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 SO2 emission amounted to a maximum of 6 ¿ 104 t d-1, 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 104 t d-1. 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 HNO3, were made for 3--6 hours every day. Annual mean concentration of SO2 was increased 3.8 times, and concentrations of SO42- were increased 1.5 and 1.7 times in aerosol and precipitation, respectively. In contrast, aerosol concentrations of NO3- and Cl- decreased under the influence of volcanic effluents. This was caused by the excess amount of SO42- formation. The produced SO42- at first exhausted gaseous ammonia to form (NH4)2SO4 aerosol and then, driving out NO3- and Cl-, 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 HNO3 and HCl might be deposited more intensively both by wet and dry deposition processes and might accelerate the environmental acidification.