Organic peroxy radical reaction with

The OH radical also reacts with non methane hydrocarbons (NMHCs) producing a variety of organic peroxy radicals (R02). H02 and CH302 react with NO producing OH and CH30, respectively (Reactions 5, 6). 

The H atoms formed in reaction 15a can react with 02 (reaction 11) to form H02. The stabilized Criegee intermediate (CH200) can participate in further reactions, some of which will result in the formation of peroxy radicals. Larger alkenes react with ozone to produce organic peroxy radicals. 

Figure 1. Hydroperoxy and organic peroxy radical concentrations as simulated for the marine boundary layer with (solid line) and without (dotted line) peroxy radical permutation reactions. (Reproduced with permission from reference 22. Copyright 1990 American Geophysical Union.)...

Hendry, D.G. and D. Schuetzle. 1976. Reactions of hydroperoxy radicals. Comparison of reactivity with organic peroxy radicals. ]. Org. Chem. 41 3179-3182. 

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. 

For measuring the steady-state concentration of organic peroxy radicals (ROO ) produced in sunlit natural waters series of antioxidants, such as poly-methylphenols, have been successfully applied as selective probe compounds (Faust and Hoigne, 1987). The rates of transformation have shown that the steady-state concentration of the apparent photooxidants increases with the amount of light absorbed by the DOM. The sink for ROO has not been identified, but kinetic evidence is that DOM, even when the DOC amounts up to 5 mgL"1, does not control the lifetime of the peroxy radicals. The following reaction scheme summarizes the results of the kinetic analysis for peroxy radical formation ... 

The methylperoxy radical can react with NO, N02, HOz radicals, and itself (also other organic peroxy radicals R02 ). The reaction with NO, analogous to reaction (6) (and 19),... 

Nitric oxide (NO) plays a central role in atmospheric chemistry, influencing both ozone cycling and the tropospheric oxidation capacity through reactions with hydroperoxy- and organic peroxy-radicals. When the NO concentration exceeds 40 pptv (pptv = parts per trillion by volume) it catalyzes the production of ozone (O3) ... 

Peroxy radicals are stable with respect to reaction with the major constituents of the atmosphere. Their chemistry is dominated by reactions with peroxy radicals, HO2 and RO2, and with the nitrogen oxides, NO, NO2, and NO3. Two recent articles review the spectroscopy, kinetics, and reaction mechanisms of organic peroxy radicals [11,12], Three of the more important reaction channels are illustrated in Figure 1. The reaction with NO2 proceeds exclusively by addition to form the corresponding alkyl-peroxy nitrate. This reaction is generally not important in determining the end products of the degradation process due to the usually rapid dissociation rate of the peroxy nitrate however, it can be relevant to dynamical issues, because it sequesters N02-... 

In remote atmospheres, where NOx concentrations are very low, reactions of peroxy radicals with HO2, or with other peroxy radicals, compete with the reaction with NOx and result in the formation of various oxidised compounds such as aldehydes, alcohols, organic acids and hydroperoxides. Several studies performed as part of LACTOZ have provided important data for modelling these processes in the atmosphere. 

The discovery of the rapid reactions of organic peroxy radicals with NO3 and the recognition of their importance in tropospheric chemistry has been a significant achievement within this project. Similarly the development of a method for the detection of ROx has provided a large number of measurements of ROx to be made in the planetary boundary layer and a few to be made in the free troposphere (Izana, Tenerife). This has allowed box models of HO2 and RO2 chemistry to be developed and the inadequacies of our current understanding to be pointed out. In conclusion the aims and objectives of LACTOZ have been well served by the achievements of this project. 

The reactions of organic peroxy radicals and HO2 are of interest for tropospheric chemistry, since they are chain carriers in VOC oxidation mechanisms, converting NO to NO2 and consequently producing O3. The reactions of RO2 with HO2 and the permutation reactions of organic peroxy radicals (RO2 + RO2 and RO2 + R 02) lead to radical termination, and may therefore significantly reduce O3 formation chain lengths, particularly at lower levels of NO. ... 

In the remote marine boundary layer, the concentrations of NO are typically very low (ca. 5-10 pptv) [47] and reactions of CH3SCH2OO with other species may represent important reaction pathways. By analogy with the oxidation processes of other alkyl peroxy radicals, reactions of CH3SCH2OO with HO2 and organic peroxy radicals deserve consideration as potentially important atmospheric processes. 

No direct studies of CH3S reactions with HO2 or organic peroxy radicals have been reported. However, because the atmospheric concentrations of peroxy radicals are much smaller than typical concentrations of O3, peroxy radicals will generally be unimportant as CH3S reaction partners. Indirect experimental evidence suggests that the methyl thiyl radical reacts with both HO2 and CH3O2 via O atom transfer [63, 64] (see Table 3). 

In the free troposphere with low NOx concentration, the chain termination reaction by the cross radical reaction between HO2 and CH3O2 formed in the oxidation of CH4 is important in additimi to the self-reaction of HO2. In the polluted atmosphere where cmicentrations of organic peroxy radicals (RO2) are high, their cross radical reactions with HO2 also need to be considered in the model calculation of photochemical ozone formation. Here, as a representative radical-radical reaction of RO2, the reaction of HO2 and CH3O2 is described. 

Ozone production in the troposphere results from the formation of 0( P)-atoms following the photolysis of NO2. The reaction of HO2 and organic peroxy radicals with NO reforms the NO2 thereby completing the cycle and enabling more ozone production ... 

Radical Scavengers Hydrogen-donating antioxidants (AH), such as hindered phenols and secondary aromatic amines, inhibit oxidation by competing with the organic substrate (RH) for peroxy radicals. This shortens the kinetic chain length of the propagation reactions. 

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