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挥发性有机物氧化生成二次有机气溶胶的研究
Alternative TitleThe studies of secondary organic aerosols formed from the atmospheric oxidation of volatile organic compounds
马朋坤
Subtype博士
Thesis Advisor束继年 ; 杨波
2018-06
Degree Grantor中国科学院生态环境研究中心
Place of Conferral北京
Degree Name理学博士
Degree Discipline环境科学
Keyword挥发性有机物,大气氧化,二次有机气溶胶,形成机理 Volatile Organic Compounds, Atmospheric Oxidation, Secondary Organic Aerosols, Formation Mechanism.
Abstract

      二次有机气溶胶(SOA)作为大气颗粒物的重要组成部分,不仅影响着大气环境和气候变化,同时也对人体健康造成危害。汽油机动车尾气和单萜烯作为大气环境中重要的人为源和天然源挥发性有机物(VOC),在大气环境中SOA的形成中起到至关重要的作用。本论文针对人为源和天然源挥发性有机物氧化生成SOA的机理进行了研究。研究内容主要包括OH启动光氧化汽油机动车尾气生成SOA的研究,臭氧氧化汽油机动车尾气生成SOA的研究和臭氧氧化α-水芹烯生成SOA的研究。研究结果将有助于了解人为源和天然源VOC在大气中氧化生成SOA的形成机理。具体研究如下:

      (1)利用真空紫外光电离质谱仪对比研究了光氧化汽油机动车尾气和C7-C9单环芳烃化合物分别形成的SOA的组分分布。通过对比汽油机动车尾气SOA和混合C7-C9单环芳烃化合物SOA的组分分布,发现两者的组分分布有极大的相似之处。同时对光氧化单个单环芳烃生成的SOA组分进行分析,研究汽油机动车尾气SOA组分的主要来源,研究表明在汽油机动车尾气SOA中分子量为98和110的SOA组分主要来自于光氧化甲苯和间二甲苯产生的,分子量为124的组分主要来自于间二甲苯,分子量为100、112、128、138和156主要来自于1,3,5-三甲苯。该研究结果表明C7-C9单环芳烃化合物是SOA的重要前体物,并且是汽油机动车尾气SOA中组分的重要来源。此外,一些新的单环芳烃SOA组分首次被VUV-PIMS发现,并对其相应的反应机理进行了推导,这有助于丰富光氧化单环芳烃生成氧化产物反应机理。

     (2)利用真空紫外光电离质谱仪对臭氧氧化4辆不同型号的轻型汽油机动车尾气生成SOA的反应进行研究。在实验中能够非常明显的观察到SOA的快速形成,且SOA主要来自于臭氧氧化脂肪族烯烃。4辆机动车尾气形成的SOA的质谱图呈现着相似的分布模式(一个规律的质量数群m/z 98、112、126 …)。基于已有的研究,本次实验中臭氧氧化机动车尾气生成的气相产物被认证为羰基产物。而本实验中SOA的形成可以用羰基化合物之间的羟醛缩合反应来解释,这是对羟醛缩合反应作为SOA形成的主要反应通道的一次直接的观察,并且对臭氧氧化机动车尾气生成的SOA组分及其形成机理进行了介绍。本次研究表明,低分子量的烯烃可能在机动车尾气形成SOA中起到至关重要的作用。更为重要的是,机动车尾气SOA的质谱图与大气中捕集的颗粒物的质谱图具有非常好的相似性,这表明羟醛缩合反应可能在大气气溶胶的形成过程中起到非常重要的作用。由于羰基产物是大气人为源和天然源挥发性有机物主要的氧化产物,因此本研究中提出的反应机理可以广泛的应用到解释大气烯烃类化合物氧化生成气溶胶的反应机理。

      (3)利用热解吸激发态质子转移质谱仪(TD-ESPT-MS)对臭氧氧化α-水芹烯生成的气相产物和SOA组分进行在线监测和分析。除了已有研究报道中被认证出的氧化产物外,在本次实验中同时有新的氧化产物被TD-ESPT-MS检测到。根据被检测到α-水芹烯SOA组分的随时间的演变趋势,将这些α-水芹烯SOA组分划分为一代产物和二代产物。同时,根据臭氧氧化α-水芹烯生成的SOA产物质谱图和气相产物质谱图的对比,将3-hydroxyl-2-isopropyl-4-oxobutanoic acid (MW, 160), 2-(formyloxy)-2-hydroperoxypropanoic acid (MW, 162)和 (2R)-2-hydroperoxy-2-(1-hydroperoxyethoxy) propanal (MW, 164) 归类为极低挥发性有机物(ELVOCs)。氧化产物acetaldehyde (MW, 44),ethaneperoxoic acid (MW, 76)和2-isopropylsuccinaldehyde (MW, 128) 随着反应时间的推移而急剧的下降,这表明这些氧化产物可能具有高的反应活性,并通过进一步的反应而在SOA老化的过程中起着重要的作用。同时对一代产物和二代产物分别对新颗粒形成阶段、生长阶段和老化阶段的SOA的贡献进行计算,发现一代产物是SOA新颗粒迅速形成的重要来源。随着反应时间的推进,在颗粒生长和老化阶段,一代产物占比逐渐缩小,二代产物占比持续增加。通过参照已知的臭氧氧化α-水芹烯的反应机理以及一代产物和二代产物随时间的演化趋势,本章同样对新发现的氧化产物的形成机理进行了推测。

Other Abstract

     Secondary organic aerosols (SOA) as an important part of atmospheric particulate matter, affecting the atmospheric environment and human health. As the major SOA precursor, gasoline vehicle exhaust and monoterpene are important volatile organic compounds in atmospheric environment, and their emissions are anthropogenic and biogenic, respectively. In this study, the SOA formation from the oxidation of anthropogenic and biogenic volatile organic compounds (VOCs) were investigated, respectively. Research includes as following: the formation of SOA from the photo-oxidation gasoline vehicle exhaust, the formation mechanism of SOA from ozonolysis of gasoline vehicle exhaust, and the formation mechanism of SOA from ozonolysis of α-phellandrene. The detailed findings are shown as follows.

         To further explore the composition and distribution of SOA components from the photo-oxidation of light aromatic precursors (toluene, m-xylene, and 1,3,5-trimethylbenzene (1,3,5-TMB)) and idling gasoline exhaust, a vacuum ultraviolet photoionization mass spectrometer (VUV-PIMS) was employed. Peaks of the molecular ions of the SOA components with minimum molecular fragmentation were clearly observed from the mass spectra of SOA, through the application of soft ionization methods in VUV-PIMS. The experiments comparing the exhaust-SOA and light aromatic mixture-SOA showed that the observed distributions of almost all the predominant cluster ions in the exhaust-SOA were similar to that of the mixture-SOA. Based on the characterization experiments of SOA formed from individual light aromatic precursors, the SOA components with molecular weights of 98 and 110 amu observed in the exhaust-SOA resulted from the photo-oxidation of toluene and m-xylene; the components with a molecular weight of 124 amu were derived mainly from m-xylene; and the components with molecular weights of 100, 112, 128, 138, and 156 amu were mainly derived from 1,3,5-TMB. These results suggest that C7-C9 light aromatic hydrocarbons are significant SOA precursors and that major SOA components originate from gasoline exhaust. Additionally, some new light aromatic hydrocarbon-SOA components were observed for the first time using VUV-PIMS. The corresponding reaction mechanisms were also proposed in this study to enrich the knowledge base of the formation mechanisms of light aromatic hydrocarbon-SOA compounds.

      To provide more information on this issue, the formation of SOA from ozonolysis of four light-duty gasoline vehicle exhaust systems was investigated with a VUV-PIMS. Significant SOA formation was observed and the SOA were manily aliphatic alkenes. PI mass spectra of the SOA from all vehicles show a similar spectral patterns (a regular mass group with m/z at 98, 112, 126 …). Interestingly, most carbonyl products of aliphatic alkenes observed as major gaseous products have specific molecular weights, and the main formation pathway of SOAs can be explained well using aldol condensation reactions of these carbonyls. This is a direct observation of the aldol condensation as a dominated pathway for SOAs formation, and the first report on the composition and formation mechanism of the SOAs from the ozonolysis of gasoline vehicle exhaust is given. The study reveals that low molecular weight alkenes may play a more significant role in vehicle-induced SOAs formation than previously believed. More importantly, the PI mass spectra of SOAs from vehicles show similarities to the field aerosol sample mass spectra, suggesting the possible significance of the aldol condensation reactions in ambient aerosol formation. Since carbonyls are a major degradation product of biogenic and anthropogenic VOCs through atmospheric oxidation processes, the mechanism proposed in this study can be applied more generally to explain aerosol formation from the oxidation of atmospheric hydrocarbons.

        To further explore the composition and formation mechanism of SOA from the ozonolysis of α-phellandrene, a thermal desorption excited-state proton transfer mass spectrometry (TD-ESPT-MS) was employed for the first time to characterize the particle-phase products online. Based on the clear temporal evolutions of the SOA oxidation products along with the particle formation, the products were classified as first- and second-generation products, respectively. Moreover, based on the comparison the mass spectrum of the gas- and aerosol-phase product the species 3-hydroxyl-2-isopropyl-4-oxobutanoicacid(MW,160),2-(formyloxy)-2-hydroperoxypropanoicacid(MW,162), and (2R)-2-hydroperoxy-2-(1-hydroperoxyethoxy) propanal (MW, 164) were classified as the ELVOCs. Based on the calculated contribution of the first- and second-generation products to the total SOA mass during NPF, growth, and aged of α-phellandrene-SOA, the first-generation products are believed to be a significant source for the rapid nucleation of new particles observed in this work. Along with the passage of the reaction time, the proportion of the first-generation products decrease and the second-generation products increase in the SOA growth and aged stage. By referring the known ozonolysis mechanism and the temporal evolution of the first- and second-generation products, the formation mechanism of the new observed oxidation products that contributed to the SOA components was also proposed.

Pages124
Language中文
Document Type学位论文
Identifierhttp://ir.rcees.ac.cn/handle/311016/41504
Collection大气环境科学实验室
Recommended Citation
GB/T 7714
马朋坤. 挥发性有机物氧化生成二次有机气溶胶的研究[D]. 北京. 中国科学院生态环境研究中心,2018.
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