|Alternative Title||Study on Photochemical Degradation of Toluene in the Atmosphere Based on Smogchamber Experiment|
|Place of Conferral||北京|
挥发性有机化合物（Volatile Organic Compounds，简称VOCs）是我国发生复合污染的重要前驱物，会与多种氧化剂以及大气颗粒物发生光化学反应形成二次有机污染物。作为VOCs中光反应活性较高的一类苯系物，甲苯排放源广危害大毒性高，以甲苯为典型的挥发性有机污染物，研究大气中存在的气态氧化剂对甲苯的光降解以及甲苯降解过程对大气性质影响的相互作用，对于大气污染防治具有重要的意义。鉴于此，本研究搭建了室内烟雾箱反应器，开展了甲苯在气态氧化剂中的光化学反应，研究了甲苯与臭氧以及一氧化氮的光反应动力学，并通过高分辨质谱对反应产物中可能存在的大分子量化合物进行了鉴定。为模拟实际大气甲苯光降解反应，本研究还引入了实际大气重霾天和普通天作为反应基质，测定两种体系中甲苯的反应动力学参数，并对其中存在的细颗粒物的微观形貌、化学元素组成和元素结合态进行了扫描电镜、透射电镜和X射线能谱的表征，来分析细颗粒物对实际大气中甲苯光降解反应的影响。本研究得到以下结论：
1. 利用烟雾箱微型反应系统，结合热脱附装置联用气相色谱质谱对清洁基质中甲苯的光降解反应转化率及动力学分析，表明清洁体系中一氧化氮在308 K时相比于278 K对甲苯的转化表现明显，而不同温度下臭氧体系中甲苯的转化率没有明显的差距。在不同温度下对单甲苯、甲苯/臭氧、甲苯/一氧化氮三种体系中反应物甲苯进行一级反应动力学的拟合，温度为308 K时反应速率常数分别为0.19 d-1、0.27 d-1、1.1 d-1，其半衰期分别为3.65 d、2.57 d、0.63 d；温度为278 K时反应速率常数分别为0.12 d-1、0.17 d-1、0.14 d-1，其半衰期为13.86 d、4.08 d、4.95 d。此外在甲苯/臭氧和甲苯/一氧化氮体系还分别对臭氧和一氧化氮的衰减进行了一级反应动力学拟合，在308 K时，臭氧和一氧化氮的反应速率常数分别为1.14 d-1和0.59 d-1，在278 K时分别为1.09 d-1和0.21 d-1。采用（+）APPI-FT-ICR-MS对三种体系甲苯光降解转化的大分子量产物进行分析，基于DataAnalysis软件对可匹配分子式的化合物进行鉴定，结果表明三种体系中甲苯的光降解均可能会产生大分子量产物，以碳氢氧化合物为主。对三种体系进行芳香性分析发现三者在DBE（Double Bond Equivalent，双键当量）分布上具有相似性，并且均在DBE为6时具有较高的相对相应强度。
2. 对引入实际大气的冬季重霾天和普通天颗粒物进行透射电镜和扫描电镜表征，发现形貌均较为丰富，可分为块状、椭球状、球型以及混合聚集状。其中扫描电镜显示普通天颗粒物粒径分布在10 μm至50 nm之间不等，且多观察到粒径较大的颗粒物，而粒径较小的颗粒物多附着在大颗粒物表面。重霾天颗粒物多以纳米级球型细颗粒物团簇在一起形成花状或枝串状。重霾天和普通天颗粒物元素均以C、O、Si为主，还含有Mg、Al、S、Cl、K等地壳含量较为丰富的元素以及Fe、Ni等过渡金属元素。其中重霾天颗粒物碳元素含量相对较高，其丰度可高达42%。而对重霾天和普通天形貌相似的团簇状集合体进行透射电镜能谱表征发现重霾天颗粒物除了含有较高的碳元素还混合有Tm、Nd等稀土金属。对同为类球状的颗粒物进行能谱分析发现其可富含不同元素Fe、Ca、Si、C，表明同为类球状颗粒物化学成分不同且内部结构略有差异。此外矿物颗粒、金属颗粒以及有机组分可发生混合形成具有核膜结构的复杂颗粒物。雾霾天和普通天O1s的XPS分析表明，雾霾天活性氧物种相对普通天含量较高，而S2p的XPS谱图表明雾霾天只有SO42-的特征峰，普通天出现SO42-、HS-和S2-三种形态的特征峰，说明雾霾天颗粒物表面氧化性较强，还原态硫物种可能均被氧化为硫酸盐形态。
3. 在对以上引入室内实际大气重霾天和普通天存在的细颗粒物进行表征的基础上，分析了两种反应基质下甲苯的光降解转化效率并对两种体系进行了准一级反应动力学常数的实验测定。对实际体系中浓度增量为500 ppb的甲苯分析其308 K条件下的降解转化效率发现，反应初期12 h两种基质中甲苯的转化率区别不明显，均能达到10%左右。而4 d后重霾基质下的甲苯转化率明显高于普通天基质达到80%左右,而普通天转化率则达到50%左右。对普通天\甲苯、重霾天\甲苯做准一级反应动力学的拟合，308 K时两种体系的反应速率常数分别为0.20 d-1和0.40 d-1，其半衰期分别为3.47 d和1.73 d。采用（+）APPI-FT-ICR-MS对四种体系的化合物进行分析，结果表明在加入甲苯后普通天体系和重霾天体系DBE值分别分布在0~24和0~25之间。对普通天和雾霾天基质及增加一定浓度甲苯后体系中DBE值与碳数的点状图分布的分析有普通天、普通天/甲苯、重霾天、重霾天/甲苯四种体系的DBE/CN比率为0.4418~0.9184、0.4969~0.6813、0.4332~1.069和0.5155~0.7068。该比率在一定程度上反应了体系中的芳香度变化范围。
Volatile Organic Compounds is an important kind of precursor of the combined pollution in China. The secondary organic aerosols are mainly formed through the reactions between the VOCs and a variety of oxidants and atmospheric particulates. As one typical aromatic hydrocarbon of higher photo-reactivity, toluene was emitted from wide sources with great harm and highly toxicity. The research of the interaction between the photo-degradation of toluene and its effect on atmospheric properties has an important significance for the prevention and control of atmospheric pollution. In view of this, this study built indoor smog chamber reactor for a series of experiments to simulate the photochemical reaction of toluene in gas phase oxidant. The photo-reaction kinetics for toluene was studied with ozone and nitrous oxide, and the reaction products of the macromolecular aromatic hydrocarbons were identified through high resolution mass spectrometry. To stimulate photo-degradation of toluene under the real circumstance, the real atmosphere was introduced into the reactor as reaction substrate. The reaction kinetics constants of toluene in real atmosphere were experimentally determined, also the microscopic appearance and chemical composition of particulate matters in the normal and heavy haze day was characterized by scanning electron microscopy and transmission electron microscopy to understand the influence of the particulate matters to the photo-degradation of incremental toluene under the actual substrate. The conclusions are as follows:
1. Combined with the thermal desorption device and the gas chromatography mass spectrometry, the toluene conversion rate and reaction kinetics were analyzed. The results showed that nitric oxide oxidation with toluene was more obvious in 308 K compared to 278 K, while there was no significant difference in the toluene/ozone oxidation with temperature variation. As for reaction kinetics, the single-toluene、toluene/ozone、toluene/nitric oxide systems were fitted to the pseudo first order kinetics under different temperature. The first order reaction rate constant were 0.19 d-1、0.27 d-1、1.1 d-1, the half-life were 3.65 d、2.57 d、0.63 d respectively in 308 K, while the values were 0.12 d-1、0.17 d-1、0.14 d-1, and 13.86 d、4.08 d、4.95 d respectively in 278 K. The same fitting were done to the depletion of ozone and nitric oxide in the toluene/ozone and the toluene/nitric oxide systems with 1.14 d-1、 0.59 d-1 in 308 K and 1.09 d-1、 0.21 d-1 in 278 K as a result. The larger molecular weight products in the three systems were analyzed with (+)APPI-FT-ICR-MS, the matched molecular formula was identified with the DataAnalysis software, indicating the abundant species generated from the photo-degradation of toluene within three systems, which was mainly composed of hydrocarbon and oxygen. Except that, an analysis of the aromaticity of the three kinds of system shows the similarity on the distribution of double bond equivalent, with a high relative strength of DBE for six.
2. The characterization were conducted to the introduced actual atmosphere in the winter heavy haze day and normal day. The morphology was found abundant, which could be divided into block 、ellipsoid、spherical and mixed aggregates. The particle size of normal day varies from 10 μm to 50 nm. The proportion of particles with larger particle size was relatively higher and the particles with smaller particle size tended to attach to the surface of larger ones. While in the heavy haze days, the particulate matter was mainly composed of nano-sized pellet clusters, which form the flowerlike or twiglike structure. The particulate matter collected in heavy haze day and normal day were mainly composed of C、O and Si, as well as the rich crustal content element as Mg、Al、S、Cl and K, and transition metal elements as Fe and Ni. In the haze day, the carbon content of particulate matter is relatively high with its abundance as much as 42%. The further TEM spectrometry to the similar cluster morphology aggregates in the haze day and the normal day shows an addition mixture with rare earth metal as Tm and Nd. An energy spectrum analysis of the observed results showed the different internal structure and chemical composition with the same spherical kind of particles which contained Fe、Ca、Si、C respectively. Mineral particles, metal particles and organic components could form into complex nuclear membrane structures. The XPS spectrum analysis showed that the reactive oxygen species content within the haze day particles was relatively higher, the S2p XPS spectrum showed that the haze day contained only the SO42- characteristic peak, while SO42-、HS- and S2- were all appeared within the normal day, illustrated the stronger oxidizing surface of haze day , oxidized the reduced sulfur to sulfates.
3. Combined with the thermal desorption device and the gas chromatography mass spectrometry, the photo-degradation of toluene in two systems including a typical heavy haze day and normal day were analyzed in detail. The first order reaction kinetics constants were experimental determined in the actual systems. With the toluene concentration increment of 500 ppb, its degradation efficiency were found to be unobvious under the temperature of 308 K in the early 12 hours. And the conversion reached 10% approximately both. While the conversion rate of the heavy haze day matric tended to be as high as 80% four days later, exceeded the normal day as low as 50%. Reaction kinetics fitting results showed that the first order reaction kinetics rate constants were 0.20 d-1 and 0.40 d-1 respectively, the half-life in the process of simulated atmospheric response in heavy haze day and normal day were 3.47 d and 1.73 d, respectively. The FT-MS results showed that the DBE values varied within 0~24 and 0~25 under the normal air and the heavy haze air with the addition of toluene to the substrate. The ratio analysis of DBE/CN to the matrix as the normal day、normal day/toluene、heavy haze day、heavy haze day/toluene were in the range of 0.4418~0.9184、0.4969~0.6813、0.4332~1.069 and 0.5155~0.7068 respectively. The ratios might reflect the range of aromatic variation to some extent.
|刘雅露. 基于烟雾箱实验模拟大气中甲苯的光化学降解反应研究[D]. 北京. 中国科学院生态环境研究中心,2018.|
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