Reactions of RO2 radicals

As E- and do not differ greatly, the relative importance of (-6) and (9) is not markedly influenced by temperature, and the relative yields of O-heterocyclic compounds (from OOOH homolysis) and conjugate al-kenes are similarly little affected between 600 and 800 K. This is particularly noticeable [17] with the larger alkanes where O-heterocyclic compounds and conjugate alkenes are formed in similar quantities. Fish [76] was the first to develop the extensive peroxy radical isomerization and decomposition (PRID) theory which was used to explain the many oxygenated compounds in alkane oxidation. Fish also considered the possibility of group transfer in RO2 radicals, for example 

Energetically less favourable, no real evidence has been found for its occurrence. Usually it has been introduced into mechanisms to explain the formation of lower oxygenated products, but it is far more likely that they are formed in secondary processes from the primary products. 

The internal H atom transfer (9) is extremely important in the chemistry of engine efficiency and knock, as illustrated later, and in particular the competition between reactions (IOC) and (37) is the key to the amount of branching that occurs in low-temperature ignition. 

Baldwin and Walker have obtained quantitative experimental data for reaction (9), the kinetically significant reaction (along with reaction (6)) in the formation of oxiranes, oxetanes, tetrahydrofuranes and tetrahydro-pyranes in the generalized sequence. 

With R and RO2 fully equilibrated in the temperature region 600-800 K, and considerable evidence that A ioc k.g [23,78], the rate of formation of O-heterocycle is given by equation (1.12). 

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In the atmosphere peroxy radicals react with NO, NO2, HO2 radicals and other peroxy radicals (R 02). The importance of these reactions is dictated by the abundances of NO, NO2, and HO2 radicals and by the rates of the reactions of RO2 radicals with these species. In the troposphere the concentrations of NO, NO2, and HO2 vary widely, however, for the present purposes reasonable average concentrations are approximately (2.5—10) x 10s cm-3. Under atmospheric conditions, typical rate constants for the reactions of RO2 radicals with NO, NO2, and HO2 radicals lie in the ranges (8-20)xlO-12, (5-10) xlO 12, and (5-15)xl0 12 cm3molecule 1 s, respectively [4]. Hence, on average these reactions are of comparable importance in the atmospheric fate of RO2 radicals. On a local scale one reaction may dominate because of variation in the concentrations of NO (NO and NO2) and HO2 radicals. Thus, in remote marine locations with low NO levels, reaction of RO2 radicals with HO2 will be much more important than in urban air masses with high NO concentrations. 

In polluted areas, reactions of RO2 radicals with nitrogen oxides are of major importance. The reaction of RO2 with NO results in the production of NO2 and hence formation of ozone in the troposphere, and the reaction with NO2 forms temporary reservoir peroxynitrate species. Much attention has been given in LACTOZ to the kinetics of these reactions, which has contributed to substantial improvement of the kinetic data base. 

A few cross-reactions of RO2 radicals with the acetylperoxy radical have been investigated ... 

As emphasised above, reactions of RO2 radicals with NO are important propagating reactions, resulting in the formation of NO2 which, on photolysis, yields O3. The reactions of a series of peroxy radicals, derived from both... 

Oxy radicals (RO) are important intermediates in all VOC oxidation chains. They are formed in the chain propagating channels of the reactions of RO2 radicals with NO and in the self-reactions of RO2. Three principal reaction pathways for RO radicals have been identified under atmospheric conditions ... 

A few cross-reactions of RO2 radicals with the acetylperoxy radical have been investigated CH3O2, C2H5O2 and C-C6H11O2. The reactions are very fast, with rate constants equal to 10 " cm molecule" s". This is even true for the secondary radical C-C6H11O2, in spite of the much lower rate constant observed for the selfreaction (4 X 10 " cm molecule" s ). This may be important since the acetylperoxy radical is also an abundant radical in the atmosphere and, if all cross-... 

The reaction of RO2 radicals with NO proceeds through two channels ... 

TABLE 1. Absolute rate constants for reaction of peroxyl radicals (RO2 ) with TPMD... 

The OOQOOH radical may isomerize further, similar to the reactions of RO2. The isomer-ized product decomposes into a ketohydroperoxide5 species and one OH radical. The keto-hydroperoxide is fairly stable below about 800 K, but at higher temperatures it decomposes to yield two additional radicals [426]. Thus it is not until this final decomposition step of the ketohydroperoxide that chain branching is finally achieved in the low-temperature mechanism, yielding three radicals from the initial peroxide radical. 

It is well established that the reaction of peroxy radicals with NO2 proceeds by a simple association mechanism in which an alkyl peroxy nitrate is formed. The exothermicity associated with formation of the RO2 - NO2 bond resides initially in the RO2NO2 molecule. Collisional deactivation is required to remove this excess energy. 

With further temperature increase, two features dominate. The ratio [R]/[R02] rises further as (6) becomes increasingly reversible, so that reactions of R radicals rather RO2 influence the oxidation. Of these the most important is the overall reaction (5A) which gives the... 

Alkoxy (RO ) radicals are formed in the reaction of alkyl peroxy (RO2 ) radicals with NO, reaction 5.59a. Subsequent reactions of alkoxy radicals determine to a large extent the... 

The results of LACTOZ have provided an extended kinetic data base for the following classes of reactions reactions of OH with VOCs, reactions of NO3 with VOCs and peroxy radicals, reactions of O3 with alkenes, reactions of peroxy radicals (self reactions, reaction with HO2, other RO2, NO, NO2), reactions of alkoxy radicals (reactions with O2, decomposition, isomerisation), thermal decomposition of peroxynitrates. Photolysis parameters (absorption cross-section, quantum yields) have been refined or obtained for the first time for species which photolyse in the troposphere. Significantly new mechanistic information has also been obtained for the oxidation of aromatic compounds and biogenic compounds (especially isoprene). These different data allow the rates of the processes involved to be modelled, especially the ozone production from the oxidation of hydrocarbons. The data from LACTOZ are summarised in the tables given in this report and have been used in evaluations of chemical data for atmospheric chemistry conducted by international evaluation groups of NASA and lUPAC. 

The sequence of chemical reactions leading to oxidative degradation and ozone formation in the atmosphere is often determined by the relative rates of competing reaction pathways at several critical points in the degradation. The protocol adopted was therefore to establish these critical points and emphasis was then placed on provision of data allowing the relative importance of these competing processes to be quantitatively defined for a range of VOCs. These critical points were found to lie for example in the reactions of RO2 and RO radicals. 

Hydroxyl radical attack on VOCs is the primary source of RO2 radicals in the troposphere, and consequently the rates of the OH + VOC reactions have a strong influence on the local rate of O3 formation (Fig. 1). Moreover, the mechanism of the OH reaction determines the structure of the peroxy radical formed, and hence, provides information on its subsequent degradation pathway. ... 

The rates and mechanisms for OH attack on simple organic compounds have been established for sometime, and any uncertainty in their atmospheric lifetimes largely resides in the value of the concentration of the OH radical in the atmosphere. In LACTOZ, effort has been concentrated on oxidation mechanisms, particularly on the reactions of RO2 and RO radicals, for understanding the degradation of small organic compounds. 

Alkyl nitrates, formed in a secondary channel of the reaction of RO2 with NO, are another "stable" form of NOy. LACTOZ has provided a comprehensive data set for the formation of nitrates, mainly from the higher molecular weight VOCs. In addition, the rates of degradation of organic nitrates, by photolysis and by their reaction with OH radicals, can be determined from LACTOZ results, although some uncertainties remain in the production and loss of alkyl nitrates. 

The initial attack of radicals on VOCs gives rise to the formation of RO2 radicals which can undergo a variety of reactions. These reactions may lead to propagation of the radical chain, e.g. formation of RO, or termination of the chain leading to radical loss and formation of stable products, e.g. hydroperoxides, peroxynitrates and nitrates. Propagation is necessary for ozone production, and thus these reactions are central to the scientific aim of LACTOZ quantification of tropospheric ozone budget. 

Of these objectives, only the first is related to overall OH radical reaction rate studies. The latter two rely largely on the full complexity of the oxidation chain including reactions of RO2 and RO radicals as well as their interactions with NOx. 

CH3O2 is the most abundant alkylperoxy radical in hydrocarbon-rich atmospheres. It is of interest to assess the role of RO2 + CH3O2 cross-reactions since a series of rate constants has been obtained for such reactions, either resulting from indirect determinations (secondary reactions of RO2 + RO2 reactions) or from direct measurements. Rate constants for this class of reactions are reported in Table 8. 

In the first of these processes, the carbonyl compound generated contains the same number of carbon atoms as the parent VOC. Formation of HO2 radicals leads to subsequent NO to NO2 conversion associated with HOx radical recycling. The second process (dissociation) gives rise to breakdown of the carbon chain, and as a consequence the resulting carbonyl compound is of smaller size than the present VOC and reflects oxidative breakdown of the VOC. The third process (isomerisation) may lead to several NO to NO2 conversion as the carbon atoms in the molecule are oxidised by internal H abstraction. The alkyl radicals generated in both dissociative and isomerisation reactions lead to formation of RO2 radicals and hence enhanced NO to NO2 conversion. Thus, the three alternative reaction pathways for oxy radicals have a large influence on the total net O3 generation. 

Peroxynitrates are formed in the troposphere by the recombination of RO2 radicals and NO2. The relevant chemical reactions which determine the importance of peroxynitrates as temporary reservoirs of NOx are the following ... 

As NO3 is photolysed rapidly during the day, its maximum concentration occurs at night. The reaction of NO3 with volatile organic compounds (VOC) is therefore anticipated to represent a night-time source of RO2 radicals. 

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. ... 

Self-reactions of peroxy radicals may occur in the atmosphere under low NO concentrations, particularly for the fastest cases, where rate constants may reach values as high as 10" cm molecule" s". In addition, self-reactions must be well characterised before studying other reactions of peroxy radicals, particularly those with HO2 and CH3O2. The general mechanism for RO2 self-reactions is the following ... 

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