DEFINING THE CHEMICAL CHARACTER OF AEROSOLS FROM THE ATMOSPHERE OF THE MEDITERRANEAN-SEA AND SURROUNDING REGIONS

Atmospherically-transported trace metals can play an important role in biogeochemical cycles in the Mediterranean Sea. However, although the magnitude of trace metal air to sea fluxes is initially dependent on the metal concentrations in the air, it is the ''chemical character'' of the aerosol which constrains the fate of the metals in sea water; f or example, the solubility of trace metals is considerably greater fro m anthropogenic than from crustal components. To a first approximation the chemical composition of the Mediterranean particulate aerosol is controlled by the extent to which an anthropogenic-rich ''background'' material, having a mainly European origin, is perturbed by mixing wit h crustal components having a desert origin. These perturbations offer an environmentally meaningful index which can be used to define the ' 'chemical character'' of the Mediterranean particulate aerosol in term s of the manner in which trace metals are partitioned between the anth ropogenic and crustal components. This ''chemical character'' concept was assessed using: a) a total of 83 particulate aerosols collected on a W --> E transect across the Mediterranean Sea and surrounding regio ns; and b) particulate aerosol populations from a number of adjacent c oastal seas. The crustal component in the samples was assumed to have the composition of the average crust and trace metals in excess of the ir crustal metal:Al ratios were assigned to a European aerosol ''backg round'' component (EABC). It was found that the average Zn:Cu, Zn:Ni, Zn:Cr and Zn:Mn ratios in the EABC indicate that it has a composition which is generally similar to the average anthropogenic background emi ssions of trace metals over Europe. The extent to which the EABC is af fected by mixing with crustal material in the particulate aerosols was estimated using enrichment factors in which Al is employed as the sou rce indicator; thus, EF(crst) = (C(xp)/C(Alp))/(C(XC)/C(Alc)), in whic h C(XP) and C(Alp) are the concentrations of an element x and Al, resp ectively, in the aerosol, and C(XC)c and C(Alc) are their concentratio ns in average crustal material. Elements which have EF(crust) values < 10 in a particulate aerosol are assumed to have a significant crustal source and are termed non-enriched elements (NEEs), and those which h ave EF(crust) > 10 are assumed to have a significant non-crustal sourc e and are termed anomalously enriched elements (AEEs). For a particula te aerosol composed of crustal and anthropogenic components the EF(cru st) values of the AEEs will be reduced as the proportions of crustal m aterial increase. When this is expressed in the form of an EF diagram, in which the EF(crust) values of a trace metal in a particulate aeros ol population are plotted against the concentrations of Al, the partic ulate aerosols from the atmosphere of the Mediterranean Sea and surrou nding regions can be described in terms of the extent to which the EAB C is mixed with, and diluted by, crust-rich components. Under certain conditions the ''mixing relationship'' is distorted by relatively larg e inputs of either: a) crust-rich components, or b) urban-rich materia l having a local origin. However, the overall ''chemical character'' o f the aerosols can be assessed on the basis of the EF(crust) values of the AEEs Cd, Pb, Zn, Cu, Ni and Cr.