Theoretical Investigation On The Formation Mechanism Of Opahs And Npahs From Atmospheric Reactions Of Typical Pahs

Polycyclic aromatic hydrocarbons?PAHs?are a series of organic compounds with fused aromatic rings.Although the carcinogenic and mutagenic potency varies between PAH congeners,they are present in the atmosphere as a mixture with substantial toxic hazards.Although often recognized as persistent organic pollutants?POPs?,PAHs are reactive with OH,NO₃ and O3 in the atmosphere.Oxidized products,most notable being mostly quinones and nitrated compounds are formed.Some of these quinones and nitrated compounds are potent mutagens and carcinogens.To the best of our knowledge,the experimental and theoretical data addressing the oxidation and transformation mechanism of gas phase PAHs containing 3 or more aromatic rings in the atmosphere are sparse.Furthermore,the transformation of PAHs and the formation of mutagens and carcinogens from the transformation of PAHs are associated with the trace gases,e.g.water and alcohols,in the atmosphere,which is rarely reported.This study aimed at investigating the oxidation and transformation mechanisms of gas phase PAHs with the effect of trace gases in the atmosphere by theoretical calculation.This study can explain the previous experiments and offer some guidance to future experimental and theoretical investigations.We hope this study would contribute to more effective control strategies of the PAHs pollution and the atmospheric chemistry.1 Mechanism of atmospheric oxidation of phenanthrene initiated by OH/NO₃ radicals in the presence of O2 and NOxThe present work investigated OH/NO₃ radical-initiated atmospheric degradation of phenanthrene?Phe?in the presence of O2 and NOx.The possible reaction mechanism was elucidated by density functional theory?DFT?calculations.The rate constants of elementary reactions were assessed by Rice-Ramsperger-Kassel-Marcus?RRKM?theory.?1?Assuming typical tropospheric 12 h daytime average concentrations of OH is 2×106 molecule cm-3,the atmospheric lifetime of Phe relative to gas-phase reactions with OH is estimated to be 4.6 h,based on the calculated overall rate constant of 3.02x10-11 cm3 molecule-1 s-1 at 298 K and 1 atm.Assuming typical tropospheric 12 h nighttime average NO₃ concentration is 5×108 molecule cm-3,the atmospheric lifetime of Phe relative to gas-phase reactions with NO₃ is estimated to be 1.8 h,based on the calculated overall rate constant of 3.04×10-13 cm3 molecule-1 s-1 at 298 K and 1 atm.?2?The OH/NO₃ addition to 9,10-bond of Phe is the most favorable pathway for the reaction of Phe with OH/NO₃ radicals,due to the strong olefinic character and thus higher reactivity of 9,10-bond of Phe.Phe-OH and Phe-NO₃ intermediates are unlikely to proceed unimolecular isomerization in the atmosphere.Instead,they prefer to associated with O2/NO₂.Syn-Phe-OH-O2 formation is more favorable over anti-Phe-OH-O2 formation in OH initiated reactions.Conversely,anti-Phe-NO₃-02 formation is more favorable over syn-Phe-NO₃-02 formation in NO₃ initiated reactions.?3?Calculations show that the main products of OH radical-initiated atmospheric degradation of Phe are a series of ring-retaining and ring-opening oxygenated PAHs containing phenanthrol,phenanthones,phenanthrenequinone,and dialdehydes.The main products of NO₃ radical-initiated atmospheric degradation of Phe are 10-?nitrooxy?-10-hydro-phenanthrene-9-one,2,2'-diformylbiphenyl,9,10-phenanthrenequinone,9-fluorenone and dibenzopyranone.10-?nitrooxy?-10-hydro-phenanthrene-9-one and 2,2'-diformylbiphenyl are classified as first-generation products which are subject to secondary reactions to produce 9,10-phenanthrenequinone,9-fluorenone and dibenzopyranone.2 The effect of some-OH containing trace gas on the atmospheric formation of nitrated polycyclic aromatic hydrocarbons from OH/NO₃ initiated reactions of the corresponding PAHsWe theoretically investigated the catalyzed formation of nitro-polycyclic aromatic hydrocarbons?NPAHs?from the gas-phase OH initiated reactions of naphthalene,anthracene,fluorene,fluoranthene,phenanthrene,acenaphthene,acenaphthylene and pyrene.The catalytic effect of water,H₂SO4,HNO₃,HCOOH,CH₃COOH,CH₃OH and CH₃CH₂OH on NPAH formation from OH initiated reactions of the corresponding PAHs were analyzed both energetically and kinetically.The effect of water,CH₃OH and CH₃CH₂OH on NPAH formation from NO₃ initiated reactions of the corresponding PAHs was also investigated.?1?The rate constants of catalyzed or uncatalyzed water elimination of PAH-OH-NO₂ intermediates can be used to kinetically evaluate the atmospheric formation of NPAHs from OH initiated reactions of the corresponding PAHs.H₂SO4,HNO₃,HCOOH,CH₃COOH,CH₃OH and CH₃CH₂OH can lower the activation energy of the dehydration step to promote NAPH formation by OH initiated reactions of the corresponding PAHs.However,their specific catalytic efficiencies depend on multiple factors including the rate constants of catalyzed dehydration from reactant complexes?RCs?,the equilibrium constants for the formation of RCs and the specific concentration of the catalysts.?2?The present study suggests that the potential of NPAH formation in the polluted days with high RH and elevated concentrations of pollutants will be substantially higher than in clean days.Under typical atmospheric conditions,water,H₂SO4,HNO₃,HCOOH,CH₃COOH,CH₃OH and CH₃CH₂OH can promote NAPH formation by OH initiated reactions of the corresponding PAHs.Among these substances,water is the major factor facilitating NAPH formation,while alcohols and acids can contribute more than water NPAH formation in heavily polluted atmosphere or near the emission sources where the concentration of the acids and alcohols are significantly elevated.?3?This study also highlights the significant role of catalyzed NPAH formation from dehydration of para-PAH-OH-NO₂ intermediates,which may change the specific isomer distribution of the formed NPAH in comparison with that of uncatalyzed reactions.NPAH formation from para-PAH-OH-NO₂ contributes 82.60-99.99%of the total pseudo first-order rate constants of NPAH formation from catalyzed dehydration of all the PAH-OH-NO₂ intermediates,whilst para-PAH-OH-NO₂ intermediates cannot dehydrate without catalysts.?4?Water,CH₃OH and CH₃CH₂OH cannot lower the activation energy of the dehydration step to promote NAPH formation by NO₃ initiated reactions of the corresponding PAHs.Water,CH₃OH and CH₃CH₂OH can only affect NPAH formation from dehydration of para-PAH-NO₃-NO₂ intermediates.