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The oxidation of 2,2,4,4,5-pentabrominated diphenyl ether(BDE99),a highly detected pollutant in environment,has been investigated both in atmosphere and water solution.The gas phase results suggest that BDE99 are feasible to react with OH radicals,leading to the formation of hydroxylated tetrabrominated diphenyl ethers(OH-TetraBDEs)and hydroxylated pentabrominated diphenyl ethers(OH-BDE99s).BDE99 are hardly to react with HO 2 radical due to the high free energies of activation and extremely positive free energy changes.The reaction parameters in water are explored using free energy perturbation(FEP)theory with the combined quantum and molecular mechanical(QM/MM)method through Monte Carlo(MC)simulations.The QM/MM/MC/FEP results indicate that water may be the catalyst for the oxidations between BDE99 and OH/HO2 radicals.All the addition reactions are barrierless and highly feasible.The total rate constant for the reaction of BDE99 with OH radical is determined to be 1.85×10-13 cm 3 molecule-1 s-1 at 298.15 K.The atmospheric lifetime of BDE99 is 131 days.The total rate constant of the reaction of BDE99 and HO2 radical is calculated to be 4.01×10-27 cm3 molecule-1 s-1.