Abstract
We developed a new method for detecting Kosa (Asian dust) days from the suspended particulate matter (SPM) concentrations measured at seven air-quality monitoring stations in western Japan. When the frequency curve of the horary SPM concentration monitored at each station from January to May showed a bimodal distribution, we assigned a threshold value to extract the distribution of higher concentration. When the days with high SPM concentration were in agreement over a wide area, we defined them as SPM Kosa days. There were 101 SPM Kosa days from 1993 to 2001. Through the analysis of the distribution of atmospheric pressure and the back trajectory analysis, we confirmed that the present method was able to detect 100 Kosa days out of 101, including the Kosa days which were not detected visually because of its weak event. There were 20 SPM Kosa days in 1999, which was the maximum number of Kosa days observed during 1993-2001.
