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Stratospheric Ozone[edit]

The topic of my essay is Stratospheric Ozone and its depletion. Ozone gas is naturally present in the earth’s atmosphere. It was discovered in the early 19th century. The formula of ozone is O3. It is naturally formed in a chemical reaction with UV light and oxygen molecules. This image shows the ozone production:

The oxygen molecules absorb the sunlight and break down into two oxygen atoms: O2→sunlight 2O Each oxygen atom can react with an oxygen molecule to form ozone: O+O2→ O3 The quantity of ozone in the atmosphere is not that big, but it has an important role for life on earth, as it absorbs the ultraviolet (UV) light from the sun, which is harmful for organisms. Most of the ozone is found in the stratosphere, as you can see in this graph. This is called the ozone layer.

In the seventies scientists discovered that chemicals such as chlorofluorocarbons break down the ozone layer.


CFCs, chlorofluorocarbons, are compounds that consist of chlorine, fluorine and carbon. They are part of the group ‘volatile organic compounds’ (VOC). The chemical structure of a few typical CFCs looks like this:

A carbon atom has four electrons in its outershell, so it needs to form four chemical bonds in order to get a full outershell. Fluorine and chlorine both have seven electrons in their outershells, so they like to form one chemical bond. You can find these compunds as a propellant in aerosol sprays, but they’re also used for refrigeration and air conditioning.

Ozone depletion[edit]

In the environment where CFCs get released, the troposphere, they have almost no impact. They slightly participate in chemical reactions that form ground level ozone. In the stratosphere on the other hand, they have an enormous impact. Because of their long lifetime they end up the stratosphere, where they destroy the ozone. This is really bad since the ozone in the stratosphere is very important: it absorbs at UV wavelengths, reflects UV light and transmits less harmful UV. It also stores energy in its chemical bonds, so it is like a warm layer over the earth. It releases this warmth when the temperature drops, so it acts like a greenhouse gas.

At first, a CFC loses its chlorine atom under the influence of sunlight. In this example carbonfluorotrichlorine turns into carbonfluorodichlorine and chlorine: CFCl3 →sunlight CFCl2+Cl Then the Cl atom reacts with and thereby destroys ozone. The products are chlorineoxide and oxygen: Cl+O3 → ClO+O2 The big issue here is that the ClO can in turn react with an oxygen atom and form an oxygen molecule and chlorine: ClO+O→ Cl+O2 In this way the chlorine is again free to destroy another ozone molecule.

The consequences of the depletion of ozone[edit]

Most of the ozone in the atmosphere, around 90%, can be found in the stratosphere. Ozone absorbs 95-99.9% of the UV light and thus forms a shield above the earth. It protects us from the most energetic UV light: UV-B and UV-C. These are really bad for you. The intensity of UV light on the earth’s surface increases as the amount of ozone in the atmosphere decreases. This is because ozone absorbs and reflects the sunlight. The intensity of the UV light increases in the regions where depletion of the ozone layer is detected. UV light damages your skin and your eyes severely. UV light doesn’t penatrate deeply into the body. It can cause skin cancer and damage cells in plants. This can have short term and long term effects. An example of a short term effect is sunburn (damage of your skin) or a tan (a reaction on the harm UV light caused) On the longer term it can damage your cells and change your DNA (skin cancer)

The hole in the ozone layer[edit]

The UV light intensity on earth has increased, because CFCs have destroyed the ozone layer severely, especially above Antarctica. There the layer is so thin that people have called it the hole in the ozone layer. The ozone-depleting substances that contain chlorine and bromine work better in a cold environment. At Antarctica there is no exchange with relatively ‘warm’ air. Because of that, iceclouds, also known as Polar Stratospheric Clouds (PSC), can form. They form a surface on which chlorine and bromine can destroy ozone. This is why there is a hole in the ozone layer above Antarctica.

The Montreal Protocol[edit]

Because of this severe threat to the ozone layer, international rules have been agreed to reduce the emission of CFCs. In 1987 the United Nations Montreal Protocol was created. It is an agreement between 191 countries to gradually terminate the use of CFCs. According to this protocol inspections are held to determine the national production and consumptions of CFCs.

Different viewpoints on the use of CFCs[edit]

From an environmental viewpoint humans have a big impact on the hole in the ozone layer. We use a lot of equipment that contain harmful CFCs. Because it is used in so many things, it’s really hard to reduce the emission. However, from a political viewpoint it’s a big issue to protect life on earth, so governments from all over the world are trying to reduce the emission and save the ozone layer, for instance with the Montreal Protocol.

Effect of the Montreal Protocol[edit]

Fortunately, the hole in the ozone layer above Antartica is starting to heal. This was measured in 2009, so 22 years after the Montreal Protocol. Detecting changes in the ozone layer is very hard, mainly because of natural variations, for instance, between summer and winter: annual swings are linked to the changes in the pattern of the stratospheric weather (when it’s cold, the destruction is more severe) The ozone layer has recovered for 15% since the late 1990s. However, even in 2085 ozone levels will still be lower than they were in the 1980s once every ten years. But it is very hard to make predictions. This is because of the climate change; you don’t know what’s going to happen. The Montreal Protocol has been a big success. The 191 countries that participated in this project have reduced their emission by 95%. Developing countries have managed a reduction of 72%. Despite these positive changes, the Montreal Protocol is still a success in the making and not a completed problem yet. In 2006 the hole in the ozone layer reached record levels. The impact of the emission of CFCs are longlasting. But we’re definitely on the right track!

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