Geology Reference
In-Depth Information
monoxide to carbon dioxide and water. Thus, although the chemical
elements are conserved, methane itself is destroyed and were it not
continuously replenished would disappear from the environment. The
breakdown of methane is an important source of water vapour in the
stratosphere, illustrating another, perhaps less obvious, connection be-
tween the cycles of different compounds.
Degradable chemicals that cease to be used will disappear from the
environment. Polychlorinated biphenyls (PCBs) are no longer used
industrially to any significant degree, having been replaced by more
environmentally acceptable alternatives. Their concentrations in the
environment are decreasing, although because of their slow degradabil-
ity (i.e. persistence), it will take many years before their levels decrease
below analytical detection limits.
The transfer of an element between different environmental compart-
ments, involving both chemical and biological processes, is termed
biogeochemical cycling. The biogeochemical cycles of the elements lead
and nitrogen will be discussed later in this chapter.
7.1.1 Environmental Reservoirs
To understand pollutant behaviour and biogeochemical cycling on a
global scale, it is important to appreciate the size and mixing times of the
different reservoirs. These are given in Table 1. The mixing times are a
very approximate indication of the timescale of vertical mixing of the
reservoir.
3
Global mixing can take very much longer as this involves
some very slow processes. These mixing times should be treated with
considerable caution as they oversimplify a complex system. Thus, for
example, a pollutant gas emitted at ground level mixes in the boundary
layer (approximately 0-1 km altitude) on a timescale typically of hours.
Mixing into the free troposphere (1-10 km) takes days, while mixing
into the stratosphere (10-50 km) is on the timescale of several years.
Thus, no one timescale describes atmospheric vertical mixing, and the
same applies to other reservoirs. Such concepts are useful, however,
Table 1 Size and vertical mixing of various reservoirs (from Brimblecombe
3
)
Mass (kg)
Mixing time (years)
4.2
10
15
Biosphere
60
5.2
10
18
Atmosphere
0.2
1.4
10
21
Hydrosphere
1600
2.4
10
22
43
10
7
Crust
4.0
10
24
4 10
8
Mantle
1.9
10
24
Core
