Analysis On Characteristics Of Atmospheric Particles PM_2.5 And The Relationship Between PM_2.5 And The Precipitation Of Xi’an

There was a PM2.5automatic monitoring station in Xi’an University of Architectureand Technology; we studied the dependence relation between spatio-temporal factors onthe average concentration and water-soluble ions of PM2.5, and also the relationshipbetween PM2.5and the rainfall in Xi’an in this paper. All data based on monitoring andexperimental test data.The average concentration of PM2.5was92.990μg/m3, The average concentrationof PM2.5was ranged from33.316μg/m3to155.164μg/m3. Characteristics of PM2.5bytime interval variation showed that higher concentrations occurred in09:00-12:00and21:00-00:00, but from15:00to18:00was the lowest concentrations of PM2.5in differentseasons and different time interval.water-soluble ions in total PM2.5were accounted for38.03%, SO42-, NO3-andNH4+were the main ions of water-soluble components in PM2.5, the change trend ofSO42-, NO3-and NH4+in winter was similar, and didn’t show the obvious trend in otherseasons. The highest concentration of SO42-, NO3-happened in09:00-12:00and21:00-24:00the lowest concentration happened in15：00-18：00. The highestconcentration of NH4+occurred in12:00-15:00in summer.According to the experimental test data of the rainfall in Xi’an, the highest pH ofthe rainfall is7.12, the lowest is4.68, and the average is5.98. The frequency of acidrain in Xi’an is16.7%. The pollution type of the acid rain is almost H2SO4-type. Theacid rain often happened in persistent rain in autumn and winter. SO42-and NO3-were the main negative ion, NH4+and Ca2+were the main positive ion.The precipitation can remove the qualitative concentration of PM2.5，the removalefficiency is23.26%,and the removal efficiency on NH4+,Ca2+and NO3-is obvious.There is a obvious relationship between pH of PM2.5and pH of the precipitation. Thereis also a evident relationship between the qualitative concentration of PM2.5，SO42-andpH of precipitation. The relationship between NO3-and the pH of precipitation is notobvious. PM2.5in Xi’an is acid, it didn’t have buffer action for precipitation, also canadd the frequency of acid rain in Xi’an.