Ozone

Ozone (O3) is a molecule made up of three oxygen atoms bonded together. Ozone has a distinctive, pungent smell. Electric motors and electric sparks can produce ozone. Most of the ozone in the stratosphere is confined to a region about 6 to 25 miles (10 to 40 km) above the Earth’s surface. This region is called the ozone layer.
The ozone layer is important because ozone molecules absorb ultraviolet radiation coming in from the Sun and prevent ultraviolet radiation from reaching the Earth’s surface. Ultraviolet radiation is damaging to plant and animal tissue and is what causes human skin to tan or burn. In fact, overexposure or long- term exposure to
ultraviolet radiation can lead to skin cancer in humans. The application of ultraviolet radiation is one method used to kill microorganisms in water during purification. In the stratosphere, ozone forms when ultraviolet radiation in sunlight causes an oxygen molecule (O2) to decompose into two oxygen ions (O–). Each of these ions reacts with an oxygen molecule to form an ozone molecule (O3). When an ozone molecule absorbs
ultraviolet radiation, it decomposes into an oxygen molecule (O2) and an oxygen ion (O–). This process continues repeatedly.

In the stratosphere there are two forms of oxygen: normal oxygen, O2, consisting of two O atoms; and ozone, O3, consisting of three O atoms. UV light is required to transform oxygen into ozone. The O-O bond of an oxygen molecule is broken by the energy from the UV light. Each O atom reacts with oxygen and forms ozone. In a similar way, ozone is destroyed by UV light. In this case the formed O atom reacts with another ozone molecule and forms two oxygen molecules O2. This process absorbes UV light and prevents it from reaching the Earth surface.

 

Good and Bad Ozone

Ozone located in the ozone layer is considered “good” ozone. The ozone layer absorbs most of the ultraviolet radiation and prevents it from reaching Earth. However, ozone found in the troposphere, the lowest level of the atmosphere, is considered “bad” ozone.

This “bad” ozone causes problems because it is highly reactive and damages plant and animal tissues. It is sometimes used to sterilize water to kill microorganisms. Ozone is one of the components of smog— air pollution— commonly seen as a haze above large cities. Ozone is also a greenhouse gas.
Ozone is produced in the troposphere when different compounds react with ultraviolet radiation that reaches that level. These compounds include hydrocarbons and nitrogen oxides, which are emitted by automobiles, gasoline vapors, fossil fuel power plants, refineries, and certain other industries. In this way, humans contribute greatly to the production of ozone in the troposphere and it is considered a pollutant.
Ozone Depletion Compounds containing chlorine, hydrogen, and nitrogen destroy
ozone naturally. Chlorine compounds in the atmosphere come from the ocean, hydrogen compounds come from decomposition of water, and nitrogen compounds come from the soil and the oceans. These different natural compounds form in different seasons and cause seasonal changes in ozone concentrations in
the troposphere. These compounds seldom affect the ozone in the ozone layer. Natural events, however, such as volcanic eruptions sometimes send these compounds into the stratosphere. The compounds then break down the ozone in the ozone layer, causing an increase in the amount of ultraviolet radiation reaching Earth.
Human activity has also had an affect on the ozone in the stratosphere. A class of compounds called chlorofluorocarbons (CFCs) is used in air conditioning, refrigeration units, and as an aerosol spray propellant. CFCs were discovered in 1920s and were thought to be safe because they were stable and chemically inert. When CFCs rise
through the troposphere, however, they are broken down by ultraviolet radiation. The chlorine released from this process destroys ozone. The reaction takes place in three steps as shown in these formulas (where CFCl3 represents chlorofluorocarbons):

CFCl₃ + UV light ➝ CFCl₂ + Cl
Cl + O₃ ➝ ClO + O₂
ClO + O ➝ Cl + O₂

The chlorine released in the third step is not broken down or changed, so it continues reacting with ozone as fast as the ozone is produced. Chlorine was not known to affect ozone in the stratosphere until 1973. Scientists found conclusive evidence of ozone depletion in 1984. Widespread depletion over the South Pole was discovered in 1985. Soon after, many CFCs were either banned or highly regulated. As a result, ozone levels in the stratosphere have begun to rise again.