Tropospheric Oxidation Chemistry

Though atmospheric composition is dominated by both oxygen and nitrogen, it is not the amount of oxygen that defines the capacity of the troposphere to oxidise a trace gas. The oxidising capacity of the troposphere is a somewhat nebulous term probably best described by Thompson. 

Atmospheric photochemistry produces a variety of radicals that exert a substantial influence on the ultimate composition of the atmosphere. Probably the most important of these in terms of its reactivity is the hydroxyl radical, OH. The formation of OH is the initiator of radical-chain oxidation. Photolysis of ozone by UV light in the presence of water vapour is the main source of hydroxyl radicals in the troposphere, viz 

Greenhouse gases Primary Pollutants Natural Biogenic Species 

Reflective Free Radical/Atom Less Reactive Radicals Reflective Aerosols 

The fate of the bulk of the 0( D) atoms produced via reaction (2.7) is collisional quenching back to ground-state oxygen atoms, viz 

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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. 

Sequential Oxidation Products from Tropospheric Isoprene Chemistry MACR and MPAN at a NOt-Rich Forest Environment in the Southeastern United States, J. Geophys. Res., 103, 22463-22471 (1998). 

Nouaime, G S. B. Bertman, C. Seaver, D. Elyea, H. Huang, P. B. Shepson, T. K. Starn, D. D. Riemer, R. G. Zika, and K. Olszyna, Sequential Oxidation Products from Tropospheric Isoprene Chemistry MACR and MPAN at a NO,-Rich Forest Environment in the Southeastern United States, J. Geophys. Res., 103, 22463-22471 (1998). 

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... 

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... 

We have also studied the behavior of gas-phase radicals, such as the hy-droperoxyl radical (HO2) [62], in water clusters which is important in atmospheric science (Figure 16.4). The hydroperoxyl radical is a major species in the HOx chemical family [2] that affects the budgeting of many chemical systems in the atmosphere. The HOx system plays a central role (along with the OH radical) in oxidative chemistry in the troposphere and ultimately controls the production rate of tropospheric ozone [7,16]. It is hence considered significant in atmospheric [2,5] and combustion chemistry [184]. Recent theoretical studies [16,17] have indicated the HO2 radical to possess stable interactions with water clusters. Such stability provides an important sink for HOx compounds [16,17,61,185] in the troposphere. As a result, the structural and dynamical features of water clusters play a vital role on HO2 related chemistry. 

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). 

Platt, U. (1995) The chemistry of halogen compounds in the Arctic troposphere, in Tropospheric Oxidation Mechanisms, K. H. Becker, ed., European Commission, Report EUR 16171 EN, Luxembourg, pp. 9-20. 

Nitric oxide (NO) plays a central role in atmospheric chemistry, influencing both ozone cycling and the tropospheric oxidation capacity through reactions with hydroperoxy- and organic peroxy-radicals. When the NO concentration exceeds 40 pptv (pptv = parts per trillion by volume) it catalyzes the production of ozone (O3) ... 

It has become increasingly apparent that omission of biogenic species from tropospheric models leads to incorrect evaluation of photo-oxidant chemistry, particularly in rural areas. LACTOZ has provided substantial new data, largely on... 

Although the specific and significant impact of aromatics upon tropospheric chemistry in urban areas is well documented [1], the exact mechanism of their tropospheric oxidation remained largely uncertain at the beginning of LACTOZ. Taking toluene as an example, the first reaction steps are as represented on the scheme of Fig. 9 [2, 3]. 

Section 18.4 In the troposphere the chemistry of trace atmospheric components is of major importance. Many of these minor components are pollutants. Sulfur dioxide is one of the more noxious and prevalent examples. It is oxidized in air to form sulfur trioxide, which, upon dissolving in water, forms sulfuric acid. The oxides of sulfur are major contributors to acid rain. One method of preventing... 

Although we wish to preserve the ozone in the stratosphere, we want to minimize its production in the troposphere, that part of the atmosphere where we live. Photochemical smog produced as a result of the action of solar radiation on the effluents from automobiles is the primary source of ozone in the troposphere. The trigger for photochemical smog is based on nitrogen oxide chemistry, so we defer our discussion of this problem until Chapter 16 on Group 5A chemistry. 

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