【作者】 陈军；

【导师】 盛六四； 刘付轶；

【作者基本信息】 中国科学技术大学 ， 同步辐射及应用， 2018， 博士

【摘要】 环已烯在自然界中可以通过人为源或自然源的排放进入到大气中,其是城市环境当中含量最多的环状烯烃,并且已经知道其在大气环境下可以氧化而生成气溶胶。研究显示在大气中环己烯可以和OH自由基、NO3自由基、O3发生化学反应。对于气溶胶的形成,先前的研究显示对于环己烯这样的环状烯烃其与O3发生反应比与OH自由基反应更容易生成气溶胶,而且在与臭氧的反应过程中往往会生成较高产率的OH自由基,而这有可能使得本来只在白天具有较高浓度的OH自由基在夜晚也维持较高浓度,从而使得在夜晚也可以发生OH自由基的氧化反应。从全球范围来看,臭氧和环状烯烃的反应因为含量相对较少,因而不会对全球的气候或者气溶胶的生成有重大影响。但是在科研领域,这些环状烯烃一直引起了科学家们的高度兴趣,因为这些环状烯烃可以作为反应更为复杂的萜烯类物质的简化模型。而萜烯类物质一直被科学家们推断是自然源中排放气溶胶的主要贡献者,但是由于其结构复杂,对于这些反应的生成物和反应机理,目前知之甚少。但是研究气溶胶的化学成分对了解气溶胶的老化过程以及预测其对于环境和人体潜在的伤害等方面都具有重要的意义。因此,阐明环己烯臭氧化反应的机理对于萜类生成气溶胶研究具有较大的借鉴意义。另一方面,环己烯是结构对称的环烯烃的典型代表,因此环己烯也一直是研究retro-Diels-Alder的重要模型。虽然之前有大量的理论和实验研究了中性环己烯或者环己烯离子的retro-Diels-Alder反应,但是一些有关环己烯解离的物理参数却没有确定。例如,先前报道的环己烯电离数据能差别比较大,特别是对其解离产生的离子,各种研究方法得到的离子出现势差别很大。此外,对于环己烯母体离子如何解离得到这些离子的具体路径也不清楚。同步辐射光电离质谱技术具有能量连续可调性,非常适合于研究小分子的光电离解离过程。此外,我们研制出基于同步辐射的热解析真空紫外光电离气溶胶质谱,能够实时在线研究气溶胶的反应产物。本文中环己烯的光电离和其臭氧化反应形成气溶胶的研究都是基于同步辐射光电离的原理来完成的。整个论文一共分为五个章节,各章节的主要内容总结如下:第一章主要是介绍了一些气溶胶相关的基本概念,包括气溶胶的来源,气溶胶的主要组分,气溶胶对于大气和人体的潜在影响等。对于一次气溶胶和二次气溶胶的概念也做了简单介绍。另外,对于气溶胶质谱的发展和各种气溶胶质谱的特点进行了简单的总结。第二章主要是简单了介绍了原子分子物理线站。对于波荡器的光谱分辨,飞行时间质谱的质量分辨,以及实验线站开展的研究工作进行了简单的小结。对光电离和气溶胶研究工作涉及的仪器做了简单介绍。第三章中我们主要介绍了环己烯光电离的工作。为了获得环己烯的电离能和其碎片离子的出现势,我们在8-15.5 eV的能量区间对环己烯进行了光电离研究。使用的仪器是自制的飞行时间质谱和一个具有三级差分系统的光电离装置,其电离源是同步辐射真空紫外光。为了解释碎片离子产生的机制,我们进行了相关的量子化学计算,将实验和理论结合起来解释环己烯的解离机制。环己烯的电离能和碎片离子的出现势是根据光电离效率曲线的测定得到的。通过实验和理论数据的比对我们提出了环己烯可能的解离机制。第四章主要是介绍使用最近研发的一套基于同步辐射光电离的热解析气溶胶质谱结合自制的光化学烟雾箱系统对环己烯臭氧化反应生成的气溶胶的化学成分进行研究。利用同步辐射光能量连续可调的特点,我们选择了合适的电离能量,得到了含碎片相对较少的气溶胶颗粒相产物的光电离质谱图。一些先前没有被报道的同分异构体和同质异构体通过光电离效率曲线得到的电离能被检测区分出来。一些产物例如乙二醛、丙酮酸等第一次在颗粒相中被检测出。同时我们推断了某些产物可能形成的反应路径。最后一章中,对于环己烯臭氧化反应的产物我们固定能量对其进行了实时的检测。对于环己烯臭氧化反应生成的二次有机气溶胶,我们提出了一种新的离线分析方法,第一次将滤膜收集的离线方式和TD-VUV-TOF-PIAMS质谱结合起来。实验结果显示,这种离线方法可以在比在线浓度低的条件下检测到大部分在线产物,同时还可能检测到部分含量很低而无法在线检测到的产物。该方法可能在今后用于在实验室内模拟二次有机气溶胶研究和检测真实的外场大气气溶胶。更多还原

【Abstract】 Cyclohexene,emitted from both natural and anthropogenic soureces,is one of the most abundant cyclic alkenes in urban aereas which are known to produce aerosol upon atmospheric oxidation.Previous studies have shown that it can readily undergo oxidation in the presence of OH radicals,NO3 radicals or O3.It has been found that the major pathway to aerosol formation for many cyclic alkenes such as cyclohexene involves reaction with O3 rather than with the OH radicals which is the important daytime oxidant.In addition,both experimental and theoretical studies have shown that the reaction of O3 with cyclic alkenes can lead to the direct formation of OH radicals,often in high yield,thus allowing the OH radical intiated oxidation of organic compounds to continue at night.On a global basis,although the reacions of O3 with cyclic alkenes are not atmospherically important,they nevertherless provide model systems for the more complex reacions of O3 with monoterpenes,which are believed to be the major contributors to SOA emitted by biogenic sources.To date,the cyclohexene ozonolysis reaction has been the subject of numerous studies.While the rate constant for this reaction is reasonabley well known,this is not the case for the chemical compostion and formation mechanism of SOA involved,which is crucial to understand the growth and aging of SOA and to predict its physical properties as well as potential health.On the other hand,cyclohexene is a prominent representative of symmetrical olefins which has been invstigated by researchers invent on understanding the intrinsic features of retro-Diels-Alder reactions.Despite considerable experimental and theoretical work performed on neutral and cationic cyclohexene,some physical properties,such as its ionization energy and the appearance energies of the fragment ions,have still not been definitely determined.A considerable discrepancy was found for the AEs of the fragment ions.The detailed mechanisms for the formation of several fragment ions and some dissociation channels are still not well understood expeimently and theoretically.Tunable VUV PI mass spectrometric technique has been widely used to provide reliable gas phase ion energetics for a wide range of molecules.In the present work,the photoionization of cyclohexene and the SOA products were investigated by the synchrotron based mass spectrometer.This paper consists of five chapters and the main contents for each chapter are summarized asfollows:The first chapter introduced some basic concepts associated with the atmospheric aerosol,including the sources of aerosols,the components involved,and its potential influence on climate and health.The concepts about sources of primary and secondary aerosol have also been summarized.Subsequently,a brief overview of common components of different aerosol mass spectrometers along with their technical description is provided here.In the second chapter,the performances of the Atomic and Molecular Physics Endstation and the experimental equipments have been introduced.The energy resolution of the undulator,the photon flux and mass resolution of the home-made mass spectrometer were presented.In the third chapter,to obtain accurate IE of cyclohexene and AEs of various fragment ions and to explore the mechanism of the comolicated photodissociation process,a photoinization study of cyclohexene was perfomed in the 8-15.5 eV energy range utilizing a three-stage differentially pumped ionizaiton chamber equipped with a reflectron time-of-flight mass spectrometer,where synchrotron radiation was used as the ionization source.Quantum chemical calculations were perfomed to obtain thermochemical data and fragmentation information.The IE of cyclohexene and AEs of its major fragment ions were acquired by measurements of photoionization efficiency spectra.In addition,the major fragmentation channels were proposed and dsicussed on the basis of experimental and theoretical data.In the fourth chapter,the molecular composition of SOA formed from ozonolysis of cyclohexene was reinvestigated by using a recently developed thermal desorption/tunable vacuum-ultraviolet photoionization time-of-flight mass spectrometer coupled with a home-made smog chamber.The photoionization mass spectra of particulate-phase products formed in the ozone-cyclohexene system were measured with fixed photon energies which were appropriate to ionize the products with little or no fragmentation.Some isobaric/isomeric species of previously undetected products were identified according to photoionization efficiency curves obtained by scanning the photon energy,enabling us to gain a deeper understanding of the reaction mechanism for cyclohexene ozonolysis.Some products such as glyoxal,acetic acid,oxoacetic acid,and malonaldehydic acid were fistly observed in the particle phase.Some possible reaction mechanisms have also been proposed.The results presented in this study will contribute to a better understanding of the SOA formed in the cyclohexen-ozone system.In the final chapter,we have investigated the ozonolysis of cyclohexene system in an attempt to shed light on the discrepancies existing in the literatures concering the low-molecular-weight components in SOA produced in this system.Compounds in the aerosol phase were on-line investigated by a recently developed TD-VUV-TOF-PIAMS mass spectrometer combined with a home-made smog chamber.In addition,an off-line method employing filter collection coupled with TD-VUV-TOF-PIAMS was tested to probe the composition of SOA produced by cyclohexene ozonolysis for the first time which may have potential applications in both field and laboratory measurements of atomospheric aerosol.更多还原