Studies on Kosa flying to Japan were performed by using the concentration of SPM monitor (DKK Inc., Model GRH-76) at 6 air pollution monitoring stations in Nara pre-1ecture during the period from 1980 to 1993. Kosa event was ableto be estimated to a certain extent when the maximum concentration of SPM in a day measured by SPM monitor of β-ray absorption method was high concentration. At Kosa event, in the comparison of SPM concentration by light scattering method with β-ray absorption method,

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... ray absorption method was able to detect Kosa event, but light scattering method was not. Criteria of Kosa event by SPM concentration measured by SPM monitor of β-ray absorption method were as follows : SPM concentration in a wide area were appeared over lOOμg/m simuト taneously or were similar pattern (correlation coefficients were significant). The case over lOOμg/m3 0f SPM concentration measured by SPM monitor of β-ray absorption method throughout the whole area of Nara prefecture were 72 days during the period from 1987to 1993. The number of Kosa event was the most (28 days) in 1988, and then became to be less year by year. It was found that the most Kosa event was in January, and the secondary was in April during the observation period in Nara.