Hydroxyl radical reaction with nitric oxide

The temperature profile strongly influences those reactions whose rate coefficients have large activation energies. As will be shown in Sections IV, V, and VI, a number of reaction paths, while dominant in the lower troposphere, lose their importance with increasing altitude as the temperature drops sharply. Particularly affected are the altitude profiles of the hydroxyl radical, formaldehyde, and nitric oxide number densities. 

Hyperoxyl radicals can be converted back to hydroxyl radicals by reaction with nitric oxide ... 

Nitric oxide is a physiological substrate for mammalian peroxidases [myeloperoxide (MPO), eosinophil peroxide, and lactoperoxide), which catalytically consume NO in the presence of hydrogen peroxide [60], On the other hand, NO does not affect the activity of xanthine oxidase while peroxynitrite inhibits it [61]. Nitric oxide suppresses the inactivation of CuZnSOD and NO synthase supposedly via the reaction with hydroxyl radicals [62,63]. On the other hand, SOD is able to modulate the nitrosation reactions of nitric oxide [64]. 

Similarly, recent experiments" have been interpreted to mean that about 10% of the reaction of hydroperoxy radical with nitric oxide gives per-nitrous add, HOONO, instead of nitrogen dioxide and hydroxyl radical. Because this reaction is of major importance, even 10% of a second channel would be important, although it has been argued that such compounds would not be sufFidently stable to accumulate in the atmosphere." Whether such peroxynitrogen compounds are stable in the gas phase and whether they can be found in the atmosphere must await further experiments. 

FIGURE 3-13 Relations between conversion of nitric oxide to nitrogen dioxide and ozone, atomic oxygen, and hydroxyl-radical reaction rate constants. Reprinted with permission from Grosjean. ... 

Recently, a novel mechanism for hydroxyl radical production, which is not dependent on the presence of transition metal ions, has been proposed (7). This involves the production of peroxynitrite (ONOO ) arising from the reaction of nitric oxide (NO ) with superoxide (OJ), as shown in the following reactions ... 

Simple radical-radical addition reactions would not be expected to show any pH dependence. The pH effects seen here (Figs. 3-6) are likely due to the ratios of ONOO" and ONOOH (pKa, 6.8) and to the greater ease with which phenoxylate anion (pK 10) undergoes one-electron oxidation reactions. Overall, the nitrated and hydroxylated products from ONOO"/ phenol reactions can be explained using Scheme 4, whereby phenol undergoes a one-electron oxidation by either radical end of ONOO" to yield phenoxyl radical, followed by concerted addition of the remaining radical end of ONOO". Increased nitrosation when both ONOO" and nitric oxide are added simultaneously can be explained in terms of one-electron oxidation of nitric oxide by ONOO" to yield NO, which directly attacks phenol. Oxidation of nitric oxide to NO is also consistent with nitric oxide... 

There is no enzymatic system to eliminate hydroxyl radical because it reacts so quickly that no enzyme could remove it fast enough to reduce its toxicity. Another very short lived but highly damaging molecule is peroxynitrite (ONOO ), which forms by direct reaction of superoxide with nitric oxide. Because enzymatic intervention is essentially futile with OH and ONOO, cells depend on SODs and catalases to prevent the formation of precursors to these toxic molecules. 

Inorganic nitrates or nitric acid are formed by several reactions in smog. Among the important reactions forming HNO3 are the reaction of N2O5 with water and the addition of hydroxyl radical to NO2. The oxidation of NO or NO2 to nitrate species may occur after absorption of gas by an aerosol droplet. Nitric acid formed by these reactions reacts with ammonia in the atmosphere to form ammonium nitrate ... 

Chemical radicals—such as hydroxyl, peroxyhydroxyl, and various alkyl and aryl species—have either been observed in laboratory studies or have been postulated as photochemical reaction intermediates. Atmospheric photochemical reactions also result in the formation of finely divided suspended particles (secondary aerosols), which create atmospheric haze. Their chemical content is enriched with sulfates (from sulfur dioxide), nitrates (from nitrogen dioxide, nitric oxide, and peroxyacylnitrates), ammonium (from ammonia), chloride (from sea salt), water, and oxygenated, sulfiirated, and nitrated organic compounds (from chemical combination of ozone and oxygen with hydrocarbon, sulfur oxide, and nitrogen oxide fragments). ... 

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,... 

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