Stratosphere, activity

Similar heterogeneous reactions also can occur, but somewhat less efticientiy, in the lower stratosphere on global sulfate clouds (ie, aerosols of sulfuric acid), which are formed by oxidation of SO2 and COS from volcanic and biological activity, respectively (80). The effect is most pronounced in the colder regions of the stratosphere at high latitudes. Indeed, the sulfate aerosols resulting from emptions of El Chicon in 1982 and Mt. Pinatubo in 1991 have been impHcated in subsequent reduced ozone concentrations (85). 

The chlorine-containing product species (HCl, CIONO2, HOCl) are "inert reservoirs" because they are not directly involved in ozone depletion however, they eventually break down by absorbing solar radiation or by reaction with other free radicals, returning chlorine to its catalytically active form. Ozone is formed fastest in the upper stratosphere at tropical latitudes (by reactions 1 and 2), and in those regions a few percent of the chlorine is in its active "free radical" form the rest is in the "inert reservoir" form (see Figure 3). 

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). 

Concern has been expressed over the destruction of ozone in the stratosphere brought about by its reactions with chlorine atoms produced from chlorofluoroalkanes that are persistent in the troposphere, and that may contribute to radiatively active gases other than COj. 

Essentially, all reactions that require the formation of a chemical bond with an activation energy of around 100 kJ mol-1 are frozen out at the surface of Titan but are considerably faster in the stratosphere, although still rather slow compared with the rates of reaction at 298 K. Chemistry in the atmosphere of Titan will proceed slowly for neutral reactions but faster for ion-molecule reactions and radical-neutral reactions, both of which have low activation barriers. The Arrhenius equation provides the temperature dependence of rates of reactions but we also need to consider the effect of cold temperatures on thermodynamics and in particular equilibrium. 

In the late 1960s, direct observations of substantial amounts (3ppb) of nitric acid vapor in the stratosphere were reported. Crutzen [118] reasoned that if HN03 vapor is present in the stratosphere, it could be broken down to a degree to the active oxides of nitrogen NO (NO and N02) and that these oxides could form a catalytic cycle (or the destruction of the ozone). Johnston and Whitten [119] first realized that if this were so, then supersonic aircraft flying in the stratosphere could wreak harm to the ozone balance in the stratosphere. Much of what appears in this section is drawn from an excellent review by Johnston and Whitten [119]. The most pertinent of the possible NO reactions in the atmosphere are... 

Ozone is main component in many oxidation processes assembled imder the term ozonation processes. In these processes ozone is applied either alone (O3 process) or with the addition of oxidant, e.g. H2O2 (O3/H2O2 process), UV radiation (explained in above subchapter), catalyst, activated carbon, ultrasoimd etc. Ozone is inorganic molecule constituted by three atoms of oxygen. It is present in nature in upper atmosphere in the form of stratospheric layer aroimd the earth, and it is formed by the photolysis of diatomic oxygen and further recombination of atomic and diatomic oxygen, shown by equations (25) and (26) [35] ... 

In spite of the occurrence of natural events such as the eruption of Krakatoa, scientists are now well aware that human activities can have serious long-term effects on the Earth s atmosphere. The hrst such effect to be noticed historically was the increase in acid precipitation resulting from the combustion of fossil fuels. Acid precipitation is also known as acid rain or acid deposition. The second, discovered in the mid-20th century, was the depletion of stratospheric ozone. More recently, atmospheric scientists established a link between so-called greenhouse gases and global climate change. 

There are numerous natural contributors of chlorine to the stratosphere, for example, volcanic eruptions. The main concern regarding ozone destruction in recent years is associated with human activities that have increased chlorine and other synthetic chemical input into the stratosphere. At the top of the list of such chemicals are chlo-rofluorocarbons, or CFCs. CFCs are compounds that contain carbon, chlorine, and fluorine they were first developed in 1928. Common CFCs are called Freons, a trade name coined by the DuPont chemical company. CFC compounds are nonreactive, nontoxic, inflammable gases. Because of their... 

As described earlier, in the stratosphere, a steady-state concentration of 03 is produced naturally by the Chapman cycle, reactions (l)-(4). Until about 1970, relatively little attention was paid to potential anthropogenic (i.e., man-made) perturbations of the stratosphere. At that time, Crutzen (1970) examined the potential role of NO and N02 formed in the stratosphere from reactions of N20 that was originally generated at the earth s surface. Because N20 is unre-active in the troposphere, it has a sufficiently long lifetime to end up in the stratosphere, where it can be converted into NO (see Chapter 12). Crutzen (1970) proposed that the NO and N02 formed from reactions of N20 can then participate in a chain reaction that destroys 03 ... 

Destruction of stratospheric ozone caused by relatively small atmospheric concentrations of chlorofluo-rocarbons has vividly illustrated the capacity of human activity to alter our atmosphere in a manner that has significant and far-ranging effects. There is similar concern for the effects of greenhouse gases on the earth s climate. 

Zhang, R., M.-T. Leu, and L. F. Keyser, Heterogeneous Chemistry of HONO on Liquid Sulfuric Acid A New Mechanism of Chlorine Activation on Stratospheric Sulfate Aerosols, . /. Phys. Chem., 100, 339-345 (1996). 

Arnold, F., and G. Hauck, Lower Stratosphere Trace Gas Detection Using Aircraft-Borne Active Chemical Ionization Mass Spectrometry, Nature, 315, 307-309 (1985). 

See Chapter 7.E for a discussion of the kinetics and Brown et al. (1999) for a recent study under low-temperature and low-pressure conditions representative of the stratosphere.) This reaction removes NO, from the ozone destruction cycle since HNO-, does not readily regenerate active forms of NO, and is removed by transport to the troposphere followed by rainout and washout. Photolysis of HNO-, to OH + NOz is... 

Increased production of oxides of nitrogen through solar proton events associated with the 11-year cycle in solar activity would be expected to be most important in the upper stratosphere, above the region where the majority of the ozone depletion was observed in addition, lower, rather than higher, concentrations of gas-phase oxides of nitrogen appear to be associated with the ozone depletion (e.g., see Noxon, 1978 McKenzie and Johnston, 1984 Thomas et al., 1988 Keys and Gardiner, 1991 and Solomon and Keys, 1992). Hence both of these explanations are not consistent with atmospheric observations. 

Through a variety of studies, it is now generally accepted that the observed losses are associated with chlorine derived from CFCs and that heterogeneous chemistry on polar stratospheric clouds plays a major role. The chemistry in this region is the result of the unique meteorology. As described in detail by Schoeberl and Hartmann (1991) and Schoeberl et al. (1992), a polar vortex develops in the stratosphere during the winter over Antarctica. The air in this vortex remains relatively isolated from the rest of the stratosphere, allowing photochemically active products to build up... 

Briihl, C P. J. Crutzen, and J.-U. GrooG, High-Latitude, Summertime N02. Activation and Seasonal Ozone Decline in the Lower Stratosphere Model Calculations Based on Observations by HALOE on UARS, J. Geophys. Res., 103, 3587-3597 (1998). 

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