O3 in the troposphere

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. 

P) combines rapidly with an oxygen molecule to form ozone and in this way NO is the most important precmrsor of O3 in the troposphere. NO and NO2 interconvert in a few minutes. In da5rtime, the lifetime of NO in the lower part of troposphere is typically 1-2 days and is determined by the oxidation of NO2 by the OH radical to produce nitric acid ... 

Tropospheric excited atomic oxygen (0( D)) is produced by the photolysis of O3. In the troposphere, this photolysis reaction occurs over a very narrow spectral interval, between 290 nm and 310 nm. The production of excited atomic oxygen decreases as the latitude increases, i.e., less incoming solar radiation for photolysis is available. The bulk of the water vapor in the atmosphere resides in the troposphere. The amount of H2O in the atmosphere is controlled by the saturation vapor pressure, which decreases with decreasing atmospheric temperature, i.e., as altitude or latitude increases. 

In addition to controlling the chemical des-tmction of OH, the oxidation of CO and CH4 by OH outlined above initiates the CO and CH4 oxidation schemes, which lead to the photochemical production of O3 in the troposphere. 

In addition to being a key player in the CO and CH4 oxidation chains leading to the chemical production of O3 in the troposphere, NO also leads to the chemical production of HNO3, the fastest growing component of acidic precipitation. NO is chemically transformed to nitrogen dioxide (NO2) and then to NO via the following reactions ... 

The presence of O3 in the troposphere is also veiy important because it generates OH radicals ... 

The O2 photodissociation and subsequent O3 formation in the lower stratosphere is the most important source of O3 in the troposphere via the stratosphere-troposphere exchange. The tropospheric net O3 formation (Chapter 5.3.7) is another important source, especially because of anthropogenic enhancement. Fig. 5.6 shows all principal oxygen reactions note that O2 photolysis can be excluded in the climate system in the more narrow sense close to the earth s surface. The exit pathways (sinks) to products (Fig. 5.6) are only possible if there are additional elements and compounds (Chapter 5.3.2.2). In a pure oxygen gas system, we only consider four reactions (5.5, 5.6, 5.13 and 5.14), forming a steady state ... 

Nitrogen oxides play an important role in the photochemistry of the troposphere, controlling the formation of tropospheric O3, affecting the concentration of the hydroxyl radical, and contributing to acid precipitation. Nitrogen dioxide (NO2) is one of the most important reactive nitrogen species its photolysis is the primary source of O3 in the troposphere (see Section 28.6 for further details on the actual chemical processes and rates). 

Why do we not want O3 in the troposphere Because that is where we Uve—along with all the other plants and animals on Earth Ozone is a powerful oxidizing agent and can be the culprit lurking behind respiratory diseases, deterioration of human-made materials such as rubber and nylon, and damage to crops and other plants. 

Fig. 4.2 Absorption spectrum of O3 in the tropospheric actinic flux region (Adapted from Orphal 2003)...

The formation of oxygen atom 0( P) in the photolysis of nitrogen dioxide (NO2) is the fundamental reaction that causes direct production of O3 in the troposphere. In this section, absorption spectrum and 0( P) production quantum yields relevant to the tropospheric photochemistry are described. 

Johnson and Marston (2008) reviewed on the reactions of unsaturated VOC with O3 in the troposphere, and Donahue et al. (2011) introduced new data and interpretation on the pressure dependence of the OH yields. 

In summary, biomass burning is a major source of many trace gasses, especially the emissions of CO2, CH4, NMHC, NO,, HCN, CH3 CN, and CH3 Cl (73). In the tropics, these emissions lead to local increases in the production of O3. Biomass burning may also be responsible for as much as one-third of the total ozone produced in the troposphere (74). However, CH3 Cl from biomass burning is a significant source for active Cl in the stratosphere and plays a significant role in stratospheric ozone depletion (73). 

The resultant O3 layer is critically important to life on Earth as a shield against LTV radiation. It also is responsible for the thermal structure of the upper atmosphere and controls the lifetime of materials in the stratosphere. Many substances that are short-lived in the troposphere (e.g. aerosol particles) have lifetimes of a year or more in the stratosphere due to the near-zero removal by precipitation and the presence of the permanent thermal inversion and lack of vertical mixing that it causes. 

Nitrogen oxides also play a significant role in regulating the chemistry of the stratosphere. In the stratosphere, ozone is formed by the same reaction as in the troposphere, the reaction of O2 with an oxygen atom. However, since the concentration of O atoms in the stratosphere is much higher (O is produced from photolysis of O2 at wavelengths less than 242 nm), the concentration of O3 in the stratosphere is much higher. 

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

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). 

Above the troposphere is the stratosphere, which extends many kilometers above the earth. Chemicals slowly migrate into the stratosphere, where there are few reactions to remove them. The stratosphere is also irradiated by high intensities of ultraviolet Hght which induces photochemical reactions such as production of O3 from O2. [In the troposphere O3 is bad, but in the stratosphere it is good because it absorbs ultraviolet radiation from the sun and prevents it from reaching the earth s surface.]... 

In this section, we use another chain reaction to show the relation between the steady-state treatment and the quasi-equilibrium treatment. The former is more general than the latter, and leads to more complete but also more complicated results. Ozone, O3, is present in the stratosphere as the ozone layer, and in the troposphere as a pollutant. Ozone production and destruction in the atmosphere is primarily controlled by photochemical reactions, which are discussed in a later section. Ozone may also be thermally decomposed into oxygen, O, although... 

NOx emissions from subsonic aircraft flying in the troposphere and the lowermost stratosphere lead to a significant increase in ozone in the upper troposphere. Emissions of NOx and H20 from supersonic aircraft cruising in the stratosphere are calculated to decrease the column abundance of O3. The effects of aircraft emissions are found to be strongly dependent on flight altitudes and on assumed emission indices for NOx. 

Dissolved metals affect the concentration of atmospheric trace gases, such as ozone, organics and sulfur compounds. Ozone is formed in the troposphere through a complex series of homogeneous reactions as shown schematically in Fig. 4. The chain represented in the figure by thick arrows involves the cooperative oxidation of organic molecules and NO with the intermediacy of HO, formed by subsequent photolysis of NO2 and O3, and HO2 radicals. 

A few key (i.e., primary or direct) photochemical reactions are the principal drivers of overall chemical reactions in the troposphere. These reactions involve primarily O3 and NO2 photolysis. Other reactions presented in Table 1 will be discussed later. 

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