The anthropogenic atmospheric elements fraction: A new interpretation of elemental deposits on tree barks

doi:10.1016/j.atmosenv.2008.11.004

Atmospheric Environment

Volume 43, Issue 5, February 2009, Pages 1124-1130

https://doi.org/10.1016/j.atmosenv.2008.11.004 Get rights and content

Abstract

The superficial deposit on the bark surface of several trees (mainly Fraxinus excelsior L.) was sampled in the experimental station of the university campus in Grenoble (France). Its composition was studied by scanning electron microscopy–energy dispersive X-ray emission (SEM–EDX) and, after digestion, by inductively coupled plasma-mass spectrometry (ICP-MS). The deposit was composed of 81.3% ± 7.4 organic matter, 9.4% ± 4.9 of geogenic minerals issued from the atmosphere (atmospheric geogenic fraction: AGF) and 9.3% ± 2.7 of a mixture of elements which was called anthropogenic atmospheric elements fraction (AAEF). The SEM–EDX analysis showed the presence of particles of geogenic compounds such as different types of silicates, phosphates, carbonates, sulphates, oxides and also particles of metals such as iron or of alloys of Fe–Zn, Fe–Ni, Ni–Cr and Ca sulphates or phosphates. Typical spheres of “fly ashes” composed of pure iron or Al-silicates were detected. Using the SEM–EDX analysis of the deposit and the average local soil composition, an empiric formula for the AGF (without polluting elements) was chosen, which presented a clear analogy with the global formula of the upper continental crust. In the same way, a formula for the pure organic matter fraction was chosen. Withdrawing the elements corresponding to these two fractions allows a tentative estimation of the content of the AAEF which was supposed to better represent the elemental anthropogenic contamination issued from the atmosphere. In the station, most of Sb, Cd, Sn, Pb, Cu, V and Zn were found in the AAEF. This AAEF composition was compared to that of the deposit in a highway tunnel where Pb and Cu were at a very high level. The meaning and the limits of the AAEF concept were critically discussed.

Introduction

For years, biomonitoring methods have been used to estimate airborne contamination and its changes over long periods of time (Al-Shayeb et al., 1995, Askoy and Öztürk, 1997, Böhm et al., 1998, El-Hasan et al., 2002, Freitas et al., 1997). For this purpose, different types of perennial plant matrices have been used, among which lichens, mosses and tree barks (Asta et al., 2002, Bargagli, 1998, De Bruin and Hackeniz, 1986, Garrec and Van Haluwyn, 2002, Walkenhorst et al., 1993).

Tree bark seems to offer a good natural bio-indicator of air pollution, as it can accumulate large amounts of atmospheric dust. Retention of suspended particles is promoted by a moist, rough, or electrically charged surface making bark a highly effective collector (Panichev and McCrindle, 2004).

During the last decades, numerous studies gave a complete picture of the global relations between the air contamination and the bark composition (Freer-Smith et al., 2005, Odukoya et al., 2000, Salma and Maenhaut, 2006, Santitoro et al., 2004, Suzuki, 2006).

For metallic contamination, barks of several tree species were commonly used (Ayrault et al., 2007, Mandiwana et al., 2006, Shin et al., 2007, Tye et al., 2006) such as Pinus nigra or Pinus sylvestris (Schulz et al., 1999, Haapala and Kikuchi, 2000, Narewski et al., 2000, Huhn et al., 1995, Harju et al., 2002, Saarela et al., 2005), olive-trees (Freitas et al., 2003, Pacheco et al., 2001, Pacheco et al., 2002, Pacheco et al., 2004; Pacheco and Freitas, 2004), Fagus sylvatica (Bellis et al., 2004), Betula sp. (Herman, 1992) and Populus nigra (Berlizov et al., 2007). However, the mechanisms and physico-chemical principles of bark deposit and accumulation for the elements of atmospheric origin remain poorly understood so far.

In a previous report, accumulation of elements inside the bark and xylem tissues of ash-tree (Fraxinus excelsior L.) was studied separately from the superficial deposit (Catinon et al., 2008). It was suggested that the superficial deposit and the internal accumulation were two different and complex processes, the limiting factors and time scales of which were not the same.

Consequently, the purpose of the present work was to establish precisely the composition of the deposit on the ash-tree bark and try to understand the main characteristics of its formation and its relations with the atmospheric anthropogenic contamination.

Section snippets

Sampling sites

The experimental site chosen for this study was the botanical garden of the LECA Laboratory in the University Campus (University Joseph Fourier, Saint Martin d'Hères, Isère, France) where numerous 11-year-old and 40-year-old F. excelsior were at our disposal. This site was at a distance of 800 m from the A41 highway Grenoble–Chambéry with 50,000 vehicles day⁻¹ traffic and at 4 km from the center of the town at an altitude of 200 m, in the Isère valley surrounded by mountains between 1500 and

SEM–EDX analysis of the superficial deposit

The collected samples of superficial deposits were a complex mixture of suspended particles and of water-soluble components.

Fig. 2 shows a SEM photograph of this deposit. The photograph background is constituted of a dark-grey matrix representing mostly the remaining organic matter. Inside this matrix, numerous more or less bright particles can been seen. Their size is between 1 and 20 μm.

The X-ray emission was established with EDX for the three larger particles in the photograph. As shown in

Organic matter in the deposit

Numerous measurements of the organic content of the bark superficial deposits have been carried out on our station, for several tree species. Organic matter was always the main component (from 70 to 90% of the dry weight). In the case of ash-trees younger than 20 years, the bark only provides very small amounts of dead cells, as it does not exfoliate. Most of the organic matter is therefore coming either from an atmospheric transfer of plant material or humic substances, or from different live

Conclusion

As a whole, it seems therefore that the free deposit cannot be representative of a full several-year-accumulation of atmospheric contaminants. It is likely to correspond to a several month accumulation period, with important differences between the bark areas washed by the rain and the areas protected from leaching. Therefore, climatic conditions certainly play an important part in the rate of deposition and leaching. Furthermore, the presence of organic matter coming from living organisms, of

Acknowledgments

SA thanks E. Douville for his assistance in ICP-MS apparatus maintenance. We thank an anonymous reviewer for his helpful comments. This is an LSCE contribution no. 3164.

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