Chemical properties of aerosols were investigated by analyzing the inorganic water-soluble content in PM2.5 collected in the downtown area of Jeju City in Jeju Island. Due to an increase in both the number of visiting tourists and the size of local population, the number of cars in this area is increasing, causing an increase in PM2.5. Eight PM2.5-bound major inorganic ions were analyzed during the summer and winter periods. The water-soluble inorganic component represents a significant fraction of PM2.5. In particular, secondary inorganic aerosols contribute 36.2% and 47.5% of PM2.5 mass in summer and winter, respectively. Nitrate concentrations increase for [NH4 +]/[SO4 2-]>1.5, and excess ammonium, which is necessary for ammonium nitrate formation, is linearly correlated with nitrate. These results are clearly observed during the winter because conditions are more conducive to the formation of ammonium nitrate. A significant negative correlation between Nitrogen Oxidation Ratio (NOR) and temperature was observed. The obtained results can be useful for a better understanding of the aerosol dynamics in the downtown area in Jeju City.