Trend in total atmospheric deposition fluxes of aluminium, iron, and trace metals in the northwestern Mediterranean over the past decade (1985–1997)

Abstract

The total atmospheric deposition of aluminium (Al), cadmium (Cd), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn) on the northwestern coast of Corsica (Pirio) was sampled for two years, and their respective concentrations were measured during that period. The sampling station was chosen for its isolation from any local and regional sources of contamination. The year-to-year variability of the total atmospheric deposition was found to be high (up to a factor of 2). Using Al as the crustal reference indicates that Pb, Cd, and Zn are mainly associated with man-made aerosols (>85%), and Fe is mainly associated with crustal aerosols (>70 %). However, our results indicate that Saharan dust is a potential source of “natural” lead, especially in the case of a major input of dust to the northwestern Mediterranean. To determine the spatiotemporal variability of the trace metals over the northwestern Mediterranean Sea, the observed fluxes were compared to those found in the past decade using the same methodology. The comparison indicated a relative homogeneity of Cu and Ni fluxes over the northwestern Mediterranean and over the past decade. The decrease found in Al and Fe since 1985 (by a factor of 4 to 6) can be related to the decrease in Saharan dust fallout. The limitation in the use of lead additives in gasoline may have resulted in a decrease in the European atmospheric lead emissions by a factor of about 6 since 1985 and in a maximum decrease in the total atmospheric flux by a factor of 12 at most if the natural interannual variability of atmospheric deposition in this region is taken into account. So the recorded decrease in the atmospheric lead flux since 1985 (by a factor of ∼30) reveals a slight local contamination at the earlier Corsican station. In the case of Cd and Zn, there was a decrease by a factor of 30 and 23, respectively, between the data obtained at La Tour du Valat (July 1988 to June 1989) and those at Pirio (March 1996 to March 1997); such a decrease cannot be the consequence of either a reduction in emissions in Europe as a whole (the factor being at most 4), or the distance from the emission sources. Hence, we can confirm that Zn and Cd fluxes at La Tour du Valat and Zn fluxes at Cap Ferrat were not representative of a long-range transport to the Mediterranean but the result of a local/regional contamination. The large decrease observed for metals (Cd, Zn, and Pb) mainly mobilized by human activities, results from a combination of the actual diminution of the concentrations due to a reduction in the emissions and the occurrence of local/regional contamination for some elements at some sampling sites.