What is ozone?

Ozone is a trace gas in the earth’s atmosphere.  The atmosphere is made of many different gases, mostly nitrogen (78%), oxygen (21%), and a little argon (almost 1%).  Trace gases like ozone and carbon dioxide make up the remaining atmosphere.  These trace gases are measured in parts per million or even parts per billion—they are present in very small amounts!  Only about three molecules of ozone exist in the atmosphere for every 10 million molecules of other gases.  Despite their rarity, they cause some very important chemical reactions. 

The oxygen you breathe is made of two oxygen atoms bonded together.  Ozone is three oxygen atoms bonded together—but the bonds between the atoms are much weaker.  So ozone atoms combine easily with other chemicals around them.  What ozone combines with depends on where it is in the atmosphere.

Ozone high up

Most ozone is found in the stratosphere, an upper layer of the atmosphere, where it protects the earth from excess solar radiation.  Ultraviolet light from the sun can cause sunburns and damage the tissues of people, animals, and plants when it reaches the earth’s surface.  Ozone works to absorb much of this ultraviolet radiation in the stratosphere, before it gets to the surface. 

Stratospheric ozone occurs naturally when ultraviolet light from the sun breaks the chemical bonds of oxygen atoms, freeing up single atoms.  Those single atoms each combine with an oxygen molecule to form ozone.

Ozone low down

Ozone in the troposphere, the lowest layer of the atmosphere, acts differently than ozone in the stratosphere.  High levels of ozone can be harmful to people, animals, and plants.  Ozone is a main ingredient in smog, where it causes eye irritation and worsens the conditions of some people suffering from asthma, bronchitis, and heart disease.  Ozone damages plant tissues in crops and forests. It can break down some materials like rubber and nylon.

Ozone also contributes to climate change. Ozone is the third most important greenhouse gas after carbon dioxide and methane, even though ozone is a much smaller component of the total atmosphere.

Some tropospheric ozone is produced naturally by downward flux from the stratosphere or during lightning strikes.  But most of it is formed by the interaction of sunlight with hydrocarbons and nitrogen oxides produced in car exhaust, fossil fuel power plant emissions, and other combustion processes.  Human activities have doubled the level of ozone in the troposphere over the last century. 

It is anticipated that ozone from human activities will continue to increase over the coming decades.  To assess the effects of future ozone levels and to develop ozone control strategies, we need models of how ozone works in the atmosphere that are as accurate as possible. Finding out how the atmosphere works is like putting together a puzzle with thousands of pieces - many teams of researchers will work on various parts until the whole picture becomes visible.  Scientists have found out a lot about how ozone acts, but a few pieces are still missing.  The Ozone and the Oceans team is working on the piece that describes how much ozone is absorbed into the ocean.

Ozone is present in two layers of the atmosphere. In the stratosphere, ozone filters out harmful ultraviolet light. But ozone in the troposphere can cause problems for animals, people, and plants. Diagram from Project LEARN: Atmospheric Science Explorers web site Cycles of the Earth and Atmosphere.