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Ozone in the Atmosphere ; Chemical Destruction of Ozone in the Stratosphere

Ozone is very rare in our atmosphere, averaging about three molecules of ozone for every 10 million air molecules. In spite of this small amount, ozone plays a vital role in the atmosphere.

Ozone is a form of oxygen that comprises three atoms (O3) rather than the two atoms (O2) found in ordinary molecular oxygen.

Ozone is mainly found in two regions of the Earth's atmosphere. Most ozone (about 90%) resides in a layer between 20 and 30 kilometers (12 and 18 miles) above the Earth's surface. This region of the atmosphere is part of the stratosphere. Within the stratosphere, the level of maximum concentration is at about 25 km (15 miles) where the average ozone concentration is 10 ppm (parts per million); that is, for every one million molecules, 10 are ozone molecules. The ozone in this region is commonly known as the ozone layer. It is important to realize that although we refer to this region of higher ozone concentrations as the ozone layer, ozone makes up only a small percentage the gas molecules in ozone layer (10 out of 1 million). The fact that there is so little ozone naturally in the stratosphere is one reason we are concerned about human activities that deplete stratospheric ozone, as this ozone is beneficial for life. The other region of elevated ozone concentration happens just above the ground surface and is mainly the result of human activity. This ozone can be harmful and is considered pollution. The figure above shows how ozone is typically distributed in the atmosphere.

The ozone molecules in the upper atmosphere (stratosphere) and the lower atmosphere (troposphere) are chemically identical because they all consist of three oxygen atoms and have the chemical formula O3. However, they have very different roles in the atmosphere and very different effects on humans and other living beings. Stratospheric ozone (sometimes referred to as "good ozone") plays a beneficial role by absorbing most of the biologically damaging ultraviolet sunlight (called UV-B), allowing only a small amount to reach the Earth's surface. The absorption of ultraviolet radiation by ozone creates a source of heat, which actually forms the stratosphere itself (a region in which the temperature rises as one goes to higher altitudes). Ozone thus plays a key role in the temperature structure of the Earth's atmosphere. Without the filtering action of the ozone layer, more of the Sun's UV-B radiation would penetrate the atmosphere and would reach the Earth's surface. Many experimental studies of plants and animals and clinical studies of humans have shown the harmful effects of excessive exposure to UV-B radiation.

Because ozone is toxic to humans and many other animals and plants, ozone near the ground surface is sometimes referred to as "bad ozone". Naturally, very little ozone is present near the Earth's surface. But, due to human activities, harmful concentrations can develop under certain conditions. The process that creates ozone near the ground is known as photochemical smog. It occurs when vehicle exhaust, other industrial chemicals, and abundant sunlight mix together in a complicated series of chemical reactions. This can be a very big problem in large cities in the summertime. Los Angeles, Phoenix, and Washington D.C. are examples of cities where ozone concentrations sometimes reach dangerously high levels. The national weather service will issue ozone warnings when ozone levels become dangerously high near the ground. The rest of this page concerns only stratospheric ozone and the issue of possible depletion of stratospheric ozone due to human activity.

Summary for Stratospheric Ozone Depletion

If you are interested, more detailed information about ozone depletion and the Antarctic ozone hole is provided by US Environmental Protection Agency Ozone Home Page.

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