Chemical Characteristics And Source Apportionment Of Water-soluble Ions In PM_2.5 In Shijiazhuang

In order to study the chemical characteristic and source of water-soluble ions in atmospheric PM_2.5 particles in Shijiazhuang,atmospheric PM_2.5 samples were collected at shijiazhuang from 2017 september to 2018 february,and 8 water-soluble inorganic ions(Cl^-、NO₃^-、SO₄^2-、K⁺、Na⁺、Ca²⁺、Mg²⁺、NH₄⁺)in PM_2.5.5 were determined by ion chromatography.First of all,the pollution episodes during study period were classified according to the officially released PM2.5 mass concentration.The variations of PM2.5 mass concentration,major gaseous pollutants（SO₂,NO₂,CO,O₃）and meteorological parameters in different pollution episodes were analyzed.Afterwards,the composition and concentration characteristics of water-soluble ions in PM_2.5,the balance of anion and cation,the chemical forms of main ions,the sulfur oxidation rate（SOR）and the nitrogen oxidation rate（NOR）were analyzed under different conditions.At the same time,ion radio analysis,positive matrix factorization model（PMF）,backward trajectory model and potential source contribution function model（PSCF）were used to analyzed the source apportionment and source contribution of water-soluble ions in PM_2.5.The main results showed that:1)In Shijiazhuang,the average mass concentration of PM2.5 during the study period was 89±57μg·m^-3,the average mass concentration of PM_2.5 in autumn was 76±38μg·m^-3,the average mass concentration of PM_2.5 in winter was 102±67μg·m^-3.The average mass concentration of PM_2.5.5 in autumn and winter all exceeded the PM_2.5 concentration limit of China(75μg·m^-3).Compared with autumn,the level of PM_2.5 in winter was higher.2)The average concentration of water-soluble ions in PM_2.5 during the study period was60.49±38.71μg·m^-3.The average concentration of 8 water-soluble ions followed the sequence of NO₃^->SO₄^2->NH₄⁺>Cl^->Ca²⁺>K⁺>Na⁺>Mg²⁺.NO₃^-,SO₄^2-and NH₄⁺（SNA）were the major component of water-soluble ions,and their concentrations accounted for78.33%of the total ion concentration.The average concentration of total ions in winter was higher than autumn,the average concentration of total ions in the pollution period(PM_2.5>75μg·m^-3)was higher than that of the clean period(PM_2.5<75μg·m^-3)and the proportion of SNA in the pollution period was also higher than that of the clean period.3)Ion balance analysis showed that PM2.5 in Shijiazhuang was alkaline.Overall,the alkalinity of PM_2.5 in winter was stronger than that of autumn.The alkalinity in the clean period was stronger than that of the pollution period.The chemical forms of NH₄⁺were mainly NH₄NO₃,NH₄HSO₄ and（NH₄）₂SO₄ in autumn,while NH₄NO₃,（NH₄）₂SO₄ and NH₄Cl in winter.The main chemical forms of NH₄⁺in the clean period was similar to autumn,the pollution period was similar to winter.The SOR and NOR values were all higher than0.10 during the study period,indicating that the atmospheric conditions were beneficial to secondary oxidation process.The SOR and NOR in autumn were higher than that of winter,the SOR and NOR in the pollution period were higher than that of the clean period.The conditions of high temperature,high humidity and low wind speed are beneficial to the increased of SOR and NOR.4)The ratio analysis of NO₃^-and SO₄^2-showed that the average value of NO₃^-/SO₄^2-in PM_2.5 during the study period was 1.42±0.58,which indicated that the ion source of PM_2.5in Shijiazhuang was mainly discharged from the mobile sources.The ratio of NO₃^-/SO₄^2-in autumn was higher than that of winter,and the ratio of NO₃^-/SO₄^2-in the pollution period was higher than that of the clean period.5)Four sources of ion in PM_2.5.5 were identified by using PMF model during the study period,separately secondary source,combustion source,industrial and soil dust.The variation in source contribution ratio showed that the proportion of secondary sources in winter was significantly reduced,and the proportion of combustion sources was significantly increased.Compared with the clean period,the secondary source and industrial source in the pollution period were significantly increased,and the proportion of soil dust was significantly reduced.6)The results of backward trajectory cluster analysis showed that airflow could be divided into four categories during the study period in Shijiazhuang.The airflow mainly came from the west and the south of Shijiazhuang in autumn,while the airflow from the southern region was significantly reduced and the airflow from the northern region was significantly increased.Compared with the clean period,increased in pollutant concentration was mainly affected by airflow from the west and south regions during the pollution period.The results of PSCF model analysis showed that the potential source areas of the 8 ions in PM_2.5 were mainly located in the southwestern part of Shijiazhuang.