ABSTRACT

Wet and dry deposits were collected using an automatic fractionation precipitation collector for three and a half years in Matsumoto city, central Japan. Because of the imperfect collection of some inorganic ions, the fraction collected as dry deposit is named “dry dish sample” in this study. The collected samples were analyzed for major cations and anions using ion chromatography. Based on the elemental ratios, Na⁺, Cl^-, and Mg²⁺ in the precipitation are considered to be of sea salt origin. However, the Cl^- found in the dry dish samples was deficient with respect to Na⁺, possibly resulting from chlorine loss from aerosols, and the Mg²⁺ content of dry dish samples is thought to be derived from the soil. Abnormally high concentrations of Ca²⁺ and SO₄^2- were observed in both the precipitation and dry dish samples for a few days after the eruption of Miyakejima volcano. The Na⁺ and Cl^- concentrations in precipitation showed high values in winter, and the yellow sand carried from China in the spring affected the concentrations of Ca²⁺ and Mg²⁺ in the precipitation, as well as those in the dry dish sample. High time-resolution sampling of the precipitation disclosed short variations in its composition, which would have otherwise escaped our attention. When continuous precipitation from stratiform clouds occurred due to a warm front, the concentration of chemical constituents in the precipitation was high in the first fraction of precipitation, but was subsequently markedly reduced. On the other hand, when intermittent precipitation was produced from cumuliform clouds due to a cold front, it became more complex: in such cases, there was no predictable pattern for the subsequent concentration fluctuations, i.e., the concentrations of the chemical constituents fluctuated within a day.