Chapman layer ozone

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. 

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. 

The simplest model for considering the stratospheric ozone layer is the Chapman oxygen-only mechanism (Figure 7.11), which describes the reactions steady-state ozone concentration as resulting from a... 

The theoretical framework just presented was first suggested by Chapman (1931). This theory provides an explanation for the behavior of the layers of ionization in the thermosphere or of photodissociation in the middle atmosphere. The production of ozone through photodissociation of molecular oxygen exhibits a maximum near 45 km dependent on the insolation. The rate of heating through absorption of ultraviolet radiation by ozone similarly leads to a maximum near the stratopause. Numerous examples of such layers can be found in the neutral and ionized atmosphere. 

Terrestrial stratospheric chemistry is closely linked to the ozone (O3) layer at 15-35 km, which shields the Earth s surface from harmful UV sunlight (X<300 nm) and dissipates the absorbed solar energy as heat. The abundance of O3 in the stratosphere is a balance between production, destruction, and lateral transport. Production and destruction of O3 in the absence of other perturbing influences is described by the Chapman cycle given in Table V. 

The photolysis of oxygen was described many years ago by Chapman (1930). The attention to the stratospheric ozone was drawn by Bates and Nicolet (1950), who presented the idea of catalytic O3 decay. However, only through the implications of manmade influences on the stratospheric ozone cyele by Crutzen (1971), Johnston (1971), Molina and Rowland (1974) as well as Stolarski and Cicerone (1974) was our attention to the stratospheric ozone layer drawn. 

It is known that measured ozone concentrations are lower than can be accounted for by the simple Chapman cycle. This has led scientists to look for other influences on the concentration of ozone. First, let s briefly consider one of the natural reactions that destroys ozone. UV radiation can break the oxygen-hydrogen bond of a water molecule in the stratosphere to generate hydrogen atoms and hydroxyl radicals ( OH). These two species are involved in many reactions, some of which actually convert Og to Og. However, this process, which scientists now believe is an efficient process above 50 km, has been occurring since the ozone layer developed, and there is little, if anything, that humans can do about it. The system has obviously attained a steady state that includes this perturbation. 

Earlier, we noted the net reaction of the Chapman cycle 3 O2 3 O2. In some ways, it appears pointless to talk about the kinetics of a reaction that has no net change. But as we have already seen, the importance of the Chapman cycle lies in the details. Without the Chapman cycle, the upper atmosphere would not have an ozone layer, and the quantity of harmful UV radiation reaching the surface of the planet would be significantly greater. This natural cycle points to the importance of the mechanism of the reaction. A reaction mechanism is a collection of one or more molecular steps that account for the way reactants become products. If the overall equation is like a parts list and a finished product, then the mechanism is like the assembly instructions. In many cases, such as the Chapman cycle, if we want to understand the kinetics of the process we must consider the reaction mechanism. What is the stepwise process of bond breaking and bond formation behind the overall reaction ... 

As described in Chap. 1, in contrast to stratospheric chemistry that can be traced back to the 1930s when the chemical theory of ozone layer formation was published by Chapman (1930), research on chemical reaction system in the tropospheric... 

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. 

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