Size Distribution Characteristics And Cause Analysis Of Atmospheric Particles And Its Water Soluble Ions Of Different Pollution Levels In Urban Beijing

To analyze the size distribution,concentration variation and pollution source of atmospheric particles and water soluble ions in Beijing,the ambient eight stage cascade impactor sampler were used to collect atmospheric particle samples at the roof of Smog Chamber Building in Chinese Research Academy of Environmental Sciences from January to December,2016,and water soluble ions of different size were determined by ICS-1000.Meantime,the pollutants data(SO₂,O₃,NO_x,CO,PM_2.5,NH₃)were measured regularly by the online monitoring system of environmental ambient conventional pollutants.The results suggested that the mean value of PM_2.5 was 79.36μg·m^-3,showing significant seasonal variation.The occurrence of high particle mass concentration often corresponded to unfavorable weather conditions,meanwhile the concentration of NOx and SO₂ were obviously higher than usual.Backward trajectory analysis demonstrated that the fine particle concentration in winter mainly affected by the air mass of south direction from Shanxi Province and Shaanxi Province,while in spring and autumn mainly affected by short-distance transmission of air mass in Jing-Jin-Ji Region,and the air mass in summer was relatively clearer compared with other seasons.The particles of four levels（clear,light pollution,moderate pollution,severe pollution）all had bimodal size distribution,with the peak value appearing at the size range of 0.7-2.1μm and>9.0μm.As the degree of pollution worsened,the concentration of fine particles increased gradually,and in severe pollution,the particles at the size range of 1.1-2.1μm were dominant.The water soluble ions concentration of fine particle in four level were 32.52μg·m^-3,62.31μg·m^-3,137.62μg·m^-3,175.29μg·m^-3respectively,accounting for 59.63%,63.33%,74.09%and 72.20%of total water soluble ions in particles.In slight pollution,moderate pollution and severe pollution,the concentrations of three secondary ions(NO₃^-,SO₄^2-,NH₄⁺)were higher than other ions,accounting for37.71%-52.64%,17.96%-28.47%and14.82%-17.97%of total water soluble ions in PM_2.1,respectively,while in coarse particles(PM_2.1-9),NO₃^-,SO₄^2-,Ca²⁺were main water soluble ions,accounting for 26.53%-38.46%,10.61%-28.83%and 1.66%-19.31%,respectively.According to the variation trend of size distribution,water soluble ions can be divided into three groups:NO₃^-,SO₄^2-,NH₄⁺and K⁺were unimodal distribution,and concentrated in accumulation mode,peaking at the size range of 1.1-2.1μm;Cl^-and Na⁺also mainly concentrated in accumulation mode,but were bimodal distribution,with the peak value at the size range of 1.1-2.1μm and 5.8-9.0μm;the third group ions were mainly at coarse mode,including Ca²⁺and Mg²⁺.In slight pollution,moderate pollution and severe pollution,the mean values of NO₃^-/SO₄^2-were 1.32、2.52 and 1.60,which were all larger than 1.It revealed that Beijing was greatly affected by mobile sources,and should pay more attention to the impact of motor vehicle emission especially in moderate pollution,while in severe pollution,the impact of stationary sources were relatively increased.The NH₄⁺was not sufficient to completely neutralize NO₃^-and SO₄^2-,so the PM_2.1 in three pollution levels were all acidic.In severe pollution,the concentration of NH₄⁺significantly increased,which caused the decrease of acidity.Meanwhile,the neutralization of Cl^-was also important and more ions had complicated reactions,which showed that the impact of dust and coal combustion during heating season were significant in severe pollution.The change of relative humidity was significant and important for secondary transformation process,especially during the formation process of NO₃^-,SO₄^2-and NH₄⁺.The characteristics of size distribution were closely related to the hygroscopic growth of particles,and the relative humidity at the range of 50-70%obviously promoted the formation of sulfate and nitrate and liquid water content of fine particles.The cause of severe pollution events had a certain relationship with the seasonal change,but each severe pollution event had its own characteristics because of different pollution source,the transportation path and pollutants concentration of air mass.Specifically,in March 3^rd,2016,biomass combustion was one of the main causes of this pollution event;the pollution process in March 16^th was most affected by vehicle exhaust;in November 26^th,the severe pollution was mainly affected by dust transportation;in December31^st,the influence of coal burning was the main cause of the pollution process,and the biomass combustion also had a certain contribution.The surrounding area of survey site and the boundary area of Shandong and Hebei Province could be considered as the main potential source areas of fine particles in severe pollution process in Beijing.