Ozone Layer

I

Introduction

Ozone Layer, layer of the chemical ozone present in the atmosphere protecting the Earth from ultraviolet radiation.

II

Composition of the Ozone Layer

Ozone is formed in the atmosphere when ultraviolet radiation from the Sun splits one oxygen molecule into two oxygen atoms (O₂). The atomic oxygen then combines with another oxygen molecule to form ozone (O₃). Most ozone found in the Earth’s atmosphere occurs in one layer in the stratosphere, between altitudes of around 20 to 50 km (12 to 30 mi). Early in the Earth’s history, the gradual build-up of this layer from oxygen released by marine life allowed life to develop on land. Today, the ozone layer helps to produce the observed vertical structure of the atmosphere, and absorbs harmful ultraviolet radiation that would otherwise damage plant and animal life (also causing skin cancer) on the Earth’s surface. In contrast, ozone close to the Earth’s surface is a health hazard, as it is one of the major constituents of photochemical smog.

III

Seasonal Changes in the Ozone Layer

The ozone layer is thinnest near the equator and thickest at the poles. Since ozone formation depends on ultraviolet radiation from the Sun, the amount of ozone present in the atmosphere at any given time and place varies. Also, the lifetime of an ozone molecule in the stratosphere is between several months and several years, so the distribution of ozone is affected by the motion of the atmosphere; ozone molecules can be transported long distances before being destroyed.

There are long-term trends in ozone. Between 1979 and 1991, the ozone in the mid-latitudes (roughly between 25° and 60°) decreased by an average of around 4 per cent per decade. In the northern hemisphere mid-latitudes, the decrease is greater in the winter and spring, and less in the summer and autumn, while the southern hemisphere mid-latitude decrease shows less seasonal variation.

IV

Degradation of the Ozone Layer

The first explanation for the presence of a layer of ozone in the atmosphere was given by the scientist Chapman in 1930, who noted that ozone is destroyed by ultraviolet, and created by a combination of one oxygen molecule with one oxygen atom. In the 1960s, however, it was realized this simple explanation could not account for the observed levels of ozone in the atmosphere. It is now known that catalytic cycles, involving other chemical species, influence the concentration of ozone. A catalytic cycle is essentially a chain of reactions:

• 1. X + O₃ → XO + O₂
• 2. XO + O → X + O₂
• Net: O + O₃ → O₂ + O₂

The first two reactions produce no overall change in the reactive chemical, X, but result in the loss of an ozone molecule. Such catalytic reactions are very important in understanding how the ozone hole is produced chemically; since X is neither created nor destroyed, one molecule of X can go through many cycles. This means that some chemical species in the atmosphere with much lower concentrations (1,000 times less) can significantly affect the amount of ozone; for example, chlorine, bromine, nitrogen, and hydrogen species. Ozone is therefore produced and destroyed by a balance between different reactions. If this balance is disturbed, large changes can be expected in ozone, and this is what occurs in the stratosphere over Antarctica to produce the ozone hole.