Student's Guide - The Ozone Hole
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The Ozone Hole

 

What is it?

The phenomenon which we call the ozone hole was first discovered in the 1980s. The ozone hole isn't actually a "hole" in the literal sense, but rather more of a wearing thin of the normal ozone levels. It is defined by the scientific community as anywhere in which the amount of column ozone in the atmosphere is under 220 Dobson Units. The ozone hole now occurs every year in Antarctica between the months of September and November, which is the South Pole's springtime. During this time, ozone levels over most of Antarctica are reduced by about 60% of their usual amount.

 

Causes of the Ozone Hole

When it was first detected in the 1980s that the minimum springtime levels of ozone were steadily dropping, great concern began to arise in the scientific community. Almost immediately, theories regarding the cause of such a phenomenon were developed. Some of these theories included a belief that changing wind conditions in the Antarctic was causing ozone-low tropospheric air to be blown up into the stratosphere. This was eventually proven false as scientists observed a lack of other gases that would have been present in the troposphere had this theory been correct. Another theory included the idea that ozone was destroyed by chemical reactions with other trace gases. In the end, it was this theory that was determined to be correct. The chemical culprit prompting these reactions was determined to be chlorine, which was being introduced into the atmosphere largely in the form of CFCs (see below). Satellite data and observations further strengthened this idea, as it was found that as ozone levels dropped, levels of ClO (chlorine monoxide - the product of a reaction between chlorine and oxygen atoms) increased.

 

What are CFCs?

CFCs, also known as chlorofluorocarbons (composed of carbon, fluorine, and chlorine) were invented in the early 1930s. Over the years, they have served many functions, including propellants for aerosols, refrigeration coolants, and electronic circuit board cleaners. There are different types of CFCs: CFC-11 and 12 for example are common refrigeration coolants. CFC-11 is also known under the brand name of Freon.

 

Why Antarctica?

After determining the cause of the problem, a question naturally arises, if most of the CFCs are released over major industrial countries such as the United States and Japan, then why is the ozone hole forming over Antarctica? The answer lies in at least two reasons.

First, when something (like a CFC molecule) is released into the air, it does not remain in the atmosphere directly above its origin. Because CFCs have a life-span of several decades, they remain intact long enough to make their journey up into the stratosphere. The key to the long life of CFCs is their non-reactivity. They don't react with other substances in the troposphere, and only break apart in the stratosphere when they are exposed to high-energy ultraviolet radiation - a process that could take up several years. Therefore, winds in the troposphere and stratosphere have sufficient time to distribute CFC molecules around the globe

Secondly, the weather conditions in the Antarctic are such that they encourage the creation of so-called polar stratospheric clouds (PSCs). These clouds form only under persistently cold conditions, which is why they are usually only found in Antarctica (PSCs can also be found in the Arctic, but because the weather is not as persistently cold, they are less common.) To understand why it is that PSCs contribute to ozone depletion, more information about the chemistry taking place in the stratosphere is needed.

As mentioned before, when CFCs enter the stratosphere, they are exposed to high-energy ultraviolet rays from the sun, which cause the chlorine (chemical symbol Cl) to break apart from the CFC molecule. One chlorine atom is capable of fragmenting over 1000 ozone molecules (for example via the reaction  Cl + O3 -> ClO +O2) before it is trapped again in stable molecules (sometimes called reservoir substances), such as chlorine nitrate  (ClONO2). This fact is interesting by itself because it explains some of the ozone decrease observed world-wide. Yet it does not explain the ozone hole, which only forms when chlorine is again released from the reservoir substances.

This is were the PSCs come into play. At the surface of these icy clouds, the reservoir substances are again transformed into more active forms of chlorine. For example, ClONO2 reacts with hydrochloride acid (HCl) to form chlorine gas (Cl2) and HNO3. During the period of complete darkness during the polar night large quantities of Cl2 can accumulate, but still only little ozone decrease is observed.

The massive destruction of ozone that finally leads to the ozone hole takes place only when the first rays of sunlight are striking the Antarctic atmosphere after the polar night, splitting Cl2 into two atoms of chlorine (Cl2-> 2 Cl). Now ozone destruction can start again via the reaction   Cl + O3 -> ClO +O2 (It goes without saying that the complete chemistry that is going on is far more complex).

As there is so much chlorine in an active form at the end of the polar night (September in Antarctica) the ozone hole can grow to a size larger than the United States. At the South Pole, ozone levels below 100 Dobson Units are now frequently observed in late September and early October. Before the ozone hole existed, typical ozone values were 300 Dobson Units.

 

Relevance to the Rest of the World

Because the location of the ozone hole is far separated from most of the human population, it would be easy to dismiss it as a phenomenon irrelevant to the rest of the world. It does, however, have a significant impact on the rest of society. First, the oceans around Antarctica are rich in life, which is threatened by increased ultraviolet radiation under the ozone hole. Secondly, when the polar vortex, which keeps ozone depleted air trapped over Antarctica during the spring, breaks up, ozone-poor air is dispersed over nearby human-inhabited areas. Such areas include Australia, New Zealand, Southern Argentina, and Southern Chile. And thirdly, there are concerns that the stratosphere over the North Pole could cool down during the next decades. If that were the case, PCS would become more frequent in the northern hemisphere, causing severe ozone depletion over Alaska, Canada, Northern Europe and Siberia. Here human settlements are far more frequent than in the Southern Hemisphere.

 

What is being done about it?

In light of the increasing evidence that man-made products were partly responsible for the newly observed ozone hole, the world agreed that measures needed to be taken. As a result, 24 nations convened in September of 1987 and designed what we know as the Montreal Protocol, a treaty intended to limit the production of CFCs and other ozone depleting substances (95 chemicals in all), to the eventual goal of ending their manufacture completely. As outlined in the original protocol, developed countries would be required to phase out CFC production completely by 1996. Developing countries were also acknowledged in the agreement, and were allotted more time to end production - they were given until 2010 to perform the same task. However, the Montreal Protocol was designed in such a way that revisions were easily accomplished, and have been. Since 1987, there have been a number of amendments to the original document, resulting in a more rapid decline of ozone depleting substances. These additions include the London, Vienna, Copenhagen, Montreal, and Beijing amendments. More information on the Montreal Protocol is given here (PDF-document).

 

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