Lower Atmosphere

Read more

Distribution & concentration (2)

If we would like to know about concentrations of less stable compounds in the atmosphere, we cannot just give a number. They depend very much on the chemistry going on in the air.

Vertical profile

Not only the horizontal distribution can vary depending on sources, sinks and different physical parameters (hours of sunshine, temperature, precipitation, wind). Most gases have also a vertical profile and often show slightly decreasing mixing ratios with increasing altitudes, in particular above the tropopause and in the first two kilometres from the planetary boundary layer (influenced by the Earth surface) to the free troposphere. An exception is ozone, which has the highest mixing ratio and concentration in the ozone layer in the stratosphere. But we need not to go that high. Most of the chemistry is taking place in the boundary layer next to the ground, where the chemical compounds are emitted.

In the graphs below you see vertical tropospheric profiles of several organic and inorganic trace gases measured by a research plane. Apart from carbon dioxide, ozone and methane typical values for mixing ratios are a few hundred ppt or a few ppb. But the ones observed here are only the more important trace gases. There are hundreds of other organic gases which have lower mixing ratios of only a few ppt.

Measured compounds:

CH₄ = methane
CO = carbon monoxide
CH₃OH = methanol
CH₃COCH₃ = acetone
HCHO = formaldehyde

O₃ = ozone
NO = nitrogen oxide
NO_y = oxidised nitrogen compounds without NO, NO₂
PAN = peroxiacetylnitrate
CN = condensation nuclei (particles)

C₂H₆ = ethane
C₂H₂ = acetylene = ethyne
C₃H₈ = propane
C₆H₆ = benzene
CH₃Cl = methyl chloride

Gases in the troposphere

To give an overview of trace gases in the troposphere and their concentrations is hardly possible. The same compound can be present in extremely low concentrations for example over the ocean and in very high concentrations in the urban environment. Also, a few dozens of gases could be regarded as important. Therefore the following table can only be a compilation of examples, which gives a reasonable range of mixing ratios (usually near the ground) for often measured compounds.

Overview of important gases in the free atmosphere:

*Gases with increasing concentration due to human influence which are relatively well mixed over the globe. Data from 2001-2003.

Mixing ratios, concentrations and different units:

Amounts of gases are often given in different units:

concentrations: molecules/cm³ or µg/m³
or mixing ratios: ppt (pmol/mol), ppb (nmol/mol), ppm (µmol/mol), % (10 mmol/mol)

Mixing ratios are often more helpful for scientist, because if air rises and expands the volume grows and therefore the concentration change, but the mixing ratio (relation of the gases) remains the same.
The conversion from one unit into the other depends on the pressure (= the altitude) and molecular weight. If we do the calculation for the Earth surface for normal pressure of about 1 bar we can express the total molecules per air volume in the following way:

1 mol = 22.4 L = 6x10²³ molecules =>
1 cm³ = 2.7x10¹⁹ molecules
1 dm³ = 1 L = 2.7x10²² molecules
1 m³ = 2.7x10²⁵ molecules

Example for a rough estimate:

2 µg/m³ = 2x10^-6 g/m³ NO₂ is a typical value for nitrogen dioxide in a non-urban area.
molecular weight M(NO₂) = 46 g/mol
This means: 2x10^-6 g/m³ = 4.3x10^-8 mol/m³ = 2.6x10¹⁶ molec/m³

So the mixing ratio is about 2.7x10¹⁶ / 2.7x10²⁵ = 10^-9 = 1 ppb

Since ozone has a similar molecular weight, M(O₃) = 48 g/mol, we can also roughly say
2 µg/m³ of ozone = 1 ppb
This calculation is valid only for the Earth surface, where we live. So, for ozone smog events in urban areas we can now calculate:
120 µg/m³ = 60 ppb -> high level
240 µg/m³ = 120 ppb -> very high level, no sports, risk for health
360 µg/m³ = 180 ppb -> extremely high level, very unhealthy for the lung, stay at home!

Related pages

You will find more explanations about concentrations and mixing ratios in
Higher atmosphere - Basics - Unit 1 - composition

About this page:

author: Dr. Elmar Uherek - Max Planck Institute for Chemistry Mainz
scientific reviewer: Dr. Rolf Sander - Max Planck Institute for Chemistry, Mainz - 2004-05-18
last published: 2004-04-06