Import of atmospheric trace metal elements in the aspe valley and somport tunnel (Pyrénées Atlantiques, France): Level of contamination and evaluation of emission sources [Apports atmosphériques des éléments traces métalliques dans la vallée d'aspe et le tunnel du somport (Pyrénées-Atlantiques, France): Niveau de contamination et évaluation des sources d'émissions]
Résumé
The objective of this study is to define a reference state of metallic contaminants for the monitoring of road traffic emissions since the opening of the Somport tunnel in the Aspe valley (Pyrénées). To establish air quality indicators, trace metal elements (TME) (Al, Na, Mg, K, Ca, V, Mn, Cr, Zn, Cu, Rb, Cd, Sn, Sb, Ba, Ce, Pb and U) and lead isotopic ratios ( 208Pb/206Pb and206Pb/207Pb) were determined in various atmospheric receptors (fresh snow, bulk deposition, atmospheric particles and epiphytic lichen). Sampling and analyses with ultra clean procedures were employed for TME quantification. The different investigated atmospheric receptors allow the integration of the atmospheric inputs from daily to pluriannual temporal scales and along spatial scales through the North-South axis of the valley and as a function of the altitude. The Aspe valley exhibits a contamination level characteristic of remote European areas and the major metallic contaminants identified are Cd, Sb, Zn, Cu, Pb and Sn. All the analyses of TME in atmospheric receptors point out a decrease of Pb and Cd emissions in the northern hemisphere. The comparison with previous studies demonstrates a significant change in lead isotope composition and both Cu and Sb increase concentrations in atmospheric receptors. These results suggest a significant contribution of both incinerators emissions and the new road traffic pollution. In addition, elementary ratios Zn/Sb vs Cu/Sb seems to discriminate the inputs from waste treatment industries in the Aspe valley. The monitoring of wet deposition and atmospheric particles indicates an additional contribution from regional sources for Zn, Pb and Cd, in the low Aspe valley. The analysis of TME in atmospheric receptors allows thus to assess the contaminant contributions (i) from local emissions of domestic heat sources, from agricultural burning practices and road traffic, and (ii) from regional anthropogenic sources of waste incinerators, industries and urban area. Furthermore, the valley is significantly influenced by wind erosion and long range transport of TME in the Northern Hemisphere. The determination of TME and Pb isotopic ratios in the Somport tunnel indicate that local road traffic emissions are characterised (i) by a significant contribution of Sb, Zn and Cu, and (ii) by a slightly radiogenic Pb isotopic composition. Cu/Sb ratio that is used to trace brake wear emission is also well correlated with the road traffic in the Somport tunnel. In summary, the actual low traffic intensity in the Aspe valley prevents from conclusive evidence of significant and widespread contamination from road traffic emissions. The spatial distribution of the TME enrichment factor in epiphytic lichen, from the main road (RN134), indicates however the dry particulate mobilisation and emission by vehicles close to the road.