FACTORS CONTROLLING THE SOLUBILITIES OF TRACE-METALS FROM NONREMOTE AEROSOLS DEPOSITED TO THE SEA-SURFACE BY THE DRY DEPOSITION MODE

Previous studies have shown that when non-remote marine aerosols are delivered to the sea surface by the 'dry' deposition mode some trace metals (e.g. Cu, Zn and Pb) are more soluble from urban-rich than from crust-rich aerosols, but that others (e.g. Mn and Cd) are significantly soluble from both aerosol types. These solubility variations are not directly related to the enrichment of the metals with respect to the crustal source and an attempt was therefore made to explain them within a speciation-mediated 'potential environmental mobility' framework, in which metals in exchangeable associations are the most mobile in aqueous media. The results showed that there is a well-developed relationship between the extent to which a trace metal is soluble in seawater and the extent to which it is held in exchangeable associations in non-remote marine aerosols. Thus, Cu, Zn and Pb are more soluble from urban-rich than from crust-rich aerosols because they are more exchangeable in character in the urban-rich types. In contrast, Mn and Cd are significantly soluble from both crust-rich and urban-rich aerosols because they are significantly exchangeable in character in both aerosol types. However, there is not a 1:1 relationship between the extent to which all trace metals are exchangeable in character and the extent to which they are soluble in seawater. For example, in one urban-rich aerosol the average ratios between the percentage of the total metal which is soluble in seawater and the percentage which is in exchangeable associations are 0.93 for Zn but only 0.63 for Pb. As a result, only about 65% of the 'potentially environmentally mobile' anthropogenic Pb delivered to the oceans via non-remote urban-rich marine aerosols will enter the dissolved trace metal pool, compared with about 90% of the anthropogenic Zn. In seawater, crustal aerosols can act as re-scavenging substrates for dissolved Zn and Pb released from urban-rich aerosols. In contrast, the dissolved Mn and Cd released from urban-rich aerosols is not re-scavenged by crustal aerosols. After their deposition to the sea surface urban-rich aerosols can therefore act as sources for dissolved Zn, Pb, Mn and Cd to the mixed layer. In contrast, whereas crust-rich aerosols are still sources of dissolved Mn and Cd, they can act as sinks for dissolved Zn and Pb in the oceanic mixed layer. The trace metal 'seawater solubility-exchangeable speciation' relationship derived in the present paper can be applied directly to coastal seas and to non-remote ocean regions, such as the North Sea and the Arabian Sea; however, for some trace metals, e.g. Fe and Pb, aerosol solubility characteristics may be different in remote open-ocean regions.