And HO2 radicals

From these relations it can be concluded that ksi > k 3 > k, which implies that the concentrations of SO4 and HO2 radicals are relatively large compared to that of OH. It is striking, however, that for kinetic reasons this mechanism does not involve termination steps (78) and (89) which are widely favoured in the chemistry of HO2 and OH radicals... 

Cooper, P. L. and Abbatt, J. P. D. Heterogeneous interactions of OH and HO2 radicals with surfaces characteristic of atmospheric particulate matter, J. Phys. Chem., 100,2249-2254,1996. 

Gratpanche, F., Ivanov, A., Devolder, P, Gershenzon, Y., and Saw-erysyn, J.-P Uptake coefficients of OH and HO2 radicals on material surfaces of atmospheric interest, 14 International Symposium on Gas Kinetics, Leeds, September, 1996. 

Kanaya, Y., Matsumoto, J., Kato, S., and Akimoto, H. Behavior of OH and HO2 radicals during the Observations at a Remote Island of Okinawa (ORION99) field campaign 2. Comparison between observations and calculations, J. Geophys. Res.-A., 106,24209-24223,2001. 

Thus, O2 " and HO2 radicals behave very differently. While the H02 radical undergoes an H-abstraction [reactions (80)-(83)] and behaves like an ordinary peroxyl radical, the 02 reaction sequence is initiated by an addition reaction [reaction (84), see below]. The H-abstraction reaction (80) is slow (k = 120 dm3 mol"1 s 1). Similar conclusions, revising an earlier report (Schulte-Frohlinde et al. 1986), have been obtained by Hildenbrand and Schulte-Frohlinde (1997) for the reaction of DNA peroxyl radicals with GSH. 

Dissolved metals affect the concentration of atmospheric trace gases, such as ozone, organics and sulfur compounds. Ozone is formed in the troposphere through a complex series of homogeneous reactions as shown schematically in Fig. 4. The chain represented in the figure by thick arrows involves the cooperative oxidation of organic molecules and NO with the intermediacy of HO, formed by subsequent photolysis of NO2 and O3, and HO2 radicals. 

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. 

A generic scheme for the atmospheric oxidation of a C2 haloalkane is given in Fig. 6. Values in parentheses are order of magnitude lifetime estimates. Reaction with OH radicals gives a halogenated alkyl radical which reacts with O2 to give the corresponding peroxy radical (RO2). As discussed in previous sections, peroxy radicals can react with three important trace species in the atmosphere NO, NO2, and HO2 radicals. 

The chalking process can be regarded as the reaction of water and oxygen to form OH and HO2 radicals under the influence of shortwave radiation and the catalytic... 

Fig. 11. Schematic view of the copper vapor laser pumped dye laser system developed at Harvard for the detection of OH and HO2 radicals in situ from balloon-borne descent probes.

Suggested reaction schemes are given in block diagram form in Fig. 20. In summary, there is good evidence for believing that, under almost all conditions so far studied, the oxidation of HCHO proceeds via HCO and HO2 radicals. HCO3 radicals may also be present, even up to 600 °C in... 

A key set of experiments have been carried out by Walker and co-workers [111] in connection with the CH2CHCH2 + HO2 reaction. Allyl and HO2 radicals were produced in three separate systems. 

Many chemical reactions occur by more than one reaction channel giving rise to corresponding different sets of products. For example, the reaction between H atoms and HO2 radicals can proceed by three paths... 

The numerical model for n-butane oxidation, by Pitz et al. [228], was used also by Carlier et al. [21] to simulate experimental studies of the two-stage combustion of n-butane at 0.18 MPa on a flat-flame burner and, following this validation, to simulate the ignition delays of n-butane in a rapid compression machine. The numerical studies of the burner experiments were extended by Corre et al. [233]. For simulations of the behaviour on a flat-flame burner the chemical model was computed in an isothermal mode, the experimental one-dimensional temperature profile being introduced as an input parameter. Among the important aims of the tests by Corre et al. [233] was the rationalization of the predicted extent of n-butane consumption throughout the development of the first (cool-flame) and second stages of combustion, with that observed experimentally. The experimental study by Minetti et al. [22, 116] included the detection and measurement of RO2 and HO2 radicals by esr, the one-dimensional spatial profiles of which were simulated by Corre et al. [233],... 

An even more marked deviation occurs when L-ascorbic acid is irradiated with Co 7-rays in oxygenated 0.8 N sulfuric acid. Here, the over-all stoichiometry is such that quantitative interpretation is impossible unless hydroxyl radicals and HO2 radicals are involved in an initial, abstraction step ... 

Calvert and McQuigg suggest that yet unknown radicals, such as OCH2O or those derived from it, formed in the 03-olefin-air mixtures may oxidize SO2 in the homogeneous reaction. It is known that OH and HO2 radicals combine rapidly with SO2. The addition products may eventually be transformed into sulfuric acid, peroxysulfuric acid, sulfates, and nitrates in a polluted atmosphere probably in a liquid phase of aerosol particles, although the detailed steps are still unknown. Finlayson and Pitts (357) believe that the oxidation of aromatic compounds by such species as OH, HO2, O3, and 0( P) may also be significant for the formation of organic aerosol. 

The OH and HO2 radicals produced do not react further with O3. Hence, the quantum yield of O3 decomposition is near unity. 

To be fully utilized, this chamber should also serve as a resource for collaborative studies where environmental chamber measurements imder highly controlled conditions would be useful. We encourage collaboration with other researchers who have instrumentation that we lack that would be useful for various studies, such as our collaboration with William Brune of Penn State University, whose HO and HO2 radical measurement instrumentation was used with the surrogate - NOx experiments carried out for the observational based methods study. This facility would also be useful as a test bed for evaluating analytical instrumentation for use in ambient monitoring, since it can produce realistic but highly characterized simulated pollution conditions for instrument evaluation and inter-comparisons. 

Meinel bands observed in the airglow see Chapter 2). At altitudes above 40 km, the reactions of the OH and HO2 radicals with atomic oxygen are important ... 

These four reactions provide important loss processes for odd oxygen. Near the tropopause, the OH and HO2 radicals also react with carbon and nitrogen monoxide ... 

Linden, L. A., Rabek, J. R, Kaczmarek, H., Kaminska, A., and Scoponi, M. "Photooxidative Degradation of Polymers by HO and HO2 Radicals Generated During the Photolysis of H2O2 FeCl3 and Fenton Reagants." Coordination Chemistry Reviews 125 (1993) 195-218. 

OH and HO2 Radicals The hydroperoxyl radical, H02, is a weak acid and on dissolution in the aqueous phase dissociates,... 

CX3CH2O radicals (X = H, Cl, or F) react predominantly with O2 to form the aldehyde and HO2 radicals. 

The negative effect of the admixture of hydrocarbons on ozone synthesis is also partially due to the formation of OH and HO2 radicals with the chain mechanism (6-82), (6-83). The major effect of hydrocarbons, however, is related to trapping of oxygen atoms by the hydrocarbon molecules ... 

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