Photochemistry of the Ozone Hole: Ultraviolet Photodissociation Cross Sections and Product Quantum Yields of ClOOCl
Date Issued
2010
Date
2010
Author(s)
Huang, Wen-Tsung
Abstract
The photolysis rate of chlorine peroxide (ClOOCl) is considered the key process in the ozone hole formation. Photolysis rate is in proportion to photodissociation cross section, which is equal to the product of absorption cross section and photodissociation quantum yield. In the past 2 decades, scientists determined the absorption cross section of ClOOCl by radiation intensity attenuation after passing through the sample cell. Since ClOOCl is extremely unstable and the laboratory synthesized sample has massive impurities, the measurements of ClOOCl absorption spectrum have large errors. Despite possible error in estimating the ClOOCl photolysis rate, accumulation of past knowledge let chemists believe that we have already understood the mechanism of the ozone hole formation. However, NASA-JPL published in 2007 their new ClOOCl absorption cross section measurements [J. Phys. Chem. A 111, 4322 (2007)], which is much smaller than that the previously accepted value. If we apply this value to the existing atmospheric chemistry model, half of the ozone depletion cannot be explained. It caused the chemists to start afresh to suspect and to carefully examine the existing photochemistry mechanism of the ozone hole. This experiment uses the brand-new method that we determined the photodissociation cross section of ClOOCl at 266 and 330 nm with molecular beam and mass-resolved detection by measuring the decrease of the ClOOCl intensity upon laser irradiation. This method may guarantee the ClOOCl signal not to be disturbed by impurity and get the most accurate value. Our value is far larger than the JPL published result. After substitution our value to the atmospheric chemistry model, it may properly explain the ozone hole formation and confirmed the existing model of the ozone hole is still valid. Since error of previously observed absorption cross section is huge, few people care about the precision level of the photodissociation product quantum yields. In 1999, CIT published a paper that after absorption ultraviolet light, photodissociation of ClOOCl produces: 2Cl + O2 and ClO + ClO two product channels [J. Phys. Chem. A 103, 1691 (1999)], the determined quantum yields are 0.9 and 0.1, respectively. Only the channel of Cl atom production will destroy the ozone. We utilize the molecular beam experiment to measure the product quantum yields at 248 nm excitation. Product velocity distributions and anisotropic parameters let us interpret the photodissociation dynamics of ClOOCl molecule. The results of this thesis may contribute to a more thorough understanding of the ozone hole formation and to the protection of the natural environment of the Earth.
Subjects
chlorine peroxide (ClOOCl)
ozone hole
photolysis rate
photodissociation cross section
photodissociaiton quantum yield
molecular beam
anisotropic parameter
photodissociation dynamics
SDGs
Type
thesis
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