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[1] Miyake volcano, which is located in the northwest Pacific Ocean 200 km south of the Tokyo metropolitan area, began to erupt on 8 July 2000. Its SO 2 emission amounted to a maximum of 6 Â 10 4 t d À1, which was about the same level as the anthropogenic emission of northeast Asia and 20 times larger than that of Japan, and is decreasing to 10 4 t d À1 . Aerosol and precipitation, together with gaseous pollutants, have been observed from 2 years before the eruption to present on a prominent mountain ridge, Happo ridge (330 km north of the volcano). Short time samplings of aerosols and gaseous acids, such as HCl and HNO 3 , were made for 3-6 hours every day. Annual mean concentration of SO 2 was increased 3.8 times, and concentrations of SO 4 2À were increased 1.5 and 1.7 times in aerosol and precipitation, respectively. In contrast, aerosol concentrations of NO 3 À and Cl À decreased under the influence of volcanic effluents. This was caused by the excess amount of SO 4 2À formation. The produced SO 4 2À at first exhausted gaseous ammonia to form (NH 4 ) 2 SO 4 aerosol and then, driving out NO 3 À and Cl À , it took their place in the aerosol and sometimes existed as sulfuric acid mist after exhausting ammonia. These facts were explained successfully by a multicomponent gas-aerosol equilibrium. The expelled HNO 3 and HCl might be deposited more intensively both by wet and dry deposition processes and might accelerate the environmental acidification.

[1] A regional-scale Eulerian Model System for Soluble Particles (MSSP) was constructed to simulate environmental changes caused by a SO 4 2À increase as the result of the eruption of Miyakejima Volcano in the northwest Pacific Ocean. The measured volcanic SO 2 emission was 9 Tg yr À1 for a year from the beginning of the eruption, July 2000. It is equivalent to 70% of global volcanic emission and 6.9% of global anthropogenic emission. Seasonal variations of the volcanic sulfate increase, and change of gas-aerosol partitioning of NH 3 and pH decrease of precipitation were studied using the MSSP model for 1 year from September 2000 to August 2001, together with observations performed at Happo Ridge observatory in the mountainous area in central Japan. In winter, northwesterly wind prevails, and volcanic SO 4 2À was mainly transported southeastward to the Pacific Ocean while volcanic SO 4 2À was transported southwestward to Japan, Korea, and Taiwan, owing to the subtropical high-pressure system over the Pacific Ocean in summer. Temporal variations of SO 4 2À concentrations and gas-aerosol equilibrium of NH 3 at Happo Ridge were well-simulated. In the plume from the Asian continent, 98.7% of total SO 4 2À was anthropogenic, and 63.5% of NH 3 existed in aerosol phase as (NH 4 ) 2 SO 4 . In the volcanic plume, 95.5% was volcanic, excessive sulfate fixed 100% of NH 3 into aerosol phase, and aerosol was strongly acidified. Modeled annual mean pH of precipitation in Japan decreased by 0.3-1.0, which is equivalent to neutralization by yellow sand.

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