Question

Why ozone layer can only form in stratosphere and not in troposphere

Answer 1

The stratosphere is the second layer of the atmosphere as we go upward. The troposphere, the lowest layer, is right below the stratosphere. The next higher layer above the stratosphere is the mesosphere. The stratosphere stretches between 15 and 50 kilometers up from the Earth's surface.

Ozone is produced naturally in the stratosphere when highly energetic solar radiation breaks down molecules of oxygen - O2 - and cause the two oxygen atoms to split apart in a process called photolysis. If a freed atom collides with another O2, it joins up, forming ozone O3. Ozone, that is relatively abundant in the stratosphere, heats this layer as it absorbs energy from incoming ultraviolet radiation from the Sun.

Temperatures rise as we move upward through the stratosphere. Such a temperature profile creates very stable atmospheric conditions, This is exactly the opposite of the behavior in the troposphere in which we live, where temperatures drops with increasing altitude. Because of this temperature stratification, there is little convection and mixing in the stratosphere, so the layers of air there are quite stable. (Hence the name stratosphere)

Beyond a certain depth into the atmosphere, there is insufficient ultra-violet light to break down oxygen any more. In an unpolluted atmosphere there is a balance between the amount of ozone being produced and destroyed and so the total concentration remains relatively constant. Ozone is unstable with respect to Oxygen and so water vapour at lower levels of atmosphere breaks down Ozone molecules. The stratosphere is very dry; air there contains little water vapor. That's why the Ozone layer stabilizes there.

(While this helps to retain Ozone in the stratosphere, it also means that other gases that get into the stratosphere can stay for long times. This is the case for Ozone destroying chemicals like chlorofluorocarbons or CFCs.)