Tropospheric oxidation, examples

CFCs are the most important, hut by no means the only, chemicals capable of destroying ozone molecules. For many years, researchers have recognized that oxides of nitrogen have the capacity both to increase and to decrease the concentration of ozone in the stratosphere. They can increase ozone concentrations in the presence of ultraviolet (uv) radiation by undergoing uv-mediated reactions similar to those that occur in the lower troposphere. For example ... 

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... 

Spatial scales characteristic of various atmospheric chemical phenomena are given in Table 1.1. Many of the phenomena in Table 1.1 overlap for example, there is more or less of a continuum between (1) urban and regional air pollution, (2) the aerosol haze associated with regional air pollution and aerosol-climate interactions, (3) greenhouse gas increases and stratospheric ozone depletion, and (4) tropospheric oxidative capacity and stratospheric ozone depletion. The lifetime of a species is the average time that a molecule of that species resides in the atmosphere before removal (chemical transformation to another species counts as removal). Atmospheric lifetimes vary from less than a second for... 

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]. 

Air pollution (qv) problems are characteri2ed by their scale and the types of pollutants involved. Pollutants are classified as being either primary, that is emitted direcdy, or secondary, ie, formed in the atmosphere through chemical or physical processes. Examples of primary pollutants are carbon monoxide [630-08-0] (qv), CO, lead [7439-92-1] (qv), Pb, chlorofluorocarbons, and many toxic compounds. Notable secondary pollutants include o2one [10028-15-6] (qv), O, which is formed in the troposphere by reactions of nitrogen oxides (NO ) and reactive organic gases (ROG), and sulfuric and nitric acids. 

Air pollution in cities can be considered to have three components sources, transport and transformations in the troposphere, and receptors. The sources are processes, devices, or activities that emits airborne substances. When the substances are released, they are transported through the atmosphere, and are transformed into different substances. Air pollutants that are emitted directly to the atmosphere are called primary pollutants. Pollutants that are formed in the atmosphere as a result of transformations are called secondary pollutants. The reactants that undergo the transformation are referred to as precursors. An example of a secondary pollutant is troposphere ozone, O3, and its precursors are nitrogen oxides (NO = NO + NO2) and non-methane hydrocarbons, NMHC. The receptors are the person, animal, plant, material, or urban ecosystems affected by the emissions (Wolff, 1999). 

It is difficult to separate out the uptake and/or reactions of N03 with water and those of N2Os. As discussed in the next section, there is an abundance of evidence that N205 is taken up by aqueous droplets and surfaces in both the troposphere and stratosphere and hydrolyzes to form HNO,. However, it appears that N03 may also be taken up, and in this case, may act as a strong oxidant in solution (see, for example, Chameides, 1986a, 1986b and Pedersen, 1995). 

Table 8.17 summarizes the rate constants and estimated tropospheric lifetimes of some of these sulfur compounds with respect to reaction with OH or NO-,. The assumed concentrations of these oxidants chosen for the calculations are those characteristic of more remote regions, which are major sources of reduced sulfur compounds such as dimethyl sulfide (DMS). It is seen that OH is expected to be the most important sink for these compounds and that NO, may also be important, for example, for DMS oxidation (see also Chapter 6.J). 

As air is transported rapidly upward, for example in a convective system, cooling occurs (see Chapter 2), leading to the condensation of water as ice crystals. Because of this removal of water as moist tropospheric air rises, air in the stratosphere is very dry, of the order of a few ppm. Some water is also produced directly in the stratosphere from the oxidation of CH4 and H2. The so-called extratropical pump then moves the air poleward and downward at higher latitudes (Path I), warming the air as it descends. 

As discussed in other chapters of this book and summarized in Chapter 16, the formation of tropospheric ozone from photochemical reactions of volatile organic compounds (VOC) and oxides of nitrogen (NC/) involves many reactions. Concentrations are therefore quite variable geographically, temporally, and altitudinally. Additional complications come from the fact that there are episodic injections of stratospheric 03 into the troposphere as well as a number of sinks for its removal. Because 03 decomposes thermally, particularly on surfaces, it is not preserved in ice cores. All of these factors make the development of a global climatology for 03 in a manner similar to that for N20 and CH4, for example, much more difficult. In addition, the complexity of the chemistry leading to O, formation from VOC and NOx is such that model-predicted ozone concentrations can vary from model to model (e.g., see Olson et al., 1997). 

Such organics are highly reactive with essentially all oxidants of tropospheric interest, including OH, 03, N03, and chlorine atoms. For example, the lifetimes of... 

Hydroxyl radical, OH, is the principal atmospheric oxidant for a vast array of organic and inorganic compounds in the atmosphere. In addition to being the primary oxidant of non-methane hydrocarbons (representative examples of these secondary reactions are given in Table 6), OH radical controls the rate of CO and CH4 oxidation. Furthermore, the OH reaction with ozone also limits the destruction of O3 in the troposphere, it also determines the lifetime of CH3CI, CHsBr, and a wide range of HCFC s, and it controls the rate of NO to HNO3 conversion. Concentration profiles for hydroxyl radical in the atmosphere are shown in Fig. 2. 

A primary example of the interplay between hydroxyl radical and ozone is provided by the photo-induced oxidation of methane in the troposphere. The overall sequence of stoichiometric reactions can be written as ... 

There are two sources of tropospheric ozone. First, transport from the stratosphere in meteorological events known as tropospheric folding in which a layer of stratospheric air is entrained in tropospheric air-flow and mixed into the troposphere. Second, peroxy radical reactions which oxidize NO to N02. For example, in the OH radical initiated oxidation of CO ... 

Most of the photo-initiated processes in the atmosphere are radical reactions. However, the hydroxyl radical ( OH) is of special significance for the chemistry of the atmosphere (Ehhalt, 1999). This reactive species is mainly responsible for the photooxidation of trace organic chemicals in the troposphere and hence for the oxidative cleansing of the atmosphere (Fabian, 1989). It is nature s atmospheric detergent (Comes, 1994, Ravishankara, et al. 1998). Furthermore, several years ago it was well established that the interaction of UV/VIS radiation and environmental pollution seems to be responsible for the dramatic forest decline that has been observed, for example in the higher areas of the Black Forest or the Ore Mountains in Germany (Schenck, 1985). 

A homogeneous catalyst exists in the same phase as the reacting molecules. There are many examples in both the gas and liquid phases. One such example is the unusual catalytic behavior of nitric oxide toward ozone. In the troposphere, the part of the atmosphere closest to earth, nitric oxide catalyzes ozone production. However, in the upper atmosphere it catalyzes the decomposition of ozone. Both of these effects are unfortunate environmentally. 

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. 

It is quite ambitious for a scientist to describe a natural phenomenon in terms of a specific reaction. The situation in the atmospheric environment is however more complicated as a variety of reactions are occurring simultaneously and a certain species may take part in different reactions affecting the relative equilibria. Most data are coming from laboratory work and experimental conditions are definitely different from the ones observed in the troposphere. As an example the mechanism of oxidation of sulphur dioxide, in gas phase is usually reported occur to a large extent through free radicals. If the presence of humidity and of particulated matter is considered, specifically in the lower part of the troposphere, definitely also heterogeneous reactions play a very important role. I feel that experiments carried on in the atmosphere yield more consistent results to elucidate the chemistry of the atmospheric environment. 

Your point is certainly well taken there are many aspects of tropospheric chemistry that are uncertain, and that involving methyl peroxide is without doubt a prime example. From my own estimation of how the methane system works in the atmosphere, I believe that a significant fraction of the methyl peroxide is removed by heterogeneous reactions before it has a chance to react. This terminates the chain making methane oxidation a net sink for OH regardless of the details of the chemistry of methyl peroxide and its daughter molecules. Nevertheless we certainly need to be aware of the many uncertainties in this chemistry. 

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