Geoscience Reference
In-Depth Information
2 Greenhouse gases
and vegetation burning) in the middle and upper
stratosphere.
In spite of their relative scarcity, the so-called
green-
house gases
play a crucial role in the thermodynamics
of the atmosphere. 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:
5
Chlorofluorocarbons (CFCs: chiefly CFCl
3
(F-12)
and CF
2
Cl
2
(F-12)) are entirely anthropogenically
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 photochemical
processes into chlorine after an estimated average
lifetime of some 65 to 130 years.
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
photosynthesis. The imbalance between emissions
and uptake by the oceans and terrestrial biosphere
leads to the net increase in the atmosphere.
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 sub-
stantial greenhouse effects. The role of
halogens
of carbon (CFCs and HCFCs) in the destruction of
ozone in the stratosphere is described below
2
Methane (CH
4
) is produced primarily through anaer-
obic (i.e. oxygen-deficient) processes by natural
wetlands and rice paddies (together about 40 per cent
of the total), as well as by enteric fermentation in
animals, by termites, through coal and oil extraction,
biomass burning, and from landfills.
7
Water vapour (H
2
O), the primary greenhouse gas,
is a vital atmospheric constituent. It averages about
1 per cent by volume but is very variable both in
space and time, being involved in a complex global
hydrological cycle (see Chapter 3).
CO
2
4H
2
→
CH
4
2H
2
O
Almost two-thirds of the total production is related
to anthropogenic activity.
3 Reactive gas species
Methane is oxidized to CO
2
and H
2
O by a complex
photochemical reaction system.
In addition to the greenhouse gases, important
reactive
gas species
are produced by the cycles of sulphur,
nitrogen and chlorine. These play key roles in acid
precipitation and in ozone destruction. Sources of these
species are as follows:
Nitrogen species
. The reactive species of nitrogen
are nitric oxide (NO) and nitrogen dioxide (NO
2
). NO
x
refers to these and other odd nitrogen species with
oxygen. Their primary significance is as a catalyst for
tropospheric ozone formation. Fossil fuel combustion
(approximately 40 per cent for transportation and 60
per cent for other energy uses) is the primary source of
NO
x
(mainly NO) accounting for ~25
10
9
kg N/year.
Biomass burning and lightning activity are other impor-
tant sources. NO
x
emissions increased by some 200
per cent between 1940 and 1980. The total source of
NO
x
is about 40
10
9
kg N/year. About 25 per cent
of this enters the stratosphere, where it undergoes
CH
4
O
2
2
x
→ CO
2
2
x
H
2
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 per cent) and industrial
processes. Other sources are transportation, biomass
burning, cattle feed lots and biological mechanisms
in the oceans and soils. It is destroyed by photo-
chemical reactions in the stratosphere involving the
production of nitrogen oxides (NO
x
).
4
Ozone (O
3
) is produced through the breakup 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
