OH radical reaction rate constants

Kwok ESC, R Atkinson, J Arey (1995) Rate constants for the gas-phase reactions of the OH radical with dichlorobiphenyls, 1-chlorodibenzo-p-dioxin, 1,2-dimethoxybenzene, and diphenyl ether estimation of OH radical reaction rate constants for PCBs, PCDDs, and PCDFs. Environ Sci Technol 29 1591-1598. 

Atkinson, R. Estimation of OH radical reaction rate constants and atmospheric lifetimes for polychlorinated biphenyls, dibenzo-/rdioxins, and dibenzofmans. Environ. Sci Technol, 21(3) 305-307, 1987a. 

OH radical reaction rate constants for phenol were estimated by De et al. (1999). 

De, A.K., Chaudhuri, B., Bhattacharjee, S., Dutta, B.K., Estimation of OH radical reaction rate constants for phenol and chlorinated phenols using UV/H202 photo-oxidation, /. Haz. Mat., 64, 91-104, 1999. 

Estimation Methods for OH Radical Reaction Rate Constants... 

Based on direct spectroscopic measurements of OH radical concentrations at close to ground level, peak daytime OH radical concentrations are typically (3-10) x 106 molecule cm-3 (see, for example, Brauers et al., 1996 Mather et al., 1997 Mount et al., 1997). A diur-nally, seasonally, and annually averaged global tropospheric OH radical concentration has been derived from the emissions, atmosphere concentrations, and OH radical reaction rate constant for methyl chloroform (CH3CC13), resulting in a 24-hr average OH radical concentration of 9.7 x 10s molecule cm 3 (Prinn et al., 1995). 

For the majority of gas-phase organic chemicals present in the troposphere, reaction with the OH radical is the dominant loss process (Atkinson, 1995). The tropospheric lifetime of a chemical is the most important factor in determining the relative importance of transport, to both remote regions of the globe and to the stratosphere, and in determining the possible buildup in its atmospheric concentration. Knowledge of the OH radical reaction rate constant for a gas-phase organic compound leads to an upper limit to its tropospheric lifetime. 

To date, OH radical reaction rate constants have been measured for 500 organic compounds (Atkinson, 1989, 1994, 1997). However, many more organic chemicals are emitted into the atmosphere, or formed in situ in the atmosphere from photolysis or chemical reactions of precursor compounds, for which OH radical reaction rate constants are not experimentally available. Thus the need to reliably calculate OH radical reaction rate constants for those organic compounds for which experimental data are not currently available. 

The method is based on the observations that gas-phase OH radical reactions with organic compounds proceed by four reaction pathways, assumed to be additive H-atom abstraction from C-H and O-H bonds, OH radical addition to >C=C< and -C=C-bonds, OH radical addition to aromatic rings, and OH radical "interaction" with N-, S-, and P-atoms and with more complex structural units such as ->P=S, >NC(0)S- and >NC(0)0- groups. The total rate constant is assumed to be the sum of the rate constants for these four reaction pathways (Atkinson, 1986). The OH radical reactions with many organic compounds proceed by more than one of these pathways estimation of rate constants for the four pathways follow. Section 14.3.5 gives examples of calculations of the OH radical reaction rate constants for the "standard" compounds lindane (y-hexachlorocyclohexane), trichloroethene, anthracene, 2,6-di-ferf-butylphenol, and chloropyrofos. 

As examples of the calculation of OH radical reaction rate constants using the method discussed above (Kwok and Atkinson, 1995), the OH radical reaction rate constants for lindane [y-hexachlorocyclohexane cyclo-(-CHCl-)6], trichloroethene (CHC1=CC12), 2,6-di-tert-butylphenol, and chloropyrofos appear below. As the section dealing with OH radical addition to aromatic rings mentions, at present the rate constant for the reaction of the OH radical with anthracene (and other PAH) cannot be estimated with the method of Kwok and Atkinson (1995). In carrying out these calculations, one first must draw the structure of the chemical (the structures are shown in the appendix to Chapter 1). Then one carries out the calculations for each of the OH radical reaction pathways which can occur for that chemical. 

To date, no experimentally measured OH radical reaction rate constant for lindane at room temperature is available in the literature for comparison. 

The OH radical reaction rate constant for 2,6-di-ferf-butylphenol is given by ... 

OH radical reaction with chloropyrofos is anticipated to proceed via OH radical addition to the pyridine ring, OH radical "interaction" with the P=S group [with a group rate constant k >P=S (Table 14.4)], and H-atom abstraction from the two -OCH2CH3 groups bonded to the P atom. The total OH radical reaction rate constant is given by ... 

Atkinson77 and Kwok et al.95 have proposed and discussed methods for the estimation of OH radical reaction rate constants for the PCBs, PCDDs and PCDFs, based on the correlation of the rate constants for OH radical addition to aromatic rings, kOH, with the sum of the electrophilic substituent constants, Ecr+.77,100 Based on a review of the literature rate constants for OH radical addition to a wide range of aromatic compounds, Atkinson77 derived the correlation,... 

Calculated using the estimated OH radical reaction rate constants given in Table 5 and assuming a 24h average OH radical concentration of 9.7 x 105 molecule cm-3.91 bUsing the experimentally measured reaction rate constants. 

Dreisbach, R.R., Shrader, A.A.I. (1949) Vapor pressure-temperature data on some organic compounds. Ind. Eng. Chem. 41,2879-2880. Eadsforth, C.V. (1986) Application of reverse-phase HPLC for the determination of partition coefficients. Pest. Sci. 17(3), 311-325. Edney, E.O., Corse, E.W. (1986) Validation of OH Radical Reaction Rate Constant Test Protocol. NTIS PB86-166 758/as. U.S. Environmental Protection Agency, Washington, D.C. 

In the atmosphere, the vapor-phase reaction of PCBs with hydroxyl radicals (photochemicaUy formed by sunlight) is the dominant transformation process (Brubaker and Hites 1998). The calculated tropospheric lifetime values for this reaction increases as the number of chlorine substitutions increases. The tropospheric lifetime values (determined using the calculated OH radical reaction rate constant and assuming an annual diurnally averaged OH radical concentration of 5x10 molecule/cm ) are 5-11 days for monochlorobiphenyls, 8-17 days for dichlorobiphenyls, 14—30 days for trichlorobiphenyls,... 

Relative rate evaluations are often used to ascertain OH reactivity information. For most volatile organics, this method should provide an OH-radical reaction rate constant within a factor of two of the expected value (22). 

While OH radical reaction rate constants have been successfully generated to extend structure-activity predictive capability for thiocarbamates and chlorinated aromatics, further e q>erimental work will be inq>ortant to assess OH reaction rates for die more complex chemistries diat con rise die majority of high-use urban and agricultural pesticides. 

The OH radical reaction rate constants with 1- and 2-nitronaphthalene and that of 2-methyl-1-nitronaphthalene are significantly lower than that for the parent PAH naphthalene [A (OH + naphthalene) = 2.3 x 10 , fe(OH- -1-methylnaphthalene) = 5.3 X 10 and fe(OH + 2-methylnaphthalene) = 5.23 x 10 cm molecule s (Calvert et al., 2002)]. As mentioned earlier for other nitro-containing compounds, this shows that substitution of an — N02 group on the aromatic ring markedly deactivates the aromatic ring to electrophilic addition of the OH radical. A similar situation is observed for the monocyclic aromatics, with the ambient temperature rate constant for the reaction of the OH radical with nitrobenzene being a factor of 10 lower than that for benzene. 

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