Abstract
In Japan, the nation-wide annual-mean concentration of suspended particulate matter (SPM) dramatically decreased, and the achievement rate of the air quality standard of SPM was considerably improved in FY1999. This study shows the actual conditions and meteorological causes of the low concentration. The nation-wide monthlymean concentration of SPM was low in April, from June to August, and in February in FY 1999. In July and August, the SPM concentration was remarkably low in Kanto, where the SPM concentration was characterized with very low concentrations continuing for as long as a week or more. Meteorological factors were analyzed for the low concentrations in Kanto in July and August, 1999. On a monthly basis, these two months of 1999 were characterized by stronger wind speeds and more precipitation than the five years around 1999, which could be causes of the low concentration. In the very low concentration weeks, however, such low concentrations were related to the mostly constant wind direction, which brought maritime air from the Pacific Ocean.
