The Earth's Climate

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Ozone

Ozone is a gas whose molecule is formed by 3 oxygen atoms. It is highly reactive, and its abundance in the atmosphere is the result of a balance between the formation and breakdown processes.
Ozone is found mainly in two atmospheric zones, and its role in relation to living organisms is beneficial in one and toxic in the other. The first zone is the stratosphere, a layer of air located between about 10 to 50 kilometers above sea level. Under the atmospheric pressure of the Earth's surface, this layer would be only about 3 millimeters thick. In the springtime, the stratospheric ozone is partially broken down in the polar regions due primarily to the particular meteorological conditions in these zones during the winter, and to the presence of chlorine and bromine atoms in the stratosphere resulting from human emissions of chlorofluorocarbons (CFCs) and halons. The destruction of the ozone layer above Antarctica in September and October is quite extensive, causing "the ozone hole." The phenomenon is also observed regularly in the Northern Hemisphere, although to a lesser degree. The ozone breakdown, which also occurs to some extent in the middle latitudes, has led to a steady depletion of the ozone layer over the past few decades. The beneficial role of stratospheric ozone arises from its capacity to absorb solar radiation in a particular ultraviolet range, UVB, thus protecting living cells on the Earth's surface against UVB rays' destructive action. Ozone is the only atmospheric compound, when it is present in sufficient quantities, capable of absorbing UVB rays in this wavelength range.

This depletion phenomenon was first noticed in the late nineteen-seventies. Following the signature of the Montreal Protocol, which called for the phasing out of CFC and halon emissions, the further breakdown of stratospheric ozone came to halt in the late nineties, although it will be several decades before the ozone layer is completely "repaired."
The second atmospheric zone where ozone is more or less uniformly present is the troposphere, the layer between the Earth's surface and an altitude of 10 to 16 kilometers. The ozone concentration here is only one-tenth that of the stratosphere. Although it is indeed the same molecule, tropospheric ozone is a pollutant. Its powerful oxidative action makes it a toxic gas when it reaches excessive levels. Vehicle exhaust fumes, which contain pollutants that undergo chemical reactions resulting in the production of ozone, are partly responsible for the increased ozone levels in the troposphere. When a certain "ozone peak" threshold is reached, public health warnings are issued and vehicle speed limits are reduced in densely populated areas.

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