Ozone

What is ozone? Ozone is a molecule made up of three atoms of oxygen. It works a lot like sunscreen, blocking out harmful ultraviolet (UV) rays from the sun. In recent years, the amount of ozone in the atmosphere has decreased. So will the Earth, and all of the life on it, get sunburned? That’s the concern, and it’s one of the reasons why ozone is being looked at a lot by scientists who study climate and changes in Earth systems.

Is ozone good or bad? That’s a trick question, because it answer is: It depends. Most ozone is found below a 30 mile (48 km) height. However, depending upon WHERE the ozone is located, it can protect or harm life.

Let’s look at the Earth’s atmosphere. It’s really a thin layer of gases surrounding the Earth. How thin? Well, imagine a basketball with one layer of aluminum foil around it. The aluminum foil represents the average thickness of the atmosphere. That’s pretty thin! And considering without an atmosphere we’d all die, it’s kind of important to take care of the atmosphere.

The atmosphere is divided into regions defined primarily by temperature. From the Earth’s surface through the bottom layer of atmosphere, called the troposphere, temperature DECREASES with altitude. Weather occurs in this layer. It’s also the layer we live in.

The next layer up is called the stratosphere. In the stratosphere, temperature INCREASES with altitude. This is because of ozone. When the ozone in this layer absorbs UV light from the sun, it increases in temperature.

In the mesosphere, ozone concentration decreases. This means there is less absorption of UV light in this layer of atmosphere. Do you think the temperature would increase or decrease? Can we make this interactive with a popup right or wrong? It decreases!

In the upper atmosphere, called the thermosphere, temperatures are HOT. This is because incoming energy from the sun heats the molecules up.

So where is ozone "good", and where is it "bad" for life? High in the stratosphere, ozone acts as a protection from UV light. Without this protective shield, all animal life (not just humans) would be more susceptible to cancer, impaired immune systems, and eye problems like cataracts. If the ozone is close to the Earth and we breathe it in, ozone can cause damage to lungs in all animals and the transpiration (breathing) process in plants.

The amount of "good" and "bad" ozone in the atmosphere depends upon the balance between the processes that make ozone and those that destroy ozone. An upset in this balance could have serious consequences for all types of life on Earth.

For those of you who want the "bad" ozone news first, click on troposphere.

For those of you who want the "good" ozone news first, click on stratosphere.

Ozone in the troposphere is "bad" for breathing, for contributing to the smog and greenhouse gases created by human activities, and it can also act as a chemical oxidant by ripping off oxygen atoms from other compounds (including you, the plants around you, other animals, etc.)! Ozone this close to the surface does not exist in high enough concentrations to shield us from UV light at this level, so its benefits are only truly seen in the stratosphere.

Let’s look first at how ozone is made.

Insert o2.swf here

Oxygen gas (two molecules of oxygen, or O₂) is present in the atmosphere. High energy UV light collides with the oxygen molecule, causing it to split into two oxygen atoms. These atoms are unstable, and they prefer being "bound" to something else. The free oxygen atoms then smash into other molecules of oxygen, forming ozone. What’s the overall reaction?

O_{1 (atom)} + O_{2 (oxygen gas)} à O_{3 (ozone)}

The ozone is destroyed in the very process that protects us from UV rays emitted by the sun. When ozone (O₃) absorbs UV light, it will split the molecule into one free oxygen atom (O₁) and one molecule of oxygen gas (O₂).

O_{3 (ozone)} à O_{1 (atom)} + O_{2 (oxygen gas)}

Ozone is valuable to us because of the way it is destroyed – it absorbs UV radiation in the process. Even low-energy radiation can split ozone.

Are there any other processes or methods that change the amount of ozone in the atmosphere?

Natural forces can alter the amount of ozone. Remember, ozone is very unstable. It reacts easily with other atoms, and will easily donate that free oxygen atom (O₁) to nitrogen gas (N₂), hydrogen gas (H₂), or chlorine (Cl). These atoms have always existed in the stratosphere, and they are released from a wide variety of sources (volcanoes, oceans, etc.)

Scientists have discovered that ozone levels change as part of natural cycles on Earth. Ozone levels vary slightly with seasonal changes (winter to summer as an example), wind circulation patterns, and solar cycles. If a large volcano erupts, it can send enough material into the stratosphere to increase the breakdown of ozone.

Natural processes appear to have regulated the balance of ozone in the stratosphere over the Earth’s lifetime. Ozone production = ozone destruction. But in the early 1970s, scientists found some startling evidence that humans were contributing to the destruction of ozone. Many technological advances of the time included chemicals containing chlorofluorocarbons (CFCs for short). CFCs contain a chlorine, fluorine, and carbon atoms that are bound together. CFCs are stable. This may be great in an appliance like a refrigerator, but this is disastrous for CFCs that find their way up into the atmosphere. CFCs do not react easily with other chemicals in the lower atmosphere (troposphere). But UV light can break up a CFC, making it highly reactive. In the lower atmosphere, CFCs are protected from UV light from the ozone layer in the stratosphere. But as CFCs rise, they move into the stratosphere. The UV light in the stratosphere breaks up the CFCs. They release chlorine, and these free chlorine atoms rip oxygen atoms off of ozone, leaving ordinary oxygen gas.

Cl + O₃ à ClO + O₂

Chlorine + Ozone à chlorine monoxide +oxygen gas

This would not be a huge problem, except for one thing: chlorine monoxide will collide with free oxygen atoms (O₁). This oxygen atom will break apart the chlorine monoxide, releasing the chlorine atom back into the stratosphere to degrade more ozone. The chlorine atoms keep cycling through the process of breaking up ozone, and it has upset the balance of the ozone system. This reaction happens over and over again, allowing one chlorine atom to destroy many ozone molecules.

ClO + O_{1 à} O₂ + Cl

Chlorine monoxide + oxygen atom à oxygen gas + chlorine atom

Put a link to cfc.swf here

So, do chlorine atoms last forever, cycling through the stratosphere and breaking up ozone? Fortunately for us, they don’t. When a free chlorine atom reacts with other gases that contain hydrogen, the chlorine will be bound to the hydrogen to form HCl (hydrochloric acid). This can be carried into the troposphere and washed away by rain or snow. If humans then quit putting CFCs into the atmosphere, the ozone layer could eventually (in theory) repair itself.

Ozone depletion is a growing problem, but by banning the use of CFCs countries of the world hope to eventually restore the ozone balance to the Earth’s atmosphere. However, right now ozone loss is more than ozone produced. The most critical area seen with decreased ozone is over Antarctica. There are stratospheric cloud and ice particles that are not present in the stratosphere in warmer regions. Reactions that destroy ozone more quickly happen on the surface of these ice particles. Ozone levels drop so low, especially in the spring, that scientists call it a "hole" in the ozone layer. This allows UV light to reach life at the surface, and this light causes damage to the genetic material (DNA) of all living things who are exposed to it for long periods of time. Because of the low temperatures on Antarctica and the minimal amount of life existing there, the area of most concern is Australia. People, plants, and other animal life in the region have experienced increased rates of cancer, cataracts, etc. Could what happened in Australia get worse? Could the Earth lose its ozone shield altogether? We can only hope and pray that we have averted the problem of losing our "sun shield" in the future. It will take time to produce enough ozone to replace what has been lost.