Peroxy radical - reaction/source

There are two sources of tropospheric ozone. First, transport from the stratosphere in meteorological events known as tropospheric folding in which a layer of stratospheric air is entrained in tropospheric air-flow and mixed into the troposphere. Second, peroxy radical reactions which oxidize NO to N02. For example, in the OH radical initiated oxidation of CO ... 

The temporal profile of peroxy radicals at night follows that of N03 during the day, there are additional sources, of course, through OH and O, reactions. Carslaw et al. (1997) suggest that the reactions of H02, and perhaps CH3SCH202 (from DMS oxidation), with NO, at night may also be important. 

Important peroxy radical sources include the reactions of the hydroxyl radical with various compounds, for example, carbon monoxide ... 

The mechanisms behind lipid oxidation of foods has been the subject of many research projects. One reaction in particular, autoxida-tion, is consistently believed to be the major source of lipid oxidation in foods (Fennema, 1993). Autoxidation involves self-catalytic reactions with molecular oxygen in which free radicals are formed from unsaturated fatty acids (initiation), followed by reaction with oxygen to form peroxy radicals (propagation), and terminated by reactions with other unsaturated molecules to form hydroperoxides (termination O Connor and O Brien, 1994). Additionally, enzymes inherent in the food system can contribute to lipid oxidization. 

Nitrogen oxides (NOx= N02 and nitrogen monoxide NO) sources are mainly emitting NO into the troposphere. Thai, NO may be converted to N02 by reaction with hydrogen peroxy radical (H02) or with higher peroxy radicals (R02), produced from hydrocarbon oxidation. 

The phenomenon of catalyst-inhibitor conversion1 2,143,356 may be understood and critical concentration of metal can be deduced by reference to Eq. (280). If decomposition of the hydroperoxide is the source of initiation, it must be formed as rapidly as it is consumed to maintain a steady rate. If termination by metal complex predominates, a steady state occurs when the right-hand side of Eq. (280) equals unity. No oxidation will occur when this quantity is less than unity. Hence, catalyst-inhibitor conversion is observed as the metal concentration is increased to the point that the chain length becomes less than unity. If termination occurs by the bimolecular reaction of peroxy radicals, a chain length of less than unity will result in the depletion of the hydroperoxide until the rate of initiation has decreased to the point where the chain length is unity again. No inhibition is expected or observed. 

In the presence of traces of transition-metal ions, lipid hydroperoxides are a continuous source for the formation of new alkoxy and peroxy radicals which initiate new chain reactions and therefore act as amplifiers for the initial free radical event. 

Another potential dark source of in the atmosphere, more particularly in the boundary layer, is from the reactions between ozone and alkenes. The ozonolysis of alkenes can lead to the direct production of the OH radical at varying yields (between 7 and 100%) depending on the structure of the alkene, normally accompanied by the co-production of an (organic) peroxy radical. As compared to both the reactions of OH and NO3 with alkenes the initial rate of the reaction of ozone with an alkene is relatively slow, this can be olfset under regimes where there are high concentrations of alkenes and/or ozone. For example, under typical rural conditions the atmospheric lifetimes for the reaction of ethene with OH, O3 and NO3 are 20 h, 9.7 days and 5.2 months, respectively in contrast, for the same reactants with 2-methyl-2-butene the atmospheric lifetimes are 2.0 h, 0.9 h and 0.09 h. 

Ripperton and Vukovich (213) suggested the existence of an atmospheric sink for ozone, possible reactions with NC. Recently Crutzen (43) suggested that reactions involving peroxy radicals may be an indirect source of ozone in the lower troposphere and that the photolysis... 

Another possible source involves the oxidation of NO by CH302, but this reaction is still in question and H02 is the major peroxy radical in the troposphere [Levy (153,154)]. 

This rearrangement proceeds via a 2,3-peroxy radical mechanism4,5 24. Radicals are probable intermediates as these reactions are initiated or accelerated by light or free radical sources (benzyl peroxides, tert-butyl hyponitrite) 2 6, are inhibited by radical scavengers (4-methyl-2,6-di-/ert-butyl phenol)6,7, and display ESR signals of allyl peroxy radicals7-9. 

The photochemical interconversion between NO and N02 was discussed in Section 5-2. During the day N02 undergoes photodissociation, forming NO and O atoms that quickly attach to molecular oxygen, producing ozone. Back-reactions of NO with ozone and with peroxy radicals generated from hydrocarbons establish a steady state between NO and N02. In the absence of local sources, their molar ratio should be given by the steady-state equation... 

This reaction is facilitated by formation of the stabilized RSSO radical that is isoelectronic with the stabilized polysulfide radical, RS3. The analogous sulfenic acids are effective radical scavengers reacting with peroxy radicals with a rate constant of KFM"1 sec1 at 60°C (16). The S-S bond in the thiolsulfinate is weak, and the corresponding bond in the thiosulfoxyl radical should be considerably less stable. Thus, the thiosulfoxyl radical may function as a source of sulfur oxides ... 

Ideally, the source of peroxy radicals must be controllable so that their formation may be stopped quickly to allow their disappearance to be followed spectrometrically. This condition is easily met when photolysis or radiolysis is used the use of metal ions [reaction (79)] requires rapid mixing and stop flow techniques [50],... 

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