Environ. Sci. Technol. 1994, 28, 1605-1608
Transient Pb Isotopic Signatures in the Western European Atmosphere Francis E. Grousset,*~tChrlstophe R. Qubtel,* Bertrand Thomas,* Patrick Buat-M(nard,t Olivier F. X. Donard,* and Alain Bucherl Departement de Ghologie et Oceanographie, URA CNRS 197, Universite Bordeaux I, Avenue des Facultes, 33405 Talence-Cedex, France, Laboratoire de Photophysique et Photochimie Mol6culaire, URA CNRS 348, Universite Bordeaux I, 351 Cours de la Liberation, 33405 Talence-Cedex, France, and Observatoire du Pic du Midi et de Toulouse, 65200 Bagnhres-de-Bigorre, France
The progressive phasing-out of lead from gasoline has resulted in a significant decrease in the global atmospheric lead burden over the last 20 years. Here we show that in Europe this change in lead concentration-determined by the analysis of aerosols-is accompanied by a systematic change in lead isotopic compositions and that there was a significant reversal in this trend during the 1970s. Such changes in isotopic signatures over time have been accurately documented in North America. We observe that European and North American records display opposite trends in isotopic composition, thus providing a powerful tool for assessing the response time of the North Atlantic environment and its surrounding continents to transient lead inputs. This database could help to better constrain the time scale of coastal and upper ocean mixing processes and of particle transport to marine sediments.
ZOO 0
Introduction 206Pb/Z07Pb
Over the last 20 years, the progressive phasing-out of lead from gasoline exhausts has resulted in a significant decrease in the global atmospheric lead burden, as recorded in Greenland ice (1, 2), in annually banded corals from Bermuda (3),in Mississippi deltaic surface sediments ( 4 ) , and for the most recent period in the Sargasso Sea waters (5). Recently, it has been shown that the concomitant shift in lead isotopic composition (6) provides a powerful tool to assess the response time of the environment of the North Atlantic and its surrounding continents to transient lead inputs. Over time, isotopic signatures from source regions have only been accurately documented in North America (3, 5, 7-10). In this paper, we reconstruct the atmospheric lead isotope distribution in western Europe during the past =15 years, so determined by the analysis of aerosols collected a t different intervals. Such a database could help to better constrain the time scale of coastal and upper ocean mixing processes and of particle transport to marine sediments. S a m p l e s and M e t h o d s
The worldwide invasion of the earth's environment by anthropogenic heavy metals is now well-documented ( 11). For the last few decades, the anthropogenic lead emitted from smelters and automobile exhausts has been dominant compared to lead coming from soil dust and volcanic gases (7). Since lead is mostly adsorbed on particles and
* Address correspondence to this author; e-mail address: grousset age0cean.u-bordeaux.fr. + DBpartement de GBologie et OcBanographie,UniversitB Bordeaux I. t Laboratoire de Photophysique e t Photochimie MolBculaire, UniversitB Bordeaux I. I Observatoire du Pic d u Midi et de Toulouse. 0013-936X/94/0928-1605$04.50/0
0 1994 American Chemical Society
T 1.18..
I Error
0
1.161.141.121.101978
1980
1982
1984
1986
1988
1990
1992
years
Flgure 1. Evolution since 1979 of concentrations (pg g-I) and isotopic composition of the European atmospheric lead. Along with our data (closed symbols), we plotted data from the literature (open symbols): aerosols from London ( 14, Paris (7, 79, 20), Belgium (7), and the northwestern Mediterranean Sea ( 75, 22). Solid curves indicate regression lines. Relative standard deviations (RSD) obtained on lead concentration measurementsare =& 10%. Mean error bars only have been calculated for our resuits (closed symbols).
scattered by aeolian transport, we decided to analyze a time series of aerosol samples to see if they record pollutant lead evolution over time. Aerosol dust samples were collected in different parts of western Europe-mostly in France-almost every year from 1979 to 1992. In order to document the regional variability of the lead composition, both urban and mountain samples were used. The urban samples were collected in Paris, Lyon, and Belgium; the mountain samples were collected in southern France (Alps, PyrBnBes, and Massif-Central). Red dusts originating from the Sahara are frequently spread over Europe (12,131and were sampled in the French Mountains almost every year, using the same clean sampling techniques. The simultaneous collection of rain samples and associated dust allowed us to collect both dry and wet fallouts. Further samples were taken from the northern Mediterranean Sea (14). Environ. Sci. Technol., Vol. 28, No. 9, 1994
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I _ _
Table 1. Lead Concentration ( r g gL) and Isotopic Ratios location
country
years
Paris Paris Munich Cardiff Londres Londres Tyrrhenian Sea Paris Wimereux Gravelines Paris Wimereux Ligurian Sea Marseilles Gravelines Alps Alps Leuven Alps Alps Alps Pyrenees Alps CBvennes Alps Alps Alps Alps Alps Alps Matmata loess
France France south Germany south England south England south England NW Med. Sea northern France northern France northern France France northern France NW Med. Sea northern France northern France southeast France southeast France Belgium southeast France southeast France southeast France southern France southeast France southern France southeast France southeast France southeast France southeast France southeast France southeast France southern Tunisia Ho1ocen e
1966 1966 1966 1968 1968 1971 1979 1981 1982 1982 1982 1983 1983 1983 1984 1987 1988 1988 1989 1989 1990 1990 1990 1991 1991 1991 1991 1991 1991 1992
Pb a
206Pb/204Pb
18.25 18.24 18.16 a 17.66 a 17.45 a 17.32 116 17.033 f 0.085 17.25 a a 17.20 a 17.35 1000 17.55 f 0.04 a 17.13 a 17.82 a 17.27 a 16.99 276 18.0160 f 0.124 593 17.8737 f 0.102 152 17.8178 f 0.128 508 17.5851 f 0.043 127 17.9644 f 0.074 75 18.2088 f 0.076 207 18.0642 f 0.141 24 18.3392 f 0.122 51 18.3106 f 0.203 32 18.3063 f 0.104 18.0004 f 0.112 90 26 a 46 a 127 18.2395 f 0.141 18.1351 f 0.134 57 7 18.3677 f 0.171
a a
207Pb/204Pb
206Pb/204Pb
a a a a a a
a
15.382 f 0.057 15.56 f 0.05 15.54 15.55 15.66 f 0.03 15.57
36.397 f 0.19 37.49 f 0.13 37.05 37.21 37.65 f 0.05 37.08
a
a
a
a
15.54 15.598 f 0.082 15.597 f 0.176 15.505 f 0.051 15.455 f 0.115 15.541 f 0.055 15.674 f 0.074 15.600 f 0.095 15.535 f 0.053 15.609 f 0.112 15.498 f 0.056 15.656 f 0.080
36.87 38.037 f 0.137 37.798 f 0.267 37.489 f 0.179 37.368 f 0.195 37.704 f 0.142 36.223 f 0.334 37.925 f 0.253 38.274 f 0.211 38.349 f 0.262 38.236 f 0.203 37.954 f 0.186
a
a
a a a a a
a 15.615 f 0.083 38.202 f 0.276 15.487 f 0.092 37.949 f 0.266 15.322 f 0.275 38.206 f 0.427
a
2OOpb/207pb
z06P b/ zosP b
1.162 1.163 1.161 1.134 1.126 1.105 1.107 f 0.006 1.101 f 0.001 1.107 1.116 1.12 f 0.001 1.1 1.149 1.118 1.093 1.1550 f 0.00231 1.1459 f 0.00229 1.1491 f 0.00230 1.1378 f 0.00228 1.1559 f 0.00231 1.1617 f 0.00232 1.1580 f 0.00232 1.1805 f 0.00236 1.1731 f 0.00235 1.1812 f 0.00236 1.1497 f 0.00230 a
2.113 2.113 2.106 2.155 2.141 a 2.132 2.173 2.154 2.145 2.145 2.165 2.108 2.137 2.170 2.1112 f 0.00032 2.1147 f 0.00032 2.1040 f 0.00032 2.1250 f 0.00032 2.0989 f 0.00031 2.0981 k 0.00031 2.0994 f 0.00031 2.0870 k 0.00031 2.0943 f 0.00031 2.0866 f 0.00031 2.1086 f 0.00032
source (ref no.)
19 19 19 18 18 18 33 20 24 24 20 24 22 22 24 this work this work this work this work this work this work this work this work this work this work this work a this work a a this work 1.1680 f 0.00234 2.0945 f 0.00031 this work 1.1710 f 0.00234 2.096 f 0.00031 this work 1.1988 f 0.00240 2.0801 f 0.00031 this work
No measurement.
Analytical work was carried out in a US.-class 1000 clean room a t the University of Bordeaux using chemical techniques currently used for mass spectrometry (15,16). Bulk subsamples weighing =50 mg (41.4-59.7 mg) were dissolved in pressurized PTFE bombs in a -6 N mixture of [HF + HC1041. Lead contents and isotope ratios were measured with an ICP-MS Elan 5000 P.E.-Sciex. The instrument was operated under standard settings in nearly clean room conditions (US.-class 10 000). We analyzed two field blanks and two chemistry blanks: they were found to be less than -3 X lo4 of the sample concentrations. We already reported low blanks (15) for the Mediterranean samples (14). Detection limits, calculated as 2 standard deviations, were all 10.1 pg L-l. Concentrations measured in the samples were all superior to 10 times their corresponding detection limits, and precision was within 10%. Lead isotope ratios were corrected for mass fractionation and were normalized to the standard NIST 981. Accuracy was found to be within 10 f 5 % . Mass 204 was counted 15 times longer than for the other masses (206, 207, and 208), due to its very low abundance in all samples. The mean standard deviation (RSD) obtained for the major ratios (206Pb/204Pb, zo7Pb/204Pb)ranged between ~ 0 . 1 % 5 and ~ 0 . 3 %(Table 11,i.e., about the same order of magnitude as RSD obtained by other authors (17). Details of the analytical methods employed will be reported elsewhere (QuBtel et al., manuscript in preparation).
Results and Discussion Our results are shown in Figure 1and Table 1. We have added data from the literature of analyzed aerosols collected over London (18),Paris (7,19,20), Belgium (21), 1606
Environ. Sci. Technol., Vol. 28, No. 9. 1994
and the northwestern Mediterranean Sea (15,22) in order to extend the data set and to better assess the western Europe variability. Lead concentrations can differ within a period of 3 years by 1order of magnitude, depending on their location: for instance, we found =lo0 pg g-1 (dry weight) in the aerosol collected in 1979over the Tyrrhenian Sea, whereas only 3 years after (in 1982)the concentration was =lo00 pg g1over Paris (20) (Figure 1). Nevertheless, there has been a general decrease in aerosol lead concentrations during the last 15 years, and since 1990 all the concentrations we measured were lower than =200 pg g'. In order to free ourselves from this geographicvariability, we analyzed the isotopic composition of aerosol lead, thereby obtaining qualitative information that is basically independent of the lead content. Both zo6Pb/204Pb and 206Pb/207Pb ratios have been increasing regularly since a t least 1979. Unfortunately, isotopic data from aerosols before this period are extremely scarce in Europe, and we can only document this earlier trend by means of 206Pb/ 207Pbratios (Figure 2). I t seems that the 206Pb/207Pb ratios decreased continuously from 1965to sometime in the 1970s to early 1980s and has been increasing ever since. The evolution that occurred in the atmosphere above the North American continent (6) is characterized by exactly the opposite trend (upper band on Figure 2), but the reversals are synchronous. Figure 3 is a plot of 206Pb/206Pb against 206Pb/z07Pb ratios for the last decade in Europe. This type of plot seems to be the more informative way of viewing the evolution over time of lead isotopes in light of historical inputs, as already demonstrated for North American records in Bermuda (3). Since the early 19809,there has been a generaldecrease of the 208Pb/z06Pbratios and an increase in the 206Pb/ 207Pbratios.
‘ 0 6 Pb /‘07Pb
1.1li
-North-Rmerican
The importance and isotopic composition of the lead in these two sources is well-documented. Lead contained in the desert-derived dusts (25, 26) accounts generally for less than 20 pg g1and is considered a minor fraction ( I I ) , as confirmed by the low scandium content of aerosols. The mean isotopic composition of pre-industrial sediments (27) provides a good estimate of the dust composition (zOsPb/zosPb 2.04 andZosPb/2O7Pb = 1.21) (3)as confirmed by the Holocene Saharan loess (Figure 3). On the other hand, anthropogenic lead concentrations are high in European aerosols, due to high lead emissions in Europe (29). Lead concentration can reach values as high as 103 pg g1in urban aerosols (20). Although the origin of the lead added to the gasoline may vary slightly from one European country to another (221,generally for many years lead from lower radiogenic Precambrian ores, mostly from the Broken Hill ores in Australia (23),has been commonly used. In the 199Os, this Australian isotopic signature was still dominant in the gasoline used in eastern Europe (17 ) . This lead has 20sPb/z06Pb ratios of m2.22 and zo6Pb/207Pb ratios of ~ 1 . 0 4(28) and has already been identified in remote Atlantic sediments (23, 30, 31) and aerosols (15, 32). The aerosols analyzed in this work as well as those analyzed by other authors are distributed along a mixing curve that links the two end members described above. The oldest aerosols ( 1979-1985) have an isotopic composition that is strongly influenced by gas-derived lead (Broken Hill, top left-hand side of the graph). Between 1985 and 1990, compositions are roughly halfway between the natural and the anthropogenic end members. For the 1986-1988 period, these values (1.133 C zo6Pb/207Pb C 1.162) are very similar to the composition attributed by Hopper et al. (17 ) to their western Europe source region (1.13 < 206Pb/z07Pb C 1.155). Finally, the most recent samples (1990-1992) display compositions that are very close to the upper crust composition, represented by the
\wn, . trend
1.09 . 1965
. . . \ . . . , . . . . , 1970
1975
. / ,
1980
e,.
. .
1985
,
,
1990
,
,
Years
Flgure 2. Plot of 20ePb/207Pb ratios versus time as recordedby European aerosols since 1965. This work (closed symbols); data from the literature ( 75, 7&20,23,24)(open symbols). Error bars are smaller than symbols. The curves have no statistical meaning, but Indicate roughly both the European trend (lower band, our work) and the synchronous North American trend (upper band, ref 6). Within the same year, for example 1988, we observe similar ratios In both town (closed square) and mountain (closed circle) samples.
Lead associated with aerosols over southern Europe may have two different origins. One fraction is the natural lead contained in crust-derived dust particles, mostly from the Sahara Desert. The other fraction is anthropogenic European and is derived mostly from gasoline consumption (15,20-24). The process that mixes these components is simple (25, 26): when Saharan dust plumes are spread out by winds over Europe, the contact between the warm Saharan air mass and the cold European air mass often results in a rain front that causes dust fallout. As the rain falls through the lower atmosphere it picks up the local anthropogenic heavy metals contained in the air. These allochthonous dust samples are soaked with local pollution (both wet and dry phases) and, therefore, record the anthropogenic lead input.
N
2 08Pb/2 06Pb
t
\
1985
2.17
2.13
2.09 Preindustrial sediments
2.05 1.03
I
I
1.05
1.07
1.09
1.1 1
1.13
1.15
1.17
1.19
1.21
206Pb/2 07Pb Flgure 3. Plot of 20sPb/20sPb versus 2osPb/207Pb ratios as recorded by European aerosols. Our work (closed symbols); data from the literature (open symbols): mean pre-industrial sediment values (23, Broken Hill ores (24,and some other aerosols (75, 78, 79, 27, 24). The isotopic composition that we obtained for a Saharan Holocene loess sample confirms the validity of using the mean composition of pre-industrial sediments (27)as the natural end member. The line linklng the two end members and aerosol samples is a polynomial regresslon line. All error bars are smaller than the symbols. Envlron. Sci. Technol., Vol. 28, No. 9, 1994
1607
pre-industrial sediments (27). This trend is characterized by some scatter that may be due both to the fact that the alkyl lead used in each European country does not have exactly the same origin and to the variability of air mass trajectories (17, 33). However, at a first order there is a general trend that records the decrease in anthropogenic lead in the southwestern European atmosphere. This decrease parallels the decrease in the production of alkyl lead in Europe, Le., the increasing use of unleaded gasoline. Moreover, it is consistent with a decrease by 1 order of magnitude in P b concentrations measured in the atmosphere of urban French towns since 1975 (Patrick Thomas, personal communication). Conclusions
A rapid isotopic evolution has thus been observed since the initiation of the phasing-out of alkyl lead in Europe toward the end of the 1970s. This decrease mirrors the increase observed in the North American environment (6) since ~1976-1978. The opposing trends are explained by the fact that North American countries used higher radiogenic lead (zo6Pb/207Pb= 1.3) as an additive to gasoline. These two different patterns allow us to differentiate easily between European and North American sources. Thanks to the performance level reached nowadays by the ICP-MS technique (high accuracy, low cost, and fast measurements of lead isotopic ratios), the monitoring of atmospheric lead should be continued during the next decade until the total phase-out of lead in gasoline is completed in Europe. Such a database would help both to ascertain the time scale of coastal and upper ocean mixing processes and the particle transport to marine sediments and biomass and to much more accurately define lead pathways through the terrestrial environment, Le., soils and aquatic ecosystems. Acknowledgments
We acknowledge help received from (Mrs. Perfus, Mr. Coin, and Mr. Didon-Lascaut and from different French Observatories (CBte d' Azur, Mercantour, CBvennes) for having collected the Saharan-derived dust and rain samples over the past few years. We thank Dr. Bruno Hamelin for his fruitful comments, Dr. Hannes Brueckner for his friendly and pertinent review of the manuscript, and Dr. A. R. Flegal and two anonymous reviewers for their reviews. This research was supported by the CNRSINSU. Literature Cited (1) Murozumi, M.; Chow, T. J.; Patterson, C. C.Geochim. Cosmochim. Acta 1969, 33, 1247-1294. (2) Boutron, C. F.; Gorlach, U.; Candelone, J. P.; Bolshov, M. A.; Delmas, R. J. Nature 1991, 353, 153-156. (3) Shen, G. T.; Boyle, E. A. Earth Planet. Sci. Lett. 1987,82, 289-304.
1608 Envlron. Sci. Technol., Vol. 28, NO. 9, 1994
(4) Trefry, J. H.; Metz, S.;Trocine, R. P.; Nelsen, T. A. Science 1985,230,439-441. (5) Sherrel, R. M.; Boyle, E. A.; Hamelin, B. J . Geophys. Res. 1992, 97, 11,257-11,268. (6) Rosman, K. J. R.; Chisholm, W.; Boutron, C. F.; Candelone, J. P.; Gorlach, U. Nature 1993, 362, 333-335. (7) Patterson, C. C.; Settle, D. M. Mar. Chem. 1987,22, 137162. (8) Sturges, W. T.; Barrie, L. A. Nature 1987, 329, 144-146. (9) Flegal, A. R.; Schaule, B. K.; Patterson, C. C. Mar. Chem. 1984, 14, 281-287. (10) Flegal,A. R.; Nriagu, J. 0.;Niemeyer, S.; Coale, K. H. Nature 1989,339, 455-458. (11) Nriagu, J. 0. Nature 1989, 338, 47-49. (12) Bucher, A.; Lucas, C. Bull. Cent. Rech. Explor. Prod. ElfAquitaine 1984, 8, 151-165. (13) Bucher, A.; Dessens, J. J. Meterol. 1992, 17, 226-233. (14) Chester, R.; Sharples, E. J.; Sanders, G.; Saydam, A. C. Atmos. Environ. 1984, 18, 929-935. (15) Hamelin, B.; Grousset, F. E.; Biscaye, P. E.; Zindler, A.; Prospero, J. J. Geophys. Res. 1989, 94, 16,243-16,250. (16) Grousset, F. E.; Rognon, P.; CoudB-Gaussen,G.; Pedemay, P. Palaeogeogr. Palaeoclimatol. Palaeoecol. 1992,93,203211. (17) Hopper, J. F.; Ross, H. B.; Sturges, W. T.; Barrie, L. A. Tellus 1991,43B, 45-60. (18) Hamilton, E. I.; Clifton, R. J. Estuarine Coastal Mar. Sci. 1979,8, 271-278. (19) Chow, T. J.; Snyder, C.; Earl, J. Proc. IAEA-SM-lSIj4, 1975, 95-108. (20) Elbaz-Poulichet, F.; Holliger, P.; Huang, W. W.; Martin, J. M. Nature 1984, 308, 405-414. (21) Petit, D.; Menessier, J. P.; Lamberts, L. Atmos. Enuiron. 1984,18, 1189-1193. (22) Maring, H.; Settle, D. M.; Buat-MBnard, P.; Dulac, F.; Patterson, C. C. Nature 1987, 330, 154-156. (23) Hamelin, B.; Grousset, F. E.; Sholkovitz, E. R. Geochim. Cosmochim. Acta 1990,54, 37-47. (24) Flament, P. These de 38me cycle.UniversitBde Lille, France, no. 1251, 1985. (25) Bergametti, G.; Dutot, A.; Buat-MBnard, P.; Losno, R.; Remoudaki, E. Tellus 1989,41, 353-361. (26) Remoudaki, E.; Bergametti, G.; Buat-MBnard, P. J . Geophys. Res. 1991, 96, 1043-1055. (27) Sun, S. S. Philos. Trans. R. SOC.London 1980,297, 409445. (28) Doe, B. R.; Stacey, J. S. Econ. Geol. 1974, 69, 757-776. (29) Pacyna, J. M. Atmos. Environ. 1984, 18, 41-50. (30) VBron, A,; Lambert, C. E.; Isley, A.; Linet, P.; Grousset, F. E. Nature 1987, 326, 278-281. (31) Lambert, C. E.; VBron, A,; Buat-MBnard, P.; Heyraud, M.; Grousset, F.; Simpson, W. Oceanol. Acta 1991,14,67-76. (32) VBron, A,; Church, T. M.; Flegal, A. R.; Patterson, C. C.; Erel, Y. J. Geophys. Res. 1993 , 98 (ClO), 18,269-18,276. (33) Grousset, F. E.; QuBtel, C.; Thomas, B.; Donard, 0. F. X.; Lambert, C. E.; Guillard, F.; Monaco, A. Mar. Chem., in
press. Received for review December 15, 1993. Revised manuscript received April 19, 1994. Accepted May 18, 1994.' @
Abstract published in Advance ACS Abstracts, June 15, 1994.
