Pollution Characteristics And Source Analysis Of Water-Soluble Ions In PM_2.5 In Lanzhou City

In this study,Lanzhou City was used as the research area to analyze the characteristics and sources of water-soluble inorganic ions in PM_2.5 in autumn and winter.The Lanzhou Jiaotong University was selected as the sampling point,and collected the atmospheric PM_2.5 samples from Sept.2017 to Feb.2018.Synchronously we collected weather factors released by air quality real-time release platform of the China Environmental Monitoring Station,i.e.wind speed,humidity,temperature,NO₂,SO₂,O₃.The main eight kinds of water-soluble inorganic ions(NO₃^-,SO₄^2-,Cl^-,NH₄⁺,Na⁺,K⁺,Mg²⁺,Ca²⁺)in PM_2.5 were measured by ion chromatography.Principal component analysis（PCA）was used to analysis the source of PM_2.5 in different periods.The main conclusions were as follows:（1）The annual change of PM2.5 mass concentration in Lanzhou City from 2013 to2017 showed a continuous downward trend,indicating that the air pollution control work in Lanzhou City has achieved results in recent years.The PM_2.5 pollution in autumn and winter showed obvious seasonal characteristics,and the average PM_2.5 concentrations were lower in autumn than those in winter.（2）The PM_2.5 mass concentrations were 53.12μg/m³ and 122.72μg/m³ on clean days and pollution days from 14 to 22 in December of 2017,respectively,which means the PM_2.5 mass concentration on the polluted days were about 2.3 times than those on the clean days.The high concentrations of polluted days were related to the higher temperatures and the lower humidity.The concentrations of water-soluble ions in PM_2.5 on polluted days were higher than those on clean days,and similarly the concentrations of Na⁺,Cl^-,NO₃^-,SO₄^2-,and NH₄⁺increased significantly in polluted days.Those were mainly related to the coal burning and the motor vehicle emissions.（3）The total variations of water-soluble inorganic ions of PM_2.5 in Lanzhou were 5.28to 173.01μg/m³,the average concentration was 38.01μg/m³,and the concentration of water-soluble inorganic ions accounted for 55.83%of PM_2.5 in the observation periods.The order of ion concentrations were NO₃^->SO₄^2->NH₄⁺>Ca²⁺>Cl^->Na+>K⁺>Mg²⁺,and the four species ions including NO₃^-、SO₄^2-、NH₄⁺and Ca²⁺were the main water-soluble ions,accounting for 61.98%⁸8.76%of total mass concentrations.（4）The correlation analysis showed that PM_2.5 was positively correlated with these water-soluble ions,and was significantly correlated with NO₃^-,SO₄^2-,and NH₄⁺,indicating secondary pollution caused by human activities were the main sources of pollution.For the balance analysis of anion and cation,the ratios of anion to cation were less than 1,the anions in autumn and winter were relatively depleted.Considering the SOR and NOR in autumn and winter,the atmosphic PM_2.5 had a relatively high secondary conversion in Lanzhou City.（5）Using to the principal component analysis method,the pollution sources of the water-soluble inorganic ions in PM_2.5.5 in autumn and winter in Lanzhou City mainly came from secondary pollution and soil sources.The average value of NO₃^-/SO₄^2-was1.56,which indicated that mobile source contributes were more to atmospheric PM_2.5than fixed source in Lanzhou City.