Gas-Phase Reaction with OH

N02 reacts readily with OH, forming nitric acid (shown as H0N02 to emphasize the source, but as HN03 in most cases throughout this book)  

The reaction is termolecular, with recommended values (DeMore et al., 1997) of k ) = 2.5 X 10-3() cm6 molecule-2 s-1, n = 4.4 for its temperature dependence, kx = 1.6 X 10- cm3 molecule-1 s-1 at 300 K, and m = 1.7 for its temperature dependence (see Chapter 5.A.2). These values of the low- and high-pressure limiting rate constants can be used as described in Chapter 5.A.2 to calculate an effective second-order rate constant at 300 K and 1 atm of 8.7 X 10 12 cm3 molecule-1 s- 1 (see Problem 4). At an OH concentration of 2 X 10f radicals cm-3, this translates to a pseudo-first-order rate constant of 1.7 X 10-5 s-1, and a lifetime of N02 with respect to this reaction of about 16 h. 

The 1997 recommendations for the OH + N02 rate constants (DeMore et al., 1997 Atkinson et al., 1997a, 1997b) may be systematically high (e.g., Donahue et al., 1997) at temperatures below 240 K. Thus, recent measurements at temperatures characteristic of the upper troposphere give rate constants that are smaller than the recommendations by 10-30% (Brown et al., 1999a Dransfield et al., 1999). In addition, 02 appears to be only about 70% as efficient a third body as N2 in the termolecular reaction. Using a modified form of the semiempirical equation for the rate constant in the falloff region (Chapter 5, Eq. (C)), which takes into account the variable collision efficiency /3, 

Brown et al. (1999a) recommend k(300 = 2.47 X 10 30 cm6 molecule-2 s-1 with n = 2.97, 300 = 1.45 X 10-11 cm3 molecule-1 s-1 with m = 2.77, and j8(N2) = 1.0, /3(02) = 0.7, and jS(air) = 0.94. The smaller values calculated for lower temperatures using this expression are important since the OH + N02 reaction converts NO, to NO, thus smaller values of this rate constant lead to increased model values of NOr/NOy in the upper troposphere and lower stratosphere. 

This reaction is primarily a daytime reaction because most OH sources are photolytic in nature. As a result, the N02 reaction with OH competes with N02 photolysis, reaction (4). As discussed in Chapter 3, a typical value of the photolysis rate constant for N02 would be kp = k4 = 7 X 10 3 s-1 at a solar zenith angle of 50° (e.g., see Fig. 3.31). Thus, the reaction with OH is not usually a dominant loss process for N02, but it is still sufficiently fast to form significant amounts of HNO, during the day, particularly in polluted regions with relatively large N02 concentrations. 

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Oxidation rate constant k = 4.58 x 10-12 cm3 molecule-1 s-1 for the gas phase reaction with OH radical at room temp. (Ohta Ohyama 1985 Atkinson 1989) ... 

Table 3 shows the atmospheric lifetime for eleven PAH with respect to gas-phase reaction with OH and NO3 radicals, O3 and N2O5. This was calculated from the estimated and calculated rate constants. It is evident that most of the nitroarenes formed under ambient atmospheric conditions were produced by reaction of PAH with OH. The PAH reaction with NO3 radical was also considered as an important step because it resulted in the formation of nitroarenes from the N2O5 reaction with gas-phase PAH. 

TABLE 3. Calculated atmospheric lifetimes of PAH due to gas-phase reactions with OH and NO3 radicals, O3 and N2O547... 

Titanium dioxide suspended in an aqueous solution and irradiated with UV light X = 365 nm) converted benzene to carbon dioxide at a significant rate (Matthews, 1986). Irradiation of benzene in an aqueous solution yields mucondialdehyde. Photolysis of benzene vapor at 1849-2000 A yields ethylene, hydrogen, methane, ethane, toluene, and a polymer resembling cuprene. Other photolysis products reported under different conditions include fulvene, acetylene, substituted trienes (Howard, 1990), phenol, 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2,6-dinitro-phenol, nitrobenzene, formic acid, and peroxyacetyl nitrate (Calvert and Pitts, 1966). Under atmospheric conditions, the gas-phase reaction with OH radicals and nitrogen oxides resulted in the formation of phenol and nitrobenzene (Atkinson, 1990). Schwarz and Wasik (1976) reported a fluorescence quantum yield of 5.3 x 10" for benzene in water. 

Chemical/Physical. Products identified from the reaction of toluene with nitric oxide and OH radicals include benzaldehyde, benzyl alcohol, 3-nitrotoluene, p-methylbenzoquinone, and o, m, and p-cresol (Kenley et ah, 1978). Gaseous toluene reacted with nitrate radicals in purified air forming the following products benzaldehyde, benzyl alcohol, benzyl nitrate, and 2-, 3-, and 4-nitro-toluene (Chiodini et al., 1993). Under atmospheric conditions, the gas-phase reaction with OH radicals and nitrogen oxides resulted in the formation of benzaldehyde, benzyl nitrate, 3-nitrotoluene, and o-, m-, and p-cresol (Finlayson-Pitts and Pitts, 1986 Atkinson, 1990). 

Chemical/Physical. Under atmospheric conditions, the gas-phase reaction with OH radicals and nitrogen oxides resulted in the formation of p-tolualdehyde (Atkinson, 1990). Kanno et al. (1982) studied the aqueous reaction of p-xylene and other aromatic hydrocarbons (benzene, toluene, o-and /n-xylene, and naphthalene) with hypochlorous acid in the presence of ammonium ion. They reported that the aromatic ring was not chlorinated as expected but was cleaved by chloramine forming cyanogen chloride. The amount of cyanogen chloride formed increased at lower pHs (Kanno et al, 1982). Products identified from the OH radical-initiated reaction of p-xylene in the presence of nitrogen dioxide were 3-hexene-2,5-dione, p-tolualdehyde, and 2,5-dimethylphenol (Bethel et al., 2000). 

Air calculated tropospheric lifetime of 8-17 d due to calculated rate constant of gas-phase reaction with OH radical for dichlorobiphenyls (Atkinson 1987) ... 

Air calculated tropospheric lifetime of 8-17 d due to calculated rate constant of gas-phase reaction with OH radicals for dichlorobiphenyls (Atkinson 1987) the tropospheric lifetime of 3.4-7.2 d based on the experimentally determined rate constant for gas-phase reaction with OH radicals for dichlorobiphenyls (Kwok et al. 1995)... 

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