Hydroxyl radical stratosphere

Typical precautions with trichloroethylene are summarized in Table 5.52. An important factor is that the vapours are much heavier than air they will therefore spread and may accumulate at low levels, particularly in undisturbed areas. Because of its volatility, releases to the environment usually reach the atmosphere. Here it reacts with hydroxyl or other radicals (estimated half-life for reaction with hydroxyl radicals is less than a week) and is not therefore expected to diffuse to the stratosphere to any significant extent. There is some evidence for both aerobic and anaerobic biodegradation of trichloroethylene. 

Hexachloroethane is quite stable in air. It is not expected to react with hydroxyl radicals or ozone in the atmosphere or to photodegrade in the troposphere (Callahan et al. 1979 Howard 1989). Degradation by photolysis may occur in the stratosphere. 

Only an excited singlet oxygen atom could react with water at stratospheric temperatures to form hydroxyl radicals. 

Environmental Fate. The fate of bromomethane in the environment is dominated by rapid evaporation into air, where it is quite stable (EPA 1986b). The rates of volatilization from soil and water have been studied and are known with reasonable precision (although such rates are typically site-specific) (Jury et al. 1984 Lyman et al. 1982). The rates of breakdown by hydrolysis, reaction with hydroxyl radical, and direct photolysis in the stratosphere have also been estimated (Castro and Belser 1981 Davis et al. 1976 Robbins 1976). Further studies to improve the accuracy of available rate constants for these processes would be helpful, but do not appear to be essential in understanding the basic behavior of bromomethane in the environment. 

CHC1F2. These HCFCs do react with atmospheric hydroxyl radicals, shortening their lifetime so that they do not reach the stratosphere. The problem with the HCFCs is that they cannot be used in older appliances that were designed for CFCs. When CFCs will no longer be found in the market, the older appliances will need to be replaced by new ones designed for HCFCs. 

Hydrochlorofluorocarbons (HCFCs) have also been implicated in the depletion of stratospheric ozone, but are largely destroyed in the lower atmosphere by reaction with hydroxyl radicals. 

Here, we focus our attention on the interplay that exists between solvation processes and ultrafast redox reaction in the vicinity of the strong oxidant hydroxyl radical (OH). This diatomic radical represents one of the most efficient oxidant of cellular components (proteins, lipids, DNA), contributes to Haber-Weiss reaction and plays some important role in fundamental radiation or stratospheric chemistry. Presently, we have investigated short-time water caging effect on transient electron delocalization-relocalization in the vicinity of nascent aqueous OH radicals. This specific electronic channel is represented by Eq.(l). 

Although the troposphere has the characteristic of containing a high relative concentration of water vapor (10 5-10-2), the stratosphere is dry and the water vapor concentration is only a few parts in a million. However, the oxidation of methane by hydroxyl radical must be intro-... 

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. 

In the troposphere ozone is produced by photodissociation (equation 3) not of 02 but of N02, initiated with wavelengths <410 nm, followed by recombination (equation 2) with 02. In the upper troposphere and lower stratosphere the photolysis of ozone (equation 4) yields excited-state oxygen atoms which react with water to produce hydroxyl radicals (equation 5). These are crucial to the removal of organic compounds from the troposphere, since they readily abstract hydrogen atoms (equation 6) to yield organic radicals which subsequently undergo further oxidative degradation. 

The oxidation scheme for halomethanes not containing a hydrogen atom is similar to that for those which do, except that it is not initiated by tropospheric reaction with hydroxyl radicals, since the fully halogenated methanes are unreactive. Consequently, substantial amounts of CFCs and halons are transported intact up into the stratosphere, where they absorb UV radiation of short wavelength and undergo photodissociation (equation 36) to a halogen atom and a trihalomethyl radical. The halogen atom Y may enter into catalytic cycles for ozone destruction, as discussed in the introduction. 

On the basis of ratios of C and C present in carbon dioxide, Weinstock (250) estimated a carbon monoxide lifetime of 0.1 year. This was more than an order of magnitude less than previous estimates of Bates and Witherspoon (12) and Robinson and Robbins (214), which were based on calculations of the anthropogenic source of carbon monoxide. Weinstock (250) suggested that if a sufficient concentration of hydroxyl radical were available, the oxidation of carbon monoxide by hydroxyl radical, first proposed by Bates and Witherspoon (12) for the stratosphere, would provide the rapid loss mechanism for carbon monoxide that appeared necessary. By extension of previous stratospheric models of Hunt (104), Leovy (150), Nicolet (180), and others, Levy (152) demonstrated that a large source of hydroxyl radical, the oxidation of water by metastable atomic oxygen, which was itself produced by the photolysis of ozone, existed in the troposphere and that a chain reaction involving the hydroxyl and hydroperoxyl radicals would rapidly oxidize both carbon monoxide and methane. It was then pointed out that all the loss paths for the formaldehyde produced in the methane oxidation led to the production of carbon monoxide [McConnell, McElroy, and Wofsy (171) and Levy (153)1-Similar chain mechanisms were shown to provide tropospheric... 

The major sink would appear to be the oxidation of CO by hydroxyl radicals. The calculated sink for this process gives a lifetime [Levy (152,154)] in harmony with that predicted from radiocarbon data [Weinstock (250) and Weinstock and Niki (251)]. Ingersoll and Inman (107) and Inman, Ingersoll, and Levy (108) have shown that soil bacteria can destroy CO and suggested that it may be an important sink. Pressman and Warneck (196) calculated the sink due to the transport of CO into the stratosphere, where it is then oxidized to CO2, and found that it was small. 

In particular, the excited oxygen atom quickly attacks methane, water vapor or molecular hydrogen present in stratosphere, see Figure 3.1, producing hydroxyl radicals according to reactions [4]... 

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 ... 

Ozone is of major interest to tropospheric chemists for two reasons It leads to the production of hydroxyl radicals, and it is a greenhouse gas. For many years, tropospheric ozone was believed to be chemically inert. Scientists held that the ozone present in the troposphere was formed initially in the stratosphere — where ultraviolet radiation is of high enough energy to dissociate oxygen — and mixed down into the troposphere. It was postulated further that ozone was removed from the atmosphere primarily by reacting with Earth s surface. 

As we noted in Section 4.01.1, the ability of the troposphere to chemically transform and remove trace gases depends on complex chemistry driven by the relatively small flux of energetic solar UV radiation that penetrates through the stratospheric O3 layer (Levy, 1971 Chameides and Walker, 1973 Crutzen, 1979 Ehhalt et al., 1991 Logan et al, 1981 Ehhalt, 1999 Crutzen and Zimmerman, 1991). This chemistry is also driven by emissions of NO, CO, and hydrocarbons and leads to the production of O3, which is one of the important indicators of the oxidizing power of the atmosphere. But the most important oxidizer is the hydroxyl free radical (OH), and a key measure of the capacity of the atmosphere to oxidize trace gases injected into it is the local concentration of hydroxyl radicals. 

Most of the releases of carbonyl sulfide to the environment are to air, where it is believed to have a long residence time. The half-life of carbonyl sulfide in the atmosphere is estimated to be 2 years. It may be degraded in the atmosphere via a reaction with photochemically produced hydroxyl radicals or oxygen, direct photolysis, and other unknown processes related to the sulfur cycle. Sulfur dioxide, a greenhouse gas, is ultimately produced from these reactions. Carbonyl sulfide is relatively unreactive in the troposphere, but direct photolysis may occur in the stratosphere. Also, plants and soil microorganisms have been reported to remove carbonyl sulfide directly from the atmosphere. Plants are not expected to store carbonyl sulfide. 

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