Ozone, photolysis decomposition

The rate of ozone photolysis increased with increasing light intensity, ozone concentration and pH, and decreased with increasing inorganic carbon concentration. As the formation of HO is tied to ozone decomposition, this model can be extended to predict the oxidation rates of water contaminants by HO generated in the process. 

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. 

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

The quantum yield of ozone decomposition at 334 nm (L2) is 4, indicating that one of the products must be an excited species capable of decomposing 0 further. The primary process of the 0 photolysis at 334 nm occurs according to the reactions ... 

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. 

Takenaka and Lemal subsequently undertook an extensive study of the perfluorobenzene oxide (3)/perfluorooxepin (4) system.13,14 After considerable effort, benzene oxide 3 was successfully prepared by ozonization of tricyclic precursor 5 and photolytic decomposition of the resultant ozonides. Oxepin 4 was not observed directly, but was estimated by NMR computer simulations to be present in small proportion (3 %) at 55"C. The principal reaction of 3 is its rearrangement to cyclohexadienone 2, which occurs at room temperature in polar solvents, by heating in nonpolar solvents, or in the presence of Lewis acids. Photolysis of 3 with benzophenone as triplet sensitizer also produces 2. Furthermore, attempts to trap the oxepin, for example by treatment with bromine in the dark, result in cyclohexadienone 2. 

There would appear to be no difference in the reactions expected photo-chemically as the primary products are H atoms and OH radicals in both the electric discharge and on irradiation. Chen and Taylor (22) state that there is no evidence for oxygen atoms either in the photolysis or in the decomposition of water vapor in an electric discharge. However, the secondary formation of O atoms (2) and the formation of ozone (31, 50) in an electric discharge through water vapor have been demonstrated. It might be expected that under the proper experimental conditions similar results could be obtained photochemically. 

Flash photolysis studies of mixtures of ozone and hydrogen (6,56) have shown that the reaction of 0(1D) atoms with hydrogen yields vibra-tionally excited OH radicals. Studies of ozone and hydrogen mixtures in the visible (35) and in the ultraviolet (94) have shown that water is formed and that the rate of ozone decomposition is increased in the presence of hydrogen. 

Ozonation processes are rather complex, due to the high instability of ozone in aqueous solutions. Ozone absorbs UV photons with the maximal absorption at 253.7 nm. The decomposition of ozone under UV radiation typically occurs through three reactions direct photolysis, direct ozonation, and reactions between hydroxyl radicals and hydrogen peroxide as shown in the following reactions ... 

Ozone decomposition process by photolysis at 253.7 nm. (From Peyton, R. and Glaze, W., Environ. Sci. Technol., 22, 761-767, 1988. With permission.)... 

The effect of UV light intensity on the decomposition of organics by the UV/ ozone process was studied by Ku and Shen (1999). The removal efficiency of three chloroethenes increased with increasing light intensity and could be as high as 95%. The treatment efficiency of chloroethenes by the UV/ozone process was found to be much higher than for direct photolysis under various UV light intensities. 

When pH increased from 2 to 7, the removal rate of fluorene increased however, a subsequent increase in pH from 7 to 12 reduced the removal efficiency back to about the rate at the pH of 2. The increase of pH leads to an increase in the hydroxyl-ion-catalyzed decomposition of ozone into hydroxyl radicals however, the amount of ozone available to undergo direct photolysis and produce hydrogen peroxide will decrease with increasing pH. Eventually more hydroxyl radicals will be produced, which is particularly important at pH 12 (Beltran et al., 1995). The rate of oxidation of fluorene is given by ... 

Hanst and Calvert64 photooxidized azomethane at room temperature proceeding to about 50% decomposition. They put forward the idea that ozone was an intermediate in the oxidation and showed that it was removed rapidly only when photolysis was taking place. However, this merely proves that ozone will react with one of the radicals produced in the system. Similarly, the test which they used for ozone (tetramethyl-ethylene) was not shown to be specific to ozone and, indeed, is not likely to be. They were unable to detect methyl hydroperoxide in these experiments under conditions in which it was shown to be peculiarly stable at 200°C. for several hours. Formic acid was shown to be the major product of the prolonged further oxidation of formaldehyde in the presence of 1 atm. of oxygen (initially). 

Laboratory experience had convinced chemists earlier that the Chapman mechanism needed a supplement of additional reactions. In 1960, McGrath and Norrish discovered the formation of OH radicals in the reaction of water vapor with 0( D) atoms generated by the photolysis of ozone, and they proposed a chain decomposition of ozone by water radicals. Meinel (1950) had previously demonstrated the existence of OH in the upper... 

The results of LACTOZ have provided an extended kinetic data base for the following classes of reactions reactions of OH with VOCs, reactions of NO3 with VOCs and peroxy radicals, reactions of O3 with alkenes, reactions of peroxy radicals (self reactions, reaction with HO2, other RO2, NO, NO2), reactions of alkoxy radicals (reactions with O2, decomposition, isomerisation), thermal decomposition of peroxynitrates. Photolysis parameters (absorption cross-section, quantum yields) have been refined or obtained for the first time for species which photolyse in the troposphere. Significantly new mechanistic information has also been obtained for the oxidation of aromatic compounds and biogenic compounds (especially isoprene). These different data allow the rates of the processes involved to be modelled, especially the ozone production from the oxidation of hydrocarbons. The data from LACTOZ are summarised in the tables given in this report and have been used in evaluations of chemical data for atmospheric chemistry conducted by international evaluation groups of NASA and lUPAC. 

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