An investigation is carried out for the roles of the synoptic meteorology in governing $SO_2$ concentrations in Seoul during the wintertime. This study has used the daily records of wind and temperature measured at the Korea Meteorological Administration in Seoul. A one-dimensional diffusion model has been used for investigating the influence of the mixing height on the diurnal variation of concentration. The day to day variations of the concentration are well correlated with those of wind and temperature. The diurnal variation of the concentration is dictated by the variation of mixing height. It is also found that mesoscale wind field is required to explain the mesoscale distribution of the concentration.