ozone-layer chemistry

ozone-layer chemistry Ozone, a gas composed of three oxygen atoms (O_3(g)), protects the Earth's surface from harmful ultraviolet radiation from the Sun. More than 90 per cent of the ozone in the atmosphere resides in the upper atmosphere, the stratosphere.

Ozone is formed as a result of the dissociation of molecular oxygen by ultraviolet radiation in the stratosphere to generate oxygen free radicals (O^•):O_2(g) + energy → O^• + O^•.

These free radicals may then combine with another molecule of oxygen to form ozone:O_2(g) + O^• + M → O_3(g) + M,

where M is another molecule which acts simply to remove energy from the system and stabilize the ozone molecule that is generated.

Ozone is destroyed naturally in two main ways:(1) The ozone formed is rapidly broken up by ultraviolet (UV) light:O_3(g) → O_2(g) + O^•generating a molecule of oxygen and an oxygen free radical; or(2) an ozone molecule combines with an oxygen free radical to form two molecules of oxygen:O_3(g) + O^• → 20_2(g).Ozone can also be destroyed in other ways by man-made chemicals that have been released into the atmosphere. Most problematic are chlorofluorocarbons (CFCs). Chlorofluorocarbons are volatile compounds, used as refrigerants, which have escaped into the atmosphere. With time these chemicals have reached the upper atmosphere, where they can react with ozone and destroy it. Concern with CFCs, which persist for long periods of time in the atmosphere, grew and led to international regulation in the form of the Montreal Protocol which (as amended in 1990) imposed a complete ban on CFC production by the year 2000. Chlorine gas (Cl_2(g) is released into the atmosphere as a component of volcanic gases. If chlorine molecules undergo photodissociation (break-up by light) two chlorine free radicals are generated (Cl^•) which may then cause the destruction of ozone:O_3(g) + Cl^• → ClO + O_2(g)ClO + O^• → Cl^• + O_2(g).

The net reaction isO^• + O_3(g) → 20_2(g).

Ozone can also be destroyed by reactions with nitrous oxide emissions from aircraft. This, once thought to be a major factor responsible for ozone depletion, is now considered to be less significant than the other mechanisms.

In recent years ‘holes’ in the ozone layer have been reported to develop each spring over the polar regions. These holes result from the formation of a body of still, cold air over the poles during the winter months. Temperatures within this cold air may drop to below −80 °C. If this occurs, reactions take place which generate forms of chlorine that become active ozone depleters when sunlight returns in the spring.

Elizabeth H. Bailey