Composition of the stratosphere

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Topic: Upper atmosphere. Basics.
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Most of the compounds released at the Earth's surface do not reach the stratosphere, instead they are:

  • decomposed by the main tropospheric oxidants (hydroxyl radicals - OH, nitrate radicals - NO3, ozone - O3)
  • broken down by sunlight
  • deposited back to the surface of the Earth in rain or as particles
  • trapped in the cold tropopause.

Because the temperature trend between the troposphere and the stratosphere reverses, there is almost no air exchange between these two layers.  Mixing of air in the troposphere takes hours to days whereas mixing in the stratosphere takes months to years.

One of the consequences of this lack of mixing between the troposphere and the stratosphere is that the water vapour content of the stratosphere is very low.  Typical mixing ratios (see below for definition) are in the range of 2 - 6 ppm (parts per million) compared to 100 ppm in the upper troposphere and 1,000 - 40,000 ppm in the lower troposphere, close to the surface of the Earth. This means that stratospheric clouds form very rarely and only if temperatures are so low that ice crystals grow.  These conditions generally only occur in the polar regions.  However, increasing water vapour concentrations due to emissions from aeroplanes and higher temperatures due to tropospheric warming below may lead to more polar stratospheric clouds being formed in the future.

polar stratospheric clouds

1. Polar stratospheric clouds over Kiruna / Sweden.
source: MPI Heidelberg

Inorganic compounds in the stratosphere

Stratospheric chemistry is dominated by the chemistry of ozone.  Between 85 and 90% of all the ozone in the atmosphere is found in the stratosphere. Ozone is formed when sunlight breaks down molecular oxygen (O2) in the stratosphere into oxygen atoms (O).  The highly reactive oxygen atoms then react with more molecular oxygen to form ozone (O3).   Most of the other gases in the stratosphere are either really long lived compounds emitted originally into the troposphere (such as the chlorofluorocarbons - CFC's) or are brought in by severe volcanic eruptions (generally sulphur containing compounds and aerosols).  So inorganic compounds such as ozone, nitrogen oxides, nitric acid, sulphuric acid, halogens and halogen oxides from CFC's are the dominant chemicals in the stratosphere.

Volcanic eruptions

Severe volcanic eruptions can inject large quantities of gases and particles directly into the stratosphere. These gases include the halogen containting acids, hydrochloric acid (HCl) and hydrofluoric acid (HF) and sulphur dioxide (SO2) which is converted to sulphuric acid (H2SO4), one of the compounds responsible for cloud formation.  The particles emitted include silicates and sulphates and these absorb sunlight in the stratosphere. Volcanic eruptions can, therefore, lead to a temporary warming in the stratosphere and a temporary cooling in the troposphere.  These effects on temperature can last around 1 - 2 years.  If the eruption is large enough, such the eruption of Mt. Pinatubo in the Philippines in June 1991, the effect can be seen over the whole hemisphere.

Mt Pinatubo eruption

2. Eruption of Mt. Pinatubo Philippines in June 2001.
source: Cascades Volcano Observatory USGS Photo by Rick Hoblitt.

Understanding concentrations and mixing ratios

We can express the amount of a compound in the atmosphere in two ways, relative and absolute:
a) mixing ratio = the fraction of the compound as a proportion of all the air molecules present. If there are 40 ozone molecules in 1 million air molecules the mixing ratio is 40 ppm (parts per million). This is relative.
b) concentration = the concentration of the molecules of the compound in a certain volume of air. If there are 100 molecules of ozone in one cubic meter of air, the concentration is 100 molecules m-3. This is absolute.

If you know the air pressure, it is possible to convert between the two units.

Pressure decreases with altitude, i.e. the higher we go in the stratosphere, the fewer molecules there are in each unit volume of air.  This means that if the absolute amount of ozone remains the same as the altitude increases, the mixing ratio for ozone also increases.

We can explain this general principle very simply. In a certain volume (light blue box) there is a certain number of air molecules (blue) and a certain number of ozone molecules (red). The number of air molecules decreases with altitude.

illustration of concentration and mixing ratio

3. Here the number of ozone molecules remains constant with altitude. As the total number of air molecules decreases with altitude, the ozone mxing ratio increases with altitude (see below).

illustration of concentration and mixing ratio

3. a) Simple ozone profile for the example above. The total concentration of air is given in blue, the ozone concentration in red and the ozone mixing ratio (% ozone) is shown in green. Since the number of ozone molecules stays constant but the total air concentration decreases, the mixing ratio increases with altitude.

illustration of concentration and mixing ratio

4. Here the absolute number of ozone molecules decreases in parallel with the decrease in the number of air molecules. As a result, the mixing ratio remains constant as the altitude increases.

illustration of concentration and mixing ratio

4. a) Simple ozone profile for the example above. The total concentration of air molecules is given in blue, the ozone concentration in red and the ozone mixing ratio (% ozone) in green. As the ozone concentration decreases in parallel with the decrease in the total concentration of air molecules, the ozone mixing ratio is constant with altitude.

Between the ground and the lower stratosphere, ozone mixing ratios tend to increase with altitude as ozone concentrations remain nearly constant but air becomes thinner.  In the lower stratosphere, ozone concentrations increase with altitude (the example below shows an increase of a factor of eight) increasing ozone mixing ratios further.  It is only above the ozone layer that mixing ratios are approximately constant with altitude.

ozone profile

5. Figure showing how the ozone mixing ratio and ozone concentration changes with altitude.
source: adapted from IUP Bremen.

About this page:
author: Dr. Elmar Uherek - Max Planck Institute for Chemistry, Mainz, Germany.
scientific reviewing: Dr. John Cowley- Max Planck Institute for Chemistry, Mainz - 2004-05-04
educational proofreading: Michael Seesing - Univ. of Duisburg, Germany.  Dr. Ellen K. Henriksen - Univ. of Oslo, Norway.  Yvonne Schleicher - Univ. of Erlangen-Nürnberg, Germany.
last published: 2004-05-05

Last modified: Sunday, 2 May 2021, 3:35 AM