Ozone Chapman mechanism

Reactions 1 to 4 are known collectively as the Chapman mechanism (first outlined by Sidney Chapman (1) in 1930. They basically explain how ozone can exist in the stratosphere in a dynamic balance it is continuously being produced by the action of solar ultraviolet radiation on oxygen molecules and destroyed by several natural chemical processes in the atmosphere. 

This simple oxygen-only mechanism consistently overestimates the O3 concentration in the stratosphere as compared to measured values. This implies that there must be a mechanism for ozone destruction that the Chapman model does not account for. A series of catalytic ozone-destroying reactions causes the discrepancy. Shown below is an ozone-destroying mechanism with NO/NO2 serving as a catalyst ... 

The rate of photolysis, J, depends on the absorption cross-section, a, the number density, the scale height and the angle, all of which are unique properties of a planetary atmosphere. For the Earth and the Chapman mechanism for ozone the O3 concentration maximum is 5 x 1012 molecules cm-3 and this occurs at 25 km, shown in Figure 7.12, and forms the Chapman layer structure. 

The reality of using thermodynamics is somewhat simpler than the preceding derivations imply. Consider the reaction we have been using in the formation of ozone in the Chapman mechanism ... 

Although the Chapman mechanism models the general shape of the profile of stratospheric ozone correctly, it seriously overestimates its concentration. To account for this discrepancy, a catalytic process which increases the rate of step 4 occurs (Figure 7.12). Trace amounts of radicals present in the stratosphere reduce natural ozone levels below those predicted by the Chapman scheme ... 

Reactions 2-167 to 2-170 constitute the Chapman mechanism for the creation and destruction of ozone in the unpolluted stratosphere. 

These four reactions constitute the Chapman mechanism for establishing the abundance of ozone in the stratosphere. However, the abundance of ozone is dictated also by other loss mechanisms that mimic reaction 4. 

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... 

Deviation from the Chapman mechanism. It was recognized by Nicolet (740) that the observed Oa concentrations were much less than the calculated values even near the stratopause where the physical transport processes are not important (see Fig. VIII 10). He suggested that to explain the observed ozone concentration, the effective value of k44, the rate constant for the destruction of Oj, must be much larger than that given in (VIII-44g) for a pure 02- N2 atmosphere. 

Johnston, Crutzen, and others have also recognized that the natural ozone balance in the stratosphere cannot be explained on the basis of the Chapman mechanism and air motions. Johnston (542) has concluded that the calculated ozone destruction rate based on the Chapman reactions and air motions can explain only 20% of the natural destruction rate. About 80% of ozone produced by sunlight must be destroyed by a mechanism other than (VIII-43) and (VII1-44). 

On the simplest possible level, the ozone in the stratosphere is maintained by the Chapman mechanism, (13), (16), (22-24) plus the catalytic cycles which in cryptic form are shown as (25)-(30)... 

The ozone destruction is mainly caused by its own photolysis (Chapman-mechanism,13)) ... 

Laboratory experience had convinced chemists earlier that the Chapman mechanism needed a supplement of additional reactions. In 1960, McGrath and Norrish discovered the formation of OH radicals in the reaction of water vapor with 0( D) atoms generated by the photolysis of ozone, and they proposed a chain decomposition of ozone by water radicals. Meinel (1950) had previously demonstrated the existence of OH in the upper... 

In these reactions NO is not consumed while it destroys ozone. Rather, NO acts as a catalyst to ozone destruction in a pure oxygen atmosphere. Because it is faster, the catalytic cycle proceeds several times during the same time interval in which the 03 loss reaction of the Chapman mechanism occurs once. 

Above the troposphere, in the stratosphere and mesosphere, the gas density becomes increasingly more pronounced, and the thermal conditions become subject to complex variations, both temporal and spatial. At a height of 25-35 km the oxygen molecules are split by the influence of solar UV radiation to form ozone in accordance with the Chapman mechanism, which is responsible for tbe absorption of 97% of harmful ultraviolet solarradiation. Devoidof this shield, life on the terrestrial surface would have been doomed to extinction (see Chapter 6). 

The principal reactions in the photochemical process of formation and destruction of ozone are reactions (l)-(4), collectively known as the Chapman mechanisms... 

Describe the Chapman mechanism for ozone formation-destrnction processes in the stratosphere. 

Does the Chapman mechanism present the correct description of ozone chemistry in the stratosphere and what are the limitations of this mechanism ... 

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. 

FIGURE 5.5 Comparison of stratospheric ozone concentrations as a function of altitude as predicted by the Chapman mechanism and as observed over Panama (9°N) on November 13, 1970. 

Since most (over 99%) of the odd oxygen is in the form of O3, the Chapman mechanism predicts that local stratospheric ozone concentrations are proportional to the square root of the O2 photolysis rate. At night, reactions 4.1 and 4.3 cease, but reactions 4.2 and 4.4 persist. Atomic oxygen concentrations fall rapidly, with the net effect that reactions 4.2 and 4.4 more or less balance each other so that diurnal variations in stratospheric O3 are small. 

The ozone destruction processes that must be added to the Chapman mechanism take the form of a catalytic cycle ... 

The chlorine not only destroys the ozone that is produced from reaction (2) in the Chapman mechanism, but it also inhibits the formation of ozone by sequestering oxygen atoms in the form of chlorine monoxide (CIO). Under certain meterological conditions the CIO is itself stored in the form of chlorine nitrate, to be re-released when the meterology changes. This is what makes the ozone depletion process so effective in forming the antarctic ozone hole, for example. 

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. 

Chapman (1930a, b) showed successfully that the characteristics of the height of the ozone layer and ozone density in the earth s atmosphere can be described by assuming only oxygen is present as a reactive species in the atmosphere, and this reaction scheme is called pure oxygen theory or Chapman mechanism. Photolysis of O2 occurs by the solar radiation of wavelength shorter than 242 nm (see Sect. 4.3.1). Only the photolytic process. 

Figure 8.1 shows the comparison of ozone density profile in the mid-latitude between the calculated and observed values. The horizontal lines with the observation curve represent the observed range of ozone density. It can be seen that the Chapman mechanism predicts the maximum of the ozone layer at around 25 km and its ozone density almost properly. However, there are two points of deviation of the theoretical curve from the observation. One is the overestimation of the density by a factor of two above the altitude of ozone maximum and the theoretical value of the altitude of the maximum is higher than observation by a few km. Another is that in the lower stratosphere to the troposphere, the theoretical value of ozone decreases rapidly whereas the observed value keeps nearly a constant value. 

Fig. 8.1 Comparison of vertical ozone profiles caleulated by the pure oxygen atmosphere themy (Chapman mechanism) and by observation (Adapted from Shrmazaki et al. 1987)...

The chemistry of the stratospheric ozone will be sketched with a very broad brush in order to illustrate some of the characteristics of catalytic reactions. A model for the formation of ozone in the atmosphere was proposed by Chapman and may be represented by the following "oxygen only" mechanism (other aspects of... 

In the present-day atmosphere ozone forms into layers and this was first explained by Chapman who proposed a photolysis mechanism for ozone formation. Chapman s mechanism is a simple steady-state production of ozone and led to the concept of odd oxygen. The odd-oxygen reaction scheme is shown in Table 7.4. 

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