Three-year determination of trace metals and the lead isotope ratio in rain and snow depositions collected in Higashi–Hiroshima, Japan

Abstract

The concentrations of 14 elements (Al, Ba, Ca, Cd, Cu, K, Mg, Mn, Na, Ni, Pb, Sr, V and Zn) and five ions (Cl −, NO 3 −, SO 4 2−, NH 4 + and H +) in wet depositions are reported. Samples were collected in Higashi–Hiroshima in western Japan for 3 years (1995–1997). The characteristic variations of these species were investigated in terms of the source of atmospheric heavy metals. The elements were classified into three categories by principal component analysis; constituents from an anthropogenic source (Cd, Cu, Pb, V, Zn, NO 3 −, SO 4 2−, NH 4 + and H +), elements associated with crustal source (Al, Ba, Ca, and Mn) and elements from sea salts (Na, Mg and Cl −). The depositions and concentrations of the elements associated with soil dust were peculiarly enhanced in the spring. The Kosa phenomenon, in which yellow soil dust was transported from Asian deserts by strong westerly winds in spring, is a possible reason of increased deposition and concentration of the elements. Soil-derived elements were washed out at the early stage of a precipitation event, and their concentrations were mostly controlled by dilution. For the anthropogenic elements, such as Cd, Pb and Zn, no systematic seasonal variation in wet deposition was observed. Log–log plots of precipitation volume and the concentration of anthropogenic elements suggested that the concentrations of these elements were controlled not only by dilution but also by other factors. Lead isotope ratios changed in winter. The changes must be associated with pollutant-enriched aerosols and snow particles transported from the Asian continent by strong northwest monsoons, since lead isotope ratios in aerosols transported from Asian continent are different from those in Japanese aerosol. The chemical species in snow depositions collected between the Sea of Japan and the Seto Inland Sea were also determined to evaluate the long-range transport of pollutants from the Asian continent in winter.