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Topic: Lower atmosphere. Basics.
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Tropospheric air is a mixture of a few dominant gases and many many trace gases, some of which are rather important for our climate.

The gas phase

The most obvious feature of the air air is that we cannot see it!  But if something is invisible this does not mean that it does not exist.  Dew on the grass in the morning disappears as the sun comes up.  The water droplets don't disappear by magic, they simply evaporate and change from the liquid phase to the gas phase.  This change between a visible and an invisible state is most easily understandable for water.

1. Dew rising in the morning - Virgen valley Osttirol.  Photograph: Elmar Uherek.

Dry air is made up of about 78% nitrogen, 21% oxygen and 1% Argon. These gases can also occur as liquids but it takes temperatures below -150°C for this to happen and we never observe such low temperatures naturally.  So air always exists as a gas and is invisible to our eyes.

2. The major components of the atmosphere - nitrogen (N2), oxygen (O2) and argon (Ar).  Author: Elmar Uherek.


When we see pictures of sand storms in the Sahara it's very obvious that there is a lot of sand and dust particles in the air. The same is true in cities where industrial processes and car exhausts emit particles into the air.  Little particles are even found in air over really remote places such as Antarctica or over the middle of the oceans.

3. Electron microscope image of aerosol particles collected from the atmosphere above the Mediterranean Sea.  Author: Research Group Dr. Helas, MPI Mainz.

Particles can either be directly emitted into the atmosphere or can be formed by chemical reactions in the air.  They are extremely important to our climate, they are essential for cloud formation and they can prevent solar radiation from reaching the surface of the Earth.

Water vapour

When we talk about the composition of air we generally mean dry air and ignore any water it contains.  The main gases (nitrogen, oxygen, and argon) make up nearly 100% of the composition of dry air.  The most important of the trace gases is carbon dioxide which makes up 0,04% of the air. Other gases occur in much smaller amounts.  The amount of water vapour in the air is really variable, making up between 0.1% and 4% of tropospheric air, depending on the climatic conditions.  Cold air can hold much less water vapour than warm air.

4. Global overview of the total water vapour column in July 1989.  Source: NASA water vapour project NVAP. /NVAP/nvapcira.html

Trace gases

Many climate processes are controlled by the levels of trace gases in the atmosphere, rather than the major constituents.  These gases are present in very low amounts, i.e. a few molecules in one million or even one billion air molecules.  To describe this, we often use the unit ppm (parts per million) so a trace gas with a concentration of 1 ppm means that there is just one molecule of the gas in every 1,000,000 air molecules (the more scientific unit is 1 µmol mol-1, we will talk more about atmospheric gas concentration units later). 

Levels of carbon dioxide, a very important greenhouse gas have increased from 280 ppm in preindustrial times to about 400 ppm now (2015) and predictions are that these concentrations will continue to rise due to human activities, the most important of which is fossil fuel combustion.  Two other important greenhouse gases are methane (1.8 ppm) and ozone (varying around about. 0.04 ppm).  In addition, there are thousands of organic and inorganic gases which are emitted into the air from plants (imagine the smell of flowers) or during industrial procedures (think about solvents) or are formed during chemical processes in the atmosphere. These gases all play a part in the complex chemistry which goes on in the lowest layer of the atmosphere, the troposphere.


Related pages:

Find out more about the amounts of trace gases in our atmosphere:
Lower Atmosphere - More - Unit 4

Find out more about particles
Clouds and Particles - Basics - Unit 2


About this page:

author: Dr. Elmar Uherek - Max Planck Institute for Chemistry -  Mainz / Germany
1. sci. reviewing: Dr. Katja Mannschreck - GAW Hohenpeissenberg - 2003-08-07
2. sci. reviewing: Dr. Gerd Folberth - Meteorological Service of Canada / Univ. of Victoria - 2003-08-10
edu. reviewing: Michael Seesing - Uni Duisburg - 2003-07-02
last published: 2004-06-12

Остання зміна: вівторок 25 червня 2019 20:04 PM