The impact of volcanic gases from Miyake island on the chemical constituents in precipitation in the Tokyo metropolitan area

The volcano on Miyake Island first erupted in July 2000 and continuous emission of volcanic gas from the collapsed caldera has been observed from the middle of August 2000. The large volcanic emission of SO 2 had a strong influence on Tokyo metropolitan area, which is located approximately 150 km north of Miyake Island. We measured major ions in precipitation and dry deposition samples which had been collected at five sampling sites (Yokohama, Kashiwa, Fujisawa, Yokosuka, and Hachioji) in the Tokyo metropolitan area for 12 years since 1990. We have evaluated quantitatively the impact of the volcanic SO 2 gas emitted from Miyake Island on the Tokyo metropolitan area by comparing depositional ionic constituents in the volcanic degassing period (from September 2000 to August 2001) with those in the normal period of the past 10 years (September 1990 to August 2000). nss-SO 4 2− concentrations in precipitation at the sampling sites in the Tokyo metropolitan area were 59.5–77.0 μeq/L during the degassing period, and 33.3–44.1 μeq/L during the normal period, respectively. The difference of nss-SO 4 2− concentrations between the two periods was statistically significant. In contrast, no significant differences were observed in the concentrations of the other major ions (NH 4 +, nss-Ca 2+, Cl −, and NO 3 −) between the two periods. The impact of volcanic degassing from Miyake Island on the ionic concentrations in the precipitation of the Tokyo metropolitan area was seen only in the H + and nss-SO 4 2− concentrations. The annual wet deposition amount of volcanic nss-SO 4 2− into the Tokyo metropolitan area has been quantitatively estimated. The annual wet deposition amounts were calculated as 701±277 Meq/year (22.4±8.9 kt SO 2/year) on the total area of the Tokyo metropolitan area (14,000 km 2). The wet deposition amount of nss-SO 4 2− corresponds to only 0.15% of the total annual amount of volcanic SO 2 (15 Mt/year) emitted from Miyake Island from September 2000 to August 2001. This estimation is consistent with a previous study which reported that 0.3% of the atmospheric discharge of SO 2 from the volcano in Mt. Etna in Sicily was deposited by precipitation to the surrounding area (1200 km 2). The annual wet deposition amount of volcanic SO 2 to the Tokyo metropolitan area was at the same order of the magnitude (37±15%) as the annual anthropogenic SO 2 amount (60 kt/year) emitted from the Tokyo metropolitan area.