【作者】 刘磊；

【导师】 李卫军；

【作者基本信息】 山东大学 ， 环境科学， 2019， 博士

【摘要】 近年来,随着经济的高速发展以及工业化和城市化进程的加快,我国的大气污染已经由最初的煤烟型污染转化为以工业、交通、燃煤、扬尘等多污染源为主的复合型污染,而华北地区的大气污染问题尤为突出。针对华北地区的雾霾成因,科研人员经过大量研究逐渐认识到高强度的大气污染物排放是内因,不利的气象条件和地形因素是外因。但是雾霾的形成涉及到不同污染物之间复杂的化学转化以及内因和外因之间的相互反馈,其形成机制尚存在很多争议。目前的研究主要采用全样分析手段对排放源及大气中颗粒物的组分进行分析,会掩盖颗粒物的形貌、组分和混合状态等特征,不利于追溯大气中一些痕量颗粒物组分的来源和揭示颗粒物的老化机制。本文以华北地区频繁发生的雾霾事件作为研究对象,首先利用单颗粒分析方法(TEM-EDS)对不同排放源排放的一次颗粒物的形貌、组分和混合状态等特征进行分析,构建了排放源单颗粒特征数据库;其次分别在华北地区的典型农村和城市设置采样站点,利用全样分析和单颗粒分析方法对PM2.5及其各组分(包括水溶性无机离子(WSII)、有机碳(OC)、元素碳(EC)和痕量金属)的浓度水平和污染特征以及气溶胶单颗粒的类型、组分、混合状态和粒径分布等特征进行了分析,确定了雾霾期间气溶胶颗粒物的来源及其在本地累积和区域传输过程中的老化机理,为解释华北地区的雾霾形成机制提供了依据;最后利用TEM-EDS对泰山山顶云雾过程中采集的云滴残余颗粒(cloud RES)和云间隙颗粒(cloud INT)的形貌、组分、粒径和混合状态等特征进行了分析,探讨了华北地区重污染条件下人为源颗粒物对气溶胶-云相互作用的影响。本文的主要结论如下:(1)利用TEM-EDS对排放源排放的一次颗粒物的形貌、组分和混合状态进行分析,结果表明作物秸秆在闷烧阶段排放的颗粒物主要为有机颗粒(OM),但是在明火燃烧阶段则以烟尘(soot)、OM和富钾(K-rich)的内混颗粒(soot-OM-K)为主。此外野草燃烧还会排放少量的富氯颗粒(Cl-rich)。苹果木在闷烧阶段主要排放球形OM颗粒,在明火燃烧阶段主要排放OM-K和soot-OM-K颗粒,在燃尽阶段则主要排放被OM包裹的簇状soot颗粒(soot-OM)。梨木和竹子等硬木在明火燃烧阶段排放的是以soot为主的颗粒物,而柏木和松木等软木在明火燃烧阶段排放的颗粒物则以OM为主。纸箱在明火燃烧阶段排放的颗粒物以OM为主,并且部分OM与富硫颗粒(S-rich)内混在一起。泡沫盒、塑料制品和橡胶轮胎等易燃材料在明火燃烧阶段排放大量的链状soot颗粒,且塑料制品和橡胶轮胎燃烧排放的soot表面被少量的OM所包裹。含有溴代阻燃剂的材料,如印刷电路板和铜芯电缆等在明火燃烧阶段排放的颗粒物主要为OM和富溴(Br-rich)内混的颗粒。此外,印刷电路板燃烧排放的颗粒物中部分含有Zn和Pb等有毒有害重金属。内蒙古不粘煤在明火燃烧阶段排放的颗粒物为黑色球形OM颗粒,而在燃尽阶段则为半透明状OM颗粒。山东肥煤在明火燃烧阶段排放的颗粒物主要为OM且部分OM与soot发生内混,在燃尽阶段则主要为黑色球形OM。重型柴油车排放的颗粒物均为链状soot且表面被OM包裹。轻型汽油车排放的颗粒物大部分是被OM包裹的链状soot颗粒,此外有少部分的soot被S-rich包裹。通过建立排放源数据库可以为追溯大气中颗粒物的来源以及揭示颗粒物的老化机制提供依据。(2)华北地区冬季观测期间PM2.5样品的全样分析结果表明,固城站点和济南站点的PM2.5平均质量浓度分别为258.83±148.31和134.29±80.01 μgm-3,前者是后者的近2倍。固城站点的总水溶性离子平均浓度为64.84±37.37μg n-3,是济南站点(50.64±33.39 μg m-3)的1.3倍。固城站点和济南站点NO3-/SO42-比值分别为1.17和1.68,说明济南站点受机动车等移动源的影响更大,而固城站点主要受燃煤等固定源影响。固城站点OC和EC的平均浓度分别为69.88和6.92μgm-3,分别是济南站点OC(16.90μg m-3)和EC(2.96 μg m-3)平均浓度的4.1倍和2.3倍。固城站点的OC/EC比值为10.59,可以推断冬季农村地区燃煤取暖做饭所排放大量的一次有机物是固城碳质气溶胶的主要来源,而济南OC/EC比值为6.29,碳质气溶胶受市区机动车尾气排放的影响更大一些。在固城站点和济南站点Ti、Mn、Fe、Cu具有相当的浓度水平,而Zn和Pb在固城站点的浓度是济南站点的2倍。Fe在两个站点的六种元素中均是浓度水平最高的元素。钢铁和有色金属冶炼等工业排放是这些痕量元素的主要来源,同时民用煤的燃烧也会排放Zn、Pb等元素。通过对不同污染阶段主要组分(SO42-、NO3-、NH4+、OM、EC)对PM2.5的贡献进行分析发现,在固城站点当污染程度由清洁条件向重度污染变化时,有机物的相对占比先降低后升高,但在四个不同污染条件下有机物对PM2.5的贡献均占主导地位,在济南站点随着污染程度的加重,有机物对PM2.5的贡献逐渐降低,而二次离子逐渐占主导地位。因此推断固城站点民用燃煤排放大量的有机物对雾霾的形成贡献最大,而二次离子的生成是推动济南站点雾霾发展的主要因素。(3)针对华北地区一次严重雾霾事件过程的单颗粒分析结果表明,固城站点由清洁阶段向重污染阶段快速转化过程中一次OM/soot颗粒的占比迅速升高。但是随着污染程度的进一步加重,OM/soot的占比不断下降,而OM/soot与S-rich内混颗粒(S-OM/soot)的占比则不断升高,说明在重污染期间SO2和NOx等气态前体物向二次无机气溶胶不断转化并且与OM/soot发生内混。在北京站点,来自南部输送的OM/soot颗粒是造成北京由清洁阶段向污染阶段转化的主要原因,在输送过程中部分OM/soot颗粒与S-rich内混发生老化,并且随着污染程度的加重S-OM/soot的占比不断升高。济南站点由于没有受到固城颗粒物输送的影响,主要颗粒类型为S-rich,受OM/soot等一次颗粒物的影响较小。随着污染程度的加重,固城站点和北京站点颗粒物的粒径分布峰值分别由污染初期的380 nm和350 nm增加到重度污染期的600 nm和550 nm。这是由于大量的二次气溶胶生成并包裹在OM和soot等一次颗粒物表面导致颗粒物粒径增大。通过与排放源数据库进行对比,发现农村地区的燃煤取暖和做饭等活动所排放的OM和soot等一次颗粒物对华北地区灰霾的早期形成阶段具有重要贡献,而一次颗粒物可以为S02和NOx的非均相化学反应提供反应界面,促进了硫酸盐和硝酸盐的生成,使一次颗粒物发生老化形成“核-壳”结构,促进了重污染期间PM2.5的爆发性增长。(4)泰山站点云雾事件中单颗粒样品的分析结果表明,在清洁期(PM2.5<15μg m-3),颗粒类型以S-rich为主(78%),而在污染期大量的人为源难熔颗粒物(refractory颗粒,例如soot、fly ash、metal等)以及它们与S-rich的内混颗粒(即S-refractory)占比显著增高并占主导地位。这表明在污染期燃煤电厂、工厂和汽车尾气等排放的人为源颗粒物能够被山谷风抬升到山顶导致灰霾事件的发生。Cloud INT中S-rich颗粒占主导地位,占比为61%,而cloud RES中占主导的是S-refractory颗粒,占比高达76%,是cloud INT中S-refractory颗粒占比(22%)的3.5倍。同时26%的cloud RES内混含有两种以上的难熔颗粒(即S-fly ash/metal-soot),而在cloud INT中这一比例仅为3%。此外,cloud INT中外混的难熔颗粒占比为l17%,但是cloud RES中并没有发现外混的难熔颗粒。据此我们推测外混的难熔颗粒虽然不能直接活化为云滴,但是在云雾过程中云滴可以像收集器一样去捕集清除大气中的难熔颗粒并形成组分复杂的“脏云滴”,而这一现象在针对清洁的大陆和海洋上空的云雾事件研究中并没有报道过。我们的发现对于研究重污染地区黑碳颗粒对云辐射强迫的影响以及云滴中飞灰和铁氧化物是否释放出Fe、Mn等过渡金属离子从而促进SO2的液相催化氧化反应等具有重要指导意义。更多还原

【Abstract】 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 μg 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 μg m-3,which was 1.3 times higher than that(50.64±33.39 μg 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 μg m-3,which were 4.1 and 2.3 times higher than OC(16.90 μg m-3)and EC(2.96 μg 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 μg 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.更多还原