Hydroxyl radicals, ozone destruction

An example of great environmental interest is the catalytic mechanism for ozone destruction by the hydroxyl radical, which is believed to be... 

Li, and Cal profiles ai altitudes of Xrt to I00 km. The method also has been useful lor studying ihe hydroxyl free radical (OH), This radical is of principal inlerest because or ihe cataly tic role which it exerts in atmospheric chemistry. The OH radical, along with chlorine and nitrogen oxides, is involved in the ozone destruction cycle. 

The oxidation scheme for halomethanes not containing a hydrogen atom is similar to that for those which do, except that it is not initiated by tropospheric reaction with hydroxyl radicals, since the fully halogenated methanes are unreactive. Consequently, substantial amounts of CFCs and halons are transported intact up into the stratosphere, where they absorb UV radiation of short wavelength and undergo photodissociation (equation 36) to a halogen atom and a trihalomethyl radical. The halogen atom Y may enter into catalytic cycles for ozone destruction, as discussed in the introduction. 

Hydroxyl radical, OH, is the principal atmospheric oxidant for a vast array of organic and inorganic compounds in the atmosphere. In addition to being the primary oxidant of non-methane hydrocarbons (representative examples of these secondary reactions are given in Table 6), OH radical controls the rate of CO and CH4 oxidation. Furthermore, the OH reaction with ozone also limits the destruction of O3 in the troposphere, it also determines the lifetime of CH3CI, CHsBr, and a wide range of HCFC s, and it controls the rate of NO to HNO3 conversion. Concentration profiles for hydroxyl radical in the atmosphere are shown in Fig. 2. 

The combination of photo-Fenton and ozonation results in an important enhancement of the destruction efficiency of organic compounds like phenol [96], 2,4-D [97], aniline or 2,4-chlorophenol ([33] and references therein). As mentioned in Sect. 2.5.1, metal ions catalyze ozone decomposition. In the dark, Fe(II) catalyzes O3 degradation giving the ferryl intermediate (Fe02+, see Sect. 2.6.9), which can directly oxidize the organic pollutant or evolve to a hydroxyl radical ... 

In low-NO c conditions, HO2 can react with ozone leading to further destruction of ozone in a chain sequence involving production of hydroxyl radicals. 

Potential pathways for the destruction of HMSA in cloudwater or fogwater include reactions with 03(aq), H202(aq), and the hydroxyl radical OH(aq). Hoigne et al. (1985) observed no direct reaction between ozone and HMSA. HMSA is also resistant to oxidation by H202 (Kok et al. 1986). The reaction between HMSA and OH results in the production of the SO, radical... 

Ozone is formed by the photolytic decomposition of NO2 yielding oxygen radicals and by the reaction sequence NO2 — HNO3 — NO3 O3. In particular, the reaction of NO2 with hydroxyl radicals to form HNO3 increases ozone concentration because two radicals, NO2- and -OH, which catalyze ozone decomposition, are removed. Other radicals are also important for ozone destruction in the stratosphere, especially chlorine oxides see Chlorine, Bromine, Iodine, Astatine Inorganic Chemistif). The mechanism of ozone destruction is complicated as there many compounds involved. Chlorine nitrate and dinitrogen pentoxide can act as reservoir species for CIO-, NO2-, and NO3- radicals. 

Photolytic Ozonation has been known for over a decade as a powerful water treatment process for the destruction of organic compounds, but its mode of action has not been understood. It is shown using kinetic arguments and data from scavenging experiments that photolysis of aqueous ozone yields hydrogen peroxide, which then reacts with further ozone in a complex series of reactions to yield hydroxyl radical. The proposed mechanism is shown to explain pH behavior of model compound destruction in earlier data. 

Hydroxyl radicals (OH) and nitric oxide (NO) radicals can promote the destruction of ozone. 

In this model, for every methane molecule which reacts, the sequence leads to 4 ozone and 2 hydroxyl radicals, extra. Formation of ozone in the lower troposphere is therefore catalysed by photochemical oxidation of organic molecules, but it does require comparatively high levels of NO (mixing ratio > 5 — 10 x 10 ) to be present. If it goes to completion, OH can react further with CO to make CO2 thus completing the oxidation of methane (Scheme 5.2). At low NO levels, the net reaction is the destruction of ozone via the reaction with CO [Scheme 5.2b]. 

The dominant reactions forming and removing nitrogen oxides and chlorine radicals in the stratosphere were given in Figs. 1 and 2. There it can be seen that the number of hydroxyl radicals present controls the balance between ozone destruction by nitrogen oxides and by chlorine radicals because the reaction... 

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