotopic compositions, (3) mountain: heterogeneous Pb concentrations but more radiogenic and homogeneous Pb isotopic composition. Other metals (Zn, Cu, Cd, As), when normalised one to another, are not fractionated between these zones and display homogeneous relative abundance along the altitudinal sections of both sites. Variation of Pb/Pb ratios with altitude is interpreted in terms of mixing of at least two pollution sources: one being the petrol (leaded and/or unleaded) combustion, and the other being of industrial origin. The latter is characterised by a more radiogenic isotopic composition. The Pb isotopic composition of #ue gas residues from di!erent municipal solid waste combustors in the Rhine valley and in other areas of France would suggest that these plants might be an important source of industrial Pb in the atmosphere. If the average industrial Pb in France has a Pb/Pb close to 1.15, between 60 and 80% of the total Pb in lichens from the Rhine valley would come from gasoline combustion, whereas 85}90% of the Pb would have an industrial origin in lichens from higher altitude in the Vosges mountains. Although lichens from the Alps were collected at higher altitude, the percentage of industrial Pb for these lichens would be slightly lower (65%). Major winds and convection winds in the di!erent valleys must then play an important role in term of distribution of atmospheric Pb in function of altitude. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Atmosphere; Lead; Metals; Lichens; Pollution
1. Introduction * Corresponding author. Fax: #33-03-8351-1798. E-mail addresses: [email protected] (F.J. Doucet), [email protected] (J. Carignan). Present address: Department of Chemistry, Birchall Centre for Inorganic Chemistry and Materials Science, Keele University, Sta!ordshire ST5 5BG, UK.
Since the middle of the 20th century, industrial production (e.g. metal re"ning, waste incineration burning of coal and wood) and vehicular tra$c (leaded petrol combustion) have released large amounts of heavy metals (Pb, Cu, Zn, Cd, Ni) into the atmosphere (e.g. Patterson and Settle, 1987; Nriagu and Pacyna, 1988).
1352-2310/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 5 1 0 - 0
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Two approaches have been used to identify sources of metals and other pollutant elements in the atmospheric environment. One involves identifying di!erences in the multi-elemental composition of atmospheric aerosols (e.g. Narita et al., 1999) and precipitation (e.g. Fujita et al., 2000). The other approach exploits di!erences in the lead isotopic composition (e.g. Hopper et al., 1991; Monna et al., 1997; Aberg et al., 1999; VeH ron et al., 1999; Chiaradia and Cupelin, 2000). However, it is believed that the two approaches together are necessary for a more precise identi"cation of atmospheric pollution sources (e.g. Sturges and Barrie, 1989; Halstead et al., 2000; Simonetti et al., 2000). The serious increase in world-wide environmental contamination caused by trace metals is well documented (e.g. Nriagu and Pacyna, 1988; Nriagu, 1989). Over the past two decades, the progressive restriction on lead addition in gasoline has resulted in a signi"cant decrease of the global atmospheric lead burden, as recorded in ice from Greenland (Murozumi et al., 1969; Boutron et al., 1991), in corals from Bermuda (Shen and Boyle, 1987), in Mississippi deltaic surface sediments (Trefry et al., 1985), lake and estuary sediments (Lucotte et al., 1995; Gobeil et al., 1995), herbarium specimens (Weiss et al., 1999) and in the Sargasso Sea waters (Sherrel et al., 1992). There has been a general variation in aerosol lead isotopic signatures and lead concentration in France during the last 15 yr (VeH ron et al., 1987; Grousset et al., 1994), interpreted as resulting from the decrease of atmospheric pollution by Pb. According to Grousset et al. (1994), the Pb isotopic signatures of aerosols over southern Europe reveal the presence of two di!erent sources: one fraction would be the natural lead contained in crust-derived particles, mostly from the Sahara Desert; the other fraction would be anthropogenic European derived mostly from gasoline consumption. A feature of this previous study is that the analysed aerosols were sampled in both urban and mountainous sites. However, data available in the literature on the composition of aerosols prior to ca. 1985 concern mostly samples collected in urban area, and most samples collected after 1985 are from remote areas (see the review in Grousset et al., 1994). Although VeH ron et al. (1999) have documented such evolution of Pb isotopic ratios for aerosols collected in Northern France between 1983 and 1994, some studies showed that chemical and isotopic gradients of atmospheric Pb may exist between urban and rural areas of North America (Sturges and Barrie, 1989; Carignan and Doucet, 1997). The present study aims to verify any variation in the distribution of atmospheric metals on a small geographic scale, in particular in relation to the altitude of the sampling site. For this, epiphytic lichens collected on tree branches were used as natural "lters of atmospheric matter, since epiphytic lichens derive moisture and nutrients exclusively from air. Indeed, the use of epiphytic
lichens as bio-monitors for trace element air pollution is widely acknowledged (e.g. Lawrey and Hale, 1988; Rope and Pearson, 1990; Sloof and Wolterbeek, 1991; Lawrey, 1993; Carignan and GarieH py, 1995). Therefore, it is assumed that the chemical composition of epiphytic lichens can be used to estimate that of the atmosphere. The principal accumulation mechanisms are via extracellular complexing of metals by ion-exchange processes, by particle entrapment and by direct uptake of gases (Deruelle and Lallemant, 1983).
2. Samples and methods During Fall 1996, epiphytic lichen samples were collected on tree branches along altitudinal sections from two areas of France, the Vosges mountains in the NorthEast (Haut-Rhin) and the Alps in the Centre}East (HauteSavoie). The "rst area (the Vosges) is located West of the Valley of the Rhine River. Lichens have been collected between altitudes of 200 and 1300 m. Samples were collected along a small road called `la Route des Cre( tesa, from the valley (200 m in altitude) to the `col de la Schluchta and the `col du Bonhommea. The second area (the Alps) is located close to the Annecy Lake. Samples were collected along a small local road on the East #ank of the Semnoze mountains between altitudes of 1700 and 1000 m. A sample was also collected in the valley, south of the Annecy Lake in a recreation park (450 m in altitude). With the exception of the valleys themselves, these two areas are mostly uninhabited, and remote from industries and highways or national roads. The epiphytic lichens used for analysis were fruticulose specimens of Evernia sp. and/or Usnea sp. The lichens were collected from trees near the tip of tree branches less than a few millimeters in diameter, whenever possible. This was done to obtain lichens that were exposed to the atmospheric signal for a time period as short as possible (i.e., less than a few years). Samples were taken with pre-cleaned plastic tweezers, and transferred to sealedplastic bags. In the laboratory, the lichen samples were immediately separated from their substratum, warmdried at 1053C for 4 h, and stored in hermetic vessels. Grinding and homogenisation of lichen samples is di$cult and need a lot of manipulation. Because of possible contamination during the preparation, the samples were not powdered. Between 50 and 100 mg of an aliquot of small lichen branches were directly transferred to Te#on威 capsules and digested in a mixture of HNO , H O and HF. Results obtained on two aliquots of three di!erent lichen samples yielded sometimes small di!erences in concentrations (5}25% relative di!erences) but very similar metal/metal ratios. Heavy metal concentrations were measured using a Perkin-Elmer ICP-MS SCIEX ELAN 5000. Reagent blanks were carried through all procedures and were negligible in all cases.
F.J. Doucet, J. Carignan / Atmospheric Environment 35 (2001) 3681}3690
using anion-exchange chromatography (Manhe`s et al., 1980). All lead isotope ratios were measured using a TIMS Finnigan MAT 262, operated in multi-collection mode. A mass fractionation correction of 0.12% amu\ was applied to all ratios. Repeated measurements of the NBS 981 Pb standard material yielded uncertainties (2) lower than 0.1, 0.2, 0.3, and 0.4% for the Pb/Pb, Pb/Pb, Pb/Pb, and Pb/Pb ratios, respectively.
Table 1 Certi"ed values and measured concentrations of the BCR-CRM 482 lichens standard Elements
Certi"ed values (ppm)
Measured values (ppm)
Al Cd Cu Ni Pb Zn
1103$24 0.56$0.02 7.03$0.19 2.47$0.07 40.9$1.4 100.6$2.2
1187$15 0.46$0.04 6.83$0.19 2.33$0.17 39.6$0.6 102.5$1.7
3683
3. Results and discussion
Each value is the mean of four replicates. Concentrations are reported on a dry weight base. The $ values are S.Ds.
All the results on trace element concentrations and Pb isotopic compositions measured in lichens are reported in Table 2.
The accuracy of the method was veri"ed by applying it to a lichen standard reference material (BCR-CRM 482: Community Bureau of Reference CRM 482; Table 1). Lead separation for isotopic measurements was done
3.1. Pb isotope and trace element variations with altitude Fig. 1 shows the evolution of the Pb/Pb ratios measured in lichens in relation to their altitude of
Table 2 Trace element concentrations (ppm) and Pb isotopic composition in lichens from the studied areas Altitude (m)
Lichen
Vosges area 900 1100 900 510 1100 800 560 510 800 200 855 1050 630 430 900 510 700 1300 900 320
Evernia sp. Evernia sp. Usnea sp. Evernia sp. Evernia sp. fruticuleux Evernia sp. tapis (chou) fruticuleux Evernia sp. Evernia sp. Evernia sp. Folioses Evernia sp. Evernia sp. Usnea sp. Evernia sp. Evernia sp. Evernia sp. Mixed
Alps area Valley 1075 1300 1400 1400 1468 1468 1700 1700
Evernia sp. Evernia sp. Evernia sp. ? Evernia sp. Evernia sp. Usnea sp. Usnea sp. Evernia sp.
n.d.: not determined.
Pb/Pb
Pb/Pb
1.145
2.113
1.140 1.147
2.119 2.112
1.141 1.147 1.127 1.147
2.118 2.113 2.131 2.113
1.141 1.144 1.136 1.147 1.146 1.146 1.145
2.118 2.116 2.123 2.111 2.112 2.113 2.114
1.128 1.140
2.132 2.118
1.136
2.123
1.136 1.133
2.123 2.125
Al
As
Cd
Cr
Cu
Ni
Pb
Zn
46 131 38 90 296 271 213 333 221 1625 238 298 175 258 208 69 237 751 208 563
0.37 0.23 0.29 0.14 0.47 0.41 0.37 0.26 0.70 1.11 0.56 0.58 0.49 0.39 0.26 0.13 0.51 1.48 0.39 0.50
0.23 0.24 0.19 0.25 0.43 0.33 0.13 0.31 0.46 0.38 0.55 0.87 0.16 0.18 0.02 0.12 0.24 0.91 0.15 0.44
1.31 0.90 0.31 0.60 1.27 0.67 0.85 1.01 0.95 3.73 0.53 1.31 0.65 0.42 0.10 n.d. 0.72 1.65 1.40 1.65
4.22 2.44 2.76 1.59 4.82 4.27 3.41 3.29 4.80 11.97 3.57 4.02 5.34 2.67 1.85 1.53 3.55 6.69 2.26 5.15
0.89 0.68 0.48 0.32 1.21 0.53 0.69 0.48 0.92 3.00 2.97 1.25 0.86 0.84 0.81 0.32 0.66 2.41 0.89 2.58
8.6 10.7 5.5 2.6 13.4 5.5 4.5 5.0 10.9 69.8 13.2 15.9 9.4 3.7 2.9 4.7 5.1 23.5 3.6 8.2
61.6 21.6 15.8 29.2 63.2 60.8 30.3 21.7 40.0 72.5 32.6 59.6 36.6 21.4 16.1 12.1 21.1 67.4 25.3 30.3
460 311 259 260 270 472 219 87 341
0.80 0.33 0.19 0.81 0.68 0.32 0.45 0.78 0.76
0.72 0.27 0.24 0.44 0.40 0.22 0.36 0.26 0.44
6.01 1.47 0.58 1.29 0.48 1.70 0.92 2.58 2.51
7.58 3.51 2.67 5.35 5.46 3.70 4.21 3.68 8.36
7.56 5.42 4.84 22.6 5.31 5.23 5.34 4.40 6.44
17.8 5.6 5.1 8.7 13.3 8.0 6.8 5.9 28.3
72.0 49.6 32.8 55.0 74.3 56.1 51.4 43.5 112
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F.J. Doucet, J. Carignan / Atmospheric Environment 35 (2001) 3681}3690
Fig. 1. The Pb/Pb ratios measured in lichens against the altitude of the sampling site. (a) cross section in the Vosges Mountains; (b) cross section in the Alps (Haute Savoie).
sampling. Lichens sampled in the valleys of both studied areas are characterised by the lowest Pb/Pb ratio, having a composition close to that of leaded (Pb/ Pb"1.109$0.004; average for six di!erent distributors) and unleaded (Pb/Pb"1.124) gasoline used in France (Roy, 1996). On the other hand, samples collected at altitude are characterised by higher Pb/Pb ratios, which suggest the contribution of more radiogenic sources for atmospheric Pb. The isotopic composition of lichens from the Haute Savoie area do not vary systematically with altitude from 1000 to 1700 m; they rather yield relatively homogeneous Pb/Pb ratios (1.136$0.003). Lichens from the Vosges area display an evident correlation between their isotopic composition and the altitude at which they were sampled. Lichens sampled between 700 and 1300 m have a very homogeneous composition with an average Pb/Pb ratio of 1.146$0.001. Lichens located at mid-altitude, from 350 to 500 m, are less homogeneous in composition (Pb/Pb ratio of 1.141$0.003), and may represent a transition zone between the valley signal and that of higher altitude. The variation of the Pb isotopic composition of lichens with altitude may then
result from a mixing of at least two sources of Pb. The least radiogenic one, at the valley bottoms, is probably Pb emitted from petrol combustion, and the more radiogenic one, at altitude, is of an unknown origin. Either the radiogenic source is not homogeneous in composition at the regional scale (Pb/Pb of 1.146$ 0.001 and 1.136$0.003 for the Vosges and the Alps, respectively), or the proportions of the sources in altitude are di!erent at the two localities. One way to discriminate the contribution of anthropogenic sources with respect to natural sources for heavy metals in the environment is to normalize metal concentrations to Al concentration and to compare the ratio obtained to that of upper crustal rocks. The contribution of anthropogenic sources is expressed as the enrichment factor (EF"[metal/Al] /[metal/Al] ) of a given metal. According to the metal EF's (Table 3), between 98.5 and 99.9% of the total Pb measured in lichens would be in excess relative to crustal Pb. This means that the isotopic composition of lichens may directly be taken as that of atmospheric pollution Pb and that variations of Pb/Pb ratios re#ect di!erent proportions of two or more pollution sources with distinctive lead isotopic composition. The Pb and Al concentrations measured in lichens (Vosges) in function of their sampling altitude are reported in Fig. 2(a). From the valley (200 m) to an altitude of about 400}500 m, both Pb and Al concentrations decrease systematically. From 400 to 1300 m, Al concentrations stay relatively constant (mean$S.E: 240$50 ppm) whereas Pb concentrations, on average, increase systematically from (3}5) to 15}20 ppm. Unfortunately, the Pb/Al ratios in lichens are not correlated with altitude, probably because of the scatter of the data in the di!erent trends. This means that the shift in isotopic composition do not simply represent the fading out of one of the source (Pb from the combustion of gasoline in the valley for example). Other metals also having high EF's, such as Cd and Zn, have a concentration distribution similar to that of Pb in Fig. 2(a), whereas As and Cu display a behaviour rather similar to Al (see Table 3 for metal EF's). Cr and Ni are metals having the lowest EF's and no particular correlation is observed between their concentrations and the altitude. Fig. 2(b) presents Pb concentrations in correlation with the Pb isotopic composition for lichens sampled in the Vosges area. The data were divided according to their observed distribution in the Pb/Pb vs. altitude diagram of Fig. 1: valley, transition zone and mountains. Lichens from the transition zone have a large range of isotopic composition but fairly uniform and low Pb concentrations (3$1.5 ppm). On the other hand, lichens from the mountain zone have homogeneous isotopic ratios and present a wide range of Pb concentrations (3.5}23.5 ppm). Assuming that all the lichens have accumulated the atmospheric signal for about the same period of time, and that they have similar EF's, their total Pb concentration
F.J. Doucet, J. Carignan / Atmospheric Environment 35 (2001) 3681}3690
3685
Table 3 Enrichment factors measured in lichens from the studied areas Altitude (m)
Lichen
EF-As
EF-Cd
EF-Cr
EF-Cu
Vosges area 900 1100 900 510 1100 800 560 510 800 200 855 1050 630 430 900 510 700 1300 900 320
Evernia sp. Evernia sp. Usnea sp. Evernia sp. Evernia sp. fruticuleux Evernia sp. tapis (chou) fruticuleux Evernia sp. Evernia sp. Evernia sp. folioses Evernia sp. Evernia sp. Usnea sp. Evernia sp. Evernia sp. Evernia sp. mixed
428 95 410 86 86 81 93 42 169 37 125 105 149 80 68 103 115 106 100 48
4173 1484 3994 2288 1192 987 493 766 1704 194 1899 2401 750 579 91 1427 831 994 592 634
65 16 18 15 10 6 9 7 10 5 5 10 9 4 1 n.d. 7 5 16 7
295 60 234 57 52 51 51 32 70 24 48 43 98 33 29 71 48 29 35 29
Alps area Valley 1075 1300 1400 1400 1468 1468 1700 1700
Evernia sp. Evernia sp. Evernia sp. ? Evernia sp. Evernia sp. Usnea sp. Usnea sp. Evernia sp.
94 56 39 168 135 36 109 481 120
1291 698 749 1396 1201 377 1357 2484 1067
30 11 5 11 4 8 10 68 17
53 36 33 66 65 25 62 136 79
EF-Ni
EF-Pb
EF-Zn
78 21 51 14 16 8 13 6 17 7 50 17 20 13 16 19 11 13 17 18
747 329 582 117 181 81 84 61 199 173 223 214 216 57 55 273 86 126 69 59
1516 187 471 367 242 254 161 74 205 51 155 226 237 94 88 199 101 102 138 61
66 70 75 350 79 45 98 203 76
156 72 79 135 198 68 125 270 334
177 180 143 240 311 134 266 565 373
n.d.: not determined.
would then re#ect the total atmospheric #ux. Hence, the altitudinal section of the Vosges Mountains may be divided in three di!erent domains, not only by the isotopic composition of lichens but also in terms of global atmospheric #uxes. These domains are: (i) the valley, having the highest #uxes for all the studied metals; (ii) a transition zone, having the lowest #uxes for many studied metals; (iii) the mountain, having intermediate metal #uxes but homogeneous Pb isotopic composition suggesting a single source. The distribution of di!erent metal/metal ratios measured in lichens is presented in relation with the altitude at which they were sampled in Figs. 3(a)}(c). At both localities, Vosges and Alps, Cu/As and Zn/Cd ratios yield similar average values and present no systematic in function of altitude (Figs. 3(b) and (c)). On the other hand, Zn and Cu normalised to Pb concentrations form a convex pattern when plotted against altitude. This pattern is
particularly visible for the Vosges samples, for which a second-degree regression "t is shown in Fig. 3(a). This distribution is somewhat the inverse of the one obtained in Fig. 2(a) for Pb concentrations against altitude, and is probably controlled by the largest variation of the Pb #ux in comparison to other metals or to the decoupling of sources. In any case, the di!erent sources enlightened by Pb isotopes do not seem to be discriminated by metal/metal ratios. 3.2. Sources of metals in lichens Relative metal abundance could be used to trace sources of emissions. However, there are a lot of processes between the locus of emission and the precipitation and accumulation of metals that may lead to fractionation of elements. For example, these processes may be (1) various atmospheric residence times, (2) di!erent solubility of
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F.J. Doucet, J. Carignan / Atmospheric Environment 35 (2001) 3681}3690
Fig. 2. (a) Pb and Al concentrations (ppm) in lichens from the Vosges Mountains in function of altitude. (b) Pb concentration vs. Pb/Pb with di!erent domains as outlined from the Pb/Pb vs. altitude of Fig. 1.
elements, and (3) fractionation during up-take and bioaccumulation. One way to estimate if bio-accumulation processes fractionate metals from each other between the atmosphere and the lichens would be to compare their chemical composition to that of rainwater. For example, Zn/Pb, Cu/Pb, and Zn/Cd ratios measured in lichens (Fig. 3) yield average and variation of ratios very similar to those measured in rain samples from Paris (Roy, 1996) and Nancy in NE France (Kazpard, 1997). On the other hand, although the Cu/As ratio measured in lichens for both studied areas is homogeneous, this ratio is highly heterogeneous in rain samples (2}400; Kazpard, 1997) and average at more than 50. However, the median value yield Cu/As of 9 in rain from Nancy, which is much closer to the Cu/As ratio measured in lichens (9}9.5; Fig. 3(b)). This suggests that the lichen's chemical composition represents that of the rain and that no signi"cant bio-accumulation fractionation occurs within the variation observed. Except for Zn, metal/metal ratios (Pb, Cd, Cu) measured in lichens are also in the range of ratios calculated from total atmospheric #uxes estimated from Pacyna (1984) for Europe, Nriagu and Pacyna (1988) and Nriagu
Fig. 3. Relative abundance of metals measured in lichens in function of altitude of sampling. Lines through data points in Fig. 3(a) are second degree "ts showing variations in the Zn/Pb and Cu/Pb ratios observed for the Vosges samples. These variations are mainly attributed to the larger variation of Pb concentrations along the sections.
(1989) for the global #uxes and Simonetti et al. (2000) for NE North America. The Zn/Pb ratio calculated from estimated global emissions (0.4$0.27; Nriagu and Pacyna, 1988), is much lower than Zn/Pb ratios measured in lichens from France and precipitation samples from France and North America (5}10 on average; Simonetti et al., 2000; Kazpard, 1997). Furthermore, total #uxes of Zn and Pb calculated recently for NE North America would correspond to Zn/Pb ratios of 80 to more
F.J. Doucet, J. Carignan / Atmospheric Environment 35 (2001) 3681}3690
than 500 (Simonetti et al., 2000). High depositional budgets of Zn in the late 1990s may re#ect either an increase in atmospheric industrial emissions (since 1980s), a lower residence time in the atmosphere, or particular chemical speciation which a!ects the deposition of Zn from the lower troposphere (Simonetti et al., 2000). Anyhow, if the di!erence between the Zn/Pb ratio measured in precipitation and the one calculated using total #uxes is simply due to the importance of dry deposition of Zn, this would imply that the uptake of elements by lichens fractionate the dissolved and particulate fractions from `weta and `drya deposition. This may have implications for the interpretation of Pb isotopes measured in lichens if dissolved and particulate Pb in rainwater are not in isotopic equilibrium. Roy (1996) have shown that dissolved and total (dissolved#particulate) Pb phases in a given rain sample from Paris yield the same isotopic composition, the dissolved phase representing between 4 and 65% of the total Pb. We will then make the assumption that dissolved and particulate Pb in rainwater are e!ectively in isotopic equilibrium and that the composition of lichens represent that of total atmospheric Pb for a given area. In Fig. 4, the Pb isotopic composition of lichens from this study is compared with data reported in the literature: (i) wet and dry aerosols from di!erent areas of western Europe (Grousset et al., 1994); (ii) dry aerosols (1982}1995) from northern France (VeH ron et al., 1999); (iii) lichens sampled close to main roads in France (Innocent et al., 1996); (iv) rainwater (1993) sampled in Paris (Roy, 1996). The isotopic composition of lead ores from Broken Hill, Australia, used as anti-knock additives in French gasoline (80% of the total Pb added in 1994}1995; VeH ron et al., 1999), measured Pb isotopic ratios in French gasoline (Roy, 1996), the isotopic composition of Saharan dust (Grousset et al., 1994) and pre-industrial marine sediments (Sun, 1980) and preliminary results on #y ashes from di!erent waste combustors in France have also been reported in Fig. 2. As stated in the introduction, Grousset et al. (1994) have documented a general variation in lead isotopic compositions measured in aerosols over the last 15 yr in France re#ecting a decrease of the atmospheric pollution over this time scale. A detailed study of VeH ron et al. (1999) reported similar variations for aerosols sampled in northern France between 1982 and 1995. These authors attributed this shift in isotopic composition to the more and more importance of industrial sources relative to the burning of leaded gasoline. The isotopic ratios measured in lichens appears signi"cantly less radiogenic than those obtained by Grousset et al. (1994) on aerosols sampled in remote areas (French Alps) for the 1990s period, but in the range of aerosols collected in remote areas between 1985 and 1990 (Grousset et al., 1994), of aerosols collected in the more industrialised area of Northern France in 1994 (VeH ron et al., 1999) and of rainwater collected in
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Fig. 4. Pb}Pb isotope diagram comparing lichens data with rainwater and aerosols data from the literature (references are indicated in the legend). Fields for the Broken Hill ore (Doe and Stacey, 1974; Pb additive in gasoline), for Saharan Holocene loess (Grousset et al., 1994) and pre-industrial sediments (Sun, 1980) and for #y ashes from waste combustors in France (our preliminary unpublished data) were reported for comparison.
Paris in 1993 (Roy, 1996). Lichens collected near valleys in the Vosges and Alps areas have a Pb isotopic composition towards that of lichens sampled near main roads elsewhere in France (Innocent et al., 1996) and towards that of unleaded and leaded gasoline used in France (Roy, 1996). Lichens sampled in altitude, in particular those from the Vosges mountains, have isotopic compositions very similar to that of #ue-gas residues (#y ashes) from di!erent municipal solid waste combustors in the Rhine valley and in other areas of France (Fig. 4). Lead, and other metals, emitted from these waste combustors is then a possible major end-member to account for the Pb isotopic variation observed in Fig. 4, not only for the Vosges area but also in the Paris and Northern France regions. As di!erent possible sources have Pb isotopic compositions falling on the single trend de"ned in Fig. 4, it is di$cult to estimate the various proportions of one and the others. Although VeH ron et al. (1999) have shown deviation from this trend for aerosols sampled near some industrial plants in the Nord-Pas de Calais region (France), it might be useful to put in correlation the isotopic composition and the Pb concentration of the di!erent samples. Fig. 5 presents the lichen's data (this study and Innocent et al., 1996 and unpublished data) and aerosol's data from Grousset et al. (1994) in a Pb/Pb vs. 1/Pb diagram. Fields for upper crust, Broken Hill ore, leaded gasoline in France and #y ashes from waste combustors are reported for comparison. In such a diagram, mixing between two sources having di!erent Pb concentrations and Pb isotopic compositions should de"ne a straight line. Lichens do not de"ne a mixing but rather form a sub-horizontal array
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F.J. Doucet, J. Carignan / Atmospheric Environment 35 (2001) 3681}3690
then record an average `industriala Pb-signal, independently of their concentrations. This industrial signal (with Pb/Pb"1.15) is probably the non-radiogenic end-member recorded by aerosols (Grousset et al., 1994). This value of 1.15 is compatible with the isotopic ratios measured in recently collected (1992) sphagnum mosses in Switzerland (Weiss et al., 1999) and in #y ashes collected from an incinerator in Geneva (Switzerland; Chiaradia and Cupelin, 2000). Supposing that the variation in Pb isotopic compositions observed for lichens collected in the Vosges and the Alps areas is the result of mixing between two major sources of Pb: (1) gasoline combustion and (2) industrial Pb emitted from waste combustors (or other plants), it would then be possible to calculate relative contributions of these sources in relation to the altitude of sampling. If the average industrial Pb in France has a Pb/Pb close to 1.15, between 60 and 80% of the total Pb in lichens from the Rhine valley would come from gasoline combustion, whereas 85}90% of the Pb would have an industrial origin in lichens from higher altitude in the Vosges mountains. Although lichens from the Alps were collected at higher altitude, the percentage of industrial Pb for these lichens would be slightly lower (65%). Major winds and convection winds in the di!erent valleys must then play an important role in terms of distribution of atmospheric metals in function of altitude. Fig. 5. Data for lichens from France (this study and Innocent et al., 1996 and unpublished data) and for aerosols from France (Grousset et al., 1994) in a Pb/Pb vs. 1/Pb diagram. Fields for upper crust (Taylor and McLennan, 1995; Sun, 1980), Broken Hill ore (Doe and Stacey, 1974), Saharan Holocene loess (Grousset et al., 1994), leaded gasoline in France (Roy, 1996) and #y ashes from waste combustors in France (our preliminary unpublished data) are also reported in the diagram. Linear arrays obtained with aerosols data and lichens data both intersect the #y ashes "eld at Pb/Pb"1.15.
suggesting a single source independently of the concentration. Only the most Pb-rich lichens ('10 ppm) have compositions approaching the leaded gasoline "eld. Although aerosols (1987}1992) data de"ne a linear array, it is evident that their composition does not result from a mixing between Pb from gasoline combustion (or Pb from Broken Hill ore) and natural crustal Pb. Only an aerosol from Paris collected in 1982 and one from the NW Mediterranean Sea collected in 1979 have Pb isotopic composition within the leaded gasoline "eld. An interesting fact is that the trends obtained with data for the 1987}1992 aerosols and for lichens both intersect the Pb "eld for waste combustors (Pb/Pb"1.15). On the contrary to aerosols collected in remote areas in France between 1987 and 1992, there is no evidence for any crustal component in lichens collected in di!erent regions of France between 1995 and 1997. Most lichens
4. Conclusions Trace metal concentrations and lead isotopic compositions have been measured in epiphytic lichens sampled along altitudinal sections in the Vosges mountains and the Alps, in the area of Annecy (Haute Savoie), France. The Pb isotopic ratios measured are correlated to the altitude of sampling particularly for lichens from the Vosges mountains. For most metals, excess relative to crustal abundance is measured in lichens suggesting that these are derived from anthropogenic activities. Based on Pb isotopes and Pb concentrations, the altitudinal section in the Vosges may be divided into three di!erent zones: (1) valley: Pb-rich and non-radiogenic ratios, (2) transition: low-Pb and intermediate isotopic compositions, (3) mountain: heterogeneous Pb concentrations but more radiogenic and homogeneous Pb isotopic composition. However, when normalised to each other, the other metals (Zn, Cd, Cu, As) have relatively homogeneous ratios along the sections and do not discriminate the di!erent zones. Except for Zn, the relative abundance of metals in lichens is very similar to that measured in rainwater and that calculated from estimated total atmospheric #uxes. The variation of Pb isotopic composition of lichens with altitude is interpreted as the result of mixing between Pb from gasoline combustion, (60}80% of the total
F.J. Doucet, J. Carignan / Atmospheric Environment 35 (2001) 3681}3690
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