Hydroxyl radical primary species

Little is known concerning the chemistry of nickel in the atmosphere. The probable species present in the atmosphere include soil minerals, nickel oxide, and nickel sulfate (Schmidt and Andren 1980). In aerobic waters at environmental pHs, the predominant form of nickel is the hexahydrate Ni(H20)g ion (Richter and Theis 1980). Complexes with naturally occurring anions, such as OH, SO/, and Cf, are formed to a small degree. Complexes with hydroxyl radicals are more stable than those with sulfate, which in turn are more stable than those with chloride. Ni(OH)2° becomes the dominant species above pH 9.5. In anaerobic systems, nickel sulfide forms if sulfur is present, and this limits the solubility of nickel. In soil, the most important sinks for nickel, other than soil minerals, are amorphous oxides of iron and manganese. The mobility of nickel in soil is site specific pH is the primary factor affecting leachability. Mobility increases at low pH. At one well-studied site, the sulfate concentration and the... 

Berdnikov VM, Bazhin NM, Fedorov VK, Polyakov OV (1972) Isomerization of the ethoxyl radical to the a-hydroxyethyl radical in aqueous solution. Kinet Catal (EngITransI) 13 986-987 Bonifacic M, Asmus K-D (1984) Adduct formation and absolute rate constants in the displacement reaction of thiyl radicals with disulfides. J Phys Chem 88 6286-6290 Bonifacic M, Mockel H, Bahnemann D, Asmus K-D (1975a) Formation of positive ions and other primary species in the oxidation of sulphides by hydroxyl radicals. J Chem Soc Perkin Trans 2 675-685... 

Among the oxygen-derived free radicals, the species of primary concern include superoxide anion (O -), hydrogen peroxide (H202), and hydroxyl radical (OH-). The superoxide is further converted in peroxynitrite (ONOO-) by reacting with nitric oxide. 

The main source of oxygen in prebiotic planetary atmospheres is ablation of cometary material. The primary source of various oxygen species in all atmospheric photochemical reactions is water. Water molecules are easily photolyzed by the solar Lyman aline (121.6nm) and on irradiation within the water UV absorption bands (140-190 nm).The main products are hydroxyl radicals, oxygen atoms in their ground (3P) and excited ( D) states, and hydrogen atoms [14],... 

Lipid peroxidation may beinitiated by any primary free radical which has sufficient reactivity to extract a hydrogen atom (Fig. 2.10) from a reactive methylene group of an unsaturated fatty acid. For example, species such as hydroxyl radicals OH, alkoxyl radicals RO peroxyl radicals ROO and alkyl radicals R may be involved. The formation of the initiating species is accompanied by bond rearrangement that results in stabilization by diene conjugate formation. The lipid radical then takes up oxygen to form the peroxyl radical. Peroxyl radicals can... 

A varying and much more complex mechanistic situation exists in heterogeneous photocatalysis (Fig. 5-13). With respect to the transient oxygen species, comparable overall oxidation reactions are usually observed, but the set of primary reactive oxygen species is slightly different. It is commonly assumed, that superoxide radical anions and hydroxyl radicals are the primary species formed after photogeneration of the electron-hole pair of a semiconductor catalyst in the presence of water and air (Serpone, 1996). In the presence of ozone, ozonide radical anions or are formed by fast electron transfer reaction of superoxide radical anions with O3 molecules. The combination Ti02-03-UV/VIS is called photocatalytic ozonation (Kopf et al., 2000). For example, it was applied for the decomposition of tri-chloroethene in the gas phase (Shen and Kub, 2002). 

Well studied primary reactive species in radiation- or photo-initiated reactions of auxiliary oxidants in an aqueous phase are hydrated electrons (eaq), hydrogen atoms (H ) and hydroxyl radicals ( OH), the last being by far the most important ones in photo-initiated AOPs. The formation and reactivity of ejq and of H were described by Hart and Anbar (1970) and by Buxton et al. (1988). Hydrated electrons can be produced by VUV photolysis of water, by photolysis of aqueous solutions of [FelCNlq]" or of V with formation of [Fe(CN)5] and il2, respectively (cf. Buxton et al, 1988). 

Reactions with hydroxyl radicals are considered one of the most efficient ways used by the atmosphere to remove natural and anthropogenic trace gases in the atmosphere [28,29]. Previous kinetics [27,30-32] and modeling studies [2, 33-35] have shown that the primary atmospheric sink of bromopropane is the reaction with OH and that it has an atmospheric lifetime of 10-16 days. Further studies have determined that the lifetime of short-lived species, such as... 

There is a variety of processes that act to remove a trace gas from either the troposphere or the stratosphere. For oxygenated VOCs the main tropospheric loss occurs via gas phase oxidation reactions involving OH, O3, NO and Cl radicals, and photolysis. However, the hydroxyl radical is the most important oxidizing species in the global troposphere [16-19]. As a result of its role in initiating the majority of oxidation reaction chains, the OH radical is the primary cleansing agent for the lower atmosphere and has been called the "tropospheric vacuum cleaner" [20]. The dominant production cycle for tropospheric OH involves the reaction of 0( D), produced from the photolysis of O3, with H2O ... 

Methane is oxidized primarily in the troposphere by reactions involving the hydroxyl radical (OH). Methane is the most abundant hydrocarbon species in the atmosphere, and its oxidation affects atmospheric levels of other important reactive species, including formaldehyde (CH2O), carbon monoxide (CO), and ozone (O3) (Wuebbles and Hayhoe, 2002). The chemistry of these reactions is well known, and the rate of atmospheric CH4 oxidation can be calculated from the temperature and concentrations of the reactants, primarily CH4 and OH (Prinn et al., 1987). Tropospheric OH concentrations are difficult to measure directly, but they are reasonably well constrained by observations of other reactive trace gases (Thompson, 1992 Martinerie et al., 1995 Prinn et al., 1995 Prinn et al., 2001). Thus, rates of tropospheric CH4 oxidation can be estimated from knowledge of atmospheric CH4 concentrations. And because tropospheric oxidation is the primary process by which CH4 is removed from the atmosphere, the estimated rate of CH4 oxidation provides a basis for approximating the total rate of supply of CH4 to the atmosphere from aU sources at steady state (see Section 8.09.2.2) (Cicerone and Oremland, 1988). 

Photochemistry of the OH radical controls the trace gas concentration. The photochemistry of the free hydroxyl radical controls the rate at which many trace gases are oxidized and removed from the atmosphere. Processes that are of primary importance in controlling the concentration of OH in the troposphere are indicated by solid lines in the schematic diagram those that have a negligible effect on OH levels but are important because they control the concentrations of associated reaction and products are indicated by broken lines. Circles indicate reservoirs of species in the atmosphere arrows indicate reactions that convert one species to another, with the reactant or photon needed for each reaction indicated along each arrow. Multistep reactions actually consist of two or more sequential elementary reactions. HX = HQ, HBr, HI, or HF. CxHy denotes hydrocarbons. (From Chameides and Davis, Chem. Eng. News 60 (40) 38-52, 1982. Copyright American Chemical Society.)... 

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