Trace metals in bulk precipitation and throughfall in a suburban area of Japan

Throughfall and bulk precipitation samples were collected monthly for 1.5 years over bare land and under canopies of Japanese cedar (Cryptomeria japonica), Japanese red pine (Pinus densiflora), Japanese cypress (Chamaecyparis obtusa), and bamboo-leafed oak (Quercus myrsinaefolia) in a suburban area of Japan. Samples were analyzed for dissolved Al, Mn, Fe, Cu, Zn, Ag, In, Sn, Sb and Bi by ICP-AES and ICP-MS. The metal concentrations were higher in throughfall, especially that of C. japonica, than bulk precipitation. Enrichment ratios (ERs: ratios of metal concentrations in throughfall to those in bulk precipitation) ranged from 2.5 (Zn) to 5.3 (Ag) (3.9 on average), and ERs for slightly soluble metals were generally higher than those for easily soluble metals. Concentrations of Mn, Fe, Cu, and Zn accounted for 99% of the total concentration of heavy metals in rainwater, whereas those of rare metals such as Ag, In, Sn, and Bi totaled <0.23%. Average concentrations of rare metals were 0.002 and 0.010 μg l−1 for Ag, 0.001 and 0.005 μg l−1 for In, 0.062 and 0.21 μg l−1 for Sn, and 0.006 and 0.023 μg l−1 for Bi in bulk precipitation and throughfall, respectively. The metal concentrations in rainwater were negatively correlated to the volume of rainwater, indicating that washout is the main mechanism that incorporates metals into rainwater. From the enrichment factors, that is, (X/Al)rain/(X/Al)crust, metals other than Fe were shown to be more enriched in rainwater than in the Earthʹs crust, including those present as a result of leaching from soil dust (Mn) and from anthropogenic sources (Cu, Zn, Ag, In, Sn, Sb, and Bi).