Ozone photolysis mechanism

In the present-day atmosphere ozone forms into layers and this was first explained by Chapman who proposed a photolysis mechanism for ozone formation. Chapman s mechanism is a simple steady-state production of ozone and led to the concept of odd oxygen. The odd-oxygen reaction scheme is shown in Table 7.4. 

Consider the water photolysis mechanism for the formation of oxygen atoms and ozone, shown below ... 

A knowledge of the kinetics of the decomposition of ozone is essential for the understanding of the chemistry of some important processes which occur in earth s atmosphere. Yet, in spite of numerous studies and the structural simplicity of ozone, the mechanism of its ultraviolet photolysis is still uncertain. Electronically and vibrationally excited species are involved in ozone decomposition and the current knowledge of the chemical behavior of such intermediates is still in its infancy. 

Precise determinations of quantum yields in dry ozone photolysis would be of considerable value in clarifying the possible mechanisms. If an energy or photon chain exists, there is no reason why the quantum yield should be limited to about 6. It should be possible to use experimental conditions where it would be much higher. If an energy or photon chain does not exist, the quantum yield should not exceed 4 or, perhaps, 6. It should be emphasized that the most precise work in the ultraviolet yields a maximum quantum yield of 6 and that an error of 2 in this value would be possible. 

Table 6.6 gives the quantum yields for the ozone photolysis as a function of wavelength [270].) This produces a density of approximately 1010 (a 1A ) molecules cm-3 in the normal atmosphere at altitudes in the vicinity of 50 km [155, 156], Another mechanism that may contribute, particularly in the disturbed atmosphere [157], is... 

Figure 5.19. Cyclic mechanism for ozone photolysis ("advanced oxidation") in the presence of a generalized oxidizable substrate HRH. Du = "utilized ozone dose. From Peyton et al. (1988). Reprinted by permission of Lewis Publishers.

In 1983, Peyton and Glaze (J ) presented experimental evidence which showed that ozone photolysis at 25 i nm in aqueous solution resulted directly in the production of hydrogen peroxide. Further studies (jt j 1 jt), and reexamination of older data ( ) have confirmed these conclusions. The resulting mechanism for photolytlc ozonation is discussed below. 

From this mechanism, the overall quantum yield of O3 decomposition is tf> = A. Ozone photolysis occurs in two stages a very fast process (50 ps), followed by a slow process lasting many milliseconds. 

Oxidant Formation. The role of HO. in controlling the time-scale and severity of tropospheric oxidant pollution may be seen from the parameterization of O Brien and co-workers (75,76). The simplest possible mechanism for oxidant (Le. ozone, PAN, H2O2, etc.) formation consists simply of the reaction of an individual NNlHCj with HO. to convert the NMHCj to a generic product(s) PRODj, followed by removal of the product by HO. (PROD photolysis may be important, but is ignored here)... 

The rate of photolysis, J, depends on the absorption cross-section, a, the number density, the scale height and the angle, all of which are unique properties of a planetary atmosphere. For the Earth and the Chapman mechanism for ozone the O3 concentration maximum is 5 x 1012 molecules cm-3 and this occurs at 25 km, shown in Figure 7.12, and forms the Chapman layer structure. 

To investigate the role of water photolysis in ozone production in a prebiotic atmosphere, the following mechanism can be explored ... 

Photolysis Abiotic oxidation occurring in surface water is often light mediated. Both direct oxidative photolysis and indirect light-induced oxidation via a photolytic mechanism may introduce reactive species able to enhance the redox process in the system. These species include singlet molecular O, hydroxyl-free radicals, super oxide radical anions, and hydrogen peroxide. In addition to the photolytic pathway, induced oxidation may include direct oxidation by ozone (Spencer et al. 1980) autooxidation enhanced by metals (Stone and Morgan 1987) and peroxides (Mill et al. 1980). 

The production of 02CA9) on photolysis at A = 2537 A of oxygen-ozone mixtures, observed by Izod and Wayne,70 is probably explained by the energy-transfer mechanism... 

Vallance-Jones and Gattinger72,95 were able to reconcile intensities and decay rates for the 1.27 and 1.58 [x infrared atmospheric bands on the basis of a mechanism in which 02(1A9) was produced principally in the primary photolysis of ozone. The best fit to the data was obtained for an excitation mechanism... 

The Chapman mechanism. The mechanism of ozone formation and destruction in the stratosphere was first formulated by Chapman (205) in 1930. He did not consider the effects of minor constituents and physical transport processes that have since been recognized as important factors to explain the discrepancy between the calculated results and the actual observation. According to his mechanism, ozone is formed by the photolysis... 

Nitrogen dioxide is about 20 to 50% of the total nitrogen oxides NO, (NO, NOz, HN03, N2Os), while CIO represents about 10 to 15% of the total chlorine species CIO, (Cl, CIO, HCI) at 25 to 30 km. Hence, the rate of ozone removal by CIO, is about equal to that by NO, if the amounts of NO, are equal to those of CIO,. According to a calculation by Turco and Whitten (981), the reduction of ozone in the stratosphere in the year 2022 with a continuous use of chlorofluoromethanes at present levels would be 7%. Rowland and Molina (843) conclude that the ozone depletion level at present is about 1%, but it would increase up to 15 to 20% ifthechlorofluoromethane injection were to continue indefinitely at the present rates. Even if release of chlorofluorocarbons were stopped after a large reduction of ozone were found, it would take 100 or more years for full recovery, since diffusion of chlorofluorocarbons to the stratosphere from the troposphere is a slow process. The only loss mechanism of chlorofluorocarbons is the photolysis in the stratosphere, production of HCI, diffusion back to the troposphere, and rainout. 

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