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High PM_2.5 concentration exceeding Japanese ambient criteria (daily mean 35μg/m^3) were observed in late March 2018 over the wide area of Japan, and high PM_2.5 concentration lasted for about a week.... Aerosol observations and simulation by Chemical Transport Model illustrated the cause of this episode. In this episode, blocked high pressure existed over Japan, and it made subsidence inversion layer. Subsidence inversion layer suppressed the vertical diffusion, so pollutions were accumulated under this layer. Both particle SO_4^2- and NO_3- are transported from outside of Japan, but SO_4^2- was mainly transported from China and NO_3- was also from Korea. Simulated concentrations of PM_2.5 well explained the time and space variation of observation data. But when effect of transboundary air pollution became high, simulated concentrations of PM_2.5 tended to be larger than observation, because anthropogenic emission data used was 2008 year base (REAS2008) and did not reflect recently decrease of Chinese emission. The model sensitivity analysis showed that SO_4^2- was mainly transported from outside of Japan. NO_3- concentration is effected by both the transboundary air pollution and local emission. For rural site like Oki, NO_3- is mainly controlled by the transboundary air pollution, while for urban site like Osaka, local emission is more important than transboundary air pollution.show more
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