Distribution & concentration (2)

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1. The main oxidants in the troposphere and how we observe them
Oxidation in the Atmosphere
Night-time conditions and chemistry
Measurement techniques - spectroscopy
Worksheet 1.1
Worksheet 1.2
2. Radiation, greenhouse gases and the Greenhouse Effect
The Earth's radiation budget and the Greenhouse Effect
The Greenhouse gases - carbon dioxide and methane
Water vapour and clouds
Worksheet 2.1
Worksheet 2.2
3. More on ozone and fire
Why is ozone dangerous?
Understanding tropospheric ozone abundance
Fire chemistry and its global importance
Worksheet 3.1
Worksheet 3.2
Worksheet 3.3
4. Gases in our atmosphere
Distribution & concentration (1)
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
NOy = 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
formula |
mixing ratio |
|
nitrogen |
N2 |
78.08 % |
oxygen |
O2 |
20.95 % |
argon |
Ar |
0.93 % |
water vapour |
H2O |
0.1 - 4 %
|
carbon dioxide |
CO2 |
385 ppm* |
carbon monoxide |
CO |
50 - 200 ppb |
methane |
CH4 |
1.7-1.8 ppm* |
hydrogen |
H2 |
0.5 ppm
|
ozone |
O3 |
10 -100 ppb
|
hydroxy radical |
OH |
< 0.01 - 1 ppt |
nitrogen dioxide |
NO2 |
1 - 10 ppb |
nitrogen oxide |
NO |
0.1 - 2 ppb |
nitrous oxide |
N2O |
320 ppb* |
nitrate radical |
NO3 |
5 - 450 ppt |
nitric acid |
HNO3 |
0.1-50 ppb |
ammonia |
NH3 |
< 0.02 - 100 ppb |
sulphur dioxide |
SO2 |
1 ppb (background)
|
formaldehyde |
HCHO |
0.5 - 75 ppb |
formic acid |
HCOOH |
< 20 ppb |
acetone |
CH3COCH3 |
0.1 - 5 ppb |
isoprene |
C5H8 |
< 1 - 50 ppb |
monoterpenes |
- |
< 100 ppt |
carbonyl sulfide |
COS |
500 +/- 50 ppt |
CFC11 |
CCl3F |
251* |
CFC12 |
CCl2F2 |
538* |
*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