Particulate matter with diameters less than 2.5 microm (PM2.5) and ranging between 10 to 2.5 microm (PM10-2.5) were simultaneously collected at four air-quality monitoring stations in the Taichung area of central Taiwan during the period of February 12 to 22, 2004. Two different types of PM10 episodes, a nonlocal dust-storm episode and a local episode, were observed in the present study. High concentrations of coarse aerosols occurred during the dust-storm episode, whereas high concentrations of fine aerosols were present during the local episode. Relatively high levels of Na+, Mg2+, Ca2+, and Cl- in coarse aerosols were observed during the dust-storm episode. Very high concentrations of secondary aerosols (NH4+, SO4(2-), and NO3-) in fine aerosols were observed during the local episode. The nitrate ion demonstrated the greatest increase in the ratios of ionic species to PM2.5 and ionic species to PM10-2.5 during the local episode. Significantly high ratios (0.444) of NO3- to NO2 in fine aerosols were present during the local episode, indicating that the relatively high formation rate of NO3 was one of the important factors leading to the increase of the NO3 to PM2.5 ratio during the local episode. Results also showed that an abundant quantity of fine ammonium nitrate was formed during the local episode, and chloride depletion probably was the major pathway to form coarse NaNO3 during this episode.