Distribution & concentration (2)

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It's impossible to give average atmospheric concentrations for the less stable compounds in the air.  Their concentrations depend strongly on the chemical conditions in the air at a particular time.

Vertical profiles

Sources, sinks and physical conditions (hours of sunshine, temperature, rainfall, wind strength and direction) affect both the horizontal and vertical distributions of a gas.  Vertical profiles often show slightly decreasing mixing ratios with increasing altitude particularly in the free troposphere and the stratosphere.  Ozone is an exception to this and has its highest mixing ratios and concentrations in the ozone layer in the stratosphere.  Most of the chemistry in the atmosphere happens much lower in the atmosphere, in the planetary boundary layer.  This is the layer of the atmosphere which is directly affected by the surface of the Earth and is where the chemical compounds are emitted.

1. Mixing ratios of different compounds during the night close to the ground.  The ratios are not measured but are the result of a chemical model which allows us to look at how the mixing ratios change over time.  Authors: Andreas Geyer, Shuihui Wang and Jochen Stutz.

The graphs below show the vertical profiles for several organic and inorganic trace gases in the troposphere measured by a research aeroplane.  Apart from carbon dioxide, ozone and methane, the typical values for mixing ratios are a few hundred parts per trillion (ppt) or a few parts per billion (ppb). The compounds shown are the more important trace gases, there are hundreds of other organic gases in the troposphere which have mixing ratios of just 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 = chloromethane = methyl chloride

2. a-c) Vertical profiles of various organic gases and a few inorganic gases.  The values were measured from a research aeroplane over the Mediterranean sea during the MINOS field campaign in August 2001. The strong black lines show the median vertical profiles, the thinner black lines give the standard deviation. Grey squares show values from canister samples. Red dashed lines and red squares are from another flight and give you an idea of how the values vary within a few days. Data and figures from: J. Lelieveld and co-authors.

Gases in the troposphere

Its very difficult to give an overview of the concentrations of trace gases in the troposphere.  The same compound can be present at extremely low concentrations, for example, over the ocean and at very high concentrations in the urban environment.   There are also many different gases which play an important role in the troposphere.  So the following table just gives a few examples of the average mixing ratios near the ground for commonly measured compounds.

Overview of important gases in the free atmosphere:

name

*The concentrations of gases with a star have increased as a result of human activity and are relatively well mixed over the globe. The percent values of the main components of air are given for dry air. Data from 2005-2007.

Mixing ratios, concentrations and different units:

Amounts of gases are often given in different units:

concentrations: molecules cm^-2 or µmol m^-3
or mixing ratios: ppt (pmol mol^-1), ppb (nmol mol^-1), ppm (µmol mol^-1), % (10 mmol mol^-1)

Mixing ratios are often more helpful for scientists. When air rises, it expands in volume and, as a result, the concentration of the gas changes.  The mixing ratio (relative proportion of the gas to the total number of air molecules), however, remains the same. 

Conversion from one unit to the other depends on the pressure (= the altitude) and the molecular weight of the compound. If we do the calculation for the surface of the Earth at a normal pressure of about 1 bar we can express the total molecules per volume of air in the following way:

1 mol = 22.4 L = 6x10²³ molecules =>
1 cm³ = 2.7 x 10¹⁹ molecules
1 dm³ = 1 L = 2.7 x 10²² molecules
1 m³ = 2.7 x 10²⁵ molecules

A rough estimate:

2 µg m-3 = 2 x 10-6 g m-3 NO2 is a typical value for nitrogen dioxide in a non-urban area.
the molecular weight of NO2 = 46 g mol-1
This means: 2 x 10-6 g m-3 = 4.3 x 10-8 mol m-3 = 2.6 x 1016 molecules m-3

So the mixing ratio is about 2.7 x 10¹⁶ / 2.7 x 10²⁵ = 10^-9 = 1 ppb

Since ozone has a similar molecular weight, M(O₃) = 48 g mol^-1, we can also say roughly that;
2 µg m^-3 of ozone = 1 ppb

This calculation is valid only for surface of the Earth where we live. So for ozone smog events in urban areas we can now calculate:
120 µg m^-3 = 60 ppb -> high levels
240 µg m^-3 = 120 ppb -> very high levels, no sports, risky for health
360 µg m^-3 = 180 ppb -> extremely high levels, very unhealthy for the lungs, stay at home!

Related pages

There is more about concentrations and mixing ratios at:
Upper 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