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 (SO 4 2– ) concentrations surpassed 15 µg m –3 in spring 2012. In this study, source apportionment for these high concentrations was conducted using an air quality model with the tagged tracer method, and the main source was identified as volcanoes in March and as anthropogenic emissions from China in April. In March, the prevailing northerly wind transported a volcanic SO 2 plume with a low conversion ratio to Cape Hedo. The impacts of 15 volcanoes in Japan were estimated, and a substantial impact from Sakurajima, which accounted for more SO 2 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 the volcanic SO 2 emission amounts and injection heights, revealing the importance of the injection height in addition to the SO 2 emission amount. Throughout April, contributions from anthropogenic emissions from China were found; hence, this source was further divided into 31 provincial scales. Shandong and Jiangsu Provinces, which are the first and seventh largest emission sources in China, respectively, were identified as significant sources at Cape Hedo. These sources showed day-to-day variation in their contributions, and the highest contribution from Shandong Province occurred on April 23, whereas that from Jiangsu Province occurred on April 22.
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