Kinetics of OH radical reactions with dibenzo-p-dioxin and selected chlorinated dibenzo-p-dioxins

The pulsed laser photolysis/pulsed laser-induced fluorescence (PLP/PLIF) technique has been applied to obtain rate coefficients for OH + dioxin (DD) (k1), OH + 2-chlorodibenzo-p-dioxin (2-CDD) (k2), OH + 2,3-dichlorodibenzo-p-dioxin (2,3-DCDD) (k3), OH + 2,7-dichlorodibenzo-p-dioxin (2,7-DCDD) (k4), OH + 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD) (k5), OH + 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TCDD) (k6), and OH + octachlorodibenzo-p-dioxin (OCDD) (k7) over an extended range of temperature. The atmospheric pressure (740 ± 10 Torr) rate measurements are characterized by the following Arrhenius parameters (in units of cm3 molecule−1 s−1, error limits are 1σ):k1(326-907K)=(1.70±0.22)×10-12exp(979±55)/T,k2(346-905K)=(2.79±0.27)×10-12exp(784±54)/T,k3(400-927K)=(1.83±0.19)×10-12exp(742±67)/T,k4(390-769K)=(1.10±0.10)×10-12exp(569±53)/T,k5(379-931K)=(1.02±0.10)×10-12exp(580±68)/T,k6(409-936K)=(1.66±0.38)×10-12exp(713±114)/T,k7(514-928K)=(3.18±0.54)×10-11exp(-667±115)/T.The overall uncertainty in the measurements, taking into account systematic errors dominated by uncertainty in the substrate reactor concentration, range from a factor of 2 for DD, 2-CDD, 2,3-DCDD, 2,7-DCDD, and 2,8-DCDD to ± a factor of 4 for 1,2,3,4-TCDD and OCDD. Negative activation energies characteristic of an OH addition mechanism were observed for k1-k6. k7 exhibited a positive activation energy. Cl substitution was found to reduce OH reactivity, as observed in prior studies at lower temperatures. At elevated temperatures (500 K < T < 500 K), there was no experimental evidence for a change in reaction mechanism from OH addition to H abstraction. Theoretical calculations suggest that H abstraction will dominate OH reactivity for most if not all dioxins (excluding OCDD) at combustion temperatures (>1000 K). For OCDD, the dominant reaction mechanism at all temperatures is OH addition followed by Cl elimination.