The major conversion processes of SO$_2$ to sulfate are reactions in gaseous, aqueous phase and on dust surface. Using the measurement data in Ganghwa, the background area of metropolitan Seoul Area, the relative contiribution of the conversion processes are estimated. Generally, aqueous cloud if the most important conversion path followed by dust surface, gas, and aqueous aerosol. Importance of conversion on dust surface increases for the dust storm period. The total conversion rate values over the metropolitan Seoul area are between 1.5 and 8.8$ imes$10$^{-11}$ mole m$^{-3}$ air.