Sources,Physicochemical Characteristics,and Aging Mechanism Of Individual Aerosol Particles In North China Plain

In recent years,owing to the rapid development of economy and the acceleration of industrialization and urbanization,large quantities of aerosol particles from anthropogenic sources are released into the atmosphere,which lead to the frequent occurrence of haze-fog episodes in China,especially in North China Plain(NCP).In order to elucidate the severe haze-fog formation in China,many studies have been conducted and the scientists already realized that high-intensitive pollutant emissions are internal causes,while unfavorable meteorological and topographic conditions are external causes.However,because the haze-fog formation involves complex chemical transformation among different pollutants and feedbacks between internal and external factors,the haze-fog formation mechanism in NCP is still controversial.Current researchs mainly use various bulk methods to characterize the concentration and composition of primary particles from emission sources and ambient particles in the atmosphere.However,they cannot provide the detailed information about the morphology,composition,and mixing state of individual particles,which is critical to track the emission sources of ambient particles and reveal the atmospheric aging process of primary particles.In this study,to better understand the formation mechanisms of the frequent occurred haze-fog episodes in NCP,we firstly characterized the morphology,composition,and mixing state of individual particles from different emission sources using a transmission electron microscope equipped with an energy-dispersive X-ray spectrometer(TEM-EDS),and therefore constructed an individual particle database of emission sources.Secondly,filed observations were carried out in typical rural and urban sites in NCP during wintertime.The concentrations of PM2.5 and its components(water-soluble inorganic ions(WSII),organic carbon(OC),elemental carbon(EC),trace metals)were ananlyzed by bulk methods.The type,morphology,composition,size,and mixing state of individual particles were characterized by single particle analysis.We confirmed the sources of ambient particles and the aging mechanism of primary particles during haze episodes,which is helpful for elucidating the formation mechanism of haze-fog in NCP.Finally,the morphology,composition,size,and mixing state of the cloud droplet residual(cloud RES)and interstitial(cloud INT)particles which were collected in the cloud events at the top of Mt.Tai were analyzed by TEM-EDS.The effects of anthropogenic particles on aerosol-cloud interactions under heavy pollution conditions in NCP were explored.The main conclusions in this study are listed as follows:(1)The morphology,composition,and mixing state of primary particles were investigated by TEM-EDS.The results showed that particles from crop residue burning were mainly organic matter(OM)in smoldering phase,whereas soot-OM internally mixed with K-rich in flaming phase.Besides,wild grass buring in flaming phase released some Cl-rich-OM/soot particles.Apple wood burning emitted spherical gel-like OM in the smoldering phase,but irregular OM internally mixed with KCl and soot in the flaming phase.In the burn-out phase,the particles were mainly cluster-like soot particles with OM coating.Interestingly,particles from hardwood(pear wood and bamboo)and softwood(cypress and pine wood)burning were mainly soot and OM in the flaming phase,respectively.Cardboard burning in the flaming phase mainly released OM particles and a small number of them were internally mixed with S-rich particles.The combustion of foam boxes,rubber tires,and plastic bottles/bags in the flaming phase released large amounts of soot internally mixed with a small amount of OM.The brominated flame retardant-containing materials,such as printed circuit boards and copper-core cables,emitted large amounts of OM with Br-rich inclusions in the flaming phase.In addition,the printed circuit board combustion released toxic metals containing Pb,Zn,Sn,and Sb.Bituminous coal from Inner Mongolia mainly emitted spherical dark OM particles in the flaming stage and gel-like OM particles in the burn-out phase.Bituminous coal from Shandong mainly emitted OM particles and parts of them were internally mixed with soot in the flaming phase,and gel-like OM particles were emitted in the burn-out phase.Particles emitted by heavy-duty diesel tucks were mainly chain-like soot particles coated by OM.Most of the particles emitted by light-duty gasoline cars were chain-like soot particles coated by OM,and a small number of particles were soot coated by S-rich.The results are important to document properties of primary particles from combustion sources,which can be used to track the sources of ambient particles and to reveal their aging process in the atmosphere.(2)The bulk analysis of PM2.5 samples collected during the whole observation period in the NCP showed that the average mass concentrations of PM2.5 at Gucheng and Jinan sites were 258.83±148.31 and 134.29±80.01 ?g m-3,respectively.The average mass concentration of PM2.5 at Gucheng site was nearly twice as high as that at Jinan site.The average concentration of total WSII at Gucheng site was 64.84±37.37 ?g m-3,which was 1.3 times higher than that(50.64±33.39 ?g m-3)at Jinan site.The NO3-/SO42-ratios of Gucheng and Jinan sites were 1.17 and 1.68 respectively,which indicates that Jinan site was mainly affected by mobile sources such as motor vehicles,while Gucheng site was mainly affected by fixed sources such as coal combustion.The average concentrations of OC and EC at Gucheng site were 69.88 and 6.92 ?g m-3,which were 4.1 and 2.3 times higher than OC(16.90 ?g m-3)and EC(2.96 ?g m-1)at Jinan site,respectively.The OC/EC ratio of Gucheng site was 10.59.It suggests that the abundant primary organic particles emitted from coal-fired heating and cooking in rural areas in winter was the main source of carbonaceous aerosol at Gucheng.While the OC/EC ratio of Jinan was 6.29,which indicates that the carbonaceous aerosol was mainly affected by vehicle exhaust emissions in urban areas.The concentrations of Ti,Mn,Fe,and Cu at Gucheng and Jinan sites were comparable,while the concentrations of Zn and Pb at Gucheng site were twice as high as those at Jinan site.Among the six elements in the two sites,Fe was the most abundant element.Industrial activities such as smelting of steel and non-ferrous metals were the main sources of these trace elements.Besides,residential coal burning could also emits trace elements such as Zn and Pb.By calculating the contribution of main components to PM2.5 in different pollution periods,we found that the relative proportion of OM decreased first and then increased when the pollution level changed from the clean period to the severe pollution period at Gucheng site,but OM still played a dominant role in PM2.5 under all four different pollution periods.At Jinan site,when the pollution level became worse,the contribution of OM to PM2.5 decreased gradually.On the contrary,the contribution of secondary ions increased and dominated.Therefore,we conclude that the large amount of organic matter emitted by household coal burning at Gucheng site contributes most to the formation of haze,whereas the transformation of secondary ions is the main factor promoting the development of haze at Jinan site.(3)The single particle analysis of individual particle samples collected during one severe haze spisode in the NCP showed that the proportion of OM/soot in Gucheng site increased rapidly in the rapid transformation from clean period to heavy polluted period.However,with the further aggravation of pollution level,the proportion of OM/soot decreased,while the proportion of S-OM/soot increased.This indicates that gaseous precursors such as SO2 and NO.transformed into secondary inorganic aerosols and internally mixed with OM/soot during the severe pollution.At Beijing site,the transportation of OM/soot particles from the south region was the man reason for the transformation from clean period to polluted period.During the transportation,some OM/soot particles internally mixed S-rich particles,which led to the aging of OM/soot particles.With the development of pollution level,the proportion of S-OM/soot increased at Beijing site.Because Jinan site was not affected by the transportation of OM/soot particles originated from Gucheng site,the main particle type was S-rich at Jinan site with a low proportion of OM/soot.As the pollution level increasing,the peak of the particle size distribution at Gucheng and Beijing sites increased from 380 nm and 350 nn at the beginning of pollution to 600 nm and 550 nm at the severe pollution level,respectively.This could be attributed to the formation of large amounts of secondary aerosols which coatted on the surface of primary particles such as OM and soot,and resulted in the increase of particle size.Compared with the individual particle database of emission sources,we found that primary OM and soot particles emitted from coal-fired heating and cooking activities in rural areas contributed significantly to the early stage of haze formation in NCP.Thereafter,these primary particles could provide reaction interface for heterogeneous chemical reactions of SO2 and NOx,which promoted the formation of secondary sulfate and nitrate,led to the aging of primary particles through the formation of"core-shell" structures by secondary aerosols coating on them,and finally caused the explosive growth of PM2.5 during severe pollution.(4)The analysis of individual particles collected in the cloud events at Mt.Tai showed that in the clean period(PM2.5<15 ?g m-3),individual particles were dominated by S-rich particles(78%),whereas the fraction of refractory particles(e.g.,soot,fly ash,metal)and their mixtures with S-rich particles(defined as S-refractory)increased significantly and dominated during the polluted periods.This suggests that anthropogenic pollutants from tall stacks of coal-fired power plants and heavy industries and vehicular exhaust in cities could be lifted to the summit of Mt.Tai under the prevailing southerly winds in summer.TEM observstions showed that 61%of cloud INT were S-rich particles,which were dominated in cloud INT.However,S-refractory particles were dominetaed in cloud RES.The percentage of S-refractory in cloud RES was 76%,which was 3.5 times more than 22%in cloud INT.Furthermore,26%of cloud RES contained two or more types of refractory particles(i.e.S-fly ash/metal-soot)but only 3%of cloud INT did.In addition,the externally mixed refactory particles accounted for 17%of cloud INT,but externally mixed refactory particles were not found in cloud RES.We speculate that although externally mixed refactory particles cannot be activated to form cloud droplets directly,cloud droplets could act as a collector to scavenge abundant refractory particles and therefore become an extremely complicated mixture.These complicated cloud droplets have not been reported in clean continental or marine air before.Our findings provide an insight into the potential impacts on cloud radiative forcing from black carbon and metal catalyzed reactions of SO2 in micro-cloud droplets containing soluble metals released from fly ash and metals over polluted air.