Atmospheric Oxidation Reaction Of Polychlorinated Biphenyls And Alkenes With OH Radicals

Polychlorinated biphenyl?PCBs?,alkenes and other substances have a wide range of applications in human activities,but with the development of human activities a large number of emissions into the environment,causing serious environmental and health problems.Both compounds belong to volatile organic pollutants,among which polychlorinated biphenyls are also known as persistent organic pollutants?POPs?due to their strong persistence in the environment..They can all be emitted into the atmosphere in gaseous form.Due to the existence of OH,NO₃,O₃,Cl and other active oxidants in the atmosphere,these organic compounds are prone to atmospheric oxidation reactions in the complex atmosphere,resulting in the increase of ozone in the atmosphere and the formation of secondary organic aerosols.Therefore,the study of their reaction mechanism and gas reaction kinetics in the atmosphere will help us to understand the chemical process of the atmosphere and improve the pollution of the atmosphere.The atmospheric oxidation reaction of pollutants in the atmosphere and the reaction pathways are very complex and diverse,and the products or intermediates generated are difficult to capture due to their short life or very low yield,so it is difficult to predict the reaction mechanism and the structure or yield of products only by relying on conventional experimental equipment.The theoretical calculation method can predict the intermediate structure and product yield in the reaction process,as well as the branch ratio of different reaction paths,and realize the simulation and prediction of the whole mechanism of atmospheric oxidation reaction,which plays a crucial role in the research of atmospheric chemistry.Therefore,in this paper,we study the atmospheric oxidation mechanism of PCBs and OH radicals in the atmosphere by using quantum chemistry and gas reaction kinetics.In this paper,we use the M06-2X to optimize the geometry of reactants,intermediates,products and transition states.Then the electron energy of the optimized structure is calculated by ROCBS-QB3 and the complete base set model.Finally,according to the potential energy surface and electron energy of the reaction,the rate coefficients of each step in the reaction are calculated by combining with the classical transition state theory,and the branch ratios of the main reaction channels and the final products are predicted.The main results are as follows:?1?Oxidation of Biphenyl Initiated by OH RadicalUnder atmospheric conditions,reaction of BP with OH proceeds mainly by OH addition to C2,C3 and C4 position of biphenyl,forming three BP-n-OH radicals,namely R2,R3 and R4.Subsequently,Rn reacts with O₂ to form peroxyradicals.Total effective bimolecular rate formed from the reaction with OH radicals are R2-1OO?R2-3OO and R2-5OO,and the cyclic radical R2-13OO-s would reacts similarly as the dicyclic radical formed in the oxidation of benzene.In presence of high NO concentrations,possible product channels include phenylglyoxal+butenedial and 1-phenyl butenedial+glyoxal.For BP-4-OH,the reaction with O₂ proceeds mainly by addition to C3/C5 position of phenyl ring,almost forming the cyclic intermediate R4-35OO-s and R4-35OO-s would transform to 2-phenylglyoxal+butenedial.For BP-3-OH,the reaction with O₂ proceeds mainly by addition to C2 and C5position of phenyl ring,mainly forming R3-24OO-s,and the cyclic R3-24OO-s would transform similarly as R2-13OO-s to 2-phenylglyoxal+butenedial and 2-phenyl butenedial+glyoxal.The total effective rates of removal by reacting with atmospheric oxygen by R2,R3,and R4,respectively.The reactions of R2 and R4 with O₂ would react with NO₂ in polluted regions.Reaction of BP-2OH and BP-4OH with NO₂ likely forms 3-nitrobiphenyl.Here,the yield of 2-HO-BP and 4-HO-BP are predicted to be 0.63 and 0.16 respectively.?2?Oxidation of polychlorinated biphenyl initiated by OH radicalOH addition occurs dominantly at non-chlorinated ortho-sites?2-,6-,2'-,and 6'-?,the CH sites with two neighbouring chlorination,and the CH sites with one neighbouring and one para-Cl,and formed a large number of PCB-OH adduct.,which is consistent with experimental observations.For PCBs considered here,fractions of OH addition to ortho-positions are usually among the highest.The peroxy radicals PCB-OH-OO would have unimolecular channels.The product channels with highest effective rate coefficients are2HO-BP+HO₂ and R-1'5'OO-s for PCB1-6'OH,2'HO-PCB2+HO₂ and R-15OO-s for PCB2-6OH,and 2'HO-PCB3+HO₂ and R-1'3'OO-s for PCB3-2'OH.Further transformation of cyclic peroxy radicals should be similar to that of BP-2OH-3OOs.For PCB2-6OH,we expect high yield of 5-Nitro-PCB2.Several PCB-OH radicals react with O₂ at rather low effective bimolecular rate coefficients.Their reaction with NO₂ in the polluted atmosphere may produce nitro-PCBs with stronger carcinogenic properties than PCB,and cause significant impact to the environment.?3?Reaction of alkene with OH radical in the Gas PhaseIn the experiment of this paper,the OH radical generating source by photolysising HONO gas was builted,and a gas-path device for the reaction of alkene with OH radicals.HONO gas can be generated at a steady flow rate and the concentration reaches 2?3 ppm.The whole experiment was carried out in the gas path system.We investigated the effects of different flow rates and carbon numbers in the reaction of 1-alkenes with OH radicals under NO conditions,as well as the effects of different temperatures on the reaction.In the experiment,the nitrogen oxide analyzer was used to monitor the concentration changes of NO and NO₂ in the reaction system online in real time.The results showed that the amount of NO consumed by 1-alkenes reaction increased with carbon number,and the effect of 1-alkenes flow rate on the reaction was not obvious.Due to the great instability of the experimental device,the deviation of the result is large.