Reactions of HO2, CH3O2 Radicals

reaction of HO2 and O3 is important in the stratosphere as a reaction to craivert HO2 to OH in the HOx cycle. In the troposphere, it is a main reaction to convert HO2 to OH in the marine boundary layer and free troposphere where the concentration of NO is low. 

As for the reaction rate constants, the NASA/IPL panel evaluation No. 17 (Sander et al. 2011) recommends the Arrhenius formula, 

Wang et al. (1988), Herndon et al. (2001), etc. The rate constants at 298 K is 5 57 (298 K) = 2.0 X 10 cm molecule s, which is relatively slow as compared to the reaction of OH + O3 (see 5.2.1) by an order of magnitude. Furthermore,, the Arrhenius plot shows a curved feature as shown in the above formula, which is more pronounced under 250 K. This implies the activation energy decreases at low temperature. 

As for the reaction mechanism this reaction, Sinha et al. (1987) detected OH by LIF in the experiments using isotope labeled H 02 and 03, and found that most of OH (75 10 %) is OH. This means that the hydrogen abstraction by O3 is the main reaction. Nelson and Zahniser (1994) determined the formation ratio of OH and OH by LIF in a similar experiment, which showed the H-atom abstraction by O3 occurs 94 5 and 85 5 % at 226 K and 355 K, respectively, indicating that the temperature dependence is small. 

Quantum chemical calculation for the reaction of HO2 and O3 has been performed to show that the energy barrier for the pathway of H-atom abstraction by forming HO3 via O3 — HO2 complex is lower than that of O-atom abstraction via O3 — O2H, which agrees well with the branching, ratio obtained by the experiments (Xu and Lin 2007 Varandas and Viegas 2011). 

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The overall reaction sequence leading to CO2 formation, through the HCHO and CO intermediate "stable products, is shown in Figure 5.2. When NO, levels are sufficiently high that reaction of the peroxy radicals HO2 and CH3O2 with NO predominates over peroxy radical self-reactions, the methane oxidation chain depicted in Figure 5.2 can be written as... 

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. 

The comparison of the reaction rates of O3, HO2, and CH3O2 with olefin, paraffin, and NO reveals that the predominant reactions of these reactive species are the oxidations of NO [(VI11-I8), (VIll-21a), and (Vlll-24)]. The major destruction processes of olefin are the reactions with O3 and with OH. (The rate of olefin destruction is proportional to the rate constant times the concentration of the active species.) The destruction process of olefins by HO2 is less important and those by O atoms and CH3O2 radicals are also minor. 

The reaction of CH3O2 with the HO2 radical leads to the formation of methyl hydroperoxide,... 

Self-Reaction Kinetics. Of all peroxy radical reactions, the self-reaction between two identical peroxy radicals is perhaps the most studied. The measurement of peroxy radical UV absorption cross sections, discussed above, often occurs under the assumption that all the chlorine or fluorine atoms produced by photolysis are converted quantitatively into peroxy radicals however, this assumption must be corrected for by the loss of peroxy radicals from self-reaction. Furthermore, studies of RO2 -b NO or RO2 -f HO2 reactions usually take place at sufficiently high RO2 concentrations to require knowledge of the self-reaction rate constant, in order to interpret the results of the kinetics measurements. Both concerns make laboratory studies of peroxy self-reaction kinetics an important issue. In contrast, the steady-state atmospheric concentrations of HCFC-based peroxy radicals are probably too small for their self-reactions to be relevant to atmospheric chemistry. In this context, the most important peroxy-peroxy radical reactions would be between the HCFC-based peroxy radicals and CH3O2, but such reactions have not been studied to date. 

Reactions of NO3 with HO2 and simple RO2 radicals (CH3O2, C2H5O2 and CH3C(0)02) have been investigated within LACTOZ (Table 3), as possible chain propagation steps in the mechanism suggested for the night-time oxidation of VOCs (see above). 

Kinetic studies have been performed on the individual steps occurring in the NO3 and OH initiated oxidation of VOCs. The studied reactions include essentially reactions of NO3 with alkenes, di-alkenes and dimethyl sulfide (DMS), reactions of NO3 with intermediate peroxy radicals (HO2, CH3O2, C2H5O2) and reactions of OH with methane and oxygenated VOCs (ethers, alcohols). The rate constants for these reactions have been measured, and mechanistic information has been determined. The experimental methods used were discharge-flow reactors coupled with mass spectrometry, electron paramagnetic resonance (EPR), laser-induced fluorescence (LIF) analysis and the laser photolysis associated with LIF analysis. The discharge-flow LIF and laser photolysis LIF experiments have been especially developed for these studies. 

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 ... 

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). 

Typical O3 mixing ratios are 10 ppbv and 30-50 ppbv over the clean ocean and the clean terrestrial, respectively, while those of peroxy radicals are 10 pptv, three orders of magnitude lower than O3. However, the rate constants of the reaction of NO with the peroxy radicals, e.g. HO2, CH3O2, are 8 x 10 cm molecule s which is nearly three orders of magnitude larger than that of NO and O3,... 

Figure IV-G-2. Simplified oxidation mechanism for Cj F2x+iCHO illustrating the three possible mechanisms for perfluorocarboxylic acid formation A, reaction of hydrate with OH B, reaction of perfluoroacylperoxy with HO2 radicals and C, reaction of perfluoroalkyl peroxy with a-hydrogen-containing peroxy radicals (e.g., CH3O2).

Horie, O., and GK. Moortgat (1992a), Reactions of CH3C(0)02 radicals with CH3O2 and HO2 between 263 and 333 K. A product study, J. Ghem. Soc., Faraday Trans., 1992, 3305-3312. 

The two major tropospheric sinks of OH are the reactions with CO and CH4. In the clean Southern Hemisphere, CO and CH4 account for up to 50% each of the total OH loss, and HO2 and CH3O2 are the predominant forms of peroxy radicals formed (Reactions 3, 4, respectively). 

A rate of production analysis shows that radical production occurs primarily via 0(xD)+H20, but with a significant contribution to HO2 from HCHO photolysis. OH reacts mainly with CO and CH4, followed by HCHO, H2, O3 and CH3OOH with minor contributions from NMHCs. At the low NO concentrations encountered on these clean days, radical-radical reactions dominate the loss of peroxy-radicals resulting in a reduced chain propagation via CH3O2+NO and HO2+NO and in a very short chain length ( 0.14), calculated as the rate of HC>2 OH conversion divided by the total radical production rate. 

However, several complications arise in hydroxyl chemistry when one considers the ultimate fate of the hydrogen and methyl radicals formed in these two reactions. Both radicals combine rapidly with molecular oxygen to form hydroperoxyl radicals (i.e. HO2 and CH3O2). But the hydroperoxyl radical can regenerate OH ... 

Production of O3 in the CH4 oxidation chain is interrupted if the peroxy radicals HO2 and CH3O2 react with something other than NO, for example, themselves, NOj, or O3 itself, or if NO< is removed from the active cycle by reaction with OH to form HNO3. For the HO2 radical, besides reaction 5.46, another important reaction is... 

At one atmosphere total pressure these reactions are at or close to the high pressure limit with rate constants of 6.0 x 10" cm molecule s" Reasonable estimates of the global average tropospheric NO2, HO2, and RO2 (represented by CH3O2) are 2.5 x 10, 10, and 1.3 x 10 cm [5]. Based upon the data base for other peroxy radicals we estimate that the halogenated peroxy radicals studies have ks = 6 X 10", 4 = 6 X 10 , and 5 = 4 x 10 cm molecule" s" The atmospheric lifetimes of the halogenated RO2 radicals with respect to reactions (3), (4) and (5) are likely to be of the order of 11, 3, and 32 minutes, respectively. 

Egsgaard, H., Carlsen, L. Experimental evidence for the gaseous HS03 radical. The key intermediate in the oxidation of SO2 in the atmosphere. Chem. Phys. Lett. 148,537-540 (1988) Elrod, M.J., Ranschaert, D.L., Schneider, N.J. Direct kinetics study of the temperature dependence of the CH2O branching channel for the CH3O2-1-HO2 reaction. Int. J. Chem. Kinet. 33,... 

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