Geoscience Reference
In-Depth Information
box 2.1
significant
20th-c. advance
OZONE IN THE STRATOSPHERE
Ozone measurements were first made in the 1930s. Two properties are of interest: (i) the total ozone in an atmospheric
column. This is measured with the Dobson spectrophotometer by comparing the solar radiation at a wavelength where
ozone absorption occurs with that in another wavelength where such effects are absent; (ii) the vertical distribution
of ozone. This can be measured by chemical soundings of the stratosphere, or calculated at the surface using the
Umkehr
method; here the effect of solar elevation angle on the scattering of solar radiation is measured. Ozone
measurements, begun in the Antarctic during the International Geophysical Year, 1957-58, showed a regular annual
cycle with an austral spring (October-November) peak as ozone-rich air from mid-latitudes was transported poleward
as the winter polar vortex in the stratosphere broke down. Values declined seasonally from around 450 Dobson units
(DU) in spring to about 300 DU in summer and continued about this level through the autumn and winter. Scientists
of the British Antarctic Survey noted a different pattern at Halley Base beginning in the 1970s. In spring, with the
return of sunlight, values began to decrease steadily between about 12 and 20 km altitude. Also in the 1970s, satellite
sounders began mapping the spatial distribution of ozone over the polar regions. These revealed that low values formed
a central core and the term “Antarctic ozone hole” came into use. Since the mid-1970s, values start decreasing in late
winter and reach minima of around 100 DU in the austral spring.
Using a boundary of 220 DU (corresponding to a thin, 2.2-mm ozone layer, if all the gas were brought to sea level
temperature and pressure), the extent of the Antarctic ozone hole at the end of September averaged 21 million km
2
,
during 1990-99. This expanded to cover 27 million km
2
by early September in 1999 and 2000.
In the Arctic, temperatures in the stratosphere are not as low as over the Antarctic, but in recent years ozone
depletion has been large when temperatures fall well below normal in the winter stratosphere. In February 1996, for
example, column totals averaging 330 DU for the Arctic vortex were recorded compared with 360 DU, or higher, in
other years. A series of mini-holes was observed over Greenland, the northern North Atlantic and northern Europe
with an absolute low over Greenland below 180 DU. An extensive ozone hole is less likely to develop in the Arctic
because the more dynamic stratospheric circulation, compared with the Antarctic, transports ozone poleward from
mid-latitudes.
spring (October), when temperatures decrease to 185 to
195 K, 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. Nevertheless, 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 equilibrium above about 40 km. How-
ever, the ozone mixing ratio is at its maximum at
about 35 km, whereas maximum ozone concentration
(see Note 1) occurs lower down, between 20 and 25 km
in low latitudes and between 10 and 20 km 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 sealevel (at normal sea-level temperature
and pressure) ozone would contribute only about 3 mm
to the total atmospheric thickness of 8 km (Figure 2.3).
6 Variations with latitude and season
Variations of atmospheric composition with latitude and
season are particularly important in the case of water
vapour 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
