Study Of Environmental Scanning Electron Microscopy On Individual Haze Particles In Beijing

In recent years,haze pollution in Beijing has drawn great attention in the world.The haze weather caused not only visibility reduction but also severe health impact.It has become an urgent need of the moment to carry out research on the morphological characteristics and composition of haze particles,as well as their physical and chemical properties.Our studies on haze pollution situation in Beijing have started in 2013.The morphology observation,composition analysis,cathodoluminescece(CL)and hygroscopicity test on individual haze particles were performed based on an integrated high resolution environmental scanning electron microscopy(ESEM)instrument.In this paper,we analyzed about 1100 haze particles,including PM10,PM2.5 and total suspended particulates.The statistical data of component analysis showed that the weight percentage of S in particles decreased in years,whereas the weight percentage of N showed a flat trend.Classification statistics of the particles showed that the(Si,Al,O)-rich particles(mostly aluminosilicate),Ca-rich particles(mostly carbonates),Si-rich particles(mostly quartz)and other natural mineral dust particles accounted for 29% of the total simple.(S,N)-rich particles from anthropogenic sources(possibly secondary reaction particles)accounted for about 25%;Ti,Fe,Cr,Mn,Zn and other metal based particles,accounting for about 21%;fly ash particles accounted for approximately 17%;soot particles accounted for about 8%.Among them,the natural mineral dusts showed irregular shape and were usually several microns in size.Spherical fly ash from anthropogenic sources ranged from tens of nanometers to a few microns in size.Soot particles shaped like chains are around several nanometers to tens of nanometers in size.The secondary particles of mainly sulfate and nitrate showed particle sizes from tens of nanometers to several microns.Their formation endured stages of varied atmospheric reactions,resulting in complex shapes such as flies strips,spheres,cylinders,lamellar needles,etc.Natural mineral particles,anthropogenic particles,and secondary particles usually form complex mixtures.The luminescent properties of haze particles were studied by ESEM-CL.Their possible phases and origins were inferred by CL and EDS results.The oxide-based mineral dust of haze particles give out CL emission in visible light range.The nitrates and ammonium salts from secondary reactions showed a strong CL emission in the ultraviolet band.The particles like soot,fly ash,Fe-rich and Ti-rich give out only weak or undetectable CL emissions.The mixed haze particles showed broadened spectra and attenuation of the light intensity.In the wet environment of ESEM,we could adjust the relative humidity(RH)by changing the sample chamber water vapor pressure and sample temperature.The step controlling of RH could reach an accuracy up to 0.3%.In situ observation of water absorption,water-uptake,and morphology change for the haze particles were carried out during a wetting cycle of water condensation and vaporization.In addition,the deliquescence-crystallization process of pure salt particles is observed in the same wetting cycles.The deliquescence relative humidity of NH4NO3,NaCl and(NH4)2SO4 were determined to be 77.3%,79.8%,and 83.8%,respectively.The typical haze particles containing both water-soluble and non-water-soluble components would show water-uptake characteristics at namely RH greater than 100%.When aluminosilicate mineral dust,metal particles,fly ash,soot,and other water-insoluble particles were mixed with the soluble components,there would be hydrophilic and water-uptake characteristics.Sulfates,nitrates,and ammonium salts(mostly secondary particles)are usually soluble or slightly soluble,showing significant water-uptake characteristics and morphological changes in wetting cycles.The microscopy analsysis of individual haze particles in Beijing as well as the studies on their cathodoluminescence and hygroscopicity would definitly provide information on their morphology-composition-properties relationships,and therefore improve the knowledge on the characteristics,formation mechanisms,and control of haze particles.