Kinetics of the ClO Self-Reaction and 210 nm Absorption Cross Section of the ClO Dimer
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Abstract:
The kinetics of the dimerization of ClO radicals, ClO + ClO + M → Cl2O2 + M (1a), and the 210 nm absorption cross sections of the ClO dimer have been studied using the technique of flash photolysis with UV absorption spectroscopy, over the temperature range 183−245 K and pressure range 25−700 Torr. ClO radicals were generated following the photolysis of Cl2/Cl2O/N2 mixtures and were quantified via their differential absorption between the peak of the (12,0) band of the (Ã ← X̃) transition at 275.2 nm and the adjacent minimum to higher wavelengths, while Cl2O2 formation was simultaneously monitored at 210 nm. ClO differential absorption cross sections were measured under identical conditions to the kinetic experiments by four separate calibration schemes. The rate coefficient measured at the lower temperatures studied (T < 200 K) was found to be up to 40% faster than extrapolation of previous results would suggest, with the limiting low- and high-pressure rate coefficients for reaction 1 in nitrogen determined to be k0 = (1.59 ± 0.60) × 10-32 × (T/300)-4.50±0.98 molecules-2 cm6 s-1 and k∞ = (1.36 ± 0.22) × 10-12 × (T/300)-3.09±0.40 molecules-1 cm3 s-1 respectively, obtained with Fc = 0.6. The corresponding value for k0 in air is k0(atm) = (1.49 ± 0.56) × 10-32 × (T/300)-4.50±0.98 molecules-2 cm6 s-1. The 210 nm absorption cross section of Cl2O2 was measured following time-resolved monitoring of its formation via ClO radical association, and a mean value of (2.94 ± 0.86) × 10-18 molecule-1 cm2 obtained, temperature independent (to within ± 15%) between 183 and 245 K. Errors are combined systematic uncertainties and 2 standard deviations statistical variation.Keywords:
Flash photolysis
Absorption cross section
Rate constants are reported for the reactions OH+OH → H2O+O (where k1 is the rate constant) and OH+OH+N2 → H2O2+N2 (where k5 is the rate constant) measured in a room temperature flash photolysis experiment. Hydroxyl radicals were produced by the photodecomposition of water vapor, and time resolved absorption spectroscopy was utilized to determine the absolute OH concentration. Specific rate constants obtained are k1=2.1±0.2×10−12 cm3/sec and k5=2.5±0.3×10−31 cm6/sec.
Flash photolysis
Hydroxyl radical
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Rate constants for the reactions of hydroxyl radicals with CH3CF3 (HFC-143a) and CHF2CH2CF3 (HFC-245fa) have been measured using the flash photolysis resonance fluorescence technique over the temperature range 273−370 K. A data analysis procedure is presented which should minimize rate constant errors introduced by the possible effects of radical diffusion. The following Arrhenius expressions have been derived for the reactions of OH with CH3CF3 and CHF2CH2CF3, respectively (in units of cm3 molecule-1 s-1): × 10-12 exp{−(1979 ± 65)/T} and × 10-12 exp{−(1331 ± 43)/T}. With these values, the atmospheric lifetimes for the two HFCs have been estimated to be 51 and 7.4 years, respectively. An error analysis is presented from which the rate constant uncertainty at any temperature can be calculated.
Flash photolysis
Atmospheric temperature range
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Flash photolysis
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Citations (47)