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NIRE Annual Report
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 1999

A Numerical Analysis of the Diffusion of Volcanic Gases

Atmospheric Environment Division
Environmental Assessment Department
Objectives
In 1997, the accidents caused by volcanic gases occurred three times and 10 people were killed in Japan. Each accident was caused by different type of gases, CO2 at Mt. Hakkouda, H2S at Mt. Adatara1), and SO2 at Mt. Aso2), respectively. These volcanic gases have larger density than air in the normal temperature. Because the density depends on the concentration and temperature, the diffusion process of the volcanic gas is much complicated particularly on the complex terrain. Here the diffusion of volcanic gases from Mt. Aso (in Kumamoto prefecture) and Mt. Adatara (in Fukushima prefecture) was investigated by a numerical model with the aid of wind tunnel and water tank experiments.
Results
It is necessary to acquire the information about the emission rate and the location of the source points, in order to execute the numerical analysis. The emission rate of SO2 from Mt. Aso under stable volcanic conditions can be estimated as 40t/day3) . The source point locates in the south wall of the first crater. But those information at Mt. Adatara is hardly got. The numerical model used in the calculation was NIRE-MM4), which is usually used for the analysis of air pollution. The fine mesh topographic data were specially organized by the Geographical Survey of Japan. The results of the calculation at Mt. Aso showed that the high concentration area often extends to outside of the crater in downstream side particularly under the northerly wind condition. Then the area where tourists can enter suffers from high concentration (Fig. 1). In case of Mt. Aso, strong wind condition does not always mean relatively safe condition against the volcanic gas. On the other hand, the accident at Mt.Adatara occurred under calm condition. Although there are some visible source points of the gas in the bottom of the crater of Mt. Adatara (Numa-no-Taira), the results of numerical analysis did not show the high concentration at the point on the surrounding wall of the crater (about 50m above the bottom of the crater), where the accident occurred. It may be probable that there should be an invisible source of the gas near the accident point.
Fig.1. Calculated surface concentration of SO2(ppb) under NNE wind (7ms-1)
Selected Publications
1) URL:http://www.geo.nihon-u.ac.jp/tchiba/adatara.html.
2) URL:http://www.mri-jma.go.jp/Dep/sv/4ken/sv4lab.html.
3) Ohta, K. et al., Report of intensive observation of Mt. Aso No. 2(1981), 85-88,(1984).
4) Kondo, H., Description of NRIPR mesoscale model, Technical Report of National Research Institute for Pollution and Resources No. 44, (1989).
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