A regional-scale Eulerian Model System for Soluble Particles (MSSP) was constructed to simulate environmental changes caused by a SO42- increase as the result of the eruption of Miyakejima Volcano in the northwest Pacific Ocean. The measured volcanic SO2 emission was 9 Tg yr-1 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 NH3 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 SO42- was mainly transported southeastward to the Pacific Ocean while volcanic SO42- was transported southwestward to Japan, Korea, and Taiwan, owing to the subtropical high-pressure system over the Pacific Ocean in summer. Temporal variations of SO42- concentrations and gas-aerosol equilibrium of NH3 at Happo Ridge were well-simulated. In the plume from the Asian continent, 98.7% of total SO42- was anthropogenic, and 63.5% of NH3 existed in aerosol phase as (NH4)2SO4. In the volcanic plume, 95.5% was volcanic, excessive sulfate fixed 100% of NH3 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.