Abstract. Effect of the exclusion of crustal ions (Ca²⁺, Mg²⁺, and K⁺) in estimating water content of PM_2.5 is investigated using a gas/aerosol equilibrium model, SCAPE 2 (Simulating Composition of Atmospheric Particles at Equilibrium 2), (Kim et al., 1993; Kim and Seinfeld, 1995; Meng et al., 1998), for the particles collected at polluted city, Seoul, and clean background offshore, Gosan, Korea. Measurement data show higher concentration of all inorganic species at Seoul, up to 5 times higher in average equivalent concentration especially for volatile species. At both sites there were sufficient equivalent fractions of t-NH₃ (total ammonia = NH₃+NH₄⁺ to neutralize acidic species such as t-H₂SO₄ (total sulfuric acid = H₂SO₄+SO₄²⁻), t-HNO₃ (total nitric acid = HNO₃+NO₃⁻) and t-HCl (total hydrochloric acid = HCl+Cl⁻). t-NH₃ and t-HNO₃ were higher at Seoul while t-H₂SO₄ was higher at Gosan. With respect to the estimated water content differences between with and without crustal ions, all the samples are classified into 3 cases; increased, decreased, and constant water content. We identified that change in inorganic composition contributes to increase of water content for aqueous aerosols. And those inorganic compositions vary differently according to ambient atmospheric composition. Meanwhile, aerosol phase shifting from the aqueous to solid phase is the main contributor to decrease of water content and binary salt composition change in the solid phase appears as no change of water content.