OH reaction kinetics and atmospheric lifetime estimates for several hydrofluorocarbons

Abstract

The room-temperature rate constants for the reaction of the OH radical with 10 hydrofluorocarbons have been measured using the discharge flow technique with laser-induced fluorescence detection of the OH radicals. The rate constants at T = 298 K are CF3CH2CH3 (kII = (42 ± 4.3) × 10-15 cm3 molecule-1 s-1), CF3CH2-CHF2 (kII = (6.6 ± 0.7) × 10-15 cm3 molecule-1 s-1), CF3CHFCH2F (kII = (14.8 ± 1.7) × 10-15 cm3 molecule-1 s-1), CHF2CHFCHF2 (kII = (16.0 ± 2.3) × 10-15 cm3 molecule-1 s-1), CF3CH2CF3 (kII = 0.57-0.22+0.11 × 10-15 cm3 molecule-1 s-1), CF3CHFCHF2 (kII = (5.3 ± 0.7) × 10-15 cm3 molecule-1 s-1), CF3CH2CH2CF3 (kII = (7.0 ± 0.7) × 10-15 cm3 molecule-1 s-1), CF3CF2CH2CH2F (kII = (42.2 ± 3.1) × 10-15 cm3 molecule-1 s-1), CF3CH2CF2CH2CF3 (kII = (2.62 ± 0.19) × 10-15 cm3 molecule-1 s-1), CF3CF2CH2CH2CF2CF 3 (kII = (8.3 ± 0.9) × 10-15 cm3 molecule-1 s-1). In addition, the temperature dependence of the rate coefficient was measured for two of the above hydrofluorocarbons. In both cases the temperature dependence is well described by a simple Arrhenius expression, k(T) = A exp[-E/(RT)]. We find that A = 1.74 × 10-12 cm3 molecule-1 s-1 and EIR = 1108 K for CF3CF2CH2CH2F and that A = 1.23 × 10-12 cm3 molecule-1 s-1 and EIR = 1833 K for CF3CH2CF2CH2CF3. The temperature dependent rate coefficients are used to derive atmospheric lifetimes for CF3CF2CH2CH2F and CF3CH2CF2CH2CF3 of 1.2 and 23 years, respectively. We estimate the atmospheric lifetimes of the remaining HFCs based on their estimated OH reaction rates at T = 277 K. The rates at T = 277 K are derived from the room temperature reaction rates and estimates of the activation energies for these reactions. The OH reaction rates of this group of closely related compounds provide a data base for the testing of several OH reaction rate estimation schemes. © 1995 American Chemical Society.