In this post I shall be explaining:
- Why absorbing too much ultraviolet radiation is harmful
- How the ozone layer protects us from absorbing too much ultraviolet radiation
- How the ozone layer is depleted by CFCs
Why absorbing too much ultraviolet radiation is harmful
When ultraviolet rays from the sun hits the skin, its energy is absorbed by the molecules in the skin causing the molecules in the skin to vibrate more vigorously. Sometimes, the UV ray is powerful enough such that it causes electrons to be detached from the molecules causing them to become ions.
This results in damage to the cells in the body. When the DNA in the cell is damaged, it may lead to abnormal pattern of cell division thus causing cancer.
However, UV rays do not carry the same amount of energy. Generally speaking, ultraviolet radiation from the sun can be divided into 3 bands, UVA, UVB, and UVC.
UVC has the highest energy as it has the highest frequency [thus UVC is the most harmful] while UVA has the lowest energy as it has the lowest frequency.
Fortunately, most of the UVB and UVC do not reach the Earth due to the ozone layer.
How does the ozone layer protect us from UVB and UVC?
Many students have the misconception that the ozone layer acts as some kind of giant shield that reflects the UV radiation back into outer space. This is not true. The ozone layer absorbs the UVC and UVB radiation as seen in the reaction sequence below.
UVC which has the highest amount of energy is able to break the strong double covalent bond between the oxygen atoms in an oxygen molecule. (Remember, energy is absorbed during bond breaking!) This forms 2 oxygen atoms (The fancy JC term for this is called “free radical” – but this term just refers to any atom/molecule/ion with unpaired valence electrons.) The dot that you see beside the oxygen atom in the equation above is just to signify that the oxygen atom is a free radical, i.e. it has unpaired valence electrons. See below for electronic structure of oxygen atom.
The oxygen atom is obviously unstable since it does not have a stable electronic structure, so it will react with another oxygen molecule to form ozone O3. The full equation with the various electronic structures is shown below
Ozone molecule is less stable as compared to oxygen molecule due to the dative covalent bond. Hence, the ozone molecule can be decomposed by the less energetic UVB radiation to form oxygen molecule and an oxygen atom (free radical).
The oxygen atom, in its quest for stability then reacts with another ozone molecule to form 2 oxygen molecules. And the cycle would repeat itself again from step 1.
There are 2 key points to note:
- Concentration of ozone in the atmosphere remains relatively stable. This is because, the 2 ozone molecules formed in step 2 are broken down in steps 3 and 4 respectively
- The continuous formation and destruction of ozone molecules helps absorb the harmful UVC and UVB radiation from the sun before it reaches the earth (Step 1 and 3).
How is the ozone layer depleted by CFCs?
At low altitudes, CFCs are stable, because the UVA rays that reach the earth’s surface do not have enough energy to break the C – Cl or the C – F bonds in CFCs. Therefore, the CFCs will diffuse to the higher altitudes. However, at high altitudes, the higher energy UVB/UVC rays are able to break the C – Cl bond to produce a chlorine atom. (Now, to be more precise there is a 2 stage reaction but I’m skipping one step to keep things short, since that step will not significantly alter the story anyway)
The chlorine atom is unstable and hence reacts with the ozone molecule to form oxygen gas and chlorine oxide which is free radical. See below for equation with electronic structures drawn in.
The chlorine oxide free radical reacts with another ozone molecule to form oxygen gas and a chlorine atom. Hence steps 2 and 3 will keep repeating resulting in a vicious cycle which destroys thousands of ozone molecules.
So, you may ask me, what is the problem? The problem is that steps 2 and 3 here result in the destruction of ozone molecules but without absorbing any harmful UVB unlike the previous mechanism! Hence, UVB radiation is are able to reach the earth and give sunbathers skin cancer.
This vicious cycle is only broken when the chlorine atom reacts with water vapour in the air to form hydrogen chloride which has a stronger bond and cannot be broken down by UVB or UVC radiation. (Note: There are other reactions that also remove the chlorine atom but let’s keep it simple)
Phew, we have finally reached the end of this long and convoluted story.. Thank you for taking the time and effort to read it! If you didn’t understand a thing, it’s alright; this story will not be tested in the exam! But it is an interesting story nonetheless, no? Hasta La Vista Baby!