Monthly Characteristics of Rainwater Chemistry at a Coastal Site in Southwestern Japan

Monthly characteristics of rainwater chemistry at a coastal site in southwestern Japan were examined based on an eight year record. In the period November-May when rain was mainly caused by cyclones, the monthly mean concentrations of nss-SO 4 2− , NO 3 − , NH 4 + , nss-Ca 2+ , Na + and Cl − over the eight years were 25.1–57.8, 9.9–25.0, 11.3–31.4, 5.5–18.7, 24.2–154.9 and 30.0–178.5 μeq L −1 , respectively. In June and July when rain was mainly caused by stationary fronts, i.e. Meiyu fronts, the concentrations were 14.4–20.7, 7.2–9.5, 7.7–12.9, 4.1–6.8, 21.7–33.6 and 26.4–40.5 μeq L −1 , respectively. In August and September when typhoons contributed substantial rainfall, the respective concentrations of Na + and Cl − were as high as 97.7–105.3 and 116.8–122.9 μeq L −1 , while the concentrations of other ions were low. These results indicate a large variation of monthly rainwater chemistry, which is basically dependent on the synoptic weather patterns causing rain. From later autumn to early spring, rain contains ions in high concentration and large variation ranges. In the Meiyu season, rain contains less ions which vary in a range much smaller than that in later autumn-early spring. In summer and autumn, the concentrations are low, except Na + and Cl − which can be large due to typhoons’ contribution.