Rate constants were measured for gas-phase reactions of the hydroxyl radical (OH) with six polycyclic aromatic hydrocarbons (PAH) and four polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F). These OH reaction rate constants were determined in helium at about 1 atm over various temperature ranges between 306 and 405 K. The experiments were carried out in a small, heated quartz reaction chamber sampled by on-line mass spectrometry, and OH was produced by the photolysis of O 3 in the presence of H 2 O. Arrhenius regressions were performed with the rate constants of each compound, and the temperature dependencies were found to be slight to nonexistent. The OH reaction rate constants of PCDD/F were in agreement with those predicted by a structure-reactivity method. The resulting rate constants at 298 K (in units of 10 -12 cm 3 s -1 ) were as follows:
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-pdioxin at 373-432 K using a heated quartz reaction chamber. The photolysis of O 3 in the presence of H 2 O and the photolysis of H 2 O 2 (both at λ ) 254 nm) served as OH sources. An extrapolation using the Arrhenius equation gives a 1,2,3,4tetrachlorodibenzo-p-dioxin-OH reaction rate constant of 8.5 × 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 particlephase removal processes of higher chlorinated ones.
Rate constants for the gas-phase reactions of the hydroxyl radical (OH) with Rand γ-hexachlorocyclohexane (Rand γ-HCH) and hexachlorobenzene (HCB) were measured over the temperature range 346-386 K. The experiments were carried out in He diluent gas at approximately 1 atm, in a 160-mL quartz chamber. OH was produced by the photolysis of ozone in the presence of H 2 O, and reactants in the chamber were monitored by online mass spectrometry. The rate constants measured at elevated temperatures were extrapolated by the Arrhenius equation, and OH reaction rate constants at 298 K were estimated for each compound. These rate constants at 298 K (in units of 10 -13 cm 3 s -1 ) were R-HCH, 1.4; γ-HCH, 1.9; and HCB, 0.27. Atmospheric lifetimes based on OH reactions (τ OH ) were also estimated (in units of days): R-HCH, 120; γ-HCH, 96; and HCB, 940. These relatively high atmospheric lifetimes indicate that these compounds can be transported great distances through the atmosphere before removal processes associated with the global distillation effect.
Our laboratory recently measured the gas-phase reaction rate constants of polychlorinated biphenyls (PCBs) with the hydroxyl radical (OH) and concluded that OH reactions are the primary removal pathway of PCBs from the atmosphere. With the reaction system previously employed for kinetics, we have now investigated the products of these PCB-OH reactions. Experiments were carried out in either air or He as the diluent gas at approximately 1 atm in a 160-mL quartz chamber. Temperatures ranged between 318 and 363 K in order to enhance the vapor pressures of these less volatile compounds. OH was produced in situ by the photolysis of ozone in the presence of H 2 O. Reaction products from biphenyl, the three monochlorobiphenyls, and five dichlorobiphenyls were extracted from the chamber, derivatized by diazomethane, and analyzed by gas chromatographic mass spectrometry (GC/MS). Experiments gave benzoic acid and the appropriate chlorinated benzoic acids in significant product yields (8-17%).
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