The chemical composition of $PM_{2.5}$ such as ${SO_4}^{2-},;{NO_3}^-,;Cl^-,;{NH_4}^+,;Ca^{2+},;K^+,;Na^+,;Mg^{2+}$, OC, and EC and the concentrations of reactive trace gases including $O_3,;CO,;NOx,;SO_2,;and;H_2O_2$ were measured at Gosan in Jeju Island during March $13{sim}30$, as a part of the Atmospheric Brown Clouds-East Asian Regional Experiment 2005(ABC-EAREX2005). The average mass concentrations of $PM_{2.5}$ was 27.3 ${mu}g/m^3$, of which OC showed the highest concentration as 4.22 ${mu}g/m^3$ and nss ${SO_4}^{2-}$ was the second highest as 3.34 ${mu}g/m^3$. During that period, average concentrations of CO and $O_3$ was about 300 ppbv and 56 ppbv, respectively. For the whole experiment, the correlations of CO with ${SO_4}^{2-}$ and EC were very good, which suggests that CO can be used as tracer for the formation of fine aerosols. Several pollution and dust episodes were identified by the enhancement of CO, OC, EC, nss ${SO_4}^{2-},;or;Ca^{2+}$ concentrations or their ratios. In conjunction with factor analysis, air trajectory analysis, and comparison with emission inventories, these results indicate the spring aerosols collected at Gosan was strongly influenced by Asian outflows.