Geology Reference
In-Depth Information
chapter. In general the kinetic or reaction rate approach will be most
appropriate to the study of short-lived reactive substances, while the
equilibrium approach will be more applicable to long-lived substances.
7.2 RATES OF TRANSFER BETWEEN ENVIRONMENTAL
COMPARTMENTS
7.2.1 Air-Land Exchange
The land surface is an efficient sink for many trace gases. These are
absorbed or decomposed on contact with plants or soil surfaces. Plants
can be particularly active because of their large surface area and ability
to absorb water-soluble gases. The deposition process is crudely de-
scribed by the deposition velocity, n d ,
Flux ð mgm 2 s 1 Þ
Atmospheric concentration ð mgm 3 Þ
n d ð cm s 1 Þ¼
The term flux is analogous to a flow of material, in this case expressed as
micrograms of substance depositing per square metre of ground surface
per unit time. In the case of rough surfaces the square metre of area
refers to the area of hypothetical horizontal flat surface beneath the true
surface rather than the sum of the area of all the rough elements such as
plant leaves, which make up the true surface.
Since the deposition process itself causes a gradient in atmospheric
concentration, n d is defined in relation to a reference height, usually 1 m,
at which the atmospheric concentration is measured. For reasons de-
scribed later, n d is not constant for a given substance, but varies
according to atmospheric and surface conditions. However, some typ-
ical values are given in Table 2, which exemplify the massive variability.
Table 2 Some typical values of deposition velocity
Deposition velocity (cm s 1 )
Pollutant
Surfaces
SO 2
Grass
1.0
SO 2
Ocean
0.5
SO 2
Soil
0.7
SO 2
Forest
2.0
O 3
Dry grass
0.5
O 3
Wet grass
0.1
O 3
Snow
0.1
HNO 3
Grass
2.0
CO
Soil
0.05
Aerosol (42.5 mm)
Grass
0.1
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