Atmospheric Oxidation Mechanism And Kinetics Of Naphthalene Initiated By Chlorine Radicals (·Cl)

The high oxidation ability and new findings of chlorine radicals（·Cl）in mid-continental areas increase the importance of·Cl in transforming organic pollutants.Moreover,the reaction mechanism of atmospheric organic pollutants initiated by·Cl is not always the same as corresponding one initiated by hydroxyl radicals（?OH）,the most important oxidant in the atmosphere,implying that the atmospheric fates and environmental risks of organic pollutants initiated by·Cl may differ from those by?OH.Therefore,more research should be performed on the atmospheric transformation of organic pollutants initiated by·Cl.Naphthalene,an important organic pollutant in the atmosphere,is the highest concentration of polycyclic aromatic hydrocarbons in the urban atmosphere.In previous studies,atmospheric oxidation pathways of naphthalene initiated by?OH have been studied extensively.However,·Cl-initiated atmospheric oxidation pathways and kinetics is not clear.Herein,a quantum chemical method[ωB97XD/6-311++G（3df,2pd）//ωB97XD/6-31+G（d,p）]and kinetics modeling was employed to investigate·Cl-initiated atmospheric oxidation of naphthalene.The specific investigated contents and results as follows:（1）Initial reactions with·Cl and subsequent reactions of the important radicals of naphthalene formed in the initial steps were investigated.Results show that the initial step is dominanted by·Cl addition to the C₅-position of naphthalene,forming radicals?C₁₀H₈Cl（R₁）,followed by O₂ addition to the C₁ and C₆ position of R₁ to form peroxy radicals（RO₂?,R₁-1OO-s/a and R₁-6OO-s/a,s/a=syn/anti,where syn and anti correspond to the O₂ additions from the same and oppsite site of the direction of·Cl addition）depending on the attacking direction of O₂.In the atmosphere,the formed four RO₂·will mainly react with HO₂?and NO to form hydroperoxide,alkoxy radicals（RO?,R₁-2O-s/a and R₁-6O-s/a）and organonitrate.The RO?can undergo ring-closure to form bi-cyclic radicals（R₁-21O-s/a and R₁-61O-s/a）in the subsequent reactions.（2）Subsequent reactions of bi-cyclic radicals（R₁-21O-s/a and R₁-61O-s/a）and the aquatic toxicity of the important closed-shell products formed from·Cl-initiated reactions of naphthalene are clarified.It was found that O₂ can further addition to the C₆ position of R₁-21O-s/a radicals and C₂ position of the R₁-61O-s/a radicals,to generate new RO₂?that mainly transform into several products including new RO?hydroperoxide(C₁₀H₉O₃Cl)and organic nitrates(C₁₀H₈NO₄Cl)via in their reactions with NO and HO₂?.The formed new RO?will undergo cyclization,hydrogen transfer and dissociation reactions leading to new C-centered radicals.Based on the aquatic toxicity data of hydroperoxide and organic naitrate formed from·Cl-initiated reactions of naphthalene by ECOSAR program,it was found that the acute and chronic toxicity of hydroperoxy and organic nitrates to fish,green algae,and daphnia are stronger than parent naphthalene,and its toxicity increases with the oxidation process.More importantly,the acute and chronic toxicity of the closed-shell products of naphthalene initiated by·Cl are more toxic than the corresponding products initiated by?OH.In summary,based on the computational simulation methods,this study fully reveals the microscopic mechanism,kinetics and aquatic toxicity of important closed-shell products of·Cl-initiated reactions of naphthalene.The results are important for evaluating the fate and environmental risks of naphthalene,and enriching the knowledge of·Cl chemistry.