Mechanistic And Kinetic Study On The Atmospheric Reaction Of Typical Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons(PAHs)are typical atmospheric pollutants with two or more benzene rings.Due to their activity in the atmosphere,gaseous PAHs can easily react with activated oxidants,such as OH radical,Cl radical and NO3 radical.In the oxidation processes,low toxic and non-toxic products may be formed,and secondary pollutants with acuter toxicity may also be produced,such as oxygen-ated PAHs(OPAHs)and nitro-PAHs(NPAHs).Compared with their parent PAHs,the secondary pollutants have stronger carcinogenicity and mutagenicity.Therefore,at-mospheric oxidation of PAHs has attracted much attention in recent decades.Chlorinated PAHs(ClPAHs)are derivatives of PAHs whose one or more hydro-gen atoms are substituted by chlorine atoms.Due to the similar structures and for-mation mechanism with dioxins,some of ClPAHs have stronger toxicity and carcino-genicity compared with their parent PAHs.About 81%of atmospheric ClPAHs exist in the gas phase.The main sources of CIPAHs are industrial production,waste incin-eration and PAHs oxidation processes.ClPAHs can cause environmental pollution in different regions through long-distance atmospheric transmission.Therefore,investi-gating the atmospheric oxidation of ClPAHs is of great significance for evaluating their environmental risks.1 Atmospheric Oxidation of PAHs and ClPAHs Initiated by OH RadicalThe atmospheric oxidation processes of indene,fluorene,chrysene,triphenylene,1-chloronaphthalene initiated by OH radical in the presence of O2 and NO were sim-ulated using quantum chemical calculation and the rate constants were determined at 298 K and 1 atm.In the reaction of indene,the oxidation products include hydroxy-indene,indenone,1,2-diformylcyclopentadiene and ring-opening dialdehydes.The branch ratio of H abstraction by OH radical at carbon 7 is 35.29%,which is dominant in the initial reaction.The 7-indene radical is the precursor of various PAHs,indicat-ing that the atmospheric oxidation of indene will increase its environmental risks.The reaction rate constant of indene with OH radical was calculated to be 1.04×10-10 cm3 molecule-1 s-1 and the lifetime of indene determined by OH radical was 2.74 h.In the reaction of fluorene,the addition reaction of OH radical leads to the formation of hy-droxyfluorene and a series of ring-opening products,while the H-abstraction reaction results in the formation of 9-fluorenone,which has considerable potential to form dibenzo-p-dioxin and nitrofluorenone.The reaction rate constant of gaseous fluorene with OH radical was calculated to be 2.29×10-11 cm3 molecule-1 s-1 and the atmos-pheric lifetime of fluorene determined by OH radical was 12.51 h.In the reaction of chrysene,the oxidation products include nitrochrysene,hydroxychrysene,hydroxy-chrysenone,phenanthrene-1,2-dicarboxaldehyde and 11-benzo[?]fluorenone.Nitro-chrysene is easier to be produced than the nitro-products of PAHs with 1-3 rings,suggesting that chrysene,which has four rings,is easier to be nitrated.Water can hardly catalyze the H-shift reaction.The reaction rate constant of gaseous chrysene with OH radical is 4.48×10-11 cm3 molecule-1 s-1 and the lifetime of atmospheric chrysene determined by OH radical is 6.4 h.In the reaction of triphenylene,the C1 of triphenylene molecule is more active than the C2,thus,the addition of OH radical has high selectivity in the initial reaction.The degradation products of triphenylene in-clude nitrotriphenylene,hydroxytriphenylene and phenan-threne-9,10-dicarboxaldehyde.2-nitrotriphenylene has large yield in the degradation process,however,it is strongly mutagenic.Triphenylene,which has four rings,is more easier to be nitrated.Water can hardly catalyze the H-shift reaction.The rate constant of gaseous triphenylene reacting with OH radical is 7.17×10-12 cm3 mole-cule-1 s-1 and the lifetime of atmospheric triphenylene determined by OH radical is 1.66 days,indicating that triphenylene can react with oxidants to form secondary pol-lutants.Therefore,triphenylene has high environmental risks that cannot be ignored.In the reaction of 1-chloronaphthalene,OH radical tends to add to carbons 4 and 5,forming intermediates that will subsequently react with O2 and finally produce a se-ries of ring-opening dialdehydes.The intramolecular H-shift reaction will be affected by tunneling effect,but it is still a favorable reaction pathway.The rate constant for the reaction of gaseous 1-chloronaphthalene with OH radical is 1.94×10-11 cm3 mole-cule-1 s-1 and its lifetime determined by OH radical is 0.62 day.Compared with naph-thalene,the substituent group Cl atom of 1-chloronaphthalene decreases its reactivity and makes it more persistent in the atmosphere.2 Atmospheric Oxidation of PAHs Initiated by Cl and NO3 RadicalsIn this study,fluorene was chosen as the representative,of which the reactions with Cl and NO3 radicals as well as the subsequent reactions were investigated.The reaction of fluorene with Cl radical leads to the formation of 9-fluorene radical,which is the precursor of many secondary pollutants,such as fluorenone and diben-zo-p-dioxin.Atmospheric Cl radical will increase the generation of 9-fluorene radical by 6%-29%.The formation yield of 9-fluorene radical in the oxidation of fluorene initiated by atmospheric OH radical and Cl radical is 18%-22%.Therefore,Cl radical plays a significant role in the atmospheric oxidation of fluorene.In the reaction of fluroene with NO3 radical,NO3 addition is the main reaction pathway with no barrier.In the presence of O2 and NO,the atmospheric oxidation process of fluorene initiated by NO3 radical eventually forms nitrofluorene,1,4-fluorenequinone and ring-opening dialdehydes.The reaction rate constant of gaseous fluorene with NO3 radical was calculated to be 1.52×10-14 cm3 molecule-1 s-1 and the atmospheric lifetime of fluorene determined by NO3 radical was 1.52 days.