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; for example, the solubility of trace metals is considerably greater from 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 with 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 terms of the manner in which trace metals are partitioned between the anthropogenic 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 regions; and b) particulate aerosol populations from a number of adjacent coastal seas. The crustal component in the samples was assumed to have the composition of the average crust and trace metals in excess of their crustal metal:Al ratios were assigned to a European aerosol ''background'' 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 emissions of trace metals over Europe. The extent to which the EABC is affected by mixing with crustal material in the particulate aerosols was estimated using enrichment factors in which Al is employed as the source indicator; thus, EF(crst) = (C(xp)/C(Alp))/(C(XC)/C(Alc)), in which C(XP) and C(Alp) are the concentrations of an element x and Al, respectively, in the aerosol, and C(XC)c and C(Alc) are their concentrations 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 have EF(crust) > 10 are assumed to have a significant non-crustal source and are termed anomalously enriched elements (AEEs). For a particulate aerosol composed of crustal and anthropogenic components the EF(crust) values of the AEEs will be reduced as the proportions of crustal material 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 aerosol population are plotted against the concentrations of Al, the particulate aerosols from the atmosphere of the Mediterranean Sea and surrounding regions can be described in terms of the extent to which the EABC is mixed with, and diluted by, crust-rich components. Under certain conditions the ''mixing relationship'' is distorted by relatively large inputs of either: a) crust-rich components, or b) urban-rich material having a local origin. However, the overall ''chemical character'' of the aerosols can be assessed on the basis of the EF(crust) values of the AEEs Cd, Pb, Zn, Cu, Ni and Cr.