Aerosol samples were collected by high-volume-samplers at 11 sampling points in Seoul, Taegu, Kwangju, Ulsan and Pusan, Korea during Kosa periods in 1995. The water soluble inorganic ions in the aerosol samples were measured and compared with those in rain water samples collected at Seoul, in 1995 and 1993. Calcium ion concentration was evaluated in two parts, one which actually contributed to the neutralization of rain and/or cloud water and the other which did not. The former should consist of carbonate, the latter, presumably, of sulfate and/or nitrate. The ratio of Ca2+ concentration derived from calcite to total Ca2+ concentration in rain water during Kosa periods, was estimated. As a result, it was suggested that almost of Ca2+ in the rain water was derived from CaCO3 in Kosa aerosols. On the other hand, it was considered that the Kosa aerosols collected on the ground had reacted with acidic materials on the filter. Sea-salt components were not correlated with Ca2+ derived from Kosa aerosols collected at sampling points located in the east side of Korea, but were correlated with that collected at sampling points located in the west side.