Carbon monoxide, tropospheric radical

Photolysis of an aqueous solution containing chloroform (314 pmol) and the catalyst [Pt(cohoid)/Ru(bpy) /MV/EDTA] yielded the following products after 15 h (mol detected) chloride ions (852), methane (265), ethylene (0.05), ethane (0.52), and unreacted chloroform (10.5) (Tan and Wang, 1987). In the troposphere, photolysis of chloroform via OH radicals may yield formyl chloride, carbon monoxide, hydrogen chloride, and phosgene as the principal products (Spence et al., 1976). Phosgene is hydrolyzed readily to hydrogen chloride and carbon dioxide (Morrison and Boyd, 1971). 

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. 

Photolytic. Irradiation of vinyl chloride in the presence of nitrogen dioxide for 160 min produced formic acid, HCl, carbon monoxide, formaldehyde, ozone, and trace amounts of formyl chloride and nitric acid. In the presence of ozone, however, vinyl chloride photooxidized to carbon monoxide, formaldehyde, formic acid, and small amounts of HCl (Gay et al, 1976). Reported photooxidation products in the troposphere include hydrogen chloride and/or formyl chloride (U.S. EPA, 1985). In the presence of moisture, formyl chloride will decompose to carbon monoxide and HCl (Morrison and Boyd, 1971). Vinyl chloride reacts rapidly with OH radicals in the atmosphere. Based on a reaction rate of 6.6 x lO" cmVmolecule-sec, the estimated half-life for this reaction at 299 K is 1.5 d (Perry et al., 1977). Vinyl chloride reacts also with ozone and NO3 in the gas-phase. Sanhueza et al. (1976) reported a rate constant of 6.5 x 10 cmVmolecule-sec for the reaction with OH radicals in air at 295 K. Atkinson et al. (1988) reported a rate constant of 4.45 X 10cmVmolecule-sec for the reaction with NO3 radicals in air at 298 K. 

The temperature and density structure of the troposphere, along with the concentrations of major constituents, are well documented and altitude profiles have been measured over a wide range of seasons and latitudes for the minor species water, carbon dioxide, and ozone. A few profiles are available for carbon monoxide, nitrous oxide, methane, and molecular hydrogen, while only surface or low-altitude measurements have been made for nitric oxide, nitrogen dioxide, ammonia, sulfur dioxide, hydrogen sulfide, and nonmethane hydrocarbons. No direct measurements of nitric acid and formaldehyde are available, though indirect information does exist. The concentrations of a number of other important species, such as peroxides and oxy and peroxy radicals, have never been determined. Therefore, while considerable information concerning trace constituent concentrations is available, the picture is far from complete. 

On the basis of ratios of C and C present in carbon dioxide, Weinstock (250) estimated a carbon monoxide lifetime of 0.1 year. This was more than an order of magnitude less than previous estimates of Bates and Witherspoon (12) and Robinson and Robbins (214), which were based on calculations of the anthropogenic source of carbon monoxide. Weinstock (250) suggested that if a sufficient concentration of hydroxyl radical were available, the oxidation of carbon monoxide by hydroxyl radical, first proposed by Bates and Witherspoon (12) for the stratosphere, would provide the rapid loss mechanism for carbon monoxide that appeared necessary. By extension of previous stratospheric models of Hunt (104), Leovy (150), Nicolet (180), and others, Levy (152) demonstrated that a large source of hydroxyl radical, the oxidation of water by metastable atomic oxygen, which was itself produced by the photolysis of ozone, existed in the troposphere and that a chain reaction involving the hydroxyl and hydroperoxyl radicals would rapidly oxidize both carbon monoxide and methane. It was then pointed out that all the loss paths for the formaldehyde produced in the methane oxidation led to the production of carbon monoxide [McConnell, McElroy, and Wofsy (171) and Levy (153)1-Similar chain mechanisms were shown to provide tropospheric... 

There are over 70 alcohols in the atmosphere as a result of biogenic and anthropogenic emissions [67]. For example methanol and ethanol [68-70] have been used as fuels additives to reduce automobile emissions of carbon monoxide and hydrocarbons [71], in particular ethanol has been used in Brazil as a fuel for over 20 years [72]. 1-Propanol is widely used as a solvent in the manufacturing of different electronic components. The high volatility of these compounds causes their relative abundance in the troposphere and makes it relevant to determine their degradation pathways. During daytime the major loss process for alcohols is their reaction with OH radicals [68]. Accordingly, several experimental [69,70,73-84] and theoretical [85-88] kinetic studies of alcohols -F OH reactions have been performed. 

Carbon monoxide is oxidized in the troposphere ((133) and (134)). With a high concentration of nitric oxide in the troposphere, reactions (135) and (136) take place. This sequence is a formation of ozone catalyzed by nitric oxide. If the nitric oxide concentration is too low, the perhydryl radicals decompose ozone to form hydroxyl radicals (136). Ozone and peroxyacylnitrates PAN are the major toxins of smog. Peroxyacylnitrates are formed from aldehydes in a reaction catalyzed by nitric oxide. 

The hydroxyl radical so produced is the major oxidising species in the troposphere, and a complete picture of its chemistry holds the key to furthering progress in understanding tropospheric chemistry. The chemistry discussed in detail elsewhere, is of course very complex. To take, for example, the cycle of reactions with carbon monoxide, which may be net producers or destroyers of tropospheric ozone depending upon the concentration of oxides of nitrogen present. In the presence of NO, the cycle (16)-(20) occurs, without loss of OH or NO, whereas at low NO concentrations, the cycle (17), (18) and (21), again without loss of OH. 

Methane is oxidized primarily in the troposphere by reactions involving the hydroxyl radical (OH). Methane is the most abundant hydrocarbon species in the atmosphere, and its oxidation affects atmospheric levels of other important reactive species, including formaldehyde (CH2O), carbon monoxide (CO), and ozone (O3) (Wuebbles and Hayhoe, 2002). The chemistry of these reactions is well known, and the rate of atmospheric CH4 oxidation can be calculated from the temperature and concentrations of the reactants, primarily CH4 and OH (Prinn et al., 1987). Tropospheric OH concentrations are difficult to measure directly, but they are reasonably well constrained by observations of other reactive trace gases (Thompson, 1992 Martinerie et al., 1995 Prinn et al., 1995 Prinn et al., 2001). Thus, rates of tropospheric CH4 oxidation can be estimated from knowledge of atmospheric CH4 concentrations. And because tropospheric oxidation is the primary process by which CH4 is removed from the atmosphere, the estimated rate of CH4 oxidation provides a basis for approximating the total rate of supply of CH4 to the atmosphere from aU sources at steady state (see Section 8.09.2.2) (Cicerone and Oremland, 1988). 

Chlorine atoms are thus released and are available to attack the environmental ozone. The free radical, [CCIF j] , reacts with dioxygen to release [CIO] and form COF j. Further dissociation of COFj results in the generation of carbon monoxide, and fluorine atoms, which are ultimately precipitated as HF in the troposphere [36b],... 

Carbon monoxide (CO) is also formed in aquatic environments from the photochemical degradation of DOM [3,4,8,22,94-105]. Strong gradients of CO have been observed in the lowest 10 metres of the atmosphere over the Atlantic Ocean [97]. The samples nearest the ocean surface were some 50 ppb higher than at the 10-metre altitude-sampling inlet. This implies that the ocean is a source of CO to the atmosphere and that this source can increase the atmospheric concentration. CO is reactive in the troposphere and thus its emissions from the ocean may influence the hydroxyl radical (OH) and ozone concentrations in the marine atmospheric boundary layer that is remote from strong continental influences. 

PCBTF received an exemption from VOC regulations based on the fact that its atmospheric hydroxyl radical reaction rate is slower than that of ethane [25]. A VOC is defined as any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic add, metallic carbides or carbonates, and ammoniiun carbonate, which partidpates in atmospheric photochemical reactions [26]. A VOC exemption petition for BTF was filed with the EPA on March 11,1997. Volatile organic compounds (VOCs) emission is controlled by regulation in efforts to reduce the tropospheric air concentrations of ozone. 

It is important to note that certain cycles can lead to ozone formation this phenomenon is readily observed in the troposphere (see Box 5.4), which is rich in anthropogenic hydrocarbons and nitrogen oxides. In the free troposphere and lower stratosphere, the conversion of NO to NO2 by peroxy radicals (HO2 and CH3O2) produced by the oxidation of methane and carbon monoxide, followed by the photodissociation of NO2 leads to the formation of O3. The complete chains are the following (Crutzen, 1974) ... 

A fairly general treatment of trace gases in the troposphere is based on the concept of the tropospheric reservoir introduced in Section 1.6. The abundance of most trace gases in the troposphere is determined by a balance between the supply of material to the atmosphere (sources) and its removal via chemical and biochemical transformation processes (sinks). The concept of a tropospheric reservoir with well-delineated boundaries then defines the mass content of any specific substance in, its mass flux through, and its residence time in the reservoir. For quantitative considerations it is necessary to identify the most important production and removal processes, to determine the associated yields, and to set up a detailed account of sources versus sinks. In the present chapter, these concepts are applied to the trace gases methane, carbon monoxide, and hydrogen. Initially, it will be useful to discuss a steady-state reservoir model and the importance of tropospheric OH radicals in the oxidation of methane and many other trace gases. 

The OH radicals play an important role not only in the removal but also in the formation of CO, so that their content in the troposphere may affect the total amount of the atmospheric carbon monoxide. They react with methane and with organic radicals, forming CO ... 

Hydrojg l radical is most frequently removed from die troposphere by reaction with methane or carbon monoxide ... 

Other trace gases such as carbon monoxide (CO) strongly affect the tropospheric concentrations of the highly reactive hydroxyl (OH) radical through the reaction... 

For photolyses at wavelengths available in tropospheric sunlight (A. >290 nm), primary photodecomposition modes (I) and (II) dominate the products are carbon monoxide and a methylene radical (CH2) formed in either the singlet state, d A, or the triplet state, X Bi. In this discussion, the singlet and triplet states are designated as CH2, and CH2, respectively. 

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