Kinetics of the OH + ClOOCl and OH + Cl2O reactions: experiment and theory.

The rate coefficients for the reactions OH + ClOOCl --> HOCl + ClOO (eq 5) and OH + Cl2O --> HOCl + ClO (eq 6) were measured using a fast flow reactor coupled with molecular beam quadrupole mass spectrometry. OH was detected using resonance fluorescence at 309 nm. The measured Arrhenius expressions for these reactions are k5 = (6.0 +/- 3.5) x 10(-13) exp((670 +/- 230)/T) cm(3) molecule(-1) s(-1) and k6 = (5.1 +/- 1.5) x 10(-12) exp((100 +/- 92)/T) cm(3) molecule(-1) s(-1), respectively, where the uncertainties are reported at the 2sigma level. Investigation of the OH + ClOOCl potential energy surface using high level ab initio calculations indicates that the reaction occurs via a chlorine abstraction from ClOOCl by the OH radical. The lowest energy pathway is calculated to proceed through a weak ClOOCl-OH prereactive complex that is bound by 2.6 kcal mol(-1) and leads to ClOO and HOCl products. The transition state to product formation is calculated to be 0.59 kcal mol(-1) above the reactant energy level. Inclusion of the OH + ClOOCl rate data into an atmospheric model indicates that this reaction contributes more than 15% to ClOOCl loss during twilight conditions in the Arctic stratosphere. Reducing the rate of ClOOCl photolysis, as indicated by a recent re-examination of the ClOOCl UV absorption spectrum, increases the contribution of the OH + ClOOCl reaction to polar stratospheric loss of ClOOCl.

Ultraviolet absorption spectrum of chlorine peroxide, ClOOCl.

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.

Kinetics and product studies of the reaction of Br, Cl, and NO with ClOOCl using discharge-flow mass spectrometry.

The kinetics of the reactions Cl + ClOOCl --> products, Br + ClOOCl --> products (7), and NO + ClOOCl --> products have been studied as a function of temperature at 1 Torr total pressure using a discharge-flow mass spectrometric technique. The measured rate coefficients, expressed in Arrhenius form, are k5 = (7.60 +/- 0.56) x 10(-11)exp((65.4 +/- 17.9)/T) cm3 s(-1) for 217 K < T < 298 K, and k7 = (5.88 +/- 0.50) x 10(-12) exp(-173 +/- 20/T) cm3 s(-1) for 223 K < T< 298 K. The observed temperature dependencies of these rate coefficients indicate a common direct halogen atom abstraction mechanism. Mass spectral product studies indicate that BrCl is the only major Br-containing product from reaction. Extensive product studies were used to estimate that the peroxide form of the ClO-dimer is formed in > 90% yield from the ClO self-reaction. No evidence for the formation of ClOClO and ClClO2 was observed. No reaction between NO and ClOOCl was observed and an upper limit of k8 < 1 x 10(-15) cm3 s(-1) for 220 K < T < 298 K was determined.

Kinetics of the reactions of the CHBr2 and CHBr2O2 radicals with O2 and NO.

When bromoform (CHBr3) is photolyzed at 266 or 303 nm in the presence of O2 and NO, the formation of secondary Br atoms is observed. By following the rate of growth of this secondary Br atom signal as a function of conditions, rate constants have been determined for the reactions CHBr2 + O2, CHBr2 + NO (both pressure-dependent), and CHBr2O2 + NO (k(2a) = (1.74 +/- 0.16) x 10(-11) cm3 molecule(-1) s(-1) at 23 degrees C). By measuring the amplitude of the secondary Br signal compared to the primary Br formed in the initial photolysis, it is established that the CHBr2O radical spontaneously decomposes to form CHBrO + Br at least 90%, and probably 100%, of the time, in agreement with previous work and with recent ab initio calculations. A survey of four other polybrominated methanes, CH2Br2, CHClBr2, CF2Br2, and CBr4, shows that they all generate secondary Br atoms when photolyzed at 266 nm in the presence of O2 and NO, suggesting that their reaction sequences are similar to that of bromoform.