Geology Reference
In-Depth Information
the trace gas as a volume mixing ratio. Thus, 1 cm
3
of pure sulfur dioxide
dispersed in 1 m
3
of polluted air would be described as a concentration of
1 ppm. Reference to the gas laws tells us that not only is this one part per
10
6
by volume, it is also one molecule in 10
6
molecules and one mole in
10
6
moles, as well as a partial pressure of 10
6
atm. Additionally, if the
temperature and pressure of the airmass change, this affects the trace gas
in the same way as the air in which it is contained and the volume-mixing
ratio does not change. Thus, ozone in the stratosphere is present in air at
considerably higher mixing ratios than in the lower atmosphere (tropo-
sphere), but if the concentrations are expressed in mgm
3
they are little
different because of the much lower density of air at stratospheric
attitudes. Chemical kineticists often express atmospheric concentrations
in molecules per cubic centimetre (molec cm
3
), which has the same
problem as the mass per unit volume units.
Worked Example
The concentration of nitrogen dioxide in polluted air is 85 ppb. Express
this concentration in units of mgm
3
and molec cm
3
if the air
temperature is 201C and the pressure 1005 mb (1.005
10
5
Pa). Relative
molecular mass of NO
2
is 46; Avogadro number is 6.022
10
23
.
The concentration of NO
2
is 85 mLm
3
.At201C and 1005 mb,
85 mLNO
2
weigh 46
85
10
6
22
:
41
273
293
1005
1013
¼
161
10
6
g
NO
2
concentration
¼
161 mgm
3
This is equivalent to 161 pg cm
3
, and
161 pg NO
2
contain 6
:
022
10
23
161
10
12
46
¼
2
:
1
10
12
molecules
and NO
2
concentration
¼
2.1
10
12
molec cm
3
.
1.4.2 Soils and Waters
Concentrations of pollutants in soils are most usually expressed in mass
per unit mass, for example, milligrams of lead per kilogram of soil.
Similarly, concentrations in vegetation are also expressed in mg kg
1
or
mgkg
1
. In the case of vegetation and soils, it is important to distinguish
