Wet deposition of trace metals in Singapore

The concentrations of 12 trace metals (Al, Cd, Cr, Cu, Co, Fe, Mn, Ni, Pb, Zn, V, and Ti) in wet depositions are reported. Eighty four rainwater samples were collected using an automated wet-only sampler in Singapore for one year (2000) and subjected to chemical analysis using ICP-MS. Based on the volume-weighted mean concentrations measured, the trace metals were classified into three groups: Al and Fe with an average concentration of larger than 15 mug L-1, Cr, Cu, Mn, Ni, Pb, Zn, V, and Ti with concentrations between 1 and 10 mug L-1, and finally Co and Cd with concentrations lower than 1 mug L-1. Element enrichment factors were calculated to distinguish between natural and anthropogenic sources. The calculation of crustal enrichment factors with Al as the reference element indicated that while Ti, Fe and Mn originated from crustal sources, the remaining trace metals (Cd, Cr, Co, Cu, Ni, Pb, Zn and V) were mainly derived from anthropogenic sources. The removal of the trace metals from the atmosphere by precipitation was influenced by the rainfall amount as well as pH. The magnitude of the measured average annual wet deposition fluxes of Al, Fe, and combustion-generated elements such as V, Ni, and Cu is higher than that reported for other sites outside Singapore, owing to abundant rainfall throughout the year in this region.