Theoretical Studies On The Atmospheric Degradation Mechanism And Kinetics Of Halogenous And Oxygenous Volatile Organic Compounds

The atmospheric volatile organic compounds have significant implications on the on environment where we are living.For example,halogenated alkanes,alcohols,esters and ether compounds in the troposphere have been widely concerned by researchers.These species can cause environmental problems such as greenhouse effect,ozone layer damage and poor air quality,which can directly or indirectly cause irreparable harm to human health.Therefore,it is necessary to carry out a detailed study and accurate prediction of the main behavior and atmospheric lifetime of these species in the atmosphere.The reactions of volatile organic compounds with the active radicals such as OH,Cl,and NO₃ are deemed to the primary degradation pathways,and thus the atmospheric lifetimes of volatile organic compounds can be estimated.In this paper,we have adopted DFT and ab initio methods to investigate the mechanisms and kinetic properties of the reactions between the above species and free radicals,the rate constants and branching ratios in a wide temperature range are also calculated.In addition,the subsequent behaviors of the primary product radicals?such as alkyl radical,alcoxyl radical,and peroxy radical?are further explored in detail.The main conclusions are following:1.The dual-level direct dynamics method is adopted to investigate the multichannel reactions of the small molecules iodide of CH₃I and C₂H₅I with NO₃ radical.The equilibrium geometries and frequencies are optimized and calculated at the B3LYP level using the6-311++G?d,p?basis set for C,H,N,O and cc-pVTZ-PP basis set for iodine.The single point energy calculations are carried out by the methods of CCSD?T?and QCISD?T?with the basis sets of aug-cc-p VTZ-PP for iodine atom and 6-311++G?d,p?for the remaining atoms.The aug-cc-pVTZ-PP and cc-pVTZ-PP basis sets included an effective-core-potential?ECP?for iodine to decrease the cost of computation and describe the relativistic effects.There are two types for the title reactions,i.e.hydrogen abstraction and displacement.The rate constants and branching ratios for different channels are calculated.The NO₃-determined atmospheric lifetime of CH₃I and C₂H₅I is respectively 3.07 h and 5.86 h,showing that CH₃I and C₂H₅I can be degraded into active iodine compounds within a short time in the nighttime.The formed active iodine compounds can be transformed into IO radical in the troposphere,which is the important scource of atmospheric IO radical.2.The oxidation mechanisms and dynamics of CF₃COOCH₃,CF₂HCOOCH₃,CF₃COOCH₃,CF₃COOCH₂CH₃,and?CH₃?₃CCOOCH₃ reacions with Cl or OH are theoretically studied.In order to distinguish the effects of different methods,we have used four different methods of B3LYP,MP2,BHandHLYP,and M06-2X with the 6-311G?d,p?basis set to optimize the geometric configuration and calculate the harmonic frequencies,and then by means of the CCSD?T?and QCISD?T?/6-311++G?d,p?//B3LYP/6-311G?d,p?level,we have obtained the accurate energy information.We have also computed the rate constants and the product branching ratios in the temperature region of 200–1000 K,the calculated rate constant is in good agreement with the corresponding experimental data at room temperature.In addition,the results show that the harmful specie of?CH₃?₃CCOONO₂ may be the stable product in the presence of NO and O₂.Taking reaction of?CH₃?₃CCOOCH₃ with OH as an example,we theoretically explored the effect of water and formic acid molecules,the results show that the water and formic acid molecules can promote the degradation of?CH₃?₃CCOOCH₃.We have also estimated the global warming potentials relative to CO₂ of the above species to assess their environmental impacts.3.Using the B3LYP and MP2/6-311G?d,p?method,the geometric configuration optimization and frequencies calculation are carried out which are involved in the reactions of CF₃CH₂CH₂CH₂OH/CF₃CF₂CF₂CH₂OH with OH and Cl.More accurate energy information is obtained from the CCSD?T?and QCISD?T?/6-311++G?d,p?level.The mechanisms and kinetic content for the above reactions are also investigated.The kinetic isotope effects for reactions CF₃CH₂CH₂CH₂OH/CF₃CD₂CD₂CD₂OD+OH and CF₃CH₂CH₂CH₂OH+OH/OD are discussed to provide valuable information.We have analyzed the primary degradation pathway and products in the presence of O₂ and NO.The global warming potentials and atmospheric lifetimes for the CF₃CH₂CH₂CH₂OH and CF₃CF₂CF₂CH₂OH are also computed.In conclusion,the atmospheric influences of the hydrofluoroethers are evaluated at the theoretical level.4.Theoretical investigation on the degradable mechanism and kinetics of the?CF₃?₂CFOCH₃,?CF₃?₂CHOCH₃,?CF₃?₂CHOCF₃,and CF₃OCF₂CF₂H by OH radical and Cl atom are performed at the B3LYP and M06-2X method with the 6-311++G?d,p?basis sets,the high level energy information is obtained at the CCSD?T?//B3LYP/6-311++G?d,p?level.In the meantime,taking?CF₃?₂CHO?O?CH and?CF₃?₂CFO?O?CH as an example,we investigated the atmospheric degradation mechanism and kinetics of the oxidation products of hydrofluoroethers.The rate constants are computed by using the canonical variational transition state theory with small curvature tunneling.In addition,the atmospheric behaviors of the alkyl radical?which is formed from the reaction of radical with HFEs and its oxidation products?with the presence of O₂ and NO are studied,the results show that the peroxynitrites and organic nitrates are respectively the main products.In the end,the atmospheric lifetimes and global warming potentials relative to CO₂ are predicted to assess the environmental implications of above species.5.The Cl-initiated oxidation of?CH₃?₃CC?O?F,?CH₃?₃CC?O?Cl,and?CH₃?₃CC?O?Br is studied at the CCSD?T?/aug-cc-pVDZ//B3LYP/6-311++G?d,p?level.Using canonical variational transition state theory with small curvature tunneling,the rate coefficients between the chlorine atom and?CH₃?₃CC?O?F?k₁?,?CH₃?₃CC?O?Cl?k₂?,and?CH₃?₃CC?O?Br?k₃?are calculated within 250–1000 K.The initial degradation mechanism and the secondary reactions for the hydrogen abstraction intermediates are studied involving NO and O₂molecules in the atmosphere.The organic nitrate,which is the important components of secondary organic aerosol organic aerosol,can be formed in this reaction cycle.