Ozone formation and destruction

Appreciation of interactive processes that have been outlined has been able to illuminate discussion of mechanisms for reactions as diverse as the acidification of water masses, climate alteration, ozone formation and destruction, and the possible enviromnental roles of trichloroacetic acid and nitroarenes. 

The Chapman mechanism. The mechanism of ozone formation and destruction in the stratosphere was first formulated by Chapman (205) in 1930. He did not consider the effects of minor constituents and physical transport processes that have since been recognized as important factors to explain the discrepancy between the calculated results and the actual observation. According to his mechanism, ozone is formed by the photolysis... 

Most ozone is formed near the equator, where solar radiation is greatest, and transported toward the poles by normal circulation patterns in the stratosphere. Consequently, the concentration is minimum at the equator and maximum for most of the year at the north pole and about 60° S latitude. The equilibrium ozone concentration also varies with altitude the maximum occurs at about 25 km at the equator and 15-20 km at or near the poles. It also varies seasonally, daily, as well as interannually. Absorption of solar radiation (200-300 nm) by ozone and heat liberated in ozone formation and destruction together create a warm layer in the upper atmosphere at 40-50 km, which helps to maintain thermal equilibrium on earth. 

The parameters which influence the ozone formation and destruction (e.g. UV-radiation intensity, oxygen partial pressure and total pressure) are not constant throughout the atmosphere. Therefore, the ozone density varies with the altitude of the atmospheric level in a characteristic pattern with a significant maximum in the range of 15—25 km above the earth surface (see Fig. 2 right). 

The classical ideas on ozone formation and destruction in the stratosphere are discussed on the basis of Paetzold s work as summarized by Junge (1963). As Chapman (1930) demonstrated the formation of ozone is initiated by the following photochemical processes ... 

Reactions (1) and (2) represent ozone formation, while (3) and (4) are the reverse process. Oxygen atom (0) is very reactive, and consequently has a short lifetime in the stratosphere. Sunlight drives both ozone formation and destruction and this means that all four reactions will halt at sunset, and so during the nighttime the(03> is essentially the same as at the end of the day. 

Until about 1964, the Chapman mechanism was thought to be the principal set of reactions governing ozone formation and destruction in the stratosphere. First, improved measurement of the rate constant of reaction 4 (above) indicated that the reaction is considerably slower than previously thought, leading to larger abundances of 03 as predicted by (5.10)—(5.12). Then, measurements indicated that the actual amount of ozone in the stratosphere is a factor of 2 less than what is predicted by the Chapman mechanism with the more accurate rate constant of reaction 4 (Figure 5.5). It was concluded that significant additional ozone destruction pathways must be present beyond reaction 4. 

In 1930, the English geophysicist Sydney Chapman (1888-1970) worked out the cycle of ozone formation and destruction in the stratosphere. There are four chemical reactions in the cycle ... 

The Chapman mechanism for ozone formation and destruction is incomplete. There are compounds from natural sources—N O from agriculture, and OH from water, for example—that contribute to ozone s destruction. In an impoUuted stratosphere, a balance between formation and destruction is maintained. Nothing can be done to promote the formation of ozone in the stratosphere. Several pollutants, however, can very effectively promote the destruction of ozone. 

For the discussion of the formation and destruction of ozone in (In stratosphere it is convenient to define the photodissociation coefficient generally denoted by J (in units ofsec 1). J is the probability of dissocial mn of a molecule per second by light absorption. 

There are various approaches to parameterizing the process of formation and destruction of the ozone layer. The difficulty of deriving dynamic models of the ozone cycle in the atmosphere has to do with the participation in the cycle of more than 75 chemical reactions, a qualitative and quantitative description of which is impossible without deriving detailed models of the many minor gas components of the atmosphere. Nevertheless, there are empirical models of the ozone layer, which make it possible, under the present climatic situation, to obtain adequate spatial distributions of ozone. For instance, Bekoryukov and Fedorov (1987) derived a simple empirical model of total ozone content confirmed by observational data for the Southern Hemisphere ... 

MFDO Modeling the formation and destruction of ozone by taking account of all flight corridors over the territory in question. 

The dominant loss of OH radicals is reaction with CO and organic compounds such as CH4, both reactions produce peroxy radicals. Peroxy radicals play a key role in atmospheric chemistry. They are intimately involved in the formation and destruction of ozone and in the photooxidation of all organic compounds in the atmosphere [4], The lifetime of OH radicals with respect to reactions Eq. 3 and Eq. 8 is of the order of a second and in the day-time a steady state condition is established. The OH radical concentration in the atmosphere varies with location, time of day, season, and meteo-... 

The ozone content of the atmosphere is held nearly constant by a dynamic equilibrium determined by the rates of formation and destruction, respectively. 

What actually happens is that NO, destroys ozone at high altitude but forms it at low altitude. As a result, whether there is a net increase or decrease in total ozone depends on the altitude at which NO, is injected. The transition from ozone formation to destruction appears to occur somewhat below 20 km, the precise level depending on details of the mathematical model (Johnston and Podolske, 1978). Thus the flight altitude of a supersonic transport is critical NO, from sources at the Earth s surface always seems to increase ozone (Turco et ai, 1978). 

The next section presents the major photochemical processes affecting the formation and destruction of ozone in a pure oxygen atmosphere . 

Table 3-1. Globally Integrated Rates of Ozone Formation and Ozone Destruction (Johnston, 1975)...

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