Methyl radical tropospheric

A low specific rate (large residence time), however, does not mean that this pathway is unimportant. On the contrary, because of the large CH4 concentration in the air, nearly homogeneously distributed in the whole troposphere (Chapter 2.8.2.4), CH4 controls to a large extend the background OH concentration and tropospheric net O3 formation. The methyl radical CH3 rapidly reacts with O2, producing the meth-ylperoxo radical = 1.2 10" cm molecule" s" at 298 K) ... 

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 transformation of arenes in the troposphere has been discussed in detail (Arey 1998). Their destruction can be mediated by reaction with hydroxyl radicals, and from naphthalene a wide range of compounds is produced, including 1- and 2-naphthols, 2-formylcinnamaldehyde, phthalic anhydride, and with less certainty 1,4-naphthoquinone and 2,3-epoxynaphthoquinone. Both 1- and 2-nitronaphthalene were formed through the intervention of NO2 (Bunce et al. 1997). Attention has also been directed to the composition of secondary organic aerosols from the photooxidation of monocyclic aromatic hydrocarbons in the presence of NO (Eorstner et al. 1997) the main products from a range of alkylated aromatics were 2,5-furandione and the 3-methyl and 3-ethyl congeners. 

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. 

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). 

In such models the OH concentration field is computed using measured or estimated concentration fields of the precursor molecules and photon flux data. The resulting OH field is then tuned such that it correctly predicts the lifetime of methyl chloroform (CH3CCI3) with respect to OH radical attack. From measurements of the atmospheric turnover time of CH3CCI3 (4.8 years) [20], its lifetime with respect to loss in the stratosphere (45 years), and its lifetime with respect to loss in the oceans (85 years) the tropospheric lifetime of CH3CCI3 with respect to OH radical attack has been inferred to be 5.7 years [17,21], Methyl chloroform is the calibration molecule of choice because it has a long history of precise atmospheric measurements, it has no natural sources, its industrial production is well documented, and because the kinetics of reaction Eq. 20 are well established, feo = 1.8 x 10-12 exp(- 1500/T) cm3 molecule-1 s-1 [22]. 

Alkyl halides (RX X = Cl, I) are an important source of halogens in the atmosphere. The major tropospheric sinks of these compounds are photolysis (RBr, RI) and reaction with OH radicals. In the case of alkyl iodides (RI) relative kinetic studies of their OH reactions in photoreactors are complicated by fast reactions with the 0( P) atoms generated by the photochemical OH radical sources. Figure 1 below shows a In-ln plot of the kinetic data from an experiment performed in a large photoreactor to determine the OH rate coefficient for the reaction OH + CH3CH2CH2I relative to OH + ethene using the photolysis of methyl nitrite (CH3ONO) as the OH radical source. A recent example of the implementation of the relative kinetic technique for the determination of OH radical rate coefficients in a photoreactor can be found in Olariu et al. (2000). 

An example in which formation of a carbon radical is not the initial reaction is provided by the atmospheric reactions of organic sulfides and disulfides. They also provide an example in which rates of reaction with nitrate radicals exceed those with hydroxyl radicals. 2-dimethylthiopropionic acid is produced by algae and by the marsh grass Spartina alternifolia, and may then be metabolized in sediment slurries under anoxic conditions to dimethyl sulfide (Kiene and Taylor 1988), and by aerobic bacteria to methyl sulfide (Taylor and Gilchrist 1991). It should be added that methyl sulfide can be produced by biological methylation of sulfide itself (HS ) (Section 6.11.4). Dimethyl sulfide — and possibly also methyl sulfide — is oxidized in the troposphere to sulfur dioxide and methanesulfonic acids. 

A likely source of active iodine is provided by the photolysis of methyl iodide (CH3f) and perhaps of other iodocarbons. As the atmospheric lifetime of CH3f is relatively short (a few days), the tropospheric abundance of this compound is generally lower than 10 pptv (Moyers and Duce, 1972 Singh et al., 1983 Atlas et al., 1993) although local maxima are found over the productive regions of the ocean (Oram and Penkett, 1994). If iodine atoms are released above the tropopause, they react with ozone to form the iodine monoxide radical... 

Methyl chloride reacts with OH radicals by hydrogen abstraction in the same way as methane, except that the rate coefficient is about five times greater. From the known rate coefficient and the usual assumption of an average OH number density of 5 x 105 molecules/cm3 in the troposphere, one infers an atmospheric lifetime for CH3C1 of about 1.8 yr. The uniform distribution of methyl chloride in the troposphere would be incompatible with such a relatively short lifetime, if the substance were mainly human-... 

As we have mentioned already, the Cl/ClO cycle is also important in stratosphere. The natural source of atomic chlorine. Cl, is methyl chloride gas, CH3CI, produced at the Earth s surface, mainly in the oceans as a result of the interaction of chloride ions with decaying vegetation. Only a portion of methyl chloride gas is destroyed in the troposphere. When intact molecules of it reach the stratosphere, they react photochemically decomposed by UV-C or attacked by OH radicals (reactions (15) and (16))... 

Under tropospheric conditions, the methyl peroxy radical can react with NO, N02, H02 radicals, and itself the reactions with NO and H02 are the most important. The reaction with NO leads to the methoxy radical, CH30, and N02 ... 

PROBABLE FATE photolysis information is lacking, probably unimportant, appreciable photodissociation may occur in stratosphere, photooxidation half-life in air 61.3-613 days oxidation information lacking, probably unimportant, in troposphere oxidation by hydroxyl radicals for formyl chloride and other products is an important fate hydrolysis slow hydrolysis, unimportant in comparison to volatilization, first-order hydrolytic half-life 292 days at pH 7 volatilization volatilization to the atmosphere is rapid and is a major transport process for removal of methyl chloride, evaporation from water 25°C of 1 ppm solution 50% after 27 min, 90% after 91 min volatilization half-life in a typical river 2-1 hr sorption no data is available, sorption onto sediments and suspended particulates probably unimportant biological processes data is lacking, biodegradation and bioaccumulation are not expected to be important fates... 

PROBABLE FATE photolysis volatilized methyl bromide should photodissociate above the ocean layer, probably not significant in aquatic systems, reaction with photochemi-cally produced hydroxyl radicals has a half-life from 0.29-1.6 yrs, direct photolysis is the dominant fate in the stratosphere, but is not expected to be important in the troposphere oxidation atmospheric photooxidation by hydroxyl radicals releases inorganic bromide which is carried... 

Methyl peroxynitrate, CH3OONO2, thermally dissociates back to the reactants with a lifetime with respect to thermal decomposition of 1 second at room temperature and atmospheric pressure, which increases to 2 days for the temperature and pressure conditions in the upper troposphere (Atkinson et al., 1989 Atkinson, 1990). Methyl peroxynitrate can act as a temporary reservoir of NO2 and CH3O2 radicals in the upper troposphere. 

This class of organic compounds is exemplified by acetone and its higher homologues. As for the aldehydes, photolysis and reaction with the OH radical are the major atmospheric loss processes (Atkinson, 1989). The limited experimental data available indicate that, with the exception of acetone (see Figure 5.11), photolysis is probably of minor importance. Reaction with the OH radical is then the major tropospheric loss process. For example, for methyl ethyl ketone the OH radical can attack any of the three carbon atoms that contain hydrogen atoms ... 

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