Study Of The Photochemical Reaction Mechanism Of N（Ⅲ）（H₂ONO⁺、HONO And NO₂^-） And Typical Volatile Chlorinated Aromatic Hydrocarbons In The Atmospheric Aqueous Environment

Volatile chlorinated aromatic hydrocarbons（VOCs）are one of the chlorine-containing volatile organic compounds（Cl-VOCs）,which can cause distortion,cancer,mutation for human beings and cause a series of environmental problems.Nitrous acid（HONO）is an important pollutant in the troposphere and is widely distributed in the atmosphere.It can absorb ultraviolet light and dissociate to generate HO^·radicals,which plays an important role in the conversion and removal of most water-soluble pollutants in the atmosphere.Therefore,o-dichlorobenzene（o-DCB）,2,4,6-trichlorophenol（2,4,6-TCP）and 4-chlorinated biphenyls（4-PCB）were selected as target pollutants,and the effects of pollutant concentration,pH value,HONO concentration and chloride ion concentration on the photoinduced reaction are investigated with HONO as the source of HO^·radicals.The properties of transient intermediate products are analyzed by means of steady state experiment and laser flash photolysis,and the photochemical reaction mechanism is proposed.The main conclusions are as follows:（1）UV lamp irradiation experiments show that the degradation rate of o-DCB was greatly affected by the initial concentration of o-DCB,pH value,and N（Ⅲ）concentration,while it is less affected by chloride ions.Laser flash photolysis experiments show that o-DCB and HO^·radicals react to form o-DCB-OH adducts,and the secondary reaction rate is（2.69±0.3）×10⁹ L mol^-1 s^-1.Subsequent adducts decay in many ways,among which the reaction with N（Ⅲ）and oxygen is considered to be the main decay route of o-DCB-OH adducts.GC-MS analysis results show that the steady-state products mainly include o-chlorophenol,2,3-dichlorophenol,2,3-dichloro-1,4-benzoquinone,2-chloro-3-nitrophenol,2,3-dichloronitrobenzene and 2,3-dichloro-4-nitrophenol.（2）The contribution of different forms of N（Ⅲ）to the degradation of 2,4,6-TCP when the pH of the atmospheric aqueous phase changes.It was found that the reaction with HONO is dominant in the pH range of 1.5 to 3.8.When pH<1.5 or pH>3.8,H₂ONO⁺and NO₂^-is dominate respectively.The total secondary reaction rate of HO^·radical and2,4,6-TCP measured by transient competitive kinetics method is（1.1±0.2）×10¹⁰ L mol^-1s^-1,hydroxyl radical and 2,4,6-TCP,and the addition reaction rate of is（9.27±0.2）×10⁹L mol^-1 s^-1,so it is believed that about 82%of the HO^·radicals attack the benzene ring of 2,4,6-TCP.（3）Through transient experiments,it could be known that HO^·radicals produced by HONO photolysis would attack the benzene ring of 4-PCB to form 4-PCB-OH adducts,and the secondary reaction rate is（9±1）×10⁹ L mol^-1 s^-1.Subsequently,the 4-PCB-OH adduct will continue to react with HONO and O₂,and its second-order rate constants are（5.3±0.1）×10⁶ L mol^-1 s^-1 and（4.9±0.2）×10⁶ L mol^-1 s^-1,respectively.Due to the electron withdrawing effect of the chlorine atom,the electron cloud density of the substituted benzene ring will be reduced,so most HO^·will be added to the benzene ring not substituted by the chlorine atom with a higher probability to form 4-chlorobiphenyl-4-alcohol.