Oxidants in the Troposphere

Although ozone is quite reactive, most photochemical oxidations in the atmosphere (as in surface waters) involve still more reactive free radicals. Free radicals are species that contain an unpaired electron therefore, the reaction of any free radical with a chemical other than another free radical still leaves an electron unpaired. The newly formed free radical may readily react with another chemical, forming yet another free radical, and so on. An example of such a free-radical chain reaction is a combustion flame. A free radical is destroyed only when reaction with another free radical causes the two unpaired electrons to pair with each other. 

Examples of free radicals in the atmosphere include the hydroxyl radical (OFF), hydroperoxy radical (HOp), and alkoxy and alkyl peroxy radicals (RO and R02% respectively, where R is an alkyl group). The most important of these free radicals in the oxidation of atmospheric chemicals is the hydroxyl 

Many reduced gases and vapors released by both human activities and natural ecosystems are eventually oxidized in the atmosphere by free-radical chain reactions initiated by OH. Rate constants for the reaction of OH with several organic vapors are shown in Table 4-12. 

From Table 4-12, the reaction rate of carbon disulfide with OH is 29 X 10 cm3 per molecule per second. Assuming a concentration of OH- in 

2 Production of Photochemical Smog The Ozone-NOx-Hydrocarbon Connection 

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Burns with a blue flame releasing carbon dioxide and sulfur dioxide (Windholz et al, 1983). Oxidizes in the troposphere forming carbonyl sulfide. The atmospheric half-lives of carbon disulfide and carbonyl sulfide were estimated to be approximately 2 yr and 12 d, respectively (Khalil and Rasmussen, 1984). 

Numerous field studies of the rate of S02 oxidation in the troposphere have shown that the oxidation rate depends on a number of parameters. These include the presence of aqueous phase in the form of clouds and fogs, the concentration of oxidants such as H202 and... 

The aldehyde HC(0)CFC12 formed in the oxidation of HCFC-141b is expected to photolyze in the troposphere, forming in part CFHC12 (Wallington et al., 1994a), which is itself oxidized in the troposphere by reaction with OH. By analogy to the photolysis of... 

From Table VIII-2 one can see that this is not the case. The mixing ratios of CO and 02 are only about 0.1% of the amount of C02 that would be produced in only 2 years. Considerable efforts have been devoted to explain this unusual stability of C02 in Mars [see Hunten (491)]. Based on the abundant water vapor and HO (H, HO, H02) in the Martian atmosphere, McElroy et al. (675, 677) present a mechanism involving an HO cycle for the catalytic oxidation of CO to C02 similar to the one proposed for NO oxidation in the troposphere [see Section (VII 1-2.3), p. 333],... 

In the troposphere reaction (2a) is approximately 10 times slower than (2b). However, it is of great importance because excited atomic oxygen (0 D) reaction with water vapor is the major source of hydroxyl radical (OH), the main oxidant in the troposphere. 

The importance of NMHC on tropospheric chemistry is demonstrated since consideration of NMHC oxidation in the troposphere is improving the agreement between model results and observations (Figure 3). 

Grennfelt P, Schjoldager J (1984) Photochemical oxidants in the troposphere a mounting menace. Ambio 13 61-67... 

For the major atmospheric oxide of nitrogen—nitrous oxide—the source is biological activity at the surface, and the sink is transport into the stratosphere, where it is destroyed by photodissociation and reaction with 0( D). There are no important photochemical reactions for nitrous oxide in the troposphere. 

Nitric oxide in the troposphere (ti/2 about Iday) stems from bacteria, lighming and for the main part from emissions of combustion engines. 

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. 

This shorthand version of the mechanism assumes that one odd hydrogen radical (OH) is consumed per SO2 oxidized. This approach allows modellers to simulate SO2 oxidation in the troposphere. 

Logan J.A., Nitrogen oxides in the troposphere Global and regional budgets. Advances in Chemistry, in press (1983). 

I would like to resurrect the question that Dr. Chameides raised about the changing rate of oxidation in the troposphere. That to me is the central issue, and if we cannot partition this point between increasing production and decreasing rate of oxidation, what do you think is the approach that should be taken in order to determine what that spot is. 

Hebestreit K., Honninger G., Stutz J., Alicke B., and Platt U. (2000) Measurements of halogen oxides in the troposphere. Geophys. Res. Abstr. 2, 1061. 

The formation of microbe-generated S species in the global ocean is still uncertain. Some estimates deviate from 6x10 tons/yr. (Lein el al, 1988) up to 48 x 10 tons/yr. (Zehnder et al, 1980) for sulfur release rate from the ocean surface. The uncertainty relates mainly to unknown rates of H2S oxidation in the troposphere that is coming back to the ocean with atmospheric precipitation. 

Pierotti, D., R. A. Rasmussen, and R. Chatfield (1978). Continuous measurements of nitrous oxide in the troposphere. Nature 274, 574-576. 

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. 

Dimethyl sulfide — and possibly also methyl sulfide — is oxidized in the troposphere to sulfuric and methanesulfonic acids, and it has been suggested that these compounds may play a critical role in promoting cloud formation... 

The oxidizing properties of the troposphere have a strong influence on the lifetime of chemical compounds in the atmosphere, and hence on the probability for a molecule to reach the middle atmosphere. Most hydrocarbons, for example, including hydrogenated halocarbons, are efficiently destroyed by the OH radical in the troposphere before they can penetrate into the stratosphere. Compounds that are not oxidized in the troposphere (e.g., chlorofluorocarbons) or weakly oxidized (e.g., methane) reach the stratosphere more easily. 

Figure 9-6 summarizes our current understanding of the chemical reactions involving nitrogen oxides in the troposphere. Photolytically induced... 

There are no sinks for nitrous oxide in the troposphere. Instead, all of it rises eventually in the stratosphere where each molecule absorbs UV light and decomposes, usually to N2 and atomic oxygen (90%) or reacts with atomic oxygen (10%). More details about the role of N2O as ozone-depleting species are contained in Chapter 6. 

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