Hydroxyl radicals abundance

Mohler O. and Arnold F. (1992). Gaseous sulfuric acid and sulfur dioxide measurements in the Arctic troposphere and lower stratosphere Implications for hydroxyl radical abundances. Ber. Bunsenges. Phys. Chem., 96, 280-283. 

Arnold, F., G. Knop, and H. Ziereis. 1986. Acetone measurements in the upper troposphere and lower stratosphere — implications for hydroxyl radical abundances. Nature 321 505-507. 

Table 4. Median Concentration of the Ten Most Abundant Ambient Air Hydrocarbons in 39 U.S. Cities and Their Reactivity with Hydroxyl Radical...

In a high-temperature atmosphere created by the combustion of a host hydrocarbon fuel, there will be an abundance of hydroxyl radicals. Thus, boron monoxide reacts with hydroxyl radicals to form gaseous metaboric oxide HOBO. 

Because of the reactivity of the hydroxyl radical, and the fact that the ingredients are inexpensive, the Fenton reaction is used on a commercial scale to treat waste water. The Fenton reaction can also occur with copper as the transition metal. Given that Fe2+, Cu+, and H202 are abundantly present in biological systems, hydroxyl radicals can be generated via the Fenton reaction in vivo. Reviews by Schiitzendubel and Polle (2002) and by Valko et al. (2005) describe the impact of ROS in plants and humans, respectively. 

Although there is still debate as to whether hydroxyl radicals or ferryl species are the key oxidants in Fenton systems, most literature reports on the mechanisms of degradation of organic compounds invoke the hydroxyl radical. Based on the reports discussed above, it seems likely that hydroxyl radical is a major oxidant during Fenton degradations. Although ferryl ions or other highly oxidized forms of iron may occur, either to a limited extent or more abundantly under specific conditions, this section will deal with documented reaction pathways and kinetics for hydroxyl radical or species assumed to be hydroxyl radical. The reader should keep in mind that ferryl pathways may need to be considered under certain conditions. 

In the case of semiconductor assisted photocatalysis organic compounds are eventually mineralized to carbon dioxide, water, and in the case of chlorinated compounds, chloride ions. It is not unusual to encounter reports with detection of different intermediates in different laboratories have been observed. For example, in the degradation of 4-CP the most abundant intermediate detected in some reports was hydroquinone (HQ) [114,115,123], while in other studies 4-chloro-catechol, 4-CC (3,4-dihydroxychlorobenzene) was most abundant [14,116-118, 121,163]. The controversy in the reaction intermediate identification stems mainly from the surface and hydroxyl radical mediated oxidation processes. Moreover, experimental parameters such as concentration of the photocatalyst, light intensity, and concentration of oxygen also contribute in guiding the course of reaction pathway. The photocatalytic degradation of 4-CP in Ti02 slurries and thin films... 

There is abundant evidence that activated oxygen species [i.e., singlet oxygen ( 02), peroxy radicals ( OOR), superoxide anion ( (V), and hydroxyl radicals ( OH)] are involved in carcinogenesis. Potentially, they act both in initiation and in promotion and progression. For example,... 

It has been shown that loss of a hydroxyl radical from metastable p-nitromethyl benzoate ions involves abstraction of an ortho hydrogen by the nitro group [474]. From metastable ion abundance measurements on the 3-d, and 2-dj compounds, an average isotope effect /OH//OD of 1.41 was determined. On the basis of an intermolecular isotope effect, it has been further suggested that this loss of a hydroxyl radical from the p-nitromethyl benzoate ion is also preceded by hydrogen transfer from the methyl group to the ring [474], The ratio (M— OH)+/M+ for the d3 -isomer was found to be less than half the same ratio for the unlabelled isomer. 

The superoxide anion radical and hydrogen peroxide are not particularly harmful to cells. It is the product of hydrogen peroxide decomposition, the hydroxyl radical (HO ), that is responsible for most of the cytotoxicity of oxygen radicals. The reaction can be catalyzed by several transition metals, including copper, manganese, cobalt, and iron, of which iron is the most abundant in the human body (Reaction 2 also called the Fenton reaction). To avoid iron-catalyzed reactions, iron is transported and stored chiefly as Fe(III), although redox active iron can be formed in oxidative reactions, and Fe(III) can be reduced by semiquinone radicals (Reaction 3). 

It has been reported that formic acid is the most abundant carboxylic acid in the troposphere [127], In wet deposition, along with acetic acid, it accounts for up to 18% of the total acidity in rain in some areas [128]. The main chemical sink for atmospheric carboxylic acids is their reaction with the hydroxyl radical. 

One potential source of hydroxyl radicals is depicted in Figure 6. The reduced form of some transition metals, the most notable and abundant being iron, can react... 

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). 

In biological systems the hydroxyl radical can lead to damage such as lipid peroxidation and DNA breakage (van Maanen et al. 1999). In support of this hypothesis, iron is often the most abundant transition metal in atmospheric particles, and there are numerous reports that particulate Fe(III) can be reduced to Fe(II) as a result of atmospheric reactions (Faust 1994). 

We have shown above that dissolution rates of multiple oxides can be related to the abundance and speciation of hydrogen and hydroxyl radicals at different metal centers at the surface. Since dissolution of most complex oxides is nonstoichiometric, the identity of these centers varies as a function of time and experimental conditions. The selective removal of some cations from the solid surface creates a reacted layer that is depleted in those elements that dissolve rapidly (i.e, modifying cations during basalt dissolution or sodium, calcium, and aluminum in the case of feldspars). As steady-state dissolution is controlled by the dismantling of these altered layers, it is critical to know their chemical characteristics and to identify the main mechanisms that control their formation. Two important findings obtained via microbeam techniques will be presented here. 

The most abundant carbon-containing compound in the stratosphere and mesosphere is carbon dioxide (CO2). By interacting with infrared radiation, this gas plays an important role in the thermal budget of the atmosphere, and the 30% increase in its concentration resulting mainly from fossil fuel burning has provided a significant forcing to the climate system of about 1.5 Wm 2 (IPCC, 2001). Carbon dioxide does not play any substantial role in the chemistry of the atmosphere except in the lower thermosphere, where its photolysis is an important source of carbon monoxide (CO). This latter gas, which is also released at the Earth s surface by incomplete combustion (pollution) and is partially transported to the stratosphere, is converted to CO2 by reaction with the hydroxyl radical (OH). 

It has been shown that loss of a hydroxyl radical from metastable p-nitromethyl benzoate ions involves abstraction of an ortho hydrogen by the nitro group [474]. From metastable ion abundance measurements on the 3-di and 2-di compounds, an average isotope effect/qh/fon... 

In studies of metastable ions of acetic acid, the loss of a hydroxyl radical has been used as an internal reference [see Sect. 7.5.3(b)] to justify intermolecular comparisons on loss of water [407, 753]. The ratio of metastable ion abundances, /h,o/ oh. with CH3COOH was... 

Aromatic compounds are of great interest in the chemistry of the urban atmosphere because of their abundance in motor vehicle emissions and because of their reactivity with respect to ozone and organic aerosol formation. The major atmospheric sink for aromatics is reaction with the hydroxyl radical. Whereas rate constants for the OH reaction with aromatics have been well characterized (Calvert et al. 2002), mechanisms of aromatic oxidation following the initial OH attack have been highly uncertain. Aromatic compounds of concern in urban atmospheric chemistry are given in Figure 6.16. 

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