Palladium in a mountain ecosystem (Aspe Valley, Pyrenees, France)
Résumé
Mobility is one of the most important factors for modern industrial societies in the globalized economy of today. About 90% of the total passenger transport (in passenger per km) and 40% of the freight transport (in tons per km) occurs at the surface, and most of this is powered by fossil fuel combustion engines (Schultz et al., 2004). Road traffic contributes to the deterioration of air quality. The key primary emissions from the transport sector that may have an impact on air quality are: carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOX), sulphur dioxide (SO2), particulate matter and air toxics (including organic compounds and metals). Fuel additives, lubricating oils, anti-freeze compounds and the degradation of the different vehicle components are the emission sources of several trace metals into the environment (Dannecher et al., 1990). Vehicles fitted with catalytic converters significantly minimize undesirable emissions, such as CO, HC, NOX and particulate matter produced during fossil fuel combustion. Catalytic converters reduce about 90% of these pollutants and transform them into more innocuous carbon dioxide, nitrogen and water. Platinum, palladium and rhodium, known as platinum group elements (PGE), are the active components of catalytic converters. Both platinum and palladium are used to oxidize CO and HC. Rhodium must be present in order to reduce NOX (Lox and Engler, 1999). However, due to surface abrasion and deterioration, catalytic converter release PGE into the environment causing their deposition near roads, in vegetation and in other environmental compartments as a result of wind and water transport (Moldovan et al., 2002). A great number of studies have reported the dispersion and accumulation of PGE in numerous environmental matrices. Platinum is the element most studied, while palladium and rhodium have been monitored in more recent research projects. The presence of PGE in urban environments has been fully investigated, but very little research has been performed in rural areas and/or mountain regions. The Aspe Valley, located in the Pyrenees National Park (Atlantic Pyrenees), is crossed by the RN134 French national road, and due to the low traffic density existing nowadays it could be considered as a pristine area. The opening of the Somport Tunnel, connecting France and Spain, in January 2003 will clear the frontier points of Irun-Biriatou (Atlantic Pyrenees) and La Junquera-Pertuis (Mediterranean Pyrenees) (Etchelecou et al., 2001). On the contrary, the Somport Tunnel will result in a significant increase of road traffic on the Aspe Valley, especially of heavy-duty vehicles, with an expected traffic density of 2000 vehicles per day, and thus it will add a new source of local pollution. Furthermore, the geographical characteristics of the valley together with the atmospheric conditions of the area involve little dispersion of the pollutants and therefore their deposition on the adjacent environment, representing an important impact on the air quality of the valley. Information concerning air quality in the Pyrenees Mountains is very scarce. During the last two years, a research project has been focussed on the study of the air quality of the Aspe Valley ecosystem and its relation with road traffic (Veschambre et al., 2003). The main objectives of the study were: · To develop reliable ultra-clean sampling and analytical methods for the determination of trace metals (Al, Ca, Mg, V, Cr, Mn, Cu, Zn, Rb, Cd, Sn, Sb, Ba, Ce and Pb), platinum group elements (Pt, Pd and Rh) and Pb isotope ratios in air particles, bulk wet deposition, snow and epiphytic lichens. · To study the dispersion of persistent pollutants in the Aspe Valley. · To evaluate the road transport impact on a mountain ecosystem. Air particles and bulk wet deposition were monitored for temporal variations, meanwhile fresh snow (short event) and epiphytic lichens (long term study) were used for spatial variations. Within this chapter the presence of palladium in the four sets of environmental samples under study from the Aspe Valley will be presented and discussed.