Hydroxyl radical ozone

Two of the strongest chemical oxidants are ozone and hydroxyl radicals. Ozone can react directly with a compound or it can produce hydroxyl radicals which then react with a compound. These two reaction mechanisms are considered in Section A 2.1. Hydroxyl radicals can also be produced in other ways. Advanced oxidation processes are alternative techniques for catalyzing the production of these radicals (Section A 2.2). 

Emitted ethene is distributed primarily into the atmosphere and reacts with photochemically reactive hydroxyl radicals, ozone, and nitrate radicals, with half-lives ranging from 1.9, 6.5, and 190 days, respectively. Biodegradation in water occurs with half-lives in the range of 1-28 days, or under anaerobic conditions, 3-112 days. Bioaccumulation in aquatic organisms is not expected to occur, based on ethene s high vapor pressure and log octanol/wa-ter partition coefficient. 

If released to the atmosphere, o-limonene is expected to rapidly undergo gas-phase oxidation reactions with photochemically produced hydroxyl radicals, ozone and, at night, with nitrate radicals. Limonene can react with ozone, forming submicron particulates that could impact asthmatics and those with other respiratory ailments. 

Atmospheric chemistry, Hydroxyl radicals. Ozone, Photoreactors, Photosmog... 

Detailed reviews of the reactions of hydroxyl radicals, ozone, and nitrate radicals with different classes of organic compounds are available. In addition, lUPAC sponsors a continuing program to evaluate and compile kinetic information on these reactions and these reports are published in the Journal of Physical Chemical reference Data. The analysis lists among other data the preferred rate constants and where possible, information on temperature effects. A recent analysis focuses primarily on reactions of hydroxyl and nitrate radicals. This extensive database provides an opportunity for developing systematic approaches to predicting reaction rates. 

In the atmosphere, there are three reactants that determine the rate of degradation of organics hydroxyl radicals, ozone, and nitrate radicals. The hydroxyl radical is by far the most important, since it reacts with most organics, with the exception of fully halogenated compounds (no hydrogen to abstract). Ozone is important for only a small group of compoimds, i.e., acetylenics and olefins. Nitrate radicals are only important at night and only react rapidly with a few classes of chemicals (e.g., phenols, mercaptans). 

Chemistry of the atmosphere review of gas-phase reactions radical reactions and thermodynamics chlorine radicals and the ozone layer , CFCs and other ozone depleting contaminants, catalysis on condensed phases hydroxyl radical, ozone production, proton abstraction, VOCs, NOx, and photochemical smog (four lectures)... 

Vapor-phase solvents can dissolve into water vapor, and be subject to hydrolysis reactions and ultimately, precipitation (wet deposition), depending on the solubility of the given solvent. The solvents may also be adsorbed by particulate matter, and be subject to dry deposition. Lyman asserted that atmospheric residence time cannot be directly measured that it must be estimated using simple models of the atmosphere. Howard et al. calculated ranges in half-lives for various organic compounds in the troposphere, and considered reaction rates with hydroxyl radicals, ozone, and by direct photolysis. 

In the presence of oxygen, strong oxidizers are formed, such as atomic oxygen, hydroxyl radicals, ozone, and so on, which lead to VOCs oxidation [56]. 

In the presence of water vapor, oxygen atoms formed by uv radiation react to form hydroxyl radicals (35), which can destroy ozone catalyticaHy. 

Aqueous Phase. In contrast to photolysis of ozone in moist air, photolysis in the aqueous phase can produce hydrogen peroxide initially because the hydroxyl radicals do not escape the solvent cage in which they are formed (36). 

Hydrogen peroxide is photolyzed slowly to hydroxyl radicals, which decompose ozone. 

Oxygen Compounds. Although hydrogen peroxide is unreactive toward ozone at room temperature, hydroperoxyl ion reacts rapidly (39). The ozonide ion, after protonation, decomposes to hydroxyl radicals and oxygen. Hydroxyl ions react at a moderate rate with ozone (k = 70). 

Effect of Hydroxyl Radicals on Ozone Depletion. Hydroxyl radicals, formed by reaction of ( D) oxygen atoms with water or CH, can destroy ozone catalyticahy (11,32) as shown in the following reactions. 

Table I. Trace gas rate constants and lifetimes for reaction with ozone, hydroxyl radical, and nitrate radical. Lifetimes are based upon [O3]=40ppb [HO ]=1.0x10 molecules cm (daytime) [NO3 ]=10ppt (nighttime).

Figure 4-13 shows an example from a three-dimensional model simulation of the global atmospheric sulfur balance (Feichter et al, 1996). The model had a grid resolution of about 500 km in the horizontal and on average 1 km in the vertical. The chemical scheme of the model included emissions of dimethyl sulfide (DMS) from the oceans and SO2 from industrial processes and volcanoes. Atmospheric DMS is oxidized by the hydroxyl radical to form SO2, which, in turn, is further oxidized to sulfuric acid and sulfates by reaction with either hydroxyl radical in the gas phase or with hydrogen peroxide or ozone in cloud droplets. Both SO2 and aerosol sulfate are removed from the atmosphere by dry and wet deposition processes. The reasonable agreement between the simulated and observed wet deposition of sulfate indicates that the most important processes affecting the atmospheric sulfur balance have been adequately treated in the model. 

The reaction of volatile chlorinated hydrocarbons with hydroxyl radicals is temperature dependent and thus varies with the seasons, although such variation in the atmospheric concentration of trichloroethylene may be minimal because of its brief residence time (EPA 1985c). The degradation products of this reaction include phosgene, dichloroacetyl chloride, and formyl chloride (Atkinson 1985 Gay et al. 1976 Kirchner et al. 1990). Reaction of trichloroethylene with ozone in the atmosphere is too slow to be an effective agent in trichloroethylene removal (Atkinson and Carter 1984). 

Reactions in the troposphere are mediated by reactions involving hydroxyl radicals produced photochemically during daylight, by nitrate radicals that are significant during the night (Platt et al. 1984), by ozone and, in some circumstances by 0( P). 

The concentrations of all these depend on local conditions, the time of day, and both altitude and latitude. Values of ca. 10 molecules/cm for OH, 10 -10 ° molecules/cm for NO3, and ca. 10 molecules/cm for ozone have been reported. Not all of these reactants are equally important, and the rates of reaction with a substrate vary considerably. Reactions with hydroxyl radicals are generally the most important, and some illustrative values are given for the rates of reaction (cmVs/molecule) with hydroxyl radicals, nitrate radicals, and ozone (Atkinson 1990 summary of PAHs by Arey 1998)... 

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

Combined treatment of atrazine with ozone and H2O2 resulted in retention of the triazine ring, and oxidative dealkylation with or without replacement of the 2-chloro group by hydroxyl (Nelieu et al. 2000). Reaction with ozone and hydroxyl radicals formed the analogous products with the additional formation of the acetamido group from one of the N-alkylated groups (Acero et al. 2000). 

Gas-phase reactions have been carried out in 160 mL quartz vessels, and the products analyzed online by mass spectrometry (Brubaker and Hites 1998). Hydroxyl radicals were produced by photolysis of ozone in the presence of water ... 

Atkinson R, Carter WPL, Aschmann SM, et al. 1985. Atmospheric fates of organic chemicals Prediction of ozone and hydroxyl radical reaction rates and mechanisms. Research Triangle Park, NC U.S. Environmental Protection Agency, Office of Research and Development. 

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