< Previous | Contents | Next >
There are three types of stratospheric clouds. They are:
1. Nacreous clouds extend from 10 to 100km in length and several kilometers in thickness. They are also called ‘mother-of-pearl’ clouds due to their glow with a seashell like iridescence.
There is a correlation exist between the cycle of ozone depletion and the presence of polar stratospheric clouds (PSCs) i.e. the ice particles of the cloud provided substrates for chemical reactions which freed chlorine from its reservoirs. Usually the reaction between HCl and ClONO2 is very slow, but this reaction occurs at a faster rate in the presence of a suitable substrate which is provided by the stratospheric clouds at the poles.
HCl + Chlorine nitrate —► molecular chlorine
It results in the formation of molecular chlorine and nitric acid. The molecular chlorine formed in the above reaction can be broken down to atomic chlorine and the ozone depletion reaction would continue. The PSCs not only activate chlorine, but they also absorb reactive nitrogen. If nitrogen oxides were present they would combine with chlorine monoxide to form a reservoir of chlorine nitrate (ClONO2).
Dimer of chlorine monoxide: Stratospheric chlorine monoxide reacts with itself forming a dimer Cl2O2. This dimer is easily dissociated by sunlight, giving rise to free chlorine atoms which can further react to destroy ozone.
Every spring, a hole as big as the USA develops in the ozone layer over Antarctica, in the South Pole. A smaller hole develops each year over the Arctic, at the North Pole. And there are signs that the ozone layer is getting thinner all over the planet.