Intensive observation campaigns approximately 1 week long were conducted periodically from March 2010 to November 2015 at Cape Hedo, Okinawa, Japan. The maximum daily mean sulfate aerosol (SO42-) concentrations surpassed 15 μg m-3 in springtime 2012. In this study, the sources of the high SO42- concentrations were estimated by using the air quality model with the tagged tracer method. The main source of the high SO42- concentrations in March was volcanoes and that in April was anthropogenic emissions from China. In March, the prevailing northerly wind transported a volcanic SO2 plume with a low conversion ratio to Cape Hedo. The impacts of 15 volcanoes in Japan were estimated in this study, and a substantial impact of Sakurajima, which accounted for more SO2 than anthropogenic emissions from Japan, was found. Because the model had difficulty capturing the highest concentration, three sensitivity simulations were performed to consider the uncertainty of volcanic SO2 emission amounts and injection heights, revealing the importance of the injection height in addition to SO2 emission amount. Throughout April, source apportionments from anthropogenic emissions from China were found; hence, the source was further divided into 31 provincial scales. Shandong and Jiangsu provinces, which are the first and seventh largest emission sources in China, were identified as important sources at Cape Hedo. These sources showed day-to-day variation, and the highest contribution from Shandong province was on April 23, whereas that from Jiangsu province was on April 22.