Ozone in Two Places
The type of oxygen we are familiar with is diatomic oxygen, O2, the one we breathe. There is another molecular form of oxygen known as ozone, given by the chemical formula O3. It is now common to hear about the hole in the ozone layer. The thinnest layer of ozone is located in the Antarctic, where most of the frozen ice and snow is located. Ozone is beneficial when it is in the upper atmosphere and harmful when there is too much close to the surface of the earth on a hot day. Those of us who live in highly populated areas have probably heard of ozone days when the air quality is bad.
The ozone layer in the upper atmosphere is beneficial and protects the earth from the ultraviolet light of the sun. Ozone protects the earth in the upper atmosphere by the equilibrium of bond breaking and bond formation. Ozone and diatomic oxygen are in flux back and forth in the upper atmosphere. The suns rays break the oxygen bonds in ozone creating oxygen free radicals. In general, free radials are "bad" molecules. They cause very reactive and very random uncontrolled chemical reactions. Oxygen free radicals interact with diatomic oxygen to form ozone again. This radical reaction is in balance if no other free radicals are around. However, when aerosols are used that contain chlorofluorocarbons (CFCs) this increases the amount of free radicals like chlorine and fluoride. These free radicals break down the ozone molecule and do not allow it to go back to ozone readily. This is what happens in the upper atmosphere.
In contrast, in the lower atmosphere that we breathe daily there is an abundance of ozone on hot days. Car exhaust indirectly produces ozone at lower atmosphere levels. Ozone is produced in a similar type of radical reaction at the lower atmosphere level. Ozone at lower atmospheric levels is a greenhouse gas. It traps heat where it should not be and reduces air quality.
When you think about ozone remember that it is being depleted in the upper atmosphere and produced in the lower atmosphere. We need ozone in the upper atmosphere to absorb light from the sun, but until recently we were burning a hole up there slowly due to CFCs. Where we do not need ozone is in our cities, where smog produces unhealthy air quality and traps heat by our cities where it should be. The use of chemicals by humans without knowing the unintended consequences is an important example of how we impact the environment around us. At one time we used many CFCs in spray cans but we learned from the ozone example that this was bad chemistry for our atmosphere. Because we have stopped the use of CFCs the ozone layer has started to recover. In the a world with vast chemical knowledge available, we should learn from our chemical mistakes and try to remediate them as quickly as possible to keep our world clean and inhabitable.