Geoscience Reference
In-Depth Information
Additional important cycles involve chlorine
(ClO) and bromine (BrO) chains at various
altitudes. Collisions with monatomic oxygen may
re-create oxygen (see Figure 2.2B ), but ozone
is mainly destroyed through cycles involving
catalytic reactions, some of which are photo-
chemical associated with longer wavelength ultra-
violet radiation (2.3-2.9μm). The destruction of
ozone involves a recombination with atomic
oxygen, causing a net loss of the odd oxygen. This
takes place through the catalytic effect of a radical
such as OH (hydroxyl):
decrease of stratospheric ozone over Antarctica
(see Box 2.1 ). A mechanism that may enhance the
catalytic process involves polar stratospheric
clouds. These can form readily during the austral
spring (October), when temperatures decrease to
185-195K, permitting the formation of particles
of nitric acid (HNO 3 ) ice and water ice. It is
apparent, however, that anthropogenic sources of
the trace gases are the primary factor in the ozone
decline. Conditions in the Arctic are somewhat
different, as the stratosphere is warmer and there
is more mixing of air from lower latitudes. Never-
theless, ozone decreases are now observed in the
boreal spring in the Arctic stratosphere.
The constant metamorphosis of oxygen to
ozone and from ozone back to oxygen involves a
very complex set of photochemical processes,
which tend to maintain an approximate equilib-
rium above about 40km. However, the ozone
mixing ratio is at its maximum at about 35km,
whereas maximum ozone concentration (see
Note 1) occurs lower down, between 20 and 25km
in low latitudes and between 10 and 20km in
high latitudes. This is the result of a circulation
mechanism transporting ozone downward to
levels where its destruction is less likely, allowing
an accumulation of the gas to occur. Despite the
importance of the ozone layer, it is essential to
realize that if the atmosphere were compressed to
sea level (at normal sea-level temperature and
pressure) ozone would contribute only about
3mm to the total atmospheric thickness of 8km
( Figure 2.3 ).
H + O
HO 2
HO 2 + O → OH + O 2
net: 2O
O 2
OH + O → H + O 2
The odd hydrogen atoms and OH result from the
dissociation of water vapor, molecular hydrogen
and methane (CH 4 ).
Stratospheric ozone is similarly destroyed in
the presence of nitrogen oxides (NO x , i.e., NO 2
and NO) and chlorine radicals (Cl, ClO). The
source gas of the NO x is nitrous oxide (N 2 O),
which is produced by combustion and fertilizer
use, while chlorofluorocarbons (CFCs), manu-
factured for 'freon', give rise to the chlorines.
These source gases are transported up to the
stratosphere from the surface and are converted
by oxidation into NO x , and by UV photo-
decomposition into chlorine radicals, respectively.
The chlorine chain involves:
2 (Cl + O 3
ClO + O 2 )
ClO + ClO → Cl2O 2
and
Cl + O 3
6 Variations with latitude and
season
ClO + O 2
OH + O 3 → HO 3 + 2O 2
Both reactions result in a conversion of O 3 to O 2
and the removal of all odd oxygens. Another cycle
may involve an interaction of the oxides of
chlorine and bromine (Br). It appears that the
increases of Cl and Br species during the decades
1970-1990 are sufficient to explain the observed
Variations of atmospheric composition with
latitude and season are particularly important in
the case of water vapor and stratospheric ozone.
Ozone content is low over the equator and
high in subpolar latitudes in spring (see Figure
2.3 ). If the distribution were solely the result of
photochemical processes, the maximum would
occur in June near the equator, so the anomalous
 
 
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