Chemistry Reference
In-Depth Information
The temperature declines linearly with height
because the main heating mechanism is through
absorption of solar energy by the planet's surface,
followed by conduction and convection. Pressure
declines logarithmically with height and the vast
majority of the mass of the atmosphere is contained
in its lowest layer, which is also the initial recipient
of almost all the pollution.
Above the troposphere is the stratosphere. Here,
heating is due to absorption of solar UV by oxygen
and ozone molecules, which increases with increas-
ing distance from the planet's surface. Temperature,
therefore, increases with height and radiation inten-
sity. With hotter, less-dense strata on top there is less
convection and the effect of the Earth's rotation also
is felt less strongly at this greater distance. This leads
to less mixing and a layered structure. Some pollu-
tion is introduced directly into the stratosphere by
jet aeroplane exhausts and naturally by volcanoes.
Pollution also crosses the boundary from the tropo-
sphere. The latter normally is very slow but can
increase where tropospheric 'folding' occurs.
The atmosphere is the least massive of the Earth's
environmental compartments, being only 0.2% of
the mass of the hydrosphere (oceans plus terrestrial
waters) and 0.02% of the mass of the crust. It weighs
Pressure (Pa)
10
-6
10
-4
10
-2
10
0
10
2
10
4
200
1
8
0
160
Temperature
140
120
Turbopause
100
Mesopause
8
0
Mesosphere
60
Stratopause
40
Stratosphere
Pressure
Tropopause
20
Troposphere
100
200
300
400
500
600
700
Temperature (K)
Fig. 3.1
The vertical structure of the atmosphere and
associated temperature and pressure variations.
Table 3.1
Composition of unpolluted air
Gas
Formula
Concentration (ppm)
Approximate residence time
Nitrogen
N
2
7
8
0
8
20
10
6
years
Oxygen
O
2
209 450
5000 years
Argon
Ar
9 340
Not cycled
Carbon dioxide
CO
2
360
100 years
Neon
Ne
1
8
.1
8
Not cycled
Helium
He
5.24
Not cycled
Methane
CH
4
1.70
15 years
Krypton
Kr
1.14
Not cycled
Hydrogen
H
2
0.50
10 years
Nitrous oxide
N
2
O
0.31
120 years
Carbon monoxide
CO
0.12
65 days
Xenon
Xe
0.09
Not cycled
Ozone
O
3
0.01
100 days
Nitric oxide/nitrogen dioxide
NO/NO
2
10
-
6
/10
-
2
1 day
Sulfur dioxide
SO
2
10
-
3
-10
-
2
10 days
Ammonia
NH
3
10
-
4
-10
-
3
5 days
Nitric acid
HNO
3
10
-
5
-10
-
3
1 day
Water vapour
H
2
O
Variable between 5 000 and 40 000 ppm
Source: Refs 1 and 2.
