Polychlorinated dibenzo-p-dioxins and dibenzofurans: Gas phase hydroxyl radical reactions and related atmospheric removal

Gas-phase reactions with the hydroxyl radical (OH) are expected to be an important removal pathway of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) in the atmosphere. Our laboratory recently developed a system to measure the rate constants of the gas-phase reactions of OH with semivolatile organic compounds using on-line mass spectrometry. We have now incorporated electron capture mass spectrometry (EC-MS) into this system to increase its sensitivity to PCDD/F, which tend to have low vapor pressures. OH reaction rate constants were determined in helium for 1,2,3,4-tetrachlorodibenzo-p-dioxin at 373-432 K using a heated quartz reaction chamber. The photolysis of O-3 in the presence of H2O and the photolysis of H2O2 (both at lambda = 254 nm) served as OH sources. An extrapolation using the Arrhenius equation gives a 1,2,3,4-tetrachlorodibenzo-p-dioxin-OH reaction rate constant of 8.5 x 10(-13) cm(3) s(-1) at 298 K, which is in excellent agreement with the value predicted by a structure-activity method. The predicted OH reaction rate constants for tetra- through octachlorodibenzo-p-dioxin and dibenzofuran isomers were used in a simple model of the atmospheric removal of PCDD/ F. The results of our model indicate that atmospheric removal is a combination of gas-phase removal processes of lower chlorinated dioxins and furans and particle-phase removal processes of higher chlorinated ones.