Nitrate Radical NO

While these reactions are much slower than the corresponding OH reactions, the nighttime peak concentrations of NO, under some conditions are much larger than those of OH during the day, 400 ppt vs 0.4 ppt. Even given the differences in concentration, however, as seen from the lifetimes in Table 6.1, the nitrate radical reaction is still relatively slow. While the removal of the alkanes by NO, is thus not expected to be very significant under most tropospheric conditions, reaction (20) can contribute to HNO, formation and the removal of NOx from the atmosphere. 

The rate constants for the simple alkanes can be empirically fit by assigning rate constants to the primary and secondary C-H groups, along with sub- 

TABLE 6.3 Rate Constants at Room Temperature for Reaction of NO, Radicals with Alkanes  

However, this approach is not accurate for branched chain alkanes (Atkinson, 1997). 

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Finally, the oxidation of nitrate anions leads to nitrate radicals (NOs ) that add to olefins (Scheme 14) [37]. These oxidations were carried out at a platinum anode using constant current conditions, an undivided cell, a mixed MeCN H2O Et20 solvent system, and LiN03 as the electrolyte. The initial oxidation led to a nitrate product that was not stable and hence... 

N02 also reacts with O, forming the nitrate radical, NO, ... 

The nitrate radical NO, formed in reaction 4.23 is, during daytime, rapidly photoly/.ed (at a rate of about 0.2 s ). There are two channels for photolysis (see Table 3.3),... 

Secondary organic aerosol material is formed in the atmosphere by the mass transfer to the aerosol phase of low vapor pressure products of the oxidation of organic gases. As organic gases are oxidized in the gas phase by species such as the hydroxyl radical (OH), ozone (O3), and the nitrate radical (NO 3), their oxidation products accumulate. Some of these products have low volatilities and condense on the available particles in an effort to establish equilibrium between the gas and aerosol phases. 

These alternate mechanisms are exemplified by the oxidation of propylene in acetic acid. With lithium perchlorate as the supporting electrolyte, direct electron transfer from the alkene occurs to form an allylic carbenium ion, e.g., mechanism (b) above. However, in the presence of lithium nitrate, oxidation of the nitrate ion has been shown to occur preferentially and the resulting nitrate radical, NO 3, reacts with the alkene, i.e., mechanism (c). 

Nitrate radicals are formed from NO that is produced during combustion processes and these are significant only during the night in the absence of photochemically produced OH radicals. They are formed by the reactions... 

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. 

The formation of the Wheland intermediate from the ion-radical pair as the critical reactive intermediate is common in both nitration and nitrosation processes. However, the contrasting reactivity trend in various nitrosation reactions with NO + (as well as the observation of substantial kinetic deuterium isotope effects) is ascribed to a rate-limiting deprotonation of the reversibly formed Wheland intermediate. In the case of aromatic nitration with NO, deprotonation is fast and occurs with no kinetic (deuterium) isotope effect. However, the nitrosoarenes (unlike their nitro counterparts) are excellent electron donors as judged by their low oxidation potentials as compared to parent arene.246 As a result, nitrosoarenes are also much better Bronsted bases249 than the corresponding nitro derivatives, and this marked distinction readily accounts for the large differentiation in the deprotonation rates of their respective conjugate acids (i.e., Wheland intermediates). 

Proof of Constitution.—Reference to Table XII will show that all of the methods mentioned above as being suitable for the synthesis of 4,6-dimethylglucose have been correlated by means of crystalline derivatives. If the tosyl, benzyl and nitrate radicals can be regarded as being non-migratory (and there is no evidence to the contrary) then the course of the synthesis from 4,6-acetals (the constitution of which has been proved by their conversion into 2,3-dimethyl-D-glucose) leaves no doubt that the methyl groups occupy positions 4 and 6. 

For example, the nitrate radical, which plays an important role in nighttime chemistry (see Chapter 6), absorbs light in the red region of the visible (600-700 nm). The electronically excited state formed on light absorption can dissociate into either N02 + O or into NO + 02, or it can fluoresce ... 

Alkylperoxy radicals (R02) in the atmosphere react primarily with NO, H02, and other R02. Reaction with the nitrate radical, N03, at night, has been recently recognized as being important as well. A less important reaction is that with N02. There are two excellent reviews of R02 chemistry by Lightfoot et al. (1992) and Wallington et al. (1992), including the thermochemistry, spectroscopy, kinetics, and mechanisms. The reader should consult these for references to the original work in this area. 

As seen in Table 6.1, the reactions of the nitrate radical with the simple aromatic hydrocarbons are generally too slow to be important in the tropospheric decay of the organic. However, one of the products of the aromatic reactions, the cresols, reacts quite rapidly with NO,. o-Cresol, for example, reacts with N03 with a room temperature rate constant of 1.4 X 10 " cm3 molecule-1 s-1, giving a lifetime for the cresol of only 1 min at 50 ppt N03. This rapid reaction is effectively an overall hydrogen abstraction from the pheno-... 

Because of the rapid photolysis of NO, during the day (see Chapter 4.G), competing reactions of NO, are important primarily at night. Some of the most important dark reactions are discussed in the following sections. For detailed treatments of nitrate radical chemistry, the reader should consult the extensive reviews by Wayne et al. (1991) and Atkinson (1991). 

FIGURE 7.12 Comparison of measured (Platt et al., 1984) and predicted nitrate radical concentrations at Edwards Air Force Base, California, May 23-24, 1982. The dashed line is the model prediction without the unimolecular decomposition of NO, and the solid line is that with the decomposition (adapted from Russell et at., 1986). 

In the radiolysis of water-alkaline glasses containing nitrate ion, NO , the anion radical NO - is formed. This possesses strong electron-donor... 

As Barr et al. (2003) pointed out, the importance of such emissions is determined mainly by their impact on the three processes taking place in the atmosphere. The first consists in that such NMHCs as isoprene form in the course of carboxylization in plants and contribute much thereby to the formation of biospheric carbon cycle. The second process is connected with NMHCs exhibiting high chemical activity with respect to such main oxidants as hydroxyl radicals (OH), ozone (03), and nitrate radicals (N03). Reactions with the participation of such components result in the formation of radicals of alkylperoxides (R02), which favor efficient transformation of nitrogen monoxide (NO) into nitrogen dioxide (N02), which favors an increase of ozone concentration in the ABL. Finally, NMHC oxidation leads to the formation of such carbonyl compounds as formaldehyde (HCHO), which stimulates the processes of 03 formation. Finally, the oxidation of monoterpenes and sesquiterpenes results in the intensive formation of fine carbon aerosol with a particle diameter of <0.4 pm... 

The two processes likely to remove dinitrocresols from the atmosphere are reactions with hydroxyl and nitrate radicals (Atkinson et al. 1992). No experimental kinetic data are available for these two reactions (Grosjean 1991). The rate constant for the gas phase reaction of dinitrocresols with OH radicals is 3.0x10 cm /molecule-second (Grosjean 1991). Using the method of Atkinson (1988), the estimated rate constant for this reaction is 2.1x10 cm /molecule-second. Based on an average ambient atmospheric concentration of OH radicals in the northern hemisphere of 5x10 radicals/cm... 

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