Initial Degradation Mechanism And Synchrotron Radiation Studies Of Typical Unsaturated Volatile Organic Compounds In Atmosphere

The degradation of volatile organic compounds（VOCs）in the atmosphere is closely relevant to the formation of secondary organic aerosol（SOA）and ozone.The initial degradation reaction of VOCs is one of the important topics in atmospheric chemistry research,and understanding its chemical kinetics is crucial to study the formation mechanism of subsequent oxidation products and air pollutants.Experimental and theoretical studies of atmospheric reaction kinetics are the basis for exploring the initial degradation mechanism of VOCs.Experimental detection（such as the experiment based on vacuum ultraviolet synchrotron radiation photoionization mass spectrometry）can qualitatively and quantitatively analyze intermediate products,thereby predicting and verifying the reaction mechanism.With the help of high-level quantum chemical and kinetic calculations,the results of the theoretical study can be used to validate the experimental results from the scale of molecular reactions.Furthermore,it has been widely acknowledged that theoretical calculations have output valuable kinetic parameters for atmospheric modeling.In the present thesis,the reaction kinetics of the initial degradation mechanism of typical unsaturated VOCs has been investigated from experimental and theoretical perspectives.Most VOCs are initiated by reacting with hydroxyl radicals（OH）in the atmosphere.Toluene and styrene are two of the most important non-methane VOCs,which are harmful to the health of humans and plants.The potential energy surface of toluene/styrene+OH covers the OH-addition,H-abstraction and addition-dissociation reaction pathways.In this thesis,the potential energy surface of toluene+OH reaction was was built at the high level of quantum chemical methods,based on which the rate coefficients and branching ratios were obtained by the RRKM（Rice-RamspergerKassel-Marcus）/master equation simulation.During the kinetic calculations,the lowfrequency vibrational modes of the toluene-OH pre-reaction Van der Waals complexes exhibit translational properties,which were mimiced by the particle-in-a-box model.The computed rate coefficients were compared with previous experimental and theoretical results.The branching ratio information reveals the dominant products and predicts the types of peroxy radicals（RO₂）formed in the subsequent oxygenation reactions.Our results indicate that both high-level quantum chemical calculations for the crucial barrier heights and appropriate treatments for the anharmonicity determine the accuracy of the computed total rate coefficients and branching ratios.Compared with toluene+OH,the reaction pathways of styrene+OH are more complex.Previous experiments on the total rate coefficients of styrene+OH show large deviations.Only one theoretical study is available to the best of our knowledge.In this thesis,the reaction kinetics of styrene+OH was extensively studied with high-level quantum chemical methods combined with RRKM/master equation simulations.In particular,we carried out reduced density gradient analysis for the formation of pre-reaction Van der Waals complexes,and examined their influence on the reaction kinetics.Finally,the calculated rate coefficients were used to estimate the atmospheric lifetime of styrene and its effect on ozone formation.The branching ratio information was used to predict the dominant products of the initial degradation reaction and evaluate their subsequent effects on SO A formation.In addition to OH radical,O₃ is an important atmospheric oxidant as well,especially for alkenes.Regarding the fate of ethylene（the simplest alkene）in the atmosphere,Criegee intermediates are formed during the ozonolysis of ethylene,leading to the generation of active radicals including RO₂.As an important intermediate in the atmosphere,the detection of RO₂ is of great significance for predicting the detailed mechanism of the ethylene+O₃ system.However,the active RO₂ radical is difficult for experimental measurement.In this study,the formed RO₂ species in the ethylene ozonation reaction were analyzed by the vacuum ultraviolet synchrotron radiation photoionization mass spectrometry.The precise ionization energies obtained from high-level quantum chemical calculations were compared with experimental observations to determine the structures of RO₂ and analyze possible reaction paths.By comparison of the relative intensities of corresponding hydroperoxides in mass spectrums with and without excessive n-C₄H₁₀ as a scavenger,the existence and proposed formation channels of characterized peroxy radicals are supported qualitatively.Four kinds of RO₂ were identified in this study providing valuable clues for the construction of a complex alkene ozonation reaction mechanism.RO₂ plays a key role in atmospheric chemistry and is usually considered to undergo bimolecular reactions with NO,HO₂,or other RO₂ radicals.In recent years,with a new atmospheric autoxidation（the oxidation cycle with OH consumption and production）proposed,rapid H-shift of RO₂ as a key step of this mechanism has also drawn widespread attention.However,available studies mostly focused on the structural characteristics required for rapid H-shift and the impact on the formation of highly oxidized molecules.The atmospheric formation of the precursors for such ROO radicals has been overlooked.Long-chain alkenes and aldehydes with weak tertiary CH bonds are initially oxidized to form substituted alkyl radicals,which lead to the generation of RO₂ by O₂ addition.Such RO₂ radicals are supposed to be able to undergo rapid H-shift reactions.Substituted alkyl radicals as the precursors of RO₂,their atmospheric yields directly affect the occurrence of rapid H-shift reaction.To clarify the yield of substituted alkyl radicals and analyze their influence on subsequent reactions,two typical VOCs（3-methyl-1-hexene and 2-methyl-pentaldehyde）that satisfy the relevant structural characteristics were selected.We subsequently computed the bond dissociation energy,constructed the potential energy surface for the reaction with OH and performed the RRKM/master equation simulations.The results suggest that the initial oxidation reaction between VOCs and OH can consume or destruct the weak tertiary C-H bond,as a result,the yield of substituted alkyl radicals（corresponding to the rapid H-shift reaction）is less than 10%.Hence,the initial oxidation reaction of long-chain alkenes and aldehydes VOCs with OH largely restricts the possibility of the subsequent rapid H-shift reaction of RO₂.The kinetic information obtained in this study quantitatively explained the influence of RO₂ precursor on the intramolecular H-shift reaction,and established a connection between the initial oxidation reaction of VOCs and the atmospheric autooxidation mechanism.