The Physical Chemistry Of The Ozone Layer

Ozone owes its existence to the impact of ultraviolet radiation on oxygen molecules in the stratosphere, with the main production taking place in tropical regions where radiation levels are high (Rodriguez 1993). Oxygen molecules normally consist of two atoms, and in the lower atmosphere they retain that configuration. At the high energy levels associated with ultraviolet radiation in the upper atmosphere, however, these molecules split apart to produce atomic oxygen (see Figure 6.1). Before long, these free atoms combine with the available molecular oxygen to create triatomic oxygen or ozone. That reaction is reversible. The ozone molecule may break down again into its original components, molecular oxygen and atomic oxygen, as a result of further absorption of ultraviolet radiation, or it may combine with atomic oxygen to be reconverted to the molecular form (Crutzen 1974). The total amount of ozone in the stratosphere at any given time represents a balance between the rate at which the gas is being produced and the rate at which it is being destroyed. These rates are directly linked; any fluctuation in the rate of production will be matched by changes in the rate of decay until some degree of equilibrium is attained (Dotto and Schiff 1978). Thus, the ozone layer is in a constant state of flux as the molecular structure of its constituents changes.

The role of ultraviolet radiation and molecular oxygen in the formation of the ozone layer was first explained by Chapman in 1930. Later measurement indicated that the basic theory was valid, but observed levels of ozone were much less than expected, given the rate of decay possible through natural processes. Since none of the other normal constituents of the atmosphere, such as molecular oxygen, nitrogen, water vapour or carbon dioxide was considered capable of destroying the ozone, attention was eventually attracted to trace elements in the stratosphere. Initially, it seemed that these were present in insufficient quantity to have the necessary effect, but the problem was solved with the discovery of catalytic chain reactions in the atmosphere (Dotto and Schiff 1978). A catalyst is a substance which facilitates a chemical reaction, yet remains itself unchanged when the reaction is over. Being unchanged, it can go on to promote the same reaction again and again, as long as the reagents are available, or until the catalyst itself is removed. In this form of chain reaction, a catalyst in the stratosphere may destroy thousands of ozone molecules before it is finally removed. The ozone layer is capable of dealing with the relatively small amounts of naturally occurring catalysts. Recent concern over the thinning of the ozone layer has focused on anthropogenically produced catalysts (see Figure 6.2), which were recognized in the stratosphere in the early 1970s, and which have now accumulated in quantities well beyond the system's ability to cope.

The Basic Survival Guide

The Basic Survival Guide

Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

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