Investigations On The Mechanisms Of Atmospheric Photooxidation Of Typical Monocyclic Aromatic Hydrocarbons:implications For O₃ Formation

As typical volatile organic compounds in urban atmosphere,monocyclic aromatic hydrocarbons（MAHs）have high reactivity and ozone（O₃）formational potential.MAHs can contribute to tropospheric O₃ and fine particle through complex photochemical reactions.At present,the atmospheric oxidation mechanism of MAHs still has problems such as the difficulty of accurate measurement of oxidation intermediates,and the great controversy over the branch ratio and main products of the oxidation reactions.Therefore,the model simulation value and the actual observation value of key products cannot be closed,affecting our accurate assessment of the contribution of MAHs to air pollution.In this thesis,the photooxidation reactions of seven common MAHs（toluene,o-xylene,m-xylene,p-xylene,1,2,3-trimethylbenzene,1,2,4-trimethylbenzene and 1,3,5-trimethylbenzene）were systematically studied by combining quantum chemistry and atmospheric photochemical model simulation.The photooxidation mechanism and atmospheric lifetime of typical MAHs were elucidated,the response relationship between the structure and reaction mechanism of MAHs was indentified,and the effects of atmospheric photooxidation reactions of seven MAHs on O₃formation under typical pollution conditions were evaluated.This thesis provides data support for the subsequent simulation of atmospheric photochemical models.The main results of this thesis are summarized as follows:（1）The initial addition reaction of MAHs initiated by OH radicals were four pathways:ipso-,ortho-,meta-and para-OH addition.The ipso-OH addition pathway is OH attacks theα-site on the aromatic ring to generate the ipso-OH adduct,and the others pathways are OH attacking theβ-site on the aromatic ring to generate the ortho/meta/para-OH adducts.Mechanistic results show that the ipso-OH addition pathway of toluene,o-xylene,p-xylene,1,2,3-trimethylbenzene and 1,2,4-trimethylbenzene is the main reaction pathway,and the main products are ipso-OH adducts.m-Xylene and 1,3,5-trimethylbenzene are dominated by ortho-OH addition reaction,and ortho-OH adducts are the dominant intermediate products.Kinetic calculations show that the reaction rate of OH addition pathway increases with the degree of methylation on the structure of MAHs,so the MAHs with higher methylation degree have stronger atmospheric reactivity.In MAHs dominated by ortho-para-CH₃ groups,theα-site in the structure is more easily attacked by OH,and the activity of the ipso-OH addition reaction increases with the increase of the number of-CH₃ groups in MAHs.However,the-CH₃ groups on MAHs that are in the meta position of each other can effectively activate theβ-site and promote the ortho-OH addition reaction.The subsequent reactions of the main intermediates（ipso/ortho-OH adducts）in the above-mentioned seven MAHs reactions were further explored,O₂ could initiate the addition and H-abstraction reactions of ipso/ortho-OH adducts.For m-xylene,the ortho-OH adduct mainly undergoes O₂ addition reaction to generate peroxy radical,followed by bridging reaction to generate ortho-bicyclic radical（BR）with a yield of about70%.Similarly,for toluene,o-xylene,p-xylene,1,2,3-trimethylbenzene and 1,2,4-trimethylbenzene,the O₂ addition reaction of the ipso-OH adducts produces ipso-BR,and the yields are all greater than 63%.For 1,3,5-trimethylbenzene,its ortho-OH adduct is rapidly attacked by O₂ and undergoes H-abstraction reaction to formate 2,4,6-trimethylphenol with a yield of 79%.Expect for 1,3,5-trimethylbenzene,the yields of the phenolic products for other six MAHs were all less than 37%.It indicates that theα-sites on the ipso/ortho-OH adduct structure are easily affected by the steric hindrance effect of the-CH₃ group,resulting in lower O₂ oxidation reactivity.（2）Based on the above-mentioned photooxidation mechanism of the seven MAHs,this thesis uses an atmospheric photochemical reaction model to simulate the effect of the photooxidation of the seven MAHs on O₃ generation under urban pollution conditions.The simulation results show that seven MAHs can generate HO₂ and a series of RO₂ radicals during the photooxidation reaction during the day.The HO₂ and RO₂ radicals generate O₃ by reacting with nitric oxide（NO）,and the O₃ contribution from the HO₂+NO reaction is greater than60%.The average daytime O₃ production rates of the seven MAHs were:3.32 ppbv/h for toluene,5.10 ppbv/h for o-xylene,6.88 ppbv/h for m-xylene,4.96 ppbv/h for p-xylene,10.21ppbv/h for 1,2,3-trimethylbenzene,12.06 ppbv/h for 1,2,4-trimethylbenzene and 31.24 ppbv/h for 1,3,5-trimethylbenzenes.The increase in methylation degree on MAHs is beneficial to O₃generation.Further analysis of the influence of the structural differences of MAHs on the formation rate of RO₂ and phenolic products during the photooxidation reaction.It showed that MAHs with a higher degree of methylation had a faster RO₂ formation rate,which promoted the RO₂+NO reaction to generate O₃.However,MAHs with mutual meta-CH₃groups have a faster generation rate of phenolic products,which corresponds to the photooxidation reaction mechanism of the above seven MAHs.The effect of MAHs precursor concentration on O₃ generation was further explored.When the MAHs/NO_x initial concentration ratio decreased from 3.25 to 0.5,the O₃ generation rate of toluene decreased from 21.87 ppbv/h to 4.76 ppbv/h.For the other six MAHs,the rate of O₃ generation decreased by 29.2-51.8%with the decrease of the precursor concentration ratio,and the reduction amount decreased with the increase of methylation degree.Therefore,for toluene and xylene,which are more abundant in urban areas,the contribution to O₃ can be more effectively reduced by reducing the concentration of precursors.Using the branching ratios of the above seven MAHs photooxidation mechanisms,it was found that compared with the original MCM v3.3.1chemical mechanism,the branching ratio of the phenolic pathway of toluene increased from18%to 31%,and the generation rate of O₃ decreased by 35%.When the branching ratios of the phenolic pathway of o-xylene,m-xylene and p-xylene are doubled,the O₃ generation rates are reduced by 17%.The branching ratio of the RO₂ pathway of 1,3,5-trimethylbenzene decreased from 79%to 19.4%,resulting in a 6.5%reduction in the O₃ generation rate.This change reflects the combined effects of lower concentrations of RO₂ and HO₂ resulting from the oxidation of MAHs due to a higher branching ratio of the phenolic pathway.The photooxidation mechanism of MAHs proposed in this thesis can provide a theoretical basis for the reactions of related species,and the kinetic data of MAHs also provide solid data support for the subsequent atmospheric photochemical model simulation.