Oxidation tropospheric

Oxidant Formation. The role of HO. in controlling the time-scale and severity of tropospheric oxidant pollution may be seen from the parameterization of O Brien and co-workers (75,76). The simplest possible mechanism for oxidant (Le. ozone, PAN, H2O2, etc.) formation consists simply of the reaction of an individual NNlHCj with HO. to convert the NMHCj to a generic product(s) PRODj, followed by removal of the product by HO. (PROD photolysis may be important, but is ignored here)... 

Methane sulfonic acid is produced by tropospheric oxidation of methyl sulfides, and there are naturally occurring sulfonates including derivatives of taurine and of glucose-6-sulfonate (sulfoquinovose),... 

Fritz B, Lorenz K, Steinert W, et al. 1982. Laboratory kinetic investigation of the tropospheric oxidation of selected industrial emissions. In Comm. Eur. Communities, Eur 7624. Phys Chem Behav Atmos Pollut, 192-202. 

Finlayson-Pitts, B. J., Chlorine Atoms as a Potential Tropospheric Oxidant in the Marine Boundary Layer, Res. Chem. Interned., 19, 235-249 (1993). 

As we shall see in the following sections, these observations are readily understood in terms of the kinetics and mechanisms of oxidation of S02. The oxidation of S02 occurs in solution and on the surfaces of solids as well as in the gas phase. Indeed, under many conditions typical of the troposphere, oxidation in the aqueous phase provided by clouds and fogs predominates, consistent with the observed dependence on these factors. The presence of oxidizers to react with the S02 is, of course, also a requirement hence the dependence on 03 (which is a useful surrogate for other oxidants as well) and sunlight, which is needed to generate significant oxidant concentrations. 

Barnes, I., K. H. Becker, and I. Patroescu, The Tropospheric Oxidation of Dimethyl Sulfide A New Source of Carbonyl Sulfide, Geophys. Res. Lett., 21, 2389-2392 (1994b). 

TABLE 13.7 Typical Organic Products of the Tropospheric Oxidation of Some of the CFC Replacement Compounds... 

Mathur, R., K. L. Schere, and A. Nathan, Dependencies and Sensitivity of Tropospheric Oxidants to Precursor Concentrations over the Northeast United States A Model Study, J. Geophys. Res., 99, 10535-10552 (1994). 

The importance of photochemical destruction in the 03s tropospheric budget implies that the lifetime of 03s is coupled to the chemical production and destruction of 03. Consequently, the simulated tropospheric budget of 03s may be affected directly by differences in the simulated chemistry. For example, simulations with a pre-industrial and a present-day emission scenario or with and without representation of NMHC chemistry will produce different estimates of the tropospheric oxidation efficiencies [39, 40]. However, our simulations indicate only small effects on the calculated 03s budget [6]. Figure 5 presents the simulated zonal distribution of 03s, the chemical destruction rate, of ozone (day"1) and the chemical loss of 03s (ppbv day 1) for the climatological April. The bulk of the 03s in the troposphere resides immediately below the tropopause, whereas the ozone chemical destruction rate maximizes in the tropical lower troposphere (Figures 5a and 5b). Hence, most 03s is photochemically destroyed between 15-25 °N and below 500 hPa. This region... 

Thompson. A. M., R. W. Stewart, M. A. Owens, and J. A. Herweke, 1989 Sensitivity of tropospheric oxidents to global chemical and climate changes. Atmos. Environ., 23,519-532. 

Many of HCFC or HFC alternatives to CFCs are two-carbon compounds, and are therefore haloethanes of general formula CX3CX2H, where each X may be any of H, F or Cl. These compounds should be susceptible to tropospheric degradation by hydroxyl-mediated abstraction of the hydrogen atom which each one contains, and should not reach the stratosphere. This strategy is effective if the tropospheric lifetime of the HFC or HCFC is short relative to its rate of transport into the stratosphere. Obviously it is important to consider whether tropospheric oxidation of these compounds may lead to the production of any other halogenated species which might themselves be transported into the stratosphere. 

Tropospheric oxidation of the so-called reduced sulfur , mostly dimethylsul-fide, hydrogen sulfide, and dimethyldisulfide, is thought to proceed via several... 

As both reactions (2.37) and (2.19) lead to the oxidation of NO to NO2 the subsequent photolysis leads to the formation of ozone (see reactions (2.2) and (2.20)). The individual reaction mechanism depends on the identity of the organic compounds and the level of complexity of the mechanism. Although OH is the main tropospheric oxidation initiator, reaction with NO3, O3, 0( P) or photolysis may be an important loss route for some NMHCs or the partially oxygenated products produced as intermediates in the oxidation (see reaction (2.38)). 

Though photochemistry does not take place at night, it is important to note, within the context of tropospheric oxidation chemistry, the potential for oxidation chemistry to continue at night. This chemistry does not lead to the production of ozone, in fact the opposite, but has importance owing to the potential for the production of secondary pollutants. In the troposphere, the main night-time oxidant is thought to be the nitrate radical formed by the relatively slow oxidation of NO2 by O3, viz. 

Dhar and Ram (50) found formaldehyde in rain water and estimated a tropospheric mixing ratio of 0.7 ppb, while Cauer (36) measured a mean value of 0.4 ppb. Lodge and Pate (160) obtained an average value of 1.1 ppb for the total aliphatic aldehydes in surface air in the tropics. Levy (152) proposed the formation of formaldehyde via the tropospheric oxidation of methane and calculated (155) an upper limit of 1 ppb for the mixing ratio, with an altitude profile for a summer midlatitude decreasing from 0.6 ppb at the ground to less than 0.01 ppb in the upper troposphere, where methane oxidation is very slow (154). 

The primary tropospheric oxidants are OH, O3, and NO3, with "OH and O3 reactions with hydrocarbons dominating primarily during daytime hours, and NO3 reactions dominating at night. Rate constants for the reactions of many different aromatic compounds with each of the aforementioned oxidants have been determined through laboratory experiments [16]. The rate constant data as well as atmospheric lifetimes for the reactions of toluene, m-xylene, p-xylene, m-ethyl-toluene, and 1,2,4-trimethylbenzene appear in Table 14.1. Only these particular aromatic compoimds will be discussed in this review paper, since much of the computational chemistry efforts have focused on these compounds. When considering typical atmospheric concentrations of the major atmospheric oxidants, OH, O3, and NO3 of 1.5 x 10, 7 x 10, and 4.8 x 10, molecules cm , respectively [17], combined with the rate constants, it is clear that the major atmospheric loss process for these selected aromatic compounds is reaction with the hydroxyl... 

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). 

Gramatica, P Pilutti, P. and Papa, E. (2004a) A tool for the assessment of VOC degradability by tropospheric oxidants starting from chemical structure. Atmos. Envir., 38, 6167-6175. 

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