To better understand the characteristics and transformation mechanisms of secondary inorganic aerosols,hourly mass concentrations of water-soluble inorganic ions(WSIIs)in PM_(2.5)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 related parameters were characterized.The(TNH_(3))Rich was also defined to describe the variations of the excess NH_(3)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 PM_(2.5)has increased markedly in recent years,especially under polluted conditions.In the four seasons,only a small portion of NO_(2)in the atmosphere was converted into total nitrate(TNO_(3)),and more than 80%of TNO_(3)occurred in the form of nitrate due to the abundant ammonia.The concentration of total ammonia(TNH_(3))was much higher than that required to neutralize acid gases,and most of the TNH_(3)occurred as gaseous NH_(3).The nitrous acid(HONO)concentration was highly correlated with NH_(3)concentration and had increased significantly in Beijing compared with previous studies.The total chloride(TCl)was the highest in winter,andε(Cl^(-))was more sensitive to variations in the ambient temperature(T)and relative humidity(RH)thanε(NO_(3)^(-)).
A MOUDI-120 sampler was used in Beijing to collect multi-stage samples in the summer and winter of 2013 to 2015. Thirty-three sample sets were collected during the daytime,nighttime, and different pollution levels. The actual relative humidity in the impactors was calculated for the first time. The carbonaceous components（organic and elemental carbon,OC and EC, respectively） and water-soluble inorganic ions（Na~＋, NH4~＋, K~＋, Mg^（2＋）, Ca^（2＋）, Cl^-, NO3^-,and SO4^（2-）） were analyzed in each sample. The characteristics of the mass concentration distribution and charge balance were discussed. On the basis of relative humidity in the impactors, aerosols less than 1.0 μm were sampled under relatively dry conditions in most cases. The concentration levels for the chemical species were higher in the winter than in the summer. Three modes（condensation mode, droplet mode, and coarse mode） could be identified from the distributions of NH4~＋, NO3^-, SO4^（2-）, Cl^-, K~＋, OC and EC. The distribution characteristics for the pollution dissipation process were different from the pollution accumulation process. NO3^-and NO2^-contributed most of the negative electric charges in the stage below 0.1 μm. In the condensation mode, the cations were dominated by NH4~＋, which was sufficient to balance the anions. In the droplet mode of the heavily polluted samples,the ammonium was not sufficient to balance the anions. In the coarse mode, the positive electric charges were primarily composed of metal cations. The analyzed anions were not sufficient to neutralize the measured cations.
