A3.8 Ozone Depletion


Ozone is a form of oxygen. It is three molecules of Oxygen bound together (O3) [as opposed to the Oxygen we breathe which is two molecules (O2)]. Ozone is colourless and has a very harsh odour.

Most of the atmosphere’s ozone occurs in the upper atmosphere called the stratosphere. The stratosphere ranges from an altitude of 10km to 50km and it lies above the troposphere (or lower atmosphere). Stratospheric ozone shields us from the harmful effects of the Sun’s ultra-violet radiation. The structure of the atmosphere is illustrated below:

Source: http://www.ozone-hole.org.uk

Although ozone can be found through the entire atmosphere, the greatest concentration occurs at an altitude of about 25 km. This band of ozone-rich air is known as the “ozone layer”. Stratospheric ozone blocks harmful solar radiation (the ‘good ozone’ mentioned earlier).

Ozone also occurs in very small amounts in the troposphere. It is produced at ground level through a reaction between sunlight and volatile organic compounds (VOCs) and nitrogen oxides (NOx). This is an air pollutant and as discussed earlier, is ‘bad ozone’. This type of ozone will not be discussed further in this section.

Most of the ozone in the stratosphere is formed over the equatorial belt, where the level of solar radiation is greatest. It is transported by latitudinal air movements towards polar latitudes (and therefore the amount of stratospheric ozone above a location on the Earth varies naturally with latitude and season). Under normal circumstances highest ozone values are found over the Canadian Arctic and Siberia, whilst the lowest values are found around the equator.

The ozone layer acts as Earth’s sunscreen, providing an invisible filter to help protect all life forms from the Sun’s damaging ultraviolet (UV) rays. Most incoming UV radiation is absorbed by ozone and prevented from reaching the Earth’s surface. Without the ozone layer, UV radiation would not be stopped from entering the Earth’s atmosphere and arriving at the surface.