Direct Observations Of Organic Aerosols In Common Wintertime Hazes In North China Plain

Organic aerosols influence the physical and chemical properties of atmospheric particles(e.g.,size,light-absorptivity,and hygroscopicity);they directly affect visibility and climate by scattering and absorbing solar radiation.Many studies have focused on the physicochemical properties of aerosol particles in unusually severe haze episodes instead of the more freqent and less severe hazes.Understanding the physical and chemical properties of organic aerosol particles in these haze days is important to evalate their influences on climate and human.Consistent with this lack of attention,the morphology and mixing state of organic matter(OM)particles in the frequent light and moderate(L&M)hazes in winter in North China Plain(NCP)have not been examined,even though OM dominates these fine particles.In order to characterize organic aerosol particles in NCP,we carried out one regional field experiment at an urban site(Jinan,S1),a mountain site(Tai,S2),and a background island site(Changdao,S3)in NCP,individual particle samples were collected on December 13-23,2014.Moreover,we collected PM2.5 filter samples during the same period in these smpling sites.In the present work,morphology,mixing state,and size of organic aerosols in the L&M hazes were systematically characterized with the comparisons among three sampling sites using transmission electron microscopy(TEM)coupled with energy-dispersive X-ray spectrometry(EDX),atomic force microscopy(AFM)and nanoscale secondary ion mass spectrometry(nanoSIMS).Besides,we analyzed the source and aging process of organic aerosol particles in NCP.Water-soluble ions,OC and EC concentrations in PM2.5 were measured in both haze and clean days by an ion chromatography system(Dionex ICs-90)and an OCEC analyzer(Sunset Lab).TEM/EDX identified six types of particles during the haze episodes:sulfates(including K-rich sulfate and ammonium sulfate),metal,fly ash,mineral,soot,and OM particles.We found that OM-containing particles were most abundant in all the haze samples,accounting for 70%of the 5090 analyzed particles.Based on their morphology,the OM particles were divided into six different types:spherical(type 1),near-spherical(type 2),irregular(type 3),domelike(type 4),dispersed-OM(type 5),and OM-coating(type 6).In the three sampling sites,type 1-3 of OM particles were most abundant in the L&M hazes and most of them were internally mixed with non-OM particles.Based on the high resolution of TEM,we found that 86%OM particles were internally mixed with non-OM particles(e.g.,soot,mineral,metal,fly ash,and sulfate particles)at three sites.Based on their morphological mixing state,we discriminated four OM internally mixed particles:OM-soot,OM-mineral,OM-fly ash/metal,and OM-sulfate particles.Our results showed that 83%of type 1-4 OM particles were attached to soot,mineral,sulfate,and metal particles,only 17%of type 1-4 OM particles were externally mixed particles,and all the type 5-6 OM were internally mixed with sulfate particles.Aerosol particles collected at S2 and S3 display a similar size peak at?400 nm,much smaller than the peak at 600 nm at the S1 site.This result indicated that sizes of locally emitted OM-containing particles were much larger than the long-range transported OM-containing particles.We further obtained the size distribution of type 1-3 OM particles which displayed similar peaks around 350 nm at all three sampling sites.This result suggests that the OM sources were similar in the same haze layer over the NCP.In this study,we discussed the sources of the abundant organic aerosol particles in L&M haze days in NCP.We believe that the abundant type 1-3 OM particles were emitted from coal combustion or biomass burning for household heating and cooking in wintertime.In particular,the abundant near-spherical OM particles with higher Sph and lower AR indicated that these OM particles formed in cooling process after polluted plumes emitted from coal combustion and biomass burning.To evaluate OM sources in this study,we compared ratios of Si,O,and C in individual OM particles collected in haze and fresh OM particles from corn stalks and coal combustion conducted in the laboratory.Based on the Si-O-C ratio in OM particles,we estimated that 71%of type 1-3 OM particles were associated with coal combustion.Our result suggested that coal combustion in residential stoves was a widespread source from urban to rural areas in the NCP.Furthermore,average OM thickness which correlated with the age of the air masses in type 6 particles only slightly increased from S3 to S2 to S1,suggesting that the photochemistry and heterogeneous reactions between aerosols and gases were much slower in L&M hazes which were usually in dry(relative humidity<60%)comdition.This is consistant with our result that we found higher number and mass fractions of abundant type 1-3 primary OM particles and lower type 6 secondary OM particles in the L&M hazes.Thermoduric properties of some typical OM particles were studied in this study.We found that the shape of type 1 OM particles relatively stable after the heating process of 300 degrees and 500 degrees.The size of OM-soot particles and the thickness of type 6 OM particles were decreased after heating process.Type 4 and type 5 OM particles include volatile and non-volatile OM under the mentioned temperature in this thesis.As for the inorganic components which mixed with OM particles were evaporated in the process of heating analysis.This thesis studied the physical and chemical properties of organic aerosol particles in detail information collected in the North China Plain(NCP),which concluded that the direct emissions from coal stoves without any pollution controls in rural areas and urban outskirts mainly contributed into the regional L&M haze episodes in the NCP.This finding provides important information on the different haze formations and evaluate their atmospheric impacts on climate and human health.