jp067660w_si_001.pdf (77.36 kB)
Ultraviolet Absorption Spectrum of Chlorine Peroxide, ClOOCl
journal contribution
posted on 2007-05-24, 00:00 authored by Francis D. Pope, Jaron C. Hansen, Kyle D. Bayes, Randall R. Friedl, Stanley P. SanderThe 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 °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 86o, 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.
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20 km altitudeproduct ClOOClzenith anglechlorine peroxideUltraviolet Absorption Spectrumband strengthsChlorine Peroxide300 nmconditions representativephotolysis ratespeak absorbancespath celltemperature profilesnovel fashionspectrum resultozone depletionClOOClThe photolysisO 3ab initio calculationsCl 2CF 2Climpurity levelslaser photolysisJPL
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