Investigation Of The Photoionization Of Cyclohexene And Secondary Organic Aerosol Formed From The Ozonolysis Of Cyclohexene

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.