Lead ISOTOPES in the seasonal snow cover in the French Alps.

Audrey VEYSSEYRE^1,*, Christophe FERRARI ^1,2, Andreas BOLHÔFER³, Kevin ROSMAN³ and Claude BOUTRON^1,4

¹ Laboratoire de Glaciologie et Géophysique de l’environnement du CNRS, 54 rue Molière, B. P. 96, 38402 Saint Martin d’Hères, France.

² Institut des Sciences et Techniques de Grenoble, Université Joseph Fourrier de Grenoble, 28 Avenue Benoît Frachon, B.P. 53, 38041 Grenoble , France.

³Department of Applied Physics, Curtin University of Technology, Perth 6845, Western Australia.

⁴ Unités de Formation et de Recherche de Mécanique et de Physique, Université Joseph Fourier de Grenoble (Institut Universitaire de France), B.P. 68, 38041 Grenoble, France.

* Corresponding author : e-mail : audrey@glaciog.ujf-grenoble.fr

SUMMARY

Fresh snow samples, collected in 15 remote locations of the French Alps between November 1998 and April 1999, have been analysed for Pb isotopes, by Thermal ionisation mass spectrometry. ²⁰⁶Pb/²⁰⁷Pb ratios display great variations from 1.1371 to 1.1628. These results require the involvement of at least two anthropogenic end-members. The results indicate that there is no evidence of a long range atmospheric transport of pollutants. As a consequence of the phasing-out of lead in gasoline, the importance of automobile emissions has decreased, and as a result, other sources such as natural and/or industrial, can be identified using isotopic analysis. Regional sources have been characterised and their signature are identified in our samples, without, however, determining clearly the relative contribution of each source.

Introduction

Combustion of leaded gasoline was considered for a long time to be the major source of Pb contamination (Pacyna et al., 1984; Nriagu, 1990). However, since 1975 in the United States, and the mid-1980s in Europe, legislative measures have lead to a continuous reduction of lead in gasoline. As a consequence of this phasing-out, the abundance of lead in the atmosphere has decreased (Boutron et al., 1991; Rosman et al., 1993), but this element still remains mainly anthropogenic in atmospheric aerosols.

Investigations of the isotopic composition of Pb in the atmosphere are characterised in terms of isotope abundance ratios which are different in different regions of the world, due mainly to anthropogenic Pb emissions. For instance Rosman et al.(1993) have recently identified US and Eurasian emissions in central Greenland, while Hopper et al.(1991) and Aberg et al. (1999) focussed on Europe. This paper presents a study of snow precipitation composition carried out in a weakly contaminated ecosystem, in the French Alps.

Material and methods

From November 1998 to April 1999, fresh snow samples were collected in fifteen remote alpine sites whose altitude ranged from 1540 to 2700m, with an average altitude of 1900m. Precise location of these sites is given on figure 1 and Table 1.

Samples were retrieved kilometres away from cities, industries, highways and main roads, in order to obtain a regional atmospheric signal.

After each snow event, fresh snow samples were collected using LDPE tubes. These tubes had been extensively cleaned with ultrapure nitric acid and water as described in detail in Boutron (1990) and Ferrari et al. (2000). At the Laboratoire de Glaciologie et Géophysique de l'Environnement (LGGE), snow samples were melted inside a clean bench (class 100) (Ferrari et al., 2000). Aliquots were prepared in acid cleaned LDPE bottles, then frozen, and transferred to the Centre of Excellence in Mass Spectrometry (CEMS) at Curtin University (Australia) where the isotopic ratios and concentrations were measured.

Results and discussion

The samples analysed in this work are distributed within the region formed by the signature of the three regional end-members (natural, waste incineration and gasoline, see figure 2). The aim of this study is to determine the relative contribution of each source.

During the past 15 years, leaded gasoline signature has remained quite low in France (²⁰⁶Pb/²⁰⁷Pb <1.10). In 1981-1989, the isotopic composition of urban aerosols was in good agreement with the gasoline values, but a shift occurred in the past 5 years. In French urban areas the signature increased from 1.115 in 1981-1989 to 1.143 in 1993-95 (Veron et al., 1999). Isotopic composition of atmospheric lead in French urban areas was no longer representative of automobile emissions solely but derived from a mixing of automobile and industrial lead.

We can assume that automobile emissions are not mainly responsible for lead contamination in the region of Grenoble, however, they can't be neglected.

On the basis of a two end-members mixing, the ²⁰⁶Pb/²⁰⁷Pb ratio of 1.1193 for Grenoble's filter collected in 1999 can be explained as a mixture of 45% of gasoline combustion (²⁰⁶Pb/²⁰⁷Pb = 1.0775) and 55% of industrial emissions such a refuse incinerators (²⁰⁶Pb/²⁰⁷Pb: 1.142 to 1.160), which are present in the region (one is located less than 1km away from the sampling site in Grenoble).

On the other hand, values observed in Alps samples, ranging from 1.1371 to 1.1607, lie in the industrial range (waste incinerators), and could be explained according two hypothesis:

1) If we transpose to rural area the observations made close to the highway (Deboudt et al., 1999), automobile emissions are likely to be negligible in our sampling sites, which are located far away from main roads. Therefore, isotopic signature will still be a mixture of gasoline combustion and industrial emissions (waste incinerators) but this latter is responsible for a larger contribution, as we can observe a higher ²⁰⁶Pb/²⁰⁷Pb ratio. This hypothesis is enhanced when looking at figure 2 where waste incinerators signature lie just above on the Alps domain.

2) Isotopic composition observed in our samples represents a mixture of industrial, automobile and natural emissions. Automotive emissions accounts for a smaller contribution than in urban area, but the increase of the ²⁰⁶Pb/²⁰⁷Pb ratio is also explained by the crustal contribution (²⁰⁶Pb/²⁰⁷Pb: 1.19 to 1.21).

References

Åberg G., Pacyna J.M., Stray H. and Skjelvale B.L. (1999). Atmos. Env. 33: 3335-3344.

Boutron, C.F. (1990) Fresenius Z. Anal. Chem. 337: 482-491.

Ferrari, C.P., Moreau, A.L. and Boutron, C.F. (2000) Fresenius. Z. Anal. Chem. 366:433-437.

Hopper J.F., Ross H.B., Sturges W.T. and Barrie L.A. (1991). Tellus 43B: 45-60.

Nriagu J.O. (1990). The Science of the total Environment, 92: 12-28.

Pacyna J.M., Semb A., Hanssen J.E. (1984) Tellus 36B: 163-178.

Rosman K.J.R., Chisholm W., Boutron C.F., Candelone J.P., Gorlach U. (1993) Nature, 362: 333-335.

Deboudt K., Flament P., Weis D., Mennessier J.-P. and Maquinghen P. (1999) The Sci. Total Environ. 236: 57-74.

Veron A., Flament P., Bertho M.-L., Alleman L., Flegal R. and Hamelin B. (1999) Atmos Environ 33: 3377-3388.

Table 1: Location and altitude of the sampling sites

Site No	Location	Altitude (m)
1	Chablais	1540
2	Chamonix	1655
3	Mont Blanc	1850
4	Aravis	1750
5	Haute Maurienne	2300
6	Maurienne	2240
7	Chartreuse	1700
8	Grandes Rousses	1750
9	Belledonne	1800
10	Vercors	1800
11	Alpe du Grand Serre	1750
12	Oisans	1700
13	Pelvoux	1700
14	Champsaur	1800
15	Queyras	2700

Figure 1. Location of the fifteen sampling sites for this study in the French Alps. Location names are listed in Table 1.

Figure 2. Isotopic composition of lead in terms of ²⁰⁶Pb/²⁰⁷Pb versus ²⁰⁸Pb/²⁰⁷Pb ratios in aerosols (black triangle) and snow samples (empty square) collected in the French Alps, petrol, industrial and natural environments.