Oxygen ozone layer formation

Both processes are switched on by the absorption of short-wavelength radiation X < 240 nm for H20 and X < 230 nm for C02. On the assumption that H atoms escape from the atmosphere, there is a net gain in oxygen to the atmosphere. Reactions of O atoms and 02 chemistry would then lead to the formation of a small ozone layer with a low ozone concentration. 

It is a matter of conjecture as to whether sunlight was involved in the buildup of early organic molecules which eventually formed DNA, RNA and proteins. Photosynthetic bacteria may well have been the very first independent life forms, and from the time of the growth of green plants the atmosphere must have become gradually richer in oxygen, since it appears as a byproduct in the major process of photosynthesis. It is generally accepted that early life developed in the oceans, and it may be surmised that its eventual development on land was made possible by the formation of the protective ozone layer in the upper atmosphere. 

The formation of the stratospheric ozone layer can be understood most simply on the basis of a reaction model composed of a minimum set of four elementary processes (a) the dissociation of oxygen molecules by solar radiation in the wavelength region 180-240 nm (b) the attachment of oxygen atoms to molecular oxygen, leading to the formation of ozone (c) the photodissociation of ozone in the Hartley band between 200 and 300 nm and (d) the destruction of ozone by its reaction with oxygen atoms. The reactions may be written... 

The reactions of the ozone cycle account for some, but not all, of the facts about the ozone layer. Many chemical reactions occur that involve substances other than oxygen. We must also consider the effects of turbulence and winds that mix up the stratosphere. A complicated picture results. The overall result of ozone formation and removal reactions, coupled with atmospheric turbulence and other factors, is to produce the upper-atmosphere ozone profile shown in FIGURE 18.4, with a maximum ozone concentration occurring at an altitude of about 25 km. This band of relatively high ozone concentration is referred to as the ozone layer or the ozone shield. ... 

The prime driver of the chemical system of the earth s atmosphere is photochemical reactions caused by solar radiation. The atmosphere of the earth is divided into levels called the troposphere, stratosphere, mesosphere and thermosphere from nearest the ground to farthest according to the characteristics of the temperature gradient as shown in Fig. 3.1. The cause of the temperature inversion in the stratosphere, which characterizes the earth s atmosphere, is the formation of an ozone layer by the photolysis of oxygen, one of the major components of the atmosphere. In this chapter, the spectrum of solar radiation, actinic flux, and so on, that is necessary to calculate the photolysis rate of atmospheric molecules are explained. 

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