The Influence of Saharan and Middle Eastern Desert-Derived Dust on the Trace Metal Composition of Mediterranean Aerosols and Rainwaters: An Overview

The particulate aerosol over the Mediterranean Sea consists largely of a ‘background’ of European-derived anthropogenic-rich material which has a trace metal composition similar to that over other European coastal seas, but which is subject to sporadic inputs of crustal-rich material from Saharan and Middle East desert sources which are often transported in the form of dust ‘pulses’. The input of Saharan dust results in increases in the total concentrations of crust-controlled trace metals (NEEs), such as Al and Fe, and to decreases in the values of AEEs, such as Cu, Zn and Pb, in the aerosol. Variations in the concentrations of trace metals at any one site in the Mediterranean Sea are largely controlled by: (i) the magnitude of the inputs of crust-rich dusts which are mixed with the anthropogenic-rich ‘background’ material, and (ii) precipitation scavenging, which removes both anthropogenic and crust-rich material from the air.The effects that the inputs of desert-derived crustal dusts have on the Mediterranean Sea aerosol are transmitted to the marine biogeochemical cycles via the deposition of the aerosol to the sea surface. The magnitude of the air to sea fluxes are dependent on the concentrations of the trace metals in the air, but the fates of the metals are dependent on their solid state speciation in the aerosols and the mode by which they are deposited to the sea surface. The solid state speciation signatures of some trace metals are different in the anthropogenic and crust-dominated aerosol ‘end-members’. In particular, larger fractions of the total concentrations of Cu, Zn and Pb are present in potentially mobile, i.e. sea water and rain water soluble, associations in anthropogenic-dominated aerosols. Following ‘dry’ deposition to the mixed layer the trace metals Cu, Zn and Pb are therefore considerably less soluble from crust-dominated than from anthropogenic-dominated aerosols. In the ‘wet’ deposition mode, the scavenging of desert-derived dusts affects both the pH and the trace metal composition of Mediterranean rainwaters. In general, the progressive scavenging of desert-derived dusts results in; (i) an increase in the pH of the rain water solutions, (ii) an increase in the total concentrations of crust-derived metals, such as AI, and a decrease in the EF_crust values of anthropogenic-derived metals, such as Pb and Zn, and (iii) a decrease in the extent to which both crustal and anthropogenic trace metals are soluble in the rain waters from the parent aerosols as a result of the increasing pH of the solutions.