Mixing States And Formation Mechanisms Of S-Rich Particles In Summer And Winter In Beijing

Since the heavy pollution episode occurred in January 2013,PM2.5 pollution has become one of the most important environmental problems in China.It has drawn national attention because of its negative influences on human health and social economy.One of the districts with high concentration of PM2.5 in global scale appears in China area,especially in Jing-Jin-Ji district.The research on PM2.5 source emissions,formation mechanisms and pollution accelerations during heavy episodes have been the essential references and evidences for measures and policies of scientific haze control.This study established a method of size-fractionated filter sampling and a method of particles' analyses through the parameters' optimization,which were suitable for SEM/FESEM-EDX.In comparison with the traditional method of ion chromatography for sulfate quantifying,this method can avoid the information loss of sulfate micro existence forms during the sample extraction,and be convenient for the research on the formation mechanisms of different sulfates in micro conditions.Combined with on-line observation,we recognized and analyzed the S-rich components in PM2.5 during the typical heavy episodes of summer and winter in Beijing.Through the comparisons between micro data and macro ones in experiments,we summarized the characteristics and evolvements of S-rich particles.Meanwhile,we explored the physicochemical factors on its formations and also speculated its possible mechanisms.The results of on-line observation for typical pollution episodes in Beijing showed that the secondary formation process of sulfate in PM2.5 was a pivotal driving force to pollution occurrence.At the same time,based on the micro morphologies and element distributions in microscopies' experiments,we recognized four major existence forms of sulfate,i.e.,secondary CaSO4,S-rich mineral particles,S-rich mineral droplet and sulfate-organic carbon mixed particles.Through the pollution characteristics of different sulfates during heavy episodes,including relative abundance,S atom concentration and size distribution,we found that S-rich mineral particle was the most abundant type in summer episode,while sulfateorganic mixed particle was the most abundant one in winter episode.As the pollution became heavier,the intensities of secondary liquid formation of all types of sulfates increased synchronously and their average particle sizes enhanced gradually.However,these sulfates exhibited the different micro reaction modes and degrees in different seasons.Sulfate-organic mixed particle,showing the different mixing states in summer and winter,was the most special existence form for all sulfates.In summer,organic matters can be absorbed by sulfates to mainly form this type of particle and their biding sites and areas in individual particle can increase continuously with pollution accumulation.On the contrary in winter,sulfate can condense on the surface of organic matter to mostly produce this particle and these two parts can enhance simultaneously with pollution aggravation.The seasonal differences on mixing states and secondary formation mechanisms of sulfate-organic mixed particles are the consequences from the joint effects of gas precursors,atmospheric relative humidity and temperature,acidity,diameter and superficial area of individual particle.