Ozone oxidation path

Oxidation path Usually direct aqueous molecular ozone oxidation Primarily hydroxyl radical oxidation... 

In air, in the absence of additives, NO removal takes place by oxidation [56-58], Wu et al. [56] observed that the concentration of ozone generated in the corona discharge in the presence of NO is significantly smaller as compared to the ozone formed in air without NO under the same conditions. Thus, they concluded that NO oxidation by ozone and atomic oxygen is an important reaction path for the NO conversion. 

Hydrogen abstraction by RH02 could also participate in the process of initiating a chain of thermal oxidation reactions (pathy). In aqueous systems, cations will further react by solvolysis, and superoxide anion will readily disproportionate to yield H202 (path i). This is in contrast to the fate of superoxide anions in ozonation advanced oxidation processes (AOPs), where they react primarily with ozone to produce hydroxyl radical. This description of the chemical pathways of UV/H202 oxidation of organics illustrates that, when oxygen is present, the major paths directly or indirectly create more... 

A theoretical investigation showed that the most favourable unimolecular decomposition path of primary fluorozonide is a concerted cleavage to carbonyl oxide and formyl fluoride. The secondary fluorozonide decomposition takes place most readily in a stepwise manner initiated by the 0-0 bond rupture.153 DFT calculations have shown that ozone-difluoroethylene reactions are initiated by the formation of van der Waals complexes and then yield primary ozonides, which rapidly open to carbonyl oxide compounds. The formation of primary ozonide has been predicted to be the rate-controlling step of the oxidation process.154... 

Mechanisms of Aldehyde Oxidation. There must be at least two paths for oxidation of aldehyde to acid, and at least one of these must be temperature dependent. One pathway is the ozone-oxygen oxidation of aldehydes to peracids (14). However, peracid can also serve as an oxidizing agent for aldehyde. In the oxidation of acetaldehyde, Reaction 3 is thought to occur (14). 

On the basis of ratios of C and C present in carbon dioxide, Weinstock (250) estimated a carbon monoxide lifetime of 0.1 year. This was more than an order of magnitude less than previous estimates of Bates and Witherspoon (12) and Robinson and Robbins (214), which were based on calculations of the anthropogenic source of carbon monoxide. Weinstock (250) suggested that if a sufficient concentration of hydroxyl radical were available, the oxidation of carbon monoxide by hydroxyl radical, first proposed by Bates and Witherspoon (12) for the stratosphere, would provide the rapid loss mechanism for carbon monoxide that appeared necessary. By extension of previous stratospheric models of Hunt (104), Leovy (150), Nicolet (180), and others, Levy (152) demonstrated that a large source of hydroxyl radical, the oxidation of water by metastable atomic oxygen, which was itself produced by the photolysis of ozone, existed in the troposphere and that a chain reaction involving the hydroxyl and hydroperoxyl radicals would rapidly oxidize both carbon monoxide and methane. It was then pointed out that all the loss paths for the formaldehyde produced in the methane oxidation led to the production of carbon monoxide [McConnell, McElroy, and Wofsy (171) and Levy (153)1-Similar chain mechanisms were shown to provide tropospheric... 

As pointed out by Le Bras and Platt (1995), the reaction BrO -b CIO is 4 times faster than BrO -b BrO, making ozone destruction even more efficient if significant amounts of both halogen oxides are present. Other reaction paths for the BrO -b CIO reaction yield BrCl and OCIO (e.g., Sander et al., 2000), whereas one channel of the BrO -b lO reaction and one channel of the selfreaction of 10 yield OIO (Gilles et al., 1997 Bedjanian et al., 1998 Misra and Marshall, 1998 Bloss etal., 2001 Rowley etal., 2001). In the selfreaction of 10 also I2O2 can be formed. Formation of OBrO was found to be unimportant (e.g., Rattigan et al., 1995 Rowley et al., 2001). 

Recently, a number of municipalities have become so concerned with the problem that they have installed automatic ozone recorders at strategic locations. Some of these instruments are based on the chemical determination of ozone by oxidation of potassium iodide, and colorimetric or electrometric measurement of the extent of the reaction. Others are spectrophotometric instruments a few are based on rubber cracking. The value of the chemical determinations of ozone in the presence of the oxidizing or reducing substances present in polluted atmospheres is questionable. Spectrophotometric methods require a light path of a few hundred feet and cannot be moved easily from one location to another. Determination by rubber cracking is... 

One molecule of ozone liberates one molecule of iodine and one molecule of oxygen when the pH of the medium is 9.0. The value obtained with ozone and iodide at pH 9.0 is a compromise between the rate of decomposition of the ozone and the oxidation of iodide by different reaction paths leading to stoichiometry different from that assumed. At pH 2, more iodine is liberated which may be ascribed to either oxidation by the by-product oxygen liberated in the reaction even though normal molecular oxygen generally does not cause this reaction, or to a second reaction path which is acid-catalyzed. 

Since the steady-state O3 concentration achieved in the fast cycle is proportional to the ratio of [N02 to [NO], the effect of the slow CO cycle is to slowly convert NO to NO2 and therefore to increase the steady-state O3 concentration. Thus, because of the rapidity of the NO2/O3 cycle, an independent path that changes the ratio of [NO2] to [NO] indirectly controls the ozone concentration. It is common to refer to such oxidation chains that are driven by sunlight as photooxidations. 

The photochemistry of the ethane and higher hydrocarbon oxidation in the atmosphere follows similar reaction paths as for methane, although reactions occur faster because of the higher reactivity of these molecules [33, 36]. In the case of ethane, there can be a net production of five ozone molecules per ethane molecule consumed, if sufficient NO is present in the atmosphere. The cycle of reactions, cycle C4, that produces ozone from ethane is shown in Box 5.3. The compound peroxyacetyl nitrate, CH3(C=0)02N02, which appears in C4 is a strong phytotoxicant and air pollutant, better known by the acronym PAN [37]. The compound, CH2O, is formaldehyde and CH3CHO is acetaldehyde. 

The other radical mechanisms are much less well defined. The problem they present is typified by the common observation that almost any mixture of fluorescent or potentially fluorescent compounds and powerful oxidants such as peroxides or ozone will produce measurable and indeed sometimes easily visible light emission. Some of these mechanisms will, by presently obscure and circuitous routes, produce intermediates which can react by the CIEEL path. This may be the case for the Grignard reaction (p. 28). However there are cases such as the reaction between per-acids and fluorescers (p. 42) for which no mechanism can be envisaged as yet. 

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