Environ. Sci. Technol. 2008, 42, 692–698
Tracing of Industrial Aerosol Sources in an Urban Environment Using Pb, Sr, and Nd Isotopes MAJDI LAHD GEAGEA, PETER STILLE,* FRANÇOIS GAUTHIER-LAFAYE, AND MAURICE MILLET EOST: Centre de Géochimie de la Surface, CNRS-UMR 7517, 1 rue Blessig, 67084 Strasbourg, France
Received July 11, 2007. Revised manuscript received November 14, 2007. Accepted November 19, 2007.
A comprehensive Pb-Sr-Nd isotope tracer study of atmospheric trace metal pollution has been performed in the urban environment of Strasbourg-Kehl. Filter dust of the principal pollutant sources (waste incinerators, thermal power plant and steel plant) and soot of car and ship exhausts have been analyzed. In addition tree barks (as biomonitors) and PM10 have been analyzed to trace and determine the distribution of the pollution in the environment. The industrial sources have highly variable Nd values (-9.7 and -12.5 for incinerators and -17.5 for steel plant). Much higher Nd values have been found for soot of car exhausts (-6 and -6.9). These high values make the Nd isotope system a powerful tool for the discrimination of traffic emissions but especially for the identification of diesel derived particles in the urban environment. The 206Pb/207Pb isotope ratios of gasoline are low (1.089) compared to diesel soot (1.159). The 206Pb/207Pb ratios of 1.151–1.152 for the steel plant and 1.152 for the solid waste incinerator are close to the Pb isotope ratio of diesel. The 87Sr/ 86Sr isotope ratios of the principal industrial sources vary significantly: 0.7095 for the domestic solid waste incinerator, 0.709 for the steel plant, and 0.7087 for car exhaust soot. PM10 aerosols collected in the urban center of Strasbourg show the influence of the pollutant sources at 3-7 km distance from the center. Most of the aerosols Pb isotopic compositions suggest Pb admixtures from at least three sources: a natural background and in function of the wind direction the domestic waste incinerator (S-wind) or the steel plant and the chemical waste incinerator (NE-wind). The traffic contribution can only be estimated with help of Nd isotopes. Therefore the clear identificationofdifferentpollutantsourcesintheurbanenvironment is only possible by combining the three different isotope systems and is based on the fact that significant differences exist between the Pb, Sr, and Nd isotope ratios of the natural atmospheric background and pollutants containing Pb, Sr, and Nd of industrial origin with similar variable 206Pb/207Pb, 87Sr/ 86Sr, and 143Nd/144Nd.
Introduction The chemical composition of the atmosphere is affected especially in the Northern Hemisphere by increasing concentrations of heavy metals due to both natural and * Corresponding author phone: +33(0)390240434; fax: +33(0) 390240402; email:
[email protected]. 692
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anthropogenic emissions. These pollutants are concentrated in submicrometer aerosols, which can be transported over long distances before their deposition (1–3). Grousset and Biscaye (3) showed in their review paper that Sr-, Nd-, and Pb-isotope ratios are the most powerful tracers to identify natural dust sources and to identify their transport patterns. It has also been shown that Pb isotope ratios provide a very useful and effective method of monitoring and tracing of anthropogenic atmospheric pollution (4–12). However, Sr and Nd isotopes have very seldom been used for environmental studies although, for instance, Sr has been shown to be useful since Sr isotopes allow the origin of base cations in rainwater to be traced (13–16) and, more indirectly, in combination with Pb isotopes allow tracing of the sources of atmospheric heavy metals (17, 18). Nd isotopes and rare earth elements (REE) have been shown to be powerful tools in combination with Sr isotopes for the characterization and quantification of atmospheric REE in soils and surface waters (19, 20). In previous studies, tracing is done by sampling rain and snow (21) or biomonitors such as lichens (22, 23). Only recently has the first comprehensive tracer study of atmospheric trace metal pollution by a steel plant been successfully performed using tree barks as biomonitors and Pb-, Sr-, and Nd-isotope ratios (206Pb/207Pb, 87Sr/86Sr, 143Nd/144Nd) and REE as environmental tracers (24). This is of importance since Pb isotope ratios alone did not allow such distinction and industrial emissions of, e.g., waste incinerator and steel plant have similar Pb isotopic compositions and plot e.g. on a 206Pb/207Pb vs 208Pb/207Pb diagram between leaded gasoline and natural Pb. However, as will be shown in the present study, their Nd isotopic compositions (143Nd/144Nd) are significantly different. Thus, the possibility of the identification of different anthropogenic emissions is given by the combined application of the three isotope systems, and it is based on the existence of significant Sr, Pb, and Nd isotopic differences between natural background material and pollutants containing Pb, Sr, or Nd of industrial origin with likewise variable 87Sr/86Sr, 206Pb/207Pb or 143Nd/144Nd isotope ratios. Lahd Geagea et al. (24) have further shown that aerosol samples have isotopic compositions very similar to tree bark if both have been collected at the same sampling site. In the present study we analyzed the heavy metal pollution in Strasbourg (France) and Kehl (Germany) (Figure S1, Supporting Information (SI)) with help of tree barks as biomonitors and Sr, Pb, and Nd isotopes as environmental tracers. This region suffers from substantial air pollution due to the presence of different industrial parks, heavy road and river traffic, and the topography of the Rhine valley, which provides a particularly unfavorable situation in respect of ventilation and dispersion of pollutants (prevailing wind comes from SW). We focused especially on the isotopic characterization of the domestic waste incinerator and the chemical and industrial waste incinerator on the French bank of the river Rhine as well as on the traffic emissions. During transport of the emitted aerosols their original Pb, Nd, and Sr isotopic source signatures become obscured by mixing processes with aerosols from several other industrial and natural sources. Thus, at a final depositional site, e.g., in the downtown area of Strasbourg or Kehl, the particulate matter might be a pollutant derived from several sources and, therefore, containing isotopic signatures resulting from mixing of the different aerosols (10, 25, 26). We, therefore, collected PM10 aerosol samples (18.3.
FIGURE 5. 208Pb/206Pb vs 206Pb/207Pb diagram for aerosols collected at the botanical institute. The data are compared with isotopic compositions of samples from other municipal waste incinerators in France (greyisch field) (4). combustion derived soot particles (Figure 1B) and steel plant derived aerosols which have been deposited in this periurban environment after a 10 km flight in the atmosphere (Figure 1B). Another periurban-type bark sample has been collected on the east-side of the city of Kehl in the Memorial park (site no. 25). It is the only sample with Sr and Nd isotopic compositions closest to those of the average “natural” background composition (Figure 1A). Its 206Pb/207Pb isotope ratio (1.151), however, is close to that of the waste incinerator (1.152). Aerosols collected in Auenheim (site no. 3; 0.9 km NE steel plant) and Kehl (site no. 29; 4 km SSW steel plant) allow under SW and NNE wind conditions, respectively, to clearly recognize the steel plant derived aerosols (Figure 1). The bark samples collected in the public park on both banks of the river Rhine (site nos. 26 and 27) show very similar Sr, Pb, and Nd isotopic compositions. Both sites are situated 200 m south of a traffic burden bridge crossing the Rhine and on a SW-NE axis between steel plant and chemical waste incinerator on one side (both ca. 3 km NNE) and the domestic waste incinerator on the other side (5 km SSW). Their Pb isotopic compositions and Nd values are very similar to those of particles derived from the domestic waste incinerator (Figure 1B). However, their 87Sr/86Sr ratios are slightly more radiogenic possibly due to the additional presence of a natural background-derived component much better visible in the Memorial park. Traffic burden environments (site no. 31: in the center of Strasbourg; site no. 30: close to bridge connection between Strasbourg and Kehl) have specific isotopic signatures slightly different from those of soot particles from car exhausts. This is due to the fact that vehicles do not only emit soot particles but also particles from abrasion. Whereas emissions of exhaust particles are subjected to regulatory control and decrease with time, nonexhaust emissions are difficult to control and are not regulated. Main nonexhaust particle 696
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sources relating to road traffic are tire, brake, and clutch wear, as well as road surface alteration and resuspension of road dust. The isotopic decoupling between exhaust particles and total traffic derived particles is visible in Figure 1. Origin of Aerosols Collected in the Center of Strasbourg. Aerosols (PM10) have been collected during 5 month on the roof of the botanical institute (ca. 40 m height) in the center of Strasbourg (SI Figure S1 and SI Table S2). This type of experiment did not allow us to collect enough Nd for precise isotope measurements. However, first preliminary Pb isotope data are available of aerosols collected under prevailing south, north-northeast, and west wind conditions. Their 206Pb/ 204 Pb and 207Pb/204Pb isotope ratios are shown in Figure 4 and compared with the principal pollutive sources, the steel plant (4.5 km distance; site no. 1), the chemical waste incinerator (3.5 km distance; site no. 22), the domestic waste incinerator (7 km distance; site #10) and the traffic (Figure 4). Most of the samples plot in a triangle defined by the steel plant (S), the two waste incinerators (I) and a radiogenic Pb background composition (N). The samples collected under prevailing west wind conditions can isotopically not be influenced by components (I) and (S). They are principally more radiogenic than the remaining samples and plot on a line connecting a radiogenic natural component (N) with a less radiogenic gasoline or industry derived Pb component (Table S1) (dotted line; Figure 4). Under prevailing north-northeast wind conditions the chemical waste incinerator (site no. 22), the steel plant (site no. 1), the thermal power plant (site no. 33) and the natural background might particularly determine the Pb isotopic compositions of the aerosols as indicated by the distribution of the corresponding data points (small filled circles) in the triangle SIN. The presence of particles from the domestic waste incinerator can be excluded. One sample collected on a Sunday has comparatively high 206Pb/204Pb isotope ratios and is close to the natural background composition (a; Figure
4) of the Rhine valley (31). The remaining samples point to interaction and mixing of particles originating principally from the steel plant, chemical waste incinerator and possibly diesel soot (dotted lines). The impact of diesel and gasoline soot from car and ship exhausts on the aerosol composition remains difficult to evaluate. This question might be solved in future studies with help of especially Nd isotope tracers. The phenomenon that the “Sunday” sample plots close the “background” isotopic composition of the Rhine valley (206Pb/ 204Pb > 18.4) (31) or to values of preindustrial (natural) Pb ratios (207Pb/204Pb:15.64; 206Pb/204Pb:18.73 (10);) might indicate that on this Sunday the air was neither polluted by industrial activities nor by traffic. The air carrying these aerosols shows lowest particle concentrations (SI Table S2). The aerosol samples collected under prevailing south wind conditions can not be isotopically influenced neither by the chemical waste incinerator nor by the steel plant. The distance to the urban incinerator is the double of what it is to the steel plant, and therefore, the air masses coming from the urban incinerator might have exchanged with other not yet identified industrial sources. Nevertheless the south wind aerosol samples have Pb isotopic compositions rather close to those of the waste incinerator as indicated by the dotted field in Figure 4. The close relationship of the aerosols collected under S wind conditions to the municipal waste incinerator is also recognizable in the 208Pb/206Pb vs 206Pb/207Pb diagram (Figure 5) where the isotopic compositions of the aerosol samples are compared with those of the waste incinerators and the steel plants filter dust; also shown are the isotope ratios of the bark samples collected NE of the incinerator and of other municipal waste incinerator samples (4). Most of the aerosol samples collected under south wind conditions plot close to or in the field of waste incinerators and close to the ESP line. The two samples collected under west wind conditions have high 206Pb/207Pb and low 208Pb/206Pb ratios compared to those collected under south or southwest wind conditions. The aerosol sample collected under southwest wind conditions mainly contains waste incinerator derived Pb. The filter dust of the waste incinerator and the steel plant have very similar 208Pb/206Pb and 206Pb/207Pb isotope ratios. Nevertheless, the aerosol samples of the steel plant emissions arriving under northeast wind conditions at the botanical institute point in contrast to most of the south wind aerosols to an important admixture of a natural dust component with high 206Pb/207Pb and low 208Pb/206Pb ratios (e.g., average modern crustal Pb (34)). Thus, it appears that the northeast and west winds contain less industry derived Pb and more crust derived (natural) Pb than the south and southwest winds. Similarly south winds appear to be generally more enriched in particles than northeast and west winds (SI Table S2).
Acknowledgments We thank S. Wörner (town of Kehl) for many important discussions and very helpful assistance during sample collection. F. Hofmann gave us an introduction into bark sampling using patented drill machine. The hospitality at the branch of Isotope Geology of the University of Bern and the great help of Th. Nägler during the Nd isotope measurement on the MC-ICPMS are gratefully acknowledged. We thank R. Boutin, J.-J. Frey, B. Kiefel, and Th. Perrone of the Centre de Géochimie de la Surface at Strasbourg for their technical assistance and analytical work, and “ASPA” is thanked for supplying the aerosol collectors. We are also grateful to A. Bouzeghaia for drawing the map. Comments from two anonymous reviewers were most helpful. This study has been financially supported by the town of Kehl (Germany), by REALISE (Réseau Alsace de Laboratoires en Ingénierie et Sciences pour l’Environnement, France) and a Lebanese CNRS grant to M.L.G. This is EOST contribution 2007.206-UMR7517.
Supporting Information Available The map Figure S1 and Tables S1 and S2 are shown in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.
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