【作者】 张帆；

【导师】 王祖武；

【作者基本信息】 武汉大学 ， 环境工程， 2014， 博士

【摘要】 作为中国中部地区中心城市,武汉近年来经济发展迅速,区域影响力不断增强,但空气污染问题较为突出,灰霾污染频繁发生,并主要以细颗粒物(PM25)为首要大气污染物。本研究旨在通过对武汉PM25的化学特性及其在典型灰霾事件中污染特征的研究,为武汉PM2.5污染防治对策的制定提供理论依据。自2012年8月至2013年7月,采用大流量采样器在武大(混合功能区)和天虹(高新产业区)两个站点采集了PM2.5样品,并用离子色谱、热光透射碳分析仪、电感耦合等离子体质谱等仪器对PM2.5所含主要组分(水溶性离子、碳质组分、元素组分)进行了化学分析。基于以上样品采集及化学分析等基础工作,本论文首先研究了武汉PM2.5浓度的时间变化、化学组成及污染水平,然后分别分析了水溶性离子、碳质组分、元素组分等三大组分的化学特性,最后研究了武汉两次典型灰霾过程中PM25的污染特征并探讨了霾污染的防治对策。主要研究内容及成果包括以下几个方面：(1)武大、天虹两站点PM25的年均浓度分别为106.47μg/m3和114.86μg/m3,分别是国家二级标准(GB3095-2012)的3.04倍和3.28倍。两站点间PM2.5的化学组成具有相似性,各组分按比重从高到低均依次为POM(有机颗粒物)、S042-、矿物尘、NO3、NH4+、EC(元素碳)、C1-、微量元素,这种相似性和多组分性表明武汉PM2.5污染具有城域性和复合性特征。在季节分布上,PM2.5及所含大部分组分浓度呈现冬季高、夏季低的季节性变化特征。武汉、金沙区域大气本底站等地区间的比较性分析表明,华中区域的硫酸盐气溶胶污染较为突出。(2)SO42、NO3、NH4+、Ca2+、K、Cr、Mg2+、F是武汉PM25中的主要水溶性离子,它们占武大、天虹两站点PM2.5的比重可分别达43.72%和41.82%。二次离子(SO42、N03、NH4+)是最重要的水溶性离子,也是影响各地区PM2.5酸碱程度最重要的因素,其中SO42-对增强金沙区域大气本底站PM25的酸性具有决定性的影响。在水溶性离子中,SO42-所占比重较高,其浓度在武汉和华中区域大气本底站均居较高的水平；颗粒态NH4NO3的不稳定性是导致武汉PM25中水溶性离子结构呈季节性变化的关键原因,其中气温和气压是影响气态NH3和HNO3向颗粒态NH4NO3转化的重要因素；地壳元素离子Ca2+占水溶性离子的比重仅次于二次离子(SNA),其浓度分布受局地排放源的影响较为明显,这表明建筑扬尘污染在武汉己非常突出；K+、Cl-浓度的季节性变化较为显著,分别与生物质燃烧污染、燃煤污染的季节性特征密切相关。(3)本研究中的碳质组分(TC)由有机碳(0C)和元素碳(EC)组成。武大、天虹两站点PM2.5中TC的年均浓度分别为18.89μg/m3和20.47μg/m3,其中OC占TC的比重分别为89.51%和87.47%。地区间OC/EC的比较分析表明,城市气溶胶中的OC/EC值比区域背景点小,城市气溶胶中较低的OC/EC值与机动车、餐饮油烟等污染源中较高的EC排放量有关,而区域背景点气溶胶中较高的OC/EC则与有机气溶胶在长距离迁移过程中被充分氧化或“老化”有关。武汉两站点PM25中的二次有机碳(SOC)浓度较高,占OC的比重可达50%以上；与一次有机碳(POC)相比,SOC与PM25的变化过程更为密切相关。(4)本研究中的元素组分由4种地壳元素和16种微量元素组成。武大、天虹两站点中地壳元素占PM2.5比重的年均值为7.80%和8.90%,占元素组分比重则分别达92.31%和92.59%,Zn、Pb、Mn、As等4种元素占16种微量元素的年均比重可达80%以上。富集因子(EF)分析表明,两站点PM2.5中的Li、Cr、Ni、Sr、 Cs、Ba、Mn、Mg、K等元素主要来源于土壤源；主要来自人为源的元素在武大站点的富集程度排序从高到低依次为Cd、Se、Ag、Pb、Zn、As、T1、Ca、Cu、 Ga,在天虹站点则为Cd、Se、Pb、Ag、Zn、As、Tl、Ca、Cu、Se、 Pb、Ag等四种元素在两站点中处于高度富集水平(EF>100)。二次离子(SNA)、二次有机碳(SOC)均与武汉PM2.5的变化过程密切相关,而元素组分与PM2.5间变化过程的联系却较不紧密,且元素组分的局地性污染特征较为明显。(5)在秋季典型灰霾过程中(2012.10.08-2012.10.17),影响武汉的气团主要来自东北和西北两个方向,两者分别占气流轨迹总数的59%和41%,其中西北气团影响下的霾污染较为严重,这与西北气团较缓的运行速度及其途经的污染带有关；生物质燃烧和固定源排放是秋季灰霾形成的主要原因,高含量的硫酸盐气溶胶是秋季灰霾中PM2.5的显著特征,且SO42-是导致此过程中PM2.5浓度呈“爆发式”增长的较重要组分；冷锋降雨过程是消除此次霾污染的主要气象原因。(6)在冬季典型灰霾过程中(2013.01.09-2013.02.06),影响武汉的气团主要来自东部、东北和西北三个方向,三者分别占气流轨迹总数的60%、24%和16%,其中霾污染期间主要受东部气团和西北气团的影响,正常天气期间则主要受东北气团的影响；固定源和移动源排放是冬季灰霾形成的主要原因,高含量的二次无机气溶胶是冬季灰霾中PM2.5的显著特征,SO42-和N03-是导致此过程中PM2.5浓度呈“爆发式”增长的较重要组分,重污染期间移动源对颗粒物的相对贡献呈显著增强的趋势,冬季霾天气下PM2.5的酸性比秋季更强；自西北方向经长距离迁移抵达武汉的干冷气团的影响是导致1月24日武汉PM2.5浓度骤降的气象原因,除Ca2+外,其它主要组分浓度均呈较大幅度的下降；自1月27日开始,PM2.5所含组分浓度呈逐步下降趋势,这与武汉周边地区间大气环流的增强有关；人工增雨前较低的空气湿度及增雨后的弱风条件是导致人工增雨后霾污染依然严重的重要气象原因；东南气团影响下有利的大气扩散条件和较强的暖锋降雨过程是消除此次霾污染的重要气象原因。高湿度下大量二次离子的非均相生成导致雨停后高湿度空气中PM2.5浓度仍呈高值。(7)加强对SO2、NOx、VOCs等污染物排放的协同控制并建立武汉及周边的区域联防联控机制是实现霾污染控制的有效途径。更多还原

【Abstract】 As the key city of Central China, Wuhan has witnessed rapid economic development and expanded the regional influence in recent years. However, the air pollution in Wuhan is serious to a large extent and haze events happen frequently. PM2.5(fine particles) becomes the premier pollutant of atmospheric pollution. The paper aims to study the chemical characteristics of PM2.5and some typical haze events and help the government to develop effective pollution control strategies in Wuhan.PM2.5samples were collected at a mixed-use sampling site (WD site) and a sampling site (TH site) in the high-tech industrial park from August2012to July2013by using two high volume samplers. The samples, composed of water soluble inorganic ions, carbonaceous species and elements, were analyzed by IC, TOT and ICP-MS, respectively. Based on sample collection and chemical analysis, the paper first studied the temporal variations, chemical components and pollution levels of PM2.5, then analyzed the chemical characteristics of water soluble inorganic ions, carbonaceous species and elements, then did research on the pollution characteristics of PM2.5in two typical haze events and proposed the countermeasures for haze pollution control.The main contents and conclutions were obtained as follows:(1)The annual mean concentrations of PM2.5were106.47μg/m3and114.86ug/m3at WD and TH site, exceeding the annual PM2.5regulation (35μg/m3) of Chinese National Ambient Air Quality Standards (GB3095-2012) by factors of3.04and3.28, respectively. The chemical compositions of PM2.5at WD site were similar to those at TH site and the fractions of the major components at both sites were in the following order of POM>SO42>crustal dusts>NO3">NH4+>EC>Cl>trace elements, suggesting that the PM2.5pollution in Wuhan was urban and complex. The PM2.5and its major components at both sites exhibited strong seasonal variations, consistently with lowest values in summer and highest in winter. Based on comparative analysis among Wuhan, Jinsha(a regional background site in Central China) and other cities, sulphate aerosol pollution was a serious problem in Central China.(2)Water soluble inorganic ions(WSIIs) in PM2.5were composed primarily of SO42-, NO3-, NH4+, Ca2+, K+, Cl", Mg2+and F, accounting for43.72%and41.82%of PM2.5mass at WD site and TH site, respectively. NO3", SO42-and NH4+were the most important water soluble inorganic ions, representing the secondary ionic aerosols (SNA). SNA was the major fator influencing on the acidity of PM2.5in many regions and SO42-determined the acidity of PM2.5in Jinsha. SO42-appeared high concentrations in Wuhan and Jinsha, which possessed upper proportion in WSIIs. Particulate-phase NH4NO3played a key role in the seasonal variations of ionic compositions because of its instability and its formation from gas-phase NH3and HNO3was strongly influenced by temperature and pressure. As a crustal element, the ratio of Ca2+to WSIIs was lower than SNA but higher than other ions which were affected by local emissions, showing that dust pollution was serious in Wuhan. The dynamics of K+and Cl" showed clear seasonal variations, closely related to biomass burning and coal combustion, respectively.(3)Carbonaceous species(TC) consitsted of organic carbon(OC) and elemental carbon(EC). The annual mean concentrations of carbonaceous species were18.89μg/m3and20.47μg/m3at WD site and TH site, respectively. And OC concentration contributed89.51%of TC mass at WD site and87.47%at TH site, respectively. Based on comparative analysis of OC/EC among different regions, OC/EC in urban aerosols was lower than that in rural, due to the high EC emissions in cities and the heavy oxidation of OC in countrysides. There had always been secondary organic carbon (SOC) in PM2.5which accounted for more than half of the OC. Compared with primary organic carbon (POC), SOC was more linked to PM2.5dynamics.(4)In this study, the detected elements consisted of4crustal elelments and16trace elements. The crustal elements were important parts of PM2.5, contributed7.80%of PM2.5at WD site and8.90%at TH site, respectively. In addition, the crustal elements dominated the detected elements in PM2.5, accounting for92.31%of these elements at WD site and92.59%at TH site, respectively. The total concentrations of Zn, Pb, Mn and As accounted for more than80%of the trace elements. Enrichment factors(EF) analysis indicated that Li, Cr, Ni, Sr, Cs, Ba, Mn, Mg and K were mainly from crustal sources. And Cu, Ga, Ag, T1, Ca, As, Zn, Pb, Se and Cd were enriched and associated with anthropogenic sources. The EF values of the elements related to human activities were in the following order of Cd> Se> Ag> Pb> Zn> As> Tl> Ca> Cu> Ga at WD site. And the similar results at TH site were Cd> Se> Pb> Ag> Zn> As> Tl> Ca> Cu>Ga. It was worth noting that the EF values of Cd, Se, Pb and Ag were higher than100. Compared with the elements, SNA and SOC were closely related to the formation, transportation and removal of PM2.5in Wuhan.(5)During a typical haze episode from October8to17,2012, Wuhan was mainly influenced by airmasses from northeast and northwest which accounted for59%and41%of the total airmass trajectories, respectively. The air pollution influenced by the northwest was more serious, due to the low speed of airmass and more pollution emissions along the airmasses’ pathway. Biomass burning and stationary emissions contributed a lot to the formation of the autumn haze. PM2.5was characterized by high sulfate content and sulfate played a key role in the surge of PM2.5concentrations during the autumn haze. Frontal rainfalls were the main cause of cutting haze pollution load.(6)During a typical haze episode from January9to February6,2013, Wuhan was mainly influenced by air masses from northwest, northeast and east, accounting for16%,24%and60%of the total airmass trajectories, respectively. On haze days, PM2.5concentrations in Wuhan were mainly affected by air masses from northwest and east, while the PM2.5showed no significant difference between both air types. Both of mobile sources and stationary emissions contributed a lot to the formation of the winter haze. PM2.5was characterized by high SNA content. SO42-and NO3-played a key role in the surge of PM2.5concentrations during the winter haze. The cold and dry airmass, transported long distances from northwest, resulted in the significant decline of PM2.5and all of the major PM2.5-related species except Ca2+in January24. All of the major PM2.5-related species had declined gradually with the strengthening of atmospheric circulation around Wuhan since January27. As a matter of meteorology, the artificial precipitation failed to remove haze pollution, due to relative low air humidity before rain and weak wind conditions after rain. While the normal rainfall contributed a lot to the removal of haze, attributed to favorable atmospheric diffusion and higher precipitation influenced by airmasses from southeast. Similar to the autmnn haze event, the formation of SNA was accelerated through heterogeneous reactions in the wet weather after rain.(7)The cooperative pollution control of SO2, NOx and VOCs and the establishment of regional cooperation should be taken to control haze pollution.更多还原