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To better understand the characteristics and transformation mechanisms of secondary inor-ganic aerosols,hourly mass concentrations of water-soluble inorganic ions(WSIIs)in PM2.5 and their gaseous precursors were measured online from 2016 to 2018 at an urban site in Beijing.Seasonal and diurnal variations in water-soluble ions and gaseous precursors were discussed and their gas-particle conversion and partitioning were also examined,some re-lated parameters were characterized.The(TNH3)Rich was also defined to describe the varia-tions of the excess NH3 in different seasons.In addition,a sensitivity test was carried out by using ISORROPIA Ⅱ to outline the driving factors of gas-particle partitioning.In Beijing,the relative contribution of nitrate to PM2.5 has increased markedly in recent years,especially under polluted conditions.In the four seasons,only a small portion of NO2 in the atmo-sphere was converted into total nitrate(TNO3),and more than 80％of TNO3 occurred in the form of nitrate due to the abundant ammonia.The concentration of total ammonia(TNH3)was much higher than that required to neutralize acid gases,and most of the TNH3 occurred as gaseous NH3.The nitrous acid(HONO)concentration was highly correlated with NH3 con-centration and had increased significantly in Beijing compared with previous studies.The total chloride(TCl)was the highest in winter,and ε(Cl-)was more sensitive to variations in the ambient temperature(T)and relative humidity(RH)than e(NO3-).