Theoretical Study On Atmospheric Oxidation Mechanism Of Typical Monoterpenes Compounds

Monoterpenes?MTs?are an important source of volatile organic compounds?VOCs?.Because of their biogenic nature,emission of MTs to the atmosphere is highly uncertain and uncontrollable.Oxidation of MTs results in high yields of secondary organic aerosol due to?extremely?low volatility of their products.Therefore,it is of great practical significance to clarify their atmospheric oxidation mechanism in order to assess their impact on the environment and to formulate pollution prevention measures.Therefore,supplementing and perfecting their atmospheric oxidation mechanism is one of the top priorities in recent studies.However,due to the complexity of the reaction,attempts on atmospheric oxidation mechanisms of MTs were hindered by the experimental measurements in several aspects.?1?The free radical intermediates produced in the reaction are difficult to detect due to their short lifetime,low concentration and structural complexity;?2?Many of intermediate products and final products are so complicated with different functional groups that they can easily elude from current detection techniques.Low carbon balance was usually found in experimetal studies of oxidation of MTs,e.g.,less than 10%in the reaction of limonene with O₃.?3?The OH radical generated in the reaction may cause secondary reactions,affecting qualitative and quantitative determination of products.Desipte of a large amount of studies,the oxidation mechanisms of MTs remain highly uncertain,even for the most-studied?-and?-pinenes.Alternatively,theoretical study is becoming more and more important in elucidating reaction mechanisms.Theoretical calculations can be used to identify the important free radical intermediates from their reaction kinetics and then identify the important reaction pathways and predict the reaction products as well as their formation yields.In this thesis,high-level quantum chemical calculations,coupled with kinetic calculations with transition state theory?TST?and unimolecular rate theory?RRKM?were used to study the atmospheric oxidation mechanisms of sabinene and 3-carene.Main reaction channels and product yields were predicted and compared with the available experimental results.Main results of this thesis are as follows:1.Atmospheric Ozonolysis Mechanism of SabineneSabinene is one of the monoterpenes of biogenic origin in the atmosphere.Ozonolysis is one of important oxidation removal for sabinene in the atmosphere during daytime,leading to formation of secondary organic aerosol.In this study,we investigate the mechanism of gas-phase ozonolysis of sabinene using quantum chemistry and kinetic calculations.The reaction starts with the formation of four primary ozonides?POZs?,which decompose to primary product channels as CH₂OO+sabinaketone and CH₂O+two Criegee intermediates?CI-1 and CI-2?with branching ratios of 17%,45%,and 38%for sabinaketone?CI-1 and CI-2,respectively,at 298 K and 760 Torr.Calculations showed that the stabilized CI-1 would undergo rapid intramolecular H-shift to a vinyl hydroperoxide?VHP?at a rate of²700 s^–1 followed by rapid decomposition to OH radical and a vinoxy-type radical?VTR?and CI-2 would slowly isomerize to dioxirane at a rate of 0.97 s^–1.In the atmosphere CI-2 would instead react with water and water dimer,forming?-hydroxyalkyl hydroperoxides??HAHPs?,which would decompose to sabinaketone and H₂O₂ via heterogenerous processes.Reaction of CI-2 with SO₂would also be significant in dry and cold atmosphere.The yield of sabinaketone of 47%,from primary POZ decomposition and secondary reactions of?HAHPs,agrees with the previously measured values of 35⁵0%.OH radical,formed from CI-1,could reach 44%,agreeing with previously reported value of?33±6?%.Further reaction of VTR radical would form highly-oxidized multifunctional products?HOMs?containing carboxylic and/or carbonyl groups which might contribute substaintially to SOA formation.2.Atmospheric Oxidation Mechanism of Sabinene Initiated by OH RadicalThe atmospheric oxidation mechanism of sabinene initiated by the OH radical has been studied using quantum chemistry calculations at the CBS-QB3 level and reaction kinetic calculations using transition state theory and unimolecular rate theory coupled with collisional energy transfer.The oxidation is initiated by OH radical additions to the CH₂=C<bond with a branching ratio of??92-96?%,while all the hydrogen atom abstractions count for??4-8?%of branching ratio,which was estimated by comparing the rate coefficients of the reactions of sabinene and sabinaketon with the OH radical.Addition of OH to the=C<carbon forms radical adduct Ra,while addition of OH to the terminal CH₂=carbon forms radical adduct Rb,which would break the three-membered ring promptly and almost completely to radical Re.RRKM-ME calculations obtained fractional yields of 0.40,0.09,and 0.51 for radicals syn-Ra,anti-Ra,and Re,respectively,at 298 K and 760 Torr.In the atmosphere,the syn/anti-Ra radical would ultimately transform to sabinaketone in the presence of ppbv levels of NO,while in the transformation of the Re radical,both bimolecular reactions and unimolecular H-migrations could occur competitively for the peroxy radicals formed.The H-migrations in peroxy radicals result in the formation of unsaturated multifunctional compounds containing>C=O,–OH and/or–OOH groups.Formation of sabinaketone from syn-and anti-Ra and formation of acetone from Re are predicted with yields of?0.37 and?0.38 in the presence of high NO,being larger than while in reasonable agreement with the experimental values of 0.19-0.23 and of 0.21-0.27,respectively.3.Ozonolysis Mechanism of 3-Carene in the AtmosphereThe gas-phase ozonolysis mechanism of 3-carene is investigated using high level quantum chemistry?CCSD?T?-F12a?and kinetic calculations.The reaction flows the Criegee mechanism with an initial addition of O₃ to the>C=C<bond,followed by a chain of unimolecular isomerizations,as 3-Carenne+O₃?POZs?Primary Ozonides??CIs?Criegee Intermediates,4 conformers??Ps?products?.In the course of reaction,the large excess energy retained in POZs*lead to prompt unimolecular processes in POZs*,CIs*,and Ps*,and only⁴%of CIs*could be stabilized by collision at 298 K and 760 Torr.From RRKM-ME calculations,The VHPs*would further dissociate to Vinoxy-type radical and OH radical,SOZs*would isomerize to 3-caronic acid,and Dioxirane*would be collisionally stabilized.The fractional yield of OH radical,in the range of 0.56 to 0.59,agrees reasonably with the previously measured value of 1.06?with an uncertainty factor of 1.5,revised to 0.6?.