The flash photolysis resonance fluorescence technique has been utilized to determine the Arrhenius parameters for several reactions between the hydroxyl radical and halogenated hydrocarbons. The reactions studied, and their corresponding Arrhenius expressions in units of cm3 molecule™1 s 1 are shown: (1) OH + CH2C1F -CHC1F + H20, ki = (2.84 ± 0.3) X 10™12 exp(-(1259 ± 50)/T) K; (2) OH + CHC12F -CC12F + H20, k2 = (1.87 ± 0.2) X 10 12 exp(-(1245 ± 26)/T) (245-375) K; (3) OH + CHC1F2 -» CC1F2 + H20, k3 = (9.25 ± 1.0) X 10™13 exp(-(1575 ± 71 )/T) (250-350) K; (4) OH + CH3CCI3 -CH2CC13 + H20, fc4 = (3.72 ± 0.4) X 10™12 exp(-(1627 ± 50)/T) (260-375) K; (5) OH + CH3CF2C1 -> CH2CF2C1 + H20, h = (1.15 ± 0.15) X 10™12 exp(-(1748 ± 30)/T) (273-375) K; (6) OH + CF2C1CFC12 -products, k6 < 3 X 10 16. Tropospheric lifetimes have been calculated for the above species by combining the rate constant data with global seasonally and diurnally averaged hydroxyl radical concentrations.
The technique of flash photolysis–resonance fluorescence has been utilized to study the temperature dependences of two chlorine atom reactions of considerable fundamental importance to stratospheric chemistry. These reactions have been studied using a wide range of experimental conditions to ensure the absence of complicating secondary processes. The reactions of interest with their corresponding rate constants are expressed in units of cm3 molecule−1 s−1: Cl+O3→ClO O2 (k1), ΔU°298=−164 kJ mol−1, k1 = (3.08±0.30) ×10−11 exp[−(576±60/RT)], (220–350) K; Cl+CH4→CH3+HCl (k2), ΔU°298=−164 kJ mol−1, k2 = (7.44±0.75) ×10−12 exp[−(2437±110/RT)], (218–401) K. In addition, the followong reaction was studied at 300 K: Cl+H2O2→HCl+HO2 (K3), ΔU°298=−56.8 kJ mol−1, k3?5.8×10−13 (±factor 2.0), 300 K. A direct implication of the new rate data is the need to revise downward by a factor of 2.4 to 3 the magnitude of the ozone perturbation predicted by earlier model calculations due to the presence of ClOx species in the stratosphere.
Reported in this study are temperature dependent rate data for the reaction of OH with the partially halogenated methane species CH3Cl, CH2Cl2, CHCl3, and CH3Br. The nominal temperature range covered was 245–375 K. The appropriate Arrhenius expressions are kA= (1.84±0.18) ×10−12 exp[−(2181±70/RT)], kB= (4.27±0.63) ×10−12 exp[−(2174±161/RT)], kC= (4.69±0.71) ×10−12 exp[(2254±214/RT)], kD= (7.93±0.79) ×10−13 exp[−(1766±116/RT)]. Units are cm3 molecule−1⋅s−1. No simple correlations between Eact and C–H bond strengths were found. The impact of these halogenated species on stratospheric ozone is also discussed.
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