Atmospheric reactions radicals

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. 

Air calculated lifetime of 3 d due to reaction with OH radical, assuming an average daytime atmospheric OH radical concn of 1 x 106 molecule/cm3 (Atkinson et al. 1984) ... 

The sulfate and nitrate content of atmospheric particles comes primarily from the conversion of sulfur dioxide and nitrogen dioxide. Photochemi-cally initiated atmospheric reactions and transient free radicals are... 

Photolytic. Grosjean (1997) reported a rate constant of 1.87 x lO " cm /molecule-sec at 298 K for the reaction of 2-ethoxyethanol and OH radicals in the atmosphere. Based on an atmospheric OH radical concentration of 1.0 x 10 molecule/cm , the reported half-life of methanol is 0.35 d (Grosjean, 1997). Stemmier et al. (1996) reported a rate constant of 1.66 x 10 " cm /molecule-sec for the OH radical-initiated oxidation of 2-ethoxyethanol in synthetic air at 297 K and 750 mmHg. Major reaction products identified by GC/MS (with their yields) were ethyl formate, 34% ethylene glycol monoformate, 36% ethylene glycol monoacetate, 7.8% and ethoxyacetaldehyde, 24%. 

Tuazon et al. (1984a) investigated the atmospheric reactions of TV-nitrosodimethylamine and dimethylnitramine in an environmental chamber utilizing in situ long-path Fourier transform infared spectroscopy. They irradiated an ozone-rich atmosphere containing A-nitrosodimethyl-amine. Photolysis products identified include dimethylnitramine, nitromethane, formaldehyde, carbon monoxide, nitrogen dioxide, nitrogen pentoxide, and nitric acid. The rate constants for the reaction of fV-nitrosodimethylamine with OH radicals and ozone relative to methyl ether were 3.0 X 10 and <1 x 10 ° cmVmolecule-sec, respectively. The estimated atmospheric half-life of A-nitrosodimethylamine in the troposphere is approximately 5 min. 

As shown, peroxy radical chemistry plays a substantial role in low-temperature combustion as opposed to the alkoxy radical chemistry of high-temperature combustion. Thus, the peroxy radicals constitute an important class of reactive intermediates with significant implications for low temperature combustion and atmospheric reactions. 

While most studies have focused on the pyrolytic unimolecular decomposition of these monoheteroaromatic compounds, our group has explored their oxidative decomposition. As with benzene, where phenylperoxy radical plays a major role in dictating oxidation pathways, we hypothesize that the peroxy radicals derived from heteroaromatic rings are reactive species of considerable interest for combustion and atmospheric reactions. 

An example of the application of transition state theory to atmospheric reactions is the reaction of OH with CO. As discussed earlier, this reaction is now believed to proceed by the formation of a radical adduct HOCO, which can decompose back to reactants or go on to form the products H + COz. For complex reactions such as this, transition state theory can be applied to the individual reaction steps, that is, to the steps shown in reaction (15). Figure 5.3 shows schematically the potential energy surface proposed for this reaction (Mozurkewich et al., 1984). The adduct HOCO, corresponding to a well on the potential energy surface, can either decompose back to reactants via the transition state shown as HOCO./ or form products via transition state HOCO,/. ... 

In addition, in the nighttime atmosphere, reaction of the R02 radicals with N03 may occur as discussed earlier. As a result, the products of the isoprene-N03 reaction in the atmosphere will depend on the concentrations of NO, N03, H02, and R02. 

N02 has also been shown to react with 1,1-dimethyl-hydrazine in air, forming HONO and tetramethyltetra-zine-2, (CH3)2NN=NN(CH3)2 (Tuazon et al., 1983b). The reaction is also proposed to involve abstraction of a hydrogen from the weak N-H bond by N02, forming HONO. The tetramethyltetrazine-2 is hypothesized to be formed by the addition of N02 to the (CH3)2NNH radical, followed by decomposition to (CH3)2N2 + HONO and the self-recombination of the (CH3)2N2 radicals (Tuazon et al., 1982). The apparent overall rate constant for the reaction was 2.3 X 10 17 cm3 molecule-1 s-1 so that the lifetime of 1,1-dimethylhy-drazine at an N02 concentration of 0.1 ppm would be 5 h. Since the lifetimes with respect to 0.1 ppm 03 or 1 X 106 OH radicals cm-3 are 7 min and 6 h, respectively, the reaction of N02 can contribute to the atmospheric reactions of the hydrazine only at low 03 levels. 

Atkinson, R., Atmospheric Reactions of Alkoxy and /3-Hydroxy-alkoxy Radicals, Ini. J. Chem. Kinet., 29, 99-111 (1997b). 

Finally, while several volatile and semivolatile PAHs, e.g., naphthalene, the methylnaphthalenes, phenan-threne, pyrene, and fluoranthene, are not significant mutagens or carcinogens (hence not included in Table 10.13), they are precursors to powerful direct bacterial mutagens formed in gas-phase atmospheric reactions with hydroxyl during the day and nitrate radicals at night (see Section F). Furthermore, 2-nitrofluoranthene,... 

FIGURE 10.22 Direct mutagenicity of ambient particles (mutagen density, rev m-3, TA98, —S9) as a function of ambient concentrations of 2-nitropyrene, a directly mutagenic product of a gas-phase atmospheric reaction initiated by OH radical attack on pyrene. Samples collected at six sites in California with different types of emissions ( ) Glendora (O) Yuba City ( ) Concord ( ) Mammoth Lakes ( a ) Oildale ( ) Reseda (see Fig. 10.22) (adapted from Atkinson et al., 1988a). 

Reiner, T., M. Hanke, and F. Arnold, Atmospheric Peroxy Radical Measurements by Ion Molecule Reaction-Mass Spectrometry A Novel Analytical Method Using Amplifying Chemical Conversion to Sulfuric Acid, J. Geophys. Res., 102, 1311-1326 (1997). 

Niki, H., J. Chen, and V. Young, Long Path-FTIR Studies of Some Atmospheric Reactions Involving CF,00 and CF,0 Radicals, Res. Chem. Interned., 20, 277-301 (1994). 

Most of the reactions the inorganic chemist encounters in the laboratory involve ionic species such as the reactants and products in the reactions just discussed or those of coordination compounds (Chapter 13). However, in the atmosphere there are many free radical reactions initiated by sunlight. One of the most important and controversial sets of atmospheric reactions at present is that concerning stratospheric ozone. The importance of ozone and the effect of ultraviolet (UV) radiation on life has been much discussed. Here we note briefly that only a small portion of the sun s spectrum reaches the surface of the earth and that parts of the UV portion that are largely screened can cause various ill effects to living systems. 

Two examples serve to illustrate these photochemical relationships. One member of the family of atmospheric peroxy radicals is the peroxy acetyl radical, CH3C(0)02. In at least the warm portion of the troposphere, PAN is near thermal equilibrium with the peroxy acetyl radical and NO . The equilibrium constant for this reaction has been measured in laboratory studies. Therefore, if concentrations of both PAN and N02 are measured, the concentration of these radicals can be calculated from the equilibrium constant and the ratio of the two nitrogen species as shown in Figure 2. The... 

Many of the atmospheric reactions leading to the formation of HO and HOO radicals have been adopted for laboratory FTIR-based studies. Namely, HO radicals are produced photochemically in the atmosphere from 03 in the presence of water vapor, and also from atmospheric products such... 

In the atmospheric free radical reactions involving hydrocarbon species, molecular products of interest are formed via either radical chain propagation or termination steps. 

Ethers are susceptible to attack by halogen atoms and radicals, and for this reason they are not good solvents for radical reactions. In fact, ethers are potentially hazardous chemicals, because in the presence of atmospheric oxygen radical-chain formation of peroxides occurs, and peroxides are unstable, explosion-prone compounds. This process is called autoxidation and occurs not only with ethers but with many aldehydes and hydrocarbons. The reaction may be generalized in terms of the following steps involving initiation... 

Peroxy radicals are intermediates in the atmospheric oxidation of air pollutants and in oxidation reactions at moderate temperatures. They are rapidly formed from free radicals by addition of 02. Free radicals in the atmosphere are quantitatively converted to R02 with a half-time of about 1 fis. The peroxy radicals are then removed by reaction with other trace species. The dominant pathways are reactions with NO and NOz. Only a few peroxy radicals have been detected with a mass spectrometer, and extensive research on these reactions has been done by UV absorption spectroscopy with the well-known and conveniently accessed band in the 200- to 300-nm region. Nevertheless, FPTRMS has been used for some peroxy radical kinetics investigations. These have usually made use of the mass spectrometer to observe more than one species, and have given information on product channels. The FPTRMS work has been exclusively on atmospheric reactions of chlorofluoromethanes and replacements for the chlorofluoromethanes. 

These data and the resulting mechanism are consistent with earlier observations of smog chamber processes conducted in the presence and absence of added S02, as discussed in references (2 61. In the atmosphere, reaction (4) is expected to be the primary fate of SO3 due to the presence of large quantities of water and the presumed lack of other important reactions for SO3. Under atmospheric conditions, this mechanism suggests that HO radicals are effectively converted to HO2 radicals, if reaction (3) is sufficiently fast and there... 

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