Theoretical Investigation On The Reaction Mechanism And Kinetics Of Several VOCs In The Troposphere

With the acceleration of economic development and the reform of the economic system,the composition of volatile organic pollutants(VOCs)in urban ambient air has become more and more complex,and the concentration has gradually increased,which has played an increasingly important role in atmospheric chemical reactions The importance of biogenic volatile organic pollutants(BVOCs)has also received increasing attention.These substances undergo migration and transformation in the atmosphere,affecting the feedback effect of atmospheric chemistry on the biosphere The use of atmospheric oxidants to study the degradation of atmospheric pollutants has received widespread attention in experiments and practical applicationsIn this paper,we selected volatile organic pollutants formaldehyde and acetaldehyde,biogenic volatile organic pollutants camphene and trans-2-hexenal as research objects,and used a variety of gas molecules and oxidants to study the mechanism of sulfur dioxide with formaldehyde and acetaldehyde,the reaction mechanism of Cl with camphene and the reaction mechanism of NO3 with trans-2-hexenal,providing a basis for the reactions of pollutants.In this paper,we used quantum chemistry theory,through density functional method(DFT).The reaction mechanism of sulfur dioxide with formaldehyde and acetaldehyde was studied at CCSD(T)/aug-cc-pVTZ//M06-2X/6-311++G(d,p)level,the other two chapters were at 6-311++G(3df,3pd)//M06-2X/6-311++G(d,p)level.The reaction rate constants were calculated by the transition state theory.The following conclusions are obtained:1.A theoretical study on the formation and oxidation mechanism of hydroxyalkysulfonate in the atmospheric aqueous phaseHydroxymethanesulfonate(HMS)is an important organosulfur compound in the atmosphere.In this work,we studied the formation mechanism of HMS via the reaction of formaldehyde with dissolved SO2 using the quantum chemistry calculations.The results show that the barrier(9.70 kcal mol-1)of the HCHO+HSO3-reaction is higher than that(1.60 kcal mol-1)of the HCHO+SO32-reaction,indicating that the HCHO+Sk32-reaction is easier to occur.For comparison,the reaction of acetaldehyde with dissolved SO2 also was discussed.The barriers for the CH3CHO+HSO3-reaction and CH3CHO+SO32-reaction are 16.60 kcal mol-1,2.50 kcal mol-1,respectively,indicating the reaction with SO32-is easier to occur.And the result suggested that the reactivity of HCHO with dissolved SO2 is higher than that of CH3CHO.The further oxidation of CH2(OH)SO3-1 and CH3CH(OH)SO3-by an OH radical and O2 showed that the SO5·-radical can be produced.2.Mechanism and kinetic studies of camphene with Cl radicalThe reaction process between camphene and Cl radical was calculated.Camphene can react with Cl through two kinds of ways:Cl addition and H abstraction ways,the reactions are exothermic,and the Cl addition way is more important.Further addition pathways show that in the presence of NO and O2,the main products are acetone and formaldehyde,the results are consistent with the experimental observations.The rate constants of the addition pathways of camphene and Cl under the condition of 298 K are 3.12 × 10-10 cm3 molecule-1 s-1 and 3.34 × 10-10 cm3 molecule-1 s-1,and the reaction rate decrease with increasing temperature.Comparing the rate constants of the addition pathways and the abstraction pathways can further illustrate the importance of the addition channels.By comparing the reaction of camphene with Cl radical,OH radical and O3,we can find that the fastest reaction rate is camphene with Cl radical.3.Mechanism and kinetic studies of trans-2-hexenal with NO3 radicalThe mechanism of the degradation reaction of trans-2-hexenal induced by NO3 radical in the atmosphere was studied.Trans-2-hexenal can react with NO3 radicals through two ways:NO3 addition and H abstraction ways.In the gas phase,the intermediate obtained after the addition reaction of trans-2-hexenal and NO3 is further oxidized in the presence of O2 and NO and NO2.The main products are CO,HO2 radical and aldehydes.The intermediates obtained after the H abstraction pathways can further react with O2 and NO in the atmosphere.The main products are CO2 and peroxyacyl nitrate(PAN).These results are consistent with experimental predictions under the conditions of 298 K and 1 atm,the total reaction rate of trans-2-hexenal induced by NO3 is 2.27 × 10-13 cm3 molecule-1 s-1.In 298 K,the rate of H abstraction on the aldehyde group is the fastest,and the H abstraction ways at the remaining alkyl positions is not as important as the aldehyde group position.Compared with the NO3 addition and H abstraction ways,the H abstraction way is dominated because of the formation of PAN,which is consistent with the experimental result.And in a temperature range of 200-400 K,the rate constant shows a decrease with increasing temperature.By comparing the reaction rate constants of trans-2-hexenal with OH radical and O3,the rate constants of trans-2-hexenal and OH were found the fastest,but NO3 radical should not be ignored because of its importance at night.