Hydroxyl radical concentration

Trace-gas Lifetimes. The time scales for tropospheric chemical reactivity depend upon the hydroxyl radical concentration [HO ] and upon the rate of the HO/trace gas reaction, which generally represents the slowest or rate-determining chemical step in the removal of an individual, insoluble, molecular species. These rates are determined by the rate constant, e,g. k2s for the fundamental reaction with HO, a quantity that in general must be determined experimentally. The average lifetime of a trace gas T removed solely by its reaction with HO,... 

The lifetime of T is an inverse function of the hydroxyl radical concentration [HO>] and the rate constant kj for its reaction with a particular trace gas... 

The dominant transformation process for trichloroethylene in the atmosphere is reaction with photochemically produced hydroxyl radicals (Singh et al. 1982). Using the recommended rate constant for this reaction at 25 °C (2.36x10 cm /molecule-second) and a typical atmospheric hydroxyl radical concentration (5x10 molecules/cm ) (Atkinson 1985), the half-life can be estimated to be 6.8 days. Class and Ballschmiter (1986) state it as between 3 and 7 days. It should be noted that the half-lives determined by assuming first-order kinetics represent the calculated time for loss of the first 50% of trichloroethylene the time required for the loss of the remaining 50% may be substantially longer. 

The most reliable kinetic data are for atmospheric oxidation by hydroxyl radicals. These data are usually reported as second-order rate constants applied to the concentration of the chemical and the concentration of hydroxyl radicals (usually of the order of 10s radicals per cm3). The product of the assumed hydroxyl radical concentration and the second-order rate constant is a first-order rate constant from which a half-life can be deduced. 

Phenol is released into the air and discharged into water from both manufacturing and use. Based on its high water solubility (see Table 3-2) and the fact that it has been detected in rainwater, some phenol may wash out of the atmosphere however, it is probable that only limited amounts wash out because of the short atmospheric half-life of phenol. During the day, when photochemically produced hydroxyl radical concentrations are highest in the atmosphere, very little atmospheric transport of phenol is likely to occur. 

For the best data available for the hydroxyl radical concentration and the rate constants, the ratio has the values... 

SinghHB. 1977. Atmospheric halocarbons Evidence in favor of reduced average hydroxyl radical concentration in the troposphere. Geophys Res Lett 4 101-104. 

Chemical/Physical. Chemical oxidation of mono-, di-, and trichlorophenols using Fenton s reagent were investigated by Barbeni et al. (1987). To a 70-mL aqueous solution containing 4-chlorophenol thermostated at 25.0 °C was added ferrous sulfate and hydrogen peroxide solution (i.e., hydroxyl radicals). Concentrations of 4-chlorophenol were periodically determined with... 

Altshuller, A. P., Ambient Air Hydroxyl Radical Concentrations Measurements and Model Predictions, JAPCA, 39, 704-708 (1989). 

Blake, N. J., S. A. Penkett, K. C. Clemitshaw, P. Anwyl, P. Lightman, A. R. W. Marsh, and G. Butcher, Estimates of Atmospheric Hydroxyl Radical Concentrations from the Observed Decay of Many Reactive Hydrocarbons in Well-Defined Urban Plumes, J. Geophys. Res., 98, 2851-2864 (1993). 

Davenport, J. E., and H. B. Singh, Systematic Development of Reactive Tracer Technology to Determine Hydroxyl Radical Concentrations in the Troposphere, Atmos. Environ., 21, 1969-1981 (1987). 

Prinn, R.G. et al. (1995) Atmospheric trends and lifetime of trichloroethane and global average hydroxyl radical concentrations based on 1978 -1994 ALE/G AGE measurements. Science, 269,187. 

Similarly, the change of hydroxyl radical concentration should also equal zero ... 

Then, the final form of hydroxyl radical concentration can be expressed as follows ... 

OH is the steady-state hydroxyl radical concentration (M). PCB-OH is the hydroxylated PCB radical concentration (M). 

Values of the hydroxyl radical concentration for each oxidation system. (From Beltran-Heredia, J., Torregrosa, J., Domingues, J.R., and Peres, J.A., Chemosphere, 42, 351-359, 2001. With permission.)... 

Simons, J.H., Mausteller, J.W. (1952) The properties of w-butforane and its mixtures with n-butanc. J. Chem. Phys. 20, 1516-1519. Singh, H.B. (1977) Atmospheric hydrocarbons Evidence in favor of reduced average hydroxyl radical concentrations in troposphere. Geophys. Res. Lett. 4, 101-104. 

In this latter case, one of the terms of the denominator in Eq. (5) contains the concentration of hydrogen peroxide because this also plays a role as a scavenger. Thus, from Eq. (75), it follows that an increase in hydrogen peroxide concentration leads to a decrease in hydroxyl radical concentration and hence, a decrease of rR. 

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