A - THE ATMOSPHERE
Introduction
Radiation and The Atmosphere
Ozone 
Halogenoalkanes
Problems in the Troposphere
Ozone is present in the atmosphere in only tiny amounts dispersed among other gases. It is a very reactive gas and powerful oxidising agent. It is a nuisance in the troposphere. In the stratosphere it protects us by absorbing ultra-violet radiation.
Ozone is formed when an oxygen atom reacts with an oxygen molecule.
O + O2 O3
Ultra-violet radiation or an electric discharge split up (dissociate) an oxygen molecule (bond energy = 498 kJ mol-1). In the stratosphere, oxygen atoms are formed by photodissociation of oxygen molecules.
O2 + hν O + O
The oxygen atoms could recombine to reform oxygen molecules or combine with ozone to give oxygen molecules.
When ozone absorbs radiation of a certain frequency, some molecules undergo photodissociation and split up again.
O3 + hν O2 + O
This is the reaction which is responsible for the screening effect of ozone.
The above reactions indicate that ozone is being made and destroyed all the time. There is a steady state situation. However, the current situation is that ozone is being destroyed faster than it is being produced. It has been found that the chlorine atom, Cl reacts with ozone.
Cl + O3 ClO + O2 (1)
The ClO formed is another reactive radical and can react with oxygen atoms:
ClO + O Cl + O2 (2)
We have, therefore two reactions competing to remove ozone.
O + O3 2O2 (3)
and
Cl + O3 ClO + O2 (1)
Reaction 1 takes place 1500 times faster than reaction 3, even though the concentration of Cl atoms is much lower than the concentration of O atoms.
Reactions 1 and 2 shew that Cl is regenerated: it is acting as a catalyst.
Other radicals in the stratosphere can remove ozone:
- Hydroxyl radicals formed by the reaction between oxygen atoms and water.
HO + O3 HO2 + O2
The HO2 radicals can then react with oxygen atoms to regenerate HO radicals. - Nitrogen monoxide reacts with ozone to form nitrogen dioxide and oxygen.
NO + O3 NO2 + O2
