High PM 2.5 concentrations (daily mean exceeding 50 µg/m 3 ) were observed from July 16 to 21, 2018, over a wide area of Japan. The surface level aerosol observation data by the Aerosol Chemical Speciation Analyzer (ACSA-14) were used for the analysis of the PM 2.5 compositions, and the Community Multi-scale Air Quality (CMAQ) were used for the detailed analysis of the observation data. Our findings from this study can be summarized as follows: 1) The PM 2.5 was mainly composed of SO 4 2− . SO

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... emitted from the Sakurajima volcano is converted to SO 4 2− over the East China Sea, then transported to the Japan Sea region along the marginal flow of the Pacific High. SO 4 2− over the Japan Sea, then penetrates to the San-in and the Hokuriku area by the sea breeze. 2) A sensitivity analysis determined that the volcanic SO 2 contribution was 80% at the sea near Sakurajima, 70% at the Hokuriku and off the coast of the San-in area. 3) The Process analysis of CMAQ showed that SO 4 2− near Sakurajima was formed by a gas phase reaction in the daytime and the liquid phase reaction in the clouds, then transported out by horizontal advection and diffusion. SO 4 2− at the Japan Sea increased by horizontal advection and diffusion. It indicated that SO 4 2− formed near Sakurajima was transported to the Japan Sea.