I Introduction
Today we are going to talk about the ozone hole. There’s a good chance you got a sunburn in the last few weeks and so you know by experience how important it is to block out the ultraviolet radiation from the sun. Luckily, most of the radiation is blocked by our dear earth’s ozone layer already. Problem is there’s a hole in it. You probably have heard of it before, but do you know what caused it, how politicians tried to stop it and why they created a new problem by doing so? You will find out soon!
II What is the ozone layer
Earth’s atmosphere is divided in several regions, from the troposphere where we live, ’till the exosphere where space begins. The ozone layer can be found in the lower part of the stratosphere, between 20 to 30 km above the earth’s surface (see fig 1).
1. Position of the ozon layer in our atmosphere
The ozone layer gets its name from ozone gas, an allotropic form of the element oxygen, which is best known in the form of O2, the oxygen we breath. Ozone in the Earth’s stratosphere is created by ultraviolet light striking oxygen molecules containing two oxygen atoms (O2), splitting them into individual oxygen atoms, which is highly reactive and combines with unbroken O2 to create ozone, O3. The ozone molecule is also unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O2 and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle, thus creating an ozone layer in the stratosphere. The two gases are always converting into each other, continually “reapplying” the earth’s sunscreen (see fig 2 ).
2. The ozone-oxygen cycle
However, ozone gas doesn’t just coat our earth’s stratosphere. It can be found on the earth’s surface as well. The majority of tropospheric ozone formation occurs when nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs) react in the atmosphere in the presence of sunlight. Therefore it causes a lot of trouble in regions with a lot industry and traffic during the summer months. In contrast with the ozone layer, tropospheric ozone is bad for us, causing damage at the respiratory system.
III What is the ozone hole
Each year for the past few decades during the Southern Hemisphere spring, chemical reactions involving chlorine and bromine cause ozone in the southern polar region to be destroyed rapidly and severely. This depleted region is known as the “ozone hole” (see fig.3 – the bluer the thinner).
3. Comparison of thickness of the ozone layer above the antarctic continent
The primary cause of ozone depletion is the presence of chlorine-containing source gases (primarily CFCs and related halocarbons). When they reach higher levels in our atmosphere, the presence of UV light makes these gases to dissociate, releasing chlorine atoms, which then go on to catalyze ozone destruction. For example:
- Cl + O3 → ClO + O2: The chlorine atom changes an ozone molecule to ordinary oxygen
- ClO + O3 → Cl + 2 O2: The ClO from the previous reaction destroys a second ozone molecule and recreates the original chlorine atom, which can repeat the first reaction and continue to destroy ozone.
The Cl-catalyzed ozone depletion can take place in the gas phase, but it is dramatically enhanced in the presence of polar stratospheric clouds (PSCs). These polar stratospheric clouds(PSC) form during winter, in the extreme cold. Polar winters are dark, consisting of 3 months without sunlight. The lack of sunlight contributes to a decrease in temperature and the polar vortex traps and chills air. These low temperatures form cloud particles. These clouds provide surfaces for chemical reactions whose products will, in the spring lead to ozone destruction.
IV What they tried to do about it and how that created a new problem
This problems didn’t get by unnoticed, and in 1987, the Montreal Protocol was signed, controlling the emissions of CFCs, which where taught to be one of the major causes of ozone depletion. They were banned and scientist are measuring small recovering of the ozone layer, although it will take many years before all the CFCs have worked out completely. It was thus a good choice to ban the CFCs, but laws made it be replaced by HCFCs. This are in fact very damaging greenhouse gases. So they did solve the problem, but created a new one as well.
Sources:
http://ozonewatch.gsfc.nasa.gov/
http://science.howstuffworks.com/environmental/green-science/ozone-layer.htm
http://en.wikipedia.org/wiki/Tropospheric_ozone#Health_effects
