Student's Guide - Ozone
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What is ozone?

Ozone is a molecule composed of three oxygen atoms. The chemical symbol of ozone is O3 as the symbol of oxygen atoms is O.  

Most ozone found in our atmosphere is formed by an interaction between oxygen molecules and ultraviolet radiation emitted by the sun. (Oxygen molecules make up about 21% of all gases in the earth's atmosphere; they consist of two atoms of oxygen and are therefore labeled O2). 

When ultraviolet (UV) radiation hits an oxygen molecule, it may cause the molecule to break apart into single atoms of oxygen (O2 + UV -> O + O). These atoms are very reactive, and a single oxygen atom can combine with a molecule of oxygen to form ozone (O2+ O -> O3).

The individual ozone molecules make up what we call "the ozone layer". It is not, however, an especially thick layer - there are usually less than three ozone molecules for every ten million molecules of air. That is why ozone is known as a trace gas, meaning that only trace (very small) amounts are present. We are fortunate that these small amounts make a big difference.


Where is it found?

The ozone layer is a concentration of ozone in the stratosphere, an area about 10 to 50 kilometers above the earth's surface. In this layer, there is about 90% of the planet's ozone. Stratospheric ozone is a natural gas that stops most of the sun's ultraviolet radiation from reaching the surface of the earth. A diminished ozone layer allows more radiation to reach the earth's surface, where it has many harmful effects.

The layer of the atmosphere below the stratosphere is called the troposphere. That's the region where the weather takes place. About 10% of all existing ozone is found in this level. Its presence has two reasons. First, ozone from the stratosphere occasionally "strays" from the stratosphere into the troposphere. However, a great deal of tropospheric ozone has its source right here on earth. It is a product of complex chemical reactions that partly involve components of car exhaust and wastes from power plants (for example carbon monoxide and nitrogen oxides). Thus, ozone is often a key component of industrial smog, a common problem in many cities.


What does it do?

Ozone is a key element of the protection of life on earth from solar UV radiation. It is able to absorb much of the sun's harmful ultraviolet radiation, thus preventing its penetration to the earth's surface. In a sense, the ozone layer can be thought of as a UV filter for the earth. Without the ozone layer, the earth would have little defense against these harmful rays. The details of how it works are rather simple: When the ultraviolet rays emitted by the sun pass through the ozone layer, they may hit ozone molecules. Rays with the strongest energy break apart the bonds holding the oxygen atoms together, thus absorbing the energy and splitting the molecule. 

Ozone found in the lowest levels of the atmosphere can have harmful effects on humans. People with asthma or other breathing problems are especially prone to its effects.


How is it measured?

There are special instruments to measure the thickness of the ozone layer, see the section about instrumentation.

Scientists use a unit called the Dobson Unit (abbreviated DU) to describe the amount of ozone in the atmosphere. They calculate how thick the ozone layer would be if all ozone in the atmosphere were compressed into a single layer at zero degrees Celsius and sea level atmospheric pressure (that is 1013 mbar). To get a better understanding, picture a column extending through the ozone layer. That column would capture a certain number of ozone molecules. However, these ozone molecules would be widely spread out throughout the column. Now let's say that you were to take all the ozone molecules caught within the column and compress them to sea level pressure, and then measure the thickness in millimeters. Multiply the number by 100, and you have the thickness of the ozone layer expressed in  Dobson Units! The average thickness is about 300 DU, which equals a three millimeter (or 0.12") thick layer of compressed ozone. Yes, the ozone layer is really rather thin!


How is it distributed over the globe?

Just as UV levels vary according to different factors, ozone levels follow a similar course. A distinct ozone pattern exists over the entire globe. There is less ozone over the equator than over other parts of the world, exempting the seasonal ozone hole in Antarctica. Surprisingly, even though the ozone levels are consistently low in the tropics, it is here that most of the ozone production takes place. This is because the amount of ozone produced is directly related to the level of ultraviolet radiation, which is highest in the tropics. On the other hand, processes that destroy ozone are also more effective above the equator, and that leads to the observed relatively small ozone amounts there. Ozone at mid-latitudes (that is were the United States and Europe are located) is partly produced in the tropics and then transported to those latitudes. Because of these transport processes there is some seasonal variation in the ozone layer's thickness with higher values during spring than in fall.

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