The authors conducted on-line measurements of water soluble inorganic ions and their gas phase precursors for more than 30 days at Wangdu, Baoding City in summer in 2014 with a Gas and Aerosol Collector (GAC). It was found that, the observation site was always subjected with an ammonia rich conditions; the averaged PM_2.5 concentration was 68.2 μg/m3, of which the detected SO₄^2-, NO₃^-, Cl^-, NH₄⁺ and K⁺ by GAC was 12.6, 8.5, 1.4, 11.7 and 0.7 μg/m³, respectively, consisted 51% of the total mass of PM_2.5. All these observed pollutants showed distinct diurnal variations: SO₂, SO₄^2-, NO₃^-, NH₄⁺ and Cl^- showed a morning maximum, HCl and HNO₃ presented an afternoon peak, and NH₃ possessed a day-night variations. As depicted by the analysis of Sulfur and Nitrogen Oxidation Rates (SOR, NOR), fast transformations of trace gases into secondary inorganic aerosols were indicated. The averaged value of SOR and NOR are 0.43 and 0.22, respectively. The variation of SOR and NOR inferred that both gas phase and aqueous oxidations contributed to the fast transformation that formed water soluble inorganic salts.