Hydroxyl radical phase

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). 

Free-radical substitutions of heterocyclic compounds have been carried out with alkyl, aryl, and hydroxyl radicals in solution and with halogen atoms in the gas phase. Of these, arylations have been the most extensively investigated. 

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. 

Atkinson R. 1985. Kinetics and mechanisms of the gas-phase reactions of hydroxyl radical with organic compounds under atmospheric conditions. Chem Rev 85 69-201. 

The degree of dissociation is very small but the diphenylcyanomethyl radical is sufficiently reactive to induce polymerization in styrene. Methyl radicals or hydrogen atoms bring about polymerization of vinyl monomers in the gas phase.Hydrogen peroxide in the presence of ferrous ions initiates polymerization in the aqueous phase or in aqueous emulsions through generation of hydroxyl radicals according to the Haber-Weiss mechanism... 

For polychlorinated biphenyls (PCBs), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 d for 3-chlorobiphenyl to 34 d for 236-25 pentachlorobiphenyl (Anderson and Hites 1996). It was estimated that loss by hydroxy-lation in the atmosphere was a primary process for the removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a hydroxyl radical at the 1-position followed by transannular dioxygenation at the 2- and 5-positions followed by ring fission (Brubaker and Hites 1998). Reactions of hydroxyl radicals with polychlorinated dibenzo[l,4]dioxins and dibenzofurans also play an important role for their removal from the atmosphere (Brubaker and Hites 1997). The gas phase and the particulate phase are in equilibrium, and the results show that gas-phase reactions with hydroxyl radicals are important for the... 

Considerable attention has been directed to the formation of nitroarenes that may be formed by several mechanisms (a) initial reaction with hydroxyl radicals followed by reactions with nitrate radicals or NO2 and (b) direct reaction with nitrate radicals. The first is important for arenes in the troposphere, whereas the second is a thermal reaction that occurs during combustion of arenes. The kinetics of formation of nitroarenes by gas-phase reaction with N2O5 has been examined for naphthalene (Pitts et al. 1985a) and methylnaphthalenes (Zielinska et al. 1989) biphenyl (Atkinson et al. 1987b,c) acephenanthrylene (Zielinska et al. 1988) and for adsorbed pyrene (Pitts et al. 1985b). Both... 

Brubaker WW, RA Hites (1997) Polychlorinated dibenzo-/)-dioxins and dibenzofurans gas-phase hydroxyl radical reactions and related atmospheric removal. Environ Sci Technol 31 1805-1810. 

Bunce NJ, L Liu, J Zhu, DA Lane (1997) Reaction of naphthalene and its derivatives with hydroxyl radicals in the gas phase. Environ Sci Technol 31 2252-2259. 

Zielinska B, J Arey, R Atkinson, PA McElroy (1989) Eormation of methylnitronaphthalenes from the gas-phase reactions of 1- and 2-methylnaphthalene with hydroxyl radicals and NjOj and their occurrence in ambient air. Environ Sci Technol 23 723-729. 

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 ... 

The rate of gas phase reaction of pollutants with chlorine radicals is 10 to 100 times faster than with hydroxyl radicals, and we [1, 2] have proposed Cl as the smface active species responsible for the rate enhancement observed on addition... 

The hydroxyl radicals produced by water molecule radiolysis react promptly with oxidizable species such as SOg in air and form a condensed phase. These molecules further coagulate and become ultrafine particles. The radon concentration in the... 

It has been found that the activity which is conventionally referred to as the "unattached" fraction is actually an ultrafine particle aerosol with a size range of 0.5 to 3 nm. The hydroxyl radical from water molecule radiolysis is a key element to the particle formation mechanism. By injecting different concentrations of S02 into the test chamber, a possible particle formation mechanism has been suggested as follows Oxidizable species such as S02 reacts promptly with hydroxyl radicals and form a condensed phase. These molecules coagulate and become ultrafine particles. 

Gusten, H., Filby, W.G. and Schoaf, S., Prediction of Hydroxyl Radical Reaction Rates with Organic Compounds in the Gas Phase, Atmospheric Environ., 15 1763-1765 (1981). 

It is also possible that COS might be a precursor for CS2 in soil and vegetation chemical analogies have been proposed.12 The reverse reaction, CS2—>COS, occurs in our oxidative atmosphere via hydroxyl radical and other gas phase oxidants. 

Extensive research has been conducted into the atmospheric chemistry of organic chemicals because of air quality concerns. Recently, Atkinson and coworkers (1984, 1985, 1987, 1988, 1989, 1990, 1991), Altshuller (1980, 1991) and Sabljic and Glisten (1990) have reviewed the photochemistry of many organic chemicals of environmental interest for their gas phase reactions with hydroxyl radicals (OH), ozone (03) and nitrate radicals (N03) and have provided detailed information on reaction rate constants and experimental conditions, which allowed the estimation of atmospheric lifetimes. Klopffer (1991) has estimated the atmospheric lifetimes for the reaction with OH radicals to range from 1 hour to 130 years, based on these reaction rate constants and an assumed constant concentration of OH... 

Air half-life of 2.4-24 h based on photooxidation rate constant of 3.2 x 1012 cm3 mol1 s 3 for the gas-phase reaction with hydroxyl radical (Darnall et al. 1976) ... 

Atkinson, R. (1989) Kinetics and Mechanisms of the gas-phase reactions of the hydroxyl radical with organic compounds. J. Phys. Chem. Data Monograph No.l. 

Dilling, W.L., Gonsior, S.J., Boggs, G.U., Mendoza, C.G. (1988) Organic photochemistry. 20. A method for estimating gas-phase rate for reactions of hydroxyl radicals with organic compounds from their relative rates of reaction with hydrogen peroxide under photolysis in 1,1,2-trichlorotrifluoroethane solution. Environ. Sci. Technol. 22, 1447-1453. 

Guesten, H., Filby, W.G., Schoop, S. (1981) Prediction of hydroxyl radical reaction rates with organic compounds in the gas phase. Atmos. Environ. 15, 1763-1765. 

Doyle, G.J., Lloyd, A.C., Darnall, K.R., Winer, A.M., Pitts, J.N. Jr. (1975) Gas phase kinetic study of relative rates of reaction of selected aromatic compounds with hydroxyl radicals in an environmental chamber. Environ. Sci. Technol. 9, 237-241. 

Klamt, A. (1993) Estimation of gas-phase hydroxyl radical rate constants of organic compounds from molecular orbital calculations. Chemosphere 26, 1273-1289. 

Kwok, W.S.C., Atkinson, R. (1995) Estimation of hydroxyl radical reaction rate constants for gas-phase organic compounds using a structure reactivity relationship an update. Atmos. Environ. 29, 1685-1695. 

Wallington, T.J., Neuman, D.M., Kurylo, M.J. (1987) Kinetics of the gas phase reaction of hydroxyl radicals with ethane, benzene, and a series of halogenated benzenes over the temperature range 234 -38 K. Int. J. Chem. Kinet. 19, 725-739. 

Oxidation rate constant k, for gas-phase second order rate constants, koH for reaction with OH radical, kNC,3 with N03 radical and krj, with 03 or as indicated, data at other temperatures see reference photooxidation tA = 1111-38500 h, based on measured rate constant for reaction with hydroxyl radical in water (Radding et al. 1976 quoted, Howard et al. 1991) k(aquatic) fate rate of 50 L mol 1 s 1 with t,A = 1600 d (Callahan et al. 1979)... 

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