What exactly is “Ozone Layer”
The ozone layer acts like a shield in the upper atmosphere (the stratosphere), to protect life on Earth from harmful ultra-violet (UV) radiation. The Earth's atmosphere is divided into several layers. The lowest region, the troposphere, is about 10 kilometers the Earth's surface high in altitude. To explain in simple words, like Mt. Everest, the tallest mountain on our planet, is only about 9 km high, similarly, troposphere is another 1 km high. Virtually all human activities occur in troposphere.
The next layer, the stratosphere, continues from 10 km to about 50 km. Most commercial airline traffic occurs in the lower part of the stratosphere.
Ozone is a molecule containing three oxygen atoms. It is blue in color and has a strong odor. Normal oxygen, which we breathe, has two oxygen atoms and is colorless and odorless. Most atmospheric ozone is concentrated in a layer in the stratosphere, about 15-30 kilometers above the Earth's surface. Ozone is much less common than normal oxygen. Out of each 10 million air molecules, about 2 million are normal oxygen, but only 3 are ozone.
However, even the small amount of ozone plays a key role in the atmosphere. The ozone layer absorbs a portion of the radiation from the sun, preventing it from reaching the planet's surface. Most importantly, it absorbs the portion of ultraviolet light called UVB. Overexposure to UV radiation can cause a range of health effects, including more skin cancers and premature aging, eye damage (including cataracts), reduced plant and animal productivity, poorer air quality, damage to plastics, impact on climate and suppression of the immune system and damage sensitive crops, such as soy-beans, and reduce crop yields.
At any given time, ozone molecules are constantly formed and destroyed in the stratosphere. The total amount, however, remains relatively stable. The concentration of the ozone layer can be thought of as a bucket of water under running tap. Although water is constantly flowing in and out, the depth of bucket remains constant.
While ozone concentrations vary naturally with sunspots, the seasons, and latitude, these processes are well understood and predictable. Scientists have established records over several decades that each natural reduction in ozone levels has been followed by a recovery. Recently, however, convincing scientific evidence has shown that the ozone shield is being worn out very fast as compared to the changes due to natural processes.
Ozone can be “good” or “bad” for people’s health and the environment, depending on its location in the atmosphere. “Good” ozone is produced naturally in the stratosphere and is “good” because it blocks harmful UV radiation from reaching the Earth’s surface where it can harm people and ecosystems. “Bad” ozone is an air pollutant found at ground level and is “bad” because it is harmful to breathe and can damage crops, trees, and other vegetation. Ground-level ozone is a main component of urban smog.
Reasons for Ozone Depletion
The Earth's Ozone Layer protects all life from the sun's harmful radiation. But human activities have damaged this shield. The most common of these are chemicals like chlorofluorocarbons (CFCs), halons, hydrochlorofluorocarbons (HCFCs) and methyl bromide used in refrigeration, air-conditioning, fire-fighting, metal-cleaning, foam-blowing, soil fumigation and so on.
All of these compounds have atmospheric lifetimes long enough to allow them to be transported by winds into the stratosphere. Because they release chlorine or bromine when they break down, they damage the protective ozone layer.
The CFCs are so stable that only exposure to strong UV radiation breaks them down. When that happens, the CFC molecule releases atomic chlorine. One chlorine atom can destroy over 100,000 ozone molecules. The net effect is to destroy ozone faster than it is naturally created.
It is important to note here that Chlorine from swimming pools, industrial plants, sea salt, and volcanoes does not reach the stratosphere. Chlorine compounds from these sources readily combine with water and they rain out of the earth very quickly. In contrast, CFCs are very stable and do not dissolve in rain. Thus, there are no natural processes that remove the CFCs from the lower atmosphere. Over time, winds drive the CFCs into the stratosphere. Large fires and certain types of marine life produce one stable form of chlorine that does reach the stratosphere. However, numerous experiments have shown that CFCs and other widely-used chemicals produce alarming 84% of the chlorine in the stratosphere, while natural sources contribute only 16%.
The World's Reaction
In the 1980s, scientists observed a thinning of the ozone layer over Antarctica, and people began thinking of it as an “ozone hole.” More research has shown that ozone depletion occurs over every continent.
Research has also shown that ozone depletion occurs over the latitudes that include North America, Europe, Asia, and much of Africa, Australia, and South America. Over the U.S., ozone levels have fallen 5-10%, depending on the season. Thus, ozone depletion is a global issue and not just a problem at the South Pole.
In 1987, leaders from many countries came together to sign a landmark environmental treaty, the Montreal Protocol on Substances That Deplete the Ozone Layer. Today, more than 190 countries—including the United States—have ratified the treaty. These countries are committed to taking action to reduce the production and use of CFCs and other ozone depleting substances.
The U.S., in cooperation with over 160 other countries- under Montreal Protocol, is phasing out the production of ozone-depleting substances in an effort to safeguard the ozone layer.
As a result of the Protocol, the total consumption of CFCs has fallen by more than 80 per cent since 1986. The consumption of CFCs, carbon tetrachloride, methyl chloroform and hydrobromofluorocarbons (HBFCs) was completely phased out by developed countries in 1996, except for a consumption of around 15,000 tons for essential uses approved by the parties.
Atmospheric scientists have detected the results of this reduction on the abundance of these chemicals in the atmosphere. The ozone layer has not grown thinner since 1998 over most of the world, and it appears to be recovering because of reduced emissions of ozone-depleting substances. They predict that, given the full implementation of the Montreal Protocol by all the countries, the ozone layer will begin its recovery within a few years and complete it by the middle of the twenty-first century.
Some Common ways by which contributions has been made to reduce depletion in Ozone Layer:
- In Fire protection to save property and lives: In Fire Extinguishers, use of dry chemicals (instead of using Halons-earlier) which don’t deplete Ozone Layer.
- In household and car air conditioners: use of HFC’s (instead of using CFC’s earlier). HFC’s are far less damaging to the ozone layer than CFCs, and HFC’s are not ozone-depleting substances
- In degreasing use of water soluble compounds (instead of using methyl chrolorform or, CFC’s earlier)
For the past 200 years, the burning of fossil fuels, such as coal and oil, and deforestation has caused the concentration of heat-trapping “Greenhouse gases” to increase significantly into our atmosphere. These gases prevent heat from escaping to space, somewhat like glass panels of a glasshouse.
Greenhouse gases are necessary to life because they keep the planet’s surface warmer then it otherwise would be. But as the concentration of these gases increase in the atmosphere, the Earth’s temperature is climbing the past levels. According to data, the warmest global temperatures on record have occurred in the past 15 years. Most of the warming in recent past is due to the result of human activities. Phasing out ozone-depleting substances in order to protect the ozone layer directly benefits the Earth’s climate.
Saloni Jain : A Shining Star of East Delhi
Images/illustrations by Saloni Jain