We determined Asian dust days by constructing the standard of Asian dust using PM$_{10}$ concentrations ($geq$150 $mu extrm{g}$m$^{-3}$ , 24-hr average) and TOMS aerosol index ($geq$0.7) for 5 years (1998-2002), and grouped Into long-lasted cases (LLCs, $geq$4 days) and short-lasted cases (SLCs, $leq$2 days) concerning the mean lasting time (about 3 days) of Asian dust. Further we performed the specific analyses associated with concentration variations and synoptic conditions by using PM$_{10}$ and TOMS data, weather maps during the dusty cases (LLCs and SLCs). As a result, the LLCs (9 cates) had large variations of PM$_{10}$ concentration as the mean of 131.1$mu extrm{g}$m$^{-3}$ and the maximum mean of 379.8$mu extrm{g}$m$^{-3}$ , and showed dominant features the continuous passage of deep trough caused by blocking effect and weak trough (56%, 5 cases) over Korea. The SLCs (11 cases) had relatively small variations of PM$_{10}$ concentration as the mean of 133.3$mu extrm{g}$m$^{-3}$ and the maximum mean of 247.2$mu extrm{g}$m$^{-3}$ , and showed passage of one weak trough (64%, 7 cases) over Korea. Thereafter, the case studies (April 7-13, 2002 of LLC and March 23-24, 2000 of SLC) performed by the simulation of MM5 with meteorological variables of the horizontal wind, potential temperature, isentropic potential vorticity, and helped to better understand the features of synoptic conditions in connection with the concentration variations for each case.