2018 Volume 53 Issue 4 Pages 120-129
High concentration episodes of PM2.5 have been occuring in any seasons. The formation process of these high concentrations occurring in west-east Japan have been intensively studied, but the characteristic process in summer cases has not been systematically analyzed. The present study extracted six events in the summers between 2010–2015 in the Tokyo metropolitan area, and using hourly monitoring data, analyzed the bulk behaviors of PM2.5 in parallel with Ox in the area and major urbanized prefectures in central and western Japan. Every event, spanning two to six days, coincided with a period of a series of clear sky days when the sea breeze system developed, and at the same time, with a very calm period roughly corresponding to a westerly geostrophic wind of 5 m/s or lower. A backward trajectory analysis then suggested the polluted air mass migrated eastward from western Japan in a synoptic-scale view. Even under a similarly calm condition, southerly geostrophic winds gave trajectories that reached the area of interest from the south after a westward migration, and caused no PM2.5 event.
