CH3O2 with radicals, reactions

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. 

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,... 

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... 

Considerable progress has been achieved in the past two years in the determination of rate constants for RO2 + radical reactions. Prior to this, A (H02 + HO2O was known with some precision, and estimates for CH3O2 + radical reactions were available from studies of highly complex photochemical oxidations. ... 

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. 

A very sensitive and specific method of CH3O2 detection has been developed in the project. It is based on REMPI-MS techniques employing laser irradiation at two wavelengths. In EI-MS studies radical reaction of CH3O2 with other radicals were investigated including the species NO3, halogen atoms, CIO and BrO. 

The reaction of CH3O2 with halogen atoms X (X = F, Cl, Br) constitutes an interesting source of Criegee radical formation in flow systems. 

The experimental data agree rather well with the simulations as is evident from the above Figures 3 and 4, in which the symbols represent the experimental results while the lines correspond to the simulations. The dependence of ethane and ethylene selectivity on the methane-to-oxygen ratio at a total pressure of 41 kPa and 1044 K as simulated by the described procedure is shown in Figures 6 and 7. From the comparison of experimental and simulated results it turns out that model A is equivalent to model B for temperatures above 1000 K. For lower temperatures model A gives too high C24- selectivities. This indicates that the CH3O2 radical reaction path is important below that temperature. 

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. 

However, the subsequent transformations of these radicals are less evident. Kinetic simulations of the DMTM process in the framework of the mechanism developed by Vedeneev with co-workers [63—66], have demonstrated that an important role is played not only by the interaction of this radical with molecular products to form peroxides, the decomposition of which provides an effective branching in this reaction — no less important are the radical—radical reactions involving CH3O2. 

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). 

Br-atom initiated oxidation of dimethyl sulfide (DMS) in a large-volume reaction chamber gave SO2, CHsSBr, and DMSOJ A rapid addition of Br atoms to DMS takes place, forming an adduct that mainly reforms reactants but also decomposes unimolecularly to form CHsSBr and CH3 radicals. DMSO is formed from the reaction of BrO radicals with DMS. The reaction CH3O2 + Br CH3O + BrO is postulated as the source of BrO radicals. 

This reaction competes favorably with other CH3O2 reactions, such as (R16) and (R20), and offers a fast pathway to the methoxy radical (CH3O). In a similar reaction, nitric oxide converts the hydroperoxy radical (HO2) to the more reactive hydroxyl radical,... 

Akimoto and coworkers detected S2 following 248 nm photolysis of DMDS, implying that CH3 radicals are also produced (22). Low levels of CH3O2 and H02 radicals that could be produced in the C.W. experiments would not be removed by reactions with NOx but rather sustain a chain process leading to S02production. 

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 ... 

Chemical oxidation of NO to NO2 occurs rapidly through reaction with ozone as well as in daylight with photochemically generated peroxy radicals (HO, CH3O2 ecc.) according to this reaction... 

The ozone formation process is almost always initiated by a reaction involving a primary hydrocarbon (abbreviated here as RH), other organic or CO with the OH radical. The reaction with OH (Equation (1)) removes a hydrogen atom from the hydrocarbon chain, which then acquires O2 from the atmosphere to form a radical with the form RO2. For example, methane (CH4) reacts with OH to form the RO2 radical CH3O2 propane (CsHg) reacts to form C3H7O2, etc. The equivalent reaction for CO (Equation (2)) forms HO2, a radical with many chemical similarities to the various RO2 radicals ... 

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... 

Extrapolation of this mechanism to stratospheric conditions, where the radical combination reactions will be very unlikely. Is very uncertain at best. It Is probably that radicals of the type CF2CIO2 will react with NO (analogous to CH3O2) ... 

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). 

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