Estimation of the seasonal variation of particulate nitrate and sensitivity to the emission changes in the greater Seoul area

Seasonal variation of fine particulate nitrate and sensitivity of fine particle mass concentration to the emission changes of VOCs and NO_x were estimated in the greater Seoul area. SBOX, a photochemical box model was used to obtain the total nitric acid (HNO₃^T) concentration, and SCAPE2, a gas/particle equilibrium model was used to determine the partitioning of nitric acid/nitrate and particulate water content. Most HNO₃^T existed as nitrate except in summer (∼60%) since there was enough ammonia to form particulate ammonium nitrate. In summer, high temperature was favorable to gaseous nitric acid. Also, because of average relative humidity (RH) higher than the deliquescence points of most salts, water content in summer was higher than those in other seasons by a factor of two. For all seasons, fine particle mass concentration (the sum of ion concentrations and water content) increased until considerable amount of NO_x emissions was reduced. This phenomenon is a typical example of the so-called 'NO_x disbenefits' that has been discussed in relation to the abatement of ozone pollution.