The paradox of ozone

Ozone can be a problem gas: in the lower atmosphere, there may be too much of it and it is an indicator of pollution; in the upper atmosphere, there is not enough to block undesirable solar radiation. In both cases, the problem results from anthropogenic contamination of the air.

Oxygen is a basic building block of our planet. The crust, the oceans, and the atmosphere all contain important proportions of oxygen. Free oxygen, which is only present in the atmosphere, occurs as the diatomic molecule O2. In some cases, as a result of various natural or artificial causes, oxygen may occur as a triatomic molecule or ozone (O3).

When normal diatomic oxygen molecules reach the stratosphere, they are exposed to high-energy ultraviolet radiation, resulting in the formation of ozone. Ozone in the stratosphere filters out an important part of the ultraviolet solar spectrum. Without this protective layer, the amount of ultraviolet radiation reaching the Earth’s surface would increase to the detriment of all living organisms, including humans. The main effects would be at the molecular level, resulting in genetic malformations, cancers, and other diseases.

The concentration of ozone in the stratosphere has been gradually decreasing (despite seasonal variations), particularly over both polar regions. In Antarctica, where the process has received more attention, an “ozone hole” was observed in the early 1980s. More recently, an “Arctic hole” has also been found. In other parts of the ozone layer, there is also widespread thinning, which is becoming significant enough to affect biological activities.

The culprits identified as being responsible for this change are the chlorofluorocarbons (CFC-11 and CFC-12) contained in aerosol sprays, refrigerants, solvents, and foams. About 1 million tonnes of CFCs are emitted into the air every year. They remain in the atmosphere for 60 to 100 years - the current atmospheric concentration of chlorine is about 3 parts per billion (ppb) (Graedel and Gutzen 1989).

In the early 1970s, it was already possible to detect CFCs in Antarctica (Lovelock 1988). At that time, the concentration in the southern hemisphere was about 40 parts per trillion (ppt) and 50 to 70 ppt in the northern hemisphere. The threat to the ozone layer was not yet recognized. In 1974, Rowland and Molina (see Lovelock 1988) developed the hypothesis that CFCs were a source of chlorine and, therefore, a threat to the ozone layer. Since then, considerable scientific research has been done and, although not unanimously, it is generally believed that CFCs are indeed having deleterious effects on the ozone layer.

At ground level, ozone is a secondary photochemical oxidant, which is formed as a result of various human activities, including automobile engine combustion. Although it is not part of the emissions themselves, ozone is formed as an immediate result and is an important component of smog. Contamination in urban and industrial areas can be measured in terms of “ozone concentrations.” The gas is a clear indicator of air quality: the more ozone occurring in the lower atmospheric layers, the more contaminated the air. An improvement in air quality in large metropolitan areas will be accompanied by a reduction in the concentration of ozone. Ozone, itself, at low concentrations, is not a toxic gas, but its presence reveals that pollution emissions are taking place.