The photolysis of chlorine peroxide (ClOOCl) is understood to be a key step in the destruction of polar stratospheric ozone. This study generated and purified ClOOCl in a novel fashion, which resulted in spectra with low impurity levels and high peak absorbances. The ClOOCl was generated by laser photolysis of Cl2 in the presence of ozone, or by photolysis of ozone in the presence of CF2Cl2. The product ClOOCl was collected, along with small amounts of impurities, in a trap at about -125 degrees C. Gas-phase ultraviolet spectra were recorded using a long path cell and spectrograph/diode array detector as the trap was slowly warmed. The spectrum of ClOOCl could be fit with two Gaussian-like expressions, corresponding to two different electronic transitions, having similar energies but different widths. The energies and band strengths of these two transitions compare favorably with previous ab initio calculations. The cross sections of ClOOCl at wavelengths longer than 300 nm are significantly lower than all previous measurements or estimates. These low cross sections in the photolytically active region of the solar spectrum result in a rate of photolysis of ClOOCl in the stratosphere that is much lower than currently recommended. For conditions representative of the polar vortex (solar zenith angle of 86 degrees, 20 km altitude, and O3 and temperature profiles measured in March 2000) calculated photolysis rates are a factor of 6 lower than the current JPL/NASA recommendation. This large discrepancy calls into question the completeness of present atmospheric models of polar ozone depletion.

Relative rate experiments were used to measure ratios of rate constants as a function of temperature for the reactions of OH with propane, n-butane, n-pentane, n-hexane, cyclopropane, cyclobutane, cyclopentane, cyclohexane, and dimethyl ether. To assure internal consistency, ratios were measured for seventeen reactant pairs among these reactants. All of the derived rate constants are based on an absolute rate constant of the OH + C2H6 reaction using k(ethane) = 1.0 × 10-11 exp(−1094/T) cm3/molecule s. The rate constants obtained are as follows. propane:  1.29 × 10-11 exp(−730/T), k(298 K) = 1.11 × 10-12. n-butane:  1.68 × 10-11 exp(−584/T), k(298 K) = 2.37 × 10-12. n-pentane:  1.94 × 10-11 exp(−494/T), k(298 K) = 3.70 × 10-12. n-hexane:  2.60 × 10-11 exp(−480/T), k(298 K) = 5.19 ×10-12. cyclopropane:  5.15 × 10-12 exp(−1255/T), k(298 K) = 7.64 × 10-14. cyclobutane:  1.62 × 10-11 exp(−611/T), k(298 K) = 2.08 × 10-12. cyclopentane:  2.57 × 10-11 exp(−498/T), k(298 K) = 4.83 × 10-12. cyclohexane:  3.58 × 10-11 exp(−500/T), k(298 K) = 6.69 × 10-12. dimethyl ether:  1.51 × 10-11 exp(−496/T), k(298 K) = 2.86 × 10-12. These results are compared with previous literature data and are discussed in terms of trends in preexponential factors and activation energies. Also, rate constants and Arrhenius parameters are derived for methylene groups in the alkanes and cycloalkanes. In the low temperature regime, the present data illustrate a persistent discrepancy between absolute and relative rate measurements. The relative data show less curvature at low temperatures, and can be adequately fit with two-parameter Arrhenius expressions.

While generating the CH2OO molecule by reacting CH2I with O2, significant amounts of the OH radical were observed by laser-induced fluorescence. At least two different processes formed OH. A fast process was probably initiated by a reaction of vibrationally hot CH2I radicals. The second process appeared to be associated with the decay of the CH2OO molecule. The addition of molecules known to react with CH2OO increased the observed decay rates of the OH signal. Using the OH signals as a proxy for the CH2OO concentration, the rate constant for the reaction of hexafluoroacetone with CH2OO was determined to be (3.33 ± 0.27) × 10(-11) cm(3) molecule(-1) s(-1), in good agreement with the value measured by Taatjes et al.1 The rate constant for the reaction of SO2 with CH2OO, (3.53 ± 0.29) × 10(-11) cm(3) molecule(-1) s(-1), showed no pressure dependence over the range of 50-200 Torr and was in agreement with the value at 4 Torr reported by Welz et al.

Rate constants for the reaction of four different butyl radicals with molecular oxygen have been measured at room temperature. The radicals were generated by flash photolysis and their time decay was followed with a photoionization mass spectrometer. The radical concentrations were kept low to avoid complications from radical–radical reactions. Radical lifetimes were long, up to 50 msec, thus assuring that thermalized radicals were being studied. The rate constants, in units of 10−11 cm3 molecule−1 sec−1, are: n-butyl (0.75±0.14); s-butyl (1.66±0.22); t-butyl (2.34±0.39); 3-hydroxy s-butyl (2.8±1.8). No pressure dependence of the rate constants was observed over the range 1 to 4 Torr. In the absence of O2, the butyl radicals decay mainly by loss on the quartz surface of the reaction cell, with sticking coefficients in the range of 10−2 to 10−3. The Adiabatic Channel Model can predict the approximate absolute values of these rate constants using reasonable molecular parameters, but it fails to reproduce the observed trend of rate constants with radical ionization potential.

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Steady state concentrations of formyl radicals were measured with a photoionization mass spectrometer. The reaction of ethylene with oxygen atoms in a system free of OCHO+Oas was suggested by Groth and coworkers in 1938. The rate constant for this reaction was calculated to be (5.7±1.2)×10