Alkylperoxy radicals nitrate radical reactions

Alkylperoxy radicals (R02) in the atmosphere react primarily with NO, H02, and other R02. Reaction with the nitrate radical, N03, at night, has been recently recognized as being important as well. A less important reaction is that with N02. There are two excellent reviews of R02 chemistry by Lightfoot et al. (1992) and Wallington et al. (1992), including the thermochemistry, spectroscopy, kinetics, and mechanisms. The reader should consult these for references to the original work in this area. 

In summary, alkyl radicals, R, are all converted to alkylperoxy radicals, R02, in the troposphere. R02 reacts with NO, except in very remote regions where the concentration of NO is of the order of 40 ppt or less. The reaction converts NO to N02 and hence acts as a source of O-, via the N02 photolysis. For the larger alkylperoxy radicals, a substantial portion of the R02 + NO reaction also gives the stable alkyl nitrate,... 

The aUcyl radicals formed by the abstraction reaction react exclusively with O2 in the atmosphere to form alkylperoxy radicals (pathways (c) (h)), and most of them are then transformed to alkoxy radicals by oxidizing NO to NO2 in the presence of NOx (pathways (d) and (i)). However, some alkylperoxy radicals react with NO by the recombination isomerization reactions (e) and (j), to form alkyl nitrate (see Sect. 5.3.3). Although the yields of 2-butyl and 1-butyl nitrate are not large in the case of 2-butyl and 1-butyl radicals, 0.083 and <0.04, respectively, the production yields of alkyl nitrates increase with the increase of carbon number of alkyl radicals as described in Sect. 5.3.3, and it is as large as >0.2 for the Cg, and C7 secondary n-aUcyl radicals (2-hexyl, 3-hexyl, 2-heptyl, and 3-heptyl radicals) (Lightfoot et al. 1992 Arey et al. 2001). Since these reactions act as termination reactions for the OH chain reaction, they are important as parameters in determining the ozone formation efficiency in model calculations. The production yields of RONO2... 

The 2-butoxy, and 1-butoxy radicals formed in the reactions of 2-butyl and 1-butyl peroxy radicals with NO (path (d) and (j)), react with O2 to produce HO2 together with carbonyl compounds such as methyl ethyl ketone and butanal (path (g) and (1)), and complete the HO cycle. Under the typical NO concentrations in the polluted atmosphere, the formation rate of alkyl nitrates by the reactions of alkoxy radicals with NO2 is negligible, the recombination isomerization reaction by the alkylperoxy radicals with NO is the major pathway as the formation process of alkyl nitrate. 

Reaction Scheme 7.2 summarizes the reaction mechanism for 1-butene (l-C4Hg) as an example of alkenes. The hydroxyalkyl radicals formed by the pathways (a) and (b) is a kind of alkyl radicals mentioned in the previous Sect. (7.2.2), and exclusively forms hydroxyperoxy radicals by the reaction with O2 in the atmosphere. From the hydroxyperoxy radicals, oxyradicals (hydroxybutoxy radicals) and NO2 (pathways (d), (k)), and partially hydroxybutyl nitrate (pathways (e), (1)) are produced by the reaction with NO as in the case of alkylperoxy radicals described in the previous paragraph. The yields of hydroxylalkyl nitrates are 2-6 % for C4-C6 alkenes (O Brien et al. 1998), which is about half of those for alkyl nitrates from the alkoxy radicals. Hydroxy alkoxy radicals formed in pathways (d) and (k) are known to follow the three reaction pathways, unimolecular decomposition ((g), (n)), H-atom abstraction by O2 ((h), (o)), and dihydroxyl radical formation by isomerization (p), corresponding to reactions (7.25), (7.24) and... 

The reaction with NO leads to the formation of CO2 and a methyl radical that is oxidized to formaldehyde by reactions (16)-(20). In addition, the oxidation of CH3 regenerates HO c so that the oxidation cycle continues. Association with NO2 produces peroxyacetyl nitrate (PAN). Its lifetime is longer than that of alkylperoxy nitrates, but strongly temperature dependent, ranging from 1 hr at 298 K to 140 d at 250 K. Thus, PAN can be transported over a great distance before undergoing thermal decomposition. Under conditions of lowNOj concentrations acetyl peroxy radicals interact also with HO2 radicals... 

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