7. What is the ozone hole?

7. What is the ozone hole?

Ozone (O3) is a greenhouse gas in the stratospheric level, between 10 and 50 km above Earth’s surface. Ozone has a concentration of 2 – 8 ppm, while the remaining 210,000 ppm are taken up by oxygen (O2). Ozone in the upper atmosphere absorbs short-wave ultraviolet (UV) rays between 240 and 160 nm. This high energy radiation can produce ozone from oxygen, also known as photochemical reaction. The process of ozone creation and destruction is called Chapman cycle. Although ozone is created primarily at tropical latitudes, large-scale air circulation cells in the lower stratosphere move ozone towards the poles, where its concentration increases.

The ozone in the stratosphere absorbs a large part of the solar high energy radiation, which is harmful to organisms. The UV radiation can change chemical structure of molecules due to its high energy and cause gene mutations. The stratospheric level acts like a barrier for the dangerous radiation emitted by the sun. Ozone can also be found at ground level, which is a human health irritant and component of smog. This tropospheric ozone is produced by anthropogenic sources and has nothing to do with the “ozone hole”.

The term “ozone hole” refers to the depletion of the protective ozone layer in the stratosphere over Earth’s polar regions (see figure below). The concentration of ozone in this area is lower, which causes more high-energy radiation to pass. This increase in UV-radiation causes harm to the organisms there, resulting in health problems, from eye damage to skin cancer.

Why is ozone depleting?

The thinning of the ozone layer is caused by increasing concentrations of ozone depleting chemicals – chlorofluorocarbons (CFCs) and, to a lesser extent, halons. These chemicals can remain in the atmosphere for decades to over a century. These gas molecules were found in refrigerators or aerosol-cans, but their use was prohibited with the Montreal protocol in the 1980s. During the dark polar winters, stratospheric clouds form due to very low temperatures. These clouds last until spring and create the condition for drastic ozone destruction, since they provide a surface for chlorine to change into its ozone destroying form.

The CFCs attach to the polar stratospheric clouds until they are released in spring. When released, these ozone destroying molecules attach to the ozone and break their molecular bonds. After this process, the molecular structure of ozone is altered and the ability to absorb UV radiation is reduced. Due to the CFC molecules, the concentration of ozone at the poles has decreased significantly and ultimately resulted in a “hole”. The ozone reduction in the Arctic is comparable to that in the Antarctic. Ozone can also be removed by aerosols emitted from non-anthropogenic sources. Volcanic eruptions can emit sulfate particles into the stratosphere, which have a similar effect to the CFCs.

Figure 5: False-color view of total ozone over the Antarctic pole. Purple and blue represent areas where there is the least ozone, yellows and reds where there is more ozone. Source: NASA Ozone Hole Watch.

Does the ozone hole affect climate change?

The continuously increasing greenhouse effect prevents heat from the lower atmosphere to traverse into the stratosphere. This effect occurs due to an increasing amount of greenhouse gases, which absorb the heat radiation in the troposphere and reflect it back to the surface. The troposphere then acts like a heat barrier, preventing the heat from rising to the stratosphere, which in response is cooling down. In a cooler stratosphere, ozone loss creates a cooling effect that results in further ozone depletion. UV radiation releases heat into the stratosphere when it reacts with ozone. With less ozone, there is less heat released, amplifying the cooling in the lower stratosphere and enhancing the formation of ozone-depleting polar stratospheric clouds, especially near the South Pole.

With less ozone in the stratosphere, the more UV radiation passes through the stratosphere to the surface. However, UV radiation plays only a small role in global warming, because the concentrations in the solar radiation are too low to contribute to a significant warming effect. The net effect of UV-radiation is to cool the stratosphere rather than warming the troposphere. Therefore, the ozone hole does not contribute to the effect of global warming. Yet the depletion of ozone is problematic, since the increasing amount of UV-radiation may affect the health of organisms.

What scientists have uncovered recently, however, is that the ozone hole has been affecting climate in the Southern Hemisphere. That’s because ozone is also a powerful greenhouse gas and destroying it has made the stratosphere over the Southern Hemisphere colder. The colder stratosphere has resulted in faster winds near the pole, which somewhat surprisingly can have impacts all the way to the equator, affecting tropical circulation and rainfall at lower latitudes. The ozone hole is not causing global warming, but it is affecting atmospheric circulation.





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