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Lower Atmosphere
Basics |
Tropospheric Ozone
Ozone is probably the most famous gas in climate science. Why?
It is a very contradictory gas, sometimes needed, sometimes not. Some people complain, because of the ozone smog. In the worst case you are not allowed to go by car, because too much ozone may be formed. |
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On the other hand people complain, because the ozone layer becomes thinner and the ozone is urgently needed in order to prevent us from skin cancer. And finally some scientists even tell us, ozone is a greenhouse gas and contributes to Earth warming. Who is wrong???
Actually, all of them are right.
Here, we will look at how ozone close to the ground is harmful to plants and humans and acts as a greenhouse gas. In the topic 'Higher Atmosphere' we will study the role of ozone as a shield against ultraviolet radiation. We will see that the same gas can play several roles.
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1. a-e) A chronic damage is a typical example for the negative impact of ozone on leafs. Here: Prunus serotina (autumn cherry) 0%, 4,4%, 7,8%, 12,3% and 24,5% damage.
source: Innes, Skelly, Schaub - Ozon, Laubholz- und Krautpflanzen, ISBN 3-258-06384-2, Copyright by Haupt Verlag AG / Switzerland
Please click images to enlarge them! (about 25 KB)
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Ozone is a gas with different properties. Some of them are helpful, some of them are not. Ozone is found in different layers of the atmosphere: The ozone layer is located in the stratosphere. We find high ozone concentrations between 25 and 40 km altitude. There, ozone is helpful and prevents us from skin cancer. But in this chapter we talk about the troposphere. A little bit of ozone is needed in the troposphere, in order to start processes of chemical cleaning, but not too much. During the last decades the ozone concentration has continuously increased in the troposphere. It has increased in average and sometimes, during ozone smog events, it was so high in certain areas, that it was dangerous for our health.
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2. Tropospheric ozone has been continuously increasing since the first measurements 1870. The diagramm shows ozone levels in fractions of ozone per one billion volumen fractions of air (parts per billion volume = ppbv).
composed by Valérie Gros, MPI Mainz, adapted from:
Marenco et al., 1992 (Long term evolution of ozone at the mid-latitudes of the Northern Hemisphere, European Geophysical Society, XVII General Assembly, 6-10 April 1992, Edinburgh).
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Danger for the respiratory system
Ozone is a reactive and irritant gas and in high concentrations it leads to respiratory problems. It can cause inflammation in the lungs and bronchia, which are usually reversible. Our body tries to protect the alveoli in the respiratory system from ozone. But if less ozone can penetrate, also less oxygen is let in ... the oxygen supply is too low, the heart has to work too much. In particular people, who already have health problems (e.g. asthma), are in danger. In the worst case ozone can be the elicitor of death.
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A form of oxygen
Ozone is a special form of oxygen. Normal oxygen (O2) consists of two oxygen atoms, ozone consists of three (O3), is less stable and more reactive, we could also say aggressive. Ozone is able to destroy organic material as explained for example in 'More - Unit 3'. This is why it attacks the human organism and also the vegetation, as shown in figure 1 in the case of leaves.
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3. Three forms of oxygen, but of completely different stability. The arrow shows increasing reactivity.
image: Elmar Uherek
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Table: forms of oxygen in the air
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form of oxygen |
number of atoms |
chemical stability |
appearance in air |
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atomic oxygen |
1 atom |
unstable / very reactive |
slightest traces |
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'normal' oxygen |
2 atoms |
stable |
21% of the air |
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ozone |
3 atoms |
rather stable / reactive |
10 - 100 ppb | |
ppb = parts per billion
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A greenhouse gas
Finally, like carbon dioxide (CO2) and methane (CH4), also ozone in the troposphere absorbs the heat radiation coming from the Earth and contributes to the greenhouse effect. It is the third most important greenhouse gas. The additional radiative forcing  F since the industrialisation (about 1750) is a measure for the human contribution to Earth warming. The diagramme shows the impact of tropospheric ozone compared to other greenhouse gases.
For all these reasons we do not want to have high ozone concentrations in the troposphere.
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Calculate the global warming contribution!
The diagramme above shows the contribution of several greenhouse gases to the positive change of the radiative forcing F at the tropopause given in Wm-2. The IPCC report (chapter 6.2.1 TAR scientific basis) tells us about the related change of the average Earth surface temperature TS.The relationship is roughly:
TS / F = 0,5 °C / Wm-2
Can you calculate the temperature change TS caused by cabon dioxide, methane, ozone and nitrous oxide?
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About this page:
author: Dr. Elmar Uherek - Max Planck Institute for Chemistry, Mainz
scientific reviewing: Dr. Rolf von Kuhlmann, Max Planck Institute for Chemistry, Mainz
educational proofreading: Michael Seesing - Uni Duisburg - 2003-07-02
revised and last published: 2004-04-30
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