Ozone layer holes

Measurements of ozone (O3) concentrations in the atmosphere are of particular importance. Ozone absorbs strongly in the ultraviolet region and it is this absorption which protects us from a dangerously high dose of ultraviolet radiation from the sun. The vitally important ozone layer lies in the stratosphere and is typically about 10 km thick with a maximum concentration about 25 km above the surface of the earth. Extreme depletion of ozone in a localised part of the atmosphere creates what is known as an ozone hole. 

Ozone has received increased attention for its occurrence and function in the Earth s atmosphere.For example the decreasing ozone concentration in the stratospheric ozone layer, becoming most obvious with the Antarctic ozone hole. 

The ozone (03) layer aver the southern hemisphere stratosphere in August 2007. The bar at the bottom indicates the color-coding used to indicate the thickness of the layer. The thickness is measured in Dobsons (= 0.01 mm thick). The normal ozone layer for the stratosphere is 360 Dobsons (color coded as green). The ozone "hole" shown in pink is 200-220 Dobsons. The "hole" will increase when another reading is taken in September. 

Ever) year our planet is bombarded with enough energy from the Sun to destroy all life. Only the ozone in the stratosphere protects us from that onslaught. The ozone, though, is threatened by modern life styles. Chemicals used as coolants and propellants, such as chlorofluorocarbons (CFCs), and the nitrogen oxides in jet exhausts, have been found to create holes in Earth s protective ozone layer. Because they act as catalysts, even small amounts of these chemicals can cause large changes in the vast reaches of the stratosphere. 

Mario Molina and Sherwood Rowland used Crutzen s work and other data in 1974 to build a model of the stratosphere that explained how chlorofluorocarbons could threaten the ozone layer. In 1985, ozone levels over Antarctica were indeed found to be decreasing and had dropped to the lowest ever observed by the year 2000, the hole had reached Chile. These losses are now known to be global in extent and it has been postulated that they may be contributing to global warming in the Southern Hemisphere. 

Events that take place on a grand scale often can be traced to the molecular level. An excellent example is the depletion of the ozone layer in the Earth s stratosphere. The so-called ozone hole was first observed above the Antarctic in the 1980s and is now being observed above both the Arctic and Antarctic poles. The destruction of ozone in the stratosphere is caused primarily by reactions between chlorine atoms and ozone molecules, as depicted in our molecular inset view. 

The chlorine atoms in the upper atmosphere come from the breakdown of CF2 CI2 and other similar chlorofluorocarbons (CFCs), known commercially as Freons. Production of these compounds was more than one million tons in 1988, largely for use in relrigerators and air conditioners. Once released into the atmosphere, CFCs diffuse slowly upward in the atmosphere until they reach the ozone layer. There, ultraviolet light Irom the sun splits off chlorine atoms. These react with ozone, with dramatic results. Annual ozone decreases have exceeded 50% above Antarctica. The background photo shows the Antarctic hole (red-violet) on September 24, 2003. 

In 1985 a hole was discovered in the ozone layer above Antarctica. 

There is an agreement that a hole (really a thin area) in the ozone layer over the Earth s polar regions, particularly Antarctica, changes in size over periods of time. The ozone is produced over the tropics and spreads to the polar areas. But not all scientists agree on the associated causes, seriousness, dangers involved, and remedies. 

The ozone layer is dynamic and unpredictable, which means it is constantly changing and seems to change in ways we cannot yet understand. The large thin area (hole) over Antarctica seems not only to move hut also to become larger and then smaller. The ozone layer is thickest over the poles of the Earth, yet it is mostly produced over the equator. 

The history of ozone depletion took a dramatic turn in 1985 when J. C. Farman at the BAS Halley Bay station announced that ozone levels over the Antarctic had decreased by more than 40 percent between 1977 and 1984. Farman explained that ozone levels had fallen so low that one could say that a "hole had formed in the ozone layer above the South Pole. In 1984, that "hole covered an area of more than 15 million square miles (40 million square kilometers), equal to the size of the continental United States. Clearly, ozone depletion was not a long-term problem about which scientists could debate for the next century or so. It was an issue that demanded quick attention and action. 

CFCs released to the atmosphere evenmally find their way up to the stratosphere where they destroy the ozone layer which protects the Earth s surface from harmful ultra-violet radiation. During the last decades, the ozone layer has been severely depleted, both over the Antarctic region where the ozone hole now appears annually, but also over the northern hemisphere. Ozone depletion up to 40% has been recorded in each of the last three years over Northern Europe. 

Stratospheric O3 depletion, commonly known as the ozone hole, is caused by the release into the atmosphere of certain manmade substances that destroy the ozone (the good ozone) at high altitude. Because of the thinning of the ozone layer,... 

Environmental concerns are very important. Some gronps promote the pnrchase and nse of green products. Other groups act as watchdogs, calling public attention to mannfactnrers and corporations they perceive to be lagging in environmental responsibility. It is not easy on anyone nsing hazardous solvents, especially if they are released into the environment and can be linked to holes in the ozone layer or to ecosystem disturbances. 

Figure 6.2 The hole in the ozone layer is represented in purple here.

Nobody takes responsibility for the unexpected side-effects of technology no one has to account for their actions, pay compensation or suffer punishment when it is suddenly discovered that refrigerants have caused a hole in the ozone layer or that fire retardants are present in human breast milk and may be affecting the development of our children. There is a lack of responsibility as accountability. 

Third, even where it is clear that a particular type of chemical has caused a particular incident of damage, and we can identify who was causally responsible for the existence of that chemical (if not necessarily its presence in a particular location), the first condition, that the conduct transgressed a norm, is often not met. This is because those norms require only that foreseeable consequences are taken into account one is not responsible for consequences that could not have been foreseen. Thus Du Pont, the main manufacturer of chlorofluorocarbons (CFCs) — non-toxic and non-flammable chemicals that were used as a refrigerant and aerosol propellant for many decades — have not been held to account for the hole in the ozone layer caused by those chemicals because at the time the key decisions to manufacture them were made, in the 1930s, these effects could not have been predicted (Colborn et al, 1996, pp243-245). 

In 1974, F. Sherwood Rowland and Mario Molina, who shared the 1995 Nobel Prize in Chemistry with Crutzen, showed that chlorine from photolyzed chlorofluorocarbons (CFCs) such as CF2C12 and CFCI3, which were used as supposedly inert refrigerants, solvents for cleaning electronic components, plastic foam blowing agents, and aerosol spray propellants, can also catalyze ozone loss. Subsequently, the chlorine monoxide molecule CIO, which is involved in the chlorine-catalyzed ozone destruction cycle, has been shown to be present in the holes in the ozone layer and to correlate inversely with... 

There has been an unprecedented level of international cooperation toward banning ozone-destroying chemicals. The first major step was the signing of the 1987 Montreal Protocol on Substances That Deplete the Ozone Layer, which called for the reduction of CFC production to one-half of 1986 levels by the year 1998. However, only a few years after the protocol was ratified, in 1990, scientists confirmed that the CFC problem was much more serious than they believed in 1987 when the protocol was drafted. The protocol was soon amended to call for cessation of all CFC production by 1996. Even with the protocol in place and the continued cooperation of all signatory nations, however, the ozone-destroying actions of CFCs will be with us for some time. Atmospheric CFC levels are not expected to drop back to the levels found before the ozone hole was formed until sometime in the 22nd century. 

Now we turn to the origin of the rate laws themselves. How do molecules of ozone change into molecules of oxygen What atomic interactions turn a mixture of fuel and air into carbon dioxide and water when it ignites in an engine What is really going on in terms of atoms high in the atmosphere as CFC molecules punch holes in the ozone layer Chemical kinetics and particularly the rate law for a reaction provide the kind of information we need to build a model of the reaction at a molecular level. 

In 1985 large holes were discovered in the ozone layer over Antarctica, Australasia and Europe (Figure 7.12). Scientists think that these holes have partly been produced by chemicals called chlorofluorocarbons or CFCs. CFCs were developed as refrigerants in fridges back in the 1950s. They have also been used in air conditioning systems, in aerosol cans (as propellants) and in the manufacture of expanded plastics such as polystyrene. CFCs are very unreactive molecules. They escape into the atmosphere and, because of their inertness, remain without further reaction until they reach the... 

Figure 7.12 Diagram showing the hole in the ozone layer over Antarctica...

Large holes have recently been discovered in the ozone layer over Antarctica, Australasia and Europe (Figure 11.5, p. 173). Scientists think that these holes have been produced by CFCs such as... 

In the 1980s, scientists studying the atmosphere over Antarctica shocked the world by announcing they had found a "hole" in the ozone layer over that continent. The startling discovery led to an international treaty called the Montreal Protocol that banned the use of CFCs in most products. Nearly all of the world s countries have signed the treaty, leading to a sharp decrease in the amount of ozone-eating CFCs in the atmosphere. 

---