RAIN WATER AEROSOL TRACE-METAL RELATIONSHIPS AT CAP-FERRAT - A COASTAL SITE IN THE WESTERN MEDITERRANEAN

The factors controlling the distributions of the trace metals Al, Co, Ni, Cu and Pb in a series of 25 individual rain water samples collecte d at Cap Ferrat, a site on the Western Mediterranean coast, are interp reted in relation to aerosols taken simultaneously at the same site. T he trace metal chemistry and pH of the rain waters are constrained by the scavenged aerosols, which are composed of a mixture of urban-domin ated (European) and crust-dominated (Saharan) components. Thus, the pH values of the rain waters, which range between 3.95 to 6.77, reflect the type of aerosol scavenged from the air; urban-dominated aerosol co mponents giving rise to acidic rains, and crust-dominated aerosol resu lting in neutral to basic rains. The average solubilities of the trace metals in the rain waters increase in the order Al (17%), Co (36%), N i (53%), Pb (65%) and Cu (76%). The particulate <----> dissolved speci ation of the non-crust-dominated metals Cu and Pb varies with pH, bein g more soluble at lower pH values, and exhibits the classical pH 'adso rption edge'. However, the pH of rain can Vary during an individual ra in event in response to the sequential scavenging of crust-dominated a nd urban-dominated aerosol components. As a result, the solubility of non-crust-dominated trace metals, such as Pb, can also vary sequential ly during an individual rain event; the maximum solubility being relat ed to a 'dip' in pH associated with the scavenging of urban-rich aeros ol components, followed by a return to the initial pH as the pH-influe ncing components are exhausted. Data from the present study therefore indicate that the pH-controlled trace metal solubility relationship re ported for individual rain events can also occur sequentially in the s ame event. The particulate material in the rain waters does not contai n the relatively high concentrations of Ni, Cu and Pb found in the par ent aerosols, and its composition approaches that of crust-dominated a erosols transported to the Mediterranean. Data from the present study, together with those for other Western European coastal locations, ind icate that there is a Pb-Cu fractionation between aerosols and rainwat ers which results in a significantly greater fraction of the aerosol P b, relative to Cu, escaping precipitation scavenging in the coastal zo ne and so becoming available for long-range atmospheric transport. (C) 1997 Elsevier Science B.V.