Winter and summer samples were collected from January 3 to February 7, 1997 and July 26 to September 11, 1997, respectively, in Seoul. This study was to characterize the concentrations of the annular denuder system (ADS) were $HNO_3$, $HNO_2$, $SO_2$ and $NH_3$ in the gas phase, and $PM_{2.5}$ ($d_p$〈2.5$mu extrm{m}$), $SO_4^{2-}$, NO3-, and $NH_4^+$ in the particulate phase. All chemical species monitored from this study showed statistical seasonal variations except for $SO_4^{2-}$ . Nitric acid (HNO3) and ammonia ($NH_3$) exhibited substantially higher concentrations during the summer, while nitrous acid ($HNO_2$) and sulfur dioxide ($SO_2$) concentrations were higher during the winter. $PM_{2.5}$, $NO_3^-$, and $NH_4^+$ were higher leves in the winter. High correlations were found among $PM_{2.5}$, $NO_3^-$ $SO_4^{2-}$ and $NH_4^+$ during two seasons.