Geoscience Reference
In-Depth Information
their predominance, these gases are of little
climatic importance.
biological mechanisms in the oceans and soils.
It is destroyed by photochemical reactions in
the stratosphere involving the production of
nitrogen oxides (NO
x
).
4
Ozone (O
3
) is produced through the break up
of oxygen molecules in the upper atmosphere
by solar ultraviolet radiation and is destroyed
by reactions involving nitrogen oxides (NO
x
)
and chlorine (Cl) (the latter generated by
CFCs, volcanic eruptions and vegetation
burning) in the middle and upper stratosphere.
5
Chlorofluorocarbons (CFCs: chiefly CFCl
3
(F-
12) and CF
2
Cl
2
(F-12)) are entirely anthro-
pogenically produced by aerosol propellants,
refrigerator coolants (e.g., 'freon'), cleansers
and air-conditioners, and were not present in
the atmosphere until the 1930s. CFC molecules
rise slowly into the stratosphere and then move
poleward, being decomposed by photochem-
ical processes into chlorine after an estimated
average lifetime of some 65-130 years.
6
Hydrogenated halocarbons (HFCs and HCFCs)
are also entirely anthropogenic gases. They
have increased sharply in the atmosphere
over the past few decades, following their use
as substitutes for CFCs. Trichloroethane
(C
2
H
3
Cl
3
), for example, which is used in
dry-cleaning and degreasing agents, increased
fourfold in the 1980s and has a seven-year
residence time in the atmosphere. They
generally have lifetimes of a few years, but still
have substantial greenhouse effects. The role of
halogens
of carbon (CFCs and HCFCs) in the
destruction of ozone in the stratosphere is
described below.
Water vapor (H
2
O), the primary greenhouse
gas, is a vital atmospheric constituent. It averages
about 1 percent by volume but is very variable
both in space and time, being involved in a com-
plex global hydrological cycle (see Chapter 3).
2 Greenhouse gases
In spite of their relative scarcity, the so-called
greenhouse gases
play a crucial role in the thermo-
dynamics of the atmosphere (see
Box 2.1
). They
trap radiation emitted by the earth, thereby
producing the
greenhouse effect
(see Chapter 3C).
Moreover, the concentrations of these trace gases
are strongly affected by human (i.e., anthro-
pogenic) activities:
1
Carbon dioxide (CO
2
) is involved in a complex
global cycle (see 2A.7). It is released from the
earth's interior and produced by respiration of
biota, soil microbia, fuel combustion and
oceanic evaporation. Conversely, it is dissolved
in the oceans and consumed by plant photo-
synthesis. The imbalance between emissions
and uptake by the oceans and terrestrial
biosphere leads to the net increase in the
atmosphere
2
Methane (CH
4
) is produced primarily through
anaerobic (i.e., oxygen-deficient) processes by
natural wetlands and rice paddies (together
about 40 percent of the total), as well as by
enteric fermentation in animals, by termites,
through coal and oil extraction, biomass
burning and from landfills.
CO
2
+ 4H
2
→
CH
4
+ 2H
2
O
Almost two-thirds of the total production is
related to anthropogenic activity.
Methane is oxidized to CO
2
and H
2
O by a
complex photochemical reaction system.
CH
4
+ O
2
+ 2
x
→ CO
2
+ 2
x
H2
where
x
denotes any specific methane
destroying species (e.g.H, OH, NO, Cl or Br).
3
Nitrous oxide (N
2
O) is produced primarily
by nitrogen fertilizers (50-75 percent) and
industrial processes. Other sources are trans-
portation, biomass burning, cattle feed lots and
3 Reactive gas species
In addition to the greenhouse gases, important
reactive gas species
are produced by the cycles of
