Role of desert aerosol in metal fluxes in the Mediterranean area

The chemical composition of aerosols and precipitation collected over Sardinia primarily reflect Saharan and European inputs. European background aerosols in Sardinia show a 3- to 25-fold decrease in EF(crust) values for Cd, Zn, Pb, and Cr compared with coastal stations in the Western Mediterranean. Partitioning of total atmospheric deposition between soluble and insoluble phases shows that Al, Si, Fe and Pb originating from the atmosphere are mostly in an insoluble form. For Ca, Cd and Cr metals, the dissolved fraction represents 50-90% of the total input. Aerosol dissolution experiments performed at particle concentrations ranging from 3 to 300 mg l(-1) show that Cd and Pb have low solubilities at high mass particle concentrations. The solubility of Pb increases with EF(crust) values and the finest grain-size of the aerosols (<1 mu m), and is also affected by pH. Dissolution of Al and Fe decreases significantly from 30% (13% for Fe) at aerosol loads lower than 5 mg l(-1) to less than 1% for both metals at total aerosol concentrations greater than 100 mg l(-1). The fluxes (dry + wet) of metals in Sardinia show that similar to 30% of the Cd and Pb to more than 80% of the Fe and Al come from the Sahara. (C) 1999 Elsevier Science Ltd. All rights reserved.